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Experimental data on the liquid-to-vapor homogeneous-nucleation temperature, which provides a fundamental limit on the maximum attainable superheat, are presented for liquid nitrogen in the temperature range 77.4 K & T, (124.6 K, where T, is the saturated bath temperature of liquid nitrogen. These data were obtained from a transient-heating experiment and are relevant to the design of superconducting systems using the recently discovered high-temperature superconductors and liquid nitrogen as the coolant.

An equation of general applicability is proposed to describe the surface tension of simple liquids over an extended temperature range. This equation incorporates the quantum effects on the exponent μ, which characterizes the temperature dependence of surface tension. Far from the critical region, μ decreases with increasing de Boer parameter and appears to reach a constant value μ≃1 for He4 and He3. It is pointed out that for quantum liquids μ crosses over to the nonquantum mechanical value of μ≃1.28 as the critical point is approached. The surface tension and its temperature dependence are predicted for hydrogen isotopes such as T2, DT, D2, HT, and HD.

We have investigated the transient transfer of heat from a crystal of bismuth into a bath of liquid helium I in response to a step-function current applied to the crystal. The superheat temperature ΔT of the helium in contact with the crystal was monitored through the magnetoresistance of the bismuth which is strongly temperature dependent at liquid-helium temperatures. It is found that as a function of time, the transient superheat temperature can rise above (i.e., ’’overshoot’’) its steady-state value. Of particular interest is the fact that, within a certain range of bath temperatures and heat fluxes, these transient overshoots develop quite sharp peaks at a superheat temperature ΔTmc which is independent of the applied heat flux. This overshooting of the transient superheat temperature is discussed in terms of the delayed onset of nucleation. In particular, it is suggested that the sharp overshoots result from the rapid onset of homogeneous nucleation. The temperatures associated with the sharp overshoots are compared with theoretical estimates of the homogeneous nucleation temperature for liquid helium.