Magnetic refrigeration
To achieve refrigeration, the material is allowed to radiate away its heat while in the magnetized hot state.
[7] Praseodymium alloyed with nickel (PrNi5) 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.
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.
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.
In practice, the magnetic field is decreased slowly in order to provide continuous cooling and keep the sample at an approximately constant low temperature.
The magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems but can be much larger for ferromagnets that undergo a magnetic phase transition.
First order phase transitions are characterized by a discontinuity in the magnetization changes with temperature, resulting in a latent heat.
[11][13] Gadolinium and its alloys undergo second-order phase transitions that have no magnetic or thermal hysteresis.
[3] These materials exhibit the magnetic shape memory effect and can also be used as actuators, energy harvesting devices, and sensors.
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.
Unfortunately, the small magnitudes of nuclear magnetic dipoles also makes them less inclined to align to external fields.
Research and a demonstration proof of concept device in 2001 succeeded in applying commercial-grade materials and permanent magnets at room temperatures to construct a magnetocaloric refrigerator.
As of 2013 this technology had proven commercially viable only for ultra-low temperature cryogenic applications available for decades.
At year-end, Cooltech Applications announced that its first commercial refrigeration equipment would enter the market in 2014.
At the 2015 Consumer Electronics Show in Las Vegas, a consortium of Haier, Astronautics Corporation of America and BASF presented the first cooling appliance.
[25] One year later, in September 2016, at the 7th International Conference on Magnetic Refrigeration at Room Temperature (Thermag VII)] held in Torino, Italy, Cooltech Applications presented the world's first magnetocaloric frozen heat exchanger.
One year later, in September 2018, at the 8th International Conference on Magnetic Refrigeration at Room Temperature (Thermag VIII]), Cooltech Applications presented a paper on a magnetocaloric prototype designed as a 15 kW proof-of-concept unit.
[28] At the same conference, Dr. Sergiu Lionte announced that, due to financial issues, Cooltech Applications declared bankruptcy.
[29] Later on, in 2019 Ubiblue company, today named Magnoric, is formed by some of the old Cooltech Application's team members.
In 2019, at the 5th Delft Days Conference on Magnetocalorics, Dr. Sergiu Lionte presented Ubiblue's (former Cooltech Application) last prototype.
[31] Thermal and magnetic hysteresis problems remain to be solved for first-order phase transition materials that exhibit the GMCE.
Vapor-compression refrigeration units typically achieve performance coefficients of 60% of that of a theoretical ideal Carnot cycle, much higher than current MR technology.
The GeoThermag system showed the ability to produce cold water even at 281.8 K in the presence of a heat load of 60 W. In addition, the system has shown the existence of an optimal frequency f AMR, 0.26 Hz, for which it was possible to produce cold water at 287.9 K with a thermal load equal to 190 W with a COP of 2.20.
[5] 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.[36] Between 1933 and 1997, advances in MCE cooling occurred.
[37] 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.
[7] 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.
[12] Since then, hundreds of peer-reviewed articles have been written describing materials exhibiting magnetocaloric effects.
