One-Step Sintering Process for the Production of Magnetocaloric La(Fe,Si)₁₃-Based Composites

Abstract

A one-step sintering process was developed to produce magnetocaloric La(Fe,Si)₁₃/Ce-Co composites. The effects of Ce₂Co₇ content and sintering time on the relevant phase transformations were determined. Following sintering at 1373 K/30 MPa for 1–6 h, the NaZn₁₃-type (La,Ce)(Fe,Co,Si)₁₃ phase formed, the mass fraction of α-Fe phase reduced and the CeFe₇-type (La,Ce)(Fe,Co,Si)₇ phase appeared. The mass fraction of the (La,Ce)(Fe,Co,Si)₇ phase increased, and the α-Fe phase content decreased with increasing Ce₂Co₇ content. However, the mass fraction of the (La,Ce)(Fe,Co,Si)₇ phase reduced with increasing sintering time. The EDS results showed a difference in concentration between Co and Ce at the interphase boundary between the 1:13 phase and the 1:7 phase, indicating that the diffusion mode of Ce is reaction diffusion, while that of Co is the usual vacancy mechanism. Interestingly, almost 100 % single phase (La,Ce)(Fe,Co,Si)₁₃ was obtained by appropriate Ce₂Co₇ addition. After 6 h sintering at 1373 K, the Ce and Co content in the (La,Ce)(Fe,Co,Si)₁₃ phase increased for larger Ce₂Co₇ content. Therefore, the Curie temperature increased from 212 K (binder-free sample) to 331 K (15 wt.% Ce₂Co₇ sample). The maximum magnetic entropy change (−ΔS_M)^max decreased from 8.8 (binder-free sample) to 6.0 J/kg·K (15 wt.% Ce₂Co₇ sample) under 5 T field. High values of compressive strength (σ_bc)^max of up to 450 MPa and high thermal conductivity (λ) of up to 7.5 W/m·K were obtained. A feasible route to produce high quality La(Fe,Si)₁₃ based magnetocaloric composites with large MCE, good mechanical properties, attractive thermal conductivity and tunable T_C by a one-step sintering process has been demonstrated.

1. Introduction

Climate change is of high global significance, resulting in high interest in more efficient cooling systems. Therefore, there is considerable research interest in the emerging attractive topic of magnetic refrigeration (MR) [1,2]. Advances in magnetocaloric materials (MCMs) are urgently needed to realize the promise of MR. MCMs which exhibit a giant magnetocaloric effect (MCE) include Gd₅(Si_xGe_1−x)₄ [3,4], MnFe(P,As) [5], Mn(Fe,Co)Ge [6], LaFe_13−xSi_x [7,8], and Heusler alloys [9], which undergo a first-order magnetic transition (FOMT) near their Curie temperatures (T_Cs). Recently, other materials, such as RE₂ZnMnO₆ (RE = Gd, Dy, and Ho) perovskites [10], RE₂FeAlO₆ (RE = Gd, Dy, Ho) oxides [11], RE₆₀Co₂₀Ni₂₀ (RE = Ho and Er) amorphous ribbons [12], LaFe₁₂B₆-based materials [13], and rare earth-based intermetallic compounds [14], were found to exhibit outstanding magnetocaloric performance at cryogenic temperatures. The development of these materials can promote the development of magnetic refrigeration technology.

NaZn₁₃-type La(Fe,Si)₁₃ compounds have attracted considerable attention as magnetocaloric materials due to their large magnetocaloric effect (MCE) values, relatively low cost, tunable Curie temperature (T_C), and environmental friendliness [1,7,15]. However, these compounds have some disadvantages [16,17,18]: First, it is difficult to form single-phase La(Fe,Si)₁₃ material. An ingot of La(Fe,Si)₁₃ has to be annealed above 1273 K for several days or even a month to obtain a single phase [18]. Second, the T_C of LaFe_13−xSi_x (1.0 ≤ x ≤ 1.6) compounds is less than 210 K, which is too low for near room temperature magnetic cooling. Third, these compounds show a first-order magnetic transition (FOMT), which has a desirable large magnetic entropy change, but is accompanied by thermal and magnetic hysteresis and relatively large undesirable changes in crystal symmetry and/or volume [17]. The magneto-volume effect can lead to pulverization of bulk LaFe_13−xSi_x magnetocaloric materials when the magnetic field is varied during the MR process [17]. The brittleness and magnetic volume change reduce formability, limiting the progress of this material for commercial applications.

