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Scripta Materialia 69 (2013) 485–488

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A new approach to prepare sphericalLa–Fe–Si–Co magnetocaloric refrigerant particles

Jian Liu,a,b,⇑ Pengna Zhang,c Fuping Daid and Aru Yana,b

aKey Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering,

CAS, Ningbo 315201, People’s Republic of ChinabZhejiang Province Key Laboratory of Magnetic Materials and Application Technology,

CAS, Ningbo 315201, People’s Republic of ChinacNanjing University of Information Science and Technology, Nanjing 210044, People’s Republic of China

dNorthwestern Polytechnical University, Xi’an 710072, People’s Republic of China

Received 16 April 2013; revised 22 May 2013; accepted 7 June 2013Available online 14 June 2013

The drop-tube solidification technique has been employed as a novel approach to prepare spherical LaFe11Co0.8Si1.2 magnetoc-aloric particles, which have been shown to be highly suitable for active magnetic regenerator systems. The rapidly solidified particleshave an average value of the size distribution of about 300 lm, a perfectly spherical shape and are essentially void free. Only a verybrief annealing at 1373 K for 1 h was required to obtain the La(Fe,Si)13 phase. A large magnetic entropy change of 15 J kg�1 K�1

caused by a field change of 5 T was observed around room temperature for annealed spheres.� 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: La–Fe–Si–Co spheres; Magnetocaloric effect; Drop-tube solidification; Annealing

Magnetic cooling is a novel, energy-efficient andenvironmentally friendly technology that aims to replaceconventional vapor-compression technology for air-con-ditioning, space heating and food refrigeration [1]. It isbased on the magnetocaloric effect (MCE), which occursin magnetic materials, in particular those undergoing amagnetic or magnetostructural phase transition [2].Room-temperature giant magnetocaloric compounds,such as Gd5Si4�xGex, LaFe13�xSixH, MnFeP(As,Ge)and Ni–Mn-based Heusler alloys are the focus of re-search and their development towards practical applica-tions is being pursued [3–7]. Very recently, the technicaldetails of routes to process and upscale magnetic refrig-erants, such as La(Fe,Si)13, have been reviewed [8]. Inprinciple, an active magnetic regenerator (AMR) ma-chine requires thin plates or spherical particles with anoptimal geometry to achieve the best thermal transportproperties from the magnetic refrigerants to the heat-ex-change medium, thus increasing the operating frequency

1359-6462/$ - see front matter � 2013 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2013.06.009

⇑Corresponding author at: Key Laboratory of Magnetic Materialsand Devices, Ningbo Institute of Material Technology and Engi-neering, CAS, Ningbo 315201, People’s Republic of China. Tel.: +860574 86683062; e-mail: liujian@nimte.ac.cn

and the cooling power. La–Fe–Si-based thin plates<1 mm thick, which are most often fabricated by pow-der metallurgical processing, are difficult to shape andmachine due to the intrinsic brittleness of the NaZn13-type phase (namely the 1:13 phase) [9]. Also, due to ef-fect of demagnetization, the significant dependence ofthe MCE on the orientation of thin plates has to be ta-ken into account when designing AMR devices [10]. Theuse of spherical MCE particles is another important ap-proach for improving the heat transfer efficiency of theAMR cycle. The optimum size of spherical particles is200–400 lm in order to obtain a large specific surfacearea, and simultaneously to avoid any drop in pressure[11]. The plasma rotating electrode process (PREP)has been used to produce spherical La(Fe,Co,Si)13 par-ticles 200–1000 lm in size, with an average size of500 lm [12,13]. However, the cost of fabrication ofspherical particles by PREP is high due to the expensiveand sophisticated equipment required. In addition, theLa(Fe,Co,Si)13 spherical particles produced by PREPneed to be homogenized for more than 10 days to obtainthe 1:13 phase [14]. The gas atomization process has alsobeen employed to produce spheres, e.g. Er3Ni, but isprone result in internal porosity especially for particle

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Figure 2. SEM backscattered electron images of spherical LaFe11-

Co0.8Si1.2 particles with diameters of 500 lm (a, b) and 200 lm before(c, d) and after (e, f) 1373 K/1 h annealing. As a comparison, themicrostructures of bulk sample before (g) and after 1373 K/14 daysannealing (h) are also included.

