CA1054674A - DENSE CERAMIC BODIES CONTAINING POLYCRYSTALLINE .beta."-ALUMINA - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for preparing dense, ceramic polycrystalline bi- or multi-metal oxide bodies comprising: (A) disposing the bi- or multi-metal oxide in powder form within a hot pressing means adapted to simultaneously apply heat and pressure to said powder and spacing said powder from the pressure applying surfaces of said hot pressing means by interposing between said surfaces of said powder spacers formed of densified polycrystalline bi- or multi-metal oxide of the same composition as said powder; and (B) heating said powder within said hot pressing means to a temperature of at least 1250°C and applying a pressure of between about 3000 psi and about 10,000 psi for greater than about one (1) minute while maintaining said temperature. The method of the invention is particularly suited for the preparation of polycrystalline .beta."-alumina containing bodies exhibiting near theoretical density, low porosity, high strength and low electrical resistance. Such polycrystalline .beta."-alumina containing bodies are ideally suited for use as reaction zone separators or solid electrolytes in certain electrical conversion devices.

Description

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The invention herein described was made in the course of or under a Contract or subcontract thereunder with the National Science Foundation, U.S.A.
This application relates -to a method of preparing dense, ceramic polycrystalline bi- or multi-metal oxide bodies. This application further relates to a method of preparing dense, high strength Beta-type alumina bodies.
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this application relates to the preparation of polycrystalline ~"-alumina containing bodies and still more particularly, this application relates to a method of preparing polycrystalline ~"-alumina containing bodies exhibiting near theoretical density, low porosity, high strength and low electrical resistance. The polycrystalline bi- or multi-metal oxide bodies made in accordance with the method of this invention could have many applications which will be apparent to those skilled in the art. However, these dense, high strength poly-crystalline bodies exhibiting low electrical resistance, inparticular the polycrystalline ~"-alumina containing bodies of the invention, are particularly suited for use as reaction zone separators or solid electrolytes in certain electrical conversion devices.
As mentioned above, the method of this invention relates to the preparation of polycrystalline bi- or multi-metal oxide bodies, many of which are useful as solid electro-lytes in the preparation of energy conversion devices. The particular family of polycrystalline bi- or multi-metal oxides with which the method of this invention is most effective is Beta-type-alumina ceramic and Beta-type~alumina ceramic which has been substituted or otherwise modified. Beta-type-alumina, or sodium Beta-type-alumina, is a material which may be ~ ~
thought of as a series of layers of alumina oxide IA12O3) held ~ -apart by columns of linear Al-O bond chains with sodium ions ~ `
occupying sites between the aforementioned layers and columns.
This type of material is widely used in the manufacturing of refractory bricks for lining furnaces subject to corrosion from a basic melt and/or slag.
The bi- and multi-metal oxides exhibiting the B"-alumina crystalline lattice make effective separators and/or .

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solid electrolytes for use in energy conversion devices particularly those employing molten metal and/or molten metal salts as reactants.
In the operation of such energy conversion devices, the cations such as sodium in the polycrystalline bi- or multi-metal oxide, or some other cation which has been sub-stituted for sodium in part of in whole, migrate in relation to the crystal lattice as a result of effects caused by an electric field. Thus~ the solid ceramic electrolytes for which the method of this invention is particularly suited provide selective cationic communication between anodic and cathodic reaction zones of the energy conversion devices and are essentially impermeable to the fluid reactants employed in the device when the reactants are in the elemental, compound or anionic stateO Among the energy conversion devices in which the particular pol~crystalline bi- or multi-metal oxides are useful are: (1) primary batteries employing electrochemically reactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte or reaction zone separators; (2) secondary batteries employing molten, electrochemically reversibly reactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte or reaction zone separator; (3) thermo-electric 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 is converted to ionic form, passed through the polycrystalline wall or inorganic membrane and reconverted to elemental form; and (4) thermally regenerated fuel cells.
As discussed above, the polycrystalline bi- or multi-metal ceramic bodies for which the method of this ~ .
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inventlon is particularly helpful in forming is a ceramic containing the ~"-alumina crystalline lattice~ i.e., seta-type-alumina and substituted Beta-type-alumina or Beta-type-alumina which has been otherwise modified. The large number of such known polycrystalline materials and some of the elec-trical conversion devices in which they may be employed is a solid electrolyte or reaction zone separator are dis-closed in the following U.S. Patents: 3,404,035; 3,404,036;
3,413,150; 3,446,677; 3,458,356; 3,~68,709; 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 materials dis-closed in these patents and for which the method of this invention is suitable are the following:
(1) Standard Beta-type-alumina exhibiting the above-discussed crystalline structure comprising a series of layers of aluminum oxide held apart by columns of linear Al-O bond chains with sodium ions occupy- ;
ing sites between the aforementioned layers and columns. As discussed in the aforementioned ~ .:
patents, seta-type-alumina is formed ~rom compositions comprising at least about 80% by weight of aluminum oxide and between about 5 and about 15 weight per-cent, preferably between about 8 and about 11 weight -~, percent, of sodium oxide. There 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 characteristic x-ray diffraction pattern. ~-alumina is one crystalline form which may be represented - . .
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by the formula Na2O llA12O3. The second crystalline form is ~"-alumina which may be represented by the formula Na2O-6A12O3. It will be noted that the ~"
crystalline form of the Beta-alumina contains approximately twice as such soda ~sodium oxide) per unit weight of material as does the ~-alumina.
It is the ~"-alumina crystalline structure which is preferred for the formation of solid electrolytes or reaction zone separators for energy conversion - devices. In fact, if the less desirable ~ form is present in appreciable quantities in the final ceramic, certain electrical properties of the body `
will be impaired,

