CA1099497A - METHOD OF PREPARING DENSE, HIGH STRENGTH, AND ELECTRICALLY CONDUCTIVE CERAMICS CONTAINING .beta.- ALUMINA - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Methods of preparing d dense and strong polycrystalline .beta."-alumina-containing ceramic body exhibiting an electrical resistivity for sodium ion conduction at 300°C of 9 ohm-cm or lower obtained directly after sintering and a controlled fine microstructure exhibiting a uniform grain size under 50 micrometers. The invention more particularly relates to methods of uniformly distributing selected metal ions having a valence not greater than 2, e.g.
lithium or magnesium, uniformly throughout the beta-type alumina composition to be sintered to form .beta."-alumina. This uniform distribution allows more complete conversion of .beta.-alumina to .beta."-alumina during sintering.
As a result, the polycrystalline .beta." - alumina -containing ceramic bodies obtained by the methods of this invention exhibit high density, low porosity high strength, fine grain size (i.e. no grains over 25-50 micrometers with an average size under 5-10 micrometers), low electrical resistivity and a high resistance to degradation by water vapor in an ambient atmosphere.
- The invention herein described was made in the course of or under a contract or subcontract thereunder with the National Science Foun-dation.

Description

~ U.S~ 658,160 'SPECIF'~:~'AT'T'ON
This application relates to methods for preparing dense, strong, ~"-alumina containing ceramic bodies having a low elec-trical resistivlty for sodium ion conduction. More particularly, this application relates to methods for preparing polycrystalline B"-alumina-containing bodies exhibiting low porosity, small grain size, near theoretical density, low electrical resistivity, high strength and a high resistance to degradation by water vapor under ambient conditione. Still more particularly, this appli-cation relatee to methods for preparing polycrystalline ~"-alumina-containing bodies which are ideally suited for use as reaction zone separators or solid electrolytes in certain electrical conver~ion devicee.
Among the polycrystalline bi- or multi-metal oxides which are mo5t useful for use in electrical conversion devices, particularly tho~e employing molten metal and/or molten metal salts as reactants, are those in the family of beta-aluminas, all of which exhibit a generic cryetalline structure which is readily identifiable by X-ray diffraction. Thus, beta-type 2Q alumina or sodium beta-type alumina i8 a material which may be thought of as a eeries of layere of aluminum oxide (A12O3) held apart by columns of linear Al-O bond chains with sodium ions occupying eites between the aforementioned layers and oolumns. Ni~ous beka-type alu~ pol~ysti~line ~ate~ e~iting thie generic crystalline structure-arë disciosed in the following U.S. Patents 3,404,035; 3,404,Q36; 3,413,150; 3,446,667;
3,458,356; 3,468,709; 3,468,7}9; 3,475,220; 3,475~223; 3,475,225;
3,535,163; 3,719,531; and 3,811,943.
Among the numerous polycrystalline beta-type alumina materials disclosed in theee patents and which may be processed in accordance with the methods of this invention are those which are modified by the addit~n of a minor proportion by weight of metal ions having a valence not greater th2n ~ (23 such that the mcdified beta-type alu~na ox~osition ox~rises a ma~or proportion by weight of ions of al~n~m and _~_ ~

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~9949~

o~y~en and a minorproportion byweightofmetalionsin crystallattice combination alongwith cationswhichmigrate in relationto the crystal latt;ce asa resultof anelectric field, the preferred embodiment being wherein the metal ion having a valence not greater than two

(2) is either lithium or magnesium or a combination of lithium and magnesium. These metal may be included in the composition in the form of lithi~m oxide or maynesium oxide or mixtures thereof in amounts ranging from about 0.1 to about 5 weight-percent, pre-ferably from about 0.1 to about 1.5 weight-percent. This type of modified beta-type alumina is more thoroughly discussed in U.S.
Patents 3,475,252 and 3,535,163 mentioned above. Such lithia and magnesia-stabilized beta-alumina are preferred compositions for the preparation of beta-type alumina bodies demonstxating the ~"
~rystal structure.
The energy conversion devices for which the dense poly-crystalline ~-alumina containing bodies of this invention are particularly useful as reaction zone separators or solid elec-trolytes are disclosed in some detail in the aforementioned patents. In the operation of such energy conversion devices, the cations such as sodium in the ~"-alumina, or some other cation which has been substituted for sodium in part or in whole, migrates in relation to the crystal lattice as a result of effects caused by an electric field. Thus, the solid ceramic electrolyte made by the process of this invention is part.icularly suited since it provides selective cationic communication between the anodic and cathodic reaction zones of the energy conversion devices and lG99497 ls essentially impermc-cble to the Jll~id re~ctar~s ,1~p] oyed in the device when the reactants ale in the e].ernental, cc.npound or anionic state. ~mong the er.ergy conversion devices in which the particular polycrystalline ~"-alumina containing ceramics are useful are:
(1~ primary batteries employing electrochemically ~eactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte or reaction zone scparator;
(2) secondary batteries employing molten, electro-chemically reversible reactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte or reaction zone separator;

