Session: 17-06: Symposium Steinfeld - Radiative and materials characterization and solar technology development

Paper Number: 142455

142455 - A New Approach for Direct Measurement of Spectral Emissivity at Ultra-High Temperatures 

Abstract: 

We report a new approach and experimental apparatus for direct measurement of the spectral emissivity of surfaces at ultra-high temperatures. Spectral emissivity ε_λ(λ,T) is a property of fundamental interest, from which many other radiative surface properties including absorptance and reflectance can be determined. There are three main approaches for experimentally measuring a surface’s emissivity: 1) calorimetric; 2) radiometric; and 3) reflection-based methods. Of these, the radiometric approach is the most robust, since it measures emission directly (as opposed to reflection-based methods) and allows spectral measurement (unlike the calorimetric approach). The basic principle of the radiometric approach is to measure the spectral radiation emitted by a heated sample using a spectrometer. Because the spectrometer can only provide a relative spectrum, it is necessary to use a suitable referencing method to determine the absolute spectral emissivity. Existing approaches perform this referencing by comparing the spectrum emitted by the sample to that of a blackbody cavity. This can be achieved either by flipping the beam path between a separate sample and blackbody reference (comparison method), or by integrating a blackbody into the sample design (integrated blackbody method), for example by enclosing the sample inside a cavity and shifting the sample to the top or bottom of the cavity to perform the sample and reference measurement, respectively. In either case, the main experimental challenge is maintaining isothermality and identical temperatures for both sample and reference. This is exacerbated by the fact that the sample (typically a material coupon) and the reference (a blackbody cavity) are geometrically dissimilar. Due to the strong dependence of spectral emission on temperature, this leads to a large propagated uncertainty in the emissivity measurement.

To circumvent this, we introduce a self-referencing approach which removes the need for a separate blackbody reference. The self-referencing approach makes use of the slope of the emissive power vs. temperature curve to directly extract the spectral emissivity of the sample. This significantly simplifies the experimental apparatus since the need for sample/reference swapping or beam flipping is negated, and, moreover, the accuracy of the measurement is improved since errors induced by nonisothermality are significantly reduced. In this talk, we report the theoretical development of the method, as well as the design, fabrication, and experimental demonstration of a laboratory apparatus which implements the proposed method. The apparatus comprises a high vacuum chamber equipped with electrical, water cooling, instrumentation, and optical (infrared) feedthroughs, which houses an in-house-built sample heater stage capable of heating samples to a maximum temperature of 2000 K. Radiation emitted by the hot sample is collected by an off-axis parabolic mirror which collimates the beam and delivers it to a Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70) which extracts the relative emission spectrum covering wavelengths from 0.5 to 20 μm. Fine control of the sample heater allows scanning the sample surface temperature about a target temperature from which the spectral emissivity can be computed from a plot of relative signal vs. temperature.

This method and apparatus enable accurate determination of spectral emissivity for temperatures ranging up to 2000 K which assist in the development of next generation refractory materials for applications including: concentrating solar power; thermal energy storage; and high-temperature industrial processes.

Presenting Author: Thomas Cooper York University

Presenting Author Biography: Dr. Cooper is Associate Professor in the Department of Mechanical Engineering at the Lassonde School of Engineering, York University, Toronto. There he leads the CooperLab, which conducts fundamental and applied renewable energy research. With a current focus on solar thermal technology, Dr. Cooper's research spans the fields of thermal science, optics, and materials to develop new pathways and devices for transforming sunlight into useful forms, including electricity, heat, clean water, and renewable fuels, also touching on complementary fields including energy storage, advanced thermal insulation materials, and advanced thermal characterization techniques. Dr. Cooper received his Dr. sc. and M. Sc. from ETH Zurich in 2014 and 2010 respectively, and his B.A.Sc. from the University of Toronto in 2008. Prior to joining Lassonde, Dr. Cooper was a postdoc at MIT, where his research targeted the development of nanoporous materials with tailored optical and thermal properties for solar energy applications.

Authors:

Anahita Pirvaram York University
Ikbal Kabir York University
Philipp Good Synhelion SA
Gianluca Ambrosetti Synhelion SA
Thomas Cooper York University