Considerable research has been performed to overcome these shortcomings. Short time annealing produced a single phase material in melt-spun samples [19]. Combining kilogram-scale strip-casting with short time annealing has been deployed for mass production of LaFe_13−xSi_x-based magnetic cooling materials [20]. Elemental substitution of Fe (by Co [18,21,22], Si [23], etc.) or La (by Ce [24]) can improve magnetocaloric performance. T_C can be raised from ~200 K to room temperature or even higher by addition of interstitial atoms like H [23,25,26,27]. Hydrogenation of LaFe_13−xSi_x is the most effective method to shift T_C to room temperature while maintaining a large magnetocaloric effect [28]. Quaternary La(Fe,Co,Si)₁₃ compounds can increase T_C and change the magnetic transition from first-order to second-order, accompanied by lower MCE.

LaFe_13−xSi_x bulk composites can be prepared through powder metallurgy technology using LaFe_13−xSi_x particles and binder powder as raw materials. Although LaFe_13−xSi_x is hard and brittle, particle fracture can be avoided by adding low hardness binders during the powder metallurgy process. Additionally, the magneto-volume effect can be reduced by pulverization of the material [29]. Various binder materials, such as polymers [30], high thermal conductivity metals [31], and low melting alloys [32,33], have been studied to obtain high performance using relatively low sintering temperatures. High-temperature sintering will lead to metallurgical reactions between the binder and the particles, it will also significantly decrease the mass fraction of the 1:13 phase and lower the MCE of the composites [31]. The magnetocaloric properties of the composites arise from the La-Fe-Si based alloy. Addition of binder material reduces the mass fraction of magnetocaloric material and decreases the MCE of the composites.

La-Co alloys [34,35] have been used as binder due to the low enthalpy of formation of the LaCo₁₃ phase (–5.79 kJ/mol) [36], lower La-Fe binary phase content and the low melting point of a La-based eutectic alloy [37]. Based on grain boundary diffusion (GBD) theory [38], Co diffusion occurs through annealing after hot pressing [34,39]. During annealing, Co can diffuse into the 1:13 matrix, influencing the magnetic and magnetocaloric properties and promoting the formation of the 1:13 phase. The binder content and annealing parameters play an important role in the phase formation process. For example, addition of more than 5 wt.% of La-Al alloy was detrimental to 1:13 phase formation in La-Fe-Si/La-Al composites [40]. Our previous work [41] showed that excess addition of Ce-Co alloy binder would result in the formation of a new phase, which disappeared with increasing heat treatment time. However, the sintering temperature is below the melting point of Ce₄₀Co₆₀ (1323 K), resulting in a porous structure and insufficient mechanical properties. The large pressure of hot pressing (600 MPa) and long diffusion annealing times (up to 24 h) are unfavorable for industrial production. To overcome these limitations, we combined the advantages of hot pressing and diffusion annealing in this work. We developed a novel process to prepare La-Fe-Si-based composites by sintering to shorten the processing time and improve the compressive properties. The effect of Ce₂Co₇ binder on the formation of the 1:13 phase, thermal conductivity, mechanical, magnetic and magnetocaloric properties of LaFe_11.6Si_1.4/Ce₂Co₇ composites was studied. The diffusion mechanism of Ce and Co was also investigated. The T_C increased from ~200 K to near room temperature. High values of the compressive strength (σ_bc)^max of up to 450 MPa and of the thermal conductivity (λ) of up to 7.5 W/m·K were successfully obtained.

2. Experimental Section

Annealed LaFe_11.6Si_1.4 flakes were prepared following the procedure described in our previous work [41]. The Ce₂Co₇ compounds were prepared by an arc melting process with pure Ce (≥99.5 wt.%) and Co (≥99.9 wt.%). The annealed LaFe_11.6Si_1.4 flakes were mechanically milled to powders with a particle size in the range of 45 to 100 μm. Ce₂Co₇ fine powders were prepared with a particle size smaller than 30 μm. The LaFe_11.6Si_1.4 and Ce₂Co₇ powders were homogenously mixed with 0, 5, 10, and 15 wt.% content of Ce₂Co₇ powder, followed by sintering under uniaxial stress of 30 MPa at 1373 K to produce cylindrical samples with 15 mm in diameter and 5 mm in height.

