K. A. Gschneidner

Magnetocaloric effect and magnetic refrigeration

The phenomenon of the magnetocaloric effect along with recent progress and the future needs in both the characterization and exploration of new magnetic refrigerant materials with respect to their magnetocaloric properties are discussed. Also the recent progress in magnetic refrigerator design is reviewed.

Tunable magnetic regenerator alloys with a giant magnetocaloric effect for magnetic refrigeration from ∼20 to ∼290 K

A giant magnetocaloric effect (ΔSmag) has been discovered in the Gd5(SixGe1−x)4 pseudobinary alloys, where x⩽0.5. For the temperature range between ∼50 and ∼280 K it exceeds the reversible (with respect to alternating magnetic field) ΔSmag for any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large ΔSmag in Gd5(SixGe1−x)4, is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from ∼30 to ∼276 K by adjusting the Si:Ge ratio without losing the giant magnetic entropy change.

Description and performance of a near-room temperature magnetic refrigerator

Magnetic refrigeration has been viewed as primarily a cryogenic technology because the necessary high magnetic fields are most easily provided by superconducting magnets. However, some of the largest magnetocaloric effects are exhibited by gadolinium-based alloys near room temperature. Ames Laboratory and Astronautics Corporation of America have been collaborating to apply such materials to large-scale commercial and industrial cooling near room temperature. Astronautics has designed and operated a reciprocating magnetic refrigerator that uses water as a heat transfer fluid. The device uses the active magnetic regeneration concept of recent cryogenic devices, but in contrast to the cryogenic case, the heat capacity of the fluid in the pores of the regenerator bed is comparable to that of the solid matrix. Using a 5 T field, the refrigerator reliably produces cooling powers exceeding 500 watts at coefficients of performance of 6 or more. This record performance puts magnetic refrigeration in a class with the best of current technology, vapor cycle refrigeration, without having to use volatile, environmentally hazardous fluids.

Magnetocaloric effect from indirect measurements: Magnetization and heat capacity

Accurate values for the magnetocaloric effect can be obtained from both magnetization and heat-capacity data. A reliable estimate of the experimental errors in the calculated magnetocaloric effect can be made from the known experimental errors of the measured physical properties. Attempts in the past to simplify the basic thermodynamic relation to allow the calculation of the adiabatic temperature change from the heat capacity at constant field and the magnetic entropy change calculated from the magnetization data fail because the assumption that heat capacity is magnetic-field independent is erroneous. A suitable approach to carry out these calculations from the combined heat capacity and magnetization data is suggested.

Effect of alloying on the giant magnetocaloric effect of Gd5(Si2Ge2)

Abstract A study of the effect of different alloying additions substituting for nonmagnetic Si and Ge on the giant magnetocaloric effect observed for the compound Gd5(Si2Ge2) revealed that small amounts (∼ 0.33 at%) of Ga raise the Curie temperature and preserve the giant magnetocaloric effect. Larger quantities of Ga lead to a significant reduction in the magnetocaloric effect, but the Curie temperature continues to rise. Many 3d-metal and p-element additions also reduce the magnetocaloric effect in Gd5(Si2Ge2) due to the changes in the thermodynamic nature of the magnetic phase transition. In general, all alloying additions raise the Curie temperature of the parent Gd5(Si2Ge2).

The giant magnetocaloric effect of optimally prepared Gd5si2Ge2

The appropriate heat treatment of the Gd5Si2Ge2 alloy prepared from high-purity Gd results in a considerable enhancement of its magnetocaloric effect. The maximum magnetic entropy change increases by ∼80% (from ∼20 to 36 J/kg K) and the adiabatic temperature change increases by ∼55% (from ∼11 to 17 K) for a magnetic field change from 0 to 50 kOe when compared to the as arc-melted material. The magnetic ordering temperature is slightly reduced from ∼277 K in the as arc-melted material to ∼272 K in the fully homogenized and annealed Gd5Si2Ge2. The behavior of the isothermal magnetization as a function of magnetic field changes to some extent, while the heat capacity anomaly at the phase transition temperature is considerably sharper. The 1570 K heat treatment results in the partial ordering and redistribution of the Si and Ge atoms between their corresponding crystallographic sites in the monoclinic Gd5Si2Ge2-type structure and to the removal of the polymorphic orthorhombic Gd5Si2Ge2 phase with the Gd5Si4-t...

