CA1087374A - METHOD FOR PREPARING SHAPED, GREEN CERAMIC COMPACTS FROM HIGHLY FLOWABLE AND FILLABLE POWDERS CONTAINING .beta.-AND /OR .beta."-AL.SUB.2O.SUB.3 - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A stabilized slurry of Beta-type alumina powder, a method of stabilizing said slurry of Beta-type alumina and a method of pro-ducing minute, spherical agglomerates of said Beta-type alumina by spray drying said stabilized slurry. Said stabilized slurry exhibits a prolonged viscosity sufficiently low to accomodate spray drying techniques to produce said minute spherical agglomerates capable of rapid filling in ceramic forming pressing machinery. Beta-type alumina powder is mixed with water and other conventional additives to form a slurry mixture. A small amount of a polycarboxylic acid containing 3 or more carbon atoms and being capable of chelating with aluminum ion is added as a dispersing agent to inhibit any substantial increase in viscosity. The maintenance of moderate to low viscosity permits application of such slurry to above processes for forming Beta-type alumina agglomerates which rely for formation on a low viscosity, uniformly dispersed stabilized slurry.
The invention herein described was made in the course of or under a contract or subcontract thereunder with the National Science Foundation.

Description

11 ~87374 - m is application relates to a stabilized slurry of Beta-type alumina powder as well as to a method of stabilizing said slurry and to a method of spray drying said stabilized slurry to produce a highly flowable and fillable powder suitable for use in automatically-filled die cavity pressing machinery, the pressed articles from said machinery to be sinterable to dense beta-type alumina ceramic articles of suitably low electrical resistivity.
More particularly, this application relates to a method for preparing flowable powder which, after pressing, may be fired to form high density ~" or ~-alumina containing ~ ceramic bodie~ suitable for use in a variety of electrical ; convers~on devices. Still more parti~ularly, this appli-cation relates to a method for preparing flowable powders, which, after pressing, may be fired by conventional sin-tering techniques to form shaped, polycrystalline ~" or ~-alumina containing bodies which are ideally 3uited for use as reaction zone separators or solid electrolytes in certain electrica} conversion devices.
Among the polycrystalline bi- or multi-metal oxides which are most suitable for use in electrical con-version devices, particularly those employing molten metal and/or molten metal 8alt~ a~ reactants, are those in the family of Beta-aluminas, all of which exhibit a generic crystalline structure which is readily identifiable by X-ray diffraction. Thus, ~eta-type-alumina ~sometimes referred to as sodium-Beta-type-alumina) is a material which may be thought of as a series of layers o aluminum oxide ~A1203) held apart by columns of linear Al-0-bond ,, .

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chains with sodium ions occupying sites between the afore-mentioned layers and columns. Numerous Beta-type-alumina polycrystalline materials exhibiting this generic crystalline structure are disclosed in the following U.S.
Patents, 3~404,035, 3,404,036; 3,413,150; 3,446,677;
3,458,356; 3,468,709; 3,468,719; 3,475,220; 3,475,223;
3,475,225; 3,535,163, 3,719,531: and 3,811,943.
Among the numerous polycrystalline ~eta-type-alumina materials disclosed in those patents and which may be processed in accordance with the method of this invention are the following:
(1) Standard Beta-type-aluminas which exhibit the above-discus~ed crystalline structure comprising a series of layers of aluminum oxide held apart ~y columns of linear Al-O bond chains with sodium ions occupying sites between the aforementioned layers and aolu~J As dis-cussed in the aforementioned patents, Beta-type-alumina i8 formed from compositions comprising at least about 80 percent by weight, preferably at least about 85 per-cent by weight of aluminum oxide and between about 5 andabout 15 percent by weight, preferably between about 8 and about 11 percent by weight sodium oxide. ~here are two well known crystalline forms of Beta-type-alumina, both of which demonstrate the generic Beta-type-alumina crystalline structure discussed hereinbefore and both of which can easily be identified by their own characteristic X-ray diffraction pattern. ~-Alumina is one crystalline form which may be repressented by the formula Na20,11A1203.
~he second crystalline form is ~"-alumina which may be represented by the formula Na20.6A1203. It will be noted the the ~" crystalline form of Beta-type-alumina contains approximately twice as much soda ~sodium oxide) per unit weiyht of material as does the B-alumina. It i8 the ; ~"-alumina crystalline structure whi¢h is preferred for the formation of solid electrolytes or reaction zone separators for electrical conversion devices becauQe of its inherent lower electrical resistivity.

