CA1114151A - Production of polycrystalline titanium dioxide ceramic - Google Patents

Abstract

A B S T R A C T
Polycrystalline titanium dioxide ceramic members are obtained by (A) forming a slurry comprising titanium dioxide and said metal species, said metal species being present in an amount to provide from 0.01 to 8 atomic percent of said metal species in said ceramic member, said slurry being formed either (i) by dissolving in a solvent selected from water and aliphatic alcohols the pentafluoride salt of said metal species in an amount adapted to provide from .01 to 8 atomic percent of said metal species in said ceramic member and then adding titanium dioxide powder in either the rutile or anatase form to the solution to produce a slurry; or (ii) dispersing a mixture of a pentachloridediethyl-etherate complex of said metal species and titanium dioxide powder in either the rutile or anatase form to form a slurry, said slurry components being included in amounts adapted to provided from .01 to about 8 atomic percent of said metal species in said ceramic member;
(B) Drying said slurry to powder form at a temperature at which evaporation of said metal species is avoided;
(C) Green forming said powder to the desired shape;
(D) Heating the shaped green body to 350°C. at a rate of at least 10°C. per minute; or in the case where slurry of step A is formed using the pentachloride dietherate complex heating to 500°C. at a rate at least 40°C. per minute;
(E) Sintering said shaped green body at a tem-perature of at least 1330°C.; and optionally (F) Annealing said sintered shaped body in a re-ducing atmosphere having an oxygen partial pressure of 10-5 to 10-25 atmospheres at a temperature of from about 850°C.
to 1400°C. to increase the conductivity of said ceramic member.

Description

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The inventio~ h~rein de~cribed w~ made in the course of or und~r a contract or subcontract with the National Science FoundatisnO
This invention relate~ to an improved electrlcAlly conductiYe current collactor ~u~t~ble for use in high te~p ~rature application~ in tho pre30nce of ~orra~ive environ-m~nt~.
More particularly thi~ inven~on ralate~ to a m~thod f~r manufactuxing ~ high str~ngth el~ctronic~lly condùetlve polycxy~talline titanium dioxide c~ramlo m~mbBr u~eful in ~n electronlcally condu~t~ve curront coll~otor or current collector/çont~inor for u~e in energy aonvorslo~
d~vla~s ~uch ~9 the ~odlum-~ul~ur batt~y.
Suoh curr~nt collec~ors ar~ crib~d ~nd cl~l~ad~
ln.Appllcation No. 272017 from whlch th~ p~aoa~t a~pllc~t~on : has b~n dividsd.
Thor~ aro ~ numb~r of e~lectr~oal applicatlon~ ln-volvlng v~lou~ ~n~rgy conv~xsloll tovl~ ln whlch th-, cnvl~onma~t to ~hl~h tho curr~nt coll~ctor of th~ ~o~lc~ 1 - 20 axpoo~d i~ ~xtrQm~ly co~roolv~D For oxamplo i~ rgy con-var~lon ~lc-a of th~ typo ~omp~s1~g a molton ~athodl~
x~actant ~u~h a~ sodlum.poly~ulf~d~ th~ ~12i~tlon of a ~u~t~ çu~r~n~ ~olloctor ~B wall as ~ ~u~tablo ~ontain~r h~ n a ~ou~oo o~ aon~lder~bl~ conc~rn.
On~ of ~ho prlmo o~nd~dato~ to date for u3e a3 a . , .

