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• Thermal transport across metal interfaces was measured at the non-equilibrium states. • The measured h ee of Au/Pt matches well with the predicted value by electronic DMM. • Oxide layer significantly hinder electron thermal transport across metal interfaces. Interfaces play a critical role in nanoscale thermal transport, and understanding thermal transport mechanisms across metal/metal interfaces at the non-equilibrium states is urgently needed for highly integrated electronic structures or devices. In this work, the electronic interfacial thermal conductance h ee of two metal interfaces, Au/Pt and Au/Al, was measured by time domain thermoreflectance (TDTR) at the non-equilibrium states and an extended two-temperature model was applied to well describe the interactions of electrons between the metal films. For the Au/Pt structure, the measured h ee is 4.4 ± 1.8 GWm−2K−1, which is consistent with the predicted value by the electronic diffuse mismatch model (DMM). While for the Au/Al structure, the measured h ee (0.13±0.05 GWm−2K−1) is an order of magnitude smaller than that of Au/Pt and the predicted value of the electronic DMM. The results indicate that even the extremely thin oxide layer at the Au/Al interface can significantly hinder the electron thermal transport across the metal interface. Overall, our findings contribute to revealing the non-equilibrium interfacial thermal transport mechanisms and pave the way for designing the highly integrated electronic devices. [ABSTRACT FROM AUTHOR]

• The interfacial thermal conductance of GaN/AlN is investigated by both MD simulations and Monte Carlo simulations based on first-principles calculations. • Interfacial phonon modes are the additional channels for interfacial thermal transport. • Modal temperature and wave-resolved spectral heat flux provide modal-level and frequency-level acknowledge. Interfacial thermal conductance (ITC) is of critical importance for GaN-based device performance. GaN/AlN is a key component in GaN-based devices. Therefore, systematically investigating the interfacial thermal transport between GaN and AlN is of great significance for the thermal management of GaN-based devices. The temperature dependence of ITC across GaN/AlN interfaces are investigated using nonequilibrium molecular dynamics (NEMD) and Monte Carlo simulations based on first-principles calculations. Interestingly, the calculated ITC is much larger than that of GaN/metal, GaN/Si and GaN/SiC. Structural similarity and interfacial phonon modes are used to reveal the underlying mechanism. Moreover, the modal temperature and wave-resolved spectral heat flux are calculated to understand the interfacial thermal nonequilibrium and the dominant thermal transport channel, respectively. This study explores the modal-level and frequency-level mechanisms for interfacial thermal transport between GaN and AlN, which provides effective understanding in thermal management of GaN-based devices. [ABSTRACT FROM AUTHOR]

• The N-face GaN optical phonon branch is localized more strongly than Ga-face. • The interface phonon of GaN/Al x Ga 1- x N is significantly localized. • The interface phonon dispersion curve is elevated. • Lattice distortion leads to a drift of the interface phonon branch. The phonon transport processes within a few atomic layers near heterointerfaces play a crucial role in thermoelectric applications such as nitride-based high electron mobility transistors (HEMTs) and light-emitting diodes (LEDs). However, a systematic investigation of their localized transport characteristics remains unexplored. This article employs the Interfacial Phonons method based on first-principles calculations to extract interface phonon information for GaN/Al x Ga 1- x N heterostructures. Molecular dynamics simulations are utilized to compare the phonon behaviors between Ga-face and N-face GaN-based heterointerfaces. The research extensively investigates the dependence of interfacial thermal transport on the concentration of Al atoms under ordered substitutional doping and the strength of uniaxial compressive strain. The research revealed that both Ga-face and N-face GaN/Al x Ga 1- x N heterointerfaces exhibit phonon branch drift. The differences in charge distribution at the polar surfaces lead to varying degrees of Brillouin zone distortion. The vibrational modes of the atoms at the N-face GaN interface are more compact and localized than those of Ga-face GaN, resulting in a stronger degree of localization for the optical phonon branches. When varying the doping concentration of Al atoms, some phonon branches at the GaN interface appear outside the band structure of the intrinsic bulk and surface states, exhibiting strong localized characteristics. The introduction of Al atoms at the heterointerface induces additional phonon scattering, and the lattice vibration mismatch between Al and Ga atoms results in a non-uniform residual stress field at the interface, further enhancing phonon-interface scattering. Moderate compressive strain-induced lattice distortion promotes the overlapping of phonon modes at the interface, allowing some localized phonons to serve as interface transport modes. This optimization enhances phonon transport at the heterointerface. This research provides valuable insights into a comprehensive understanding of interfacial thermal transport in GaN-based semiconductors. Moreover, it offers useful theoretical guidance for thermal management through the manipulation of phonon wave properties. [ABSTRACT FROM AUTHOR]

