Formation of Molybdenum Borides and Their Composites by Combustion Synthesis with Solid-phase Displacement Reactions
許瑋升、葉俊良
E-mail: 9607731@mail.dyu.edu.tw
ABSTRACT
An experimental study on formation of molybdenum boride (MoB) was conducted by self-propagating high- temperature synthesis (SHS) using compacted samples from elemental powder mixtures. Upon ignition, the reaction was characterized by a planar combustion front propagating along the sample in a self-sustaining manner. In the synthesis of monolithic MoB, effects of the initial sample density and preheating temperature on combustion characteristics and the composition of final products were studied.
Flame-front propagation velocity and combustion temperature were found to increase with initial sample temperature and density.
Based upon the measured data, the activation energy of combustion synthesis of MoB was determined as 80.44 kJ/mol. For the reactant compacts with Mo:B = 1:1, the XR D analysis identified formation of α-MoB as the dominant phase and Mo2B as the intermediate phase. Formation of molybdenum borides of four different phases, including Mo2B, MoB2, Mo2B5, and MoB4, was performed by self-propagating high-temperature synthesis (SHS ) from the reactant compacts composed of MoO3, Mo, and B powders. The solid-phase displacement reaction of MoO3 with boron was involved in the SHS process. The effect of starting stoichiometry on flame-front propagation velocity, combustion temperature, and product composition was studied. In all cases, experimental observations show a planar reaction front propagating in a stable and self-sustaining manner. During the reaction, misty boron oxide in the form of small liquid droplets was produced in the displacement reaction and ejected from the sample. Thus, the end product was significantly expanded. A decrease in the displacement reaction taking place during viithe SHS process reduces the heat flux to the synthesis reaction,thus resulting in lower combustion temperatures and reaction front velocities. The hihest combustion temperature was observed in the synthesis of MoB2, followed by Mo2B. The reaction temperature of Mo2B5 is comparable to that of MoB4. Based upon the XRD analysis, formation of MoB2 and Mo2B5 as the dominant boride phases in the end products was successful. It was found a large amount of Mo left in the synthesis of Mo2B. However, the powder compact prepared for the production of MoB4 yielded mostly Mo2B5.
Keywords : Molybdenum borides, SHS, Displacement reaction, Flame-front velocity, Combustion temperature.
Table of Contents
封面內頁 簽名頁 授權書 ...iii 中文摘要 ...iv 英文摘要 ...vi 誌謝 ...viii 目錄 ...ix 圖目錄 ...xii 表目錄
...xv 符號說明 ...xvi 第一章緒論 ...1 1.1 研究背景
...2 1.2 文獻回顧 ...2 1.2.1 自持傳遞燃燒高溫合成法相關文獻 ...2 1.2.2 硼化鉬之相 關文獻 ...4 1.2.3 置換反應之相關文獻 ...4 1.2.4 氧化鋁之相關文獻 ...5 1.2.5 矽化鉬之相關 文獻 ...5 1.3 研究目的 ...6 第二章實驗方法 ...7 2.1 試片
...7 2.1.1 硼化鉬燃燒合成 ...7 2.1.2 各組態硼化鉬與Al2O3 複合材料合成 ...8 2.1.3 MoB-MoSi2 複合材料燃燒合成 ...9 2.1.4 MoSi2 -Al2O3 複合材料燃燒合成 ...9 2.2 燃燒室主體
...10 2.3 資料擷取系統 ...10 2.4 影像擷取系統 ...11 2.5 產物分析 ...12 第三章結果與討論 ...13 3.1 燃燒合成不同組態硼化鉬 ...13 3.1.1 MoB ...13 3.1.1.1 固相火焰觀察 ...13 3.1.1.2 火焰傳遞速度 ...14 3.1.1.3 溫度量測 ...15 3.1.1.4 產物活化能與分析 ...16 3.1.2 Mo2B、MoB2、Mo2B5 與MoB4 ...16 3.1.2.1 固相火 焰觀察 ...16 3.1.2.2 火焰鋒面傳遞速度與溫度 ...17 3.1.2.3 產物分析 ...18 3.2 燃燒合成複合 材料 ...20 3.2.1 硼化鉬與Al2O3 複合材料 ...20 3.2.1.1 固相火焰探討 ...20 3.2.1.2 產物分析 ...21 3.2.2 MoB-MoSi2 複合材料 ...22 3.2.3 MoSi2 -Al2O3 複合材料 ...23 第四章結論 ...25 參考文獻 ...29 附錄 ...34
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