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%.     

Improving the Dynamic Emissivity Measurement Above 1000 K by Extending the Spectral Range

To improve the dynamic emissivity measurement, which is based on the laser-flash method, an array spectrometer is characterized regarding its spectral radiance responsivity for a spectrally resolved emissivity measurement above \(1000\,\)K in the wavelength range between \(550\,\)nm and \(1100\,\)nm. Influences like dark signals, the nonlinearity of the detector, the size-of-source effect, wavelength calibration and the spectral radiance responsivity of the system are investigated to obtain an uncertainty budget for the spectral radiance and emissivity measurements. Uncertainties for the spectral radiance of lower than a relative \(2\,\%\) are achieved for wavelengths longer than \(550\,\)nm. Finally, the spectral emissivity of a graphite sample was determined in the temperature range between \(1000\,\)K and \(1700\,\)K, and the experimental data show a good repeatability and agreement with literature data.     

Microsecond Laser Polarimetry for Emissivity Measurements on Liquid Metals at High Temperatures—Application to Tantalum

The aim of this work was to determine accurate and reliable thermophysical properties of liquid tantalum from melting up to temperatures of 5000 K. Temperature measurements on pulse-heated liquid metal samples reported by different authors have always been performed under the assumption of a constant emissivity over the whole liquid range because of the lack of data for liquid metals. The uncertainty in temperature measurement is reduced in this work by the direct measurement of emissivity during the experiments. The emissivity measurements are performed by linking a laser polarimetry technique with the established method for performing high speed measurements on liquid tantalum samples at high temperatures during microsecond pulse-heating experiments. A set of improved thermophysical properties for liquid tantalum, such as temperature dependences of normal spectral emissivity at 684.5 nm, heat capacity, enthalpy, electrical resistivity, thermal diffusivity, and thermal conductivity, was obtained.     

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.     

Linearity Study of a Spectral Emissivity Measurement Facility

Linearity is one of the important characteristics of a spectral radiation measurement facility. Basically, it depends on the linearity of the spectral responsivity of the detector and amplifier at different wavelengths. As spectral emissivity is measured over wide wavelength and temperature ranges and the detection system has significant drift and noise, it is not easy to measure the linearity of this facility accurately using only one standard radiator. A simple double-blackbody method has been adopted to simulate reference emissivity samples and test the linearity of the spectral emissivity measurement facility developed at the National Institute of Metrology. Good linearity results were obtained from 3 μm to 15 μm. This method minimizes the influence of drift on the emissivity measurement over a wide ratio of measurement signals and wide spectral range.     

Determination of Opaque Material Temperature from the Spectral Distribution of Emitted Radiation Intensity with Unknown Emissivity

The thermodynamic temperature of an opaque material with unknown emissivity was determined from the recorded distribution of spectral intensities of the emitted radiation. The initial system of equations was derived in the logarithmic form in accordance with the Planck formula. At the first stage, the spectral dependence of the material emissivity was investigated with a special function—relative emissivity. Based on the analysis of spectral dependences of this function at different reference temperatures, a parametric model for the material emissivity was chosen. At the second stage, the desired parameters of the chosen model of spectral emissivity and the thermodynamic temperature of the material were determined. An example of the determination of a temperature of a tungsten sample from the spectral distribution of emitted radiation intensities is given. A method for narrowing the range, which includes the thermodynamic temperature of the material, is presented.     

Effect on the Spectral Emissivity of SPHC Steel by Surface Oxidization

Surface oxidation can dramatically change the spectral emissivity of a metallic surface that is maintained at an elevated temperature over a period of time. Many kinds of metallic specimens, in particular for steel in use in industrial conditions, are usually heated in an oxidizing environment at high temperatures for a relatively long time. For this reason, in this paper, the SPHC steel was chosen as the specimen to investigate the effect on the spectral emissivity by surface oxidization over the temperature range from 800 K to 1000 K. The experimental setup for the spectral emissivity measurement operates at a wavelength of $1.5\,\upmu $ 1.5 μ m with a bandwidth of 20 nm. The temperature of the sample surface is determined by averaging two R-type platinum–rhodium thermocouples. The radiant energy from the surface of the SPHC specimen is received by an infrared detector. On the one hand, it is found in a detailed study that the spectral emissivity varies as the temperature for a given heating time; on the other hand, it was investigated that the spectral emissivity varies as the heating time at a given temperature. We find that the relationship between the spectral emissivity and the temperature can still be fitted to an exponential function by the least-squares method. The peculiar behavior of the spectral emissivity is discussed at an early stage when the oxidation film on the surface of the SPHC sample is grown. The conclusion is drawn that the contribution to the spectral emissivity by the surface oxidation mainly comes from the oxidation growth during the first 3 h.     

