Thermal Boundary Resistance Research Articles

Reduced thermal resistance of amorphous Al2O3 thin films on β-Ga2O3 and amorphous SiO2 substrates via rapid thermal annealing

The impact of rapid thermal annealing (1000 °C for 1 min) on the thermal transport properties of amorphous alumina (a-Al2O3) thin films grown by atomic layer deposition on β−Ga2O3 and amorphous silica (a-SiO2) substrates is determined using frequency-domain thermoreflectance measurements. The annealing more than doubles the a-Al2O3 thermal conductivity for both substrates (1.54 ± 0.13 to 3.14 ± 0.27 W m−1 K−1 for β−Ga2O3 and 1.60 ± 0.14 to 3.87 ± 0.33 W m−1 K−1 for a-SiO2) while keeping the film amorphous. The thermal conductivity increase is attributed to partial recrystallization and off-gassing of embedded impurities. Annealing halves the thermal boundary resistance of the a-Al2O3/a-SiO2 interface (10.5 ± 1.0 to 4.47 ± 0.42 m2 K GW−1), which is attributed to compositional mixing and structural reorganization that are enabled by the elastic matching of these two materials. The thermal boundary resistance of the a-Al2O3/β−Ga2O3 interface is not affected by annealing due to the elastic mismatch. Reducing the thermal resistance of a-Al2O3 dielectric films and adjacent interfaces by annealing will promote lateral heat spreading adjacent to hot spots and improve device longevity.

Conformal BN coating to enhance the electrical insulation and thermal conductivity of spherical graphite fillers for electronic encapsulation field

Herein, we provided a structural design strategy of constructing boron nitride (BN) conformal coating on the surface of the spherical graphite (SG) particles via precursors deposition and high-temperature pyrolysis method to achieve improved electrical insulation. The dissolved and reprecipitated BN precursors were uniformly deposited on the surface of polydopamine-modified SG through strong adhesion and hydrogen bonding of polydopamine. Then, the homogeneous and dense BN ceramic coating was formed on the surface of SG via slow pyrolysis at high temperature. The complete encapsulation of BN coating prevents the SG particles directly contacting with each other in the silicon rubber (SR) matrix to retain electrical insulation of SR composites. Furthermore, the BN coating flattens the uneven surface of SG so that the fillers are better infiltrated by the SR matrix. The small specific surface area of SG@BN filler particles is beneficial to significantly reduce the generation of the filler/matrix interfaces and weaken phonons scattering, thus decreasing the thermal boundary resistance. As a result, the thermal conductivity of prepared SG@BN/SR composites is up to 3.65 W m−1 K−1, which is 20.2-fold of that of pure SR with 0.18 W m−1 K−1. And the SG@BN/SR composites exhibit a satisfactory breakdown voltage of 5.6 kV mm−1, which is much higher than 1.1 kV mm−1 for the original SG/SR composites. This strategy is worth using for reference to extend the application of graphite-type fillers in the electronic encapsulation field.

Multifunctional nanocomposites reinforced by aligned graphene network via a low-cost lyophilization-free method

Compared with a disordered network, the ordered framework is more beneficial to improving the multifunctional properties (including thermal, electrical, and mechanical properties) of epoxy-based composites. However, current methods for building aligned structures are both energy and time-consuming on account of the tedious and lengthy lyophilization process. Herein, a new strategy is proposed to obtain low-density graphene aerogels (GAs) with long-range oriented structures through a lyophilization-free processes. Due to the minimum thermal boundary resistance, the prepared aligned graphene/epoxy composite exhibits an ultrahigh thermal conductivity of ∼11.6 W/(m·K) at a graphene content of 1.84 vol%, i.e., an enhancement efficiency of 3640% per 1 vol%. In addition, the compressive strength of the composite is increased by 2.6 times compared to epoxy (from 0.29 MPa to 0.76 MPa). Because the composite shows a modulus as low as 0.5–1.0 MPa, it has excellent deformability and compression performance. Moreover, the obtained 3-mm-thick composite achieves an electromagnetic interference shielding effectiveness (EMI SE) of 40 dB due to the interconnected graphene network structure, which meets the requirements of commercial EMI shielding applications. Hence, this strategy can be used to synthesize aligned graphene/epoxy composites with excellent multifunctional performances, which can be simultaneously used as elastic thermal interface materials (TIMs) and EMI shielding materials in the fields of advanced electronic packaging.

