【作者】 孙银宝；

【导师】 韩杰才； 张宇民；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 氮化硅陶瓷是结构陶瓷中综合性能最好的材料之一,它既具有优于一般陶瓷材料的机械性能、很高的热稳定性,又有较低的介电常数,其抗雨蚀、沙蚀能力优于其它天线罩材料。以氮化硅为基体的多孔陶瓷复合材料具有较低的介电常数、较低的密度,能够满足高速飞行器飞行过程中的信号传递功能,又能够通过多体系材料设计与相关试验克服氮化硅陶瓷本身的脆性弱点。本文以氮化硅、氮化硼以及二氧化硅为初始原料,主要从介电性能理论预测、材料体系组成试验、制备工艺研究、性能影响因素试验分析等四个方面对氮化硅基多孔陶瓷复合材料展开研究。首先,通过对电磁波在材料中的传播分析,深入研究了材料与结构透波影响因素。并进一步通过蒙特卡洛理论与有限元方法相结合,运用计算结构电容的方法对多孔陶瓷复合材料的相对介电常数进行了理论预测计算,通过拟合分析以及与相对介电常数串联模型、并联模型以及对数模型比较,从而给出了三相多孔材料相对介电常数预测公式。通过有限元数值模拟手段分析了介电常数对结构透波性能的影响。研究结果表明,氮化硅基多孔复合材料在电磁理论分析时隶属于连续随机材料,马尔可夫近似过程以及微扰法能够描述其微观透波过程,材料本身的介电常数对其组成结构透波性能影响最大。三相复合材料体系相对介电常数满足对数模型,建立的三相多孔复合材料相对介电常数预测公式与有限元方法的计算结果相一致。数值模拟结果显示材料介电常数越大,结构微波透过率越低,这与在天线罩材料设计中选用介电常数较低的材料体系理论相符合。其次,针对氮化硅、氮化硼以及二氧化硅成型为混合均匀复合材料问题,对初始原料的粒径、形貌进行了试验研究。并且分别通过添加炭粉以及淀粉作造孔剂的初步试验制备,确立了以淀粉作为造孔剂和粘接剂,采用淀粉固结工艺制备氮化硅基多孔复合材料坯体工艺过程。就淀粉固结工艺,试验研究了原材料的Zeta电位以及混合浆料的流变特性,建立了混合陶瓷浆料体系的流变模型,分析了分散剂PEI对混合陶瓷浆料体系粘度的影响,并最终确立了氮化硅、氮化硼、二氧化硅以及水的最佳配比。分别采用真空烧结、气氛烧结以及常压烧结工艺制备氮化硅基多孔陶瓷复合材料,分析了每种工艺制备的多孔陶瓷物相组成,通过烧结后样品的表观分析,最终确定低温常压烧结为本研究材料体系的烧结工艺。再次,通过试验以及理论分析深入研究了淀粉固结化学机理以及淀粉固结工艺制备氮化硅基多孔复合材料坯体的过程,得出了陶瓷浆料与玉米淀粉混合体系中水含量以及温度机制对坯体成型的影响,通过多次试验确定了坯体成型的最佳条件为水含量为50%,成型过程采用90℃1h,而后直接进行脱水。对氮化硅基多孔陶瓷材料的常压烧结过程进行了探讨,其烧结原理为氮化硅在低温时体系中氮化硅氧化产生液相,而氮化硼则没有氧化,产生的液相二氧化硅作为高温粘接剂能够促进多孔陶瓷成型,并使其具有一定强度。XRD以及SEM测试分析了保温时间对氮化硅基多孔陶瓷材料成型的影响,试验分析结果表明,烧结体在1100℃保温时间为5h时,烧结情况最好,并且连接紧密,微观结构无缺陷。最后,通过试验对显气孔隙率对氮化硅基多孔复合材料力学性能的影响以及显孔隙率、孔隙尺寸分布、频率以及温度等因素对氮化硅基多孔复合材料介电性能的影响进行了讨论分析。结果表明,显气孔隙率越大,其力学性能越低,并给出了相应的显气孔隙率与力学性能关系模型。在一定温度以及频率下,显气孔隙率对介电常数在一定范围内影响很大,能够降低成型的烧结体的介电常数,在该条件下影响介电常数的另一因素是孔隙尺寸分布,孔隙尺寸分布范围大,体系介电常数会有一定程度提高,以RIR法计算的物相质量组成为条件,对多孔陶瓷体系的介电常数进行了理论与试验结果对比,建立的预测公式与试验结果相一致。在室温,高频段的条件下,多孔氮化硅陶瓷复合材料介电常数随频率改变变化很小。在低频段下,多孔氮化硅陶瓷复合材料介电性能受温度的影响很大,温度升高,介电常数以及介电损耗显著升高。对低频下多孔氮化硅复合材料介电常数的变化规律以及原因进行了试验及理论分析,排除了相转变对介电常数的影响。极化理论,具体为德拜驰豫理论很好的解释了多孔氮化硅陶瓷复合材料在低频段介电性能随温度变化原因。更多还原

【Abstract】 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.更多还原