Thermal Diffusivity Measurements of Candidate Reference Materials by the Laser Flash Method

The National Metrology Institute of Japan (NMIJ) in AIST has investigated the laser flash method in order to establish a thermal diffusivity standard for solid materials above room temperature. A uniform pulse-heating technique, fast infrared thermometry, and a new data analysis method were developed in order to reduce the uncertainty in thermal diffusivity measurements. The homogeneity and stability of candidate reference materials such as isotropic graphite were tested to confirm their qualification as thermal diffusivity reference materials. Since graphite is not transparent to both the heating laser beam and infrared light for thermometry, the laser flash method can be applied to graphite without black coatings. Thermal diffusivity values of these specimens with different thicknesses, were measured with changing heating laser pulse energies. A unique thermal diffusivity value can be determined for homogeneous materials independent of the specimen thickness, by extrapolating to zero heating laser pulse energy on the plot of apparent thermal diffusivity values measured with the laser flash method as a function of heating laser pulse energy.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

Sondermetalle und ihre Anwendung im Chemieapparatebau

Refractory Metals and their Application in the Chemical Process Industry Special metals, such as titanium, zirconium and tantalum, are being used increasingly for chemical plant. The exceptional resistance of special metals to many corrosive chemicals - they show it even at high temperatures and pressures - arises not from natural immunity but from the formation of a protective oxide passive layer on the metal surface. Special metals are well suited for welding. Their reactions with gases of the atmosphere must be taken into account though. Welding is therefore possible only under inert gas or a high vacuum. Similarly, alloying with iron-based materials during welding must be avoided under all circumstances. It should be taken into consideration that the melting point of tantalum, for example, is about twice as high as that of steel. Tantalum and niobium are machined with high-speed cutting steels; the cutting speed and cutting angle are similar to those used for stainless steels. In detail, the outstanding properties of special metals in chemical plant are as follows:

• — the stability of titanium under oxidizing conditions
• — the stability of zirconium under reducing and alkaline conditions
• — the resistance of molybdenum to hydrofluoric acid and fluoride
• — the stability of tantalum under oxidizing and reducing conditions.

In pure mineral acids the passive behaviour generally improves in the order titanium - zirconium - tantalum. Except where molybdenum is concerned, the medium should not contain fluoride. The material with the widest range of applications is tantalum. The addition of niobium as an alloying element leads to favourably priced but similarly resistant materials whose prospects of becoming established in the chemical industry and playing a part similar in importance to that of tantalum itself are good.     

Criteria for Selecting Substrates for Nb<Subscript>3</Subscript>Sn Electrodeposition

We studied the effect of substrate material on the microstructure and properties of Nb₃Sn coatings produced by electrochemical coreduction of Nb and Sn ions in the cathodic zone in molten salts. The results demonstrate that continuous superconducting Nb₃Sn coatings can be produced on molybdenum, tantalum, niobium, copper, nickel, Invar and the 60% Ni + 22% Mo + 12% Fe alloy. The coatings grown on molybdenum, tantalum, and niobium have the highest purity. To ensure good superconducting properties of electrodeposited coatings, preference should be given, all other factors being the same, to substrate materials with thermal expansion coefficients close to or smaller than that of Nb₃Sn.     

Oxygen adsorption on the (110) face of tantalum,niobium, molybdenum and tungsten single crystals

Preliminary surface studies using work function techniques are reported for the (110) face of the refractory metals tantalum, niobium, molybdenum and tungsten, both in the clean state and during the introduction of pure oxygen gas at room temperature. The conclusions of Haas et al from LEED measurements on the same group are substantiated. The surface potential changes as a function of exposure for tantalum and niobium are very similar but quite different from molybdenum and tungsten which are also similar. The latter show a two phase adsorption, both of which are electronegative, the former an initial electronegative phase followed by a pressure dependent electropositive phase. Solution of oxygen into the bulk probably takes place for tantalum and niobium to give rise to the electropositive state. Differences in atomic size are small for all four metals and thus the observed groupings are probably a consequence of chemical effects and similarities existing between members of groups VB and VIB.     

