Magnetocaloric Composites Research Articles

La-Fe-Si magnetocaloric composite with anisotropic microstructure prepared by hot pressing

In this study, we have proposed an anisotropic microstructure in La-Fe-Si magnetocaloric composites for thermal management. This is vital to the rapid heat exchange and high working efficiency in magnetic refrigerator system. We demonstrate the anisotropic microstructure in composites can be fabricated via powder metallurgy in this work. By adjusting the particle size of the 1:13 phase, the introduced reinforcing phase with rheological property can effectively deform and anisotropic microstructure can be obviously observed. This can be attributed to the tailored stress distribution in cubic-anvil-type pressure apparatus. The element diffusion between the two phases was also discussed in this paper, taking into account the influence of particle size. Such anisotropic microstructure causes some other anisotropic physical properties in the composite, such as mechanical strength. The compressive measurements along the axial direction exhibit superior mechanical properties (~ 3.5% for strain and ~ 350 MPa for strength) compared to those along the radial direction. Attributed to the stress buffer-effect provided by the introduced ductile phase, magnetocaloric effect (magnetic entropy change ~ 7.8 J/kg K) can be maintained in the La-Fe-Si composite.

New design of La(Fe, Co, Si)13 magnetocaloric composites using Gd as a binder

We propose a new type of magnetocaloric composite that excludes the conventional use of non-magnetocaloric materials as the binder. Spark plasma sintering was adopted to make a fully dense La(Fe, Co, Si)13/Gd composite at low temperatures (450–600 °C), which shows relatively high thermal conductivity and compressive strength compared with the counterparts of other bonded magnetic refrigeration composites. The mechanism of high thermal conductivity and compressive strength were discussed. The diffusion processes between La(Fe, Co, Si)13 and Gd were also analyzed, though it has a limited influence on the magnetocaloric effect of the composite. The results suggest that using Gd as the metal binder is promising for improving the performance of present magnetic refrigeration composites. It also provides an important approach for the design of new magnetocaloric materials.

Enhancement of the thermal conductivity of a near room temperature magnetocaloric composite using graphene-like hybrid nanosheets derived from organic waste

Polymer matrix composites, fabricated to counter the inherent brittleness of magnetocaloric Heusler alloys, suffer from low thermal conductivity. Here, we demonstrate a low-cost, scalable route towards developing thermally conductive, mechanically robust near-room-temperature magnetocaloric composites by incorporating graphene-like hybrid nanostructures chemically synthesized from discarded sugarcane. Micron-sized particles obtained by manually grinding Ni50.2Mn36.7Sn13 ribbons possessing a strong magnetostructural transformation near room-temperature were chosen as the active magnetocaloric fillers. Both the functional fillers were incorporated into a polysulfone matrix by solution casting. Large values of isothermal entropy change ∼ 0.43 and -0.46 J/kg.K were observed for a ΔH = 2T, driven by two successive first and second-order transformations within the alloy fillers. Additionally, an enhanced value of the in-plane thermal conductivity ∼ 3.06 ± 0.4 W/m.K was observed in the composites owing to the formation of efficient thermal bridges/pathways by the graphene-like hybrid nanostructures, rendering them promising candidates for magnetic refrigeration applications.

Synthesis and properties of La-Fe-Si/La-Co magnetocaloric composites

LaFe11.8Si1.2/16wt%La65Co35 samples were prepared by spark plasma sintering (SPS) and subsequent annealing. The La65Co35 binder increased the desired 1:13 phase content in these samples during annealing at 1323 K. Interestingly, the α-Fe phase formed between the LaFe11.8Si1.2 particles and the La65Co35 binder and then disappeared as the annealing time extended. The sample annealed for 0.5 h had a distinct core-shell structure with Co in the shell, a desirable table-like (−∆SM)−T curve was obtained and its RC (153 J·kg−1) was 107% higher compared to the sample before annealing. The diffusion of Co from La3Co to the 1:13 phase was hampered by the growth of La5Si3 phase. Relatively large (−∆SM)max of 8.51 J·kg−1·K−1 (Δμ0H=2 T) and (σbc)max of 374 MPa for the sample annealed for 24 h were obtained. These results indicated that the method combining SPS and diffusion annealing is promising to prepare La-Fe-Si based magnetocaloric composites with good properties.

