High-temperature, normal spectral emittance of silicon carbide based materials

Abstract

An emissometer was designed and constructed to measure the normal, spectral emittance of opaque solids over the spectral range from 1 to 8 /Jtm for temperatures ranging from 500 to 1500°C using an integral blackbody technique. The emissometer is gas-tight so that the gas environments surrounding the sample can be controlled and emittance data can be collected as a function of exposure time to a specific environment at a particular temperature. An FT-IR spectrometer collects the blackbody and sample infrared signals, which are ratioed to calculate the material's emittance. A computer code was developed to check the validity of the assumptions associated with the measurement technique. The emittance of silicon carbide in the form of a-SiC (Hexoloy SA) and TiB 2 -toughened SiC (Hexoloy ST) was collected over the spectral and temperature ranges given above. The emittance for Hexoloy SA was measured in the as-received condition and after sample exposure at 1300°C to carburizing, oxidizing, and low-pressure nitrogen environments. Emittance data for Hexoloy ST were collected for samples in the as-received condition and for samples exposed to oxidizing environments. The surface morphology and composition of the samples were characterized using SEM, EDX, and X-ray diffraction techniques. NomenclatureCi, C 2 , C 3 = correction factors for systematic errors, -d = layer thickness, imm E = emissive power, W/m 2 FJ_J = view factor, from A t to Aj H = irradiation, W/m 2 / = radiation intensity, W/m 2 sr n = index of refraction, -S = signal strength, mV § -unit direction vector, -T = temperature, K e -emittance 0 = polar angle, rad A = wavelength, /jum </> = azimuthal angle, rad Subscripts b = blackbody bg = background / = incident ra = measured s = sample ss = stainless steel drop tube A = spectral value, per unit wavelength

Survey of emissivity measurement by radiometric methods

“…Such blackbodies are formed directly in the volume of the analyzed sample [3,22] or the sample can be inserted into the cavity of the blackbody [1,26,44]. Application of integrated blackbodies for the measurement of spectral directional emissivity is not frequent.…”

Accurate determination of the total hemispherical emittance and solar absorptance of opaque surfaces at elevated temperatures

“…In order to avoid the challenges of an indirect measurement, radiometric methods are used to directly measure the spectral directional emittance of opaque or partially transparent samples using a spectrometer by comparing their emission spectra to that of a blackbody reference [21][22][23][24][25][26]. The experimental systems used for these techniques are typically quite complex; their three major challenges are measuring only the radiation emitted by the sample, ensuring sample temperature uniformity, and accurately measuring the sample temperature.…”

Investigation of High-Temperature Normal Infrared Spectral Emissivity of ZrO2 Thermal Barrier Coating Artefacts by the Modified Integrated Blackbody Method

“…Although the temperature drop when the sample was pushing from the cavity center to opening has been ignored in the literatures [ 11 , 12 ], this temperature drop was measured and analyzed in this paper and this will help to improve the accuracy of measurement results. The process of pushing the sample pushed from the cavity center to the opening can be discretized into n rings of equal area, and the boundary layer at the cavity opening can be considered as a room temperature blackbody.…”

“…It applies to the case where samples under certain emissivity conditions are inserted into a cavity with a large depth to diameter ratio, such that the cavity formed by the sample surface coupling with the cavity wall can be considered as an effective blackbody, i.e., with an emissivity close to 1. In 1994, Postlethwait et al [ 11 ] from Pennsylvania State University (PSU) proposed the high temperature spectral emissivity measurement method using this principle for the first time and set up an emissivity measurement device using the vertical free falling sample method for the temperature range of 500 °C to 1000 °C. From 2004 to 2010, Herdrich et al [ 12 , 13 , 14 ] from the Institut für Raumfahrtsysteme (IRS), University of Stuttgart, designed and built an emissivity measurement device for thermal protection materials using the integrated blackbody method and adopted a narrow-band thermometer as the radiance measurement device for the study of plasma wind motion.…”