Evaluation of Thermophysical Properties of Functionally Graded Materials

In this work, by considering four-layered functionally graded material (FGM) specimens of Cu/Ni and PSZ/NiCrAlY, the transient characteristics and homogeneity of heat conduction media have been studied. The thermal diffusivities of the considered specimens have been measured by the laser flash method. As the temperature response curve of a FGM is very similar to that of a homogeneous material, it is difficult to distinguish a FGM from a homogeneous material by the shape of the temperature responses. Therefore, the thermal diffusivity obtained from the half-time method is usually taken as the corresponding value of the thermal diffusivity. The apparent thermal conductivity, obtained from the corresponding value of the thermal diffusivity and the average of the heat capacity of each layer, is different from the effective thermal conductivity, obtained from the sum of the heat resistances of each layer. As the values of the heat capacity of materials exist over a certain range, and the heat capacity distribution can be predicted when the materials in a FGM are known, the amount of error that will be caused when the effective thermal conductivity is replaced by the apparent value can be determined. Also, the heterogeneity of a FGM, based on an evaluation of thermophysical properties, has been discussed.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui,P. R. China.     

On the behaviour of body-centred cubic metals to one-dimensional shock loading

The response of metallic materials to shock loading, like all loading regimes, is controlled largely by factors operating at the microscopic or atomic levels. Over the past few years, face-centred cubic (fcc) metals have received a level of attention where the role of features such as stacking fault energy and precipitation hardening have been investigated. We now turn our attention to body-centred cubic (bcc) metals. In the past, only tantalum, tungsten, and their alloys have received significant attention at high strain-rate conditions due to their use by the ordnance community. In particular, this investigation examines the shear strength of these materials at shock loading conditions. Previous results on tantalum, tungsten, and a tungsten heavy alloy are reviewed, and more recent experiments on niobium, molybdenum, and Ta–2.5 wt% W presented. Results are discussed in terms of known deformation mechanisms and variations of Peierl’s stress.     

Analysis of interaction between reactants and substrate materials during the growth of tantalum coatings

We have analyzed interaction between reactants and substrate materials during the reduction of tantalum bromide with cadmium in the course of tantalum deposition on Cu, Fe, Mo, and Ni metallic substrates and the 25Kh3M3NBTsA, Ni₃Al, VKNA, and ZhS32 alloys. The results indicate that molybdenum and nickel substrates are the most stable to the reactants used in this study; copper and iron experience etching; and aluminum, chromium, and titanium are capable of competing with cadmium as a reducing agent. The presence of carbon in the substrate material leads to the formation of tantalum carbide in the metal–coating interfacial layer. In the absence of aluminum, oxygen impurities are likely to be removed as oxybromides. Optimal materials for the fabrication of reaction chambers for tantalum deposition are nickel-coated high-temperature steels.     

Hochschmelzende Metalle und ihre Legierungen

Refractory metals and alloys. The application of refractory metals such as niobium, tantalum, molybdenum, tungsten, chromium and rhenium at high temperatures is discussed on the basis of a compilation of their physical, chemical and technological properties. The present state of knowledge in this field indicates, that the high temperature strength needed can be obtained with some of the refractory alloys also at temperatures much higher than 1000°C. However, at present, the oxydation resistance has not yet been improved far enough to allow the use of these materials in air.     

Effect of cold deformation and annealing on the microstructure of electron-beam-evaporated NiZrO3 condensates

The optical properties of thin films of vanadium, niobium, tantalum, chromium, molybdenum and tungsten in the range 0.32–5.6 eV are reported.A Newton-Raphson numerical analysis of the data obtained from three suitably chosen photometric measurements yielded the thickness of the films and the real and imaginary parts of the dielectric constant. The optical conductivity was deduced and compared with the experimental results obtained by others from measurements on bulk monocrystalline or polycrystalline samples.The results obtained from both types of measurement on five metals agree well. For tantalum the discrepancies can possibly be explained by the small grains present in our samples (which were deposited at room temperature) and by the presence of β-tantalum.A very satisfactory interpretation of the conductivity is given in terms of interband transitions in the visible range 1–5.6 eV.     

