Most recent developments in the production, properties and applications of borides and carbides are reviewed. The reactivity and sintering of finely-divided boron carbide with metal additives has been investigated. The additives (Fe, T i , Zr, V, Nb, Ta, Mo, W and Al) generally promote sintering of the boron carbide. Their effectiveness is reduced occasionally when there is some surface activation caused by the metals reacting vjith the boron carbide to form metal borides and carbides of different, crystal lattice type and molecular volume. The more metallic character of the bonding in the metal borides and carbides enhances surface and crystal lattice diffusion at the grain boundaries of the more covalent boron carbide. Iron is much more effective than the other metal addives tested in promoting sintering of the boron carbide at 1800 degree C since it forms the lowest-melting borides. It also enhances sintering during hot pressing of boron carbide at this temperature.

AN ABSTRACT OF THE THESIS OF Lisa Rajan, for the Master of Science degree in Mechanical Engineering, presented on April 1, 2011, at Southern Illinois University Carbondale. TITLE: THEORETICAL AND EXPERIMENTAL STUDIES ON THE FORMATION OF METAL BORIDES MAJOR PROFESSOR: Dr. Rasit Koc Formation studies were done on six metal borides, TiB2, ZrB2, HfB2, AlB2, CaB6, and LaB6. The goal of this research is to research materials used for coatings on bipolar plates used in polymer electrolyte, PE, fuel cells. Theoretical calculations were done with formation equations from these six materials. Software used was HSC Chemistry version 5.0. Each equation was fully entered, and the range of formation temperature was narrowed till a value was obtained. Experimental work was done on TiB2 using thermogravimetric analysis and x-ray diffraction. Studies were done on carbon coated TiO2 precursors mixed with B4C and mixed carbon and TiO2 precursors mixed with B4C. The studies show that TiB2 is a very excellent material and easy to make. ZrB2, HfB2 and CaB6 are also easily processed and available for use in the market. AlB2 needs more research so that effective conclusions can be made about its formation temperature and suitable methods of production. Although made by methods uncommon to those studied in this work, LaB6 is already in commercial production and used in electron microscopes. Examination of previous experimental studies shows that the reduction reaction methods and formation of powders in an autoclave are the most consistent methods of metal boride formation. Calculations in the HSC 5.0 software confirmed that. When reactions for these methods were done, they consistently produced precise formation temperatures. Several variations of the combined reduction reaction were found in the literature, and all produced the same theoretical formation temperature. Molar amounts did not affect this temperature. Each method produced submicron particle powders. Thermodynamic data was useful in determining the certainty of each reaction being done experimentally. Most reactions were thermodynamically possible with a few exceptions. Change in enthalpy, ∆H, change in entropy, ∆S, change in Gibb's free energy, ∆G, and the formation constant K were all factors that were calculated and used to describe metal boride formation. The experimental studies showed that coated precursors have a lower temperature at which the reaction begins. The process is more gradual for coated precursors, although both coated and mixed precursors reach a pure TiB2 phase by 1400°C. The mixed precursor had a higher phase of purity than the coated precursor at 1400°C.

