Thermal resistance of pressed contacts of aluminum and niobium at liquid helium temperatures

We examine the resistance to heat flow across contacts of mechanically pressed aluminum and niobium near liquid helium temperatures for designing a thermally conducting joint of aluminum and superconducting niobium. Measurements in the temperature range of 3.5–5.5 K show the thermal contact resistance to grow as a near-cubic function of decreasing temperature, indicating phonons to be the primary heat carriers across the interface. In the 4–14 kN range of pressing force the contact resistance shows linear drop with the increasing force, in agreement with the model of micro-asperity plastic deformation at pressed contacts. Several thermal contact resistance models as well as the phonon diffuse mismatch model of interface thermal resistance are compared with the experimental data. The diffuse mismatch model shows closest agreement. The joints are further augmented with thin foil of indium, which lowers the joint resistance by an order of magnitude. The developed joint has nearly 1 K thermal resistance at 4.2 K, is demountable, and free of the thermally resistive interfacial alloy layer that typically exists at welded, casted, or soldered joints of dissimilar metals.     

Thermal contact conductance characterization via computational contact homogenization: A finite deformation theory framework

In order to predict the macroscopic thermal response of contact interfaces between rough surface topographies, a computational contact homogenization technique is developed at the finite deformation regime. The overall homogenization framework transfers macroscopic contact variables, such as surfacial stretch, pressure and heat flux, as boundary conditions on a test sample within a micromechanical interface testing procedure. An analysis of the thermal dissipation within the test sample reveals a thermodynamically consistent identification for the macroscopic thermal contact conductance parameter that enables the solution of a homogenized thermomechanical contact boundary value problem based on standard computational approaches. The homogenized contact response effectively predicts a temperature jump across the macroscale contact interface. The strong dependence of this homogenized response on macroscale solution variables of interest is demonstrated via representative three‐dimensional numerical investigations. The proposed contact homogenization framework is suitable for the analysis of similar energy transport phenomena across heterogeneous contact interfaces where the investigation of the sources for energy dissipation is of concern. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal conductance measurements of bolted copper to copper joints at sub-Kelvin temperatures

We have measured the thermal contact conductance of several demountable copper joints below 1 K. Joints were made by bolting together either two flat surfaces or a clamp around a rod. Surfaces were gold plated, and no intermediate materials were used. A linear dependence on temperature was seen. Most of the measured conductance values fell into a narrow range: 0.1-0.2 W K⁻¹ at 1 K. Results in the literature for similar joints consist of predictions based on electrical resistance measurements using the Wiedemann-Franz law. There is little evidence of the validity of this law in the case of joints. Nevertheless, our results are in good agreement with the literature predictions, suggesting that such predictions are a reasonable approximation.     

直接冷却中氮化铝与无氧铜低温真空接触热导的实验研究

以5W／20K小型G-M制冷机为冷源,对低温下氮化铝（AlN）与无氧铜（OFHC）界面的接触热导进行了实验研究和分析。在45～140K内,氮化铝／无氧铜界面接触热导随温度的升高而增大,同时亦随接触压力的增加而增大。实验中同时得到了氮化铝在低温下的热导率,随温度的升高,氮化铝热导率值逐渐增大。就氮化铝低温热导率及氮化铝／无氧铜接触界面热阻随温度变化规律进行了微结构机理分析。     

Joining highly conductive and oxidation resistant silver-based electrode materials to silicon for high temperature thermoelectric energy conversions

Joining silicon thermoelectric elements using silver-based alloys and adhesive was investigated. Selective etching silicon with HF and KOH was performed to increase the interface area. Physical vapor deposition was used to coat Ti, Cr, Pt and Ag on silicon surface to form transition layers for the enhancement of interface bonding. Sound joints using the silver adhesive were obtained and they can withstand the highest temperature of 925 °C. Contact resistance of the joints under both thermal cycling and isothermal heat treatment was measured from 500 °C to 920 °C. It is found that the contact resistance of the silver/silicon joints is about 1 Ω at room temperature. At the elevated temperature of 920 °C, the contact resistance is less than 2.5 Ω. We conclude that the silver adhesive has excellent adhesion to silicon surface and the contact resistance is considerably low. Therefore, it is suitable for joining silicon thermoelectric elements for energy conversion at high temperatures.     

