Printing Process And Properties Of Direct Ink Writing Si₂N₂O-based Ceramics

Si₂N₂O ceramics possess great application potential in the field of electromagnetic wave-transparent materials due to its stable chemical properties,excellent dielectric properties and mechanical properties.However,limiting by the inherent brittleness and hardness of Si₂N₂O ceramics,the high cost and low efficiency of the preparation of complex components restrict its further application and development.Direct ink writing（DIW）can utilize the slurries with low organic content as inks to form ceramic green bodies with complex structures efficiently,and shorten the cycle of design-manufacturing-test-application for products.In this paper,DIW is utilized to explore the fabrication of the Si₂N₂O ceramic components.Based on the design and optimization of high-performance water-based inks,the evolution of rheological properties of inks was revealed.The influences of the raw material composition on the phase composition,microstructure and properties of the printed samples were investigated.And the effects of volume solid content,polar solvent（n-hexanol,H₂O）ratio and NH₄H₂PO₄ dosage on the rheological properties of tetraethylene glycol dimethyl ether（TGE）paste and the strengthening mechanism of NH₄H₂PO₄were studied.The storage modulus（G’）and yield stress（τ₀）of water-basedα-Si₃N₄/SiO₂ pastes are greatly increased due to the intensification of particle flocculation caused by the increase of bulk solid content of powders.The paste with a volume solid content of22.51vol.%possesses a G’around～10⁵Pa and a τ₀ of 264Pa.After PVA is introduced into the paste,the cross-linking of hydrogen bonds of PVA increases the shear resistance,which further increases the rheological characteristic values of slurry.Theτ₀ of ink with a PVA content of 2.91wt.%is 563Pa.The degree of flocculation of slurries is influenced by different mass ratio ofα-Si₃N₄and SiO₂due to the change of volume solid content and hydration reaction ofα-Si₃N₄.Theτ₀（15）of inks prepared by theα-Si₃N₄/SiO₂ powders with mass ratios of 8:2,7:3 and 6:4 are between 10² to10³Pa,respectively.The printing results of green bodies with complex geometric structures indicate the excellent forming ability of water-based ink.The purity of Si₂N₂O was the highest when the mass ratio ofα-Si₃N₄/SiO₂ powders was 7:3.The densification and grain growth were controlled by solution-diffusion-reprecipitation of liquid phase sintering,which resulting in a rigid skeleton structure with pores constructed by grains.When the mass ratio was 6:4,the formation of Si₂N₂O and β-Si₃N₄ was inhibited due to the increase of liquid phase viscosity and the low concentration of N ions.And the densification was controlled by diffusion,theα-Si₃N₄and SiO₂ partilces with higher thermal expansion coefficients can strengthen the matrix by formation of radial tension stress σ_r and tangential compressive stress σ_f for SiO₂.The history of water treatment promotes the condensation reaction ofα-Si₃N₄ and SiO₂ to form O-Si-N bonds to inhibit crystallization of SiO₂.The samples fabricated water-based inks show excellent mechanical and dielectric properties,the flexural strength and dielectric constant are74.1 to104.9MPa and≤4.68,respectively.High porosity Si₂N₂O sample with a porosity of 42.1%and a higher purity was prepared by reducing the content of the sintering addatives in the rawpowders.Its maximum dielectric constant in each frequency band is 3.96,but the flexural strength is greatly reduced to 55.1MPa.The β-Si₃N₄ were introduced to strengthen the sample wiht high porosity based on water-based ink.The G’andτ₀（15）of inks decreased with the increase ofβ-Si₃N₄ contents.The relative strength of the（101）,（210）and（002）crystal surfaces of the samples were different,in parallel and normal printing directions showed that confirming that theβ-Si₃N₄ were oriented along the printing direction.The mechanical properties of the samples were improved by the reinforcement of β-Si₃N₄,and the flexural strength of the composites with 10wt.% β-Si₃N₄ reached to 81.5MPa（P plane）and 82.3MPa（N plane）.In order to improve the water resistance of porous materials with complex structures,SiO₂-sol impregnation-heat treatment was used to seal the pores of samples.The heat-treated samples in N₂ and air atmosphere all exhibit excellent sealing effect,and the bridging effect of oxidation reaction of Si₂N₂O in air makes the defects of samples less,which results in a better waterproof performance.TGE,a organic solvent with a high boiling point,was utilized to overcome the challenges of clogging nozzle of water-based inks due to rapid evaporation of water.Due to the low cohesion energy,the ceramic particles in TGE will flocculate to form a strong gel structure through hydrogen bonds.After adjusting the flocculation degree of ink by using the polar solvent 1-hexanol,the DIW ink with a yield stress of 542Pa was prepared.But the solvent volatilizes slowly in the drying process of large size green body,and it is also easy to crack and damage.Therefore,the ink with fractional evaporation solvent was optimized by water and TGE with different volume ratio.When the volume ratio of water and TGE was 3:1,theτ₀（15）was 454Pa.The inks prepared by introducing different concentrations of NH₄H₂PO₄ into the above ink still have excellent printability.The sintered sample prepared with 0.1g/m L NH₄H₂PO₄has excellent comprehensive properties.