Photoacoustic methodologies for thermal diffusivity measurements for highly opaque single-layered materials

The analytical solution for the one-dimensional heat diffusion problem, involving a harmonic heat source in a single layer, is used to provide of photoacoustic self-normalized methodologies for thermal diffusivity measurements for highly opaque materials. The self-normalized procedure involves the photoacoustic phase lag between the rear and front configurations. Three methodologies are described; two of them involving linear fits in the photoacoustic thermally thick and thermally thin regimes. Comparison between the theoretical normalized equations and the corresponding normalized experimental data allows for the development of criteria on the selection of an appropriate modulation frequency range where a reliable analysis can be done.     

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4^′-octyloxybiphenyl (8OCB) and 4-cyano-4^′-heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases. Received: 28 March 2001 / Revised version: 8 June 2001 / Published online: 18 July 2001     

Thermoacoustic theory of graphene films considering heat transfer of substrate materials

Based on thermoacoustic theory, a coupled thermal-mechanical model for graphene films is established, and the analytical solutions for thermal-acoustic radiation from a graphene thin film are obtained. The sound pressure of the graphene film generator on different substrates is measured, and the measurement data is compared with the theoretical results. The frequency response from the experimental results is consistent with the theoretical ones, while the measured values are slightly lower than the theoretical ones. Therefore, the accuracy of the proposed theoretical model is verified. It is shown that thermal-acoustic radiation from a graphene thin film reveals a wide frequency response. The sound pressure level increases with the frequency in the low frequency range, while the sound pressure varies smoothly with frequency in the high frequency range. Thus it can be used as excellent thermal generator. When the thermal effusivity of the substrate is smaller, then the sound pressure of grapheme films will be higher.Furthermore, the sound pressure decreases with the increase of heat capacity per unit area of grapheme films. Results will contribute to the mechanism of graphene films generator and its applications in the design of loudspeaker and other related areas.     

考虑基底热传导的石墨烯薄膜热声理论

主要基于热声效应对石墨烯薄膜发声进行理论研究。首先建立了石墨烯薄膜耦合热振动模型,推导出了石墨烯薄膜发声器的声压表达式。在此基础上,进行了不同基底石墨烯薄膜发声器的声压测试,并将测试值与理论计算结果对比,二者随频率变化趋势基本吻合,测试值略低于理论值,验证了推导出的声压表达式的正确性。研究表明石墨烯薄膜发声器有很宽的频域响应,在低频段声压级随频率增大而增大,在高频段响应平稳,具有作为优秀的热致发声器的潜力。基底材料蓄热系数越小,石墨烯薄膜的声压值越大;声压级随薄膜热容量的增大而减小。研究结果对于石墨烯的发声机制探索及其在扬声器设计等方面的应用具有指导意义。      

Measurement of thermophysical properties of YBa2Cu3O7−x thin film using picosecond thermoreflectance technique

Simultaneous determination of thermophysical properties of YBa₂Cu₃O_7?x thin film has been carried out using the optical pump–probe method. The thermal diffusivity and thermal effusivity were analyzed from the same picosecond thermoreflectance data. The thermal conductivity and specific heat were then derived from the measured values of the thermal diffusivity and effusivity. The thermal diffusivity, thermal effusivity and thermal conductivity obtained compared favorably with those reported in the literature.     

Heat treatment-induced bond layer diffusion and re-crystallization in copper carbon interface systems measured by modulated IR radiometry

Copper-carbon interface systems with additional Mo bond layers in the range of 25 nm to 200 nm have been analyzed with respect to their effective thermal depth profiles before and after heat treatment using modulated IR radiometry. Comparing the inverse calibrated modulated IR phase lags before and after heat treatment, several effects can be identified: – (1) The effusivity of the interface layer, which – due the contact resistance between the two elements copper and carbon – is rather low before heat treatment, increases considerably with heat treatment. – (2) This effect is accompanied by an increase of the thermal diffusion time of the interface layer, relying on the diffusion of Mo and Cu particles. – (3) The sputter-deposited copper films, which before heat treatment can be characterized as effective multi-layer structures, re-crystallize with heat treatment and show modulated IR phases, which are characteristic for thermally homogeneous thin films. – (4) The thermal diffusion times of the Cu films decrease considerably with heat treatment due to increased thermal diffusivities, and – (5) the thermal effusivities of the Cu films increase with heat treatment.     

Determining the thermophysical properties of Al-doped ZnO nanoparticles by the photoacoustic technique

The thermal conductivity and specific heat capacity of undoped and Al-doped (1-10 at.%) ZnO nanoparticles prepared using the solvent thermal method are determined by measuring both thermal diffusivity and thermal effusivity of a pressed powder compact of the prepared nanoparticles by using the laser-induced photoacoustic technique. The impact of Al doping versus the microstructure of the samples on such thermal parameters has been investigated. The results reveal an obvious enhancement in the specific heat capacity when decreasing the particle size, while the effect of Al doping on the specific heat capacity is minor. The measured thermal conductivities are about one order of magnitude smaller than that of the bulk ZnO due to several nested reducing heat transfer mechanisms. The results also show that Al doping significantly influences the thermal resistance. Using a simple thermal impedance model, the added thermal resistance due to Al dopant has been estimated.     

