Excellent Magnetocaloric Performance of the Fe87Ce13−xBx (x = 5, 6, 7) Metallic Glasses and Their Composite

The novel Fe87Ce13−xBx (x = 5, 6, 7) metallic glass (MG) ribbons were fabricated in this work. The compositional dependence of glass forming ability (GFA), magnetic and magnetocaloric properties of these ternary MGs, and the mechanism involved was investigated. The GFA and Curie temperature (Tc) of the MG ribbons were found to improve with the boron content, and the peak value of magnetic entropy change (−ΔSmpeak) reaches a maximum of 3.88 J/(kg × K) under 5 T when x = 6. Based on the three results, we designed an amorphous composite that exhibits a table-shape magnetic entropy change (−ΔSm) profile with a rather high average −ΔSm (−ΔSmaverage~3.29 J/(kg × K) under 5 T) from 282.5 K to 320 K, which makes it a potential candidate for the highly efficient refrigerant in a domestic magnetic refrigeration appliance.


Introduction
Recently, the demand for refrigeration has risen sharply because of the ongoing and extremely high-temperature weather. However, conventional refrigeration technology can make this problem worse due to the generation of ozone-depleting gases by refrigerants. A novel refrigeration technology, namely magnetic refrigeration (MR, which has the advantages of high efficiency, environmental friendliness, great stability and reliability, low noise, and easy miniaturization [1][2][3][4][5]), may be a better choice. The MR technology was first used by Gauque in 1933 to achieve ultra-low temperatures below 1 K and has been applied to the low and medium temperature regions up to now [6,7], although it is still in the research stage at high temperatures, especially for civil applications at room temperature. One of the key issues affecting the civil application of MR is the magnetocaloric effect (MCE) performance of refrigerants.
Although a large number of compounds with ultrahigh magnetic entropy change (−∆S m ) have been developed, their narrow working temperature ranges make them difficult to synthesize a composite material with a flattened −∆S m curve [8][9][10][11], which is of great significance for the optimization of cooling efficiency in the Ericsson cycle [12]. Additionally, the presence of magnetic hysteresis and thermal hysteresis in these compounds prevents them from operating at high frequencies with fast response. On the contrary, the metallic glasses (MGs) that exhibit a broader −∆S m peak (−∆S m peak ) without magnetic hysteresis and thermal hysteresis are preferred, such as Gd-based and Fe-based MGs [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. The tunable Curie temperature (T c ) in a wide range of composition adjustments is beneficial for constructing the flattened −∆S m curve [15][16][17]. Furthermore, the advantages of outstanding mechanical, physical, and chemical properties can also ensure that MGs have a longer service life and numerous application scenarios [27,28]. For example, Zr-based or Ti-based MGs with both ultra-high strength and plasticity have made significant progress in key structural materials for military and aerospace equipment; Ca-based, Mg-based, or Fe-based MGs possess good biocompatibility and comprehensive mechanical properties and are expected to be used in artificial bones.
