The synthesis of substances under high static pressures

Abstract

Phase transitions of many substances occur under extremely high pressures and temperatures. The development of such conditions is possible within a limited reaction vessel volume of a special high pressure apparatus. In order to obtain pressures of 5 to 8 GPa in a reaction volume a high-pressure apparatus is loaded by anvils of hydraulic presses whose working force is 1 to 10 MN. The heating of substances up to 1500–2000 K is carried out by direct current transmission or a reaction vessel is provided with special resistors. The high pressure apparatus (HPA) consists of a number of elements whose yield and heat conductivities differ. This defines the pressure and temperature gradients within the reaction vessel volume and throughout the HPA as a whole. The dimensions and the design pecularities of HPA substantially affect its temperature and pressure fields. The basic principles of the theory of large plastic deformations and those of the mechanics of deformation in solids are considered. The temperature and pressure fields in the HPA reaction volume of model shape and dimensions are defined as a result of solving associated non-linear non-stationary problems of electro- and heat-conductivity in HPA and those of thermal plasticity by the method of finite elements. The probable arrangement and volume of various phases (initial low pressure phase, synthesized high pressure phase and those of the solid and melted metal solvent) were investigated. The characteristics allowing the quantitative evaluation of the efficiency of the selected pressure level within the reaction vessel prior to the heating cycle were proved to be valid.