The near-field radiative heat transfer (NFRHT) between two semi-infinite α -MoO 3 biaxial crystals is investigated numerically based on the fluctuation–dissipation theorem combined with the modified 4 × 4 transfer matrix method in this paper. In the calculations, the near-field radiative heat flux (NFRHF) along each of the crystalline directions of α -MoO 3 is obtained by controlling the orientation of the biaxial crystals. The results show that much larger heat flux than that between two semi-infinite hexagonal boron nitride can be achieved in the near-field regime, and the maximum heat flux is along the [001] crystalline direction. The mechanisms for the large radiative heat flux are explained as due to existence of hyperbolic phonon polaritons (HPPs) inside α -MoO 3 and excitation of hyperbolic surface phonon polaritons (HSPhPs) at the vacuum/ α -MoO 3 interfaces. The effect of relative rotation between the emitter and the receiver on the heat flux is also investigated. It is found that the heat flux varies significantly with the relative rotation angle. The modulation contrast can be as large as two when the heat flux is along the [010] direction. We attribute the large modulation contrast mainly to the misalignment of HSPhPs and HPPs between the emitter and the receiver. Hence, the results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

Monohydric alcohols have been used as promising phase change materials (PCMs) for low-temperature latent heat storage. However, the heat storage/retrieval rates are limited due to the low thermal conductivity of such alcohols. In this work, nonequilibrium molecular dynamics (NEMD) simulations were performed to study the microscopic heat conduction in example monohydric alcohols, i.e., 1-dodecanol (C 12 H 26 O), 1-tetradecanol (C 14 H 30 O), and 1-hexadecanol (C 16 H 34 O). A simplified ideal crystal model was proposed to exploit the potential for improving the thermal conductivity of monohydric alcohols. The effect of ideal crystalline structures, especially the contribution of the hydroxyl group, on the microscopic heat conduction process was analyzed. The thermal conductivity of the ideal crystals of the various monohydric alcohols was predicted to be more than twice as compared to that of their respective solids. The major thermal resistance in the ideal crystals was found around the molecular interfaces, as a result of the excellent heat conduction performance along the linear molecular chains. The calculated vibrational density of states (VDOS) and interfacial heat transfer were then investigated. When the interfaces are surrounded by hydroxyl groups as walls, strong hydrogen bond (HB) interactions were observed. The interfacial heat transfer coefficient of the ideal crystalline structures of 1-tetradecanol was found to reach up to ∼735.6 MW/m 2 W. It was elucidated that the high interfacial heat transfer rate is clearly related to the stronger intermolecular interactions.

This paper presents significant advances in the analytical calculation of the low-temperature lattice thermal conductivity in finite crystals. It shows that an accurate prediction of the direction-dependent lattice thermal conductivity can be obtained at low temperatures when Houston's method is used to account for the anisotropy of the Brillouin zone in the calculation of the phonon spectrum. It also provides an approach to predict from a spatial-dependent Boltzmann equation the rate at which phonons are scattered by the sample boundary in the presence of intrinsic scattering mechanisms, which is crucial for the calculation of the lattice thermal conductivity in finite crystals.

The main objective of this study is to examine the frosting characteristics affected by the surface wettability. Two different copper surfaces – bare and nano structured - were prepared for the experiments. Their static contact angles are 74° (bare: without surface treatment) and 154° (nano-structured), respectively. The temperature of the copper substrate was measured by using resistance temperature detector (RTD) sensors embedded inside small holes drilled at 1 mm underneath the surface. During the phase change, the temperature of the copper substrates remained -7.8±0.6°C and the ambient temperature was set as 24±0.5°C with the relative humidity of 45%. Images were captured by using the CMOS camera with the 5 second time interval. Film condensation occurred because of higher wettability of the bare copper surface. Film condensates were frozen at the early stage and frost crystal grew in the vertical direction. On the other hand, dropwise condensates formed on the nano-structured copper surface remained as the supercooled liquid phase for 44 minutes owing to its low wettability. After 4 minutes, frosting on the bare copper substrate was triggered and propagated until it covered the whole surface. The frosting was significantly delayed on the superhydrophobic copper surface due to the lower surface free energy. The different porous media composed of frost which directly influence the heat transfer characteristics was formed on each surfaces. Therefore, additional investigation for heat transfer phenomenon on superhydrophobic surface should be conducted.

