Interfacial Thermal Resistance Research Articles

Molecular dynamics study on the role of hydrogen bonds and interfacial heat transfer between diverse silica surfaces and organic liquids

The understanding of interfacial heat transfer mechanism is increasingly significant since interfacial thermal resistance plays an important role in thermal management as the size of electronic devices decreases. In this work, interfacial heat transfer between five diverse silica surfaces and two organic liquids was studied using the molecular dynamics method. The two organic liquids are triacontane and triacontanol, which are either hydrogen bond incapable or capable, respectively. Silica surfaces with silanols show better thermal transport ability with triacontane/triacontanol because of a vibration matching effect. Increase of silanol area number density enhances the interfacial heat transfer for silica–triacontanol systems but has little effect for silica–triacontane systems because triacontane is hydrogen bond incapable. However, even for silica–triacontanol, the improvement of interfacial heat transfer does not scale proportionally to silanol area number density, since silanol–triacontanol and silanol–silanol hydrogen bonds are competing. The effective hydroxyl density is proved to reasonably explain this effect. Furthermore, temperature affects the interfacial heat transfer of silica-triacontanol systems vitally because both the number and lifetime of hydrogen bonds are reduced at higher temperature. The vertical orientation of triacontanol adsorbed onto the interface brings efficient heat paths via hydrogen bonds and alkyl backbones, which causes the interfacial molecular layers of triacontanol to have higher thermal conductivity in the direction normal to the interface.

High Performance Epoxy Composites Containing Nanofiller Modified by Plasma Bubbles.

The thermal conductivity of polymer materials is a fundamental parameter in the field of high-voltage electrical insulation. When the operating frequency and power for electrical equipment or electronic devices increase significantly, the internal heat will increase dramatically, and the accumulation of heat will further lead to insulation failure and serious damage of the whole system. The addition of filler with high thermal conductivity into polymer is a common solution. However, the interfacial thermal resistance between filler and bulk materials is the major obstacle to improve thermal conductivity. Herein, in order to reduce the interfacial thermal resistance, nanofillers are modified by plasma technology. The surface modification of nano-Al2O3 is carried out using plasma bubbles with three atmospheres (Ar, Ar+O2, air) as well as coupling agent. The situation of surface grafting before and after the modification is characterized using FTIR, XPS, and SEM. The effect of the mechanism of modification on the thermal conductivity and reaction pathway is investigated. The results showed that the thermal conductivity after plasma modification is increased significantly. Especially, the thermal conductivity is increased by 35% for the sample modified by Ar+O2 atmosphere. This results because more hydroxyl is introduced on the filler surface by the plasma bubbles, which enhance the interface compatibility between filler and epoxy. In addition, surface insulation performance for the modified samples also is enhanced by 14%. This is associated with the change of surface resistance and trap distribution. These results provide potential support for the development of fabrication for high performance epoxy composites.

What quantity of charge on the nanoparticle can result in a hybrid morphology of the nanofluid and a higher thermal conductivity?

One of the most important mechanisms for promotion of the thermal conductivity(TC) of nanofluid is aggregation of nanoparticles. In present work, a hybrid method of multi-particle collision dynamics and molecular dynamics (MPCD-MD) is employed to evaluate the aggregation morphology and TC of Cu-H2O nanofluid. Numerical simulations for various quantity of charges (QoC) on nanoparticles are conducted, and three kinds of morphology of nanoparticles distribution, whole aggregation morphology at lower QoC, hybrid morphology at moderate QoC and fully dispersed morphology at greater QoC, are obtained for the first time in microscopic simulations. For hybrid morphologies, the fractal dimension first drops and then rises with increasing QoC, since aggregates can break up into several clusters at relatively low QoC and can be fully dispersed at relatively high QoC. Instead, the TC first increases and then decreases, and the TC can reach a peak at a certain QoC. The underneath mechanism should be a balance between the effects of nanolayers and Kapitza resistance. This can provide helpful guidance to prepare nanofluids with higher TC.

