Magnetocaloric Refrigeration Research Articles

Performance assessment of a rotary active magnetic regenerator prototype using gadolinium

• The cooling performance of a multi-bed AMR device with gadolinium is presented. • A cooling power of 818 W with a COP of 4.2 was achieved over a 10 K span. • The device can establish a 16.6 K span starting from room temperature in 25 min. • Active valve control can increase cooling power and COP by more than 70%. • A maximum second-law efficiency of 39.2% was obtained at a span of 7.3 K. We present the experimental results for a rotary magnetocaloric prototype that uses the concept of active magnetic regeneration, presenting an alternative to conventional vapor compression cooling systems. Thirteen packed-bed regenerators subjected to a rotating two-pole permanent magnet with a maximum magnetic field of 1.44 T are implemented. It is the first performance assessment of the prototype with gadolinium spheres as the magnetocaloric refrigerant and water mixed with commercial ethylene glycol as the heat transfer fluid. The importance of various operating parameters, such as fluid flow rate, cycle frequency, cold and hot reservoir temperatures, and blow fraction on the system performance is reported. The cycle frequency and utilization factor ranged from 0.5 to 1.7 Hz and 0.25 to 0.50, respectively. Operating near room temperature and employing 3.83 kg of gadolinium, the device produced cooling powers exceeding 800 W at a coefficient of performance of 4 or higher over a temperature span of above 10 K at 1.4 Hz. It was also shown that variations in the flow resistance between the beds could significantly limit the system performance, and a method to correct those is presented. The performance metrics presented here compare well with those of currently existing magnetocaloric devices. Such a prototype could achieve efficiencies as high as conventional vapor compression systems without the use of refrigerants that have high global warming potential.

Magnetic properties and promising magnetocaloric performances in the antiferromagnetic GdFe2Si2 compound

The magnetocaloric (MC) effect-based solid-state magnetic refrigeration (MR) technology has been recognized as an alternative novel method to the presently commercialized gas compression technology. Searching for suitable candidates with promising MC performances is one of the most urgent tasks. Herein, combined experimental and theoretical investigations on the magnetic properties, magnetic phase transition, and cryogenic MC performances of GdFe2Si2 have been performed. An unstable antiferromagnetic (AFM) interaction in the ground state has been confirmed in GdFe2Si2. Moreover, a huge reversible cryogenic MC effect and promising MC performances in GdFe2Si2 have been observed. The maximum isothermal magnetic entropy change, temperature-averaged entropy change with 2 K lift, and refrigerant capacity for GdFe2Si2 were 30.01 J kg−1 K−1, 29.37 J kg−1 K−1, and 328.45 J kg−1 at around 8.6 K with the magnetic change of 0–7 T, respectively. Evidently, the values of these MC parameters for the present AFM compound GdFe2Si2 are superior to those of most recently reported rare-earth-based MC materials, suggesting the potential application for active cryogenic MR.

High‐Efficiency Power Generation Device of Magnetic Declination Thermoelectric Material and Multisource Coordination Optimization of Distribution Network