(2) Boron oxide s2O3 modified Beta-type-alumina wherein ~` about 0.1 to about 1~ 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-aluminas wherein the sodium ions of the composition are replaced in part or in whole with other positive ions which are preferably ; alkali metal ions.
~4) Beta-type-aluminas which are modified by the addition of a minor proportion by weight of metal ions having a valence not greater than 2 such that the modified Beta-type-alumina composition com-prises a major proportion by weight of ions of aluminum and oxygen and a minor proportion by weight of metal ions in crystal lattice combination along with cations which migrate in relation to th~
crystal lattice as a result of an electric field, ~; :
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the preferred embodiment being wherein the metal ion having a valence no greater than 2 i5 either lithium or magnesium or a combination of lithium -and magnesium; these metals may be included in the composition in the form of lithium oxide or magneslum oxide or mixtures thereof 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 U.S. Patents 3,475,225 and 3,535,163 mentioned above; such lithia and magnesia stabilized Beta-aluminas are preferred compositions for the preparation of Beta- ~ .
type-alumina bodies demonstrating the ~" crystal ~`
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- structures. The Beta-type--aluminas most preferred `; -~
for processing in accordance with this invention includes lithium oxide as the modifier. ;~
Many of the bi- or multi-metal oxides are con-ventionally formed into densified, polycrystalline bodies by sintering techniques. While these techniques are often used, they suffer from certain disadvantages in that often-times the bi- or multi-metal oxide reactants, decompose or are otherwise modified at the temperature necessary for sintering and, therefore, the desirable final structure and properties of the ceramic may not be obtained. For example, it has been found that if sodium oxide and alumina (constituents of Beta-type-alumina) are caused to react ;
below about 1550~C, the primary product is ~"-alumina, a crystalline compound which as mentioned above is readily 30 identified by a characteristic x-ray diffraction pattern. ~ ~
It has also been found that if ~"-alumina is heated to temp- ;

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eratures above 1550C, temperatures to which it is necessary to heat the composition in order to obtain the densified, sintered polycrystalline bodies useful in energy conversion devices, the ~"-alumina decomposes forming ~-alumina, the other crystalline form which is also easily identified by x ray diffraction and distinguishable from the ~"-alumina.
secause sintering is a desirable technique for preparing ceramic bodies of various shapes, methods were sought to modify the composition in order to stabilize the ~"-alumina crystalline structure at temperatures necessary to effect sintering. Thus, it has been found that if small amounts of -lithium oxide are added to the mixtures of sodium oxide and aluminum and the mixture is heated ~"-alumina is formed, but the ~"-alumina containing the small amount of lithium oxide may be heated to 1700C without decomposition to the ~-alumina crystalline form. It is for this reason that the last of the ~ ;
four (4) types of polycrystalline Beta-type-aluminas dis-cussed above, i.e., multi-metal polycrystalline oxides con-taining small amounts of the metal ions having a valence not greater than 2, such as magnesium or lithium ions, is a pre-ferred ceramic for use in the preparation of electrical con-version devices. It is these compositions which may be most readily prepared at h~gher temperatures and still be main-tained in the ~"-alumina crystalline form.
~ otwithstanding the improvements brought about by the inclusion of small amounts of materials such as lithium oxide or magnesium oxide or mi~tures thereof, sintering con-tinues to ha~e certain disadvantages for the preparation of dense, polycrystalline bi- or multi-metal oxides. While additives assist in improving the electrical properties, i.e., lowering the electrical resistance, the final fired _ 7 _ . , , ~ . ' ' 67g~
ceramics tend to be somewhat porous, demonstrating one to three percent porosity even with optimum samples, and demonstrate a duplex grain ' ' '.' ;~ . ' - 7a -. ' ' ' . ' : , ~`