A self-healing polyvinyl alcohol-based composite with high thermal conductivity and excellent mechanical properties

Conventional thermally conductive polymeric composites are prone to mechanical failures when deployed in dynamic environments, which results in loss of functionality and reduced service life. Herein, self-healing thermally conductive composites are fabricated by dispersing polydopamine (PDA) modified boron nitride (BN) and borate ions into polyvinyl alcohol (PVA) matrix. Due to the decreased thermal interface resistance and formation of thermal conduction channels, the 13 vol% BN-PDA/PVA composite endows an enhancement of 562% in thermal conductivity (1.007 W/mK) comparing with pure PVA (0.152 W/mK). In addition, the hydrogen bonds between BN-PDA and PVA chains and the borate bonds in dynamic borax/PVA networks impart a high self-healing efficiency of more than 80% and excellent mechanical properties in BN-PDA/PVA composites. This study provides a straightforward method for the preparation of thermal interface materials with fast recovery and long service life for next generation electronic packaging applications.

Decoding the phonon transport of structural lubrication at silicon/silicon interface

Although the friction characteristics under different contact conditions have been extensively studied, the mechanism of phonon transport at the structural lubrication interface is not extremely clear. In this paper, we firstly promulgate that there is a 90°-symmetry of friction force depending on rotation angle at Si/Si interface, which is independent of normal load and temperature. It is further found that the interfacial temperature difference under incommensurate contacts is much larger than that in commensurate cases, which can be attributed to the larger interfacial thermal resistance (ITR). The lower ITR brings greater energy dissipation in commensurate sliding, and the reason for that is more effective energy dissipation channels between the friction surfaces, making it easier for the excited phonons at the washboard frequency and its harmonics to transfer through the interface. Nevertheless, the vibrational frequencies of the interfacial atoms between the tip and substrate during the friction process do not match in incommensurate cases, and there is no effective energy transfer channel, thus presenting the higher ITR and lower friction. Eventually, the number of excited phonons on contact surfaces reveals the amount of frictional energy dissipation in different contact states.

Modelling the anisotropic thermal conductivity of 3D logpile structures

Assessing and predicting the thermal conductivity (κ) of macroporous materials is particularly important for additive manufactured 3D structures, as they offer considerable potential for tuning architectures and properties. In this work, finite element methods (FEM) are used to simulate the transient plane source test in 3D logpile lattices, approaching the effect of interfacial thermal contact resistances on κ measurement. Besides, the influence of different geometrical parameters (rod diameter, macropore size and overlapping between rods in consecutive layers) and the κ of the strut material on the anisotropic thermal conductivity of 3D lattices are investigated. The models are validated using experimental data for 3D composite structures of γ-Al2O3 with graphene nanoplatelet contents up to 18 vol%, as well as with reported data for alike scaffolds, all printed by robocasting. Results have strong impact for the application of novel 3D structures in energy production and storage, catalysis and heat transfer-related fields.

Interface thermal resistance of micron-thin film

The utilize of multilayer stacked structure dielectric is very common in electrical engineering, such as thin film capacitor wound by multilayer dielectric, and oil paper cable insulation layer formed by multilayer oil paper winding. Interface thermal resistance between dielectric layers are crucial thermo-physical parameters, which are very important for the thermal management of electric devices to improve the operating stability. However, there are some difficulties in precisely measuring thermal resistance parameters between micron-thin films by conventional ways. In this paper, a novel method is proposed for measuring thermal resistance parameters between films based on the thermal pulse method, which is normally used for mapping space charge or electric field distribution. The sample in this paper is of a multilayer structure containing at least one insulating dielectric film. The parameters of the interface thermal resistance of the multilayer structure are obtained by simulating and analyzing the thermal response current characteristics of the dielectric film. To study the characteristics of interface thermal resistance, several different thin film interfaces were designed by changing the type of thermal interface materials and the roughness of the contact surface. The obtained data of the interface thermal resistance show well consistence with the theoretical expectation.

