Magnetic Properties of Mix-Bonded La

Abstract

La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3 master alloys were prepared by medium frequency induction furnace, then annealed, saturatedly hydrogenated, and finally crushed into powders. The multiple components of La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 (x=0.23, 0.26, 0.29, 0.32, wt%) powders with the Curie temperature (T_C) interval of 5 K were mix-bonded by epoxy resin to extend the full width at half maximum of magnetic entropy of alloy. The magnetic properties of the mix-bonded specimens were measured by VersaLab and adiabatic temperature change direct test device. The maximal magnetic entropy change of the mix-bonded specimens is decreased, whereas the full width at half maximum of magnetic entropy and the relative cooling power are increased, compared with those of the single-component-bonded specimens. The maximal relative cooling power is 139.2 J/kg for the four-component-bonded specimen.

Keywords

magnetic materials; magnetocaloric effect; relative cooling power; rare earths

Science Press

As La(Fe, Si)₁₃ materials have characteristics of large magnetocaloric effect, continuously adjustable Curie temperature (T_C), and low cost, they are regarded as one of the most promising magnetocaloric materials for room-temperature refrigeration^[1-3]. However, T_C of the La(Fe, Si)₁₃ parent alloys is too low for room temperature magnetic refrigeration^[3]. Although T_C of La(Fe, Si)₁₃ materials can increase to room temperature by partial substitution of Co for Fe, their magnetocaloric effect decreases sharply and the magnetic phase transition changes from the first-order to second-order^[4]. It is found that after hydrogen absorption, the T_C of La(Fe, Si)₁₃ materials is increased and the large magnetocaloric effect of first-order transition remains^[5-10]. Unfortunately, the application of La(Fe, Si)₁₃H_y materials in magnetic refrigerator is restricted due to their poor mechanical properties and narrow full width at half maximum of magnetic entropy (∆T). In order to improve the mechanical properties, Zhang et al^[11] prepared the epoxy resin-bonded LaFe_11.7Si_1.3-C_0.2H_1.8 material and found that the maximal compressive strength of 162 MPa is achieved when the content of epoxy resin is 3wt%. Pan et al^[12] bonded the La(Fe, Si)₁₃ hydrides with sodium silicate. Besides, the low melting point metals, such as Sn^[13], In^[14,15], Pb-Bi-Cd^[15], and Bi_32.5Sn_16.5In₅₁^[16], have been widely used as the metal adhesive to bond the La(Fe, Si)₁₃ hydrides. However, the broadening of the ∆T of La(Fe, Si)₁₃ hydrides is rarely studied.

In this research, various La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_y alloys of the first-order phase transition with the Curie temperature interval of 5 K were mix-bonded by epoxy resin to expand the ΔT. The magnetic properties of the composites were investigated.

1 Experiment

The industrially pure raw materials (99.5wt% La, 99.5wt% Ce, 99wt% Fe, 99.7wt% Mn, 99.9wt% Si) were cast to prepare the La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3 master alloys by medium frequency induction furnace. Afterwards, the alloys were annealed at 1363 K for 144 h in Ar atmosphere. The bulk alloys were crushed into irregular particles and hydrogenated in a hydrogen atmosphere of 0.13 MPa at 593 K for 5 h until they were hydrogen-saturated. Then the La_0.8Ce_0.2Fe_11.7-x-Mn_xSi_1.3H_1.8 alloys were obtained and subsequently ground into powders with particle size<0.15 mm. All the powders consisting of different components were bonded by 1.8wt% epoxy resin, then pressed into cylinders at the pressure of 557 MPa, and finally annealed at 175 °C for 45 min. The La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloys with Mn contents of x= 0.23, 0.26, 0.29, 0.32 (wt%) are denoted as specimen A, B, C, and D, respectively. The composition of mix-bonded La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloys is listed in Table 1.

Table 1  Components of mix-bonded La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloys (wt%)

Mix-bonded specimen	Component				Epoxy resin
	A	B	C	D
MAB	49.10	49.10	-	-	1.8
MBC	-	49.10	49.10	-	1.8
MCD	-	-	49.10	49.10	1.8
MABC	32.73	32.73	32.73	-	1.8
MBCD	-	32.73	32.73	32.73	1.8
MABCD	24.55	24.55	24.55	24.55	1.8

The phase components of specimens were determined by X-ray diffraction (XRD). The magnetic properties were measured by VersaLab with VSM option, and the magnetic entropy changes were calculated by Maxwell relation. The adiabatic temperature changes were tested by the home-made adiabatic temperature change direct test device. The compressive strength was tested by the electronic universal testing machine (WDW3200).

