Preparation And Properties Of Porous BN/Si₃N₄Composite Ceramics

Aerospace wave-transparent materials have many function characteristicssuch as wave-transparency, heat insulation, carrying capacity, and erosionresistance, which allow them to protect the spacecrafts from the influence ofharsh environments. With the rapid development of aircrafts and the increase offlight speed, materials are required to possess a number of critical properties suchas good mechanical and dielectric properties, excellent thermal resistance andrain erosion resistance, have a light weight, and so on. Silicon nitride is one ofthe most promising ceramic materials for the applications of wave-transparentmaterials, due to its high mechanical strength, good thermal shock resistance andexcellent erosion resistance. However when silicon nitride is used as high-machmissile radome, the dielectric properties are poor relatively. Boron nitride hasbetter thermal stability, a lower dielectric constant than silicon nitride, and it hasmore stability of dielectric properties at high temperatures. The downside is thatthe mechanical properties and rain-erosion resistance of BN are poor. In order toimprove the dielectric properties of Si₃N₄ceramics, BN can be introduced intothe Si₃N₄matrix to fabricate BN/Si₃N₄wave-transparent composites withexcellent properties.Porous composite ceramics have low dielectric constant and low density.They can not only satisfy the need to transmit signals with high speed, but alsoare able to overcome the brittleness of ceramic by designing and testingmulti-system materials. In this work porous BN/Si₃N₄composite ceramics with highporosity, high strength and excellent dielectric properties have been synthesized bygel-casting process.Parameters in the preparation process were thoroughly investigated andoptimized; the TBA-based gel-casting process has been improved to become a maturepreparation technique in preparing porous BN/Si₃N₄composite ceramics. The resultsshowed that the slurry with1.44g/l dispersant added had the best stability. Thecatalyst （MBAM） and initiator （APS） aqueous solution of the best content was1.5wt% and30wt%（to monomer）, respectively. The green body was moisturizing dried at40°C in N2environment.Secondly, porous BN/Si₃N₄composite ceramics was fabricated by dry-pressingmethod, pore-forming agent method and TBA-based gel-casting process. It was foundthat the advantages of TBA-based gel-casting in fabricating porous BN/Si₃N₄composite ceramics could be ascribed to its feasibility in obtaining a homogeneousand controllable structure. And also, through TBA-based gel-casting we could gethigh porosity. When the porosity was36.5%, the flexural strength was136±5.35MPaand when the porosity was77.4%, the dielectric constant was1.74.At a fixed solid loading of15vol%of initial slurry, the effect of the content ofsintering additives Y2O3-Al2O3and SiO2and sintering temperature on the propertieswas analyzed. The results showed that the porosity increased with the higher amountsof sintering additives. A uniform pore size distribution was observed, and the medianpore size was micron or submicron. The flexural strength and dielectric constant ofporous BN/Si₃N₄ceramics increased when the content of sintering additives increased.When the content of Y2O3-Al2O3was5wt%, the content of SiO2was7.5wt%, andthe flexural strength was65.84±3.08MPa. At room temperature, porous BN/Si₃N₄composite ceramics had excellent and stable dielectric properties at7¹8GHzfrequency. The dielectric constant was2.39, and the dielectric loss stayed between1.5³.5×10^-3. The porous BN/Si₃N₄composite ceramics had excellent dielectricproperties at high temperatures.Furthermore, the effect of BN contents on the microstructure and properties ofporous BN/Si₃N₄composite ceramics was analyzed. The porosity, microstructure,mechanical properties, and dielectric properties of BN particles and BN whiskerswere compared in porous BN/Si₃N₄composite ceramics. When the content of BNincreased the porosity of BN/Si₃N₄composite ceramics increased while the dielectricconstant and flexural strength decreased. The porous Si₃N₄composite ceramics withBN whiskers had better dielectric and mechanical properties.Lastly, the wave-transparent mechanism was investigated. The dielectricconstant of porous BN/Si₃N₄composite ceramics was decreased while theporosity increased. The relationship between the dielectric constant and porositywas consistent with Lichtencker’s logarithmic equation （ε）. Phase composition was another important factor on dielectric properties. The increase of BN with alow dielectric constant could decrease the dielectric constant of the composites.The dielectric constant of BN/Si₃N₄composites could be calculated by themultiphase Lichtencker’s logarithmic equation （ε）. Furthermore, we discussed theinfluence of the grain boundary phase, the pore structure and impurity ondielectric properties especially dielectric loss. The content of the grain boundaryphase decrease was beneficial for the dielectric loss. The samples with biggerpore size had small dielectric loss.