⇦Chinese Physics Letters, 2020, Vol. 37, No. 1, Article code 017501 Table-Like Large Magnetocaloric Effect in the Misch Metal $R$Si Compound * Ruo-Shui Liu (刘若水)1†, Jun Liu (刘俊)1,2†, Li-Chen Wang (王利晨)1†, Zheng-Rui Li (李峥睿)1, Xiang Yu (俞翔)1, Yan Mi (米岩)1, Qiao-Yan Dong (董巧燕)1, Kai Li (李凯)3, Dan-Li Li (李丹丽)3, Chen-Hui Lv (吕晨辉)1, Li-Feng Liu (刘丽峰)1, Shu-Li He (贺淑莉)1** Affiliations 1Department of Physics, Capital Normal University, Beijing 100048 2State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 3Department of Chemistry, Capital Normal University, Beijing 100048 Received 16 July 2019, online 23 December 2019 *Supported by the National Natural Science Foundation of China under Grant Nos. 51701130, 51571146, and 51771124.
†These authors contributed equally to this work.
**Corresponding author. Email: shulihe@cnu.edu.cn Citation Text: Liu R S, Liu J, Wang L C, Li Z R and Yu X et al 2020 Chin. Phys. Lett. 37 017501    Abstract Magnetic properties and the magnetocaloric effect (MCE) of the $R$Si ($R$ = Ce, Pr, Nd) compounds made of Misch metal (MM) are investigated. Two transitions are found at 12 K and 38 K. Field variation generated large MCE and two peaks are found in the magnetic entropy change ($\Delta S$) curves, which correspond to the two transition temperatures. The maximum values of the magnetic entropy changes ($\Delta S$) are found to be $-5.1$ J/(kg$\cdot$K) and $-9.3$ J/(kg$\cdot$K) for the field ranges of 0–2 T and 0–5 T, respectively. The large $\Delta S$ as well as ultra-low price of MM make (MM)Si a competitive magnetic refrigerant candidate for low temperature in Eriksson cycle. DOI:10.1088/0256-307X/37/1/017501 PACS:75.30.Sg, 75.50.Cc, 75.60.Ej © 2020 Chinese Physics Society Article Text The magnetocaloric effect (MCE) was considered as one of the most important fundamental thermodynamic effects, which could induce temperature changes of magnetic material by exposing the material to a changing magnetic field.[1–3] Magnetocaloric effect technology has the outstanding merits of being highly energy-efficient and eco-friendly compared with traditional vapor compression refrigeration, which promotes it to play an important role in the refrigeration area.[1–5] Nowadays, La(Fe,Si)$_{13}$,[6] MnAs$_{1-x}$Sb$_{x}$,[7] MnFeP$_{1-x}$As$_{x}$[8] and Ni-Mn based Heusler alloys[9] have the potential application of working at room temperature, which were extensively reported one after the other, but only the paramagnetic salts such as Gd$_{3}$Ga$_{5}$O$_{12}$, GdLiF$_{4}$ or GdF$_{3}$ have been commercially used in the low temperature refrigeration area.[10,11] Rare earth base materials with low transition temperatures also exhibit bright application prospects in the low temperature refrigeration area. Lots of materials were reported, among which (ErAl$_{2})_{0.312}$(HoAl$_{2})_{0.198}$(Ho$_{0.5}$Dy$_{0.5}$Al$_{2})_{0.490}$ and RCo$_{2}$ were highly concerned.[12,13] The Eriksson cycle was considered as the most suitable mode for the magnetic refrigeration machine, for which materials with table-like MCE property and wide refrigeration temperature range were desired.[12] (ErAl$_{2})_{0.312}$(HoAl$_{2})_{0.198}$ (Ho$_{0.5}$Dy$_{0.5}$Al$_{2})_{0.490}$ was designed and used for the Eriksson cycle, and considerable effort was obtained.[12] Except for this, great effort has been devoted to investigate the table-like MCE materials. In general, the maximum values of magnetic entropy changes ($\Delta S$) correspond to the transition temperatures. Multiple transitions could generate several magnetic entropy change peaks, and a table-like MCE phenomenon would be observed. The table-like MCE was pivotal to the Eriksson cycle used in the magnetic refrigeration machine. It is difficult to get multiple transitions in a pure phase material, and many researchers have found ways to acquire this. In our previous works, $R$Si ($R$ = Ce, Pr, Nd) with low cost and excellent MCE performance has been generally studied.[13–15] After that, we also devoted energy to improve the MCE performance of these materials to give impetus to their application. The MM is mainly composed by La, Ce, Pr, Nd rare earth and some trace elements, as list in Table 1. The price of the MM is much lower than the rare earth sample substance. Depending on this, we expect to make an $R$Si sample using the Misch metal that can exhibit multiple phases with different transition temperatures, which could generate the table-like MCE phenomenon. For generality, the magnitude of MCE is characterized by the isothermal magnetic entropy change or the adiabatic temperature change under the variation of the magnetic field.[1–9] In this work, (MM)Si is prepared and MCE is characterized by the isothermal magnetic entropy changes. Two peaks are found in the magnetic entropy change curves and table-like MCE property is discovered, which indicates its potential applications in the near future. Taking into consideration of the comparison between the MM and rare earth sample substance, three samples were prepared by the arc melting method and named as S1, S2 and S3. The specific compositions of the three samples are listed in Table 1. Except for the Misch metal, the purity of all the constituent materials is better than 99.9 wt%. Stoichiometric starting raw materials were weighted and the ingots were obtained on a water-cooled copper crucible under protection of high-purity argon atmosphere. There was 3 wt% excessive rare earth added to make up the weight loss during the arc melting. The samples were turned over and re-melted six times to ensure homogeneity. The as-cast ingots were sealed in a quartz tube filled with high-purity argon atmosphere and then annealed at 1073 K for four weeks.
