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

Abstract

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.

Article Highlights

La-Fe-Si magnetocaloric composite with an anisotropic microstructure was successfully fabricated via hot pressing in a cubic-anvil-type pressure apparatus.

Deformation of the reinforcing phase and anistropic microstructure in composite are well-related to the effectively tailored stress distribution, which is achieved by tuning particles size of 1:13 phase.

This study unveils the potential to tailor microstructure by utilizing physical property (rheology) of reinforcing phase and changes of external field change (such as pressure).

Introduction

The emerging magnetic cooling technology has shown significant potential as an energy-efficient alternative to the conventional vapor-compression technology [1, 2]. Ascribed to giant magnetocaloric effect, Gd₅Si_4-xGe_x, LaFe_13-xSi_x(H), MnFeP(As,Ge) and Ni-Mn-based Heusler alloys are widely used for the applications of room temperature magnetic refrigeration [3–6]. Based on the magnetocaloric effect (MCE), magnetocaloric materials convert energy from magnetic work (field change) to thermal energy (temperature change). In order to achieve large-scale commercial implement of magnetic refrigerator, the requirements have been proposed regarding both inherent properties of magnetocaloric materials and designs of prototype system as well [7].

An active magnetic regenerator (AMR) cycle has been utilized to exploit MCE towards a wider temperature span in magnetic refrigerator prototypes [8–10]. In an AMR magnetic refrigeration system, the solid refrigerant is subjected to a magnetic field. As the refrigerant is magnetized isothermally, the heat is released from refrigerant and transfers to heat exchange fluid, which is known as the hot side [11]. During demagnetization process, refrigerant absorbs heat through the heat exchange fluid on the cool side. The pumped heat exchange fluid circulates through the heat exchanger and serves as the intermediate medium for heat transfer. As a result, there is a need for rapid and efficient heat exchange between the solid refrigerant and the pumped heat exchange fluid, which requires anisotropic thermal transfer perpendicular to heat fluid. Accordingly, the efficiency of the AMR-type refrigerator depends on the shape of the magnetic refrigerants as well as their magnitudes of MCEs and thermal conductivity. This issue triggered the research into the designs of refrigerant shape, pursuing higher heat transfer efficiency and energy convention in magnetic refrigerators [12]. Magnetic refrigerants have been shaped into forms with a larger specific heat transfer area, such as micro-sized spherical particles, thin plates or wires [13–15]. This allows for more efficient heat exchange and higher working efficiency in refrigerator prototype [16, 17]. Another idea is to enhance heat transfer in directions perpendicular to the fluid by introducing a reinforcing phase and constructing an anisotropic structure [18, 19].

Reinforcing phases were introduced to fabricate magnetocaloric composites to promote practical applications in recent years, which aims to bypass some inherent disadvantages (poor thermal and mechanical properties) of magnetocaloric alloys [20–25]. Such multi-component framework provides the potential to create anisotropic structures in composites. In our previous research, we explored how the shape of the reinforcing phase influences the thermal conductivity of La(Fe, Si)₁₃ composite using Nielsen model [26]. Weise et al. observed texture in the polymer-bonded La(Fe, Co, Si)₁₃ composite prepared by conventional cold-pressing method [18]. Through numerical simulation of particle arrangements, reorientation of particles along the axis leads to a significant increase of thermal conductivity in comparison to the natural plane-anisotropy [18].

When choosing a reinforcing phase, not only physical characteristics (incl. magnetic, thermal and strength properties) during the magnetic phase transition process, but also performances of the reinforcing phase during the preparation process should be taken into consideration [20]. During the hot pressing process, the temperature was typically raised above the melting point of the reinforcing phase, which may lead to hydrogen evaporation and new phases formation [27–29]. Proper fluidity of the reinforcing phase is essential to protect the integrity of the main phase and act as a buffer during hot pressing [30].

In this study, we have successfully developed a La-Fe-Si magnetocaloric composite with anisotropic microstructure through hot pressing in a cubic-anvil-type pressure apparatus, a widely used industrial processing method. High entropy alloy CrMnFeNi was selected as reinforcing phase, known for its superior excellent malleability and mechanical strength [31]. The influence of particle size on deformation of reinforcing phase was studied here, by utilizing rheology of the reinforcing phase. The study uncovered the anisotropic physical response to changes in the external field, such as mechanical properties. The magnetic and magnetocaloric properties of the composites were investigated, by taking into account particle size and element diffusion during the hot pressing. This work unveils possibility to tailor microstructure and fabricate anisotropic magnetocaloric composite through introducing a reinforcing phase in experiments.

