Magnetic and nuclear cooling

PRESENTATION ONPRESENTATION ON

MAGNETIC & NUCLEAR COOLINGMAGNETIC & NUCLEAR COOLING

PRESENTED BYPRESENTED BY

PADMADHAR GARGPADMADHAR GARG

DEPARTMENT OF MECHANICAL ENGINEERING,DEPARTMENT OF MECHANICAL ENGINEERING,

MALVIYA NATIONAL INSTITUTEMALVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUROF TECHNOLOGY JAIPUR

It is possible to attain a temperature of about It is possible to attain a temperature of about 0.8K (-272.2C) through the lowering of pressure 0.8K (-272.2C) through the lowering of pressure over liquid helium.over liquid helium.

Giauque and Debye proposed the adiabatic Giauque and Debye proposed the adiabatic demagnetization of magnetic salt for attaining demagnetization of magnetic salt for attaining the lower temperature.the lower temperature.

The lowest recorded temperature as low as The lowest recorded temperature as low as 0.001K may be obtained by adiabatic 0.001K may be obtained by adiabatic demagnetization of certain paramagnetic salt demagnetization of certain paramagnetic salt previously cooled by liquid helium and subjected previously cooled by liquid helium and subjected to a strong magnetic field. to a strong magnetic field.

Content:Content:

>>Basic principles of magnetic refrigerationBasic principles of magnetic refrigeration

>Thermodynamic cycle>Thermodynamic cycle

>>Materials : Working materialsMaterials : Working materials

>Nuclear demagnetization>Nuclear demagnetization

>Advantages>Advantages

Introduction : PrincipleIntroduction : Principle

Mageto calorific effect is the basic principle on which the Mageto calorific effect is the basic principle on which the cooling is achieved.cooling is achieved.

All magnets bears a property called Currie effect i.e. If a All magnets bears a property called Currie effect i.e. If a temperature of magnet is increased from lower to higher range temperature of magnet is increased from lower to higher range at certain temperature magnet looses the magnetic field. at certain temperature magnet looses the magnetic field.

Currie temperature. Depends on individual property of each Currie temperature. Depends on individual property of each material.material.

As Energy input to the magnet is increased the orientation of the As Energy input to the magnet is increased the orientation of the magnetic dipoles in a maget starts loosing orientation. And vice a magnetic dipoles in a maget starts loosing orientation. And vice a versa at currie temperature as magnet looses energy to the versa at currie temperature as magnet looses energy to the media it regains the property. media it regains the property.

Thermo dynamic cycleThermo dynamic cycle

Adiabetic Demagnetization apparatusAdiabetic Demagnetization apparatus

Details Of the Thermodyanamics Cycle Details Of the Thermodyanamics Cycle

PROCESS IS SIMILAR TO GAS COMPRESSION AND EXPANSION PROCESS IS SIMILAR TO GAS COMPRESSION AND EXPANSION CYCLE AS USED IN REGULAR REFRIGERATION CYCLE. CYCLE AS USED IN REGULAR REFRIGERATION CYCLE.

Steps of thermodynamic cycle - Steps of thermodynamic cycle -

Adiabatic magnetization Adiabatic magnetization

Isomagnetic enthalpic transfer Isomagnetic enthalpic transfer

Adiabatic demagnetization Adiabatic demagnetization

Isomagnetics entropic transferIsomagnetics entropic transfer

Adiabatic magnetization Adiabatic magnetization

Procedure to be followed :Procedure to be followed :

Substance placed in insulated environment. Substance placed in insulated environment.

Magnetic field +H increased. Magnetic field +H increased.

Magnetic dipoles of atoms to align, therebyMagnetic dipoles of atoms to align, thereby material decreases.material decreases.

Total Entropy of the item is not reduced, and Total Entropy of the item is not reduced, and item heats upitem heats up

Isomagnetic enthalpic transferIsomagnetic enthalpic transfer

Added heat removed by fluid, gas – Added heat removed by fluid, gas – gaseous or liquid helium gaseous or liquid helium

Magnetic field held constant to prevent the Magnetic field held constant to prevent the dipoles from reabsorbing the heat. dipoles from reabsorbing the heat.

After a sufficient cooling magnetocaloric After a sufficient cooling magnetocaloric material and coolant are seperated material and coolant are seperated

Adiabatic DemagnetizationAdiabatic Demagnetization

Substance returned to another adiabatic ( insulated ) conditionSubstance returned to another adiabatic ( insulated ) condition

Entropy remains constantEntropy remains constant

Magnetic field is decreased, Magnetic field is decreased,

Thermal energy causes the magnetic moments to overcome Thermal energy causes the magnetic moments to overcome the field and sample cools ( adiabatic temperature change )the field and sample cools ( adiabatic temperature change )

Energy transfers from thermal entropy to magnetic entropy Energy transfers from thermal entropy to magnetic entropy ( disorder of the magnetic dipoles )( disorder of the magnetic dipoles )

Isomagnetic entropic transferIsomagnetic entropic transfer

Material is placed in thermal contact with the Material is placed in thermal contact with the environment being refrigerated.environment being refrigerated.