• Ultra-low lattice thermal conductivity of BaAg 2 SnSe 4 with the inclusion of the temperature-induced phonon coherent resonance effect is firstly captured. • Coupled with particle-like and wave-like propagation of phonon excitations successfully resolve the discrepancy between the theoretical and experimental κ L. • The results unequivocally verify the authenticity of the two-channel transport phenomenon driven by temperature in the case of superior thermoelectric materials. Phonon heat transfer by thermoelectric modules undoubtedly plays a pivotal role for the future waste heat recovery and utilization. BaAg 2 SnSe 4 , a prototypical glasslike compound among quaternary non-centrosymmetric chalcogenides, demonstrates remarkable thermoelectric performance due to its exceptionally low lattice thermal conductivity (κ L). Even with a comprehensive grasp of the underlying cause behind ultra-low κ L and the subsequent heightened thermoelectric conversion efficiency, the temperature-dependent lattice dynamics and glass-like attributes governing κ L continue to pose unresolved questions. In this study, we investigate the phonon thermal transport properties of quaternary BaAg 2 SnSe 4 by incorporating the influence of phonon coherent resonance, thereby departing from the conventional Peierls's framework. Based on the temperature dependence of the interatomic force constants and coherences' contribution to total lattice thermal conductivity (κ L (C)), we intriguingly observe that the particle-like driven lattice thermal conductivity (κ L (P)) and κ L (C) have the quite significant and dominant role for describing ultralow phonon transport behaviors in the middle-high-temperature regions. Specially, enhanced phonon anharmonicity coupled with excited coherent phonons due to increasing temperature yields two close values, with 700 K values of 0.16 and 0.13 W/mK predicted for BaAg 2 SnSe 4 , respectively, accelerating its application in thermoelectric field. Furthermore, we also find that the off-diagonal terms of heat flux operators in κ L exist distinction for different phonons, with zero contribution modes exhibiting temperature-independent phonon coherence effect. Temperature response of the force constants tensors and coherent resonance proposed in our work can be employed as a powerful tool, which would substantially be expected to inspire a broad revisiting phonon transport study in various device applications, including advanced thermal barrier coatings and superior thermoelectrics. [Display omitted] [ABSTRACT FROM AUTHOR]

• For the first time, first-principles calculations are used to predict the lattice thermal conductivity of free-standing monolayer XS2 (X = mo and W) and monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates. • The lattice thermal conductivity of monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates show a significant decrease, and the decrease is even greater for the monolayer XS2 supported on ZnO (0001) substrates, reaching about 65–70 %. • The substrate drives softening of the acoustic branch, making the low-frequency phonon group velocity decrease and the three-phonon scattering enhance. • From the perspective of atomic bonding, the substrate breaks the symmetry of bonding of monolayer XS2, which leads to the increase of phonon anharmonicity and ultimately to a decrease in thermal conductivity of monolayer XS2. Thermoelectric materials have properties such as direct conversion of waste heat into electricity and environmental friendliness, which make them promising for renewable energy utilization, however, the lower conversion efficiency limits the large-scale application of thermoelectric materials. Monolayer transition metal dichalcogenides (TMDs) are candidates for thermoelectric materials due to their high power factor, but their higher thermal conductivity (TC) limits the further improvement of the figure of merit (ZT). Here, we employ first-principles calculations and solve the Boltzmann transport equation (BTE) to compare the lattice TC of free-standing monolayer XS 2 (X = Mo and W) and monolayer XS 2 supported on SiC (0001) and ZnO (0001) substrates. We find that the lattice TC of monolayer XS 2 supported on SiC (0001) and ZnO (0001) substrates are sharply reduced. Especially supported on the ZnO (0001) substrate, the lattice TC of XS 2 reduce by 65 %–70 %, which is expected to improve the thermoelectric properties of monolayer XS 2. In addition, we report the underlying cause of this significant reduction in TC. Firstly, the substrate drives softening of the acoustic branch, which leads to a decrease of phonon group velocity in the low-frequency and an enhancement of the three-phonon scattering. Then, the asymmetric bonding of XS 2 is induced by the substrates, which enhance phonon anharmonicity and reduce the phonon lifetime. Our results demonstrate that the lattice TC of monolayer XS 2 can be tuned by adjusting the substrate, which has certain reference values for the thermoelectric design of XS 2. SiC (0001) and ZnO (0001) substrates break the symmetry of the monolayer XS 2 (X = Mo and W) bonding, resulting in an increase in phonon anharmonicity, which reduces the contribution of the low-frequency acoustic branch of monolayer XS 2 to the thermal conductivity. [Display omitted] [ABSTRACT FROM AUTHOR]