Highly loaded magnetocaloric composites by La(Fe,Si)13H powder dedicated to extrusion-based additive manufacturing applications

Additive manufacturing is attracting increasing interest in the field of magnetic refrigeration to solve the problem of the brittleness of magnetocaloric alloys. The formulation of a composite based on a thermoplastic binder loaded with La(Fe,Si)13H magnetocaloric powder and then its shaping by an innovative additive manufacturing process were investigated in this work. To best preserve the magnetocaloric properties of the powders, the composite must have the highest powder mass fraction and the most suitable rheological behaviour for 3D printing from pellets. The thermo-rheological characterizations of the powders, the constituents of the binders, the binders and the composites were performed. Except for the powder, these behaviours were modelled as a function of the powder's shear rate, temperature and volume fraction leading to the identification of their different parameters. Criteria encompassing more parameters are defined to select the formulation of the composite with its forming parameters (i.e. from its elaboration to its printing). They lead to the selection of a powder, binder and composite components as well as some process parameters that can be optimised (e.g. maximum powder loading rate, nozzle temperature, shear rate). Particular attention is paid to the homogeneity of the composite and the non-degradation of the magnetocaloric properties throughout the fabrication process (i.e. the preservation of hydrogen in the La(Fe,Si)13H powder) which places this work in the context of 4D printing. Different parts (small 0.6 mm thick plates and characterization samples) of highly charged magnetocaloric powder (89% by mass) were printed. Their characterization reveals favourable properties, such as a porosity of 10.8 ± 2% present in the binder (and a global porosity of 5.4 ± 1% in the composite) in the form of pore size <2.10−3 mm3 and with a few voids of 4.10−3 mm3 obtained by X-ray tomography, and, high magnetocaloric properties (Δs = 9.3 J·K−1·kg−1). Other characterizations reveal less favourable mechanical properties, such as a yield strength of 0.7 MPa at room temperature which is highly dependent on the latter rising to an acceptable level of 5 MPa at −20 °C. A comparison with extruded strips of the composite of the same formulation but with lower porosity (i.e. 4% in the binder and overall 2% in the composite) shows an approximately 5-fold higher yield strength and the same initial Young's modulus, which determines the gap for improvement of the overall printing process used in this study.

Enhanced mechanical strength in hot-rolled La-Fe-Si/Fe magnetocaloric composites by microstructure manipulation

La-Fe-Si-based alloys possess a giant magnetocaloric effect due to the sharp itinerant-electron metamagnetic transition. However, the relatively poor mechanical strength of these alloys impedes their practical application in the magnetic refrigeration system. In this study, we employed hot rolling to manipulate the microstructure, and thus to improve the mechanical strength of LaFe11.6Si1.4/Fe composites. The microstructure and magnetic phase transition behavior have been systematically investigated by X-ray tomography, high-resolution transmission electron microscope and digital image correlation technique. We found that the bending strength of the hot-rolled LaFe11.6Si1.4/Fe reached up to 176 MPa, which is about 26% higher than the non-rolled one. The refinement of the α-Fe particles plays a key role in hindering crack propagation and enhancing the mechanical strength of the hot-rolled sample. Additionally, the reduction of the La-rich phase and large-size pores is found to be beneficial to the mechanical strength enhancement. The hot-rolled composite shows a sharp metamagnetic transition with a maximum magnetic entropy change of 17 J kg−1 K−1 for 2 T magnetic field change. This study demonstrates that microstructure manipulation based on hot rolling is a feasible approach to improve the mechanical strength of La-Fe-Si materials.

