Showing papers on "Thermal contact conductance published in 2013"

Thermal conductivity and phonon transport in suspended few-layer hexagonal boron nitride.

Abstract: The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m(-1) K(-1), approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m(-1) K(-1) at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.

Three-Dimensional Thermal Modeling of a Lithium-Ion Battery Considering the Combined Effects of the Electrical and Thermal Contact Resistances between Current Collecting Tab and Lead Wire

Abstract: This paper reports on a three-dimensional thermal modeling approach for a lithium-ion battery (LIB). The combined effects of the thermal and electrical contact resistances between the current collecting tab of an LIB cell and the lead wire connecting the cell to an external cycler are considered explicitly in addition to the heat generated as a result of electrochemical reactions and ohmic heating in the electrode region of the battery cell. The effect of electrical contact resistance is taken into account when calculating current collecting tab heating, and the effect of thermal contact resistance is included in the heat flux boundary condition at the contact area between the current collecting tab and the lead wire. The three-dimensional thermal modeling is validated by comparing the modeling results with experimental temperature distributions from IR images during discharge in an LIB cell.

The influence of interface bonding on thermal transport through solid–liquid interfaces

Abstract: We use time-domain thermoreflectance to show that interface thermal conductance, G, is proportional to the thermodynamic work of adhesion between gold and water, WSL, for a series of five alkane-thiol monolayers at the gold-water interface. WSL is a measure of the bond strength across the solid-liquid interface. Differences in bond strength, and thus differences in WSL, are achieved by varying the terminal group (ω-group) of the alkane-thiol monolayers on the gold. The interface thermal conductance values were in the range 60–190 MW m−2 K−1, and the solid-liquid contact angles span from 25° to 118°.

Thermal conductivity from approach-to-equilibrium molecular dynamics

Abstract: We use molecular dynamics simulations to study the thermal transport properties of a range of poor to good thermal conductors by a method in which two portions are delimited and heated at two different temperatures before the approach-to-equilibrium in the whole structure is monitored. The numerical results are compared to the corresponding solution of the heat equation. Based on this comparison, the observed exponential decay of the temperature difference is interpreted and used to extract the thermal conductivity of homogeneous materials. The method is first applied to bulk silicon and an excellent agreement with previous calculations is obtained. Finally, we predict the thermal conductivity of germanium and α-quartz.

Thermal transport into graphene through nanoscopic contacts.

Abstract: Superior thermal conductivity of graphene is frequently reported and used to justify its technical relevance for ultimately scaled devices. However, this extraordinary property is size dependent, and understanding of graphene's thermal properties in the quasiballistic thermal transport regime is lacking. To overcome this limitation, we directly probe local heat transfer into graphene by high-resolution scanning thermal microscopy on amorphous silicon oxide (${\mathrm{SiO}}_{2}$) and crystalline silicon carbide (SiC). We quantify thickness-dependent thermal resistance modulations at sub-10-nm lateral resolution and thermal sensitivity for the individual atomic layers. On ${\mathrm{SiO}}_{2}$, we observe a decrease of thermal resistance with increasing number of graphene layers. We attribute this trend to the spreading of heat using the thickness dependence of graphene's thermal conductivity. On SiC, the heated tip-sample contact is scaled below the phonon mean free path of both the graphene and its supporting substrate. Consistently, we find the thermal interface resistances of the graphene top and bottom contacts dominating thermal transport.

The effect of thermal contact resistance on the thermosetting pultrusion process

Abstract: In the present study the control volume based finite difference (CV/FD) method is utilized to perform thermo-chemical simulation of the pultrusion process of a composite rod. Preliminary, the model is applied for a simple setup without die and heaters and the results match well with those obtained experimentally in the literature. In order to study the effects of the thermal contact resistance (TCR), which can also be expressed by the heat transfer coefficient (HTC), on the pultrusion process, a cylindrical die block and heaters are added to the original problem domain. The significance of using the TCR in the numerical model is investigated by comparing constant and variable TCR (i.e. position dependent) at the interface. Results show that the use of a variable TCR is more reliable than the use of a constant TCR for simulation of the process.

