Structure, Magnetic And Magnetocaloric Properties Of Gd-Co(Fe)-M Series Of Alloys

Magnetic refrigeration technology with the advantages of high efficiency, energy conservation, environmentally safe is considered to be a broad prospect of application of refrigeration technology. In recent years, magnetocaloric materials as one of the core technologies for magnetic refrigeration has become a hotspot in the research of the magnetism and magnetic materials. Based on Gd-base amorphous alloy possessing tunable magnetic phase transition temperature and large magnetocaloric effect, in this paper, different phase composition and microstructure of Gd-Co(Fe)-M series of alloys were obtained through melt-spinning or melt-spining followed by crystallization heat treatment. The structure, thermal stability, magnetic and magnetocaloric properties of the series of alloys were systematically investigated by X-ray Diffraction(XRD), Transmission Electron Microscope(TEM), Differential Scanning Calorimetry(DSC), and Physical Property Measurement System(PPMS). The main research results are as follows.The effect of different types of elements such as non-3d element Al, Si, Zr, Nb and 3d element Mn, Fe, Ni substitution for part of Co in Gd55Co45 binary alloy on the thermal stability and magnetocaloric effect of Gd-Co-M rapid quenching alloys were studied. The XRD results show that only Gd55Co35Al10 rapid quenching alloy is composed of amorphous and nanocrystalline, the other alloys are amorphous structure. TEM results show that Gd nanocrystalline less than 5 nm precipitates in Gd55Co35Fe10 amorphous matrix.The addition of Si to Gd-Co-M as-spun alloy is most beneficial to the promotion of thermal stability. The Curie temperature(TC) of Gd-Co-M series of rapid quenching alloys can be tuned in the temperature range of 148 ~ 268 K by different alternative elements. All alloys undergo second order phase transition from ferromagnetic(FM) to paramagnetic(PM) in the vicinity of TC. Among them, the TC of Gd55Co35Fe10(~268 K) is very close to the room temperature. Under the magnetic field change of 2T, the maximum magnetic entropy changes(–?SM)max of the above mentioned alloys range from 1.72 to 4.65 J/kg K. Even though the(–?SM)max of Gd55Co35Fe10 is the smallest, its refrigeration capacity(RC) reaches maximum value among those alloys to 266 J/kg due to its larger value of the full width at half maximum(δTFWHM) of the magnetic entropy change.Gd55Co35Al10 and Gd55Co35Fe10 were melt-spinning quenched, followed by crystallization heat treatment, the results showed that Gd2Co2 Al and Gd2 Al phases were in-situ precipitated in the matrix of the Gd55Co35Al10 amorphous alloy. A table-like magnetic entropy change peak ranging from about 70 to 140 K, and the platform is about 70 K. In an applied field of 2T, the biggest relative refrigeration capacity value(RCP) of 260.3 J/kg in the sample was obtained after 603 K/30 min crystallization heat treatment. As for Gd55Co35Fe10 rapid quenching alloy, both GdFeCo and Gd3 Co phases were precipitated after crystallization heat treatment at 595 K and 643 K for 30 min, respectively. Two transition points(165 K and 268 K) appear in the sample annealed at 595 K for 30 min, while approximate table-like magnetic entropy change peak is obtained under the field change of 2T. However, only one transition point, i.e. 130 K, occurs in sample annealed at 643 K for 30 min. The values of(-ΔSM)max, δTFWHM, and RCP for the samples annealed at 595 K and 643 K for 30 min, are 1.26 and 2.09 J/kg K, 197 and 54 K, 248.2 and 112.9 J/kg, respectively. Even though the sample annealed at 595 K for 30 min possesses relatively smaller(-ΔSM)max, its δTFWHM is almost four times as large as that of the sample annealed at 643 K for 30 min, resulting in larger relative refrigeration capacity.With increasing temperature, α-Gd, GdFe2 and Gd5Si3 phases were precipitated sequentially in Gd65Fe25Si10 rapid quenching alloys; while α-Gd, Gd Fe2 and GdZn phases were precipitated sequentially in Gd65Fe25Zn10 as-spun alloys with α-Gd precipitation. XRD and DSC analysis results showed that the addition of Si is more conducive to improving glass forming ability in Gd65Fe25Si10 and Gd65Fe25Zn10 as-spun alloys. The TC of Gd65Fe25Si10 and Gd65Fe25Zn10 rapid quenching alloys are 250 and 285 K, respectively. Under the magnetic field change of 2T, the values of(-ΔSM)max, δTFWHM and RC for Gd65Fe25Si10 and Gd65Fe25Zn10 rapid quenching alloys are 1.99 and 2.31 J/kg K, 99 and 185 K, 172.1 and 280.5J/kg, respectively; while the values of(-ΔSM)max for Gd65Fe25Si10 and Gd65Fe25Zn10 as-spun alloys after fully crystallization treatment are 2.43 and 4.06 J/kg K.