First-principles calculations are performed to study the absorption spectra and electron-vibration coupling of titanium-doped sapphire (Ti:Al 2 O 3 ). Geometry optimization shows a local structure relaxation after the doping of Ti. Electronic band structure calculation shows that five additional dopant energy bands are observed around the band gap of Al 2 O 3 , and are attributed to the five localized d orbitals of the Ti dopant. The optical absorption spectra are then predicted by averaging the oscillator strength during a 4 ps first-principles molecular dynamics (MD) trajectory, and the spectra agree well with the experimental results. Electron-vibration coupling is further investigated by studying the response of the ground and excited states to the E g vibrational mode, for which a configuration coordinate diagram is obtained. Stokes shift effect is observed, which confirms the red shift of emission spectra of Ti:sapphire. This work offers a quantitative understanding of the optical properties and crystal-field theory of Ti-doped sapphire. The first-principles calculation framework developed here can also be followed to predict the optical properties and study the electron-vibration coupling in other doped materials.

We present in this paper numerical simulation results of the thermal radiative properties of a two-dimensional (2D) rectangular SiC grating atop a photonic crystal (PC). The results show that surface phonon polaritons (SPhPs) can be excited by both TE and TM waves when they are scattered by the 2D grating. Excitation of SPhPs, PC modes, and magnetic polaritons (MPs), and interactions between them give rise to great enhancement of the emissivity. Distinct effects of the grating geometry on the resonance of SPhPs, PC modes, and MPs were revealed, which suggest a way to effectively manipulate the emissivity by tuning the structure's geometry. Furthermore, the results indicate that quasi-diffuse emissivity of the structure can be obtained for both TE and TM waves.

Recently, several significant progresses have been made on the studies of extracellular and intracellular ice formation based on high-speed camera and cryomicroscope. This experimental methodology could accurately capture the rapid formation process of ice crystals at microscale. However, quantitative interpretation on such phase change behavior still reserved a tough issue. Here, in this paper, we quantitatively studied the ice crystals growth in three kinds of cryoprotectants like dimethyl sulfoxide (DMSO), sucrose, and trehalose via high-speed camera, cryomicroscope as well as the proposed data processing method. Several critical impact factors such as the concentration of cryoprotectants and the cooling rate have been investigated. Particularly, an efficient image processing technology has been developed to quantify the growth rate and morphology of the ice crystals. The results indicate that the species and concentration of cryoprotectants and the cooling rate could significantly affect the growth rate and morphology of ice crystals. DMSO is better than trehalose and sucrose as cryoprotectant because of the molecular structure. This work established a new methodology to quantify the ice crystals growth and would enhance current understanding of the factors for ice crystals formation. It is also expected to help optimize the cryopreservation process in the near future.

Longitudinal and transverse modes of thermal disturbances in isotopically pure alkali-halide crystals are derived from phonon hydrodynamics. Guyer-Krumhansl (GK) model of phonon scattering is first recovered by relating the first and second viscosity to the relaxation times. Helmholtz potentials are then introduced to split the two modes from the heat flux vector. It has been found that the scalar potential coincides with the heat equation in the dual-phase-lag model, while the vector potential describes dispersive transverse phonons and introduces a new type of equation in microscale heat transfer.

