On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

Summary By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the ?fractional heat loss? in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems. This work was supported by the Gruppo Nazionale di Struttura della Materia of the Consiglio Nazionale delle Ricerche and by the Centro Interuniversitario di Struttura della Materia of the M.P.I., Roma (Italia).     

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     

Photothermal measurements in determination of the thermal diffusivity of SiC

In this work a measuring method for determination of the thermal diffusivity of SiC substrates commonly used in electronic industry is proposed. Such substrates are (300 900) μm thick plates, The method bases on generation of temperature field disturbance in the sample by modulated light beam. Fitting of theoretical curves to the ratio of signals measured in two experimental geometries as a function of modulation frequency allows determination of the thermal diffusivity and optical absorption of the sample. Optical absorption is also measured independently using typical spectrometer. Results obtained for a few SiC substrates confirm usability of proposed method for investigation of such samples. Determined thermal diffusivities of polycrystalline samples are of the order of (0.2 0.5) cm² s ^{- 1}. The thermal diffusivities of single crystals are considerably higher and reach (1.3 1.5) cm² s^-1.     

Infrared thermography protocol for simple measurements of thermal diffusivity and conductivity

An accurate characterization of thermal properties requires the knowledge of both thermal diffusivity and conductivity. Most of the time, the thermal conductivity have to be measured with complicated setups. In this paper, we show that a combination of two experiments carried out with the same setup – stimulated infrared thermography – allows to estimate straightforwardly and quickly the absolute value of the thermal diffusivity and the thermal conductivity relatively to a reference material.     

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.     

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.     

Validation of hard tooth tissue thermal diffusivity measurements applying an infrared camera

The problem concerns investigations of thermophysical properties of small biological specimens. Results of experimental verification of some thermal diffusivity data of hard tooth tissue are presented and discussed. The measurements have been performed on two sliced tooth specimens of different thicknesses. The temperature changes of one flat surface have been recorded applying an infrared camera while the opposite surface has been monotonously heated. The results of measurements have been analysed in order to identify the differences related to variations in thermal properties of hard tooth tissues. Distinct differences between heated enamel, crown dentine and root dentine structures have been revealed. Lack of analytical heat transfer formulae restricted the analysis to qualitative aspects of the problem. Conclusions concerning possible quantitative calculations and the methodology of such an analysis have been formulated.     

Theoretical analysis of PPE measurements in liquids using a thermal-wave cavity

The photopyroelectric measurements in a thermal-wave cavity (PPE) were analyzed with a conventional one-dimensional approach and a three-dimensional approach. The calculations show that the dimensionality of the thermal-wave field in the cavity depends on the boundary conditions and the beam size of the applied laser. The study resulted in identifying ranges of heat transfer rates and cavity configurations for which accurate quantitative measurements of the thermal diffusivity of intracavity fluids can be made within the far simpler, but only approximate, one-dimensional approach conventionally adopted by users of thermal-wave cavities.     

Thermal diffusivity measurements in porous ceramics by photothermal methods

Received: 16 September 1996/Accepted: 6 January 1997     

Measurement of thermal diffusivity by magnetic resonance imaging

Nuclear magnetic resonance (NMR) may be used for monitoring temperature changes within samples based on measurements of relaxation times, the diffusion coefficient of liquids, proton resonance frequency or phase shifts. Such methods may be extended to the explicit measurement of the thermal diffusivity of materials by NMR imaging. A method based on measuring nuclear spin phase shifts or changes in the equilibrium nuclear magnetization has been developed for measuring transient thermal diffusion effects and thermal diffusivity with potential applications in NMR thermotherapy and materials science. In this method, a thermal pulse is applied to a medium, and the resultant temporal variations of the nuclear spin phase or of the magnitude of the nuclear magnetization produced by the thermal pulse are monitored at a spatial distance. The results obtained on common fluids agree well with the data from other methods.     

Simultaneous measurements of thermal diffusivity, thermal conductivity and thermopower with application to copper and ceramic superconductors

A home made experimental set-up allows us to measure the thermal conductivity, the thermopower and the thermal diffusivity simultaneously in the temperature range (20-300 K). Therefore the specific heat can be deduced. The role of a radiation shield is shown to be relevant. Tests of the system are made on a 99.9% pure Cu sample and two polycrystalline cuprate ceramics for illustration. Without any complicated optimisation, the technique indicates much promise already due to its efficiency and rapidity. Received 9 November 1998 and Received in final form 12 July 1999     

Elastic modulus and in-plane thermal diffusivity measurements in thin polyimide films using symmetry-selective real-time impulsive stimulated thermal scattering

We describe a technique for extending the utility of the real-time Impulsive Stimulated Thermal Scattering (ISTS) method for thin film characterization. Using weakly absorbed excitation pulses, we show how to selectively drive acoustic waveguide modes that are unobservable when strongly absorbed pulses are used. The ability to excite and monitor these modes is important because it allows for a significant increase in the experimental sensitivity to the film longitudinal velocity. This arrangement also greatly simplifies determination of the in-plane thermal diffusivity. The technique is illustrated through study of unsupported polyimide films with six different thicknesses.     