Table 1. Compositions of $R$Si series samples.
Element S1(wt%) S2(wt%) S3(wt%)
La 28.27 28.27
Ce 50.46 50.46 50.46
Pr 5.22 5.22 5.22
Nd 15.66 15.66 15.66
Sm 0.05
Fe 0.037
Mg 0.057
Si 0.016
Zn 0.01
W 0.01
C 0.01
Cu 0.01
Ti 0.01
Ga 0.01
Pb 0.01
Mo 0.007
Cr 0.03
D8 x-ray diffractometer(XRD) from Bruker Inc by using Cu $K\alpha$ radiation was employed to determine the lattice parameter and phase composition. The ingots were milled to powder before the measurement. The DC magnetization as functions of temperature and magnetic field was measured on a small piece of the sample using the commercial superconducting quantum interference device magnetometer (SQUID, Quantum Design).
cpl-37-1-017501-fig1.png
Fig. 1. XRD patterns of $R$Si series compounds at room temperature.
Figure 1 shows the standard $\theta $–$2\theta$ powder x-ray diffraction patterns for S1, S2 and S3 samples collected at room temperature. The black, blue and red patterns correspond to the S1, S2 and S3 samples, respectively. It is revealed that the three patterns are almost the same and all the peaks could be indexed, indicating that all the samples crystallized in an orthorhombic FeB-type structure (space group: $Pnma$, No. 62) as reported before.[16,17] Figure 2 shows the temperature dependence of magnetization ($M$–$T$) both under zero-field cooling (ZFC) and field-cooling (FC) modes with a magnetic field of 0.01 T, in order to determine the transition temperatures of the three samples. The S1 sample as seen in Fig. 2(a) shows two phase transitions at 12 K and 46 K, respectively. However, S2 and S3 prepared by the rare earth sample substance only show one phase transition temperature at 12 K and 38 K, respectively, as shown in Figs. 2(b) and 2(c). As is expected, multiple transitions appear in the (MM)Si samples. For the S2 and S3 samples, single pure phase as (La, Ce, Pr, Nd)Si and (Ce, Pr, Nd)Si may be obtained. It is interesting that S1 and S2 almost have the same compositions, but with such different properties. A similar result was reported in (MM$_{0.3}$Nd$_{0.7})$–Fe–B sintered magnets.[18] The reason was also not declared in that work. To our knowledge, the reason may be attributed to the existence of the trace elements and the symbiosis nature of the MM. In addition, more detail and deeper analysis will be required to figure out how the Misch metal worked in the (MM)Si sample. Considering the XRD patterns shown in Fig. 1, there may be two explanations for the two transitions in S1. First, there may exist several phases with different transition temperatures in the (MM)Si sample, which induces multiple transitions in the $M$–$T$ curve. The existing phases in S1 all crystallized in a pure FeB-type structure with almost the same patterns, so that it is difficult to distinguish them from the XRD data. For the second explanation, there may be only one pure phase in the (MM)Si sample, which shows two transitions. The fact that several transitions exist in one pure phase has been reported in other rare earth alloys.[19,20]
cpl-37-1-017501-fig2.png
Fig. 2. The temperature dependence of magnetization curves of $R$Si series samples (a) S1, (b) S2, and (c) S3.
Figure 3 exhibits the initial magnetization curves ($M$–$H$) for the S1, S2 and S3 samples with magnetic fields up to 5 T, respectively. Figure 4 presents the Arrott plots ($M^{2}$ vs $H/M$) for the S1, S2 and S3 samples, which are derived from the $M$–$H$ data in Fig. 3. The positive slopes of the three samples in the Arrott plots confirm the occurrence of second-order phase transition in the three samples according to the Banerjee criterion.[21]
cpl-37-1-017501-fig3.png
Fig. 3. Initial magnetization curves of $R$Si series samples (a) S1, (b) S2, and (c) S3.
cpl-37-1-017501-fig4.png
Fig. 4. Adiabatic magnetization curves and the corresponding Arrott plots of $R$Si series samples (a) S1, (b) S2, and (c) S3.
