Ultrahigh-Pressure Experiments on Thermal Transport—Linking Theory to Applications

Ultrahigh-pressure experiments reveal significant changes in the stability, elasticity and transport properties of crystalline and fluid matter, challenging first-principles theory and helping in the development of new materials. Compression into the high energy-density regime (pressures exceeding 10^11 Pa or 10^6 atmospheres) causes electronic as well as crystal-structural transitions due to the thermodynamic perturbation being comparable to valence electron energies (~ eV): insulators become metals (and vice-versa), ions change shape as well as size, and chemical bonding is altered. Recently, time-domain thermoreflectance (TDTR) and other pulse-probe measurements have been integrated with diamond cells to quantify thermal transport under static high pressures. The connection between thermal and electron transport is currently receiving particular attention, the results to date being in agreement with established theory of electron and phonon scattering, and providing important constraints on our planet’s early evolution. Future experiments using dynamic compression may allow measurements to be extended into the realm of atomic pressures (10¹³ Pa), at which the structure of the atom is fundamentally altered and compressional energies exceed core-electron values (keV). Experiments at such extreme conditions are part of a strategy for advancing theory and producing new materials of interest under practical, near-ambient conditions.