CA1265914A - Heating aluminum nitride with reducing additives to produce high thermal conductivity ceramics - Google Patents

Abstract

HIGH THERMAL CONDUCTIVITY CERAMIC BODY
ABSTRACT OF THE DISCLOSURE

A process for producing an aluminum nitride ceramic body having a composition defined and encompassed by polygon JKLM but not including line MJ of Figure 4 and a thermal conductivity greater than 1.00 W/cm?K at 25°C, preferably greater than 1.42 W/cm?K at 25°C, which comprises forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point L to less than point J of Figure 4, said compact having an equivalent % composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, heating said compact up to a temperature at which its pores remain open reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O
and N is defined and encompassed by polygon JKLM but not including line MJ of Figure 4, and sintering said deoxidized compact at a temperature of at least about 1860°C producing said ceramic body.

Description

~D-16,3 HIGH THERMAL CONDUCTIVITY CERAMIC BODY

The present invention relates to the production of a liquid phase sintered polycrystalline aluminum nitride body having a thermal conductivity higher than 1.00 W/cm~K
at 25C and preferably higher than 1.42 W/cm-K at 25C. In one aspect of the present process, aluminum nitride is deoxidized by carbon to a certain extent, and then it is further deoxidized and/or sintered by utilixing yttrium oxide to produce the present ceramic.
A suitably pure aluminum nitride single crystal, containing 300 ppm dissolved oxygen, has been measured to have a room temperature thermal conductivity of ~.8 W/cm-K, which is almost as hi~h as that of BeO single crystal, which is 3.7 W/cm~K, and much higher than that of ~-A12O3 single crystal, which is 0.44 W/cm-K. The ~hermal conductivity of an aluminum nitride single crystal is a strong function of dissolved oxygen and decreases with an increase in dissolved oxygen content. For example, the thermal conductivity of aluminum nitride single crystal having 0.8 wt% dissolved oxygen, is about 0.8 W/cm-R.
Aluminum nitride powder has an affinity for oxygen, especially whèn its surface is not covered by an oxide. The introduction of oxygen into the aluminum nitride lattice in aluminum nitride powder results in the formation of Al vacancies via the e~uation:

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~ RD-15,3~1 3N-3 ' 30-2 ~ V (1) (N-3) (N-3) (A1 3) Thus, the insertion of 3 oxygen atoms on 3 nitrogen sites will form one vacancy on an aluminum site. The presence of oxygen atoms on nitrogen sites will probably have a neglig-ible influence on the thermal conductivity of AlN. However, due to the large difference in mass between an aluminum atom and a vacancy, the presence of vacancies on aluminum sites has a strong influence on the thermal conductivity of AlN
and, for all practical purposes, is probably responsible for all of the decrease in the thermal conductivity of AlN.
There are usually three different sources of oxygen in nominally pure AlN powder. Source #l is discrete particles of A1203. Source #2 is an oxide coating, perhaps as A1203, coating the AlN powder particles. Source #3 is oxygen in solution in the AlN lattice. The amount of oxygen present in the AlN lattice in AlN powder will depend on the method of preparing the AlN powder. Additional oxygen can be introduced into the AlN lattice by heating the AlN powder at elevated temperatures. Measurements indicate that at -1900C the AlN lattice can dissolve ~1.2 wt% oxygen. In the present invention, by o~ygen content of AlN powder, it is meant to include oxygen present as sources #1, #2 and #3.
Also, in the present invention, the oxyqen present with AlN
powder as sources #1, ~2 and ~3 can be removed by utilizing free carbon, and the extent of the removal of oxygen by carbon depends largely on the composition desired in the resulting ~intered body.
According to the present invention, aluminum nitride powder can be processed in air and still produce a ceram~c ~ody having a thermal conductivity greater than 1.00

-2-,~

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: ' ' ~''''': : ' ~;5~4 P~ 16,3~1 W/cm-K at 25~C, and prefera~ly greater than 1.42 W/cm~K
at 25~C.
In one embodiment of the present ;.nvention, the aluminum nitri.de in a compact comprised of particulate aluminum nitride of known oxygen content, free carbon and yttrium oxide, is deoxidized by carbon to produce a desired equivalent composition of Al, N, Y and 0, and the doxidized compact is sintered by means of a liquid phase containing mostly Y and 0 and a smaller amount of Al and N.
Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjuinction with the figures accompanying and forming a part of the specification in which:
FIGURE 1 is a composition diagram (also shown as Figure 1 in U. S. Pat. 4,547,741 issued Oct. 15, 1985 and assigned to the assignee herein) showing the subsolidus phase equilibria in the reciprocal ternary system comprised of AlN, ~, Y203 and A1203.
Figure l is plotted in equivalent % and along each axis of ordinates the equi.valent % of oxygen is shown (the equivalent % of nitrogen is 100% minus the equivalent ~
of oxygen). Along the axis of abscissas, the equivalent % of yttrium is shown (the equivalent % of aluminum is 100% minus the equivalent ~ of yttrium). In Figure 1, line ABCDEF but not lines CD and EF encompasses and defines the composition of the sintered body of U.S.
Patent 4,547,741. Figure 1 also shows an example of an ordinates-joining straight line ZZ' joining the oxygen contents of an YN additive and an aluminum nitride powder. From the given equivalent % of yttrium and ~1 at any point on an ordinates-joining line passing through the polygon ABCDEF, the required amounts of yttrium additive and AlN for producing the composition of that point on the ordinates-joining line can be calculated;

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FIGURE 2 is an enlarged view of the section of Figure 1 showing the composition of the polycrystalline body of United States Patent 4,547,741.
FIGURE 3 is a composition diagram showing the subsolidus phase equilibria in the reciprocal ternary system comprised of AlN, YN, Y203 and A1203. Figure 3 is plotted in equivalent % and along each axis of ordinates the equiva-lent % of oxygen is shown (the equivalent % of nitrogen is 100% minus the equivalent % of oxygen). Along the axis of abscissas, the equivalent % of yttrium is shown (the equiva-lent % of aluminum is 100% minus the equivalent % of yttri-um). In Figure 3, line, i.e. polygon, J~LM but not includ-ing line MJ encompasses and defines the composition of the sintered body produced by the present process;
FIGURE 4 is an enlarged view of the section of Figure 3 showing polygon JKLM; and FIGURE 5 is a photomicrograph (magnified lOOOX) of a polished cross-section of a polycrystalline body produced by the present process wherein the lighter colored phase is AlN and the darker colored phase is a second phase comprised of Y203 and Y4A1209.
Figures 1 and 3 show the same composition diagram showing the subsolidus phase equilibria in the reciprocal ternary system comprised of AlN, YN, Y203 and A1203 and differ only in that Figure 1 shows the polygon ABCDEF of of the sintered body of--U.S. Pat. 4,547,741. Figure 1 also the polygon JKLM. The composition defined and encompassed by the polygon ABCDEF includes the composition o~ the polygon JKLM.
Figures 1 and 2 were developed algebraically on the basis of data produced by forming a particulate mixture of YN of predetermined oxygen content and AlN powder of predstermined oxygen content, and in a few instances a , ' , ''~' . . , . ' ' P~-16,~8i ~65~

mixture of AlN, YN and Y203 powders, under nitrogen gas, shaping the mixture into a compact under nitrogen gas and sint~ring the compact for time periods ranging from 1 to 1.5 hours at sintering temperatures ranging from about 1860C to about 2050C in nitrogen gas at ambient pressure. More specifically, the entire procedure ranging from mixing of the powders to sintering the compact formed therefrom was carried out in a nonoxidizing atmosphere of nitrogen.
Polygon JKLM of Figures 3 and 4 also was developed algebraically on the basis of data produced by the examples set forth herein as well as other experiments which included runs carried out in a manner similar to that of the present examples.
The best method to plot phase equilibria that involve oxynitrides and two different metal atoms, where the metal atoms do not change valence, is to plot the composi-tions as a reciprocal ternary system as is done in Figures 1 and 3. In the particular system of Figures 1 and 3 there are two types of non-metal atoms (oxygen and nitrogen) and two types of metal atoms (yttrium and aluminum). The Al, Y, oxygen and nitrogen are assumed to have a valence of +3, +3, -2, and -3, respectively. All of the Al, Y, oxygen and nitrogen are assumed to be present as oxides, nitrides or oxynitrides, and to act as if they have the aforementioned ~S valences.
The phase diagrams of Figures 1 to 4 are plotted in equivalent percent. The number of equivalents of each of these elements is equal to the number of moles of the particular element multiplied by its valence. Along the ordinate is plotted the number of oxygen e~uivalents multi-plied by 100% and divided by the sum of the oxygen equiva-lents and the nitrogen equivalents. Along the abscissa is plotted the number of yttrium equivalents multiplied by 100%

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RD 16,381 and divided by the sum of the yttrium equivalents and the alumlnum equivalents. All compositions of Figures 1 to 4 are plotted in this manner.
Compositions on the phase diagrams of Figures 1 to 4 can also be used to determine the weight percent and the volume percent of the various phases. For example, a particular polnt in the polygon JKLM in Figure 3 or 4 can be used to determine the phase composition of the polycrystalline body at that point.
Figures 1 to 4 show the composition and the phase equilibria of the polycrystalline body in the solid state.
In United States Patent 4,547,741 entitled "High Thermal Conductivity Aluminum Nitride Ceramic Body" issued October 15, 1985 in the names of I. C~
Huseby and C. F. Bobik and assigned to the assignee hereof there is disclosed the process for producing a polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line ABCDEF but not including lines CD and EF of Figure 1 therein ~also shown as prior art Figure 1 herein), a porosity of less than about 10~ by volume of said body and a thermal conductivity greater than 1.0 W/cm.K at 22C
which comprises forming a mixture comprised of aluminum nitride powder ~nd an yttrium additive selected ~rom the group consisting of yttrium, yttrium hydride, yttrium nitride and mixtures thereof, said mixture having a composition wherein the equivalent % of yttrium, aluminum, nitrogen and oxygen is defined and encompassed by line ABCDEF but not including lines CD and EF in Figure 1, shaping said mixture into a compact, and sintering said compact at a temperature ranging from about 1850C to about 2170C in an atmosphere selected , . `

P~-15,3al ~6~

from the group consisting of nitrogen, argon, hydrogen and mixtures thereof to produce said polycrystalline body United States Pat. 4,547,741 also discloses a polycrystalline body having a composition comprised of from greater than about 1 6 e~uivalent % yttrium to about 19 75 equivalent % yttrium, from about 80 25 equivalent % aluminum up to about 98 4 equivalent % aluminum, from greater than about 4 0 equivalent % oxygen to about 15 25 equivalent %
oxygen and from about 84 75 equivalent % nitrogen up to about 96 equivalent % nitrogen United-States Pat. 4,547,741 also discloses a polycrystalline body having a phase composition comprised of AlN and a second phase containing Y and 0 wherein the total amount of said second phase ranges from greater than about 4 2% by volume to about 27 3% by volume of the total volume of said body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than l 0 W/cm-K at 22C
Briefly stated, the present process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line, i e polygon, JKLM but not including line MJ of Figures 3 or 4, a porosity of less than about 10% by volume, and preferably less than about 5% by volume, of said body and a thermal conductivity greater than 1 00 W/cm-K at 25C, and prefera-bly greater than 1 42 W/cm-K at 25C, comprises the steps (a) forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide or a precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C to about lOOO~C to free carbon and gaseous ~, .;~

, ~ :

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product of decomposition which vaporizes away, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point L up to point J of Figures 3 or 4, i.e. from greater than about 2.5 equivalent ~ to about 4.9 equivalent % yttrium and from about 95.1 e~uivalent % to less than about 97.~ eguivalent % aluminum, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon JKLM of Figures 3 or 4, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, (c) heating said compact in a nitrogen-containing 1~ nonoxidizing atmosphere at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon JKLM but not including line MJ of Figure 3 or 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of atleast about L860C, and preferably at lea~t about 1890C, producing said polycrystalline body.
In the present process, the composition of the deoxidized compact in equivalent % is the same as or does not differ significantly from that of the resulting sintered body in e~uivalent %.
In the present invention, oxygen content can be determined by neutron activation analysis.