XRD characterization was carried out using X-ray diffractometer (Rigaku D/max-2200/PC, Tokyo, Japan) with Cu-Kα₁ radiation (λ = 1.54056 Å) at a scan rate of 2°/min. The phase compositions and microstructures were analyzed by a thermal field emission scanning electron microscope (SEM, FEI Nova Nano SEM 430, Davis, CA, USA) equipped with an energy-dispersive spectrometer (EDS) attachment. Measurements of thermal conductivity and magnetic properties were carried out using a Physical Property Measurement System (PPMS-9, Quantum Design, San Diego, CA, USA)). The compressive strengths of the samples were measured by a mechanical testing system (Shimadzu AG-100NX, Kyoto, Japan). The porosity (P) of the composites were determined by the following equation: P = 1 − ρ/ρ₀, where ρ is the effective density and ρ₀ is the theoretical density. The effective density (ρ) of each sample was tested by the Archimedes method. The theoretical density (ρ₀) of each sample was calculated by the following equation: ${\rho }_0=\frac{100}{{{\displaystyle \sum }}_i\frac{m_i}{{\rho }_i}}$, where m_i was the mass of the corresponding phase in the 100 g sample and the ρ_i was the density of the specific phase.

3. Results and Discussion

3.1. Phase Analysis

Figure 1a shows the Rietveld refined X-ray diffraction (XRD) patterns of LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10, and 15) composites sintered at 1373 K for 6 h. LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 5, 10, and 15) sintered composites exhibited a majority 1:13 phase. The α-Fe phase content decreased, corresponding to the appearance of a CeFe₇-type phase (1:7 phase) and a La₂O₃ phase. For composites sintered at 1373 K for 6 h, higher Ce₂Co₇ binder content (less than 15 wt.%) can promote formation of the 1:13 phase and decrease the α-Fe phase content (Figure 1a and

Table 1. The weight percentages of phases in the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) sintered composites and fit coefficient.

Sample	Sintering@1373 K		Weight Percentage (wt.%)			Fit Coefficient
			1:13 phase	α-Fe Phase	1:7 Phase	Rp	Rwp	Rexp	χ2
Annealed LaFe11.6Si1.4 flakes			84.69(5)	15.31(2)		0.95	1.62	0.80	4.16
LaFe11.6Si1.4/x wt.%Ce2Co7 composites	x = 0	1 h	89.36(5)	10.64(2)	/	0.95	1.44	1.10	1.71
		3 h	87.74(5)	12.26(2)	/	1.28	1.73	1.42	1.49
		6 h	89.26(5)	10.82(2)	/	0.87	1.36	1.01	1.80
	x = 5	6 h	93.21(5)	5.90(1)	0.89	0.87	1.20	1.01	1.43
	x = 10	1 h	87.28(5)	4.72(1)	8.00	1.08	1.46	1.26	1.33
		3 h	87.02(5)	6.28(1)	6.70	0.98	1.27	1.23	1.07
		6 h	93.93(5)	3.30(1)	2.77	0.90	1.19	0.98	1.46
	x = 15	6 h	88.29(5)	4.96(1)	6.75	1.19	1.53	1.09	1.98

).

For example, LaFe_11.6Si_1.4/5wt.%Ce₂Co₇ composites contained 93.21 wt.% 1:13 phase, 5.90 wt.% α-Fe phase and a small amount of 1:7 phase (0.89 wt.%). However, when the addition of Ce₂Co₇ binder rose to 10 wt.% or more, the mass fraction of 1:7 phase increased. Moreover, the mass fraction of 1:7 phase in the LaFe_11.6Si_1.4/10 wt.%Ce₂Co₇ composites decreased with an increase in sintering time. As reported earlier [41], the 1:7 phase occurred at grain boundaries after annealing for a short time. Clearly, the 1:7 phase content after short time annealing increased for larger binder content.

To investigate the kinetics of diffusion mode of Ce and Co, binder-free and 10 wt.% Ce₂Co₇ composites sintered at 1373 K for 1, 3 and 6 h were prepared. The Rietveld refined. XRD patterns are shown in Figure 1b,c, respectively. The weight percentages of the phases obtained from the Rietveld refined XRD data are listed in Table 1. The phase composition of annealed LaFe_11.6Si_1.4 flakes was 84.69(5) wt.% 1:13 phase, and 15.31(2) wt.% α-Fe phase. There were no peaks of the La₂O₃, LaFeSi (1:1:1), and (La,Ce)₂(Fe,Co,Si)₁₇ (2:17) phases (Figure 1) in the XRD patterns due to the limitation of the resolution of normal XRD, indicating the low content of these phases in the sintered composites.

