Thermal Conductivity of Solid Hydrogen at Low Temperature

This dissertation presents the results of the study of properties of solid hydrogen at very low temperatures by means of thermal conductivity, CW NMR, and specific heat measurements. The experiment consists of two major parts: the study of quantum diffusion in low ortho concentration solid, and the study of the orientational order-disorder phase transition in high ortho concentration solid. The effect of quantum diffusion on the thermal conductivity and the NMR signal of isolated ortho hydrogen molecules were measured on a solid sample with ortho concentration ranging from 4% to 0.5%, in the temperature range 0.1K < T < 2.0K. It was found that at high temperatures (T > 0.25K), a hydrogen solid containing ortho molecule clusters has lower thermal conductivity than when it contains random distributed ortho molecules. At lower temperatures, the opposite was observed. The relaxation time for isolated ortho molecules to form larger clusters, as measured from NMR signal in a sample under 90atm pressure, was found to be the same as that observed in samples under zero pressure measured in previous experiment. This similarity of the relaxation time indicates that the main mechanism of quantum diffusion in solid hydrogen is the resonant ortho-para conversion, and the quantum tunneling process does not play a major role as it does in solid helium. For the first time, thermal conductivity in the orientational ordered phase was measured, and was found to have very large values. The pure ortho hydrogen solid may have a thermal conductivity comparable with that of a pure para hydrogen solid. The observed thermal conductivities at constant concentration were found to fall into two distinguishable branches. CW NMR spectra and the specific heat of the solid were measured simultaneously with the thermal conductivity. The width of the order-disorder phase transition was also studied and discussed.