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By weight % or % by weight of a component herein, it is meant that the total weight % of all the components is 100%.
By ambient pressure herein, it is meant atmospher-ic or about atmospheric pressure.
By specific surface area or surface area of a powder herein, it is meant the specific surface area accord-ing to BET surface area measurement.
Rriefly stated, in one embodiment, the present process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line, i.e. polygon, UMJV but not including line MJ of Figures 3 or 4, a porosity of less than about 10%
by volume, and preferably less than about 2% by volume, of said body and a thermal conductivity greater than 1.00 W/cm^K at 25C, and preferably greater than 1.42 W/cm-K at 25C, comprises the steps:
(a) forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide or a precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C to about lOOO~C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific surface area greater than about 20 m /g, the aluminum nitride powder in said mixture having a spec fic surface area ranging from about 3.4 m2/g to about 10 m /g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the e~uivalent % of yttrium and aluminum ranges ~rom point U up to point J
of Figures 3 or 4, i.e. from greater than about 2.5 e~uivalent % to about 4.35 equivalent % yttrium and from ....

., ~ .. . . .

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about 95.65 equivalent % to less than about 97.5 equivalent % aluminum, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon J~LM of Figures 3 or 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, (c) heating said compact at ambient pressure in a nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350C
to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a compo-sition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon UMJV but not including line MJ of Figure 3 or 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume ~itrogen at a temperature ranging from about 1900C to about 2050C, preferably from about 1900C to about 1960C, producing said polycrystalline body.
Briefly stated, in another embodiment, the present process for producing a sintered polycrystalline aluminum .

P~-lS ~i , ~Z~5~

nitride ceramic body having a composition defined and encompassed by line, i.e. polygon, JKLM but not including line MJ of Figures 3 or 4, a porosity of less than about 10%
by volume, and preferably less than about 5% by volume, of said body and a thermal conductivity greater than 1.00 W/cm-K at 25C, and preferably greater than 1.42 W/cm-K at 25C, comprises the steps:
(a) processing an aluminum nitride powder into a compact for deoxidation by free carbon by providing a starting oxygen-containing aluminum nitride powder having an oxygen content up to about 4.4% by weight of said aluminum nitride powder, forming a mixture comprised of said aluminum nitride powder, yttrium oxide or precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C to about 1000C to free carbon and gaseous product of decompo-sition which vaporizes away, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point L up to point J of Figures 3 or 4, i.~. from greater than about 2.5 equivalent % to about 4.9 eguivalent %
yttrium and from about 95.1 equivalent % to less than about 97.5 equivalent % aluminum, said compact having an equiva-lent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon JKLM of Figures 3 or 4, during said processing said aluminum nitride picking up oxygen, the oxygen content of said aluminum nitride in said compact before said deoxidation by carbon ranging from greater than about 1.0% by weight, and preferably greater than about 1.40% by weight, up to about 4.5% by weight of said aluminum nitride, , :, :
, , .
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(b) he~ting said compact in a nonoxidizing atmosphere at a temperature up to about 1200C thereby providiny yttrium oxide and free carbon, (c) heating said compact in a nitrogen-containing nonoxidizing atmosphere at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon JKLM but not including line MJ of Figure 3 or 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of atleast about 1860C, and preferably at least about 1890C, producing said polycrystalline body.
Briefly stated, in another embodiment, the present process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon UMJV but not including line MJ of Figures 3 or 4, a porosity of less than about 10% by volume, and preferably less than about 2% by volume, of said body and a thermal conductivity greater than 1.00 W/mc~K at 25~C, ~5 and preferably greater than 1.42 W/cm-K at 25C, eomprises the steps:
(a) processing an aluminum nitride powder into a compact for deoxidation by free carbon by providing a starting aluminum nitride powder having an oxygen content ranging from greater than about 1.0% by weight to less than about 4% by weight of said aluminum nitride powder, forming a mixture comprised of said aluminum nitride powder, yttrium oxide or precursor therefor, and a carbonaceous additive , :
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selected from the group consisting of free carbon, a carbon-aceous organic material and mixtures thereof, said carbon-aceous organic material thermally decomposing at a temperature ranging from about 50C to about 1000C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific ~urface area greater than about 20 m /g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 10 m2/g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J of Figures 3 or 4, i.e. from greater than about 2.5 equivalent % to about 4.35 equivalent %
yttrium and from about 95.65 equivalent % to less than about 97.5 equivalent % aluminum, said compact having an equiva-lent % composition o~ Y, Al, o and N outside the composition defined and encompassed by polygon JKLM of Figures 3 or 4, during said processing said aluminum nitride picking up oxygen, the oxygen content vf said aluminum nitride in said compact before said deoxidation by carbon ranging from greater than about 1.40% by weight up to about 4.5% by weight of said aluminum nitride and being greater than said oxygen content of said starting aluminum nitride powder by an amount ranging from greater than about 0.03% by weight up to about 3% by weight of said aluminum nitride, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, (c) heating said compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by ~olume nitrogen at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby " .
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reacting said free carbcn with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon UMJV but not including line MJ of Figure 3 or 4, said free carbon being in an amount which produc~s said deoxidized compact, and (d) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about l900~C to about 2050~C, preferably from about 1900C to about 1960C, producing said polycrystalline body.
In another embodiment of the present process, said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges between points L
and J but does not include points L and J of Figure 4, said yttrium in said compact ranging from greater than about 2.5 equivalent % to less than about 4.9 equivalent %, said aluminum in said compact ranging from greater than about 95.1 equivalent % to less than about 97.5 equivalent %, and said sintered body and said deoxidized compact are comprised of a composition wherein the e~uivalent percent of A1, Y, 0 and N is defined and encompassed by polygon JKLM but does ~5 not include lines MJ and LK of Figure 4.
In yet another embodiment of the present process, said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point L to point K of Figure 4, said yttrium in said compact ranges from about 4.4 equivalent % to about 4.9 equivalent %, said aluminum in said compact ranges from about 95.1 e~uivalent %
to about 95.6 equivalent %, and said sintered body and said deoxidized compact are comprised of a composition wherein . . .
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: .
. .

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~ G-16,3~1 the equivalent percent of Al, Y, Q and N is defined by line LK of Figure 4.
The calculated compositions of particular points in Figures 3 or 4 in the polygon JKLM are shown in Table I
as follows:
TA~LE I
Composition (Equivalent %) Vol ~ and (Wt ~ of Phases*
10Point Y Oxygen AlN Y203 Y4A1209 J 2.5 4.194.0(91-9) ~ 6.0( 8.1) K 4.4 4.492.4(8~.7)7.6(11.3) L 4.9 4.991.6(87.6)8.4(12.4) M 4.0 6.390.6(87.4) - 9.4(12.6) U 4.35 5.7591.1 4.5 4.4 V 3.1 4.2 93.2 3.4 3.4 * - Wt % is given in parentheses, Vol % is given without parentheses The polycrystalline aluminum nitride body produced by one embodiment of the present process has a composition defined and encompassed by polygon, i.e. line, JKLM but not including line MJ of Figures 3 or 4. The sintered polycrys talline body of polygon JKLM but not including line MJ of Figures 3 or 4 produced by the present process has a compo-sition comprised of ~rom greater than about 2.5 equivalent %
yttrium to about 4.9 eguivalent % yttrium, from about 95.1 equivalent % aluminum up to about 97.5 equivalent % alumi-num, from greater than about 4.1 equivalent % oxygen to less than about 6.3 equivalent % oxygen and from greater than .

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about 93.7 equivalent % nitrogen up to about 95.9 equivalent % nitrogen.
Also, the polycrystalline body defined and encom-passed'by polygon JKLM but not including line MJ of Figure 3 or 4 is comprised of an AlN phase and a second phase con-taining yttrium and oxygen. The second phase can be Y203 or a mixture of Y2O3 and Y4A12Og which always contains Y2O3 in at least a trace amount, i.e. at least an amount detectable by X-ray diffraction analysis, and the second phase ranges from greater than about 6% by volume to less than about 9.4%
by volume of the total volume of the sintered body.
As can be seen from Table I, the polycrystalline body closest to point M composition would have the largest amount of second phase present.
In another embodiment, the polycrystalline alumi-num nitride body produced by the present process has a composition defined and encompa~sed by polygon, i.e. line, UMJV but not including line MJ of Figures 3 or 4. The sintered polycrystalline body of polygon UMJV but not including line MJ of Figures 3 or 4 produced by the present process has a composition comprised of from greater than about 2.5 equivalent % yttrium to about 4.35 equivalent %
yttrium, from about 95.65 equivalent % aluminum up to about 97.5 equivalent % aluminum, from greater than about 4.1 equivalent % oxygen to l~ss than about 6.3 e~uivalent %
oxygen and from greater than about g3.7 equivalent % nitro-gen to less than about 95.9 equivalent % nitrogen.
Also, the polycrystalline body defined and encom-passed by polygon UMJV but not including line MJ of Figure 3 or 4 is comprised of an AlN phase and a second phase com~
prising a mixture of Y203 and Y4A1209. This second phase mixture ranges from greater than about 6% by volume to less than about 9.4% by volume of the total volume of the . , :, . , :, ~ , ., ,, :
.. . . .

2~-16,3~1 sintered body, wherein Y2O3 is always present in at least a trace amount, i.e. at least an amount detectable by X-ray diffraction and wherein the Y4Al~Og phase is always present in an amount greater than about 3.4% by volume of the S sintered body.
In one embodiment, the present polycrystalline body has a composition defined and encompassed by polygon JKLM but not including lines MJ or LK of Figures 3 or 4, i.e. it has a composition comprised of from greater than about 2.5 equivalent % yttrium to less than about 4.9 equivalent % yttrium, from greater than 95.1 equivalent %
aluminum to less than about 97.5 equivalent % aluminum, from greater than about 4.1 equivalent % oxygen to less than 6.3 equivalent % oxygen and from greater than about 93.7 equiva-lent % nitr~gen to less than about 95.9 equivalent % nitro-gen. In this embodiment, the phase composition of the sintered body is comprised of AlN and a second phase com-prised of a mixture of Y2O3 and Y4A12Og. This second phase mixture ranges in amount from greater than about 6% by volume to less than about 9.4% by volume of the body and always contains Y2O3 and Y4Al2Og at least in a trace amount, i.e. at least in an amount detectable by X-ray diffraction analysis.
In another embodiment, the present process pro-duces a sintered body defined by line LK of Figure 4 which has a phase ~omposition comprised of AlN and Y2O3 wherein the Y203 phase ranges from about 7.6% by volume to about 8.4% by volume of the body. Line LK of Figure 4 has a composition comprised of from about 4.4 equivalent % to about 4.9 equivalent % yttrium, from about 95.1 e~uivalent %
to about 95.6 equivalent % yttrium, from about 4.4 eguiva-lent % to about 4.9 equivalent % oxygen and from about 95.l equivalent % to about 95.6 eguivalent % nitrogen.