2 Results and Discussion

Fig.1 shows the XRD patterns of La_0.8Ce_0.2Fe_11.7-xMn_x-Si_1.3H_1.8 alloys with x=0.23, 0.26, 0.29, 0.32. It can be seen that all the specimens have the main cubic phase of NaZn₁₃ structure, and a small amount of impurity phase α-Fe can be detected.

Fig.1  XRD patterns of specimen A, B, C, and D at room temperature

Fig.2 shows the relationship between the magnetization M and temperature T of various La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloys at the magnetic field of 0.05 T in the warming process. The T_C is obtained by the valley bottom values of dM/dT-T curves. It is known that the T_C of La(Fe_xSi_1-x)₁₃ alloys can be reduced through the partial substitution of Mn for Fe or Ce for La^[17], so the T_C of La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloys is relatively lower than that of La(Fe, Si)₁₃H_1.8 alloy. The T_C interval of specimen A, B, C, and D is 5 K, as listed in Table 2. The specimen M_ABC and M_BCD both show the distinctive magnetic transitions. In order to further widen the half peak width of the magnetization of alloys, specimen A, B, C, and D are mix-bonded, and therefore four magnetic transition points can be observed for the specimen M_ABCD, which are basically consistent with those of the single-component-bonded specimen A, B, C, and D.

Fig.2  Relationships between magnetization and temperature of different specimens: (a) single-component specimens A~D; (b) two-component-bonded specimens M_AB, M_BC, and M_CD; (c) three-component-bonded specimens M_ABC and M_BCD; (d) four-component-bonded specimen M_ABCD

Table 2  Magnetic transition temperatures T_C of different La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloy specimens (K)

A	B	C	D	MAB	MBC	MCD	MABC	MBCD	MABCD
273	278	283	288	277	282	287	276, 282	283, 288	272, 276, 283, 287

The area of magnetic phase transition from the ferromagnetic state to the paramagnetic state of the mix-bonded specimens is larger than that of the single-component-bonded specimens, indicating that the maximal magnetic entropy change of the mix-bonded specimens reduces, compared with that of the single-component-bonded specimens. With increasing the components in mix-bonded specimens, the temperature range of magnetic phase transition becomes wider, and the maximum magnetic entropy change is decreased.

Fig.3 exhibits the magnetization isotherms and the corresponding Arrott plots of different La_0.8Ce_0.2Fe_11.7-xMn_x-Si_1.3H_1.8 alloys. According to the magnetization isotherms, there is no obvious change of magnetic moments for the single-component-bonded specimens. The magnetic moment changes more and more slowly with increasing the magnetic field for the two-component-bonded specimens, implying that the field-induced first-order phase transition is weakened and a smaller hysteresis of 2.68 J/kg is achieved, compared with that of the single-component-bonded specimens (3.66 J/kg for specimen C; 5.15 J/kg for specimen D). It is found that the hysteresis caused by the magnetic field cycles can reduce the refrigeration capacity^[18]. Hence, the reduction in hysteresis for the mix-bonded specimens is favorable for the magnetic refrigeration. According to the s-d orbit of electrons, the slop of the Arrott plots can be used to describe the types of the magnetic phase transition. If the slop is positive, the phase transition is second-order; if the slop is negative or the inflection point exists, the phase transition is first-order^[19]. The Arrott plots in Fig.3d and 3e have obvious inflection points, indicating that the field-induced first-order phase transition occurs. The inflection point in Fig.3f is not obvious, indicating that the first-order phase transition is weakened in the two-component-bonded specimen M_CD.

Fig.3  Isothermal magnetization curves at magnetic strength of 0~2 T (a~c) and Arrott plots (d~f) of specimen C (a, d), specimen D (b, e), and specimen M_CD (c, f)

The maximum magnetic entropy ΔS_M of mix-bonded specimens at the magnetic strength of 0~2 T can be calculated based on the isothermal magnetization curves by Maxwell relation^[20], as follows:

$$\mathrm{\Delta }{S}_{\mathrm{M}}(T,H)={\int }_{\mathrm{0}}^H{(\partial M/\partial T)}_H\mathrm{d}H$$

(1)

where T is the temperature; H is the magnetic field intensity; M is the magnetization. Table 3 shows the calculation results of maximum magnetic entropy of different La_0.8Ce_0.2-Fe_11.7-xMn_xSi_1.3H_1.8 alloys. The maximal magnetic entropy changes of all the mix-bonded specimens are decreased, compared with those of the single-component-bonded specimens. The average ΔS_M of two-component-bonded specimens, three-component-bonded specimens, and four-component-bonded specimens is decreased by about 6.7%, 18.6%, and 35.2%, compared with that of the corresponding single-component-bonded specimens, respectively. The decreased ΔS_M of the mix-bonded specimens is caused by the weakening of the magnetic transformation.