The first‐order phase transition compounds have attracted widespread attention from refrigeration enterprises and scientists due to their giant magnetocaloric effect and refrigeration temperature region spanning room temperature. Magnetic substances are crystal structures composed of magnetic ions or atoms with a certain thermal motion or vibration. This research mainly discusses the multisource coordination optimization of magnetic declination thermoelectric material efficient power generation device and distribution network. In this study, large doses of MnFe(P,Si) compounds with different ratios were prepared. The reason for choosing a large dose is that the properties and structures of the samples prepared in a large amount and a small amount will be more or less the same for the samples fired in the same proportion and in the same process. At the same time, mass production also prepares for future mass production. Thermomagnetic power generation described in this study is a form of power generation that converts thermal energy directly into electrical energy. The conversion of thermal energy to electric energy is realized by the change of magnetization in the closed coil by the thermomagnetic material in the magnetic field environment, and then, the change of the magnetic flux in the closed coil is realized. The distribution network refers to the power network that receives electrical energy from the transmission network or regional power plants and distributes it locally through distribution facilities or distributes it to various users step by step according to the voltage. It is composed of overhead lines, cables, towers, distribution transformers, isolating switches, reactive power compensators, and some ancillary facilities. A network that plays an important role in distributing electrical energy in the power grid. This research starts with the active distribution network scheduling priority and the distribution network scheduling coordination control framework. Combined with the scheduling and operation characteristics of each component in the distribution network, the coordinated optimization scheme of the distribution network and the source‐network load‐storage coordinated control framework are investigated and analyzed. The scheduling resources of the model include the power of the substation flowing into the distribution network, the output of the adjustable distributed power source, the output value of the energy storage element, the closing status of the tie switch, and the section switch. In the study, it was found that when the Fe content was 0.63, the Curie temperature of the compound could reach 273 K at the highest. Scientifically designing a thermomagnetic power generation demonstration device is an important part of the realization of thermomagnetic power generation. The magnetic declination thermoelectric materials designed in this study will help to improve the power generation efficiency.

Evaluation of Magnetocaloric Behavior and Refrigeration Performance of Gd3co10al87 Nanostructured Thin Film Alloy with Multiple Curie Temperatures

Evaluation of Magnetocaloric Behavior and Refrigeration Performance of Gd3co10al87 Nanostructured Thin Film Alloy with Multiple Curie Temperatures

Room temperature Magneto-Caloric Refrigerator: system level analysis

A system consisting of magneto-caloric fluid, which releases (absorbs) heat in the presence (absence) of a magnetic field, is used to achieve refrigeration. Gadolinium is used as a magneto-caloric material. The influence of using water and galinstan as heat transfer liquid, on the refrigerator performance is numerically analysed by varying the mass flow rate of the magneto-caloric fluid, and MCM volume fraction (0.4 to 0.6). A cooling load upto 20 W at room temperature is obtained over a temperature difference of 0.5 K.

Tailoring the magnetocaloric, magnetic and thermal properties of Dy6(Fe,Mn)X2 intermetallics (X[dbnd]Sb, Te, Bi)

The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (XSb, Bi, Te) intermetallics (space group P6¯2m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129–370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with antiferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents β,γ,δ related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for μ0ΔH = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (μ0ΔH = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie temperature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.

Magnetic and magnetothermal properties of the GdTi0.05Fe0.95-xMnxSi canted ferrimagnets

The GdTi0.05Fe0.95-xMnxSi, х = 0–0.95 intermetallic compounds with a tetragonal crystal structure of the CeFeSi type (space group P4/nmm, N 129, tP6) have been studied. The lattice parameter c and the Curie temperature TC increase quickly, whereas the lattice parameter a is almost unchanged in the system when the content of Mn increases. The saturation magnetization, magnetocaloric effect MCE and refrigerant capacity RC change non-monotonically with a dip at intermediate compositions that is presumably due to the canted ferrimagnetic structure of these compounds. The MCE and RC vary in a field changing to 17 kOe within the small ranges of 1.6–2.7 J/kgK and 60–117 J/kg around TC, respectively, while TC changes in a wide range around the room temperature from 160 to 360 K.

The spin reorientation and improvement of magnetocaloric effect in HoCr1-xGaxO3 (0 ≤ x ≤ 0.5)

