Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation. Our study provides important new insights into heat transport and phonon scattering mechanisms in graphite and other anisotropic van der Waals solids

Assessing the Performance of Recent Density Functionals for Bulk Solids

We assess the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and non-metals. The functionals tested are the modified Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For completeness, we also test more-standard functionals: the local density approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria (TPSS) meta-GGA. We find that the recent density functionals for solids reach a high accuracy for bulk properties (lattice constant and bulk modulus). For the cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is actually better for the alkali metals and alkali halides. For fair comparison of calculated and experimental results, we consider the zero-point phonon and finite-temperature effects ignored by many workers. We show how Gaussian basis sets and inaccurate experimental reference data may affect the rating of the quality of the functionals. The results show that PBEsol and AM05 perform somewhat differently from each other for alkali metal, alkaline earth metal and alkali halide crystals (where the maximum value of the reduced density gradient is about 2), but perform very similarly for most of the other solids (where it is often about 1). Our explanation for this is consistent with the importance of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil

Force-matched embedded-atom method potential for niobium

Large-scale simulations of plastic deformation and phase transformations in alloys require reliable classical interatomic potentials. We construct an embedded-atom method potential for niobium as the first step in alloy potential development. Optimization of the potential parameters to a well-converged set of density-functional theory (DFT) forces, energies, and stresses produces a reliable and transferable potential for molecular dynamics simulations. The potential accurately describes properties related to the fitting data, and also produces excellent results for quantities outside the fitting range. Structural and elastic properties, defect energetics, and thermal behavior compare well with DFT results and experimental data, e.g., DFT surface energies are reproduced with less than 4% error, generalized stacking-fault energies differ from DFT values by less than 15%, and the melting temperature is within 2% of the experimental value.Comment: 17 pages, 13 figures, 7 table

Thermodynamic properties and structural stability of thorium dioxide

Using density functional theory (DFT) calculations, we have systematically investigated the thermodynamic properties and structural stabilities of thorium dioxide (ThO$_2$). Based on the calculated phonon dispersion curves, we calculate the thermal expansion coefficient, bulk modulus, and heat capacities at different temperatures for ThO$_2$ under the quasi-harmonic approximation. All the results are in good agreement with corresponding experiments proving the validity of our methods. Our theoretical studies can help people more clearly understand the thermodynamic behaviors of ThO$_2$ at different temperatures. In addition, we have also studied possible defect formations and diffusion behaviors of helium in ThO$_2$, to discuss its structural stability. It is found that in intrinsic ThO$_2$ without any Fermi energy shifts, the interstitial Th$_i^{4+}$ defect other than oxygen or thorium vacancies, interstitial oxygen, and any kinds of Frenkel pairs, is most probable to form with an energy release of 1.74 eV. However, after upshifting the Fermi energy, the formation of the other defects also becomes possible. For helium diffusion, we find that only through the thorium vacancy can it happen with the small energy barrier of 0.52 eV. Otherwise, helium atoms can hardly incorporate or diffuse in ThO$_2$. Our results indicate that people should prevent upshifts of the Fermi energy of ThO$_2$ to avoid the formation of thorium vacancies and so as to prevent helium caused damages.Comment: 11 pages, 11 figure

Thermoelastic dissipation in inhomogeneous media: loss measurements and displacement noise in coated test masses for interferometric gravitational wave detectors

The displacement noise in the test mass mirrors of interferometric gravitational wave detectors is proportional to their elastic dissipation at the observation frequencies. In this paper, we analyze one fundamental source of dissipation in thin coatings, thermoelastic damping associated with the dissimilar thermal and elastic properties of the film and the substrate. We obtain expressions for the thermoelastic dissipation factor necessary to interpret resonant loss measurements, and for the spectral density of displacement noise imposed on a Gaussian beam reflected from the face of a coated mass. The predicted size of these effects is large enough to affect the interpretation of loss measurements, and to influence design choices in advanced gravitational wave detectors.Comment: 42 pages, 7 figures, uses REVTeX

Coil optimization for electromagnetic levitation using a genetic like algorithm

he technique of electromagnetic levitation (EML) provides a means for thermally processing an electrically conductive specimen in a containerless manner. For the investigation of metallicliquids and related melting or freezing transformations, the elimination of substrate-induced nucleation affords access to much higher undercooling than otherwise attainable. With heating and levitation both arising from the currents induced by the coil, the performance of any EML system depends on controlling the balance between lifting forces and heating effects, as influenced by the levitation coil geometry. In this work, a genetic algorithm is developed and utilized to optimize the design of electromagnetic levitation coils. The optimization is targeted specifically to reduce the steady-state temperature of the stably levitated metallic specimen. Reductions in temperature of nominally 70 K relative to that obtained with the initial design are achieved through coil optimization, and the results are compared with experiments foraluminum. Additionally, the optimization method is shown to be robust, generating a small range of converged results from a variety of initial starting conditions. While our optimizationcriterion was set to achieve the lowest possible sample temperature, the method is general and can be used to optimize for other criteria as well

Thermal Conductivity of Isotopically Enriched 28Si Revisited

The thermal conductivity of isotopically enriched 28Si (enrichment better than 99.9%) was redetermined independently in three laboratories by high precision experiments on a total of 4 samples of different shape and degree of isotope enrichment in the range from 5 to 300 K with particular emphasis on the range near room temperature. The results obtained in the different laboratories are in good agreement with each other. They indicate that at room temperature the thermal conductivity of isotopically enriched 28Si exceeds the thermal conductivity of Si with a natural, unmodified isotope mixture by 102 %. This finding is in disagreement with an earlier report by Ruf et al. At 26 K the thermal conductivity of 28Si reaches a maximum. The maximum value depends on sample shape and the degree of isotope enrichment and exceeds the thermal conductivity of natural Si by a factor of 8 for a 99.982% 28Si enriched sample. The thermal conductivity of Si with natural isotope composition is consistently found to be 3% lower than the values recommended in the literature

Dynamical properties of Au from tight-binding molecular-dynamics simulations

We studied the dynamical properties of Au using our previously developed tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K were determined by computing the dynamical-matrix using a supercell approach. In addition, we performed molecular-dynamics simulations at various temperatures to obtain the temperature dependence of the lattice constant and of the atomic mean-square-displacement, as well as the phonon density-of-states and phonon-dispersion curves at finite temperature. We further tested the transferability of the model to different atomic environments by simulating liquid gold. Whenever possible we compared these results to experimental values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical Review