BASICS OF MAGNETIC REFRIGERATION & MATERIALS By RV KALMEKAR Roll No- 10MS60-D05

1. BASICS OFMAGNETIC REFRIGERATION & MATERIALS ByRV KALMEKARRoll No- 10MS60-D05

2. Difference Between Optical & Thermal Camera

3. Second law of thermodynamics • Clausiusstatement • No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature • heat doesn't flow from cold to hot without work input

4. Magnetic refrigeration • dU=Tds-PdV-MdH • M- magnetic moment, • H=magnetic field • Magnetic refrigeration was the first method developed for cooling below about 0.3 K • Magnetic refrigeration is a cooling technology based on the magneto caloric effect • A strong magnetic field is applied to the refrigerant

5. High dT/dM • substantial for normal ferromagnets which undergo a second order magnetic transition • Which have no magnetic or thermal hysteresis involved • Eventually paramagnetic salts become either diamagnetic or ferromagnetic, limiting the lowest temperature which can be reached using this method • new alloy, composed of cobalt, manganese, silicon and germanium that can be used for magnetic refrigeration. This has made the use of the expensive material gadolinium redundant, and made the creation of domestic magnetic refrigerators possible • Magnetic fields above 2 T are difficult to produce with permanent magnets and are produced by a superconducting magnet (1 T is about 20,000 times the Earth's magnetic field)

6. Working definition : A system which undergoes magnetic field induced first order magneto-structural phase transition involving large increase in magnetization

7. Magnetocaloric Effect & Magnetic Refrigerator Ordered spins Magnetic refrigerationis an economic and environmentally sound alternative tovapour-cycle refrigerators and air conditioners. It offers considerable operating cost savings by eliminating the most inefficient part of the existing refrigerators – the compressor It uses a solid refrigerant and a common heat transfer fluids (e.g. water, air or helium gas) with no ozone-depleting and global-warming effects. This solid refrigerant heats up when magnetized and cools down when demagnetized -- magnetocaloric effect (MCE). Random spins The stronger the MCE the higher the efficiency of the device

8. Magnetic refrigeration is a cooling technology based on the Magneto Caloric Effect • This technique can be used to attain extremely low temperatures (well below 1 kelvin)

9. Construction • Components required for construction • Magnets • Hot Heat exchanger • Cold Heat Exchanger • Drive • Magneto caloric wheel

10. A brief history of magnetocaloric effect Magnetocaloric Effect A magnetic solid heats up when magnetized and cools down when demagnetized . Originally discovered inironby E Warburg in1881. Thermodynamics ofMCEwas understood independently by Debye (1926) and Giaque( 1927).Both of them suggested that MCE could be used to reach low temperature in a process known asadiabatic demagnetization. An adiabatic demagnetization refrigerator was constructed and utilized byGiauque and McDougal (1933)to reach 0.53, 0.34 and 0.25K starting at 3.4 , 2.0 and 1.5K, respectively using a magnetic field of0.8T and 61g of Gd2(So4)3.8H2Oas the magnetic refrigerant.

11. Steps of thermodynamic cycle - • Adiabatic magnetization • Isomagnetic enthalpic transfer • Adiabatic demagnetization • Isomagnetic entropic transfer

12. Adiabatic Magnetization • The substance is placed in an insulated environment and magnetic field is applied. • The increasing external magnetic field (+H) causes the magnetic dipoles of the atoms to align. • As a result material’s magnetic entropy and heat capacity decreases. • The net result is that the item heats up (T + ΔTad).

14. Working Materials • Magneto caloric effect is an intrinsic property of magnetic solid. • Ease of application and removal of magnetic effect is most desired propery of material. It is individual characteristics and strongly depends on : • Curie temperature • Degree of freedom for magnetic dipoles during ordering and randomization of particals.

15. The originally suggested refrigerant was a paramagnetic salt, such as cerium magnesium nitrate. • Gadolinium and its alloys are the best material available today for magnetic refrigeration

16. Two quantitative characteristics of MCE : Isothermal entropy change, SM(T)H = HI HF (M(T,H)/ T)HdH Adiabatic temperature change, Tad(T)H = HI HF(T/C(T,H))H (M(T,H)/ T)HdH The material should havelarge (M(T,H)/ T)H andlow C at the temperature of interest.

17. Large (M(T,H)/ T)H LargeMCE (M(T,H)/ T)His large for a ferromagnetnear its Curie temperature( TC ). So, one needs a ferromagnetic material with TCaroundRT. Gd with TC around 290K is a potential MCE material for RT refrigeration. In 1998 Ames Lab. Iowa + Astronautics Technology Center in Madison, Wisconsin, USA demonstrated a magnetic refrigeration unit that operated atroom temperature. However, the device required a cryogenically cooled superconducting magnet, making it impractical for homes. Also Gd is costly!

18. Materials with giant MCE: Newer Promise Gd5(GexSi1-x)4 alloys ; discovered in late 1990s. TCaround room temperature ;MCE2-4 times larger than Gd. Giant MCEin Gd5(Ge,Si)4 is correlated with afirst order magneto-structural transition.

19. The magneto caloric effect is an intrinsic property of a magnetic material. • 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 • Gadolinium and its alloys are the best material available today for magnetic refrigeration near room temperature since they undergo second-order phase transitions which have no magnetic or thermal hysteresis involved • Gd5(SixGe1 − x)4, La(FexSi1 − x)13Hx and MnFeP1 − xAsx alloys are some of the most promising substitutes for Gadolinium and its alloys

20. Super conducting magnets Most practical magnetic refrigerators are based on superconducting magnets operating at cryogenic temperatures (i.e., at -269 C or 4 K) These devices are electromagnets that conduct electricity with essentially no resistive losses The superconducting wire most commonly used is made of a Niobium-Titanium alloy.

21. Advantages of Magnetic Refrigeration • MG do not use hazardous or environmentally damaging chemicals • Purchase cost may be high, but running costs are 20% less than the conventional chillers • Ozone depleting refrigerants are avoided in this system, hence it more eco-friendly

22. Advantages of Magnetic Refrigeration High efficiency: As the magneto caloric effect is highly reversible, the thermo dynamic efficiency of the magnetic refrigerator is high. It is somewhat 50% more than Vapor Compression cycle. Reduced operating cost: As it eliminates the most inefficient part of today’s refrigerator i.e. compressor. The operating cost reduces as a result. Reliability: - Due to the absence of gas, it reduces concerns related to the emission into the atmosphere and hence is reliable one.

23. CONCLUSION • Magnetic refrigeration is a technology that has proven to • be environmentally safe. Computer models have shown • 25% efficiency improvement over vapor compression • systems. • In order to make the Magnetic Refrigerator commercially • viable, scientists need to know how to achieve larger • temperature swings and also permanent magnets which can • produce strong magnetic fields of order 10 tesla • There are still some thermal and magnetic hysteresis problems to be • solved for the materials that exhibit the MCE to become really • useful

24. Applications A rotary AMR liquefier: The Cryofuel Systems Group is developing an AMR refrigerator for the purpose of liquefying natural gas. A rotary configuration is used to move magnetic material into and out of a superconducting magnet. This technology can also be extended to the liquefaction of hydrogen.

25. Future Applications • At the present stage of the development of magnetic refrigerators with • permanent magnets, hardly any freezing applications are feasible • Some of the future applications are • Magnetic household refrigeration appliances • Magnetic cooling and air conditioning in buildings and houses • Central cooling system • Refrigeration in medicine • Cooling in food industry and storage • Cooling in transportation • Cooling of electronic equipments

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