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(2) Boron oxide B2O3 modified Beta-type-alumina wherein about ~.1 to about 1 weight percent of boron oxide is added to the composition. This modification of the Beta-type-alumina is more thoroughly discussed in afore-mentioned U S. Patent 3,404,036.

(3) Substituted Beta-type-alumina wherein the sodium ions of the composition are replaced in part or in whole with other positive ions which are preferably alkaline metal ions.

(4) Beta-type-alumina which is modified by the addition of a minor proportion by weight of a metal ion having a valence not greater than 2 such that the modified Beta-type-alumina composition comprises a major proportion by weight of ions of aluminum and oxygen and a minor pro-portion by weight of mètal ions in crystal lattice com-bination along with cations which migrate in relation to the crystal lattice as a result of an electric field, the preferred em~odiment being wherein the metal ion having a valence not greater than 2 is either lithium or mag-nesium or a combination of lithium and magnesium. ~hesemetals may be included in the composition in the form of lithium oxide or magnesium oxide or mixtures thcreof in amounts ranging from about 0.1 to about 5 weight percent, preferably from about 0.1 to about 1.5 weight percent.
This type of modified Beta-type-alumina is more thoroughly discussed in V.S. Patent 3,475,225 and 3,535,163 mentioned above. Such lithia and magne3ia stabilized ~-aluminas are preferred compositions for the preparation of Beta-type-alumina bodies demonstrating the ~" crystal structure.
me energy conversion devices for which the bodies prepared from compacts made in accordance with this invention are particularly useful as solid electrolytes are disclosed in some detail in the aforementioned patents.
In the operation of such energy conversion devices, the catiDns, such as sodium in the ~"-alumina, or some other cation which has been substituted for sodium in part or ~ in whole, migrate in relation to the crystal lattice as :`
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a result of ef~ects caused by an electric field. Thus, the solid ceramic electrolytes which may be prepared from the dense, green ceramic compacts made by the method of this invention are particularly suited to provide selective cationic communication between the anodic and cathodic reaction zonès of the energy conversion device6 and are essentially impermeable to the fluid reactant~ employed in the device when the reactants are in the elemental, compoundor anionic states. Among the energy conversion devices in which ~"-alumina containing ceramics are use-ful are: (l) primary 4atterie~ employing electrochemically reactive oxidants and reductants in contact and on opposite sides of the solid electrolyte or reaction zone separators;
t2) secondary batteries employ~ng electrochemically re-versably reactive oxidants and reauctants in contact with and on opposite sides of the solid electrolyte or reaction zone separator; (3) thermo-electric ge~erators 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 re-converted to elemental form; and (4~ thermally regenerated fuel cells.
~rior art techniques for the preparation of shaped, dense green ceramic bodies suitable for forming ~"-containing ceramic bodies have typically involved pouring a fine powder with poor flow characteristics, this ~ powder having a composition comprising at least about 80 30 weight percent of aluminum oxide and between about 5 and about 15 weight percent of sodium oxide, into a suitably shaped mold and then is~statically pressing at high pres-sure ~typically around 60,000 psi~.
In order to obtain uniform filling of the iso-static pressing mold by the prior art powder it has been found necessary to cause vibration of the mold during powder addition or after powder addition is complete, or, ., .