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current collector or current collector/container for such devices have been certain metals. However, metal ~ystems, both pure and alloyed, often exhibit the phenomenon of severe plastic deformation under stresses (external and from their own weight). For this rèason and because of severe corrosion problems, many metals are not practical for u~e in ~uch high temperature or corrosive (oxidative) environments.
Since the thermodynamic stability of ceramic materials such as oxides and sulfide~ in the pre~ence of corrosive environme~t~ is well established ànd since it is also known that the thermodynamic stability of such materials is maintained to temperatures much higher than is com-patible for metal systems, it ha5 been sugge~ted to employ a ceramic coating on a metal which is used as the load bearing element of the current collector or container.
Where a metal system operates as the load bearing element and includes a protective covering separating the metal from the corrosive substance, the l3elect$0n of a suitable covering must be made from materials which ~1) are non-corrosive and impermeable to the corrosive sub~tance, (2) adhere well under c~nditions of thermal cycling and (3) have suf~icient electronic conductivi~y. Oftentims~ a thermal expansion mismatch between the attached metal and ceramic covering re~ults in fractures, microcrack~ and eventual spalling of the coating from the metal surface.
In addi~ion to mechanic~l incompatibility, conventional me~hod of applying the ceramic coating such as by anodizing often result in an insulative rather than a con-ductive coating.
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In summary, the concurrent development of the requisite non-corrosive character, good adherence, and adequate con-ductivity in a coating which will be mechanically compatible :.
under recurring cycles of thermal expansion has long ~::
presented a difficult challenge in this field of art.
In view of the above discussed inherent limitations : .
of current collecting systems comprising a metal load bearing element with a corrosion resistant ceramic coating, ~ -the u~a of corrosion resistant ceramic per se has been ~uggested. However, the vast majority of useful ceramics are electrical insulators, thus making them unsuitable for ~-current collection purposes. Of course, some ceramics are known to be conductive in the metallic sense, but are not economically attractive, A large class of ceramics can be made moderately cond~ctive, but with conductivities whi~h are much less than metals. Consequently, a curr~nt collector constructed of an electronically conductive ceramic will exhibit a much ; .
higher resistance than that of a si.milarly shaped metal current collector.
According to the present invention there i5 providea a method for manufacturing a high strength, non-corrosive, electronically conductiv~ polycrystalline .
titanium dioxide ceramic mambex exhibiting high resistance to thermal shock and uniform grain size with an average size of less than about 25 micrometers and consistin~ of titanium dioxide in the rutlle crystallographic form doped with a homogeneously distributed ionic metal species selected from tantalum and niobium, comprising:
(A) forming a slurry comprising titanium dioxide and . . ~ , . .

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said metal specie~, said metal specie~ being pre~ent in an amount to provide from 0.01 to 8 atomic percent of sai~
metal species in said ceramic member, said slurry being formed either (i) by dissolving i.n a solvent ~elected from water and aliphatic alcohols the pentafluoride salt of said metal species in an amount adapted to provide from about .01 to about 8 atomic percent of caid metal ~pecies in said cerami~
member and then adding titanium dioxide powder in either the rutile or anatase form to the 801ution of (1~ to produce a mixed slurry; or (ii) di~persing a mixture of a pent~chlorido-dle~hyl-etherat~ complex of ~aid metal 3pecie~ and tltanium dioxld~
powder in either the rutile or anatas~ ~orm to form a slurry, said slurry components bolng ~nalud~d in amount#
adap~ed to provide from about .O:L to about 8 atomic p~rc~nt of said metal species in 9aid cernmic memb~rt (~) Drylng said ~lurry to powdor ~orm at a t~mpQrature adapted to avold ~v~poration of l3aid metal apecl~
~0 (C) ~reen form~ng ~ld powld~r to th~ d~lr~A ~hapet (D~ Heating the ~haped gre2n body to about 350~C. ~t a rate of at le~t about 10C. por minute; or ln th~ ca5a where slurry of s~ep A is formed u~ing the pentachloride di~th~r~t~ compl~x heating to about 500C. at a rate at le st 40C. par mlnute (E) Sin~aring sald ~haped gr~en body at a temperatur~
of at least about 1330C.; and ~ (F) ~nne~ling said sintered shaped body ~n a reducing atmo~phere having an oxygeh partlal pres~ure of about 10 5 to about 10 25 atmo~pheres at a temperature of from about .~ , .

