Measurement of surface tension of tantalum by a dynamic technique in a microgravity environment

A dynamic technique has been used in a microgravity environment to measure the surface tension of tantalum at its melting point. The basic method involves resistively heating a tubular specimen from ambient temperature to temperatures above its melting point in about 1 s by passing an electrical current pulse through it, while simultaneously measuring the pertinent experimental quantities with millisecond resolution. A balance between the magnetic and the surface tension forces acting on the specimen is achieved by splitting the current after it passes through the specimen tube and returning a fraction of the current along the tube axis and the remaining fraction concentrically outside the specimen. Values for surface tension are determined from measurements of the equilibrium dimensions of the molten specimen tube and the magnitudes of the currents. Rapid melting experiments were performed during microgravity simulations with NASA's KC-135 aircraft and the results were analyzed, yielding a value of 2.07±0.06 N · m^–1 for the surface tension of tantalum at its melting point. Conditions for improving specimen stability during temperature excursions into the liquid phase are discussed.     

A dynamic technique for measuring surface tension at high temperatures in a microgravity environment

The feasibility of a dynamic technique for measuring surface tension of liquid metals at high temperatures in a microgravity environment is considered. The basic method involves heating a tubular specimen resistively from ambient temperature through its melting point in about l s by passing an electrical current pulse through it, while simultaneously recording the pertinent experimental quantities. Static equilibrium for the molten specimen may be achieved (at least for a short time) in a microgravity environment by splitting the current after it passes through the specimen tube and returning a fraction along the tube axis and the remaining fraction outside the specimen. Adjustments to the current split enable a balance between the magnetic and surface tension forces acting on the specimen. Values for surface tension are determined from measurements of the equilibrium dimensions of the molten specimen tube, and the magnitudes of the currents. Preliminary rapid melting experiments, performed during microgravity simulations with NASA's KC-135 aircraft, yield a value for the surface tension of copper at its melting point which is in reasonable agreement with literature data.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Mathematical Models for Pulse-Heating Experiments

Accurate measurements of thermophysical properties at high temperatures (above 1000 K) have been obtained with millisecond pulse-heating techniques using tubular specimens with a blackbody hole. In the recent trend toward applications, simpler specimens in the form of rods or strips have been used, with simultaneous measurement of the normal spectral emissivity using either laser polarimetry or integrating sphere reflectometry. In these experiments the estimation of the heat capacity and of the hemispherical total emissivity is based on various computational methods that were derived assuming that the temperature was uniform in the central part of the specimen (long thin-rod approximation). The validity of this approach when using specimens with large cross sections (rods, strips) and when measuring temperature on the specimen surface must be verified. The application of the long thin-rod approximation to pulse-heating experiments is reconsidered, and an analytical solution of the heat equation that takes into account the temperature dependence of thermophysical properties is presented. A numerical model that takes into account the temperature variations across the specimen has been developed. This model can be used in simulated experiments to assess the magnitude of specific phenomena due to the temperature gradient inside the specimen, in relation to the specimen geometry and to the specific thermophysical properties of different materials.     

A dynamic technique for measurements of thermophysical properties at high temperatures

A technique is described for the dynamic measurement of selected thermophysical properties of electrically conducting solids in the range 1500 K to the melting temperature of the specimen. The technique is based on rapid resistive selfheating of the specimen from room temperature to any desired high temperature in less than 1 s by the passage of an electrical current pulse through it and on measuring the pertinent quantities, such as current, voltage, and temperature, with millisecond resolution. The technique was applied to the measurement of heat capacity, electrical resistivity, hemispherical total emissivity, normal spectral emissivity, thermal expansion, temperature and energy of solid-solid phase transformations, melting temperature, and heat of fusion. Two possible options for the extension of the technique to measurements above the melting temperature of the specimen are briefly discussed. These options are: (1) submillisecond heating of the specimen and performance of the measurements with microsecond resolution, and (2) performance of the experiments in a near-zero-gravity environment with millisecond resolution.Paper presented at the Japan-United States Joint Seminar on Thermophysical Properties, October 24–26, 1983, Tokyo, Japan.     

Effects of Surface Morphology and Composition Modification on Spectral Emissivity and Thermal Diffusivity Profile at High Temperatures

A new all-spectroscopic method for depth-resolved thermal diffusivity measurement of metallic specimens has been demonstrated. The method entails measurement of the mass entrained into a laser-produced plasma (LPP) plume in such a manner that the plume is representative of the specimen in elemental composition. Both the abundance of matter and its elemental composition are measured by time-resolved spectroscopy for each LPP plume. In order to delineate the morphology versus composition basis of the depth dependence, a new study on a Nichrome ribbon specimen heated by ohmic heating in a vacuum is presented. A set of depth-resolved thermal diffusivity measurements is carried out, while noting the attendant changes in the spectral emissivity and elemental composition at succeeding ablation layers. Additional measurements are carried out after the specimen has been treated under varying heating conditions. Preferential diffusion of chromium at high temperatures has been found to contribute to the dynamics of surface thermophysical properties at high temperatures. Representative LPP ablation is well suited for removal of surface impurities prior to thermophysical property measurements by the pulse heating technique.     

