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The thermal conductivities along the basal and hexagonal directions of α -quartz silica, the low-temperature form of crystalline SiO2, are predicted from the solution of the Boltzmann transport equation combined with the van Beest, Kramer, and van Santen potential for the temperature up to 900 K and the pressure as high as 4 GPa. The thermal conductivities at atmospheric pressure, which show a negative and nonlinear dependence on temperature, are in reasonable agreement with the experimental data. The influence of pressure on thermal conductivity is positive and linear. The pressure (P) and temperature (T) dependences of the thermal conductivity (λ) in basal and hexagonal directions are fitted to a function of the form λ = (b + c P) T a. The thermal conductivity, influenced by temperature and pressure, is analyzed based on phonon properties, including spectral thermal conductivity, dispersion relation, phonon density of states, phonon lifetime, and phonon probability density distribution function. [ABSTRACT FROM AUTHOR]

Owing to nano-MoS2's excellent anti-friction and anti-wear properties, nano-MoS2, which can act as a nano-additive in lubricating oil or solid lubricants, is believed to have great potential in the lubrication of power machinery and moving parts of a spacecraft. The molecular dynamics method was used to construct a rough surface and a multi-level asperity structure to simulate starved lubrication before oil film breakdown, and the lubrication mechanism of MoS2 as a nano-additive or directly coated on the textured surface could reduce the friction coefficient and wear was explained from the atomic perspective. Simulations showed that the multilayer MoS2 played a role of load-bearing at light load or low velocity, and slipped into the grooves to repair the surface under heavy load or high velocity. Even if local asperity contact occurs, MoS2 nanoparticles could accelerate the detachment of the initial asperity contact to prevent large-scale adhesion. The MoS2 nanoparticles transformed the pure liquid oil film into a liquid–solid composite oil film, which was more suitable for lubrication under heavy load and high velocity because it increased the contact area, protected the friction surface and prevented asperity contact. The proposed lubrication mechanism contributes to understanding the frictional properties of layered nanomaterials under extreme conditions and provides a reference for further application of MoS2 materials in the field of lubrication. [ABSTRACT FROM AUTHOR]

With excellent physical and chemical properties, 2D TMDC materials have been widely used in engineering applications, but they inevitably suffer from the dual effects of strain and device size. As typical 2D TMDCs, HfSe2 and ZrS2 are reported to have excellent thermoelectric properties. Thermal transport properties have great significance for exerting the performance of materials, ensuring device lifetime and stable operation, but current research is not detailed enough. Here, first-principles combined with the phonon Boltzmann transport equation are used to study the phonon transport inside monolayer HfSe2 and ZrS2 under tensile strain and finite size, and explore the band structure properties. Our research shows that they have similar phonon dispersion curve structures, and the band gap of HfSe2 increases monotonically with the increase of tensile strain, while the bandgap of ZrS2 increases and then decreases with the increase of tensile strain. Thermal conductivity has obvious strain dependence: with the increase of tensile strain, the thermal conductivity of HfSe2 gradually decreases, while that of ZrS2 increases slightly, and then gradually decreases. Reducing the system size can limit the contribution of phonons with a long mean free path, significantly decreasing thermal conductivity through the controlling effect of tensile strain. The mode contribution of thermal conductivity is systematically investigated, and anharmonic properties including mode and frequency-level scattering rates, group velocity and Grüneisen parameters are used to explain the associated mechanism. Phonon scattering processes and channels in various cases are discussed in detail. Our research provides a detailed understanding of the phonon transport and electronic structural properties of low thermal conductivity monolayers of HfSe2 and ZrS2, and further completes the study of thermal transport of the two materials under strain and size tuning, which will provide a foundation for further popularization and application. [ABSTRACT FROM AUTHOR]