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P~l 850C. to about 1400C. to increase the conductivity of said ceramic member.
The invention will be more fully understood from the following detailed description of the invention taken in conjunction with the drawings in which the Figure shows a graph depicting the reduction in resistivity achieved in rutile titanium dioxide by annealing.
Preparation of highly corrosive oxide ceramics is accomplished in the art by four commonly accepted methods:
tl) intrinsic high conductivity, (2) reduction of the oxide ceramic causing a deficiency in oxygen ions and subsequent electrical compensation by the addition of conducting elect~ons, t3) controlled addition of an ionic species difering from the solute cationic species in both con-stitution and electric charge, the added speci~s occupying an interstitial crystal site, c~arge neutrality con- ;
siderations creating conducting electrons and higher con-ductivity, and (4) controlled addition of an ionic species difering from the solute cationic species in both con-stitution and electric charge, the added species occupying by substitution the sites of the parent cationic species with charge neutrality considerations producing conducting electrons.
Intrinsic high conductivity is exhibited by ruthenium oxide, a compound normally considered uneconomical because of the rare occurrence of ruthenium in nature.
Methods (2) and t3) normally result in the creation of charged, mobile atomic entities which can move easily under the force due to an electric field. Method (4) offers the 3~ greatest promise for applicability in the current collector ~ , :
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described in that the addition of impurity ions in a sub-stitutional manner usually produces a nearly immobile impurity except at very high temperatures.
Three economically viable metal oxides which may be made conducting and which are economically viable because of natural abundance are calcium titanate (CaTiO3), strontium titanate (SrTiO3) and one of the derivatives of both titanates, titanium dioxide (TiO2) in the rutile crystallographic form. Common substitutional additive ions for all of these oxides include iron in the +3 oxidation state¢and aluminum in the ~3 oxidation state. Greater electronic conductivity increase may bP accomplished by the addition of an ionic metallic species having a stable valence in said ceramic of at least ~5. Tantalum in the +5 oxidation state or niobium in the -~5 oxidation state are preferred because of the solubility of these elements and because the charge carriers created from the niobium or tantalum impurity additions remain nearly free for electronic current flow.
By far the most common prlor art method of adding niobium or tantalum to these metal oxideq, when the resulting ceramic is to be polycrystalline, i~ the simple mixture of fine powders of niobium pentoxide (Nb205~ or tantalum pentoxide (Ta205) with fine powders of the solute subqtance CaTiO3, SrTiO3 or TiO2. Subsequent processing by commonly known arts of pressing the mixed powders into green ceramic form and sintaring at a suitable temperature yield a black, dense ceramic w~th conductivity drastically enhanced over the pure ceramic.
Grain size is generally very nonuniform coincident _7_ - ~
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with the inhomogeneous distribution of the impurity in the solute. That is, the tantalum or niobium is not distributed in a homogeneous fashion, the large ceramic grains contain smaller amounts, the smaller grains existing as such because tantalum and niobium tend to serve as grain growth inhibi-tors. Excessive heating at very high temperatures may be employed to further homogenization but at the expense of further grain growth and excessive processing costs.
Barium titanate, homologous to calcium titanate and strontium titanate, may be formed as a powder and in-timatèiy mixed with tantalum oxide or niobium oxide by a gel process deRcribed in U.S. patent 3,330,697. Even if this method is extendable to CaTiO3, SrTiO3 and TiO2, a need exi~ts to simplify the powder preparation proce~s ~or economlc reasons when large quantities of conducting ceramic are required.
Relative abundance of CaTiO3, SrTiO3 and TiO2 makes these materials attractive for use in corrosion resistance, electronically conducting current collectors provided these materials may b~ processed at low CQSt and exhibit electronic conductivities adequate for the planned use of the current collector. Of special interest i~ the use of electronically conducting Ca~iO3, SrTiO3 and ~iO2 as materials for a current coll~ctor or current collector/container for the ~odium-~ul~ur battery, such as is disclosed in ~he U.S.
patents 3,404,035 an~ 3,468,70g.
The above ceramic oxideR not only may be made electronically conducting in the sense that resistivities at room temperatur~ are less than 50 ohm-centimeters but 0 also are known to be resistant to corrosive attack by . . . - . , . : :

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commercial and/or electrolytic grade sodium polysulfides at 400C or below.
Disc shaped samples of about one and one~fourth inch diameter and one-eight inch thickness have been formed and sintered for the following chemical compositions and te~ted for corrosion resistance.
1) TiO2 (rutile) containing 1% tantalum

2) SrTiO3 reduced in water vapor atmospheres

3) CaTiO3 containing 3.0% iron

4) LaO 84Sr0 16CrO3 with no other additives.
Also, a single crystal sample of Tio2 (rutile) con-taining 0.05 percent tantalum has been subjected to the corrosion tests above, which are performed by the method of recording th~ initial ~ample weight, subjecting the sample to the sodium polysulfides (in either commercial grade or electrolytic grade quality) at 400C by immersion for 14 days and subsequently weighing the cleaned samples after immarsion for detection of weight 1089 or weight gain due to corrosive reactions with the ~odium polysulfides. All o the above named samples exhibited either no weight change or a very small change after ~he above tests (see ~able belo~) lndicating g~od corro ion resistance to these liquids.