Superior properties of LaFe11.8Si1.2/La65Co35 magnetocaloric composites processed by spark plasma sintering

A series of LaFe11.8Si1.2/La65Co35 bulk composites were fabricated by spark plasma sintering (SPS) followed by diffusion annealing. The effect of sintering temperature (1073–1223 K) on the magnetocaloric and other properties of LaFe11.8Si1.2/La65Co35 bulk composites were investigated. The 1:13 phase content in all the annealed samples was ∼90 wt%. The mechanical and magnetocaloric properties of these annealed bulk composites could be improved by tuning the sintering temperature. The Curie temperature (TC) and maximum magnetic entropy change (−ΔSM)max of the annealed samples varied in the range of 211–228 K and 5.56–9.33 J kg−1 K−1, respectively. Selected samples exhibited excellent mechanical and magnetocaloric properties. The (−ΔSM)max value was 9.33 J kg−1 K−1 at TC of 217.8 K. Its maximum compressive strength (σbc)max value was 540 MPa at a strain of 3.48%, which exceeded the mechanical properties of most La–Fe–Si based bulk composites prepared by hot-pressing sintering (HPS). Thus, La–Fe–Si based bulk composites with good comprehensive properties could be obtained by combining SPS and diffusion annealing.

Novel magnetocaloric composites with outstanding thermal conductivity and mechanical properties boosted by continuous Cu network

Magnetic refrigeration based on the magnetocaloric effect is expected to trigger technological revolution in the refrigeration industry due to the merits of high energy efficiency and complete elimination of greenhouse gas emissions. However, the implementation of this emerging technology is hindered by some challenges in the magnetocaloric materials, e.g., low thermal conductivity (λ) and poor mechanical properties. This paper reports a novel magnetocaloric composite that is characterized by continuous Cu networks within the (Mn,Fe)2(P,Si) magnetocaloric matrix. This unique microstructure is designed with the aid of finite-element simulations and experimentally realized by hot pressing the (Mn,Fe)2(P,Si)/Cu core/shell powders. High-resolution transmission electron microscope revealed good interfacial coherence between the matrix and the binder, which is highly desirable to reduce the interfacial thermal resistance. The magnetocaloric composites with such a novel microstructure exhibit a high λ of 20.4 Wm−1K−1 and a large maximum compressive strength of 570 MPa, which are the best comprehensive properties for room-temperature magnetocaloric materials ever reported. Consequently, this work offers a promising way to tackle the bottleneck problems of low thermal conductivity and the brittleness of the magnetocaloric materials, which may accelerate the practical application of magnetic refrigeration.

Graphene‐Based Magnetocaloric Composites for Energy Conversion

Herein, a simple, versatile, and cost‐effective method to fabricate innovative thermal conductive magnetocaloric (MC) composites, which offers a smart solution to manufacture active elements with desired geometries, overcoming the current thermal and mechanical limits of the most studied MC materials, is presented. The composite is prepared by embedding powder of a MC material in an epoxy matrix enriched with a graphene‐based material, obtained by thermal exfoliation of graphite oxide. The graphene‐enriched composite shows a significant improvement of the MC time response to the magnetic field, due to the formation of a 3D network that bridges the MC particles and reduces the metal–matrix contact resistance, thus creating a percolation path for an efficient heat transfer. Because of the simplicity and scalability of the preparation method and the great enhancement in response time, these new functional composites represent an important step for the effective application of MC materials in thermomagnetic devices for the energy conversion.

Microstructure, magnetism and critical behavior of hot pressed Ni-Mn-Ga/Al magnetocaloric composites with enhanced thermal conductivity and mechanical properties