Compared with the annealed flakes, the 1:13 phase content in the binder-free sample sintered at 1373 K for 1~6 h (Figure 1b) slightly increased, accompanied by a decrease in α-Fe phase content. This indicated that additional formation of the 1:13 phase occurred during sintering. The XRD patterns of LaFe_11.6Si_1.4/10 wt.%Ce₂Co₇ composites sintered at 1373 K for 1, 3 and 6 h are displayed in Figure 1c. When the sintering time was extended from 1 h to 6 h, the 1:7 phase content reduced from 8.00 wt.% to 2.77 wt.% (Table 1), indicating that increased high-temperature sintering time can significantly reduce the 1:7 phase content.

3.2. Microstructure Evolution

The microstructure of the composites sintered at 1373 K for 6 h is shown in Figure 2 for various values of Ce₂Co₇ binder content. For the binder-free sample, the black area is the α-Fe phase, the dark gray area is the 1:13 phase (matrix phase), and white grains consist of the La₂O₃ phase (not detected in the XRD results owing to its low mass fraction). After sintering at 1373 K for 6 h, a high mass fraction of the 1:13 phase and a small α-Fe phase content (Table 1) were obtained. It was reported that the temperature range for thermal decomposition (TD) reaction for La(Fe,Si,Co)₁₃ based compounds is 873–1173 K [42]. A lamellar structure, usually observed upon decomposition of the 1:13 phase was not seen, indicating no decomposition of the 1:13 phase occurred during sintering of the binder-free LaFe_11.6Si_1.4 composites [43]. The sintering temperature was near the formation temperature of the 1:13 phase, thus, a large amount of precipitation was observed (Figure 2b). The α-Fe phase (black areas), the LaFeSi (1:1:1) phase (white grains inside the 1:13 particles) and the La₂O₃ phase (white grains besides the α-Fe phase), shown in Figure 2b, were observed. As previously reported [42,43], the white phase at the particle interfaces (Figure 2a) is believed to be a La- and Si-rich phase.

From the Ce-Co binary phase diagram [44], the melting point of Ce₂Co₇ alloy is 1418 K, which is 45 K higher than the sintering temperature (1373 K). Therefore, the soft Ce₂Co₇ alloy binder and LaFe_11.6Si_1.4 particles could be bonded by solid phase sintering to obtain a highly dense microstructure under a pressure of 30 MPa [45,46].

For the LaFe_11.6Si_1.4/5 wt.%Ce₂Co₇ sintered composites (Figure 2c,d), the fraction of α-Fe phase (black area), La- and Si-rich boundary phase and La₂O₃ phase (white area) decreased markedly. A small amount of CeFe₇-type phase (light grey) appeared in the binder bonded composites. The formation enthalpy of CeCo₇ (−10.704 kJ/mol calculated by the modified Miedema (ZSL’s Model) [36] is more negative than that of LaCo₁₃ (−5.79 kJ/mol) [34], favoring the formation of the 1:7 phase. The EDS map results of the LaFe_11.6Si_1.4/5 wt.%Ce₂Co₇ sintered composites are shown in the inset of Figure 2c, uniform diffusion of Co occurred in the 1:13 phase. However, there are some regions of La and Ce enrichment, accompanied by up to ~68 at.% for oxygen (Figure 2d). Owing to the limitation of EDS in quantifying the concentration of light elements, this value for oxygen must be regarded as a rough estimate. These La- and Ce-rich regions can correspond to the (La,Ce)₂O₃ phase. Ce is mainly found in the 1:7 phase, a small amount of Ce is found in the 1:13 phase. Sintering at 1373 K for 6 h cannot produce a uniform distribution of Ce or eliminate the 1:7 phase.

Interestingly, the LaFeSi (1:1:1), La-, and Si-rich phases are virtually absent in LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 5, 10, 15) composites sintered at 1373 K for 6 h. The chemical compositions of the 1:7 and 1:13 phases and lattice constant (a) of 1:13 phase for the sintered composites are listed in

Table 2. Lattice constant (a), phase composition, Curie temperature (T_C), maximum magnetic entropy change ((−ΔS_M)^max), and relative cooling power (RCP) of LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) composites sintered at 1373 K for 1~6 h.

