Shixian Liu

This study investigates the dual nature of phonons—encompassing both particle-like and wave-like behaviors—and their roles in thermal transport within pillared graphene nanoribbons (PGNRs). Monte Carlo simulations are employed to evaluate how the presence of pillars affects the thermal conductivity of graphene nanoribbons (GNRs), revealing that pillars significantly reduce thermal conductivity by enhancing phonon-boundary scattering, thereby emphasizing particle effects. A comparison with molecular dynamics simulations enables quantitative assessment of the respective contributions of particle and wave phenomena to the observed reduction in thermal conductivity. Notably, as the width of PGNRs decreases, the influence of wave effects initially increases and then diminishes, suggesting a saturation behavior. Furthermore, this study introduces and evaluates the concept of phonon resonance hybridization depth in PGNRs.

In this study, the temperature fluctuations in three-dimensional confined nanostructures (quantum dots) of germanium, silicon, and diamond were calculated using the \(T^{3/2}\)model based on the particle-in-a-box (PIAB) approach and compared with the Debye \(T^3\)model. The analysis focused on quantum dots with sizes ranging from 1 to 100 nm. According to the PIAB \(T^{3/2}\)model, temperature fluctuations decrease as the temperature decreases, consistent with the principles of statistical physics. In contrast, the Debye \(T^3\)model predicts an increase in temperature fluctuations with decreasing temperature, contradicting the principles of statistical physics. These results emphasize the significant impact of quantum confinement and highlight the limitations of the Debye \(T^3\)model in describing nanoscale systems. Furthermore, distribution diagrams illustrating temperature fluctuations as functions of size and temperature were established for the first time. Based on these diagrams, clear boundaries were defined for the temperature and thermophysical property ranges where reliable predictions can be made.

This study introduces a novel method for calculating the thermal conductivity of graphene using a Monte Carlo approach to evaluate anisotropic three-phonon interactions. The phonon dispersion relation is derived using a force constant model that incorporates up to fifth-order nearest-neighbor interactions, while the phonon density of states (DOS) is computed via a generalized Gilat-Raubenheimer method. A quantitative relationship for the scaling exponent of the specific heat capacity at low temperatures is established, emphasizing the unique two-dimensional characteristics of graphene. To address anisotropic effects, the Monte Carlo approach efficiently identifies three-phonon combinations that adhere to the conservation laws of energy and momentum. The findings highlight the pivotal role of anisotropic phonon interactions in graphene’s thermal conductivity. The thermal conductivity values obtained through the iterative method exhibit strong agreement with previous three-phonon calculations, thereby validating the model. Nevertheless, discrepancies with experimental data suggest that incorporating higher-order phonon processes, such as four-phonon scattering, may further improve predictive accuracy.

We estimate the thermal properties of unsmooth Si nanowires, considering key factors such as size (diameter), surface texture (roughness) and quantum size effects (phonon states) at different temperatures. For nanowires with a diameter of less than 20 nm, we highlight the importance of quantum size effects in heat capacity calculations, using dispersion relations derived from the modified frequency equation for the elasticity of a rod. The thermal conductivities of nanowires with diameters of 37, 56, and 115nm are predicted using the Fuchs– Sondheimer model and Soffer’s specular parameter. Notably, the roughness parameters are chosen to reflect the technological characteristics of the real surfaces. Our findings reveal that surface texture plays a significant role in thermal conductivity, particularly in the realm of ballistic heat transfer within nanowires. This study provides practical recommendations for developing new thermal management materials.

The results of the validation of the STEG code using the experiments on two-phase flow across horizontal tube bundles are presented. The experiments are carried out on two water–air SG models, consisting of a transparent vessels, inside of which there is a tube bundle, a perforated sheet and a bead separating the upstream section of the circulation circuit of the model from the downstream one. The void fractions and water velocities are measured in different locations of the models. The brief description of the STEG code based on the 3D two-fluid model is presented. A set of interfacial drag correlations, which was recently developed by the authors, are used in the validation calculations. The peculiarities of the spatial two-phase flows have been established, and a quantitative comparison with experimental data has been performed. Good agreement between the calculated and experimental data is obtained.