Novel microstructure and large magnetocaloric effect in La2Fe11Si2 magnetic refrigerant

Materials Letters 134 (2014) 87–90

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Novel microstructure and large magnetocaloric effect in La2Fe11Si2 magnetic refrigerant Mingxiao Zhang a,b, Jian Liu a,b,n, Chun He a,b, Aru Yan a,b a Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

art ic l e i nf o

a b s t r a c t

Article history: Received 24 April 2014 Accepted 8 July 2014 Available online 15 July 2014

A study of the microstructure and magnetocaloric effect in bulk La2Fe11Si2, as an off-stoichiometric NaZn13-type La(Fe,Si)13 composition, was carried out. The microstructure and phase constitution strongly rely upon heat treatment processing, especially on annealing temperature. Upon annealing at 1423 K, a unique morphology containing granular-like La(Fe,Si)13 grains with a high fraction of
80 vol. %, surrounded by Cr5B3-type La5Si3 phase was observed. A large entropy change of 14.5 J/kg K in 2 T was achieved in this bulk sample after annealing for a relatively short time (e.g. 24 h). In addition, the bulk sample also exhibits pronounced first-order magnetic transition behavior with zero thermal and magnetic hysteresis. The syntheses of bulk La2Fe11Si2 alloy with good magnetocaloric performance by simplified preparation process are encouraging for future development of LaFe-based magnetocaloric alloys as a magnetic refrigerant for magnetic cooling system applications. & 2014 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Magnetocaloric Microstructure Magnetic entropy change Hysteresis

1. Introduction Based on magnetocaloric effect (MCE), magnetic refrigeration as a novel, energy-efficient and environmentally friendly technology aims to replace conventional vapor/compression technology for airconditioning, space heating and refrigeration of food. Among these reported magnetic refrigerants [1,2], NaZn13-type La(Fe,Si)13 alloys exhibit a large entropy change (ΔS) and adiabatic temperature change in moderate magnetic fields [3–5], which is associated with an itinerant-electron metamagnetic (IEM) transition, i.e., the fieldinduced first-order transition from paramagnetic to ferromagnetic state [6]. Besides, La(Fe,Si)13-based alloys are of low cost, nontoxicity, and easiness of tuning Curie temperatures (TC) [7–9]. Thus, La(Fe,Si)13-based alloys are considered to be one of the most promising material candidates as magnetic refrigerants over a broad temperature range. In LaFe13 xSix compounds, an evolution of the magnetostructural transition from first-order to second-order can take place with increasing Si content. As a result, the entropy change decreases rapidly when xZ1.6 [5]. Therefore, it is necessary to decrease the Si content in La(Fe, Si)13 compounds for a large MCE.

However, the NaZn13-type single phase (i.e. the 1:13 phase) is difficult to form for these alloys with low Si content. To obtain a single 1:13 phase, the as-cast ingots have to be subjected to a homogenization treatment around 1300 K for one or more weeks followed by quenching [10,11]. Alternatively, Liu et al. [12] reported that a homogenous microstructure and large MCE can be obtained by annealing LaFe11.6Si1.4 bulks at elevated temperature at 1573 K only for 1 h. Unfortunately, such a process always brings about the giant thermal and magnetic hysteresis [13,14]. Compared to traditional preparing method, a melt spinning technique has been proposed to produce La(Fe,Si)13 ribbon materials by a short time (e.g. a few hours) heat treatment [15]. Although melt spinning considerably simplifies the annealing process, it has difficulties for mass production and also ribbon samples cannot be directly used in refrigerator machines. In this study, we report a new composition of La2Fe11Si2. This La2Fe11Si2 alloy exhibits a pronounced first-order magnetic transition without hysteresis and large ΔS by reducing long-time annealing.

2. Experiments n Corresponding author at: Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. Tel.: þ 86 574 86683062; fax: þ86 574 87911392. E-mail address: [email protected] (J. Liu).

http://dx.doi.org/10.1016/j.matlet.2014.07.060 0167-577X/& 2014 Elsevier B.V. All rights reserved.

Alloy ingot with a nominal composition La2Fe11Si2 was prepared by arc melting the mixtures of pure La (99.9 mass %), Fe (99.99 mass %) and Si (99.99 mass %) under a purified argon atmosphere. Each ingot was remelted at least for four times to

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1:13 1:1:1

5:3

Fe 20 m

20 m

5:3

Fe

1:13

1:13

5:3

100 m

100 m

Fig.1. SEM images in BSE mode for La2Fe11Si2 alloys subjected by different annealing processes: (a) as-arc melted, (b) 1323 K for 24 h, (c) 1423 K for 24 h and (d) 1523 K for 12 h.

