瞬态热线法测量复合材料导热系数的方法

通过比较导热系数测试方法,结合非平面式薄片状复合材料试样结构特点,确定采用瞬态热线法测量复合材料的纵向和横向导热系数。简要介绍瞬态热线法测量原理,探讨复合材料试样与传感器的放置方式、试样外表面粗糙状况、测试重复性对导热系数测量结果的影响,得到复合材料纵向和横向导热系数。     

浅谈应用材料导热系数的测定方法

随着“双碳”目标的提出，应用材料的导热性能是评定该材料是否符合节能理念的重要指标之一。导热系数的测量则是对材料导热性能的重要衡定，如何选择合适的测量方法得到准确的导热系数非常关键。本文结合以往经验，对防护热板法、热流计法、圆管法、热脉冲法、热线法、激光闪射法以及瞬态平面热源法的测试原理、仪器设备、影响因素以及测试改进措施进行了综述，为应用材料的测定方法选择提供参考。对特定的材料在合理的适用范围内选择何种测试方法进行了展望。     

碳酸二乙酯液相导热系数实验研究

为了给可替代清洁燃料的研究提供急需的基础物性数据,利用瞬态单热线法对温度区间为250～390K、压力区间为0.1～30MPa的碳酸二乙酯（DEC）的导热系数进行了实验研究。将实验数据拟合成关于温度和压力的导热系数方程,并分析了碳酸二乙酯导热系数与温度和压力的关系,方程采用完全三次多项式形式。结果表明,碳酸二乙酯的导热系数随温度升高而减小,随压力的升高而增大。实验数据与拟合方程计算值的最大偏差为-1.94%,绝对平均偏差为0.84%。另外,文中对改进的瞬态单热线测量导热系数实验装置以及所采用的测量系统进行了说明。     

氧化硅纳米流体的导热性能研究

采用瞬态热线法测量SiO2-水和SiO2-乙二醇两种纳米流体在不同体积分数和温度下的导热系数,分析研究纳米SiO2体积含量、温度及悬浮稳定性对纳米流体导热系数的影响。研究结果表明,纳米流体的导热系数显著高于基体液体的导热系数,且随着粒子体积分数的增大和温度的升高均有所提高;对于悬浮稳定性越好的纳米流体,它的导热系数增加量越大。     

测定材料高温导热系数的非稳态热线法

本文介绍材料高温导热系数的新型测定方法——非稳态热线法的原理,并研究使用国产仪器实现这种方法。结果表明,与国外仪器测定结果比较,符合良好。自行设计的加热炉设备为全耐火纤维炉衬、升降式结构、具有快速升温优良性能的节能电炉。用Mittenbuhler热线方法测定了四种低导热材料(聚胺酯硬质泡沫塑料、加气混凝土、轻质耐火浇注料以及轻质粘土砖),并且探索了测定高导热材料的平行热线法,测定了镁砖的导热系数,结果是令人满意的。     

甲基叔丁基醚饱和液相导热系数的实验研究

利用瞬态双热线法测量了243—403 K温度范围内饱和液相甲基叔丁基醚的导热系数,并将实验数据拟合为温度的关联式。实验数据与导热系数关联式计算结果的标准偏差和最大偏差分别为0.30%和0.89%,导热系数的合成标准不确定度小于±1.0%。甲基叔丁基醚导热系数的实验研究为正在进行的甲基叔丁基醚替代物筛选提供急需的基础热物性数据,对改进汽油、柴油质量,提高油品的环境友好性及推广使用清洁燃料方面有重要价值。     

用并联模型法测定泡沫塑料的导热系数

稳态热流计法操作简单,广泛用于测量保温材料的导热系数。但由于仪器设备和方法的限制,难以测试异型材料的导热系数。分别采用并联模型法和直接测量法分析了挤塑聚苯乙烯泡沫、硬质聚氨酯泡沫、橡塑泡沫在平均温度10、15、25℃下的导热系数。结果表明:在不同平均温度和不同材料中,采用并联模型法所计算的导热系数与测试结果误差较小,说明并联模型法可用于不规则泡沫塑料导热系数的测试。     