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, 873~4 preferrably, both. For acceptably dense and uniform filling with the prior art powder a total time of mold filling exceeding five minutes has been typically used.
The uniformity of filling is necessary becaUse thlckness variations typically occur in the isostatically pressed solid electrolyte or reaction zone separator green ceramic body if the mold has been filled non-uniformly prior to isostatic pressing. These thickness variations give rise to corresponding regions of minimum thickness in the sin-tered solid electrolyte or reaction zone separator part.Such regions of minimum thickness offer low resistance paths for sodium ion conduction. Such low electrical resistance paths may lead to premature failure of the solid electrolyte or reaction zone separator part under conditions of service in energy conversion devices.
The prior art process, including mold vibration, while entirely suitable for forming high quality compacts, is time consuming and not readily adaptable to large scale commercial production of compacts. In order to ; 20 make the aforementio~ed electrical energy conversion devices cost compet~tive with other electrical conversion devices, it is, among other things, necessary to devise a rapid and efficient method for the production of suchc~cts.
The wall thickness of solid electrolyte or re-action zone separator parts should be large enough to give reasonable mechanical strength to the part but small enough to present an acceptably low resistance to sodium ion conduction. A practical desirable wall thickness has been found to be about 1 to 3 millimeters. This wall thickness after sintering dictates a width for the corres-ponding dimension of the opening of the isostatic pressing mold of about 2.5 to about 7.5 millimeters. That this mold opening width is typical is seen by inspection of, for example, U.S. Patent 3,903,225. The mold opening width dimension corresponding to wall thickness after sin-tering given in the example in U.S. Patent 3,903,225 is 3.15 millimeters.

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` ~87374 The above considerations show the need for a flowable powder. A flowable powder is defined for the purposes of this patent to be one which will ~low con-tinuously and unaided under the influence of gravity through a 2.5 millimeter or smaller orifice in a stem-less funnel without the application of vibration to the funnel and without other application of energy such as might be supplied by a current of gas This definition of flowability is consistent in principle with that given in Metal Powder Industries Federation Standard No. 03, as revised 1972. It is, more particularly, con-sistent with that given in Degussa Technical Bulletin No. 31, "Aerosil~ for Improving the Flow ~ehavior of Powder Substances". A series of stemless glaæs funnels of different exit orifice diameters is described in the latter publication. Funnels of this design, rQferred to hereafter as sand timers, have been used to assess the flowability of powders produced by the prior art and of powders produced by this invention.
The flowability test using sand timers is an i indirect measure of the frictional and cohesive forces ; acting between particles of a powder. Such forces not only impede the flowa~ility of a powder but also limit the mass of powder per unit volume of container which may be achieved by adding a standard mass of powder to a ~ standard container and vibrating said container in a -~ standard manner until the powder ceases to settle. Therate at which the powder achieves its final mass per volume value and the final mass per volume value itself are both useful indirect measures of the frictional and cohesive forces between powder particles a~ to how these forces affect the ability of a powder to effectively fill a container rapidly. A powder which will rapidly achieve a relatively high mass per volume of container under such ; 35 conditions is defined for the purposes of this patent as being a fillable powder.
The prior art of spray drying as applied to . .

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Such prior art slurry stabilizations have typically employed adjustment of the pH of aqueous slurries to between 3 and 9. Deflocculants of ~he polyelectrolyte type such as gum arabic and Darvan(R~No. 7 (R.T.
Vanderbilt Company, Inc., Norwalk, Conn.) have also been used.
Such techniques are ineffective in the case of Beta-type-alumina slurries due to the relatively high pH, greater than 13, which can be produced by the leaching of basic components from the powder into the aqueous phase during the course of slurry preparation and stabilization. Such very basic (caustic) conditions make neutralization of the excess basicity impractical due to the high percentage of acid anion which i9 required, which would, in turn, l~wer the density of the individual spray dried agglomerate particles by introducing excessive voidage into them. Furthermore, the neutralization of this high basicity speeds the leaching of further quan-tities of ba ic components from the powder, leading to rapid re-establishment of the original basic conditions.
Those skillea in the art to which this invention pertain~ will readily recognize that such slurry sta-bilization techniques which are useful for alpha-alumina slurries would not be expected to be useful for Beta-type alumina slurries ina6much as Bçta-type alumina slurries are chemically distinct from alpha-alumina slurries.
Structurally, the distinction between ~età-type alumina and other species of the generic class of aluminas is the presence of substantial sodium ion with the A1203 primary constituent. Unfortunately, the genealogy of nomenclature for the A1203 materials developed outside the framework of any unified nomenclature system.
One compound of the A1203 class was first identified as Alpha-alU~, being pure A1203 with an hexagonal crystalline struc~ure. The later discovery ,; , ~. ~ . ,~ ;

