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.     

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.     

Radiance temperatures (in the wavelength range 525 to 906 nm) of vanadium at its melting point by a pulse-heating technique

The radiance temperatures (at six wavelengths in the range 525 to 906 nm) of vanadium 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 the specimen radiance temperatures every 0.5 ms with a high-speed six-wavelength pyrometer. Melting was manifested by a plateau in the radiance temperature-vs-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 vanadium as determined by averaging the results at each wavelength for 16 specimens (standard deviation in the range 0.3 to 0.4 K. depending on the wavelength) are 2030 K at 525 nm, 1998 K at 622 nm, 1988 K at 652 nm, 1968 K at 714 nm, 1935 K at 809 nm, and 1900 K at 906 nm. Based on estimates of the random and systematic errors that arise from pyrometry and specimen conditions, the resultant uncertainty (2 SD level) in the reported values is about ±7 K at each wavelength.     

Radiance temperatures (in the wavelength range 519–906 nm) of tungsten at its melting point by a pulse-heating technique

Radiance temperatures (at six wavelengths in the range 519–906 nm) of tungsten 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 the specimen radiance temperatures every 0.5 ms with a high-speed six-wavelength pyrometer. Melting 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 tungsten were determined by averaging the results at each wavelength for 10 specimens (standard deviation in the range 0.5–1.1 K, depending on the wavelength) as follows: 3319 K at 519 nm, 3236 K at 615 nm, 3207 K at 652 nm, 3157 K at 707 nm, 3078 K at 808 nm, and 2995 K at 906 nm. Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the total uncertainty in the reported values is about ±7 K at 653 nm and ± 8 K at the other wavelengths.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Radiance temperatures (in the wavelength range 522–906 nm) of niobium at its melting point by a pulse-heating technique

Radiance temperatures (at six wavelengths in the range 522–906 nm) of niobium 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 the specimen radiance temperatures every 0.5 ms with a high-speed multiwavelength pyrometer. Melting was manifested by a plateau in the radiance temperatureversus-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 (standard deviation of an individual temperature from the mean: 0.1–0.4 K). The melting-point radiance temperatures for niobium were determined, by averaging the results at each wavelength for 10 specimens (standard deviation: 0.3 K), as follows: 2497 K at 522 nm, 2445 K at 617 nm, 2422 K at 653 nm, 2393 K at 708 nm, 2337 K at 809 nm, and 2282 K at 906 nm. Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the total error in the reported values is about 5 K at 653 nm and 6 K at the other wavelengths.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.     

Radiance Temperature and Normal Spectral Emittance (in the Wavelength Range of 1.5 to 5 μm) of Nickel at its Melting Point by a Pulse-Heating Technique

The radiance temperature of nickel at its melting point was measured at four wavelengths (in the nominal range of 1.5 to 5 μm) by a pulse-heating technique using a high-speed fiber-coupled four-channel infrared pyrometer. The method was based on rapid resistive self-heating of a specimen from room temperature to its melting point in less than 1 s while simultaneously measuring the radiance emitted by it in four spectral bands as a function of time. A plateau in the recorded radiance-versus-time traces indicated melting of the specimen. The melting-point radiance temperature for a given specimen was determined by averaging the temperature measured along the plateau at each wavelength. The results for several specimens were then, in turn, averaged to yield the melting-point radiance temperature of nickel, as follows: 1316 K at 1.77 μm, 1211 K at 2.26 μm, 995 K at 3.48 μm, and 845 K at 4.75 μm. The melting-point normal spectral emittance of nickel at these wavelengths was derived from the measured radiance in each spectral band using the published value of the thermodynamic (true) melting temperature of nickel.     

Radiance temperatures (in the wavelength range 523–907 nm) of group IVB transition metals titanium,zirconium, and hafnium at their melting points by a pulse-heating technique

The melting-point radiance temperatures (at six wavelengths in the range 523–907 nm) of the Group IVB transition metals titanium, zirconium, and hafnium 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 the specimen radiance temperatures every 0.5 ms with a high-speed six-wavelength pyrometer. Melting 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 each metal were determined by averaging results for several specimens at each wavelength as follows: Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the combined uncertainty (95% confidence level) in the reported values is about ±8K at each wavelength.     