LaFe11.6Si1.4/Ce2Co7 Composites		1:13 Phase a (Å)	Phase Composition		TC (K)	(−ΔSM)max J/(kg·K) ΔH = 2 T (5 T)	RCP J/kg ΔH = 2 T (5 T)
Binder Content	Sintering		1:7 Phase	1:13 Phase
Binder- free	1373 K/1 h	11.49282(6)	------	LaFe10.95Si1.59	206	7.5 (/)	122.4 (/)
	1373 K/3 h	11.49015(9)	------	LaFe10.73Si1.57	210	5.0 (/)	106.6 (/)
	1373 K/6 h	11.49209(9)	------	LaFe10.53Si1.60	212	5.2 (8.8)	146.7 (339.0)
5 wt.%	1373 K/6 h	11.49190(0)	La0.26Ce0.74Fe6.02Co0.56Si0.93	La0.87Ce0.13Fe9.70Co0.56Si1.35	247	4.0 (8.6)	158.4 (428.7)
10 wt.%	1373 K/1 h	11.49271(9)	La0.28Ce0.72Fe5.75Co1.27Si0.87	La0.89Ce0.11Fe9.71Co0.91Si1.35	275	0.9 (/)	------
	1373 K/3 h	11.49559(8)	La0.27Ce0.73Fe6.14Co0.82Si0.90	La0.83Ce0.17Fe9.82Co0.95Si1.29	296	2.2 (/)	185.2 (/)
	1373 K/6 h	11.49301(7)	La0.27Ce0.73Fe6.32Co0.72Si1.04	La0.82Ce0.18Fe9.24Co0.91Si1.34	291	3.9 (7.6)	209.2 (457.7)
15 wt.%	1373 K/6 h	11.51400(9)	La0.28Ce0.72Fe5.71Co0.98Si0.91	La0.75Ce0.25Fe9.10Co1.30Si1.28	331	3.2 (6.0)	178.3 (399.2)

. The 1:13 phase, with a composition of La_0.87Ce_0.13Fe_9.70Co_0.56Si_1.35, La_0.82Ce_0.18Fe_9.24Co_0.91Si_1.34, and La_0.75Ce_0.25Fe_9.10Co_1.30Si_1.28 was obtained in sintered composites with 5 wt.%, 10 wt.%, and 15 wt.% Ce₂Co₇, respectively.

On the other hand, in sintered composites with 10 wt.% and 15 wt.% Ce₂Co₇, the fraction of 1:7 phase (light gray) increased remarkably, accompanied by a decrease in the α-Fe phase content. With 10 wt.% Ce₂Co₇, the content of Ce and Co in the 1:13 phase reached 1.34 at.% and 7.51 at.%, respectively. With 15 wt.% Ce₂Co₇, the values of Ce and Co content in the 1:13 phase matrix were 1.89 at.% and 10.26 at.%, respectively. This change in the composition of the 1:13 phase illustrates that diffusion of Ce and Co increased for larger Ce₂Co₇ binder content. The lattice parameter a of the 1:13 phase is similar in the sintered samples with 0–10 wt.% Ce₂Co₇. However, the a of the 1:13 phase increased for 15 wt.% Ce₂Co₇ samples. For the 1:7 phase, the Co content increased while the Ce fraction in the 1:7 phase was almost unchanged with increasing Ce₂Co₇ content. The 1:7 phase is an iron-poor type 2:17 phase, these two phases have the same crystal structure [47]. For the 2:17 phase in the composite, the atom ratio of (Fe + Co + Si) to (La + Ce) is 8.5. The EDS results of the 1:7 phase include the contribution from the 2:17 phase, thus the ratio is not always equal to 7 (Table 2).

3.3. Kinetics of Diffusion of Ce and Co

To understand the diffusion of Ce and Co, the binder-free and 10 wt.% Ce₂Co₇ composites were studied. Backscattered SEM micrographs of the composites with and without 10 wt.% Ce₂Co₇ sintered at 1373 K for 1, 3, and 6 h are shown in Figure 3. For the binder-free samples (Figure 3a–c), with increasing sintering time from 1 to 6 h, the grain boundaries (white phase) are more obvious. It is likely that a La- and Si-rich phase formed at the particle boundaries of the 1:13 phase during sintering [43], similar to the microstructure of Nd₂Fe₁₄B magnets produced by liquid phase sintering (LPS). Sintering occurred at approximately the temperature of 1:13 phase formation, resulting in 1:13 phase formation [20]. The La- and Si-rich phases were distributed at particles boundaries while the α-Fe phase was inside the 1:13 particles [42]. The fraction of α-Fe phase in the 1:13 particles decreased, while the size of the α-Fe phase grains increased. The EDS results show that the content of Fe in the 1:13 phase matrix decreased, indicating that the 1:13 phase formed in the binder-free samples by the following reaction [48]:

