Simultaneous measurement of thermal properties by thermal probe using stochastic approximation method

A novel thermal probe method is proposed for the simultaneous measurement of the thermal properties by the Monte Carlo stochastic approximation method. In this method, thermal capacity of probe and thermal contact resistance between probe and sample are considered. An experimental system is set up with the method to validate the measurement accuracy of the method. The thermal properties of several liquid samples as well as solid samples are measured. The results show that: (1) the thermal conductivity and the volumetric heat capacity can be measured with an error of less than 1.2% and 3% respectively, therefore, the measurement accuracy by the method is much higher than the conventional method and (2) the thermal contact resistance has a great effect on thermal conductivity for solid sample, while little influence on thermal conductivity for liquid sample and volumetric heat capacity.     

Photopyroelectrical measurement of the thermal properties of knitted textile samples. Influence of composition,structural parameters and water content

The amplitude and phase of the PPE signal for a strongly scattering sample were calculated using a four-flux model. The sensitivity of both PPE amplitude and phase with respect to the thermal properties and water content of the sample were calculated together with the correlations between the parameters. We show that the water content of a sample can be estimated from the signal amplitude, provided the availability of an a priori knowledge of the optical parameters' dependence on the water content. Once the optical characterization is completed, the phase signal can be used for the thermal properties evaluation. The experimental values of the thermal parameters depend on composition, structure and water content. The difference between the results obtained by the photothermal method and those obtained by DSC and Alambeta methods is discussed.     

基于热反射法的微纳结构热扫描技术研究

随着微电子及微纳加工技术的飞速发展,电子器件的发热和热管理成为制约器件性能进一步提升的关键因素,微纳结构的热物性是电子器件热设计中最关键的参数之一,因而对微纳结构材料的热物性进行测量,对于微纳器件和材料的热设计和热优化具有极为重要的意义。基于光热反射法,利用双波长飞秒激光抽运探测热反射系统对微纳结构材料的热物性进行了测量。此外,还通过样品表面的微区热扫描,实现了对微纳结构形貌和热性质的对比和表征,并分析了系统的空间分辨率。     

孔隙率与含水率对砂质土样导热系数的影响

鉴于研究岩土体导热系数的变化规律及影响因素,对岩土的导热理论和工程实践的现实意义,利用热探针测定了不同孔隙率和含水率条件下的砂质土样导热系数,分析其变化规律,并用1stOpt软件得到孔隙率、含水率与导热系数的拟合公式。计算结果与试验结果表明,导热系数随孔隙率的增加而减小,随含水率的增加而增大,且在一定含水率下,导热系数随孔隙率的增加呈线性减小,孔隙率为0.468~0.511时,导热系数降幅为20.19%;在一定孔隙率下,导热系数随含水率的增加呈非线性增长,含水率0~10%时,导热系数增幅为338.38%,含水率10%~15%时,导热系数的增幅为8.83%。     

光学热带法测量固态材料热物性研究

针对接触式瞬态热带法测量导热系数时,加热丝和样品间接触热阻,会影响实验测量结果以及对固体样品形状大小要求较高的现状,根据瞬态热带法原理,本文提出了一种光学瞬态热带法来测量固体材料的导热系数。采用连续激光为加热源,通过透镜将光斑放大并聚焦照射在样品表面,实现样品非接触式测量。构建二维导热模型,采用红外热像仪记录样品表面温升随时间的变化关系,根据导热理论模型求出待测样品的热扩散系数及导热系数。以K9和石英玻璃为样品对本套测量方法进行验证,制备并测量了纯石蜡、0.5%和1%石墨烯-石蜡的固态复合相变材料的导热系数,探讨了影响实验结果的潜在因素。     

NON-FOURIER HEAT CONDUCTION PHENOMENA IN POROUS MATERIAL HEATED BY MICROSECOND LASER PULSE

Experiments on porous material heated by a microsecond laser pulse and the corresponding theoretical analysis are carried out. Some non-Fourier heat conduction phenomena are observed in the experimental sample. The experimental results indicate that only if the thermal disturbance is strong enough (i.e., the pulse duration is short enough and the pulse heat flux is great enough) is it possible to observe apparent non-Fourier heat conduction phenomenon in the sample, and evident non-Fourier heat conduction phenomenon can only exist in a very limited region around the thermal disturbance position. The hyperbolic heat conduction (HHC) equation and the dual-phase lag (DPL) model are employed, respectively, to describe the non-Fourier heat condution process happening in the experimental sample, and the finite-difference method (FDM) is used to solve them numerically. The numerical solutions show that both the HHC equation and the DPL model can predict the non-Fourier heat conduction phenomenon emerging in the experimental sample qualitatively. Moreover, if τ_q and τ_T are assumed to have suitable values, the theoretical result of the DPL model is more agreeable to the experimental result.     

