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%.     

Ageing evaluation of thermal barrier coatings by thermal diffusivity

Ceramic thermal barrier coatings (TBC) are widely applied for protecting from combustion gases hot path components of gas turbines for both aero- and land-based applications. In order to prevent the detachment of TBC, it would be essential to monitor their degradation in terms of sintering kinetic. As sintering strongly affects also the thermal diffusivity of TBC, the idea is to measure the latter parameter to account for the former. The technique to measure thermal diffusivity using pulsed thermography is described, together with the model that leads to the identification of TBC diffusivity. Tests and results on specimens artificially aged are reported.     

基于反射式脉冲红外热成像法的定量测量方法研究

本文提出了一种利用反射式脉冲热成像法测量缺陷深度、热扩散系数或缺陷界面热波反射系数的方法. 首先,介绍了脉冲热成像法的基本原理以及定量测量算法.其次,利用304不锈钢制作了平底孔试件 并预埋了四种不同物质并进行了实验,给出在不同条件下对缺陷深度、热扩散系数或缺陷界面热波 反射系数测量的结果.实验结果显示实际测量值与其他方法测量值基本符合, 误差范围在±5%以内, 并讨论了影响测量精度的原因.     

Analysis of melt ejection during long pulsed laser drilling

In pulsed laser drilling, melt ejection greatly influences the keyhole shape and its quality as well, but its mechanism has not been well understood. In this paper, numerical simulation and experimental investigations based on 304 stainless steel and aluminum targets are performed to study the effects of material parameters on melt ejection. The numerical method is employed to predict the temperatures, velocity fields in the solid, liquid, and vapour front, and melt pool dynamics of targets as well. The experimental methods include the shadow-graphic technique, weight method, and optical microscope imaging, which are applied to real-time observations of melt ejection phenomena, measurements of collected melt and changes of target mass, observations of surface morphology and the cross-section of the keyhole, respectively. Numerical and experimental results show that the metallic material with high thermal diffusivity like aluminum is prone to have a thick liquid zone and a large quantity of melt ejection. Additionally, to the best of our knowledge, the liquid zone is used to illustrate the relations between melt ejection and material thermal diffusivity for the first time. The research result in this paper is useful for manufacturing optimization and quality control in laser-material interaction.     

Thermal conductivity and thermal diffusivity of the R134a refrigerant in the liquid state

Thermal conductivity and thermal diffusivity of “ozone-safe” refrigerant R134a in liquid state within the range of temperatures 295.9–354.9 K and pressures from the liquid — vapor equilibrium line up to 4.08 MPa have been studied by high-frequency thermal-wave method. The experimental uncertainties of the temperature, pressure, thermal conductivity and thermal diffusivity measurement errors were estimated to be 0.1 K, 3 kPa, 1.5 and 2.5 %, respectively. Values of thermal conductivity and thermal diffusivity of liquid R134a on saturated line have been calculated. Approximation dependences for thermal conductivity and thermal diffusivity within the whole studied range of temperatures and pressures as well as on the saturated line have been obtained. The work was financially supported by the Russian Foundation for Basic Research (grant No. 07-08-00295-a).     

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.     

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.     

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.     

On the solidification front of a dilute binary alloy: Thermal diffusivity effects and breathing solutions

The asymptotic equation derived in [6],

, to describe the interfacial structure of the solidification front of a dilute binary alloy in the limit in which the solute rejection coefficient is close to unity, is demonstrated to be also valid to lowest order when the thermal diffusivities in the liquid and in the solid are unequal. At higher order nonequal thermal diffusivity effects contribute an integral term. A uniformly valid approximation for the interfacial structure is obtained. The integral term is shown to be capable of producing slowly travelling waves or breathing solutions along the interfacial front when the thermal diffusivity of the liquid is sufficiently larger than the thermal diffusivity of the solid. This slowly oscillatory behavior may signal the proximity of a chaotic region located within the region of linear instability. Latent heat of fusion effects are also discussed.     

Thermal Diffusivity and Thermal Conductivity of Neodymium in the Temperature Range 293 to 1773 K

Thermal conductivity and thermal diffusivity weremeasured for the first time by the laser flash method in the temperature range of thermal diffusivity of neodymium from 293 to 1773 K, including regions of phase transitions. The results are compared with the available literature data. Reference tables for neodymium heat transfer coefficients are designed for scientific and practical use. The possibility to predict the thermal conductivity of neodymium liquid and β-phase using the Wiedemann–Franz law is shown.     

