基于热成像的材料热扩散率测量方法研究

将一种基于热成像的薄片材料热扩散率测量方法的理论模型由二维拓展到三维,以适用于更大厚度材料的热扩散率测量。指出该方法适用的特征条件为:Z向空间导数值与材料表面和周围环境的温度差值为线性关系。仿真分析了传热距离与激励时间对特征条件的影响以及相关的信噪比问题,并在304不锈钢材料实测实验中依照仿真分析设置传热距离和激励时间以满足特征条件,结果显示厚度为1 mm和2 mm的不锈钢材料测量偏差均在3.0%以内,厚度为5 mm的不锈钢材料测量的偏差为4.1%,从而扩大了该测量方法的适用范围。     

基于脉冲涡流热成像的面内方向性热扩散率测量

针对导电材料面内方向性热扩散率的测量,提出脉冲涡流热成像法这一新方法。该方法采用感应式脉冲线激励源,在导电试件表面形成沿一定方向的感应涡流,实现局部热激励,在非稳态条件下实现了材料面内方向热扩散率的测量;简单调节试件与线感应激励线圈的角度,就可以快速无损非接触地测量试件在垂直线圈方向上的热扩散率值。对感应线激励下面内热传导及高斯温度分布进行了分析,分别对AISI304不锈钢、纯铁、纯镍3种材料的热扩散率进行了测量,测量结果与手册值相符,偏差小于9.0%,相对扩展不确定度分别为3.18%,3.72%,3.70%。     

利用红外热成像技术测算碳纤维材料的热扩散系数

利用红外热成像技术测算了5种碳纤维材料的热扩散系数，并分析了影响测量准确度的因素。结果表明，利用红外热成像技术可以方便、快速地测算出碳纤维材料的导热性能参数值，从而为这类材料导热参数的测算提供了一种新颖、可行的方法。     

基于LabVIEW的热扩散率测量系统

对中国计量科学研究院的热扩散率测量系统的数据采集硬件进行了改进与提高,并以LabVIEW为主要开发工具,编写了一套新的数据采集处理软件,增加了频谱分析、数字滤波及各项实验数据修正等功能.与改进之前相比,该系统的测量重复性由2%提高到3 ‰,测量效率和准确度得到较大提高.     

红外热成像技术在零件无损检测中的发展和应用现状

红外热成像技术是一种通过外加激励获得零件表面温度场分布,并从热像图中提取零件损伤信息的无损检测技术,它具有快速、实时、非接触等优点,研究和应用前景广阔。综述了红外热成像无损检测技术的发展现状和应用实例,通过对比3种常用的红外热成像技术:脉冲红外热成像、超声红外热成像和锁相红外热成像,认为超声锁相红外热成像技术具有其他几种技术无法比拟的优势,并从检测工艺和信号处理等方面提出了改进此技术的具体措施。     

石墨材料热扩散率的评价：微结构和热扩散率的关系

石墨材料热扩散率的评价——微结构和热扩散率的关系前言以往，关于固体的热扩散率，计量研究所一般采用瞬时激光法从室温到高温进行高精度的测定。本文介绍另一种关于热扩散率的测定方法，测量精度有所提高。即以组成试料的全部材料作为对象，由点到线和面开展测定。石墨...     

固体材料热扩散率测量系统研究

研制了基于激光闪光法的热扩散率测量装置,利用机械泵和分子泵对炉内抽真空,之后对炉膛进行加热,采用大功率脉冲激光仪激励被测样品,样品受激光照射的面温度升高,热量开始传导向另一面,红外探测器实时记录此温升变化过程,温升信号经放大器放大后转换成电压信号输入至采集卡,采集卡与电脑软件进行通讯,自行编写的测量软件实时计算处理数据,分析得到样品的热扩散率值。利用该系统对标准样品在300～1300℃温度范围内进行了实验,测量重复性小于0.85%。     

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。     

激光闪光法测量材料热扩散率的漏热修正

采用激光闪光法测量材料的热扩散率，其中对漏热的修正目前主要有参数估计法、Cowan修正、ASTM修正三种方法。本文对这三种方法分别进行了介绍，并在中国计量科学研究院最新建立的激光热导仪上对PTB提供的标准试样奥氏体不锈钢进行测量，对测量结果利用这三种方法进行修正比较，发现这三种方法各有其优缺点以及各自适用的温度范围，为选择合适的漏热修正方法提供了依据。     

薄膜热扩散率及耦合层厚度的光声检测

从一维四层煤质模型出发，在只考虑试样吸收光的假设下，得出传声器检测时空气中的光声信号的相位响应。由此对厚度为２０μｍ的铝膜和４μｍ的镍膜的热扩散率进行了测定，并对薄膜与背衬之间的油耦合层作了估算。实验结果表明光声技术不仅可用来检测分层媒质表面层材料的热物性，而且还有可能估测内层煤质的厚度或热扩散率。     

Measurement of the In-plane Thermal Diffusivity of Materials by Infrared Thermography

