Title of Invention

A PROCESS FOR THE PREPARATION OF MgO STABILIZED BETA ALUMINA

Abstract

An improved process for the preparation of MgO stabilised beta alumina having the general formula M n+- β˝/ β-alumina where M n+ represents Na+, Sr2+ or Ba2+, useful as solid electrolyte in power sources, chemical sensor, lower temperature production of high-pure sodium and in molten metal processing. The essential steps of the process are preparing and compacting an appropriate microwave active precursor of β˝/ β -alumina and subjecting the compacted product to heating to get MgO stabilized beta aluminas.

Full Text

F O R M - 2 THE PATENTS ACT, 1970 (39 OF 1970) COMPLETE SPECIFICATION (See Section 10 rule 18 1. TITLE OF INVENTION /Novol microwave active precursors of p"Vp-alumina, a process for its preparation and an improvod process for the preparation-of-MgO stabilized-fi /ft-alumina using microwave ladiatton A process for the preparation of mgo stabilised deta alumina 2. DEPARTMENT OF ATOMIC ENERGY, represented by its Secretary, A department of the Govt, of India, having its office at Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg, Mumbai - 400 039, Maharashtra, India. - 9 JUL 2001 Granted The following specification particularly describes the nature of the invention and the manner in which it is to be performed. This invention relates to a new microwave active precursors of P"VP-alumina. The new precursor is useful for the preparation of, MgO stabilised β"/β-alumina., This invention also relates to a process for the preparation of the new precursor . This invention also relates to an improved process for the preparation of MgO stabilized β"/β-alumina using microwave radiation. The MgO stabilised beta-aluminas prepared by the process of the present invention have the general formula Mn+- β"/β-alumina where Mn+ represents Na+, Sr2+ and Ba2+ and the corresponding material is an Mn+-ion conducting solid electrolyte. They are useful as solid electrolytes in Power Sources viz Sodium-Sulphur (Na-S) and Na-Metal/Metal chloride batteries (for electric vehicle and load levelling applications), chemical sensors viz. COx and SOx sensors for monitoring toxicity in the ambient atmosphere, lower temperature production of high purity sodium and in molten metal processing. As explained above, the invention also relates to novel microwave active precursors of β"/β-alumina (T-alumina, a polymorph of alumina, along with soda and magnesia; T-alumina, a polymorph of alumina, along with strontia and magnesia; T-alumina, a polymorph of alumina, along with baria and magnesia ) useful for the preparation of MgO stabilised β"/β-aluminas mentioned above and a process for the preparation of the said precursors. The preparation of dense p"Vp-aluminas in the desired shape for the above indicated applications is conventionally carried out in two steps namely a) the synthesis of phase-pure material and b) compaction to the desired shape followed by sintering. The preparation of Mn+- β"/β-alumina (Mn+ = Na+, Sr2+ and Ba2+) is conventional carried out by the solid-state reaction of a-alumina with Na2C03; a-alumina withSrCO3 ; a-alumina with BaC03 along with MgO or Li20 stabilizer. The use of MgO compared to Li20 is advantageous as MgO stabilised Mn+- β"/β-aluminas (Mn+ =Na+, Sr2+ and Ba2+) are less moisture-sensitive. In the case of Na-β"-alumina, synthesis through solid state reaction route conventionally known as the ceramic route has the disadvantage that a two phase mixture of p and P"-alumina is formed by the reconstructive transformation of the hexagonal close packed stacking sequence of the oxygen ions of α-alumina to the cubic close packed stacking sequence of the oxygen ions in the spinel blocks of the β - alumina. The amount of P"-phase in the biphasic mixture can be enhanced by various techniques such as post-sinter anneal (see the article of A.D.Jatkar, LB.Cutler and R.S.Gordon in eds. R.M.Fulrath and J.A.Pask, Ceramic microstructures (Westview Press, Boulder, CO, 1978), p.414), two peak firing ( see the article of J.H.Duncan and W.G.Bugden, Special Ceramics 7 (1981) 221.) and seeding of the reaction with pre-reacted β"-alumina (see the article of A.V.Virkar, M.L.Miller, J.B.Cutler and R.S.Gordon, US Patent 3 113 928 (1978)). Though there are various techniques to enhance the amount of P" in the biphasic mixture as mentioned above, repeated grinding, compaction and heat treatment at temperatures in excess of 1873 K for several hours are required for obtaining a phase-pure material. Besides being cost and energy intensive the prolonged heat treatment will lead to loss of Na20 from β"-alumina resulting in significant variation in composition. The loss of Na2O will thus lead to a decrease ln Na -ion conductivity (the most salient feature among the desired properties of the material for the above mentioned industrial applications). Moreover, it will lead to grain growth, which has a deleterious effect on ionic conductivity as well as mechanical strength of the finished product, which has also to be taken care of them. In addition to the conventional ceramic route, synthesis of P"-alumina through soft-chemistry routes viz. alkoxide hydrolysis (see the article of P.Morgan, Mater.Res.Bull. 11 (1976) 233), sol-gel