(3) thermoelectlic generators wherein a temperature and pressure differential is maintained between anodic and cathodic reaction zones and/or between anode and cathode and a molten alkaline metal which is converted to ionic form, passed through the polycrystalline ~"-alumina-containing ceramic wall or inorganic ^' membrane and reconverted to elemental form; and

(4) thermally regenerated fuel cells.
A review of the operation of such electrical conversion devices as disclosed in the aforementioned patents and detailed at length in the literature should make it clear that the conductive ceramic material employed in the preparation of these devices should have a low electrical resistivity, preferably from about 3 to about 9 ohm-cm, and a strength as high as possible, preferably from about 20,000 psi to about 50,000 psi. (50 kpsi) In particular the solid electrolytes to be used in the sodium sulfur battery must meet stringent requirements with respect to physical and mechanical properties for the attainment of good performance and long service life. The strength prcperties for example should be at least 20 kpsi, preferably as high as 40-50 kpsi, and the resistivity should be` as low as about 3 to 9 ohm-cm 1~)99497 at the opera~iny temperature of the cell. It is, therefore, desirable to produce a ceramic electrolyte of such low resistivity and high strength by a commercially practical process.
It is well known that the physical and mechanical properties of ceramics depend on microstructure and composition among other parameters. For example, it is well known that grain size should be as small as possible.
It is also well known to those familiar with the fabrication of polycrystalline ceramic materials that the technique of hot pressing or pressure sintering, which permits densification to be achieved at lower temperatures, leads to the formation of fine-grained materials which are completely dense (i.e. contain no porosity). Ceramic materials in this form normally possess the highest mechanical strengths which can be achieved in a poly-crystalline body.
Previous work has shown that fracture strengths (obtained on bar specimens under four-point loading conditions) between 30,000 and 40,000 psi can be achieved in polycrystalline ~"-alumina which has been hot pressed at 1400C and annealed at lower temperatures. Also, it has been shown that in dense polycrystalline alumina the fracture strength begins to deteriorate rapidly when the largest grain size in the distribution of grain sizes exceeds about 125 micrometers in size. Thus, for high strengths in polycrystalline ~"-alumina it is essential that microstructures be produced with small averages grain sizes (< 10 micrometers) so that no grains are present in the distribution which exceed about 125 micrometers in size. While such fine grain sizes are possible using hot pressing technique, such processes are expensive and thus not practical for commercial production of large numbers of ~"-alumina bodies. A common objective, therefore, in the fabrication of ~"-alumina ceramic is to achieve by a less expensive technique, such as conventional sintering, bodies with 1(199497 fine-grained microstructures and corresponding]y high mechanical stengths.
It is known by those skilled in the art that in a reactive sintering process such as the one used for forming ~"-alumina, accompanying exaggerated grain growth may depend on composition as well as phase distribution. Prior art techniques for the preparation of conductive beta-type alumina ceramic either produce materials of high strength and high electrical resistivity or materials of low strength and low electrical resistivity, depending upon the time and temperature of sintering.
Those bodies which have been sintered at higher temperatures and for longer periods of time in order to lower the resistivities often tend to exhibit a duplex grain structure with grains ranging up to 150 to 200 micrometers or even more in size. Such a duplex grain structure has a deleterious effect on fracture strength elastic modulus and fracture toughness of the sintered body. Those bodies which have been sintered at lower temperatures of for shorter periods of time, on the other hand, while demonstrating a small grain size which is desirable for maintaining the strength or the body, do not exhibit the low electrical resistivity which is desired, presumably because substantial amounts of ~alumina remain unconverted to the ~" crystalline form.
Still another prior art technique has been developed wherein a rapid sintering step at temperatures between 1520 and 1650C is followed by a prolonged (1-40 hours) thermal anneal at temperatures between about 1300C and about 1500C. In this ~ process a low resistivity ~"-A12O3 ceramic with ; 30 -4a-1~9~97 a much finer grain size and a reasonable strength can be pro-duced. The post-sintering treatment normally at a temperature lower than that used for sintering provides for a more complete conversion to Bll-Al2o3 while at the same time reduces the occurrence of exce~sive deleterious grain growth. Such a tech-n~ue of sintering and annealing is disclosed in U.S. Patent 3,903,225, in which a beta alumina ceramic after sintering at a temperature between 1500 and 1900C. for less than 3 minutes is sub~ected to a heat treatment at a temperature between 1200aC.
and 1600~C. but at least 50C below the minimum sintering temp-erature used. Thi~ heat treatment is for at least one hour but may be 24 hours or longer and results in an appreciable red~ction of the electrical re~istivity of the material. While this tech-nique is an improvement over previous methods, it is desirable to eliminate the need for the post-sintering heat soak from the point of view of increased production rates.
It iB therefore an ob~ective of this invention to pre-pare dense, polycrygtalline ceramics exhibiting (1) a fine, more uniform microstructure and greater strength properties than can be achieved by the above sinter plus anneal process, ~2~ low porosity, and ~3) an electrical resistivity for sodium ion con-duction at 300C. under 9 ohm-cm without the necessity of a po8t-sinter annealing step.
It i6 a further object of this invention to prepare 8uch 2~ a dense, high strength, fine microstructured polycrys~alline cexamicin which the resi~tivity may be lowered still further below 9 ohm-cm by a subse~uent annealing step of less than 1 hour in durationO
It is a-~till further ob~ect of this invention to pre-pare such a den~e, high strength, fine microstructured polycry~-talline ceramic having properties comparable to those achieved~y the expensive fabrication technique of hot-pressing.
It is an even still fur~r ob~ect of this invention ~ pr ~ re such a dense, high strength, fme microstruc~red polycrystalline ceramic whic~
i8 esgentially single phase Bll-Al2o3 and very resistive to ~echani~ degrada-tion by water vapor under ambient conditions.

