Influence of particle size on the mechanical properties and

Journal of Magnetism and Magnetic Materials 463 (2018) 23–27

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Research articles

Influence of particle size on the mechanical properties and magnetocaloric effect of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites

T

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X.C. Zhonga,e, , X.L. Fenga,b, J.H. Huangc, Y.L. Huangd, Z.W. Liua, R.V. Ramanujane,f a

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China Guangzhou Special Pressure Equipment Inspection and Research Institution, Guangzhou 510633, PR China c Baotou Research Institute of Rare Earth, Baotou 014030, PR China d School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China e School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore f Singapore-HUJ Alliance for Research and Enterprise (SHARE), Nanomaterials for Energy and Energy-Water Nexus (NEW), Campus for Research Excellence and Technological Enterprise (CREATE), Singapore 138602, Singapore b

A R T I C LE I N FO

A B S T R A C T

Keywords: Magnetocaloric effect Microparticle Metal matrix composite Compressive strength Mechanical properties

The particle size dependence of the mechanical properties and the magnetocaloric effect (MCE) in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites were studied. The compressive strength (σbc) was in the range of 180–200 MPa for composites with particle sizes less than 180 μm, which is much higher than the compressive strength of larger size powders (136 MPa). When the particles were larger than 45 μm, the observed maximum magnetic entropy change (−ΔSM)max of 7.66–7.99 J/(kg⋅K) shows that surface/interface anisotropy effects have a negligible impact on MCE. The adiabatic temperature change (ΔTad) increased from 1.74 [email protected] T, for particles in the size range of 0–45 μm, to 1.91 [email protected] T for particles in the size range of 45–100 μm. The ΔTad was in the range of ∼2.0 [email protected] T when the particle size increased from 100 to 250 μm. Magnetic hysteresis in these second-order phase transition alloys showed negligible change in the particle size range of 0–250 μm. These results are useful of La(Fe,Si)13-based compounds for magnetocaloric applications.

1. Introduction In recent years, there has been extensive research on magnetic refrigeration technology, since there are several benefits, including energy efficiency and environmental friendliness, compared with the conventional gas compression-expansion technology [1–3]. High-performance magnetocaloric materials are a fundamental part of achieving efficient cooling in magnetic refrigeration technology. Many giant magnetocaloric effect (GMCE) materials, such as Gd5(Ge1−xSix)4 [4], La (Fe,Si)13 [5], MnAs1−xSbx [6], MnFeP0.45As0.55 [7] and Ni-Mn-based Heusler compounds [8], exhibiting a first-order magnetic transition (FOMT) have been developed. Among these cubic NaZn13-type La(Fe,Si)13 compound has many advantages, such as relatively low material cost, non-toxicity of the constituent elements and excellent values of the magnetocaloric effect (MCE) [9,10]. However, besides the issues of the low Curie temperature (TC) of the La(Fe,Si)13 compound, the thermal/magnetic hysteresis and poor mechanical properties urgently need to be resolved. The TC can be elevated by introducing interstitial C or H atoms [11–13]. In addition, the substitution of Co for Fe atoms increases TC [14,15]. But the FOMT

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becomes weak and the maximum magnetic entropy change (−ΔSM)max decreases with the increase of Co content in La(Fe,Co)13−xSix alloys [14,15]. Opposite to the substitution of Co for Fe in La(Fe,Si)13 compounds, the substitution of R (R = Ce, Pr, Nd) for La not only results in a remarkable enhancement in (−ΔSM) but also decreases the TC of compounds [16–18]. The thermal/magnetic hysteresis can be reduced by changing the order of the phase transition to a second order magnetic transition (SOMT) [9,10,16]. Another approach to lower thermal/magnetic hysteresis is realized by reducing particle size. Lower particle size will increase surface area and partially eliminate internal strain and grain boundaries [19,20]. Poor mechanical properties can be attributed to large lattice contraction in the vicinity of TC [5] and low ductility of the NaZn13-type phase [21], which results in volume change and microcracks in the bulk La (Fe,Si)13 alloy. Therefore, much effort has been devoted to the processing method, e.g. by epoxy bonding [22–25], hot pressing (HP) [21,26–29], and powder metallurgy [30–33] to improve mechanical properties. However, the poor thermal conductivity of the epoxy resin is an undesirable factor from the viewpoint of engineering applications [25]. To simultaneously improve thermal conductivity and mechanical

Corresponding author at: School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China. E-mail address: [email protected] (X.C. Zhong).

https://doi.org/10.1016/j.jmmm.2018.05.033 Received 6 April 2018; Received in revised form 11 May 2018; Accepted 11 May 2018 Available online 12 May 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.