Weight Change in .
Sodium Sulfide at 400C. (PCT) Material Form after 14 davs ~ . .
CaTiO3 + 3,0% Fe Sintered -1.45 SrTiO3 - xed~ced Sintered -.141 SrTiO3 Sintered -S.0 TiO2 + 0.5% Ta Single Crystal 0 ~i2 ~ 1.0% Ta Sintered 0 ;~ 9- ' "

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The ideal electronically conductive ceramics for use in this invention are the tantalum and niobium doped titanium dioxides prepared in accordance with the method of this invention which was briefly described above and will be more fully discussed hereinafter.
As mentioned previously the ideal high strength, noncorrosive, electronically conductive polycrystalline ceramic for use in preparing the current collectors are prepared in accordance with either one of two variants of a basic method. The ceramics are titanium dioxide in the rutile crystallographic form which is doped with a homogeneouqly distributed ionic metal species selected from tantalum and niobium. This homogeneous distribution of dopant i8 a result of the processes employed and results in ceramics having excellent properties including high resistance to thermal shock, uniform grain size and an average grain size of less than about 25 micrometers, The first of the two proce~ses comprises:
A) The formation of a slurry o~ titanium dioxide in either the rutile or anatase crystallographic forms in water solution (or aliphatic alcohol solution) of tantalum or nio~ium pentafluoride. The weights of titanium and th~
pentafluoride salt constituent will be determined by the desired final concentration of dopant in the titanium dioxide. In general, it is desirable to provide from abou~
.01 to about 8 atomic percent of the ionic metal species in the sintered ceramic. It will be appreciated that the amount of dopant which may be added with continuous ~lectrical property enhancement of the ~iO2 due to homogeneous tantalum or niobium distribution i5 ultimately limited by the 1 0-- , ~L$~
solubility of tantalum or niobium in TiO2, but certainly quantities of up to 3 percent Ta or Nb on a titanium ion basis is possible. For no tantalum or niobium percentage should the sum of all percentages of cationic impurities of normal ionic charge less than +4 or greater than +5 be greater than about 0.10 percent and concurrent with this restriction, the sum of all percentages of cationic impurities of ionic charge less than +4 or greater than ~5 should not exceed 10 percent of the added tantalum con-centration. Impurity levels greater than these mentioneds_rve to drasticalIy limit the attainable conductivity resulting in a highly inefficient use of the tantalum panta-fluoride addition~.
B) The slurry is next dried to powder at a tem-lS perature adapted to avoid evaporat:ion of the metal species.The drying of the slurry is generally accomplished by slowly heating to a temperature o not more than 110C and should be accompli~hed in a time of not rnore than 10 hours.
Stirring of the slurry enhances the rate o~ drying. During thiq process addition of a suitable binder useful in green forming of the ceramic body may be accomplished. Penta-fluoride saltq of the metal ion which remains after drying, melt~ at temperatures less than 100C and the ~apor pressure of the substance rises to an unreasonably high value before the drying slurry reache~ 130C. Long term heating at temperatures above 120C results in the evaporation loss of the pentafluorlde when experimental conditions are equivalent to open container heating.
G) The powder with water removed is pressed into a ~uitable or desired form and sintered at elevated temperatures in air or oxygen with critical heating rate of at least 10C per minute maintained between ambient and 350C to prevent loss of tantalum or niobium via a vaporization process. Above 350C oxidation of tantalum or niobium and fluorine to succeedingly more stable oxides occurs, each succeeding form being less susceptible to vapori2ation loss until the final form of tantalum or niobium pentoxide is reached and which is stable to vaporization loss throughout common sintering temperature ranges. A wide range of final sintering temperatures and holding times at the sintering temp2rature may be useful to those skilled in the arts of ceramic processing. A temp-erat~re of at least about 1330C and more preferably a range of about 1380C to about 1440C may be used. ~owever, the preferred conditions for optimal densification, homogenization of the tantalum or niobium ion by diffusion into the rutile powders and minimization of uniform grain size i~ about 1400C ~or approximately 3 hours. A minimum temperatUrQ of 1330C is required ~or reasonable rates of homogenization. These above stated conditions apply to the processing regardles~ of the quantity of tant~lum or niobium fluoride added to the rutile. The resulting dense material may be cooled to room temperature from 1400C in as little ~s 10 minutes for samples containing 1 percent tantal~m or niobium. Cooling rate~ must decrease for samples with decreasing pe~centages of tantalum additive.
The above stated m~thod is conductive to the fab-rication of highly conductive tantalum or niobium doped rutile ~n a batch sintering mode and may be subjected to 0 many minor modifications to suit available processing ' ~ .

