CA1065580A - Methods for producing beta-alumina composites - Google Patents

Abstract

Abstract of the Disclosure Methods are provided for producing a hot-pressed beta-alumina composition. The hot-pressed composition consists essentially of M2O?nAl2O3, wherein n is a positive integer between about 3 and about 12 and M is selected from Na and K, and features a flexural strength of at least about 45,000 psi as measured by ACMA Test No. 2, and high trans-missability to light.

Description

s~

rleld of the Invention .
This invention relates to improved ionically conductive crystalline beta-alumina composltions and methods for preparing them. More specifically, the invention relates to a method for produ`cing ionically conductive crysta:Lline beta-alumina from a finely divided amorphous powder.

Prior Art Beta-alumina is a well-known commercially available material which has found widespread use as a cation conductor in devices which are electrically or electrochemically actuated, and which is particularly useful for ~orming half-cell separators in batteries employing a molten alkali as a reactant. Crystalline beta-alumina has a hexagonal type structure consisting of spinel blocks of oxygen in which the aluminum is situated in the same positions as magnesium and aluminum in a magnesium aluminate spinel. The spinel blocks are separated by a NaO mirror plane. The distance between the two oxygen mirror planes distinguishes ~-alumina from ~'-alumina. In ~-alumina, the distance between the mirror planes is 11.23 ~; in ~"-alumina the distance is about doubled.
Beta alumina is generally prepared commercially by heating an appropriate mixture of sodium carbonate and aluminum oxide to somewhere between 1550 and 1800C. In U.S. Patent No. 3,475,225, issued to G.J. Tennenhouse, typical temperatures of about 1700C for sintering mixtures of sodium and aluminum oxides are reduced to temperatures between 1000 and 1600C by using pressures ranging from about 5000 psi to about 110,000 psi.
U.S. Patent Nos. 3,1313238 and 3,437,724 are typical of disclosures of hot-presslng techniques for forming crystalline compositions. However, none of these are used to form beta-alumina.

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The present invention results in improved crystalline beta-alumina which is produced at temperatures and pressures ` which are substantially lower than those employed in the prior art. In addition, the method of this invention utilizes standard equipment and readily available materials in the production of the crystalline beta-alumina which has improved physical characteristics when compared with beta-alumi.na produced by prior art methods.
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Summary of the Invention ~: 10 In accordance with this invention, a hot pressed beta-alumi.na composition is provided which consists essentially ~.
~of M20~nA1203, wherein n is a positive integer between about 3 and about 12 and M is an alkali metal selected from Na and K. The novel beta-alumina composition of this invention features the following physical properties; ionic conductivity, hieh theoretical density, and a flexural strength of at least about 45,000 psi when measured by ACMA Test No. 2. The . beta-alumina compositions`of this invention also have a high ~ ~: transmissability to light.
.~20: : ~ The ionically conductive crystalline beta-alumina ~ eompositlon of this invention is prepared and formed into a .
-composite by a method which comprises: -~ mlxing and reacting together a solution aluminum ;~ ~ alcoholate and an aqueous solution of an alkali metal salt;

~ `coprecipitating as a gelatinous mass aluminum hydroxide ~ and the alkali metal salt;
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drying the gelatinous mass coprecipitated in the previous step;

: grinding the dried ~elatinous mass into a fine 30 amorphous powder; andhot pressing the resulting powder to yield a beta-aiumina composite.

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In a preferred embodiment, this invention provides a hot-pressed ionical~y conductive crystalline beta-alumina composition consisting essentially of Na20 5A1203. The hot pressed Na20-5A12O3 beta~alumina composite has essentially theoreti~al density and is nearly transparent.
Beta-alumina composites produced in accordance with the teachings of this invention possess the advantage that standard hot pressing equipment can be used to attain the temperatures and pressures required, thus eliminating the need for specially designed apparatus capable of wi~hstanding the high temperatures and pressures necessary for the methods of the prior art.

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Brief Description of the Drawin~s Fig. 1 is a graph of the infrared transmission spectra of Na2O 5A12O3-Fig. 2A is an X-ray diffraction Pattern of a hot pressed composite of Na2O 5A12O3 taken parallel to the direction of hot pressing.
Fi~. 2B is an X-ray diffraction pattern of a hot pressed composite of Na2O-5A12O3 taken perpendicular to the direction of hot pressing. ~r Detailed Description of the Invention The beta-alumina composites of this invention are made by vacuum hot pressing a finely divided homogeneous beta-alumina powder. The finely divided homogeneous beta~
alumina powder is prepared by first coprecipitating a mixture of hydrated aluminum oxide and an alkali metal salt from solutions of components which, when heated, give rise to a - volatile by-product. To a solution of an aluminum alcoholate 3 is added an aqueous solution of the alkali metal salt. A