ensure its chemical homogeneity. The as-cast samples were placed in corundum crucible and sealed in a quartz tube evacuated up to about 10  4 Pa. The samples were subsequently annealed at 1323, 1423 and 1523 K for 12 or 24 h, followed by quenching into ice water. The microstructure was observed by scanning electron microscopy (SEM, FEI QUANTA 250 FEG) with an Energy Dispersive Spectrometer (EDS) in backscattered electron mode. The chemical composition analysis was carried out using EDS and Electron Microprobe Analyses (EMPA, JEOL JXA-8100). The crystal structure was characterized by X-ray diffraction (XRD, Bruker D8, Cu Kα radiation). Magnetization vs. temperature (M–T) and magnetization vs. field (M–H) curves were carried out using a Quantum Design MPMS SQUID vibrating sample magnetometer.

3. Results and discussion The as-cast La2Fe11Si2 sample shows the dendritic morphology, as shown in Fig. 1a. The EDS results show that the black area represents the α-Fe phase, and the interdendritic area consists of LaFeSi (i.e. the 1:1:1 phase, gray area) and La5Si3 (i.e. the 5:3 phase, white area) phases. It should be noted that the La5Si3 phase was rarely found in stoichiometric La–Fe–Si alloys, meaning that Larich compositions favor the stability of the La5Si3 phase. This agrees with the La–Fe–Si equilibrium phase diagram [16]. After annealing at 1323 K for 24 h, the amount of the α-Fe phase significantly decreases and the La(Fe,Si)13 matrix phase (the 1:13 phase) appears, as shown in Fig. 1b. At the same time, 5:3 and 1:1:1 phases were still maintained. Upon a higher temperature annealing at 1423 K for 24 h, we observed a novel microstructure that the large amount of granular-like 1:13 phases were mainly surrounded by the La5Si3 phase, whereas only a little fraction of

the α-Fe phase was presented (Fig. 1c). In addition, a small amount of La oxides was detected within the area of the La5Si3 phase, which might be due to the high temperature annealing and/or the surface polishing [17]. For this sample, the EMPA result shows that the composition of the 1:13 phase is La7.5Fe80.59Si11.91 (correspondingly La1.05Fe11.33Si1.67) in atomic percent. The presence of Si-rich La5Si3 phase results in the matrix being depleted of Si. At even higher temperature annealing at 1523 K for 12 h, the coarsening of α-Fe particles became pronounced (Fig.1d), being similar to the case of LaFe11.8Si1.2 [11]. It is worth mentioning that the formation of 1:13 grains in the present La-rich samples is rather rapid compared with LaFe11.6Si1.4 bulk. By even shorter time annealing for 12 h at 1423 K, the nearly same morphology was observed (not shown here). This indicates that the atomic diffusion rate in the 1:13 phase for La-rich alloys is much faster than that for stoichiometric La–Fe–Si alloys. We believe that the present high fraction of La5Si3 phase in the initial as-cast state plays an important role to drastically accelerate the reaction process to form the 1:13 phase. On the other hand, the high-temperature annealing condition does assist to enhance the atomic diffusivity in the 1:13 phase. Fig. 2a shows XRD patterns of La2Fe11Si2 alloys annealed at different temperatures. The Rietveld refinement confirms that the constituent phases of the as-cast sample are α-Fe, Cu2Sb-type LaFeSi and Cr5B3-type La5Si3 phases. The cubic NaZn13-type 1:13 and La5Si3 phases were detected in all the annealed samples. For the sample annealed at 1323 K, the XRD patterns are identified as α-Fe, LaFeSi, La5Si3 and 1:13 phases. As annealing temperature (Ta) increases to 1423 K, the α-Fe peak and LaFeSi almost disappeared. With further increasing Ta, the intensity of the α-Fe peak obviously increases. All these suggest that 1423 K is an optimal annealing temperature. The small amount of La oxides cannot be detected due to the limit of XRD resolution. These results are in a good

Intensity (a.u.)

As-cast

1323 K

24 h

1423 K

24 h

1523 K

12 h

-Fe Cu Sb-type LaFeSi NaZn -type La(Fe,Si) Cr B -type La Si

Fraction of each phase (vol.%)

M. Zhang et al. / Materials Letters 134 (2014) 87–90

80 La(Fe,Si)13 phase

60

40 La5Si3 phase

20

-Fe

0

LaFeSi phase

As-cast

20 25 30 35 40 45 50 55 60 65 70 75 80

89

1300

1400

1500

Annealing temperature (K)

2 theta (deg.)

Fig. 2. (a) XRD patterns of as-cast and annealed La2Fe11Si2 alloys. The vertical bars below the diffraction patterns represent the calculated Bragg reflection positions of observed phases: α-Fe, LaFeSi with a Cu2Sb-type structure, La(Fe,Si)13 with a NaZn13-type structure and La5Si3 with a Cr5B3-type structure. (b) Volume fraction of constituent phases as a function of annealing temperature (closed symbols determined by Rietveld refinement, open symbols determined by SEM).

