Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys

Accepted Manuscript Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys Lingtong Zhou, Yongbai Tang, Yungui Chen, Huaqiang Guo, Wenkai Pang, Xi Zhao PII:

S1002-0721(18)30197-2

DOI:

10.1016/j.jre.2018.01.009

Reference:

JRE 142

To appear in:

Journal of Rare Earths

Received Date: 22 June 2017 Revised Date:

23 January 2018

Accepted Date: 24 January 2018

Please cite this article as: Zhou L, Tang Y, Chen Y, Guo H, Pang W, Zhao X, Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys, Journal of Rare Earths (2018), doi: 10.1016/j.jre.2018.01.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys Lingtong Zhou, Yongbai Tang*, Yungui Chen, Huaqiang Guo, Wenkai Pang, Xi Zhao College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China

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Abstract: Composite magnetic refrigerants were prepared by physical mixing LaFe11.6Si1.4Hy alloys with different Curie temperatures (TC). The magnetocaloric effect (MCE) and refrigerant capacity (RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The results indicate the experimental magnetic entropy change (-∆SM)-T curve corresponds reasonably with the (–∆SM)-T curve calculated by the linear combination of (–∆SM)-T curves of the single material. An optimal mixing ratio can make the composite magnetic refrigerant possess a table-like (–∆SM)-T curve which is beneficial to magnetic Ericsson cycle. When three LaFe11.6Si1.4Hy alloys with different TC are mixed, the full width at half maximum (∆TFWHM) of (–∆SM)-T curves is about 48.7 K and the RC is about 177.76 J/kg under a magnetic field change of 2 T. The composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys can be promising candidates for near room temperature magnetic refrigeration and the work will be helpful to develop novel composite magnetic refrigerants with table-like MCE and large RC. Keyword: Magnetic refrigeration; Magnetocaloric effect; Table-like; LaFe11.6Si1.4Hy; Composite refrigerant; Rare earths

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1. Introduction Magnetic refrigeration, which is based on the magnetocaloric effect discovered by Weiss and Piccard in 1917 [1] , was a new cooling technology. Compared with traditional compression refrigeration, magnetic refrigeration is environmentally friendly and more efficient, so it has attracted wide attention from all over the world [2–4]. In recent years, owing to the damage of traditional organic refrigerant (CFCs) to ozone layer and the advent of global warming, room-temperature magnetic refrigeration has wide application prospect in civil refrigeration field [5]. Especially, with the discovery of the giant MCE in Gd5Si4–xGex [6–9], LaFe13–xSix [10-15], MnFeP(Ge, Si, As) [16-18] and Ni-Mn-X (X = Sn, In, Ga and Sb) [19-23] which greatly promoted the practical application of room-temperature magnetic refrigeration, room-temperature magnetic refrigeration has become a new research hotspot . For room-temperature magnetic refrigeration system, Ericsson cycle should be chosen because Erickson-cycle can overcome the influence of large lattice entropy at high temperature and possesses large cooling temperature range. For the magnetic refrigeration system based on Ericsson cycle, the magnetic refrigerant needs a table-like MCE which means the –∆SM of the magnetic refrigerant should be constant and high in the operating temperature range [24,25]. Unfortunately, traditional room-temperature magnetic refrigeration materials, such as Gd5Si4–xGex [6–9] and MnFeP(Ge, Si, As) [16–18], possess the narrow operating temperature range as a result of the sharp magnetic entropy change –∆SM peak. In other words, the –∆SM values of these magnetic refrigerant materials drop rapidly when the temperature deviates from TC. So these traditional room-temperature magnetic refrigeration materials are not applicable to Ericsson cycle. To break through this limitation, composite magnetic refrigeration materials composed of a few ferromagnetic materials with different TC have been extensively investigated. Tian et al [26] have prepared Fe78–xCexSi4Nb5B12Cu1 (x = 0–10) composite materials by gluing the Fe78–xCexSi4Nb5B12Cu1 alloy ribbons layer by layer or mixing the alloy powders with varied Ce contents and the composites possessed a table-like MCE (the –∆SM value remained approximately constant in a wide temperature span over 80 K) and large RC values (>370 J/kg at a field change of 0–5 T). Kim et al.[27] have reported that a Mn5–xGe3(Co, Fe)x composite consisting of physical mixture of Mn5Ge3, Mn5.1Ge2.9, Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3, generated a table-like (–∆SM)-T curve which leads to a wide operating temperature range of 45 K and enhanced RC value of 52 J/kg at a field change of 0–1 T. La(Fe, Si)13-based compounds，one of the most promising materials for room-temperature magnetic refrigeration, have been highly studied for their advantages of low cost, giant MCE and adjustable TC in a large temperature range [10–15]. However, La(Fe,Si)13-based compounds also possess sharp –∆SM peak which extremely restricts the practical application. In this work, LaFe11.6Si1.4Hy alloys with different TC were prepared by the high-temperature hydrogenation method [28,29]. On this foundation, composite magnetic refrigerants composed of a physical mixture of LaFe11.6Si1.4Hy alloys with different TC were prepared to broaden the operating temperature range and consequently increase the refrigerant capacity. An optimal compound ratio made the composite magnetic refrigerant based on LaFe11.6Si1.4Hy alloys possess a table-like MCE and enhanced RC, which was applicable for Ericsson cycle. 2. Experimental Foundation item: Project supported by the Key Project of National Natural Science Foundation of China (grant number 51176065). * Corresponding author. College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China. E-mail address: [email protected] (Y.B. Tang); Tel.: 028-85405670

