Zhilin Xie

High temperature Ir segregation in Ir–B ceramics: effect of oxygen presence on stability of IrB2 and other Ir–B phases

The formation of IrB2, IrB1.35, IrB1.1 and IrB monoboride phases in the Ir–B ceramic nanopowder was confirmed during mechanochemical reaction between metallic Ir and elemental B powders. The Ir–B phases were analysed after 90 h of high energy ball milling and after annealing of the powder for 72 h at 1050°C in vacuo. The iridium monoboride (IrB) orthorhombic phase was synthesised experimentally for the first time and identified by powder X-ray diffraction. Additionally, the ReB2 type IrB2 hexagonal phase was also produced for the first time and identified by high resolution transmission electron microscope. Ir segregation along disordered domains of the boron lattice was found to occur during high temperature annealing. These nanodomains may have useful catalytic properties.

Hexagonal OsB2 reduction upon heating in H2 containing environment

Abstract The stability of hexagonal ReB2 type OsB2 powder upon heating under reforming gas was investigated. Pure Os metal particles were detected by powder X-ray diffraction starting at 375°C and complete transformation of OsB2 to metallic Os was observed at 725°C. The mechanisms of precipitation of metallic Os is proposed and changes in the lattice parameters of OsB2 upon heating are analysed in terms of the presence of oxygen or water vapour in the heating chamber. Previous studies suggested that Os atoms possess (0) valence, while B atoms possess both (+3) and (−3) valences in the alternating boron/osmium sheet structure of hexagonal (P63/mmc, No. 194) OsB2; if controllable method for Os removal from the lattice could be found, the opportunity would arise to form two-dimensional (2D) layers consisting of pure B atoms.

Aluminium magnesium boride: synthesis, sintering and microstructure

ABSTRACT AlMgB14 ceramics were reported as high-hardness materials over a decade ago. While different synthesis routes for processing of AlMgB14 ceramics were reported in the past, however the synthesis routes are still not optimised and present a significant challenge to the manufacturers. In this work six different synthesis routes were explored for the synthesis of AlMgB14 powder. The synthesised compositions were characterised by XRD, where weight fractions of each phase were calculated by Rietveld refinement. The bulk ceramics were sintered using powder with the highest yield (93.2%) of AlMgB14 phase by spark plasma sintering at 1315°C and 50 MPa. Both phase composition and microstructure of the sintered AlMgB14 were characterised by XRD and SEM/EDS, which revealed the existence of AlMgB14, MgAl2O4 and a small amount of unreacted Al. Hardness and indentation fracture resistance of AlMgB14 ceramics were measured to be 26.7 ± 2.2 GPa and 5.59 ± 0.42 MPa m1/2, respectively by Vickers indentation technique.

In search of the elusive IrB{sub 2}: Can mechanochemistry help?

We produced hexagonal ReB2-type IrB2 diboride and orthorhombic IrB monoboride phases, that were previously unknown and saw them produced by mechanochemical syntheses. High energy ball milling of elemental Ir and B powder for 30 h, followed by annealing of the powder at 1050 °C for 48 h, resulted in the formation of the desired phases. Both traditional laboratory and high resolution synchrotron X-ray diffraction (XRD) analyses were used for phase identification of the synthesized powder. Additionally, scanning electron microscopy and transmission electron microscopy were employed, along with XRD, to further characterize the microstructure of the phases produced.