Study on a Thermal-diffusivity Standard for Laser Flash Method Measurements

The National Metrology Institute of Japan (NMIJ) of AIST has been studying the laser flash method in order to establish an SI traceable thermal- diffusivity standard. Key technologies have been developed to reduce the uncertainty in laser flash measurements. In the present study, an uncertainty evaluation has been carried out on the laser flash measurement method in order to determine the thermal diffusivity value of IG-110, a grade of isotropic high-density graphite, as a candidate reference material. The thermal diffusivity measured by the laser flash method is derived from a specimen thickness and a heat diffusion time. And a laser flash measurement is carried out at a given temperature. The measurement system is composed of three sections corresponding to each measured quantity: length, time, and temperature. Therefore, we checked and calibrated our measurement system, and estimated the uncertainty of measurement results for the case of a grade of isotropic graphite.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

基于激光闪光法的立式热扩散率测量装置研究

介绍了基于激光闪光法的立式热扩散率测量装置,利用脉冲激光对样品进行均匀加热,使样品内部产生一维热流,并通过红外探测器测量样品温升信号,采用立式真空加热炉控制测量温度环境,实现室温至1600℃的热扩散率测量。用该装置测量厚度为1. 1 mm,直径为10 mm的不锈钢样品,测量结果与PTB参考数据的偏差小于1%。     

Cu2S相变过程中热扩散系数的精确测量和解析

材料发生相变时, 其结构和物理性能可能会发生剧烈的变化。采用激光闪射法测量热扩散系数时, 激光照射样品可能会伴随有光吸收/发射现象以及温度的显著升高, 导致其测量值偏离真实值。本工作以Cu₂S为研究对象, 发现激光照射样品后, 光吸收/发射的影响很小可以忽略, 但样品温度的升高则会明显影响热扩散系数的测量。通过构建具有不同石墨层厚度的石墨/Cu₂S双层结构, 利用石墨层减弱激光照射时Cu₂S样品的温度增加幅度, 成功使热扩散系数出现显著降低的起始温度接近采用DSC测量材料发生相变的起始温度。本研究进一步建立了石墨/Cu₂S双层结构样品的热流输运模型, 从石墨/Cu₂S双层结构样品的实验测试热扩散系数中解析出了Cu₂S在相变区间的本征热扩散系数。本工作对于理解和精确表征具有相变特征的离子导体热电材料、光敏、热敏材料的热扩散系数具有重要的意义。     

Experimental Verification to Obtain Intrinsic Thermal Diffusivity by Laser-Flash Method

There is a need to obtain highly reliable values of thermophysical properties. The thermal conductivity of solids is often calculated from the thermal diffusivity, specific heat, and density, respectively, measured by the laser-flash method, differential scanning calorimetry, and Archimedes’ method. The laser-flash method is one of the most well-known methods for measuring the thermal diffusivity of solids above room temperature. This method is very convenient to measure the thermal diffusivity without contact in a short time. On the other hand, it is considered as an absolute reference measurement method, in particular, because only measurements of basic quantities such as time, temperature, length, and electrical quantities are required, and because the uncertainty of measurement can be analytically evaluated. However, it could be difficult in some cases to obtain reliable thermal-diffusivity values. The measurement results can indeed depend on experimental conditions; in particular, the pulse heating energy. A procedure to obtain the intrinsic thermal-diffusivity value was proposed by National Metrology Institute of Japan (NMIJ). Here, “intrinsic” means unique for the material, independent of measurement conditions. In this method, apparent thermal-diffusivity values are first measured by changing the pulse heating energy at the same test temperature. Then, the intrinsic thermal diffusivity is determined by extrapolating these apparent thermal diffusivities to a zero energy pulse. In order to verify and examine the applicability of the procedure for intrinsic thermal-diffusivity measurements, we have measured the thermal diffusivity of some materials (metals, ceramics) using the laser-flash method with this extrapolation procedure. NMIJ and Laboratoire National de Metrologie et d’essais (LNE) have laser-flash thermal-diffusivity measurement systems that are traceable to SI units. The thermal diffusivity measured by NMIJ and LNE on four materials shows good agreement, although they used different measurement systems and different analysis methods of the temperature-rise curve. Experimental verification on the procedure was carried out using the measured results. Some problems and considered solutions for laser-flash thermal-diffusivity measurements are discussed.     