高温超导直接冷却中AlN与Bi-2223间界面热阻的实验研究

基于微结构低温工程学，提出三维低温界面层的概念，指出界面热阻是制冷机直接冷却超导磁体需要解决的关键技术之一。以GM制冷机为冷源，按稳态热流法原理测量了Bi-2223、AIN的热导率及它们之间的低温界面热阻。在0.15MPa-0.55MPa压力范围内，AIN和Bi-2223间的界面热阻随界面层温度和接触压力的升高而降低，并随接触界面处温度的不同表现出不同的变化率。当界面层Bi-2223侧温度为55K时，在0.5469MPa的接触压力作用下，Bi-2223和AIN间的界面热阻是厚度为10mm的AIN垫片体积热阻的38.86倍，是接触压力0.2281MPa时界面热阻的38.7％。     

Experimental Investigation of Thermal Contact Conductance: Variations of Surface Microhardness and Roughness

Surface roughness parameters were measured using nine different sampling intervals. The results showed that the roughness of profile was nearly a constant at all sampling intervals; the slope, peak curvature and density of the profile decreased with increasing sampling interval; and the distribution of the peak height was different from the profile height distribution and was not Gaussian. It was found that the surface microhardness value, under small loads, varied with the indentation size and the surface preparation procedure. A model was developed which used a discretization method to obtain the contact radius and area in each section of the surface summit height distribution curve. This model eliminated some of the assumptions made by previous models and can be used to predict the thermal contact conductance without assuming the asperity deformation mode and surface summit height distribution. The predicted thermal contact conductance values were in good agreement with experimental results.     

Estimating thermal contact resistance using sensitivity analysis and regularization

Characterization of the thermal contact resistance is important in modeling of multi-component thermal systems which feature mechanically mated surfaces. Thermal resistance is phenomenologically quite complex and depends on many parameters including surface characteristics of the interfacial region and contact pressure. Although most studies seek a single value as a function of these parameters, in general, the contact resistance is non-uniform over the interface. In this paper, a technique is developed for extracting non-uniform contact resistance values from experiments in two-dimensional configurations. To begin, a two-dimensional model problem is formulated for a known contact resistance between two mated surfaces. An inverse problem is devised to estimate the variation of the contact resistance by using the BEM to determine sensitivity coefficients for specific temperature measurement points in the geometry. Temperature measured at these discrete locations can be processed to yield the contact resistance between the two mating surfaces using a simple matrix inversion technique. The inversion process is sensitive to noise and requires using a regularization technique to obtain physically possible results. The regularization technique is then extended to a genetic algorithm for performing the inverse analysis. Numerical simulations are carried out to demonstrate the approach. Random noise is used to simulate the effect of input uncertainties in measured temperatures at the sensors.     

真空低温下螺钉压紧的Cu-Cu界面间接触热阻的实验研究

为满足航天器热控设计的需要,实验测量了真空条件下接触面温度110 K时螺钉压接的Cu-Cu界面间的接触热阻,比较了不同的螺钉预紧力矩以及不同的导热填料对接触热阻的影响。实验数据表明,界面接触热阻随着螺钉预紧力矩增大成一阶指数衰减,导热填料为铟箔时界面热阻显著的减小,接触热阻最低可以达到2.0×10-5K.m2/W。     

Oxygen partial pressure dependence of surface tension and its temperature coefficient for metallic melts: a discussion from the viewpoint of solubility and adsorption of oxygen

Surface tension of molten Si, Ag, and Fe–18Cr–8Ni alloy (Type-304 stainless steel) were measured as a function of the temperature and oxygen partial pressure of an ambient atmosphere by an oscillating drop method using electromagnetic levitation, which assures measurement over a wide range of temperatures below and above the melting point and of oxygen partial pressure. For molten silicon, the lower limit of measured temperature was determined by equilibrium oxygen partial pressure for SiO₂ formation; as oxygen partial pressure increases, undercooled conditions necessary for taking measurements become smaller. This is attributed to the low solubility of oxygen in molten silicon. For Ag, however, surface tension can be measured in a wide range of temperature and oxygen partial pressures due to the high solubility of oxygen. A boomerang-shaped behavior of surface tension was observed for Ag in an atmosphere with high oxygen partial pressure; surface tension has a maximum value when plotted against temperature. This boomerang-shaped behavior is attributed to desorption of oxygen at high temperature; surface tension increases with increasing temperature. The boomerang-shaped behavior was also observed for Fe–18Cr–8Ni alloy.     