A combined transient and brief steady-state technique for measuring hemispherical total emissivity of electrical conductors at high temperatures: Application to tantalum

A new method for measuring hemispherical total emissivity of electrically conducting materials at high temperatures (above 1500 K) using a feedback-controlled pulse-heating technique has been developed. The technique is based on rapid resistive self-heating of a solid cylindrical specimen in vacuum up to a preset high temperature in a short time (about 200 ms) and then keeping the specimen at that temperature under steady-state conditions for a brief period (about 500 ms) before switching off the current through the specimen. The specimen is maintained at constant temperature with a feedback control system which controls the current through the specimen. The computer-controlled feedback system operates a solid-state switch (composed of field-effect transistors). The sensing signal for the feedback is provided by a high-speed optical pyrometer. Hemispherical total emissivity is determined at the temperature plateau from the data on current through the specimen, the voltage drop across the middle portion of the specimen, and the specimen temperature using the steady-state heat balance equation based on the Stefan-Boltzmann law. The true temperature of the specimen is determined from the measured radiance temperature and the normal spectral emissivity: the latter is obtained from laser polarimetric measurements. The experimental quantities are measured and recorded every 0.2 ms with a 12-bit digital oscilloscope. To demonstrate the feasibility of the technique, experiments were conducted on a tantalum specimen in the temperature range 2000 to 2800 K. The results on hemispherical total emissivity are presented and are compared with the data given in the literature.     

Measurement of the heat of fusion of tantalum by a microsecond-resolution transient technique

The heat of fusion of tantalum was measured using a microsecond-resolution pulse-heating technique. The technique is based on rapid (about 100-s) resistive self-heating of a specimen by a high-current pulse from a capacitor discharge system and measuring the current through the specimen, voltage across the specimen, and radiance temperature of the specimen as functions of time. Melting of a specimen is manifested by a plateau in the radiance temperature versus time function. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements gave a value of 34.8 kJ · mot^– for the heat of fusion of tantalum, with a total uncertainty of ±6%. Electrical resistivity of solid and liquid tantalum at its melting temperature was also measured.     

Radiance Temperatures and Normal Spectral Emittances (in the Wavelength Range of 1500 to 5000 nm) of Tin, Zinc, Aluminum, and Silver at Their Melting Points by a Pulse-Heating Technique

The radiance temperatures at four wavelengths (in the range of 1500 to 5000 nm) of tin, zinc, aluminum, and silver at their respective melting points were measured by a pulse-heating technique using a high-speed fiber-coupled four-wavelength infrared pyrometer. The method is based on rapid resistive self-heating of a sample from room temperature to its melting point in less than 1 s while measuring the radiance emitted by it in four wavelength bands as a function of time. A plateau in the recorded radiance-versus-time traces indicates melting of the sample. The melting-point radiance temperatures for a given sample are determined by averaging the measured temperatures along the plateau at each wavelength. The melting-point radiance temperatures for each metal are, in turn, determined by averaging results for several samples. The normal spectral emittances at the melting transition of each metal are derived from the measured radiances at each wavelength and the published values of the thermodynamic (true) melting temperatures.     

Radiance Temperatures (in the Wavelength Range 521 to 1500 nm) of Rhenium and Iridium at Their Melting Points by a Pulse-Heating Technique

The melting-point radiance temperatures (at seven wavelengths in the range 521 to 1500 nm) of rhenium and iridium 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 the specimen radiance temperature every 0.5 ms with two high-speed pyrometers. Melting 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 each metal were determined by averaging results for several specimens at each wavelength. The results are as follows. Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the expanded uncertainty (two standard-deviation level) in the reported values is ±8K.     

A Multiwavelength Reflectometric Technique for Normal Spectral Emissivity Measurements by a Pulse-Heating Method

A new technique has been developed for the direct measurement of the normal spectral emissivity at several wavelengths in pulse-heating conditions, adding some novel features to previous versions of this type of apparatus. Pulse-heating experiments were performed on niobium strip specimens, taking the specimen from room temperature to the melting point using rapid resistive self-heating. The normal spectral emissivity was measured at three wavelengths by a multi-wavelength reflectometric technique. At the same time, the radiance temperature was measured at the same wavelengths by a high-speed pyrometer from approximately 1100 K to the melting point. Details of the method, the measurement apparatus, and the calibration technique are described. Preliminary results for the normal spectral emissivity of niobium at 633, 750, and 900 nm over a wide temperature range are presented.     

