Thermal conductivity of AZ91 magnesium integral foams measured by the Transient Plane Source method

The thermal conductivity of a collection of magnesium integral foams has been measured by using the Transient Plane Source (TPS) method. The results have shown a power-trend dependency with bulk density as the existing models predict. Additionally, micro-computed tomography (μCT) studies have been carried out on selected samples whose thermal conductivity values slightly deviate from the fitted curve to inspect the density distribution. Differences have been explained in terms of the local average density obtained by μCT in the volume covered by the heat flux. These results have revealed the high accuracy of the TPS method when it is combined with micro-tomographic techniques.     

平面絷原法测量均质固体的蓄热系数

本建立了平面热源法实验条件下的非稳态导热的数学模型,应用拉普拉斯变换和热四端网络法,求解了实验测量系统的解析模型和各种相应的简化模型,根据参数估计的基本原理,对模型中的待估计做了灵敏度分析。建立了实验测量条纹,可以满足20－200℃环境下固体材料蓄热系数测量的需要。测量了常温下几种材料的蓄热系数,测量结果与热线法实验结果相吻合。     

瞬态热丝法测量纳米流体的导热系数

推导了用双铂丝测量流体导热系数的瞬态热丝法的实验关联式,并据此设计实验装置,分析误差因素,给出了系统误差修正系数.用该装置测量了几种纳米流体的导热系数并作了相关分析.     

Determining Thermal Conductivity and Thermal Diffusivity of Low-Density Gases Using the Transient Short-Hot-Wire Method

The transient short-hot-wire method for measuring thermal conductivity and thermal diffusivity makes use of only one thermal-conductivity cell, and end effects are taken into account by numerical simulation. A search algorithm based on the Gauss–Newton nonlinear least-squares method is proposed to make the method applicable to high-diffusivity (i.e., low-density) gases. The procedure is tested using computer-generated data for hydrogen at atmospheric pressure and published experimental data for low-density argon gas. Convergence is excellent even for cases where the temperature rise versus the logarithm of time is far from linear. The determined values for thermal conductivity from experimental data are in good agreement with published values for argon, while the thermal diffusivity is about 10 % lower than the reference data. For the computer-generated data, the search algorithm can return both thermal conductivity and thermal diffusivity to within 0.02 % of the exact values. A one-dimensional version of the method may be used for analysis of low-density gas data produced by conventional transient hot-wire instruments.     

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.     

瞬态热线法导热系数测量实验数据处理方法的研究

为克服瞬态热线法导热系数测量中实验数据处理一般方法的弊端,将数值模拟引入实验数据处理过程,并通过比较理论计算曲线与实验曲线的符合程度来获得最终的实验结果。通过不同方法对实验数据处理结果的比较分析表明,所使用的方法可以更好地处理瞬态热线法导热系数测量数据,同时,与传统方法相比,采用较少的数据点即可得到正确的结果。研究结果不仅可以改进瞬态热线法导热系数实验数据的分析方法,而且对实验系统的设计与搭建也有借鉴意义。     

Improvements in the Dynamic Plane Source Method

Three sources of errors in the extended dynamic plane source (EDPS) method caused by the discrepancy between experiment and model are analyzed. The source effect is eliminated by introducing the nuisance parameter R ₀ and the surface effect by a surrounding vacuum. The original model assumes a constant heating power but a constant current is used in the experiment. Suppression of this effect leads to a new solution of the heat equation designated as the constant-current model. The measurements on polymethylmetacrylate (PMMA) in vacuum, evaluated by the constant-current model, provided results of λ = 0.191 W.m⁻¹.K⁻¹ and a = 0.118 ×10⁻⁶ m ².s ^{− 1}, which are in good agreement with published values. The total standard uncertainty was estimated as 1.5 % for both thermophysical properties.     

Correlations for the Thermal Conductivity of Metals as a Function of Temperature

The objective of this work was to find a suitable correlation that best fits the thermal conductivity of metals as a function of temperature. It was found that a multiple linear regression model of the form k=aT ^b e ^cT e ^d/T gives the smallest deviations from experimental data among various models. The coefficient of determination, R ², lies between 0.97 and unity, except for thorium (R ²=0.86). The average of the absolute relative error, AARE, in the predicted data is less than 4.75%, except for iron (about 11.7%), and the overall AARE for all data points is about 1.4%.     

磁性液体在均匀磁场中的瞬态双热线导热系数的测试

将封有聚α-烯烃合成油基磁性液体的两玻璃管放置于磁场中,为消除磁场力、重力所引起的磁性液体自然对流的影响,消除端部效应,研制了磁性液体在均匀磁场中瞬态双热线导热系数的实验测量系统,经与蒸馏水、乙醇标准样品的导热系数测量比较,实验装置有较高的测量精度。实验测量了不同方向的均匀磁场对不同体积浓度的磁性液体导热系数的影响。结果显示,当磁场方向与热通量方向一致时,磁场显著强化磁性液体的导热系数,其导热系数随磁场强度的增加而近似线性增加,且体积浓度越大增加量越大;当磁场方向与热通量方向垂直时,磁性液体的导热系数随磁场强度的变化不明显。     

Minimization of Measurement Errors of the Coefficients of Heat Conductivity and Thermal Diffusivity of Thermal Insulating Materials by the Plane Pulsed Heat Source Method

Measurement Techniques - Mathematical models have been developed for the calculation of measurement errors of the coefficients of the heat conductivity and thermal diffusivity of thermal insulating...     