Pulsed thermoreflectance imaging for thermophysical properties measurement of GaN epitaxial heterostructures.

Multilayer heterostructures composed of a substrate and an epitaxial film are widely utilized in advanced electronic devices. However, thermal bottlenecks constrain their performance and reliability, and efficient approaches to comprehensively measure the thermophysical properties of heterostructures are urgently needed. In this work, a pulsed thermoreflectance imaging (PTI) method is proposed, which combines the transient temperature mapping of thermoreflectance thermal imaging with transient pulsed excitation. By executing merely three transient tests, six thermophysical properties, including the film thermal conductivity and specific heat capacity, the substrate thermal conductivity and specific heat capacity, the film-substrate thermal boundary resistance, and the equivalent thermal conductivity of the insulating layer, can be simultaneously measured in a heterostructure sample. The proposed method applies a pulsed current excitation to a metal heater line on the sample surface and utilizes the thermoreflectance thermal imaging system to measure the temperature of different spatial regions on the sample surface at different time windows. The temporal and spatial variation information of the temperature field is then extracted and combined with finite element method inversion calculation to obtain the thermophysical properties of heterostructures. To validate the accuracy and reliability of this method, we conducted measurements on a GaN-on-SiC heterostructure sample and obtained thermophysical properties consistent with the representative literature data that have previously been reported. The proposed PTI method, characterized by its high sensitivity, demonstrates good efficiency and reliability in conducting comprehensive thermophysical property characterization of GaN epitaxial heterostructures.

Environment-friendly efficient thermal energy storage paradigm based on sugarcane-derived eco-ceramics phase change composites: From material to device

Latent heat thermal energy storage (LHTES) technology can well alleviate the imbalance between intermittent energy supply and demand. However, the low thermal conductivity and poor shape stability of phase change materials (PCMs) seriously limit their practical applications. Here, sugarcane-derived biomimetic SiC ceramics are proposed for fast and efficient thermal energy storage. After loading paraffin, the composite phase change materials (CPCMs) demonstrate a high thermal conductivity of 10.34 W/mK and a high energy density of 151.20 kJ/kg at a porosity of 85%, outperforming state-of-the-art ceramics-based CPCMs. This benefits from continuous SiC skeletons composed of tightly stacked grains, so that both boundary and contact thermal resistances are reduced even at a high porosity. No prominent decay of thermal conductivity and energy storage density after 500 charging-discharging cycles, as well as good leakage resistance, confirm the good cyclic stability of proposed CPCMs. High-performance CPCMs are further packed into a fixed-bed LHTES device and investigated both experimentally and numerically. The melting time of LHTES device is prominently reduced by 44.3% benefiting from synergy of high thermal conductivity and non-coaxial arrangement of packed CPCMs cells. This work opens a new route for rapid thermal energy storage based on sugarcane-derived biomimetic materials.

On the Continuum Fallacy: Is Temperature a Continuous Function?