processing (see the article of Chandry and R.M.Cannon, Mater.Sci.Res. 11 (1978) 443), co-precipitation (see the article of Pekarsky and P.S.Nicholson, Mater.Sci.Res. 15 (1980) 1517) and solution combustion technique (see the article of Tom Mathews, Mater.Sci. Engg. B78 (2000) 39) are reported. The alkoxide hydrolysis technique yields always biphasic mixtures of β and β" at all soaking temperatures ranging from 1223 to 1773 K and the subsequent conversion to β or β" (with Mg) is slow unless a eutectic intervenes. In case of the solution combustion technique, the stable phases for Na1+yMgyAl11-yO17 in the composition ranges 0.45 Therefore, though this method of synthesis yields a single-phase material at very low temperatures such as 773 K, still a high temperature heat treatment for obtaining the required β"-structure becomes mandatory. The other soft-chemistry routes namely sol-gel processing and co-precipitation techniques yielded single-phase material only after prolonged heat treatments for several hours (5 to 9 h) at temperatures in excess of 1823 K. These processes too are energy intensive because of the time and high temperatures involved. A lower temperature synthesis technique using low cost hydroxyl aluminas as precursor material has been developed and patented (see J.H.Duncan, P.Barrow, A,van Zyl and A.I.Kingon, UK Patent 2-175 582A (1986)). This technique was later confirmed by Van Zyl (see A.van Zyl, Solid State Ionics 86-88 (1996) 883). In this technique, the precursor along with Na2CO3 and the stabilizer are ball milled and made in to a slurry followed by freeze drying. The freeze-dried powders were then compacted and heat-treated at 1473 K for 24 hours to yield phase-pure β"-alumina. Although a phase-pure material is obtained at temperatures as low as 1473 K, the processing time is relatively high (24 h) and hence energy intensive. Through this route, β"-alumina can be synthesized at temperatures as low as 1473 K the precursor has to be maintained at 1473 K for at least 24 h in order to obtain phase pure β"-alumina. Though the temperature of synthesis was reduced, the time of anneal is still high making the process energy intensive. Therefore, the synthesis through any procedure be it conventional or through soft chemistry routes will surely be energy intensive. Though synthesis through one of the soft chemistry routes namely the urea combustion may yield β-alumina as a product at temperatures as low as 773 K, still a further heating, to 1823 K or above for a few hours is warranted for its conversion to P" structure. Other soft chemistry routes need heat treatment in excess of 1823 K for longer periods to yield phase pure p"-alumina. The advantages of soft chemistry route over conventional solid-state synthesis route are better homogeneity, lower processing temperatures and shorter times. However, single-phase P"-alumina is obtained only at temperatures in excess of 1823 K. Such a high temperature heating in turn inevitably leads to all of the problems addressed above. For fabrication of dense electrolyte membranes of required shape and properties, the synthesized powder has to be cast into desired shape and sintered at very high temperatures (T > 1873 K) for several hours (t ~30 h). The sintering process is time consuming and is energy intensive predominantly because of the high temperatures involved.. To circumvent this problem, a microwave-assisted sintering process has been carried out recently (see the article of H.S.Kalsi, M.Dutta, S.K.Sharda, G.K.Padam, N.K.Arora and B.K.Das, J.Mater.Sci.Lett. 19 (2000) 1119). In this technique, a biphasic mixture of p and P" -alumina , synthesised using conventional ceramic process were compacted to the desired shape and subjected to microwave radiation. It is reported that the sintering could J be achieved at temperatures as low as 1573 to 1673 K in 3 h. However, the final product was a biphasic mixture of β and β" -alumina like the starting material (detected by X-ray \ diffraction analysis). Hitherto, the divalent (Sr2+ and Ba2+) beta-alumina membranes of desired geometry and properties are fabricated by conventional ceramic technique (see the article of G.He, T.Goto, T.Narushima and Y.Iguchi, Solid State Ionics 124 (1999) 119; S.Yamaguchi and Y.Iguchi and A.Imai, Solid State Ionics 40/41 (1990) 87 and S.Yamaguchi, K.Kimura, M.Tange and Y.Iguchi and A.Imai, Solid State Ionics 26 (1988) 183) or ion-exchanging Na+ in dense Na-beta-alumina membranes, with Sr2+/Ba2+ ions in a molten-salt bath of Sr/Ba chlorides or nitrates ( see the article of G.C.Farrington and B.Dunn, Solid State Ionics 7 (1982) 267 and J.Kirchnerova, A.Petric, A.D.Pelton and C.W.Bale, Mater.Res.Bull. 26 (1991) 909). In the former case, very high temperatures (T > 1900 K) are required for sintering to high densities. In the latter case, a pre-fabricated high density Na-β-alumina membrane is required for ion exchange. Hence, both the techniques are energy intensive. For all practical applications, single phase Mn+- β"/β-alumina (Mn+ = Na+, Sr2+ and Ba2+) powder has to be compacted into desired geometry and sintered well to have the desired properties. The beta aluminas do not sinter well at temperatures below 1873 K. In order to obtain a high-density material, sintering