Introduction: What is Cryogenics?
This is another one of those areas that people have heard about, but have a tough time defining. It is a very important area in basic science, engineering, food, metallurgy, manufacturing, to name but a few.
It deals with cold and colder, essentially everything below about -150 °C or 123 K.
In this range, the International Temperature Scale of 1990 (ITS-90) has seven fixed, primary defining points.
Indeed, one of the features of ITS-90 was the extension of the low end of the scale from the previous lower limit of 13.81 K (-259.43°C), the low limit of IPTS-68, to 0.5 K (-272.65 °C).
Most people associate this region with liquid nitrogen and liquid oxygen, some even with liquid hydrogen and liquid helium, for these are some of the gases used as cooling agents or propellants in many space vehicle rocket engines.
Much of the work reported in the popular press does indeed revolve around work in these areas, e.g. 77 K to 4 K.
There are many many faces to cryogenics from the liquid oxygen plant next to a steel mill (that condenses oxygen from the air and supplies to the Basic Oxygen furnaces for making steel) to the special research apparatus used in studying materials properties at temperatures below that of liquid Helium.