基于激光热成像技术的耐火材料导热系数测试新方法

为了满足耐火材料对导热系数测试快速、准确的需求,提出了基于激光加热耐火材料、结合红外热成像技术的材料导热系数测试新方法——激光热成像法,并与现有国家标准的闪光法测试导热系数常用的耐驰公司激光导热仪FLASHLINE-500进行数据对比,探讨了两种不同方法测试不同成分耐火材料导热系数的差异。结果表明:相比闪光法,采用激光热效应结合红外热成像技术的新测试方法能快速、准确地测量出不同耐火材料的导热系数;对材料的导热情况显示直观、明确,测试方法操作简单,抗干扰能力强,大大降低了测试的成本投入。     

串联模型法测量泡沫塑料导热系数

采用串联模型热流计法,即通过叠加试样间接测量的方式,对硬质聚氨酯泡沫塑料、橡塑泡沫塑料及挤塑聚苯乙烯泡沫塑料在平均温度10℃、15℃、25℃下的导热系数进行了测试,并分析比较了导热系数测试值与模型计算值。结果表明:在不同平均温度及材料条件下,导热系数的串联模型计算值与测试值的误差均较小,因此该串联模型可用于薄层材料导热系数的测试。     

串联模型热流计法测薄层泡沫塑料导热系数

稳态热流计法因方法简单,广泛应用于测量保温材料的导热系数。但因仪器设备和方法的限制,难以测试薄层材料的导热系数。本文分别以聚苯乙烯泡沫和硬质聚氨酯泡沫为研究对象,首先在不同平均温度下测试不同厚度聚苯乙烯材料的导热系数,再利用串联模型法推导计算多层材料的导热系数。为了验证串联模型的可靠性和可重复性,按模型对聚苯乙烯泡沫及聚氨酯泡沫试样重复测试,并对测试结果和计算结果进行比较,结果表明,利用串联模型法计算所得的导热系数值与实测值误差小,重复性好。     

Measurements of the thermal conductivity and thermal diffusivity of polymers

In this paper, the thermal conductivity and thermal diffusivity of nine polymers were measured by using the transient short‐hot‐wire method. The corresponding specific heat was measured with a commercial Differential Scanning Calorimeter (DSC). The effects of temperature on the thermal conductivity, thermal diffusivity, and the product of density and specific heat are further discussed. The results show that the transient short‐hot‐wire method can be used to measure the thermal conductivity, thermal diffusivity, and the product of density and specific heat of polymers within uncertainties of 3%, 6%, and 9%, respectively.     

A method for measuring the thermal conductivity of isotropically compressed powders

This paper proposes a method for measuring the thermal conductivity of compressible powders, in particular of powders which do not permit a calculation of sphere models. The apparatus, which is briefly duscussed, is designed to compress powders hydrostatically up to 600 bars while measuring the thermal conductivity with a hot wire lance. With this range of compression it is possible to determine the thermal conductivity for almost every plastic powder, non-compressed powders as well as quasi-solid materials. Experimental results are given for polytetrafluorethylene (PTFE) and polyethylene (PE) powders. The effect of crystallization on thermodynamic properties of polyethylene under high pressure is discussed.     

Thermal properties of hydrating calcium aluminate cement pastes

As the hydration of calcium aluminate cements (CAC) is highly temperature dependent, yielding morphologically and structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. Thermal diffusivity and thermal conductivity during CAC hydration was investigated by a transient method with a numerical approach and a transient hot wire method, respectively. For hydration at 15 °C (formation of mainly CAH₁₀), thermal diffusivity shows a linear decrease as a function of hydration degree, while for hydration at 30 °C there is a linear increase of thermal diffusivity. Converted materials exhibited the highest values of thermal diffusivities. The results on sealed converted material indicated that thermal conductivity increased with an increase in temperature (20-80 °C), while thermal diffusivities marginally decreased with temperature. The Hashin-Shtrikman boundary conditions and a simple law of mixtures were successfully applied for estimating thermal conductivity and heat capacity, respectively, of fresh cement pastes.     

