Oxidation Kinetics and Microstructural Characterization of LaB6 in Oxygen-Rich Condition
| Autor: | Chung-Han Wen, 溫宗翰 |
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| Rok vydání: | 2003 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 91 This work has investigated the oxidation behavior and microstructure evolution of LaB6 in the temperature range of 700oC-1185oC, and the better oxidation resistance of LaB6-ZrB2 composite. From the results measured by TGA (thermogravimetric system), the initial oxidation shows parabolic mass gain with increasing temperature in the range of 800~945oC, and the activation energy of oxidation of 24015 kJ/mol in air, and 20010 kJ/mol in pure oxygen in the range of 700oC~825oC. The first oxidation product is La-B-O glass. After oxidizing for a period of time, it will transform to La(BO2)3 crystals and B2O3 glass. When the temperature exceeds 995oC, the other reaction, evaporation of B2O3, obviously dominates the oxidation mechanism that results in the mass loss by deviation from parabolic behavior. In the later stage of the oxidation, the mass change stops because of the protection of oxide scale La(BO2) 3. While the testing temperature is higher than the melting temperature of La(BO2) 3 at 1141oC, the sample surface melts and produces porosity that offers channels for the oxidizing reaction. The microstructure of the La(BO2) 3 appears in columnar and equi-axial shapes if calcined at 800oC and 945oC respectively. The microstructural evolution of the LaB6-ZrB2 composite shows that ZrB2 fibers perform better oxidation resistance by heat treatment at 945oC than the LaB6 matrix. The surface morphologies, oxidizing mechanism, and the formation of porosity and crystalline phase are discussed in this report. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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