Study on thermal diffusivity of materials by laser photothermal reflection technique

A method of measuring thermal diffusivity of materials at room temperature by photothermal reflection technique is described. An intensity-modulated Ar+ laser beam is used as incident light. The beam is focused to about 1 mm diameter spot and illuminates the sample surface. HgCdTe infrared detector is used to receive photothermal signal. Using this technique, the photothermal signals are experimentally measured as the function of different frequencies. The thermal diffusivities can be obtained by fitting the experimental data. On the other hand, the thermal diffusivities of one-way composite and orthogonal symmetric arranged composites Al2O3/Al are measured in transverse, longitudinal and arbitrary directions. The results show that the diffusivity of one-way material decreases with the increase of the measurement angle; the diffusivity of orthogonally arranged material almost keeps the same when measurement angle changes.     

Characterization of thermal diffusivity of micro/nanoscale wires by transient photo-electro-thermal technique

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step cw laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A dc current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, one can measure the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers. For nonconductive wires, a thin (∼ nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10^-5 m²/s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10^-7 m²/s. PACS 78.20.Nv; 42.62.-b; 65.80+n; 66.30.Xj     

Thermal diffusivity study of cheese fats by thermal lens detection

In this paper we used thermal lens spectrometry to determine the thermal diffusivity of cheese fats. We have used equal concentrations of cheese fats from oaxaca, chihuahua, gouda, manchego and mozzarella cheeses at 42°C temperature. The two lasers mismatched mode experimental configuration was used with a He-Ne laser, as a probe beam and an Ar⁺ laser as the excitation one. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression to the experimental data in order to obtain the thermal diffusivity of the cheese fat samples. This measured thermal property may contribute to a better understanding of the cheese fats quality, which is very important in food industry.     

激光输出不稳定性对激光与物质热作用的影响

研究了激光与物质相互作用中激光输出不稳定性对材料中温度场颁的影响以及减少这种影响的途径。首次在激光热传导方程中引入噪声项,推导了噪声影响下该激光热传导方程的解。研究发现,温度场的分布受噪声的影响,材料表面温度的涨落较大,而材料的深处涨落较小;温度场的涨落还和激光加热的过程有关,随激光加热时间的增长,温度场的涨落增大。此外,材料的热传导系数、热扩散率和发射率均对温度场的涨落有影响。文中还提出了减小激     

The effects of experimental parameters in the thermal diffusivity measurements of oriented polymer films using mirage effect: Computer simulation

Computer simulations have been carried out to study the effects of the experimental parameters when the mirage method has been applied to thermal diffusivity measurements of oriented polymer films. The parameters under study are the thermal diffusivity of the fluid surrounding the sample, the modulation frequency and the radius of the heating beam, the height and the radius of the probe beam, and the sample thickness and thermal diffusivity. Proposals for the optimum parameter values to maximize the measurement sensitivity for the sample diffusivity are made and the difficulties arising from the low diffusivity of the samples are described. It is also concluded that because the thermal properties of the fluid surrounding the sample have a strong contribution to the mirage signals, the signals do not include any simple feature corresponding to the sample diffusivity. Therefore it should be determined from the entire measurement data using regression methods.     

The reliability of optical fiber-TWRC technique in liquids thermal diffusivity measurement

The conventional thermal-wave resonator cavity (TWRC) technique was modified by using an optical fiber as both to transmit light beam and to produce thermal wave. This technique also known shortly as OF-TWRC was used to measure liquid thermal diffusivity in a thermally thick condition. The stability of the pyroelectric signal amplitude was good over long time duration. The thermal diffusivity values of various liquids obtained by this technique are in agreement with those of literature indicating this technique is reliable as compared to the conventional TWRC technique.     

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.     

The influence of the coupling fluids and of the pyroelectric transducer on low-temperature photopyroelectric studies

Thermal diffusivity of various compounds used as coupling fluids in photopyroelectric experiments have been measured at room temperature and at 80 K. Measurements on high vacuum silicone grease have also been performed as a function of temperature and an increase in the thermal diffusivity values with decreasing temperature has been found. Two phase transitions in the LiTaO₃ pyroelectric transducer have been observed at 200 and 225 K.     