Experimental

LaFe_11.6Si_1.4 were prepared by induction melting in an argon atmosphere using rare materials La (99.9 wt.%), Fe (99.9 wt.%) and Si (99.99 wt.%). The ingots were then sealed into quartz ampoules in vacuum and annealed at 1100 °C for 7 days before being quenched into iced water. Coarse La-Fe-Si powders with sizes of 54–75 μm and fine powders smaller than 54 μm were selected for comparison. CrMnFeNi powders with a size of 54–100 μm were milled from ribbon and introduced as reinforcing phase. The weight ratio of La-Fe-Si to reinforcing phase was maintained at 4:1 for the hot pressing process. The homogeneously mixed powders were pre-pressed into cylindrical pellets with a diameter of 10 mm at room temperature. The pellet was put into graphite oven, with MgO filling in the gaps. The graphite tube oven, containing the cylindrical sample, was then inserted into a cubic pyrophyllite. The configuration of the assembled cubic pyrophyllite is illustrated in Fig. 1a and b, showing both a normal section and an inside view of the setup, separately. With the load of oil pressure on the pyrophyllite, the samples was pressed at 1050 °C under a pressure of 4 GPa for 15 min. The schematic drawing of the cubic-anvil-type high-pressure apparatus and the pressure cell can be easily found everywhere [32].

Fig. 1 [Images not available. See PDF.]

Sketch diagram of a a normal section and b an insight view of the assembled cubic pyrophyllite

The microstructure was observed using a QUANTA FEG 250 Scanning Electron Microscope (SEM). The phases composition was examined by Energy Dispersive Spectrometer (EDS) equipped in SEM. Measurements of mechanical properties were carried out in a universal testing machine. The specimens with size of 3 × 3 × 6 mm³ for mechanical measurements were obtained by wire cutting. The magnetic properties were investigated using a Quantum Design Physical Property Measurement System (PPMS) equipped with a Vibrating Sample Magnetometer (VSM) module.

Results and discussion

Horizontal and longitudinal section of cylindrical composite with fine La-Fe-Si powders (< 54 μm) are displayed in Fig. 2a and b, respectively. The homogeneous pressure distribution on the horizontal section have been previously reported [32]. It is evident from the images that the reinforcing phase (dark area) is well connected with La-Fe-Si particles (gray area), indicating a good wettability between two phases during hot pressing. From SEM observation, the secondary phase is determined to account for 24.6% on horizontal section, slightly higher than that on longitudinal section (22.9%). Such obscure discrepancy from observation of the SEM images hints there is not an obvious anisotropic microstructure in the composite. With increasing 1:13 particle size, horizontal and longitudinal sections of the magnetocaloric composite are respectively shown in Fig. 2c and d for comparison. The oblate shape of reinforced phase can be observed in Fig. 2c, with the area expanded to 40.3%. In contrast, the reinforced phase appears in a long bar shape and only accounts for 17.6% (Fig. 2d). This microstructural comparison suggests that the reinforcing phase has elongated along the radial direction, as visually depicted in Fig. 4a. The shape deformation and anisotropic distribution of reinforcing phase can be ascribed to higher pressure along the axial direction. The filled MgO adjunct to the cylindrical sample with higher modulus than that of pyrophyllite can more effectively transfer the pressure, which causes more effective pressure along the axial direction of sample.

Fig. 2. [Images not available. See PDF.]

Horizontal a and longitudinal b section for composite with fine La-Fe-Si powders. Horizontal c and longitudinal d section for composite with coarse La-Fe-Si powders

The anisotropic microstructure with reinforcing phase deformation has been rarely reported in La-Fe-Si composites prepared by hot pressing. The inherent characteristic of high tenacity for reinforcing phase at high temperature can be one important reason [20]. The dispersed ductile phase with low elasticity modulus can consume some energy during plastic deformation and deform easily during the hot pressing. La-Fe-Si phase in major proportion takes the main responsibility of stress transfer in the composites during hot pressing, which can be ascribed to continuous distribution and inherent brittleness as well. The coarse La-Fe-Si particles with larger size can transfer the load more effectively, with fewer energy consumption on less interface area. Thus, the anisotropic pressure was transferred by coarse La-Fe-Si phase to the reinforcing phase, which contributed to the deformation of reinforcing phase and such anisotropic structure.

The phase composition detected by EDS is displayed in Fig. 3. A small amount of Mn (1.2 wt.%) was detected in 1:13 phase (Fig. 3a) for the composite with fine La-Fe-Si particle, which diffused from the reinforcing phase during hot pressing. Due to the lack of data available on the diffusion coefficient of atoms in high-entropy alloy into 1:13 phase, we assess the diffusion velocity by considering the diffusion activation energy of Cr, Mn and Ni into γ-Fe. Mn is more easily to diffuse, due to lower diffusion activation energy into γ-Fe (276 kJ/mol) than that of Ni (282.5 kJ/mol) and Cr (335 kJ/mol). As the size of La-Fe-Si particles increases, there is a noticeable absence of Mn content in the La-Fe-Si phase, indicating a densification process without diffusion (see Fig. 3c). The contact surface area between coarse La-Fe-Si particles and reinforcing phases was reduced, which inhibits the diffusion of Mn element. The diffusion of Si element from fine 1:13 particles (Fig. 3b) into the reinforcing phases is more pronounced compared to that from coarse 1:13 particles (Fig. 3d), which can also be ascribed to the lager interface area. The Si diffusion results in a decrease of Si content in the 1:13 phase, typically leading to a lower Curie temperature.

Fig. 3. [Images not available. See PDF.]

