X – Ray powder diffraction study of leucite crystallisation

M. Novotna¹, J. Maixner²

¹Inst. of Inorganic chemistry AS CR,250 68 Øež u Prahy, Czech Republic

²Inst. of Chemical Technology in Prague Technická 5, 166 28 Praha 6, Czech Republic

Leucite based materials are being a subject of an extensive scientific interest in recent years, especially in a field of dental prosthesis. Presence of leucite (KAlSi₂O₆), exhibiting high values of a thermal expansion coefficient, in a porcelain matrix enables its fusion to metal reinforcement. In addition thanks to its phase-transition behaviour leucite could potentially toughen leucite-glass (porcelain) systems and so lead to dental materials having high fracture toughness.

As the amount, average crystal size and structure of the leucite phase directly affect thermal and mechanical properties of the final composite material; suitable technology of its preparation is necessary to be developed.

A new preparation procedure of leucite based on its crystallisation from an amorphous precursor was studied. The precursor was synthesized from a gel containing aluminium, amorphous silica and potassium hydroxide solution in hydrothermal conditions at 200°C for 2h. Leucite was subsequently prepared by calcinations of obtained powders.

Crystallisation of leucite was monitored at 800, 900 and 1000°C using X-ray powder diffraction analysis. At high temperatures the stable form of leucite is cubic (high leucite), and as it cools below a temperature of about 630°C there is a displacement phase transformation to tetragonal (low leucite). As this transformation is substantial for the toughening mechanism in leucite dental porcelains the process of leucite transformation was studied using X-ray diffractometer equipped with high temperature cell.

Acknowledgements.

This work was supported by GA CR Grant 104/03/0031.