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gelatinous precipltate of hydrated aluminum oxide and the alkali metal salk results. After separating the gelatinous precipitate from the supernatent liquid and drying it, the dried precipitate is heated at a temperature of at least about L~oooc but which is less than about 12~0C, for a time period of from 1 to 16 hours, thus removing undesired volatiles. The temperature and leng~h of time used in any particular application is dependent upon the particular alkali metal salt being used. The resulting amorphous material is then ground to a fine powder. The powder should be ground to a particle size of less than about 50 micrometers, and preferably less than 1 rnicrometer or submicron.
Various alkali metal salts can be used in the above-described process including, for example, the bicarbonate, acetate, hydroxide, nitrate and carbonate of sodium and potassium. The beta-alumina powders thus produced have the formula M20 nA1203 wherein M is the alkali metal ion and n is an integer from about 3 to about 12 depending upon the , ratio of reactants used.

Before hot pressing, the powder can be placed in a - : .
pressing cyllnder and cold pressed at a pressure of at least about 4000 psi and preferably in the range of from about 4000 to about 16,000 psi. This insures intimate contact between the powders and prevents vacuum removal of loose powder.
Whether cold préssed or not, the powder is then placed in the ~pressing apparatus, and the apparatus is assembled and connected ;;~ to a vacuum system. Thereafter the powder is heated to a .
~irst temperature of from about 1150C to about 1400C, and preferably at least about 1200C, while a vacuum is drawn 30~-and maintained. Upon reaching the selected temperature, an initial pressure of at least about 4000 psi is applied while maintaining the vacuum. This pressure is maintained during a holding period, during which the powder can be further heated until it reaches a second temperature slightly higher than the first temperature ~65S80 y about 100C. The initial pressure is held for a time period of at least about 5 minutes, preferably at least about 10 minutes. The pressure is then increased to a higher pressure of at least about 20,000 psi, advantageously at le~ast about 25~000 psi. The compressed powder is maintained under vacuum at this temperature and pressure for a length of tirne of at least about 10 minutes, preferably at least about 20 mlnutes. Then the compressed powder is cooled to a temperature below about 1150QC and preferably to about 1000C whereupon the vacuum is released and the pressing cylinder is backfilled with nitrogen. The compressed powder is then cooled further and the pressure is released, yielding an lonically conductive crystalline beta-alumlna composite.
Preferably the temperature is cooled to about 800C before the pressure is released. It should be noted that the particular temperatures, pressures, and time periods used for the hot pressing process are generally dependent upon the composition of the powder and the amount of powder being ; hot pressed.
In another embodiment of the above-described process, after the compressed powder is heated to the first temperature, preferably about 1200C, and the initial pressure is applied, the compressed powder is further heated to a higher temperature, for example 1300C, whereupon the process is continued as described above.
The beta-alumina composites produced in accordance with the teachings of this invention exhibit spinel crystal structure as determined by X-ray diffraction patterns. ~lthough hot pressed from an amorphous powder, all compositions produced ionically conductive crystalline composites having conduc-tivities in the range of from about the order of 10 3 to about the order of 10 6 (ohm-cm) 1 The beta-alumina composites Or this invention further exhibited a flexural strength o~ at .

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least about 45,000 psi as measured by ACMA Test No. 2 and a high transmissabilit~y to light. More specifically, the transmission of incident radiation is at least about 70%
when the wavenumber of the incident radiation is between about 2200 and about 380o cm 1 The density of` the sample is also high, approaching that of the calculated or theoretical density for a single crystal of beta-alumina having the same chemical composition. Specific examples have a density at least as high as 93% that of theoretical.
The invention will be further illustrated hy the following examples.
,, Example 1 In a 4 liter beaker, 408.4 g (2.0 mole) aluminum isopropoxide was dissolved in a mixture of 1750 ml of benzene and 850 ml of isopropanol. To this was added, with stirring, a solution containing 17.6 g (0.44 mole) sodium hydroxide dissolved in 500 ml water. The resul~ing gel was stirred for 5 minutes, allowed to stand for 1 hour, then filtered by suction and dried at 180C for about 16 hours ~overnight). The easiiy ; 20 friable amorphous product weighed 191 g.
Thirty grams of this dried gel, contained in a fused alumina crucible, was placed in a muffle furnace at 400C. The furnace temperature was raised to 900C and the mixture held at 900C for 4 hours. An amorphous beta-alumina having the composition Na2O 4A12O3 was produced. After cooling, the amorphous material was ground for 2 hours with a mullite mortar and pestle~ using a Fisher Automatic Mortar Grinder.
The material was then ready for hot pressing.
It should be noted that about 10 percent excess 30 sodium hydroxide was used in Example 1 because it was found that, when using the hydroxide of an alkali metal, approximately 10 percent of the hydroxide does not precipitate but remains in the filtrate.

