Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique
can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.
The effect was first observed by French physicist P. Weiss and Swiss physicist A. Piccard in
1917. The fundamental principle was suggested by P. Debye and W. Giauque. The first working
magnetic refrigerators were constructed by several groups beginning in 1933. Magnetic
refrigeration was the first method developed for cooling below about 0.3K.
The magnetocaloric effect The magnetocaloric effect is a magneto-thermodynamic
phenomenon in which a temperature change of a suitable material is caused by exposing
the material to a changing magnetic field. This is also known by low temperature physicists
as adiabatic demagnetization. In that part of the refrigeration process, a decrease in
the strength of an externally applied magnetic field allows the magnetic domains of a magnetocaloric
material to become disoriented from the magnetic field by the agitating action of the thermal
energy present in the material. If the material is isolated so that no energy is allowed tomigrate
into the material during this time, 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 of a ferromagnetic material, 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 increases when it enters certain magnetic
fields. When it leaves the magnetic field, the temperature drops. The effect is considerably
stronger for the gadolinium alloy Gd 5Si
2Ge 2). Praseodymium alloyed with nickel (PrNi
5) has such a strong magnetocaloric effect that it has allowed scientists to approach
to within one milliKelvin, one thousandth of a degree of absolute zero.
Equation The magnetocaloric effect can be quantified
with the equation below:
where T is the temperature, H is the applied magnetic field, C is the heat capacity of
the working magnet and M is the magnetization of the refrigerant.
From the equation we can see that magnetocaloric effect can be enhanced by:
applying a large field using a magnet with a small heat capacity
using a magnet with a large change in magnetization vs temperature, at a constant magnetic field
Thermodynamic cycle
The cycle is performed as a refrigeration cycle that is analogous to the Carnot refrigeration
cycle, but with increases and decreases in magnetic field strength instead of increases
and decreases in pressure. It can be described at a starting point whereby the chosen working
substance is introduced into a magnetic field, i.e., the magnetic flux density is increased.
The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated
environment. Adiabatic magnetization: A magnetocaloric
substance is placed in an insulated environment. The increasing external magnetic field causes
the magnetic dipoles of the atoms to align, thereby decreasing the material's magnetic
entropy and heat capacity. Since overall energy is not lost and therefore total entropy is
not reduced, the net result is that the item heats.
Isomagnetic enthalpic transfer: This added heat can then be removed by a fluid or gas —
gaseous or liquid helium, for example. The magnetic field is held constant to prevent
the dipoles from reabsorbing the heat. Once sufficiently cooled, the magnetocaloric substance
and the coolant are separated. Adiabatic demagnetization: The substance is
returned to another adiabatic condition so the total entropy remains constant. However,
this time the magnetic field is decreased, the thermal energy causes the magnetic moments
to overcome the field, and thus the sample cools, i.e., an adiabatic temperature change.
Energy transfers from thermal entropy to magnetic entropy.
Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material
from reheating. The material is placed in thermal contact with the environment to be
refrigerated. Because the working material is cooler than the refrigerated environment,
heat energy migrates into the working material. Once the refrigerant and refrigerated environment
are in thermal equilibrium, the cycle can restart.
Applied technique The basic operating principle of an adiabatic
demagnetization refrigerator is the use of a strong magnetic field to control the entropy
of a sample of material, often called the "refrigerant". Magnetic field constrains the
orientation of magnetic dipoles in the refrigerant. The stronger the magnetic field, the more
aligned the dipoles are, corresponding to lower entropy and heat capacity because the
material has lost some of its internal degrees of freedom. If the refrigerant is kept at
a constant temperature through thermal contact with a heat sink while the magnetic field
is switched on, the refrigerant must lose some energy because it is equilibrated with
the heat sink. When the magnetic field is subsequently switched off, the heat capacity
of the refrigerant rises again because the degrees of freedom associated with orientation
of the dipoles are once again liberated, pulling their share of equipartitioned energy from
the motion of the molecules, thereby lowering the overall temperature of a system with decreased
energy. Since the system is now insulated when the magnetic field is switched off, the
process is adiabatic, i.e., the system can no longer exchange energy with its surroundings,
and its temperature decreases below its initial value, that of the heat sink.