α-Fe + LaFeSi → La(Fe,Si)₁₃

(1)

For the 10 wt.% Ce₂Co₇ samples (Figure 3d–f), the content of the 1:7 and α-Fe phases reduced with increasing sintering time, which is consistent with the results in Figure 1 and Table 1. The α-Fe phase in the 1:13 phase reduced and eventually disappeared with increasing sintering time. The increase of Ce, Co content in the 1:13 phase (Table 2) indicated that Ce, Co diffusion increased for larger sintering time. As sintering time increased, the α-Fe phase content decreased while the content of Fe in the 1:7 phase increased. In sintered composites with 10 wt.% Ce₂Co₇ binder, the ratio of (La,Ce) to (Fe,Co,Si) in the 1:13 phase was higher than that of binder-free samples.

In the La(Fe,Si)₁₃ unit cell, La atoms occupy 8a sites, Fe atoms occupy 8b (FeI) and 96i (FeII) positions in the ratio of 1:12. Si atoms occupy Fe positions [49,50]. The similar electronic structure of Co and Fe atoms or Ce and La atoms implies that Co atoms will occupy the position of FeII atoms, and Ce atoms will substitute the position of La atoms. In all Ce₂Co₇ bonded sintered composites, the relative atomic content of La and Ce atoms for the 1:7 phase remains unchanged at ~27:73, but the relative content of Co, Fe, and Si is changed (Table 2). The SEM micrographs of composites with 10 wt.% Ce₂Co₇ sintered at 1373 K for 6 h is shown in the inset of Figure 3f. The line scan of the interphase boundary between the 1:13 phase and the 1:7 phase showed a sudden change in Ce content and little change in Co concentration. Thus, the diffusion of Ce and Co in the 1:13 phase occurs by different mechanisms. Co diffuses by the usual vacancy diffusion mechanism in the 1:13 phase. On the other hand, Ce diffusion is by reaction diffusion. The Ce content in the 1:13 phase is mainly due to the reaction between the Ce-rich (La,Ce)(Fe,Co,Si)₇ phase and the La(Fe,Co,Si)₁₃ phase, resulting in the formation of (La,Ce)(Fe,Co,Si)₁₃ phase. Therefore, Ce is mainly enriched in 1:7 phase but Co is uniformly distributed.

3.4. Thermal Conductivity, Mechanical and Magnetic Properties

The thermal conductivity of the studied samples in this work was measured at room temperature. The porosity, thermal conductivity, and mechanical properties (Figure 4) of the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10, and 15) composites sintered at 1373 K for 6 h are listed in

Table 3. Porosity, thermal conductivity, and mechanical properties of LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) composites sintered at 1373 K for 6 h.

Sample	Porosity (%)	λ (W/m·K)	(σbc)max (MPa)	Strain (%)
Binder free	5.14	13.71	458	5.0
5 wt.%	4.02	7.81	474	4.7
10 wt.%	4.78	8.21	530	6.1
15 wt.%	4.56	7.50	457	4.7

. For the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10, and 15) composites, structures with density up to 95% were successfully obtained. This high density resulted in a high value of compressive strength (σ_bc)^max of up to 450 MPa and high thermal conductivity of up to 7.5 W/m·K. These composites exhibited markedly higher (σ_bc)^max value than those of low melting point alloy bonded composites fabricated by low temperature hot pressing (200 MPa) [32].

