Probe beam size effect on the measurement of the distance between the probe beam and the sample in photothermal deflection

A distance-scan method to determine the distance between the probe beam and sample, which is not easily measured exactly, in photothermal deflection (PD) was reported, with which the distance and thermal diffusivity of the deflecting medium can be simultaneously measured. Probe beam size effect (PBSE) on PD phase signal was quantitatively analyzed to clearly show its physical meaning. The measured distance was experimentally verified as correct and reliable, and the measured thermal diffusivities of N2 and CO2 are in good agreement with the literature values. They could not be precisely measured by phase signal without considering the PBSE.     

Thermal characterization of film-on-substrate system by photothermal method and genetic algorithm

For a film-on-substrate system, in the case of thin films with lower thermal diffusivities compared to substrates, the phase characterization of the photothermal signals is analyzed. Moreover, the numerical estimations of multiparameter are performed, which show the feasibility of simultaneous determination of the thermal diffusivities of the film and the substrate, as well as the thermal interface resistance of the film/substrate. Because the thermal diffusivity of the thin-film and the thermal interface resistance may be highly correlated, a genetic algorithm is used as an estimation method for the determination of the thermal properties of thin films with low thermal diffusivity.     

Noncontact measurement of the thermal diffusivity of IR semi-transparent and semiconducting n-CdMgSe mixed crystals by means of the photothermal radiometry

In this paper we present results of noncontact measurements of the thermal diffusivity of infrared semi-transparent n-CdMgSe mixed semiconductor crystals by means of the photothermal radiometry (PTR) in a transmission configuration. In order to overcome an influence of the infrared semi-transparency and plasma waves on the PTR signal from n-CdMgSe mixed crystals the samples were covered by thin aluminum foils on both sides. The thermal diffusivities of n-CdMgSe mixed crystals were estimated from PTR phase frequency characteristics using a well-known formula. It was found that the obtained results are underestimated in comparison to thermal diffusivities estimated from the PPE (photopyro-electric) measurements. A three layer model of a PTR signal was applied in order to estimate an error in determination of the thermal diffusivity of a sample caused by aluminum foils.     

光热偏转检测技术自动测量的研究

介绍了光热偏转技术自动测量材料热物性的原理及实验方法。根据光热偏转检测理论建立了一套自动检测系统,从自动控制、数据采集和数据处理等方面实现了光热偏转检测技术的自动化测量。设计出了步进电机自动控制和数据处理软件。利用该系统对钛酸锶钡材料的热扩散率进行了测量,发现随着掺杂量铈的增加,钛酸锶钡的热扩散率逐渐减小,当掺杂量铈增加到一定值时,其热扩散率又开始增加。     

A simple theoretical extension to the analysis of photothermal deflection signal for low thermal diffusivity evaluation

A modified amplitude method to analyze the photothermal probe beam deflection signal for the determination of low thermal diffusivity values of materials is proposed. This simple theoretical model, which is an extension of the amplitude method proposed by Quelin et al., takes into account the dependence of the photothermal signal on the height of the probe beam above the sample surface which affects mirage measurements when the thermal diffusivity of the coupling medium is greater than that of the sample. The present work is similar to the modification to the phase method proposed by Bertolotti et al. for determination of low thermal diffusivity. The method can be applied irrespective of whether the sample is optically transparent or optically opaque and is independent of thickness.     

利用激光光热技术研究钛酸钡材料的导热性能

利用激光光热偏转技术测量了钛酸钡材料的热扩散率。根据所测量材料的光热光偏转信号,通过最小二乘法中的非线性拟合,直接拟合出了材料的热扩散率,克服了光热偏转技术中“Mirage effect”步骤多、计算复杂、误差大的缺点。测量了在不同成型压力和不同烧结温度下钛酸钡材料的热扩散率,得到了热扩散率随成型压力和烧结温度的变化规律。对实验结果进行了分析和讨论。     

Photothermal probe beam deflection measurement of thermal diffusivity of atmospheric air

We have used a modified photothermal probe beam deflection system with a back pumping configuration for the measurements of the temperature-dependent thermal diffusivity of atmospheric air. The results are consistent and reasonably well agree with the literature values. The measured thermal diffusivity values are the same for the same measuring temperature regardless of the beam offsets and the deflecting surface temperatures. For the gas with known temperature-dependent thermal diffusivity, this method can be used to deduce the temperature of a gas from the measured thermal diffusivity value.     

An analytical model for thermal diffusivity measurements of film substrate composite using the mirage technique

The thermal diffusivity h of a thin film on a substrate is measured by using the mirage technique. The photothermal deflection of the probe beam is caused by the heat field and the substrate, heated by the pump beam. From the experimental data a two-dimensional algorithm is proposed to obtain the measurements of the diffusivity of film and substrate in one set of mirage detection.     

Pulsed-laser Pumped Photothermal Deflection Spectroscopy for Liquid Thermal Diffusivity Measurement

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Pulsed-laser Pumped Photothermal Deflection Spectroscopy for Liquid Thermal Diffusivity Measurement

The pulsed-laser pumped photothermal deflection spectroscopy (PPDS) is applied to measure liquid thermal diffusivity in a modified collinear configuration. Here the pulsed beam is regarded as in the form of a Dirac delta function. The experiment setup used is described. Measurement result for the thermal diffusivity of a liquid sample(magnta/ethanol saturated solution) is reported. The measurement error is less than 3%.     