A transient method using the Laplace transform for estimation of the in-plane thermal diffusivity of low conductive materials is presented. The temperature field of the sample is measured by infrared thermography. The main interest of the technique proposed here is to not require a knowledge of the stimulation and boundary conditions by using two reference temperature profiles. The parameter estimation is implemented in the time domain by an inverse technique using numerical Laplace inversion and convolution products. A sensitivity study has been carried out to optimize the choice of the two reference profiles. The effect of a space varying heat transfer coefficient on the estimated values of the unknown parameters has also been evaluated. Finally, the apparatus is described and experimental results obtained for a low conductive material like a vitroceramic are shown.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

In-plane Thermal Diffusivity Measurement of Thin Samples Using a Transient Fin Model and Infrared Thermography

A method for determining the in-plane thermal diffusivity of planar samples was constructed. The time-dependent temperature field of the sample heated at one edge was measured with an infrared camera. The temperature fields were averaged for different times over a narrow strip around the center line of the sample, and the temperature profiles for varying time were fitted by a solution to a corresponding one-dimensional heat equation. Heat losses by convective and radiative heat transfer were both included in the model. Two fitting parameters, the thermal diffusivity and the effective heat-loss term, were obtained from time-dependent temperature data by optimization. The ratio of these two parameters was also extracted from the steady-state temperature profile. The method was found to give good and consistent results when tested on copper and aluminum samples.     

Measurements of Thermal Diffusivity for Thin Slabs by a Converging Thermal Wave Technique

The measurement of thermal diffusivity for thin slabs by a converging thermal wave technique has been studied. Temperature variation at the center of the heat source ring that is produced by a pulsed high-power laser is detected by an infrared detector. A computer program based on the finite difference method is developed to analyze the thermal diffusivity of the slabs. Materials of both high thermal diffusivity (CVD diamond wafer) and low thermal diffusivity (stainless-steel foil) have been used for the measurements. The measurements have been performed by varying the size and the thickness of specimen. The converging thermal wave technique has proved to be a good method to measure the thermal diffusivity of a CVD diamond without breaking the wafer into small specimens. The technique can be applied for a small slab if the diameter of the slab is two times larger than that of the heat source ring. The sensitivity of thickness in measuring the thermal diffusivity is low for ordinary CVD diamond. The use of the converging thermal wave technique for nonhomogeneous, nonuniform, and anisotropic materials has been accomplished by applying the finite difference method.     

Thermal Diffusivity Measurements on Insulation Materials with the Laser Flash Method

An iterative approach is adopted to determine the thermal diffusivity of the xonotlite-type calcium silicate insulation material with very low thermal conductivity. The measurements were performed with a conventional laser flash apparatus by rear-face detection of the temperature response of the three-layered sample, where the insulating material is sandwiched between two iron slices. In the evaluation of the thermal conductivity, the theoretical curve is fitted to the complete temperature–time curve, instead of just using the t _1/2 point. The theoretical model is based on the thermal quadrupole method. The nonlinear parameter estimation technique is used to estimate simultaneously the thermal diffusivity, heat transfer coefficient, and absorbed energy. Based on experimental results, the optimal thickness range of the insulation material in the sample is indicated as 1.6 to 1.9 mm. The effects of the uncertainties of the thicknesses, contact resistance, and thermophysical properties of the three layers on the measurement uncertainty are estimated, giving an overall uncertainty in the thermal conductivity of approximately 7.5%.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

Investigation of Uncooled Microbolometer Focal Plane Array Infrared Camera for Quantitative Thermography

Thermal nondestructive evaluation has shown promise as a potential NDE technology for next generation US Army rotorcraft structures because it is rapid, noncontacting, and able to inspect complex geometries. To successfully apply thermal inspection systems for field use, the cost and size must be lowered. The infrared camera is a major factor contributing to the overall cost of commercially available thermal inspection systems. Recent advances in uncooled microbolometer focal plane array detectors have resulted in low cost, small size/weight, and low power consumption cameras. These attributes make this technology well suited for portable low cost thermal inspection systems. The purpose of this paper is to investigate the capabilities of the new microbolometer infrared cameras for quantitative thermal nondestructive evaluation. Quantitative thermal diffusivity and thickness images are obtained by minimizing the squared difference between the data and a thermal model on samples with fabricated defects. Critical infrared camera features such as spatial and temperature resolution, detector response time, and detector stability are studied by comparing results to a conventional thermal imaging camera using a cooled InSb focal plane array detector. Finally several techniques are presented to improve the camera’s performance. These techniques include temporal background subtraction, use of a synchronized electronic shutter system, and cyclic flash heating.     

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

The National Metrology Institute of Japan (NMIJ) in AIST has investigated the laser flash method in order to establish a thermal diffusivity standard for solid materials above room temperature. A uniform pulse-heating technique, fast infrared thermometry, and a new data analysis method were developed in order to reduce the uncertainty in thermal diffusivity measurements. The homogeneity and stability of candidate reference materials such as isotropic graphite were tested to confirm their qualification as thermal diffusivity reference materials. Since graphite is not transparent to both the heating laser beam and infrared light for thermometry, the laser flash method can be applied to graphite without black coatings. Thermal diffusivity values of these specimens with different thicknesses, were measured with changing heating laser pulse energies. A unique thermal diffusivity value can be determined for homogeneous materials independent of the specimen thickness, by extrapolating to zero heating laser pulse energy on the plot of apparent thermal diffusivity values measured with the laser flash method as a function of heating laser pulse energy.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.