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, ~ 6,3 In the present process, the aluminum nitride powder can be of commercial or technical grade. Specifical-ly, it should not contain any impurities which would have a significantly deleterious effect on the desired properties of the resulting sintered product, and preferably it is greater than about 99% pure AlN excluding oxygen. The starting aluminum nitride powder used in the present process contains oxygen generally ranging in amount up to about 4.4%
by weight, and usually ranging from greater than about 1.0%
by weight to less than about 4.0% weight, i.e. up to about ~% by weight. Typically, commercially available aluminum nitride powder contains from about 1.5 weight % (2.6 equiva-lent %) to about 3 weight % (5.2 equivalent %) of oxygen and such powders are most preferred on the basis of their substantially lower cost.
The oxygen content of aluminum nitride is deter-minable by neutron activation analysis. -Generally, the present starting aluminum nitride powder has a specific surface area which can range widely, and generally it ranges up to about 10 m2/g. Frequently, it has a specific surface area greater than about 1.0 m2/g, and more frequently of at least about 3.0 m2/g, usually greater than about 3.2 m2/g, and preferably at least about 3.4 m2/g.
Generally, the present aluminum nitride powder in the present mixture, i.e. after the components have been mixed, usually by miLling, has a specific surface area which oan range widely, and generally it ranges to about 10 m2/g.
Freguently, it ranges from greater than about 1.0 m2/g to about lO m /g, and more fre~uently from about 3.2 m2/g to about 10 m2/g, and preferably it ranges from about l.5 m2/g to about 5 m2/g, and in one embodiment it ranges f~om about

3.4 mZ/g to about 5 m2/g, according to BET surface area measurement. Specifically, the minimum sintering .

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~5~

temperature of a given composition of the present invention increases with increasing particle size of the aluminum nitride.
Generally, the yttrium oxide (Y203) additive in the present mixture has a specific surface area which can range widely. Generally, it ranges from greater than about 0.4 m2/g to about 6.0 m2/g, preferably from about 0.6 m2/g to about 5.0 m2/g, more preferably from about 1.0 m2/g to about 5.0 m2/g, and in one embodiment it is greater than 2.0 10 m2/g In the practice of this invention, carbon for deoxidation of aluminum nitride powder is provided in the form of free carbon which can be adde~ to the mixture as elemental carbon, or in the form of a carbonaceous additive, for example, an organic compound which can thermally decom pose to provide free carbon.
The present carbonaceous additive is selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof. The carbonaceous organic material pyrolyzes, i.e. thermally decomposes, completely at a temperature ranging from about 50C to about lOOODC to free carbon and gaseous product of decomposition which vaporizes away. In a preferred embodiment, the carbonaceous additive is free carbon, and preferably, it is graphite.
High molecular weight aromatic compounds or materials are the preferred carbonaceous organic materials for making the present free carbon addition since they ordinarily give on pyrolysis the required yield of particu-late free carbon of submicron si7e. Examples of such aromatic materials are a phenolformaldehyde condensate resin known as Novolak which is soluble in acetone or higher alcohols, such as butyl alcohol, as well as many of the related condensation polymers or resins such as those of .~

;' ' ' ~ ~
' ' ' ~ ' ' "~ ' ~-15,3 resorcinol~formaldehyde, aniline-formaldehyde, and cresol-formaldehyde. Another satisfactory group of materials are derivatives of polynuclear aromatic hydrocarbons contained in coal tar, such as dibenzanthracene and chrysene. A
preferred group are polymers of aromatic hydrocarbons such as polyphenylene or polymethylphenylene which are soluble in aromatic hydrocarbons.
The present free carbon has a specific surface area which can range widely and need only be at least sufficient to carry out the present deoxidation. Generally, it has a specific surface area greater than about 10 m2/g, preferably greater than 20 m2/g, and more preferably greater than 150 m /g, according to BET surface area measurement to insure intimate contact with the AlN powder for carrying out its deoxidation.
By processing of the aluminum nitride powder into a compact for deoxidation by free carbon, it is meant herein to include all mixing of the aluminum nitride powder to produce the present mixture, all shaping of thé resulting mixture to produce the compact, as well as handling and storing of the compact before it is deoxidized by carbon.
In the present process, processing of the aluminum nitride powder into a compact for deoxidation by free carbon is at least partly carried out in air, and during such processing of the aluminum nitride powder, it picks up oxygen from air usually in an amount greater than about 0.03% by weight of the aluminum nitride, and any such pick up of oxygen is controllable and reproducible or does not differ signifi-cantly if carried out under the same conditions. If de-sired, the processing o~ the aluminum nitride powder into acompact for deoxidation by free carbon can be carried out in air.

.

.

.

P.~-15,3~, In the present processing of aluminum nitride, the oxygen it picks up can be in any form, i.e. ~t initially may be oxygen, or initially it may be in some other form, such as, for example, water. The total amount of oxygen picked up by aluminum nitride from air or other media generally is less than about 3.0% by weight, and generally ranges from greater than about 0.03% by welght to less than about 3.0%
by weight, and usually it ranges from about 0.1% by weight to about 1% ~y weight, of the total weight of the aluminum nitride. Generally, the aluminum nitride in the present mixture and compact prior to deoxidation of the compact have an oxygen content of less than about 4.5% by weight, and generally it ranges from greater than about 1.00% by weight to less than about 4.5% by weight, and usually it ranges from greater than 1.40% by weight to less than about 4.5% by weight, and more usually it ranges from about 1.9% by weight to about 4.0% by weight, of the total weight of aluminum nitride.
In a compact an aluminum nitride containing oxygen in an amount of about 4.5% by weight or more generally is not desirable.
In carrying out the present process, a uniform or at least a significantly uniform mixture or dispersion of the aluminum nitride powder, yttrium oxide powder and carbonaceous additive, generally in the form of free carbon powder, is formed and such mixture can be formed by a number of techniques. Preferably, the powders are ball milled preferably in a liquid medium at ambient pressure and temperature to produce a uniform or significantly uniform dispersion. The milling media, which usually are in the form of cylinders or balls, should have no significant deleterious effect on the powders, and preferably, they are comprised of polycrystalline aluminum nitride or steel.

` '' . ~: ~ ';
: .
~ : `

P~v-1~,3~1 Generally, the milling media has a diameter of at least about 1/4 inch and usually ranges from about 1/4 inch to about l/2 inch in diameter. The liquid medium should have no significantly deleterious effect on the powders and preferably it is non-aqueous Preferably, the liguid mixing or milling medium can be evaporated away completely at a temperature ranging from above room or ambient temperature to below 300~C leaving the present mixture. Preferably, the liquid mixing medium is an organic liquid such as heptane or hexane. Also, preferably, the liquid milling medium contains a dispersant for the aluminum nitride powder thereby producing a uniform or significantly uniform mixture in a significantly shorter period of milling time. Such dispersant should be used in a dispersing amount and it should evaporate or decompose and evaporate away completely or leave no significant residue, i.e. no residue which has a significant effect in the present process, at an elevated temperature below 1000C. Generally, the amount of such dispersant ranges from about 0.1% by weight to less ~han about 3% by weight of the aluminum nitride powder, and generally it is an organic liquid, preferably oleic acid.
In using steel milling media, a residue of steel or iron is left in the dried dispersion or mixture which can range from a detectable amount up to about 3.0% by weight of the mixture. This residue of steel or iron in the mixture has no significant effect in the present process or on the thermal conductivity of the resulting sintered body.
The liquid dispersion can be dried by a number of conventional techniques to remove or evaporate away the liquid and produce the present particulate mixture. I~
desired, drying can be carried out in air. Drying of a milled liquid dispersion in air causes the aluminum nitride to pick up oxygen and, when carried out under the same .

'~

RD-iF`, ~-1 conditions, such oxygen pick up is reproducible or does not differ significantly. Also, if desired, the dispersion can be spray dried.
A solid carbonaceous organic material is prefera-bly admixed in the form of a solution to coat the aluminumnitride particles. The solvent preferably is non-aqueous.
The wet mixture can then be treated to remove the solvent producing the present mixture. The solvent can be removed by a number of techniques such as by evaporation or by freeze drying, i.e. subliming off the solvent in vacuum from the frozen dispersion. In this way, a substantially uniform coating of the organic material on the aluminum nitride powder is obtained which on pyrolysis produces a substan-tially uniform distribution of free carbon.
The present mixture is shaped into a compact in air, or includes exposing the aluminum nitride in the mixture to air. Shaping of the present mixture into a compact can be carried out by a number of technigues such as extrusion, injection molding, die pressing, isostatic pressing, slip casting, roll compaction or forming, or tape casting to produce the compact of desired shape. Any lubricants, binders or similar materials used to aid shaping of the mixture should have no significant deteriorating effect on the compact or the present resulting sintered body. Such shaping-aid materials are preferably of the type which evaporate away on heating at relatively low temperatures, preferably below 4~0C, leaving no significant residue. Preferably, the compact has a porosity of less than 60% and more preferably less than 50% to promote densification during sintering.
If the compact contains carbonaceous organic material as a source of free carbon, it is heated at a : temperature ranging from about 50~C to about 1000C to ,,.

; , ~ ,.

~ ,3~'~
_ _ _ 3~

pyroly~e, i e. thermally decompose, the organic material completely producing the present free carbon and gaseous product of decomposition which vaporizes away. Thermal decomposition of the carbonaceous organic material is S carried out, preferably at ambient pressure, in a non-oxidizing atmosphere. Preferably, thermal decomposition is carried out in an atmosphere selected from the group con-sisting of nitrogen, hydrogen, a noble gas such as argon and mixtures thereof, and more preferably it is nitrogen, or a mixture of at least about 25% by volume nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon and mixtures thereof. In one embodiment, it is a mixture of nitrogen and from about 1% by volume to about 5% by volume hydrogen.
The actual amount of free carbon introduced by pyrolysis of the carbonaceous organic material can be determined by pyrolyzing the organic material alone and determining weight loss. Preferably, thermal decomposition of the organic material in the present compact is done in the sintering furnace as the temperature is being raised to deoxidizing temperature, i.e. the temperature at which the resulting free carbon reacts with the oxygen content of the AlN.
Alternately, in the present process, yttrium oxide can be provided by means of an yttrium oxide precursor. The term yttrium oxide precursor means any organic or inorganic compound which decomposes completely at a temperature below about 1200C to form yttrium oxide and by-product gas which vaporizes away leaving no cuntaminants in the sintered body which would be detrimental to the thermal conductivity.
Representative of the precursors of yttrium oxide useful in the present process is yttrium acetate, yttrium carbonate, -2~-R~~16,~i yttrium oxalate, yttrium nitrate, yttrium sulfate and yttrium hydroxide.
If the compact contains a precursor for yttrium oxide, it is heated to a temperature up to about 1200C to thermally decompose the precursor thereby providing yttrium oxide. Such thermal decomposition is carried out in a non-oxidizing atmosphere, preferably at ambient pressure, and preferably selected from the group consisting of nitro-gen, hydrogen, a noble gas such as argon and mixtures thereof. Preferably, it is nitrogen, or a mixture of at least about 25% by volume nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon and mixtures thereof. In one embodiment, it is a mixture o nitrogen and from about 1% by volume to about 5% by volume hydrogen.
The present deoxidation of aluminum nitride with carbon, i.e. carbon-deoxidation, comprises heating the compact comprised of aluminum nitride, free carbon and yttrium oxide at deoxidation temperature to react the free carbon with at least a sufficient amount of the oxygen contained in the aluminum nitride to produce a deoxidized compa~t having a composition defined and encompassed by polygon JKLM but not including line MJ of Figures 3 or 4.
This deoxidation with carbon is carried out at a temperature ranging from about 1350C to a temperature at which the pores of the compact remain open, i.e. a temperature which is sufficient to deoxidize the compact but below the pore closing temperature, generally up to about 1800C, and preferably, it is carried out at from about 1600C to 1650C.
The carbon-deoxidation is carried out, preferably at ambient pressure, in a gaseous nitro~en-containing nonoxidizing atmosphere which contains sufficient nitrogen ~ . , .