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The LaFe11.6Si1.4 alloys were prepared by arc melting under the protection of high-purity argon atmosphere. The purities of raw materials La, Fe and Si were 99.4 wt%, 99.9 wt% and 99.9999 wt%, respectively. Considering the loss of lanthanum, an excess of 4% of La was systematically added. In order to ensure homogeneity, the alloy buttons were re-melted five times. Then the as-cast LaFe11.6Si1.4 alloy was annealed in a ZM-40-16 vacuum molybdenum wire furnace at 1473 K for 5 h to obtain NaZn13-type main phase [30,31]. The annealed LaFe11.6Si1.4 alloy buttons were ground into powder samples by mechanical shattering and the particle size was limited in the range of 0.15–0.45 mm by the standard sample sieve. In order to obtain LaFe11.6Si1.4Hy alloy powders with different TC, these LaFe11.6Si1.4 alloy powder samples were hydrogenated in high-purity H2 atmosphere (≥99.999 wt%) of 0.6 MPa at 433, 453 and 483 K, respectively, for 1 h to saturate by the PCTPro-2000 Sieverts Gas Sorption Analyzer (made by Hy-Energy Company). The composite magnetic materials were made through ultrasonic mixing the LaFe11.6Si1.4Hy alloy powders with different TC and alcohol was used as the dispersant in the ultrasonic mixing process to prevent oxidation. Magnetic properties were measured by a vibrating sample magnetometers (VSM, Quantum Design). The Curie temperature (TC) was obtained from the magnetization-temperature (M–T) curve and the magnetic entropy change (∆SM) was calculated from isothermal magnetization curve through the Maxwell relation [32,33]. Peak temperature is the temperature for the maximum magnetic entropy change (–∆SM)max. The full width at half maximum (∆TFWHM) is the difference between Thot and Tcold, where Thot and Tcold correspond to the two temperatures at which the ∆SM T RC = ∆S (T )dT value is half of the peak value. Furthermore the refrigerant capacity (RC) is calculated with

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3. Results and discussion 3.1 Phase structure analysis and hydrogen absorption kinetics performance Fig. 1 shows the X-ray diffraction (XRD) patterns of the annealed LaFe11.6Si1.4 alloy and three hydrides. As can be seen from Fig. 1, the major phase of LaFe11.6Si1.4 alloy was cubic NaZn13 structure with minor α-Fe phase and the major phase was still reserved after hydrogen absorption. The hydrogen absorption kinetics performance of the three LaFe11.6Si1.4 alloys are shown in Fig. 2. The hydrogen saturation time and the hydrogen absorption capacity of the three LaFe11.6Si1.4 alloys decreased with the increase of treatment temperature. The hydrogen content y of the LaFe11.6Si1.4 alloys hydrogenated at 433, 453 and 483 K were 1.16, 1.03 and 0.92, respectively. So three hydrides were LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92, respectively.

