Large adiabatic temperature rise above the water ice point of a minor Fe substituted Gd50Co50 amorphous alloy
Introduction
Magnetic refrigeration (MR) based on the magneto-caloric effect (MCE) of magnetic materials has attracted increasing interests because it is more compact, more effective, safes for the environment and has lower energy consumption than the traditional vapor-cycle refrigeration [1], [2], [3], [4], [5]. In the last two decades, MR working materials have been studied intensively and numerous MCE alloys have been developed [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Among these MCE materials, some intermetallic compounds such as Gd5(Si2Ge2), MnFeP0.45As0.55, MnAs1-xSbx and Ni52.6Mn23.1Ga24.3, show a sharp magnetic entropy change (− ΔSm) peak due to their first order magnetic phase transition [5], [6], [7], [8], [9]; in contrast, metallic glasses and some crystalline alloys (e.g., Gd, Gd6Co2Si3 and so on) exhibit a broadened -ΔSm peak because they undergo a second order magnetic phase transition [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Except for the relatively lower peak values of -ΔSm (− ΔSmpeak), almost all the features of metallic glasses are superior to crystalline alloys: ultrahigh refrigeration capacity (RC), which is several times higher than that in crystalline alloys; tunable Curie temperature (Tc) without dramatic deterioration of MCE within a large compositional range; low hysteresis loss and low current eddy loss; excellent mechanical properties and good corrosion resistance [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. It is therefore important to improve the -ΔSmpeak values of amorphous MCE alloys, especially near room temperature.
Although some of the Gd-based metallic glasses exhibit higher -ΔSmpeak and even much higher RC than those of the pure Gd, their Tc values are far from room temperature [15], [16], [17], [18], [19], [20]. The -ΔSmpeak values of Fe-based amorphous alloys, however, are not high enough for use as magnetic refrigerants even though they exhibit a Tc around room temperature [12], [13], [14]. Currently, we have prepared Gd50Co50 binary amorphous ribbons with excellent magneto-caloric properties near the freezing temperature of water [21]. This binary metallic glass exhibits a large -ΔSmpeak and adiabatic temperature rise (ΔTad) peak at about 267 K. On the other hand, the Gd48Co52 binary amorphous ribbons exhibit a ΔTad peak comparable to that of Gd50Co50 metallic glass above the ice point of water, but is hard to be fabricated due to its poor glass forming ability (GFA) [22]. Although the Tc of binary Gd50Co50 amorphous alloy has been successfully improved to nearly 290 K by adding 5% (at. %) Fe as a replacement of Co in the binary glass forming alloy, the -ΔSmpeak of the Gd50Co45Fe5 decreased dramatically [23]. Considering the application of metallic glasses as magnetic refrigerants for a household refrigerator, it is more important to develop amorphous MCE alloys with a high -ΔSmpeak value above the freezing temperature of water. In the present work, we add small amount of Fe as a replacement for Co in the Gd50Co50 binary amorphous alloy in an attempt to improve the Tc to the temperature to above the ice point of water, and at the same time keep the ΔTad of the Gd50Co48Fe2 metallic glass comparable to the that of the Gd50Co50 binary amorphous alloy. The magnetic properties as well as the magneto-caloric behavior of the Gd50Co48Fe2 amorphous alloy were studied in detail.
Section snippets
Experimental procedure
A Gd50Co48Fe2 ingot was prepared by arc-melting a mixture of Gd, Co and Fe metals with purities above 99.9% (at. %) and re-melting for least four times in a water cooled copper crucible under a Ti-gettered argon atmosphere. Gd50Co48Fe2 as-spun ribbons were prepared by melt-spinning on a single copper wheel with a linear speed of about 30 m/s under a pure argon atmosphere. The amorphous structure of the Gd50Co48Fe2 as-spun ribbon was ascertained by a Rigaku D\max-2550 X-ray diffractometer (XRD)
Results and discussion
Fig. 1 shows the XRD pattern of the Gd50Co48Fe2 and Gd50Co50 as-spun ribbons. The Gd50Co50 as-spun ribbon was also prepared at a surface speed of 30 m/s. The ribbons show the typical amorphous structures of a broadened hump without any sharp peaks of crystalline phases on the XRD patterns. The glass transition and crystallization behavior, as typical characteristics of amorphous alloys, are also found in the DSC trace of the as-spun Gd50Co48Fe2 ribbon, as shown in the inset of Fig. 1. The Gd50Co
Conclusions
In summary, we obtained Gd50Co48Fe2 amorphous alloys with improved Tc above the ice point. The -ΔSmpeak of the Gd50Co48Fe2 amorphous ribbon, which is closely related to its Tc value according to mean field theory, is found to be slightly lower than that of the Gd50Co50 amorphous ribbon. The n -T curve of the Gd50Co48Fe2 glassy ribbon illustrates the typical magneto-caloric behavior of soft magnetic metallic glasses. The maximum ΔTad of Gd50Co48Fe2 metallic glass is larger than those of other
Acknowledgments
The work described in this paper was supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 511212).
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