Influence of pH on the property of apatite-type lanthanum silicates prepared by sol-gel process

Abstract

The apatite-type lanthanum silicates with formula La_9.33Si₆O₂₆ are prepared by sol-gel process. The homogeneity of the sol affected by pH value of the solution is investigated. The viscosity of the sols slightly increases first and then increases abruptly because the predominant reaction mechanism changes from hydrolysis reaction to condensation reaction. In addition, the onset time of the increase for the viscosity shortens from pH 1 to pH 4. The gelation time decreases with increasing pH of the solution. Therefore, the pH of the sols should be less than 4 to form gel. The sol with initial pH 2 shows maximum value of zeta potential and maximum stability. For the sample with initial pH 2, pure apatite-type lanthanum silicates La_9.33Si₆O₂₆ have been successfully prepared after the dried gel is calcined at 1 000 °C. In addition, this sample sintered at 1 550 °C exhibits the highest ionic conductivity. The activation energies are all less than 0.90 eV.

Keywords

sol-gel process / pH / X-ray diffraction / ionic conductivity

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