Thermal Diffusivity of Film/Substrate Structures Characterized by Transient Thermal Grating Method

Transient thermal grating method is used to measure the thermal diffusivity of absorbing films deposited on transparent substrates. According to periodically modulated dielectric constant variations and thermoelastic deformations of the thin films caused by the transient thermal gratings, an improved optical diffraction theory is presented. In the experiment, the probing laser beam reflectively diffracted by the thermal grating is measured by a photomultiplier at different grating fringe spaces. The thermal diffusivity of the film can be evaluated by fitting the theoretical calculations of diffraction signals to the experimental measured data. The validity of the method is tested by measuring the thermal diffusivities of absorbing ZnO films deposited on glass substrates.     

Photothermal techniques with a top-hat beam excitation

A flatly topped circular beam (“top-hat beam”) is employed as the excitation beam for photothermal deflection and surface thermal lens techniques. The Fresnel diffraction model is applied to describe the photothermal signals. The theoretical comparison between a Gaussian beam and a top-hat beam excited photothermal signal amplitudes shows that the use of the top-hat beam excitation improves the measurement sensitivity of the photothermal techniques. Experimental results for both photothermal deflection and surface thermal lens are presented. The potential applications of the top-hat beam excited photothermal techniques are highlighted.     

Study of the Thermal Diffusivity of Optical Coating by Photo－thermal Deformation Technique

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Thermal properties of compressed expanded graphite: photothermal measurements

The thermal diffusivity and the thermal conductivity of compressed expanded graphite (CEG) samples were investigated by photothermal measurements in two geometries differing by a place of temperature disturbance detection. This disturbance can be detected on a surface opposite to the one at which the disturbance was generated (rear detection) or on the same surface (front detection). A measurement based on the rear detection allowed us to determine the effective thermal diffusivity of the sample, while the method with front detection gives the possibility of analysis of homogeneity of the sample. It is shown that the thermal diffusivity of CEG strongly depends on its apparent density. Moreover, CEG samples reveal anisotropy of the thermal properties. The thermal diffusivity in the direction parallel to the compacting axis is lower than the one in the direction perpendicular to it. The parallel thermal diffusivity decreases with growing apparent density, while the perpendicular thermal diffusivity significantly grows when the apparent density grows. The perpendicular thermal conductivity exhibits the same behavior as the perpendicular thermal diffusivity. The parallel thermal conductivity slightly grows with growing density and then reaches a plateau. The anisotropy of CEG samples grows with growing apparent density and vanishes for low-density samples. The photothermal measurement with front signal detection revealed that the CEG samples are non-homogeneous in the direction of the compacting axis and can be modeled by a two-layer system.     

光热偏转技术测量Sr_(2-x)Sm_xFeMoO_6材料的热扩散率

采用固体烧结法制备了Sr2-xSmxFeMoO6(x=0,0.03,0.05,0.08,0.10,0.13,0.15,0.20)多晶结构样品,用X射线衍射对材料的结构进行了检测。通过光热偏转技术方法对该材料的热扩散率进行了研究,给出了掺杂比例与材料热扩散率的关系曲线,结果发现Sr2FeMoO6样品随着参杂Sm浓度的增加,结构发生变化的同时,样品的热扩散率也随着波动变化。这与文献[1]中给出的结论有明显不同,从声子、电子、自旋及其相互作用角度揭示了与文献[1]不同的根本原因;并且从x射线和热扩散率两个角度给出了样品Sr2-xSmxFeMoO6结构转变的参杂比例在10—13%的范围,说明光热偏转技术方法是研究参杂镧系元素的双钙钛矿结构和电子参杂效应的可行有效方法。     

Photothermal deflection studies on heat transport in intrinsic and extrinsic InP

Laser induced transverse photothermal deflection technique has been employed to determine the thermal parameters of InP doped with Sn, S and Fe as well as intrinsic InP. The thermal diffusivity values of these various samples are evaluated from the slope of the curve plotted between the phase of photothermal deflection signal and pump-probe offset. Analysis of the data shows that heat transport and hence the thermal diffusivity value, is greatly affected by the introduction of dopant. It is also seen that the direction of heat flow with respect to the plane of cleavage of semiconductor wafers influences the thermal diffusivity value. The results are explained in terms of dominating phonon assisted heat transfer mechanism in semiconductors . PACS 78.20.Nv; 66.30.Xj; 61.72.Vv     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

横向激光光热偏转相位法测量金刚石薄片的热扩散率

本文介绍横向激光光热偏转装置的优化设计及其利用相位信息测量CVD金刚石薄片的热扩散率,并和脉冲激光光热辐射方法测量的结果相比较.     

牙齿组织光热动态特性仿真与试验研究

调制激光作用牙齿组织发生散射形成光子密度波, 而由于光热效应产生热波, 基于一维介质辐射传输漫射近似方程与一维热传导方程建立了调制激光作用牙齿组织半透明混合介质的一维热波数学模型. 利用该模型仿真分析了牙齿龋损特性参数(牙釉质龋损层光吸收系数、散射系数、热扩散系数及龋损深度)对光热辐射动态响应特性的影响与规律. 利用红外探测器(HgCdTe, 2–12 μm)记录808 nm半导体激光激发牙齿组织产生的热波信号, 由锁相放大器计算热波信号的幅值与相位. 通过频率扫描试验获得了牙齿组织的光热动态响应, 利用多参数最佳统计拟合方法得到了牙齿组织特性参数. 结果表明光热辐射测量对牙齿组织不均匀性和龋损特性均具有较高敏感性与特异性.