arXiv subjects

Xueyan Zhu

Publications and source records attributed to Xueyan Zhu.

5 recordsLinked to original sources

arXiv2025

Revisiting the Mechanisms of Thermal Transport in Vacancy-Defective Silicon

Understanding heat conduction in defective silicon is crucial for electronics and thermoelectrics. Conventional understanding relies on phonon gas picture, treating defects as scattering centers that reduce phonon lifetimes without altering frequencies and group velocities. We go beyond phonon gas picture by employing Wigner transport equation to investigate heat conduction in vacancy-defected silicon. Our findings reveal that while thermal conduction in pristine silicon stems mainly from particle-like propagation of vibrational modes, wave-like tunnelling becomes increasingly significant in the presence of vacancies. Contrary to the conventional belief that defects only perturb mode lifetimes, we demonstrate that vacancies also diminish velocity operators, a dominant factor in thermal conductivity reduction, surpassing even the effect of lifetime shortening. Furthermore, incorporating anharmonic frequencies and interatomic force constants shows that while anharmonicity suppresses thermal conductivity in pristine silicon, this effect weakens with vacancy concentration and reverses to enhance conductivity. These findings challenge conventional knowledge and provide new insights into thermal conduction in defective materials.

cond-mat.mtrl-sci↗

arXiv2022

Effect of anharmonicity on the thermal conductivity of amorphous silica

Proper consideration of anharmonicity is important for the calculation of the thermal conductivity. However, how the anharmonicity influences the thermal conduction in amorphous materials is still an open question. In this work, we uncover the role of anharmonicity on the thermal conductivity of amorphous silica (a-SiO2) by comparing the thermal conductivity predicted from the harmonic theory and the anharmonic theory. Moreover, we explore the effect of anharmonicity-induced frequency shift on the prediction of the thermal conductivity. It is found that the thermal conductivity calculated by the recently developed anharmonic theory (quasi-harmonic Green-Kubo approximation, QHGK) is higher than that by the harmonic theory developed by Allen and Feldman. The use of anharmonic vibrational frequencies also leads to a higher thermal conductivity compared with that calculated using harmonic vibrational frequencies. The anharmonicity induced frequency shifts is a mechanism for the positive temperature dependence of the thermal conductivity of a-SiO2 at higher temperatures. Further investigation on mode diffusivity suggests that although anharmonicity has larger influence on locons than diffusons, the increase of the thermal conductivity due to the anharmonicity is mainly contributed by the anharmonicity induced increase of the diffusivity of diffusons. Finally, it is found that the cross-correlations between diffusons and diffusons contribute most to the thermal conductivity of a-SiO2, and the locons contribute to the thermal conductivity mainly through collaboration with diffusons. These results offer new insights into the nature of the thermal conduction in a-SiO2.

cond-mat.mtrl-sci↗

arXiv2020

UO2/BeO interfacial thermal resistance and its effect on fuel thermal conductivity

UO2/BeO interfacial thermal resistance (ITR) is calculated by diffuse mismatch model (DMM) and the effects of ITR on UO2-BeO thermal conductivity are investigated. ITR predicted by DMM is on the order of 10-9 m2K/W. Using this ITR, UO2-BeO thermal conductivities are calculated by theoretical models and compared with experimental data. The results indicate that DMM prediction is applicable to the interface between UO2 and dispersed BeO, while not applicable to the interface between UO2 and continuous BeO. If the thermal conductivity of UO2 containing continuous BeO was to be in agreement with experimental data, its ITR should be on the order of 10-6 - 10-5 m2K/W. Therefore, the vibrational mismatch between UO2 and BeO considered by DMM is the major mechanism for attenuating the heat flux through UO2/dispersed-BeO interface, but not for UO2/continuous-BeO interface. Furthermore, it is found that the presence of ITR leads to the dependence of the thermal conductivity of UO2 containing dispersed BeO on BeO size. With the decrease in BeO size, UO2-BeO thermal conductivity decreases. When BeO size is smaller than a critical value, UO2-BeO thermal conductivity becomes even smaller than UO2 thermal conductivity. For UO2 containing continuous BeO, the thermal conductivity decreases with the decrease in the size of UO2 granule surrounded by BeO, but not necessarily smaller than UO2 thermal conductivity. Under a critical temperature, UO2-BeO thermal conductivity is always larger than UO2 thermal conductivity. Above the critical temperature, UO2-BeO thermal conductivity is larger than UO2 thermal conductivity only when UO2 granule size is large enough. The conditions for achieving the targeted enhancement of UO2 thermal conductivity by doping with BeO are derived. These conditions can be used to design and optimize the distribution, content, size of BeO, and the size of UO2 granule.

physics.app-ph↗

arXiv2017

New zirconium hydrides predicted by structure search method based on first principles calculations

The formation of precipitated zirconium (Zr) hydrides is closely related to the hydrogen embrittlement problem for the cladding materials of pressured water reactors (PWR). In this work, we systematically investigated the crystal structures of zirconium hydride (ZrHx) with different hydrogen concentrations (x = 0~2, atomic ratio) by combining the basin hopping algorithm with first principles calculations. We conclude that the P3m1 ζ-ZrH0.5 is dynamically unstable, while a novel dynamically stable P3m1 ZrH0.5 structure was discovered in the structure search. The stability of bistable P42/nnm ZrH1.5 structures and I4/mmm ZrH2 structures are also revisited. We find that the P42/nnm (c/a > 1) ZrH1.5 is dynamically unstable, while the I4/mmm (c/a = 1.57) ZrH2 is dynamically stable.The P42/nnm (c/a < 1) ZrH1.5 might be a key intermediate phase for the transition of γ->δ->ε phases. Additionally, by using the thermal dynamic simulations, we find that δ-ZrH1.5 is the most stable structure at high temperature while ZrH2 is the most stable hydride at low temperature. Slow cooling process will promote the formation of δ-ZrH1.5, and fast cooling process will promote the formation of γ-ZrH. These results may help to understand the phase transitions of zirconium hydrides.

cond-mat.mtrl-sci↗

arXiv2016

Effect of vacancies on the mechanical properties of zirconium: An ab initio investigation

It is well known that the irradiation-induced defects strongly influence the mechanical properties of zirconium (Zr) or its alloys in nuclear reactors. However, how the point defect changes the mechanical properties has been rarely studied. Here, we systematically investigated the effect of vacancies on the mechanical properties of alpha-Zr based on density functional theory (DFT). Both uniformly distributed vacancies and vacancy clusters were considered. Our results reveal that the existence of vacancy will reduce the bulk modulus, while enhance the shear and Young's moduli. Based on these moduli, the ductility and hardness were further calculated. With the introduction of vacancy, the ductility decreases, but the hardness increases. However, when the vacancy concentration is larger than a critical value, a rise in the ductility and a reduction in the hardness occur, which indicates the degeneration of the material. Moreover, it was found that the vacancies lead to a more isotropic distribution of Young's modulus in 3D space. To further investigate how the clustering of vacancies influences the mechanical properties, the most stable configurations of di- and trivacancy clusters have been predicted, which correspond to the most compact distribution of vacancies. Compared with the uniform distribution of vacancies, clustering of vacancies will strengthen the above changes of elastic moduli, ductility and hardness. Our results indicate that the increase of the vacancy concentration may be the basic cause for the changes of the mechanical properties under irradiation, while the formation of vacancy clusters intensifies these changes.

cond-mat.mtrl-sci↗