GRAIN SIZE CONTROL IN SITU AT HIGH PRESSURES AND HIGH TEMPERATURES IN A DIAMOND-ANVIL CELL

The grain size distribution and the character of individual grain boundaries in microcrystalline networks play a significant role in material properties, such as melting temperature, diffusion coefficients, resistivity, optical absorption, elastic constants, phase transformation pressure, and so on. The effects of pressure, temperature and quench rate on crystal growth from different starting materials have been studied in the diamond-anvil cell with synchrotron x-ray diffraction techniques. Pressure-induced structural phase transformations at room temperature can produce different phenomena: single-crystal growth, grain size reduction and amorphization. At ambient temperature, grain size is difficult to control because it mostly depends on the type and structure of starting materials and weakly on high-pressure parameters (hydrostaticity, compressibility etc). However, using high-temperature heating (annealing and/or melting), grain size can be modified in-situ at high pressure. For example, varying the cooling speed of a molten sample at high-pressure nanocrystalline (quenching in less than 1 ms) or microcrystalline materials (cooling rate under 10000 deg per second) can be synthesized. In this study, we show that the grain size of materials may be controlled in-situ at high pressures over the entire range of the length scale of crystallinity: single crystal, micro-/nano-crystalline and amorphous materials within a volume commensurate with the size of the probing x-ray beam. The structure refinement of high pressure samples from x-ray diffraction data can be significantly improved by controlling grain size by selecting the structure of starting materials and following certain high pressure-temperature-time paths.