Page 532 - Mechanical Engineers' Handbook (Volume 4)

8 Superconductivity and Its Applications 521

through 2015. Total demand for U.S. helium is nearly constant at about 3 Bcf/yr (in 1994).
Private industry supplies about 89% of this market, the rest coming from the stored govern-
ment supply. The estimated He resources in helium-rich natural gas in the United States is
about 240 Bcf as of 1994. With the stored He, this makes a total supply of about 270 Bcf,
probably enough to supply the demand until the middle of the 21st century. Eventually
technology will be needed to economically recover He from more dilute sources.
The liquefaction of He, or the production of refrigeration at temperatures in the liquid
He range, requires special techniques. He, and also H , have negative Joule–Thomson co-
2
efficients at room temperature. Thus cooling must first be done with a modified Claude
process to a temperature level of 30 K or less. Often expanders are used in series to obtain
temperatures close to the final temperature desired. An expansion valve may then be used
to effect the actual liquefaction. Such a process is shown in Fig. 42. The goal of this process
is the maintenance of a temperature low enough to sustain superconductivity (see below)
using a conventional low-temperature superconductor. Since such processes are usually small,
and since entropy gains at very low temperature are especially damaging to process effi-
ciency, these processes must use very small T’s for heat transfer, require high-efficiency
expanders, and must be insulated nearly perfectly. Note that in heat exchanger X4 the T at
the cold end is 0.55 K.

8 SUPERCONDUCTIVITY AND ITS APPLICATIONS
For normal electrical conductors the resistance decreases sharply as temperature decreases,
as shown in Fig. 43. For pure materials this decrease tends to level off at very low temper-
atures. This results from the fact that the resistance to electron flow results from two factors:
the collision of electrons with crystal lattice imperfections and electron collisions with the
lattice atoms themselves. The former effect is not temperature dependent, but the latter is.
This relationship has, itself, proven of interest to engineers, and much thought and devel-
opment has gone toward the building of power transmission lines operating at cryogenic
temperatures and taking advantage of the reduced resistance.

8.1 Superconductivity
In 1911 Dr. Onnes of Leiden was investigating the electrical properties of metals at very
low temperatures, helium having just been discovered and liquefied. He was measuring the
resistance of frozen mercury as the temperature was reduced into the liquid He range. Sud-
denly the sample showed zero resistance. At first a short circuit was suspected. However,
very careful experiments showed that the electrical conductivity of the sample had dropped
discontinuously to a very low value. The phenomenon of superconductivity has since been
found to occur in a wide range of metals and alloys. The resistance of a superconductor has
been found to be smaller than can be measured by the best instrumentation available. Possibly
it is zero. Early on this was demonstrated by initiating a current in a superconducting ring
which could then be maintained, undiminished, for months.
The phenomenon of superconductivity has been studied ever since in attempts to learn
the extent of the phenomena, to develop a theory that will explain the basic mechanism and
predict superconductive properties, and to use superconductivity in practical ways.
On an empirical basis it has been found that superconductors are diamagnetic, that is,
they exclude a magnetic field, and that they exist within a region bounded by temperature
and magnetic field strength. This is shown in Fig. 44. In becoming superconductive a material
also changes in specific heat and in thermal conductivity.