apparatus. Samples of dried TiO2 powders in both the rutileand anatase forms and under several forming conditions may be processed with the method described including about 1 atom percent of Ta and produce very nearly identical values for properties of electrical conduckivity (i.e. about 1 mho/cm), uniform grain size less than 25 micrometers, fracture strength of 18,000 psi or greater and density greater than 9B percent of the theoretical density of Tio2 in the rutile ~orm. Some variations of processing methods, starting TiO2 powders and green forming methods are discussed in ~pecific examples will make this point clear.
The second method of processing TiO2 and small percentages of tantalum or niobium additive into a highly conducting, thermally shock resistant ceramic with uniform grain size o~ less than 5 micrometerC and fracture strength of 18,000 psi or greater comprises:
A) The addition of tantalum or niobium penta-chloride to diethyl ether to form the molecular complex tantalum or niobium pentachloride di.ethyletherate, with any excess ether to serve as a liquid into which Tio2 powder in eith~r the rutile or anatase form may ~e stirred to form a 31urry. Con3tant stirring causes the ether to evaporate rapidly a~ room ~emperature leaving essentially dry, mixed powders to which a binder may be added. This simple method is ideally suited for rapid, ~ontlnuous raw material prep-arat~on. At this stage of processing, the concen~ration of tantalum or niobium is adjusted by varying the amount of tantalum or n~obium pentachoride used to react with the diethyl ether. As in the first method, it will be appreciated that the amount of tantalum or niobium which may ' ' ' ' : . , be added with continuous electrical conductivity enhancement of the rutile due to homogeneously distributed tantalum or niobium i5 ultimately limited by the solubility of the metal species in TiO2, but certainly quantities of up to 3 atomic percent are possible. For no tantalum or niobium percentages should the sum of all percentages of cationic impurities of normal ionic charge less than +4 or greater than ~5 exceed 10 perc~nt of the added tantalum or niobium concentration.
Impurity levels greater than these mentioned serve to ~-drastically limit the attainable conductivity resulting in a highly inefficient use of the tantalum or niobium penta-chloride additions. The resultin~ mixed powders may not be dried at temperatures exceeding 65C for times exceeding a few minutes due to the loss of tantalum or niobium by vaporization o ~aOC13, a product of the tantalum chloride diethyletherate decomposition at 65C or the comparable niobium compound.
B) The powder with a suitable binder is pressed into a suitable form and sintered a~t elevated temperatures in air or oxygen with a critical heating rate of at least 40C per minute maintained between ambient and 500C to prevent loss of tantalum or niobium via vaporization processes. ~bove 500C, more stable oxides of tantalum or niobium and chlorine form and are less suscQptible to vaporization, the final form being tantalum or niobium pentoxide which i3 stable to vaporization loss throughout the common sintering temp~rature ranges. The methods, rAnges of sinterlng conditions, cooling rates and properties are very similar to the first method o~ preparation except 0 that the grain size is uniform at 5 micrometers or less and has fracture strength of greater than 18,000 p8i.
For elther o the above method~, in order to produc~ rutile doped with tantalum or niobium or repe~table elec~rical characteri~tics an anneal of tha caramic in a relativ21y low oxygen pressure environment is useful.
Such a reducing treatment serves to increa~e the conductivity ef the ~ample to valuec as high aq 5 (ohm-cm1 1 ~t room temperature for material containing about 0.5~ tantalum and 9 (ohm-cm) at room temperature for m~t~rial contalnlng 1 atom percent tantalum. The approxlmate rang~ o~ useful oxygen partial presiure iq from 10 5 to 10 25 atmooph~r~
(optimum being 10 10 to 10-2 ~tmcsph0r~s) with the ann~llng temperature ranging from a~out 850C ~o ~bout 1400~C. ~t the lower temperature of B50C, th~ ann~ ho~ld b~
out for appxoxlm~tely 3 hour~ whll~ at 1400-C ~pproxl~
1/2 hour ~hould b~ adequatQ~ ~hoge ~klll~d ln tha art ca~
ea~ily determine optimum time~ fo~ ~n~Als carria~ out ln the middle of tha above temperatur0 rang~. A~t~r ~uch ~n anneal, tha conductivity of the cex~mic con~lnlng from O. 1 to 3.0 atomlc p~rcent of tantalum or nioblum ln th~ t~mp~
ar~tuE~ r~nge of ambion~ to 350~ 1~ ral~tlv~ly ~n~p-n~nt o~ temp~r~turo.
The Figure i~ a graph sho~in~ the relationship between the resisti~i~y of the ceramic and the anne~}ing ~tep.
EXAMP~E 1 ~n ord~r to produ~e 10~.28 g~am~ of Tio2 powd2r-cont~inlng 1 p~rcont Ts catlons, 3.49 ~ram~ o~ tantalum panta~luorid~ o~! 99~ purity on a me~ b~ d~ssolved ln 20 ml o~ wat-r at about 25C in a poly~thylen~ b~3aker.
Th~ aolutlon i~ a~d to an ~th~l alcohol ~lurry cont~ir,ing .

