Determination of Thermal Conductivity of Silicate Matrix for Applications in Effective Media Theory

Silicate materials have an irreplaceable role in the construction industry. They are mainly represented by cement-based- or lime-based materials, such as concrete, cement mortar, or lime plaster, and consist of three phases: the solid matrix and air and water present in the pores. Therefore, their effective thermal conductivity depends on thermal conductivities of the involved phases. Due to the time-consuming experimental determination of the effective thermal conductivity, its calculation by means of homogenization techniques presents a reasonable alternative. In the homogenization theory, both volumetric content and particular property of each phase need to be identified. For porous materials the most problematic part is to accurately identify thermal conductivity of the solid matrix. Due to the complex composition of silicate materials, the thermal conductivity of the matrix can be determined only approximately, based on the knowledge of thermal conductivities of its major compounds. In this paper, the thermal conductivity of silicate matrix is determined using the measurement of a sufficiently large set of experimental data. Cement pastes with different open porosities are prepared, dried, and their effective thermal conductivity is determined using a transient heat-pulse method. The thermal conductivity of the matrix is calculated by means of extrapolation of the effective thermal conductivity versus porosity functions to zero porosity. Its practical applicability is demonstrated by calculating the effective thermal conductivity of a three-phase silicate material and comparing it with experimental data.     

Two Effective Thermal Conductivity Models for Porous Media with Hollow Spherical Agglomerates

Based on the microstructure features of xonotlite-type micro-pore calcium silicate, two unit cell models, the point-contact hollow spherical model and the surface-contact hollow cubic model, are developed. As one of several excellent insulation materials, xonotlite is represented as porous media with hollow spherical agglomerates. By one-dimensional heat conduction analysis using theunit cell, the effective thermal conductivity of xonotlite is determined. The results show that both of the models are in agreement with experimental data. The algebraic expressions based on the unit cell models can be used to calculate the effective thermal conductivity of porous media that have similar structure features as xonotlite.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

Determination of Moisture Diffusivity as a Function of Both Moisture and Temperature

The effect of moisture and temperature on liquid water transport in porous media was studied. Specimens of autoclaved aerated concrete were subjected to one-sided water penetration in isothermal conditions at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. After specified time intervals, moisture profiles were determined gravimetrically. The moisture diffusivity was calculated for a particular temperature as a function of moisture content, using an inverse analysis. The results demonstrate the dependence of the moisture diffusivity on the moisture content and the temperature of the samples. The moisture diffusivity for high moisture content can be as much as one order of magnitude greater than for the lowest moisture content studied. The moisture diffusivity was found to increase by as much as a factor of two when the temperature is increased from 20 °C to 80 °C.     

Effect of Moisture Content on Thermal Properties of Porous Building Materials

The thermal conductivity and specific heat capacity of characteristic types of porous building materials are determined in the whole range of moisture content from dry to fully water-saturated state. A transient pulse technique is used in the experiments, in order to avoid the influence of moisture transport on measured data. The investigated specimens include cement composites, ceramics, plasters, and thermal insulation boards. The effect of moisture-induced changes in thermal conductivity and specific heat capacity on the energy performance of selected building envelopes containing the studied materials is then analyzed using computational modeling of coupled heat and moisture transport. The results show an increased moisture content as a substantial negative factor affecting both thermal properties of materials and energy balance of envelopes, which underlines the necessity to use moisture-dependent thermal parameters of building materials in energy-related calculations.     

Modeling of Effective Thermal Conductivity of Dunite Rocks as a Function of Temperature

The thermal conductivity, thermal diffusivity, and heat capacity per unit volume of dunite rocks taken from Chillas near Gilgit, Pakistan, have been measured simultaneously using the transient plane source technique. The temperature dependence of the thermal transport properties was studied in the temperature range from 303 K to 483 K. Different relations for the estimation of the thermal conductivity are applied. A proposed model for the prediction of the thermal conductivity as a function of temperature is also given. It is observed that the values of the effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 9%.     

