High-Accuracy Emissivity Data on the Coatings Nextel 811-21, Herberts 1534, Aeroglaze Z306 and Acktar Fractal Black

The Physikalisch-Technische Bundesanstalt determined the directional spectral emissivities of several widely used black coatings: Nextel 811-21, Herberts 1534, Aeroglaze Z306 and Acktar Fractal Black. These are and were often applied in different industrial and scientific applications. The measurements are taken angularly resolved over a range from \(10{^{\circ }}\) to \(70{^{\circ }}\). They cover the temperature range typical for the application of the respective coating and a wide wavelength range from \(4~\upmu \hbox {m}\) to \(100~\upmu \hbox {m}\). The respective directional total emissivities and hemispherical total emissivities are given as well. The measurements were taken under vacuum at the reduced background calibration facility to achieve low uncertainties and avoid atmospheric interferences. Additionally, some measurements were taken with the emissivity measurement setup in air.     

Evaluation of Blackbody Cavity Emissivity in the Infrared Using Total Integrated Scatter Measurements

Deviations from ideal blackbody (BB) behavior can be characterized by a BB’s effective emissivity. The cavity emissivity is most often obtained through a model, given a particular set of input parameters associated with the BB cavity geometry and surface optical properties. It can also be measured directly (radiance) or indirectly (reflectance). A study of BB cavity emissivity using the reflectance method is presented. Several types and designs of blackbody cavities, including those from fixed-point and water bath BBs, using our infrared total integrated scatter (ITIS) instrument for emissivity evaluation are examined. The emissivity is characterized as a function of position on the output aperture, as well as a function of output angle. The measurements have revealed emissivity values, both significantly greater than, and in confirmation of, modeling predictions. For instance, the emissivities of three fixed point BB cavity designs were found to vary significantly despite modeling predictions in the design process of similar behavior. Also, other BB cavities that exhibited poor emissivity performance were re-painted and re-machined, in one case more than once, before the predicted performance was achieved.     

Phase Dependence (Liquid/Solid) of Normal Spectral Emissivities of Noble Metals at Melting Points

Normal spectral emissivities of liquid and solid Cu, Ag, and Au have been determined at their melting (freezing) points in the visible region using a cold crucible as the heating method. The use of the cold crucible enables the solidification front to be moved on the molten metal surface slowly enough to measure the emissivities of liquid and solid phases separately at the freezing point. Combined standard uncertainties of the spectral emissivities and wavelengths have been estimated. In silver, the spectral emissivity obtained for the liquid is systematically larger than that for the solid over the visible region, which is consistent with the prediction from a classical free-electron model. In copper and gold, the spectral emissivities at wavelengths around their absorption edges do not change for the solid-to-liquid transition. The wavelength range where the emissivity of copper is independent of the phase is unexpectedly broad (the width is greater than 40 nm), which differs significantly from classical experimental studies on the so-called X-point in the emissivity of copper. A qualitative explanation is provided for the difference in the phase dependence (liquid/solid) of the emissivity between copper and gold.     

Thermophysical Properties of Solid Phase Zirconium at High Temperatures

This paper presents experimental results on the thermophysical properties of relatively pure polycrystalline zirconium samples in the solid phase from room temperature up to near the melting point. The specific heat capacity and specific electrical resistivity were measured from 290 to 1970 K, the hemispherical total emissivity from 1400 to 2000 K, the normal spectral emissivity from 1480 to 1940 K, and the thermal diffusivity in the range from 290 to 1470 K. From these data, the thermal conductivity and Lorentz number were computed in the range from 290 to 1470 K. For necessary corrections the most recent values of the linear thermal expansion from the literature have been used. Subsecond pulse calorimetry for measuring heat capacity, specific electrical resistivity, and both emissivities and the laser flash method for measuring thermal diffusivity were applied. Samples in the form of a thin rod and in the form of a thin disk were used in the first and second methods, respectively. Measurement uncertainties were generally about 3% for heat capacity, 1.6% for specific electrical resistivity, 3–10% for the two emissivities, and from less than 1% up to 6% for thermal diffusivity. All the results are discussed in reference to available literature data.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Radiation characteristics of a high-emissivity cylindrical-spherical cavity with obscuration