Current views on thermal conductivity and diffusivity measurements of oxide melts at high temperature

There have been various attempts to measure thermal conductivity of relatively low conductive materials such as oxides and nitrides, at high temperature. However, their accuracy was not sufficient to allow quantitative discussion mainly because of experimental difficulties, particularly at sample temperature above 1000 K, for several reasons. For example, the radiative heat flow is known to play a significant role at higher temperature, but the separation of such a component from the measured heat flows has only been established recently. An attempt will be made in this paper to review current information on thermal conductivity measurement of oxide melts at high temperature above 1000 K. Thus, this review is primarily concerned with the several methods for determining thermal conductivity of high temperature melts including their respective merits and demerits. A new laser-flash method proposed in the early 1960's has received much attention for determining thermal conductivity of high temperature melts and is an even better technique than the conventional techniques such as the hot-wire method. Although some advantages of the laser-flash method are readily apparent, major progress has been obtained only in the last few years and new information is now available. Therefore, it will be the authors' intention to provide the principle and the new data processing for the laser-flash method, including some selected examples of its application to determining thermal conductivity of oxide melts at high temperature. The relevant significant quantities such as the radiative heat transfer coefficient are also discussed, in order to facilitate the understanding of the present status of fundamentals and the powerful link between physical constants such as the reflective index of sample material and the required formulae for determining thermal conductivity of high temperature melts by the laser-flash method.     

Investigation of poling field effects on PVDF pyroelectric detectors: Photoacoustic thermal diffusivity measurements

The influence of poling fields on the electrical and thermal properties of polyvinylidene fluoride (PVDF) pyroelectric detectors is discussed. It is shown that both the dielectric constant and the thermal diffusivity exhibit a monotonically increasing behavior with the poling field. The detectors were fabricated by solution casting of the PVDF films followed by thermal poling. It is concluded that the predominant crystal phase present in our films is the -form. Except for the dielectric breakdown field, all the electrical and thermal properties of these films were found to be in good agreement with the commercially available ones. The thermal diffusivity was measured using the so-called open photoacoustic cell method.     

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.     

Determination of thermal diffusivity of suspended porous silicon films by thermal lens technique

An alumina suspension containing 21 vol.% solids was made to flow through a needle at rates between 10^-12–10^-11 m³s^-1 and was subjected to electrostatic atomization at different applied voltages in the range 5–13 kV. The resulting modes of atomization were studied. The stable cone-jet mode was first obtained at 7 kV and 2.2×10^-12 m³s^-1 and the effect of increasing flow rate and applied voltage on the jet diameter was investigated. Using a pointed ground electrode the alumina droplets produced by the jet in the stable cone-jet mode were printed according to a pre-determined architecture. Alumina relic diameters in the print were <35 m. PACS 81.05.Je; 81.20.Ev; 81.20.Rg; 81.15.Pq; 47.27.Wg     

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.     

GaP材料热导率的测量

针对目前用于半导体材料掺杂浓度纵向深度分布测量用的方法，如Ｃ－Ｖ法、电化学法等方法对样品造成的损伤或污染，本文提出一种非接触式的光热法测量技术。并结合室温下光热法测得的实验数据计算了ＧａＰ∶Ｎ多层材料中不同杂质浓度下的热导率，获得热导率随半导体内掺入的杂质浓度增加而减小的关系。这一结果与现有用光热法测量单层结构的半导体材料得到的规律相符合，从而表明了用光热法对层状半导体材料进行掺杂浓度纵向分布测量的可行性；同时还讨论半导体内临近层间结晶程度的差异对光热信号幅度造成的影响。     

PTR技术测量材料的热扩散系数的线性化方法

报道了一种用光热辐射 (PTR)技术测量不透明材料热扩散系数的优化方法。从光热辐射理论出发 ,在一定条件下 ,推导出位相信号和调制频率的关系表现为线性关系 ,分析实验数据得出材料的热扩散系数。     

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.