.
~:, ~' , :, ' ' ~--1~,3~1 ~65~

to facilitate the deoxidation of the aluminum nitride. In accordance with the present invention, nitrogen is a re-quired component for carrying out the deoxidation of the compact. Preferably, the nitrogen-containing atmosphere is nitrogen, or it is a mixture of at least about 25% by volume of nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon, and mixtures thereof.
Also, preferably, the nitrogen-containing atmosphere is comprised of a mixture of nitrogen and hydrogen, especially a mixture containing up to about 5% by volume hydrogen.
The time required to carry out the present carbon-deoxidation of the compact is determinable empirically and depends largely on the thickness of the compact as well as the amount of free carbon it contains, i.e. the carbon-deox-idation time increases with increasing thickness of thecompact and with increasing amounts of free carbon contained in the compact. Carbon-deoxidation can be carried out as the compact is being heated to sintering temperature provid-ed that the heating rate allows the deoxidation to be completed while the pores of the compact are open and such heating rate is determinable empirically. Also, to some extent, carbon deoxidation time depends on deoxidatlon temperature, particle size and uniformity of the particulate mixture of the compact, i.e. the higher the deoxidation temperature, the smaller the particle size and the more uniform the mixture, the shorter is deoxidation time. Also, to some extent, deoxidation time depends on its final position on the phase diagram, i.e. as line LK is approach-ed, deoxidation time increases. Typically, the carbon-deox-idation time ranges ~rom about ~ hour to about 1.5 hours.
Preferably, the com~act is deoxidized in thesintering furnace by holding the compact at deoxidation temperature for the re~uired time and then raising the . .

P.~-i6,3~1 _ _ ~265~

temperature to sintering temperature. The deoxidation of the compact must be completed before sintering closes off pores in the compact preventing gaseous product from vapor-izing away and thereby preventing production of the present sintered body.
In the present deoxidation with carbon, the free carbon reacts with the oxygen of the aluminum nitride producing carbon monoxide gas which vaporizes away. It is believed that the following deoxidation reaction occurs wherein the oxygen content of the aluminum nitride is given 23 3C ~ N2 ~ 3C0(g) 1 2AlN (2) In the deoxidation effected by carbon, gaseous carbon~containing product is produced which vaporizes away thereby removing free carbon.
If the compact before deoxidation is heated at too fast a rate through the carbon-deoxidation temperature to sintering temperature, and such too fast rate would depend largely on the composition of the compact and the amount of carbon it contains, the present carbon-deoxidation does not occur, i.e. an insufficient amount of deoxidation occurs, and a significant amount of carbon is lost by reactions (3) and/or (4).

C t AlN ' AlCN(g) (3) C + 1/2 N2 ~ CN(g) (4) The specific amount of free carbon reguired to produce the present deoxidized compact can be determined by ~ -27-' :- - ' . .
:: . - ~ , : , "
.. . .

P.~-15,331 ~S~

a number of techniques. It can be determined empirically.
~referably, an initial ~pproximate amount of carbon is calculated from Equation (2), that is the stoichiometric amount for carbon set forth in Equation (2), and using such approximate amount, the amount of carbon re~uired in the present process to produce the present sintered body would require one or a few runs to determine if too much or too little carbon had been added. Specifi~ally, this can be done by determining the porosity of the sintered body and by analyzing it for carbon and by X-ray diffraction analysis.
If the compact contains too much carbon, it will be diffi-cult to sinter and will not produce a sintered body having a porosity of less than about 10% by volume, and preferably less than about 5% by volume of the sint0red body, or it will contain carbon in an excessive amount. If the compact contains too little carbon, X-ray diffraction analysis of the resulting sintered body will not show any Y203 phase and that its composition is not defined or encompassed by th~
polygon JKLM not including line MJ of Figure 4.
The amount of free carbon used to carry out the present deoxidation should produce the present deoxidized compact leaving no significant amount of carbon in any form, i.e. no amount of carbon in any form which would have a si~nificantly deleterious effect on the sintered body. More specifically, no amount of carbon in any form should be left in the deoxidized compact which would prevent production of the present sintered body, i.e. any carbon content in the sintered body should be low enough so that the sintered body has a thermal conductivity greater than 1.00 W/cm~K at 25C.
Generally the present sintered body may contain carbon in some form in a trace amount, generally less than about .08%
by weight, preferably in an amount of less than about .065%
by weight, and more preferably less than about .04% by ,: :
, :.

R~-;6,~1 weight, and most prefera~ly less than .03% by weight of the total weight of the sintered body.
A significant amount of carbon in ani form rem~in-ing in the sintered body significantly reduces its thermal conductivity. An amount of carbon in any form greater than about 0.065% by weight of the sintered body is likely to significantly decrease its thermal conducti~ity.
Generally, the oxygen content of the AlN in the deoxidized compact is at least about 10% by weight lower than the oxygen content of the starting aiuminum nitride.
Also, îor a given system, with increasing oxygen content in the deoxidized compact, sintered bodies with decreasing thermal conductivity will be produced.
The deoxidized compact is sintered at a tempera-ture which produces a sintered body having a porosity ofless than about 10% by volume of the body and generally such temperature is at least about 1860~C, and preferabLy at least about 1890C. More specifically, the present deoxidi-~ed compact is densified, i.e. li~uid-phase sintered, at a minimum temperature generally ranging from about 1860C, and preferably from about 1890C to about 2010C or higher, depending largely on its composition and particle size, to a maximum temperature generally of about 2050C to produce the present polycrystalline body. To carry out the present liguid phase sintering, the present deoxidized compact contains su~ficient equivalent percent of Y and 0 to form a sufficient amount of li~uid phase at sintering temperature to densify the carbon-deoxidized compact to produce the present sintered body. The particular densification, i.e.
sintering, temperature is determinable empirically and depends largely on the compositio~ of the deoxidized com-pact, i.e. the amount of liquid phase it generates. Specif-ically, for a sintering temperature to be operable in the .~
.~ .

, :
, ~D-16,3~1 5~

present invention, it must generate a'c least suffi_ient liquid phase in the particular composition of the deoY~idized compact to carry out the present liquid phase sintering to produce the present product. For a given composition, the lower the sintering temperature, the smaller is the amount of liquid phase generated, i.e. densification becomes more difficult with decreasing sintering temperature. However, a sintering temperature higher than about 2050C generally provides no significant advantage.
In the polygon JKLM of Figure 4, the minimum sintering temperature increases from about 1860~C, and preferably from about 1890C, as the composition moves from line MJ toward line LK, and at line LK, the minimum sinter-ing temperature is about 1960C. In the polygon JKLM of Figure 4, as the composition moves from line MJ toward line LK, an increasing minimum sintering temperature is reouired to generate sufficient liquid phase to carry out the present sintering to produce a sintered body having a poroslty of less than about 10% by volume, and pre~erably less than about 5% by volume, of said sintered body. Specifically, the minimum sintering temperature is dependent largely on the composition (i.e. position in the Figure 4 phase diagram), the green density of the compact, the particle size of aluminum nitride, and to a lesser extent the particle size of yttrium oxide and carbon. The minimum sintering temperature increases as the composition moves from MJ to LK and as the particle size of aluminum nitride and to a lesser extent yttrium oxide and carbon, increases.
At MJ, or a composition represented by a point or points next to MJ within polygon JKLM, the minimum sintering temperature varies from about 1860C for the particle size combination of aluminum nitride, yttrium oxide and carbon of about 5.0 m2/g, 2.8 m2/g and 200 m2/g, respectively, to - . : ; .
,, .

: . :

~-i5,3~i ~5~

about 1890~C for the particle size combination of aluminum nitride, yttrium oxide and carbon of about 0.5, 0.5 and 20 m2/g, re~pectively. At LK, the minimum sintering temperature varies from about 1960DC for the particle size combination of aluminum nitride, yttrium oxide and carbon of about 5.0 m2/g, 2.8 m2/g and 200 m2/g, respectively, to about 2000~C for the particle size combination of aluminum nitride, yttrium oxide and carbon of about 1.5 m2/g, 0.6 m2/g and 20 m2/g, respectively.
For the composition defined and encompassed by polygon UMJV of Figure 4, for an aluminum nitride powder in the final mixture or compact having a specific surface area of at least about 3.4 m~/g, the minimum sintering tempera-ture is about 1900C to produce a sintered body having a porosity of less than about 2% by volume of said sintered body.
The deoxidized compact is sintered, preferably at ambient pressure, in a gaseous nitrogen-containing nonoxid-izing atmosphere which contains at least sufficient nitrogen to prevent significant weight loss of aluminum nitride. In accordance with the present invention, nitrogen is a neces-sary component of the sintering atmosphere to prevent any significant weight loss of AlN during sintering, and also to optimize the deoxidation treatment and to remove carbon.
Significant weight loss of the aluminum nitride can vary depending on its surface area to volume ratio, i.e. depend-ing on the form of the body, for example, whether it is in the form of a thin or thick tape. As a result, generally, significant weight loss of aluminum nitride ranges from in excess of about 5% by weight to in excess of about 10% by weight of the aluminum nitride. Preferably, the nitroyen-containing atmosphere is nitrogen, or it is a mixture at least about 25% by volume nitrogen and a gas sçlected from .: .
, :; . .
` ' ' ~ '; ' ' ' ' ;:

..
, : ~ ' ~ 16,3~i the group c~nsisting of hydrogen, a noble gas such as argon and mixtures thereof. Also, preferably, the nitrogen-containing atmosphere is comprised of a mixture of nitrogen and hydrogen, especially a mixture containing from about 1%
by volume to about 5% by volume hydrogen.
Sintering time is determinable empirically.
Typically, sintering time ranges from about 40 minutes to about 90 minutes.
In one embodiment, i.e. the compositicn defined by polygon JKLM but not including lines MJ and LK of Figure 4, where the aluminum nitride in the carbon-deoxidized compact contains oxygen, the yttrium oxide further deoxidizes the aluminum nitride by reacting with the oxygen to form Y4A1209, thus decreasing the amount of oxygen in the AlN
lattice to produce the present sintered body having a phase composition comprised of AlN and a second phase mixture of Y203 and Y4A1209.
In another embodiment, i.e. line LK of Figure

4,where the aluminum nitride in the carbon-deoxidized compact contains oxygen in an amount significantly smaller than that of polygon JKLM but not including lines MJ and LK
of Figure 4, the resulting sintered body has a phase compo-sition comprised of AlN and Y203.
In one embodiment of the present process for producing the sintered body having a composition defined and encompassed by polygon UMJV but not including line MJ of Figure 4, the aluminum nitride powder in the present mix-ture, i.e. the dried mixture after milling to form such miXture, has a surface area ranging from about 3.4 m2/g to about 4.8 m2/g. In this embodiment, the aluminum nitride in the compact before deoxidation has an oxygen content gener-ally ranging from about 1.7% by weight to about 4.3% by weight, and usually from about 1.9% by weight to about 4.0%

.~

. :
~ ' - ~

~-i6,~, ~2655~

by weight, of such aluminum nitride, the deoY.idized compact as well as the resulting sintered body is comprised of from greater than about 2.5 equivalent % to about 4.35 equivalent % yttrium, from about 95.65 equivalent % to less than about 97.5 equivalent % aluminum, from greater than about 4.1 equivalent % to less than about 6.3 e~uivalent % oxygen and from greater than about 93.7 equivalent % to less than about 95.9 equivalent % nitrogen, the sintering temperature ranges from about 1900C to about 1960C, and the sintering atmos-phere is nitrogen, or a mixture of nitrogen and from about1% by volume to about 5% by volume hydrogen. The resulting sintered body has a porosity of less than about 2% by volume of the body and a thermal conductivity greater than 1.42 W/cm K at 25C. The sintered body of this embodiment has a phase composition comprised of AlN and a second phase mixture of Y203 and Y4Al209 wherein the total amount of such second phase mixture ranges from greater than about 6% by volume to less than about 9.4% by volume of the body and wherein Y203 is always present in at least an amount detect-able by X-ray diffraction analysis and Y4Al209 is always present in an amount of at least about 3.4% by volume of the body.
In another embodiment of the present process, the aluminum nitride powder in the présent mixture has a surface 25 area greater than about 3.2 m2/g to about 6.0 m2/g, the sintering temperature ranges from about 1900C to about 2050C, and the sintering atmosphere is nitrogen, or a mixture of nitrogen and rom about 1% by volume to about 5%
by volume hydrogen, to produce a sintered body having a carbon content of less than .04% by weight of the body, a porosity of less than about 2% by volume, and a thermal conductivity greater than about 1. 53 W/cm-K at 25C.