Photothermal techniques applied to the study of thermal properties in biodegradable films

The objective of the present work was to determine the thermal diffusivity and effusivity of biodegradable films by using photothermal techniques. The thermal diffusivity was studied by using the open photoacoustic cell technique. On the other hand the thermal effusivity was obtained by the photopyroelectric technique in a front detection configuration. The films were elaborated from mixtures of low density polyethylene (LDPE) and corn starch. The results showed that at high moisture values, the thermal diffusivity increased as the starch concentration was higher in the film. However at low moisture conditions (<9%) the thermal diffusivity diminished when the starch content in the sample was increased. On the other hand the thermal effusivity has a behavior in opposition to the thermal diffusivity. The thermal effusivity increased with the increase of the starch content in the film, at low extrusion moisture conditions (6.55%). As the moisture and starch concentration in the films were increased, the thermal effusivity diminished.     

Contrast-enhanced photothermal imaging for the evaluation of thermal transport parameters of liquids

In this article, we discuss the application of a baseline-suppressed, contrast-enhanced laser photothermal imager as a sensitive effusivity sensor for liquids, for the first time. We analyze the sources of errors associated with a conventional phase-comparison technique for evaluating the thermal transport parameters of liquids as a function of the pump power. Weak signal at lower powers and convection currents at higher powers are found to be the principal agencies deteriorating the sensitivity. Enhanced sensitivity has been achieved by using lower pump power and a signal baseline-suppressing common-mode-rejection-demodulation technique. The strength of thermoelastic vibration is small due to the low linear thermal expansion coefficient of the silicon nitride plate, which is the optical absorber. The approach has been extended for analyzing the effusivity modification of a water–methanol mixture as a function of methanol volume. The method is capable of detecting less than 1% change in effusivity for the mixture, which is 300–400% enhancement compared to the capability of conventional phase comparison. PACS 78.20.Nv; 81.70.Cv     

Non-stationary heat conduction of a porous medium

The thermal diffusion process is examined for a porous sample with idealized arrangement and form of the pores, when its surface is illuminated by a modulated light. A formula for the frequency dependence of the average surface temperature is derived. It is shown that it depends on the porosity, the form of the pores, and the ratio between a characteristic pore size and the thermal wavelength. In the limiting cases of low frequency of modulation and low porosity the results agree well with those quoted in the literature. The frequency dependence of the surface temperature of a microporous rubber sample, glass filtering crucibles, and leather samples have been measured by a PA cell and compared with the analytical results. The influence of various processes on the heat diffusion in porous media is discussed.     

Determination of thermal effusivity of solids by a photoacoustic scanning technique

A new method is proposed to determine the thermal effusivity of solid samples using a one dimensional photoacoustic scanning technique. The method employs a sample configuration in which the backing for a good light absorber layer is changed from a reference sample to the unknown sample by scanning the absorber surface with an incident modulated light beam. From the measured phase difference or amplitude ratio one can determine the thermal effusivity of the unknown sample, knowing the effusivity of the reference sample. The Rosencwaig-Gersho theory of photoacoustic effect has been extended to the present experimental situation and expressions have been derived for photoacousitc phase difference and amplitude ratio as the backing is changed. Values calculated using these expressions are found to agree well with measured values for different sample combinations except in amplitude ratio values when the thermal effusivities of the samples differ very widely. The reason for this disagreement is discussed.     

Thermal analysis of Polymer Dispersed Liquid Crystal under electric field using photopyroelectric technique

In this paper the first measurement of effective thermal parameters (thermal diffusivity, effusivity, conductivity and heat capacity) of Polymer Dispersed Liquid Crystal (PDLC) composites using the photopyroelectric (PPE) calorimetry is reported. The PPE technique is used in the standard “back” configuration and the cell has been designed for allowing the application of an electric field to the sample. Results show a dependence of the thermal parameters on the applied electric field which is explained by the reorientation of the liquid crystal molecules inside the droplets.     

Theory of thermal wave interference induced by laser action on a normally cut layered structure

The exact analytical solution for the thermal wave multiple reflections and interference in a normally cut layered structure is derived. The analysis predicts the existence of the new characteristic spatial scale in the considered phenomena—the length of thermal wave synchronism, which depends on the modulation frequency of the laser-induced head flux and the thermal diffusivities of the alternating layers. Both the amplitude and the phase of the photothermal response exhibit pronounced variation with modulation frequency when the length of the thermal wave synchronism is close to the spatial period of the structure.     

