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Jon Binner and Ji Zou School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK (Received 1 March 2016; accepted 16 May 2016)

This paper critically evaluates methods used to synthesize boride compounds with emphasis on diborides of the early transition metals. The earliest reports of the synthesis of boride ceramics used impure elemental powders to produce multiphase reaction products; phase-pure borides were only synthesized after processes were established to purify elemental boron. Carbothermal reduction of the corresponding transition metal oxides emerged as a viable production route and continues to be the primary method for the synthesis of commercial transition metal diboride powders. Even though reaction-based processes and chemical synthesis methods are mainly used for research studies, they are powerful tools for producing diborides because they provide the ability to tailor purity and particle size. The choice of synthesis method requires balancing factors that include cost, purity, and particle size with the performance needed in expected applications.

I. INTRODUCTION

Transition metals (TMs) form a tremendous variety of boride compounds ranging from boron-rich dodecahedron or icosahedral structures such as ScB12 and YB25 to metal-rich close-packed compounds such as W2B5 and TiB. From the larger family of boride compounds, the early TMs all form metal-rich refractory borides, Table I,1–21 although synthesis methods described herein are applicable to other boride compounds with different stoichiometries and structures, including ternary borides. From the larger family of metal-rich TM borides, the paper focuses on methods to produce Ti, Zr, and Hf diborides, which share the AlB2 structure with other TM diborides including CrB2, TaB2, and NbB2. The TM diborides have a complex mix of bond types resulting in a remarkable combination of metal-like and ceramic-like properties.22–25 The AlB2 structure consists of layers of close-packed metal atoms that alternate with boron in graphite-like sheets. Metallic bonding in the TM layers leads to high electrical (107 S/m for ZrB2)26,27 and thermal (;130 W/m K for ZrB2)27
29 conductivities, while strong covalent bonding in the B layers gives high hardness (;33 GPa for TiB2)30 and elastic modulus (;560 GPa for TiB2)31 values. Strong cohesion between Contributing Editor: Tina Salguero Address all correspondence to this author. a) e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/. DOI: 10.1557/jmr.2016.210 J. Mater. Res., 2016

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TM and B layers is the result of complex bonding that includes ionic, hybridized covalent, and metallic character.32,33 The same characteristics (high melting temperature, strong covalent bonds) that give TM boride compounds attractive properties also result in challenges