The thermal conductivity of these composites are comparable or larger than the values for hot-pressed La_0.7Ce_0.3Fe_11.45Mn_0.15Si_1.4/13.5%Fe (7.5 W/m·K) [51] and LaFe_11.6Si_1.4H_y/Sn (7.9 W/m·K) [32], but smaller than those of hot-pressed La_0.8Ce_0.2(Fe_0.95Co_0.05)_11.8Si_1.2/Sn₄₂Bi₅₈ (10.72–19.64 W/m·K) [33], LaFe₁₁Co_0.8Si_1.2/Al (9.9~17.0 W/m·K) [52], and La_0.7Ce_0.3Fe_11.48Mn_0.12Si_1.4H_1.8/20 wt.%In (11.5 W/m·K) [53]. As La(Fe,Si)₁₃ and residual Ce-Co phase possess different thermal conductivities, thermal conductivity of sintered composites may be improved by even longer sintering time. The strain values are larger than 4.7%, which is about 4.5 times the values of sintered LaFe_11.7Co_1.3Si composites [42]. Thus, the ductility of the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) composites sintered at 1373 K for 6 h is markedly improved. These improved properties are mainly due to the stress distribution in the α-Fe and 1:7 phases and the improved quality of the sintered compacts during high temperature sintering. High ultimate compressive stress and better elongation can lead to better machinability.

The normalized M_norm–T (M/M_max–T) curves upon cooling process under 0.05 T, normalized magnetization temperature derivative curves (dM_norm/dT–T) measured during cooling under an applied field of 0.05 T and the (−ΔS_M)–T curves measured under applied field changes of 0–2 and 0–5 T for the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) composites sintered at 1373 K for 6 h are shown in Figure 5 and Figure 6, respectively. The isothermal magnetization curves for the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ were measured with an increasing magnetic field in a wide temperature range. The sweep rate of the field was slow enough to ensure that the data were recorded in an isothermal process. The value of the isothermal magnetic entropy change −ΔS_M(T, H) is given by the Maxwell relationship:

$$-\Delta S_{\mathrm{M}}\left(T,H\right)={{\displaystyle \int }}_0^H\left(\partial M/\partial T\right)dH$$

(2)

Another parameter called RCP (Relative Cooling Power) can be employed to evaluate the refrigeration efficiency. RCP was defined as the product of (−ΔS_M)^max times the full temperature width at half maximum of the peak (RCR = (−ΔS_M)^max × δT_FWHM) [54]. The values of T_C, (−ΔS_M)^max and RCP of the studied composites are presented in Table 2.

The binder-free sintered composites show higher T_C (212 K) than that of the annealed LaFe_11.6Si_1.4 flakes (196 K). The sintering temperature is close to the optimum formation temperature of the 1:13 phase [55], which may bring about 1:13 phase formation and decrease the α-Fe phase content (Table 1), resulting in the formation of a nonstoichiometric Fe deficient 1:13 phase (Table 2). This Fe-deficient 1:13 phase has elevated T_C (Table 2), which weakens the first order magnetic transition (FOMT) and lowers the (−ΔS_M)^max [7,18,34]. The binder-free composites exhibited the maximum magnetic entropy change (−ΔS_M)^max of 5.2 and 8.8 J/kg·K under applied field changes of 0–2 and 0–5 T, respectively (Table 2).

The T_C of the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x= 5, 10, and 15) composites increased linearly with larger Ce₂Co₇ content due to the higher Co content diffused into the 1:13 phase. Sintering caused Ce and Co to diffuse from the Ce₂Co₇ binder to the 1:13 particles through the particle boundaries. It changed the chemical composition of the 1:13 phase, the 1:13 phase content, and the magnetocaloric performance. As calculated from the results in Table 2, the content of Ce and Co in the 1:13 phase increased from 1.03 to 1.97 at.% and 4.44 to 10.25 at.%, respectively. The T_C of sintered composites increased from 247 K to 331 K due to the substitution of Fe by Co in the 1:13 phase. Under the applied field changes of 2 T and 5 T, the (−ΔS_M)^max values of the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ composites with x = 0, 5, 10 and 15 were 5.2, 4.0, 3.9, 3.2 J/kg·K and 8.8, 8.6, 7.6, 6.0 J/kg·K, respectively. RCP values of these composites were 146.7, 158.4, 209.2, and 178.3 J/kg and 339.0, 428.7, 457.7, and 399.2 J/kg, respectively (Table 2).

