Thermal Properties of Anisotropic and/or Inhomogeneous Suspended Thin Films Assessed via Dual-Side Time-Domain Thermoreflectance: A Numerical Study

Time-domain thermoreflectance (TDTR) is a powerful method for measuring thermal properties, such as thermal conductivity and thermal boundary resistance, of a broad variety of thin-film materials and interfaces. Dual-side TDTR, in which measurements are performed on the top and bottom sides of a suspended region of a thin film of interest, has recently emerged as an effective way to investigate the thermal properties of a film that is thermally anisotropic and/or inhomogeneous. Despite its experimental versatility, dual-side TDTR has yet to be fully interrogated. In this work, we examine the thermal conductivity and boundary resistance of anisotropic and/or inhomogeneous suspended thin films, extracted by dual-side TDTR on these films via numerical simulation. We start from a simple case of an anisotropic or inhomogeneous suspended membrane and then consider the combined case where the suspended membrane is both anisotropic and inhomogeneous. Taken together with analysis of measurement sensitivity, we aim to provide a general guideline for data extraction methodologies for dual-side TDTR on anisotropic and/or inhomogeneous suspended thin films.     

Analysis of transient heat transfer in multilayer thin films with nonlinear thermal boundary resistance

Transient energy transport in thin-layer films with a nonlinear thermal boundary resistance is analyzed theoretically within the framework of the dual-phase-lag heat conduction model. An iterative finite difference numerical method is used and is verified using a derived semi-analytical solution of the problem. Effects of the thermo-physical properties on energy transport when a two-layer film is exposed to a thermal pulse of certain duration and strength are presented. The thermal boundary resistance, the heat flux and temperature gradient phase lags and the thermal conductivities and heat capacities all are important factors that characterize energy transport through the interface and the temperature distribution in the two layers. The maximum interfacial temperature difference that takes place in the transient process of thermal pulse propagation is found to be the proper choice to measure the perfect-ness of the interface with a finite thermal boundary resistance. The results show that even with high values of the thermal boundary resistance the maximum interfacial temperature difference can be very small when the thermal pulse propagates from a high-thermal conductivity and heat capacity layer to a low-thermal conductivity and heat capacity layer. For a certain range of the thermal conductivities and heat capacities, the maximum interfacial temperature difference approaches zero even with high values of the thermal boundary resistance. Thermal conductivities and heat capacities are much more important in characterizing transient heat transfer through the imperfect interface than the phase lags of the heat flux and temperature gradient.     

Time dependent heat transfer in Hele-Shaw slots

Time dependent heat transfer in high Prandtl number free convection is studied in a Hele-Shaw slot with low heat conductivity side walls. Temperature fields are visualized by holographic interferometry. The time dependent Nusselt number is evaluated at the centreline of the slot from a time series of interferograms. The time dependent thermal boundary layer thickness δ reflects the oscillatory behaviour of the flow as well as the plume release process from the thermal boundary layers. The heat transfer analysis shows that unstable horizontal thermal boundary layers are the cause of time dependent high Prandtl number convection in Hele-Shaw slots.     

Interface Structure Influence on Thermal Resistance Across Double-Layered Nanofilms

This work investigated the interface influence on the thermal resistance across double-layered thin films by non-equilibrium molecular dynamics (NEMD) with Lennard-Jones potential. Layer A is a solid argon with a face-centered cubic structure and Layer B is obtained by changing atomic mass only. A flat interface is formed when each of the contacting atomic planes from the two layers has the same kind of atoms. A staggered interface is obtained by mixing atoms A and B around the interface region. The temperature profile, vibration amplitude, and structure factor are studied to observe the interface effects. It is found that the thickness of the staggered atomic layer has significant influences on the normal thermal conductivity. With a staggered interface thickness of two atomic planes, the normal thermal conductivity is sharply increased. Further increasing the staggered thickness will gradually decrease the normal conductivity. This result suggests a possibility to control the thermal conductivity of the double-layered structure by engineering its interface condition.     