., , . . .` ' .
- ::

P~ a ~2~5~

In another embodiment of the present invention, the aluminum nitride powder in the present mixture has a surface area ranging from greater than about 3.2 m2/g to about 6 m2/g, the sintering temperature ranges from about 1950C to about 2050C, and the sintering atmosphere is nitrogen, or a mixture of nitrogen and from about 1% by volume to about 5% by volume hydrogen, to produce a sintered body having a carbon content of less than .04% by weight of the body, a porosity of less than about 2% by volume of the body and a thermal conductivity greater than about 1.64 W/cmoK at 25C.
The present sintered polycrystalline body is a pressureless sintered ceramic body. By pressureless sinter-ing herein it is meant the densification or cons~lidation of the deoxidized compact at ambient pressure, i.e. without the application of mechanical pressure, into a ceramic body having a porosity of less than about 10% by volumé, and preferably less than about 5% by volume.
The polycrystalline body of the present invention has the appearance of having been liquid-phase sintered.
I.e., it sinters due to the presence of a liquid phase, that is liquid at the sintering temperature and is rich in yttrium and oxygen and contains some aluminum and nitrogen.
Substantially all of the AlN grains are rounded or signifi-cantly or substantially rounded and have a smooth surface,i.e. they have the appearance of a liquid-phase sintered ceramic. In the present polycrystalline body, the AlN
grains have about the same dimensions in all directions, and are not elongated or disk shaped. Generally, the AlN phase has an average grain size ranging from about 1 micron to about 20 microns. An intergranular second phase of Y203 or a mixture of Y203 and Y4A1209 is present along some o~ the AlN grain boundaries. The morphology of the microstructure :.~
:: :

- : -: , , .
~-: . ' ':

_D-16,3&' indicates that this intergranular second phase was a liquid at sintering temperature.
The present sintered body has a porosity of less than about 10% by volume and generally less than about 5% by volume of the sintered body. Prefe.ably, the present sintered body has a porosity of less than about 2% and most preferably less than about 1% by volume of the sintered body. Any pores in the sintered body are fine sized, and generally they are less than about l micron in diameter.
Porosity can be determined by standard metallographic procedures and by standard density measurements.
In the present invention, the thermal conductivity of a given composition increases with increasing sintering temperature in the range between about 1890C and 1950C.
Also, the thermal conductivity of a given sintered body increases with decreasing porosity.
The present process is a control process for producing a sintered body of aluminum nitride having a thermal conductivity greater than 1.00 W/cm-K at 25C, and preferably greater than 1.42 W/cm-K at 25C. Generally, the thermal conductivity of the present polycrystalline body is less than that of a high purity single crystal of aluminum nitride which is about 2.8 W/cm-K at 25C. If the same procedure and conditions are used throughout the present process, the resulting sintered body has a thermal conduc-tivity and composition which is reproducible or does not differ significantly.
In the present process, aluminum nitride picks up oxygen in a controllable or substantially controllable manner. Specifically, if the same procedure and conditions are used in the present process, the amount of oxygen picked up by aluminum nitride is reproducible or does not differ significantly. Also, in contrast to yttrium, yttrium , , ":~
` `' ~-i6,3~:
~265~1~

nitride and yttrium hydride, yttrium oxide or the present precursor therefor does not pick up oxygen, or does not pick up any significant amount of oxygen, from air or other media in the present process. More specifically, in the present process, yttrium oxide does not pick up any amount of oxygen in any form from the air or other media which would have any significant effect on the controllability or reproducibility of the present process. Any oxygen which yttrium oxide might pick up in the present process is so small as to have no effect or no significant effect on the thermal conductiv-ity or composition of the resulting sintered body.
Examples of calculations for equivalent % are as follows: ^
For a starting AlN powder weighing 8g.0 grams measured as having 2.3 weight % oxygen, it is assumed that all of the oxygen is bound to Al as A1203, and that the measured 2.3 weight % of oxygen is present as 4.89 weight %
A1203 so that the aluminum nitride powder is assumed to be comprised of 84.65 grams AlN and 4.35 grams A1203.
A mixture is formed comprised of 89.0 grams of the starting AlN powder, 9.7 grams of Y203 and 1.25 grams free carbon.
During processing, this AlN powder picks up additional oxygen by reactions similar to (4) and now contains 2.6 weight % oxygen.

2 AlN ~ 3H20 ' A1203 + 2NH3 The resulting compact now is comprised of the following composition:

~`
., . .

:: .
: - : - ~ : : . . , .
' ~:

RD-16,3cl ~Z~5~

89.11 grams AlN powder containing 2 6 weight % oxygen, (84.19g AlN + 4.92g Al2O3), 9.7 grams Y2O3 and 1.25 grams Carbon.

During deoxidation of the compact, all the carbon is assumed to react with Al2O3 via reaction ~5) A12O3 + 3C + N2 ' 2AlN + 3CO~g) ~5) In the present invention, the carbon will not reduce Y2O3, but instead, reduces Al2O3.
After reaction (5) has gone to completion, the deoxidized compact now is comprised of the following compo-sition which was calculated on the basis of Reaction (S):

86.43 grams AlN powder containing 0.6 weight % Oxygen (85.04 grams AlN + 1.38~grams Al2O3) and 9.7 grams Y2O3 From this weight composition, the composition in equivalent % can be calculated as follows:

Wt (g) Moles Eauivalents AlN 85.04 2.075 6.224 A123 l.38 1.356 x 10 2 8.138 x 10 2 Y203 9 7 4.296 x 10 2 0.258 TOTAL EQUIVALENTS = 6.S64 V = Valence M = Moles =
: MW
: ; MW = molecular weight 2S Eg - E~uivalents `~ -37--`'' .

. . .
, .. . . : . - . ~ , .
. :

~ 6,3~;
~Z~S~

Eq = M X V
Valences: Al + 3 Y ~ 3 Eq % Y in deoxidized compact =
no.Y eauivalents x 100% (6) no.Y equivalents + no.Al equivalents = 0.258 x 100% = 3-93%
6.564 Eq % O in deoxidized compact =
no O equivalents _ x 100% (7) no. O equivalents + no.N equivalents = ~.13~ x 10 + 0-258 x 100% = 5.17% (8) 6.564 This deoxidized compact as well as the sintered body contains about 3.93 equivalent % Y and about 5.17 equivalent % Oxygen.
To produce the present sintered body containing 4.0 equivalent % Y and 5.0 equivalent % O, i.e. comprised of 4 equivalent % Y, 96 equivalent % Al, 5.0 equivalent % O and 95 equivaIent % N, using an AlN powder measured as having 2.3 weight % Oxygen (4.89 weight % Al203), the following calculations for weight % from equivalent % can be made:

100 grams = weight of AlN powder x grams = weight of Y2O3 powder z grams = weight of Carbon powder : Assume that during processing, the AlN powder picks up:additional oxygen by reaction similar to (g) and in :
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the compact before deoxidation now contains 2.6 weight %
oxygen (5.52 weight % Al203) and weighs 100.12 grams 2AlN ~ 3H20 ~ Al203 + 2NH3 (9) After processing, the compact can be considered as 5 having the following composition:

Weight (g) Moles Equivalents AlN94.59 2.308 6.923 Al23 5-53 0.0542 0.325 Y203 x4.~29 x 10 3x 0.02657x 10 C z .0833z During deoxidation, 3 moles of carbon reduce 1 mole of A1203 and in the presence of N2 form 2 moles of AlN
by the reaction:

2 3 N2 2AlN 3C0 (10) 15 After deoxidation, all the carbon will have reacted and the compact can be considered as having the following composition:

Weight (g) Moles Equivalents AlN94.59 t 2.275z 2.308 + 0. 05551z 6.923 + 0.1665z Al2035. 53 - 2. 830z 0. 0542 - 0.02775z 0.325 0.1665z Y2o3 x 4.429 x 10 3 x 0.02657 x ::
T = Total E~uivalents ~ 7.248 + 0.02657 x Equlvalent Fraction of` Y = 0.04 = 0.02657 x ~11) .

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~-'6,3&1 Equivalent Fraction of 0 =0.05=0.325-0.1665Z + 0.02657x (12) Solving Equations (11) and (12) for x and z:
x = 11.37 grams of Y2O3 powder z = 1.50 grams of free carbon A body in a form or shape useful as a substrate, i.e. in the form of a flat thin piece of uniform thickness, or having no significant difference in its thickness, usually referred to as a substrate or tape, may become non-flat, for example, warp, during sintering and the resulting sintered body may require a heat treatment after sintering to flatten it out and make it useful as a sub-strate. This non-flatness or warping is likely to occur in the sintering of a body in the form of a substrate or tape having a 'hickness of less than about .070 inch and can be eliminated by a flattening treatment, i.e. by heating the sintered body, i.e. substrate or tape, under a sufficient applied pressure at a temperature in the sintering tempera-ture range for the particular composition of the body for a period of time determinable emperically, and allowing the sandwiched body to cool to below its sintering temperature before recovering the resulting flat substrate or tape.
Specifically, in one embodiment of this flattening process, *he non-flat substrate or tape is sandwiched between two plates and is separated from such plates by a thin layer of AlN powder, the sandwiched body is heated to the sintering temperature for its composition, i.e. from about 1860C to about 2050C, in the ~intering atmosphere, preferably at ambient pressure, i.e. the nitrogen-containing ` 30 ~non-oxidizing atmosphere useful for sintering, under an applied pressure at least sufficient to flatten the body, ~: :
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generally at least about .03 psi, for a time period sufficient to flatten the sandwiched body, and then the sandwiched body is allowed to cool to below its sintering temperature before it is recovered.
One embodiment for carrying out this flattening treatment of a sintered thin body or substrate tape compris-es sandwiching the sintered non-flat substrate or tape between two plates of a material which has no significant deleterious effect thereon such as molybdenum or tungsten, or an alloy containing at least about 80% by weight of tungsten or molybdenum. The sandwiched substrate or tape is separated from the plates by a thin layer, preferably a discontinuous coating, preferably a discontinuous monolayer, of aluminum nitride powder preferably just sufficient to prevent the body from sticking to the surfaces of the plates during the flattening heat treatment. Generally, the applied flattening pressure is at least about .03 psi. A
decrease in flattening temperature requires an increase in flattening pressure or flattening time. Typically, for example, heating the sandwiched sintered body at the sinter-ing temperature under a pressure of from about .03 psi to about .5 psi for 1 hour in nitrogen produces a flat body useful as a substrate, especially as a supporting substrate for a semiconductor such as a silicon chip.
The present invention makes it possible to fabri-cate simple, complex and/or hollow shaped polycrystalline aluminum nitride ceramic articles directly. Specifically, the present sintered body can be produced in the form of a useful complex shaped article without machining such as an impervious crucible, a thin walled tube, a long rod, a spherical body, a tape or a hollow shaped article. The dimensions of the present sintered body differ from those of ,.

