The magnetocaloric effect (MCE, from magnet and calorie) is a phenomenon in which certain materials warm up when a magnetic field is applied. The warming is due to changes in the internal state of the material, which releases heat. When the magnetic field is removed, the material returns to its original state, reabsorbing the heat, and returning to original temperature. This can be used to achieve refrigeration, by allowing the material to radiate away its heat while in the magnetized hot state. Removing the magnetism, the material then cools to below its original temperature.
The effect was first observed in 1881 by German physicist Emil Warburg, followed by French and Swiss physicists Pierre Weiss and Auguste Piccard in 1917. The fundamental principle was suggested by American chemists Peter Debye (1926) and William Giauque (1927). The first working magnetic refrigerators were constructed by several groups beginning in 1933. Magnetic refrigeration was the first method developed for cooling below about 0.3 K (the lowest temperature attainable before magnetic refrigeration, by pumping on vapors).
The magnetocaloric effect can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.
History
The effect was first observed by German physicist Emil Warburg in 1881 Subsequently by French physicist Pierre Weiss and Swiss physicist Auguste Piccard in 1917.
Major advances first appeared in the late 1920s when cooling via adiabatic demagnetization was independently proposed by chemistry Nobel Laureates Peter Debye in 1926 and 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 (1 T is about times the Earth's magnetic field).
Room temperature devices
Recent research has focused on near room temperature. Constructed examples of room temperature magnetic refrigerators include:
+ Room temperature magnetic refrigerators Sponsor Location Announcement date Type Max. cooling power (W) Max ΔT (K) Magnetic field (T) Solid refrigerant Quantity (kg) COP (-) Ames Laboratory/Astronautics Ames, Iowa/Madison, Wisconsin, US February 20, 1997 Reciprocating 600 10 5 (S) Gd spheres Mater. Science Institute Barcelona Barcelona, Spain May 2000 Rotary ? 5 0.95 (P) Gd foil Chubu Electric/Toshiba Yokohama, Japan Summer 2000 Reciprocating 100 21 4 (S) Gd spheres University of VictoriaRowe A.M. and Barclay J.A., Adv. Cryog. Eng. 47 995 (2002). Victoria, British Columbia Canada July 2001 Reciprocating 2 14 2 (S) Gd & L.B. AstronauticsZimm C, Paper No K7.003 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) Madison, Wisconsin, US September 18, 2001 Rotary 95 25 1.5 (P) Gd spheres Sichuan Inst. Tech./Nanjing UniversityWu W., Paper No. K7.004 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) Nanjing, China 23 April 2002 Reciprocating ? 23 1.4 (P) Gd spheres and Gd5Si1.985Ge1.985Ga0.03 powder Chubu Electric/ToshibaHirano N., Paper No. K7.002 Am. Phys. Soc. Meeting March 4, Austin, Texas, Yokohama, Japan October 5, 2002 Reciprocating 40 27 0.6 (P) L.B. Chubu Electric/Toshiba Yokohama, Japan March 4, 2003 Rotary 60 10 0.76 (P) L.B. 1 Lab. d'Electrotechnique Grenoble Grenoble, France April 2003 Reciprocating 8.8 4 0.8 (P) Gd foil George Washington University US July 2004 Reciprocating ? 5 2 (P) Gd foil AstronauticsZimm C, Paper No. K7.003 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) Madison, Wisconsin, US 2004 Rotary 95 25 1.5 (P) Gd and GdEr spheres / University of Victoria Victoria, British Columbia Canada 2006 Reciprocating 15 50 2 (S) Gd, and pucks 0.12 University of Salerno Salerno, Italy 2016 Rotary 250 12 1.2 (P) Gd 0.600 mm spherical particles 1.20 0.5 - 2.5 MISiS Tver and Moscow, Russia 2019 High speed rotary ? ? ? Gd bricks of two types, cascaded 1maximum cooling power at zero temperature difference (ΔT=0); 2maximum temperature span at zero cooling capacity (W=0); L.B. = layered bed; P = permanent magnet; S = superconducting magnet; 3 COP values under different operating conditions
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 on June 9, 1997. Since then, hundreds of peer-reviewed articles have been written describing materials exhibiting magnetocaloric effects.
Process
The MCE 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 (phonons) present in the material. If the material is isolated so that no energy is allowed to (re)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 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 . Praseodymium alloyed with nickel () 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 following equation:
where is the adiabatic change in temperature of the magnetic system around temperature ; is the applied external magnetic field; is the heat capacity of the working magnet (refrigerant); and is the magnetization of the refrigerant.
From the equation we can see that the magnetocaloric effect can be enhanced by:
a large field variation a magnet material with a small heat capacity a magnet with large changes in net magnetization vs. temperature, at constant magnetic field
The adiabatic change in temperature can be seen to be related to the magnet's change in magnetic entropy () since
This implies that the absolute change in the magnet's entropy determines the possible magnitude of the adiabatic temperature change under a thermodynamic cycle of magnetic field variation.
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 (yet) and therefore total entropy is not reduced (according to thermodynamic laws), the net result is that the substance is heated (). 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 (). : The substance is returned to another adiabatic (insulated) 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 (and entropy) transfers from thermal entropy to magnetic entropy, measuring the disorder of the magnetic dipoles. Extract of page 99 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 (by design), 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 (ADR) 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 (effectively) lost some of its internal degrees of freedom. If the refrigerant is kept at a constant temperature through thermal contact with a heat sink (usually liquid helium) 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 (the heat sink), and its temperature decreases below its initial value, that of the heat sink.