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~xample 2 _.
Beta-alumina powder prepared according to the method described in Example 1 was sieved through 270 mesh (U.S. Standard), and placed in the pressing cylinder between pyrolytic graphite discs and cold pressed at 5000 psi. The cylinder containing the powder was then placed in the pressing apparatus, and the apparatus was assembled and connected to a vacuum system.
The powder was heated to 1200C for a time period 10 Of abuut 30 minutes while the vacuum was maintained below 150 microns. No pressure was applied until the powder reached 1200C, at which time 4000 psi was applied. Heating ~.
continued until the temperature reached 1300C. The powder was then held at 1300C and 4000 psi for 10 minutes. The pressure was next increased to about 25000 psi and held for 20 minutes while the temperature remained at about 1300C.
The heat was shut orf and when the temperature had cooled to about 1000C the vacuum was shut off and the apparatus backfilled with N2. When the temperature was approximately 20 800C the applied load was removed. The apparatus was disassembled when the temperature approached ambient, and the pressed Na2O 5A12O3 disc was then removed from the pressing cylinder.

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Example 3 -In a manner similar to that of Example 1, beta-alumina powders were prepared using sodium bicarbonate, sodium acetate, sodium nitrate, potassium carbo~ate and potassium hydroxide. Except when using potassium hydroxide, the alkali metal salts were mixed in the exact proportion in which they were desired in the final product. When using potassium hydroxide, 10 percent excess hydroxide was used as in Example 1.

655~30 ' _xample 4 Sample Na20~5A1203 dlscs were prepared according to the methods described in Examples 1 and 2 except using ; sodium carbonate as the alkali metal salt and hot pressing at a maximum pressure of 20,000 psi. The resulting composite disos were then physically characterized.
The infrared spectra were measured using a Beckman 21A spectrophotometer. The density of the samples was determined by a hydrostatic weighing technique. The Knoop hardness was measured using a Tukon testing machine'with a lloo-g load. The çoefficient of thermal expansion was obtained using a Leitz dilatometer on'samples 10 mm long x 3 mm square. The modulus of rupture was calculated from loads measured on an Instron testing machine by the procedure described in AC~A Test No. 2.
Samples 0.18 cm square x 2.54 cm long were tested using a three-point bending fixture~with a 1.8 cm span. A head speed of 0.13 cm/min was used. Tests were run both parallel and perpendicular to the pressing direction. ~h,e fracture surfaces were examined,using a scanning electron microscope with the sample positioned at 4$.
A typical infrared transmission spectrum is shown in Fieure 1, for a thickness of 1.5 mm. As can be seen, the sample is free of impurity absorption bands frequently found in single crystals that have b'een exposed to moist air. Note particularly the absence of D(OH) absorption at 3100 cm 1, X-ray 'diffraction analysis was made both parallel and perpendicular to the direction of hot pressing. Figure 2A shows a typical X-ray diffraction pattern parallel to the direction of hot pressing and Fig. 2B shows a typical,X-ray diffraction pattern perpendicular 30 to the direction of hot pressing. Enhancement of the refraction peaks as seen in Fig. 2B indicate a preferred crystalline orientation.

Other physical properties of the samples are tabulated below in Tabies 1 and 2.

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Table_l Properties of Na20-5A1203 Knoop hardness 1120-1200 400-g load Coeff. of expansion 7.03 x 10-6 25-800C
in./in./C

Density 3.257 g/cc Ionic conductivity 7.5 x 10 1 at 25C
(ohm cm) (parallel face) Table 2 Modulus of Rupture (Flexural Strength) Load applied to surface Load applied to surface parallel in direction perpendicular in direction of pressing _ of pressin~ _ Sample No. Sample No.

1 53,636 psi 2 58,252 psi 4 46,120 3 58,463 46,120 9 56,820 : ~: 6 52,075 -After the pressed discs of Na20.5A1203 were ground and polished, they were highly translucent to visible light.
In fact, they were nearly transparent in that overhead lights could clearly be seen through them.
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Example 5 Na20 nA1203 powders wherein n=4, 6 and 11 wereproduced in a manner similar to to Example 1 except that~after drying, samples were heated at 400, 900 and 1200C for various lengths of time before grinding and hot pressing. All powders made by heating at 400 or 900C were amorphous. The hot-pressing steps of Example 2 were followed to make composites for measuring physical properties, except that the temperature was not increased above 1200C during the application of pressure.
The results are tabulated below in Tables 3, 4 and 5.