Constructing “sesame‐biscuit” structured hybrids with SiC and Ag particles for enhancing thermal conductivity and dielectric constant of elastomer composites

AbstractWith the development of multifunctional and miniaturized integrated circuits, polymeric materials with large dielectric constant and high thermal conductivity have major significance in modern electronic industry. In this work, “sesame‐biscuit” structured thermally conductive hybrids were prepared by polydopamine (PDA) modification and in‐situ electroless silver (Ag) plating on the surface of silicon carbide (SiC), referred as SiC‐P‐Ag. Then, the SiC‐P‐Ag was incorporated into epoxidized natural rubber (ENR) matrix to yield polymeric composites (ENR/SiC‐P‐Ag) with excellent thermal and dielectric properties. It was found that PDA improved the interfacial compatibility and reduced the interface thermal resistance between SiC and ENR matrix. In addition, the formation of heat conduction channels and conductive Ag nanoparticles caused an increase in thermal conductivity of the 50 vol% ENR/SiC‐P‐Ag composite to 0.5655 W/(mK), which was 540% that of the pure ENR (0.1048 W/(mK)). Meanwhile, the 50 vol% ENR/SiC‐P‐Ag composite showed a low AC conductivity of 2.98 × 10−11 S/cm and a high dielectric constant of 35 at 10 Hz. It is noteworthy that this simple and efficient route for preparing ENR/SiC‐P‐Ag composites can also be applied in other fillers for simultaneously enhancing dielectric constant and thermal conductivity of polymeric composites.

Tunable nonlinear conductive behavior without percolation threshold and high thermal conductivity of epoxy resin/SiC ceramic foam co-continuous phase composites

Smart dielectrics with self-adaptive capabilities can exhibit desirable electric field-grading performance as the applied electric field exceeds a critical value. However, the conventional approaches to such dielectrics need heavy doping rate, which will not only increase the interface thermal resistance and limit the improvement of thermal conductivity, but also severely sacrifice the mechanical property. In this contribution, a new type of electric field-grading co-continuous phase composite (EP/SiCcf) composed of epoxy resin and SiC ceramic foam was prepared to realize tunable nonlinear conductive performance, while simultaneously improving thermal and mechanical properties. Results show that there is no percolation threshold for all EP/SiCcf composites. The volume loadings of EP/SiCcf composites range from 8.7 vol% to 15.6 vol%, while the nonlinear coefficient subjected to potential barrier height increases from 2.1 to 4.5 and the switching field tuned by barrier width decreases from 1008 V/mm to 686 V/mm. The EP/SiCcf40 still exhibits sharp thermal conductivity enhancement of about 1000% and glass transition temperature enhancement of 10.8 °C. The surface temperature fluctuation over time during heating and cooling has illustrated the prospective application of thermal management capability. In addition, the dynamic mechanical analysis reveals that all EP/SiCcf composites have the significantly improved storage modulus and crosslinking density ascribed to the intact SiC skeleton. The novel co-continuous phase composite provides a new approach for global enhancement of smart dielectric composites in potential applications.

Research on nano-scale AlN nucleation layer growth and GaN HEMT characteristics based on MOCVD technology

The AlN nucleation layers of different thicknesses were grown on a 100 mm 4H-SiC substrate in a low-pressure MOCVD. By using in-situ high-temperature etching of the substrate steps, the AlN nucleation layer could achieve layered growth as soon as possible, and finally achieve 11.6 nm. The complete AlN film was prepared, and the surface morphology and interface quality of AlN were analyzed with atomic force microscope and transmission electron microscope. A 1.8 μm thick GaN HEMT epitaxial material was prepared based on the ultra-thin AlN nucleation layer. The (002) and (102) plane rocking curves of GaN had a FWHM of 138arcsec and 231arcsec, and the electrons mobility of the heterojunction two-dimensional electron gas reached 2200 cm2/V·s. The AlN interface thermal resistance measured by the high-precision 3ω method was reduced to 5.53 × 10−9 m2K/W.

