Magnetocaloric Properties of Melt-Extracted Gd-Co-Al Amorphous/Crystalline Composite Fiber

Abstract

In this work, a series of Gd-based amorphous/crystalline composite fibers (ANCFs) were prepared by regulating the Gd content in Gd-Co-Al alloys using the melt-extracted method. Compared to the amorphous alloy, the ANCFs display excellent magnetic refrigeration capacity (RC). Among them, Gd₈₅Co₅Al₁₀ ANCF had the largest RC (841 J kg⁻¹) and the widest working temperature range (245 K). Compared with Gd₇₀Co₁₀Al₂₀, RC and working temperature range increased by 56% and 119%, respectively. This superior property is attributed to the ideal coupling between the amorphous phase and the crystalline. This result opens a new door to optimize the magnetic refrigeration capacity by controlling the amorphous crystalline composite structure.

1. Introduction

Unlike the conventional gas-compression refrigeration, magnetic refrigeration has attracted much attention for being an ideal candidate in refrigeration because of its advantages in environmental friendliness and energy efficiency [1,2,3,4]. Magnetocaloric effect (MCE) is a thermal response caused by the change of magnetic entropy for a varying magnetic field. Magnetocaloric materials with specific magnetocaloric properties are the core issue for their actual applications, e.g., adjustable magnetic transition temperature (T_C), large magnetic entropy change (−ΔS_M), large RC, and wide working temperature range [5,6]. Previous works show that the rare earth (RE)-based alloys usually perform better in MCE than transition metals such as Fe- or Mn-based alloys [1,2,3,5,7,8,9,10,11,12,13,14,15,16,17,18,19]. For example, RE-based amorphous alloys show reduced Curie temperature (T_C), a wider working temperature range, and higher RC [20], which bestows them with superior potential refrigerants for cryogenic refrigeration, such as at liquid nitrogen temperatures or liquid helium temperatures [6,21]. However, the reduced T_C limits their refrigeration ability near room temperature [15,20,22,23].

To enhance the temperature range of magnetocaloric materials, the other phases with different T_C were induced [6,24,25,26]. For example, the composite of Gd₆₅Mn₂₅Si₁₀ amorphous ribbons and Gd sheets induced by hot-pressing displays a table-like magnetic entropy change in a wide temperature range 73 K [25]. However, without metallurgical grade bonding, composites prepared by physical compounding methods like hot-pressing have weakened strength and thermal conductivity. On the other hand, the amorphous/crystalline composites can also be achieved by the composition design or a part-crystallization annealing for amorphous alloys [6,21,26]. In these processes, the metallurgical grade-binding between different phases can be achieved, which promotes the refrigeration abilities of samples. Besides, the melt extraction method with high heat transfer capacity was used to prepare the fibers, which have relatively high magnetic, thermal, electrical and mechanical properties [6,27,28,29].

In this work, a series of Gd-Co-Al amorphous/nanocrystalline composites fibers (ANCFs) have been prepared by the melt-extraction method. The optimal refrigeration ability was determined by the wide working temperature range from 50 K to 295 K, large −ΔS_M (8.7 J kg⁻¹ K⁻¹) and RC (841 J kg⁻¹) when the Gd content is at 85%. Besides, the optimal refrigeration ability of Gd₈₅Co₅Al₁₀ originates from the coupling effect between crystalline Gd and an amorphous matrix. This result suggests that the amorphous/crystalline composite formed by composition design has potential to be a new method to fabricate superior magnetocaloric materials.

2. Experimental Details

The master alloys with nominal composition of Gd₇₀Co₁₀Al₂₀, Gd₈₀Co₁₀Al₁₀, Gd₈₅Co₅Al_10, and Gd₈₀Co₅Al₁₅ (at.%) were fabricated by arc melting pure Gd, Co and Al in a Ti-gettered argon atmosphere. In addition, the master alloys were prepared for micron fibers by the melt-extraction method (10 m/s). The phase compositions were examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation, and the specimen was scanned over a 2θ range from 20° to 90° at 6°/min. The surface and cross-section of the fibers were observed by scanning electron microscopy (SEM, EVO18, ZEISS, Berlin, Germany) at operating voltages of 20 kV. The microstructures were examined by transmission electron microscopy (TEM, Themo Fisher, Talos F200×, Waltham, MA, USA) at operating voltages of 200 kV, bright field image (BF) and selected-area electron diffraction (SAED) of the different electron beam incident axis were used to investigate the microstructure of the ANCF specimen. The magnetic properties were then measured using the Magnetic Property Measurement System (MPMS, Quantum Design Int., San Diego, CA, USA).