Radiance Temperatures (in the Wavelength Range 527 to 1500 nm) of Palladium and Platinum at Their Melting Points by a Pulse-Heating Technique

The radiance temperatures (at seven wavelengths in the range 527 to 1500 nm) of palladium and platinum at their respective melting points were measured by a pulse-heating technique. The method, based on rapid resistive self-heating of a specimen from room temperature to its melting point in less than 1 s, used two high-speed pyrometers to measure specimen radiance temperatures every 0.5 ms during the heating and melting period. Melting 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 each metal as determined by averaging the results for several specimens at each wavelength are as follows. Based on uncertainties arising from pyrometry and specimen conditions, the expanded uncertainty (two-standard deviation level) is about ±7 K for the reported values in the range 527 to 900 nm and about ±8 K for the reported values at 1500 nm.     

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.     

Thermophysical Properties of Solid and Liquid 90Ti–6Al–4V in the Temperature Range from 1400 to 2300 K Measured by Millisecond and Microsecond Pulse-Heating Techniques

The heat capacity and electrical resistivity of 90Ti–6Al–4V were measured in the temperature range from 1400 to 2300 K by two pulse-heating systems, operating in the millisecond and microsecond time regimes. The millisecond-resolution technique is based on resistive self-heating of a tube-shaped specimen from room temperature to melting in less than 500 ms. In this technique, the current through the specimen, the voltage drop along a defined portion of the specimen, and the temperature of the specimen are measured every 0.5 ms. The microsecond-resolution technique is based on the same principle as the millisecond-resolution technique except for using a rod-shaped specimen, a faster heating rate (by a factor of 10,000), and faster data recording (every 0.5 s). Due to the rapid heating with the microsecond system, the specimen keeps its shape even in the liquid phase while measurements are made up to approximately 300 K above the melting temperature. A comparison between the results obtained from the two systems with very different heating rates shows significant differences in phase transition and melting behavior. The very high heating rate of the microsecond system shifts the solid–solid phase transition from the (+) to the phase to a higher temperature, and changes the behavior of melting from melting over a temperature range to melting at a constant temperature like an eutectic alloy or a pure metal.     

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.     

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.     

A microsecond-resolution transient technique for measuring the heat of fusion of metals: Niobium

A microsecond-resolution pulse-heating technique is described for the measurement of the heat of fusion of refractory metals. The method is based on rapid resistive self-heating of the specimen by a high-current pulse from a capacitor discharge system and measurement of the current through the specimen, the voltage across the specimen, and the radiance temperature of the specimen as a function of time. Melting of the specimen is manifested by a plateau in the temperature versus time function. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements gave a value of 31.1 kj · mol^–1 for the heat of fusion of niobium, with an estimated maximum uncertainty of ±5%. Electrical resistivity of solid and liquid niobium at its melting temperature was also measured.     

Measurement of the radiance temperature (at 655 nm) of melting graphite near its triple point by a pulse-heating technique

Measurements of the radiance temperature of graphite at 655 nm have been performed in the vicinity of its triple point by means of a rapid pulse-heating technique. The method is based on resistively heating the specimen in a pressurized gas environment from room temperature to its melting point in less than 20 ms by passing an electrical current pulse through it and simultaneously measuring the radiance temperature of the specimen surface every 120 s by means of a high-speed pyrometer. Results of experiments performed on two different grades of POCO graphite (AXM-5Q1 and DFP-1) at gas pressures of 14 and 20 MPa are in good agreement and yield a value of 4330±50 K for the radiance (or brightness) temperature (at 655 nm) of melting graphite near its triple point (triple-point pressure, 10 MPa). An estimate of the true (blackbody) temperature at the triple point is made on the basis of this result and literature data on the normal spectral emittance of graphite.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Radiance temperatures (at 658 and 898 nm) of niobium at its melting point

Radiance temperatures (at 658 and 898 nm) of niobium at its melting point were measured by a pulse-heating technique. A current pulse of subsecond duration was imparted to a niobium strip and the initial part of the melting plateau was measured by high-speed pyrometry. Experiments were performed with two techniques and the results do not indicate any dependence of radiance temperature (at the melting point) on initial surface or system operational conditions. The average radiance temperature at the melting point of niobium is 2420 K at 658 nm and 2288 K at 898 nm, with a standard deviation of 0.4 K at 658 nm and 0.3–0.6 K at 898 nm (depending on the technique used). The total uncertainty in radiance temperature is estimated to be not more than ±6 K. The results are in good agreement with earlier measurements at the National Institute of Standards and Technology (USA) and confirm that both radiance temperature and normal spectral emissivity (of niobium at its melting point) decrease with increasing wavelength in the region 500–900 nm.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.