The effect of non-magnetic Ga3+ ions doping at HoCrO3 (HoCr1-xGaxO3, 0 ≤ x ≤ 0.5) on the crystal structure, electronic structure, Raman spectra, and magnetic behaviours were systematically investigated. Most strikingly, a new spin-reorientation transition (Γ2 → Γ4 →Γ2) is triggered by Ga3+ doping in HoCrO3, and the spin configuration of Γ4 is obviously displayed in ZFC and FC curves when x > 0.2. According to the XRD analysis, Ga3+ doping reduces the octahedral distortion and changes the lattice parameters, CrO and HoO bond lengths, and a turning point occurs at x = 0.2. Raman spectra further give evidence of the structural changes and suggest that spin-phonon coupling can be mediated by Ga3+ doping, illustrating the weakened effect of Ga3+ doping on the Ho and Cr interaction. The dilution of the competition between CrCr canted antiferromagnetic coupling and Ho-Cr antisymmetric coupling results in the appearance of spin configuration Γ4. Moreover, the Griffiths phase can also be observed and gradually suppressed with doping. These complex magnetic transitions in HoCr1-xGaxO3 are given in a magnetic phase diagram. From the application point of view, Ga3+ doping also improves the magneto-caloric effect (MCE) and refrigerant capacity (-ΔSM = 10.74 J/KgK, and the RC is 416 J/K at 7 T and 10 K for HoCr0.6Ga0.4O3), which sheds some light on how to optimize the MCE properties.

Low-field magnetocaloric effect in La0.75Ca0.25–x Na x MnO3 (0 ≤ x ≤ 0.10) perovskite

ABSTRACT In our present work, La0.75Ca0.25–x Na x MnO3 (0 ≤ x ≤ 0.10) samples were synthesized by the flux method. The structural and the magnetocaloric properties were studied. The crystallographic study indicates that our samples crystallize in the orthorhombic structure, with Pbnm space group. The magnetocaloric properties have been theoretically studied and compared with the experimental results, under low magnetic field of 0.05 T. It has been found that the simulated data M(T), −ΔSM (T), ΔT ad and ΔCP,H (T) are in good agreement with the experimental one. In addition, the magnetocaloric effect of Na+ in our system is important and beneficial for manipulating magnetocaloric refrigeration.

Martensitic Transformation and Magnetocaloric Effect in Co-Doped Ni–Mn–In Ferromagnetic Shape Memory Alloy

In this work, we present the detailed results of magnetic, structural, and magnetocaloric measurements in Ni-Mn-In-based Heusler alloys, namely, Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">51</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">32.8</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16.2</sub> , Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50.2</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33.6</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16.2</sub> , and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">51</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33.4</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15.6</sub> . The characteristics of magnetostructural coupling play an important role in the magnetic field-driven behavior of these alloys. A small variation in the stoichiometry composition broadens the magnetic entropy change curve and shifts the critical temperatures by as much as 48 K. The effect of cobalt doping on the magnetic, structural, and magnetocaloric properties in these multifunctional alloys is investigated through characterizing Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">45</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> samples. Cobalt doping affects the martensitic and magnetic transformation temperatures and enhances the ferromagnetic properties of the parent austenitic phase. Therefore, the magnetization change accompanying the martensitic transformation is significantly improved, resulting in a large magnetic entropy change of 21.6 J kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 1.5 T. Additionally, Co doping broadens the operating temperature window to 21 K around room temperature. Consequently, a significant effective refrigeration capacity of 254 J kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for 1.5 T is obtained which is comparable or even superior to that of the high-performance, though expensive, rare-earth-based magnetocaloric refrigerants. The high performance, low cost, nontoxicity, and tunability make these alloys of great interest and promising prospect for an affordable magnetic refrigeration system.

Simulator development of a rotary magnetocaloric refrigerator by stepwise regenerator modelling approach

Magnetic refrigeration has been seeing as a promising technology based on the magnetocaloric effect. One of its great advantages is it offers smaller global environmental impact relating to conventional refrigeration. Modeling and simulation of such processes can provide important data in the development and optimization of the experimental devices. Among existing designs, the rotary refrigerators presents several challenges in terms of complexity when comparing to reciprocating ones, which is compensated by several properties. A novel full process simulator of a magnetocaloric refrigerator processes was implemented to study the processes performance over different conditions. A stepwise modeling approach was applied, simplifying the phenomena of heat transfers. Gd was chosen as refrigerant material because of its magnetic transition temperature. The routine considered a rotating clockwise Gd wheel with an anticlockwise closed flow loop of water, which percolates the six Gd porous beds, hot and cold heat exchanger. The simulator was able to represent the transient aspects as well as the steady state conditions of the processes, considering both time performance and numerical stability. The inversion in heat transfer profiles along the process was used as a limit in the calculation of the maximum heat transfer absorption in the cold exchanger.