Element contents of 1:13 (a, c) and reinforcing (b, d) phases in composites containing fine and coarse La-Fe-Si particles

Compressive strength in both the axial and radial directions was analyzed on specimens measuring 3 × 3 × 6 mm³, as illustrated in case 1 and case 2 in Fig. 4a, respectively. During compressive test, the pressure was applied along the long axis direction of cuboid, as shown by arrows. Three specimens were utilized for measurements in each case, with the stress–strain curves depicted in Fig. 4b. Higher strain (~ 3.5%) and strength (~ 350 MPa) can be achieved when measuring along the axial direction (case 1). In case 2, strain and strength along the radial direction were reduced to approximately 2.2% and 150 MPa, respectively. The improved mechanical properties observed in case 1 are likely attributed to a more densified structure that is achieved through higher effective pressure along the axial direction during the hot pressing process. The sudden failure observed in stress–strain curves of composites demonstrates a clear indication of brittle fracture. The cleavage surface of La-Fe-Si particles in the fracture morphology shown in the inset of Fig. 4b confirms the significant role of La-Fe-Si in determining the fracture mode and pressure transfer. Furthermore, cracks on the interface tend to easily initiate and propagate. During compressive measurement, there is a larger interface area along the pressure in case 2 (as indicated by arrows in Fig. 4a), which could also contribute to the failure of composite.

Fig. 4. [Images not available. See PDF.]

a Illustration of specimens in cylinder composite (gray) for mechanical measurements along axial (blue, case 1) and radial (red, case 2) directions, with deformed reinforcing phase (yellow) inside. b Stress–strain curves characterized in both cases. The inset in (b) is the fracture surface of sample

The thermal magnetization (MT) curves of the composites at the magnetic field of T are displayed in Fig. 5. In comparison to the La-Fe-Si block pressed from pure coarse particle, there is a descend (~ 5 K) of Curie temperature in composite containing coarse 1:13 phase particle. The slight Si element diffusion from 1:13 phase should be the main reason for the Curie temperature decreasing. The hysteresis observed in the composite can be attributed to the internal pressure generated during the hot pressing process. For composite containing fine 1:13 phase powder, the curie temperature descend is elevated to approximately 50 K. More obvious Si content descend and Mn appearance can be predominant reason. In the field magnetization (MH) curve of pure coarse La-Fe-Si powders (Fig. 5b), the immediately initial abrupt magnetization at high temperatures indicates the existence of α-Fe, which precipitated from 1:13 phase during hot pressing. With the introduction of reinforcing phase, there is also precipitated α-Fe indicated by initial magnetization. For the composites with fine La-Fe-Si powders (Fig. 5d), the more obvious initial magnetization can result from severer decomposition of fine 1:13 phase during hot pressing.

Fig. 5. [Images not available. See PDF.]

a MT curves of hot pressed pure La-Fe-Si alloy, composite containing coarse and fine La-Fe-Si powders. MH curves for these samples are correspondingly displayed in (b, c and d)

To study the magnetocaloric effect of the composites, the magnetic entropy change (ΔS) was calculated by the Maxwell equation and shown in Fig. 6. Compared with hot-pressed pure La-Fe-Si alloy (ΔS≈12 J/kg K) in Fig. 6a, composite containing coarse La-Fe-Si powder shows an obvious decrease of ΔS (≈ 7.8 J/kg K) (Fig. 6b). The dilution of reinforcing phase (25 wt.%) should take the main responsibility for the MCE degradation. For the composite with fine 1:13 particle, there is more significant decline of ΔS (Fig. 6c). Such severe MCE degradation can be ascribed to Mn existence and size effect of fine 1:13 powder as well [33, 34].

Fig. 6. [Images not available. See PDF.]

Magnetic entropy change of hot pressed La-Fe-Si alloy with pure coarse powders (a), composite containing coarse (b) and fine (c) La-Fe-Si powders

Conclusion

The microstructue, magnetic and magnetocaloric properties of La-Fe-Si composites have been systematically investigated. With increasing the size of La-Fe-Si particles, anistropic pressure can be transferred during La-Fe-Si phase more effectively, which results in the deforming of reinforcing phase. Such anistropic microstructure is also well-related to the rheological property of reinforcing phase. Large particle sizes are crucial in effective stress transfer under hot pressing, as well as impeding of element diffusion between the two phases. The magnetic entropy change in the composite with coarse La-Fe-Si particles was found to be higher than that with fine La-Fe-Si powders. This work provides ideas to fabricate magnetocaloric composite with anisotropic microstructure by utilizing pressure, which is achieved in the process of hot pressing.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 52001167, No. 52371189), Jiangsu Provincial Natural Science Foundation of China (Grant No. BK20200477) and Postdoctoral Research Foundation of China (2022M721617).

Author contributions

Yanyan Shao: draft manuscript preparation, review and editing; Siyu Cheng: investigation, data collection, analysis and interpretation of results; Pengwei Guo: samples preparing and data collection; Feng Xu: review and editing.

Data availability

Data is provided within the manuscript or supplementary information files.

Declarations

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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