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2. Experiments

properties of the composites, metal-bonded magnetocaloric composites with high thermal conductivity metallic binder matrix were studied [26–29]. The results showed that bimodal metal-bonded La(Fe,Mn,Si)13Hx composite plates, with a thickness of 0.3 mm, had a thermal conductivity of 7–7.5 W/(m⋅K) and a flexural strength of greater than 100 MPa (as determined by three-point bending experiments) [28]. Surprisingly, a high compressive strength of 303 MPa and good thermal conductivity of 19.64 W/(m⋅K) were obtained for bulk composites prepared by HP a mixture of 15 wt% of Sn42Bi58 powders with La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 magnetocaloric particles, under a pressure of 1 GPa at ∼410 K for 90 s [29]. However, the starting powders in these reports differ from one another. Hence, it is difficult to compare these processing methods and the dependence of MCE on the order of the transition. Skokov et al. [34] investigated the optimum grain size of the powder used for compaction, the results reveal that particle size greater than 100 μm shows better MCE. Liu et al. [10] also explored the effect of particle size on the magnetic transition, magnetic entropy change and thermal/magnetic hysteresis. The results showed that the magnetic transition was remarkably changed from first to second order by drastically reducing the average particle size from 160 μm to less than 5 μm. Correspondingly, the magnetic entropy change and thermal/magnetic hysteresis were reduced [10]. The dependence of hysteretic behavior and magnetocaloric effect in first order La(Fe,Si)13-based alloy systems on particle size has also been studied by Hu et al. [19] and Huang et al. [20]. The results showed that hysteresis loss and the magnetic entropy changes decreased with lower particle size. For La0.7Ce0.3Fe11.6Si1.4C0.2, the reduced ratio of hysteresis loss could be as high as 61% as the sample was ground from bulk (∼945 μm) to small particles (20–50 μm) [19]. A hysteresis reduction of 70% was attained in La0.5Pr0.5Fe11.4Si1.6 when the bulk sample was ground into fine powder (average size of 2 μm) [20]. In La-Fe-Si-based compounds, the smaller particle size, the lower is the MCE and the hysteresis [10,19,20]. However, the first order magnetostructural transition and large MCE in LaFe11.6Si1.4Hy/Sn composites, with LaFe11.6Si1.4Hy particles in a size range of 50–75 μm, are recovered by hot pressing [26]. It is clear that the effect of particle size on the hysteretic behavior and magnetocaloric effect of La-Fe-Si-based composites is complex. Recently, Pulko et al. [25] reported the epoxybonded La-Fe-Co-Si magnetocaloric plates consisting of La-Fe-Co-Si particles of various size fractions. The results showed that the higher filling factor could be achieved by using a mixture of several particle size fractions, which has beneficial influence on the thermal conductivity. The measured ΔT values in the range of 1.9–2.4 K (Δμ0H = 1.9 T) and low thermal conductivity values in the range of 1.09–2.68 W/(m⋅K) (at TC = 293 K and 0 T) were obtained in four Amerlock epoxy-bonded La-Fe-Co-Si magnetocaloric plates. Radulov et al. [28] reported a thermal conductivity of 7–7.5 W/(m⋅K) and a flexural strength of greater than 100 MPa obtained in bimodal metalbonded La(Fe,Mn,Si)13Hx composite plates using Bi32.5Sn16.5In51 metal binder. However, there are few reports on the influence of particle size on the mechanical and magnetocaloric properties of La-Fe-Si-based composites. Therefore, it is necessary to obtain detailed information on the relationship between the starting powders size and the magnetocaloric effect and mechanical properties. In this paper, based on our previous work [35,36] and the fact that tin has a low melting point (Tm = 505 K) and high thermal conductivity (λ = 66.6 W/(m⋅K)), annealed strip-cast La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 flakes with second order transition character were selected and hot pressed with Sn powder. These flakes were crushed to a size of less than 250 μm and divided into four parts. The particle size dependence of the mechanical properties and the magnetocaloric effect in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites was then studied in detail.