Spatially Resolved Measurements of Thermal Stresses by Picosecond Time-Domain Probe Beam Deflection

Measurements of thermally induced strains with micron-scale lateral spatial resolution, picosecond time resolution, and sub-picometer vertical sensitivity are achieved using a newly developed experimental method, time-domain probe beam defection (TD-PBD). TD-PBD is a pump-probe optical technique that combines an ultrafast laser oscillator as the light source, high frequency (10 MHz) modulation of the pump beam, and a wide range of time delays (0–4 ns) between the pump and probe. Deflections of the probe beam are measured by a position sensitive detector and an rf lockin amplifier. The beam-deflection data are analyzed using a detailed model of heat transport and thermally generated stresses and strains. Comparisons between the model and the data enable quantitative measurements of the coefficient of thermal expansion with a spatial resolution of 4 µm.     

Optical measurement of thermal contact conductance between thin,waferlike solid samples by using laser AC heating and reflectance thermometry: Reconsideration of the loading method for the samples

A noncontact optical technique for measuring the thermal contact conductance between two thin, waferlike solid samples was developed. In this technique, one solid surface is heated with a modulated laser beam; the corresponding temperature modulation of the other solid surface across their interface is monitored by using the reflectance of a probe laser beam. Each sample can become slightly bent if its edge is compressed by the sample holder, so the contact pressure between the samples (in the range of 0.8 to 10 MPa) was calculated by elastic and plastic numerical analyses. By using the calculated contact pressure, the correlation between contact pressure and thermal contact conductance could be determined more accurately. Also, the appropriate thickness of the glass plates used to fix the samples was derived by calculating the thickness where the local contact pressure is almost the same as the averaged pressure. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 498–512, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10049     

Evolution thermique d'une sonde électrostatique dans un gaz raréfié et partiellement ionisé

Based on the classical Langmuir theory, an electrostatic probe can be used in an ionized gas to measure electron temperature, electron density and electron distribution function. In many experimental cases the characteristic curve (applied potential vs total current collected) presents a complex and reproducible hysteresis effect. Consequently, electron measurements are impossible. The hysteresis effect decreases when the probe potential frequency increases and is related to the thermal probe evolution. The thermal probe evolution is determined by a numerical method taking into account plasma energy flux, radiation and probe conduction. The numerical results show that the hysteresis effect appears simultaneously with a thermal probe hysteresis.     

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.     

Effective thermal parameters of layered films: An application to pulsed photothermal techniques

Pulsed photothermal techniques provide useful methods based on linear relations between measurable quantities to obtain the thermal diffusivity and thermal conductivity of homogeneous materials. In this work, the effective thermal parameters of two-layered films are defined starting from an homogeneous layer which at the surfaces, produces the same temperature fluctuations and the same photothermal signal that the composite heated by a fast pulse-laser. Our theoretical model predicts that the effective thermal parameters of the layered system can only be calculated in the limit when the laser pulse duration is smaller tan the characteristic time of each layer, respectively. The temperature distribution is calculated in each layer by using the Fourier integral and the time-dependent one-dimensional heat diffusion equation with appropriate boundary conditions according to the experimental conditions. Within this approximation, we found an analytical expression for both, the effective thermal diffusivity and thermal conductivity which depend significantly on the thickness and the thermal parameters of each film.     

Soot particle sizing during high-pressure Diesel spray combustion via time-resolved laser-induced incandescence

Single-pulse time-resolved laser-induced incandescence (TiRe-LII) signal transients from soot particulates were acquired during unsteady high pressure Diesel combustion in a constant volume cell for typical top dead center conditions during a Diesel engine cycle. Measurements were performed for initial gas pressures between 1 and 3 MPa, injection pressures between 50 and 130 MPa and laser probe timings between 5 and 16 ms after start of fuel injection. In separate experiments and for the same cell operating conditions gas temperatures were deduced from spectrally resolved soot pyrometry measurements. Implementing the LII model of Kock et al. [Combust. Flame 147 (2006) 79-92] ensemble mean soot particle diameters were evaluated from least-squares fitting of theoretical cooling curves to experimental TiRe-LII signal transients. Since in the experiments the environmental gas temperature and the width of an assumed particle size distribution were not known, the effects of the initial choice of these parameters on retrieved particle diameters were investigated. It is shown that evaluated mean particle diameters are only slightly biased by the choice of typical size distribution widths and gas temperatures. For a fixed combustion phase mean particle diameters are not much affected by gas pressure, however they become smaller at high fuel injection pressure. At a mean chamber pressure of 1.39 MPa evaluated mean particle diameters increased by a factor of two for probe delays between 5 and 16 ms after start of injection irrespective of the choices of first-guess fitting variables, indicating a certain robustness of data analysis procedure.     

