Optimal ferrofluids for magnetic cooling devices

Superior passive cooling technologies are urgently required to tackle device overheating, consequent performance degradation, and service life reduction. Magnetic cooling, governed by the thermomagnetic convection of a ferrofluid, is a promising emerging passive heat transfer technology to meet these challenges. Hence, we studied the performance metrics, non-dimensional parameters, and thermomagnetic cooling performance of various ferrite and metal-based ferrofluids. The magnetic pressure, friction factor, power transfer, and exergy loss were determined to predict the performance of such cooling devices. We also investigated the significance of the magnetic properties of the nanoparticles used in the ferrofluid on cooling performance. γ-Fe2O3, Fe3O4, and CoFe2O4 nanoparticles exhibited superior cooling performance among ferrite-based ferrofluids. FeCo nanoparticles had the best cooling performance for the case of metallic ferrofluids. The saturation magnetization of the magnetic nanoparticles is found to be a significant parameter to enhance heat transfer and heat load cooling. These results can be used to select the optimum magnetic nanoparticle-based ferrofluid for a specific magnetic cooling device application.

Electrical Properties of Superconductive Thick Film Wires Obtained from a Melt Process

YBaCuO superconductive thick film wires were fabricated by employing a melt process with a peak temperature of 1100 °C. Transition temperature and peak critical current density of these YBaCuO superconductive thick film wires were 90 K and 3.5 × 104 A/cm2, respectively. Their magnetic lev-itation force measured at a temperature of 77 K with a permanent magnet was 65.45 N during magnetic cooling. The repulsion force in the case of field cooling was 10.12 N. A permanent magnet with surface magnetism of 5.25 kG was used to cool down superconductive specimens, from which magnetic force of 15.62% of peak magnetic field was trapped. A single crystal YBaCuO superconductive thick film wire was obtained after coating powders of raw materials from a melt process employed for the fabrication of YBaCuO superconductive thick film wire.

Icosahedron [Cl12]12- Templated Gigantic Gd158Co38 Nanocluster with the Largest Ln158 Core for Magnetic Cooling

The synthesis of large nano-sized cluster-molecules is a goal that synthesists and structural scientists have been pursuing, as well as a huge challenge. Herein, the largest 3d-4f metal clusters Cl12@Gd158Co38 and Br12@Gd158Co38 until now are obtained through the “multi-anions-template” strategy, with a protein-sized metal frame (ca. 4.3 × 3.6 × 3.5 nm3). Different from the mixed distribution of 3d and 4f metals and the hollow structure in the previous giant 3d-4f clusters, for the dense core-shell structure Cl12@Gd158Co38 and Br12@Gd158Co38, the Ln158 core with the highest Ln nuclearity number is induced by icosahedra-shaped templates [Cl12]12- or [Br12]12-, while 3d metals (Co) are distributed on its periphery. Their appearances point out a new structure type of non-open giant Ln-based clusters (metal number > 100) and provide an ideal model for studying the multi-level assembly of complex macromolecules. Additionally, Cl12@Gd158Co38 shows the largest magnetic entropy change (-∆Smmax = 46.95 J kg-1 K-1 under 2.0 K and ΔH = 7 T) among reported high-nuclearity 3d-4f clusters.

Magnetocaloric Effect in Nanosystems Based on Ferromagnets with Different Curie Temperatures

The magnetocaloric effect in nanosystems based on exchange-coupled ferromagnets with different Curie temperatures is calculated within the mean-field theory. Good agreement between the results of the mean-field theory and the Landau theory, valid near the critical phase transition temperature, is demonstrated for a flat-layered Fe/Gd/Fe structure. We show that a high magnetic cooling efficiency in this system is attainable in principle and prove the validity of the Maxwell relation, enabling an experimental verification of the predictions made. The theory developed for flat-layered structures is generalized to a granular medium.

Ultrafast Spin Dynamics

Ultrafast spin dynamics is one of the key aspects in the realization of ultrafast spintronic devices. Moreover, it has attracted much attention due to important fundamental spin phenomena existing on nano-, pico-, and femtosecond timescales. The most important interaction of magnetism is, of course, an exchange interaction. However, in practice, spin generally exists interacting with a lattice and an electron in the form of solid. Therefore, a fundamental understanding of ultrafast spin dynamics is involved in various physical phenomena, such as spin-orbit interactions and optomagnetism. In this article, recent trends in ultrafast spin dynamics research are discussed. Ultrafast spin dynamics, which started with the observation of ultrafast demagnetization/remagnetization triggered by femtosecond laser pulses, has been intensively investigated so far. The magnetooptical Kerr effect and X-ray magnetic circular dichroism techniques are introduced as two of the main experimental techniques for exploring ultrafast spin dynamics. THz emission and ultrafast magnetic cooling phenomena are briefly introduced as examples of recent topics in the field of ultrafast spin dynamics.

Performance Assessment and Layer Fraction Optimization of Gd–Y Multilayer Regenerators for Near Room-Temperature Magnetic Cooling

An experimental and numerical assessment of multilayer active magnetic regenerators (AMR) composed of gadolinium (Gd) and gadolinium–yttrium (Gd–Y) alloys (Gd[Formula: see text]Y[Formula: see text], Gd[Formula: see text]Y[Formula: see text] and Gd[Formula: see text]Y[Formula: see text]) is presented. First, by calculating the adiabatic temperature change and the isothermal entropy change from the experimental data for the above materials, we show that, with reasonable accuracy for engineering design purposes, these properties can be determined by shifting the properties of pure Gd to the Curie temperature of the Gd–Y alloy — a common but not yet validated assumption in the design of Gd–Y AMRs with a low Y content. Next, we show that the optimal Gd–Y layer fraction in multilayer AMRs can be determined using the figure of merit known as the material refrigerant capacity (RC), which agrees well with the results from a more complex one-dimensional thermal non-equilibrium porous medium AMR model. Finally, the performance of the latter model is verified against the experimental cooling power data for two- and three-layer Gd–Y regenerators at temperature spans of 25, 30 and 35[Formula: see text]K.