The pool boiling heat transfer characteristics of smooth single crystal and densely packed cylindrical cavity surfaces were investigated using two highly wetting fluids, perfluoro-n-hexane (FC-72) and n-hexane. Three single crystal copper surfaces and five undoped single crystal silicon surfaces with different plane orientations were considered. In addition, silicon surfaces with densely packed cylindrical cavities with diameters ranging from 9 to 75 μm, depth ranging from 9 to 20 μm, and spacing ranging from 75 to 600 μm were tested for comparison. It is observed that the copper single crystal surfaces show increasing heat transfer coefficient with decreasing atomic planar density. The single crystal silicon surfaces show increasing heat transfer coefficient with increasing atomic planar density. Plausible molecular scale mechanisms are discussed. In contrast, the silicon surfaces seeded with cylindrical cavities having diameters of 27 μm or less generally yield higher heat transfer coefficients than the single crystal silicon surfaces. A decrease in the cavity spacing results in a larger number of cavities on the surface, and the heat transfer coefficient increases as a result. Cavity depths of 6 and 20 μm result in the same heat transfer coefficient irrespective of cavity diameter. The nucleation site density for the cylindrical cavity surfaces is measured and reported at low superheat using a novel imaging technique.

Accurate thermal conductivity values are essential for the successful modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure the thermal conductivity of these systems, as well as the thermal conductivity itself, varies with the device materials, fabrication processes, geometry, and operating conditions. In this study, the thermal conductivities of boron doped single-crystal silicon microbridges fabricated using silicon-on-insulator (SOI) wafers are measured over the temperature range from 80 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and either 50 or 85 μm wide. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry (SSERT) and optical time-domain thermoreflectance (TDTR). A thermal conductivity of 77 Wm−1 K−1 is measured for both microbridge widths at room temperature, where the results of both experimental techniques agree. However, increasing discrepancies between the thermal conductivities measured by each technique are found with decreasing temperatures below 300 K. The reduction in thermal conductivity measured by TDTR is primarily attributed to a ballistic thermal resistance contributed by phonons with mean free paths larger than the TDTR pump beam diameter. Boltzmann transport equation (BTE) modeling under the relaxation time approximation (RTA) is used to investigate the discrepancies and emphasizes the role of different interaction volumes in explaining the underprediction of TDTR measurements.

Spectral and directional control of thermal emission holds substantial importance in different kinds of applications, where heat transfer is predominantly by thermal radiation. Several configurations have previously been proposed, like using gratings, photonic crystals (PCs) and resonant cavities. In the present work, we investigate the thermal radiative properties of a microstructure consisting of a SiC grating on a photonic crystal. The emissivity of the microstructure is calculated with the rigorous coupled-wave analysis (RCWA) algorithm as a function of the angular frequency and the emission angle. The results reveal that thermal emission from the microstructure can exhibit very novel feature compared to those previously studied. Especially, significantly enhanced thermal emission can be achieved in a broad spectral band due to excitation of surface photon polaritons (SPhPs), PC modes, magnetic polaritons (MPs) and the coupling between them. We show that it is possible to flexibly control the thermal emission feature by adjusting the microstructure's dimensional parameters properly.

We present a continuum hybrid phonon gas model to describe transient ballistic-diffusive heat transport. In this model, heat energy is carried by a mixture of longitudinal and transverse phonon gases so that the distinction between longitudinal and transverse phonon excitations is taken into account. This new model is validated by the successful reconstruction of benchmark cases of heat-pulse experiments in NaF, which have never been completely reconstructed before. It is elucidated how thermal pulses are transmitted by longitudinal and transverse phonon gases. This model not only helps us yield new insight in transient ballistic-diffusive heat conduction mechanisms but also provides numerical tools to study transient ballistic-diffusive heat conduction in nanoelectronic and modern optoelectronics.

Complex macroscale and microscale heat and mass transfer phenomena are encountered in thermal energy storage and transport systems. Those systems involving ice slurries and nanoemulsions of phase change materials can be used for either cooling or heating applications or both, which can contribute to the reduced usage of electricity during peak hours. But heat and mass transfer and stability issues are encountered in the production, transport and storage of the heat storage media. In this paper, both the heat transfer enhancement effect and detrimental effects such as Ostwald ripening and supercooling are discussed along with the flow properties.

