| The wave-transmitting material was a set of wave, bearing, heat insulation and corrosion resistance and other functions in one multi-functional dielectric material. The Si3N4ceramic had excellent comprehensive performance, good mechanical properties and thermal stability, which was currently the most promising inorganic radome materials, becoming a hotspot in the research field of radome. But the poor brittleness and toughness, high dielectric constant and dielectric loss angle tangent hindered the large-scale application of Si3N4ceramics. BN was the the popular candidates for radome materials after Si3N4. BN ceramics had better thermal stability than the Si3N4ceramic (decomposition temperature under nitrogen or inert gas conditions up to2640℃), lower dielectric constant (ε=4.5) and dielectric loss (tgδ=0.3×10-3). Its drawback was the low strength and hardness, and intolerant of sediment erosion and rain erosion. Combining Si3N4and BN, you could give full play to the advantages of both to meet the thermal, mechanical and electrical performance of high-speed flight conditions for radome requirements. Porous ceramics could be regarded as a complex of ceramic and air, and the dielectric constant of air is very low, so the porous ceramics made of BN/Si3N4ceramics could not only retain the original excellent performance of the silicon nitride, but also have a relatively low density and dielectric constant and dielectric loss.In this paper, porous BN/Si3N4ceramics were prepared with the adding pore-forming agent method and pressure sintering process. Respectively, the impact of the pore-forming agents on the organizational structure and properties of the Si3N4matrix material, the impact of the content of the BN particles, the particle size of the pore-forming agents and the content of β-Si3N4to the organizational structure and performance of porous BNP/Si3N4ceramics, the impact of the particle size of β-Si3N4, the pickling time of a-Si3N4and the sintering aids to the organizational structure and performance of porous BNw/Si3N4ceramics were studied.In this paper, benzoic acid, stearic acid, ammonium bicarbonate, sodium carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), graphite and phenolic resin as pore-forming agents were added to the Si3N4matrix to make porous Si3N4ceramics. Comprehensive comparison added to samples of different pore-forming agents microstructure, mechanical properties and dielectric properties, benzoic acid was chosen as the final pore-forming agent. When the apparent porosity was12.67%, the flexural strength, the fracture toughness, the dielectric constant and the dielectric loss tangent were316.49MPa,5.22MPa m1/2,5.74,0.016respectively.The addition of the BN particles hindered the densification of Si3N4matrix material. With the increase of the content of the BN particles, the apparent porosity of porous BNp/Si3N4ceramics significantly increased and the mechanical properties decreased, but the dielectric properties improved with the decrease of dielectric constant and dielectric loss tangent. When the content of the BN particles in porous BNp/Si3N4ceramic was10wt.%, the apparent porosity was20.08%, the bending strength, fracture toughness, the dielectric constant and dielectric loss tangent were190.1MPa,4.16MPa·m1/2,5.14and0.0085respectively.The particle size of the pore-forming agents could cause great impact on the performance of porous BNp/Si3N4ceramics. With the particle size increase of the pore-forming agents, the flexural strength reduced from245.4MPa to190.1MPa, an reduction of29.1%; and the fracture toughness reduced from4.30MPa·m1/2to4.16MPa m1/2, an reduction of3.4%. Although the dielectric constant and dielectric loss tangent reduced slightly, the overall performance was poor. So when the pore-forming agent particle size was smaller, the performance of composite ceramics were better.The small β-Si3N4particles were added to the a-Si3N4raw materials to act as the seed in the process of phase transformation and grain growth, resulting in uniform distribution of high-aspect ratio rod-like β-Si3N4grains.The addition of β-Si3N4particles could effectively reduce the number of defects and optimize the organizational structure. When the content of β-Si3N4particles added to the porous BNp/Si3N4ceramics reached5wt.%, the apparent porosity was23.45%, the flexural strength, the fracture toughness, the dielectric constant and the dielectric loss tangent were158.1MPa,3.35MPa·m1/2,4.65,0.0058respectively. When the content of β-Si3N4added to the samples reached more than10wt.%, the grain would become coarse, the stoma increased evidently and the performance dropped.Combining the excellent performances of the BN material and the whisker material, BN whisker material had advantages of low dielectric constant, high thermal stability and mechanical strength. The addition of BN whiskers as a reinforcing phase to Si3N4matrix material could make the Si3N4material not only retain high mechanical properties, but also possess a low dielectric constant and dielectric loss tangent. The fine-grained β-Si3N4particles were added to the a-Si3N4raw materials to prepare porous BNw/Si3N4ceramics. The composite ceramics added β-Si3N4particles with median particle size of0.5μm had more long and uniformly distributed columnar grains. When the apparent porosity was19.75%, the flexural strength, the fracture toughness, the dielectric constant, the dielectric loss tangent were193.1MPa,4.21MPa m1/2,5.18,0.0088respectively. The composite ceramics porosity increased with the increase of the particle size of the added β-Si3N4, the long columnar grains reduced gradually, the grain size increased gradually and mechanical properties decreased in the samples.In this paper YAG, MgO+YAG, La2O3+YAG, Sm2O3+YAG, Dy2O3+YAG five systems of sintering aid were selected. Effect of different sintering additive systems and the component ratios to the properties of porous BNw/Si3N4ceramics were studied. Effect of La2O3+YAG composite sintering aids to promote the role of porous BNw/Si3N4ceramics densification was strongest.When the content of La2O3in the total amount of sintering aids was16.67wt.%, the most dense composite ceramics could be got. When the apparent porosity was13.48%, the bending strength,the fracture toughness, the dielectric constant, and dielectric loss tangent were282.6MPa,4.9MPam1/2,5.87,0.0152respectively.In this paper, the porous BN/Si3N4ceramics had excellent mechanical properties and dielectric properties and could be used for the preparation of the radome wave-transparent materials. |
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