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Magnetocaloric refrigeration is a solid-state cooling approach that promises high energy efficiency and low environmental impact. It remains uncompetitive with conventional vapor-compression technologies due to lack of high-performing materials that exhibit large magnetocaloric effects in low magnetic fields. Here we report a game-changing enhancement of the magnetocaloric response in a transition-metal-based Ni–Co–Mn–Ti. Mechanically and chemically stable rapidly solidified ribbons exhibit magnetic entropy changes as high as ∼ 27 J⋅kg−1K−1 for a moderate field change of 2 T, comparable to or larger than the best known materials for near-room temperature applications. The ribbons can be easily manufactured in large quantities and the transition temperature can be adjusted by varying Co concentration.

Magnetic cooling has been researched as an alternative near room-temperature refrigeration technology for the past two decades. However, one of its greatest limitations is the lack of materials which can be properly shaped for optimal thermal-hydraulic performance while maintaining a substantial magnetocaloric effect at moderate fields (i.e., between 1 and 2 T) and remaining mechanically (and chemically) stable. In this paper, we thoroughly characterized a commercially accessible La(Fe,Mn,Si)13H y material (available as spheroidal granules), in terms of its magnetocaloric properties and thermal-hydraulic performance in an Active Magnetic Regenerator (AMR) device. The regenerator bed built from epoxy-bonded spheroidal particles endured dozens of hours of operation in AMR cycles without any noticeable degradation of their mechanical integrity, thanks to a comparatively larger α − Fe content and granule porosity. As for the magnetic cooling performance, the AMR reached zero-span specific cooling capacities as high as 300 W kg−1. A 1-D two-temperature approach AMR model predicted the performance data with average deviations smaller than 7% for the zero-span specific cooling capacity and 5% for the AMR pressure drop.

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.

We present an experimental and numerical comparison of epoxy bonded multi-layered La(Fe,Si,Mn)13Hy active magnetic regenerators. First, no-load tests were performed on four regenerators with two layers of material and varying amounts of epoxy (from 1 wt. % to 4 wt. %) in order to find the amount of epoxy necessary to maintain the mechanical integrity of the regenerators. As the second part of the study, experimental results of two regenerators with five and nine layers are compared to predictions from the one-dimensional numerical model. A maximum temperature span, ΔTspan, over 20 K was measured and it is effectively equal for both regenerators. The numerical modelling was generally in good agreement with experimental results.

The magnetocaloric performance of La 0.67 Ca 0.27 Sr 0.06 Mn 1.05 O 3 is investigated as a function of the powder grain size and also as a function of decoration of grains with highly conductive silver particulates as a coating layer. We demonstrate that the thermal and electrical conductivities can be significantly modified by the Ag-particle coating when the material is examined in sintered pellet form and we compare results with a second manganite composition La 0.67 Ca 0.33 MnO 3 with significantly smaller grain size. However, we find that this microstructural engineering does not improve the performance of the active magnetic regenerator cycle using the silver decorated material in powder form. The regenerator performance is improved by the reduction of the powder grain size of the refrigerant which we attribute to improved thermal management due to increased surface to volume ratio.

Bonded plates made by hot pressing La0.85Ce0.15Fe11.25Mn0.25Si1.5Hy particles and resin have been tested as active magnetic regenerators in a small scale magnetocaloric device. Firstly the plates were carefully characterised magnetically and thermally. The plates were prepared with 5 wt% resin, and from density measurements it was found that the volume ratio of the magnetocaloric material in the plates was 0.53, due to the resin and porosity. The best operating conditions for the plate regenerator were determined at which a temperature span of 6.4 K was measured along the plates.

In this contribution, aiming to scale-up the production of the material to be used as a magnetic refrigerant, important results are reported regarding the preparation of La(Fe, Si) $_{13}$ compounds by a novel powder metallurgical approach. Additionally, with this new processing route, near net shape components can be fabricated. Based on the ability of some rare-earth compounds to absorb and desorb hydrogen in a controllable manner, an La-based alloy, obtained by conventional casting, was used as a precursor material for this approach. Due to the high content of La-rich phase (LaFeSi), it was possible to decrepitate the ingot into small pieces via interstitial hydrogen insertion. The decrepitated material was further milled, sieved and subsequently pressed into cylinders. A further consolidation step was carried out by sintering. During sintering, simultaneously, the microstructure was homogenized and the porosity controlled. After a 6 h heat-treatment at 1423 K, the samples approached single-phase La(Fe, Si) $_{13}$ , with a relative phase amount of 99 wt%. The residual porosity, inherent of powder metallurgy, plays an important role in a final hydrogenation step necessary to tune the transition temperature. This transition-temperature, around which the magnetocaloric effect is useful for refrigeration, was verified via DSC, while direct measurements of the adiabatic temperature change, $\Delta T_{\rm ad}$ , were carried out to quantify the magnetocaloric effect.

We report a thorough study of the thermal hysteretic behaviour of a single phase sample of the magnetocaloric material La0.67Ca0.33MnO3. Previous reports in the literature have variously found hysteretic and non-hysteretic behaviour. We show the importance of measuring under carefully defined heating and cooling procedures. Careful analysis of the specific heat, measured at five different temperature ramp rates, and the magnetic entropy change indicates that there is no observable hysteresis, even though the behaviour of both quantities is consistent with a first-order phase transition. We discuss the reasons for this and for the differing results previously found.