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3.2 Curie temperature (TC) and maximum magnetic entropy change (–∆SM)max Fig. 3 shows the dM/dT-T curves and (–∆SM)-T curves for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92. The TC and (–∆SM)max of the three LaFe11.6Si1.4Hy alloys were 306, 290, 276 K and 7.35, 7.93, 6.22 J/(kg·K), respectively. The TC of the three LaFe11.6Si1.4Hy alloys dropped as the the hydrogen content y decreased, which was consistent with the result reported by Zheng et al.[29].

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3.3 Composite magnetic refrigerants with LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03 Fig. 4 shows the dM/dT-T curve of a composite magnetic refrigerant composed of LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03 mixing in equal molar ratio. In Fig. 4, the composite showed two distinctive magnetic transitions and the temperature of magnetic transitions well matched the TC for each LaFe11.6Si1.4Hy alloy. Fig. 5 shows a series of (–∆SM)-T curves of the composite materials consisting of the two LaFe11.6Si1.4Hy alloys with different ratios at a magnetic field change of 2 T. In order to compare composite materials with single LaFe11.6Si1.4Hy alloy, the individual (–∆SM)-T curves for the two LaFe11.6Si1.4Hy alloy are also included in Fig. 5. All the composite materials possessed the lower peak –∆SM values but the flatter (–∆SM)-T curves. It indicated that the operating temperature range of all the composite materials had been widened. Moreover, through comparing the (–∆SM)-T curves of all the composite materials, it can be seen that the smooth degree of these curves changed with the mixing ratio. When the composite material was mixed in equal molar ratio, the (–∆SM)-T curve was flattest but a peak still existed around 294 K. Thus, the smooth degree of the (–∆SM)-T curve needed to be further tuned by changing the ratio of the two LaFe11.6Si1.4Hy alloys in the composite magnetic refrigerant in order to obtain a table-like (–∆SM)-T curve which was applicable for the Ericsson cycle. According to previous studies [27], since the alloy powders with different TC were physically mixed, the (–∆SM)-T curve of the alloy powder mixtures should be simple arithmetic average of the individual (–∆SM)-T curves of the two LaFe11.6Si1.4Hy alloys. Based on it, we can get the optimal mixing ratio which makes the powder mixture possess a table-like (–∆SM)-T curve. In order to make the composite possess a table-like (–∆SM)-T curve, the (–∆SM) values of the composite material at the peak temperature of the single component material need to be equal. According to this, two equations could be obtained as follows [34]:

 x1 (−∆S M )(a, T1 ) + x2 (−∆S M )(b, T1 ) = x1 (−∆SM )(a, T2 ) + x2 (−∆S M )(b, T2 )   x1 + x2 = 1

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Where a and b are LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03, respectively; x1 and x2 are the mole fraction of a and b; T1 and T2 are the peak temperature of a and b; (–∆SM)(a, T1) is the (–∆SM) value of a at T1, (–∆SM)(b, T1) is the (–∆SM) value of b at T1, (–∆SM)(a, T2) is the (–∆SM) value of a at T2 and (–∆SM)(b, T2) is the (–∆SM) value of b at T2. Combining the two equations and the parameters listed in Table 1, the calculated optimal mixing molar ratio of LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03 was 1.2:1. Fig. 6(a) shows (–∆SM)-T curves of single LaFe11.6Si1.4Hy alloy and the composite material mixed in the optimal ratio. As expected, the practical (–∆SM)-T curve of the composite material possessed approximately equal (–∆SM) values at the peak temperature of the single component. Although there was still a small depression around 300 K (about 0.32 J/(kg·K)), the practical (–∆SM)-T curve was almost flat in the temperature range of 290–305 K. So a table-like (–∆SM)-T curve was obtained, which will be beneficial to enhance the RC and be suitable for the Ericsson cycle. In addition, as shown in Fig. 6(b), the practical (–∆SM)-T curve calculated from the M-H curve of the composite material corresponded reasonably with the (–∆SM)-T curve calculated by the linear combination of the individual (–∆SM)-T curves from single component material, which indicated that the MCE performance of the composite magnetic refrigerants can be predicted by the simple linear combination of the individual experimental (-∆SM)-T curves from single component material. Table 2 lists ∆TFWHM and RC values of the two LaFe11.6Si1.4Hy alloys and the composite. The ∆TFWHM and RC value of the composite material mixing in the optimal ratio were 33.6 K (283.3–316.9 K) and 173.04 J/kg, respectively. Compared with LaFe11.6Si1.4H1.16, the composite possessed the flatter (–∆SM)-T curve, so the ∆TFWHM expanded by 12.1 K. Although the (–∆SM)max reduced, according to the calculation formula of RC, the RC value still enhanced by 15.01 J/kg.

