Determination of total hemispherical emissivity utilizing an in-situ calibrated, coupled experimental–simulative electron beam calorimetric approach

Abstract

The accurate determination of the total hemispherical emission coefficient (THEC) of materials is essential for modelling radiative heat transfer in high-temperature vacuum environments. This study presents a rapid steady-state calorimetric method to determine the THEC using a vacuum electron beam (EB) facility. A graphite-coated Inconel 718 foil with a thickness comparable to the electron penetration depth was used as the emission target, allowing temperature equalization within seconds. Surface temperatures were recorded via a calibrated two-colour thermal imaging system. An in-situ calibration was conducted, using the melting point of pure copper as a fixed reference. Numerical simulations were employed to validate both the emissivity determination and the temperature calibration methodology. EB heating efficiency was determined at $150keV$ in a combined experimental–simulative approach using Monte Carlo simulations and backscattered electron intensity capture. Temperature calibration showed high reproducibility with a median absolute deviation of $\pm 0.4\%$ and a half range of $\pm 1.1\%$ at $1085\circ C$, and was shown to be transferable to other materials. EB heating efficiency slightly decreased from $0.832\pm 0.003$ at room temperature to $0.825\pm 0.002$ between $800\circ C$ to $1140\circ C$. THEC values between $0.88$ and $0.92$ with a maximum half range of $\pm 0.06$ were obtained in the range of $840\circ C$ to $1110\circ C$ for graphite-coated Inconel 718 with a median roughness of $R_a=1.2\mu m$. Reflected radiation from the emission target and thermal losses from conduction and sublimation were assessed and found negligible. Results were consistent with literature, particularly under high surface roughness or pre-oxidized conditions.