Chemical reaction assisted transient liquid phase bonding of alumina in combination with cold isostatic pressing

Abstract

A new method of transient liquid phase (TLP) bonding of alumina specimens has been developed using a mixture of aluminium powder and silica powder as insert materials. A chemical reaction of aluminium with silica occurs in the inter layer to produce alumina and silicon. Some of the specimens were subjected to cold isostatic pressing (cipping) before bonding to improve the bonding strength. Specimens with an interlayer of powder mixture were joined for Al/SiO₂ ratios of 1 : 0.84 and 1 : 0.42, but did not join for an interlayer with a theoretical ratio of 1 : 1.67. When specimens were subjected to cipping before bonding, bonds were far stronger than bonds without cipping in a temperature range from room temperature to elevated temperatures above the melting point of aluminium. In the mechanical test (bending test), fracture occurs at the boundary between the alumina matrix and the interlayer at room temperature, and in the interlayer at temperatures above the melting point of aluminium.     

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.     

A dynamic technique for measuring normal spectral emissivity of electrically conducting solids at high temperatures with a high-speed spatial scanning pyrometer

A dynamic (subsecond) technique is described for measuring normal spectral emissivity of electrically conducting solids at high temperatures, primarily in the range 1800 K up to near their melting point. The basic method involves resistively heating a tubular specimen from ambient temperature through the temperature range of interest in less than 1 s by passing an electrical current pulse through it, while using a high-speed spatial scanning pyrometer to measure spectral radiance temperatures along a 25-mm length on the specimen. This portion of the specimen includes a small rectangular hole that approximates a blackbody cavity. Measurements of spectral radiance temperature of the specimen surface as well as specimen true temperature enable the determination of the normal spectral emissivity of the surface via Planck's law. The applicability of the technique is demonstrated by measurements performed on molybdenum in the range 1900–2850 K.     

一种新型热物性测量系统的研究

设计了一种动态测定物质相变点附近热物性的新型检测系统,通过这一检测系统对熔点温度较高的金属及合金相变点附近的热物性进行了测定,填补了国内外相关文献的空白。     

Thermophysical property measurements on molten semiconductors using 10-s microgravity in a drop shaft

The effectiveness of 10-s microgravity on thermophysical property measurements on molten materials, such as molten semiconductors, is discussed. The thermal conductivity of molten InSb was successfully measured under microgravity conditions on board the German sounding rocket TEXUS and in a drop shaft in Hokkaido, Japan. Surface tension measurements using an oscillating drop method was attempted in low gravity using a parabolic flight of the NASA KC-135 aircraft. Combined levitation and microgravity, which can provide a contamination-free and undercooled condition. is recommended as a novel approach to obtain missing thermophysical property data on undercooled melts of semiconductors.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29. 1995. Köln, Germany.     

Thermophysical Properties of Boron Carbide Irradiated by Ionizing Radiation

Differential-scanning calorimetry is used to study the thermophysical properties of boron carbide irradiated by the ionizing radiation from the ⁶⁰Co source. With increased temperature, the heat capacity and entropy values of nonirradiated and irradiated B₄C specimens increase. At high temperatures (723–1300 K), the character of variation of the enthalpy and the Gibbs’ potential of the irradiated B₄C specimen depends on the presence of oxygen. The values of the thermodynamic functions vary due to the formation of excited atoms, active centers, defects of the B₄C crystal structure, and B₄C oxidation in the presence of the air oxygen after the ampoule opening. Also possible is an increase, at 723–1300 K, in the rate of oxidation of the boron carbide surface (contacting with the air oxygen), where the defects that form upon irradiation are distributed. At temperatures above 723 K, melting of the oxygenated part (B₂O₃) in B⁴C specimens irradiated by the absorbed dose of 194 Gy is observed; that process continued until the transformation of ~26% of crystal structure into the amorphous phase at 1300 K.     

Routes To Development Of Near-Surface Alloy Composition Anomaly

Thermophysical data in the literature for metallic alloys, including single-element specimens, exhibit a high degree of variability. Of interest is if the reasons are intrinsic. That there exists as a rule a surface layer whose elemental composition differs from that of the bulk has been demonstrated. Such a near-surface composition anomaly can, in principle, account for the significant part of the variability. The method of time-resolved spectroscopy of emissions from laser-produced plasma (LPP) plumes has been the key in this new development because both the elemental composition and thermal diffusivity can be measured simultaneously. Multiple application of LPP analysis leads to depth-resolved measurements. A variety of alloy specimens have been subjected to different scenarios of thermal cycling to find that thermal cycling modifies the near-surface composition profile of a given specimen. A new study has been carried out with Wood’s alloy as a model system by forcing the specimen into melting at increasing temperature. This investigation has revealed that the near-surface composition undergoes sharp irreversible changes when the highest elemental melting point has been exceeded before the specimen is cooled to resolidification. A possible mechanism is suggested. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

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.     

