Mechanochemical synthesis of ReB 2 powder
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Mikhail Klimov Materials Characterization Facility, University of Central Florida, Orlando, Florida 32826
Helge Heinrich Physics Department, University of Central Florida, Orlando, Florida 32816
David Restrepo and Richard Blair Chemistry Department, University of Central Florida, Orlando, Florida 32816
Challapalli Suryanarayana Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816 (Received 24 May 2011; accepted 19 July 2011)
ReB2 was recently reported to exhibit high hardness and low compressibility, which both are strong functions of its stoichiometry, namely Re to B ratio. Most of the techniques used for ReB2 synthesis reported 1:2.5 Re to B ratio because of the loss of the B during high temperature synthesis. However, as a result of B excess, the amorphous boron, located along the grain boundaries of polycrystalline ReB2, would degrade the ReB2 properties. Therefore, techniques which could allow synthesizing the stoichiometric ReB2 preferably at room temperature are in high demand. Here, we report synthesis of ReB2 powders using mechanochemical route by milling elemental crystalline Re and amorphous B powders in the SPEX 8000 high-energy ball mill for 80 h. The formation of boron and perrhenic acids are also reported after ReB2 powder was exposed to the moist air environment for a 12-month period of time.
I. INTRODUCTION
Rhenium diboride (ReB2) is a boron-rich ceramic that has been receiving a lot of attention in the scientific community because of its unusual properties. It has been reported as a superhard material1–3 with a hardness of 48 GPa measured at a small applied load (0.5 N). It was also reported that it exhibits strong and highly covalent bonding, while a strong hybridization between the Re 5d and B 2p states indicates that Re–B bonds have also prevalent covalent character with some degree of ionic bonding present.4–6 Covalent boron–boron bonds are significantly stronger than the covalent Re–B bonds,7 and such difference between the B–B and Re–B bonds, as well as between nonequivalent Re–B bonds, is responsible for anisotropic compressibility and rigidity of the structure.8 The strong directional B–B bonding complimented by Re–B covalent bonds are responsible for the high resistance to elastic and plastic deformations resulting in high shear, bulk, and Young’s moduli, which are indicative of high hardness of the compound.9 It was also reported that both Re 5d and B 2p states are at the Fermi level and, hence, ReB2 exhibits a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.249 2772
J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011
http://journals.cambridge.org
Downloaded: 28 Jun 2014
metallic behavior.3,10 There have been numerous discussions in regard to the actual hardness of ReB2, which was reported in Science.1 Several reports11–13 suggest that the hardness of ReB2 was overestimated since it was measured in the region where the indentation size effect is known to exist. According to
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