Interestingly, the (−ΔS_M)^max (~5.2 J/kg·K@2 T at 212 K) of the binder-free composite was larger than that of the LaFe_11.6Si_1.4 bulk composite with particle size smaller than 100 μm (~4.0 J/kg·K@2 T at 220 K) [56], and far larger than that of La(Fe,Co)_11.4Al_1.6 alloy (2.2 J/kg·K@2 T at 205 K) [50] in a similar temperature range. The (−ΔS_M)^max value (~4 J/kg·K@2 T at 247 K) of the composite with 5 wt.% Ce₂Co₇ binder was larger than those of LaFe_11.6Si_1.4/5 wt.%Pr₄₀Co₆₀ (~2.9 J/kg·K@2 T at 247 K) [56] and Gd₅₀Co₄₈Zn₂ as-spun ribbons [57] (2.6 J/kg·K@2 T at 260 K) in a similar temperature range. Compared with Gd [58], the values of (−ΔS_M)^max (~3.9 J/kg·K@2 T at 291 K) and RCP (209.2 J/kg@2 T) of the LaFe_11.6Si_1.4/10 wt.%Ce₂Co₇ composite is smaller. For an applied field change of 2 T, the values of (−ΔS_M)^max and RCP for the LaFe_11.6Si_1.4/15 wt.%Ce₂Co₇ composite (Table 2) are much larger than those of Fe₇₇Ta₃B₁₀Zr₉Cu₁ amorphous ribbons ((−ΔS_M)^max~1.47 J/kg·K@2 T at 336 K; RCP~123.9 J/kg) in the similar temperature [59].

The Arrott plots near T_C of the LaFe_11.6Si_1.4/x wt.%Ce₂Co₇ (x = 0, 5, 10 and 15) composites sintered at 1373 K for 6 h are shown in Figure 7. According to the Banerjee criterion [60], the slopes of the Arrott plots for the composites sintered at 1373 K for 6 h are all positive. The magnetic phase transition of these sintered composites near its T_C is a second-order magnetic phase transition, which is due to Co diffusion in Ce₂Co₇ bonded composites and enrichment of Si in the 1:13 matrix for the binder-free composite. A similar behavior was observed for the binder-free and 10 wt.% Ce₂Co₇ binder composites sintered at 1373 K for 1~6 h, which also exhibited a second-order magnetic phase transition near their T_C.

The normalized M/M_max–T curves upon cooling under an applied field of 0.05 T of the binder-free and 10 wt.% Ce₂Co₇ binder composites sintered at 1373 K are shown in Figure 8. The (−ΔS_M)–T curves (0 − 2 T) are shown in the inset of Figure 8, respectively. For the binder-free composites, there is a composition change of the magnetocaloric phase (1:13 phase) after sintering. Based on earlier work [61], the decrease of Fe/Si ratio in 1:13 phase results in higher T_C and lower −ΔS_M. With increasing annealing time from 1 to 6 h, the Fe/Si ratio in the binder-free composites decreased from ~6.89 (10.95/1.59) to ~6.58 (10.53/1.60), the T_C increased from ~206 to 212 K while (−ΔS_M)^max decreased from 7.5 to 5.2 J/kg·K (ΔH = 2 T) (Table 2). The 10 wt.% Ce₂Co₇ binder composites also exhibited higher (−ΔS_M)^max due to the diffusion of Ce, which increased the Ce content in the 1:13 phase. Hence, the change of T_C and magnetocaloric properties is attributed to the formation of the 1:13 phase (Table 1) and grain boundary diffusion of Ce and Co atoms.

4. Conclusions

In summary, the diffusion of Ce and Co into the desired (La,Ce)(Fe,Co,Si)₁₃ phase and the partial substitution of La by Ce and Fe by Co could be realized by a one-step sintering of La(Fe,Si)₁₃/Ce-Co composites. The line scan of the interphase boundary between the 1:13 phase and the 1:7 phase showed a sudden change in Ce content and little change in Co concentration. Thus, the diffusion mechanism of Ce in the 1:13 phase was different from that of Co. Co diffused by the usual vacancy diffusion mechanism while the diffusion mode of Ce was by reaction diffusion. After 6 h sintering at 1373 K, a high value of compressive strength (σ_bc)^max of up to 450 MPa and a high thermal conductivity (λ) of up to 7.5 W/m·K were successfully obtained. The magnetocaloric properties of the composites could also be tuned by varying the content of Ce-Co alloy binder and the sintering time. T_C increased from 212 K to 331 K with increasing Ce₂Co₇ from 0 to 15 wt.% in LaFe_11.6Si_1.4/Ce-Co bulk composites. The magnetic entropy change decreased from 8.8 to 6.0 J/kg·K, the relative cooling power increased from ~339 to 428, 457 J/kg and then decreases to 399 J/kg under 5 T.

Thus, a feasible route to produce high quality La(Fe,Si)₁₃ based magnetocaloric composites with large MCE, good mechanical properties and thermal conductivity and tunable T_C by a one-step sintering process has been demonstrated.