The operation of a standard ADR proceeds roughly as follows. First, a strong magnetic field
is applied to the refrigerant, forcing its various magnetic dipoles to align and putting
these degrees of freedom of the refrigerant into a state of lowered entropy. The heat
sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal
contact with the heat sink is then broken so that the system is insulated, and the magnetic
field is switched off, increasing the heat capacity of the refrigerant, thus decreasing
its temperature below the temperature of the heat sink. In practice, the magnetic field
is decreased slowly in order to provide continuous cooling and keep the sample at an approximately
constant low temperature. Once the field falls to zero or to some low limiting value determined
by the properties of the refrigerant, the cooling power of the ADR vanishes, and heat
leaks will cause the refrigerant to warm up. Working materials
The magnetocaloric effect is an intrinsic property of a magnetic solid. This thermal
response of a solid to the application or removal of magnetic fields is maximized when
the solid is near its magnetic ordering temperature. The magnitudes of the magnetic entropy and
the adiabatic temperature changes are strongly dependent upon the magnetic order process:
the magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems; it can
be substantial for normal ferromagnets that undergo a second order magnetic transition;
and it is generally the largest for ferromagnets that undergo a first order magnetic transition.
Also, crystalline electric fields and pressure can have a substantial influence on magnetic
entropy and adiabatic temperature changes. Currently, alloys of gadolinium producing
3 - 4 K per tesla [K/T] of change in a magnetic field can be used for magnetic refrigeration.
Recent research on materials that exhibit a giant entropy change showed that Gd
5(SixGe 1−x)
4, La(FexSi 1−x)
13Hx and MnFeP 1−xAsx alloys, for example, are some of
the most promising substitutes for gadolinium and its alloys — GdDy, GdTb, etc. These
materials are called giant magnetocaloric effect materials.
Gadolinium and its alloys are the best material known for magnetic refrigeration near room
temperature, since they undergo second-order phase transitions that have no magnetic or
thermal hysteresis. However, substituting materials such as MnFeP
1−xAsx for rare earth metals has distinct cost advantages. The development of this technology
is very material-dependent and will likely not replace vapor-compression refrigeration
without significantly improved materials that are cheap, abundant and exhibit much larger
magnetocaloric effects over a larger range of temperatures. Such materials need to show
significant temperature changes under a field of two tesla or less, so that permanent magnets
can be used for the production of the magnetic field.
Paramagnetic salts The original proposed refrigerant was a paramagnetic
salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those
of the electron shells of the paramagnetic atoms.
In a paramagnetic salt ADR, the heat sink is usually provided by a pumped 4He or 3He
cryostat. An easily attainable 1 T magnetic field is generally required for initial magnetization.
The minimum temperature attainable is determined by the self-magnetization tendencies of the
refrigerant salt, but temperatures from 1 to 100 mK are accessible. Dilution refrigerators
had for many years supplanted paramagnetic salt ADRs, but interest in space-based and
simple to use lab-ADRs has remained, due to the complexity and unreliability of the dilution
refrigerator Eventually paramagnetic salts become either
diamagnetic or ferromagnetic, limiting the lowest temperature that can be reached using
this method. Nuclear demagnetization
One variant of adiabatic demagnetization that continues to find substantial research application
is nuclear demagnetization refrigeration. NDR follows the same principles, but in this
case the cooling power arises from the magnetic dipoles of the nuclei of the refrigerant atoms,
rather than their electron configurations. Since these dipoles are of much smaller magnitude,
they are less prone to self-alignment and have lower intrinsic minimum fields. This
allows NDR to cool the nuclear spin system to very low temperatures, often 1 µK or below.
Unfortunately, the small magnitudes of nuclear magnetic dipoles also makes them less inclined
to align to external fields. Magnetic fields of 3 teslas or greater are often needed for
the initial magnetization step of NDR. In NDR systems, the initial heat sink must
sit at very low temperatures. This precooling is often provided by the mixing chamber of
a dilution refrigerator or a paramagnetic salt.