Analysis of magnesium ferrite and nickel doped magnesium ferrite thin films grown by Spray Pyrolysis

The magnetic magnesium ferrite (MgFe2O4) with a partially inverse spinel structure has been prepared by the classic ceramic method, which have practical application in information storage systems, magnetocaloric refrigeration and magnetic diagnostics. Nickel-magnesium ferrites (Ni:MgxFe3-xO4 or Mg(1-x)NixFe2O4; 0 ≤ x < 1) are magnetic materials of technological significance due to low electrical conductivity, low magnetic coercivity and low eddy current deceives. Magnesium plays an important role in the electrical and magnetic properties of Mg(1-x)NixFe2O4. Thin films were synthesized on different substrates with Spray Pyrolysis (SP) technique. The thin films were analyzed through Atomic Force Microscopy (AFM), X-ray Diffraction (XRD) and UV–Vis double beam spectrophotometer and for magnetic properties of films were used Vibrating-sample magnetometer (VSM) technique. In this study, the film is doped with material having magnetic properties and non-magnetic material. The change in the magnetic properties of the film will be examined as a result of the doping. As a result, the magnetic hysteresis curve of the film was changed by doping. Investigation of magnetic properties is very important for magnetic device applications such as magnetic recording, in magnetic sensor technology and room temperature giant magnetoresistance (GMR) applications.

Tuning Magnetocaloric Properties for La-=SUB=-1-x-=/SUB=-Sr-=SUB=-x-=/SUB=-CoO-=SUB=-3-=/SUB=-

Low magnetic field magnetocaloric (MC) properties of La1-xSrxCoO3 (x=0.3 and 0.5) near phase transition from a ferromagnetic to a paramagnetic state were investigated. It is shown that the change of Sr content allows MC effect in La1-xSrxCoO3 to be tunable, which is more practical for construction of MC refrigeration. MC properties of the x=0.5 sample are significantly greater than that of the x=0.3 one. Furthermore, the results show that MC properties of La1-xSrxCoO3 samples are significantly larger, and comparable with some MC properties of many materials like Gd1-xCaxBaCo2O5.5 and Ge0.95Mn0.05. Keywords: magnetocaloric effect, La1-xSrxCoO3, magnetic entropy change.

Collective magnetism of a single-crystalline nanocomposite FeCoCrMnAl high-entropy alloy

We have investigated the nature of the magnetic state of a single-crystalline FeCoCrMnAl nanocomposite high-entropy alloy (HEA), composed of crystallographically oriented magnetic nanoplatelets embedded in a magnetic matrix of different magnetic order. The two-phase nanocomposite was formed by a bcc-B2 spinodal decomposition. Due to the single-crystalline nature of the material, there is no symmetry breaking of the surface atomic monolayer at the borders between the two phases and there are no interface regions between the nanoplatelets and the matrix. The material also does not exhibit grain structure, allowing for the observation of the true intrinsic magnetism of a nanocomposite HEA. Upon cooling, the predominantly Fe‒Cr‒Mn chemically disordered bcc matrix orders first at TC1≈ 425 K in an asperomagnetic-type magnetic state. Below TC2≈ 370 K, the B2 nanoplatelets that are predominantly an Al30(Co,Mn)70 pseudo-binary intermetallic compound, start to order in a ferromagnetic (FM)-type manner. We have focused to the question whether the magnetic state of the nanocomposite below TC2 is a collective state of the interacting nanoplatelets and the matrix or their coupling is weak enough that the magnetic ordering of each of them can be treated independently. Experimental results support the development of a single collective, disordered FM-type magnetic state upon cooling due to the exchange coupling between the nanoplatelets and the matrix. The nanocomposite is magnetically soft and the strong variation of the magnetization with the temperature in a large interval ΔT≈ 125 K just above room temperature due to two successive magnetic phase transitions make this material promising for the application in magnetocaloric refrigeration.

