Phase change cells and the verification of gallium as a thermal calibration reference in space

The validation of models of global climate change and accurate measurement of the atmosphere and surface temperatures require that orbital sensors have low drift rates, and are monitored or regularly recalibrated by accepted standards. Phase change materials (PCM), such as those that make up the ITS-90 standard, are the basis for international commerce and have been suggested for monitoring and recalibration of orbital temperature sensors. Space Dynamics Laboratory (SDL) and its partners have been developing miniaturized phase change reference technologies that could be deployed on an orbital blackbody for nearly a decade. A significant part of this effort has been the exploration of the behavior of gallium (Ga) and its eutectics, gallium-tin (GaSn) and gallium-indium (GaIn) in conditions expected to be encountered in this application. In this paper, these behaviors are detailed and an example of a hardware design that could be used as an infrared blackbody calibration monitor is presented. To determine if and how microgravity will affect the behavior of Ga, the authors conducted an experiment on the International Space Station (ISS) and compared the observed phase change temperature with earth-based measurements. This paper also provides a brief description of the experiment hardware, microgravity considerations, and the pre-flight, flight and post-flight data analysis.