The structure and magnetocaloric effect of rapidly quenched Gd5Si2Ge2 alloy with low-purity gadolinium

Materials Letters 61 (2007) 440 – 443 www.elsevier.com/locate/matlet

The structure and magnetocaloric effect of rapidly quenched Gd5Si2Ge2 alloy with low-purity gadolinium Tiebang Zhang a , Yungui Chen a,⁎, Baohua Teng b,1 , Yongbai Tang a , Hao Fu a , Mingjing Tu a a

School of Materials Science and Engineering, Sichuan University, Chengdu 610065, PR China b Department of Applied Physics, Sichuan University, Chengdu 610065, PR China Received 15 May 2005; accepted 20 April 2006 Available online 12 May 2006

Abstract A magnetic refrigerating material Gd5Si2Ge2 was prepared by arc-melting method under argon atmosphere with low-purity commercial gadolinium. The structure and magnetocaloric effect (MCE) of the as-cast and rapidly quenched Gd5Si2Ge2 alloys were investigated by means of Xray diffraction and magnetic measurements. The as-cast Gd5Si2Ge2 consists of Gd5Si2Ge2-type, Gd5Si4-type, Gd5(Si,Ge)3 and Gd(Si,Ge) phases from powder XRD results. The alloys quenched at a velocity of 25 m/s or 40 m/s both adopt in a single phase with orthorhombic Gd5Si4-type structure. The lattice parameters under different quenched velocities calculated by least-squares method have no significant difference. After being rapidly quenched at 40 m/s, the Curie temperature of Gd5Si2Ge2 is about 303 K and its maximum magnetic entropy change is 7.25 J/kg K (0–5 T). © 2006 Elsevier B.V. All rights reserved. Keywords: Magnetocaloric effect; Magnetic entropy change; Rapidly quench; Gd5Si2Ge2

1. Introduction The magnetic properties and crystal structure of pseudobinary Gd5Si4–Gd5Ge4 system were reported nearly 40 years ago [1,2]. Since the discovery of a giant magnetocaloric effect (GMCE) in Gd5Si2Ge2 alloy [3], there has been much interest in the Gd5 (SixGe1 − x)4 pseudobinary system [4–8]. The Curie temperature can be tuned by adjusting the ratio of Si and Ge atoms and the maximal magnetic entropy change of Gd5Si2Ge2 is about twice as large as that of conventional magnetic refrigerant Gd [3], making the alloy a promising candidate for room temperature magnetic refrigeration. The Gd5(SixGe1 − x)4 series alloys, where x ≤ 0.5, exhibit a giant magnetocaloric effect due to the first order magnetic– crystallographic phase transition [3,4]. Phase analysis of Gd5 (SixGe1 − x)4 alloys with x = 0.475 and 0.43 shows that the phase composition of the system is relatively complex [8]. Further⁎ Corresponding author. Tel.: +86 28 85405670; fax: +86 28 85407335. E-mail addresses: [email protected] (T. Zhang), [email protected] (Y. Chen). 1 Present address: School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.084

more, the phase composition of Gd5(SixGe1 − x)4, where x is near 0.5, is intensely dependent on the initial purity of elemental Gd. The Gd5(SixGe1 − x)4 alloys with x near 0.5 prepared by commercial grade (99 wt.%) [8], higher purity (99.9 wt.%) [9] and even high-purity (99.99 wt.%) Gd [10] all have one or more impurity phases, including Gd(Si,Ge), Gd5(Si,Ge)3 and orthorhombic Gd5Si4-type phases. At near room temperature, Gd5Si4-type phase maintains a typical second-order phase transition while Gd(Si,Ge) and Gd5(Si,Ge)3 phases have no phase transformation due to their very low Curie temperatures [8,11]. As a result, the before-mentioned impurity 1:1 and 5:3 phases in the Gd5Si2Ge2 alloy contribute little to its MCE. To improve the MCE of Gd5Si2Ge2, it's necessary to make the impurity phases in the alloy reduce or disappear. The appropriate heat treatment of the Gd5Si2Ge2 alloy prepared from high-purity Gd, for the removal of the polymorphic Gd5Si2Ge2 phase with the orthorhombic Gd5Si4-type structure in the as-cast alloy, results in a considerable enhancement of its magnetocaloric effect [10]. When using low-purity commercial grade Gd [12], the MCE value is significantly smaller than that first reported in 1997 [3], which is mainly due to the impurity phases discovered also in our experimental results below. In this paper, we attempt to control the phase component of Gd5Si2Ge2 alloy with commercial low-purity