has to be performed at temperatures in excess of 1873 K for several hours (30 h). Prolonged heat treatment at such high temperatures will lead to loss of volatile oxides like Na2O and grain growth resulting in lower electrical conductivity and mechanical strength consequent to which the power density, reliability and life of the devices made using this material are adversely affected. The fabrication of single phase MgO stabilized Mn+- p"Vp-alumina (Mn+ - Na+, Sr2+ and Ba2+) membranes is cost and energy intensive since temperatures in excess of 1873 K are required for sintering. There are no reports in the literature on the synthesis of MgO stabilized Mn+- β"/β-alumina (Mn+ = Na+, Sr2+ and Ba2+) using microwave radiation as energy source. There are also no reports in the literature on sintering of the above-mentioned beta aluminas, using microwave radiation to yield single-phase, high-density material. The main objective of the present invention is, therefore, to provide an improved process for the preparation of MgO stabilised Mnt-β"/β-alumina (Mn+ = Na+ Sr2+ and Ba2+) for use as solid electrolyte in power sources, chemical sensors , lower temperature production of high-pure sodium and in molten metal processing. Another objective of the present invention is to provide an improved process for the preparation of MgO stabilised Mn+- β"/β-alumina (Mn+ =Na+, Sr2+ and Ba2+) and their sintering to high density form for use as solid electrolyte in power sources, chemical sensors, lower temperature production of high-pure sodium and molten metal processing which is cost-effective as well as time saving. Still another objective of the present invention is to provide an improved process for the preparation of MgO stabilised Mn+- β"/β-alumina (Mn+ = Na+, Sr2+ and Ba2+) for use as solid electrolyte in power sources, chemical sensors, lower temperature production of high-pure sodium and molten metal processing by employing novel microwave active precursors of the above mentioned beta aluminas. Another objective of the present invention is to provide novel microwave active precursors of MgO stabilized Mn+- β"/β-alumina (Mn+ = Na+, Sr2+ and Ba2+) (t-alumina, a polymorph of alumina along with soda and magnesia; x-alumina, a polymorph of alumina along with strontia and magnesia; x-alumina, a polymorph of alumina along with baria and magnesia) which is useful for the preparation of MgO stabilised Mn+- p"/p-aluminas (Mn+ = Na+,Sr2+ and Ba2+). Still another objective of the present invention is to provide a process for the preparation of novel microwave active precursors of MgO stabilized Mn+- p"/p-aluminas (Mn+ = Na+, Sr2+ and Ba2+) (x-alumina, a polymorph of alumina along with soda and magnesia; x-alumina, a polymorph of alumina along with strontia and magnesia; x-alumina, a polymorph of alumina along with baria and magnesia) which is useful for the preparation of MgO stabilised Mn+- p"Vp-aluminas (Mn+=Na+, Sr2+ and Ba2+). Microwaves are electromagnetic waves ranging from 1 m to 1 mm in wavelength at frequencies from 0.3 to 300 GHz. Specific ceramic materials on exposure to microwave radiation absorb microwave energy and get heated up due to energy transfer that take place through the electric and magnetic field vectors of the microwave. Such energy transfer takes place if there occurs an electric field and/magnetic field coupling between the microwave radiation and the material. The temperature attained by coupling to microwave radiations is dependent on the electric and magnetic properties of the material. The substances that absorb microwave energy are called microwave active materials. The main advantages of using microwaves as energy source for processing of ceramic materials over conventional techniques are 1) rapid heating rates 2) lower power requirements 3) shorter processing times 4) fine grain size and 5) higher toughness. For the synthesis as well as sintering of β"/β-aluminas, the above advantages will lead to a) minimization of loss of volatile oxides like Na20 resulting in better conductivity (due to rapid heating rates and shorter processing times) (b) grain growth arrest leading to improved mechanical strength and ionic conductivity (due to shorter processing times and fine grain size) and c) reduction in cost, energy and time. The present invention has been developed based on the above. There are no reports in the literature on the synthesis of microwave active precursors of MgO stabilized Mn+ -β"/β-aluminas (Mn+ = Na+, Sr2+ and Ba2+) which on subjection to microwave radiation will lead to the synthesis of single phase MgO stabilized Mn+ -β"/β - aluminas (Mn+ = Na+, Sr2+ and Ba2+) and their sintering (densification of the compacted desired shape) in a single step. However, there is one report on only the sintering (densification) of a biphasic mixture of Na-P " and P-alumina (see the article of H.S.Kalsi, M.Dutta, S.K.Sharda, G.K.Padam, N.K.Arora and B.K.Das, J.Mater.Sci.Lett. 19 (2000) 1119) and none of synthesis or sintering, yielding a single phase material. Accordingly, the present invention provides a process for the preparation of MgO stabilized beta alumina having the general formula Mn+ -p "/p - alumina where Mn+ represents Na+, Sr2+ or Ba2+, useful as solid electrolyte in power