Thermal conductivity of thin single-crystalline germanium-on-insulator structures

We report on the effective cross-plane thermal conductivity of single-crystal Ge layers from 42 to 100 nm on a SiO₂/Si substrate in the temperature range 30–300 K. We observe a drastic reduction of the thermal conductivity compared to bulk germanium. A large contribution to the temperature rise in the Ge layer is due to the interfacial thermal resistance between c-Ge/SiO₂. The use of a size-dependent intrinsic thermal conductivity of the Ge layer instead of the bulk thermal conductivity improves the consistency with values of the thermal boundary resistance derived from the diffusive mismatch model. Ultrathin films of Ge suffer from a lower reduction of the thermal conductivity compared to ultrathin films of Si, which makes germanium-on-insulator structures promising candidates for devices with reduced self-heating effects compared to silicon-on-insulator structures.     

层流脉动流中平行圆柱体的温度边界层

采用数值模拟方法研究了一个平行圆柱体在层流脉动流中的温度边界层特性。数值模拟结果与实验数据一致。研究发现脉动流中平行圆柱体形成了形状不规则但相对稳定的温度边界层,并在流动方向上周期性脉动。脉动流中平行圆柱体的温度边界层平均厚度小于稳定流动下的温度边界层平均厚度,并以脉动流的频率进行脉动。此外, 脉动流中平行圆柱体的壁面温度小于稳定流动下的壁面温度,表明脉动流下圆柱体的对流传热得到了强化。在一个脉动周期内,圆柱体在后半周期的温度边界层厚度和热阻均小于前半周期的温度边界层厚度和热阻。     

Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 2: Hysteresis effect under cyclic compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires the determination of the effective thermal conductivity as well as the thermal contact resistance between the GDL and adjacent surfaces/layers. The Part 1 companion paper describes an experimental procedure and a test bed devised to allow separation of the effective thermal conductivity and thermal contact resistance, and presents measurements under a range of static compressive loads. In practice, during operation of a fuel cell stack, the compressive load on the GDL changes.In the present study, experiments are performed on Toray carbon papers with 78% porosity and 5% PTFE under a cyclic compressive load. Results show a significant hysteresis in the loading and unloading cycle data for total thermal resistance, thermal contact resistance (TCR), effective thermal conductivity, thickness, and porosity. It is found that after 5 loading-unloading cycles, the geometrical, mechanical, and thermal parameters reach a “steady-state” condition and remain unchanged. A key finding of this study is that the TCR is the dominant component of the GDL total thermal resistance with a significant hysteresis resulting in up to a 34% difference between the loading and unloading cycle data. This work aims to clarify the impact of unsteady/cyclic compression on the thermal and structural properties of GDLs and provides new insights on the importance of TCR which is a critical interfacial transport phenomenon.     

An analysis of the characteristics of the thermal boundary layer in power law fluid

This paper presents a theoretical analysis of the heat transfer for the boundary layer flow on a continuous moving surface in power law fluid. The expressions of the thermal boundary layer thickness with the different heat conductivity coefficients are obtained according to the theory of the dimensional analysis of fluid dynamics and heat transfer. And the numerical results of CFD agree well with the proposed expressions. The estimate formulas can be successfully applied to giving the thermal boundary layer thickness.     

Effective thermal parameters of layered films: An application to pulsed photothermal techniques

Pulsed photothermal techniques provide useful methods based on linear relations between measurable quantities to obtain the thermal diffusivity and thermal conductivity of homogeneous materials. In this work, the effective thermal parameters of two-layered films are defined starting from an homogeneous layer which at the surfaces, produces the same temperature fluctuations and the same photothermal signal that the composite heated by a fast pulse-laser. Our theoretical model predicts that the effective thermal parameters of the layered system can only be calculated in the limit when the laser pulse duration is smaller tan the characteristic time of each layer, respectively. The temperature distribution is calculated in each layer by using the Fourier integral and the time-dependent one-dimensional heat diffusion equation with appropriate boundary conditions according to the experimental conditions. Within this approximation, we found an analytical expression for both, the effective thermal diffusivity and thermal conductivity which depend significantly on the thickness and the thermal parameters of each film.     

Application of a variational method to flow over a flat plate in the entrance region with variable physical properties

A variational method has been used to solve the flow over a flat plate in the entrance region at constant wall temperature. The physical properties, i.e. Thermal conductivity and viscosity, were assumed to be linear functions of temperature in the study. Two coupled equations were derived from the variational formulation and then solved by the analog/hybrid computer. Consequently, momentum boundary layer thickness,thermal boundary layer thickness local Nusselt number and local friction factor were found for the flow. For the constant properties case a comparison was made between the exact solution, and results obtained using the solution approach suggested in this paper.     

