| As the development of aerospace technology and the progress of modern science technology, more strict performance requirements of high mechanical strength, low thermal conductivity and ultra-wide-band high transmission are proposed for wave-transmitting material. Si3N4 ceramics is one of the structure ceramic material which has the best performance, therefore, as a wave-transmitting material, Si3N4 has been received more and more extensive attention by domestic and foreign scholars. Aiming at the wave-transmitting material with the requirement of high transmission and high strength, this paper put forward ceramic matrix wave-transmitting material with pore gradient structure. This paper combine the cold spray technology and low-cost pressureless sintering technology at first time, and then exploit them to prepare ceramic matrix wave-transmitting material with pore gradient structure.First, SNNWs reforced Si3N4 porous ceramics with high porosity were prepared by pressureless sintering. SNNWs reforced Si3N4 porous ceramics with low dielectric constant were prepared by pressureless sintering technology at 1300 oC under nitrogen atmosphere. The effects of SNNWs content on the porosity, mechanical and dielectric properties and enhancement mechanism were studied. The results showed that the SNNWs with diameter of less than 50 nm and the length of 2.5~10 nm was found in the pore wall and within the pores formed by the burning out ot starch. The porosity of the obtained materials was in the range of 50~55 %, the bending strength was 19~39MPa and the dielectric constant was 2.9~3.3. With the increasing of SNNWs content, the bending strength increased obviously and then decreased, at the same time the porosity and the dielectric constant decreased and increased respectively. The enhancement mechanism of SNNWs reforced Si3N4 porous ceramics was pull-out effect and energy absorption effect.Then, Si3N4 ceramics coatings with different porosity, controlled thickness and porosity were prepared. Firstly, Si3N4 slurry with low viscosity and high solid loading was prepared. Using MgO-Al2O3-SiO2 as the sintering additives, Si3N4 ceramics coatings with controlled thickness and porosity were prepared by cold spray molding and pressureless sintering technology. The effect of the sintering aids content and cold spray technology parameters(spray pressure, spray distance and spray time) on surface topography, microstructure, thickness of coatings and bonding strength was studied. The results showed that when Si3N4 slurry with pH phase 8, dispersant content 0.6 wt. % and solid loading 40 wt. %, the viscosity of Si3N4 slurry with good dispersity and stability was 41mPa·s. So, it was suitable for cold spray. Optimum cold spray technology parameters was spray pressure 0.8 MPa and spray distance 12 cm. Using different sintering additives content and cold spray technology parameters, Si3N4 ceramic coatings with porosity 5~33 %, thickness 5~428 ?m and bonding strength 12~58MPa were prepared. The bonding strength decreased with the increasing of the thickness of coatings. The interface between coating and substrate combined well and without crack.At last, the transmission efficiency of wave-transmitting material with pore gradient structure was calculated by microwave transmission line theory and the four terminal network methods. The optimization design about structure was carried out. The optimization design results suggested that when the layer number was n=4,the thickness was d=2.4 mm, the thickness ratio was d1:dcoating=5:3, dielectric constant of the inside layer was ?1=3, the outside layer was ?n=6, the transmission efficiency of Si3N4 ceramics with pore gradient structure was more than 70% when the frequency was in the range of 0.5~40 GHz, satisfied the requirement of high transmission efficiency. According to the optimal design results, Si3N4 ceramics with pore gradient structure were prepared by cold spray molding and pressureless sintering technology. |
PDF Full Text Request