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URN etd-0823107-124017 Statistics This thesis had been viewed 2981 times. Download 1446 times. Author Kai-Yuan Pai Author's Email Address No Public. Department Materials Engineering Year 2006 Semester 2 Degree Master Type of Document Master's Thesis Language zh-TW.Big5 Chinese Page Count 97 Title Synthesis of High Melting Point Cr-B, Mn-B, Mo-B, and W-B Transition Metal Borides by Mechanical Solid-state Reaction Keyword Thermal analysis. Boride formation Structural evolution Mechanical solid-state reaction Mechanical solid-state reaction Structural evolution Boride formation Thermal analysis. Abstract This study chose boron element and transition elements of Cr, Mn, Mo, and W to synthesize high melting point Cr-, Mn-, Mo-, and W- borides by mechanical solid-state reaction. The powders employed in the ball milling process have compositional proportions in metal to boron ratio of 67:33, 50:50, and 20:80, respectively, in each metal-boron system. The milling was conducted for up to 60h. The structures of as-milled powders were characterized by X-ray diffraction. The thermal reactions during the heating process were analyzed by differential scanning calorimeter. The amorphous structures of the milled powders were also examined by transmission electron microscope. The as-milled 60h powders were annealed at different temperature for different times and the structural evolution of annealed powders were also characterized by X-ray diffraction. The activation energy of boride formation in each system during thermal annealing process was measured. It was found that the intensity of metallic diffraction peaks in all systems was reduced with increasing milling time. The diffraction peaks of crystalline metals disappear in a shorter milling time when more boron content is in the initial composition. Powders with compositions of Cr67B33, Cr50B50, Mo67B33, Mo50B50, W67B33, W50B50, and W20B80 form amorphous structure after milling for 60h. These powder mixtures, after being milled for 60h and subjected to subsequent annealing, can form single phase borides or boride mixtures. Which types of boride structure are formed during the milling and subsequent annealing process were determined by the initial powder compositions. Among all the compositions investigated, compositions of Cr67B33, Cr20B80, Mn67B33, Mn50B50, Mn20B80, and W20B80 have a greater tendency to form a single phase boride. It is important to found that for Mo-B and W-B systems which exhibit solubility in the intermediate compounds in the binary phase diagram, the milled powder mixtures of Mo67B33, Mo50B50, W67B33, and W50B50 decompose into metal elements when the milled powders are subjected to annealing treatment. The same phenomenon cannot be observed in Cr-B and Mn-B systems which show line compounds. Advisor Committee Hsin-Ming Wu - advisor
Files Date of Defense 2007-07-26 Date of Submission 2007-08-23