柴油机近气缸盖壁面气流速度和热边界层的研究

研究了倒拖工况下直喷式柴油机气缸盖近壁气流速度场的分布规律，并利用一维传热模型计算了气缸盖近壁的热边界居，得出了气缸盖近壁不同位置的气流速度和热边界层是不同的，速度边界层基本上在压缩中期形成并达到一定厚度，以及热边界层和速度边界层数量级相同等结论。     

Thermal transport within quantum-dot nanostructured semiconductors

In this work, we aim at exploring the effects of the germanium quantum dot (QD) layer embedded in silicon thin films on the thermal transport property in use of the non-equilibrium molecular dynamics simulation tool. An attempt is made to distinguish and understand the effect of the QDs themselves and the effect of the wetting layer on which QDs are grown. In this study, we notice as often observed a significant increase in the thermal resistance due to heterogeneous interfaces. Moreover, it is found that a simple QD interface has a thermal resistance monotonically decreasing with increasing quantum dot density. It is probably because the QDs make the transition from one material to another smoother, alleviate the acoustic mismatch, and thus assist the energy transport. When the germanium QDs together with a germanium wetting layer is inserted into a silicon material, the involved interface thermal resistance decreases first but increases later with increasing quantum dot density. The competition between the roughness effect and the wave interference effect is employed to explain this variation trend. As far as the quantum-dot superlattice thin film is concerned, we find its effective thermal conductivity decreases monotonically with increasing quantum dot density and with decreasing film thickness. In all cases, the size of quantum dots affects little on the thermal resistance/conductivity.     

Enhanced heat transfer by room temperature deposition of AlN film on aluminum for a light emitting diode package

Heat transfer is crucial to the fabrication of high efficiency light emitting diode (LED) packages. The effectiveness of the heat transfer depends on the package materials and design. This paper presents an application of high thermal conductivity aluminum nitride (AlN) films to replace low thermal conductivity epoxy resin or alumina substrates. The AlN film was directly deposited on an aluminum plate which enabled the removal of thermal interface materials (TIM) such as the adhesive thermal bonding sheets that are used in conventional metal printed circuit board (PCB)-based LED packaging process. A fully dense AlN ceramic film was successfully deposited at room temperature using the aerosol deposition method. The thermal resistance, a parameter of the heat transfer characteristic of an LED package, was measured using a thermal transient tester. The results showed that the thermal resistance of the LED package mounted on the AlN thick film was 28.5 K/W, while an LED package mounted on a conventional epoxy-based metal PCB and a PCB with thermal vias were 47.2 K/W and 36.5 K/W, respectively. This indicates that an aerosol-deposited AlN-based LED package exhibits greatly enhanced heat transfer compared to the conventional metal PCB.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 1: Effect of compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires determination of the effective thermal conductivity as well as the thermal contact resistance associated with the interface between the GDL and adjacent surfaces/layers.In the present study, a custom-made test bed that allows the separation of effective thermal conductivity and thermal contact resistance in GDLs under vacuum and ambient conditions is described. Measurements under varying compressive loads are performed using Toray carbon paper samples with a porosity of 78% for a range of thicknesses. The measurements are complemented by compact analytical models that achieve good agreement with experimental data. A key finding is that thermal contact resistance is the dominant component of the total thermal resistance; neglecting this phenomenon may result in significant errors in evaluating heat transfer rates and temperature distributions.     

The Ultrafast Laser Pump-Probe Technique for Thermal Characterization of Materials With Micro/Nanostructures

Advances in nano-electronics, nano-optics, energy harvesting materials, and nanoparticle-based photothermal therapies are motivating studies of the thermal properties of micro/nanostructures. Thus, the demands for highly sensitive and accurate thermal measurement techniques are encouraged for both fundamental studies and industrial applications. The time-domain thermoreflectance (TDTR) method, based on an ultrafast pump-probe technique, enables high-fidelity thermal measurements at the micro/nanoscale and the observation of dynamic processes with sub-picosecond time resolution. TDTR is an optical technique, capable of measuring the thermal properties of micro/nanostructures, including thermal conductivity and interfacial thermal conductance of bulk substrates, thin films, and nanoparticles, among others. Here we review some recent developments in the state-of-the-art ultrafast pump-probe method applied to study the thermal and magnetic properties of materials at the micro- and nanometer scales. We also discuss in detail improvements to this technique by presenting several example extensions to its capabilities.     