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Gd using a rapid solidification technique, the structure and magnetocaloric effect (MCE) of as-cast and rapidly quenched Gd5Si2Ge2 alloys were investigated by means of X-ray diffraction analyses and magnetic measurements. 2. Experimental details Gd5Si2Ge2 alloy was prepared by arc-melt method in an argon atmosphere with low-purity commercial Gd (99 wt.%), high-purity Si and Ge (purities both better than 99.99 wt.%). The typical impurities of the commercial grade Gd are (wt.%): O: 1500 ppm, C: 200 ppm, Fe: 300 ppm, Ca: 300 ppm, Mg: 300 ppm, Si: 100 ppm, Al: 100 ppm. During melting, the sample was homogenized by turning over the buttons and re-melted 5 times. In a 50 kPa argon atmosphere, the alloy was re-melted by arc-melting method in a copper crucible and the rapidly quenched ribbons with a thickness of ∼20 μm were prepared by the melt spinning technique with a copper wheel surface speed of 25 and 40 m/s. The X-ray powder diffraction data were collected on an X'Pert Pro MPD powder diffractometer using Cu Kα radiation. The voltage and anode current were 40 kV and 30 mA, respectively. Qualitative phase analysis was performed with the MDI software package and PDF2 database. Temperature dependence of magnetization and magnetization isotherms were performed on a vibrating sample magnetometer VSM and a MPMS-7 type superconducting quantum interference device (SQUID) magnetometer, respectively. 3. Results and discussions 3.1. Phases in as-cast and as-quenched alloys The room temperature X-ray powder diffraction of as-cast alloy is shown in Fig. 1. From qualitative analysis, the as-cast alloy contains a dominant quantity of 5:4 Gd5Si2Ge2 phase with monoclinic Gd5Si2Ge2-type and orthorhombic Gd5Si4type structure. A small quantity of impurity Gd5(Si,Ge)3, Gd(Si,Ge) phases with Gd5Si3- and GdGe-type structure are found in our experimental as-cast Gd5Si2Ge2 as observed in Gd5(SixGe1 − x)4 (x = 0.475 and 0.4875) prepared by commercial

Fig. 2. X-ray powder diffraction patterns of the Gd5Si2Ge2 alloys rapidly quenched at 25 m/s and 40 m/s. grade 99 wt.% and 99.9 wt.% Gd [8,9]. From the binary phase diagram of Gd–Ge [13], it's known that Gd5Ge4 phase is a peritectic reaction product at ∼1690 °C: L + Gd5Ge3 → Gd5Ge4. Because of the incompleteness of peritectic reaction, a part of the Gd5Ge3 phases will remain and accordingly the residual liquid will react with Gd5Ge4 phase to become GdGe phases. Gd–Si binary phase diagram [13] has a similar behavior as the Gd–Ge diagram. At present, there is no Gd–Si–Ge ternary phase diagram, and it could be presumed that Gd5(SixGe1 − x)4 series alloys may also have a peritectic reaction during solidification as Gd–Ge and Gd–Si based on the similarity of Si,Ge atoms: L + Gd5(Si,Ge3) → Gd5(Si,Ge)4. As a result of peritectic reaction, 5:3-type Gd5(Si,Ge)3, 5:4-type Gd5Si2Ge2 with monoclinic and orthorhombic structure, and 1:1-type Gd(Si,Ge) phases will be found in as-cast Gd5Si2Ge2 alloy. According to the aforementioned analysis, the impurity phases are probably the main reason for the low MCE of as-cast Gd5Si2Ge2 with commercial grade Gd in Ref. [12]. The X-ray diffraction patterns of alloys quenched at different velocities are displayed in Fig. 2, and the columns at the bottom are Bragg peaks of Gd5Si4 cited from ICDD database. The alloys quenched at 25 m/s and 40 m/s are both in polymorphic Gd5Si2Ge2 phase with the orthorhombic Gd5Si4-type structure, and the Gd5(Si,Ge)3 and Gd(Si,Ge) phases are not observed in the as-quenched alloy. The lattice parameters of as-quenched alloys are determined by full-matrix leastsquares method (Table 1). As the atomic radii of Ge is larger than that of Si (metallic radii of Ge and Si are 1.52 and 1.46 Å, respectively), the lattice constants and cell volume of the Gd5Si2Ge2 alloys in Gd5Si4-type orthorhombic structure increase a little in comparison with those of Gd5Si4. The lattice parameters at different quenching speeds have no significant difference within the resolution of powder X-ray diffraction. From the X-ray diffraction results, the rapid-quenched method is sufficient for the formation of Gd5Si2Ge2 single phase. The rapid-quench method removes the impurity Gd5(Si,Ge)3 and Gd(Si,Ge) phases in the as-cast alloy, which is an essential step to improve the MCE of low-purity Gd5Si2Ge2. The reasons in the formation of the orthorhombic Gd5Si2Ge2 single phase in the rapid-quenched alloy are discussed here. At first, the peritectic reaction could not take place at a rapidly quenching condition, which could result in the Gd5Si2Ge2 phase being formed directly from the molten alloy. On the other hand, as has been observed in the Gd5