D.C. scanning thermal microscopy: Characterisation and interpretation of the measurement

The used Scanning Thermal Microscopy (SThM) probe is a thin Pt resistance wire acting as a heat source and as a detector simultaneously. Its energetic balance is investigated by the study of the temperature profile along the probe. A theoretical approach of the measurement, based on this investigation, is then proposed. Simulations with this modelling are shown to predict how the heat, electrically produced in the probe, is dissipated in the probe-sample system. In particular, it is shown that the steady-state of conduction losses to the thermal element support varies versus the thermal conductivity of the sample and can lead to bad interpretations of the measurement.     

A.C. scanning thermal microscopy: Tip–sample interaction and buried defects modellings

Modellings describing the different thermal interactions between a sample surface and a probe in case of modulated heated probe are given and analysed. To obtain more reliable thermal data for a full calibration of thermal microscope or quantitative interpretations, these models are developed in three dimensions. The scattering of the thermal waves on buried small thermal resistances is theoretically studied. Lateral resolutions for the amplitude and phase of the probe's a.c. thermal response are obtained.     

Experimental measurements and theoretical predictions of the thermal conductivity of two phases glass beads

Effective thermal conductivities of dry and water saturated glass beads have been measured with the transient thermal probe method. The glass beads used in the experiments have particles with diameters ranging from 0.07 mm to 6 mm. The thermal probe used for the measurements has an external diameter of 1.2 mm. The effective thermal conductivities of dry and water saturated glass beads, with diameters smaller than 1.2 mm, are well predicted by a theoretical model, proposed by the first author, which uses the experimentally measured porosity. If the diameter of the glass beads is 3 and 6 mm, i.e. it is higher than the probe one; the porosity of the porous medium is higher near the probe than in the core of the medium. The effective thermal conductivity of the porous medium is then lower because air and water have a lower thermal conductivity than the solid particles of the beads. A theoretical approach has been used in order to take into account the increase of porosity near the probe. The theoretical models use the corrected porosity, which account for the probe presence, and provide good agreement with the experimental measurements.     

FINITE-ELEMENT MODELING OF HEAT FLOW IN DEEP-PENETRATION LASER WELDS IN ALUMINUM ALLOYS

A two-dimensional finite-element nonlinear transient heat conduction model was developed and used to simulate deep-penetration keyhole laser welds in aluminum alloys. The weld thermal profiles were calculated in an arbitrary reference plane as the laser beam approached and passed the plane. From the calculated thermal profiles, three-dimensional quasi-steady-state shapes of the weld pools were determined. The predicted weld bead shape and dimensions were in good agreement with the experimental results. The experimental laser welds in aluminum alloys contained large amounts of porosity. The model predicted large mushy zones for aluminum laser welds during solidification, which in turn increase the probability of porosity formation by increased bubble entrapment.     

A study of the axisymmetric thermal strain in a laser rod with longitudinal temperature rise

This paper presented a mathematical model of the axisymmetric thermal stress and thermal strain in the laser medium. Then an analytical solution was derived through the Thermoelastic Displacement Potential method using a Love Displacement Function. The analytical results show that under the traction-free boundary conditions, the longitudinal rise of fluid temperature has an unnoticeable effect on the thermal stress profile in the laser rod; however, the thermal strain field has an evident variation in the longitudinal direction, which will significantly affect the laser transmission characteristics and the beam quality. Numerical simulation was also performed for the thermal stress and thermal strain in the laser rod by using the temperature distribution obtained from the conjugate numerical simulation as the thermal load. The numerical results agree well with the analytical solution in the main section of the laser rod except that it better exhibits the end effects.     

掺杂对定形相变材料导热系数的影响

通过在定形相变材料中加添加剂改进定形相变材料的导热系数，用热针法对改性后的试样进行了测量，对添加剂种类和含量对定形相变材料导热系数的影响进行了定量分析。实验结果表明，石墨添加剂可以显著提高定形相变材料的导热系数。通过对实验数据的拟合，得到了石墨添加质量分数与材料有效导热系数问的拟合公式。     

检测流体多参数综合变化探头的研究

对电导率探头检测流体多参数综合变化的基本原理进行了分析，测定了该探头的极间距离对其输出信号的影响，得出了该探头的温度特性曲线，并通过该探头对双螺旋桨推进的水面船模尾流的测量，初步探讨了电导率探头对流体温度和扰动强度的响应特性。结果表明，操头的信号随流体温度降低而增大，并随流体扰动强度增大而降低。