sources, chemical sensors, low temperature production of high-pure sodium and in molten metal processing, the process comprising the steps of : a) preparing an appropriate microwave active precursor by homogenizing an aqeuous solution of salts of aluminium and magnesium with salts of sodium, strontium, or barium; b) adding an organic combustible complexant to the resultant solution, and refluxing the solution c) adding a gelating agent to the solution; d) allowing the solution to gel and slowly heating the gel to a temperature in the range of 973 -1173K in order to decompose the gel to precursor powder; e) compacting the precursor powder so obtained into suitable pellets at pressures in the range 0.2- 2 Gpa; and f) subjecting the compacted product to microwave heating to get MgO stabilised Mn+ - β"/β - alumina of the general formula given above. In a preferred embodiment of the present invention, the salts employed may be selected from nitrates, chlorides, acetates etc. The organic combustible complexant agent such as urea, hydrazine, polyvinyl alcohol, citric acid, adipic acid, ascorbic acid and the like may be employed. The refluxing may be done for a period in the range of 10-60 min, preferably at a range of 50 to 60 mins. The gelling agent such as polyhydric alcohols like ethylene glycol may be used. The solution is kept for gelling for a period in the range of 60 to 120 mins, preferably in the range of 100 to 120 min. The gelation may be effected at a temperature in the range of 320-370 K, preferably at a range of 360 to 370 K. The gel so obtained can be decomposed to yield the precursor in the temperature range 973-1173K. The precursor powders can be compacted to desired forms. The precursors on exposure to not clear radiation yielded high density, single-phase beta-alumina membranes. The resultant product is single-phase high-density MgO stabilized M -β"/β-alummas (Mn+= Na+, Sr2+ and Ba2+) confirmed by XRD analysis and density measurements. A flow chart showing the various steps involved in the process is given below. Fig. 1. Flow sheet for preparation of the microwave active precursor. The new soft chemistry precursor is capable of giving a phase-pure material at o temperatures 200 lower than other known precursors. The whole process (comprising synthesis and sintering) depends solely on microwave heating. The other known processes employ furnace heating. According to a feature of the present invention there is provided a novel microwave active precursors of MgO stabilised Mn+- β"/β-aluminas (Mn+ =Na+, Sr2+ and Ba2+) (T-alumina, a polymorph of alumina along with soda and magnesia; x-alumina, a polymorph of alumina along with strontia and magnesia; T-alumina, a polymorph of alumina along with baria and magnesia) According to still further embodiment of the present invention, there is provided a process for the preparation of the above said novel precursors of MgO stabilised Mn+- β"/β-aluminas (Mn+ -Na+, Sr2+ or Ba2+) which comprises homogenising an aqueous solution of salts of aluminum and magnesium and salts of sodium, strontium or barium, adding an organic combustible complexant to the resulting solution , refluxing the solution, adding a gelating agent to the solution and allowing the solution to gel, slowly heating the gel formed at a temperature in the range of 973-1173 K in order to decompose the gel and to form the precursor Description of the microwave set-up accompanying drawing The compacted disks were placed in a ceramic crucible embedded in powder bearing the same composition (precursor material). This ceramic crucible was surrounded by an insulating sheath made up of fibrous alumina, which was contained in a quartz container with a lid. The quartz container in turn with an insulating layer of fibrous alumina was placed in an alumina tube. The schematic diagram of the set-up is shown in Fig.2. of the drawing accompanying this specification. The whole set-up was placed inside the microwave oven and heating was done in stages. After one cycle of heating (approximately 30 min.), the sample was taken out, ground and repelletized for homogenization and again subjected to microwave heating. After a period of approximately 60 min., the sample was found to have a considerable shrinkage in dimensions. In order to check the homogeneity of the sintered sample, the disk was cut radially and horizontally to several pieces. Each piece was then crushed to powder and XRD pattern for the powders was obtained. The XRD pattern was similar for all the sections (the patterns for the beta aluminas obtained are shown in Figs 3-5). The pycnometric density of the sintered sample was measured using di-butyl phthalate and was determined to be 94 % of the theoretical value The details of the invention are given in the Examples provided below which are given to illustrate the invention only and therefore should not be construed to limit the scope of the invention. Example 1. 1.726 g of sodium nitrate, 1.360 g of magnesium acetate tetrahydrate and 36.792 g of aluminium nitrate nanohydrate were dissolved in double distilled water followed by a thorough homogenization of the same. The solution was saturated at 365-370 K over a hot plate. To this solution, 26.200 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 3.870 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 2. 