Table 3  ΔS_M, ∆T, and RCP of different La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 alloy specimens

Specimen	A	B	C	D	MAB	MBC	MCD	MABC	MBCD	MABCD
-ΔSM/J·kg-1·K-1	8.96	8.62	9.33	8.88	7.91	8.61	8.53	7.10	7.47	5.80
∆T/K	12.5	12.7	12.3	11.0	15.0	13.6	14.5	19.0	18.0	24.0
T1/K	270.0	275.0	280.3	286.0	272.0	276.4	282.5	270.0	277.0	270.3
T2/K	282.5	287.7	292.6	297.0	287.0	290.0	296.0	289.0	295.0	294.3
RCP/J·kg-1	112.00	109.47	114.76	97.71	118.64	117.1	123.67	134.83	134.46	139.20

In order to further study the refrigeration capacity of the materials, the relative cooling power (RCP) is calculated by Eq.(2), as follows:

$$\mathrm{R}\mathrm{C}\mathrm{P}=-\mathrm{\Delta }{S}_{\mathrm{M}}\mathrm{\Delta }T$$

(2)

where ∆T=T₂-T₁ (the values of T₁ and T₂ are listed in Table 3). Although the ΔS_M of the mix-bonded specimens decreases, compared with that of the single-component-bonded specimens, RCP of the mix-bonded specimens is higher than that of all the single-component-bonded specimens as a result of the broadening of ∆T. The average RCP of two-component-bonded, three-component-bonded, and four-component-bonded specimens increases by 10.4%, 24.1%, and 28.3%, compared with that of the single-component-bonded specimens, respectively. In addition, RCP of the four-component-bonded specimen is the largest of 139.20 J·kg^-1.

Fig.4 shows the magnetic entropy changes (∆S) of different specimens. The ∆T of specimen M_AB is 15.0 K, which is smaller than that of the superimposed specimen A and B (T_2B-T_1A=17.7 K). The ∆T of specimen M_BCD is 18.0 K, which is smaller than that of the superimposed specimen B, C, and D (T_2D-T_1B=22.0 K). The ∆T of specimen M_ABCD is 24.0 K, which is also smaller than that of the superimposed specimen A, B, C, and D (T_2D-T_1A=27.0 K).

Fig.4  Magnetic entropy changes of different specimens at magnetic strength of 0~2 T: (a) specimens A, B, and M_AB; (b) specimens B, C, D, and M_BCD; (c) specimens A, B, C, D, and M_ABCD

Fig.5 exhibits the temperature dependence of the adiabatic temperature change (∆T_ad) at the magnetic strength of 0~1.5 T.

Fig.5  Relationships between adiabatic temperature change and temperature of different specimens at the magnetic strength of 0~1.5 T: (a) single-component specimens A~D; (b) two-component-bonded specimens M_AB, M_BC, and M_CD; (c) three-component-bonded specimens M_ABC and M_BCD; (d) four-component-bonded specimen M_ABCD

The max ∆T_ad is 2.25, 2.33, 2.65, 2.45, 2.24, 2.01, 2.30, 1.67, 1.89, and 1.42 K for specimen A, B, C, D, M_AB, M_BC, M_CD, M_ABC, M_BCD, and M_ABCD, respectively. It can be seen that the max ∆T_ad of four-component-bonded specimen M_ABCD is the smallest, but its cooling temperature zone is the widest, compared with those of other specimens, which is consistent with the magnetic entropy change results.

Fig.6 shows the compressive stress-strain curve of the four-component-bonded specimen M_ABCD. An obvious yield stage before reaching the maximum compressive strength of 157 MPa can be observed, which is appliable for the magnetic refrigerator as the magnetic refrigerant.

Fig.6  Compressive stress-strain curve of four-component-bonded specimen M_ABCD

3 Conclusions

1) The first-order magnetic transformation in the mix-bonded La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 (x=0.23, 0.26, 0.29, 0.32) alloys is weakened, the hysteresis and magnetocaloric effect decrease, and the cooling temperature range is widened, compared with those of the single-component-bonded alloys.

2) The cooling temperature range of the four-component-bonded specimen is the widest, and its relative cooling power is optimal of 139.2 J·kg^-1 among all the mix-bonded specimens.

3) The mix-bonding preparation of La_0.8Ce_0.2Fe_11.7-xMn_x-Si_1.3H_1.8 alloy is an effective way to widen the cooling temperature range and to increase the relative cooling power.