Phase relationships and crystallography in the pseudobinary system Gd5Si4Gd5Ge4

A study of phase relationships and crystallography in the pseudobinary system Gd5(SixGe1−x)4 revealed: (1) that both terminal binary compounds Gd5Si4 and Gd5Ge4 crystallize in the Sm5Ge4-type orthorhombic structure, and (2) the appearance of an intermediate (ternary) phase with a monoclinic crystal structure which is similar to both Gd5Si4 and Gd5Ge4. The formation of the monoclinic phase at 0.24≤x≤0.5 [between Gd5(Si0.96Ge3.03)≅Gd5(Si1Ge3) and Gd5(Si2Ge2)] is probably due to the large difference in bonding characteristics of Si and Ge in the Gd5Si4-Gd5Ge4 pseudobinary system which limits the ability of the mutual substitution of Si for Ge and vice versa without a change of the crystal structure. For the composition Gd5(Si2Ge2) the lattice parameters of the monoclinic structure (space group P1121/a) are a=7.580865), b=14.802(1), c=7.7799(5)A, γ=93.190(4)°. A distinct difference in the magnetic behaviors of the alloys from three different phase regions in this system follows the distinct difference in the crystal structures observed for the alloys from the three phase regions.

Some common misconceptions concerning magnetic refrigerant materials

The relationships between both extensive and intensive properties quantifying the magnetocaloric effect, i.e., between the isothermal entropy change and the adiabatic temperature change, respectively, have been analyzed. An extensive measure of the magnetocaloric effect alone, without considering another important and also extensive thermodynamic property, i.e., the heat capacity, may lead to biased conclusions about the size of the magnetocaloric effect and, consequently, about the applicability of a magnetic material as a magnetic refrigerant. The near room temperature magnetocaloric properties of the colossal magnetoresistive manganites [(R1−xMx)MnO3, where R=lanthanide metal and M is alkaline earth metal] and the recently discovered Fe-based intermetallic material (LaFe11.47Co0.23Al1.3) have been reaccessed and correctly compared with those of the metallic Gd prototype. Our analysis has shown that these 3d materials are inferior to Gd by a factor of 2 or more because of the high values of the heat cap...

Magnetic refrigeration materials (invited)

Research on the magnetocaloric effect and its application for cooling near room temperature over the past few years has helped to move this phenomenon from a scientific curiosity to an emerging technology. Two of the most important advances include the demonstration which proved that it is possible to obtain significant cooling powers (600 W) at high Carnot efficiencies (60%) and with a large coefficient of performance (15) near room temperature in moderately strong magnetic fields (⩽5 T); and the discovery of the giant magnetocaloric effect in the Gd5(SixGe1−x)4 series of alloys. Also, new knowledge about the magnetocaloric effect has been gained. This includes: the relationship between the nature of the magnetic transformation(s) and the temperature dependence of the magnetocaloric effect, the entropy utilized in the magnetocaloric process, and the role of impurities on the giant magnetocaloric effect.

The room temperature metastable/stable phase relationships in the pseudo-binary Gd5Si4–Gd5Ge4 system ☆

Abstract The phase relationships, crystallography and magnetic phase diagram of the pseudo-binary Gd5Si4–Gd5Ge4 system in the as-cast state is described. Three extended solid solution regions were confirmed: the Si-rich alloys which have the orthorhombic Gd5Si4-type structure, the intermediate region alloys with the monoclinic Gd5Si2Ge2-type structure and the Ge-rich alloys which have the orthorhombic Sm5Ge4-type structure. The sample preparation techniques and the methods used for determining the phase purity of the alloys are also discussed.