Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique
can be used to attain extremely low temperatures
(well below 1 kelvin),
as well as the ranges used in common refrigerators,
depending on the design of the system.
History
The
effect was discovered in pure iron in 1881 by E. Warburg. Originally, the cooling effect varied
between 0.5 to 2 K/T. Major
advances first appeared in the late 1920s when cooling via adiabatic
demagnetization was independently proposed by two scientists: Debye
(1926) and Giauque (1927). The
process was demonstrated a few years later when Giauque and MacDougall in 1933
used it to reach a temperature of 0.25 K. Between 1933 and 1997, a number of
advances in utilization of the MCE for cooling occurred. This
cooling technology was first demonstrated experimentally by chemist Nobel
Laureate William F. Giauque and his colleague Dr. D.P. MacDougall in 1933 for cryogenic purposes
(they reached 0.25 K) Between
1933 and 1997, a number of advances occurred which have been described in some
reviews. In
1997, the first near room temperature proof
of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gschneidner, Jr. by the Iowa State University at Ames
Laboratory. This event attracted interest from scientists and companies
worldwide who started developing new kinds of room temperature materials and
magnetic refrigerator designs. Refrigerators
based on the magnetocaloric effect have been demonstrated in laboratories,
using magnetic fields starting at 0.6 T up to 10 teslas.
Magnetic fields above 2 T are difficult to produce with permanent magnets and
are produced by a superconducting magnet (1 tesla is about
20,000 times the Earth's magnetic field).
MAGNETO
CALORIC EFFECT
The
Magneto caloric effect (MCE, from magnet and calorie)
is a magneto-thermodynamic phenomenon in which a reversible change in
temperature of a suitable material is caused by exposing the material to a
changing magnetic field. This is also known as adiabatic demagnetization by low
temperature physicists, due to the application of the process specifically to
effect a temperature drop. In that part of the overall refrigeration process, a
decrease in the strength of an externally applied magnetic field allows the
magnetic domains of a chosen (magnetocaloric)
material to become disoriented from the magnetic field by the agitating action
of the thermal energy (phonons) present in the material. If the material is isolated
so that no energy is allowed to (e)migrate into the material during this time
(i.e. an adiabatic process), the temperature
drops as the domains absorb the thermal energy to perform their reorientation.
The randomization of the domains occurs in a similar fashion to the randomization
at the curie temperature, except that magnetic dipoles
overcome a decreasing external magnetic field while energy remains constant,
instead of magnetic domains being disrupted from internal ferromagnetism
as energy is added.
One
of the most notable examples of the magnetocaloric effect is in the chemical
element gadolinium
and some of its alloys.
Gadolinium's temperature is observed to increase when it enters certain
magnetic fields. When it leaves the magnetic field, the temperature returns to
normal.The effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2).
Praseodymium
alloyed with nickel
(PrNi5) has
such a strong magnetocaloric effect that it has allowed scientists to approach
within one thousandth of a degree of absolute
zero.