• Optimize the thermosyphon loop thermal performance by controlling loop wettability. • Long-term effective hydrophilic and hydrophobic modification method was determined. • The optimum wall wettability under different filling ratios was clarified. • The optimized R loop is reduced by 6.9% to 53.5% under different filling ratios. The behavior of vapor-liquid two-phase transport within the riser of the two-phase thermosyphon loop significantly impacts the heat transfer process from the evaporation to the condensation, as well as the circulation velocity, thus playing a crucial role in the heat transfer performance. However, existing research has predominantly focused on optimizing the performance of the evaporator, overlooking the optimization of vapor-liquid two-phase flow patterns and transport characteristics within the loop. This paper aims to optimize the heat transfer performance of the two-phase thermosyphon loop by controlling the wall wettability of the loop. The strategy employed involves modifying the wettability of the riser to regulate the two-phase flow within the loop, while visualization techniques are utilized to investigate the influence of riser wettability on the vapor-liquid two-phase distribution and heat transfer performance of the two-phase thermosyphon loop. The findings reveal that the impact of wall wettability on flow behavior and heat transfer performance within the loop varies significantly with changes in the filling ratio. This paper sheds some light on the riser wettability of two-phase thermosyphon loops to optimize the thermal performance. [ABSTRACT FROM AUTHOR]

Thermal transport properties are a significant criterion of performance evaluation for various fascinating nanoscale materials. In this review, we summarize the recent research progresses from China in the field of nanoscale thermal transport properties. Both experimental advances and atomic-level simulation development are reviewed for typical categories of nanoscale materials and structures, i.e. , nanotubes and nanowires, nanofilms, nano-interfaces, nano-functional materials and those involving in near field radiation. Some fascinating aspects about the frontier issues in nanoscale heat transfer are also highlighted. In particular, researches have witnessed a remarkable growth in the interface-dominated microscopic thermal transport from both molecular dynamics simulation and experimental methods. In addition, challenges and opportunities will be touched on for the emerging field of near-field radiation. Which is dominated by simulation predictions but with encouraging experimental advances. [ABSTRACT FROM AUTHOR]

• Thermal characteristics of a loop thermosyphon under anti-gravity are studied. • The operation principle under anti-gravity is discussed. • The influence of inclination angles and cooling water temperature are investigated. In various fields of thermal management and protection, such as vehicles, aircraft, and mobile electronic devices, it is crucial to transfer heat effectively from the top to the bottom. The use of a simple structure for this transfer can reduce the weight of the heat transfer system. Theoretical studies have demonstrated that the two-phase thermosyphon loop (TPTL) with a high filling ratio has the potential for excellent performance in anti-gravity applications. In this study, we investigate the thermal performance of a high filling ratio TPTL under anti-gravity conditions. Our results reveal that, contrary to previous understanding, a TPTL with a high filling ratio can effectively transfer heat in anti-gravity. However, we also found that the loop thermal resistance of TPTL under anti-gravity is greater than under gravity, and the heating surface temperature is higher. Additionally, we experimentally study the effect of inclination angles on anti-gravity performance, and our results indicate that smaller angles result in better heat transfer. Finally, we investigate the impact of cooling water temperature on thermal performance. Overall, considering the energy efficiency of the heat dissipation system and the heat transfer capacity of the device, it is not recommended to use cooling water at a lower temperature. This research sheds light on the potential for TPTLs to operate in anti-gravity conditions and supports their practical applications. [ABSTRACT FROM AUTHOR]

