MAGNETOCALORIC EFFECT

Fundamentals

 

     The Magnetocaloric Effect (MCE), first observed by French physicist P. Weiss and Swiss physicist A. Piccard in 1917 [1], is an exciting propriety of magnetic materials. This effect can be seen from either an adiabatic or an isothermal process; both due to a change of the applied magnetic field. Considering an adiabatic process, the magnetic material changes its temperature, whereas from an isothermal process, the magnetic material exchanges heat with a thermal reservoir. Figure 1 clarify these processes.

 

     From the quantitative point of view, the MCE is measured trough the magnetic entropy change DS (=DQ/T, where DQ is the amount of heat exchanged between the thermal reservoir and the magnetic material), when the isothermal process is considered; or adiabatic temperature change DT, when the adiabatic process. These quantities can also be seen when the magnetic entropy is expressed as a function of temperature for both, with and without applied magnetic field. See figure 2.

 

     It is straightforward the idea to produce a thermo-magnetic cycle based on the isothermal and/or adiabatic processes (like Brayton and Ericsson cycles - see figure 3); and indeed this idea begun in the late 1920s, when cooling via adiabatic demagnetization was proposed by Debye[2] and Giauque[3]. The process was after demonstrated by Giauque and MacDougall, in 1933; where they reach 250 mK[4]. Since then, the adiabatic demagnetization was used within some contexts; for instance, to cool NASA-XRS detectors (~1.5 K)[5]. On the other hand, room temperature magnetic cooling device technology is still in an early phase of development, with no commercially available products and only few prototypes. In August 2001, Astronautics Corporation of America, USA, announced a prototype of room temperature magnetic cooler. This machine has a cooling power of 95W, and uses as the active magnetic material Gd spheres[6]. Later, in March 2003, Chubu Electric and Toshiba, Japan, also announced a room temperature magnetic cooler prototype. This machine has a cooling power of 60W, and uses a layered bed of a Gd-Dy alloy as the active magnetic material[6]. Nowadays, some other prototypes were developed and the technology is still under development.

 

     However, nowadays, the magnetic materials available and studied by the scientific community do not have yet the needed characteristics to be used in large scale, due to technological and/or economic restrictions. For a successful application, we need a material of low cost, non-toxic, good thermal conductivity and with a huge and broad DS vs. temperature (maximum around the magnetic phase transition). In this sense, most of the research developed world wide is devoted to explore and optimize the magnetocaloric properties of known materials, as well to seek for new magnetocaloric features in new materials.

 

     In addition to the spin entropy, all of the mechanisms sensible to the magnetic field can be useful to maximize the DS, namely the lattice, charge and orbital entropies; since these entities, in certain cases, can also be ruled by the applied magnetic field. In this sense, some materials with strong electronic correlation have coupled magneto-structural transitions, maximizing the DS.

 

 

- FURTHER (SIMPLE) READING HERE AND HERE AND HERE.

- ADVANCED READINGS HERE AND HERE.

 

 

[1] A. Smith, European Physical Journal H 38 (2013) 507

[2] Debye, Ann. Phys. 81 (1926) 1154

[3] Giauque, J. Am. Chem. Soc. 49 (1927) 1864

[4] Giauque, Phys. Rev. 43 (1933) 768

[5] http://www.universe.nasa.gov/xrays/programs/astroe/eng/adr.html

[6] Gschneidner, Rep. Prog. Phys. 68 (2005) 1479     

 

 

 

 

 

Fig 1

Fig 2

Fig 3

Our research: New materials

 

One of our main expertise on this research line is to develope new materials to optimize the magnetocaloric effect for applications into cooling devices. See here further details.

 

 
 

Our research: Magnetocaloric properties of Graphene

 

When optimising magnetocaloric materials, their properties are fully optimised around the critical temperature and golden materials have both, magnetic and structural transitions. For instance, some materials optimised to work at room temperature are Gd5(Si2Ge2), La(FexSi1−x)13, Heusler alloys and all their families. However, there are also a plenty of other compounds proposed to work at low temperatures, especially intermetallics, multiferroics and molecular magnets. More recently, in spite of the absence of magnetic transition, the magnetocaloric properties of diamagnetic materials (such as Gold, Lead, Silver and graphene) have been deeply explored (further details here). In few words, those efforts describe the oscillating magne- tocaloric effect OMCE, that arises due to the crossing of the Landau levels through the Fermi level of the system by changing the applied magnetic field. This OMCE is governed by the same physical mechanism of the de Haas-van Alphen and Shubnikov-de Haas effects and has been previoulsy experimentally observed, mainly in GaAs-like materials.

 

Thus, this research line explores the magnetocaloric effect of an interesting diamagnet material: Graphenes. Several theoretical works have been done on this reserach line and those who are interestd in further details can then find these here.

Our research: Prototype

 

We are also developing a Gd-based micro-prototype to work at room temperature. Several under-graduate students are involved and tasks are: design conception, 3D printing of components, magnetic field design, automation and much more. Soon we will let here availble further details.

 

 
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