We have calculated, to first order, the apparent emissivity of the bounding diffuse surfaces of a high-emissivity cylindrical-spherical cavity enclosure. Our calculations indicate that to achieve emissivities close to a perfectly absorbing blackbody cavity along the bounding surfaces of the spherical enclosure, the radius of the sphere must be equal to or greater than a factor of 4 times the cylinder radius R(S) > or = 4R(C). Furthermore, to achieve emissivities approaching a blackbody cavity along the lower bounding surfaces of the cylindrical enclosure, the length of the cylinder must be a factor of 4 times greater than the radius of the cylinder L > or = 4R(C). In addition, we present the mathematical framework necessary to calculate radiant transfer within a cavity enclosure that contains obscuration. These results can be applied to the design of high-emissivity blackbody calibration cavities and to the reduction of stray light in terrestrial and spaceborne optical systems.     

Processing Method of Multi-Wavelength Pyrometer Data for Continuous Temperature Measurements

The processing of multi-wavelength pyrometer data is a problem that needs further improvements. The solutions developed in earlier decades generally assumed one particular mathematical relation for emissivity versus wavelength in the wavelength range of the measurements. Sometimes this assumption worked and produced acceptable results, but in many other cases this approach provided erroneous results. Individual results were strongly dependent on the assumed mathematical relation that often needed some prior knowledge of the emissivity behavior in the wavelength range. A new data processing method for a multi-wavelength pyrometer for continuous temperature measurements is presented. A linear relation between emissivity and true temperature at different wavelengths is assumed. Based on this assumption, the true temperatures and spectral emissivities at the two continuous temperature measurement points can be simultaneously calculated. Some experimental results for the practical data processing of measurements performed on a solid propellant rocket engine show that the difference between the calculated true temperature and the theoretical true temperature indicated by the rocket engine designer is within ±20 K. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

Emissivity of condensed substances from their thermal radiation spectra

A technique for measuring the temperature and spectral emissivity of condensed substances is described. With this technique, measurements can be made in any spectral range, rather than only at short wavelengths, where the Wien approximation for the Planck formula holds. In particular, in the spectral range around the peak-emission wavelength, the highest signal-to-noise ratio can typically be attained, which raises the accuracy in temperature and emissivity determination. The proposed approach to processing the experimental emissivity curves offers the possibility of analyzing the effects of the major factors (signal-to-noise ratio, spectral range of measurements, type of the model function representing the spectral emissivity, and others) on the accuracy in temperature measurements. The potentialities of the technique are demonstrated by measuring the temperatures and emissivities of W, Re, and Ta strip lamps in the spectral range 0.9 to 2.1 Μm.     

Metal–Carbon Eutectics to Extend the Use of the Fixed-Point Technique in Precision IR Thermometry

The high-temperature extension of the fixed-point technique for primary calibration of precision infrared (IR) thermometers was investigated both through mathematical simulations and laboratory investigations. Simulations were performed with Co–C (1,324°C) and Pd–C (1, 492°C) eutectic fixed points, and a precision IR thermometer was calibrated from the In point (156.5985°C) up to the Co–C point. Mathematical simulations suggested the possibility of directly deriving the transition temperature of the Co–C and Pd–C points by extrapolating the calibration derived from fixed-point measurements from In to the Cu point. Both temperatures, as a result of the low uncertainty associated with the In–Cu calibration and the high number of fixed points involved in the calibration process, can be derived with an uncertainty of 0.11°C for Co–C and 0.18°C for Pd–C. A transition temperature of 1,324.3°C for Co–C was determined from the experimental verification, a value higher than, but compatible with, the one proposed by the thermometry community for inclusion as a secondary reference point for ITS-90 dissemination, i.e., 1,324.0°C.     

Research on the Temperature and Emissivity Measurement of the Metallic Thermal Protection Blanket

The temperatures and emissivities of the metallic thermal protection blanket at (900–1,300) °C are investigated experimentally by using a multi-wavelength pyrometer. A linear relation between the emissivity and true temperature at different wavelengths is assumed. Based on this assumption, the true temperatures and spectral emissivities of the metallic thermal protection blanket at the two temperature measurement points can be calculated simultaneously. Some experimental results for the practical data processing of measurements performed on the metallic thermal protection blanket show that the difference between the calculated temperature and the temperature measured by a standard thermocouple is within  ±  10°C.     