Emissivity Measurements on Metallic Surfaces with Various Degrees of Roughness: A Comparison of Laser Polarimetry and Integrating Sphere Reflectometry

Both integrating sphere reflectometry (ISR) as well as laser polarimetry have their advantages and limitations in their ability to determine the normal spectral emissivity of metallic samples. Laser polarimetry has been used for years to obtain normal spectral emissivity measurements on pulse-heated materials. The method is based on the Fresnel equations, which describe reflection and refraction at an ideally smooth interface between two isotropic media. However, polarimetry is frequently used with surfaces that clearly deviate from this ideal condition. Questions arise with respect to the applicability of the simple Fresnel equations to non-specular surfaces. On the other hand, reflectometry utilizing integrating spheres provides a measurement of the hemispherical spectral reflectance, from which the normal spectral emissivity can be derived. ISR provides data on spectral-normal-hemispherical reflectance and, hence, normal spectral emissivity for a variety of surfaces. However, the resulting errors are minimal when both the sample and the reference have a similar bidirectional reflectance distribution function (BRDF). In an effort to explore the limits of polarimetry in terms of surface roughness, room temperature measurements on the same samples with various degrees of roughness were performed using both ISR and a laser polarimeter. In this paper the two methods are briefly described and the results of the comparison are discussed.     

Normal Spectral Emissivity of Niobium (at 900 nm) by a Pulse-Heating Reflectometric Technique

The normal spectral emissivity of niobium strip specimens was measured using a new pulse-heating reflectometric technique. The hemispherical spectral reflectivity of the surface of a strip tangent to an integrating sphere is determined by a high-speed lock-in technique. At the same time, the radiance temperature of the strip is measured by high-speed pyrometry from approximately 1000K to the melting point. Details of the measurement method and of the related calibration techniques are reported. Results of the normal spectral emissivity of niobium at 900 nm from room temperature to its melting point are presented, discussing differences related to the heating rate and to surface conditions.     

A New Method for the Determination of the Specific Heat Capacity Using Laser-Flash Calorimetry Down to 77K

A new method for evaluation of the specific heat capacity in the temperature regime between 77K and 330K using laser-flash calorimetry is presented. Usually, laser-flash calorimetry is accomplished by performing an additional laser-flash measurement on a reference specimen with a known specific heat capacity and by comparing the maximum rear-side temperatures rises. In this study, the calibration is achieved by comparison of the rear-side temperature rise to specific-heat-capacity data determined by other methods in an adjacent temperature regime. Subsequently, the thus yielded proportional factor is used for the evaluation of the specific heat capacity from laser-flash measurements at temperatures where no specific-heat-capacity data are available. The reliability of this method is shown by performing measurements on a material with known specific heat capacity, aluminum oxide. Furthermore, the specific heat capacity and thermal conductivity of borosilicate crown glass (BK7) was determined experimentally.     

Infrared Emissivity Measurements of Building and Civil Engineering Materials: A New Device for Measuring Emissivity

The knowledge of the infrared emissivity of materials used in buildings and civil engineering structures is useful for two specific approaches. First, quantitative diagnosis of buildings or civil engineering infrastructures by infrared thermography requires emissivity values in the spectral bandwidth of the camera used for measurements, in order to obtain accurate surface temperatures; for instance, emissivity in the band III domain is required when using cameras with uncooled detectors (such as micro-bolometer arrays). Second, setting up accurate thermal balances by numerical modeling requires the total emissivity value for a large wavelength domain; this is, for instance, the case for computing the road surface temperature to predict ice occurrence. Furthermore, periodical surveys of emissivity variations due to aging or soiling of surfaces could be useful in many situations such as thermal mapping of roads or building insulation diagnosis. The use of portable emissivity measurement devices is required for that purpose. A device using an indirect measurement method was previously developed in our lab; the method uses measurement of the reflectivity from a modulated IR source and requires calibration with a highly reflective surface. However, that device uses a low-frequency, thermal modulation well adapted to laboratory measurements but unfit for fast and in situ measurements. Therefore, a new, portable system which retains the principle of an indirect measurement but uses a faster-frequency, mechanical modulation more appropriate to outdoor measurements was developed. Both devices allow measurements in the broad \((1\,\upmu \) m to \(40\,\upmu \) m) and narrow \((8\,\upmu \) m to \(14\,\upmu \) m) bands. Experiments were performed on a large number of materials commonly used in buildings and civil engineering structures. The final objective of this work is to build a database of emissivity of these materials. A comparison of laboratory and on-site measurements of emissivity values obtained in both spectral bands will be presented along with an estimation and an analysis of measurement uncertainties.     