• Nano-groove structures were conducive to evaporation at low superheat. The structure hindered the heat convection of the liquid atoms near the structure wall, thus heat accumulated in the central region on the heat source, causing the liquid in this region to evaporate faster, which will ultimately contribute to nucleation. • The evaporation rate of structured surface was larger than the smooth surface, which was due to the higher heat flux even though the areas of heating source were the same. The main reason for this phenomenon was owing to the mass transport induced by the structure. • A verification model for the mass transport was constructed. The velocity of liquid transport on the structured surface was faster than the smooth surface significantly. Bubble behaviors in nucleate boiling have been continuously concerned, traditional experiment believes that micro/nano structures can improve the bubble behaviors by increasing the capillary force to enhance the heat efficiency of nucleate boiling. However, due to the scale limitation of the macro experiment, the evolution of the solid-liquid-gas interface (three-phase contact line) during the bubble formation and expansion on the structured surface is still not clear. Therefore, a three-phase contact line evolution model with a groove structure was constructed in this work. Results showed that the heat transfer performance of the structured surface was better than the smooth surface even though the heating area was the same and the structure was adiabatic. The three-phase contact line appeared on the structured surface but did not appear on the smooth surface when the superheat was low, which was due to the structure hindered the heat convection inner the liquid and enhanced the heat accumulation in a smaller region, then enhanced the evaporation. It was also found that structures were conducive to the liquid transport from the unheated area to the heating area, which hindered the expansion of the three-phase contact line and improved the evaporation of the microlayer that surrounded the three-phase contact line. These phenomena and mechanisms will contribute to explaining why some micro/nano structures can improve the expansion bubbles and reduce the departure radius, and can provide a more effective theoretical basis for designing the surface with the nanostructure. [ABSTRACT FROM AUTHOR]

• Heat transfer mechanism of TPLT with different filling ratios are revealed. • The parameter of subcooling degree is introduced to Lee model. • The heat transfer mechanism is subcooled flow boiling at high filling ratio. • The heat transfer mechanism is saturated pool boiling at low filling ratio. • For high filling ratios, the bubble size decreases with heat input increases. As an efficient heat transfer device, two-phase loop thermosyphon (TPLT) has a broad application prospect in many engineering fields, and its internal gas-liquid phase change and flow characteristics are the key factors affecting the heat transfer performance. In order to get a better thermal design and performance, it is necessary to clarify the heat transfer mechanism of the TPLT, especially under the condition of high filling ratio. In this study, the visualization experiment was combined with the CFD method to study the heat transfer and flow characteristics of TPLT. Due to the strong coupling relationship among the boiling process and the condensate process, the Lee model is optimized, and the relationship between saturation temperature and local pressure, and the parameter of subcooling degree in the Lee model were both introduced to accurately simulate the phase change process. Results shows that the optimized model has good prediction performance for the heat and mass transfer simulation of the loop thermosyphon. The two-phase flow in the loop thermosyphon changes from slug flow to churn flow with the increase of heating power at low filling ratio (50%), while the bubble size decreases and the number of bubbles increases with the increase of heating power (20W-80W) at high filling ratio (70%). The heat transfer mechanism before and after model optimization has changed, from saturated flow boiling to subcooled flow boiling at high filling ratios. At low filling ratio (50%), the fluid in heating section is saturated state, and the flow velocity at the bottom of the loop is low (0.018m/s). At high filling ratio (70%), the fluid in the heating section is supercooled state, and the fluid flow velocity at the bottom of the loop is high (0.171m/s). This indicates that the heat transfer mechanism of the loop thermosyphon at low filling ratio was saturated pool boiling, and the filmwise condensation occurs in the condensation section. However, the heat transfer mechanism is subcooled flow boiling at high filling ratio, and the gas-liquid two phases in the condensation section are in a mixed state. [ABSTRACT FROM AUTHOR]