A possibility for developing high strength soft magnetic materials in FeCo-X alloys

The effects of the combination of cold rolling and heat treatment upon mechanical and magnetic properties have been examined for the FeCo-based alloys with additive elements such as carbon, vanadium, chromium, molybdenum, tungsten, tantalum, niobium, and nickel. It is revealed that whenever alloys including such elements are subjected to cold rolling the yield strength can be raised to between 100 and 200 kg mm^–2, and that cold rolling is effective in both strengthening and in improving soft magnetic properties, even when cold rolling is followed by annealing or ageing. This suggests that these alloys can be used for lessening the weight of parts which are used as soft magnetic materials, although their properties will depend on the combinations of cold rolling and heat treatments, and/or on the added elements and the control of their amount.     

激光脉冲法研究半透明PI/SiO_2薄膜的热扩散率

激光脉冲法在应用于测量对实验探测波段红外线半透明的材料热扩散率时遇到了困难。文中提出了新的解决办法:即通过对理论探测曲线进行分析,并通过在实际探测曲线上的升温幅度和特征点计算得到了热扩散率,成功地解决了这一难题。在-73℃~290℃的范围内获得了对激光和红外线都是半透明的聚酰亚胺(PI)薄膜的热扩散率,并研究了室温下PI/SiO2复合材料的热扩散率随SiO2含量的变化规律。     

Thermal-Diffusivity Measurement of Ceramic Coatings at High Temperature using “Front-Face” and “Rear-Face” Laser Flash Methods

Thick ceramic coatings deposited by plasma spraying techniques are widely used as wear and corrosion resistant coatings at high temperature. To measure accurately the thermal diffusivity of such coatings, the diffusivimeter of LNE has been set up to allow multilayered material studies up to 1,400°C by rear-face and front-face laser flash methods. These two methods have been compared in a large temperature range by measuring the thermal diffusivity of homogeneous (Armco iron and Poco graphite) and multilayered materials (chromium oxide coating deposited on iron alloy substrate). The thermal-diffusivity values measured by using front-face and rear-face techniques are in good agreement, with a relative deviation of less than 5% depending on temperature and materials.     

直流电弧发射光谱法测定铌及铌合金中钽含量

采用直流电弧光谱仪对铌及铌合金中钽元素进行检测。通过试验确定满足分析条件的缓冲剂、样品与缓冲剂的配比及钽元素谱线并检测其精密度、准确度及检出限。结果表明,选择Li2CO3与碳粉1∶6混合作为缓冲剂,缓冲剂与样品比例为1∶2,钽元素的谱线为271.013 nm;加标回收率为94%~109%,测定值的相对标准偏差(测10次)均小于10%。     

Measurement of high temperature phase stability and thermophysical properties of alloy 740

In the present study, the characterisation of phase stability and measurement of different thermophysical properties of alloy 740 has been carried out. The transformation temperatures including liquidus/solidus and corresponding enthalpy of transformation have been measured for different phase changes up to melting using dynamic calorimetry. Further, the enthalpy increment data have been measured in the temperature range of 473–1473?K to obtain the heat capacity using static calorimetry. The present calorimetric data have been analysed in corroboration with the results obtained using JMatPro and Thermo-Calc simulation. In addition, the temperature dependence of other thermophysical properties such as thermal expansivity, density, thermal diffusivity and thermal conductivity are also measured in the range of 300–1473?K using thermomechanical analyser and laser flash method.     