Thermal conductivity of symmetrically strained Si/Ge superlattices

This paper reports temperature-dependent thermal conductivity measurements in the cross-plane direction of symmetrically strained Si/Ge superlattices, and the effect of doping, period thickness and dislocations on the thermal conductivity reduction of Si/Ge superlattices. The Si/Ge superlattices are grown by molecular beam epitaxy on silicon and silicon-on-insulator substrates with a graded buffer layer. A differential 3 ω method is used to measure the thermal conductivity of the buffer and the superlattices between 80 and 300 K. The thermal conductivity measurement is carried out in conjunction with X-ray and TEM sample characterization. The measured thermal conductivity values of the superlattices are lower than those of their equivalent composition bulk alloys.     

Study of gold nanoparticles effect on thermal diffusivity of nanofluids based on various solvents by using thermal lens spectroscopy

Dual beam thermal lens technique is used to determine the thermal diffusivity of different solvents in presence of gold nanoparticles. In this technique an Ar⁺ laser (wavelength 514 nm, power 40 mW) and intensity stabilized He-Ne laser were used as the heating source and probe beam respectively. The experimental results showed that thermal diffusivity values of the studied solvents (water, ethanol and ethylene glycol (EG)) were enhanced by the presence of gold nanoparticles.     

Theoretical and experimental studies on high-power laser-induced thermal blooming effect in chamber with different gases

High-power laser induced thermal blooming effects in a closed chamber with three different gases are investigated theoretically and experimentally in this work. In the theoretical treatment, an incompressible gas turbulent model is adopted. In the numerical simulation the gas refractive index as a function of both the temperature and pressure is taken into consideration. In the experimental study the pump-probe technology is adopted. A high-power 1064-nm fiber laser with maximum output power of 12 kW is used to drive the gas thermal blooming, and a 50-mW high-beam-quality 637-nm laser diode (LD) is used as a probe beam. The influences of the gas thermal blooming in the chamber on the probe beam wavefront and beam quality are analyzed for three different gases of air, nitrogen, and helium, respectively. The results indicate that nitrogen is well suitable for restraining thermal blooming effect for high-power laser. The measured data are in good agreement with the simulated results.     

Thermal buckling and natural vibration of the beam with an axial stick–slip–stop boundary

As a first attempt to study the dynamics of a heated structure with complicated boundaries, this paper deals with the thermal buckling and the natural vibration of a simply supported slender beam, which is subject to a uniformly distributed heating and has a frictional sliding end within a clearance. This sliding end is initially at a stick status under the friction force, but may be slightly slipping due to the thermal expansion of the beam until the sliding end contacts a stop, i.e., the bound of the clearance. The material properties of the beam are temperature-independent for low temperature, but temperature-dependent for high temperature. For each case, the analytic solutions for the critical buckling temperature and the natural frequencies of the heated beam are derived first. Then, discussions are made to reveal the effects of beam parameters, such as the ratio of beam length to beam thickness, the ratio of clearance to beam length and the temperature-dependent material properties, on the critical buckling temperature and the fundamental natural frequency of the heated beam. The study shows that both friction force and clearance have significant influences on the critical buckling temperature and the fundamental natural frequency of the beam. When the friction force is not very large, the clearance can greatly increase the critical buckling temperature. These conclusions enable one to properly design the stick–slip–stop boundary so as to improve the mechanical performance of the beam in thermal environments.     