Nonlinear Effect of Moisture Content on Effective Thermal Conductivity of Building Materials with Different Pore Size Distributions

Understanding the quantitative relationship between the effective thermal conductivity and the moisture content of a material is required to accurately calculate the envelope heat and mass transfer and, subsequently, the building energy consumption. We experimentally analyzed the pore size distributions and porosities of common building materials and the influence of the moisture content on the effective thermal conductivity of building materials. We determined the quantitative relationship between the effective thermal conductivity and moisture content of building materials. The results showed that a larger porosity led to a more significant effect of the moisture content on the effective thermal conductivity. When the volumetric moisture content reached 10 %, the thermal conductivities of foam concrete and aerated concrete increased by approximately 200 % and 100 %, respectively. The effective thermal conductivity increased rapidly in the low moisture content range and increased slowly in the high moisture content range. The effective thermal conductivity is related to the moisture content of the materials through an approximate power function. As the moisture content in the walls of a new building stabilizes, the effective thermal conductivity of normal concrete varies only slightly, whereas that of aerated concrete varies more significantly. The effective thermal conductivity of the material is proportional to the relative humidity of the environment. This trend is most noticeable when the wall material is aerated concrete.     

Experimental Analysis of Thermal Conductivity for Building Materials Depending on Moisture Content

The presence of moisture in building envelopes can have many causes and may lead to deterioration of useful thermophysical characteristics of the materials, to weakening of the building structure, and to facilitating growth of mold. The International Standard ISO 13788 establishes a calculation procedure for the determination of hygrometric characteristics of building components and materials, assuming that the influence of moisture content on the thermal field across walls, ceilings, and roofs may be neglected. However, condensed water increases the effective thermal conductivity of building materials, thus modifying the temperature profiles across the building envelope. This effect is analogous to the one due to the material aging. In this paper, the authors show the results of effective thermal-conductivity measurements in some commonly adopted building materials as a function of moisture content, in order to assess the potential significance of interstitial condensation on thermal losses and to verify if the maximum allowed moisture content reported is useful to prevent the decay of the thermal properties of building materials.     

渐进均匀化理论研究复合材料有效力学性能

本文利用渐进均匀化理论,详细叙述了求解具有周期性细观单元结构的复合材料有效力学性能的方法,并结合有限元理论和周期性边界条件,给出了求解渐进均匀化方程的过程,为实际工程问题提供了预测材料力学性能的方法.     

新型多孔介质局部含湿量测试仪的研究

采用针型电容测湿传感器和单片微处理机技术研制成一种新型多孔介质局部含湿量测试仪。本文论述该测试仪的组成、工作原理和硬、软件结构及其特点。实验结果表明，可望满足多孔介质局部含湿量测量的实用要求。     

浅谈工勘土工实验室含水率测定的方法及选择

土的含水率是土的三大物理性质指标(密度、比重、含水率)之一,结合土的密度和比重可以导算出土的干密度、饱和度、孔隙比、孔隙度等一系列指标,是土工试验中重要的实测指标之一。含水率的变化会使土的一系列物理、力学性质随之改变,影响到土饱和度的判定以及土的压缩性及稳定性的变化。因此,土的含水率不仅是土工试验检测报告不可缺少的一项指标,也是工程勘察报告的重要依据,是工程建设、设计的重要参数,它被检测的准确度直接对工程总体的质量产生重大影响。因此选择合适的含水率测定方法,确保含水率的准确性至关重要。     

浅谈烘干法水分测定仪检定中的实际操作

烘干法水分测定仪的主要操作及注意事项。     

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%.     

Effective Thermal Properties of Multilayered Systems with Interface Thermal Resistance in a Hyperbolic Heat Transfer Model

One-dimensional thermal wave transport in multilayered systems with an interface thermal resistance is studied under the framework of the Cattaneo–Vernotte hyperbolic heat conduction model, considering modulated heat excitation under Dirichlet and Neumann boundary conditions. For a single semi-infinite layer, analytical formulas useful in the measurement of its thermal relaxation time as well as additional thermal properties are presented. For a composite-layered system, in the thermally thin regime, with the Dirichlet boundary condition, the well known effective thermal resistance formula is obtained, while for the Neumann problem, only the heat capacity identity is found. In contrast, in the thermally thick case, an analytical expression for both Dirichlet and Neumann conditions is obtained for the effective thermal diffusivity of the whole system in terms of the thermal properties of the individual layers and their interface thermal resistance. The limits of applicability of this equation, in the thermally thick regime, are shown to provide useful and simple results in the characterization of layered systems and that they can be reduced to the results obtained using the Fourier approach. The role of the thermal relaxation time, the interface thermal resistance, and the implications of these results in the possibility of enhancement in heat transport are discussed.     