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,, 100.0 grams of reagent grade TiO2 powder in the anatase form while constantly stirring until the slurry becomes very viscous and nearly dryO The powder, still slightly wet, is placed in an oven at 100C for overnight drying.
To the dried powders in added 1 ml. of a binder composed of a solution of polyvinyl alcohol (PVA) in water.
The PVA-powder combination is ground in a mortar and pestle until apparently well mixed. The combination is then 3creened into nearl~ round pellet form by forcing the powder-binder co~bination through a screen of 20 wires per lineal inch. Subsequently, the powders are poured into molds and pressed at about 20,000 psi after which they are prefired ~o a temperature of 950C. During prefiring the elapsed time between ambient and 350C is less than about 35 minute~.
The heating rate in this temperature range is approximately constant; the average hèating rate i~ about 10C per minute.
The green ceramic bodies are held at 950C for approximately 3 hours and then cooled in about 4 hours to room temperature.
The prefired ceramic bodies are placed on a small brick slab which has been coated with unpressed powders of ~he ceramic body composition and placed in a sintering furnace at 1000C. The sintering furnace is composed of a tubular, nonporous aluminum oxide t~be and aluminum oxide flat shelf and electrically heated by silicon carbide rods.
The sample is introduced into the highest temperature zone of the furnace ovex a period of 5 to 10 minutes, after which the furnace is he~ted to a sintering temperature of 140QC
in about 2 t~ 3 hGur~ in air. The ceramic body is held at the sintering te~perature for 3 hours and immediately removed by pu~hing the brick sample slab to the end of the ': ' , , , . . . .. :
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, furnace tube and placing in ambient air to cool.
The ceramic bodies are placed upon powders of Ta doped rutile within a tubular furnace, the ends of which are sealed with water-cooled rubber rings. The furnace is evacuated with a mechanical pump after which nitrogen gas with less than 8 ppm oxygen is passed over metallic titanium powder and through the furnace. The furnace temperature is raised to 1200C at the point of placement of the ceramic body and at 1050C at the point of placement of the metallic titanium powder. The temperatures are maintained ~or approximately 3 ho~rs before slowly cooling over 8 hours to ambient. 1~ -The ceramic body is sectioned for density mea~ure- -ment by hydrostatic weighing and a section is removed for the addition of metal electrodes required for a determination of electrical resistivity and degradation of the metal-ceramic interface upon exposure to oxygen at various (higher than ambient) temperatures.
The conductivity o~ a sample treated by thi~
method will b~ approximately 9 mho per centimeter at room temperature, increasing to 16 mho per centimeter or greater above 300~C. The fracture strength of ceramics prepared in this fashion, as measured by the 4 point bending test is greater than 18,000 psi.
2S It is speculated that uniform grain size and high ~lectrical conductivity is largely due to the homogeneous distribution of tant~lum in the rutile phase of ~iO2.
~pparently the most critical step is the addition of a volatile ~luoride and rapid prefiring to prevent its loss through vapori2ation.