Compared with rare-earth (RE)-based MGs, Fe-based MGs show greater application value in civil aspects (such as magnetic refrigerators, air conditioners, wine cabinets, and so on) due to their lower cost and better glass forming ability (GFA), but the −∆S m peak of these Fe-based MGs is not as high as that of RE-based MGs [15][16][17][18][19][20][21][22][23][24][25][26]. Therefore, improving the −∆S m peak of the Fe-based MGs as much as possible is a top priority for the civil application of MR technology. Amongst these reported Fe-based MGs, Fe-Zr-B-based MG ribbons are the outstanding ones [15][16][17][18][23][24][25][26]. For example, the Fe-Zr-B ternary amorphous ribbons exhibited a −∆S m peak under 5 T of 2.75-3.34 J/(kg × K) at the T c ranging from 271 K to 327 K [18,24]. By minor element substitution, the −∆S m peak under 5 T of the multicomponent Fe-Zr-B-based MG ribbons was increased to a maximum value of 3.55 J/(kg × K) at 336 K [26]. Unfortunately, the values of −∆S m peak in these Fe-Zr-B-based MG ribbons are still not as high for the practical application of MR. In our recent works, we found that Fe-La/Ce-B-based glassy ribbons show rather excellent MCE near room temperature, and the −∆S m peak under 5 T of Fe 87 Ce 8 B 5 ribbon reaches 3.65 J/(kg × K) at 283 K, which is 12.3% higher than that of Fe 87 Zr 8 B 5 metallic glass and exceeds the −∆S m peak of most other Fe-Zr-B-based amorphous alloys [29,30]. The large −∆S m peak of the Fe 87 Ce 8 B 5 amorphous ribbon was attributed to the extra magnetic moment of the Ce atom, but the effect of Ce content on the −∆S m peak of the Fe-Ce-B MG ribbons and the involved mechanism is still unclear. The detailed investigation of the magnetic and magnetocaloric properties of the Fe-Ce-B ternary MG ribbons will be helpful for the clarification of the effects of the Ce content on the magnetic properties and MCE of the Fe-Ce-B MG ribbons and the further development of the multicomponent Fe-based amorphous alloys with better MCE near room temperature. As such, we prepared the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons using melt-spinning technology and systematically investigated the influences of boron on GFA, T c and −∆S m peak of the three ribbons, as well as the mechanisms involved, in this paper. Finally, an amorphous composite with a flattened −∆S m profile was designed according to the three samples.

Materials and Methods
The Fe 87 Ce 13−x B x (x = 5, 6, 7) alloy ingots were prepared by melting high-purity raw materials (≥99.9 at%) using a vacuum arc-melting furnace with the protection of Ar gas. Each ingot of the Fe 87 Ce 13−x B x (x = 5, 6, 7) master alloys was melted more than four times to ensure their compositional homogeneity. These ingots were remelted in a quartz tube again and sprayed onto the surface of a high-speed (~55 m/s) rotating copper roller to form the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons. The structure of these ribbons was characterized by a PANalytical X-ray diffractometer with Cu K α radiation. The differential scanning calorimetry (DSC) traces were measured at a heating rate of 20 K/min on a Netzsch DSC 404C calorimeter to evaluate the GFA of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons. The magnetic measurements of the samples, including hysteresis loops, magnetization vs. temperature (M-T) curves, and isothermal magnetization (M-H) curves, were performed on a vibrating sample magnetometer (VSM), which is a module of 6000 Physical Property Measurement System (PPMS, Quantum Design). Figure 1a shows the XRD patterns of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons. Only smooth diffraction humps between 40 • and 50 • and no obvious crystalline peaks were observed in these patterns, indicating the amorphous structure of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons. Figure 1b displays the DSC curves of the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons. A weak glass transition endothermic hump before a sharp crystallization exothermic peak was found in each DSC curve, which further confirms the glassy characteristics of the three ribbons. From their DSC curves, we can obtain the thermal parameters, including the onset temperature of glass transition (T g onset ) and crystallization (T x onset ), and liquidus temperature (T l ), of the Fe 87 Ce 13−x B x (x = 5, 6, 7) MG ribbons, as listed in Table 1. As a result, the reduced glass transition temperature (T rg = T g /T l ) [31] and the parameter γ (=T g /(T g + T l )) [32] can be calculated accordingly to