Depth profiling the thermal reflection coefficient of an opaque solid via an inverse thermal wave scattering theory based on the Method of Images

A new formulation of the inverse problem of depth profiling the thermal properties of an opaque solid based on one-dimensional photo-generated thermal waves is presented. The inverse problem as posed is linear in a set of lumped thermal reflection coefficients which account for the return of energy to the surface by all significant heat conduction channels. An analysis based on the Method of Images relates these coefficients to individual values of the interface thermal reflection coefficients in the material. No weak backscattering assumption is invoked to linearize the problem. The method yields a unique solution subject to a given condition of regularization. Solutions recovered by the method are stable at experimentally feasible error levels. Received: 27 September 1999 / Published online: 16 June 2000     

Multi-point side pumping scheme of fiber lasers for high-power diode arrays

A multi-point side pumping scheme of double-clad fiber lasers was presented for high-power diode arrays. It made use of multiple dielectric transmission gratings to realize high coupling efficiency and low pump light leakage ratio simultaneously. Appropriate material and practical structural parameters of the transmission gratings were chosen and designed by using a rigorous Eigen-mode theory combined with a genetic algorithm (GA) and a quasi-Newton method. The GA provided initial parameter values for the quasi-Newton method to get the final optimal results. For TE-polarized pump lights, a perfect transmission grating structure was found for side pumping and the coupling efficiency (CE) of 98% can be achieved. Meanwhile, the CE is 83% for TM-polarized pump lights. And the leakage ratios of the pump lights were only 11.3% and 4.9% for TE- and TM-polarized when a multi-point side pumping scheme was adopted.     

A fractal model for heat transfer of nanofluids by convection in a pool

Based on the fractal distribution of nanoparticles, a fractal model for heat transfer of nanofluids is presented in the Letter. Considering heat convection between nanoparticles and liquids due to the Brownian motion of nanoparticles in fluids, the formula of calculating heat flux of nanofluids by convection is given. The proposed model is expressed as a function of the average size of nanoparticle, concentration of nanoparticle, fractal dimension of nanoparticle, temperature and properties of fluids. It is shown that the fractal model is effectual according to a good agreement between the model predictions and experimental data.     

Measurement of the thermal properties of liquid mixtures using a thermal wave interferometer

In this paper we discuss the use of an alternative photothermal technique for measurements of thermal properties of liquid mixtures. The proposed technique is based upon the concept of a thermal wave interferometer. The liquid sample is confined between two thin pyroelectric detectors. One of these detectors acts as a modulated light absorber while the other is used for sensing the temperature fluctuations transmitted through the liquid layer. It is demonstrated that the proposed experimental configuration allows us to fully characterize the thermal properties of the constituent liquids. Received: 19 February 2001 / Published online: 18 July 2001     

Novel grating couplers for diode-bars multi-point side-pumping double-clad fiber

Three grating couplers for diode-bars single-point and multi-point side-pumping double-clad fiber (DCF) are presented. The principles, characteristics and coupling efficiencies of these grating couplers are described and computed by a rigorous electromagnetic theory. We show computationally that the optimized gratings can couple the high-power TE- or TM-polarized pump lights with 94.7% and 79.7% into the DCF, respectively. For an un-polarized pump light, 72.4% efficiency can be reached. We also have analyzed the leakage of pump powers for diode-bars multi-point side-pumping DCF. Besides, the fabrication tolerances and other performances of the grating couplers are analyzed.     

Photoacoustic measurements of the thermal properties of AlyGa1-yAs alloys in the region 0<y<0.5

y Ga_1-yAs alloys grown by liquid phase epitaxy on GaAs substrates, by means of the open photoacoustic cell detection technique and the temperature-rise method under continuous light illumination. The values of the thermal conductivity, diffusivity and specific heat were obtained in the 0<y<0.5 region, where the Al_yGa_1-yAs band gap is mainly direct. The technique presented here is based upon an effective sample model which is shown to be suitable for the determination of the thermal properties of two layer semiconductor specimens. Received: 15 November 1996/Accepted: 5 March 1997     

Comparison of end-pumped and multi-point pumped Yb3+-doped gain guided and index antiguided fiber laser

End-pumped and multi-point pumped Yb³⁺-doped gain guided and index antiguided (GG+IAG) fiber laser are analyzed by solving the rate equations and 3-D thermal conduction equations. Simulation results show that the multi-point pumped scheme can provide lower operating temperature and better temperature uniformity. Meanwhile, fabrication complexity, loss resulted from pump point should be taken into consideration in the practical design.     

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     

Analysis of liquid surface films by pulsed laser photoacoustic spectroscopy

The photoacoustic generation of plane acoustic waves in strongly absorbing or opaque liquids by pulsed laser radiation is discussed both experimentally and theoretically. The regimes of a confined and a free surface of the liquid are considered. The model which takes the temporal shape of the laser pulses applied in the experiments into account, implies that spectroscopic studies are feasible with direct photoacoustic generation and detection also for opaque liquids. The experiments are performed with a tunable hybrid CO₂ laser and piezoelectric detection. For the first time liquid/liquid interfaces are studied by this technique. We demonstrate that the presence of an absorbing liquid film with a thickness of >1 m on the surface of another liquid amplifies the acoustic signal which is detected in the bottom liquid. The enhancement depends on the thickness and the optical and thermal properties of the film medium. The surface layer can be analyzed on the basis of the photoacoustic spectrum. It is also shown that this non-contact method is surface-film selective and should thus prove useful for pollution analysis of liquid surfaces.     

Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

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.     

Laser manipulation of the size and shape of supported nanoparticles

Laser manipulation of the size and shape of metal nanoparticles prepared by self assembly of atoms on dielectric surfaces is discussed. The technique relies on the optical properties of the aggregates and the ability to remove atoms from their surfaces by laser induced thermal evaporation. A theoretical model which allows one to understand the basic mechanisms of the process is presented. Furthermore, experiments are reviewed which demonstrate that laser irradiation can be exploited for strong narrowing of initially broad size distributions yielding almost monodispersed samples and generation of aggregates with predetermined shape irrespective of their size. This makes possible preparation of very special surfaces with novel physical and chemical properties. Optical spectroscopy of the supported particles is demonstrated to be a very sensitive tool for characterization of such adsorbate/substrate systems, in particular for detection of laser induced modifications of the nanostructured surfaces. Finally, prospects for future experiments in this field and possible applications of the monodispersed systems are outlined. Received: 31 March 2000 / Accepted: 21 June 2000 / Published online: 7 March 2001     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Efficient design considerations for end-pumped solid-state-lasers

The temperature distribution in the gain medium and key design parameters for an ideal-four level end-pumped solid-state laser have been analyzed depending on the crystal's length, absorption coefficient, and pump beam M² factor. The optimum key design parameters and thermal focal lens are obtained by minimizing the root mean square of pump beam radius in the laser crystal. It is found that the focal thermal lens and key design parameters are dependent on the gain medium characteristics and pump beam properties as well. By considering the Poisson equation in cylindrical coordinate and Top-Hat pumping profile, an analytical formula has been derived to introduce the thermal focal length in the end-pumped lasers. A formula is also presented to relate the requirements of pump source to the gain medium properties for working laser at the design point.     

Determination of thermal conductivity of CVD diamond films via photoacoustic measurements

A novel method of determination of polycrystalline diamond films’ thermoconductive properties using the photoacoustic effect is proposed. By this method, we studied the diamond films grown on silicon substrates using chemical vapor deposition technique. A value of thermal conductivity obtained for the films was less than half that for diamond single crystal. The decrease of thermal conductivity, as well as characteristic features of optical properties of the films, is explained by the presence of a large amount of intercrystallite boundaries and other structure defects and admixures detected using Raman and photoluminescent spectroscopies. Received: 22 October 1998 / Accepted: 27 January 1999 / Published online: 28 April 1999     

二维费密液体理论(Ⅲ)——输运性质和声传播特性

本文研究了二维费密液体的输运性质和声传播特性,得到了扩散系数、粘滞系数和热导率的微观表达式以及零声和第一声的有关结果。通过对碰撞积分的讨论,认为须用一系列弛豫时间参数来描述二维液体,这与在三维情形可用一个唯象参数来描述不同。 关键词：     

Analytical prediction of forced convective heat transfer of fluids embedded with nanostructured materials (nanofluids)

Nanofluids are a new class of heat transfer fluids developed by suspending nanosized solid particles in liquids. Larger thermal conductivity of solid particles compared to the base fluid such as water, ethylene glycol, engine oil etc. significantly enhances their thermal properties. Several phenomenological models have been proposed to explain the anomalous heat transfer enhancement in nanofluids. This paper presents a systematic literature survey to exploit the characteristics of nanofluids, viz., thermal conductivity, specific heat and other thermal properties. An empirical correlation for the thermal conductivity of Al₂O₃ + water and Cu + water nanofluids, considering the effects of temperature, volume fraction and size of the nanoparticle is developed and presented. A correlation for the evaluation of Nusselt number is also developed and presented and compared in graphical form. This enhanced thermophysical and heat transfer characteristics make fluids embedded with nanomaterials as excellent candidates for future applications.      

Simultaneous ultrasound and microwave new reactor: detailed description and energetic considerations

A new reactor in which microwaves (MW), delivered by a coaxial dipole antenna, and ultrasound (US), delivered by a metallic horn, can be simultaneously used in a liquid to perform different types of processes, widely referenced in literature, is presented in detail. Calibrations of thermal energy delivered to two liquids having very different dipolar moments (i.e. water and cyclohexane) using MW and US, both separately and simultaneously, are performed by employing the traditional calorimetric method. The main results are: (i) MW and US used simultaneously increase the thermal energy delivered to the two liquids with respect to their separate use, but differently using water or cyclohexane, and (ii) the total power absorbed by polar or non polar liquids is very different, both using MW and US.     

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.     

Thermodynamic aspects of the glass transition of silicates

The marked increase in second-order thermodynamic properties observed at the glass-transition signals the onset of configurational changes in viscous liquids. Experimental determinations in the glass-transition range will illustrate this point for thermal expansivity and demonstrate that the kinetics of volume, enthalpy and structural relaxation are identical for silicate liquids. Within the framework of the Adam–Gibbs theory, heat capacity and viscosity data may be combined to calculate configurational entropies and gain insights into the potential energy barrier to viscous flow. Finally, the considerable effect of water on the glass transition temperature of geologically relevant silicates is presented.