Thermal conductivity of metallic nanoparticle using surface effect and kapitza resistance

To estimate the size dependence of thermal conductivity of metallic nanoparticles, different theoretical models and experimental techniques have been used by different groups of researchers. But it still requires an accurate model that supports the experimental results at low dimensions. A model is proposed to predict the change in thermal conductivity of metallic nanoparticles with size by extending the bond-energy model assuming core shell structure. In present work, we have taken metallic nanoparticles as samples for the verification of proposed model. It is observed that the coefficient of thermal conductivity of nanoparticles decreases by lowering the size of nanoparticles. The variation in thermal conductivity of metallic nanoparticles with their shape and size can be successfully explained by introducing the theory including the surface effect and kapitza resistance. It is found that the computed value has a deviation of maximum ±5% with experimental results which is in the permissible range of experimental measurement.

MgO/Molten salt interfacial thermal transport and its consequences on thermophysical property enhancement: A molecular dynamics study

The solid-liquid interface between molten salt and solid oxides is ubiquitous in the molten salt nanocomposite with enhanced heat storage and transfer properties for concentrating solar power (CSP) systems. Understanding the microscopic mechanisms of interfacial thermal transport on this solid-liquid interface have great significance for evaluating the effective thermal properties of nanocomposites. This study combines equilibrium and non-equilibrium molecular dynamics simulations to explore the thermal transport and its connection with interfacial microstructure at the solid MgO-molten Hitec salt interface. Simulation results indicate that the enhancement of thermal transport can be ascribed primarily to the highly ordered adsorption structure of molten salt ions formed at the MgO surface, and the effect medium model incorporated with the simulated interfacial thermal resistance can reproduce the thermal conductivity of nanocomposite accurately. Further investigation indicates that the stronger adsorption at the MgO (110) surface leads to smaller interfacial thermal resistance than the MgO (100) surface, while the heat transfer parallel to the slab does not show much difference in both surfaces. These findings have illustrated the enrichment of molten salt ions at the solid surface does have a strong enhancement effect on their heat transfer properties and it is also necessary to incorporate the contribution from this interfacial thermal transport enhancement when predicting the thermophysical properties of molten salt nanocomposite materials.

Microscopic Mechanism of Tunable Thermal Conductivity in Carbon Nanotube-Geopolymer Nanocomposites.

Geopolymer has been considered as a green and low-carbon material with great potential application due to its simple synthesis process, environmental protection, excellent mechanical properties, good chemical resistance, and durability. In this work, the molecular dynamics simulation is employed to investigate the effect of the size, content, and distribution of carbon nanotubes on the thermal conductivity of geopolymer nanocomposites, and the microscopic mechanism is analyzed by the phonon density of states, phonon participation ratio, spectral thermal conductivity, etc. The results show that there is a significant size effect in the geopolymer nanocomposites system due to the carbon nanotubes. In addition, when the content of carbon nanotubes is 16.5%, the thermal conductivity in carbon nanotubes vertical axial direction (4.85 W/(m k)) increases by 125.6% compared with the system without carbon nanotubes (2.15 W/(m k)). However, the thermal conductivity in carbon nanotubes vertical axial direction (1.25 W/(m k)) decreases by 41.9%, which is mainly due to the interfacial thermal resistance and phonon scattering at the interfaces. The above results provide theoretical guidance for the tunable thermal conductivity in carbon nanotube-geopolymer nanocomposites.