2.764 g of sodium acetate trihydrate, 1.364 g of magnesium acetate tetrahydrate and 36.792 g of aluminium nitrate nanohydrate were taken and an aqueous solution of these salts were prepared followed by a thorough homogenization of the same. The solution was saturated at 365-370 K over a hot plate. To this solution, 26.200 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 3.870 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 3. 0.5641 g of strontium nitrate, 0.5717 g of magnesium acetate tetrahydrate and 10.0 g of aluminium nitrate nanohydrate were taken and an aqueous solution of these salts were made in order to obtain strontium substituted beta alumina corresponding to the formula SrMgAl10O17. The solution was saturated at 365-370 K over a hot plate. To this solution, 6.740 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 1.0 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 4. 0.5484 g strontium acetate, 0.5717 g of magnesium acetate and 10.0 g of aluminium nitrate were taken and an aqueous solution of these salts were made in order to obtain strontium substituted beta alumina corresponding to the formula SrMgAl10O17. The solution was saturated at 365-370 K over a hot plate. To this solution, 6.740 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 1.0 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 5. 0.6967 g of barium nitrate, 0.5717 g of magnesium acetate tetrahydrate and 10.0 g of aluminium nitrate nanohydrate were taken and an aqueous solution of these salts were made in order to obtain barium substituted beta alumina corresponding to the formula BaMgAl10O17. The solution was saturated at 365-370 K over a hot plate. To this solution, 6.740 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 1.0 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 6. 0.6810 g barium acetate, 0.5717 g of magnesium acetate and 10.0 g of aluminium nitrate were taken and an aqueous solution of these salts were made in order to obtain barium substituted beta alumina corresponding to the formula BaMgAl10O17. The solution was saturated at 365-370 K over a hot plate. To this solution, 6.740 g of citric acid monohydrate was added as the complexing reagent and the solution was then refluxed for 50-60 minutes. To the refluxed solution, 1.0 g of ethylene glycol was added as the gelating agent and kept aside for gelation at 360-370 K. The temperature was then slowly increased to 423-550 K in steps. The gel so formed was then dried at 573 K and then decomposed by slow heating to the temperature range 973 to 1173 K. The resulting fine powder was characterized by XRD. The obtained precursor is microwave active. Example 7. Disks of dimensions 15-20 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor as obtained in Example 1 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Na-β"-alumina was obtained (confirmed by X-ray diffraction). The disks were sintered to 94% of theoretical density. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Example 8. Disks of dimensions 15-20 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor obtained in Example 2 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Na-P"-alumina was obtained (confirmed by X-ray diffraction). The disks were sintered to 94% of theoretical density. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Example 9. Disks of dimensions 10-15 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor obtained in Example 3 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave radiation got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Sr-p-alumina were obtained (confirmed by X-ray diffraction). The measured density of the products are 94% of theoretical density of the Sr-beta-alumina which is identical if not better than those reported to be obtained by employing conventional techniques at higher than 1900 K. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Example 10. Disks of dimensions 10-15 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor obtained in Example 4 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave radiation got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Sr-(3-alumina were obtained (confirmed by X-ray diffraction). The measured density of the products are 94% of theoretical density of the Sr-beta-alumina which is identical if not better than those reported to be obtained by employing conventional techniques at higher than 1900 K. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Example 11. Disks of dimensions 10-15 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor obtained in Example 5 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave radiation got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Ba-β-alumina were obtained (confirmed by X-ray diffraction). The measured density of the products are 91% of theoretical density of the Ba-beta-alumina which is identical if not better than those reported to be obtained by employing conventional techniques at higher than 1900 K. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Example 12. Disks of dimensions 10-15 mm in diameter and 3-4 mm in thickness were made by compaction of the fine powder of the precursor obtained in Example 6 at 300 MPa. The compacted disks were then exposed to microwave radiation. The disks on exposure to microwave radiation got heated up to high temperatures resulting in rapid simultaneous synthesis and sintering. Single phase Ba-β-alumina were obtained (confirmed by X-ray diffraction). The measured density of the products are 91% of theoretical density of the Ba-beta-alumina which is identical if not better than those reported to be obtained by employing conventional techniques at higher than 1900 K. Both synthesis and sintering were achieved in a single-step using microwave heating of the precursor in 60 minutes. Advantages of the present invention l.The process is simple and cost effective as the synthesis and sintering are achieved directly using a domestic microwave oven. 2) Less time required as the process can be completed in 2 h total (including the time taken for compaction) as compared with the ~30 h needed for fabrication through conventional processes 3) No contamination by impurity phases 4) All desirable properties of a good solid electrolyte material could be prepared in a relatively short period of time. 5) The loss of volatile oxides like Na20 was negligible (confirmed by chemical analysis). 6) Fine-grained material is obtained. 7) The measured pycnometric density of the sintered product was 94% of the theoretical, which is the same as that obtained through conventional means at temperatures in excess of 1873 K. 8) The microwave oven used is the simple domestic one 9) A novel single phase, high-density Na-(3"-alumina and Sr/Ba-β-aluminas are produced and 10) The process is commercially viable. WE CLAIM : 1. A process for the preparation of MgO stabilized beta alumina having the general formula Mn+ -|3 7p - alumina where Mn+ represents Na+, Sr2+ or Ba2+, useful as solid electrolyte in power sources, chemical sensors, low temperature production of high-pure sodium and in molten metal processing, the process comprising the steps of : a) preparing an appropriate microwave active precursor by homogenizing an aqeuous solution of salts of aluminium and magnesium with salts of sodium, strontium, or barium; b) adding an organic combustible complexant to the resultant solution, and refluxing the solution c) adding a gelating agent to the solution; d) allowing the solution to gel and slowly heating the gel to a temperature in the range of 973-1173K in order to decompose the gel to precursor powder; e) compacting the precursor powder so obtained into suitable pellets at pressures in the range 0.2-2 Gpa; and f) subjecting the compacted product to microwave heating to get MgO stabilised Mn+ - p 7p - alumina of the general formula given above. 2. A process as claimed in claim 1 wherein the salts of aluminum and salts of sodium, strontium or barium is selected from their nitrates and acetates. - 3. A process as claimed in claims 1/and 2 wherein the amount of the salts employed are in the range 12.5 to 13.18 mol % of sodium nitrate, 4.53 to 5.3 mol % of magnesium acetate and 83 to 81.53 mol % of aluminium nitrate; preferably 13.18 mol % of sodium nitrate, 5.3 mol % of magnesium acetate and 81.53 mol % aluminium nitrate; 8.3 mol % strontium nitrate, 8.3 mol % magnesium acetate and 83.3 mol % aluminium nitrate; 8.3 mol % barium nitrate, 8.3 mol % magnesium acetate and 83.3 mol % aluminium nitrate. 4. A process as claimed in claims 1 to 3 wherein the organic combustible complexing agents such as urea, hydrazine, polyvinyl alcohol, citric acid, adipic acid, ascorbic acid are used. 5. A process as claimed in claims 1 to 4 wherein the amount of complexing agent used is in the range of 0.5 to 3 moles of complexing agent per mole of cations, preferably in the range of 1 to 1.5 moles of complexing agent per mole of cation. 6. A process as claimed in claimsl to 5 wherein the refluxing is effected for a period in the range of 10-60 min, preferably at a range of 50 to 60 mins. 7. A process as claimed in claimsl to 6 wherein the gelling agent such as polyhydric alcohols like ethylene glycol is to be used. 8. A process as claimed in claimsl to 7 wherein the amount of the gelling agent used is in the range of 0.5 to 1 mole per mole of complexing agent, preferably 0.5 moles. 9. A process as claimed in claims 1 to 8 wherein the resulting solution is kept for gelling for a period in the range of 60 to 120 mins, preferably in the range of 100 to 120 min. 10. A process as claimed in claimsI to 9 wherein the gelation is effected at a temperature in the range of 320- 370 K, preferably at a range of 360 to 370 K. 11. A process as claimed in claims 1 to 10 wherein the gel is decomposed at temperatures in the range 973 to 1173 K preferably at 1173 K. 12. A process as claimed in claimsl to 11 wherein the powder is made into the form of disks. 13. A process as claimed in claim 12 where the disks are of 15-20 mm in dia and 3-4 mm 14. A process for the preparation of MgO stabilised beta alumina having the general formula Mn+- P"/p-alumina where Mn+ represents Na+, Sr2+ or Ba2+" useful as solid electrolyte in power sources, chemical sensors, lower temperature production of high-pure sodium and in molten metal processing substantially as herein described with reference to the Examples 6 to 12. Dated this 5th day of July 2001