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~Lv~ ~ 4 of additional materials containing what was thought to be a mere variance of structure of the same A1203 re-sulted in its denomination as Beta-al~mina. Had the discoverer known of the presence of Na in the crystalline ~- 5 matrix, the term would have more properly been a sodium aluminate.
Sodium aluminates are formed by reacting Na20 and A1203 to form new compounds which vary in sodium ; content from Na20 - A1203 down to Na20- 11 A1203. As would be expected with the change in molecular com-position, the Beta-type-alumina is easily distinguished from common alumina materials by dramatic and unpredict-able chemical and physical variations.
Among the more pronounced differences are the changes which occur in the liquid phas~ of the slurry comprising water and Beta-type-alumina. A reaction appears to result between the water and calcined Beta-type-alumina causing a Qtrong increase in pH a~d vis-cosity. These changes are demonstrated in the following summary, showing the effects of dispereing agents as well.
Visco9ity is measured as stated in the subject patent application.
TABLE A
Beta-type Alumina/Water Slurry 25 Slurry Mixture Viscosity (minutes from time of mixing slurrY)

5 min 30 min. 60 min.
(1) H O and 40% 14.4 17.9 19,6 Beta-alumina* sec pH = 13.1 (Act~al) pH - 7.0 ~Calculated) (2) Item (1) plus.
.0888 M Ta~tari~
Acid 10.5 sec 10.4 10.5 pH = 12.6 (Actual) pH = 2.0 (Calculated) ~ ~.85% Na2O, 0.75% Li2O, 90.48% A12O3 calcined for 2 hours at 1260C. and milled for 3 hours.

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- (3) Item ~1) plus ` .178 M HCl 12.0 sec 13.3 14.0 pH = 13.1 ~ctual) p~= 0.75 (Calculated) When Alpha-alumina i8 mixed with .103 M NaO~
under the conditions and measures of the corre~ponding item number of Table~, the vi8c08ity increase during the 60 minute period i5:
(1) 10.9 sec to 11.6 sec (2) 11.0 sec to 11.3 sec ~3) 11.0 sec to 10.4 6ec In addition to the minimal increase in visco~ity for the Alpha-alumina slurry with the passing time, there wa~ little, if any, deviation in actual pH from the theoretically calc~lated value. These values after 60 min for the respective slurriss (in .103M NaOH)of H20, Tartaric Acid and HCl were:
Actual pH Calculated p~
2012.6 13.0 3.8 3.8 1.7 1.1 It iS apparent that the significant change in viscosity of the Beta-type-alumina as compared to the alpha-alumina demonstrates the chemically uncommon char-; acter of these materials. Furthermore, the unexpected reaction of the Beta-type-alumina slurry causing gro3s deviation from the calculàted pH strengthens the con-clusion that Alpha and ~eta alumina are of a different nature. The chemical distinctions are also evidenced by a comparison of the X-ray diffraction patterns of the two materials, ~ brs which are both flowable and fillable are needed for rapid production of solid electrolyte or reaction zone separator green bodie~ with uniform wall thicknesses by isostatic pressing. The action of auto-matic isostatic pressing machinery involves allowing the .. .. .. . . .