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structure with grains up to 150 millimicrons in size. Such porosity and duplex grain structure have a deleterious effect on the fracture strength, elastic modulus and -~
fracture toughness of the sintered body.
Hot pressing is another known processing technique for the preparation of ceramic bodies which has certain advantages in that generally lowered temperatures can be used along with pressure to achieve densification of the bi or multi-metal oxide composition. The process, however, also has certain d1sadvantages, the primary one being the reaction of the ceramic composition with the surface of the hot pres-sing means which applies the pressure to the ceramic powder.
For example, when Beta-type alumina is hot pressed~ sodium oxide tends to volatilize and react with the hot pressing ~
surface, typically graphite, tending to result in a darkening ;
of the surfaces of the body and a loss of the desirable ~"-alumina crystal structure in those areas where the reaction has occurred.
The method fox the preparation of dense, polycrystal-line bi- or multi-metal oxides as disclosed and claimed hereafter obviates the deficiencies of prior art techniques ~or the preparation of these bodies and, in particular, obviates the deficiencies for the preparation of polycrystal-line ~"-alumlna bodies exhibiting near theoretical density, low porasity, high strength and low electrical resistance.
The method of this invention for preparing dense, ceramic polycrystalline bi- or multi-metal oxide bodies comprises: (A) disposing a powder composition comprising at least about 80 weight percent of aluminum oxide, between about 5 and about 15 weight percent of sodium oxide and between about 0.1 and about 5 weight percent of lithium :

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oxide, magnesium oxide or mixtures thereof within a hot ~ -pressing means adapted -to simultaneously apply heat and .`
pressure to the powder and spacing the powder from the ~ :
pressure applying surfaces of the hot pressing means by interposing, between the surfaces and the powder, spacers ~ -.
prepared from the same composition as the powder composition and having a densi.ty wh.ich is at least 15% of the theoretical density for polycrystalline ~ "-alumina, and (B) heating the :
powder within the hot pressing means to a temperature of between about 1300C and about 1450C and applying a pressure . of between about 4000 psi and about 6000 psi for greater ~ :
than about five minutes while maintaining the temperature.
A more specific and preferred embodiment of the method of this invention, which involves the preparation of :. .
polycrystalline ~ "-alumina ceram:ic bodies having a density of at least about 3.10 grams/cc, an electrical resistance at 300C of less than about 8 ohm--cm, a grain size of less than about 5 micromete.rs, substantially no porosity and a fracture strength between about 35,000 psi and about 40,000 ~ ~
~0 psi comprises: (A) disposing a composition comprising ~ ~ -between about 5 and about 15, preferably between about 8 and about 11 weight percent of sodium oxide, between about 0 and about 5, preferably between about .1 and about 5 and most preferably between about .1 and 1.5, weight percent of ~ ~-lithium oxide, magnesium oxide or mixtures thereof and at least about 80 weight percent, preferably at least about 85 weight percent, of aluminum oxide within a hot pressing means adapted to simultaneously apply heat and pressure to ~ :
said powder and spacing said powder from the pressure applying surfaces of said hot pressing means by interposing between said surfaces and said powder spacers formed of densified Beta-type alumina formed from the same composition as said powder;~ and (B) heating said powder within said hot ~31 9 ~L~S~7~