Texture and Porosity Effects on the Thermal Radiative Behavior of Alumina Ceramics

Thermal and optical properties of ceramics are dependent on radiation scattering and cannot be determined by a knowledge of their chemical composition alone, as for single crystals. In this paper, extrinsic effects, such as roughness, porosity, and texture, on the spectral emissivity of alumina ceramics are investigated. Roughness effects have an influence mainly in the opaque zone; an important porosity dependence and the presence of a critical porosity threshold were observed in the semitransparent zone. Furthermore, it was shown that two ceramics with similar total porosities, but with different textures, possess radically different emissivities, showing that grain size, pore size, and spatial repartition of the grains are also crucial for an understanding of the thermal properties of the ceramics. This study was performed at CEMTHI laboratory.     

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.     

Emissivity measurement for a carbon-carbon composite and a phenolic carbon plastic

Two equipments have been used together to determine composite-material emissivity at high temperatures; measurements are reported on the normal emissivities of carbon-based composites.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 2, pp. 308–313, August, 1989.     

Estimation of emission properties for silica particles using thermal radiation spectroscopy

This paper shows that spectrally resolved thermal radiation from silica aggregate particles can be used to extract an emissivity and a temperature in the visible regime. Emissivity of silica aggregate particles at temperatures above 2000?K is measured by the analysis of emission radiation spectra from the particles. Temperature is estimated from the relation between the emission intensity and the wavenumber. Relative emissivities at temperatures from 2150 to 2919?K are presented. Proper knowledge of optical properties for silica aggregate particles will help further the understanding of thermophysics at high temperature.     

Emissitivity spectra obtained from field and laboratory measurements using the temperature and emissivity separation algorithm

Surface emissivities play an important role in thermal remote sensing, since knowledge of them is required to estimate land surface temperature with enough accuracy. They are also important in other environmental or geological studies. We show the results obtained for the emissivity spectra of different natural surfaces (water, green, and senescent vegetation) by applying the temperature and emissivity separation (TES) algorithm to ground-based measurements collected at the field with a multiband thermal radiometer. The results have been tested with data included in spectral libraries, and rms errors lower than 0.01 have been found, except for senescent vegetation. Two methods are also proposed to apply the TES algorithm to measurements achieved in the laboratory: (i) by heating the sample and (ii) using a box with reflective walls.     

Simultaneous measurements of normal spectral emissivity by spectral radiometry and laser polarimetry at high temperatures in millisecond-resolution pulse-heating experiments: Application to molybdenum and tungsten

Spectral radiometry and laser polarimetry are two independent techniques for the measurement of spectral emissivity of materials. In this paper, a high-speed system is described for the rapid measurement of normal spectral emissivity of a specimen based on the simultaneous utilization of the two techniques. One of the goals of this work to ascertain the accuracy of the laser polarimetry technique in measurement of normal spectral emissivity at high temperatures. To accomplish this goal, the normal spectral emissivities, in the vicinity of 0.633m, of molybdenum and tungsten were measured by the two techniques over the temperature range 2000 to 2600 K. The results obtained by the two techniques are in agreement within 1%. The total uncertainty (two-standard deviation level) in measurement of emissivity by either spectral radiometry or laser polarimetry technique is estimated to be not more than + 2%.     

Emissivity and Optical Properties of Thermochromic Material La0.7Ca0.3?xSrxMnO3?(0 ≤ x ≤ 0.3)

La_0.7Ca_0.3−x Sr _x MnO₃ (0 ≤ x ≤ 0.3) (LCSMO) is a thermochromic material. The material can show a phase transition from ferromagnetic-metal to paramagnetic-insulator under the appropriate doping level (x). In this paper, LCSMO compounds were synthesized by a conventional solid-state reaction method. The normal emissivity and the spectral reflectance of LCSMO with different doping levels (x) are experimentally investigated. Based on the spectral reflectance data, the optical constants (refractive index n and extinction coefficient k) are calculated by using Kramers–Kronig (K–K) relations. Then, the emissivity changing mechanism is discussed. The experimental results show that the emissivity of LCSMO increases with increasing temperature. For the samples with x = 0.1 and 0.15, their emissivity has a sudden change in the vicinity of the phase transition temperature T _p.     