Attenuation of ultrasonic waves in V, Nb and Ta at low temperatures

Variation of ultrasonic attenuation and velocities with temperature have been evaluated in the temperature range 5-50 K due to electron-phonon interaction mechanism in transition metals vanadium, niobium and tantalum for longitudinal and shear waves. For this evaluation, we have also computed the second order elastic constants using Morse potential. Behaviour of acoustical phonons in these bcc metals is different from other normal metals, intermetallics, semimetallics and alloys. Some characteristic features of these materials connected to ultrasonic parameters are discussed.     

Investigation of the Thermal Diffusivity of Human Tooth Hard Tissue

Experiments of two kinds have been performed in which the heat diffusion effects in human tooth hard tissues have been investigated. The first one has been carried out on an incisor tooth as a whole with the use of a bath system. Experiments of the second kind have been done on slice specimens cut out of a tooth. A laser flash apparatus has been utilized. The time dependence of the temperature response has been measured using tiny thermocouples. The experimental data are then used to calculate the effective overall thermal diffusivity of the tooth structures as well as the thermal diffusivity of enamel and dentine alone. A discrepancy between the calculated results and literature data has been discussed.     

Evaporation characteristics of TeO2 in the formation of tellurium oxide thin films

Thin films of tellurium oxide have been formed by evaporating TeO₂ with different boat materials (molybdenum and tantalum) in a high vacuum and at different oxygen partial pressures. The optical and structural properties of the films have been studied. From the variations in the properties with respect to the formation conditions the evaporation characteristics of TeO₂ have been discussed.     

Cu2S相变过程中热扩散系数的精确测量和解析

材料发生相变时, 其结构和物理性能可能会发生剧烈的变化。采用激光闪射法测量热扩散系数时, 激光照射样品可能会伴随有光吸收/发射现象以及温度的显著升高, 导致其测量值偏离真实值。本工作以Cu₂S为研究对象, 发现激光照射样品后, 光吸收/发射的影响很小可以忽略, 但样品温度的升高则会明显影响热扩散系数的测量。通过构建具有不同石墨层厚度的石墨/Cu₂S双层结构, 利用石墨层减弱激光照射时Cu₂S样品的温度增加幅度, 成功使热扩散系数出现显著降低的起始温度接近采用DSC测量材料发生相变的起始温度。本研究进一步建立了石墨/Cu₂S双层结构样品的热流输运模型, 从石墨/Cu₂S双层结构样品的实验测试热扩散系数中解析出了Cu₂S在相变区间的本征热扩散系数。本工作对于理解和精确表征具有相变特征的离子导体热电材料、光敏、热敏材料的热扩散系数具有重要的意义。     

Radiance temperatures at 1500 nm of niobium and molybdenum at their melting points by a pulse-heating technique

Radiance temperatures at 1500 nm of niobium and molybdenum at their melting points were measured by a pulse-heating technique. The method is based on rapid resistive self-heating of the strip-shaped specimen from room temperature to its melting point in less than I s and measuring the specimen radiance temperature every 0.5 ms with a high-speed infrared pyrometer. Melting of the specimen was manifested by a plateau in the radiance temperature-versus-time function. The melting-point radiance temperature for a given specimen was determined by averaging the measured values along the plateau. A total of 12 to 13 experiments was performed for each metal under investigation. The melting point radiance temperatures for each metal were determined by averaging the results of the individual specimens. The results for radiance temperatures at 1500 nm are as follows: 1983 K for niobium and 2050 K for molybdenum. Based on the estimates of the uncertainties arising from the use of pyrometry and specimen conditions, the combined uncertainty (two standard-deviation level) in the reported values is ± 8 K.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Thermal Conductivity and Thermal Diffusivity of Liquid Indium–Tin Alloys

Thermal-conductivity and thermal-diffusivity coefficients of indium–tin alloys have been determined using the laser flash method over the temperature range from the liquidus line to 1173 K. Measurements were performed using the setup LFA-427 of NETZSCH company in an argon protective atmosphere, and cells were produced from molybdenum. The equations for temperature dependences of the thermal conductivity and thermal diffusivity of In–Sn alloys have been obtained. The results of measurements were compared with data available in the literature.