激光闪光法测量固体材料热扩散率的研究进展

基于瞬态原理的激光闪光法因其具有所用试样小、测试周期短等优点，在测量固体材料的热扩散率方面发挥了重要作用，应用较为广泛。根据近些年来在激光闪光法可测材料种类、关键技术问题以及优化和改进3个方面的研究进展，介绍了其应用情况，并分析总结了其研究重点、难点以及研究价值，最后讨论了激光闪光法存在的挑战和前景，为未来激光闪光法在更多领域的应用提供参考。     

International Comparison on Thermal-Diffusivity Measurements for Iron and Isotropic Graphite Using the Laser Flash Method in CCT-WG9

The first international pilot study of thermal-diffusivity measurements using the laser flash (LF) method was organized by the working group 9 (WG9) of the Consultative Committee for Thermometry (CCT) of the Bureau International des Poids et Mesures (BIPM). Four National Metrology Institutes (NMIs) participated in this comparison. Thermal-diffusivity measurements on the Armco iron and the isotropic graphite IG-110 were carried out from room temperature to about 1200 K. The sample sets consist of five disk-shaped specimens of 10 mm in diameter and (1.0, 1.4, 2.0, 2.8, and 4.0) mm in thickness, each cut from the same block of material. These sample sets were specifically prepared for the comparison and sent to the participants. In the pilot comparison, the thermal diffusivity of each sample was estimated using the LF method with a specific extrapolating procedure. This procedure has the advantage of determining the inherent thermal diffusivity of the material. The extrapolated value in a plot of measured apparent thermal-diffusivity values versus the amplitude of the output signal corresponding to the temperature rise during each measurement is defined as the inherent thermal diffusivity. The overall results showed good agreement between independent laboratories, measurement equipment, and specimen thicknesses. The thermal diffusivities of the materials were determined using our measured results. A quantitative evaluation of the variability of the data obtained by the participants has been done, by evaluating the deviations from the reference value, the Z-value, and the En-number. Some data showed a large deviation from the reference value. It was concluded that these are caused by an insufficient time response of the measurement equipment and some difficulties with changing the pulsed heating energy. The effect of the thermal expansion on the thermal diffusivity was checked. It was found that the thermal-expansion effect was very small and negligible in this case.     

Thermal Diffusivity Measurements of Refractory Metals as Candidate Reference Materials by the Laser Flash Method

A working group for standardization has organized to establish the Japanese Industrial Standard (JIS) for thermal diffusivity measurements of metals in the temperature range of 300–1700 K by the laser flash method. As candidate reference materials with high purity, high-temperature stability, and easy-to-get on a commercial basis, tantalum, niobium, and molybdenum have been selected. Thermal diffusivity values of the specimens, cut out of these materials, have been measured independently by members of the working group. Comparisons of results have been performed for different high-temperature stabilities, repeatabilities, and manufacturers, as well as by different members. Comparisons show that the measured values agree within 10% for different specimens by different institutions, and no systematic differences have been observed for materials from different manufacturers. The measured results for molybdenum specimens agree well with the recommended values of thermophysical properties of matter from the TPRC data series, and the high-temperature stability is found to be the best. The results for tantalum and niobium, however, show significant differences with those of the TPRC data series in the high-temperature range, and some further study on the stability of these materials is needed for recommending these values. As a result, molybdenum can be recommended as a reference material for practical use of the laser flash method. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 m thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.     