• Interfacial thermal transport mediated by hotspot in individual silicon-based transistor and heterojunctions was investigated based on BTE and e -MC. • Strong phonon nonequilibrium effect exists in the hotspot and LO mode phonons dominate the lattice temperature rise. • Reverse heat flux same as the temperature gradient appears near the hotspot due to the reflection of phonons at interface. • Interface phonon modes provide additional heat conduction channels in heterojunction under hotspot. Transistors are typical multi-interface nanostructures, and the heat dissipation problem has become the bottleneck restricting size reduction and performance improvement of semiconductor devices. Local high-temperature hotspot generated by self-heating effects seriously affects the stability and reliability of the devices, and the interfaces further hinder the heat conduction. Thermal analysis is complicated by subcontinuum phenomena which reshapes the hotspot region and interface-mediating. There has been less description of device thermal transport mediated by hotspot coupling with the interfaces. In this paper, based on the phonon BTE (pBTE) thermal solver and electronic Monte Carlo (e -MC) simulation, we systematically study the thermal transport behavior of 1-D Si transistor and Si-based heterojunction interfaces under the hotspot effect. Also, the influence of other scattering events is investigated by modifying the relaxation time. Inside hard hotspot, the temperature and heat flux of different phonon modes are decomposed, which shows a strong non-equilibrium effect. The contributions of non-Fourier effects are separated and quantified based on split-flux model. In heterojunction structure, the interfacial thermal resistance further increases the hotspot temperature. Interestingly, on the side of hotspot near the interface, the direction of heat flux is opposite to the direction of the temperature drop, which violates Fourier's law. Through different substrate combinations, the heat dissipation effect of diamond, BN is not excellent due to the low interfacial thermal conductance (ITC), time-dependent BTE is used to study the heating rate. Mode-dependent temperature and heat flux reveal nonequilibrium phonon transport performance. Finally, the wave-resolved spectral heat flux is calculated to explore the predominant thermal transport channel and interface phonon transport characters. Above studies provide useful insights for further improving the accuracy of transistor-level thermal simulations. [ABSTRACT FROM AUTHOR]

• The temperature dependence of interfacial thermal conductance across Al/MoS 2 /GaN and Al/graphene/GaN is measured by TDTR. • The interposition of graphene and MoS 2 can significantly enhance the contribution of low-frequency phonons. • The phonons below 10 THz are the main heat carriers across Al/MoS 2 /GaN and Al/graphene/GaN. Graphene/GaN and MoS2/GaN have important applications in optoelectronic devices. The two-dimensional (2D) materials greatly improves the device performance due to its overwhelming advantages. 2D materials introduce multiple interfaces while improving device performance. With the miniaturization of devices, the interfacial thermal transport has crucial effect on the performance and lifetime of optoelectronic devices. The interfacial thermal conductance of Al/graphene/GaN and Al/MoS 2 /GaN are measured by the time-domain thermoreflectance (TDTR) technique from 100 K to 400 K. The density of states and spectral thermal conductance are calculated based on nonequilibrium molecular dynamics (NEMD) simulations to understand the underlying thermal transport mechanism. The overlap energy is used to understand the thermal transport efficiency. Most interesting, the results are opposite to that measured by TDTR. Moreover, the spectral thermal conductance and phonon dispersion are used to understand the thermal transport mechanism in frequency domain. The results provide experimental and theoretical guidance for thermal management of optoelectronic devices. [ABSTRACT FROM AUTHOR]