In the application of magnetic refrigeration technology, magnetocaloric materials are required to possess large magnetocaloric effect and broad working temperature range with good thermal conductivity and mechanical properties. To address this elusive combination of properties, we prepared Ni-Mn-Ga/Al magnetocaloric composites via hot pressing method. Phase composition and element distribution revealed the influence of sintering temperature on composites' microstructure. The magnetic properties and magnetocaloric effect of hot pressed Ni-Mn-Ga alloy and Ni-Mn-Ga/Al composites were studied, indicating their largely broadened working temperature range of 71 K and maximum magnetic entropy change of 2.2–3.0 J/(kg K). Based on thermomagnetic measurements, critical behavior was investigated via modified Arrott plots and Kouvel-Fisher method, and mean-field theory model could best explain the samples' critical behavior. The Ni-Mn-Ga/Al composite also possessed enhanced fracture stress of 306 MPa, much higher than that of bulk Ni-Mn-Ga alloy by arc melting. The good magnetocaloric response of the composites, which is comparable or even better than that of many other Ni-Mn-Ga particle and microwire counterparts, is accompanied by improved thermal conductivity and mechanical properties, extending the applicability of magnetocaloric materials with large specific surface area. The novel findings of this systematic study offer new insights into magnetocaloric composites and provide an approach to reach a significant balance when fulfilling multiple requirements of magnetic refrigeration technology. • Hot pressed Ni-Mn-Ga/Al was thoroughly studied in view of refrigeration application. • HP samples had good MCE comparable to their counterparts with reduced length scale. • MAP and KF method were used to compute critical exponents and study critical behavior. • Order of phase transition was ascertained by both Banerjee criterion and exponent n. • Thermal conductivity and mechanical properties were improved after making composites.

Magnetocaloric performance of the three-component Ho1-xErxNi2 (x = 0.25, 0.5, 0.75) Laves phases as composite refrigerants

To date, significant efforts have been put into searching for materials with advanced magnetocaloric properties which show promise as refrigerants and permit realization of efficient cooling. The present study, by an example of Ho1−xErxNi2, develops the concept of magnetocaloric efficiency in the rare-earth Laves-phase compounds. Based on the magneto-thermodynamic properties, their potentiality as components of magnetocaloric composites is illustrated. The determined regularities in the behaviour of the heat capacity, magnetic entropy change, and adiabatic temperature change of the system substantiate reaching high magnetocaloric potentials in a desired temperature range. For the Ho1−xErxNi2 solid solutions, we simulate optimal molar ratios and construct the composites used in magnetic refrigerators performing an Ericsson cycle at low temperatures. The tailored magnetocaloric characteristics are designed and efficient procedures for their manufacturing are developed. Our calculations based on the real empirical data are very promising and open avenue to further experimental studies. Systems showing large magnetocaloric effect (MCE) at low temperatures are of importance due to their potential utilization in refrigeration for gas liquefaction.

High density La-Fe-Si based magnetocaloric composites with excellent properties produced by spark plasma sintering

La(Fe,Si)13 based magnetocaloric composites were prepared by spark plasma sintering (SPS), followed by annealing. The microstructure, mechanical and magnetocaloric properties were investigated. With increasing SPS temperature (TSPS) below 1373 K, the content of (La,Pr)(Fe,Co,Si)13 majority phase increased, while the content of minority phase α-Fe decreased. The increase of TSPS from 973 K to 1373 K greatly improved the composite density, and the density sharply increased from ∼ 68 % to 98 %. After SPS at 1273 K/5 min followed by 1323 K/24 h annealing, high content of 89.35 wt% of the desired (La,Pr)(Fe,Co,Si)13 phase was obtained in LaFe11.6Si1.4/10wt%Pr2Co7 composites. A large increase of Curie temperature (TC) from 196 K to 297 K was observed. The maximum magnetic entropy change (−ΔSM)max reached 2.3 J/(kg‧K) under 2 T magnetic field. ∼ 97 % of full density, excellent compressive strength of ∼ 1GPa and high thermal conductivity of 17.68 W/m∙K were exhibited. Thus, a route to produce high density La-Fe-Si based magnetocaloric composites with excellent mechanical properties and thermal conductivity, good (−ΔSM)max and adjustable TC by short time spark plasma sintering followed by annealing was demonstrated.