Magnetic field held constant to prevent from heating Magnetic field held constant to prevent from heating back up back up

Because the working material is cooler than the Because the working material is cooler than the refrigerated environment, heat energy migrates into the refrigerated environment, heat energy migrates into the working material ( +Q )working material ( +Q )

Once the refrigerant and refrigerated environment are in Once the refrigerant and refrigerated environment are in thermal equillibrium, the cycle begins a new thermal equillibrium, the cycle begins a new

Working MaterialsWorking Materials

Magneto caloric effect is an intrinsic porperty of magnetic solid. Magneto caloric effect is an intrinsic porperty of magnetic solid.

Ease of application and removal of magnetic effect is most Ease of application and removal of magnetic effect is most desired property of material. It is individual characteristics and desired property of material. It is individual characteristics and strongly depends on : strongly depends on :

Curie temperatureCurie temperature Degree of freedom for magnetic dipoles during ordering and Degree of freedom for magnetic dipoles during ordering and

randomization of particles.randomization of particles.

Ferrimagnets, antiferromagnets and spin glass sytems are not Ferrimagnets, antiferromagnets and spin glass sytems are not suitable for this applicationsuitable for this application

Alloys of gadolinium producing 3 to 4 K per tesla of change in Alloys of gadolinium producing 3 to 4 K per tesla of change in magnetic field are used for magnetic refrigeration or power magnetic field are used for magnetic refrigeration or power generation purposes.generation purposes.

Refrigeration Below 1KRefrigeration Below 1K

NUCLEAR DEMAGNETIZATIONNUCLEAR DEMAGNETIZATION

This type is one of the variant that continues to find substantial research This type is one of the variant that continues to find substantial research application.application.

It follows the same principle, but in this case the cooling power arises It follows the same principle, but in this case the cooling power arises from the magnetic dipoles of the nuclei of refrigent atoms rather than their from the magnetic dipoles of the nuclei of refrigent atoms rather than their electronic configuration. electronic configuration.

Since these dipoles are of much smaller magnitude, they are less prone to Since these dipoles are of much smaller magnitude, they are less prone to self alignment and have lower intrinsic minimum field. self alignment and have lower intrinsic minimum field.

This allows NDR to cool the nuclear spin system to very low temperatures, This allows NDR to cool the nuclear spin system to very low temperatures, often 1 micro Kelvin. often 1 micro Kelvin.

Magnetic fields of 3 telsa or greater are often needed for the initial Magnetic fields of 3 telsa or greater are often needed for the initial mgneization step of NDRmgneization step of NDR

Nuclear Demagnetization Process Nuclear Demagnetization Process

Steps of Nuclear DemagnetizationSteps of Nuclear Demagnetization

Nuclear demagnetization is a single-shot Nuclear demagnetization is a single-shot technique which consists of three steps:technique which consists of three steps:

Magnetization:Magnetization: Decoupling and demagnetization:Decoupling and demagnetization: Experiments:Experiments:

MagnetizationMagnetization

The nuclear spins in the material serving as a refrigerator and The nuclear spins in the material serving as a refrigerator and by applying a large magnetic field Bi, a thermodynamic spin by applying a large magnetic field Bi, a thermodynamic spin polarization is generated. The heat arising during this process polarization is generated. The heat arising during this process is drained by the cooling power of the Mixing Chamber until is drained by the cooling power of the Mixing Chamber until the system is equilibrated at base temperature Ti.the system is equilibrated at base temperature Ti.

Decoupling and demagnetization:Decoupling and demagnetization:

Without disconnecting the electrical contact, the thermal Without disconnecting the electrical contact, the thermal contact between the NR and the MC is cut by a contact between the NR and the MC is cut by a superconducting heat switch.superconducting heat switch.

In a next step, the magnetic field is reduced adiabatically to Bf In a next step, the magnetic field is reduced adiabatically to Bf . The final temperature in an ideally adiabatic process. The final temperature in an ideally adiabatic process

is is

Experiments:Experiments:

For a finite time, experiments can be performed at For a finite time, experiments can be performed at these low temperatures. The time, typically on the these low temperatures. The time, typically on the order of days or even weeks, depends on the size of order of days or even weeks, depends on the size of the polarized spin reservoir and on the heat leaking the polarized spin reservoir and on the heat leaking into the system.into the system.

Molar nuclear spin entropy versus nuclear spin temperature for differentMolar nuclear spin entropy versus nuclear spin temperature for differentmagnetic fields.magnetic fields.

Advantages of Nuclear and Magnetic Advantages of Nuclear and Magnetic RefrigerationRefrigeration

Purchase cost may be high, but running costs are Purchase cost may be high, but running costs are 20% less than the conventional chillers Thus life 20% less than the conventional chillers Thus life cycle cost is much less.cycle cost is much less.

Ozone depleting refrigerants are avoided in this Ozone depleting refrigerants are avoided in this system, hence it more eco-friendly.system, hence it more eco-friendly.

Energy saving would lessen the strain on our Energy saving would lessen the strain on our household applianceshousehold appliances

Energy conservation and reducing the energy Energy conservation and reducing the energy costs are added advantages.costs are added advantages.

ReferencesReferences

http://en.wikipedia.org/wiki/Magnetic_refrigerationhttp://en.wikipedia.org/wiki/Magnetic_refrigeration Maradan_MScThesis_FINALMaradan_MScThesis_FINAL T. Bandi, Semester thesis, University of Basel (2008).T. Bandi, Semester thesis, University of Basel (2008).