estimate the GFA of these ribbons. The    The hysteresis loops at 180 K and 380 K of the Fe 87 Ce 13−x B x (x = 5, 6, 7) glassy ribbons were measured under the maximum field of 5 T, as shown in Figure 2a-c. All glassy ribbons are paramagnetic at 380 K and soft magnetic with near zero hystereses at 180 K, which suggests that these ribbons transform from ferromagnetism to paramagnetism between 180 K and 380 K. The saturation magnetizations (M s ) at 180 K of the three samples are about 129.4 Am 2 /kg, 143.2 Am 2 /kg, and 142.1 Am 2 /kg, respectively. The M s first increases and then slightly decreases with the boron content. To determine the ferromagneticparamagnetic transition temperature of the Fe 87 Ce 13−x B x (x = 5, 6, 7) MG ribbons, we measured their M-T curves from 180 K to 380 K under 0.03 T after cooling these samples without any applied fields. As illustrated in the insets of Figure 2a-c, all M-T curves of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons show similar shapes, and T c can be deduced to bẽ 283 K for x = 5,~305 K for x = 6, and~323 K for x = 7, respectively. It was found that T c of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons almost linearly increases with the increase of boron content. The Curie temperature of the Fe 87 Ce 13−x B x (x = 5, 6, 7) glassy ribbons should also be decided by the interaction between Fe atoms because of only one electron in the 4f shell of the Ce atom. The monotonically increasing relationship between T c and composition was also observed in the Fe-Zr-B ternary amorphous ribbons, which is believed to be due to the enhanced 3d-3d direct interaction between Fe atoms by the addition of boron [18,24]. In addition, the increase of boron content (namely the reduction of Ce content) will diminish the antiferromagnetic coupling between Ce and Fe atoms, which can dilute the direct interaction between Fe atoms [30] and, thus, result in the enhancement of 3d-3d interaction between Fe atoms.  The Arrott plots can be deduced from their isothermal magnetization curves (not present in the paper) to judge the types of magnetic transition in the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons, as illustrated in Figure 3a-c. The slope of each Arrott plot at every temperature is positive, without "S" or "C"-shaped feature, indicating that both the three alloys experience a second-order ferromagnetic-paramagnetic transition near their T c [33]. The Arrott plots can be deduced from their isothermal magnetization curves (not present in the paper) to judge the types of magnetic transition in the Fe87Ce13−xBx (x = 5, 6, 7) amorphous ribbons, as illustrated in Figure 3a-c. The slope of each Arrott plot at every temperature is positive, without "S" or "C"-shaped feature, indicating that both the three alloys experience a second-order ferromagnetic-paramagnetic transition near their Tc [33].   Table 1. We can find that the value of −∆S m peak first increases and then decreases with the increase of boron content and reaches a maximum value of 3.88 J/(kg × K) under 5 T at x = 6. The variation is the same as that in M s . Although the B substitution for Ce can improve the 3d-3d interaction between Fe atoms, which is beneficial for the enhancement of −∆S m peak , the decrease of the Ce atom simultaneously reduces the total magnetic entropy of the alloy, thus resulting in the reduction of −∆S m peak , as the Ce atom also contributes to the magnetic entropy change [18,24,30]. It may be precisely the two opposing factors that lead to this variation in M s and −∆S m peak . According to the −∆S m peak under various magnetic fields, the field dependence of −∆S m peak for the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons can be investigated by constructing the ln(−∆S m peak )-ln(H) plots, as shown in Figure 4b. Each set of point plots is linearly fitted well, and the slope (expressed by n) of the linear fitting is 0.756 for x = 5, 0.759 for x = 6, and 0.750 for x = 7, respectively. The values of n are consistent with that of other MGs [16][17][18][23][24][25][26]29,30] but slightly larger than the predicted value of the mean field model [35]. This deviation is mainly caused by local inhomogeneity in MGs because the presence of short-range ordered clusters in MGs leads to a magnetic transformation in a wide temperature range [17].     [15][16][17][18][23][24][25][26]36]. A positive correlation between −∆S m peak and T c was observed in the Fe-Zr-B-based MGs, implying that the higher −∆S m peak can only be achieved at higher temperatures for the Fe-Zr-B-based MGs, which is very unfriendly for the application of magnetic refrigeration at room temperature. However, the novel Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons prepared in this work provide a better idea and exhibit better magnetocaloric performance than the Fe-Zr-B-based MGs. For example, the lowest −∆S m peak in these amorphous ribbons (~3.66 J/(kg × K) at 282.5 K for Fe 87 Ce 8 B 5 ) is still 3.1% higher than the largest −∆S m peak in the Fe-Zr-B-based MGs (~3.55 J/(kg × K), while T c reaches around 335 K [15,25]). The Fe 87 Ce 7 B 6 ribbon with the largest −∆S m peak shows a value at least 9.3% higher than that of the Fe-Zr-B-based MGs. Additionally, the −∆S m peak of the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons is much higher than that of the Fe-Zr-B-based MGs with the same T c , which can be intuitively observed from Figure 5a.  On the other hand, the T c of the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons increases from 283 K to 323 K, which perfectly matches the temperature interval of a domestic magnetic refrigeration appliance. Therefore, we can construct an amorphous composite with a flattened −∆S m profile that is the requirement of application in an Ericsson cycle according to the three ribbons [37]. The −∆S m of the composite can be calculated as follows:

Results and Discussion
where w i is the weight fraction of each component; the validity of Equation (2) has been verified in Ref. [38]. Therefore, by adjusting the weight fraction of these ribbons according to Equation (2), the amorphous composite was designed to be synthesized with 44% (wt.%) Fe 87 Ce 8 B 5 + 3% (wt.%) Fe 87 Ce 7 B 6 + 53% (wt.%) Fe 87 Ce 6 B 7 amorphous ribbons. As shown in Figure 5b, the −∆S m of the amorphous composite fluctuates with an average value of~1.20 J/(kg × K) from 275 K to 315 K (∆T table = 40 K) under 1.5 T, and 3.29 J/(kg × K) from 282.5 K to 320 K (∆T table = 37.5 K) under 5 T. Compared with the Fe-Zr-B-based amorphous composite materials, an amorphous hybrid with a larger average −∆S m (−∆S m average ) was achieved in this work [15][16][17]. In addition, the effective cooling capacity (RC eff = −∆S m average × ∆T table [39]) used to assess the actual cooling capacity of the Fe-Ce-B amorphous hybrid was calculated to be about 48.0 J/kg under 1.5 T and 123.4 J/kg under 5 T, both of which are also higher than those of the Fe-Zr-B-based amorphous composites with a similar ∆T table [15,17,39]. Furthermore, considering that the components of the amorphous composite do not contain radioactive elements, the Fe-Ce-B amorphous hybrid, thereby, shows a better application potential as highly efficient refrigerants in a domestic magnetic refrigeration appliance.

Conclusions
In summary, we successfully prepared the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons, and the influences of boron addition on their GFA, T c, and −∆S m peak were systematically investigated. The main conclusions are as follows: (i) The T rg and γ indicate that the GFA of the Fe 87 Ce 13−x B x (x = 5, 6, 7) ribbons were increased with the addition of boron; (ii) All ribbons show soft magnetic properties with negligible coercivity at 180 K, and T c of the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons is~283 K for x = 5,~305 K for x = 6, and~323 K for x = 7, respectively. The increased T c with the addition of boron may be due to the enhanced 3d-3d interaction between Fe atoms induced by the increase of boron content and the reduction of Ce content; (iii) The −∆S m peak of the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons first increases and subsequently decreases with the increase of boron content, which is supposed to be related to the competitive effect between the enhanced 3d-3d interaction by the addition of B content and the reduced total magnetic entropy by the decrease of Ce content; (iv) Both Arrott plots and the value of n at T c suggest the typical second-order magnetic transition magnetocaloric behavior of the three ribbons. All the Fe 87 Ce 13−x B x (x = 5, 6, 7) amorphous ribbons show rather excellent MCE than Fe-Zr-B-based metallic glasses.
Based on the above results, we designed an amorphous composite with a flattened (−∆S m )-T curve by using 44% (wt.%) Fe 87 Ce 8 B 5 + 3% (wt.%) Fe 87 Ce 7 B 6 + 53% (wt.%) Fe 87 Ce 6 B 7 ribbons. The high −∆S m average (~3.29 J/(kg × K)) and RC eff (~123.4 J/kg) from 282.5 K to 320 K under 5 T indicate that the hybrid is a promising candidate as a refrigerant applied in a domestic magnetic refrigeration appliance.