486 J. Liu et al. / Scripta Materialia 69 (2013) 485–488

diameters larger than 100 lm [15]. In this paper, weintroduce the drop-tube rapid solidification techniqueas a novel method suitable for the preparation of La–Fe–Si–Co spheres.

As a containerless and rapid solidification technique,drop-tube processing (DTP) has been widely used tostudy the microstructural evolution in the undercooledmelt and associated physical mechanism for magneto-strictive alloys (e.g. TbFe2) [16] and permanent magnets(e.g. Nd–Fe–B and Sm–Co) [17,18]. Through phaseselection, the peritectic phase such as u-Nd2Fe14B1 canbe directly obtained under a large undercooling statein the nonequilibrium solidification pathway [19].

Master alloys with the nominal composition LaFe11

Co0.8Si1.2 were prepared by arc-melting. About 1 g ofLaFe11Co0.8Si1.2 was put into a quartz tube with a noz-zle of about 0.3 mm in diameter at the bottom, as sche-matically shown in Figure 1a. The quartz tube was thenfixed at the top of a 3 m drop tube. The alloy was induc-tively melted and subsequently dispersed into smalldroplets by jetting with a He–Ar gas mixture. A moredetailed description of DTP can be found in Ref. [19].Microstructural observation by scanning electronmicroscopy (SEM; Hitach S4800) showed that the sizedistribution of the resulting particles is from 100 to500 lm with an average value of about 300 lm. The par-ticles possess regular spherical shapes, and exhibit nointernal porosity or broken surfaces, as seen in Fig-ure 1b. The crystal structure was characterized by X-ray diffraction (XRD; Bruker D8, Co Ka radiation).Magnetization vs. temperature (M–T) and magnetiza-tion vs. field (M–H) curves were carried out using aQuantum Design MPMS SQUID vibrating samplemagnetometer.

Figure 2a–d show SEM images of drop-tube samples500 and 200 lm in diameter. In the case of larger drop-lets (i.e. 500 lm), the phase constitution comprisinga-Fe (black area) and La-rich (white area) phases is very

200µm

quartz tube

droplets

nozzle

Cu coil

He-Ar

LaFe11Co0.8Si1.2 spheres

(a)

(b)

Figure 1. A schematic of drop-tube processing (a), and the resultingLaFe11Co0.8Si1.2 spherical particles observed by SEM secondaryelectron imaging (b).

close to the bulk sample prepared by conventional cast-ing techniques [20], also shown in Figure 2, except forthe refined dendritic morphology of a-Fe for the presentrapidly solidified droplets. We found that for these lar-ger droplets, a prolonged annealing for more than7 days was necessary for homogenization. For smallerparticles of 200 lm in size, the 1:13 phase (grey area inFig. 2d) can be clearly observed, amounting to a volumefraction of about 20%. It is well known that the degreeof undercooling of droplets during free fall significantlyincreases with decreasing droplet size. Although thetemperature of dispersed droplets is difficult to trackin DTP, it is possible to assess the undercooling usingNewtonian heat transfer theory [21]. Due to the lackof thermodynamic data for LaFe11Co0.8Si1.2, the rela-tionship between undercooling and droplet diameterfor a relevant Fe–Si system is used here: for dropletdiameters of 200 and 500 lm, the undercooling was esti-mated to be about 200 and 100 K, respectively [22].According to the ternary phase diagram of La–Fe–Si,the 1:13 phase forms at an undercooling level of at least400 K [23]. This is evidenced by our previous results ofan electromagnetic levitation experiment using LaFe11.6-

Si1.4, where the 1:13 phase was not visible with an und-ercooling of about 200 K [24]. Most probably, due tothe addition of Co expanding the 1:13 region and