S

.: -~99~97 BRIEF DESCRIPTION OF THE INVENTION
The above objects as well as other objects and purposes which will be apparent to those skilled in the art are accomplished by the processes of this invention which are based on the discovery that a finer, more uniform microstructure may be achieved in ~"-alumina sintered bodies if the metal ions having a valence not greater than 2, e.g. lithium or magnesium, are uniformly distributed throughout the beta-type alumina com-position to be sintered. Such a uniform distribution allows a more efficient and complete conversion to ~"-alumina.
The methods disclosed and claimed herein allow fab-rication by conventional sintering of polycrystalline ~"-alumina containing bodies with a high sodium ion conductivity which is comparable to or better than that obta~ned using the techniques of the prior art but with microstructures containing much finer grain sizes (i.e. maximum sizes under 25-50 micrometers and average grain sizes under 5-10 micrometers), and, as a conse-quence, fracture strengths which are comparable to those achieved by hot pressing.
Thus, the process disclosed herein provides techniques whereby conventional and rapid sintering at temperatures ~x~nd 1600C. can be used to produce very fine grained ~"-alumina ceramic bodies with fracture strengths comparable to those achieved in hot pressed material and with low sodium ion re-sistivities (< 9 ohm - cm at 300C). The average four point bend strengths of the ~"-alumina ceramic produced by the methods of this invention are over 30,000 psi. When strengths are measured by breaking small diametral segments of electrolyte tubes, average values around 39,000 psi can be achieved. Of course, it is well known to those skilled in the art that the _~ _ ,. ..

1C~99497 fracture strength of a ceramic body depends on the method of the test. Diametral strengths using sections taken from tubing are usually significantly higher than strengths measured on bars under conditions of four point loading.
The process of the present invention comprises: (a) mixing alumina and a sodium compound such as sodium carbonate with a lithium aluminate compound having the formula Li20:nA1203 wherein n=5 or more, preferably 5 to 11 in amounts stoichiometrically suited to form ~" - alumina upon sintering, (b) calcining the mixture at above about 1100C, preferably at about 1250C, (c)forming the desired green body and ~d) sinter-ing at a temperature above about 1500C, preferably between about 1500C. and about 1600C, and most preferably between about 1560 and 1600C. Near theoretical denqlty and desirable conversion to ~" - alumina will generally be achieved in less than about 10 minutes, typically between about one and about three minutes. The sintered ceramic body thereby obtained has such values of density, strength, grain size, electrical re-sistivity and resistance to mechanical degradation by water vapor as would make it ideally suited for use as a reaction zone separator or solid electrolyte in an energy conversion devic .
As mentioned above the process of this invention eliminates the need for post-sintering annealing treatment in that they produce directly after sintering a ~" - alumina con-taining ceramic body with an electrical resistivity for sodium ion conduction at 300C. of 9 ohm-cm or less, a high mechanical strength, and a fine grained microstructure. However, the sintered ~" - alumina bodies so produced may also be subjected to a post sintering anneal, generally of less than 1 hour ~(~99497 duration, in order to further lower their resistivity. Unlike the prior art technique discussed above wherein low temperature post sinter anneals of long duration (i.e., grea ~ than 1 hour) are used to avoid grain growth, our processes allow higher temperature annealing treatments to be carried out (e.g., ~
peratures of about 1475 to about 1550C. being useful) without the occurrence of any deleterious grain growth. Higher tem-peratures for annealing are, desired for rapid conversion to ~" - alumina which results in low sodium ion resisitivity.
The various embodiments of the methods of this in-vention will be more fully understood from the following detailed description of the invention.
More specifically, the invention sought to be claimed in this application comprises two alternative methods for pre-lS paring a dense~ strong, fine-grained polycrystalline ~ - alumina containing ceramic bo~y exhibiting an electrical resistivity for sodium ion conduction of 9 ohm-cm or less at 300C~ Both methods involve the improved distribution or introduction of minor constituents, particularly Li20, in the powder compact prior to sintering. The methods of this invention can also be applied to ~ - alumina compositions which contain various amounts of MgO in addition to or in place of Li2O.
A lithium aluminate compound tLi2o:nAl2o3) wherein n i8 at least 5, preferably 5 to 11 and most preferably 5 or 6, is used as the source of lithium in the preparation of the mixture of components necessary to produce ~"-alumina upon sintering. When using this method it is possible to achieve the desired product using only a short sintering time (e.g., less than about 10 minutes, but typically less than about 3 minutes at temperatures greater than about 1500C, but pre---L~--, -ferably between about 1560 and 1600C.) without the necessity of a post-qintering annealing tre'~tment.
This method of the invention is typically carried out as follows:
(A) Preparing the compound Li20:n5A1203.
(B) Preparing a mixture of a compound of sodium such as Na2CO3 and of ~~A1203 followed by a calcination step at a temperature of 1200C. to 1300C. for up to 2 to 3 hours wherein the said mixture has a composition such that when mixed with appropriate amount of Li20:nA1203 described under ~A) would correspond to a composition consistent with the formation of a ~" -alumina containing ceramic upon sintering, or mixing a com-pound of sodium such as Na2CO3, ~-A1203 and the Li20:nA1203 compound in the appropriate amounts to yield a composition con-lS si~tent with the formation of ~"-alumina upon sintering followed by a calcination step at a temperature of 1200 to 1300C. for 2 to 3 hours.
(C) Sintering the green body formed of the powder mixture prepared as described under ~B) above either encapsulated Z0 or in open air at temperatures between approximately 1560 and 1600C~ for about three minutes such that the green body having a green density of at least 50% of the theoretical density of ~"-alumina is converted on sintering to a ceramic body having a d~nsity of at lea~t 90~ of the theoretical density of ~" -alumina.
The sintering time and temperature schedules mentionedabove, although the most desirable for a production application, - are expected to be somewhat flexible, in that a shorter sinter-ing time could be used by raising the sintering temperature or a longer time may be needed at a lower temperature without _~_ q C, 1~399~9~