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EXAMPLE II
In ord~r to produce 102.29 gram~ of Tio2 powder containing, 4.5284 grams of tantalum pentachloride (TaC15) ~ -of 99.5+~ purity on a metal~ basl~ is dissolved in 20 ml of diethul e~ is a Pyrex beaker`'Pyrex" is a ~rade mark. ~ solution is added to a diethyl ether slwny of 100.0 ~x~ of reagent ~b TiO2 powder in the anatase form. The combination ~lurry i9 then stirred for several minute~ to assure mixing, Sub~equ~ntly the slurry is poured into 5 inch diamater watch gl~s~s from which the excess dlethyl ether i8 allowed to evaporate.
A~ no time i9 the temperature of th~ chemical sy~tom rai~ed abov~ ambient. . , To the dried powder3 i8 add~d about 2 ml of ~lne carbon powder which i8 mixed with a moxtar and p~tlo.
F~ollowing thls procedure ~bout 2 ml of ~thyl alcohol i~ i added and mixed with a plastlc 9poon. Tho powders with the carbon and ethyl alcohol binder axe scraen~d in~o ne~rly I .
round pellet form by forcing the powder blndar aomblna~on through a ~creen of 20 wire~psr lineal inch. The powder3 are then poured into mold~ and praEsad at pressure~ of about 20,000 psi after whlch thoy ~r~ prefired to a temperatur~
of 950C. Dur~ng p~eflring th~ elap~d tim~ b~twe~n ~mbl~nt and 950C is le~ than about 25 minuto~. Tha av~rage heating rate in thi~ temperature range ~ 8 about 38C p~r minute and great~r th~n 40C per minute between ambi~nt and 500C. The green ceramlc bodies are held at 950C for approximately 3 hour~ and then cooled in about 4 hour~ to room temperatu~e.
The p~sflred cera~ic bodies are placed in ~ small br~ck slab which h~s b~en coa~ed with unpre~sed powder~ of ~ ' :

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the ceramic body composition and p:Laced in a sintering furnace at 1000C. The sintering furnace is composed of a tubular, nonporous aluminum oxide tube and aluminum oxide flat shelf and electrically heated by silicon carbide rods.
The sample is introduced into the highest temperature zone of the furnace over a period of S to 10 minutes, after which the furnace is heated to the sintering temperature of 1400C in about 2 to 3 houxs in air. The ceramic body is held at the sintering temperature for 3 hours and immediately removed by pushing the brick sample slab to the end of the furnace tube and placing in ambient air to cool.
The ceramic bodies are placed upon powders of Ta L:
doped rutile within a tubular furnace, the ends of which are sealed with water-cooled rubber rings. The furnace is evacuated with a mechanical pump after which nitrogen gas with less than 8 ppm oxygen is passed over metallic titanium powder and through the furnace. The furnac~ temperature is raised to 1200C at the point o placement of the ceramic body and at 1050C at the point of placement of the metallic titanium powder. The temperatures are maintained for approximately 3 hours before slowly cooling over 8 hours to ambient.
The ceramic body is sectioned for density measure-ment by hydrostatic weighing and microscopic examination and ~-~
a section is removed for the addition of metal electrodes required for a determination of electrical conductivity.
The density of the ceramic body prepared in this way is greater than 4.10 grams per cubic centimeter, the electronic conductivity is greater than or about 9 mho per centimeter at 25C. The very uniform grain size in a -19- ' ceramic prepared in this fashion will be less than or about

5 micrometers and the corresponding fracture strength as measured by the 4 point bending method is greater than 18,000 psi.
It i5 speculated that small uniform grain size and relatively high electrical conductivity is largely due to the homogeneous distribution of tantalum in the rutile phase of TiO2. Apparently the most critical processing step in the formation of TaOC13 upon decompositon of tantalum penta-chloride diethyl operate 65C and the very rapid prefiring to prevent its loss through vaporization.