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3.4 Composite magnetic refrigerants with LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92 The TC of LaFe11.6Si1.4Hy alloys mentioned above were 306 and 292 K, respectively. But the materials with lower TC are required in order to obtain larger temperature span in room temperature magnetic refrigeration. So a third material, LaFe11.6Si1.4H0.92 was added to further widen the ∆TFWHM of the composite refrigerant. According to the method mentioned above and the parameters listed in Table 3, we could get the optimal mixing ratio of LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92, was the molar ratio of 0.33:0.17:0.50. According to the optimal mixing ratio of the three LaFe11.6Si1.4Hy alloys, we made the composite material including the three LaFe11.6Si1.4Hy alloys. Fig. 7 shows the dM/dT-T curves of the alloy powders mixture. The alloy powders mixture showed three distinctive magnetic transitions and the temperature of magnetic transitions well matched the TC for each LaFe11.6Si1.4Hy alloy. The (–∆SM)-T curve of the composite material was tested at a magnetic field change of 2 T. Fig. 8(a) shows the (–∆SM)-T curves of the three LaFe11.6Si1.4Hy alloys and the composite material and Fig. 8(b) shows the calculated and experimental (–∆SM)-T curves for the composite. As expected，the practical (–∆SM)-T curve calculated from the M-H curve of the composite material corresponded reasonably with the (–∆SM)-T curve calculated by the linear combination of the individual (–∆SM)-T curves and a table-like (–∆SM)-T curve was obtained in the temperature range of 275–310 K. Table 4 lists the (–∆SM)max, Temperature for (–∆SM)max, ∆TFWHM, temperature range for ∆TFWHM and RC values of the three LaFe11.6Si1.4Hy alloys and the composite. The peak (–∆SM) of the composite material was lower than that of the single compositions, but the ∆TFWHM of the composite material was wider and the RC is larger. The values of ∆TFWHM and RC reached to 48.7 K and 177.76 J/kg. Compared with the composite material consisting of two LaFe11.6Si1.4Hy alloys, the composite material consisting of the three LaFe11.6Si1.4Hy alloys possessed wider ∆TFWHM and higher RC value which achieved what we expected.

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4. Conclusions In this paper, the composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys were prepared. The magnetocaloric effect (MCE) and refrigerant capacity (RC) of these composite magnetic refrigerants were investigated by experiment and calculation. Compared with single LaFe11.6Si1.4Hy alloy, the composite magnetic refrigerants has a table-like (-∆SM)-T curve, wider ∆TFWHM and enhanced RC. Especially, when the composite magnetic refrigerant is composed of the mixed three LaFe11.6Si1.4Hy alloys with different TC, the value of ∆TFWHM and RC reach to about 48.7 K and 177.76 J/kg, respectively. Composite magnetic refrigerants broaden the operating temperature range and enhance the refrigerant capacity of LaFe11.6Si1.4Hy alloys, especially the composite refrigerants possess a table-like (–∆SM)-T curve for Ericsson cycle. So the composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys can be considered as promising candidates for near room temperature magnetic refrigeration and the work will be helpful to develop novel composite magnetic refrigerants with table-like MCE and large RC. Reference: [1] Smith A. Who discovered the magnetocaloric effect?. Eur Phys J H. 2013; 38(4): 507. [2] Brück E. Developments in magnetocaloric refrigeration. J Phys D Appl Phys. 2005; 38(23): R381. [3] Gschneidner KA, Pecharsky VK. Rare Earths and Magnetic Refrigeration. J Rare Earths. 2006;24(6): 641. [4] Zhong XC, Shen XY, Liu ZW. Magnetocaloric properties, microhardness and corrosion resistance of Gd100-xZrx alloys. J Rare Earths. 2016;34(9): 889. [5] Gschneiner KA, Pecharsky VK. Thirty years of near room temperature magnetic cooling: Where we are

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Table 1 The parameters for calculating the optimal mixing ratio of the composite with LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03 (–∆SM) value at the peak temperature of LaFe11.6Si1.4H1.03 (J/(kg·K))

LaFe11.6Si1.4H1.16

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(–∆SM) value at the peak temperature of LaFe11.6Si1.4H1.16 (J/(kg·K))

