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.
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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.
Consider an adiabatic thermally isolated sample. When the magnetic field is raised, the magnetic regions align and release entropy into the thermal vibrations heating the material. When the magnetic field is reduced, the regions drop out of alignment and absorb entropy from the thermal vibrations cooling the material.
A simple example of an ADR system consists of a paramagnetic solid connected to the object to be cooled and via a thermal switch to a heat sink.
The ADR system is cyclic. In the first part of the cycle, the paramagnetic solid is thermally isolated and a magnetic field is applied to the solid. As the field is increased, the magnetic regions in the solid start to align and the paramagnetic solid heats up.
Next, the thermal switch is connected and heat is transferred from the solid to the heat sink while the magnetic field is held constant. This reduces the temperature of the solid, back to near its starting point.
The thermal switch is now closed, isolating the solid and the magnetic field is now reduced. This is the adiabatic demagnetization portion of the cycle. As the magnetic field is reduced the paramagnetic regions become more disordered and absorb entropy from the thermal vibrations resulting in a cooling of the paramagnetic material and of the object being cooled.
In typical applications, the heat sink is a liquid helium bath and the ADR reaches temperatures down to the mK level. As cyclic devices, ADRs typically can be built to hold the object being cooled to mK temperatures for up to several days. After this time, the system is recycled with the temperature being raised up to near that of the heat sink before the cycle is repeated.
ADR systems that provide continuous cooling, generally by using more than one ADR operating in tandem, have also been developed. There is, of course, a lot of detailed design work required for successful ADRs. Commercially produced ADR systems can be purchased for laboratory work. Custom made ADRs are frequently used in space applications as they reduce the need for zero gravity fluid management at these temperatures. Shirron et al. Duval et al. Bartlett et al. Proceedings of ICEC 21 Cryogenic Society of America, Inc.
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.
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 Adiabatic Demagnetization Refrigerator (ADR):
One particular method of magnetic cooling is Adiabatic Demagnetization, which capitalizes on the paramagnetic properties of some materials to cool those materials usually in gaseous form down into the millikelvin -- or colder -- range. This method can also be used to cool solid objects, but the most drastic cooling in the fractions of a kelvin range are generally accomplished for low-density gases that have already been greatly cooled around K What are Paramagnetic Materials? When an outside magnetic field is applied to these materials, a magnetic field that is parallel to the applied field is induced. The Process of Adiabatic Demagnetization First, the sample to be cooled typically a gas is allowed to touch a cold reservoir which has a constant temperature of around K, and is often liquid Helium , and a magnetic field is induced in the region of the sample. Once the sample is in thermal equilibrium with the cold reservoir, the magnetic field strength is increased. This causes the entropy of the sample to decrease, because the system becomes more ordered as the partilces align with the magnetic field. The temperature of the sample is still the same as that of the cold reservoir at this point.
Adiabatic Demagnetization Refrigeration
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.