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Claims (10)

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

1. A method for manufacturing a high strength, non-corrosive, electronically conductive polycrystalline titanium dioxide ceramic member exhibiting high resistance to thermal shock and uniform grain size with an average size of less than 25 micrometers and consisting of titanium dioxide in the rutile crystallographic form doped with a homogeneously distributed ionic metal species selected from tantalum and niobium, comprising:
(A) forming a slurry comprising titanium dioxide and said metal species, said metal species being present in an amount to provide from 0.01 to 8 atomic percent of said metal species in said ceramic member, said slurry being formed either (i) by dissolving in a solvent selected from water and aliphatic alcohols the pentafluoride salt of said metal species in an amount adapted to provide from .01 to 8 atomic percent of said metal species in said ceramic member and then adding titanium dioxide powder in either the rutile or anatase form to the solution to produce a slurry; or (ii) dispersing a mixture of a pentachloridediethyl-etherate complex of said metal species and titanium dioxide powder in either the rutile or anatase form to form a slurry, said slurry components being included in amounts adapted to provide from .01 to about 8 atomic percent of said metal species in said ceramic member;
(B) Drying said slurry to powder form at a temperature at which evaporation of said metal species is avoided:
(C) Green forming said powder to the desired shape;
(D) Heating the shaped green body to 350°C. at a rate of at least 10°C. per minute; or in the case where slurry of step A is formed using the pentachloride dietherate complex heating to 500°C. at a rate at least 40°C. per minute; and (E) Sintering said shaped green body at a temperature of at least 1330°C.

2. A process according to claim 1, wherein after said sintering step there is effected a further step of annealing said sintered shaped body in a reducing atmosphere having an oxygen partial pressure of 10-5 to 10-25 atmo-spheres at a temperature of from 850°C. to 1400°C. to increase the conductivity of said ceramic member.

3. A method in accordance with claim 1, wherein the source of said metal species is employed in amounts sufficient to provide from .1 to 3 atomic percent of said metal species in said ceramic body.

4. A method in accordance with claim 1, wherein said shaped green body is sintered at a temperature in the range of 1380°C. to 1440°C.

5. A method in accordance with claim 1, wherein (i) said pentafloride salt is employed in amounts sufficient to provide from .1 to 3 atomic percent of said metal species in said ceramic body, (ii) said shaped green body is sintered at a temperature of about 1400°C. and (iii) said oxygen partial pressure during annealing is from 10-10 to 10-20 atmospheres.

6. A method in accordance with claim 1, wherein there is employed in the production of said slurry a pentachloridediethyletherate complex of said metal species which complex is prepared by adding a pentachloride salt of said metal species to an excess of diethylether.

7. A method of preparing dense, fine grained, ther-mally shock resistant and electronically conductive titanium dioxide in rutile form by addition 0.01 to 8 atomic percent based on metal species present of doping material selected from tantalum and niobium, comprising the steps of:
a. Dissolving the necessary amount of a pentaflouride compound of the doping material in water, b. Adding titanium dioxide powders to form a slurry, c. Drying the slurry to powder form by procedures which preclude critical evaporation of the pentafluoride, d. Forming said powder form into a suitable green body, e. Heating the green body to a temperature above 350°C. at a rate of 10°C. per minute or greater, and f. Sintering at temperatures above 1330°C. until appropriate homogenization has occurred.

8. A method as defined in claim 7, wherein the titanium dioxide is in the rutile or anatase form.

9. A method of preparing dense, fine grained, thermally shock resistant and electronically conductive titanium di-oxide in rutile form by addition of 0.01 to 8 of doping material atomic percent (based on metal species present) selected from tantalum and niobium, comprising the steps of:
a. Preparing a pentachloride-diethyletherate complex of the doping material by the addition of its pentachloride compound to diethylether, b. Adding titanium dioxide powder to form a slurry, c. Drying the slurry to powder form by procedures which preclude critical evaporation of the complex of the doping material, d. Forming said powder form into a suitable green body, e. Heating the green body to a temperature above 500°C.
at a rate of 40°C. per minute or greater, and f. Sintering at temperatures above 1330°C. until appro-priate homogenization has occurred.

10. A method as defined in claim 9, wherein the titanium dioxide is in the rutile or anatase form.