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densification, which occurs during sintering.
The present ceramic body has a number of uses. In the form of a thin flat piece of uniform thickness, or having no significant difference in its thickness, i.e. in the form of a substrate or tape, it is especially useful as packaging for integrated circuits and as a supporting substrate for an integrated circuit, particularly as a substrate for a semiconducting Si chip for use in computers.
The present ceramic body also is useful as a sheath for temperature sensors.
The invention is further illustrated by the following examples wherein the procedure was as follows, unless otherwise stated:
The aluminum nitride powder contained oxygen in an amount of less than 4% by weight.
The aluminum nitride powder was greater than 99%
pure AlN exclusive of oxygen.
In Examples 9-12 and 33-41 of Table II, the starting aluminum nitride powder had a surface area of 3.4 m /g (0.541 micron) and based on a series of deoxidations carried out with carbon powder, it was determined to have contained about 2.4 weight ~ oxygen.
In Examples 6-8, 16-22 and 29-31 of Table II and Examples 53A and 53B of Table III, the starting aluminum nitride powder had a surface area of 3.84 m2/g (0.479 micron) and contained 2.10 wt% oxygen as determined by neutron activation analysis.
In the remaining examples of Table II, the start-ing aluminum nitride powder had a surface area of 4.96 m /g(0.371 micron) and contained 2.25 wt% oxygen as determined by neutron activation analysis.

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In all of the examples of Table II, the Y~03 powder, before any mixing, i.e. as received, had a surface area of about 2.75 m2/g.
The caxbon used in all of the examples of Tables II and III was graphite. In Examples 29-31, 48, 49 and 52, the graphite had a surface area of 25 m2/g (0.13 micron), and in all of the remaining examples of Tables II and III, the graphite had a specific surface area of 200 m2/g (0.017 micron) as listed by the vendor.
N~n-aqueous heptane was used to carry out the mixing of the powders in all of the examples of Tables II
and III.
In all of the examples of Tables II and III, except Example 39, the milling media was hot pressed alumi-num nitride in the approximate form of cubes or rectangles having a density of about 100%. In Example 39, the milling media was low carbon steel in the form of spheres.
In all of the Examples of Tables II and III, the milled li~uid dispersion of the given powder mixture was dried in air at ambient pressure under a heat lamp for about 20 minutes and during such drying, the mixture picked up oxygen from the air.
In all of the Examples of Tables II and III, the dried milled powder mixture was die pressed in air at room temperature under a pressure to produce a compact having a density of roughly 55% of its theoretical density. In Examples 1-4, the die pressure was 10 Kpsi, and in all of the remaining examples it was 5 Kpsi.
In those examples of Tables II and III wherein the sintered body is given as being of A size or of B size, the compacts were in the form of a disk, in those examples wherein the sintered body is given as being of C size, the compacts were in the form of a bar, and in those examples , .~ .. ~ : .
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wherein the sintered body is given as being of D size, the compacts were in the form of a substrate which was a thin flat piece, like a tape, of uniform thickness, or of a thickness which did not differ significantly.
In all of the examples of Tables II and III, the given powder mixture as well as the compact formed therefrom had a composition wherein the equivalent % of yttriu~ and aluminum ranged from point L up to point J of Figure 4.
The equivalent % composition of Y, Al, 0 and N of the compacts of all of the Examples of Tables II and III, i.e. before deoxidation, was outside the composition defined and encompassed by polygon J~LM of Figure 4.
In all of the examples of Tables II and III, the aluminum nitride in the compact before deoxidation contained oxygen in an amount ranging from greater than about 1.4% by weight to less than about 4.5% by weight of the aluminum nitride.
The composition of the deoxidized compacts of all of the Examples of Tables II and III, except Examples 23, 25, 26, 28, 40, 41 and 52 were defined and encompassed by polygon JKLM of Figure 4 but did not include line MJ.
The examples in Tables II and III having the same number but including the letters A or B indicate that they were carried out in an identical manner, i.e. the powder mixtures were prepared and formed into compacts in the same manner and have the same composition, and the compacts were heat treated under identical conditions, i.e. the compacts were placed side by side in the furnace, and these examples numbered with an A or B may be referred to herein by their number only.
In all of the examples of Tables II and III, the same atmosphere was used to carry out the deoxidation of the compacts as was used to carry out the sintering of the ' .

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deoxidized compact except that ~he atmosphere te carry out the deoxidization was fed into the furnace at a rate of 1 SCFH to promo~e removal of the gases produced by deoxida-tion, and the flow rate during sintering was less than about .1 SCFH.
The atmosphere during heat treatment in the examples in Tables II and III was at ambient pressure which was atmospheric or about atmospheric pressure.
~ The furnace was a molybdenum heat element furnace.
The compacts were heated in the furnace to the given deoxidation temperature at the rate of about 100C per minute and then to the given sintering temperature at the rate of about 50C per minute.
In those examples where no deoxidation is given, the compacts were heated to the given sintering temperature at the rate of about 100C per minute, except in Examples 40 and 41 where they were heated to the given sintering temper-ature at a rate of about 190C per minute~
The sintering atmosphere was at ambient pressure, i.e. atmospheric or about atmospheric pressure.
At the completion of heat treatment, the samples were furnace-cooled to about room temperature.
In Tables II and III, the properties of the sintered body are given, but in those examples where there was no sintering, the properties of the deoxidized compact are given.
All of the examples of Tables II and III were carried out in substantially ~he same manner except as indicated in Tables II and III and except as indicated herein.
Carbon content of the sintered body or deoxidi2ed compact was determined by a standard chemical analysis technique.

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Based on the predetermined oxygen conte-.t of the starting AlN powders and the measured compositions of the resulting sintered bodies, as well as other experiments, it was calculated or estimated that in every example in Table II, the aluminum nitride in the compact before deoxidation had an oxygen content of about 0.3% by weight higher than that of the starting aluminum nitride powder.
Measured oxygen content was determined by neutron activation analysis and is given in wt%, which is % by weight of the sintered body.
In Tables II and III, in those examples where the oxygen content of the sintered body was measured, the equivalent % composition of the sintered body was calculated from the starting powder composition and from the given lS measured oxygen content of the sintered body. The Y, Al, N
and oxygen are assumed to have their conventional valences of: +3, l3, -3, -2, respectively.
In the sintered bodies, the amount of Y and Al was assumed to be the same as that in the starting powder.
During processing, the amount of oxygen gain and nitrogen loss was assumed to have occurred by the overall reaction:

AlN 3/202 A123 N2 (13) During deoxidation, the amount of oxygen loss and nitrogen gain was assumed to have occurred by the overall reaction:

2 3 N2 2AlN 3C0 (14) The nitrogen content o~ the sintered body was determined by measuring the initial oxygen content of the starting aluminum nitride powder and measuring the oxygen content of :
, , R~ ,3R1 ~S~

the sintered body and assuming that reactions 13 and 14 have occurred.
In Tables II and III, an approximation sign is used in front of the e~uivalent percent oxygen for sintered bodies whose oxygen content was not measured but calculated, and these calculations were based on the composition of the powder mixture as well as that of the resulting sintered body and were carried out as follows:
Examples 13A and B (85A and 85Al) are assumed to have the same equivalent percent oxygen as Example 15A
(85Dl). Example 8 (132B) is assumed to have the same equivalent percent oxygen as Example 7 (122F). Example 10 (130C) is assumed to have the same equivalent percent oxygen as Example 9 (130A). Examples llA (134A) and 12 (134B) are assumed to have the same equivalent percent oxygen as Example llB (134Al). Examples 16 (121G), 20 (131A) and 21 (131C) are assumed to have the same equivalent percent oxygen as Example 19 (121A). Example 31 (128C) is assumed to have the same equivalent percent oxygen as Example 29A
(128A). Examples 34 (140C) and 35 (140D) are assumed to have the same equivalent percent oxygen as Example 33 (140A). Example 37 (141F) is assumed to have the same e~uivalent percent oxygen as Example 36 (141D). The oxygen content of Examples 7 (122F), llB (134A1), 9 (130A), 19 (121Al), 18B (121Fl), 22 (131E), 27B (9OFl), 33 (140A), 36 (141D), 38 (142), 29A (128A) and 39 (148A)were calculated from the following equation:

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O = (2.91 R ~ 3.82) Y (15) 3.~6 where O = equivalent % oxygen Y = equivalent % yttrium R - v/o y4A129 ~ (16) v/o Y4A129 ~ v/o Y203 The equivalent % oxygen in Examples 25 (9OBl), 26 (9OD2) and 28 (9OK) are assumed to have the same equivalent percent oxygen as Example 23 (84F).
The equivalent % oxygen of Examples 43-48, 50, 51 and 53B (samples 161A, 164A, 163A, 166A, 168A, 169A, 170B, 162A and 131Dl) was calculated from the equation:

O - (2.91 R + 3.82) Y
3.86 The equivalent % oxygen of Example 52 (sample 174A) was estimated from the X-ray diffraction analysis data.
The equivalent % oxygen of Example 49 (sample 170A) was assumed to be the same as the equivalent % oxygen 20 of Example 50 (sample 170B).
Weight loss in Tables II and III is the difference between the weight of the compact after die pressing and the resulting sintered body.
Denslty of the sintered body was determined by the Archimedes method, ~ Porosity of the sintered body was determined by knowing the theoretical density o~ the sintered body on the : -48-;~ ~

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5~ 3~1 basis of its composition and comparing that to the density measured using the following equation:

porosity = ( 1 - measured density ) 100% (17) theoretical density Phase composi~ion of the sintered body was deter-mined by optical microscopy and X-ray diffraction analysis, and each sintered body was comprised in % by volume of the sintered body of an AlN phase and the given volume % of the given second phases. X-ray diffraction analysis for volume % of each second phase is accurate to about i ZO% of the given value.
The thermal conductivity of the sintered body of Example 25 (9OBl), Example 26 (gOD2) and Example 28 (9OK) was measured by laser flash at about 25C.
The thermal conductivity of the sintered body of all of the remaining examples was measured at 25C by a steady state heat-flow method using a rodshaped sample ~0.4 cm x 0.4 cm x 2.2 cm sectioned from the sintered body. This method was originally devised by A. Berget in la88 and is described in an article by G. A. Slack in the "EncyclopPadic Dictionary of Physics", Ed. by J. Thewlis, Pergamon, Oxford, 1961. In this technique the sample is placed inside a high-vacuum chamber, heat is supplied at one end by an electrical heater, and the temperatures are measured with fine-wire thermocouples. The sample is surrounded by a guard cylinder. The absolute accuracy is about + 3% and the repeatability is about + 1%. As a comparison, the thermal conductivity of an Al203 single crystal was measured with a similar apparatus to be 0.44 W/cm-K at about ~2C.
In Tables II and III, the size of the resulting sintered body i~ given as A, B, C or D. The body of A size .

