Cryocoolers by Milind D. Atrey

Author:Milind D. Atrey
Language: eng
Format: epub
ISBN: 9783030113070
Publisher: Springer International Publishing

The ADR cycle consists of four discrete steps: (1) adiabatic magnetization of the salt pill until warms it above the heat sink, at which point the heat switch is closed; (2) isothermal magnetization to full field as heat flows to the sink, at which point the heat switch is opened; (3) adiabatic demagnetizion of the salt pill until it reaches the desired operating temperature, and (4) isothermal demagnetizion as the stage absorbs heat from its load. When the field reaches a minimum value (usually zero), the cycle is repeated.

The salt pill’s cooling capacity at its operating temperature, T low, is ΔQ = n ⋅ ε ⋅ T low[s(B min, T low) − s(B max, T high)], where n is the number of moles of refrigerant, ε is the heat absorption efficiency, B max and B min are the maximum and minimum magnetic fields that define the ends of the isothermal magnetization and demagnetization steps, and T high is the temperature from which the salt pill is demagnetized (usually only slightly higher than the heat sink temperature, T sink).

Although there is no intrinsic limit on T high, there are fundamental constraints based on both magnet technologies and the properties of the magnetocaloric refrigerants. The primary limitation is the ability to create the magnetic fields needed to achieve reasonable temperature suppression during demagnetization. Progressively higher field is needed as T high increases, at the same time that the critical field of superconductors decreases. For NbTi magnets, the crossover occurs at about 6 K. Advances in Nb3Sn [22], MgB2, and HTS [19] magnets are beginning to accumulate to the point that producing the fields required for operation at higher than 6 K, and especially with modest currents (<10 A), will soon be practical. But even given this capability, at elevated temperature, the lattice heat capacity of the refrigerant becomes competitive with the magnetic component. The temperature change associated with demagnetization then becomes too small compared to absolute temperature to form the basis for practical refrigeration, and one must instead consider such techniques as active magnetic regenerative refrigeration [23] becomes more advantageous.

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