It is often argued that the indispensability of continuum models comes from their empirical adequacy despite their decoupling from the microscopic details of the modelled physical system. There is thus a commonly held misconception that temperature varying across a region of space or time can always be accurately represented as a continuous function. We discuss three inter-related cases of temperature modelling — in phase transitions, thermal boundary resistance and slip flows — and show that the continuum view is fallacious on the ground that the microscopic details of a physical system are not necessarily decoupled from continuum models. We show how temperature discontinuities are present in both data (experiments and simulations) and phenomena (theory and models) and how discontinuum models of temperature variation may have greater empirical adequacy and explanatory power. The conclusions of our paper are: a) continuum idealisations are not indispensable to modelling physical phenomena and both continuous and discontinuous representations of phenomena work depending on the context; b) temperature is not necessarily a continuously defined function in our best scientific representations of the world; and c) that its continuity, where applicable, is a contingent matter. We also raise a question as to whether discontinuous representations should be considered truly de-idealised descriptions of physical phenomena.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted employing a Gibbs excess approach - is found correlating with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices. This article is protected by copyright. All rights reserved.

Thermal Boundary Resistance: A Review of Molecular Dynamics Simulations and Other Computational Methods

Continued miniaturization of microelectronics has led to increased energy and interface density within those electronics. With each new interface, a new thermal resistor is created, preventing heat from efficiently escaping the device. This is such a problem that Kapitza resistance or thermal boundary resistance is now the dominant cause of thermal resistance in most microelectronics. Thermal boundary resistance has been studied extensively. However, thermal boundary resistance remains poorly understood. In this review, the existing literature is critically looked at, focusing on molecular dynamic simulations of the Si/Ge interface, which has become the de facto standard against which most other methods and systems are compared. As such, the volume of literature available on this system is considerably larger than any other, and the depth of analysis that can be performed is far greater. A research strategy for the field is presented to maximize progress in controlling Kapitza resistance. It is proposed that benchmark systems need to be found so that calculations can be properly verified, and that the size effects on Kapitza resistance need to be fully characterized. Finally, strong evidence is presented that first‐principles calculations offer the best chances for meaningful future progress, preferably with anharmonic contributions intact.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces.

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Effect of bias-enhanced nucleation on the microstructure and thermal boundary resistance of GaN/SiNx/diamond multilayer composites

The low effective thermal boundary resistance (TBReff) at the GaN/diamond interface is very important for the development of high-power, high-frequency and high-temperature GaN-on-diamond devices. The nucleation and growth of diamond are key processes for modulating the TBReff. This work proposed the bias enhanced nucleation technique to adjust the nucleation of GaN/SiNx/diamond multilayer composites in the MPCVD process at different bias voltages (400–700 V), thereby adjusting TBReff. Pulse bias is beneficial to a stable plasma environment and to obtain a complete GaN/diamond interface structure. The transient thermoreflectance characterization indicated that the GaN/SiNx/diamond multilayer composite prepared under 700 V bias nucleation condition had the lowest TBReff (26 ± 10 m2K/GW), whereas the GaN/SiNx/diamond multilayer composite at 600 V bias had the highest TBReff (83 ± 18 m2K/GW). The role of bias enhanced nucleation process in the TBReff was systematically analyzed by electron microscopies (TEM and SEM) and Raman spectroscopy. The GaN/SiNx/diamond multilayer composite prepared at 600 V showed a thick mixed transition layer containing multiphase structures and rough interfaces due to efficient subsurface ion implantation, resulting in high TBReff. In contrast, at 700 V, a thinner nucleation zone and smoother interface result in the lowest TBReff. This work demonstrated the potential of adjusting TBReff at the GaN/diamond interface by using bias enhanced nucleation technique to modulate the diamond nucleation and growth processes.

A Critical Review of Thermal Boundary Conductance across Wide and Ultrawide Bandgap Semiconductor Interfaces.