反射式热透镜时变信号测量固体热扩散率的研究

介绍了采用反射式热透镜时变信号测量固体热扩散率的一种简便方法,该方法结合表面热透镜技术原理,采用表面透镜的动态变化和反射的探测光发散的幅度来确定固体的热扩散率,相对于传统的热扩散率测量方法要快捷简单。对较大范围的一系列物质的热学特性进行了实验研究,证明了其实用性。     

Thermal diffusivity measurements of spherical samples using active infrared thermography

This work presents a method for measuring the thermal diffusivity of spherical samples using active infrared thermography. The principal novelty of this method lies in the deduction of an analytical model to obtain the spatial and temporal distribution of temperature in spherical samples. The model is obtained from the classical theory of heat conduction or the 3D heat diffusion equation. In order to analyze the behavior of the model, an active infrared thermography is used in order to monitor the spatial and temporal temperature distribution. Three different materials are used as spherical samples and they are heated by radiation increasing this way its temperature. The recorded data is fitted to the model by adjusting the diffusivity parameter. The results of the diffusivity values obtained using this model are consistent with those obtained from a standard thermal properties analyzer.     

SiNx薄膜热物性参数实验测量与分析研究

采用一种新的实验测量方案，将金属加热单元与温度探测单元合二为一，间接获得了在半导体和微电子学MEMS领域内有重要用途的SiN_x薄膜的导热系数、发射率、比热容和热扩散系数，并对实验结果进行了不确定度分析，为微电子电路设计和掩模成型工艺等提供了可靠的热物性数据. 实验结果表明，薄膜的导热系数、发射率、热扩散系数远比相应体材质低，而且还与温度、厚度有关，尺寸效应显著，而比热容则与体材质相差不大. 关键词： 微尺度传热 热物性参数 x薄膜')" href="#">SiN_x薄膜 测量技术     

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.     

Study of influences of laser power fluctuation on thermogram of the material

The effects of the laser beam instability on the temperature of the material are studied in this paper. The fluctuation of the temperature not only relates to the laser beam instability, but also correlates with the thermal conductivity, the thermal diffusivity, the emissivity, the laser–solid interaction time, and the laser spot diameter. The temperature fluctuation of the aluminum, the polymethylmethacrylate, the epoxylite, and the low carbon steel is measured by the infrared thermal imagers (HWRX··) to determine validity of the theory prediction. It is found that the theoretical prediction is in good agreement with the experimental results. Besides, the maximum amplitude of the temperature fluctuation is calculated and how to control the temperature fluctuation is discussed. The results show that the temperature fluctuation in the laser–solid interaction is of importance in practice.     

Burning rate of homogeneous energetic materials with thermal expansion and varying thermal properties in the condensed phase

We present a study of the one-dimensional flame structure of combusting solid propellants that focuses on the effects of thermal expansion and variable thermal properties in the condensed phase. A nonlinear heat equation is derived for a burning thermo-elastic solid with temperature-dependent specific heat, thermal expansion, and thermal conductivity coefficients. It is solved for different modelling approximations both analytically and numerically. Explicit expressions are derived for the regression rate of the propellant surface as functions of surface temperature and thermal expansion parameters. A simple one-step reaction model of the gas phase is used to study the full structure of propellent flame and illuminate the influence of temperature-dependent material properties on the regression rate, surface temperature, and flame stand-off distance. Results are displayed for HMX and compared with experimental data and numerical simulation with fair success.     

Photoelastic effect and mirage deflection in anisotropic materials

The collinear mirage technique is widely used to measure the thermal diffusivity of semi-transparent materials. However, in a recent paper [A. Salazar, M. Gateshki and A. Sánchez-Lavega: Appl. Phys. Lett. 76, 2665 (2000)], it was shown that for isotropic materials, because of the influence of photoelastic effect, the method was sensitive to the polarization state of the probe beam. The present paper extends the previous work to include anisotropic materials. In particular, we focus on the experimental conditions under which the thermal diffusivity of each crystal system can be measured using the phase method. Our theoretical model indicates that while the thermal diffusivity of isotropic materials can be measured using an unpolarized probe beam, for anisotropic materials, even the use of an unpolarized probe beam does not guarantee the validity of the method in all crystal systems. Experimental measurements performed on cubic, hexagonal and monoclinic crystals confirm the validity of the model. Received: 17 March 2001 / Accepted: 17 March 2001 / Published online: 25 July 2001     

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.