Thermophysical Properties by a Pulse-Heating Reflectometric Technique: Niobium, 1100 to 2700 K

Pulse-heating experiments were performed on niobium strips, taking the specimens from room temperature to the melting point is less than one second. The normal spectral emissivity of the strips was measured by integrating sphere reflectometry, and, simultaneously, experimental data (radiance temperature, current, voltage drop) for thermophysical properties were collected with sub-millisecond time resolution. The normal spectral emissivity results were used to compute the true temperature of the niobium strips; the heat capacity, electrical resistivity, and hemispherical total emissivity were evaluated in the temperature range 1100 to 2700 K. The results are compared with literature data obtained in pulse-heating experiments. It is concluded that combined measurements of normal spectral emissivity and of thermophysical properties on strip specimens provide results of the same quality as obtained using tubular specimens with a blackbody. The thermophysical property results on niobium also validate the normal spectral emissivity measurements by integrating sphere reflectometry.     

Extension of thermophysical and thermodynamic property measurements by laser pulse heating up to 10,000 K. I. Under pressure

The necessity for increased high-temperature data reliability and extension of thermophysical property measurements up to 5000 K and above are discussed. A new transient-type laser-autoclave technique (LAT) has been developed to extend density and heat capacity measurements of high-temperature multicomponent systems far beyond their melting and boiling points. Pulsed multibeam laser heating is performed in an autoclave under high inert gas pressure to eliminate evaporation. The spherical samples are positioned by containment-free acoustic levitation regardless of their conductive or magnetic properties. Temperature, spectral and total emittances are determined by a new microsecond six-wavelength pyrometer coupled to a fast digital data acquisition system. The density is determined by high resolution microfocus X-ray shadow technique. The heat capacity is obtained from the cooling rate. Further applications are a combination of the laser-autoclave with splat cooling techniques for metastable structure synthesis and amorphous metals research and an extension of the LAT for the study of critical phenomena and the measurement of critical-point temperatures.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.     

Method for the Thermal Characterization of PCM Systems in the Volume Range from 100 ml to 1000 ml

The storage of latent heat in phase change materials (PCM) is of great interest in many applications, for example in building applications. However, there is no standard method for the determination of the thermophysical properties of application-sized PCM specimens, i.e., specimens with sizes around 100 ml to 1000 ml. In order to close this metrological gap, a commercially available heat flow meter was modified to perform enthalpy measurements. The feasibility of this method was proven by performing comparative measurements on a stainless steel specimen using both the standard method DSC and the modified heat flow meter. Furthermore, measurements on a gypsum board with microencapsulated PCM were performed with the heat flow meter in order to determine the enthalpy. The coincidence with literature values is within ±4% which demonstrates that this method is a good choice for performing measurements on application-sized PCM specimens.     

纳米铜粉/石蜡复合相变储能材料的性能研究

针对石蜡作为固-液相变储能材料存在导热系数小、传热性能差的缺点,利用两步法制备了分散均匀稳定的纳米铜粉/石蜡复合相变材料,并研究了其热物性能。研究表明,纳米铜粉的加入能略微降低石蜡相变储能材料的相变潜热,对相变温度的影响不大,但能有效提高石蜡相变储能材料的导热系数,且使纳米铜粉/石蜡复合相变材料具有较好的热稳定性。     

Radiance Temperatures (in the Wavelength Range 530 to 1500 nm) of Nickel at Its Melting Point by a Pulse-Heating Technique

The radiance temperatures (at seven wavelengths in the range 530 to 1500 nm) of nickel at its melting point were measured by a pulse-heating technique. The method is based on rapid resistive self-heating of the specimen from room temperature to its melting point in less than 1 s and on simultaneously measuring specimen radiance temperatures every 0.5 ms. Melting of the specimen was manifested by a plateau in the radiance temperature-versus-time function for each wavelength. The melting-point radiance temperatures for a given specimen were determined by averaging the measured temperatures along the plateau at each wavelength. The melting-point radiance temperatures for nickel, as determined by averaging the results at each wavelength for 25 specimens, are: 1641 K at 530 nm, 1615 K at 627 nm, 1606 K at 657 nm, 1589 K at 722 nm, 1564 K at 812 nm, 1538 K at 908 nm, and 1381 K at 1500 nm. Based on uncertainties arising from pyrometry and specimen conditions, the combined uncertainty (two standard-deviation level) is about ± 6 K for the reported values in the range 530 to 900 nm and is about ± 8 K for the reported value at 1500 nm.     

金属及合金熔点热物性动态测试新方法研究

利用物质熔化与凝固过程中导热逆(或反)问题原理，在建立了相界面移动与两相热物理性质关系的基础上，提出了一种新颖的热物性动态测试方法，即由相变速率动态测试热物性参数。由于是对熔点高的金属进行测试，故不能采用解析求解，而是运用数值求解；并用铅、锌、铝等已知热物性的金属对此方法进行了评定，测试结果与参照值误差不超过3％；还对相变导热系数未知的铅锑、铅锡、铋锡、铝硅、铝铜5种共晶合金进行了测试，其结果具有较高的参考价值。该方法的优点在于测试过程中所求热物性参数与相界面位置是由精确的传热方程所约束，故测量较简便，结果准确、可靠、误差小，并可测得多个热物性数据。