Temperature Prediction on Substrates and integrated Circuit Chips

This work deals with application of semianalytical methods for evaluation of temperature distribution on substrates and integrated circuit chips. This approach is based on a method proposed by Hein and Lenzi in 1969 which is a combination of Fourier transform. Green's function, and surface-element methods. The application of the method has evolved from a model that predicts the steady-state temperature on one-layer structures with lead connectors (modeled as lumped thermal resistances) and planar-discrete sources to a model that includes the effects of multiple layers and anisotropic thermal conductivity. Further generalization of the method to three new cases is presented. The first includes the transient thermal behavior in the one-layer structures with planar-discrete sources and anisotropic conductivity. The second deals with the steady-periodic behavior of two-layer structures with planar-discrete-periodic sources and anisotropic conductivity. The third case solves for the steady-state temperature in multilayer structures in which the conductivity of the bottom layer is larger than that of the upper layers (i.e., copper substrate); the thermal contact resistance between the bottom layer and its adjacent is also taken into account. Comparison between the finite-element (FE) method and this method is presented for one case.     

Mean Free Path–Thermal Conductivity Accumulation Calculations for Wurtzite Gallium Nitride: Two Approaches

ABSTRACT

Understanding the mean free path distribution of the dominant heat carriers is very important in determining the ballistic to diffusive heat transport transition in nanoscale devices. This is true for the high electron mobility transistors made from GaN where both the thickness of the buffer layer and localized heating causing ballistic-diffusive heat transfer may complicate the transport properties needed to describe the device thermal response. In this work, we obtain the mean free path–thermal conductivity relation of phonons in bulk wurtzite GaN crystals using two different, ab-initio-based calculations. While the Vienna Ab-initio Simulation Package (VASP) is used in both approaches at the initial stage, the first method does not calculate the third-order force constants (FCs) and approximates the anharmonicity with a single fitting parameter in determination of discrete phonon properties thermal conductivity and relaxation time, while the second method uses third-order force constants directly. Results show that the third-order FCs are important in modeling the contribution of high-frequency optical phonons with relatively short MFPs, to the thermal conductivity of the material. Yet these effects are more significant at high temperatures and at samples without crystallographic disorders, and they can be omitted while modeling the real samples at low temperatures.     

含二氧化硅气凝胶和相变材料三层玻璃窗热性能分析

为了研究含二氧化硅气凝胶和相变材料三层玻璃窗对严寒地区建筑能耗的影响,建立了相变材料层与其他透明壁层结合发生的传热数值模型。分析了含二氧化硅气凝胶和相变材料三层玻璃窗在不同二氧化硅气凝胶厚度、导热系数和不同保温材料下的动态热调节性能,得到了含二氧化硅气凝胶和相变材料三层玻璃窗内表面热流密度和液相率随时间的变化规律。结果表明:随着二氧化硅气凝胶厚度增加,总传热量降低和液相率增加,当二氧化硅气凝胶厚度为20～30 mm时,可以实现有效的利用太阳能;随着二氧化硅气凝胶导热系数增加,总传热量升高和液相率降低;当二氧化硅气凝胶的导热系数从0.022降低到0.014 W/(m·K)时,最大液相率从0.83增加到1.00。二氧化硅作为保温层比相变材料作为保温层具有更好的保温隔热作用。     

Thermodiffusion,chemical reaction,and Hall and ion-slip impacts on MHD rotating flow past an infinite vertical porous plate

The current scrutiny explores the impacts of thermodiffusion, chemical reaction, and Hall and ion-slip impacts lying on unsteady heat and mass transport of free convective hydromagnetic flow enclosed past a semi-infinite porous plate within a gyratory frame under the accomplishment of a transverse magnetic field and convective boundary conditions. The nondimensional governing equations are solved systematically by means of the finite element method. Through the facilitation of graphical profiles, the outcomes of a variety of significant parameters within the boundary layer are addressed. In addition, the local skin friction coefficient and rates of heat and mass transports in expressions of the local Nusselt number and local Sherwood number are presented digitally in tabulation form, although it is originated that the Nusselt number and Sherwood number remain constant with varying all pertinent parameters. It is found that the porous medium impact on the boundary layer growth is significant due to the increase in the thickness of the hydrodynamic boundary layer and the decrease in the thickness of the thermal and concentration boundary layers. The resultant velocity enhances with increasing rotation, Hall and ion-slip parameters.