One-Step Sintering Process for the Production of Magnetocaloric La(Fe,Si)13-Based Composites

A one-step sintering process was developed to produce magnetocaloric La(Fe,Si)13/Ce-Co composites. The effects of Ce2Co7 content and sintering time on the relevant phase transformations were determined. Following sintering at 1373 K/30 MPa for 1–6 h, the NaZn13-type (La,Ce)(Fe,Co,Si)13 phase formed, the mass fraction of α-Fe phase reduced and the CeFe7-type (La,Ce)(Fe,Co,Si)7 phase appeared. The mass fraction of the (La,Ce)(Fe,Co,Si)7 phase increased, and the α-Fe phase content decreased with increasing Ce2Co7 content. However, the mass fraction of the (La,Ce)(Fe,Co,Si)7 phase reduced with increasing sintering time. The EDS results showed a difference in concentration between Co and Ce at the interphase boundary between the 1:13 phase and the 1:7 phase, indicating that the diffusion mode of Ce is reaction diffusion, while that of Co is the usual vacancy mechanism. Interestingly, almost 100 % single phase (La,Ce)(Fe,Co,Si)13 was obtained by appropriate Ce2Co7 addition. After 6 h sintering at 1373 K, the Ce and Co content in the (La,Ce)(Fe,Co,Si)13 phase increased for larger Ce2Co7 content. Therefore, the Curie temperature increased from 212 K (binder-free sample) to 331 K (15 wt.% Ce2Co7 sample). The maximum magnetic entropy change (−ΔSM)max decreased from 8.8 (binder-free sample) to 6.0 J/kg·K (15 wt.% Ce2Co7 sample) under 5 T field. High values of compressive strength (σbc)max of up to 450 MPa and high thermal conductivity (λ) of up to 7.5 W/m·K were obtained. A feasible route to produce high quality La(Fe,Si)13 based magnetocaloric composites with large MCE, good mechanical properties, attractive thermal conductivity and tunable TC by a one-step sintering process has been demonstrated.

Comprehensive performance of a ball-milled La0.5Pr0.5Fe11.4Si1.6B0.2H y /Al magnetocaloric composite

Due to the hydrogen embrittlement effect, La(Fe,Si)13-based hydrides can only exist in powder form, which limits their practical application. In this work, ductile and thermally conductive Al metal was homogeneously mixed with La0.5Pr0.5Fe11.4Si1.6B0.2 using the ball milling method. Then hydrogenation and compactness shaping of the magnetocaloric composites were performed in one step via a sintering process under high hydrogen pressure. As the Al content reached 9 wt.%, the La0.5Pr0.5Fe11.4Si1.6B0.2H y /Al composite showed the mechanical behavior of a ductile material with a yield strength of ∼ 44 MPa and an ultimate strength of 269 MPa accompanied by a pronounced improvement in thermal conductivity. Due to the ease of formation of Fe–Al–Si phases and the several micron and submicron sizes of the composite particles caused by ball milling process, the magnetic entropy change of the composites was substantially reduced to ∼ 1.2 J/kg⋅K–1.5 J/kg⋅K at 0 T–1.5 T.

LaFe11.6Si1.4/Pr40Co60 magnetocaloric composites for refrigeration near room temperature

The effect of Pr40Co60 addition on the structure, thermal conductivity, magnetic, magnetocaloric and mechanical properties of sintered LaFe11.6Si1.4/Pr40Co60 bulk composites were investigated. The addition of Pr–Co alloy has important effects on these properties due to the diffusion of Pr and Co into the La(Fe,Si)13 phase during sintering. After 6 h of sintering at 1373 K, these composites exhibited a gradual increase in Curie temperature (TC) from 220 K to near room temperature with relatively large maximum magnetic entropy change (–∆SM)max of 2.7–2.9 Jkg–1 K–1 under 0–2 T external magnetic field change. The diffusion of Pr and Co atoms into 1:13 majority phase facilitates 1:13 phase formation, while the Co content of the Pr-Co binder could be used to tune the TC but resulting in the reduced (–∆SM)max in the composites. The composites exhibit high compressive strength (σbc)max of up to 550 MPa and thermal conductivity (λ) of up to 10.64 Wm–1 K–1. Thus, these materials are promising candidates for near room temperature refrigeration.