Determination of the thermal conductivity of xenon-helium mixtures at high temperatures by the shock-tube method

The thermal conductivity of gases at high temperatures has been measured by the shock-tube method, which is uniquely suited to measure thermal conductivities of gases at high temperatures above 2000 K. A consistent set of thermal-conductivity data over a wide range of temperatures has been obtained from optimum combinations of shock-tube experiments at high temperatures, previously published data at lower temperatures, and a theoretical correlation of the temperature dependence. In the present study, the thermal conductivity of xenon-helium mixtures has been determined at compositions of 10 and 30 mol% xenon over the temperature range from 300 to 4800 K. Even though there is a large difference between the thermal conductivity of pure xenon and that of helium, it is interesting that the dependences of the thermal conductivity of the mixture on temperature and composition are linear. The experimental results are in good agreement with the predicted values based on the corresponding-states principle and the mixing rule. From these experimental results, interpolating the corresponding-states correlation data, we represent the equation of xenon-helium gas mixtures for thermal conductivity in terms of temperature and composition.     

Experimental study of gas flow through a multi-opening orifice

Gas flow through an orifice can be determined with high accuracy based only on the geometrical dimensions of the orifice and the upstream and downstream pressures when the flow is purely molecular. An orifice with a number of smaller openings in parallel can be used to maintain the molecular flow at higher pressure and high total conductance of the orifice. The question of how close such openings can be without influencing each other is important for practical design. This problem was studied experimentally. Changes in the total conductance versus pressure were followed for a set of multi-opening orifices with regularly arranged differently spaced circular openings. The experimental results show that no influence on the flow through a single opening can be observed at sufficiently distant opening, over the entire pressure range. At centre-to-centre distances shorter than approximately three times the diameter of the opening, notable differences in the total conductance can be seen in the pressure range where the transition from the molecular to transitional flow regime occurs.     

A Study on Thermal Contact Resistance at the Interface of Two Solids

In this paper, numerical simulations and measurements of the thermal contact conductance (TCC) at the interface between the plane ends of two cylinders in contact are carried out. The random model of surface roughness is developed, and the non-dimensional basic equations are solved based on a grid system with equi-peripheral intervals in the azimuthal direction that can express reasonably the real contact spot distribution. The effects of the contact pressure, the thermal conductivity of the interstitial medium, and the mean absolute slope of the rough surface on the TCC were clarified by using a network method. In the experiments, four pairs of brass cylinders, each of which has similar surface topology, are used for the TCC measurements. The hysteretic nature of TCC versus contact pressure was observed in the first loading cycle. The present numerical results show that the TCC increases linearly with the mean absolute slope of the surfaces even at the same mean roughness. Such a tendency agrees well with the measurements.     

Shakedown limit of rail surfaces including material hardening and thermal stresses

Sliding friction between railway wheels and rails results in elevated contact temperatures and gives rise to severe thermal stresses at the wheel and rail surfaces. The thermal stresses have to be superimposed on the mechanical contact stresses. Due to the distribution of stresses, the rail surface is generally subjected to higher stresses than the wheel surface. The elastic limit is reduced and yield begins at lower mechanical loads. During the first cycles of plastic deformation, the material hardens and residual stresses build up. The residual stresses provide the structure to shake down to pure elastic behaviour in subsequent load cycles up to a shakedown limit. The kind of hardening observed for rail steel has a considerable influence on the shakedown limit. The shakedown limit is dropped to lower mechanical loads due to the thermal stresses in the rail surface as well. This might cause structural changes in the rail material and rail damage.     

Mechanical properties of In-based eutectic alloy solders used in Josephson packaging

InBiSn and InSn eutectic alloy solders are used in the packaging of the cryogenic Josephson processor. The ductile behaviour of these alloys is important to the application of joints which experience large thermal stresses. In order to characterize the mechanical behaviour at cryogenic temperatures, tensile and shear strengths of bulk solders were measured at and below room temperature. It is found that the ultimate tensile and shear strengths of bulk solders increase as temperature decreases. At low temperatures, the ultimate tensile strength of InBiSn alloy is about three times less than that of InSn alloy. The ductility of both alloys reduces at low temperatures. The fracture surfaces of both bulk solders and solder joints having an interface material (Pd and Au thin films) to the electrical contact pads (Nb thin film) were examined using SEM. Ductile fracture mode was observed for all the specimens down to liquid nitrogen temperature.     