Analytical modeling and characterization of heat transfer in thermally conductive polymer composites filled with spherical particulates

Abstract: The continuous development of electronic devices requires optimum solutions for heat dissipation. This prompts the needs to the development of novel polymer composites that possess high thermal conductivity and high electrical resistivity. This paper discusses the challenges in this research area and some potential strategies to solve the problems. In this work, a thermal conductivity analyzer was designed and implemented to measure the effective thermal conductivity of composites with heterogeneous structures. An analytical model was developed to simulate the effect of contents of spherical fillers on the effective thermal conductivity of the composites. Using the developed model, together with experimentally-measured effective thermal conductivity of various composites, a semi-empirical approach was developed to estimate the thermal interfacial resistance at the polymer–filler interface.

Experimental determination of conduction channels in atomic-scale conductors based on shot noise measurements

Abstract: We demonstrate a general procedure for determining the conduction channels of quantum conductors from shot noise measurements. This numerical approach allows multichannel analysis which was previously limited to superconductors. Channel analysis of Ag and Au atomic contacts reveals a remarkable behavior in which the channels fully open one by one with increasing conductance. These results allow us to unambiguously distinguish between free-electron and tight-binding descriptions for the conductance of monovalent contacts. Furthermore, the channel resolution uncovers the presence of tunneling channels in parallel to the conductance through the main contact and provides a means for distinguishing between the contact conductance and tunneling contributions. Finally, unique channel distributions were found for Al and Pt contacts reflecting their distinct valence orbital structures.

Modeling Thermal Conductivity of Thermally Sprayed Coatings with Intrasplat Cracks

Abstract: Thermal spraying is one of the most important approaches for depositing thermally insulating ceramic top coatings for advanced gas turbines due to the low thermal conductivity of the coating resulting from its lamellar structure. The thermal conductivity of the coating has been explained based on the concept of thermal contact resistance and correlated to microstructural aspects such as splat bonding ratio, splat thickness, and the size of the bonded areas. However, the effect of intrasplat cracks on the thermal conductivity was usually neglected, despite the fact that intrasplat cracking is an intrinsic characteristic of thermally sprayed ceramic coatings. In this study, a model for the thermal conductivity of a thermally sprayed coating taking account of the effect of intrasplat cracks besides intersplat thermal contact resistance is proposed for further understanding of the thermal conduction behavior of thermally sprayed coatings. The effect of the intersplat bonding ratio on the thermal conductivity of the coating is examined by using the model. Results show that intrasplat cracks significantly decrease the thermal conductivity by cutting off some heat flux paths within individual splats. This leads to a deviation from the typical ideal thermal contact resistance model which presents cylindrical symmetry. Based on the modified model proposed in this study, the contribution of intrasplat cracks to the thermal resistivity can be estimated to be 42–57 % for a typical thermally sprayed ceramic coating. The results provide an additional approach to tailor the thermal conductivity of thermally sprayed coatings by controlling the coating microstructure.

Graphene-Based Nano-Electro-Mechanical Switch with High On/Off Ratio

Abstract: Locally defined nanomembrane structures can be produced in graphene films on a SiC substrate with atomic steps. The contact conductance between graphene and a metal-coated nanoprobe in scanning probe microscopy can be drastically reduced by inducing local buckling of the membranes. Repeatable current switching with high reproducibility can be realized. The on/off ratio can be varied from about 105 to below 10 by changing the contact force. At a low contact force, the contact conductance changes from 10μS (‘‘ON’’ state) to 100pS (‘‘OFF’’ state). This novel device structure could represent a new path to electrical switching at the nanoscale.

Reexamination of thermal transport measurements of a low-thermal conductance nanowire with a suspended micro-device.

Abstract: An increasingly used technique for measuring the thermal conductance of a nanowire is based on a suspended micro-device with built-in resistance thermometers. In the past, the technique has been limited to samples with thermal conductance larger than 1 × 10−9 W/K because of temperature fluctuations in the sample environment and the presence of background heat transfer through residual gas molecules and radiation between the two thermometers. In addition, parasitic heat loss from the long supporting beams and asymmetry in the fabricated device results in two additional errors, which have been ignored in previous use of this method. To address these issues, we present a comprehensive measurement approach, where the device asymmetry is determined by conducting thermal measurements with two opposite heat flow directions along the nanowire, the background heat transfer is eliminated by measuring the differential heat transfer signal between the nanowire device and a reference device without a nanowire sample, and the parasitic heat loss from the supporting beams is obtained by measuring the average temperature rise of one of the beams. This technique is demonstrated on a nanofiber sample with a thermal conductance of 3.7 × 10−10 W/K, against a background conductance of 8.2 × 10−10 W/K at 320 K temperature. The results reveal the need to reduce the background thermal conductance in order to employ the micro-device to measure a nanowire sample with the thermal conductance less than 1 × 10−10 W/K.