Documents:

638-MUM-2001-ABSTRACT(9-7-2001).pdf

638-MUM-2001-ABSTRACT(GRANTED)-(21-02-2008).pdf

638-mum-2001-cancelled pages(24-1-2005).pdf

638-MUM-2001-CLAIMS(9-7-2001).pdf

638-MUM-2001-CLAIMS(AMENDED)-(24-5-2004).pdf

638-MUM-2001-CLAIMS(GRANTED)-(21-02-2008).pdf

638-mum-2001-claims(granted)-(24-1-2005).doc

638-mum-2001-claims(granted)-(24-1-2005).pdf

638-mum-2001-correspondence(5-2-2007).pdf

638-MUM-2001-CORRESPONDENCE(IPO)-(15-4-2008).pdf

638-mum-2001-correspondence(ipo)-(2-8-2004).pdf

638-MUM-2001-DESCRIPTION(COMPLETE)-(9-7-2001).pdf

638-MUM-2001-DESCRIPTION(GRANTED)-(21-02-2008).pdf

638-mum-2001-drawing(9-6-2001).pdf

638-MUM-2001-DRAWING(9-7-2001).pdf

638-MUM-2001-DRAWING(GRANTED)-(21-02-2008).pdf

638-mum-2001-form 1(9-7-2001).pdf

638-mum-2001-form 19(30-10-2003).pdf

638-MUM-2001-FORM 2(COMPLETE)-(9-7-2001).pdf

638-MUM-2001-FORM 2(GRANTED)-(21-02-2008).pdf

638-mum-2001-form 2(granted)-(24-1-2005).doc

638-mum-2001-form 2(granted)-(24-1-2005).pdf

638-MUM-2001-FORM 2(TITLE PAGE)-(COMPLETE)-(9-7-2001).pdf

638-MUM-2001-FORM 2(TITLE PAGE)-(GRANTED)-(21-02-2008).pdf

638-mum-2001-form 3(9-7-2001).pdf

638-MUM-2001-GENERAL POWER OF ATTORNEY(24-1-2005).pdf

638-MUM-2001-GENERAL POWER OF ATTORNEY(28-10-2003).pdf

638-mum-2001-power of attorney(24-1-2005).pdf

638-mum-2001-power of attorney(27-8-2003).pdf

638-MUM-2001-SPECIFICATION(AMENDED)-(24-1-2005).pdf

abstract1.jpg