碳纤维/环氧树脂预浸带铺放应力场及制品微观结构的模拟与实验

碳纤维/环氧树脂预浸带复合材料在铺放成型时,由于树脂基体与碳纤维之间的热膨胀系数存在差异以及成型时热-力参数作用下由于纤维的变形而导致纤维与基体接触处产生应力集中等原因,在制品材料中会产生热残余应力。针对碳纤维/环氧树脂预浸带复合材料的实际结构特点,利用ABAQUS有限元软件建立含有界面的碳纤维/环氧树脂预浸带复合材料的细观代表性体积单元(Representative volume element, RVE)有限元模型,采用实验研究和有限元仿真分析的方法,研究在温度-压力参数作用下预浸带铺放制品残余应力的分布规律及影响机理。首先,建立预浸带铺放时的温度和压力模型,研究不同温度和压力参数条件下碳纤维/环氧树脂预浸带铺放制品残余应力的分布情况。其次,采用耦合降温法模拟碳纤维/环氧树脂预浸带残余应力随纤维体积含量、铺放压力以及铺放温度的变化规律,并采用扫描电镜对不同工艺参数条件下预浸带铺放制品的微观结构进行分析。通过对模拟结果进行分析比较得到各因素对制品残余应力的基本影响规律;最后进行不同温度和压力等铺放参数对预浸带铺放成型时残余应力影响的实验测试研究。      

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von –184 bis 600°C

Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm³ was investigated in tension tests in the temperature range of –184 and 600°C. Supplementary compression tests were carried out at 20°C. Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores. After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed. The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes.     

制备温度对Ag-IPMC拉伸及致动性能的影响

在不同的沉积温度-修饰温度(25℃-25℃,15℃-25℃,15℃-15℃,10℃-10℃,7.5℃-15℃,7.5℃-7.5℃)设定下,以化学沉积的方法制备银型离子聚合物金属复合材料(Ag-IPMC),并对制得试样进行了表面形貌观察以及拉伸测试和动态位移测试。结果表明: 制备温度在7.5~25℃范围内,反应温度越低,离子交换膜表面所沉积的银粒子团聚颗粒越细小。在7.5℃-7.5℃下制备试样比25℃-25℃下制备的试样团聚颗粒直径小60%。电极厚度受温度影响较大,不同温度下内、外层电极厚度差异明显(内: ~130%,外: ~70%); 制备温度在15℃-15℃的试样具有最大的弹性模量(比最小值大28%)。在实验温度范围内,最大驱动变形量随着反应温度的降低而增大,其最大值比最小值大80%。     

高温接触热阻的有限元模拟方法

基于多点接触理论建立了高温接触热阻的计算模型及其有限元格式,模型的基本尺寸取自表面粗糙度测量时的特征尺寸,该模型能有效模拟界面粗糙度、界面压力、界面温度和间隙填料热导率等参数对界面接触热阻的影响,同时也考虑了高温环境下材料热导率随温度变化的特点以及通过界面间隙的辐射换热效应。在此基础上,针对影响接触热阻的若干主要参数进行了研究。数值算例表明:该文所建立的有限元计算模型及其计算方法不仅能有效地模拟不同条件下高温接触热阻的变化规律,也为进行高温接触热阻研究提供了一种新途径。     

Critical Assessment of the Integrity of Thin Organic Films by Shearography

The temperature versus thermal deformation (strain) with respect to time, of different coating films was studied by a non-destructive technique (NDT) known as shearography. The behavior of organic coatings, i.e., epoxy, a white enamel, or a yellow acrylic lacquer on carbon steels, was investigated at a temperature range simulating the severe weather temperatures in Kuwait especially 20–60 °C differences between the daylight and the night time temperatures. The investigation focused on determining the in-plane displacement of the coatings, which amounts to the thermal deformation that results from the applied temperature range. Furthermore, the investigation also focused on determining the thermal expansion coefficients of coatings and the slope of the plot of the thermal deformation versus the applied temperature range. A critical (steady state) value of the thermal expansion coefficients of coatings was used to determine the integrity of the coatings with respect to time. This value was determined from the decreasing, time-dependent value of the thermal expansion coefficients of coatings. In fact, determination of critical (steady state) value of the thermal expansion coefficients of coatings could be accomplished independent of parameters such as UV exposure, humidity, presence of chemical species, and other parameters normally considered in conventional methods of the assessing of the integrity of coatings. By using the technique of shearography, one needs only to determine the critical (steady state) value of the thermal expansion coefficients of coatings, regardless of the history of the coating, in order to assess the integrity of coatings. These results indicate that the technique is a very useful NDT method for determining the critical value of the thermal expansion coefficients of different coatings and can be used as a 2D-microscope for monitoring the deformation of the coatings in real-time at a submicroscopic scale.