Composition

7.93

Table 2 (–∆SM)max, temperature for (–∆SM)max, ∆TFWHM, temperature range for ∆TFWHM and RC values of the LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and the composite material consisting of the two LaFe11.6Si1.4Hy alloys in a ratio of 1.2: 1 at a magnetic field change of 2 T Temperature for (–∆SM)max (K)

∆TFWHM (K)

Temperature range for ∆TFWHM (K)

RC (J/kg)

LaFe11.6Si1.4H1.16 LaFe11.6Si1.4H1.03 composite

7.35 7.93 5.15

306 292 294

21.5 19.2 33.6

296.0-317.5 281.4-300.6 283.3-316.9

158.03 152.27 173.04

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(–∆SM)max (J/(kg·K))

Table 3 The parameters for calculating the optimal mixing ratio of the composite material consisting of LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92 (–∆SM) value at the peak temperature of LaFe11.6Si1.4H1.03 (J/(kg·K))

(–∆SM) value at the peak temperature of LaFe11.6Si1.4H0.92 (J/(kg·K))

7.35 1.42 2.10

2.71 7.93 2.40

1.43 0.31 6.22

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LaFe11.6Si1.4H1.16 LaFe11.6Si1.4H1.03 LaFe11.6Si1.4H0.92

(–∆SM) value at the peak temperature of LaFe11.6Si1.4H1.16 (J/(kg·K))

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Table 4 (–∆SM)max, temperature for (–∆SM)max, ∆TFWHM, temperature range for ∆TFWHM and RC values of LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03, LaFe11.6Si1.4H0.92, and the composite consisting of the three LaFe11.6Si1.4Hy alloys with a ratio of 0.33: 0.17: 0.50 at a magnetic field change of 2 T

Composition

(–∆SM)max (J/(kg·K))

Temperature for (–∆SM)max (K)

∆TFWHM (K)

Temperature range for ∆TFWHM (K)

RC (J/kg)

LaFe11.6Si1.4H1.16 LaFe11.6Si1.4H1.03 LaFe11.6Si1.4H0.92 Composite

7.35 7.93 6.22 3.65

306 292 276 276

21.5 19.2 22.2 48.7

296.0-317.5 281.4-300.6 266.4-288.6 267.3-316.0

158.03 152.27 138.08 177.76

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Fig. 1 The XRD patterns of fore-and-aft hydrogen absorption of LaFe11.6Si1.4 alloys at different treatment temperatures

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Fig. 2 The hydrogen absorption kinetics curves of the LaFe11.6Si1.4 alloys at 0.6 MPa hydrogen-filling pressure and different treatment temperatures

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Fig. 3(a) dM/dT-T curves for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03, and LaFe11.6Si1.4H0.92; (b) (–∆SM)-T curves at a magnetic field change of 2 T for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03, and LaFe11.6Si1.4H0.92

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Fig. 4 dM/dT-T curves for the composite composed of LaFe11.6Si1.4H1.16 and LaFe11.6Si1.4H1.03 in equal molar ratio

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Fig. 5 (–∆SM)-T curves at a magnetic field change of 2 T for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and the composite consisting of two LaFe11.6Si1.4Hy alloys with different ratios

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Fig. 6(a) (–∆SM)-T curves at a magnetic field change of 2 T for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and the composite with the two LaFe11.6Si1.4Hy alloys at a ratio of 1.2: 1, and (b) calculated (linear combination) and experimental (from M-H curves) (–∆SM)-T curves at a magnetic field change of 2 T for the composite

Fig. 7 dM/dT-T curves for the composite composed of LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03 and LaFe11.6Si1.4H0.92 in a ratio of 0.33: 0.17: 0.50.

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Fig. 8(a) (–∆SM)-T curves at a magnetic field change of 2 T for LaFe11.6Si1.4H1.16, LaFe11.6Si1.4H1.03, LaFe11.6Si1.4H0.92, and the composite consisting of the three LaFe11.6Si1.4Hy alloys with a ratio of 0.33: 0.17: 0.50; (b) calculated (linear combination) and experimental (from M-H curves) (–∆SM)-T curves at a magnetic field change of 2 T for the composite

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TOC: Composite magnetic refrigerant possessed wide operating temperature range, large RC value and a table-like (–∆SM)-T curve for ericsson-cycle

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