Preparation And Properties Characterization Of Silicon Nitride Matrix Porous Microwave-Transmitting Materials

Silicon nitride ceramics are materials with the best combination properties in the structural ceramics. They have better mechanical characters, better thermal stability, and lower dielectric constant compared with other regular ceramics. Their ability of rain and sand erosion of silicon nitride are also superior to other radome materials. Silicon nitride matrix porous composites have low dielectric constant and density, which help to fulfill the request of signal transmission for flight of high-speed flying vehicle. They can also overcome the disadvantage of brittleness through the design of materials and correlated experiments. Based on the start materials of silicon nitride, silicon dioxide, and boron nitride, this research is focusing on the prediction of dielectric properties, the experiment of materials'system with different composition, the analysis of preparation technology, and influential factors on performance in study of silicon nitride matrix porous composites.First, according to the analysis results of electromagnetic wave transmission in material, the effect elements on transmission in material and structure were explored profoundly. Further coupled with Monte Carlo technique and FEM method, theoretical prediction of the relative dielectric constant of porous ceramic composites was solved by calculating the capacitance of structure. With the consideration of the format of the serial mixing model, parallel model, and logarithmic mixing rule, a new equation for predicting the dielectric constant of the three phase porous composites was built by fitting the FEM calculated data. Effects of dielectric constant on the transmission ability of microwave in structure were analyzed by FEM mehtod. The results of above analysis showed that silicon nitride matrix porous composite is a kind of continuous random material. Markov approximate process and perturbation theory could describe the process of microwave transmitting in the microscopic view of the materials. Dielectric constant of the material played the most important role in the microwave-transmitting process of the structure. The dielectric constant of three phases composite was in accord with logarithmic mixing rule, the predictor formula of solving the tri-phase porous material matched the results of calculated by FEM very well. It showed that the bigger the dielectric constant was, the better the microwave-transmitting ability of the structure was, which meets the theory for choice of low dielectric constant materials for material design of radome.Second, the particle sizes and patterns of the raw materials were studied for even molding composites of silicon nitride, boron nitride, and silicon dioxide. By the preliminary experiments of adding powdered carbon and corn starch as pore-forming agent, the corn starch was chose as both pore former and consolidator in the confectioning of porous composite bodies. Starch consolidation technique was used in the assembly progress of the porous composite green bodies. Zeta potential of the raw materials and rheological dynamics of the slurries were investigated experimentally to solve the potential problem in the starch consolidation technique. A new model for predicting rheological change of mixed slurries was deduced, and effect of PEI on the change of viscosity of ceramic slurries was explored; In the end, the best mixture ratio of silicon nitride, silicon dioxide, boron nitride, and water was fixed. The vacuum sintering, atmosphere sintering, and normal atmosphere sintering were adopted in preparing the silicon nitride matrix porous composites, respectively. Phase compositions of the porous composites prepared by these three sinter technologies were analyzed. According to the apparent patterns to the porous composites analysis, the normal atmosphere sintering technique in relative low temperature was employed in preparing our material systems.Third, starch consolidation technique chemism and the process of preparing silicon nitride porous composite green bodies using starch were explored by experiments and theoretical analysis. Effects of water content of the material systems and temperature on preparation of the bodies were studied and the optimal condition for body preparation was 50% water content, starch consolidation at 90℃for 1 h, drying and sintering in the electric muffle furnace. The process of normal atmosphere sintering for silicon nitride matrix porous composites was investigated theoretically. In normal atmosphere condition, at low temperature, silicon nitride could be oxidized, and the liquid phase generated at this process was a kind of high temperature adhesive material, which could accelerate the preparation of the porous composites and enhance the strength of the sintered body. Boron nitride could not be oxidized at this low temperature. Effect of holding time at 1100℃on the preparation of the porous composites was analyzed by XRD and SEM experiments. The results of experiments showed that, the porous composites perfomed perfect close bonding structure at the holding time of 5 h, and no flaw was found in the microstructure of the porous composites.Finally, effects of apparent porosity on the mechanical properties as well as the influence of apparent porosity, distribution of pore sizes, frequency, and temperature on the dielectric properties of the porous composites were studied by experiments and theoretical analysis. The mechanical experiments showed that, the bigger the apparent porosity was, the lower the mechanical property was. Models of the relationship between apparent porosity and mechanical property were deduced. The study on the dielectric properties indicated that the apparent porosity had effects on the dielectric constant of the porosity at constant frequency and temperature. Another factor influenced the dielectric properties of the sintered body was distribution of pore sizes; the range of distribution of pore sizes increment could result in increasing dielectric constant. Based on the phase mass percent calculated by relative intensity ratio (RIR) technique, the dielectric constant of the porous composites was predicted by the theoretical equations. The results by the MC equation were in accord with the experimental results. At room temperature, dielectric constant of the porous silicon nitride matrix composite changed little with variation of frequencies. At the range of the low frequency, effect of temperature on the dielectric constant was great, and dielectric constant and loss tangent raised with the increment of the temperature. The variation of the dielectric constant and loss tangent with temperature change was investigated by experiment and theoretical analysis, and the effect of phase transformation on the dielectric properties was eliminated. Debye relaxation theory was applied to explain the variation of the dielectric constant with temperature increment.