Enhanced pool boiling heat transfer by adding metalized diamond in copper porous materials

Pool boiling heat transfer has attracted great attentions in heat dissipation areas, because of its great heat dissipation capability. To enhance heat transfer performance, lots of special porous structures have been designed, while the thermal conductivity enhancement of materials applied in the porous structure is rarely carried out. In this work, by adding metalized diamond in copper porous structures to increase the structure thermal conductivity, good pool boiling performance was observed. We firstly treated diamond into metalized diamond by sintering nickel method, and then different diamond/copper porous tablets were prepared by cold pressing metalized diamond and electrolytic copper particles together. Then, the pool boiling curves and the corresponding bubble dynamics of the porous tablets were experimentally investigated. Results turn out that: There are good wettability and also small interfacial thermal resistances between diamond and copper in the porous tablets, because of the metalized treatment of diamond. Thus, the boiling heat transfer could be significantly increased by 25%, compared to the tablets without treatment. There is a critical size of diamond to enhance the thermal conductivity of the porous tablets, because a small size of diamond always suggests large thermal resistances between diamond and copper, while a large size of diamond particle will lead to a bad metallization of diamond. In this work, the tablet prepared with 170-mesh size diamond possesses the highest pool boiling heat transfer, which could be more than 50% times larger than that of the smooth copper surface.

Experimental Evaluation of Thermoelectric Generator Performance under Different Heat Conduction Boundary Conditions

Heat conduction boundary conditions play a crucial role in the performance of thermoelectric generators (TEG). The TEG output voltage and power were measured under constant temperature boundary and heat flux conditions to evaluate the TEG performance under different heat conduction boundary conditions. External loading pressure and thermal interface material (TIM) were applied to reduce the interfacial thermal contact resistance. In our measurement setup, a fast-response electronic load was used for the rapid current-voltage scan, which can eliminate the thermal drift caused by the Peltier effect. A guard heater arrangement is used to minimize heat loss. In constant temperature boundary conditions, reducing the thermal contact resistance can increase the effective temperature drop across the TEG module and significantly improve the output voltage and power. But in the constant heat flux conditions, since the heat flux flow through the TEG is unchanged, the temperature drop across the TEG was unaffected by the thermal contact resistance. As a result, the TEG performance was lightly influenced by the thermal contact resistance.

Polytetrafluoroethylene Nanocomposites with Engineered Boron Nitride Nanobarbs for Thermally Conductive and Electrically Insulating Microelectronics and Microwave Devices

The demands for effective thermally conductive dielectric composite materials continue to increase due to high speed and low-loss signal propagation required in microwave frequency applications. Composites of polytetrafluoroethylene (PTFE) and boron nitride are promising for these applications. Here, for the first time, recent generation surface-engineered boron nitride nanobarbs (BNNB) material was evaluated at a low filler content in PTFE to achieve the desirable high thermal conductivity and low dielectric loss requirements which are typically difficult without a strong filler–matrix interaction and high filler loading. Composites of modified BNNB and modified PTFE were also fabricated and evaluated. These composites exhibited higher thermal conductivity (1.2–1.3 W/mK) with a lower concentration of boron nitride nanoparticles than those reported in the literature. The data were analyzed by theoretical models and interfacial thermal resistance was the lowest for the composite made from the modified PTFE and modified BNNB. Additionally, the composites displayed excellent dielectric properties including a dielectric constant ≈2.3 and loss tangent of less than 0.005 at frequencies of 1–3 GHz. These results indicate that such thermally conductive and low-loss dielectric composites have significant potential as materials for thermal management and dielectric components in microelectronics, 5G, and microwave devices.

