Patent Number: 055454275
Section: description

EXAMPLE 1 In this example preparation takes place of a first aluminium butoxide solution by mixing under a dry nitrogen atmosphere 125 g of secondary aluminium butoxide Al(OC.sub.4 H.sub.9).sub.3 obtained from Aldrich Chemical Company with an ethanol quantity such that the ethanol:butoxide molar ratio is 8, the solution being mixed for 10 min. In another beaker, preparation takes place of a lithium hydroxide suspension by suspending 21.3 g of LiOH, H.sub.2 O in an ethanol quantity such that the ethanol:lithium hydroxide molar ratio is 5. The suspension is then added to the solution (the ethanol:aluminium alkoxide molar ratio then being 13) and vigorous stirring thereof is maintained for 30 min. during which the temperature rises to 80.degree. C. This gives a white solution. This is followed by the hydrolysis of the solution by adding deionized and decarbonated water in a quantity such that the water:aluminium butoxide molar ratio is 10 and in this way a viscous mixture is obtained, which is stirred for a further 10 min. The white, pasty mixture is then dried in an oven at 150.degree. C. for 2 h or in an autoclave at 250.degree. C. and under a pressure of 7 MPa. Thus, a fine beta lithium aluminate powder is obtained. Analysis of this powder by X-ray diffraction shows that it is indeed beta lithium aluminate. This powder undergoes cold isostatic pressing or moulding under a pressure of 200 MPa for 1 min. in order to form diameter 10 mm pellets. These pellets are then directly sintered in alumina crucibles at a temperature of 850.degree. C., under air and for 2 h with a heating speed of 3.degree. C./min., after embedding the pellets in a powder bed having the same composition in order to limit stoichiometry variations. X-ray diffraction analysis of the product obtained shows that it is gamma lithium aluminate. After sintering, there is a longitudinal shrinkage of 18%, but no dilatometric anomaly is detected during beta/gamma transformation. After sintering at 850.degree. C., the relative density is 70% and the microstructure corresponds to ultrafine particles of sizes below 0.1 .mu.m. EXAMPLE 2 The same operating procedure as in example 1 is used, but sintering is carried out at 1000.degree. C. for 2 hours. In this case the density is 92% and the grain size is 0.2 to 0.3 .mu.m. EXAMPLE 3 The same operating procedure as in example 1 is used, but sintering takes place at 1100.degree. C. for 2 hours. This leads to dense pellets with a density of 99% and a uniform microstructure with grain sizes of 2 to 3 .mu.m. EXAMPLE 4 The same operating procedure as in example 1 is used, but sintering takes place at 1150.degree. C. for 2 hours. This leads to grain sizes of 3 to 8 .mu.m. Thus, the choice of a sintering temperature from 850.degree. to 1150.degree. C. makes it possible to adapt the grain sizes of the gamma lithium aluminate and obtain the desired dimensions in the range 0.1 to 10 .mu.m, with a density which evolves between 70 and 100% of the theoretical density. EXAMPLE 5 Preparation of Li.sub.4.025 Al.sub.3.925 Si.sub.0.05 O.sub.8. In this example the operating procedure of example 1 is followed, but to the secondary aluminium butoxide solution in ethanol is added a tetraethoxysilane solution in ethanol prior to adding the lithium hydroxide suspension. The tetraethoxysilane solution is prepared in the following way. 1.46 ml of tetraethoxysilane is dissolved in an ethanol quantity such that the ethanol:silicon alkoxide molar ratio is 4, followed by the prehydrolysis of the alkoxide at a pH of approximately 2 for 1 hour. This solution is then added to the freshly prepared solution of sec aluminium butoxide in ethanol and then the operations take place as in example 1. By sintering at 1100.degree. C., this gives a relative density of 95% and the microstructure obtained corresponds to grains with a size of 0.3 .mu.m. Thus, the process according to the invention is very interesting because it makes it possible to eliminate the prior calcination stage for transforming beta lithium aluminate into gamma lithium aluminate and easily regulate the microstructure and density of the product obtained.