Research articles
The effect of plastic deformation on magnetic and magnetocaloric properties of Gd-B alloys

https://doi.org/10.1016/j.jmmm.2017.06.077Get rights and content

Highlights

  • A significant depression of magnetization and magnetocaloric effect was found for the plastically deformed Gd-B (0, 10, 15) alloys.

  • Magnetic and magnetocaloric properties of plastically deformed Gd-B (0, 10, 15) alloys can be completely restored by a high temperature heat treatment.

  • The proposed technique is convenient for designing and producing magnetocaloric materials in proper form for Active Magnetic Regenerative heat exchanger of magnetic refrigerator.

Abstract

We report on the magnetocaloric effect in Gd100-xBx (x = 0, 10, 15) cold rolled ribbons. A moderate entropy change ΔS = 5 J/kg·K and adiabatic change of ΔT = 4.8 K were observed for the as-cast materials in an external magnetic field of 3 T which is less by 20% in comparison with the pure gadolinium metal. It was found that a significant (up to 70%) depression of magnetization and magnetocaloric properties developed in the course of plastic deformation can completely be restored by means of a high temperature heat treatment. It is concluded that cold rolling is one promising technique for producing foil shaped magnetocaloric materials suitable for designing heat exchangers of magnetic cooling devices.

Introduction

Rare earths based compounds are one of the best materials for magnetic refrigeration [1]. Among others rare earth elements, Gadolinium stands out: it has unique properties, such as good ductility, magnetic phase transition at room temperature, high magnetic moment, etc. Moreover, Gd is the basis for many intermetallic compounds suitable for magnetocaloric applications and is typical for magnetic cooling technology and designing magnetic refrigerators [2]. Gadolinium also forms solid solutions with numerous elements, this feature has been investigated by a number of research groups [e.g. [3], [4], [5], [6], [7], [8]]. Magnetic, magnetoelastic and magnetothermal properties of polycrystalline and single-crystalline Gd have been investigated in detail in a number of works [e.g. [9], [10]].

A useful design for practical applications is an Active Magnetic Regenerator (AMR) whose cycle design needs a series of alloys with a large magnetocaloric effect (MCE) and a tunable temperature of the magnetic transition. Materials with first order magnetic (or coupled magnetostructural) phase transition possess large MCE values in a wide temperature range, but these compounds also have some serious disadvantages such as hysteresis losses, brittleness (in the case of magnetostructural phase transition) and irreversibility of the large MCE during cycling in magnetic field. From this point of view materials with second order magnetic phase transition (like Gadolinium) are advantageous as the magnetocaloric effect is reproducible during long-time cycling.

Tuning of Curie temperature of Gd can be realized via doping by other chemical elements. Usually the effect of doping is to reduce the Curie temperature. This has been observed in the case of doping by other rare earth elements like terbium, dysprosium, holmium or ytterbium [11], [12] and also in the case of doping by transitional metals [13], [14], [15], [16], [17]. Simultaneous enhancement of the Curie temperature and the magnetocaloric properties has been reported for gadolinium hydrides [18]. However, hydrides are unstable and tend to dehydride with time and temperatures above room temperature.

Just a few works have been devoted to solid solutions of gadolinium with non-metals such as boron and carbon [19], [20]. These works showed that these substitutions increase the Curie temperature and slightly enhance the MCE. It should be mentioned however, that the results reported in Ref. [21] are in contradiction with the previous works, i.e., no influence of boron on Tc and saturation magnetization was observed. Moreover, the magnetic entropy change was found to decrease with increase in boron concentration [21]. Similar results have been reported for melt spun ribbons of Gd100-xBx (x = 0, 5, 10, 15, 20 at.%) [5]. Thus, the information about magnetic and magnetocaloric properties of the Gd100-xBx system is a contradictory and should be revised.

For practical applications the magnetocaloric materials should be in the form of very thin (up to several micrometers) foils or other 2D-like structures with a large surface/volume ratio in order to enhance the heat transfer from the MCE material to a heat exchange fluid. This is extremely important for heat exchangers and the efficiency of magnetic refrigeration devices [22].

One suitable way to produce MCE materials is in the plastic deformation by cold rolling technique. This technique is very cheap and convenient for producing MCE materials in a large quantity. During plastic deformation the microstructure of treated materials refines down to the nanometer scale and, as a consequence, all physical properties are modified [23].

In this work, we report magnetic and magnetocaloric properties of a series of intermetallic alloys Gd100−xBx (x = 0, 10, 15) treated with the help of cold rolling.

Section snippets

Samples preparation

The starting material was 99.9% pure gadolinium and boron. Ingots of Gd100−xBx (x = 0, 10, 15) were prepared by arc melting in a pure argon atmosphere. Cold rolling was performed with the help of four rollers mill. The work rollers were made from tungsten carbide and have the diameter equal to 40 mm. The speed of the rolling was 0.05 m/s. The maximal stress produced by mill was 50 kN. All the specimens were treated by the cold rolling with many cycles of plastic deformation. In each subsequent

Results and discussions

Some characteristics of the as-cast ingots and the rolled materials are shown in Table 1. Width of the as-cast alloy means the height of the alloy ingot; length means the maximal diameter of the ingot. Deformations were determined as a relation between the widths of a sample before rolling and after the last rolling cycle.

X-ray pattern of as-cast ingots (Fig. 1) indicated that except solid solution of B in the α-Gd there are two additional phases, GdB2 and Gd2B5, which appear due to

Conclusion

In this work, we report on the magnetic and magnetocaloric properties of Gd-B alloys. We found that boron addition shift the Curie temperature of Gd-B alloys to lower temperatures. This observation corrects the results obtained in [21] where no change in Curie temperature was reported for the of boron concentration up to 12 at.%. Increase of concentration of the non-magnetic boron in Gd-B alloys reduces magnetic moment per unit volume of the alloys and slightly reduces exchange interactions.

Acknowledgements

The authors gratefully the financial support of Russian Science Foundation (Project #15-12-10008). SV acknowledge RFBR foundation (Project #16-07-00679) and Act 211 Government of the Russian Federation (contract № 02.A03.21.0011) for financing numerical simulation part of the work. OG is gratefully acknowledged financial support by the German federal state of Hessen through its excellence program LOEWE “RESPONSE”.

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