An improved photothermal gas analyser: Monitoring of simultaneous thermal diffusivity and thermal effusivity

In this paper we describe a new, simple and fast photothermal method for characterizing simultaneously the thermal diffusivity and thermal effusivity. The improved PTGA essentially combines a photoacoustic cell and a thermal wave pyroelectric cell enclosed in a single compact gas analyzer. The photo- acoustic cell is kept filled with nitrogen and sealed. The pyroelectric cell is also filled with nitrogen and after some warm up time, the nitrogen is exchanged to the gas of interest. It is shown that the analysis of the transient and saturation signals of both photoacoustic and pyroelectric cells are capable of measuring the thermal properties with an accuracy of 5%. The measurements were performed for hydrocarbons as ethane and propane, which are combustible gases. Knowing thermal diffusivity and effusivity, others important properties can be determinate: the thermal conductivity and the volumetric thermal capacity.     

Photopyroelectric calorimetry of liquids; recent development and applications

This paper is a synthesis of the main photopyroelectric (PPE) calorimetric techniques proposed in the last years for accurate measurements of dynamic thermal parameters of liquids. The area of interest is restricted to liquids because, due to the perfect sample-sensor thermal contact, accurate quantitative results can be obtained and consequently, intimate processes occurring in liquids (as molecular associations, structural changes in nanofluids, adulteration and spoilage of liquid foodstuffs, etc.) can be studied. The paper describes the possibilities offered by the two main used PPE detection configurations, “back” and “front”in investigating two dynamic thermal parameters: thermal diffusivity and effusivity. In the paper we analyzed only the information contained in the phase of the PPE signal, due to the fact that the phase, as a source of information, leads to more accurate results than the amplitude. A study of the accuracy of the investigations when using the chopping frequency, or sample’s thickness as scanning parameters is also made. Some basic applications concerning high-resolution measurements of thermal diffusivity and effusivity of some “special”liquid samples (magnetic nanofluids, adulterated vegetable oils) are described. A new possibility of using the thickness scan of the phase of the PPE signal of a liquid, in order to obtain the thermal parameters of a solid is analyzed.     

Photoacoustic measurement of thermal properties and thickness of buried layers in a multilayer film

The photoacoustic model of multilayer with a strong-absorbing surface layer is developed. The phase of ph0toacoustie signal is measured as a function of modulated frequency using apparatus totally controlled by a computer system. The thermal diffusivity, effusivity as well as thickness of several buried layers are obtained independently through the best fit of experimental data according to the theoretical model. The multilayers of thin metal and alloy film are investigated. This method is proved to be valuable particularly in nondestructive examination of subsurface physical properties.     

一维均匀Morse晶格体系的热流棘齿效应

本文系统研究了系统两端无平均温差时一维均匀Morse晶格中的热流棘齿效应. Morse晶格的两端分别与两个热浴相接触, 其中一端热浴温度周期调制,另一端热浴温度保持不变, 两端热浴温度长时平均相等. 数值结果表明, 当对一端热浴温度进行周期调制时, 系统中便会有稳定的定向热流产生. 通过改变调制频率和强度, 可以控制热流的大小及方向. 在合适的频率范围内, 可观察到一种非常有趣的现象——非定态负热导现象, 即系统中产生的定向热流逆着系统温度梯度方向由低温端流向高温端. 通过热波动力学分析(分析热流及温度分     

多层超薄薄膜介质光热辐射的三维理论研究

在柱坐标系下，以热传导方程及边界条件为基础，通过Hankel变换及逆变换，得到多层热单介质膜中的温度场.以特殊参数为例，对膜层为三层时的特殊情况进行讨论，得知对于高热 扩散率的薄膜材料，纵向和横向的热导率比值越大，相位的变化也越大.低热扩散率薄膜材 料，不同的纵向和横向的热导率比对光热信号的影响很小.同时，光吸收系数越小，频率变 化对相位影响越明显. 关键词： 多层 热单介质膜 温度场     

有矩形脉冲内热源的平板中非傅里叶导热研究

 对矩形激光短脉冲能量以内热源形式在有限厚绝热平板内释放所导致的非傅里叶导热过程进行了理论研究，利用格林函数与有限积分变换方法求得了绝热平板内温度分布的解析解,分析了平板内热波传播与反射行为以及温度场的特征。研究结果表明，温度波动频率取决于平板厚度和材料的热弛豫时间，而其幅值则与内热源释放区域的大小和脉冲宽度的长短有关。     

Photothermal measurement of thermal anisotropy in pyrolytic graphite

We investigated the anisotropic thermal conductivity in pyrolytic graphite by thermoreflectance. A laser-heated circular spot on a surface perpendicular to the planes developed into an elliptical temperature distribution which was recorded by a raster scanning technique at modulation frequencies ranging from 600 Hz to 100 kHz. The ratio of in-plane and perpendicular thermal conductivity was determined by fitting the phase of the temperature data with an analytical model, and was found to decrease with increasing modulation frequency. Highest conductivity values were considerably smaller than previously published data based on steady-state measurements. The frequency dependence and additional features in the phase profiles at high frequencies are discussed in view of sample surface preparation and the local nature of the thermoreflectance measurement.