3. Results and Discussion

In Figure 1a, the schematic melt-extraction method for preparing Gd-Co-Al fibers is given. The master alloy ingot was melted in a high-frequency-induced current, and the high-speed rotation Mo wheel spun out the melt and formed the fibers finally. The length of prepared fibers was about 15 cm, as shown in Figure 1b. SEM results show that the prepared fibers exhibit narrow distribution of size, and that their diameter is about 70 μm for the Gd₈₅Co₅Al₁₀ sample. Besides, the fibers were not cylindrical in a regular manner because of the surface tension during the melt-extraction process, as shown in Figure 1c–e. Notably, fibers have better application performance compared with their bulk or ribbon counterparts because their large surface area enhances the heat-exchange efficiency [27,28,29].

The XRD patterns of the Gd₇₀Co₁₀Al₂₀, Gd₈₀Co₁₀Al₁₀, Gd₈₀Co₅Al₁₅ and Gd₈₅Co₅Al₁₀ as-cast fibers is shown in Figure 2. For Gd₇₀Co₁₀Al₂₀, the XRD pattern only shows a broad hump, which indicates its pure amorphous structure. For Gd₈₀Co₁₀Al₁₀, Gd₈₀Co₅Al_15, and Gd₈₅Co₅Al₁₀, crystal peaks appear in their XRD patterns, which were contributed by Gd crystal. It is interesting that the (10$\overline{1}$3 ) peak of Gd appears in the XRD pattern for Gd₈₅Co₅Al₁₀, besides the (0002), (10$\overline{1}$0) and (20$\overline{2}$2) peaks. The above results indicate that high Gd content reduces glass-forming abilities and favors the formation of Gd crystal. In other words, the phase compositions of Gd-Co-Al alloys can be adjusted by regulating the Gd content.

The microstructures of Gd₈₅Co₅Al₁₀ were further identified by TEM and EDS (Energy Dispersive Spectroscopy) mapping. The results have shown in Figure 3a–d. As shown in Figure 3a, Gd₈₅Co₅Al₁₀ has amorphous/crystalline composite structures, in which the island-like crystal phase is embedded in the river-like amorphous matrix. The size of the crystal phase is 300~500 nm. In Figure 3b, the high-resolution TEM (HRTEM) shows the amorphous maze-like pattern and crystal lattice fringes pattern in regions A and B, respectively. Moreover, Figure 3c shows the selected-area electron diffraction (SAED) for region A, which confirmed its pure amorphous structures. Additionally, in Figure 3d, the SAED for region B is confirmed as the hexagonal Gd (10$\overline{1}$0) zonal axis. The EDS-mapping result further suggests that the nanocrystals were in the Gd phase, as shown in Figure 3e. Based on the above results, we confirmed the nature of ANCF for the Gd₈₅Co₅Al₁₀ sample. This conclusion is also applicable to Gd₈₀Co₁₀Al₁₀ and Gd₈₀Co₅Al₁₅ samples.

For these samples, the temperature dependences of magnetic behavior were measured under a 200 Oe magnetic field. As shown in Figure 4a–d, the zero-field cooling (ZFC) curves and field-cooling (FC) curves separate at about 50 K, which means that the spin freezing temperatures for all samples is about 50 K [30]. For Gd₇₀Co₁₀Al₂₀, there is only one T_C at 101 K, as shown in the insert of Figure 4a. For ANCFs there were two magnetic transition temperatures belonging to the amorphous and crystalline phases separately, as shown in Figure 4c,d. The T_C1 of the amorphous phase were 116, 98, 98 K, and the T_C2 of the crystalline phases were 271, 264, 264 K for Gd₈₀Co₅Al₁₅, Gd₈₀Co₁₀Al₁₀ and Gd₈₅Co₅Al₁₀ ANCFs, respectively. These results are also listed in

Table 1. The magnetic transition temperature (T_tran), peak value of magnetic entropy change ($-\Delta S_{\mathrm{M}}^{\mathrm{pk}}$), full width at half-maximum of −ΔS_M peak (ΔT_FWHM), refrigeration capacity (RC) under a maximum applied field of 5 T for some selected Gd-based magnetic refrigeration material.