Use of the Transient Plane Source Technique for Rapid Multiple Thermal Property Measurements

This paper describes work at NPL to evaluate the capability of the transient plane source (TPS) technique using various sensor sizes and different types of materials that include solids (Perspex, alumina, extruded polystyrene, agar gel, and ice) and liquids (water and silicone oil). The aim of the present work is to investigate use of the TPS technique on materials where probe size, contact, and internal specimen convection are potentially important issues. Following validation of the technique on the NPL solid reference materials, measurements were carried out on ice using TPS and the NPL guarded hot plate (GHP) to illustrate the probe-to-sample thermal contact resistance issue. Measurements on silicone oil were compared to GHP and the NPL transient hot wire (THW) technique where the probe size/short times are crucial. In addition, measurements on water and agar gel were made to illustrate the influence of natural convection. Although the TPS is a multi-property technique, the focus of this work was on thermal conductivity.     

Determination of Transient Thermal Interface Resistance Between Two Bonded Metal Bodies using the Laser-Flash Method

The paper presents the data reduction analysis for measurements of the transient thermal interface resistance between two bonded metal bodies using the laser-flash method. By using two different mathematical models, i.e., a two-layered and a three-layered model, whose complete analytical solutions for realistic conditions are provided, different results for final values and their uncertainties can be obtained. The analysis has been applied to experimental data measured from samples prepared with three different bonding materials, cyanoacrylate, metal epoxy resin, and silicone rubber.     

Measurement of Thermal Transport Properties with an Improved Transient Plane Source Technique

The transient plane source (TPS) technique has been revised with the aim of developing a simple and fast system to measure the thermal transport properties of materials at low temperatures, especially high-T _c superconductors. To ensure reliable results, any new system should be tested with known samples. Fused silica, 0.9% carbon steel (215/3), and halide crystals (silver chloride) were studied with the new setup to check its performance. Data were taken from room temperature down to liquid nitrogen temperature. The assembly was designed for cryogenic (79 to 300 K) measurements in an atmosphere free of humidity. Dry nitrogen gas was used as a heat transfer medium around the sample holder assembly. The measured values for thermal conductivity and thermal diffusivity of these samples are in excellent agreement with values reported earlier. The thermal conductivity and thermal diffusivity for silver chloride crystals are extended down to 80 K although recommended data were available only down to 220 K. A Ba-doped, Bi-based, high-T _c superconductor was prepared by a solid-state reaction method. The nominal composition used was Bi_1.6Pb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_y. Large-sized samples (diameter 28mm and length 11mm) are investigated for thermal transport properties.     

关于测试材料热物性热脉冲法的新探讨

建立起热脉冲法的新理论模型，分析了相应的测试技术问题。利用研制的微机测试系统 种材料的热扩散系数和导热系数进行了测定，得到了满意的结果，表明本文所提出的测试广阔进可信的。     

Application of the Three-Omega Method to Measurement of Thermal Conductivity and Thermal Diffusivity of Hydrogen Gas

Preliminary investigations have been conducted to discuss the possibility of measuring the thermal conductivity of hydrogen gas by the three-omega method. A one-dimensional analytical solution for the 3ω component is derived which includes the effect of the wire heat capacity. It is shown that it is very important to take into account the wire heat capacity in the calculation to measure the thermal conductivity of gas by the three-omega method. In contrast, the wire heat capacity is less important for the thermal conductivity of the liquid or solid phase. The importance of the wire heat capacity is found to increase with increasing frequency and decrease if the sample thermal conductivity is high. In order to measure the thermal conductivity of hydrogen gas at atmospheric pressure, a wire of diameter less than 1μm is necessary if the properties of the wire are to be neglected.     

瞬态热带恒功率法测量导热率

基于传统热桥法基本原理,提出了新的非对称结构模型--瞬态热带恒功率法用于测量材料的导热率。非对称结构模型将热源热带与温度传感热带电路彼此独立,消除了热带电阻自热以及环境温度变化带来的影响。通过短时间内快速测量恒流电路中两热带的电压差随时间的变化关系准确测量材料的导热率。依据此模型设计了热带加热片,建立了基于LabVIEW测试软件的实验装置自动测量平台,实验测量结果与参考材料导热率具有良好的一致性,相对偏差在3％左右,验证了该理论模型的有效性。     

热带法测量材料导热系数的实验研究

根据瞬态热带法的基本原理，建立了测量非导电固体材料以及松散材料导热系数的实验装置，采用热电偶直接测量热带的温度变化，整个测量系统更加方便实用、易于实现。对一批试样进行的测试结果表明，本测量装置具有较好的重复性和准确性，可应用于相关的科研部门和工业部门。     

Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.