The isothermal magnetic entropy changes are calculated from the isothermal magnetization data by employing Maxwell's relationship: $$ \Delta S=\int_0^H {({\partial M} / {\partial T})_{H} dH}.~~ \tag {1} $$ The $\Delta S$ for different magnetic field changes as a function of temperature ($\Delta S$–$T$) are shown in Fig. 5, under magnetic field changing in the range from 0 to 5 T with an increment of 1 T. It is apparent that the $\Delta S$–$T$ graph of the S1 sample show two peaks near the two phase-transition temperatures as expected, and the maximum $\Delta S$ under a 0–5 T magnetic field are $-5.1$ J/(kg$\cdot$K) and $-9.3$ J/(kg$\cdot$K), respectively. Different from the two peaks exhibited in the S1 sample, the $\Delta S$–$T$ plots of the S2 and S3 samples show only a single peak, with a value of $-6.4$ J/(kg$\cdot$K) and $-5.9$ J/(kg$\cdot$K) for magnetic changes of 0–5 T, respectively. It is worth noting that the two magnetic entropy peaks of S1 are significantly larger than that of S2 and S3 under the same magnetic field changes. In addition, the price of the Misch metal is one order of magnitude lower than that of rare earth sample substances, which is very beneficial to the large-scale commercial production of the (MM)Si magnetic refrigeration refrigerant in the future.
cpl-37-1-017501-fig5.png
Fig. 5. The $\Delta S$ curves of the $R$Si series samples (a) S1, (b) S2, and (c) S3 versus temperature.
Table 2. Phase transition temperatures and magnetic entropy changes of the $R$Si series samples.
Sample Magnetic ground state Phase transition temperature $\Delta S$ (0–2 T) $\Delta S$ (0–5 T)
CeSi AFM 6.1 K $-7.2$ J/(kg$\cdot$K) $-13.7$ J/(kg$\cdot$K)
NdSi FM 46 K $-6.8$ J/(kg$\cdot$K) $-12.4$ J/(kg$\cdot$K)
PrSi FM 52 K $-8.6$ J/(kg$\cdot$K) $-15.3$ J/(kg$\cdot$K)
S1 12/46 K 2.4/4.7 J/(kg$\cdot$K) 5.1/9.3  J/(kg$\cdot$K)
S2 12 K 3.3 J/(kg$\cdot$K) 6.4 J/(kg$\cdot$K)
S3 38 K 2.8 J/(kg$\cdot$K) 5.9 J/(kg$\cdot$K)
In summary, the magnetic and MCE properties of $R$Si series samples have been systematically studied. The three samples all crystallize in an orthorhombic FeB-type structure by arc melting and long-time annealing. The S1 sample made of the Misch metal shows two phase transitions and the corresponding magnetic entropy curves ($\Delta S$–$T$) show two peaks. The maximum $\Delta S$ of the S1 sample under 0–5 T magnetic field variation are $-5.1$ J/(kg$\cdot$K) and $-9.3$ J/(kg$\cdot$K), respectively. The single peaks of the S2 and S3 samples prepared with the rare earth sample substance under the same magnetic field change are $-6.4$ J/(kg$\cdot$K) and $-5.9$ J/(kg$\cdot$K), respectively. It is worth noting that the sample prepared by the Misch metal not only have two peaks of magnetic entropy change, but also its maximum value is larger than that of the samples prepared by the rare earth sample substance under the same magnetic field changes. A considerable MCE and one order of magnitude lower price make the Misch metal $R$Si a competitive candidate for low temperature magnetic refrigerant. All these characters of the Misch metal $R$Si are very beneficial to the large-scale commercial production as a magnetic refrigeration refrigerant in the future. References Recent developments in magnetocaloric materialsField dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy changeRecent Progress in Exploring Magnetocaloric MaterialsMagnetic properties and magnetocaloric effects in NaZn 13 -type La(Fe, Al) 13 -based compoundsGiant Magnetocaloric Effect in Gd 5 ( Si 2 Ge 2 ) Great magnetic entropy change in La(Fe, M ) 13 ( M =Si, Al) with Co dopingGiant magnetocaloric effect of MnAs1−xSbxTransition-metal-based magnetic refrigerants for room-temperature applicationsMagnetic entropy change involving martensitic transition in NiMn-based Heusler alloysResearch progress in magnetocaloric effect materialsNew application of complex magnetic materials to the magnetic refrigerant in an Ericsson magnetic refrigeratorHandbook on the Physics and Chemistry of Rare EarthsLow-temperature large magnetocaloric effect in the antiferromagnetic CeSi compoundMagnetic properties and magnetocaloric effects of PrSiMagnetic properties and magnetocaloric effect of the compound NdSiConstitution, structural chemistry and magnetism in the ternary system Ce–Ag–SiCerium–silicon systemLocal profile dependence of coercivity in (MM0.3Nd0.7)-Fe-B sintered magnetsThe magnetic properties and magnetocaloric effects in binary R – T ( R = Pr, Gd, Tb, Dy, Ho, Er, Tm; T = Ga, Ni, Co, Cu) intermetallic compoundsLarge magnetocaloric effect of Ho x Er 1-x Ni (0 ≤ x ≤ 1) compoundsOn a generalised approach to first and second order magnetic transitions
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