Junxian Zhang

XAS investigations on nanocrystalline Mg2FeH6 used as a negative electrode of Li-ion batteries

In the frame of research on new metallic hydrides as conversion reaction materials for negative electrodes of Li-ion batteries, the MgFe2H6 complex hydride has been investigated in and ex situ using XRD, XAS and magnetic measurements at different states: initially ball milled complex hydride (CH), electrochemically desorbed (ED) and thermally desorbed (TD). Fe–H bonding is clearly evidenced by EXAFS in the CH sample. It is also observed that the electrochemical reaction leads to a nanocrystalline state that needs local probe analyses to be fully investigated and interpreted. From the XAS and magnetic data, the different routes (ED and TD) for dehydrogenation of the complex hydride are compared. For both electrochemically and thermally driven reactions, the hydrogen depletion from Mg2FeH6 hydride leads to decomposition into its constituting elements Mg and Fe. However, different Fe structures are observed: bulk α-Fe and amorphous Fe nanoparticles for TD and ED samples, respectively.

The coercivity of the melt-spun Sm-Fe-Ga-C permanent magnets and the effect of additives (Nb, Cu and Zr)

Sm2Fe15Ga2C3 and Sm2Fe15Ga2M0.2C3 (M = Nb, Cu, Zr) ribbons with Th2Zn17-type structure have been synthesized by melt-spinning and subsequent annealing at temperatures above 1023 K. A nanoscale 2:17 phase with grain sizes of 50-200 nm is developed. The anisotropy constants K1 and K2 are determined in the temperature range of 150-600 K by fitting the demagnetization curve of the isotropic samples. Furthermore, the intrinsic material parameters, like the Curie temperature, exchange constant, anisotropy field and critical diameter of single domain particles are given. In order to understand the magnetic hardening mechanism, the temperature dependence of the coercivity is analysed within the framework of the micromagnetic models. It is concluded that the magnetic hardening mechanism at low temperatures up to 350 K is dominated by nucleation processes in a magnetically inhomogeneous region. The average widths of the inhomogeneities of 2 nm and 3 nm for Sm2Fe15Ga2C3 and Sm2Fe15Ga2M0.2C3 (M = Nb, Cu, Zr) are determined, respectively. At high temperature the pinning of the domain walls by grain boundaries controls the coercivity mechanism.