Theoretical and Experimental Studies on the Formation of Metal Borides

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\u27s 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