Non-contact sound velocities and attenuation measurements of several ceramics at elevated temperatures

Laser ultrasonic technique has been employed to carry out the sound velocities and attenuation measurements as a function of temperature in alumina, two kinds of silicon nitride and partially stabilized zirconia (PSZ) samples. Accuracy of the laser technique used has been checked in terms of the diffraction effect and reproducibility of the results. Results of attenuation at room temperature have been compared with quartz transducer technique. In PSZ, velocity behavior has become non-linear and also, a peak in attenuation has been observed around 500 degrees C. In one of the silicon nitride sample, which uses glassy sintering agent, attenuation has shown a sharp peak around 950 degrees C. Interestingly, when the experiment was repeated from 800 to 1000 degrees C, this anomalous attenuation peak has disappeared, leaving a background increasing towards higher temperatures.     

Reorientation measurements on tungsten isotopes

In a particle-γ coincidence experiment, a thick tungsten target, of natural isotopic abundance, was bombarded with a and ¹⁶O beams. From analysis of the deexcitation γ-rays following Coulomb excitation, the spectroscopic quadrupole moment of the second 2⁺ state (the 22' state) was determined for ¹⁸⁶W and ¹⁸⁴W. In a separate Coulomb excitation experiment a thin, isotopically enriched ¹⁸⁶W target was bombarded with ¹⁶O ions. From analysis of projectiles scattered elastically and inelastically the quadrupole moment of the 2^+' state of ¹⁸⁶W was extracted. The results of the two experiments are in good agreement. The quadrupole moment of the 2^+' state is found to be opposite in sign to that of the first 2⁺ state for both isotopes studied. However, its magnitude decreases rapidly in going from ¹⁸⁶W to ¹⁸⁶W, in contrast to the predictions of the rotation-vibration or asymmetric rotor models. The microscopic theory of Kumar and Baranger does predict the experimental trend, qualitatively. Thus the present results are interpreted as being evidence of strong coupling between β and γ degrees of freedom in the tungsten isotopes, which, according to the theory of Kumar and Baranger, is the source of the reduced value of the quadrupole moment.     

Temperature dependence of sound velocities in TTF-TCNQ

Measurements of a and b axis sound velocities in TTF-TCNQ single crystals are presented from 0–300K. The Young's modulus is larger in the a than in the b direction. The temperature dependence of the velocity is compared with heat capacity and thermal expansion data. A 1.5% increase in the velocity below 52K is attributed to the freezing out of the conduction electrons.     

Mechanical properties of vanadium carbide and a ternary vanadium tungsten carbide

Ab initio total energy calculations are performed on non-stoichiometric vanadium carbide with supercells representing vacancy concentrations of V C_0.875 and V C_0.75. The V C_0.875 supercell retains a cubic symmetry whilst in the case of the V C_0.75 supercell C vacancies located in close proximity have the lowest energy configuration and the cubic lattice slightly distorts to a monoclinic symmetry. Using a stress–strain calculational procedure, the elastic constants of both the cubic and the monoclinic systems are deduced. In all cases C vacancies decrease the elastic moduli.A similar analysis is then applied to consider when W is incorporated into VC. In this case it is found that the elastic moduli increase with W content suggesting that a V-W-C alloy could have significant potential as a novel hard material.     

Estimation of the area function of human ear canals by sound pressure measurements

A method to determine the "area function" of a duct, i.e., the variation of the cross section along the middle axis of the duct is described. It is shown that it is sufficient to measure the sound pressure at three locations in the duct to get knowledge of the area function. The calculated approximation fits the true area function the better the broader the bandwidth of the sound pressure signals is chosen. The bandwidth is bounded by the condition that no higher modes of sound propagation must be excited. In principle, the area function can be determined by solving a linear system of equations and executing a simple iterative algorithm. Some more work however is necessary to stabilize the computation against unavoidable small measuring errors. The calculated area function reproduces the acoustical transformation characteristics with good precision. In spite of this the calculated area function may disagree with the real area function in some regions, especially at the edges. Such errors occur, if the deviations of the area function from constant cross section correspond to frequencies outside the range of measurement. The method was applied to determine the area function of human ear canals and the results are presented.     

Heat transfer from small tungsten spheres into an ambient H2 atmosphere

The transfer of heat from tungsten spheres into an ambient H₂ atmosphere was investigated. The spheres had a radius between 35 and 100 µm and were heated to 1400–2000 K by a laser beam. The H₂ pressure ranged from 35 to 1000 mbar. Heat transfer by convection is effective with large spheres and high H₂ pressures. With small spheres and low H₂ pressures, a pronounced temperature discontinuity at the interface between the tungsten sphere and the ambient gas was observed. The results are relevant to the modelling of gas-phase transport phenomena in laser processing.     