Molecular dynamics studies of material property effects on thermal boundary conductance

Abstract: Thermal boundary resistance (inverse of conductance) between different material layers can dominate the overall thermal resistance in nanostructures and therefore impact the performance of the thermal property limiting nano devices. Because relationships between material properties and thermal boundary conductance have not been fully understood, optimum devices cannot be developed through a rational selection of materials. Here we develop generic interatomic potentials to enable material properties to be continuously varied in extremely large molecular dynamics simulations to explore the dependence of thermal boundary conductance on the characteristic properties of materials such as atomic mass, stiffness, and interfacial crystallography. To ensure that our study is not biased to a particular model, we employ different types of interatomic potentials. In particular, both a Stillinger–Weber potential and a hybrid embedded-atom-method + Stillinger–Weber potential are used to study metal-on-semiconductor compound interfaces, and the results are analyzed considering previous work based upon a Lennard-Jones (LJ) potential. These studies, therefore, reliably provide new understanding of interfacial transport phenomena particularly in terms of effects of material properties on thermal boundary conductance. Our most important finding is that thermal boundary conductance increases with the overlap of the vibrational spectra between metal modes and the acoustic modes of the semiconductor compound, and increasing the metal stiffness causes a continuous shift of the metal modes. As a result, the maximum thermal boundary conductance occurs at an intermediate metal stiffness (best matched to the semiconductor stiffness) that maximizes the overlap of the vibrational modes.

Interfacial Thermal Conductance Limit and Thermal Rectification Across Vertical Carbon Nanotube/Graphene Nanoribbon-Silicon Interfaces

Abstract: Various models were previously used to predict interfacial thermal conductance of vertical carbon nanotube (CNT)-silicon interfaces, but the predicted values were several orders of magnitude off the experimental data. In this work, we show that the CNT filling fraction (the ratio of contact area to the surface area of the substrate) is the key to remedy this discrepancy. Using molecular dynamics, we have identified an upper limit of thermal interface conductance for C-Si interface which is around 1.25 GW/m2K, corresponding to a 100% filling fraction of carbon nanotube or graphene nanoribbon on substrate. By extrapolating to low filling fraction (∼1%) that was measured in experiments, our predicted interfacial thermal conductance agrees with experimental data for vertical CNT arrays grown on silicon substrate (∼3 MW/m2 K). Meanwhile, thermal rectification of more than 20% has been found at these C-Si interfaces. We observed that this is strongly dependent on the interfacial temperature drop than the filling fraction. This new effect needs to be considered in future thermal interface materials design.

Fractal Prediction Model of Thermal Contact Conductance of Rough Surfaces

Abstract: The thermal contact conductance problem is an important issue in studying the heat transfer of engineering surfaces, which has been widely studied since last few decades, and for predicting which many theoretical models have been established. However, the models which have been existed are lack of objectivity due to that they are mostly studied based on the statistical methodology characterization for rough surfaces and simple partition for the deformation formats of contact asperity. In this paper, a fractal prediction model is developed for the thermal contact conductance between two rough surfaces based on the rough surface being described by three-dimensional Weierstrass and Mandelbrot fractal function and assuming that there are three kinds of asperity deformation modes: elastic, elastoplastic and fully plastic. Influences of contact load and contact area as well as fractal parameters and material properties on the thermal contact conductance are investigated by using the presented model. The investigation results show that the thermal contact conductance increases with the increasing of the contact load and contact area. The larger the fractal dimension, or the smaller the fractal roughness, the larger the thermal contact conductance is. The thermal contact conductance increases with decreasing the ratio of Young’s elastic modulus to the microhardness. The results obtained indicate that the proposed model can effectively predict the thermal contact conductance at the interface, which provide certain reference to the further study on the issue of heat transfer between contact surfaces.