Table 1 Lattice parameters of as-quenched Gd5Si2Ge2 alloys

Fig. 1. X-ray powder diffraction pattern of the as-cast Gd5Si2Ge2.

a (Å)

b (Å)

c (Å)

Volume (Å3)

Velocity (m/s)

Crystal structure

25 40

Orthorhombic 7.4890 (4) 14.7556 (2) 7.7869 (1) 860.49 Orthorhombic 7.4867 (4) 14.7688 (3) 7.7885 (7) 861.18

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Fig. 3. M–T curves of as-quenched Gd5Si2Ge2 measured at 100 Oe. (SixGe1 − x)4 system, monoclinic β-Gd5Si2Ge2 transforms into an orthorhombic paramagnetic γ-phase (Gd5Si4-type) at high temperatures, the γ–β transformation occurs at high temperature and the monoclinicGd5Si2Ge2-type structure in this alloy can be restored by heat treatment at temperatures exceeding 1200 K [14]. As the high temperature transformation of Gd5Si4-type to Gd5Si2Ge2-type [11,14,15] could not take place in rapid cooling [15], the crystal structure at high temperature is believed to be preserved during the rapidly quenched process, which is another important reason for the formation of Gd5Si4-type phase in the rapidly quenched alloys. Besides, the impurity atoms of initial Gd may also play a role in the formation of Gd5Si4-type phase, they may act as heterogeneity cores in the cooling process and influence the occupancy preference of Si,Ge atoms in the process, but the mechanism needs further studies.

3.2. Magnetocaloric effect Fig. 3 shows the temperature dependence of magnetization (M–T) measured under a dc magnetic field of 100 Oe for the rapidly quenched alloy at different velocities. The M–T curves indicate that there is a ferromagnetic–paramagnetic transition at ∼303 K in each rapidly quenched alloy. According to the magnetic phase diagram of Gd5(SixGe1 − x)4 [6], the alloy with Gd5Si4-type structure has

Fig. 5. Magnetic entropy change near TC of Gd5Si2Ge2 rapidly quenched at 40 m/s. higher TC and performs a typical second-order magnetic phase transition. Magnetic measurements confirm the results of powder X-ray diffraction that the low-purity Gd5Si2Ge2 alloys quenched at 20 and 40 m/s are both in polymorphic Gd5Si2Ge2 phase with orthorhombic Gd5Si4-type structure. Fig. 4(a) displays the magnetization isotherms of the alloy that rapidly quenched at 40 m/s between 275 and 320 K with temperature steps of 5 K, while the corresponding Arrot plots are shown in Fig. 4(b). No rapid magnetization increase in the M–H curves and a negative slope above TC are observed, which both indicate the rapidly quenched Gd5Si2Ge2 preserves a typical second-order phase transition. Based on the collected magnetization data, the magnetic entropy change obtained from the Maxwell relation is shown in Fig. 5. At 297.4 K, the maximal magnetic entropy change of the alloy quenched at 40 m/s is 7.2 J/kg K (0–5 T). For the room temperature magnetic refrigerant application, the Curie temperatures of the rapidly quenched low-purity Gd5Si2Ge2 alloys are suitable. But the experimental as-quenched alloys both adopt in orthorhombic Gd5Si4-

Fig. 4. (a) M–H curves of low-purity Gd5Si2Ge2 rapidly quenched at 40 m/s and (b) the corresponding Arrot plots near TC.

T. Zhang et al. / Materials Letters 61 (2007) 440–443 type structure and have a typical second-order phase transition and correspondingly the MCE of rapidly quenched alloys is not large enough. As the orthorhombic Gd5Si4-type phase transforms to the monoclinic Gd5Si2Ge2-type phase at a certain condition [11,14,15], it could be expected to obtain Gd5Si2Ge2-type single phase with proper treatment on the rapidly quenched alloys and this work is being taken by our research team.

4. Conclusions The rapidly quenching method significantly changes the structure of low-purity Gd5Si2Ge2, the impurity Gd5(Si,Ge)3 and Gd(Si, Ge) phases were both eliminated and the Gd5Si2Ge2 single phase with orthorhombic structure was obtained. At the field change from 0 to 5 T, the maximal magnetic entropy change of the alloy quenched at 40 m/s is 7.2 J/kg K, and its Curie temperature is about 303 K in the range of room temperature. Acknowledgements This work was supported by the National High Technology Research and Development Program of China under contract No. 2002AA324010. One of the authors (Tiebang Zhang) gratefully acknowledges Professor Jumu Zhu at Sichuan University for useful discussions.

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