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kk~ 13-, . .. . . _ , ~065580 Example 6 Na2o.nA12O3 powders wherein n was varied ~rom,3 to 12 were produced in a manner similar to that of ExampLe 1. After hot pressing at 1200C, the cond~ctivity of the resulting beta~ `
~: alumina composite was measured and its X-ray diffraction pattern was examined. The results are tabulated below in Table 6 .` - ' .

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Table 6 Na20 ~ A1203 Hot-Pressed at 1200C
Na20:A1203 (ohm-cm)~l X-ray diffraction 1:31.6 x 10 4 X
1:41.7 x 10-4 X
l:S5 x 10 4 X
65 x 10-4 X

: 10 1:73.2 x 10 5 X X
1:81.6 x 10 5 X X
1:91.6 x 10 5 X X
1:106 x 10 5 X X
~: ~ 1: 111 x 10-5 X X
1:122.8 x 10-5 . X X

Thi~ invention has been ful~y disclosed with particular references to the preferred embodiments thereof. However, it i9 understood that variations and modifications can be made without departing from the spirit and scope of this invention.

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Claims (7)

We Claim:

1. A method for making a hot pressed beta-alumina composite which comprises:
mixing and reacting together a solution of aluminum alcoholate and an aqueous solution of an alkali metal salt;
coprecipitating as a gelatinous mass aluminum hydroxide and the alkali metal salt;
drying the gelatinous mass coprecipitated in the previous step;
heating the dried ma s to a temperature between about 400°C to about 1200°C;
cooling and grinding the dried gelatinous mass into a fine amorphous powder; and hot pressing the resulting powder to yield a beta-alumina composite.

2. The method as defined in claim 1 wherein the hot pressing step includes the steps of:
loading the powder into a pressing cylinder;
and while maintaining the pressing cylinder under a vacuum, 1) gradually heating the powder to a first temperature;
2) applying an initial pressure to compress the powder after attaining the first temperature;
3) maintaining the compressed powder at at least the attained temperature and at the initial pressure for a first predetermined time period;
4) increasing the pressure to a holding pressure which is higher than the initial pressure;
5) holding the compressed powder at the attained values for a second predetermined time period;

6) cooling the compressed powder to a temperature which is lower than the first temperature whereupon the vacuum is released;
7) backfilling the pressing cylinder with an inert gas; and 8) cooling the compressed powder further to a temperature lower than that achieved by step (6), and thereafter removing the pressure.

3. The method of claim 2 wherein after the loading step, the powder is cold pressed under a pressure of at least 4000 psi.

4. The method of claim 2 wherein the first temperature is at least 1200°C and wherein, after applying the initial pressure on the compressed powder, the method further comprises the step of heating the compressed powder to attain a higher temperature which is at least 100°C higher than said first temperature.

5. The method of claim 2 wherein the first predetermined time period is at least 5 minutes and the second predetermined time period is at least 10 minutes.

6. The method of claim 1 wherein said alkali metal salt is selected from the group consisting of bicarbonate, acetate, hydroxide, nitrate and carbonate of sodium or potassium.

7. A method for making a hot pressed beta-alumina composite which comprises:
mixing and reacting together a solution of aluminum alcoholate and an aqueous solution of an alkali metal salt;

coprecipitating as a gelatinous mass aluminum hydroxide and the alkali metal salt;
drying the gelatinous mass coprecipitated in the previous step;
heating the dried mass to a temperature between about 400°C to about 1200°C;
cooling and grinding the dried gelatinous mass into a fine amorphous powder; and hot pressing the resulting powder to yield a beta-alumina composite, wherein the hot pressing step includes the steps of:
loading the powder into a pressing cylinder;
cold pressing the powder by applying a pressure of at least 4000 psi;
maintaining the pressure cylinder under a vacuum;
gradually heating the powder to a first temperature of at least 1200°C;
applying an initial pressure of at least 4000 psi to compress the powder after attaining the first temperature;
maintaining the compressed powder at at least the first temperature and at the initial pressure for at least 10 minutes;
increasing the pressure to a holding pressure of at least 20,000 psi;
holding the compressed powder at at least the first temperature and the holding pressure for at least 20 minutes;
cooling the compressed powder to about 1000°C where-upon the vacuum is released;
backfilling the pressing cylinder with an inert gas; and cooling the compressed powder further to about 800°C whereupon the pressure is removed.