adversely affecting the properties of the sintered ceramic body containing ~"-alumina. This will be apparent to those skilled in the art of sintering ceramics.
For the preparation of a Li20:nA1203 compound such as Li20:5A1203 a compound of lithium, such as LiNO3, and ~-A1203 may be either dry or wet mixed in acetone or other suitable solvent followed by a drying step in an oven at a temperature of 80 - 100C. The said mixture is then calcined at a tem-perature of 1200Co to 1300C. for two to three hours in a loosely covered zirconia crucible. Although LiNO3 has been used here, any such appropriate compound of lithium including, for example, lithium oxalate, lithium carbonate, lithium hydroxide, etc., may be used. The calcination temperature can be between 1100C. and 1400C. The calcined mixture is then milled, either in a vibratory mill or a ball mill.
For the preparation of the mix described under (B) above, Na2CO3 and ~-alumina may be mixed to yield a desired ratio 80 that upon the addition of Li20:nA1203 to this mix a composition consistent with the formation of ~" - alumina can 20 be achieved. The mixture of Na2CO3 and ~ - A1203 is then calcined at a temperature of about 1200C. to about 1300C.
for about two to three hours. The said mixture of sodium car-bonate and ~ - A1203 that has been calcined at 1200C - 1300C.
as mentioned above is then mixed with the appropriate amount of 25 Li20:nA1203 to yield a composition consistent with the formation of ~" - alumina upon sintering. The powder mixture so prepared is ready for sintering.
In an alternativ~ procedure, a sodium ~Y~und such as Na2CO3 is m~ with ~ - A1203 and Li20:nA1203 in appropriate amounts to yield a o position consistent with the formation of ~" - alumina upon sintering, foll~ by a ~cinaticn step at 1200 - 1300C. for 2 to 3 hours.
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In the subsequent sections, the method of this in-vention is discussed and illustrated with examples relevant to the production of ~" -alumina sta~ilized with less than one ~
percent by weight of lithium oxide. However, the method of this present invention will be of a far wider applicability, in that it could be utilized in the preparation of 6"-alumina containing ceramic bodies stabilized with magnesium oxide or magnesium and lithium oxides wherein magnesium may be introduced as magnesium-aluminum spinel (MgA1204). Slurry-solution ~spray) drying can easily be adapted to solutions containing soluble magnesium salts.
In the examples cited later in this section, encap-sulated green bodies are sintered usually in a molybdenum-wound resistance heating furnace i~94S~

with a closed-end alumina service tube. For open air sintering, a continuous pass-through furnace with molybdenum disilicide heating elements is used wherein the rate of heat-ing to and cooling from the sintering temperature is precise-ly controlled. Such sintering techni~ues are well known inthe art.
Figure 1 i8 a set of histograms showing the grain size distributions of partially converted and zeta-proce~sed powders as referred to in Example 8.
The following specific examples will serve to illustrate the various embodiments of the methods of this invention in great detail:

,a ..

1(~9~9497 The following specific examples will serve to illustrQ~e the various embodirnents of the methods of this inventlon in great detail:

The ~-alumina powder ls first dried at 800C. for 2 hours and is stored in a desiccator immediately after cooling.
Lithium nitrate and sodium carbonate are dried at temperatures of 120C. and 280C. respectively and are desiccated. Electrical ovens and furnaces can be used for drying purposes.
In order to prepare 100 gms. of Li2O:5A12O3, 25.57 gms.
of LiNO3 are mixed with 94.46 gms. of ~-A12O3 in a vibratory mill.
The mixture is contained in a loosely covered zirconia crucible;, after which it is calcined at 1250C. for two hours. X-ray dif-fraction revealed that the resulting powder contained essentially one hundred percent Li2O:5A12O3. To make a one hundred (100) gm.
batch of ~"-alumina of a composition 8.8 wt.% Na2O - 0.75 wt.%
Li2O - 90.45 wt.% ~-A12O3, 15.05 gms. of Na2CO3 and 13.55 gms. of Li2O:5A12O3 are mixed with 77.65 gms. of ~-A12O3 in a vibratory mill. The said mix was calcined at 1250C. for two hours in a zirconia crucible followed by a wet-milling stop using acetone as the fluid. The powder was then dried to evaporate the acetone.
Rectangular bars were preformed in a steel die followed by isostatic pressing at 5~,000 psi. T'r~e bars ~ e bisque-fired at ~900C. for 1/2 hour to hurn off any volatiles that may been picked up during storageO The bisque fired specimens were encap-sulated in platinum tubes and were sintexed ~t 1585C. and 1600C.
for times from 3 minutes up to 10 minutesO For sintering the specimens were inserted rapidly into the hot zone of the furnace (from 500C. to the sintering temperature in about 30 seconds) and after the desired sintering time, were pulled out of the hot zone to a region where the temperature was ~800C.-1000C. (The times at the sintering temperatures are slight overestimates).
The density and the resistivity were then measured.
The relevant data are given in Table II.
TABLE II

-- Sintering of ~"-Alumina Li2O:5A12O3 Composition: 8.8 Wt.% Na2O -0.75 wt.% Li2O - 90.45 wt.% A12O3 Sintering Sintering Density Temperature time % Resistivity ~C) (minutes) ~g/cc) Theoretical ohm-cm at 300C.

1600 3 3.16 97.0 7.0 1585 - 10 3.18 97.4 7.4 1585 3 3.19 98.0 7.5 .
LiNO3 and a-A12O3 were mixed in appropriate amounts to produce a composition on a molar basis of "Li2O:llA12O3". The 2S mix of LiNO3 and-a-A12O3 was calcined at 1250C. for two hours in a loosely covered zirconia crucible. The calcined powder mix was milled for two hours in a vibratory mill in an environment of acetone. Acetone was later evaporated in an oven to produce a dry powder. X-ray diffraction revealed that "Li2O:llA12O3"
contained the compound Li2O:SA12O3 and a-A12O3. Appropriate amounts of "Li2O:llA12O3", Na2CO3 and a-A12O3 were mixed to ~99497 produce a composition Gf 8.8 w~% Na2O 0~75 wt.% Li2O - 90.45 wt.% ~-A12O3. The said mix1:ure of the abQve ingredients was calcined at 1250C. for two hours in a loosely cove~ed æirconia crucible. The powder was then milled for two hours in a vibratory mill. Rectangular bars were preformed in a steel die followed by isostatic pressing at 55,000 psi. The bars were bisque-fired at 900C. for 1/2 hour to burn off any volatiles that may have been picked up during storage. The bisque-fired specimens were encapsulated in platinum tubes and sintered at 1600C. for 3 to 4 minutes by a procedure identical to the one described in Example 1. The density and resistivity were measured, after the sintering.
The data are given in Table III, TABLE III
Sintering of ~"-alumina (8.8 wt.% Na2O - 0.75 wt.% Li2O
90-45 wt.% ~-A12O3) with "Li2O:llA12O3"

Sintering Sintering Density Resistivity Temp. Time ~ohm-cm (C.) ~Minutes) g/cc %Theoret-cal at 300C.
1600 3 3.20 98.1 Not Determined 1600 3 3.18 97.5 7.3 1600 4 3.20 98.1 7.2 Green ceramic bodies of 8.8 wt.% Na2O, 0.75 wt.% Li2O
; and 90.45 wt.% ~-A12O3 were made using Li2O:SA12O3 as a source of lithium as described in Example 1. Bar specimens pressed iso-statically at SS,000 psi were heated to 400C. to burn off the binder. The specimens possessed a green density of 63% of the theoretical density of ~"-alumina (theoretical density of ~"-alumina is taken as 3.26 gms/cc). These specimens were placed on a platinum boat and pulled at various speeds through a tube furnace heated with super Kanthal heating elements to 1590C.

The furnace possessed a hot zone length of about six inches.Density and electrical resistivity at 300C. were measured on all of the ceramic bodies containing ~"-alumina sintered in a continuous pass-through mode without any encapsulation. The S relevant data are shown in Table IV.
TABLE IV
Continuous pass-through sintering in open air of ~"-alumina of composition 8.8% Na2O - 0.75% Li2O - 90.45% A12O3 made by using Li2O:5A12O3 as a source of lithium 10 Pass-Through Density Sintering Velocity Resistivity Time (inch/min g/cc %The~retical at 300C. ~Time at ohm-cm 1590C.) __ _ _ in minutç~

2.1 3.25 99.7 8.2 2.9 3.0 3.24 99.4 8.7 2.0 4.0 3.25 99.7 8.9 1.5 -Ceramic bodies containing ~"-alumina sintered for short times tl-10 minutes) as described in Examples 1 to 3 were given an annealing treatment at various temperatures for various lengths of time. The specimens were of nominally 8.8 wt.% Na2O - 0.75 wt.% Li2O - 90.45 wt.% ~-A12O3 composition. A few of the specimens were prepared as described in E~mples 1 and 2 but using "Li2O:-2S 6A12O3" as the source of lithium which was produced in a similarway as Li2O:SA12O3 and "Li2O:llA12O3" and contained a mixture of the compound Li20 5A12O3 and ~-A1203. On some of the sintered ceramic bodies containing ~"-alumina, strength was measured in four point bending on bars with a width of ~ 0.4", a thickness of about 0.1", an outer support span of 1 3/8", and an inner span between the load points of 3/8" in ambient air under a deflection rate of 0.02"/min on an Instron Universal Tçsting Machine. The densities, resistivities at 300C. (electrical), strength and microstructures are summarized in Table V on all of the ceramic bodies prepared by the techniques described in Examples 1 to 3 and later annealed.

