CLASS NOTES FOR ENGINEERING
Achieve cryogenic temperatures by
magnetic cooling. It is achievable by
demagnetization of a para magnetic
salt. Magnetic cooling gives
a temperature up to 0.001 K.
Very Low temperatures of the order of 0.001 K are obtainable with magnetic cooling. Demagnetization of a para magnetic salt produces such low temperature. Para magnetic salt used is Gadolinium-sulphate. It is paramagnetic at room temperature. Magnetic cooling uses this salt. Cool the salt to about 1 K by liquid helium at 0.2 atmospheric pressure. Now apply a strong magnetic field of 25000 Gauss intensity. The molecules of the salt act as tiny magnets. These arrange themselves in a regular arrangement. Large amount of heat energy releases. The various steps for magnetic cooling (Fig.) are as under:
(i) Cool the salt to 1 K by liquid helium at 0.2 atm (vacuum)
(II) Apply a strong magnet of 25000 Gauss intensity. Under this, salt molecules will arrange in a ordered manner. These molecules will behave as tiny magnets. Large amount of heat releases in achieving this ordered arrangement of molecules. Again cool the salt up to 1 K.
(iii) Thermally insulate the salt. Achieve it by removing liquid nitrogen and liquid hydrogen. Create almost a perfect vacuum.
(iv) Remove the magnet. Molecules come in a disorder form. These molecules need lot of energy for attaining this disorder. Salt supplies this energy. The temperature of salt plunges to a very low temperature.
(v) Measure such low temperatures using Madam Curie law.
T = C/χ
Where T is the absolute temperature
C is salt constant
χ is magnetic susceptibility of the salt
Advantages of Magnetic Cooling
(i) Magnetic refrigeration is quiet, safe, simple and compact.
(ii) Its cooling efficiency is high.
(iii) It does not have ozone depletion and Global warming.
(iv) Ozone-depleting and global warming potentials are absent.
(v) Energy requirements are less.
(vi) Maintenance free.
(vii) Has the potential to replace conventional refrigeration.
Disadvantages of Magnetic Cooling
(i) The initial cost is high.
(ii) Uses rare earth materials. There is a problem in their availability.
Practical Applications of Magnetic Cooling
(i) Used in fundamental research at sub-atomic level. Possibility of better understanding of Thermodynamic concepts like entropy.
(ii) Used in space aircrafts.
(iii) Its use possible in conventional refrigeration in future.
(iv) Use of super conducting magnets is likely to bring more applications.