Estimation of the long term average sound level from hourly average sound levels

It is shown how to estimate the long-term average sound level, L_AeqLT (for free flowing road traffic) from measurements of the hourly A-weighted equivalent sound level, L_Aeq1h. To estimate the parameters of the model which describe noise emission and attenuation, concurrent measurements of L_Aeq1h at two distances from the considered road are needed. A semi-empirical formula is derived for L_AeqLT approximation. Also the uncertainty of this approximation is given as a function the distance from the road and receiver height.     

Effects of laser ablation on cemented tungsten carbide surface quality

Although laser micromachining has been touted as being the most promising way to fabricate micro tools, there has been no proper evaluation of the effects of laser ablation on bulk material properties.     

High-pressure,high-temperature thermophysical measurements on tungsten

Abstract

Wire shaped tungsten samples (0.25 mm diameter, 40 mm length, 99.97%) are resistively pulse heated as part of a coaxially built up capacitor discharge circuit. With heating rates of more than 10⁹ K/s, temperatures up to 12000 K are reached. The tungsten wire is contained with the surrounding medium water in a high pressure vessel with sapphire windows and a maximum pressure capability of 0.5 GPa. Time correlated measurements of the current through the wire and the voltage drop across it as well as surface radiation and wire expansion can be performed simultaneously and allow the determination of thermophysical properties for the liquid tungsten (see key words). All measuring systems have rise times less than 10 ns.     

Boron carbide coating deposition on tungsten and testing of tungsten layers and coating under intense plasma load

A device intended for boron carbide coating deposition and material testing under high heat loads is presented. A boron carbide coating 5 μm thick was deposited on the tungsten substrate. These samples were subjected to thermocycling loads in the temperature range of 400–1500°C. Tungsten layers deposited on tungsten substrates were tested in similar conditions. Results of the surface analysis are presented.     

Synthesis of tungsten carbide nanopowder via submerged discharge method

Tungsten carbide nanopowder was prepared via pulsed discharge of bulk tungsten and graphite rods immersed in pure ethanol. The effect of discharge parameters on the characteristics of final products was investigated. Structural and morphological characterization of nanopowder was performed by means of X-ray diffraction analysis and transmission electron microscopy. In order to determine the feasibility of using synthesized material as an electrocatalyst, tungsten carbide nanopowder was tested for hydrogen evolution. A correlation was found between morphology of nanoparticles, their phase composition and electrocatalytic activity.     

新型圆角式高压碳化钨硬质合金顶锤的有限元分析

基于有限元法, 对新型圆角式高压碳化钨硬质合金顶锤进行了分析与研究.研究结果表明: 新型圆角式高压碳化钨硬质合金顶锤在不降低顶锤的传压效率的前提下, 能够将顶锤的使用寿命延长3.05%—16.75 %; 新型圆角式高压碳化钨硬质合金顶锤获得的极限腔体压力值可增加至6.09 GPa, 较传统顶锤(5.80 GPa)提高5%, 从而扩宽高压下功能材料的合成区间.新型圆角式高压碳化钨硬质合金顶锤的使用, 将降低六面顶液压机的使用成本, 促进高压技术和材料科学等学科的发展.     

反向碰撞法测量Sn的高压卸载声速

采用任意反射面速度干涉系统,通过对窗口碰撞面做特殊防护处理,发展了反向加载测量高压卸载声速的实验技术,解决了以往因使用缓冲层而不能得到样品材料体积声速的局限.利用该方法测量了Sn样品在37—80 GPa压力范围内的高压卸载声速,得到了较高精度的声速测量结果(纵波声速测量误差约为2%,体积声速误差约为5%),且与热力学理论计算结果一致. 关键词： 卸载声速 反向碰撞法 任意反射面速度干涉系统 Sn     

Dipole resonance scatterer of sound

A dipole resonance scatterer of sound in the form of a short (in comparison with the quarter-wave) tube closed on one side with a stretched membrane is investigated theoretically and experimentally. The coefficient of sound transmission through the cross section of a measuring tube, into which the resonance scatterer is placed, is calculated and measured. It is shown that the waveguide is totally blocked at the resonance frequency of the resonator.     

Thermoelectric measurements on tungsten at ultra low temperatures

Measurements are reported of the thermoelectric ratio G for three zone-refined tungsten samples at temperatures down to 45 mK. For the purest sample $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000)}$, G behaves simply and in accord with expectations. For two less pure samples $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000 and 22 000)}$, G behaves anomalously, becoming increasingly more negative with decreasing temperature down to at least 45 mK. This anomalous behavior is tentatively attributed to trace amounts of iron.     

基于声共振原理的管道长度测量仪

利用平面声波在管内形成驻波,根据管口声阻抗公式推导了驻波发生时相邻共振频率之差与管道长度的定量关系,利用自制装置实现了管道(直管、弯管)长度的测量,测量误差均在1.5%以下.     

A Spin—Orbit-induced surface resonance on tungsten

A three-band model tight binding Greens function formalism is used to calculate the electronic structure of the (100) surface of tungsten in the vicinity of the Fermi level. The results exhibit a large surface resonance, which provides an explanation for a surface-sensitive peak observed in recent field- and photoemission experiments.