• The original BCC unit was improved for accurate prediction of conductive melting. • The effective thermal conductivity of the modified BCC unit was analyzed. • The small porosity (< 0.9395) BCC unit is open-cell via structural modification. • The relationship between the BCC structure and its ETC was concluded. Metal foams have been widely used to enhance the heat storage performance of phase change materials (PCMs). For numerically simulating the metal foam composite PCMs, a body-centered cubic (BCC) unit has been proposed to represent the real foam structure. However, the original popular BCC unit is closed when its porosity is lower than 0.9395, which is very different from the actual metal foam. Besides, the effective thermal conductivity (ETC) of its paraffin composite is a key parameter that directly affects the melting performance. It is not always accurately predicted compared with the existing investigations. In the present work, various modified BCC units were built by regulating the structure of the original one to approach the open-cell structure of the real foam, and their melting performances characterized by the ETC were compared with the existing literature results. Through analyzing the structural parameter (γ, diameter ratio of the inscribed and corner spheres) and ETC, we found that when the porosity of the BCC unit is lower than 0.9395, improving the γ is helpful in gaining the open-cell structure to represent the actual metal foam. When the porosity is 0.876, γ ≥ 1.11474 means an open-cell structure, and the BCC unit with γ ≥ 1.062 is open-cell for the porosity of 0.909. Besides, when the porosity is larger than 0.9395, one can gradually approach the experimental ETC by improving γ. In a word, this study extends the accurate application of the original BCC unit in acting as metal foam. [ABSTRACT FROM AUTHOR]

• A large database for bubble departure frequency is amassed. • Nine machine learning algorithms are developed. • Optimal models performed better than prior universal correlations. • The effect of parameters on departure frequency was studied. Boiling is a very effective heat transfer mechanism, which has been widespread applicable to many engineering and industrial fields. Moreover, a critical parameter in nucleate boiling is the bubble departure frequency, which arrives at accumulations of heat carried out by vapor bubbles from the heating surface. However, because of the complex behaviors in the phase change process, there are few reliable approaches to predict bubble departure frequency. In this study, a consolidated dataset is utilized to predict bubble departure frequency in subcooling flow boiling using machine learning-based approaches. A consolidated dataset of bubble departure frequency, including four working fluids in subcooling flow boiling, is formed. Nine machine learning-based regression models are well discussed. As well as the input parameters, including geometric and dimensionless parameters, are also contrasted for a suitable approach. Generally speaking, the XGBoost model shows the most significant performance at predicting bubble departure frequency, better than highly reliable generalized prediction correlations. The machine learning-based approach unlocks a reliable tool for bubble departure frequency prediction. [ABSTRACT FROM AUTHOR]

• A refined scaling relation for predicting bubble departure diameter. • New engineering descriptors are identified. • Show machine learning details in white box with MSE of 0.0093. • High potential for the application to the process in subcooling flow. The accurate prediction of bubble departure in subcooling flow boiling is challenging because the complex physical conditions make it difficult to predict. In particular, it is difficult to accurately predict the bubble departure diameter, which plays an essential role in subcooled flow boiling. This work, assisted by data-driven machine learning, identifies new engineering descriptors in algebraic expressions and establishes a refined scaling relation for predicting bubble departure diameter. The accuracy of the new engineering descriptor is superior to that of the single descriptor highlighted previously. Compared with previous models, the new sure independence screening and sparsifying operator (SISSO)-refined scaling relation has a better prediction performance. The results show the Mean Squared Error (MSE) with 0.0093 and the Root Mean Squared Error (RMSE) with 0.0963 for the whole database. Our trained model realizes the prediction of the orientation angle between the heat and flow, different working fluids (excluded from the database), and different heating methods of the bubble departure diameter, which is the ability that the previous model did not have. The work shows that the SISSO-refined scaling relation becomes a robust new predicting tool for bubble departure diameter. [ABSTRACT FROM AUTHOR]

• Octadecylamine with a large latent heat is selected as PCM. • The energy storage performance of the MWCNT-enhanced CPCM is experimentally studied. • The CPCM with fast heat storage rate and large heat storage capacity is acquired. • The heat storage power is significantly enhanced by 28.60%. The efficient utilization of the latent heat thermal energy storage (LHTES) system is mainly limited by its underperforming heat storage rate and capacity. Recently, scholars utilized nanoparticles to enhance the thermal conductivity of phase change materials (PCMs). Nevertheless, simultaneous enhancements of thermal conduction and phase change enthalpy have not been realized, much less considering the convective heat transfer. In this study, we selected octadecylamine (ODA) as the PCM; multiwall carbon nanotubes (MWCNTs) were ultrasonically dispersed into the ODA to obtain the uniform-mixed nano-enhanced PCMs (NePCMs). The results show that MWCNTs are conducive to strengthening thermal conduction. Besides, the fusion enthalpy of ODA@0.05 (containing 0.05 wt% of MWCNTs) is 6.45% higher than the pure ODA. However, the addition of MWCNTs improves the viscosities of liquid NePCMs in a non-linear trend, which weakens the convective heat transfer. Considering both heat storage rate and heat storage capacity, we find that the heat storage power of the ODA@0.05 is 22.38% higher than that of the pure ODA. Besides, the heat release power is 14.00% higher than the pure ODA. Consequently, we acquired the composite ODA with fast heat storage/release power and large heat storage capacity, which is an excellent candidate for the LHTES system. This paper sheds some light on the rational utilization of nano-additives to avoid the abuse of nanoparticles. [Display omitted] [ABSTRACT FROM AUTHOR]