The strip-cast La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 flakes used in the present experiments were systematically investigated previously [35]. The as-cast flakes were annealed at 1323 K for 12 h and found to contain about 93.5 wt% of the NaZn13-type phase, determined by Rietveld refinement. The annealed flakes were then crushed and sieved into < 45, 45–100, 100–180, 180–250 μm sets (denoted as fine, small, middle and large particles, respectively). The Sn powders (> 99.9 wt% purity) are in the size range of 3–5 μm. Based on our previous work [29], bulk composites, composed of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 particles and 10 wt% metal binder (Sn42Bi58) with a low melting point, low porosity of ∼9.21% and relatively high thermal conductivity of 11.79 W/(m⋅K), were prepared. The mass ratio of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 particles (magnetic phase) and Sn powders (non-magnetic binder phase) was selected as 9:1. These powders were mixed and pressed for 2 min by a uniaxial stress of ∼900 MPa in air at 423 K into cylindrical pieces of Φ8 × 4 mm. In-situ XRD of the composite carried out at temperatures up to 423 K showed that no magnetic phase change was induced by hot pressing (data not shown here). The microstructure was investigated by the Hitachi S-3700 scanning electron microscopy (SEM). Compression tests were performed at ambient temperature using a Universal Materials Testing machine (SHMADZU AG-100NX). The adiabatic temperature changes of these composites were directly measured in a home-built instrument. Magnetic measurements were carried out using a Quantum Design PPMS-9 with a vibrating sample magnetometer attachment. The isothermal magnetic entropy change (−ΔSM) was calculated using an indirect method based on the thermodynamic Maxwell relations. 3. Results and discussion Fig. 1 shows the backscattered images (BSE) of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites for different average particle sizes. The dominant grey regions are the 1:13 phase with NaZn13-type structure, the black clusters embedded in the grey matrix are the α-Fe phase, and the white areas around the particles are Sn powder. Fig. 1(a1) and Fig. 1(b1) show that these fine (< 45 μm) and small (45–100 μm) particles were crushed by the pressure of 900 MPa, while the Sn powders are evenly distributed around the fragmented particles. The corresponding cross-sectional images (Fig. 1(a2) and Fig. 1(b2)) show a loose state, resulting from the fracture of fine and small particles. The composites with middle-size (100–180 μm) and large size (> 180 μm) particles are shown in Fig. 1(c1–c2) and (d1–d2). The surfaces of the particles were damaged, however, the interior is intact, but densification has taken place. When the particle size is larger than 180 μm (large particles) (Fig. 1(d1–d2)), the edge of the particles are broken and stripped, which results in the apparent density reduction. Apparently, the larger the particle size, the more compacting of the composites. Seen from the apparent density of the composites with different particles listed in Table 1, the change tendency of the apparent density increases firstly and then decrease, which is in good agreement with the images of the composites with different particles. Compressive stress-strain curves of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/ Sn composites are shown in Fig. 2. For composites with particle size less than 180 μm, designated as either fine, small or middle, the values of maximum compressive strength (σbc)max are in the range of 180–200 MPa, which have the similar level to that in Sn42Bi58-bonded La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 bulk composites with fine particles (< 45 μm) [29], but the values are higher than those of polymerbonded LaFe11.7Si1.3C0.2H1.8 (162 MPa) [22] and LaFe11.6Si1.4Hy/Sn (170 MPa) [26] composites. However, the (σbc)max of composites with large particles (180–250 μm) is 136 MPa. This lower (σbc)max for the composites with large particles can be ascribed to the impacts on the increased density of pores between particles and the edge of the particles are broken and stripped. On one hand, the cracks existing in the 24


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Fig. 1. BSE images of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites with different particle sizes.

phenomenon of the edge of the particles broken and stripped (Fig. 1) occur, therefore, the highest compressive strength is attained. Fig. 3 shows the temperature dependence of the magnetic entropy change (−ΔSM) of the composites, the inset shows the maximum magnetic entropy change (−ΔSM)max for different particle sizes. It is evident that the values of (−ΔSM)max for the composites increase almost with increasing average particle size. The composites with particle size smaller than 45 μm displays a smaller MCE of ∼6.38 J/(kg·K), compared to the other samples (7.66–7.99 J/(kg·K)), which may be due to the greater importance of the surface/interface anisotropy effect in

edge of the particles increase the possibility of rupture; On the other hand, the density, especially relatively large pores, increases gradually with particle size. The mass fraction of the non-magnetic binder phase (Sn powders) is unchanged, which makes stress transmission more difficult for composites with large particles, with a greater impact on the mechanical properties. All samples exhibit strain of ∼7.5% (Fig. 2), due to the densification process. The mechanical properties of the composite are not sensitive to the particle, unless the particle size is larger than 180 μm. Obviously, the composites with middle particles have the highest apparent density (listed in Table 1), but no 25


