Hydride Compressor Sorption Cooler and Surface Contamination Issues

Abstract

A continuous‐duty hydrogen sorption cryocooler is being developed for the Planck spacecraft, a mission to map the cosmic microwave background beginning in 2007. This cryocooler uses six individual compressor elements (CEs) filled with the hydriding alloy LaNi4.78Sn0.22 to provide high‐pressure (50 bar) hydrogen to a Joule‐Thomson (J‐T) expander and to absorb low‐pressure (∼0.3 bar) gas from liquid hydrogen reservoirs cooled to ∼18K. Quadrupole Mass Spectrometry (QMS) showed methane in these hydride beds after cycling during initial operation of laboratory tests of the Planck engineering breadboard (EBB) cooler. These contaminants have caused problems involving plugged J‐T expanders. The contaminants probably come from reactions with residual hydrocarbon species on surfaces inside the hydride bed. The hydride bed in each CE is contained in an annular volume called a “gas‐gap heat switch,” which serves as a reversible, intermittent thermal path to the spacecraft radiator. The gas‐gap is either “off” (i.e., its pressure <1.3 Pa), or “on” (i.e., hydrogen gas at ∼4 kPa). The hydrogen pressure is varied with an independent hydride actuator containing ZrNiHx. Early EBB cooler tests showed increasing parasitic heat losses from the inner beds, suggesting residual pressures in the gas gap during its “off” state. The pressure was shown to be due to hydrogen from outgassing from metallic surfaces in the gas gap and hydrogen permeation through the inner sorbent bed wall. This gas accumulation has serious end‐of‐life implications, as the ZrNi actuator has limited storage capacity and any excess hydrogen would necessarily affect its operation. This paper summarizes experiments on the behavior of hydrogen in the gas gap switch and formation of methane in the CE sorbent beds.