Ce travail de thèse a pour objet le développement d'une nouvelle voie de synthèse de nanomatériaux métalliques à base d'éléments légers : bore et phosphore. L'intérêt porté à ces composés s'explique par les propriétés variées qu'ils présentent, tels que la supraconductivité, la thermoélectricité ou le stockage d'énergie. Dans le cadre de ce travail, les domaines de la catalyse et de l'électrocatalyse sont explorés. Les borures de différents métaux de transition, en particulier le nickel, le palladium et un composite nickel-cobalt, ont tout d'abord été étudiés. Pour cela une synthèse a été mise au point, reposant sur la réactivité de nanoparticules métalliques avec un précurseur de bore en milieu sels fondus inorganiques. Elle a notamment permis d'obtenir des nanoparticules de borures de nickel avec un bon contrôle de composition, structure, morphologie et taille. Les propriétés de ces nanomatériaux ont par la suite été étudiées en catalyse dans la réaction d'hydrodésoxygénation, et en électrocatalyse dans les réactions de génération d'hydrogène ou d'oxygène à partir de l'eau. Enfin la réactivité du phosphore rouge en milieu sels fondus a été abordée, ouvrant ainsi une nouvelle voie vers l'élaboration de phosphures de métaux de transition. / This PhD work deals with a novel synthesis of metal boride and metal phosphide nanomaterials. Nanostructures of these solids are subject to an increasing interest due to their exciting properties for various applications fields such as superconductivity, high temperature thermoelectricity, energy conversion and storage. In this work, the catalytic and electrocatalytic properties of these nanomaterials are explored. First of all, borides of various transition metals, such as nickel, palladium or a nickel-cobalt composite are studied. To do so, a new liquid-phase synthesis was developed, based on the reactivity of already formed metal nanoparticles with a boron precursor in inorganic molten salts. This new synthesis allowed a precise control over the nanoparticle morphology, size, composition and crystalline structure. By accessing such nanoscale objects, we were able to investigate their properties and performances, especially in the fields of catalysis with the hydrodeoxygenation reaction and electrocatalysis for the hydrogen evolution reaction and oxygen evolution reaction. Finally, the reactivity of red phosphorus in molten salts was addressed, thus paving the way to the extension of this synthetic pathway to metal phosphides.

Borides are implemented in a range of industrial applications due to their unique mechanical, electrical, thermal and catalytic properties. In particular, transition metal diborides are of special interest. In the recent years, borides of rhenium (Re), osmium (Os) and iridium (Ir) have been studied as for their ultra-hardness and superior stiffness. In this dissertation, a mechanochemical method is introduced to produce rhenium diboride (ReB2) powder, a novel hexagonal osmium diboride (h-OsB2), and iridium boride powders. Densification by Spark Plasma Sintering (SPS), thermal stability and mechanical properties of h-OsB2 were also studied. 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. ReB2 powder was synthesized at low temperature using mechanochemical route by milling elemental crystalline Re and amorphous B powders in the SPEX 8000 high energy ball mill for 80 hours. The formation of boron and perrhenic acids are also reported after ReB2 powder was exposed to the moist air environment for a twelve month period of time. Hexagonal osmium diboride (h-OsB2), a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction (XRD) indicated the formation of h-OsB2 after 2.5 hours of milling, and the reaction reaches equilibrium after 18 hours of milling. The lattice parameters of the h-OsB2 are a=2.916Å and c=7.376 Å, with a P63/mmc space group. Transmission electron microscopy confirmed the appearance of the h-OsB2 phase. The thermal stability of h-OsB2 powder was studied by heating under argon up to 876 °C and cooling in vacuo down to -225 °C. The oxidation mechanism of h-OsB2 has also been proposed. The hexagonal phase partially converted to the orthorhombic phase (20 wt.%) after spark plasma sintering of h-OsB2 at 1500°C and 50MPa for 5 minutes. Hardness and Young*s modulus of the h-OsB2 were measured to be 31 ± 9 GPa and 574 ± 112 GPa, respectively by nanoindentation method. Prior to this research a number of compounds have been prepared in Ir-B system with lower than 2 boron stoichiometry, and no IrB2 phases have been synthesized experimentally. In this dissertation, three new iridium boride phases, ReB2-type IrB2, AlB2-type IrB2 and IrB have been synthesized with a similar mechanochemical method. The formation of these three phases has been confirmed by both X-ray diffraction (XRD) and transmission electron microscope (TEM) after 30 hours of ball milling and 48 hours of annealing. The IrB2 phases have hexagonal crystal structures and the new IrB phase has an orthorhombic crystal structure. The segregation of iridium from iridium borides* lattices has also been studied by high resolution TEM.

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