pressing means to a temperature of between about 1300C and about 1450C and applying a pressure of between about 4000 psi and about 6000 psi for between about 15 and about 80 minutes; and (C) exposing the hot pressed body so formed to a heat soak atmosphere adapted to prevent loss of sodium oxide and at a temperature between about 1300C and about 1500C to convert ~-alumina to ~"-alumina.
The various embodiments of the method of this in-vention will be more fully understood from the detailed description of the invention which follows when taken in combination with the drawings in which:
Figure 1 shows a vertical sectional view of a hot pressing means for use in carrying out the method of the invention;
Figure 2 illustrates thè use of a solid electrolyte ~' or reaction zone separator formed by the method of this in-vention in a simple cell wherein the densified polycrystalline ceramic of the invention provides cationic communication between and otherwise separates a liquid reactant anode and a liquid reactant-electrolyte which is in contact with a conventional cathode; and ~ -`
Figure 3 is a graph showing typical densification ;
curves obtained by performing the method of this invention ~
on a particular composition at a given pressure and various ~ `
temperatures.
Figure 1 shows a vertical cross section of a hot ~- -pressing means in which the bi- or multi-metal oxide in powder form may be processed to form a dense, polycrystalline bi- or multi~metal oxide body. The hot pressing apparatus shown in the drawing consists of an induction furnace used in conjunction with a laboratory hand press and a die ;

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assembly. As shown in Figure 1 the apparatus comprises a graphite d-e 10 which, as illustrated is cylindrical in shape with a cylindrical opening or cavity formed therein and having the same longitudinal axis as the die. It will be appreciated by those skilled in the hot pressing art, however, that the opening in the die can assume other con-figurations, e.g., openings with eliptical, square or ir-regularly shaped horizontal cross sections, depending upon the shape of the article desired. The opening or cavity of the die, which is preferably graphite, corresponds to that in die 10. Surrounding the graphite die is a layer of in-sulating material 12 which is capable of withstanding the high temperatures to which the die is subjected. The insulat-ing material shown is graphite felt although other materials can be used. The graphite felt 12 is encompassed and held in place by a quartz tube 14 which also functions as an in-sulator. The function of the insulat:ing material is to prevent the escape o~ heat from the graphite die when it is heated by induction as described hereinafter.
Disposed within the opening or cavity of die 10 are a pair of graphite plungers 16 and 18 which function as pressure exerting means, having pressure applying surfaces.
As shown, the plungers are cylindrical in shape as is the die opening. In the event that the opening is other than cylindrical as mentioned hereinabove~ then the plungers are correspondingly shaped. Also r the force exerting surfaces of the plungers can be other than flat without departing from the spirit and scope of the invention. The diameter of the plungers 16 and 18 is slightly less, e.g., from about 10 to about 40 mils, than that of the opening so that they may be movable therein. The lower plunger 18 rests on a ~5~
lower hydraulic press head 20 of an appropriate press, e.g., a Carver laboratory press, the upper head 22 of which exerts pressure against the upper plunger 16 of the die assembly.
Surrounding the die assembly is a coil 24 which is in the form of a metal tube, preferably copper tubing. The induction power supply for the induction furnace consists of a 20 kilowatt, lO kilocycle motor generator unit (not shown) which allows rapid heating with good control. A thermocouple 26 is provided to measure the temperature within the die assembly.
In the practice of the invention, the bi- or multi-metal powder 28 is disposed within the die assembly lO between plungers 16 and 18 and spaced from the pressure applying surfaces of said plungers 16 and 18 by spacers 30 which com-prise densified polycrystalline bi- or multi-metal oxide formed from the same composition as said powder 28 which is being hot pressed. By interposing the spacers 30 between the pressure applying surfaces of plungers 16 and 18 and the sample 28 to be hot pressed, the above-discussed problem of ~-~
reaction at the pressure applying surface and resultant deterioration in properties alo~g the surface of the hot ~
press body is obviated. Any reaction occurs between the ~ -composition of the spacer 30 and the graphite surfaces of plungers 16 and 18 rather than between the surfaces of the sample 28 and the graphite surfaces of the plungers 16 and ;
18. Thus, the sample ohtained is uniform in composition ;
and does not suffer from the aforementioned defects.
After disposing the sample 28 within the die assembly 10 between the spacers 30 and the plungers 16 and ~ -l~ the temperature within the die assembly, as measured by the thermocouple 26 is raised to at least about 1250C, pre-$9L67~
ferably between about 1300C and about 1500C, and most pre-ferably between about 1300C and 1450C. The hydraulic press is then activated and the upper head 22 drives plunger 16 down toward plunger 18 thus compressing the sample 28 between spacers 30. The pressure applied by the press to the sample 28 is between about 3000 psi and about 10l000 psi, preferably between about 4000 psi and about 6000 psi, and most preferably between about 4200 psi and abou~ 4600 psi.
The pressure and temperature are maintained for a time period which is equal to or greater than about one (1) minute, pre-ferably equal to or greater than about five (5) minutes and optimally between about 15 minutes and about 80 minutes.
Spacers 30 as mentioned above, comprise densified bi- or multi-metal oxides of the same composition as the sample 28 being hot pressed. Spacers 30 may be formed by merely compacting the bi- or multi-metal oxide powder. ~ ~;
However, this is not preferred inasmuch as the reaction which occurs between the pressure applying surfaces of plungers 16 and 18 will occur much more rapidly with a com-pacted powder than it will with a powder which has been pressed or sintered to a greater density. Preferably, the spacers 30 are densified to at least about 50~ of the theoretical density of the polycrystalline bi- or multi-metal oxide. Among the possible ways of preparing the -spacers 30 is hot pressing a sample of the bi- or multi-metal oxide powder in a hot pressing assembly shown in Figure 1, but without the use of spacers 30. After the sample has been removed from the hot pressing means, the surface portions thereof which have reacted with the pressure applying surfaces of the plungers 16 and 18 are removed by, for example, grinding or shaving and the sample is then cut, if .