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was in the form of a disk about .17 inch in thickne~s and about .32 inch in diameter. The body of B size was also in the form of a disk with a thickness of about 0.27 inch and a diameter of about 0.50 inch. The body of C size was in the shape of a bar measuring about 0.16 inch x 0.16 inch x 1.7 inches. The body of D size was in the form of a substrate, i.e. a thin piece of uniform thickness, or of no significant difference in thickness, having a diameter of about 1.5 inch and a thickness ranging from about .067 inch to about .020 inch.
In all of the Examples of Tables II and III
wherein the sintered body was of C size or of D size, the starting compact was separated from the molybdenum plate by a thin discontinuous layer of AlN powder.
The sintered body of Examples 33-38 exhibited some non-flatness, i.e. exhibited some warping, and each was subjected to a flattening treatment. Specifically, each of the sintered bodies was sandwiched between a pair of molyb-t denum plates. Each sandwiched sintered body was separated from the molybdenum plates by a thin discontinuous coating or monolayer of aluminum nitride powder which was just sufficient to prevent sticking of the sintered body to the plates during the flattening treatment period. The top molybdenum plate exerted a pressure of about 0.11 psi on the sintered body. Each such sandwiched sintered body was heated in n~trogen, i.e. the same atmosphere used to sinter it, to the given sintering temperature in Table II for such body where it was held for about 1 hour and then furnace cooled to about room temperature. Each resulting sintered body was flat and was of uniform thickness, i.e. its thick-ness did not differ significantly. All of these flat sintered bodies would be useful as a supporting substrate for a semiconductor such as a silicon chip.

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P.~-i6 3~1 ~59~ _ ExamDle l 1.65 grams of Y203 powder and 0.216 grams of graphite powder were added to 15 grams of aluminum nitride powder and the mixture, along with aluminum nitride milling media, was immersed in non-aqueous heptane in a plastic jar and vibratory milled in the closed jar at room temperature for about 68 hours. The resulting dispersion was dried in air under a heat lamp or about 20 minutes and during such drying, the aluminum nitride picked up oxygen from the air.
During milling, the mixture picked up 0.084 gram AlN from the AlN milling media.
E~uivalent portions of the resulting dried mixture were die pressed producing compacts.
One of the compacts was placed in a molybdenum boat and packed with powder having the same ratio of yttrium to AlN but no carbon as the compact. The compact was then heated in nitrogen to 1600C where it was held for 1 hour, and then the temperature was raised to 1900C where it was held for 1 hour. The resulting sintered body was then furnace cooled to about room temperature.
The resulting sintered body had a measured oxygen content of 3.39% by weight of the body and a carbon content of 0.047% by weight of the body.
The sintered body had an equivalent % composition comprised of 6.22% 0 (100% - 6.22%) or 93.78% N, 3.86% Y and (100% - 3.86%) or 96.14% Al. This composition lies within the polygon JKLM of Figure 4.
The sintered body was more than 99% dense and had a phase composition comprised of AlN, 1.3% by volume of Y203 and 6.6% by volume of Y4A1209. Based on other work, it is known that this sintered body would have a thermal conduc-tivity greater than 1.42 W/cm-K at 25C.

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1~5~ RD-1~,3~i Examples 2A and 2B

Two of the compacts prepared in Example 1 were placed side by side on a molybdenum plate The compacts were heated in nitrogen to 1600C
where they were held for 1 hour and then the temperature was raised to 1900C where it was held for 1 hour.
The properties of the resulting sintered bodies are given in Table II.
Examples 3A and 3B of Table II were carried out in the same manner as Examples 2A and 2B except that hydrogen instead of nitrogen was used as the atmosphere.
Examples 4A and 4B of Table II were carried out in the same manner as Examples 2A and 2B except that there was no deoxidation at 1600C for 1 hour.
Examples 5A and 5B of Table II were carried out in the same manner as Examples 4A and 4B except that hydrogen instead of nitrogen was used as the atmosphere.

Exam~le 7 1.653 grams of Y203 powder and 0.158 grams of graphite powder were added to 15.07 grams of aluminum nitride powder and the mixture, along with aluminum nitride milling media, was immersed in non-agueous heptane contain-ing oleic acid in an amount of about 0.7% by weight of the aluminum nitride in a plastic jar and vibratory milled in the closed jar at room temperature for about 18 hours. The resulting dispersion was dried in-air under a heat lamp for about 20 minutes and during such drying, the aluminum nitrid~ picked up oxygen from the air. During milling the mixture picked up 0.85 gram of AlN due to wear o~ the milling media.

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` i2~$9~ ~D-15,38i A portion of the resulting dried mixture was die pressed in air producing a compact in the shape of a bar The compact was heated to 1500C where it was held for 1/2 hour, then the temperature was raised to 1600C
where it was held for 1 hour and then the temperature was raised to 1900C where it was held for 1 hour, The properties of the resulting sintered body are given in Table II.
In Examples 6-12, 16-22, 29-31 and 33-41, oleic acid in an amount of about 0.7% by weight of the aluminum nitride was added to the heptane as a dispersant and the milling time was about 18 hours. In the remaining examples, no oleic acid was used, and the milling time was about 68 hours.
The examples in Table II were carried out in the same manner as disclosed for Examples 2A and 2B or as disclosed for Example 7 except as shown in Table II or noted herein.
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Examples 1, 2, 4A, 4B, 7-12, 14-15, 18, l9, 20, 21, 22, 29, 31 and 33-39 of Table II illustrate the present inven~ion, and the sintered bodies produced in the examples which illustrate the present invention would be useful for packaging of integrated circuits as well as for use as subs-.rates for semiconductors such as a silicon chip.
A comparison of the properties of the sintered body produced in Example l with those of Examples 2A and 2B
shows that the difference in weight loss W2S not significant and that in the present process, the compact does not need to be packed in powder, as it was in Example 1, before being sintered to prevent significant weight loss.
Based on other work and especially a comparison of Examples 2A and B wïth Examples 7 and 16, it is known that the sintered bodies of Examples 2A and B would have~a thermal conductivity greater than 1.42 W/cm-K at 25C.
A comparison of the weight 105s of bodie's of Examples 3A and B with Examples 2A and B shows that sintér-ing in hydrogen produces substantial weight loss and leaves an excessive amount of carbon in the sintered body, and illustrates the importance of the present nitrogen atmos-phere for sintering.
A comparison of the properties of the sintered bodies of Examples 4A and 4B with those of Examples 5A and 5B shows that the hydrogen atmosphere used in Examples 5A
and 5B left a much larger amount of carbon in the sintered body and also resulted in a substantially higher weight loss. Examples 5A and 5B illustrate the importance of the present nitrogen atmosphere.
In Example 6 there was no sinkering. Example 6 illustrates the importance of the present deoxidation of the compact as shown by the low carbon content of the deoxidized compact.
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~ RD-16,381 A comparison of the carbon contents of the bGdies of Examples 6 and 7 further shows the effectiveness of the present deoxidization as evident by the low carbon content of the sintered body of Example 7.
A comparison of Examples 7 and 8 shows that the higher sintering temperature in Example 8 resulted in a sintered body with a higher thermal conductivity.
A comparison of Examples 9 and 10 shows that the higher sintering temperature in Example 10 produced a sintered body with a slightly higher thermal conductivity.
A comparison of Example llA with Example 12 illustrates that the higher sintering temperature in Example 12 produced a sintered body with a higher thermal conducti-vity.
In Examples 13A and 13B, the sintering temperature was not high enough to sinter the given composition which lies between lines UV and KL of Figure 4.
Based on ot~er work and especially a comparison of Examples 14A and B with Examples 8 and 21, it is known that the sintered bodies of Examples 14A and B would have a thermal conductivity greater than 1.42 W/cm-K at 25DC.
Examples 14A and 14B illustrate the present process even though there was a small amount of YN phase formed in the sintered body. Specifically, sectioning of the sintered body of Example 14A showed that this YN phase was located only in the center of the body, i.e. the YN
phase was black and was ~urrounded by the present composi-tion which was tan. The ormation of this YN phase was due to ~he thickness of the sintered body and its composition, and it is caused by an oxygen gradient. Specifically, from the center of the sintered body the concentration of oxygen increases by a small amount and the concentration of nitrogen decreases by a small amount occasionally resulting : -iJ- i r~

in the form~tion of a small amount of YN phase in the center of the body when it has a composition in the polygon JKLM of Figure 4, which is close to line LK or on line LK.
Based on other work and especially a comparison of Examples 15A and 15B with Example 21, it is known that the sintered bodies of Examples 15A and 15B would have a thermal conductivity greater than 1.42 W/cm-K at 25C.
Example 17 illustrates the effectiveness of the present deoxidation.
Examples 18A and 18B illustrate that an atmosphere comprised of argon and at least about 25% by volume nitrogen is useful in the present process. Based on other experi-ments and especially a comparison of Examples lBA and 18B
with Example 22, it is known that the sintered bodies of Examples 18A and 18B would have a thermal conductivity greater than about 1.42 W/cm-K at 25C.
A comparison of Examples 21 and 22 shows that the higher sintering temperature in Example 21 produced a sintered body of higher thermal conductivity.
Example 23 illustrates that the lack of sufficient deoxidation of the compact and the use of an argon atmos-phere resulted in a large amount of carbon being left in the sintered body.
Examples 24A and 24B illustrate that even though there was a deoxidation of the compact, the use of the argon atmosphere resulted in a large amount of carbon being left in the sintered body and also had a composition outside polygon JKLM of Figure 4.
Example 25 illustrates that a lack of the deoxida-tion step and the use of a hydrogen atmosphere results in asintered body which had a low thermal conductivity and which contained a large amount of carbon.

:' ~ . .

i . .

~5~3~ 3P' Example 26 illustrates that even though th~re was a deoxidation step carried out, the use of the hydrogen atmosphere resulted in a sintered body which had a low thermal conductivity and which contained a large amount of carbon.
Both Examples 25 and 25 illustrate the deleterious effect of carbon on the thermal conductivit~ of the sintered body.
In Examples 27A and 27B, the sintering temperature was too low for the given composition.
Example 28 illustrates that the use of an argon atmosphere results in a sintered body having a low thermal conductivity.
Based on other experiments and a comparison of Examples 29A and 29B with Example 31, it is known that the sintered body of Examples 29A and 29B would have a thermal conductivity greater than about 1.42 w/cm-K at 25C.
Example 30 illustrates the effectiveness of the deoxidation of the compact in the present process.
Example 31 illustrates the present process.
In Examples 32A and 32B, the rate at which the compacts were heated to 2000C was insufficient to deoxidize the compacts before sintering as shown by the large amount o~ carbon remaining in the sintered body of Example 32B.
Examples 33-38 illustrate the present invention and show the production of sintered bodies especially useful as substrates for silicon chips. Based on other work and a comparison of Examples 33-3>3 wlth Examples 9 and llA, it is known that the sintered bodies of Examples 33-38 would have a thermal conductivity greater than 1.42 w/cm-K at 25C.
Example 39 illustrates the present invention.
Also, a comparison of Example 39 with Example 9 shows that _59_ : . ~ :
.