Calorimetric and transport properties of Zircalloy 2, Zircalloy 4, and Inconel 625

This paper presents the measurements and the results on thermal and electrical transport properties of three nuclear reactor cladding materials: Zircalloy 2, Zircalloy 4, and Inconel 625. Study of these materials constituted a part of the IAEA coordinated research program aimed at the generation and establishment of a reliable and complete database of the thermal properties of reactor materials. Measured properties include thermal diffusivity, specific heat, and electrical resistivity. Thermal diffusivity was measured by the laser pulse technique. Specific heat and electrical resistivity were measured using a millisecond-resolution direct electrical pulse heating technique. Thermal conductivity was computed from the experimentally determined thermal difusivity and specific heat functions and the room temperature density values. Measurements were performed in the 20 to 1500°C temperature range, depending on the material and property concerned.     

Measurement of Thermal Diffusivity of Thermal Control Coatings by the Flash Method Using Two-Layer Composite Sample1

The thermal diffusivity of brittle coatings cannot be measured by the flash method directly because of the difficulty of preparing free-standing samples. Adopting the flash method using a two-layer composite sample, it is possible to measure thermal diffusivity if the radiant pulse is well defined and good thermal contact on the interface of the composite sample can be ensured. Using an equilateral trapezoidal pulse of an Nd-glass laser measuring the dimensionless temperature history of the rear face of the sample, we determined the thermal diffusivity of thermal control coatings in the temperature range of 80 to 200°C. The results for different thicknesses of substrate showed that the thermal contact resistance of the interface can be neglected.     

Estimation of thermophysical properties of a film coated on a substrate using pulsed transient analysis

The thermophysical properties (thermal diffusivity, effusivity) of a film coated on a substrate have been measured by a pulsed transient analysis. The experimental approach is to utilize the film surface temperature decay following a heating pulse from a Q-switched Nd:glass laser. The temperature decay was measured using a HgCdTe infrared detector. Following the collection of data, a nonlinear least-squares regression was performed to estimate the optimal values of three separate thermal parameters by fitting the data to the semiinfinite substrate model solution. The model was checked systematically by analysis of the sensitivity and correlation of the three parameters, and the thermal diffusivity and effusivity ratio of the film and substrate were obtained from the optimal values of the estimated parameters.     

Thermophysical properties of MPG-6 graphite

The thermal diffusivity and heat capacity of four MPG-6 graphite samples (density from 1664 up to 1825 kg/m³) are measured within the temperature range from 293 K up to 1650 K by the following methods: the laser flash, the differential scanning calorimetry, and the adiabatic calorimeter of linear heating. The uncertainties of the data on the thermal diffusivity, heat capacity, and density were (2–4)%, (3–5)%, and 0.5%, respectively. On the basis of the measurement results, the temperature dependence of the MPG-6 thermal conductivity is calculated and a generalizing dependence is obtained which allows one to estimate the thermal conductivity of graphite of various porosity for a wide temperature range using only the data on the macroscopic density of the samples. Reference data tables have been developed for the thermal conductivity of MPG-6 graphite of various densities.     

Thermal Evaluation of Scorched Graphite-Epoxy Panels by Infrared Scanning

A simple measurement system is described for evaluating damage to graphite-epoxy panels, such as those used in high-performance aircraft. The system uses a heating laser and infrared imaging system to measure thermal performance. Thermal conductivity or diffusivity is a sensitive indicator of damage in materials, allowing this thermal measurement to show various degrees of damage in graphite-epoxy composites. Our measurements track well with heat-flux damage to graphite epoxy panels. This measurement system, including analysis software, could easily be used in the field, such as on the deck of an aircraft carrier or at remote air strips.     