The magnetocaloric effect (MCE) is the thermal response of a magnetic material to an applied magnetic field. Magnetic cooling is a promising alternative to conventional vapor compression technology in near room temperature applications and has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized. Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology.DOI: http://dx.doi.org/10.5755/j01.sace.14.1.15927

We measure the magnetocaloric effect of the manganite series ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ by determining the isothermal entropy change upon magnetization, using variable-field calorimetry. The results demonstrate that the field dependence of the magnetocaloric effect close to the critical temperature is not given uniquely by the critical exponents of the ferromagnetic-paramagnetic phase transition, i.e., the scaling is nonuniversal. A theoretical description based on the Bean-Rodbell model and taking into account compositional inhomogeneities is shown to be able to account for the observed field dependence. In this way the determination of the nonuniversal field dependence of the magnetocaloric effect close to a phase transition can be used as a method to gain insight into the strength of the spin-lattice interactions of magnetic materials. The approach is shown also to be applicable to first-order transitions.

Results are reported on the preparation of bulk parts of La(Fe,Si)13Hy via the Hydrogen-Decrepitation-Sintering-Hydrogenation (HDSH) process. Net shape parts for application in room-temperature magnetic refrigeration have been produced in only 8 h of heat treatment which is considerably faster than the conventional ingot homogenization heat treatment of 7 days. The samples produced by HDSH showed higher amounts of hydrogen than the parts hydrogenated by the conventional method of thermal homogenization (20 h at 1423 K), milling to fine powder and subsequent hydrogenation. Hydrogenation parameters play an important role for the stability of the desired La(Fe,Si)13 phase during the process. Hydrogen desorption was seen to occur at two temperature ranges as a result of internal gradients. Dissimilar amounts of α-Fe were precipitated for different hydrogenation times. As a result, parts produced via HDSH with 2 and 4 h of hydrogenation exhibited different magnetocaloric behaviours. For a hydrogenation step of 4 h, parts with a demagnetization factor of 0.49 showed an adiabatic temperature change ( Δ T ad ) higher than 1 K for a temperature range of 40 K with a maximum value of 1.57 K for an applied magnetic field of 1.75 T. As the duration of the hydrogenation step of the HDSH process decreased to 2 h, Δ T ad was larger than 1 K for a temperature range of 24 K. However the maximum value of Δ T ad at 328 K was 2.2 K, which is 37.5% larger than the maximum value for a hydrogenation period of 4 h.

We study the magnetocaloric metamagnetic transition in LaFe11.74Mn0.06Si1.20 and LaFe11.76Mn0.06Si1.18H1.65 under hydrostatic pressure up to 1.2 GPa. For both compounds, hydrostatic pressure depresses the zero field critical temperature. However, in detail, pressure influences the magnetic properties in different ways in the two compounds. In the dehydrogenated case the transition broadens under pressure whereas in the hydrogenated case the transition sharpens. In both cases thermal hysteresis increases under pressure, although with different trends. These observations suggest both intrinsic and extrinsic hysteresis loss brought about by the use of hydrostatic pressure. We explore the multicaloric field-pressure cycle, demonstrating that although the gain introduced by overcoming the magnetic hysteresis loss is closely countered by the loss introduced in the pressure cycle, there are significant advantages in that the temperature range of operation can be finely tuned and extended, and the magnetocaloric transition can operate in lower absolute applied fields (

First order magnetic phase transition materials present a large magnetocaloric effect around the transition temperature, where these materials usually undergo a large volume or structural change. This may lead to some challenges for applications, as the material may break apart during field change, due to high internal stresses. A promising magnetocaloric material is La(Fe, Mn, Si)13Hy, where the transition temperature can be controlled through the Mn amount. In this work we use XRD measurements to evaluate the temperature dependence of the unit cell volume with a varying Mn amount. The system is modelled using the Bean-Rodbell model, which is based on the assumption that the spin-lattice coupling depends linearly on the unit cell volume. This coupling is defined by the model parameter η, where for η > 1 the material undergoes a first order transition and for η ≤ 1 a second order transition. We superimpose a Gaussian distribution of the transition temperature with a standard deviation σ T 0 , in order to model the chemical inhomogeneity. Good agreement is obtained between measurements and model with values of η ∼ 1.8 and σ(T0) = 1.0 K.

Magnetic refrigerators can theoretically be more efficient than current vapor compression systems and use no vapor refrigerants with global warming potential. The core component, the active magnetic regenerator (AMR) operates based on the magnetocaloric effect of magnetic materials and the heat regeneration processes of periodic fluid blows. Magnetocaloric materials with a first order phase transition (FOPT) are suitable to realize a higher cooling capacity than commonly used gadolinium, but layering such materials is necessary, due to a large isothermal entropy change (Δ푆푚) in a narrow region around their Curie temperature. Simulations are implemented to investigate how to layer the FOPT materials for obtaining higher cooling capacity. Moreover, based on entropy generation minimization, optimization of the regenerator geometry and related operating parameters is presented for improving the AMR efficiency. In addition, simulations are carried out to investigate the potential of applying nanofluid in future magnetic refrigerators.