The emergence of wide and ultrawide bandgap semiconductors has revolutionized the advancement of next-generation power, radio frequency, and opto- electronics, paving the way for chargers, renewable energy inverters, 5G base stations, satellite communications, radars, and light-emitting diodes. However, the thermal boundary resistance at semiconductor interfaces accounts for a large portion of the near-junction thermal resistance, impeding heat dissipation and becoming a bottleneck in the devices' development. Over the past two decades, many new ultrahigh thermal conductivity materials have emerged as potential substrates, and numerous novel growth, integration, and characterization techniques have emerged to improve the TBC, holding great promise for efficient cooling. At the same time, numerous simulation methods have been developed to advance the understanding and prediction of TBC. Despite these advancements, the existing literature reports are widely dispersed, presenting varying TBC results even on the same heterostructure, and there is a large gap between experiments and simulations. Herein, we comprehensively review the various experimental and simulation works that reported TBCs of wide and ultrawide bandgap semiconductor heterostructures, aiming to build a structure-property relationship between TBCs and interfacial nanostructures and to further boost the TBCs. The advantages and disadvantages of various experimental and theoretical methods are summarized. Future directions for experimental and theoretical research are proposed.

Role of interfacial electric field on thermal conductivity of In x Al1−x N/GaN superlattice (x = 0.17)

Abstract In this paper, we report the role of the interfacial polarization electrical field in thermal conductivity of In x Al1−x N/GaN superlattice. Thermal conductivity reduction is one recent effort to improve thermoelectric device efficiency because a small reduction in thermal conductivity can enhance the figure of merit significantly. Quantum size effect and thermal boundary resistance are responsible for this reduction. The theoretical results demonstrate that the interfacial polarization electric field modifies acoustic phonon properties through elastic moduli and phonon group velocity as a result of the inverse piezoelectric effect. This enhances phonon scattering and thermal boundary resistance. Consequently, the thermal conductivity of the superlattice is reduced. Room temperature thermal conductivity is found to be 2.94 (3.35) W m−1 K−1 for In x Al1−x N/GaN superlattice (x = 0.17) in the presence (absence) of an electric field.

Cryogenic characteristics of graphene composites—evolution from thermal conductors to thermal insulators

The development of cryogenic semiconductor electronics and superconducting quantum computing requires composite materials that can provide both thermal conduction and thermal insulation. We demonstrated that at cryogenic temperatures, the thermal conductivity of graphene composites can be both higher and lower than that of the reference pristine epoxy, depending on the graphene filler loading and temperature. There exists a well-defined cross-over temperature—above it, the thermal conductivity of composites increases with the addition of graphene; below it, the thermal conductivity decreases with the addition of graphene. The counter-intuitive trend was explained by the specificity of heat conduction at low temperatures: graphene fillers can serve as, both, the scattering centers for phonons in the matrix material and as the conduits of heat. We offer a physical model that explains the experimental trends by the increasing effect of the thermal boundary resistance at cryogenic temperatures and the anomalous thermal percolation threshold, which becomes temperature dependent. The obtained results suggest the possibility of using graphene composites for, both, removing the heat and thermally insulating components at cryogenic temperatures—a capability important for quantum computing and cryogenically cooled conventional electronics.

Effect of surface wettability on specific heat capacity of nano-confined liquid

Equilibrium Molecular Dynamics (EMD) simulations are carried out to investigate the effects of wall wettability and nanogap thickness (h) on the constant volume molar heat capacity (Cv) of liquid argon entrapped between two solid copper walls at a fixed temperature of 100 K. Lennard-Jones (LJ) potential is employed to model the interaction between atoms, with nanogap thickness (h) varying between 0.585 nm to 5.85 nm. To characterize interfacial surface wettability as hydrophobic or hydrophilic, the solid-liquid interaction potential has been altered, and its effect on molar heat capacity (Cv) has been evaluated. Simulation results reveal three key findings. As the surface becomes more hydrophobic (i) both maximum heat capacity (Cv,max), and critical nanogap thickness (hc) (nanogap thickness at which maximum heat capacity is obtained) decrease; (ii) the range of nanogap thickness where the heat capacity of nanoconfined liquid surpasses that of bulk liquid diminishes; (iii) the heat capacity of nanoconfined liquid tends to match its bulk equivalent. This anomalous behavior of heat capacity with changing wall wettability is explained by variation in both vibrational (mode of phonon transportation) and configurational (density oscillation near the wall, thermal boundary resistance, molecular mobility, etc.) contribution of nanoconfined liquid.