常温黑体光谱发射率校准技术研究

为了能够实现常温状态下的黑体光谱发射率的准确测量,基于连续可调激光器,搭建了一套中红外波段黑体光谱发射率测量装置。采用自行设计10 mA恒流源对MCT探测器进行驱动,测量结果的动态范围从7.29×10⁴提高到4.32×10⁵,有效提升了探测系统的动态范围。该装置实现了覆盖光谱范围7.5～10.6μm,发射率测量量值范围0.01～0.999 9的高精度测量,最优不确定度为4.0×10^-5(k=2)。     

A Transient Method for Total Emissivity Determination

A transient method for determining the hemispherical total emissivity of solids is investigated using an emissometer recently developed at the NPL. The emissivity is calculated from measurement of the sample surface temperature coupled with a knowledge of its bulk thermal properties. This was conducted as part of the current work to validate the new NPL apparatus for high temperature emissivity measurements. A theoretical study shows that when a thermally thick sample is allowed to radiate instantaneously into a cold environment, then the resulting transient surface temperature depends solely on its hemispherical total emissivity and effusivity. This approach is used to obtain a hemispherical total emissivity value for Fecralloy steel, and it is then compared with the normal total emissivity value obtained by integration of normal spectral emissivity measurements in the wavelength range 2 to 9m.     

Experimental Verification to Obtain Intrinsic Thermal Diffusivity by Laser-Flash Method

There is a need to obtain highly reliable values of thermophysical properties. The thermal conductivity of solids is often calculated from the thermal diffusivity, specific heat, and density, respectively, measured by the laser-flash method, differential scanning calorimetry, and Archimedes’ method. The laser-flash method is one of the most well-known methods for measuring the thermal diffusivity of solids above room temperature. This method is very convenient to measure the thermal diffusivity without contact in a short time. On the other hand, it is considered as an absolute reference measurement method, in particular, because only measurements of basic quantities such as time, temperature, length, and electrical quantities are required, and because the uncertainty of measurement can be analytically evaluated. However, it could be difficult in some cases to obtain reliable thermal-diffusivity values. The measurement results can indeed depend on experimental conditions; in particular, the pulse heating energy. A procedure to obtain the intrinsic thermal-diffusivity value was proposed by National Metrology Institute of Japan (NMIJ). Here, “intrinsic” means unique for the material, independent of measurement conditions. In this method, apparent thermal-diffusivity values are first measured by changing the pulse heating energy at the same test temperature. Then, the intrinsic thermal diffusivity is determined by extrapolating these apparent thermal diffusivities to a zero energy pulse. In order to verify and examine the applicability of the procedure for intrinsic thermal-diffusivity measurements, we have measured the thermal diffusivity of some materials (metals, ceramics) using the laser-flash method with this extrapolation procedure. NMIJ and Laboratoire National de Metrologie et d’essais (LNE) have laser-flash thermal-diffusivity measurement systems that are traceable to SI units. The thermal diffusivity measured by NMIJ and LNE on four materials shows good agreement, although they used different measurement systems and different analysis methods of the temperature-rise curve. Experimental verification on the procedure was carried out using the measured results. Some problems and considered solutions for laser-flash thermal-diffusivity measurements are discussed.     

Determination of the Thermal Diffusivity of Electrically Non-Conductive Solids in the Temperature Range from 80 K to 300 K by Laser-Flash Measurement

The adoption of the popular laser-flash method at temperatures far below 300 K is restricted by the weak signal-to-noise ratio and the limited spectral bandwidth of the commonly used mercury cadmium tellurite (MCT) infrared (IR) detector used as a non-contacting temperature probe. In this work, a different approach to measure the temperature rise in pulse heating experiments is described and evaluated. This method utilizes the change of the temperature-dependent electrical resistance of a thin strip of sputtered gold for the detection of a temperature rise as it was proposed by Kogure et al. The main advantage of this method at lower temperatures is the significantly higher signal-to-noise ratio compared to the commonly used IR detectors. A newly developed laser-flash apparatus using this detection method for the determination of the thermal diffusivity in the temperature range from 80 K to 300 K is presented. To test the accuracy of the new detection method, the thermal diffusivity of a borosilicate crown glass (BK7) specimen at 300 K was determined and compared to results derived with a MCT detector. Good agreement of the derived thermal diffusivity values within 3 % was found. The thermal diffusivity of BK7 and polycrystalline aluminum nitride (AlN) was measured at temperatures between 80 K and 300 K by a laser-flash method to test the functionality of the apparatus. Finally, the thermal conductivity was calculated using values for the specific heat capacity determined by temperature modulated differential scanning calorimetry (MDSC). Comparisons with literature data confirm the reliability of the experimental setup.     

