Design,Fabrication And Properties Of Si₃N₄ Broadband Transparent Ceramic With Dielectric Gradient

With the improvement of the flying Mach number of the aircraft and the extension of the flying distance,more stringent requirements are placed on the antenna window materials,and the existing wave-transparent materials cannot meet the application requirements.Silicon nitride-based ceramics have become a new generation of high-temperature resistant wave-transparent materials after fused quartz due to their excellent mechanical and dielectric properties,good thermal shock and ablation resistance.However,the dielectric constant of dense silicon nitride material is relatively high,and hence there are still some limitations in widening the working frequency range of the radome and reducing the aiming error.Porosity is an effective way to optimize its dielectric performance.Therefore,it is of great theoretical significance and application value to carry out research on a novel porosity gradient broadband transmission/thermal insulation/load-bearing integrated Si₃N₄-based transmission material.In this study,the homogeneous and porosity gradient porous silicon nitride-based ceramics were fabricated by organic tape casting technology.Through the optimization of the casting process,high-performance casting tapes were prepared.The microstructure of porous ceramics and its influence on the mechanical and dielectric properties were investigated.Based on the simulation calculation of dielectric properties,the structure optimization of multilayer gradient porous silicon nitride material was realized,and the performance characterization of the porous silicon nitride with porosity gradient was also carried out.The rheological properties of the Si₃N₄ slurries are strongly affected by the type of solvent,type and amount of dispersant,solid loading and binder content.Polyvinylpyrrolidone（PVP）could effectively disperse Si₃N₄-based ceramic powders.The viscosity of the slurry increases with the increase of solid loading and binder content,and decreased with the ratio of its plasticizer to binder（R value）first and then increased.The optimized proportion of components is volume ratio 1:1:1 ethanol,toluene and NMP as solvent,2wt%PVP as dispersant,7wt%polyvinyl butyral（PVB）as binder,Polyethylene glycol 400（PEG400）and dibutyl phthalate as plasticizers quality with a mass ratio of 1:1,R value of 1 and a solid loading content of less than 35 vol%.The influences of pore-forming agent and sintering process on densification behavior,porosity,α→β-Si₃N₄ phase transition andβ-Si₃N₄ grain morphology are clarified.The main densification mechanisms of the porous Si₃N₄ ceramics are dissolution-precipitation and vapor-liquid-solid（VLS）growth.The increase of sintering temperature from 1800^oC to 1900^oC enhanced the dissolution-precipitation and VLS growth effects of Si₃N₄ during sintering,and realized the complete transformation ofα→β-Si₃N₄.The vapor-phase mass transfer effect could promote the in-situ growth ofβ-Si₃N₄ needle-like grains,and its aspect ratio increases with the increase of the pore-forming agent content,which is due to the promotion effect of high porosity on vapor-phase mass transfer.The obtained porous Si₃N₄ ceramics possess excellent mechanical properties.The flexural strength and fracture toughness of the material with a porosity of 33%could reach up to180MPa and 3.2MPa·m^-1/2respectively,which is mainly due to the self-toughening effects ofβ-Si₃N₄ grains.The porous Si₃N₄ ceramics also have excellent dielectric properties,and the dielectric constant decreases continuously as the content of the pore-forming agent increases.When the content of the pore-forming agent increased from 12.5vol%to 40vol%,and the dielectric constant of the porous Si₃N₄ ceramic decreased from 4.0 to 2.38.This is mainly due to the increased porosity.Based on the transmission line theory of electromagnetic wave propagation,the electromagnetic transmission efficiency of dielectric gradient porous ceramic was simulated to optimize the structure of porosity gradient porous Si₃N₄ceramic.A 11-layer porosity gradient porous Si₃N₄-based broadband transparent ceramic,with a structure factor of 0.5 and dielectric constant decreasing from 4.00 to 2.46,was prepared by organic tape casting and gas pressure sintering process.Its transmission efficiency is more than 72%in the frequency range from 2to 20GHz,which is in good agreement with the theoretical results.