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necessary, into spacers of the desired size. ~lternatively, t~e spacers may be prepared by conventional sintering techniques.
By hot pressing the bi- or multi-metal oxide powder in accordance with the aforementioned described process of this invention, it is possible to obtain dense polycrystalline bodies exhibiting uniform and small grain sizes, i.e., less than 10 micrometers; however, in some instances, it may be desirable to further process the material in accordance with the invention by adding a subsequent heat soak step whereby the material is heated in an atmosphere adapted to maintain the composition of the ceramic at a temperature between about 1200C and about 1500C, preferably between about 1300C and about 1500C for a sufficient time period, e.g., up to 24 hours, to develop the desired properties in the ceramic body. ~ `
The bi- or multi-metal oxides processed in accordance with the above-described process may be prepared for hot pressing in a conventional manner such as by calcining at a 20 temperature of between about 1000C and about 1300C. ;
As discussed in some detail hereinbefore, the pre-ferred material for processing in accordance with the process of this invention comprises at least about 80 weight percent, preferably 85 weight percent of aluminum oxide, between about 5 and about 15 weight percent, preferably between about 8 and about 11 weight percent, o sodium oxide and between about .1 and about 5 weight percent, preferably between about .1 and about 1.5 weight percent of Li2O,MgO or mixtures thereof with lithia being preferred. By the process of this invention, it is possible to prepare polycrystalline ~"-alumina containing ceramic bodies having a density of . , , . . , ~ . .:
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~rom about 3.10 grams/cc up to about 3.27 grams/cc and an electrical resistance at 300C. of less than about 8 ohm-cm, a uniform grain size of less than about 5 micrometers, sub-stantially no porosity and a fracture strength between about 35,000 psi and about 40,000 psi. These powder compositions may be prepared by calcining mixtures containing the given amounts of aluminum oxide, sodium carbonate, lithium nitrate and magnesium nitrate. Polycrystalline ~"-alumina -containing bodies produced by hot pressing demonstrate ~ ~`
excellent strength characteristics and high density; however, apparently because the thermodynamic equilibrium reaction of ~-alumina + Na2O _~ ~"-alumina is not carried to completion during hot pressing, there are amounts of ~-alumina present in the hot pressed body. This results in some deficiency in electrical properties, with the electrical resistance at 300C being higher than desired. Thus, when processing Beta-type-aluminas for use in electrical conversion devices, it is desirable to further process the hot press body by exposing it to a subsequent heat soak. By adding the afore-mentioned heat soak, it is possible to reduce the electrical resistance to desired levels by conversion of at least some B-alumina to ~"-alumina.
The energy conversion device or cell shown in Figure 2 is disclosed in U.SO Patent 3,404,036 which is re-ferred to above. The cell shown in the Figure is a secondary battery constructed of glass tubes 32 and 34, a slab of sodium-Beta-type-alumina 36 separating tubes 32 and 34 and ~ ;
affixed thereto in liquid-tight relationship by glass seals 38 and 40. The tubes 32 and 34 have an internal diameter 30 of about 12 mm. These and the glass seals 38 and 40 are constructed of a glass having a coefficient of expansion , ,, ~ . :