Application of High-Speed Laser Polarimetry to Noncontact Detection of Phase Transformations in Metals and Alloys at High Temperatures

A high-speed laser polarimetry technique, developed recently for the measurement of normal spectral emissivity of materials at high temperatures, was used to detect solid–solid and solid–liquid phase transformations in metals and alloys in millisecond-resolution pulse-heating experiments. Experiments were performed where normal spectral emissivity at 633 nm was measured simultaneously with surface radiance temperature, resistance, and/or voltage drop across the specimen. It was observed that a phase transformation, as indicated either by an arrest in the specimen radiance temperature or changes in the resistance and/or voltage drop, generally caused a change in normal spectral emissivity. Experiments were conducted on cobalt, iron, hafnium, titanium, and zirconium to detect solid–solid phase transformations. Similar experiments were also performed on niobium, titanium, and the alloy 85titanium–15molybdenum (mass%) to detect solid–liquid phase transformations (melting).     

Armco Iron Normal Spectral Emissivity Measurements

Directional spectral emissivity data in different environments are needed in a great number of scientific and technological applications. In this work, the normal spectral emissivity of Armco iron is studied as a function of temperature under a controlled atmosphere. Emissivity values are calculated by the direct radiometric method. The evolution with thermal cycling, the dependence on temperature, and the effect of surface roughness are considered. Additionally, the electrical resistivity is calculated by using the Hagen–Rubens emissivity relation. This work makes progress in the use of Armco iron as an emissivity reference.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Apparatus for Measuring Spectral Emissivity of Solid Materials at Elevated Temperatures

Spectral emissivity measurements at high temperature are of great importance for both scientific research and industrial applications. A method to perform spectral emissivity measurements is presented based on two sample heating methods, the flat plate and tubular furnace. An apparatus is developed to measure the normal spectral emissivity of solid material at elevated temperatures from 1073 K to 1873 K and wavelengths from \(2\,\upmu \hbox {m}\) to \(25\,\upmu \hbox {m}\). Sample heating is accomplished by a torch flame or a high temperature furnace. Two different variable temperature blackbody sources are used as standard references and the radiance is measured by a FTIR spectrometer. Following calibration of the spectral response and background radiance of the spectrometer, the effect of the blackbody temperature interval on calibration results is discussed. Measurements are performed of the normal spectral emissivity of SiC and graphite over the prescribed temperature and wavelength range. The emissivity of SiC at high temperatures is compared with the emissivity at room temperature, and the influence of an oxide layer formed at the surface of SiC on the emissivity is studied. The effect of temperature on the emissivity of graphite is also investigated. Furthermore, a thorough analysis of the uncertainty components of the emissivity measurement is performed.     

多波段伪装隐身涂层织物的制备研究与应用

采用着色颜料、红外低发射率填料、水性聚氨酯树脂等制备水性多波段伪装涂料,采用刮涂工艺,制备可见光/近红外/热红外多波段伪装涂层织物。通过双波段发射率测量仪、光谱反射率测试仪、光泽度仪等测试手段对涂层的红外发射率、光谱反射率、镜面光泽度等性能进行了表征,此外还对涂层的理化及环境性能进行了测试。结果表明:涂层的颜色满足《GJB 1082—1991伪装网用颜色》的要求,近红外光谱反射特性在380~1100nm范围内与应用背景基本实现了"同色同谱",不同颜色斑块的发射率梯度在0.13以上,在红外成像下能够形成有效的梯度分割,与应用背景具有较好的融合效果,具有可见光/近红外/热红外隐身性能,各项指标均达到了实用要求。     

A Fast Laser Polarimeter Improving a Microsecond Pulse Heating System

The microsecond pulse heating system has been used for more than 15 years to investigate thermophysical properties of solid and liquid metals and alloys. The only way to measure temperature in the time and temperature range of these experiments (duration of a few tens of microseconds, temperatures up to 7000 K) is optical pyrometry. The radiance temperature can be measured very accurately. However, to obtain true temperature from radiance temperature the normal spectral emissivity at the wavelength of interest of the material under investigation has to be known. Because normal spectral emissivity measurements on pulse heated liquid metals were not possible in the past, an assumption about the behavior of the emissivity in the liquid phase had to be made, which increased the uncertainty of the temperature determination. To overcome this limitation in temperature measurement, a microsecond division of amplitude polarimeter (µ-DOAP) was added to the pulse heating system. The normal spectral emissivity at 684.5 nm is derived from the measured change in the state of polarization of laser light that is reflected off the sample surface. The working principle of this polarimeter system is presented, and experimental results of the normal spectral emissivity at 684.5 nm as a function of radiance temperature at 650 nm are discussed.