The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Magnetocaloric effect: From materials research to refrigeration devices

Theoretical aspects of the magnetocaloric effect

The growth dynamics of a single-walled carbon nanotube (SWNT) is observed in real-time using an in situ ultrahigh vacuum transmission electron microscope at 650 °C. SWNTs preferentially grow on smaller sized catalyst particles (diameter ≤ 6 nm) with three distinct growth regimes (incubation, growth, and passivation). All of the observed SWNTs grow via a base-growth mechanism with C diffusion on active Ni catalyst sites. Under the same experimental conditions, formation of carbon nanocages was observed on larger Ni catalyst particles. The evolution of SWNTs or nanocages is dependent on catalyst size, and this can be rationalized from both energetics and kinetics considerations.

Direct observation of single-walled carbon nanotube growth at the atomistic scale.

Structure and magnetic characterization of amorphous and crystalline Nd–Fe–Al alloys

The structure and magnetic properties of amorphous melt-spun and subsequently crystallized GdNiAl ribbons were investigated. An amorphous phase was formed after the quenching process by melt spinning with a copper wheel having a surface speed of 30 m/s. A hexagonal phase with lattice parameters a=7.023 A and c=3.916 A was formed in the GdNiAl ribbon after annealing above its crystallization temperature. Magnetic entropy change was calculated directly from isothermal magnetic measurements. The results show that both the amorphous and annealed samples have a high magnetocaloric effect, indicating that these alloys can be considered as candidates for magnetic refrigeration applications.

Magnetic properties and magnetic entropy change of amorphous and crystalline GdNiAl ribbons

The effect of boron (B) on crystallization, glass forming ability (GFA) and magnetic properties of amorphous Fe91−xZr5BxNb4 (FZBN) was studied in the B content (XB) ranging from 0 to 36 at.%. The GFA changes with B content and reaches maximum near eutectic composition of 27.5 at.% B. Fully amorphous FZBN was prepared by melt-spinning in the B content between 5 and 30 at.%. Amorphous FZBN crystallizes in primary crystallization mode in low B content range of XB⩽20 at.%, but in eutectic mode in high B range of 20

Effects of high boron content on crystallization, forming ability and magnetic properties of amorphous Fe91−xZr5BxNb4 alloy

Crystallization processes of an amorphous (Fe0.99, Mo0.01)78Si9B13 alloy induced by mechanical milling and annealing at pressures from 0 to 7.0 GPa were studied. It is found that the milling time needed for the crystallization of the amorphous alloy and its crystallization products are related to the milling intensity. The crystallization products are an α-Fe(Mo, Si) disordered solid solution at a lower milling intensity, while at a higher milling intensity they are α-Fe(Mo, Si), Fe–Si–B, and Fe2B phases. By comparing the mechanical crystallization of the amorphous alloy with its high pressure crystallization, it is suggested that crystallization of the amorphous alloy driven by ball milling results from the simultaneous action of local pressure (4–6 GPa) and local temperature (600–700 K), which are produced by the collision of steel balls. The local pressure decreases the thermodynamic potential barrier of nucleation and increases the diffusion activation energy in the process of mechanical crystallizati...

L. Si

Papers

Structure and magnetic characterization of amorphous and crystalline Nd–Fe–Al alloys

Magnetic properties and magnetic entropy change of amorphous and crystalline GdNiAl ribbons

Effects of high boron content on crystallization, forming ability and magnetic properties of amorphous Fe91−xZr5BxNb4 alloy

Mechanism of mechanical crystallization of amorphous Fe–Mo–Si–B alloy

Boron content dependence of crystallization, glass forming ability and magnetic properties in amorphous Fe-Zr-B-Nb alloys