Page 479 - Mechanical Engineers' Handbook (Volume 4)

468 Cryogenic Systems

Figure 2 Skeletal P–H diagram.

Helium, though twice the molecular weight of hydrogen, also shows the effects of flow
molecular weight upon gross properties. The helium molecule is single-atomed and thus free
from ortho–para-type complexities. Helium was liquefied conventionally first in 1908 by
Onnes of Leiden, and the liquid phase showed conventional behavior at atmospheric pressure.
As temperature is lowered, however, a second-order phase change occurs at 2.18 K
(0.05 atm) to produce a liquid called HeII. At no point does solidification occur just by
evacuating the liquid. This results from the fact that the relationship between molecular
volume, thermal energy (especially zero-point energy), and van der Waals attractive forces
is such that the atoms cannot be trapped into a close-knit array by temperature reduction
alone. Eventually, it was found that helium could be solidified if an adequate pressure is
applied, but that the normal liquid helium (HeI)–HeII phase transition occurs at all pressures

Figure 3 Molecular configurations of (a) para- and (b) ortho-hydrogen.