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close to that o~ Beta-type-alumina, e.g., Corning 7052, Kovar. The tube 32 is partially filled with molten sodium 42 and tube 34 is filled with a molten sodium and sulphur containing reactant such as sodium pentasulfide (Na2S5) 44.
The sodium and sodium pentasulfide are maintained in a molten state by conventional heating means not shown. The air in tubes 32 and 34 may be essentially evacuated and the tubes sealed or the cell may be operated in an inert atmosphere, e.g., argon. The Beta-alumina slab 36 which may comprise a ~"-alumina containing body prepared in accordance with the process of this invention, is about 12 mm. in diameter and about 2 mm. thick with the face exposed to the reactants in each of the tubes 32 and 34 being about 1.13 cm2, assuming a completely flat surface. In this cell, the molten sodium serves both as the anodic reactant and as an electrode while the sodium and sulphur reactants serve both as the cathodic reactant and as a liquid electrolyte which is in contact with the electrode 46. Ordinarily, one would start the reaction with the cathodic reactant having a sodium to sulphur ratio of about 2:5 and terminate the cell discharge when this ratio is at least about 2:3. A copper wire lead 48 extending into the sodium electrode 42 and a stainless steel electrode 46 extending into the sodium pentasulfide 44 illustrate ends of an external circuit not ;
further shown, which may include a voltmeter, ammeter, etc. -;
In the discharge half-cycle of this cell, the sodium is attracted to the sulphur opposite the Beta-alumina membrane, gives up an electron, passes through the membrane as a sodium ion and combines with a sulfide ion formed at the cathode 46 with acceptance of an electron, thus causing an electric current to flow through the aforementioned external - 16 ~

~54Z674 circuit. Recharging is effected by pressing an external source of electric power upon the circuit with a reverse electron flow in relation to that of the discharge half-cycle.
The hot pressing method of this invention is ideally suited for the preparation of slabs of polycrystalline bi- or multi-metal oxide ceramic materials such as shown in the device of Figure 2.
The following specific examples, which will more fully illustrate the method of this invention, are directed to the processing of Beta-type-alumina compositions. However, it will be recognized by one skilled in this art that this invention, as discussed above, is applicable to a wide range of bi- and multi-metal oxide compositions.
Example 1 Sodium carbonate and lithium nitrate of reagent grade are dried at 280C and 120C respectively, and cooled and stored in desiccators. Linde "C~' alumina, the other component of the ceramic compositions is used as received and stored in plastic bags. In order to produce lO0 parts by weight of the final reactive ceramic product, 14.88 grams of sodium carbonate, 3.Ç8 grams of lithium nitrate and 90.45 grams of aluminum oxide ar~ weighed in air and dry mixed on a paint 7~ .

shakcr in polyethylene bottles containing "LUCALOX" (Trade Mark) balls. Lucalox is a transparent polycrystalline alumina. The reacted ceramic composition which will be obtained from these reactants will comprise 8.8 weight percent of sodium oxide, 0.75 weight percent of lithium oxide and 90.45 weight percent of aluminum oxide.
The mixture of reactants is next rolled out on aluminum foil and the balls removed. The composition is then placed in a covered platinum container, reacted at 1250C for one (1) hour, and cooled in air. The reacted powder is again rolled out to remove lumps, reloaded into the mixing container along with Lucalox balls and run again on the paint shaker. After this last mixing operation, the composition is again rolled to remove all of the Lucalox balls.
Seven (7) samples of the ceramic composition are hot pressed between densified spacers of the same compositian in a graphite die assembly employing graphite plungers.
Five (5) of the powders were pressed at pressures of 4260 psi for 60 minutes at temperatures of 1440C, 1417C, 1375+15C, 1350C, and 1328C, respectively. Two (2) ~!
samples were pressed at pressures of 6000 and 5970 psi and temperatures of 1350C and 1328C, respectively for 60 minutes. Figure 3 shows typical densification curves for the five (5~ samples which were pressed at 4260 psi. It will be noted that at the highest pressing temperature (1440C), densification was complete after about 35-40 minutes. At this pressure, temperatures over approximately 1375C were required to achieve complete densification in reasonable periods of time.