iL~87374 powder to flow under the influence of gravity to the isostatic pressing mold. This mold must be filled rapidly and uniformly with very little vibration, or, more usually, no vibration, Beta-type-alumina powders produced by the prior art, involving no addition of binders or addition of binders by means other than spray drying, have ex-hibited poor characteristics of flowability and fillability.
Such prior art powders are found to have rough surface textures and to contain large proportions of fine material.
These surface and size characteristics and others believed to contribute to effectively large cohesive and frictional forces between powder partioles make the prior art powders unsuitable for rapid isostatic pressing and especially unsuita~le for automatic isostatic pressing. It i8 believed, therefore, that a method for producing a highly flowable and fillable powder suitable for use in auto-matically-filled die cavity pressing machinery, the pressed articles from said machinery to be sinterable to dense Beta-type-alumina ceramic articles of suitably low electrical resistivity and suitably high uniformity of wall thickness would be an advancement in the art.
It is therefore an object of this invention to prepare a stabilized slurry cf Beta-type alumina powder which may be employed in a spray drying proces~
to produce minute spherical agglomerates of said Beta-type alumina.
It is a further object of this invention to provide a method for producing a highly flowable and fillable powder consisting predominately of ~ and/or ~"-alumina suitable for use in automatically-filled die cavity pressing machinery, the pressed articles from said machinery to be sinterable to dense Beta-type-alumina ceramic articles of suitably low electrical resistivity and suitably high uniformity of wall thickness for use as solid electrolytes or reaction zone separators in energy conversion devices.
These and various other objects, features and . , , ~. . . . " . j , . - ' -: . . ..
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--ll--advantages of the invention will be better understood from the following description taken in connection with the accompanying drawings and photographs in which:
Figure 1 is a photograph of spherical spray dried powder particles of said invention, Figure 2 iQ a plot of the volume of a four gram mass of two powders versus time which indicates the improved fillability characteristics of powders pre-pared under said invention compared to those prepared by prior art techniques.
The invention sought to be protected herein comprises in part a stabilized slurry of Beta-type alumina material and a method for stabilizing the same, said stabilized slurry having a prolonged low to moderate viscosity consistent with ceramic formation requirements in processes, such as spray drying, where the visc~sity of conventional beta-type alumina slurries is too high to yield a wor~able slurry for the particular process, In accordance with the invention the slurry comprises:
(a) A liquid phase constituent suitable for sus-pending a Beta-type alumina powder without causing sub-stantial adverse affect on chemical utility of said powder as a Beta-type alumina ceramic forming material;
(b) Beta-type alumina powder which has been calcined and appropriately deagglomerated and mixed in said liquid phase to form a finely dispersed slurry having a concentration of said powder in the range of about 30 to about 70 weight percent; and (c) sufficient dispersing agents to stabilize said slurry to desired low to moderate viscosity for a period of time adequate to enable the desired process to be accomplished, said dispersing agent being seleated from the class of polycarboxylic acids containing 3 or more carbon atoms and being capable of chelating aluminum ion.
The Beta-type aIum~na included in the slurry has a composition as discussed generally hereinbefore.
Thus, generally the composition comprises from about 0.1 ::: . . . .~
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:~87374 to about 5 weight percent lithium oxide, from about 5 to about 15 weight percent sodium oxide, and a balance of aluminum oxide.
Another embodiment of this invention relates to the spray drying of the above discussed Beta-type alumina slurries to form a highly flowable and fillable powder consisting largely of smooth spherical part~cles, It has been discovered that when preparing slurries of seta-type alumina in accordance with this invention polycarboxylic acids containing at least 3 carbon atoms and being capable of chelating aluminum ion when employed in amounts ranging from at least about 0.010 mole acid function per 100 grams of the ceramic powder, preferably in amounts ranging from about 0.010 to about 0.200 and most preferably in the range of 0.025 to 0.100 mole acid function per 100 gram~ of ceramic powder, will depress the slurry viscosity by 2-3 times for one to three hours, A~ discussed above the polycarboxylic acids useful as stabilizing agents in the slurries of this in-vention contain at least 3 carbon atoms and must be capable of chelating aluminum ion. It should be re-cognized that the chelating nature of the polycarboxylic acid is merely a parameter which defines the stabilizing agent and that such chelating function does not necessarily occur in the stabilized slurry of the invention. Although not i~tended to be limiting, the polycarboxylic acids found to be useful in the invention are those aluminum ion chelating agents containing 3 to 15, preferably 3 to 8 carbon atoms. m ese polycarboxylic acids include both aliphatic polycarboxylic acids, saturated or unsaturated, as well as olefinic and are preferably hydroxy functional, i.e., bear,at least one hydroxyl group, wi~h the most preferred agents being mono-or di-hydr~xy functional poly-carboxylic acids. Among the polycarboxylic acids whichfall within this general scope are tartaric acid, citric acid, tartronic acid, malic acid, and ~

