First-principles study on the thermodynamic and thermoelastic properties of pyrophyllite at high temperatures

Abstract

Pyrophyllite [Al2Si4O10(OH)2] is mainly used in the field of refractories and super-hard materials due to its excellent thermal properties such as good heat resistance, stable chemical properties and crystal structure stability during heating. However, its thermodynamic properties have rarely been fully described at the atomic scale. In this study, the thermodynamic and thermoelastic properties of pyrophyllite are reported in detail, based on the first principles. Firstly, the relationship between the Helmholtz free energy and volume (E−V) under different temperatures was fitted using the quasi-harmonic approximation (QHA), which confirmed the thermal expansion effect of pyrophyllite at 0–900 K. Phonon dispersion curves at different temperatures showed that pyrophyllite maintained phase stability within a given temperature range (0–900 K). Furthermore, its elastic behaviors at 0, 300, 450, 600, 750, and 900 K are discussed. As the temperature increased from 0 K to 900 K, the elastic modulus of pyrophyllite decreased to varying degrees, which indicated that high temperature weakened its ability to resist external pressure. This elastic softening phenomenon was caused by the thermal expansion effect. The changes of Paugh's modulus ratio (G/B) clearly indicated that the ductility of pyrophyllite under high temperature (up to 900 K) was greater than that at 0 K. Overall, high temperature increases the volume of pyrophyllite and decreases its strength. Thus, when pyrophyllite is used in the field of refractories, the particle size and amount of pyrophyllite should be reasonably selected to achieve the balance between the volume stability and strength of the material. These findings enrich our theoretical knowledge of pyrophyllite behavior at high temperatures at the atomic scale, which is difficult to determine accurately using physical experiments.