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Table 1 Apparent density (ρ), compressive strength (σbc), adiabatic temperature change (ΔTad), isothermal magnetic entropy change (−ΔSM), and hysteresis loss (W) of the composites with different particle sizes. Composites (μm)

ρ (g/cm3)

σbc (MPa)

ΔTad (K)

−ΔSMmax (J/kg K)

W (J/ kg)∼ 247 K

Fine (< 45 μm) Small (45–100 μm) Medium (100–180 μm) Large (180–250 μm)

6.361 6.824 6.844

193 181 198

1.74 1.91 2.01

6.38 7.66 7.99

4.32 4.30 4.40

6.803

136

2.05

7.96

3.82

Fig. 4. ΔTad−T curves of the composites with different particle sizes, measured at a maximum applied field of 1.4 T.

volume than that of Sn42Bi58 alloy with high density of 9.38 g/cm3, which results in more compact structure and larger values of (−ΔSM)max in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn bulk composites. Nevertheless, the values of (−ΔSM)max in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn bulk composites are lower than those of LaFe11.2Co0.7Si1.1 (∼9.5 J/(kg⋅K) at TC = 264 K, Δμ0H = 2 T) [18] and LaFe11.35Co0.6Si1.05 (∼13.5 J/(kg⋅K) at TC = 268 K, Δμ0H = 2 T) [38] bulk alloys. Besides the magnetic entropy change (−ΔSM), the adiabatic temperature change (ΔTad) is also a useful performance metric. Fig. 4 shows the temperature dependence of the adiabatic temperature change (ΔTad) for composites with different particle sizes. The ΔTad−T curves display the same trend as the (−ΔSM)−T curves shown in Fig. 3. A maximum ΔTad of 2.05 K was obtained for particles in size range of 180–250 μm. As the particles size decreases, (ΔTad)max also decreases. Thus, fine particles with size less than 45 μm exhibit a maximum ΔTad of 1.74 K, a reduction of about 15% in ΔTad compared to the value for large particles (180–250 μm). Actually, the trend of the (−ΔSM)max and (ΔTad)max for the composites is in good agreement with that of the apparent density of the composites with different particles. This (ΔTad)max per unit magnetic field of these samples are comparable to or even larger than those of magnetocaloric materials with TC at the similar temperature region, such as the bulk composite composed of LP La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 and 15 wt% Sn42Bi58 alloy binder (2.05 K under 1.4 T) [29] and LaFe14.3Co0.8Si1.2 bulk alloy (2.0 K under 1.5 T) [39]. The compressive strength (σbc), adiabatic temperature change (ΔTad), isothermal magnetic entropy change (−ΔSM), and hysteresis loss (W) of the composites with different particle sizes are listed in Table 1. The particle size dependence of hysteresis loss has been previously discussed [19,20]. Hysteresis loss reduces with smaller particle size in a certain size range. This approach could be effective in reducing hysteresis loss in La(Fe,Si)13. The hysteresis losses (Table 1), measured from closed M-H curve at 247 K (near the TC), indicate that hysteresis loss stabilized at 3.82–4.40 J/kg over a wide particle size range of 0–250 μm. These small hysteresis losses suggest a second order phase transition, in accordance with the result obtained from Arrott plots (not shown here). The present results suggest that the effect of particle size on hysteresis loss is lower in high magnetocaloric performance large particles.

Fig. 2. Stress-strain curves of the composites with different particle sizes.

Fig. 3. (−ΔSM)−T curves of the composites with different particle sizes under a maximum applied field of 2 T. The inset shows the (−ΔSM)max for the corresponding composites.

fine particles [37]. Surface/interface anisotropy will lead to a large pinning force, which prevents magnetization near the interface or at the interface from easily rotating towards the direction of the external magnetic field [37]. Such particle size dependence of (−ΔSM)max is useful to the optimize the particle size. Obviously, the (−ΔSM)max values of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn bulk composites are larger than those of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn42Bi58 bulk composites with 10 wt% alloy binder at the same TC (5.96 J/(kg·K)) [29]. This is ascribed to the fact that the same mass of Sn (7.28 g/cm3) has larger

4. Conclusions La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn bulk composites were prepared by grinding of strip-cast La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 flakes, followed 26


