Low magnetic field alloying elements Zn on Gd_5Si_2Ge_2 the phase transition behavior and magnetocaloric effect of the study

Title: Low magnetic field alloying elements Zn on Gd_5Si_2Ge_2 the phase transition behavior and magnetocaloric effect of the studyAuthor: Hou XuelingDegree-granting units: Shanghai UniversityKeywords: Gd_5Si_2Ge_2 alloy;; alloying elements Zn;; heat treatment;; phase transition;; magnetocaloric effectSummary:

Gd5Si2Ge2 alloy with non-toxic, the phase transition temperature close to room temperature and the giant magnetocaloric effect, much attention in recent years the magnetic refrigerant materials. However, it must be the giant magnetocaloric effect in the high cost of superconducting magnetic field (5 T -12 T) obtained under low magnetic field (1.0 T-1.7 T) can not stimulate the alloy produced under the giant magnetocaloric effect, resulting in higher costs for its application. Meanwhile Gd5Si2Ge2 alloy phase transition temperature is low compared with the room temperature, the magnetic hysteresis in the cooling process of serious, these problems have hindered the commercial future of the alloy Neodymium Magnets applications. Therefore, at lower magnetic field, how to improve Gd5Si2Ge2 alloy and magnetic phase transition temperature of thermal effects, and reduce the magnetic hysteresis become a research hotspot.

This systematic study of the elements together or separately added Zn substitution Si and Ge and their alloys in heat treatment △ H = 1.5 T magnetic field the phase transition temperature, the nature of the phase transition, magnetic phase transition in the lag, magnetocaloric effect and magnetic refrigeration capacity the impact of adding some described elements can improve the Gd5Si2Ge2 Zn alloy magnetic phase transition temperature and thermal properties of the magnetic mechanism and rules for the Gd5Si2Ge2 alloy obtained at low magnetic field giant magnetocaloric effect provides an effective method and basis. The results show that:

1 add trace elements Zn alloy Gd5Si2Ge2 can promote type monoclinic phase formation, inhibition of Gd5Si4 type quadrature-phase generation; reduce the energy required for phase change and improve the first order phase transition driving force and reduce the occurrence of a phase transition when Critical induced magnetic field, the alloy in the △ H = 1.5 T magnetic field produced giant magnetocaloric effect. In Gd5Si2-xGe2Znx and Gd5Si2-zGe2-zZn2z alloys, when x or 2z from the zero to 0.001, its maximum isothermal magnetic entropy change by 5.03 J / kg · K to 20.70 J / kg · K and 25.30 J / kg · K; phase transition temperature by 276 K to 284 K and 280 K; magnetic refrigeration capacity by 55.30 J / kg to 96.14 J / kg and 101.00 J / kg higher than the http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets current magnetic thermal properties reported in the literature Gd5Si2Ge2 and GdSiGeGa ingot alloys in high magnetic field (5 T) changes in performance under (| ΔSM | = 20.5 J / kg · K, Tc = 276 K) [4], the integrated magnetic alloy excellent thermal performance.

(2) After adding trace elements Zn, making the interaction through the conduction electrons to the Gd atoms 4 f-4 f electronic interactions between the enhanced spin wave energy increases, the alloy magnetic moment of Gd atoms increase; reduced room temperature Gd5Si2Ge2 monoclinic unit cell volume, and enhance the magnetic interaction energy between the Gd atoms, width of the cooling zone, improve the alloy a magnetic phase transition temperature and cooling capacity. In Gd5Si2Ge2-yZny alloy, when y changes from 0 to 0.011, the alloy phase transition temperature from 276 K to 288 K; magnetic refrigeration capacity from 55.30 J / kg up to 169 J / kg, the integrated magnetic alloy heat excellent performance.

3, with trace elements Zn compounds of Gd5Si2Ge2 alloying treatment, reducing the magnetic anisotropy energy of the alloy, the alloy during the phase transition in a magnetic domain wall motion or the magnetic moment of resistance decreases, a phase transition The critical magnetic field-induced decrease in the phase change alloy reduces magnetic hysteresis process, help to improve the thermal performance magnetic alloy composite.

Whether individually or simultaneously replaced Zn elements Si and Ge, are making the alloy in the 1.5 T magnetic field the phase transition temperature, the maximum isothermal magnetic entropy change, magnetic hysteresis and cooling capacity improved, but a different alternative to the magnitude of the performance is different.

4 by studying the heat treatment temperature on GdSiGeZn alloy magnetocaloric effect, and found that after adding trace elements Zn alloy can reduce the optimal heat treatment temperature, to prevent Gd5Si2Ge2 monoclinic phase at 773 K temperature of the eutectoid reaction, increase in β-phase temperature stability, reduced Gd5Si4 quadrature phase content, improve the β phase of the intrinsic magnetic properties and changes of temperature and magnetic field sensitivity, increasing the driving force of a phase transition. In Gd5Si2Ge2-yZny alloy, when y = 0.011 at, 773 K temperature heat treatment can help improve the alloy phase transition temperature, the isothermal magnetic entropy change and magnetic cooling capacity, lower level of the magnetic phase transition lag. When the heat treatment temperature is higher than 1413K, due to rapid cooling led to increased defects in alloys and phase transformation driving force decreases, the alloy phase transition temperature, the isothermal magnetic entropy change and magnetic cooling capacity tends to decrease, the magnetic hysteresis serious.

This work was developed with independent intellectual property rights GdSiGeZn alloy, so Gd5Si2Ge2 alloy in 1.5 T low magnetic field, the magnetocaloric effect increased 4-5 times, the phase transition temperature has increased by 4 ~ 6 K, changing the magnetic refrigeration technology relies on high magnetic field of application of the concept, low-priced NdFeB magnets in the magnetic field strength can be achieved under the Gd5Si2Ge2 alloy applications possible.Degree Year: 2009