A 15kW magnetocaloric proof-of-concept unit: Initial development and first experimental results

Abstract As any innovative development program, magnetic refrigeration still faces many challenges before being widely accepted as an actual replacement technology for cooling. In the last years, more than 50 different types of prototypes of magnetocaloric refrigerators have been presented in literature, with limited cooling power values and/or limited temperature span. This paper presents several assembly details and experimental results of a new proof-of-concept magnetic refrigeration prototype having both a temperature span of more than 20 K and a cooling capacity designed for around 15 kW. It has been made as a part of a pilot project for a large industrial partner to be used for an industrial process. The obtained experimental results have been measured and validated by an exterior independent body. This performance achievement, that we believe to be as the largest rotary magnetic refrigerator ever created, could be a milestone for the future of magnetocaloric chillers.

Giant low-field magnetocaloric effect and refrigerant capacity in reduced dimensionality EuTiO3 multiferroics

Engineering magnetic materials into a thin film form while preserving its excellent magnetocaloric response is essential in the development of miniature magnetic coolers. We demonstrate how this can be achieved in the case of EuTiO3 – an emerging multiferroic material. Unlike conventional cases where reduced dimensionality considerably decreased the magnetic entropy change (ΔSM) and hence the refrigerant capacity (RC), we show the large low-field enhancements of ΔSM and RC in a ∼100 nm thick nanocrystalline EuTiO3 film (ΔSM ∼ 24 J kg−1K−1 and RC = 152 J kg-1 for μ0ΔH = 2 T) relative to its single crystal counterpart (ΔSM ∼ 17 J kg−1K−1 and RC ∼ 107 J kg-1 for μ0ΔH = 2 T). The nanocrystalline EuTiO3 film is an excellent candidate for cryogenic magnetic refrigeration. From our study, a new approach for improving both MCE and RC in magnetic nanomaterials is proposed, which will stimulate further research on magnetocaloric thin films and related cooling devices.

Modeling of hydrogen liquefaction using magnetocaloric cycles with permanent magnets

Abstract Hydrogen (H2) is promising alternative to replace fossil fuels, but its transport and storage has been challenging. As H2 fuel cell vehicles are gaining traction, the infrastructure for storing large amounts of liquid H2 is needed. However, liquid H2 would suffer from boil-off loss, and traditional vapor compression refrigeration systems would not be able to economically recover the lost H2 due to the low efficiencies at cryogenic temperature. Magnetocaloric (MC) refrigeration systems could possess much higher coefficient of performance (COP) at cryogenic temperature compared to the vapor compression ones. Previous work on cryogenic MC systems, however, have only focused on large scale applications which use superconducting magnets to provide a large magnetic field but are prohibitively expensive to operate for small scale applications, such as that of a H2 refilling station. In this work, we modeled the performance of a MC refrigeration cycle using 1-Tesla permanent magnets for H2 liquefaction, with the objective of cooling H2 from 80 K (using liquid nitrogen as the heat sink) to 20 K (boiling point of hydrogen). We evaluated main performance metrics including the total work input to the refrigeration system, COP, total MCM mass in the system, and total volume of the permanent magnets, etc. Our modeling results indicate that such a permanent magnet-based MC cooling system is feasible for small-scale H2 liquefaction, with projected COP values significantly higher than those of vapor compression systems. This work provides design guidelines for future experimental efforts on permanent magnet MC cooling systems for cryogenic cooling.

