Direct Ink Writing Of SiOC Ceramics With Microwave Absorption Properties

In recent years,with the rapid development of communication technology and electronic facilities,electromagnetic interference and pollution have caused serious adverse effects in many fields such as human health,information security,and the ecological environment.Therefore,developing a lightweight and thin microwave absorbing material with wide absorption range to convert electromagnetic energy into thermal energy has become an urgent problem to be solved.3D printing technology is the use of 3D modeling software in computers to design shape and structures,and then use different technologies to quickly accurately stack and shape 3D objects with complex structures.In theory,3D printing can form any shape more flexible,precise,and diverse.If 3D printing is used instead of traditional molding methods to prepare absorbing materials,it can not only solve the limitations of traditional preparation methods in molding,but also play the role of structural absorption,achieving the integration of structure and function,making microwave absorbing materials more widely used.This article used polysiloxane as the matrix to prepare ink suitable for direct ink writing(DIW)through modification.SiOC ceramics with microwave absorption properties were prepared through polymer conversion method.The effects of different types of fillers and filler ratios on the performance of precursor inks and the microwave absorption performance of ceramics were explored,and the mechanical properties of ceramics were further improved through impregnation pyrolysis.The prepared ceramics possessed both shape diversity and functionality,and had potential application value in the field of microwave absorbing materials.Main research contents of the paper are as follows:(1)A polysiloxane precursor was prepared by DIW printing molding,and SiOC ceramics with a certain shape and structure were obtained by sintering under nitrogen atmosphere at 1000℃.The effects of nano gas-phase silica and SiC on the rheological properties of printing ink and the absorption performance of SiOC ceramic materials were studied.After viscosity test and non-Newtonian fluid equation fitting,the ink with nano fumed silica was converted from Newtonian fluid to non-Newtonian fluid with a certain pseudoplasticity.When the amount of nano fumed silica was 6.25 wt%,the fitted fluidity index n value tended to be stable.Considering the actual printing effect,it can be proved that the addition of 6.25 wt%nano fumed silica had a good printing effect.SiC had a relatively small impact on the viscosity of the ink.When the mass ratio of SiC to polysiloxane was 5:7,the viscosity and stability of the ink were the highest,which was beneficial for maintaining its shape after molding.After thermogravimetric analysis,the sintering process from room temperature to 1000℃ can be divided into four stages based on mass loss.The sintering system was established based on the thermogravimetric curve,and the sintered ceramics maintained their original shape and structure well without any defects such as deformation,cracking,or twisting.The addition of SiC effectively reduced the mass loss rate and size shrinkage rate of the sintered samples.Moreover,as the amount of SiC added increased,the mass loss rate of the samples gradually decreased and the size shrinkage rate stabilized.This indicated that SiC as a solid phase filler was beneficial for improving the dimensional stability and high-temperature stability of ceramics.When the mass ratio of SiC to polysiloxane was 5:7,the minimum reflection loss of SiOC ceramics can reach-36 dB,the effective absorption bandwidth was 2 GHz,and the corresponding material thickness was 3.5mm.This indicated that SiC had improved the dielectric properties of SiOC ceramics,making them have good absorption properties.(2)SiOC based absorbing materials were prepared by sintering at 1300℃ in a nitrogen atmosphere,using polysiloxane as the matrix precursor and 6.25 wt%nano fumed silica as the rheological modification filler.A certain proportion of SiC was used to improve the dimensional stability and high-temperature stability of ceramics.Different types of carbon materials with the same proportion were added as absorbing modifiers.The effects of microwave absorbing modifiers such as carbon black,nano carbon powder,fibrous carbon powder,and graphite on material properties were studied.Among them,carbon black had the greatest improvement in the absorption performance of SiOC ceramics.When the addition amount was all 25 wt%,the minimum reflection loss of the composite material was-45 dB,the effective absorption bandwidth was 2 GHz,and the corresponding material thickness was 3.5 mm.When the material thickness was 1.5 mm,the effective absorption bandwidth was 3.7 GHz.On the basis of the same addition of nano fumed silica and SiC,5 wt%T-ZnOw were added as physical modifiers to investigate the effect of different mass ratios of carbon black on the microwave absorption performance of SiOC ceramics.When the mass ratio of carbon black added was 30 wt%,the viscosity of the precursor ink significantly decreased,because the high solid phase content led to particle aggregation into large-sized particles,reducing particle concentration.The ceramics sintered at 1300℃ had no shape or structural defects,and the increase in particle size also increased the accumulation of pores,resulting in the formation of porous structures inside the material,improving the impedance matching characteristics of the material,and enhancing the interface polarization strength and absorption scattering path.At the same time,the increase in carbon black content increased the degree of graphitization of ceramics,improved conductivity and conductivity loss,and was conducive to improving the microwave absorption performance of ceramic.The minimum reflection loss of ceramics with 30 wt%carbon black can reach-43 dB,and the effective absorption bandwidth was 2 GHz.In higher frequency bands,the effective absorption bandwidth can reach 4.07 GHz,corresponding to material thicknesses of 1.5 and 2.0 mm,further reducing the effective absorption thickness of the material.By adjusting the thickness of the material,the absorption range can cover the entire frequency band of 3-17 GHz.(3)Two-dimensional plane figures were printed through DIW.Due to the excellent compression properties of the elastomer after printing and curing,the two-dimensional plane folding molding was carried out on this basis.After sintering at 1300℃,the ceramic can maintain its folded shape,further expanding the diversity of DIW printing and the feasibility of forming more complex shapes.In order to address the issue of insufficient mechanical properties of polymer converted DIW ceramics,polysilazane was used to vacuum impregnate and pyrolysis the ceramics,and the effect of impregnation and pyrolysis times on the mechanical properties of the ceramics was mainly investigated.According to SEM morphological analysis,the ceramics after immersion pyrolysis treatment would gradually become denser.After testing,as the density of the material increased,the apparent porosity decreased with the increase of immersion pyrolysis times.After compressive strength testing,the compressive strength of the material subjected to impregnation and pyrolysis once was 6.3 times that of the material before impregnation and pyrolysis,while the compressive strength of the material subjected to impregnation and pyrolysis three times was 12.8 times that of the material before impregnation and pyrolysis.It can be seen that impregnation and pyrolysis significantly improved the strength of the material.The absorption of DIW shaped blocks in the X-band was tested,and the results showed that the blocks subjected to vacuum impregnation and pyrolysis three times can achieve full band absorption in the X-band at thicknesses of 4.0 mm and 4.5 mm,with a minimum reflection loss of-64 dB.This was because although vacuum impregnation pyrolysis reduced the dielectric properties and loss capacity of the material itself,the overall impedance matching performance of the block was increased,and the multiple reflections inside the material enhanced the loss capacity.This indicated that the improvement in absorption performance came from the role of the structure,proving the potential of DIW printed ceramic structures for absorption.This study demonstrated the feasibility of combining DIW printing with polymer conversion method to prepare ceramic materials,and proposed various material modification methods.The prepared SiOC based ceramic materials not only had diversity in shape and structure,but also had excellent absorption performance and mechanical strength,achieving the integration of structure and function.The ceramic composite material prepared in this article has good application prospects,providing a certain experimental and theoretical basis for DIW printing of polymer based functional ceramics.