Background and overview
Niobium (NB) is a refractory metal with high melting point (2468 ℃), with moderate density, corrosion resistance, radiation resistance, high high temperature strength and good high temperature mechanical properties. The excellent properties of niobium and niobium alloys make them one of the important candidate materials for high-temperature structural parts in aviation, aerospace and nuclear industry. They can be used to manufacture key components such as rocket engines and nuclear reactors. However, its oxidation performance is poor. The "pest" oxidation phenomenon of pure niobium occurs at 600 ℃, and the oxidation degree intensifies with the increase of temperature, which eventually leads to the failure of its high-temperature performance. This disadvantage seriously restricts the application of niobium and niobium alloy. In order to improve its high-temperature oxidation resistance, surface coating protection is a practical and effective way to take into account the high-temperature mechanical properties and oxidation resistance of niobium and niobium alloys. When niobium silicide (NbSi2) is oxidized, SiO2 oxide film can be formed on the surface. The oxide film is dense and has self-healing ability and good high-temperature oxidation resistance. Therefore, NbSi2 has become the main choice of high-temperature protective coatings for niobium and niobium alloys.
1. Synthesis of niobium / niobium silicide composites Nb / Si binary alloy matrix composite is a promising high temperature structural material. In this two-phase composite, solid solution niobium provides room temperature toughness and Nb5Si3 provides high temperature strength. This composite has high thermodynamic stability and microstructure stability at high temperature. Japan ultra high temperature materials research institute and other units studied the preparation of niobium / niobium silicide composites by reactive hot pressing of ball milled powder, and studied and evaluated their microstructure and mechanical properties. In this study, ball milling process is used as a pretreatment method to improve the subsequent reaction sintering effect. The powder used for experimental research adopts elemental niobium powder (purity 99.9%, particle size < 325 mesh) and semiconductor grade silicon (particle size < 300 mesh). After mixing according to the composition ratio of nb-3.5% Si, nb-6% Si, nb-10% Si and nb-16% Si (all mols), it is milled in a planetary ball mill. The weight ratio of grinding ball (10mm diameter stainless steel ball) to mixed powder is about 4:1. The powder milled for a certain time (1 ~ 110h) is hot pressed and vacuum sintered in graphite mold (sintered at 40MPa and 1773K for 3h). The changes of microstructure and mechanical properties of hot pressed blocks after annealing were studied.
2. Prepare a ceramic aluminum plate. Raw materials with the following weight proportion: 800-1000 parts of clay, 50-100 parts of aluminum, 20-30 parts of carbon, 5-8 parts of tungsten, 3-5 parts of titanium, 8-10 parts of nickel, 10-15 parts of niobium silicide, 15-20 parts of molybdenum boride, 5-10 parts of titanium carbide, 2-5 parts of titanium nitride, 5-8 parts of molybdenum, 10-20 parts of zircon sand, 10-15 parts of glass fiber and 500-1000 parts of water. The ceramic aluminum plate has better heat resistance through the addition of carbon, tungsten, titanium, nickel, niobium silicide and molybdenum boride. In addition, titanium carbide, titanium nitride, molybdenum, zircon sand and glass fiber are added to make the ceramic aluminum plate have higher hardness, which can not only more effectively prevent the ceramic aluminum plate from being broken due to collision, but also make the ceramic aluminum plate withstand higher temperature and will not be damaged due to high temperature in the process of use, The application range of ceramic aluminum plate is increased and the use cost of ceramic aluminum plate is reduced.
3. Prepare an alumina ceramic material with good toughness. It is composed of 95-97 parts by weight of alumina, 1-2 parts of light calcium carbonate, 1.5-3.2 parts of kaolin, 0.7-1.8 parts of boehmite, 0.5-0.9 parts of zirconium nitride, 0.8-1.4 parts of chromium powder and 0.3-0.6 parts of niobium silicide. The invention also provides a preparation method of the alumina ceramic material with good toughness. The prepared alumina ceramic material with good toughness has good toughness and thermal shock resistance, and the production cost is low, which is conducive to further expand its application range and has broad market prospects.
4. Prepare a silicon germanium alloy based thermoelectric element. The silicon germanium alloy based thermoelectric element is composed of an electrode layer, a silicon germanium alloy based thermoelectric layer and a barrier layer between the electrode layer and the silicon germanium alloy based thermoelectric layer. The barrier layer is a mixture of silicide and silicon nitride, and the silicide is at least one of molybdenum silicide, tungsten silicide, cobalt silicide, nickel silicide, niobium silicide, zirconium silicide, tantalum silicide and Hafnium silicide. The silicon germanium alloy based thermoelectric element provided by the invention has good interface bonding, no crack and obvious diffusion phenomenon at the interface, low contact resistance, good thermal contact state, and can withstand long-term high-temperature accelerated test. In addition, the preparation method provided has the characteristics of simple process, high reliability, low cost, no special equipment, suitable for large-scale production, etc.
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