Toward a solid-state thermal diode for room-temperature magnetocaloric energy conversion

Thermal control elements, i.e., thermal diodes, switches, and regulators, can control the heat flow in an analogous way in how electronic devices control electrical currents. In particular, a thermal diode allows a larger heat flux in one direction than in the other. This has aroused the interest of researchers working on the thermal management of electronics, refrigeration, and energy conversion. Solid-state thermal diodes are attractive because they are silent, reliable, lightweight, and durable. While some solid-state thermal diodes have been developed at the nano- and microscale, the leap to the macroscale has yet to be made. A macroscale thermal diode would play a crucial role in the future development of applications related to caloric refrigeration and heat pumping. Additionally, the temperature changes of caloric materials (due to the caloric effect) are ideal for testing these thermal devices. This paper aims to numerically evaluate the influence of a macroscopic solid-state thermal diode in a magnetocaloric refrigeration device under transient and quasi-steady-state conditions. Materials with different temperature-dependent properties were analyzed, and the most promising ones were selected for the operating range of a magnetocaloric device (290–296 K). The highest achieved magnetocaloric thermal rectification ratio under transient conditions was up to 295-times higher than with quasi-steady-state operation. This shows that transient operation should be considered for future progress with this technology.

Impact of interface structure on functionality in hot-pressed La-Fe-Si/Fe magnetocaloric composites

Although the design of composite materials has been proposed as an effective way to improve the mechanical properties and thermal conductivity for magnetocaloric refrigerants, the interface between the constituent phases has not yet been optimized. In the present work, the La-Fe-Si/Fe composites with superior interface conditions have been fabricated by hot pressing. Due to the limited solubility of the Fe element in the La-Fe-Si phase, the introduction of the reinforcing phase (i.e. the Fe phase) only brings out thin diffusion layers that still remain NaZn13 structure. The reinforcing phase with high ductility tends to flow during hot pressing and hence fills in the pores in the composite, leading to a compact structure which is studied by 3D high resolution X-ray tomography and electron probe micro-analyzer. According to the transmission electron microscope analyses, a part of the La-Fe-Si and the Fe phases show a preferred orientation relationship. As a result, the reduced phonon scattering as well as the cohesive bonding at the interfaces favors the thermal conductivity and mechanical stability of the La-Fe-Si/Fe composites. The value of thermal conductivity reaches 7.5 W/m K and compression strength is about 300 MPa in the La0.7Ce0.3Fe11.45Mn0.15Si1.4/13.5%Fe composites. Besides, an isothermal entropy change as large as 15 J/kg K under a magnetic field change of 2 T has been obtained in these magnetocaloric composites, which is comparatively promising for magnetic refrigeration applications. Of particular interest is the integrity of the composite after hydrogenation due to the outstanding mechanical properties of La-Fe-Si/Fe composites.

Effect of carbide formation on phase equilibria and compositional modulation of transformation properties in (Mn,Fe)2(P,Si) alloys

Practical implementation of efficient magnetocaloric effect-based refrigeration and energy conversion applications requires the design of whole alloy families with narrow tolerances on the transformation properties, and therefore composition, of the component alloys. In the promising class of magnetocaloric (Mn,Fe)2(P,Si) alloys, this task of compositional tuning is made especially difficult by the observed co-existence of several P-depleted impurity phases, which for example, increases both the P content and transformation hysteresis losses of the main quaternary phase. In this work, we study the mechanisms that induce impurity phase formation in the Mn–Fe–P–Si system, and investigate the impact of sequential carbide formation on observed phase microstructure along with its effect on the composition and transformation properties of the (Mn,Fe)2(P,Si) phase. Using quantitative analyses to measure the composition within main and impurity phases in samples throughout the bulk alloy space, we establish that (1) repeated processing steps increase the content of a (Mn,Fe)9Si2 carbide phase, resulting in (2) deviations in the magnetocaloric (Mn,Fe)2(P,Si) phase from bulk nominal composition of up to ∼4 at. %. Finally, we map out (3) the dependence of transformation critical temperature, hysteresis, and enthalpy on the composition of the (Mn,Fe)2(P,Si) phase of interest. Together, these results suggest carbon impurities can have a critical impact on magnetocaloric transformation properties in (Mn,Fe)2(P,Si) alloys, since 0.3 wt % carbon content can cause critical temperatures and hystereses to deviate by more than 95 K and 8 K from desired design values.