Jean-Paul Crocombette

Multiscale modeling of the thermal conductivity of polycrystalline silicon carbide

A multiscale modeling, involving molecular dynamics and finite element calculations, of the degradation of the thermal conductivity of polycrystalline silicon carbide due to the thermal (Kapitza) resistances of grain boundaries is presented. Molecular dynamics simulations focus on the ⟨111⟩ family of tilt grain boundaries in cubic SiC. For large tilt angles a simple symmetry and shift procedure is used to generate the grain boundaries while for small angles the boundary structure is obtained by inserting arrays of edge dislocations. The energy and thermal resistances of the grain boundaries are presented. The latter are fed into a finite element homogenization procedure, which enables to calculate the effective thermal conductivity of the SiC polycrystal as a function of its average grain size. The decrease in the thermal conductivity of a polycrystal as a function of its grain size is qualitatively reproduced. However, available experimental values of the thermal conductivity of polycrystalline SiC tend ...

Atomistic simulation of the size and orientation dependences of thermal conductivity in GaN nanowires

The thermal conductivity of GaN nanowires has been determined computationally by applying nonequilibrium atomistic simulation methods using the Stillinger-Weber [Phys. Rev. B 31, 5262 (1985)] potentials. The simulation results show that the thermal conductivity of the GaN nanowires is smaller than that of a bulk crystal and increases with increasing diameter. Surface scattering of phonons and the high surface to volume ratios of the nanowires are primarily responsible for the reduced thermal conductivity and its size dependence behavior. The thermal conductivity is also found to decrease with increasing temperature and exhibits a dependence on axial orientation of the nanowires.

Molecular dynamics modeling of the thermal conductivity of irradiated SiC as a function of cascade overlap

SiC thermal conductivity is known to decrease under irradiation. To understand this effect, we study the variation of the thermal conductivity of cubic SiC with defect accumulation induced by displacement cascades. We use an empirical potential of the Tersoff type in the framework of nonequilibrium molecular dynamics. The conductivity of SiC is found to decrease with dose, in very good quantitative agreement with low temperature irradiation experiments. The results are analyzed in view of the amorphization states that are created by the cascade accumulation simulations. The calculated conductivity values at lower doses are close to the smallest measured values after high temperature irradiation, indicating that the decrease of the conductivity observed at lower doses is related to the creation of point defects. A subsequent decrease takes place upon further cascade accumulation. It is characteristic of the amorphization of the material and is experimentally observed for low temperature irradiation only.

Thermal conductivity degradation induced by point defects in irradiated silicon carbide

Irradiations are known to decrease the thermal conductivity of ceramics. This phenomenon is tackled by molecular dynamics simulation of the thermal resistance of point defects in cubic silicon carbide. The additional thermal resistivity due to point defects proves to vary linearly with their concentration. Large variations in the proportionality coefficient with the nature of the defects are observed. From these calculations, an approximate scale for the concentration of vacancies in irradiated SiC is built.

Atomistic Simulation of Amorphization Thermokinetics in Lanthanum Pyrozirconate

The kinetics of amorphization in La2Zr2O7 pyrochlore is investigated using molecular dynamics simulations. Irradiation damage is simulated by continuous accumulation of cation Frenkel pairs at various temperatures. As observed experimentally, La2Zr2O7 first transitions to the fluorite structure, independent of the temperature, and amorphization occurs at low temperatures. A model fit of the simulated dose-temperature curve reproduces experimental results in the literature, with a low temperature amorphization dose D0=1.1 displacement per cation and an activation energy Eact=0.036eV. Present simulations indicate that point defect recombination can control the temperature dependence of amorphization driven by point defect accumulation.

Study of the initial stages of defect generation in ion-irradiated MgO at elevated temperatures using high-resolution X-ray diffraction

The initial stages of defect generation in magnesia (MgO) single crystals irradiated with 1.2xa0MeV Au+ ions at 573, 773, and 1073xa0K and at different fluences have been studied. High-resolution X-ray diffraction was used to measure the irradiation-induced elastic strain. Point-defect relaxation volumes were computed using density functional theory calculations. The defect concentration was then calculated. It was found to increase with ion fluence at all temperatures, with maximum values being ~0.46xa0% at 573xa0K, ~0.24xa0% at 773xa0K, and ~0.13xa0% at 1073xa0K. The decrease in the maximum strain with increasing temperature indicates a dynamic annealing. The defect generation efficiencies were found to be very low and the values obtained were in the range of ~2.4, 1.2, and 0.6xa0% at 573, 773, and 1073 K, respectively. An annealing effect due to electronic energy deposition is suspected to explain these low values.

Comment on: Large swelling and percolation in irradiated zircon

A recent model for the large radiation-induced swelling exhibited by irradiated zircon (ZrSiO4) is partially based on results of molecular dynamics (MD) simulations of the partial overlap of two collision cascades that predict a densified boundary of polymerized silica and the scattering of the second cascade away from the densified boundary (Trachenko et al 2003 J. Phys.: Condens. Matter 15 L1). These MD simulations are based on an atomic interaction potential for zircon (Trachenko et al 2001 J. Phys.: Condens. Matter 13 1947), which, according to our analysis, only reproduces some of the crystallographic properties at equilibrium and does not adequately describe the atomic scattering physics for zircon, and on simulation methodologies for energetic events that are ill defined. In fact, the interatomic potential model used by Trachenko et al yields a significantly more rigid structure, with very high Frenkel defect formation energies and extremely low entropy and specific heat capacity. Consequently, the reported results of the cascade simulations, which are events far from equilibrium, may be artifacts of both the potential model and simulation methodologies employed. Thus, the structural changes predicted by the simulations must be viewed cautiously, as these simulation results cannot be taken as confirmation of a new scattering physics process that is the basis for the proposed swelling model. In this comment, the deficiencies in the atomic interaction potential and methodologies employed by these authors are critically reviewed, and the validity of the cascade overlap simulations and proposed physics is discussed.

Oxygen Self-Diffusion Mechanisms in Silica by First-Principles

We present an extended study on self-defects in a silica glass model, as well as the preliminary results for diffusion mechanisms in homogeneous or heterogeneous regime.