In order to examine the effect of water vapor on the possible mechanical degradation of ~"-alumina, ceramic bodies containing ~"-alumina were fabricated by the procedures described under Examples 1 and 2 followed by an annealing treatment described under Example 4. "Li2O:6A12O3" was used as a source of lithium.
In one set of experiments, the sintered and annealed specimens of composition 8.6 wt.% Na2O, 0.7 wt.% Li2O and 90.7 wt.% A12O3 were stored in an environment of one hundred (100) percent relative humidity at 25C. for up to 140 hours. The processing history and changes in resistivity at 300C. are summarized in Table IV. In this set of experiments specimens were removed from the humidity test chamber every 20 hourq to make electrical resi3-tivity measurements. The electrical resistivity data reported in ~able VI are after a storage of 140 hours. The strength data are obtained after the humidity test as well as on some control specimens before the test.

Ceramic bcdies containing ~"-alumina of composition 8.8 wt.%Na2O - 0.75 wt.% Li2O - 90.45 wt.% A12O3 were made by the techniques descxibed in Examples 1 and 2 by platinum encapsulation.
Sintering was accomplished at 1587C. for 1.5-2 minutes. The said ceramic bodies, in the form of bars, were then annealed by encap-sulating in a powder of the said composition for two days at 1250C. two days at 1330C. and for three days at 1350C. me said bars were stored in 100% relative hu~idity ch ~ er at 25C. me bars were stored at 100% relative humidity for as long as seven days without interrup~ion.

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1(~99497 The strength measurements were performed on bars after subject to humidity exposure in an ambient atmosphere at a de-flection rate of 0.02"/minO wherein the outer span was 1-3/8", the inner span was 3/8", the thickness of specimen of ~0.1" and the width of the specimens about 0.4" on an ~nstron Universal Testing machine. The relevant data are presented in Table VII.
TABLE VII
Resistance of ~"-A12O3 Ceramics to Mechanical Degradation in a Humid (110~ RH) Environment 10 No. of Days inStrength in psi Resistivity 100% Relative(average of at least) at Humidity three specimens 300C. in ohm-cm 0 29,300 3.23 1 day 33,600 4.51 2 days 30,100 4.14 3 days 26,600 4.65 4 days 31,300 6.04 7 days 30,100 5041 The properties in Table VII are improvements over prior art techniques which claim in U.S. Patent 3,765,915 that only com-pcsitions in the range: Li2O: 0.7 - 1.5 wto%, Na2O : 8.3 wt.%, and MgO : 0.5 - 2.0 wt.% are resistant to mechanical degradation by water vapor.

Green ceramic bodies of composition 8.8 wt.% Na2O-0.75 wt.% Li2O and 90.45 wt.% A12O3 were made as described in Example l.
Bar speaimens pressed isostatically at 55,000 psi were heated to 400C. to burn off the binder. The specimens were enclosed in a ~"-alumina tube of density 3.15 gms. or more having a slightly higher amount of sodium oxide. An end plug made of ~"-alumina was inserted loosley at the open end to minimize loss of volatile l~g9~97 Na2O during sintering. The ~"-alumina tube with the specimens was passed through the furnace where the temperature o~ the hot zone of the furnace was at 1590C. Table VIII gives the relevant data.
It should, of course, be noted that results comparable to those in Table VIII can be achieved by encapsulating the specimens in a bed of powder whose composition is similar to the specimens undergoing densification.
TABLE VIII
10 Sintering of ~"-A12O3 Ceramics by Encapsulation in ~"-A12O3 Tubes Pass-through Method of Velocity Time Density Resistivity Encap~(inch~minute) 1590C. (min) (g/cc) at 300C.
sulation~ ~ _ (ohm-cm) 15 ~"-alumina 2.1 2.9 3.25 6.8 " 3.0 2.0 3.24 8.7 " 4.0 1.5 3.23 8 7 Using an identical composition and the same procedures as described in Example 7, some experiments were performed by encapsulating specimens in a firing tube of ~-A12O3 ~i.ec Na2O.-9A12O3). Sintering at 1600C. for 30 minutes praduced a ceramic with a 4.4 ohm-cm electrical resistivity at 300C. ~-A12O3 tubes are equally effective as ~"-A12O3 refractory tubes in preventing

5 soda evaporation during sintering.

