LIQUEFACTION OF AIR ADIABATIC DEMAGNETISATION 1 CRYOGENICS Cryogenics

LIQUEFACTION OF AIR ADIABATIC DEMAGNETISATION 1

CRYOGENICS Cryogenics is a branch of Physics that deals with the production and effects of very low temperatures. In the early history of thermometry, ice was considered to be the coldest and its temperature was taken as the lowest temperature. It was Fahrenheit, who first experimentally demonstrated that a mixture of ice with common salt gives a lower temperature of the order of – 18 o. C. Later, temperatures lower than this temperature could be attained. The general principle of production of low temperature is to remove the heat content from a body. 2

LIQUEFACTION OF GASES For a long time it was thought that air remains in the gaseous state at all temperatures. But Andrew’s experiments on CO 2 led to the discovery of critical temperature. The critical temperature is the temperature below which a gas can be liquefied by mere application of pressure. But it cannot be liquefied above the critical temperature, however, larger may be the applied pressure. Below the critical temperature, the gas is termed as vapour and above the critical temperature it is called a gas. 3

LIQUEFACTION OF GASES So, the liquefaction of gases is linked with the production of low temperatures. The substances which are gaseous at ordinary temperatures can be converted into liquid state if sufficiently cooled and simultaneously subjected to a high pressure. There are various methods of liquefaction of gases. In this section, let us see three methods of liquefaction of gases, in detail. 4

CASCADE PROCESS The cascade process can be used to produce very low temperatures. The basic principle is that when a liquid evaporates at reduced pressure, it cools. Evaporation causes cooling, because when a liquid evaporates it takes up the latent heat either from the liquid itself or from the surrounding vessel. Oxygen and Nitrogen can be liquefied by cascade process. In this case a series of liquids with successively lower boiling point is employed, so that the desired low temperature is attained. 5

LINDE’S PROCESS – LIQUEFACTION OF AIR Linde in 1896 liquefied air using Joule – Thomson effect (or Joule – Kelvin effect) and regenerative cooling technique. Before going into detail about this process, it is essential to understand the Joule - Thomson effect and regenerative cooling technique 6

JOULE THOMSON EFFECT If a gas is allowed to expand through a fine nozzle or a porous plug, so that it issues from a region at a higher pressure to a region at a lower pressure there will be a fall in temperature of the gas provided the initial temperature of the gas should be sufficiently low. This phenomenon is called Joule – Thomson effect. 7

REGENERATIVE COOLING The principle of regenerative cooling consists in cooling the incoming gas by the gas which has already undergone cooling due to Joule – Thomson effect. 8

CONSTRUCTION AND WORKING The compresses C 1 air to a pressure of about 25 atmosphere and is passed through a tube surrounded by a jacket through which cold water is circulated. This compressed air is passed through KOH solution to remove CO 2 and water vapour. This air, free from CO 2 and water vapour is compressed to a pressure of 200 atmospheres by the compresses C 2. This air passes through a spiral tube surrounded by a jacket containing a freezing mixture and the temperature is reduced to -20 o. C 9

CONSTRUCTION AND WORKING 10

CONSTRUCTION AND WORKING This cooled air at high pressure is allowed to come out of the nozzle V 1. At V 1, Joule – Thomson effect takes place and the incoming air is cooled to -70 o. C. This cooled air is circulated back into the compresses C 2 and is compressed. It passes through the nozzle V 1 and is further cooled. Then it is allowed to pass through the nozzle V 2 from high pressure to low pressure, and is further cooled. 11

CONSTRUCTION AND WORKING Then it is allowed to pass through the nozzle V 2 from high pressure to low pressure, and is further cooled. As the process continues, after a few cycles, air gets cooled to a sufficiently low temperature well below its critical temperature of -170 o. C and after coming out of the nozzle V 2, gets liquefied and is collected in the Dewar’s Flask. The unliquefied air is again circulated back to the compresses C 1 and the process is repeated. The whole apparatus is packed with cotton wool to avoid any conduction or radiation. By applying the principle of Joule – Thomson effect and regenerative cooling, Hydrogen and Helium can also be 12 liquefied.

ADIABATIC DEMAGNETIZATION PROCESS This process is used to reduce the temperature of paramagnet it salts nearer to ‘ 0’ K. We know that the molecular dipole magnetic moments of a paramagnetic specimen are randomly oriented at thermal equilibrium. In this state there is maximum disorderliness of the system and its entropy is maximum. By the application of an external field, all the magnetic dipoles are aligned themselves in a common direction and hence there is an orderliness of the system. So, the entropy of the system decreases and there is a rejection of energy. 13

ADIABATIC DEMAGNETIZATION PROCESS The heat rejected by the specimen when it is magnetised is taken away by the surroundings and the original thermal equilibrium is restored. Therefore thermal motion of the molecules is unaffected. If the specimen is now thermally insulated from its surroundings and the external magnetic field is switched off, (i. e. adiabatically demagnetised) the magnetic dipoles again get random orientation in order to reach equilibrium which is a state of maximum disorder. Therefore the entropy of the system increases. 14

ADIABATIC DEMAGNETIZATION PROCESS When the entropy increases due to disorderly orientation of magnetic dipoles, there should be a corresponding decrease in entropy of disorderly thermal motion because the total energy in entropy during an adiabatic process should be zero. Thus there is a reduction in thermal energy of the molecules and therefore the temperature of the specimen falls. 15

ADIABATIC DEMAGNETIZATION PROCESS Usually gadolinium sulphate, which is a paramagnetic salt is used. It is placed in a tube which is immersed in liquid helium bath of about 1 K and magnetised by the application of a strong magnetic field. By insulating the tube from the surrounding bath and evacuating the tube, the specimen is adiabatically demagnetised. Now, the temperature of the specimen is very much reduced. Temperatures of the order of 0. 002 K can be attained by this process. 16

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