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.     

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     

Thermal properties of 15-mol% gadolinia doped ceria thin films prepared by pulsed laser ablation

Thermal properties of 15-mol% gadolinia doped ceria thin films (Ce_0.85Gd_0.15 O_1.925) prepared by pulsed laser ablation on silicon substrates in the temperature range 473–973 K are presented. Thermal diffusivities and thermal conductivities were evaluated using photoacoustic spectroscopy. The influence of grain size on thermal properties of the films as a function of deposition temperature is studied. It is observed that the thermal diffusivity and the conductivity of these films decreases up to 873 K and then increases with substrate temperatures. The thermal properties obtained in these films are discussed on the basis of influence of grain size on phonon scattering.     

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.     

Anisotropic thermal conduction in a polymer liquid subjected to shear flow

Flow-induced anisotropic thermal conduction in a polymer liquid is studied using force Rayleigh scattering. Time-dependent measurements of the complete thermal diffusivity tensor, which includes one off-diagonal and three diagonal components, are reported on an entangled polymer melt subjected to a uniform shear deformation. These data, in conjunction with mechanical measurements of the stress, provide the first direct evidence that the thermal conductivity tensor and the stress tensor are linearly related in a deformed polymer liquid.     

High accuracy photopyroelectric investigation of dynamic thermal parameters of Fe3O4 and CoFe2O4 magnetic nanofluids

The suitability of the photopyroelectric (PPE) calorimetry in measuring the thermal parameters of nanofluids was demonstrated. The main advantages of the method (concerning nanofluids) as compared to classical calorimetric techniques are: high sensitivity and small amount of sample required. The thermal diffusivity and effusivity of some nanofluids based on Fe₃O₄ and CoFe₂O₄ type of nanoparticles (mean diameter 6.5 nm) were investigated by using two PPE detection configurations (back and front). In both cases, the information is contained in the phase of the PPE signal. Due to the high accuracy of the results (within ±0.5%) thermal diffusivity was found to be particularly sensitive to changes in relevant parameters of the nanofluid as carrier liquid, type and concentration of nanoparticles.     

Synthesis and characteristics of form-stable n-octadecane/expanded graphite composite phase change materials

N-octadecane/expanded graphite composite phase-change materials were prepared by absorbing liquid n-octadecane into the expanded graphite. The n-octadecane was used as the phase-change material for thermal energy storage, and the expanded graphite acted as the supporting material. Fourier transformation infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal diffusivity measurement were used to determine the chemical structure, crystalline phase, microstructure and thermal diffusivity of the composite phase-change materials, respectively. The thermal properties and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the composite phase-change materials exhibited the same phase-transition characteristics as the n-octadecane and their latent heat increased with the n-octadecane content in composite phase-change materials. The SEM results showed that the n-octadecane was well absorbed in the porous network of the expanded graphite, and there was no leakage of the n-octadecane from the composites even when it was in the molten state.     

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.     

Effect of delay of the thermocapillary response of a transparent liquid layer during laser heating of the absorbing substrate

The effect of delay in the thermocapillary response of a transparent liquid layer on an absorbing substrate during its heating by a laser beam is discovered. The response is an interference pattern formed on a screen placed in the beam cross section by the beam reflected from a thermocapillary dip. It is found that the delay time of the response of a thin (h < 1.8 mm) layer of a liquid in a viscosity range of 3–6 mPa s to a 20.9-mW exciting beam from a He-Ne laser (λ = 633 nm) is proportional to the squared layer thickness with a proportionality factor depending on the viscosity and thermal diffusivity of the liquid.     

Laser-induced thermal lens study of the role of morphology and hydroxyl group in the evolution of thermal diffusivity of copper oxide

The paper explores the evolution of thermal behavior of the material by studying the variations in thermal diffusivity using the single beam thermal lens (TL) technique. For this purpose, the decomposition of Cu(OH)₂ into CuO is studied in a time range up to 120 h, by subjecting the sample to morphological, structural, and spectroscopic characterizations. The time evolution of thermal diffusivity can be divided into three regions for demonstrating the dynamics of the reaction. When the reaction is complete, the thermal diffusivity is also found to be saturated. In addition to the morphological modifications, from rods to flakes, the variations in the amount of hydroxyl group are attributed to be responsible for the enhancement of base fluid's thermal diffusivity by 165%. Thus the study unveils the role of hydroxyl groups in the thermal behavior of CuO.