SUPERCOOLING OF HYDRAZINE.
NAVAL RESEARCH LAB WASHINGTON D C
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The supercooling of hydrazine in thin-walled capillaries and as droplets suspended in silicone oil has been investigated by observing the freezing points of samples held in the cold stage of a microscope. A wide range in capillary diameters was employed, but a smooth correlation between capillary size and degree of supercooling was not obtained due to heterogeneous nucleation at the glass surface. Suspension of hydrazine droplets in silicone oil permitted the study of much smaller samples, while eliminating the nucleating effect of solid surfaces. Also, by this procedure, foreign nuclei present in the original sample were isolated in a certain few droplets, while the majority, having been freed of foreign nuclei, froze by homogeneous nucleation. Under these conditions, the maximum degree of supercooling of hydrazine was 120C. Using data from these experiments together with estimated values for the heat of fuusion and for the free energy of activation for self-diffusion, the solidliquid interfacial free energy for hydrazine was calculated from the theory of homogeneous nucleation to be 39.5 ergs sq. cm. Thus the supercooled state is a normal one that can be dealt with thermodynamically. The high degree of supercooling of small droplets of hydrazine would not be feasible with bulk hydrazine. At present, it is felt that in order to keep hydrazine in the lliquid state at low temperatuttures the depression of its freezing point by blending with other fuels would be the most reliable method. Author