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y-trihydroxyglutaric acid. An example of a polycarboxylic acid chelating agent useful in the invention and con-taining greater than 15 carbons is polya~rylic acid having a number average molecular weight ~Mn) between 4000 and 6000.
The most preferred polycarb~xylic acids falling within the class discussed above are the hyd~oxy func-tional (preferably mono-or ai hydroxy) saturated aliphatic polycarboxylic acids capable of chelating aluminum. of these acids the most preferred are citric and tartaric acids.
While the described acid chelating agents serve to reduce viscosity as discussed above, if the same ~uantity of nonchelating acid such as hydroxchloric or }5 acetic acid is added, the viscosity is not depressed significantly ~or several hours as reguired. ~his con-trast between chelating and nonchelati~g acids was un-expected, especially in view of the fact that the amount of acid added was generally only sufficient to lower the pH of the slurry to around ll.
It will be understood by those skilled in the art that the other agents added to the slurry also help to control the viscosity. It is unexpected, however, that such specific chelating acids for aluminum as for ; 25 example tartaric and citric acids would have such a dramatic effect in lowering the slurry viscosity and thus making possible a more concentrated slurry. Such a con-centrated slurry is advantageous in at least three respects: 1) it lowers the oost of spray drying a given quantity of powder since such cost is dependent on the amount of water which must be evaporated per unit weight of powder, 2) it increases the rate at which the powder may be produced, and 3) it increases the density of each individual spray dried agglomerate particle leading, in turn, to a more fillable powder as defined above.
It will be understood by those skilled in the art that the slurry viscosity is to be adjusted to some . . , ~ - . . . .

~l087374 convenient value sufficiently high to retard settling out of the suspended particles during pumping from the feed tank to the atomization device and sufficiently low to allow convenient pumping from the feed tank to the atomization device. I~ will also be understood that the slurry viscosity and surface tension may be altered to obtain optimum spray dried powder particle size dis-tribution and optimally smooth-surfaced, spherical spray dried powder particles.
The aspect of the invention as it relates to obtaining a stable slurry of Beta-type-alumina powder suitable for spray drying will now be further described by way of example.

A powder of 3eta-type-alumina was prepared' by firing an intimate mixture consisting of Li2O~as LiNO3) - 0.7~
Na2O ~ag Na2CO3) - 8.7%
A12O3 - balance at 1255C ~or 2 hours and the deagglomerating the same by dry milling for 1 hour in a point shaker with Lucalox balls. Several samples of this preparation of ~eta-type-alumina were slurried in water at a concentration of 52 percent by weight powder. Other constituents of each slurry were present in the following amounts ex-pressed as percents by weight of the Beta-type-alumina powder: 2.8% polyvinyl alcohol and a trace of 2-octanol.
In addition to the above constituents, amount~ of various acids equivalent to 0.02S mole acid function per 100g powder were added. Each slurry was made as ~uickly as possible with the binder constituent ~polyvinyl alcohol) being the last added. Viscosities and pH values of each slurry were obtained periodically. Viscositie~ are re-ported as seconds taken for a standard amount of slurry, about 10 milliliters, to flow under the influence of gravity out of a standard vessel hav,ing an orifice of about 1 millimeter, ~he results are shown in Table 1.

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~L~87374 The viscosity results obtained at 50 minutes remain sub-stantially the same for up to three hours. The results clearly sh~w a beneficial effect due to tartaric acid as opposed to no acid or the non-chelating acid~
hydrochloric and acetic.
EXAMP~E II
The results of a similar experimental series using a slurry concentration of 57 weight percent of the Beta-type-alumina prepared as in Example I are shown in Table II. The benefits of the chelating acids tartaric and citric are clearly evident.
To further illustrate the subject invention, the following detailed example of the preferred embodi-ments is presented.. All percentages are by weight based on total slurry weight Qr weight of Beta-type-alumina powder as in the preceding example. All o~ percentages are by weight based on total weight.