Use of Eutectic Fixed Points to Characterize a Spectrometer for Earth Observations

A small palm-sized, reference spectrometer, mounted on a remote-controlled model helicopter is being developed and tested by the National Physical Laboratory (NPL) in conjunction with City University, London. The developed system will be used as a key element for field vicarious calibration of optical earth observation systems in the visible-near infrared (VNIR) region. The spectrometer is hand held, low weight, and uses a photodiode array. It has good stray light rejection and wide spectral coverage, allowing simultaneous measurements from 400 to 900 nm. The spectrometer is traceable to NPL’s primary standard cryogenic radiometer via a high-temperature metal-carbon eutectic fixed-point blackbody. Once the fixed-point temperature has been determined (using filter radiometry), the eutectic provides a high emissivity and high stability source of known spectral radiance over the emitted spectral range. All wavelength channels of the spectrometer can be calibrated simultaneously using the eutectic transition without the need for additional instrumentation. The spectrometer itself has been characterized for stray light performance and wavelength accuracy. Its long-term and transportation stability has been proven in an experiment that determined the “World’s Bluest Sky”—a process that involved 56 flights, covering 100,000 km in 72 days. This vicarious calibration methodology using a eutectic standard is presented alongside the preliminary results of an evaluation study of the spectrometer characteristics.     

Radiative Properties of Uranium Dioxide Near Its Melting Point

A new technique for measuring radiative properties of nuclear fuel materials at high temperatures has been developed. The technique is based on pulse diffuse optical probing of the sample surface and on pyroreflectometry used in measuring radiative properties of refractory materials during laser heating or cooling. Pulse diffuse optical probing of the sample has been realized for the first time in subsecond pyrometry of the open surface heated by laser radiation. Such a procedure of sample irradiation during sample laser heating or cooling enables reflectivity and emissivity measurements near high temperature phase transitions to be performed in spite of possible sharp changes of the reflection indicatrix at phase transitions in the investigated material. With the method developed in this study, the spectral emissivity and reflectivity of uranium dioxide near its premelting and melting points have been measured. It has been found that condensation of the vapor plume formed above the sample during laser heating influenced the melting and boiling temperatures of uranium dioxide. The first-order phase transitions in uranium dioxide, such as solid–vapor–solid and liquid–vapor–liquid, have been observed in uranium dioxide for the first time during laser heating. Also, new data on the spectral emissivity of uranium dioxide at a wavelength of 0.644 m and in the temperature range of 2000 to 4200 K are presented.     

Preparation and characterization of KGM/CdS nanocomposite film with low infrared emissivity

In this research, novel organic-inorganic nanocomposite films of Konjac glucomannan (KGM) and CdS were prepared by one-step synthesis. As-prepared films were characterized by IR, TEM and SEM. The results indicated that hexagonal CdS nanoparticles with the sizes of 10 to 100 nm were well dispersed in KGM. The infrared emissivities of the films were characterized by IR-1 infrared emissivity instrument. As results showed, the KGM/CdS nanocomposite films had significantly lower infrared emissivity (8-14 μm), meanwhile when the size of KGM nanoparticles was between 10 and 20 nm and the mole ratio of CdS to KGM was 1.2:1, the film got the lowest infrared emissivity value of 0.011, which would be attributed to the strong synergism effect existing between KGM and CdS nanoparticles.     

Experimental measurements for studying angular and spectral variation of thermal infrared emissivity

One condition for precise multiangle algorithms for estimating sea and land surface temperature with the data from the Advanced Along Track Scanning Radiometer is accurate knowledge of the angular variation of surface emissivity in the thermal IR spectrum region. Today there are very few measurements of this variation. The present study is conducted to provide angular emissivity measurements for five representative samples (water, clay, sand, loam, gravel). The measurements are made in one thermal IR broadband (8-13 microm) and three narrower bands (8.2-9.2, 10.3-11.3, and 11.5-12.5 microm) at angles of 0 degrees-60 degrees (at 5 degrees increments) to the surface normal. The results show a general decrease in emissivity with increasing viewing angles, with the 8.2-9.2-microm channel the most sensitive to this dependence and sand the sample showing the greatest variation.