A numerical simulation study was carried out to shed light on the effects of applied crucible rotation and static magnetic field during the traveling heater method growth of bulk SiGe single crystals. The simulation results show that the application of crucible rotation weakens the radial silicon concentration gradient due to the effect of centrifugal force. The effects of applied static magnetic field direction and strength on the concentration field in the melt were also studied. It was found that the simultaneous application of crucible rotation and static magnetic field is best to grow large crystals with uniform composition. An optimum combination of crucible rotation rates and applied magnetic field strengths is determined.

Presented here is a new molecular dynamics simulation approach for visualizing multidimensional acoustic wave-packet propagation in anisotropic materials. This approach allows examination of longitudinal wave propagation in a selected frequency range and may also be extended to track transverse motions. The obtained results agree with analytical predictions and experimental measurements of quasilongitudinal wave front propagation in the literature. Additionally, spectral analysis reveals minor levels of frequency redistribution as the wave packet propagates, which is indicative of phonon-phonon scattering. The present approach provides new capabilities for phonon-focusing studies and offers an alternative to existing experimental and Monte Carlo techniques used for these studies.

The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr ∕ Si boundary. The Cr ∕ Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance ( h BD ) is measured with the transient thermoreflectance technique. Values of h BD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on h BD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on h BD . An excellent agreement is shown between the measured h BD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

During the vertical Bridgman process, a single semiconductor crystal is grown by the solidification of an initially molten semiconductor contained in an ampoule. The motion of the electrically conducting molten semiconductor can be controlled with an externally applied magnetic field. This paper treats the flow of a molten semiconductor and the dopant transport during the vertical Bridgman process with a periodic transverse or rotating magnetic field. The frequency of the externally applied magnetic field is sufficiently low that this field penetrates throughout the molten semiconductor. Dopant distributions in the crystal are presented.

Enhancement of thermal emission and control of its direction are important for applications in optoelectronics and energy conversion. A number of structures have been proposed as coherent emission sources, which exhibit a large emissivity peak within a narrow wavelength band and at a well-defined direction. A commonly used structure is the grating, in which the excited surface polaritons or surface waves are coupled with propagating waves in air, resulting in coherent emission for p polarization only. One-dimensional photonic crystals can also support surface waves and may be modified to construct coherent emission sources. The present study investigates coherent emission from a multilayer structure consisting of a SiC film coated atop a dielectric photonic crystal (PC). By exciting surface waves at the interface between SiC and the PC, coherent emission is predicted for both p and s polarizations. In addition to the excitation of surface waves, the emission from the proposed multilayer structure can be greatly enhanced by the cavity resonance mode and the Brewster mode.

This paper presents a linear stability analysis for the thermocapillary convection in a liquid bridge bounded by two planar liquid-solid interfaces at the same temperature and by a cylindrical free surface with an axisymmetric heat input. The two solid boundaries are rotated at the same angular velocity in one azimuthal direction, and a rotating magnetic field is applied in the opposite azimuthal direction. The critical values of the Reynolds number for the thermocapillary convection and the critical-mode frequencies are presented as functions of the magnetic Taylor number for the rotating magnetic field and of the Reynolds number for the angular velocity of the solid boundaries.

The present study is conducted to investigate the frost formation on a cold plate in atmospheric air flow by means of experimental and theoretical methods. In order to provide observations for the early stage of the frost growth process, a microscopic image system is used to record the pattern and the thickness of the frost layer per five seconds after the onset of frost formation. In this study, a multiple-step ascending frost growth pattern caused by melting of frost crystals at the frost surface has been observed. Effects of velocity, temperature and relative humidity of air ( V , T a , and ϕ) are studied, and the surface temperature of the cold plate T w is also varied. The considered ranges of these dominant variables are: 2 ⩽ V ⩽ 13 m/s, 20 ⩽ T a ⩽ 35 ° C , 40 percent ⩽ϕ⩽80 percent, and − 13 ⩽ T w ⩽ − 2 ° C . The theoretical model presented by Cheng and Cheng [22] for predicting the frost growth rate during the frost layer growth period is verified. Results show that the predictions of frost growth rate by the model agree with the experiment data, especially for the frost layer growth period.