Application of Contact Theory to Evaluation of Elastic Properties of Low Consolidated Porous Media

A new approach to the calculation of the elastic bulk modulus of low consolidated porous media is developed based on a physical consolidation model of rocks and the classical Hertz contact theory. The derived analytical relationships for the elastic bulk modulus, which take into account some micro-structural characteristics of packing, are compared with theoretical predictions from various micromechanics theories, Hashin-Shtrikman strict bounds as well as with experimental results available for low consolidated granular materials. The latter comparison demonstrates a good agreement.     

一种新型隔热涂料的制备及性能研究

以苯丙乳液为成膜剂,硅烷偶联剂处理后的中空玻璃微珠为功能隔热填料合成新型的隔热涂料。结果表明,用硅烷偶联剂预处理中空玻璃微珠后涂料附着力能提高1个级别;涂层导热系数随中空玻璃微珠粒径的增加呈现先减小后增大的趋势,当中空玻璃微珠的粒径为58μm左右时导热系数比其它粒径制得涂料的导热系数降低6%～28%;随涂膜厚度的增加,隔热效果增强,但当涂膜厚度增大到0.3mm时,继续增加涂膜厚度,隔热效果几乎不再增强;当中空玻璃微珠含量小于10%时,热反射率随中空玻璃微珠在隔热涂料中含量的增加显著增加,超过10%后,热反射率增加速率减缓,当达到12%时,涂料的热反射率值最高,继续增加玻璃微珠含量,热反射率几乎不增加。     

基于分形理论的开孔聚氨酯泡沫等效导热系数研究

开孔聚氨酯泡沫保温材料是一种典型的多孔介质。采用分形理论描述开孔聚氨酯泡沫材料的微尺度空间结构,建立了简化单元体模型,提出了计算其有效导热系数的分形模型,并导出了气相和固相热传导计算公式、热辐射等效导热系数计算公式、材料总有效导热系数计算公式。模型计算值与实验测量值比较具有较好的一致性,同时总结了多孔介质材料绝热性能的主要影响因素。该分析方法对新型绝热材料的研制和绝热性能的提高具有实用价值。     

Simulative Determination of Effective Mechanical Properties for Digitally Generated Foam Geometries

Metal foams constitute a promising and emerging material class in the context of lightweight construction. There exists a variety of different foam topologies, on which resulting mechanical properties depend. To maximize the potential of foams in material use under mechanical load, the present work addresses the question how different geometrical parameters influence the material behaviour. Therefore, an algorithm for digital generation and design of open pore foam structures is presented, that allows to regulate the geometry precisely. A method for retrieving effective mechanical properties from numerical simulations of compression tests in the elastic regime is introduced. Additionally, the representativeness of foam volumes considered for simulations is investigated. This yields a fully digital workflow, which enables the investigation of geometry influence on mechanical properties. This approach is used to conduct simulation studies on generated foam structures with a systematic variation of geometrical parameters. Herein, a range of effective Young's moduli varying by up to a factor of 1.3 for different foam structures at the same porosity is found. This shows a significant impact of the foam geometry on the elastic properties of metal foams. The presented methodology yields insights, which can guide design and optimization of materials for specific applications.     

Numerical Investigation of Thermal and Thermo-mechanical Effective Properties for Short Fibre Reinforced Composite

This study aims to investigate the thermal conductivity and the linear coefficient of thermal expansion for short fibre reinforced composites. The study combines numerical and statistical analyses in order to primarily examine the representative size and the effective properties of the volume element. Effects of various micromechanical parameters, such as fibre’s aspect ratio and fibre’s orientation, on the minimum representative size are discussed. The numerically acquired effective properties, obtained for the representative size, are presented and compared with analytical models.     

稀土复合氧化物陶瓷热导率理论及其热物性能

综述了稀土复合氧化物陶瓷热导率理论的发展情况和热物性能方面的研究现状,介绍了材料热导率的影响因素,分析了材料微结构对热导率的影响,总结了具有A2B2O7通式的烧绿石型和萤石型稀土复合氧化物的热物性能,以及掺杂对其热导率和热膨胀系数的影响.