BASICS OFMAGNETIC REFRIGERATION

& MATERIALS By

RV KALMEKAR

Roll No- 10MS60-D05

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DIFFERENCE BETWEEN OPTICAL & THERMAL CAMERA

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Second law of thermodynamicsClausius statementNo 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

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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

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• 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)

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Working definition :

A system which undergoes magnetic field induced first order

magneto-structural phase transition involving large increase

in magnetization

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Magnetic refrigeration is an economic and environmentally sound alternative to vapour-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.

Magnetocaloric Effect & Magnetic Refrigerator

This solid refrigerant heats up when magnetized and cools down when demagnetized -- magnetocaloric effect (MCE).

The stronger the MCE the higher the efficiency of the device

Randomspins

Orderedspins

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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)

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Construction Components required for construction

Magnets

Hot Heat exchanger

Cold Heat Exchanger

Drive

Magneto caloric wheel

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A brief history of magnetocaloric effect Magnetocaloric Effect

A magnetic solid heats up when magnetized and cools down when demagnetized .

Originally discovered in iron by E Warburg in 1881.

Thermodynamics of MCE was 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 as adiabatic demagnetization.

An adiabatic demagnetization refrigerator was constructed and utilized by Giauque 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 of 0.8T and 61g of Gd2(So4)3.8H2O as the magnetic refrigerant.

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Steps of thermodynamic cycle -

Adiabatic magnetization

Isomagnetic enthalpic transfer

Adiabatic demagnetization

Isomagnetic entropic transfer

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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).

Adiabatic Magnetization

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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 temperatureDegree of freedom for magnetic dipoles

during ordering and randomization of particals.

Working Materials

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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

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Two quantitative characteristics of MCE :

Isothermal entropy change,

SM(T)H = HI HF (M(T,H)/ T)H dH

Adiabatic temperature change,

Tad(T)H = HI HF (T/C(T,H))H (M(T,H)/ T)H dH

The material should have large (M(T,H)/ T)H and low C at the temperature of interest.

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Large (M(T,H)/ T)H Large MCE

(M(T,H)/ T)H is large for a ferromagnet near its Curie temperature ( TC ).

So, one needs a ferromagnetic material with TC around RT.

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 at room temperature.

However, the device required a cryogenically cooled superconducting magnet, making it impractical for homes. Also Gd is costly!

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Materials with giant MCE: Newer Promise

Gd5(GexSi1-x)4 alloys ; discovered in late 1990s.TC around room temperature ;MCE 2-4 times larger than Gd.

Giant MCE in Gd5(Ge,Si)4 is correlated with a first order magneto-structural transition.

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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

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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.

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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

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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.

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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

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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.

Applications

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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

Thank You