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J. Liu et al. / Scripta Materialia 69 (2013) 485–488 487

shortening the distance between the liquid and solidlines [25], we can detect the 1:13 phase for the presentCo-containing droplets upon undercooling of 200 K.Furthermore, as seen from Figure 2e and f, a very shortannealing (1373 K/1 h) was required for homogeniza-tion, and a very small fraction of secondary phaseswas observed, which is attributed to the initial appear-ance of the 1:13 phase and the refined microstructure.Such a short annealing time is comparable to that forLa–Fe–Si ribbons [26]. In contrast, the annealing timeis 14 days to obtain the pure 1:13 phase for a LaFe11-

Co0.8Si1.2 bulk sample.XRD patterns of arc-melted bulk sample, and of

drop-tube processed and annealed spherical particles(�200 lm in diameter) of LaFe11Co0.8Si1.2 are shownin Figure 3. The volume fraction of a-Fe was quantita-tively determined by Rietveld refinement to be about70%, 50% and 5% for the bulk, and particles beforeand after annealing, respectively. This agrees well withabove microstructural observation results.

From the M–T curve, as shown in Figure 4, the Curietemperature (Tc) of annealed 200 lm spherical particlesof LaFe11Co0.8Si1.2 is about 283 K. The negligible mag-netic hysteresis and non-S-shaped M–H curves indicatesecond-order transitions. According to the Maxwellequation, the magnetic entropy change, DS, of15 J kg�1 K�1 for a field change of 5 T at 280 K is al-most double that for Gd, and slightly higher than thatof LaFe11Co0.8Si1.2 melt-spun ribbons (13.5 J kg�1 K�1,5 T at Tc � 290 K) [27]. The difference in Tc betweenspheres and ribbons can be associated with the differentfraction of a-Fe. The higher volume fraction of a-Fe(�10%) in ribbon samples indicates a higher Si contentfor the 1:13 phase, leading to a higher Tc and lowerDS [27]. This is evidenced by comparing the presentmagnetic data for particles and bulk sample. As the La-Fe11Co0.8Si1.2 bulk contains almost no a-Fe (as seen inFig. 2h), a slightly lower Tc (�281 K) and higher DS(16 J kg�1 K�1 for H = 0–5 T) was obtained.

In summary, the drop-tube technique is proposed as afeasible route to prepare La–Fe–Co–Si spherical

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nsity

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.)In

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ity (a

.u.)

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.)

2theta (deg.)

Arc-melted bulk-Fe

40 50 60 70 80

Under-cooled particles d=200µm

40 50 60 70 80

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Figure 3. XRD patterns of LaFe11Co0.8Si1.2 powders ground from thearc-melted bulk and the undercooled particles with diameters of about200 lm before and after annealing.

250 260 270 280 290 300 310 320T (K)

Figure 4. (a) M–T curves for spherical LaFe11Co0.8Si1.2 particles withdiameters of about 200 lm after 1373 K/1 h annealing and bulksample after 1373 K/14 days annealing. (b) M–H curves at differenttemperatures for annealed particles. (c) Magnetic entropy changes ofannealed LaFe11Co0.8Si1.2 particles and bulk, compared with Gd.

particles. The large undercooling and rapid quenchingrate during droplet solidification are suitable for the for-mation of the 1:13 phase and for shortening the anneal-ing time. The resulting particles exhibit a perfectlyspherical shape and are essentially void free. The ob-tained diameter range is highly suitable for AMR sys-tems. A large MCE at room temperature was observed.Although this paper used La–Fe–Co–Si spherical parti-cles the method is generally applicable to other MCEmaterials. However, the drop-tube method has a muchlower yield compared to other conventional particlepreparation processes. This disadvantage might be

488 J. Liu et al. / Scripta Materialia 69 (2013) 485–488

overcome by modifying the crucible and nozzle, e.g.using a larger quartz tube and numerous nozzles. Fur-ther work on advancing DTP for upscaling the produc-tion of MCE materials is underway.

The research leading to these results has re-ceived funding from the Project of Thousand YouthTalent of China, and the Program of the InnovationTeam of Ningbo.

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