密度泛函理论研究高温高压下UO2弹性与热力学性能

采用第一性原理与准谐德拜模型研究UO2在高温高压条件下的弹性与热力学性能。UO2在高温高压下仍属离子型晶体，并且弹性性能计算表明，四角方向剪切常数在高温与高压下均保持稳定。高温下弹性常数C44没有明显变化，而高压下C44迅速增大。体积模量、剪切模量与杨氏模量均随压强增加而增大；高温条件下，体积模量、剪切模量与杨氏模量也未出现明显的降低，表明UO2在高温度高压下均可保持良好的力学性能。不同压强下，UO2定容热容均随温度迅速增大，并在1000 K 附近趋近于杜隆-佩蒂特极限。德拜温度则随温度降低，随压强升高。在低于室温条件下，热膨胀系数随温度急剧增加；温度继续增加，系数的增加趋势则逐渐变缓。计算结果还表明，UO2的热膨胀系数在相同条件下，远小于其他核材料。

Study of elastic and thermodynamic properties of uranium dioxide under high temperature and pressure with density functional theory

The elastic and thermodynamic properties of UO₂ under extreme physical condition are studied by using the density functional theory and quasi-harmonic Debye model. Results show that UO₂ is still stable ionic crystal under high temperatures, and pressures. Tetragonal shear constant is steady under high pressures and temperatures, while elastic constant C₄₄ is stable under high temperatures, but rises with pressure sharply. Bulk modulus, shear modulus and Young’s modulus increase with pressure rapidly, but temperature would not cause evident debasement of the moduli, all of which indicate that UO₂ has excellent mechanical properties. Heat capacity of different pressures increases with temperature and is close to the Dulong-Petit limit near 1000 K. Debye temperature decreases with temperature, and increases with pressure. Under low pressure, thermal expansion coefficient raises with temperature rapidly, and then gets slow at higher pressure and temperature. Besides, the thermal expansion coefficient of UO₂ is much lower than that of other nuclear materials.