CONSECUTIVE SOLID-PHASE hBN(1)-cBN-hBN(2) TRANSFORMATIONS AT 7 GPa UNDER EXTREMELY HIGH TEMPERATURE GRADIENTS

Аннотация

New technical solution for high-pressure cell (HPC) design was developed appling to toroidal type apparatus (HPA-T30). The cell assembly allows creating a significant radially directed gradient (ÑT) of around ~250 °C/mm in the reaction volume of the apparatus. From the point of view of HPHT experiment methodology this approach gives an opportunity to record structural changes and solid-phase transformations of compounds depending on temperature within the only one sample volume. The capabilities of the method were demonstrated by the example of the thermobaric behavior of graphite-like boron nitride (hBN) at a pressure of 7 GPa in a wide temperature range reaching the level of~ 3000 °C. The evolution of solid-phase transformations of primary hBN(1) followed the sequence hBN(1)-cBN-hBN(2), i.e., included the stage of reconversion of cubic cBN into secondary hBN(2) at temperatures above 2680°C. The structural features of the final phase indicated the formation of a polycrystalline structure of hBN(2) at the stages of recrystallization annealing (grain growth process) under high-temperature thermobaric triatment.