Na2CO3, LiNo3 and ~-A12O3 were mixed in the appropriate amounts to produce after calcination a mixture with composition 8.8% Na2O, 0.75% Li2O, and 90.45% A12O3 (by weight). The said mixture, referred to as an unconverted powder, was prepared by calcining at 1000C. for 2 hours and consisted of ~- al;umin~a : ~

i~99~97 and the aluminates of sodium and lithium. X-ray analysis indicated that the unconverted powder contained no ~ or ~" - alumina.
A second mixture of the same composition was prepared by calcining appropriate amounts of ~a2CO3, LiNO3 and ~-A12O3 at 1250C. for 2 hours to yield a partially converted powder con-taining ~ and ~" - alumina. This type of powder is typical of the prior art. X-ray analysis of the partially converted powder indicated that it contained approximately 40% 3" - alumina.
A third mixture of the same composition was prepared by a method identical to that described in Example (1).
Rectangular bars were preformed from all three mixtures (unconverted, partially converted, and one containing Li2O:5A12O3) by isostatic pressing at 55,000 psi. The bars were bisque-fired at 900C. prior to sintering by platinum encapsulation at 1600C.
for five t5) minutes. After sintering, sodium ion resistivities were measured at 300C. and microstructural examination was per-formed on the bodies prepared from the three powder mixtures. A
summary of the resistivities, microstructural features, and pro-cessing conditions for these three powders is given in Table IX.
As can be seèn from Table IX the lowest resistivity was achieved in the ceramic processed with the zeta lithium aluminate (Li2O:5A12O~) while the highest resistivity was found in the ceramic processed from the unconverted powder.
The resistivity of the ceramic prepared from the par-tially converted powder was comparable to the zeta-processed ceramic but possessed a microstructure with a much coarser grain structure.
The nature of the grain size distributions in sintered ceramic bodies prepared from the partially converted and zeta-processed powders is shown in Figure 1. In the ~" - alumina pre-1~9~4Y~7 pared from partially converted powders, which are typical of the prior art, grains in the grain size distribution up to 160 micro-meters are present after a 8-15 minute sinters at 1600Co However, in the zeta-processed ceramic the largest grains are only 20-25 micrometers after comparable sintering schedules.
The annealing treatment (Refer to Table X) at 1550C.
(1 hour) resulted in a further reduction in the resistivity of the ~" - alumina ceramic prepared from the three powder types.
After annealing the zeta-processed ceramic again possessed the lowest resistivity (~ 4 ohm-cm at 300C.) while the body processed from unconverted powders possessed the highest resistivity (~ 5 ohm-cm). The microstructures after annealing for the three conditions are markedly different. In the ceramic bodies pro-cessed from the unconverted and partially converted powders exten-sive exaggerated grain growth occurred during the high temperature anneal. Grains up to 150 to 300 micrometers were present in the grain size distributions. From previous work on the fracture of dense polycr~stalline ~" - alumina the fracture strengths of this material annealed at 1550C. would be well under 20,000 psi.
On the other hand, the fine grained microstructure which was pro-duced in the zeta-processed ceramic after sintering was retained after the high temperature anneal at 1550C. Thus it is clear that zeta-processed ceramics can be sintered and/or annealed at relatively high temperatures (over 1500C.) which is desirable from the point of view of conversion to ~" - alumina and hence, conductivity reduction. This high temperature processing can be accomplished without the occurrence of any deletexious grain growth which would adversely affect the fracture strength.

To illustrate the effect of microstructure on fracture - ^ . . .. .. ~ .. ....

strength, ~" - alumina electrolyte tubes of simllar composition were fabricated from the unconverted, partially converted, and zeta-processed powders. These tu~es (1.5 cm OD) were sintered by platinum encapsulation at temperatures between 1550 and 1585C. for times between S and 10 minutes and annealed for periods of time between 1 and 25 hours at temperatures between 1400 and 1475C. The particular times and temperatures foreach powder type are summarized in Table XI. This annealing at tem-peratures significantly lower than 1550C. tTable X) will permit further conversion to ~" - alumina (i.e. lower resistivities without the occurrence of any additional grain growth over that which occurred during the sintering step. Thus the fracture strengths of sintered ~" - alumina ceramics prepared from uncon-verted and partially converted powders and annealed at 1400C.
would be improved over those annealed at 1550C. ~Table X). In Table XI diametral fracture strengths taken from ~" - alumina electrolyte tubi~g are summarized for ceramic annealed at 1400C. and prepared -26a-1C~99497 U~

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Strengths of ~"- Alumina Ceramics Prepared from Unconverted, Partially Converted, and l~Zeta-Process Powders ~ ._.
Powder Composition Sintering Sintering Annealing Annealing Diametral Range Type temperature Time Tempera- Time Fracture (kpsi) C (min) ture C (hours) Strength (kpsi) .... ... ~
Unconv- 8.8% Na20- 1550 5 1400 25 22.8 17-25 erted 0.75%Li20 lO Unconv- 8.8% Na~0-erted 0.75% L120 1550 5 N0 ANNEAL 22.5 17-25 Partia-1-8.8% Na20-ly conv-0.75% Li20 1585 7 1400 8 24.0 17-30 erted Zeta 8.85% Na20-Process-0.75% Li20 1585 5-10 1475 1 38.0 27-53 es .