Improvement in mechanical and magnetocaloric properties of hot-pressed La(Fe,Si)13/La70Co30 composites by grain boundary engineering

Magnetocaloric composites La(Fe,Si)13/La70Co30 were fabricated by hot-pressing (HP) followed by annealing. With increasing La70Co30 binder content up to 16 wt%, the maximum compressive strength (σbc)max improved dramatically from 63 to 352 MPa. However, the maximum magnetic entropy change (–ΔSM)max decreased from 10.0 to 5.6 Jkg–1K−1 because of the presence of non-magnetic La70Co30. After annealing at 1323 K for 4 h the (σbc)max of HP + Annealed composites with up to 12 wt% La70Co30 doubled compared to HP samples. On the other hand, for a La70Co30 content of 16 wt%, the (σbc)max of HP + Annealed composites decreased slightly compared to that of HP samples, due to reduced α-Fe content and much greater 1:13 phase content in HP + Annealed samples. Remarkably, the (–ΔSM)max of HP + Annealed composites improved from 4.8 to 9.2 Jkg–1K−1 (under 2 T), due to 1:13 phase recombination, after annealing at 1323 K for 4 h.

Adiabatic temperature change in ErAl2/metal PIT wires: A practical method for estimating the magnetocaloric response of magnetocaloric composites

We report the adiabatic temperature change in ErAl2 magnetocaloric wires fabricated by a powder-in-tube (PIT) process. The adiabatic temperature change of the PIT wires is found to be determined by not only the volume fraction of ErAl2 core but also the magnitude relationship between the specific heat of the ErAl2 core and the metal sheath. We propose a quantitative analysis method for calculating the temperature and core volume fraction dependence of adiabatic temperature change in the PIT wire, whose formula is applicable to also various magnetocaloric composites, useful to estimate the magnetocaloric response prior to fabrication.

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.

LaFe11Co0.8Si1.2/Al magnetocaloric composites prepared by hot pressing

The microstructure, mechanical properties, thermal transfer performance, and magnetocaloric effect were investigated in LaFe11Co0.8Si1.2/10 wt% Al composites prepared by hot pressing. The Al particles can deform and form a homogenous matrix during compacting in these composites. Pure Al metal reacts slowly with the LaFe11Co0.8Si1.2 compound at temperatures near 873 K. The introduction of Al particles significantly enhances the mechanical and thermal transfer properties, while slightly degrades the magnetocaloric effect. At room temperature, aforementioned composites show maximum entropy change values of 5.1–5.9 J/kg K under a magnetic change of 2 T, high thermal conductivity of 9.9–17.0 W/m K, and high compressive strength of 106–186 MPa.

Simultaneous achievement of enhanced thermal conductivity and large magnetic entropy change in La-Fe-Si-H/Sn composites by optimizing interface contacts and hot pressing parameters

In this paper, we report on a systematic investigation on the microstructure, magnetocaloric effect and thermal transport properties of La-Fe-Si-H/Sn composites prepared by hot pressing. A continuously distributed Sn network is constructed to restore the mechanical integrity of La-Fe-Si-H hydrides. The hot-pressed composites show good magnetocaloric performance and improved thermal conductivity with optimized interface contacts of La-Fe-Si-H/Sn and hot-pressing parameters. A modified Hasselman-Johnson model is adopted in predicting the thermal conductivity of La-Fe-Si-H/Sn composites in terms of reasonable particle size, interfacial contact conditions and porosity. A large magnetic entropy change of 11 J/kg K in 2 T and high thermal conductivity of ∼9 W/mK could be simultaneously achieved in LaFe11.6Si1.4Hx/Sn. This work contributes to the research on high performance magnetocaloric composites with combined outstanding magnetocaloric and thermal transport properties.