激光脉冲法研究半透明PI/SiO_2薄膜的热扩散率

激光脉冲法在应用于测量对实验探测波段红外线半透明的材料热扩散率时遇到了困难。文中提出了新的解决办法:即通过对理论探测曲线进行分析,并通过在实际探测曲线上的升温幅度和特征点计算得到了热扩散率,成功地解决了这一难题。在-73℃~290℃的范围内获得了对激光和红外线都是半透明的聚酰亚胺(PI)薄膜的热扩散率,并研究了室温下PI/SiO2复合材料的热扩散率随SiO2含量的变化规律。     

计量专业实验室环境振源分析及处理方案

为保证精密仪器的测量精度,阐述了研究环境振动对测量带来影响的问题的必要性。对地面振源的特征进行了分类总结,根据实验室仪器和设备的允许振动要求,从实验室防震与环境振源两个方面详细分析了实验室的总体设计中应注意的问题,提出实验室无防振措施情况下的防振距离设计方案,以及实验楼和实验室的消极隔震措施和积极隔振措施设计方案,该总结和处理方案可供计量专业实验室设计与建设领域人员参考与借鉴。     

Thermal Diffusivity Measurement of High-Conductivity Materials by Dynamic Grating Radiometry

A new apparatus based on dynamic grating radiometry (DGR) to measure the thermal diffusivity of high-conductivity materials such as graphite and diamond has been developed. In the DGR method, a sample surface is heated by interference of two pulsed laser beams, and the decay of temperature at a spot on the thermal grating is monitored by an infrared detector. In the ideal case where the grating period is much smaller than the light absorption length, the thermal diffusivity parallel to the surface can be determined from the decay constant and the grating period. This paper describes a procedure to extract the thermal diffusivity parallel to the plane while eliminating the effect of anisotropy and gives results for a preliminary measurement using Zr foil. A quadratic dependence of the time constant on fringe space has been observed in the fringe space change. Data are also presented for a 0.1-mm-thick graphite sheet. The results indicate the capability of DGR to measure anisotropic high-conductivity materials.     

Thermal-Diffusivity Measurement of Ceramic Coatings at High Temperature using “Front-Face” and “Rear-Face” Laser Flash Methods

Thick ceramic coatings deposited by plasma spraying techniques are widely used as wear and corrosion resistant coatings at high temperature. To measure accurately the thermal diffusivity of such coatings, the diffusivimeter of LNE has been set up to allow multilayered material studies up to 1,400°C by rear-face and front-face laser flash methods. These two methods have been compared in a large temperature range by measuring the thermal diffusivity of homogeneous (Armco iron and Poco graphite) and multilayered materials (chromium oxide coating deposited on iron alloy substrate). The thermal-diffusivity values measured by using front-face and rear-face techniques are in good agreement, with a relative deviation of less than 5% depending on temperature and materials.     

A Numerical Simulation to Propose a Flash Method for In Situ Detection of the Thermal Diffusivity of Anisotropic Thin Film Materials

Strong anisotropy of thermal diffusivity is frequently observed in thin film materials. We propose an in situ experimental method to remotely measure radial and axial components of the thermal diffusivity. The method is based on the traditional laser flash technique but is specialized to also highly challenging experimental situations such as sample manufacture and use phase when thin films may be exposed to very high pressures or temperatures and to high temperature gradients. The method requires laser pulses of very short duration and fast measurement of transient temperature excursions in only radial directions on the surface of the thin film samples. The accuracy of the method is checked by comparison with results from a finite element calculation for a graphite sheet with high anisotropic conductivity that simulates a thermo-physical experiment.     

Measurement of thermal diffusivity of solids using infrared thermography

We report measurement of thermal diffusivity of solid samples by using a continuous heat source and infrared thermal imaging. In this technique, a continuous heat source is used for heating the front surface of solid specimen and a thermal camera for detecting the time dependent temperature variations at the rear surface. The advantage of this technique is that it does not require an expensive thermal camera with high acquisition rate or transient heat sources like laser or flash lamp. The time dependent heat equation is solved analytically for the given experimental boundary conditions. The incorporation of heat loss correction in the solution of heat equation provides the values of thermal diffusivity for aluminum, copper and brass, in good agreement with the literature values.     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.