Phonon-mediated heat transfer across sub-nanometer vacuum gaps is a possible way for heat transfer in the extreme near field. Although phonon-mediated heat transfer across various ideal and reconstructed surfaces has been examined, the effect of unilateral atomistic surface defects remains insufficiently resolved. Here, nonequilibrium molecular dynamics simulations are used to investigate phonon-mediated heat transfer across gold-vacuum-gold gaps with unilateral surface defects. The effects of defect concentration on interfacial thermal conductance and its dependence on vacuum gap are examined. The results show that increasing defect concentration leads to an approximately linear decrease in interfacial thermal conductance, while its power law dependence on distance remains nearly unchanged. Spectral analysis further shows that surface defects mainly reduce the magnitude of spectral thermal conductance, while the dominant frequency range remains essentially unchanged. Additional analyses of interaction energies and phonon densities of states are used to examine the microscopic origin of the reduced thermal conductance. These analyses indicate that defects weaken both interfacial coupling across the vacuum gap and phonon coupling near the defective surface, thereby suppressing phonon transport. These findings demonstrate the effect of realistic surface disorder on phonon transport across vacuum nanogaps and provide insight into interfacial heat transfer in the extreme near field, which are also relevant to thermal management in nanoscale devices.
As an important solution to the global shortage of fresh water, the spectrally selective radiative cooling film can provide a potential method for harvesting all-day freshwater. In this work, the spectrally selective radiative characteristics of a PMMA/SiO(2 )radiative cooling film are numerically investigated by the finite-difference time-domain (FDTD) method, and the effect of particle size, filling rate, and particle distribution on transmissivity and emissivity is discussed. In addition, the electric field distribution of the PMMA/SiO(2 )radiative cooling film is further analyzed to disclose the radiative transfer mechanism. The simulation results show that for 1-1.2 mu m SiO(2 )particle size, the transmissivity values of the radiative cooling film remain relatively high due to weak scattering. The emissivity values exhibit an obvious increase with filling rate because the addition of SiO(2 )particles accelerates the formation of electric dipole resonance and enhances the absorption ability. Compared with the uniformly arranged SiO(2 )particles, the randomly arranged SiO(2 )particles show more stable emissivity, and the corresponding emissivity in the 8-13 mu m wavelength range is 0.951. The net cooling power of the optimized PMMA/SiO(2 )can reach 110.2 W/m(2 )at the film temperature equaling to the ambient temperature. Moreover, with the increase in the non-radiative heat transfer coefficient, the enhanced convective exchange suppresses the radiative cooling effect.
As electronic devices rapidly evolve toward higher power density and integration, the importance of efficient thermal management grows significantly. In the context of nano-reinforced thermoplastic composites, establishing effective thermal path through the filler network is crucial. In this study, by optimizing the ratios of two sizes of hexagonal boron nitride (h-BN) nanosheets in the poly (ether ether ketone) (PEEK) matrix, a significant enhancement of the thermal conductivity of PEEK/h-BN composites was achieved. The thermal conductivity of a PEEK/h-BN composite containing a 3:7 weight ratio of the two sizes of h-BN(with 15 wt% overall h-BN content) achieved the highest thermal conductivity of 0.87 W/m & centerdot;K due to the formation of more complete thermal paths. Additionally, based on the structural distribution of boron nitride nanosheets (BNNSs) observed in scanning electron microscopy (SEM), the influence of heat transfer direction along single-layer BNNSs, as well as the interlayer relative displacement and torsional angle of bilayer BNNSs, on the thermal conductivity of BNNSs/PEEK composites was investigated through molecular dynamics simulations. The results show that when heat flow is transmitted along the armchair direction of BNNSs, the BNNSs/PEEK composites exhibit higher thermal conductivity. Furthermore, by regulating the microstructure of BNNSs, the interface scattering in BNNSs/PEEK composites can be effectively reduced, and the interfacial coupling of BNNSs/PEEK composites can be improved, leading to an enhancement in thermal conductivity. This study provides experimental data and theoretical support for the further development of high-performance thermal management materials.