• Hydrostatic pressure plays hugely positive role in the thermal conductivities of XC (X=Si, Ge) with zinc blende (ZB) and rock salt (RS) phases, and it also results in an enormous reduction of thermal conductivity when phase transition occurs. • The joint growth of phonon group velocity and phonon lifetime results in the obvious enhancement of thermal conductivity of SiC during the processes before and after phase transition. • During the process before / after phase transition, the abnormal increase of phonon scattering rate / little variation of phonon group velocity offsets the increase of thermal conductivity of GeC caused by phonon group velocity / phonon lifetime, which results in the smaller enhancement of thermal conductivity of GeC. • The thermal transport characteristic of GeC before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. • The tremendous reduction of thermal conductivity of SiC during phase transition is caused by the enhancement of the phonon scattering intensity, and that of GeC is leaded by the increase of the number of phonons scattering channels. In this paper, the effect of hydrostatic pressure on the thermal transport properties of silicon carbide (SiC) and germanium carbide (GeC) with zinc blende (ZB) and rock salt (RS) phases at room temperature is investigated by solving the phonon Boltzmann heat transport equation. Studies show that the thermal conductivities of the two materials increase with the raising pressure before the phase transition, however, the enhancement of GeC (40%) is much smaller than that of SiC (100%). Combined with phonon analysis, it is found that this phenomenon originates from the abnormal enhanced phonon scattering rate under high pressure, which partly offsets the increase of phonon group velocity and further suppresses the increase of thermal conductivity. Next, when the phase transition occurs, the thermal conductivities of SiC and GeC decrease sharply by 76% and 86%, respectively, which is mainly caused by the increase of the anharmonic characteristic related to phonon scattering. Finally, after the phase transition, the thermal conductivities of the two materials still show increasing trends with the raising pressure, however, the phonon group velocity of GeC does not change significantly and the variation of its thermal conductivity is mainly determined by the phonon lifetime, which is opposite to the case before phase transition, whose enhancement of thermal conductivity is mainly caused by the increase of phonon group velocity. The results indicate that GeC has a more complex phonon transport mechanism compared with SiC, i.e., its thermal conductivity before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. This study provides data and mechanism support for the regulation of thermal conductivity from the perspective of pressure-induced phase transition, and as well as supplies some guiding suggestions for engineering applications in the field of thermal transport regulation. [Display omitted] Hydrostatic pressure plays positive role in the thermal conductivities of XC (X=Si, Ge) during the processes before and after phase transition, and it also plays enormous negative role in the thermal conductivity when phase transition occurs. There are a variety of mutually restrictive phonon mechanisms existed in GeC, which can boost its application in the regulation of TC [ABSTRACT FROM AUTHOR]

