Cryogenics

Following his successful liquefaction of helium in 1908, Kamerlingh Onnes turned his attention almost immediately to studying the properties of other materials at cryogenic temperatures. The first property he investigated was the electrical resistance of metals, which was known to decrease with decreasing temperature. It was presumed that the resistance would completely disappear at absolute zero. Onnes discovered, however, that for some metals the resistance dropped to zero very suddenly at temperatures above absolute zero. The effect is called superconductivity and has some very important applications in today's world. For example, superconductors are used to make magnets for particle accelerators and for magnetic resonance imaging (MRI) systems used in many hospitals.

The discovery of superconductivity led other scientists to study a variety of material properties at cryogenic temperatures. Today, physicists, chemists, material scientists, and biologists study the properties of metals, as well as the properties of insulators, semiconductors, plastics, composites, and living tissue. In order to chill their samples they must bring them into contact with something cold. This is done by placing the sample in an insulated container, called a dewar, and cooling the inner space, either by filling it with a cryogenic liquid, or by cooling it with a cryogenic refrigerator.

Over the years, this research has resulted in the identification of a number of useful properties. One such property common to most materials that are subjected to extremely low temperatures is brittleness. The recycling industry takes advantage of this by immersing recyclables in liquid nitrogen, after which they are easily pulverized and separated for reprocessing. Still another cryogenic material property that is sometimes useful is that of thermal contraction. Materials shrink when cooled. To a point (about the temperature of liquid nitrogen), the colder a material gets the more it shrinks. An example is the use of liquid nitrogen in the assembly of some automobile engines. In order to get extremely tight fits when installing valve seats, the seats are cooled to liquid nitrogen temperatures, whereupon they contract and are easily inserted in the engine head. When they warm up, a perfect fit results.

Cryogenic liquids are also used in the space program. For example, cryogens are used to propel rockets into space. A tank of liquid hydrogen provides the fuel to be burned and a second tank of liquid oxygen is provided for combustion. A more exotic application is the use of liquid helium to cool orbiting infrared telescopes. Any object warmer than absolute zero radiates heat in the form of infrared light. The infrared sensors that make up a telescope's lens must be cooled to temperatures that are lower than the equivalent temperature of the light they are intended to sense, otherwise the telescope will be blinded by its own light. Since temperatures of interest are as low as 3K (-454°F [-270°C]), liquid helium at 1.8K (-456°F [-271°C]) is used to cool the sensors.

Cryogenic preservation has even extended to a hotly debated topic: in vitro fertilization. In vitro fertilization is a technique that improves the chances of a woman being pregnant by removing an egg and fertilizing it with a sperm, cultivating the zygote until it becomes an embryo and implanting it into the uterus of a female recipient. A recent report by clinical geneticists at the Georgia Reproductive Specialists LLC, in Atlanta published results from the first conception and delivery of non-identical twins after a successful transfer of human blastocyts (a stage during the early development of the embryo shortly after fertilization) that were cryogenically preserved at day six and seven after fertilization. Controversy in the medical community surrounds the value of cryogenically preserving later stage human blastocytes. Although using cryogenic preservation for maintaining embryo may lead to bioethical complications, the medical potential for preserving tissues or organs that thawed maitain viability has tremendous medical benefits.

Finally, the production of liquefied gases has itself become an important cryogenic application. Cryogenic liquids and gases such as oxygen, nitrogen, hydrogen, helium, and argon all have important applications. Shipping them as gases is highly inefficient because of their low densities. This is true even at extremely high pressures. Instead, liquefying cryogenic gases greatly increases the weight of cryogen that can be transported by a single tanker.