60

120

50

100 La2Fe11Si2

30

Annealed: 1423 K

20 0

10

Cooling Heating

H=0.05 T

TC=199 K

80

Step: 3 K

40

M(emu/g)

M(emu/g)

187 K

60 40

217 K

20

0 0 0.0

150 160 170 180 190 200 210 220 230

0.5

12

S (J/kgK)

8000

2

2

M (emu /g

2

)

10000

6000

1.5

2.0

14

187 K

12000

1.0

H (T) 0

T (K)

Step: 3 K

4000

1423 K

10 8

2T 1.5 T 1T 0.5 T

6 4

2000

2

0

217 K

0.00

0.02

0.04

0.06

0.08

H/M (T g/emu) 0

0 185

190

195

200

205

210

215

220

T (K)

Fig. 3. M–T (a) and M–H (b) curves for La2Fe11Si2 alloy annealed at 1423 K for 24 h. Arrott plots of La2Fe11Si2 alloy (c). ΔS was calculated from magnetization isothermals in (b) and plot as a function of temperature in field changes from 0 to 0.5, 1, 1.5 and 2 T (d).

agreement with the above SEM observation. The volume fraction of the constituent phases was determined by both microstructural analysis and Rietveld refinement, as shown in Fig. 2b. Both the results from two methods display that a maximum amount of the 1:13 phase of
80 vol.% can be obtained by annealing at 1423 K for 24 h. The magnetic properties including magnetic transition and entropy change for La2Fe11Si2 sample annealed at 1423 K are shown in Fig. 3. From the temperature dependence of magnetization (see Fig. 3a),

a sudden drop of magnetization down to zero near the TC of 199 K, being accompanied without any thermal hysteresis can be observed. From the isothermal magnetization curves in the vicinity of TC (see Fig. 3b), the typical S-shape magnetization curves as a signature of first-order transition can be observed. To further determine this, the corresponding Arrott plots are shown in Fig. 3c. The negative slope of the Arrott plots above TC confirms the first-order nature of this transition. More importantly, this transition undergoes in the absence of any magnetic hysteresis. In addition, the magnetization curves

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exhibit a good linear increase with increasing magnetic field to 2 T when the temperature is above 214 K. This further proves that there is almost no α-Fe contained in this sample, and the La5Si3 phase is nonferromagnetic. The reversible magneto-structural transition allows us to employ the Maxwell relation to correctly estimate ΔS [18]. The obtained maximum value of ΔS is 14.5 J/kg K in a field change of 0– 2 T, as shown in Fig. 3d. As a result of the field-induced IEM transition from paramagnetic to ferromagnetic state above TC, ΔS peak gradually widens with increasing magnetic field. The obtained maximum value of ΔS in 2 T is comparative to that in LaFe11.4Si1.6 bulk after annealing for 30 days (14 J/kg K) [3], because the matrix composition is quite close to LaFe11.4Si1.6. From the view of the simplification of preparation work, our La-rich La2Fe11Si2 alloy has an obvious advantage that only needs less than 1 day annealing to get 1:13 phase grains. Moreover, if subtracting the fraction of secondary La5Si3 phase, the ΔS value is expected to further increase by about 10%, up to about 16 J/kg K. The removal of the La5Si3 phase might be realized by magnetically extracting the hydrogenated 1:13 and 5:3 phase particles, due to the fact that La(Fe,Si)13Hx and La5Si3Hx are ferromagnetic and paramagnetic at room temperature, respectively. 4. Conclusion Aiming at shorten annealing time in La–Fe–Si bulks while keeping the large entropy change and small hysteresis losses, we proposed an off-stoichiometric composition for La2Fe11Si2. By optimizing annealing temperature, a novel microstructure consisting of La(Fe,Si)13 grains and intergranular La5Si3 phase was obtained. Upon annealing shorter than 1 day, the La2Fe11Si2 bulk sample exhibits a large entropy change of 14.5 J/kg K in 2 T. More importantly, the observed first order magnetic transition is accompanying with zero hysteresis. All the characterizations on this La2Fe11Si2 sample allow us to expect that the new magnetocaloric alloy finds application in fields as an advanced magnetic refrigerant.

Acknowledgment The research leading to these results has received funding from National Natural Science Foundation of China (Grant No. 51371184), Zhejiang Provincial Natural Science Foundation of China (Grant No. LR14E010001), China Postdoctoral Science Foundation (Grant No. 2013M541553), Thousand Young Talents Program of China, and the Program of the Science and Technology Innovation Team of Ningbo City (Grant No. 2012B81001).

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