Ultralow Thermal Conductivity Achieved by All Carbon Nanocomposites for Thermoelectric Applications

AbstractCarbon‐based materials are becoming a promising candidate for thermoelectricity. Among them, graphene shows limited scope due to its ultra‐high thermal conductivity (κ). To develop graphene‐based thermoelectric devices, reduction of κ is highly desired while maintaining reasonably high electrical conductivity (σ). Herein, multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) fillers are added into few layered graphene (FLG) to produce all‐carbon composites yielding ultra‐low thermal conductivity (κ) desired for thermoelectric applications. The novel preparation method of pristine FLG realizes very low κ of 6.90 W m−1 K−1 at 1248 K, which further reduces to 0.57, 0.81, and 0.69 W m−1 K−1 at the same temperature for FLG + MWCNTs, FLG + CB, and FLG + MWCNTs + CB, respectively. As‐prepared FLG composites also maintain reasonably high σ, whilst the Seebeck coefficient shows over a factor of five improvement after the inclusion of carbon‐based fillers. Consequently, the power factor (PF) is significantly improved. The ultralow κ is attributed to the increased thermal boundary resistance among graphene sheet boundaries. The realization of ultralow κ with simultaneous improvement in Seebeck coefficients and relatively small drops in σ with a facile and unique synthesis technique, highlight the potential of these composites.

Heat exchange performance of sintered fine silver powders in ultra-low temperature cooling of superfluid 4He

The aim of this study is to explore the heat exchange performance of sintered fine silver powders by measuring thermal boundary resistances of them to superfluid 4He below 700 mK. The sinters used in this study were made of pure fine silver powders having nominal particle sizes of 0.07 µm and 0.13 µm. Thermal boundary resistances of the sinters increased with decreasing temperature with a temperature dependence close to T-3 down to about 40 mK. The thermal boundary resistance of the 0.13 µm sinter was smaller than that of the 0.07 µm sinter below about 100 mK. Although the sinters have inhomogeneous and rough surfaces, no anomalous Kapitza resistance was observed at high temperatures. The thermal boundary resistances of the sinters were found to be in the same order of magnitude as that of a bulk silver. It appears that the microscopic structure of the sinter, with its large surface area, does not function efficiently to exchange heat with superfluid 4He at ultra-low temperatures. It is concluded that efficient cooling of superfluid 4He down to ultra-low temperatures requires heat exchangers with structural scales much larger than 0.13 µm.

Understanding Criticality of Thermal Performance in Thermal Interface Material Applications

Thermal interface material (TIM) is used in between a heat generating component (e.g., microelectronic packaging) and a heat spreading component (e.g., heatsink or cooling plate) to create an effective path for the heat (Phonon) to travel. Standard heatsink and heat generating component surfaces are generally uneven and rough. Actual metal to metal contact is no more than 10%. These surface imperfections allow air to get trapped in between the two surfaces. Air, being a thermal insulator, prevents the heat from dissipating and thus the device/system fails to maintain the required operating temperature to meet the reliability and functionality needed. Replacing the air with the “right” thermal interface material is the focus of this research. TIM is a composite of thermally conductive fillers dispersed in a polymer matrix. A higher filler loading causes a higher bulk conductivity. Common practice among design engineers is to utilize output from thermal modelling & simulation to specify a TIM with a certain thermal conductivity to meet the system’s thermal needs. What many engineers miss is the impact of thermal boundary resistance that could have significant effect on the overall thermal management of the design. This paper discusses how to characterize thermal performance of TIM beyond the bulk thermal conductivity. Boundary/interfacial thermal resistance and impedance will be explored as a function of TIM wetting ability, bond line thickness and surface conditions. This paper will discuss the importance of thermal conductivity, thermal resistance/impedance in a real application scenario.