Hot‐wire parallel technique: A new method for simultaneous determination of thermal properties of polymers

The hot‐wire parallel technique standardized for determining the thermal conductivity of ceramic materials was employed in the determination of the thermal properties of polymers. For these materials, additional care must be taken considering the low melting point of polymers, when compared with that for ceramic materials. Samples can be prepared either in the shape of bricks or in the shape of half‐cylinders. The thermal conductivity and the specific heat were simultaneously determined from the same experimental thermal transient, and the thermal diffusivity is derived from these properties. Five different polymers with different structures at room temperature were selected, and measurements were carried out from room temperature to approximately the maximum service operating temperature. A nonlinear least‐squares fitting method was employed in the calculations, so that all the experimental points obtained are considered in the thermal properties' calculations. The apparatus used in this work is fully automatic. The reproducibility is very good with respect to the thermal conductivity, even with a defective experimental arrangement with respect to the theoretical model. However, deviations from the theoretical model have a severe influence on the specific heat values and, consequently, on the thermal diffusivity. Experimental results were compared with those available in the literature, showing the applicability of this technique for the determination of thermal properties of polymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1779–1786, 2002     

Transient hot-strip (THS) method for measuring thermal conductivity of thermally insulating materials

The fast and precise transient hot-strip (THS) method is well suited for thermal conductivity measurements on solid materials. The THS method may, however, give large experimental errors when applied to thermally insulating materials of low heat capacity per unit volume. Models to deal with those potential error sources and some indications about the precautions to be taken in order to minimize them are described in the present work. Measurements of thermal conductivity of a styrofoam insulating material (thermal conductivity 0.036 W m^?1K^?1, density 25.4 kg m^?3) was performed to verify the models. The result obtained is in good agreement with the standard hot plate method, indicating that the THS method is also well suited for thermal conductivity measurements of thermal insulators.     

Numerical simulation of the effect of moisture on the thermal conductivity of porous ceramic materials

A simulation model to analyze the influence of moisture content on the thermal conductivity of porous ceramic materials is developed based on the numerical integration of the energy equation. The experimental technique employed for thermal conductivity measurements is the hot wire parallel technique. The numerical model proposed is checked by evaluating the thermal conductivity of a hypothetical porous ceramic material containing different concentrations of water in its structure. The behaviour of the thermal conductivity as a function of temperature, as predicted by the model proposed in this work was experimentally verified by the authors for an unfired refractory concrete. ©     

Use of the hot wire method for measuring the thermal conductivity of lightweight,fiber, and powder refractory materials

Conclusions The possibility is shown of determining the thermal conductivity by the hot wire method in the 20–1400°C range of thermal insulation materials in the form of lightweight and fiber parts and powders. The range of measured values is 0.05<<2 W/(m·K). The reproducibility of the results, the measurement of standard materials, and a comparison with results obtained by other methods are an indication of the quite high accuracy and reliability of the hot wire method, which makes it possible to recommend it as a standard.Translated from Ogneupory. No. 4, pp. 49–53, April, 1984.     

Fibrous heat-insulation materials use of the hot wire method to determine thermal conductivity of fibrous heat-insulation materials

Conclusions It is desirable to determine the thermal conductivity of materials in the form of plates 30–40 mm thick or of a layer of wool of the same thickness by the hot wire method throughout their service temperature range, beginning at room temperature.Translated from Ogneupory, No. 1, pp. 21–23, January, 1980.     

Enhancement of Thermal Conductivity with CuO for Nanofluids

The enhancement of the thermal conductivity of ethylene glycol in the presence of copper oxide (CuO) is investigated. CuO nanofluids are prepared in a two‐step method. No surfactant is employed as a dispersant. The volume fraction of CuO nanoparticles suspended in ethylene glycol liquid is below 5 vol.‐%. The crystalline phases of the CuO powders are measured with x‐ray diffraction patterns (XRD). CuO nanoparticles are examined using scanning electron microscopy (SEM) to determine their microstructure. The thermal conductivities of the CuO suspensions are measured by a modified transient hot wire method. The viscosity was measured with a viscosity instrument. The results show that CuO nanofluids with low concentrations of nanoparticles have considerably higher thermal conductivities than the identical ethylene glycol base liquids without solid nanoparticles. The thermal conductivity ratio improvement for CuO nanofluids is approximately linear with the volume fraction of nanoparticles. For CuO nanoparticles at a volume fraction of 0.05 (5 vol‐.%) thermal conductivity was enhanced by up to 22.4 %. CuO nanofluids thus have good potential for effective heat transfer applications.     

Influence of Coarse Aggregate Content on the Thermal Conductivity of Alumina–Spinel Castables

The thermal conductivity of three commercial-quality alumina-spinel castables containing 90–96 wt% A1₂O₃ was measured in the range room temperature to 1000°C by a hot wire method. Experimental results indicate that the thermal conductivity increases with an increase in coarse aggregate content; higher corundum content and purer alumina aggregates are more important than coarse aggregate content for higher thermal conductivity of alumina-spinel castables.