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In Table I, the densities and resistivities of the . . .
final, as-pressed billets are tabulated along with resist-ivities which were obtained after various heat soak treatments described therein. The Table also includes data for the samples pressed at approximately 6000 psi. These indicate that higher final densities can be achieved at lower temp-eratures (less than or equal to 1350C) provided the pressure is raised accordingly.
The resistivities at 300C after hot pressing varied between 7 and 13 ohm-cm. X-ray examination reveals that these materials are mixtures of ~- and ~"-alumina. To convert the specimens to substantially all ~"-alumina, as-pressed material was subjected to two (2) consecutive heat soak steps, one at 1300C for 22 hours followed by a second at 1450C for 20 hours. All heat soaking was accomplished by imbedding the specimens in a powder of the same composition.
In all cases, both heat soak treatments dropped the resist-ivity of the spectrum and increased the conversion to ~"-alumina.
The low temperature heat it should be noted caused no grain growth. The grain sizes for as-pressed and heat soaked (1300C) specimens was under 3 micrometers. Thus, it is clear that high resistivity in the as-pressed material is due primarily to lack of complete conversion to ~"~alumina and not to small grain size.
Further heat soaking at 1450C for approximately 20 hours resulted in additional drop in resistivity, substantially complete conversion to ~"-alumina and large amounts o~
exaggerated grain growth and formation of duplex microstructure including grains up to 250 micrometers. As expected, the 30 strength of this material (less than 14,300 psi) was much lower than that -- 19 -- ; .

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of the pressed material (greater than 30,000 psi). From a study of these samples which were subjected to extensive heat soaking, it is clear that there will be an optimum about of heat soaking for each sample which will, without undue ~xperimentation, be apparent to those skilled in the art.
ExamFle II
A ceramic composition is prepared in the manner described in Example I by mixing 14.88 grams of sodium carbonate, 3.68 grams of lithium nitrate and 90.45 grams of Meller alpha-alumina and heating in a closed platinum con--tainer for two ~2) hours at 1250C, the resultant ceramic composition comprises 8.8% sodium oxide, .75~ lithium oxide and 90.45 aluminum oxide.
The ceramic composition is hot pressed between spacers formed by densifying ceramic of the same composition in a graphite die assembly between graphite plungers at temp- ;~
eratures of 1420C, 1390C, 1350C, 1330C and 1310C, all samples being hot pressed at 6000 psi for about one (l) hour.
The hot-pressing results are summarized in Table II. Each of 20 the samples was then heat soaked at 1350C for about 20 hours.
The resistivities of all samples as pressed and after heat soak are set forth in Table II.

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Claims (7)

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

1. A method for preparing dense, polycrystalline .beta."-alumina containing ceramic bodies having a density of at least 3.1 g/cc comprising:
(A) disposing a powder composition comprising at least about 80 weight percent of aluminum oxide, between about 5 and about 15 weight percent of sodium oxide and between about 0.1 and about 5 weight percent of lithium oxide, magnesium oxide or mixtures thereof within a hot pressing means adapted to simultaneously apply heat and pressure to said powder and spacing said powder from the pressure applying surfaces of said hot pressing means by interposing, between said surfaces and said powder, spacers prepared from the same composition as said powder composition and having a density which is at least 15% of the theoretical density for polycrystal-line .beta."-alumina, and (B) heating said powder within said hot pressing means to a temperature of between about 1300°C and about 1450°C and applying a pressure of between about 4000 psi and about 6000 psi for greater than about five minutes while maintaining said temperature.

2. The method of claim 1, wherein said hot pressing means comprises an inductively heated graphite die assembly and said surfaces comprise the ends of graphite plungers aligned so as to apply pressure to a body disposed there-between.

3. The method of claim 1, wherein said spacers are prepared by sintering said powder composition.

4. The method of claim 1, wherein said spacers are prepared by hot pressing said powder composition and removing from the outer surfaces thereof any material which has reacted with said surfaces of said hot pressing means.

5. The method of claim 1, wherein said polycrystalline .beta."-alumina containing body is further processed by heat soaking in an atmosphere which will preclude the escape of sodium oxide at a temperature in the range of about 1300 to about 1500°C to convert substantially all .beta.-alumina to .beta."-alumina.

6. The method of claim 5, wherein said body is heat soaked in a closed container.

7. The method of claim 5, wherein said body is heat soaked while embedded in a powder of the same composition as that being formed.