Self-Propagating High-Temperature Synthesis With Magnesiothermit Reactions And Structural Macrokinetics Research Of TiB₂ And ZrB₂ Ceramic

For its advantages of rapid synthesis, simple process, energy saving, etc., SHS (Self-Propagating High-Temperature Synthesis) is a novel technology for preparing the advanced materials, such as ceramics, intermetallic compounds, high performance coatings and composite materials. Synthesizing ceramics by SHS with Magnesiothermit reactions uses natural oxides as the raw material, so the cost is low and the property of the product is good. Compared with SHS with common element, this technique offers obvious advantages and has been given more and more attention by researchers at home and abroad recently. However, due to some particularities of Magnesiothermit reactions, such as the quantity of releasing heat is large, the reaction process is difficult to control, the reaction temperature is high, etc., the studies on its reaction process and the formation mechanism of structure of the products are far from systematical. Therefore, the studies on the mechanism of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions are of significance to improve SHS itself and promote the wide application of TiB₂ and ZrB₂.In this paper, B₂O₃, TiO₂, ZrO₂ and Mg are used as the main raw material, the research on the characteristics of SHS process and chemical reaction process of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system has been made from the perspective of thermodynamics and Structural Macro Kinetics/structure dynamics.According to the thermodynamics theory, the free energy of reaction and the adiabatic temperature of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system have been theoretically calculated and analyzed. The calculation of the adiabatic temperature shows that the adiabatic temperatures of the two reaction systems are both higher than3000K, which are far higher than 1800K (the temperature meets the need of reaction itself). And the adiabatic temperatures of the reaction are falling with the increasing amount of diluents. The calculation of the free energy of reaction shows that the free energy is below than zero reaction can take place.The technological parameters, that is, the join effect of material proportion, the pressure of green compact and the green compact diameter and the diluents upon the process of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions are also studied in this thesis. The research on technological rules shows that the volatilization of Mg and B₂O₃ in raw material has a great effect on the purity of product. When the amount of Mg and B₂O₃ is increased, the purity of product becomes higher. In order to adjust the combustion temperature and change the morphology and particle size of product, appropriate amount of diluents MgO (0-5mol) can be added into the raw material. With the increasing amount of diluents MgO, the average particle size of product becomes smaller.The combustion wave of green compact in synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions can be successfully gained by means of combustion front quenching (CFQ) in cylindrical steel mould. And the microstructure of every region of the different reaction extent was observed. According to the relevant test and analysis on different product of different combustion front quenching areas combined with differential scanning calorimetry (DSC), the physical and chemical changes of all regions in the quenched samples during the combustion synthesis were followed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD). The systematic research on SHS of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃ system has been carried out. Studies shows that B₂O₃ is melting at623K, Mg at 923K, the reaction among them takes place from 1003K. The reaction between ZrO₂ and Mg to produce Zr happens first, and then B is gained from the reaction between B₂O₃ and Mg. At last, ZrB₂ is synthesized from the reaction between Zr and B. The results show that the reaction process of combustion synthesis ZrO₂-B₂O₃-Mg can be expressed as: (1) the preheat phase: B₂O₃ and Mg are melting, liquid Mg is permeable into molten B₂O₃ and solid ZrO₂ forming a hollow-cylinder molten ball, where the reaction takes place; (2) the early stage of reaction: the reaction between ZrO₂ particle and molten Mg occurs in the mechanism of dissolution and precipitation to produce Zr and MgO, releasing a great amount of reaction heat; (3) the middle stage of reaction: the great amount of reaction heat releasing in the early period leads to the reaction between B₂O₃ and Mg, which can form B and MgO; (4) the later stage of reaction: the reaction between Zr and B takes place and the ZrB₂ grain is formed. The reaction among ZrO₂, B₂O₃ and Mg is a complicated reaction of solid-liquid-liquid. The reaction process and structural change of the products of Mg-TiO₂-B₂O₃ system has the similarity with Mg-ZrO₂-B₂O₃ system.Thus, the diffusion-dissolution-precipitation mechanism of synthesizing TiB₂ and ZrB₂ by SHS with Magnesiothermit reactions is put forward and the relevant physics modeling is set. According to the physics modeling, we can finally get the original formula of kinetics in TiB₂ and ZrB₂ which is reacted and generated by the system of Mg-TiO₂-B₂O₃ and Mg-ZrO₂-B₂O₃.