• It was found that the position of the nucleate site varies with the spacing of structures. Then the evolutions of the three-phase contact line were also different between the cases. The mechanism of this phenomenon was analyzed by kinetic-potential energy theory, which was mainly related to the spacing of structures. • It was found that the relationship between liquid-solid contact area and the heat transfer enhancement of bubble expansion was not monotonically increasing. The evaporation of the microlayer around the three-phase contact line was more critical to CHF, which was not necessarily related to the liquid-solid contact area. • Liquid retention induced by the various height of structures also had significant effects on heat transfer of bubble expansion. The increased rate of CHF with the increase of liquid retention gradually slowed down, which was due to the limited effect of larger liquid retention on the liquid at bottom of structures. As the main factor limiting the performance of microprocessors, how to improve its heat dissipation capacity is critical. Nucleate boiling on the structured surface is a method that can improve heat transfer significantly due to two-phase flow, and bubble behaviors have been attracted much attention because of its capability of enhancement on critical heat flux(CHF). Although a large number of studies have proved that the structured surface can improve CHF by increasing the number of nucleation sites, bubble departure frequency and reducing bubble departure radius, the mechanism of solid-liquid-gas interface evolution on the structured surface is still unclear, which has significant effects on nucleate boiling. Therefore, a model of nucleate boiling of liquid on the structured surface was constructed, the processes of nucleate generated and bubble growth were observed directly by using molecular dynamics simulation. A novel phenomenon was observed in which the position and number of interfaces are mainly dependent on the restriction of potential energy generated by structures. This phenomenon affects CHF significantly due to the area of the microlayer beneath the bubble was different. In addition, the effects of liquid retention on interface and CHF were also explored, the result showed that CHF increased with the increase of liquid retention, and the increase rate of CHF was slowed down due to the more retained liquid in the structures. We believe that this research can improve the existent theory of nucleate boiling and provide a more effective theoretical basis for the design, processing and application of surface with the nanostructure. [ABSTRACT FROM AUTHOR]

A theoretical model for axial capillary flow in rectangular microgrooves was presented with the assumptions that the meniscus radius in the cross section is approximately constant, and the height of the centerline film recedes along the axial direction with a convex form. Experimental data obtained by Micro-PIV verify the assumptions. Comparison with the experimental data shows that the model is accurate to predict the axial profile of the liquid film for inclined rectangular microgrooves. [ABSTRACT FROM AUTHOR]

• Heat transfer mechanism of TPLT with high filling ratio is revealed. • Pool boiling is changed into flow boiling with filling ratio increases. • Heat transfer performance positively correlates with the circulation velocity. • For high filling ratios, thermal resistance decreases with heat input increases. The two-phase loop thermosyphon (TPLT) with a high filling ratio is considered an efficient heat transfer device and therefore has a good application prospect in the field of high heat flux dissipation engineering. However, the heat transfer mechanism of the TPLT with high filling ratios remains elusive. In this study, we establish a computational fluid dynamics (CFD) model of the loop thermosyphon. In order to accurately simulate boiling and condensation processes, we take into account the relationship between saturation temperature and pressure in the phase change model. There were two flow patterns, i.e., the bubbly flow and the slug flow, under low filling ratios in the heating section, in which the pool boiling dominates the main heat transfer mechanism, and dropwise condensation occurred in the condensation section. The driving force to propel the liquid during the vapor bubbles rise cannot be transferred to the liquid in the right tube, resulting in lower flow velocity. In stark contrast, the single-phase flow and the bubbly flow occurred periodically in the loop thermosyphon with high filling ratios, in which the dominant heat transfer mechanism is the flow boiling. The rising bubbles promote the clockwise flow of liquid in the loop like a pump. Because of the switch of the heat transfer mechanism, the average flow velocity increased, and the thermal resistance decreased. The minimum thermal resistance was 0.15 K/W under a filling ratio of 75%, and the maximum average velocity of the fluid was 0.19 m/s. Furthermore, we found a strong coupling relationship between the heat transfer performance and the flow characteristics of the loop thermosyphon. For low filling ratios, local dry-out occurred in the loop thermosyphon under a high input heat flux, leading to an increase in the thermal resistance. For high filling ratios, the average velocity of the fluid was augmented with the increase in heating power, while the liquid was capable of being replenished quickly in the heating section to avoid the dry-out. Therefore, the loop thermosyphon with high filling ratios enables a better heat transfer performance under a high heat flux than those with low filling ratios. [ABSTRACT FROM AUTHOR]

• Kelvin cell structure was numerically analyzed to evaluate heat transfer performance. • Optimized Kelvin cell skeletons improved heat transfer performance at high velocity. • Elliptical skeletons reduced pressure drop and turbulence at the throat. • Elliptical skeleton achieved uniform velocity distributions. The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) j v / f of the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the j v / f of the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices. [ABSTRACT FROM AUTHOR]