No.	Composition	Ttran (K)	$-{\Delta }{\mathit{S}}_{\mathbf{M}}^{\mathbf{p}\mathbf{k}} \left(\mathbf{J}{\mathbf{kg}}^{-1}{\mathbf{K}}^{-1}\right)$	ΔTFWHM (K)	RC (J kg−1)	Ref.
1	Gd70Co10Al20	101	6.36	112	538	This work
2	Gd80Co5Al15	116/271	4.06/2.64	220	622	This work
3	Gd80Co10Al10	98/264	4.31/3.29	207	625	This work
4	Gd85Co5Al10	98/264	4.24/5.07	245	841	This work
5	Gd4Co3	208	7.2	~	547	[31]
6	Gd55Al25Co20	100	9.7	~	652	[15]
7	Gd50Co45Fe5	289	3.8	~	521	[12]
8	Gd53Al24Co20Zr3	93	9.4	~	590	[13]
9	Gd5Si2Ge2	276	18.6	~	306	[32]
10	Gd	294	10.2	~	410	[33]

.

The magnetocaloric properties were investigated by measuring the isothermal magnetization curves for these fibers. In Figure 5a,b, the M-H curves and Arrot plot for Gd₈₅Co₅Al₁₅ ANCF are given. In Figure 5a, magnetization continually changes following the changes of an applied magnetic field; thus there were no features of field-induced phase transition, which is the characteristic of first-order magnetic transition [28,34]. In Figure 5b, the positive slopes further confirmed the second-order magnetic phase transition of both the amorphous and crystalline phases for ANCFs [35]. The second-order magnetic transition had almost zero magnetic and thermal hysteresis, benefiting from a wide working temperature range [34,36]. These features ensure the magnetocaloric performances for ANCFs.

The ΔS_M in the different applied magnetic fields can be calculated using the following equation [37]:

$$\Delta S_M(T,H)=\frac{{\displaystyle {\int }_0^{H}M(T_i,H)}dH-{\displaystyle {\int }_0^{H}M(T_{i+1},H)}dH}{T_i-T_{i+1}}$$

(1)

Figure 6a–d show the temperature dependences of −ΔS_M in the different magnetic fields. In Figure 6a, for Gd₇₀Co₁₀Al₂₀, there is only one −ΔS_M peak around 100 K due to its pure amorphous structures, and the peak value is 6.36 J kg⁻¹ K⁻¹ (H = 5 T). For ANCFs, two −ΔS_M peaks belong to the amorphous and crystalline phases, respectively. With the increase of Gd content, the peak values of −ΔS_M near 100 K decreases gradually. Meanwhile, the peak values of −ΔS_M around 270 K were strengthened with the increase of the crystalline Gd phase. At H = 5 T, the first −ΔS_M peak values were 4.31 J kg⁻¹ K⁻¹, 4.06 J kg⁻¹ K⁻¹ and 4.24 J kg⁻¹ K⁻¹, and at the second −ΔS_M peak values were 3.29 J kg⁻¹ K⁻¹, 2.64 J kg⁻¹ K⁻¹ and 5.07 J kg⁻¹ K⁻¹ for Gd₈₀Co₁₀Al₁₀, Gd₈₀Co₅Al₁₅ and Gd₈₅Co₅Al₁₀ ANCFs, respectively. These values are listed in Table 1. The above results show that the amount of Gd crystalline phases of ANCFs can enhance the RC near room temperature. Especially, Gd₈₅Co₅Al₁₀ ANCF not only had the highest −ΔS_M peak value but also the crystalline phase had a similar −ΔS_M peak value to that of the amorphous phase, thus the ratio of amorphous and crystalline phases is optimal in Gd₈₅Co₅Al₁₀ ANCF. Because of this optimal ratio, Gd₈₅Co₅Al₁₀ ANCF keeps a large magnetic entropy change in 50~300 K, thus Gd₈₅Co₅Al₁₀ ANCF is promising for a large application temperature span. Although the −ΔS_M peak values decreased due to the composite structure for ANCF, it is worth noting that such values were still larger than those of transition metals [38,39,40,41,42].

