John Weisend, was originally published in the Spring issue of Cold Facts as part of his series, Defining Cryogenics. Reaching temperatures below 1K requires different techniques than the various helium gas cycles found in large scale refrigeration plants and small cryocoolers. This technique takes advantage of the fact that the entropy of paramagnetic materials in a magnetic field is lower than when no field is present. The lower entropy comes from the magnetic regions in the paramagnetic material being aligned and thus more ordered in the presence of a magnetic field. A more ordered solid has lower entropy. In effect, the ADR transfers entropy between the random thermal vibrations of the paramagnetic material and the alignment of the magnetic regions.
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No matter when we put anything warm inside, the refrigerator will immediately start cooling it down. Likewise, any heat that leaks in through the insulation goes right back out. But into the room it goes, because energy cannot be destroyed. The ADR does not run continuously.
It stores the heat that it absorbs, both heat from cooling warm objects and heat that leaks in. The part of the ADR that stores the heat is called the "salt pill". Often, the material is one of the general class of materials called "salts", which includes table salt as well as many other chemicals.
They need a much colder heat sink to dump the heat. In a paramagnetic substance, each molecule acts like a tiny electromagnet, with the electrons playing the part of tiny electic currents. In nonparamagnetic substances, the fields of the various electrons all cancel each other out, leaving the molecule with no overall field.
An ADR salt pill, then, is like a group of microscopic magnets all packed in together. To get a mental image, you might try picturing a tiny compass needle attached to each molecule. The magnetic moment of a compass needle, for instance, is along the direction of the needle.
A salt pill would thus be like an array of tiny compass needles. In this diagram, we imagine that a weak magnetic field in the vertical directon has been applied to the salt pill. A real compass needle and the magnetic moment of a paramagnetic molecule are similar in some ways and different in others. Here are two similarities: Both the compass needle and the magnetic moment of the molecule tend to line up with an applied field.
Both can be pushed away from the direction of the applied field. It may not seem like it takes much energy to push a compass needle, but the amount it takes to push a molecular magnetic moment out of alignment is so small that the random molecular vibrations of heat energy are often enough to do it.
If you push a compass needle away from pointing north, it will swing back as soon as you let go of it. But if the magnetic moment of a paramagnetic molecule gets pushed away from the direction of the applied field, it might stay out of alignment for some time. The microscopic magnetic moment of the paramagnetic molecule can only point at certain angles to the applied field.
D This diagram is a bit oversimplified. D The behavior of the molecular magnetic moments seems really strange to those of us used to everyday things like compass needles. The salt pill can absorb heat because of the strange properties of the molecular magnetic moments. On the microscopic scale, heat energy consists of random vibrations of molecules. When the applied magnetic field is weak, there is enough energy in the random thermal vibrations to knock a molecular magnetic moment out of alignment with the field.
Thus, the energy that was heat energy gets changed into magnetic energy of the molecules. As the salt pill absorbs more and more heat energy, more and more of the molecular magnetic moments get knocked out of alignment with the applied magnetic field.
Instead of being all lined up, as in the diagram above, the spins are pointing every which way, like this: D At this point, the heat must be dumped. To dump the heat, the ADR operator does two things. One step is increasing the applied magnetic field, the other is turning on the heat switch that connects the salt pill with the helium coolant bath. When the magnetic field is turned up, that increases the amount of energy the molecular magnetic moments must have to stay out of alignment with the field.
When the field becomes high enough, the molecular magnetic moments give up their energy and flip back in line with the magnetic field. As the energy gets dumped by the molecular magnetic moments, it converts back into random molecular motion, i.
A heat switch does for heat what an electrical switch does for electricity. When you want the heat to be able to flow, you turn the heat switch on. When you want to block the flow of heat, you turn the heat switch off. When enough heat has flowed to the coolant bath, the operator turns off the heat switch, then reduces the magnetic field.
Once again, the amount of energy needed to knock a molecular magnetic moment out of alignment is small enough that random thermal vibrations have enough energy. Thus, the molecular moments begin absorbing heat, and the salt pill cools, starting another cycle.
No matter when we put anything warm inside, the refrigerator will immediately start cooling it down. Likewise, any heat that leaks in through the insulation goes right back out. But into the room it goes, because energy cannot be destroyed. The ADR does not run continuously. It stores the heat that it absorbs, both heat from cooling warm objects and heat that leaks in. The part of the ADR that stores the heat is called the "salt pill".
The Adiabatic Demagnetization Refrigerator (ADR):
Current research has been used to describe alloys with a significant magnetocaloric effect in terms of a thermodynamic system. Such materials need to show significant temperature changes under a field of two tesla or less, so that permanent magnets can be used for the production of the magnetic field. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms. In a paramagnetic salt ADR, the heat sink is usually provided by a pumped 4 He about 1. The minimum temperature attainable is determined by the self-magnetization tendencies of the refrigerant salt, but temperatures from 1 to mK are accessible. Dilution refrigerators had for many years supplanted paramagnetic salt ADRs, but interest in space-based and simple to use lab-ADRs has remained, due to the complexity and unreliability of the dilution refrigerator.
GSFC Adiabatic Demagnetization Refrigerator
See Article History Adiabatic demagnetization, process by which the removal of a magnetic field from certain materials serves to lower their temperature. This procedure, proposed by chemists Peter Debye and William Francis Giauque independently, , provides a means for cooling an already cold material at about 1 K to a small fraction of 1 K. The mechanism involves a material in which some aspect of disorder of its constituent particles exists at 4 K or below liquid helium temperatures. Magnetic dipoles—i. Under these conditions the dipoles occupy these levels equally, corresponding to being randomly oriented in space. When a magnetic field is applied, these levels become separated sharply; i. If the magnetic field is applied while the paramagnetic salt is in contact with the liquid helium bath an isothermal process in which a constant temperature is maintained , many more dipoles will become aligned, with a resultant transfer of thermal energy to the bath.