52~ powder, 0.025 mole acid/lOOg powder Time from Binder Addition Viscosity Acid lmin) pH ~ec) ; None 5 12.15 19 SO 12.51 25 25 dl-Tartaric 5 11.52 16 11.54 16 HCl 5 11.05 23 ll.S7 20 Acetic 5 9.85 21 SO 11.58 22 :: ,.
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TA~LE II
57% powder, 0.025 mole acid/lOOg powder Time from Binder Addition Viscosity 5 Acid (min) E~ ~sec) dl-Tartaric 5 11.42 37 11.57 33 Citric S 11.20 45 11.82 40 HCl 5 10.10 95 11.69 50 Acetic 5 9.83 1~7 11.65 74 EXAMPLE III
A powder of Beta-type-alumina was prepared by firing an intimate mix consisting of: Li20 ~as LiN03) -0.7%
Na20 ~as Na2C03)--8 7 A1203 - balance The mix was fired at 1250C. for 2 hours and then deaggl~merated ~y sha~ing 500 grams of the fire~ mix 50 high purity aluminum oxide sphere6 of 1 cm diameter.
Flowability of this and subsequent powders was as6essed by determining whether the powder would flow through sand timers having the various orifice sizes 18, 12, 8, 5 and 2.5 millimeters as described previously. The smallest orifice size, in millimeters, through which a given powder will flow is defined as the flowability index. If the powder will not flow through any of the aforementioned ~ sand timers it is assigned the flowability index "no flow".
; 30 The fired mix powaer, after deagglomeration, was characterized as follows:
particle shape - irregular particle surface - rough approximate means size - 6~m ~by weight, Coulter Counter) flowability index - no flow The fired mix powder was spray d~ied from an aqueous slurry of 54.5~ fired mix powder, 2.8~ polyvinyl . ~

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alcohol, 1.2% polyethylene glycol, 2.1% tartaric acid, and a trace of 2-octanol, The spray dryer used was of the 2-fluid, concurrent type. The resulting spray dried powder is shown in Figure 1. This powder was char-acterized as follows:shape - predominately spherical surface - smooth approximate means size - lOO~m (by number, microscopically) flowability index - 2.5 By way of contrast, a sample of fired mix powder which had been coated with 1% polyvinylbutyral binder by evaporation of a slurry of the powder in an acetone binder solution was characterized as follows:
shape - irregular surface - rough approximate mean size - 15~m (by number, micro-scopically) flowability index - 18 The last described powder is typical of the prior art.
The fillability, as previously defined, of prior art powder and of powder made according to the present invention was determined by noting the volume of a bed of 4.0g of powder in a graduated volumetric cylinder of lOcm total volume both after filling and after application of a reproducible frequency and amplitude of vibration to the cylinder. The results for the two powders are shown in Figure 2. The powder ma~e according to this invention is clearly superior in fillability both initially and after any given time of vibration.
Powder made according to the present invention has been filled into an is~static pre~sing mold within 10 seconds, i~ostatically pressed at 55,000 psi, prefired to 650C. to eliminate volatile organic constituents, and sintered for 30 minutes at 1620C. to obtain a uniform-walled body with density 96.7% of theoretical and an ,. . . - . . , ., ., ~ .; ... , . :

-l87374 electrical (sodium ion conduction) resistivity of 3.5 ohm-cm. (~t 3Q0~.) .
EXAMPLE IV
The technique of slurry spray drying was also extended to powder in which zeta ~lithium) alumina (Li20.SA1203) is substituted for ~iNo3 as the source of Li20. The stabilized slurry contained 46.5% by weight of a powder in which the lithia-alumina ratio was 1 to

6 (i.e. Li20.5A1203 plus some excess A1203) plus Na20.5A1203 soda component in a ratio tc give a nominal 9.0% Na20-0.8% Li20-90.2% A1203 by weight composition.
The ~lurry was stabilized with 2.1% tartaric acid, 1.2~
polyethylene glycol and 2.8% polyvinyl alcohol, all based on the powder. The spray dried powder possQssed excellent flowability characteristics ~index = 2.5) suitable for i~ostatic pressing. This material, after isostatic pre~sing at 55 kpsi, could be sintered to 2 density of 97% (1620C. - 30 min) and an electrical resistlvity for sodium ion conduction of 4.3 ohm-cm ~300~C~.
While there has been shown and described what are at present consi~ered the preferred embodiments of the invention, it will be o~vious to those skilled in the art that various changes and modifications may-be made therein without departing from the scope o~ the invention as defined by the appended claims. Likewise, the slurry application defined herein is merely exemplary of numerous conventional processes which require a regulated viscosity within the defined range.