P~-'5,3~i _ ~L~6~
the sintered body of Example 39 must have contained an amount of carbon close to 0.02% by weight.
In Examples 40 and 41, the compact was heated directly to the given sintering temperature at a rate of about 190C per minute. Although the sintered bodies of Examples 40 and 41 show a relatively small amount of carbon, the aluminum nitride in these examples was not sufficiently deoxidized as shown by X-ray diffraction analysis of the sintered body of Example 40 which showed YA103 phase and no Y203 phase. This indicates that the high rate of heating favored reactions (3) and/or (4) and that the reactions of the present process did not occur to a sufficient extent.
Exam~le 4?

The sintered body of Example 2B (Sample 84Bl) was chemically analyzed for Y, Al and nitrogen. The sintered body of Example 2A (Sample 84B) was analyzed for oxygen by neutron acti~ation analysis. The results were as follows:

Exam~le Weight ~ Equivalent %
2B (84Bl) Y 7.59 3.79 2B (84Bl) Al 58.56 96.22 2B (84Bl) N 29.51 94.58 2A (B4B)Oxygen2.90 5.42 Total = 98.56 This is very close to the calculated equivalent percent data for Example 2A (Sample 84B) given in Table II
of 5.31 e~uivalent percent oxygen and 3.86 equivalent percent Y.
Table III shows additional examples.
Specifically, Table III shows the composition of the powder '' .

~ ,381 ~6~

mixture, i.e. powders added, in each example as well as the specific surface area of some of the powders added.
In all of the examples of Table III, oleic acid in an amount of about 0.7% by weight of the aluminum nitride was added to the heptane as a dispersant and the milling time was about 18 hours.
The examples in Table III were carried out in substantially the same manner as disclosed for Examples 2A
and 2B or as disclosed for Example 7 except as shown in Table III or noted herein.

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~ 16,381 All of the examples of Table ~II illustrate the present invention except examples 49 and 52. r~le sintered bodies produced in the examples in Table III
which illustrate the present invention would be useful for pac~aging of integrated circuits as well as for use as substrates for semiconductors such as a silicon chip. Specifically, Examples 43-46, 48, 50, 51 and 53A and B sho~l the operability of various sizes of aluminum nitride and yttrium oxide powders.
Examples 47 and 51 show the operability of a precursor for yttrium oxide. Examples 53A and 53B show the operability of an atmosphere comprised of a mixt~re of nitrogen and hydrogen.
In Example 49 the sintering temperature was too low for the particular composition and particle size.
In Example 52 too much carbon was added for this particular composition.
In United States Patent Number 4,478,785 entitled HIGH THERMAL CONDUCTIVITY ALUMINUM NITRIDE
CERAMIC BODY (issued October 23, 1984) which issued in the names of Irvin Charles Huseby and Carl Francis Bobik and assigned to the assigne hereof, which discloses the process comprising forming a mixture comprised of aluminum nitride powder and free carbon wherein the aluminum nitride has a predetermined oxygen content higher than about 0.8~ by weight and wherein the amount of free carbon reacts with such oxygen content to produce a deoxidized powder or compact having an oxygen content ranging from greater than about 0.35% by weight to about 1.1% by weight and which is at least 20% by weight lower than the predetermined oxygen content, heating the mixture or a compact thereof to react the carbon and oxygen producing the deoxidized aluminum nitride, and ~ : -~ - ., .
: , , .;

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~ 16,381 sintering a compact of the deoxidized aluminum nitride producing a ceramic body having a density greater than 85% of theoretical and a thermal conductivity greater than 0.5 W/cm K at 22C.

: ~ - 64 -.

Claims (23)

RD-16,381 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a sintered polycrys-talline aluminum nitride ceramic body having a composition defined and encompassed by polygon JKLM but not including line MJ of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-contain-ing aluminum nitride powder, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point L up to point J of Figure 4, said yttrium ranging from greater than about 2.5 equivalent % to about 4.9 equivalent %, said aluminum ranging from about 95.1 equivalent % to less than about 97.5 equivalent %, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, (b) heating said compact in a nitrogen-containing nonoxidizing atmosphere at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O
and N is defined and encompassed by polygon JKLM but not including line MJ of Figure 4, said free carbon being in an amount which produces said deoxidized compact, and (c) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of at least about 1860°C producing said polycrystalline body.

RD-16,381

2. The process according to claim 1 wherein said nitrogen-containing atmosphere in step (b) contains suffi-cient nitrogen to facilitate deoxidation of the aluminum nitride to produce said sintered body.

3. The process according to claim 1 wherein said nitrogen-containing atmosphere in step (c) contains suffi-cient nitrogen to prevent significant weight loss of said aluminum nitride.

4. The process according to claim 1 wherein said process is carried out at ambient pressure.

5. The process according to claim 1 wherein the aluminum nitride in said compact in step (a) before said deoxidation of step (b) contains oxygen in an amount ranging from greater than about 1.0% by weight to less than about 4.5% by weight of said aluminum nitride.

6. The process according to claim 1 wherein said aluminum nitride in step (a) has a specific surface area ranging up to about 10 m2/g and said free carbon has a specific surface area greater than about 10 m2/g.

7. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J of Figure 4, said yttrium in said compact ranging from greater than about 2.5 equivalent % to about 4.35 equivalent %, said aluminum in said compact ranging from about 95.65 equivalent % to less than about 97.5 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the RD-16,381 equivalent percent of Al, Y, O and N is defined and encompassed by polygon UMJV but does not include line MJ of Figure 4.

8. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges between points L
and J but does not include points L and J of Figure 4 said yttrium in said compact ranging from greater than about 2.5 equivalent % to less than about 4.9 equivalent %, said aluminum in said compact ranging from greater than about 95.1 equivalent % to less than about 97.5 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined and encompassed by polygon JKLM
but does not include lines MJ and LK of Figure 4.

9. The process according to claim 1 said mixture and said compact have a composition wherein the equivalent %
of yttrium and aluminum ranges from point L to point K of Figure 4, said yttrium in said compact ranging from about 4.4 equivalent % to about 4.9 equivalent %, said aluminum in said compact ranging from about 95.1 equivalent % to about 95.6 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line LK of Figure 4.

10. The process according to claim 1 wherein the sintering temperature ranges from about 1900°C to about 2050°C.

RD-16,381

11. A process for producing a sintered polycrys-talline aluminum nitride ceramic body having a composition defined and encompassed by polygon UMJV but not including line MJ of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-contain-ing aluminum nitride powder, yttrium oxide, and free carbon, said free carbon having a specific surface area greater than about 20 m2/g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 10 m2/g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J of Figure 4, said yttrium ranging from greater than about 2.5 equivalent % to about 4.35 equivalent %, said aluminum ranging from about 95.65 equivalent % to less than about 97.5 equivalent %, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, (b) heating said compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equiva-lent % of Al, Y, O and M is defined and encompassed by polygon UMJV but not including line MJ of Figure 4, the RD-16,381 aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (c) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 2% by volume nitrogen at a temperature ranging from about 1900°C. to about 2050°C. producing said polycrystalline body.

12. A process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon JKLM but not including line MJ of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 X/cm?K at 25°C. which comprises the steps:
(a) forming a mixture comprised of an oxygen-containing aluminum nitride powder, yttrium oxide or precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixture thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50°C. to about 1000°C. to free carbon and gaseous product of decomposition which vaporizes away, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point L
up to point J of Figure 4, said yttrium ranging from greater than about 2.5 equivalent % to about 4.9 equivalent %, said aluminum ranging from about 95.1 equivalent % to less than about 97.5 equivalent %
aluminum, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, RD-16,381 (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200°C thereby providing yttrium oxide and free carbon, (c) heating said compact in a nitrogen-containing nonoxidizing atmosphere at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O
and N is defined and encompassed by polygon JKLM but not including line MJ of Figure 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of at least about 1860°C producing said polycrystalline body.

13. The process according to claim 12 wherein said nitrogen-containing atmosphere in step (c) contains suffi-cient nitrogen to facilitate deoxidation of the aluminum nitride to produce said sintered body.

14. The process according to claim 12 wherein said nitrogen-containing atmosphere in step (d) contains suffi-cient nitrogen to prevent significant weight loss of said aluminum nitride.

15. The process according to claim 12 wherein said process is carried out at ambient pressure.

16. The process according to claim 12 wherein the aluminum nitride in said compact in step (a) before said deoxidation of step (c) contains oxygen in an amount ranging RD-16,381 from greater than about 1.0% by weight to less than about 4.5% by weight of said aluminum nitride.

17. The process according to claim 12 wherein said aluminum nitride in step (a) has a specific surface area ranging up to about 10 m2/g and said free carbon has a specific surface area greater than about 10 m2/g.

18. The process according to claim 12 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J of Figure 4, said yttrium in said compact ranging from greater than about 2.5 equivalent % to about 4.35 equivalent %, said aluminum in said compact ranging from about 95.65 equivalent % to less than about 97.5 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equiva-lent percent of Al, Y, O and N is defined and encompassed by polygon UMJV but does not include line MJ of Figure 4.

19. The process according to claim 12 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges between points L
and J but does not include points L and J of Figure 4, said yttrium in said compact ranging from greater than about 2.5 equivalent % to less than about 4.9 equivalent %, said aluminum in said compact ranging from greater than about 95.1 equivalent % to less than about 97.5 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined and encompassed by polygon JKLM
but does not include lines MJ and LK of Figure 4.

RD-16,381

20. The process according to claim 12 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point L to point K of Figure 4, said yttrium in said compact ranging from about 4.4 equivalent % to about 4.9 equivalent %, said aluminum in said compact ranging from about 95.1 equivalent % to about 95.6 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line LK of Figure 4.

21. The process according to claim 12 wherein the minimum sintering temperature ranges from about 1900°C to about 1960°C.

22. A process for producing a sintered polycrys-talline aluminum nitride ceramic body having a composition defined and encompassed by polygon UMJV but not including line MJ of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-contain-ing aluminum nitride powder, yttrium oxide or precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50°C to about 1000°C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific surface area greater than about 20 m2/g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 10 m2/g, shaping said mixture into a compact, said mixture RD-16,381

Claim 22 - Continued:

and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J
of Figure 4, said yttrium ranging from greater than about 2.5 equivalent % to about 4.35 equivalent %, said aluminum ranging from about 95.65 equivalent % to less than about 97.5 equivalent %, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200°C thereby providing yttrium oxide and free carbon, (c) heating said compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equiva-lent % of Al, Y, O and N is defined and encompassed by polygon UMJV but not including line MJ of Figure 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a RD-16,381 temperature ranging from about 1900°C to about 2050°C
producing said polycrystalline body.

23. A process for producing a sintered polycrys-talline aluminum nitride ceramic body having a composition defined and encompassed by polygon UMJV but not including line MJ of Figure 4, a porosity of less than about 5% by volume of said body and a thermal conductivity greater than 1.42 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-contain-ing aluminum nitride powder, yttrium oxide, and free carbon, said free carbon having a specific surface area greater than about 20 m 2/g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 10 m2/g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges from point U up to point J of Figure 4, said yttrium ranging from greater than about 2.5 equivalent % to about 4.35 equivalent %, said aluminum ranging from about 95.65 equivalent % to less than about 97.5 equivalent %, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon JKLM of Figure 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, (b) heating said compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said RD-16,381

Claim 23 continued:
deoxidized compact having a composition wherein the equivalent % of Al, Y, O and N is defined and encompassed by polygon UMJV but not including line MJ of Figure 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.4% by weight to less than about 4.5% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (c) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1900°C.
producing said polycrystalline body.