Detailed consideration of the electron-phonon thermal conductance at metal-dielectric interfaces

Abstract: The effect of electron-phonon coupling on thermal conductance across metal-dielectric interfaces remains inconclusive The leading model employs the phonon thermal conductivity of the metal that is difficult to estimate We remove this difficulty by obtaining the conductance directly from the Bloch-Boltzmann-Peierls formula, describing the matrix element using a deformation potential that can be estimated from the electrical resistivity data We report calculations up to 500 K to show that electron-phonon coupling is not a major contributor to the thermal resistance across metal-dielectric interfaces Our method advances understanding of the role of electron-phonon coupling in limiting thermal transport near metal interfaces

A numerical study on the influence of insulating layer of the hot disk sensor on the thermal conductivity measuring accuracy

Abstract: A numerical study on the influence of the insulation layer of the hot disk sensor on the thermal conductivity measuring accuracy has been conducted. It is found that the influences of the thermal contact resistance and the insulating layer could be excluded in the transient plane source method. Both the kapton5501 and the mica5082 sensor could measure stainless steel and ceramic with a deviation less than 3% while the deviation increases to 54.2% of silica aerogel because of the large heat loss proportion through the mica5082 sensor side. The simulation proved that the heat loss through sensor side and accuracy could be improved by increasing the radius of the sensor.

Numerical modelling of the effective thermal conductivity of heterogeneous materials

Abstract: The effective thermal conductivity of several series of polymer composite materials constituting two polymeric matrixes: ethylene vinyl acetate (EVA) filled with micrometric particles of barium tit...

Multi-physics model of a thermo-magnetic energy harvester

Abstract: Harvesting small thermal gradients effectively to generate electricity still remains a challenge. Ujihara et al (2007 Appl. Phys. Lett. 91 093508) have recently proposed a thermo-magnetic energy harvester that incorporates a combination of hard and soft magnets on a vibrating beam structure and two opposing heat transfer surfaces. This design has many advantages and could present an optimum solution to harvest energy in low temperature gradient conditions. In this paper, we describe a multi-physics numerical model for this harvester configuration that incorporates all the relevant parameters, including heat transfer, magnetic force, beam vibration, contact surface and piezoelectricity. The model was used to simulate the complete transient behavior of the system. Results are presented for the evolution of the magnetic force, changes in the internal temperature of the soft magnet (gadolinium (Gd)), thermal contact conductance, contact pressure and heat transfer over a complete cycle. Variation of the vibration frequency with contact stiffness and gap distance was also modeled. Limit cycle behavior and its bifurcations are illustrated as a function of device parameters. The model was extended to include a piezoelectric energy harvesting mechanism and, using a piezoelectric bimorph as spring material, a maximum power of 318 μW was predicted across a 100 kΩ external load.

Thermal shock resistance of solids associated with hyperbolic heat conduction theory

Abstract: The thermal shock resistance of solids is analysed for a plate subjected to a sudden temperature change under the framework of hyperbolic, non-Fourier heat conduction. The closed form solution for ...

Thermal Shock Resistance of Ultra-High-Temperature Ceramics Under Aerodynamic Thermal Environments

Abstract: A model of thermal stress field from literature with temperature-dependent thermophysical properties is updated using an appropriate expression of thermal strain due to free thermal expansion. The thermal shock resistance of the ultra-high-temperature ceramic plate under aerodynamic thermal environments is then studied by combining the proposed analytical method of heat for thermal shock. The numerical simulation is conducted to examine the theoretical model. The results from the model agree well with those from the simulation. The study shows that for the given material and thermal shock initial temperature the same heat transfer condition (product of surface heat flux and plate thickness) results in the same critical failure temperature difference. The critical failure time is inversely proportional to the square of the surface heat flux and is proportional to the square of the plate thickness, that is, the ultra-high-temperature ceramic plate has the same critical failure dimensionless time. The therma...

Thermal accumulation and tracking failure process of BN-filler epoxy-matrix composite

Abstract: Epoxy Resin is widely used for insulation in electric devices, in spite of its weak ability of thermal conduction. One approach to obtain a higher thermal conductivity is to introduce inorganic fillers which have high thermal conductivity. The purpose of this paper is to study the effect of boron nitride (BN) particles on thermal breakdown tracking process. In this paper, epoxy samples with BN particles were prepared with different weight ratios. In order to investigate the relationship between thermal accumulation and tracking failure process, tracking failure process under different frequency of pulse discharge was investigated by an infrared thermal imager from the front and back side. Obtained results showed that with increasing the concentration of BN particles, the time to tracking failure increased. The added BN fillers have a very significant inhibitory effect on thermal accumulation. The improvement of thermal conduction plays an important role in the results of tracking failure. High frequency discharges are more likely to cause the thermal accumulation, which is unfavorable to the reliability of materials. It is concluded that the tracking failure performance could be improved by reducing the thermal accumulation.