Enhancing thermal conductivity of epoxy composites via f‐BN@f‐MgO hybrid fillers assisted by hot pressing

AbstractHigh thermal conductive polymeric composites are extremely desired for the thermal management of electronic devices due to the rapid development of the modern microelectronic industry. Herein, the functionalized boron nitride (BN) and magnesium oxide (MgO) hybrid fillers (f‐BN@f‐MgO) were synthesized and used to prepare the enhanced thermal conductive epoxy (EP) composites through the hot‐pressing method. The results demonstrated that the covalent binding of BN and MgO in the hybrid fillers reduced the interface thermal resistance effectively between fillers and matrix and the hot‐pressing induced force facilitated the construction of the continuous thermal conduction paths. Consequently, the as‐prepared epoxy composite at 40 wt% hybrid fillers loading had high thermal conductivity (TC) (1.97 W/[m·K]), outstanding insulating performance (6.9 × 1015 Ω cm) and excellent thermal stability. Furthermore, a probable thermal conduction mechanism was proposed to illustrate the high TC of the epoxy composite. Therefore, this study provides a new approach to preparing epoxy composites with outstanding performances.

Insights and theoretical model of thermal conductivity of thermally damaged hybrid steel-fine polypropylene fiber-reinforced concrete

The use of hybrid steel-fine polypropylene fiber-reinforced concrete (HFRC) may be an effective strategy for avoiding concrete thermal spalling under fire loading. The thermal conductivity of HFRC is one key parameter for the fire resistance assessing. In this research, the thermal conduction behavior of HFRC is experimentally and numerically studied to obtain a thorough understanding on its thermal degradations and their mechanisms after high-temperature exposure. New mechanisms are discovered to explain the remarkable reduction of thermal conductivity. In addition to the effect of moisture loss and porosity increase, the combined effect of heat bridge and interfacial thermal resistance (ITR) is found to be the dominant mechanisms. Moreover, the evaporation of polypropylene fibers adhering to the coarse aggregates enhances the ITR effect. Based on these mechanisms, a multi-scale homogenization method is also proposed to evaluate the thermal conductivity of thermally damaged HFRC, whose accuracy are demonstrated by the excellent agreements between the experimental data and the numerical results.

Data-driven thermal and percolation analyses of 3D composite structures with interface resistance

Data-driven thermal and percolation analyses are conducted to elucidate the effects of various characteristics on the effective thermal conductivity of complex 3D composite structures. These characteristics include the thermal and geometric properties of the composite constituents, the interface resistance, and the existence of percolation paths. A series of voxel-wise microstructure samples with various characteristics are generated. Their effective thermal conductivities are evaluated using a diffuse-interface-based computational homogenization method. A voxel-based algorithm is employed to identify the potential percolation paths in the structures. The homogenization results show particularly significant effects of the percolation path in composite samples with higher aspect ratios and interface resistances. The importance of different thermal and geometric features to the effective thermal conductivity is analyzed using a data-driven sensitivity study. The analysis also demonstrates that the particle volume fraction and interface thermal resistance are the most influential characteristics for determining the effective thermal conductivity. Finally, employing a surrogate-based classification model, microstructures with and without percolation can be distinguished with an accuracy of 93%.

Construction of thermal conduction networks and decrease of interfacial thermal resistance for improving thermal conductivity of epoxy natural rubber composites

Timely dissipation of accumulated heat in modern microelectronic equipments becomes an important strategy to ensure their operating stability and lifetime. Herein, alumina (Al2O3) and boron nitride (BN) are first modified by polyrhodanine (PRd), which then utilized to construct thermal conduction networks in epoxy natural rubber (ENR) composites, in which two-dimensional BN platelets act as bridges between zero-dimensional Al2O3 particles. During crosslinking process, the –NC–SH structures in PRd react rapidly with ENR chains and form strong covalent bonds, lead to an improved filler dispersion of polymeric composites and decreased interfacial thermal resistance between thermally conductive filler and ENR matrix. Finally, a relatively high thermal conductivity of 0.5147 W/(m·K) is obtained for ENR composites, which is 370% that of pure ENR (0.1390 W/(m·K)). Overall, construction of thermal conduction networks and decrease in interfacial thermal resistance of polymeric composites offer an effective approach to design and fabricate high-performance thermal management materials.