RC is an important parameter indicating the efficiency of magnetic refrigerant. Here RC is obtained by using the following equation:

$$RC={\displaystyle {\int }_{T_1}^{T_2}\Delta S_M}(T)dT$$

(2)

where T₁ and T₂ are identified by the temperature values at half height of magnetic entropy change peaks. For amorphous Gd₇₀Co₁₀Al₂₀, the RC is 538 J kg⁻¹. For Gd₈₀Co₁₀Al₁₀, Gd₈₀Co₅Al₁₅ and Gd₈₅Co₅Al₁₀ ANCF, the RCs were 622 J kg⁻¹, 625 J kg⁻¹, and 841 J kg⁻¹, respectively. Compared to Gd₇₀Co₁₀Al₂₀, the RC of ANCF is significantly improved due to the formation of crystalline Gd. Gd₈₅Co₅Al₁₀ ANCF shows the best performance, which is an increase of about 56% compared to pure amorphous Gd₇₀Co₁₀Al₂₀. Just as important, the range of working temperature range for Gd₈₅Co₅Al₁₀ can be as wide as 245 K from room temperature to liquid nitrogen temperature, which was also the best performance and was about 119% higher than Gd₇₀Co₁₀Al₂₀. Such a performance also proved the optimal ratio of amorphous and crystalline phases in Gd₈₅Co₅Al₁₀ ANCF.

To further distinguish the contributions from both phases in ANCF, the −ΔS_M-T curves were fitted by the Gaussian peak function. The reliability of the fitting results was confirmed by the coefficient of determination R² (≥0.917). In Figure 7a, there is only one fitted peak belonging to the amorphous phase of Gd₇₀Co₁₀Al₂₀. For ANCF, the contribution from the amorphous phase decreased with increasing Gd content, while the contribution from the crystalline phase increased, as shown in Figure 7b–d. In the Gd₈₅Co₅Al₁₀ ANCF, amorphous and crystalline phases comparably contributed to magnetocaloric performance.

The fitted peak value of −ΔS_M and RC for both phases are shown in Figure 8a,b. The −ΔS_M and RC of Gd₇₀Co₁₀Al₂₀ were completely in the amorphous phase. It was obvious that with the increase of Gd content, the −$\Delta S_{\mathrm{M}}^{\mathrm{pk}}$ and RC of crystal Gd gradually increased, which further indicated the positive correlation between the Gd content in alloy and the crystal Gd content in ANCFs. Once the Gd crystalline phase appeared, the magnetic entropy changed peaks and RC was enhanced obviously, which reflected the coupling effect between the amorphous and Gd phase improved the refrigeration ability. For the best magnetocaloric performance of Gd₈₅Co₅Al₁₀ ANCF, the magnetic entropy change peaks were 4.0 J kg⁻ K⁻ and 4.7 J kg^−a K^−a, and the RC values were 477 J kg^−J and 364 J kg^−J for amorphous and crystalline phases, respectively. In other words, the optimal ratio of amorphous and crystalline phases was established in Gd₈₅Co₅Al₁₀ ANCF.

To analyze the MCE performance characteristics of ANCFs, the RC and T_C of typical Gd-based comparisons are summarized in Figure 9, including Gd-based metallic glasses (MG), Gd-based crystals, and Gd₈₅Co₅Al₁₀. These results are also listed in Table 1. The Gd₈₅Co₅Al₁₀ ANCF possessed the largest RC of 841 J kg⁻¹ and widest working temperature range. Specifically, the wide working temperature window (from 50 K to 295 K) of Gd₈₅Co₅Al₁₀ is critical for its application in refrigerators. This makes refrigeration applicable at both room temperature and liquid nitrogen temperature.

4. Conclusions

By adjusting the Gd content, the transition of Gd-Co-Al magnetic refrigeration material from amorphous fiber to ANCF was realized. We found that the RC of Gd₈₅Co₅Al₁₀ reaches 841 J kg⁻¹ and the temperature range extended from 50 K to 295 K, which contributed to excellent MCE. This result suggests that the amorphous/crystalline composite, produced by controlling the Gd content, is a superior method to obtain a promising magnetic refrigerant from room temperature to liquid nitrogen temperature, which is not only for achieving giant RC but also for obtaining a wide temperature range.