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Claims (16)

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

1. A stabilized slurry of Beta-type alumina material have a prolonged low to moderate viscosity consistent with ceramic formation requirements in processes where the viscosity of conventional beta-type alumina slurried is too high to yield a workable slurry for the particular process, said slurry comprising, a. a liquid phase constituent suitable for suspend-ing a Beta-type alumina powder without causing substantial adverse affect on chemical utility of said powder as a Beta-type alumina ceramic forming material, b, Beta-type alumina powder which has been calcined and appropriately deagglomerated and mixed in said liquid phase to form a finely dispersed slurry having a concentration of said powder in the range of 30 to 70 weight percent, and c. a dispersing agent to stabilize said slurry to the desired low to moderate viscosity for a period of time adequate to enable the desired process to be accomplished, said dispersing agent being selected from the class of polycarboxylic acids containing at least 3 carbon atoms and being capable of chelating aluminum ion and being included in an amount ranging from at least about 0.010 mole acid function per 100 grams of said Beta-type alumina powder.

2. A stabilized slurry in accordance with claim 1, wherein said liquid phase comprises water.

3. A stabilized slurry in accordance with claim 1, wherein said polycarboxylic acid dispersing agent is in-cluded in said slurry in an amount ranging from about 0.010 to about 0.200 mole acid function per 100 grams of said Beta-type alumina powder.

4. A stabilized slurry in accordance with claim 1, wherein said polycarboxylic acid dispersing agent contains 3 to 15 carbon atoms.

5. A stabilized slurry in accordance with claim 1, wherein said polycarboxylic acid dispersing agent contains 3 to 8 carbon atoms.

6. A stabilized slurry in accordance with claim 1, wherein said polycarboxylic acid dispersing agent is selected from the group consisting of (i) saturated aliphatic poly-carboxylic acids, (ii) olefinically unsaturated aliphatic polycarboxylic acids and (iii) aromatic polycarboxylic acids.

7. A stabilized slurry in accordance with claim 6, wherein said polycarboxylic acid contains hydroxy functionality.

8. A stabilized slurry in accordance with claim 7, wherein said polycarboxylic acid dispersing agent is mono or dihydroxy functional.

9. A stabilized slurry in accordance with claim 8, wherein said polycarboxylic acid dispersing agent is select-ed from the group consisting of tartaric acid and citric acid.

10. A stabilized slurry in accordance with claim 1, wherein said polycarboxylic acid dispersing agent is present in an amount ranging from about 0.025 to about 0.100 mole acid function per 100 grams of said Beta-type alumina powder and is selected from the group consisting of hydroxy function-al saturated aliphatic polycarboxylic acids.

11. A stabilized slurry in accordance with claim 10, wherein said polycarboxylic acid dispersing agent is select-ed from the group consisting of tartaric acid and citric acid.

12. A stabilized slurry in accordance with claim 1, wherein said slurry includes a binder.

13. A method for producing a highly flowable and fillable Beta-type alumina powder comprising spray drying the stabilized Beta-type alumina slurry claimed in any one of claims 1, 2 or 3.

14. A method for producing a highly flowable and fillable Beta-type alumina powder comprising spray drying the stabilized Beta-type alumina slurry claimed in any one of claims 4, 5 and 6.

15. A method for producing a highly flowable and fillable Beta-type alumina powder comprising spray drying the stabilzed Beta-type alumina slurry claimed in any one of claims 7, 8 and 9.

16. A method for producing a highly flowable and fillable Beta-type alumina powder comprising spray drying the stabil-ized Beta-type alumina slurry claimed in any one of claims 10, 11 and 12.