Channel Temperature Analysis and Nonlinear Thermal Model of AlGaN/GaN HEMTs Including Steady-State and Transient

The thermal analysis of GaN-based high electron-mobility transistors (HEMTs) is carried out from steady-state and transient aspects, respectively. An improved closed-form nonlinear thermal model based on device structure is proposed. In the steady-state case, the plane layer is added on the basis of the original partition, and the effect of thermal boundary resistance on channel temperature is also considered. In addition, the operating temperature of each gate finger is predicted based on the thermal coupling, and the nonlinear thermal model is extended to the transient by combining the thermal time constant. The validity of our presented model is verified by comparing the simulation results with the experimental data. Furthermore, this model is also embedded in the surface-potential-based current model to verify the self-heating effect in the circuits.

Assessments of the heat transfer performance of a novel continuous oscillatory baffled crystallizer via two-fluid model

An Eulerian-Eulerian two-fluid model which incorporates the kinetic theory of granular flow, the energy balance equations, was firstly applied to investigate the heat transfer performance of solid–liquid two-phase flow in continuous oscillatory baffled crystallizers (COBC). The results show that the novel COBC (#6) has both excellent heat transfer and particle suspension performance, as compared with other COBCs, indicating that importance of optimizing the chamber connection structure of COBCs. Then, the COBC #6 was further selected to study the effects of operating conditions and particle physical properties on its heat transfer performance. With increase of oscillation Reynolds number (Reo), the radial mixing of fluid gradually reaches the peak, resulting in the increasing rate of Nusselt number (Nu) gradually decreases. Compared with single-phase flow, the existence of solid particles can produce shear stress on the wall surface, reducing the thickness of heat transfer boundary layer and thermal resistance, thereby increasing the heat flux. With increase of particle size or volume fraction, such effect becomes more obvious, which significantly enhances heat transfer. The novel designed COBC can be operated in the optimized Reo to achieve good heat transfer performance and low power consumption.

Research Progress in Capping Diamond Growth on GaN HEMT: A Review

With the increased power density of gallium nitride (GaN) high electron mobility transistors (HEMTs), effective cooling is required to eliminate the self-heating effect. Incorporating diamond into GaN HEMT is an alternative way to dissipate the heat generated from the active region. In this review, the four main approaches for the integration of diamond and GaN are briefly reviewed, including bonding the GaN wafer and diamond wafer together, depositing diamond as a heat-dissipation layer on the GaN epitaxial layer or HEMTs, and the epitaxial growth of GaN on the diamond substrate. Due to the large lattice mismatch and thermal mismatch, as well as the crystal structure differences between diamond and GaN, all above works face some problems and challenges. Moreover, the review is focused on the state-of-art of polycrystalline or nanocrystalline diamond (NCD) passivation layers on the topside of GaN HEMTs, including the nucleation and growth of the diamond on GaN HEMTs, structure and interface analysis, and thermal characterization, as well as electrical performance of GaN HEMTs after diamond film growth. Upon comparing three different nucleation methods of diamond on GaN, electrostatic seeding is the most commonly used pretreatment method to enhance the nucleation density. NCDs are usually grown at lower temperatures (600–800 °C) on GaN HEMTs, and the methods of “gate after growth” and selective area growth are emphasized. The influence of interface quality on the heat dissipation of capped diamond on GaN is analyzed. We consider that effectively reducing the thermal boundary resistance, improving the regional quality at the interface, and optimizing the stress–strain state are needed to improve the heat-spreading performance and stability of GaN HEMTs. NCD-capped GaN HEMTs exhibit more than a 20% lower operating temperature, and the current density is also improved, which shows good application potential. Furthermore, the existing problems and challenges have also been discussed. The nucleation and growth characteristics of diamond itself and the integration of diamond and GaN HEMT are discussed together, which can more completely explain the thermal diffusion effect of diamond for GaN HEMT and the corresponding technical problems.