Commercial development Research and a demonstration proof of concept
in 2001 succeeded in applying commercial-grade materials and permanent magnets at room temperatures
to construct a magnetocaloric refrigerator On August 20, 2007, the Risø National Laboratory
at the Technical University of Denmark, claimed to have reached a milestone in their magnetic
cooling research when they reported a temperature span of 8.7 K. They hope to introduce the
first commercial applications of the technology by 2010.
As of 2013 this technology had proven commercially viable only for ultra-low temperature cryogenic
applications available for decades. Magnetocaloric refrigeration systems are composed of pumps,
motors, secondary fluids, heat exchangers of different types, magnets and magnetic materials.
These processes are greatly affected by irreversibilities and should be adequately considered. At year-end,
Cooltech Applications announced that its first commercial refrigeration equipment would enter
the market in 2014. Current and future uses
Thermal and magnetic hysteresis problems remain to be solved for first-order phase transition
materials that exhibit the GMCE. One potential application is in spacecraft.
Vapor-compression refrigeration units typically achieve performance coefficients of 60% of
that of a theoretical ideal Carnot cycle, much higher than current MR technology. Small
domestic refrigerators are however much less efficient.
In 2014 giant anisotropic behaviour of the magnetocaloric effect was found in HoMn
2O 5 at 10 K. The anisotropy of the magnetic
entropy change gives rise to a large rotating MCE offering the possibility to build simplified,
compact, and efficient magnetic cooling systems by rotating it in a constant magnetic field.
History The effect was discovered using iron in 1881
by German physicist Emil 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 Peter Debye in 1926 and chemistry Nobel Laureate William
F. Giauque in 1927. It was first demonstrated experimentally by
Giauque and his colleague D. P. MacDougall in 1933 for cryogenic purposes when they reached
0.25 K. Between 1933 and 1997, advances in MCE cooling occurred.
In 1997, the first near room-temperature proof of concept magnetic refrigerator was demonstrated
by 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.
A major breakthrough came 2002 when a group at the University of Amsterdam demonstrated
the giant magnetocaloric effect in MnFe(P,As) alloys that are based on abundant materials.
Refrigerators based on the magnetocaloric effect have been demonstrated in laboratories,
using magnetic fields starting at 0.6 T up to 10 T. Magnetic fields above 2 T are difficult
to produce with permanent magnets and are produced by a superconducting magnet.
Room temperature devices Recent research has focused on near room temperature.
Constructed examples of room temperature magnetic refrigerators include:
In one example, Prof. Karl A. Gschneidner, Jr. unveiled a proof of concept magnetic refrigerator
near room temperature on February 20, 1997. He also announced the discovery of the GMCE
in Gd 5Si
2Ge 2 on June 9, 1997. Since then, hundreds of
peer-reviewed articles have been written describing materials exhibiting magnetocaloric effects.
See also Electrocaloric effect
Thermoacoustic refrigeration Dilution refrigerator
References
Further reading Lounasmaa, Experimental Principles and Methods
Below 1 K, Academic Press. Richardson and Smith, Experimental Techniques
in Condensed Matter Physics at Low Temperatures, Addison Wesley.
Lucia, U. General approach to obtain the magnetic refrigeretion ideal Coefficient of Performance
COP, Physica A: Statistical Mechanics and its Applications, 387/14 3477–3479, doi:10.1016/j.physa.2008.02.026;
see also http:arxiv.org1011.1684 External links
NASA – How does an Adiabatic Demagnetization Refrigerator Work ?
What is magnetocaloric effect and what materials exhibit this effect the most?
Magnetocaloric materials keep fridges cool by C. Wu
Ames Laboratory news release, May 25, 1999, Work begins on prototype magnetic-refrigeration
unit. Magnetic refrigerator successfully tested
Refrigeration Systems Terry Heppenstall's notes, University of Newcastle upon Tyne
XRS Adiabatic Demagnetization Refrigerator Executive Summary: A Continuous Adiabatic
Demagnetization Refrigerator Origin and tuning of the magnetocaloric effect
in the magnetic refrigerant Mn1.1Fe0.9(P0.8Ge0.2) Magnetic technology revolutionizes refrigeration
Evaluation of thermodynamic quantities in magnetic refrigeration