The structural failure of copper-freezing-point blackbodies for radiation thermometry: analysis and design improvements

Abstract

Copper-freezing-point blackbodies used in radiation thermometry should preferably provide sufficiently long service. A blackbody of this sort commonly comprises a highly pure copper sample surrounding a graphite cavity inside a graphite shell closed at both ends by graphite components. We have found that the linear expansion of the solid copper sample in heating from room temperature to the melting point marginally exceeds the linear contraction of the molten sample in cooling to room temperature, presumably due to the gravitational forces that tend to flatten the sample during solidification. The excess expansion causes various stresses in the graphite components. Some of them will eventually fail structurally after a number of cycles of melting and freezing of the sample. This paper presents experimental results of the excess expansion, delineates the various stresses and suggests design improvements for longer service.