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1~99497 from unconverted and partially converted prior art powders. As expected higher strengths t~22,000 psi) were achieved in the lower temperature anneal. However, these strengths are markedly lower than those obtained in the zeta-processed electrolyte tubing (~38,000 psi). The high strengths observed in the zeta-process ceramic are now comparable to those which have been achieved in hot-pressed ceramics developed in our laboratory.
The excellent physical properties and microstructures of the zeta-processed ceramics are believed to be the result of improved distribution of lithium in the ~" - alumina ceramic.
This improved distribution leads to enhanced conversion to ~" -alumina during calcination, sintering, and annealing and, at the same time, prevents the occurrence of any deleterious grain growth which adversely affects the mechanical properties.

~?

Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for preparing a dense, strong polycrystalline .beta."-alumina-containing ceramic bGdy exhibiting (i) an electrical resistivity for sodium ion conduction at 300°C. of less than or equal to 9 ohm-cm, and (ii) a fine, uniform microstructure and resultant fracture strength comparable to that achieved by hot pressing techniques, comprising:
(A) preparing a powder mixture comprising alumi-num oxide, sodium oxide and lithium oxide in stoichiometric portions required to produce .beta."-alumina upon being heated to crystal forming temperatures, and lithium oxide being present in an amount ranging from .1 to 5 weight per cent of the total of said mixture and being provided by a lithium aluminate compound having the formula:
Li2O:n Al2O3 wherein n equals at least 5;
(B) green forming said prepared mixture to the desired shape;
(C) sintering the green formed body at a tem-perature above 1500°C. until desirable conversion to .beta."-alumina and near theoretical density for same are achieved.

2. A method in accordance with claim 1, wherein in step A said powder mixture is prepared by calcining a mixture of alumina, sodium oxide or a sodium salt cap-able of decomposing to form sodium oxide and lithium aluminate compound in such amounts that the reaction mixture will provide the necessary stoichiometric amounts of alumina, sodium oxide or lithium oxide to form .beta."-alumina at a temperature above about 1100°C.

3. A method in accordance with claim 1, wherein in step A said powder mixture is prepared by calcining a mixture of said alumina and said sodium oxide or a sodium salt capable of decomposing to form sodium oxide, said sodium salt being present in an amount to provide the necessary stoichiometric amount of sodium oxide, said calcination being at a temperature above about 100°C. and then mixing said lithium aluminate compound therewith.

4. A method in accordance with any one of claims 1, 2 and 3, wherein said green formed body is sintered at a temperature between about 1500° and 1600°C.

5. A method in accordance with claim 1, wherein said green formed body is sintered at a temperature between about 1560° and about 1600°C. for less than about 10 minutes.

6. A method in accordance with claim 5, wherein said body is sintered for between about 1 and about 3 minutes.

7. A method in accordance with any one of claims 1, 2 and 3, wherein said lithium aluminate compound is selected from compounds in which n equals from 5 to 11.

8. A method in accordance with any one of claims 1, 2 and 3, wherein said lithium aluminate com-pound is represented by the formula Li2O:5Al2O3.

9. A method in accordance with any one of claims 1, 2 and 3, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a tem-perature of less than about 1550°C. to further reduce its electrical resistivity to sodium ion conduction.

10. A method in accordance with claim 1, wherein (i) said lithium aluminate compound is represented by the formula Li2O:5Al2O3 and (ii) said green formed body is sintered at a temperature between about 1560°
and about 1600°C. for less than about 10 minutes.

11. A method in accordance with claim 10, wherein said powder mixture is prepared by calcining a mixture of said alumina, said sodium salt and said lithium aluminate compound at a temperature above about 1100°C.

12. A method in accordance with claim 10, wherein said powder mixture is prepared by calcining a mixture of said alumina and said sodium salt at a temperature above about 1100°C. and then mixing said lithium aluminate compound therewith.

13. A method in accordance with claim 10, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a temperature of less than about 1550°C.

14. A method in accordance with claim 2, wherein said green formed body is sintered at a temperature of between about 1500°C. and about 1600°C. for less than 10 minutes.

15. A method in accordance with claim 14, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a temperature of less than about 1550°C. to further reduce its electrical resistivity to sodium ion conduction.

16. A method in accordance with claim 14, wherein (i) said lithium aluminate compound is represented by the formula Li2O:5Al2O3 and (ii) said green formed body is sintered at a temperature between about 1560°C. and about 1600°C. for less than about 10 minutes.

17. A method in accordance with claim 16, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a temperature of less than about 1550°C. for less than about one hour to further reduce its electrical resistivity to sodium ion conduction.

18. A method in accordance with claim 3, wherein said green formed body is sintered at a temperature of between about 1500°C. and about 1600°C. for less than 10 minutes.

19. A method in accordance with claim 18, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a temperature of less than about 1550°C. to further reduce its electrical resistivity to sodium ion conduction.

20. A method in accordance with claim 3, wherein (i) said lithium aluminate compound is represented by the formula Li2O:5Al2O3 and (ii) said green formed body is sintered at a temperature between about 1560°C.
and about 1600°C, for less than about 10 minutes.

21. A method in accordance with claim 20, wherein said .beta."-alumina containing ceramic body is subjected to a post-sinter anneal at a temperature of less than about 1500°C. for less than about one hour to further reduce its electrical resistivity to sodium ion conduction.