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by low-temperature hot pressing along with Sn powder. The effect of particle size on the mechanical properties and magnetocaloric effect were investigated. When the particle size is in the range of 0–180 μm, the compressive strength (σbc) of the composite was in the range of 180–200 MPa. However, a further increase of particle size to 180–250 μm, decreases the strength by ∼30% to 136 MPa. (−ΔSM)max of 6.38 J/(kg⋅K) was obtained in the composite for particle size less than 45 μm, which is much lower compared with the composite prepared from larger particles. The (−ΔSM)max of 7.66–7.99 J/(kg⋅K) reveals that the influence of surface/interface anisotropy effect has little impact on MCE when the particles are larger than 45 μm. The adiabatic temperature change (ΔTad) shows a considerable increase at a particle size of 45 μm. However, this increase reduced rapidly with increasing particle size. The magnetic hysteresis shows a relatively small change for particle sizes in the range of 0–250 μm. Overall consideration for selecting the particle size, a size distribution of 100–180 μm were used which are in the suggested range for good mechanical properties and magnetocaloric performance. These results will be useful to the select the optimum particle size for the practical applications of La(Fe,Si)13based compounds.

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Acknowledgements This work was partly supported by the National key Research and Development Project of China (Materials genome project) (Grant No. 2017YFB0702703), the National Natural Science Foundation of China (Grant Nos. 51261001, 51564037), the Natural Science Foundation of Guangdong Province (Grant No. 2017A030313317) and the Fundamental Research Fund for the Central Universities (Grant No. x2cl/D2170630). This Research is also conducted by Singapore-HUJ Alliance for Research and Enterprise (SHARE), Nanomaterials for Energy and Energy-Water Nexus (NEW), Campus for Research Excellence and Technological Enterprise (CREATE), Singapore 138602. Dr. Zhong Xichun also thanks the China Scholarship Council ([2017] 3059, No.201706155021) for financial support to visit Nanyang Technological University, Singapore. References [1] C. Zimm, A. Jastrab, A. Sternberg, V. Pecharsky, K. Gschneidner Jr., M. Osborne, I. Anderson, Description and performance of a near-room temperature magnetic refrigerator, Adv. Cryog. Eng. 43 (1998) 1759–1766. [2] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsokol, Recent developments in magnetocaloric materials, Rep. Prog. Phys. 68 (2005) 1479–1539. [3] O. Gutfleisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater. 23 (7) (2011) 821–842. [4] V.K. Pecharsky, K.A. Gschneidner Jr., Giant magnetocaloric effect in Gd5(Si2Ge2), Phys. Rev. Lett. 78 (1997) 4494–4497. [5] F.X. Hu, B.G. Shen, J.R. Sun, Z.H. Cheng, G.H. Rao, X.X. Zhang, Influence of negative lattice expansion and metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6, Appl. Phys. Lett. 78 (23) (2001) 3675–3677. [6] H. Wada, Y. Tanabe, Giant magnetocaloric effect of MnAs1−xSbx, Appl. Phys. Lett. 79 (20) (2001) 3302–3304. [7] O. Tegus, E. Brück, K.H.J. Buschow, F.R. de Boer, Transition-metal-based magnetic refrigerants for room temperature applications, Nature 415 (2002) 150–152. [8] S. Esakki Muthu, M. Kanagaraj, S. Singh, P.U. Sastry, G. Ravikumar, N.V. Rama Rao, M. Manivel Raja, S. Arumugam, Hydrostatic pressure effects on martensitic transition, magnetic and magnetocaloric effect in Si doped Ni-Mn-Sn Heusler alloys, J. Alloys Compd. 584 (2014) 175–179. [9] B.G. Shen, J.R. Sun, F.X. Hu, H.W. Zhang, Z.H. Cheng, Recent progress in exploring magnetocaloric materials, Adv. Mater. 21 (2009) 4545–4564. [10] J. Liu, J.D. Moore, K.P. Skokov, M. Krautz, K. Löwe, A. Barcza, M. Katter, O. Gutfleisch, Exploring La(Fe, Si)13-based magnetic refrigerants towards application, Scr. Mater. 67 (2012) 584–589. [11] J. Chen, H.W. Zhang, L.G. Zhang, Q.Y. Dong, R.W. Wang, Magnetic entropy change and magnetic phase transition of LaFe11.4Al1.6Cx (x=0-0.8) compounds, Chin. Phys. 15 (4) (2006) 845–849. [12] Y.F. Chen, F. Wang, B.G. Shen, F.X. Hu, Z.H. Chen, G.J. Wang, J.R. Sun, Large magnetic entropy change near room temperature in the LaFe11.5Si1.5H1.3 interstitial

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