Preparation And Electromagnetic Wave Absorption Properties Of Silicon Carbide And Its Composites

Microwave absorption materials not only have a special strategic position in military,but also become more and more popular in civil field.With the rapid development of science and technology,for absorbing wave materials,in addition to the features of strong absorption,broadband,light-weight and a thinner layer,there are more requirements for multi-spectrum,high-intensity,high-temperature resistance and radiation resistance.Silicon carbide(SiC),as an absorbing wave material,has a series of advantages such as low density,small thermal expansion coefficient,high-strength,chemical stability,anti-oxidation,etc.However,its dielectric and microwave absorption properties need to be strengthened.In this paper,taking SiC nanowires,organosilicon precursors derived SiOC ceramics(PDCs-SiOC)and SiC composites as research objects,the preparation,electromagnetic properties,microwave absorption and energy loss mechanism of these materials were studied.SiC nanowires were grown on the graphite and other substrates by chemical vapor deposition using the cross-linked products of hydrogen silicone oil(H-PSO)and divinylbenzene(DVB)or tetravinyl tetramethylcyelo tetrasiloxane(D4).The effects of reaction temperature,precursor particle size,growth matrix,polymer composition and catalyst on the growth of SiC nanowires were systematically studied,and the preparation process of SiC nanowires was optimized.It was found that the temperature and the growth substrate are the key factors influencing the growth of SiC nanowires.When the preparation temperature is above 1500?,the SiC nanowires on the surface of graphite matrix have high crystallinity,high purity,good linear structure and high production.Activated carbon atoms and coarse porous structure on the surface of the graphite plate contribute to the formation of SiC crystal nucleus,thus,the production of SiC nanowires can be increased.The rich defect dipole polarization,interfacial polarization,multiple reflection loss and conductance loss caused by the three-dimensional network of SiC nanowires endow them with excellent dielectric and microwave absorption properties.At 2?1 8GHz,when the ratio of SiC nanowires to paraffin is 3:7,the real and imaginary parts of the dielectric are about 15 and 4,respectively.The minimum reflection loss at 17.28 GHz is-52.4dB,and the thickness is only 1.29mm.Thennogravimetry and the oxidation test proved that the initial oxidation temperature of the SiC nanowires in the air is about 850?.The cross-linked products of H-PSO-DVB or H-PSO-D4 can not only provide gas source for the growth of SiC nanowires,but also serve as the precursor of SiOC ceramics.The polysiloxane(PSO)derived SiOC ceramics are easy to prepare and low-cost,however,the dielectric property of unmodified PSO derived ceramics is poor and the microwave absorption strength is low.In this paper,six kinds of polymer precursors with different Si,C,O and H contents were prepared by adjusting the ratio of H-PSO to DVB or D4.Combined with changing the treatment temperature,the chemical structure and phase of the SiOC ceramics were adjusted successfully.There are significant differences between the H-PSO-DVB and H-PSO-D4 precursors derived SiOC ceramics in yield,element composition and crystallization:ceramic yield of H-PSO-D4 precursors is higher,when the treatment temperature is 1500?,ceramic yield is about 70%,while the ceramic yield of H-PSO-DVB precursors at the same temperature is less than 50%;increasing the DVB content can improve the carbon content in SiOC ceramics,while the Si and O contents are higher in SiOC ceramics derived from H-PSO-D4;the crystallization temperature of H-PSO-DVB precursors is lower than that of H-PSO-D4 precursors.The microstructure of SiOC ceramics was observed by HRTEM,and the ceramic transform process was analyzed:when the temperature is low,the ceramics were composed of amorphous SiOC phase and turbostratic carbon;with the increase of temperature,short-range ordered regions were formed in the amorphous SiOC,and gradually grew into SiC nanocrystals of about 2nm in size;with the further increase of temperature,the SiC nanocrystals grew into large size of SiC grains of 5?10nm,and the random graphite structure of free carbon was more obvious,the SiOC ceramics are composed of SiC grains,turbostratic carbon and amorphous regions.In order to improve the crystallinity of SiOC ceramics and the content of SiC nanowires in the products,ferrocene was added to the polymer precursor.The phases,chemical compositions,microstructure and thermal stability of the Fe-containing precursor derived ceramics were studied.Fe catalyst assisted precursor derived ceramics followed the solid-liquid-solid mechanism,which reduced the crystallization temperature and increased the content of turbostratic carbon and SiC nanowires.In order to further improve the SiC nanowires content,Fe-containing precursor was coated on the surface of graphite oxide sheet,then treated at 1500?.The Fe catalyst reduced the energy required for the growth of SiC nanowires,and the graphite sheets created more space for the growth of SiC nanowires.The dielectric and microwave absorption properties of SiOC ceramics prepared at different process conditions were studied,and the relationship between the preparation process,structure and properties of SiOC ceramics was established.The mechanisms of electromagnetic wave loss in SiOC ceramics are polarization relaxation loss and conductance loss:at lower processing temperature,lots of short-range ordered regions were embedded in the amorphous SiOC,resulting in abundant dielectric relaxation loss,and with the growth of SiC grains,the dielectric loss was gradually decreased;when the content of turbostratic carbon in SiOC ceramics is high,micro-current will be generated,resulting in conductance loss,and the SiOC prepared at 1400? has higher dielectric parameters compared with samples prepared at other temperatures due to the more turbostratic carbon.When the H-PSO and DVB ratio in precursor is 2:1,the microwave absorption of SiOC prepared at 1000? can reach-27.1dB.The real parts and imaginary parts of these SiOC ceramics after oxidized at 400? were 4.49?7.45 and 1.74?2.18,respectively,and the microwave absorption can reach-15.2dB.SiC nanowires modified with Co-Ni-P nano-particles and Co-P thin films were prepared by electroless plating.The composites with three-dimensional network structure have both electrical and magnetic losses.The Co-Ni-P alloys on the surface of SiC nanowires are amorphous spheres.The Co-Ni-P/SiC composites are rich in dielectric loss,conductance loss and magnetic loss,which endow the composites with excellent microwave absorption up to-45dB.The Co-P alloys are nanocrystalline and form a dense film on the surface of SiC nanowires.Due to the poor impedance matching,microwave absorption of Co-P/SiC is only-16.18dB.Conductive polyaniline(PANI)coated SiC nano-powder composites were prepared by in-situ polymerization and the thickness of PANI layer is controllable.The dielectric and microwave absorption properties of SiC/PANI with different composite ratios at 2?18GHz were studied.The real and imaginary parts of the dielectric constant are positively correlated with the PANI content.When the content of PANI is 22.1wt%,the microwave absorption of the composites can reach-51.34dB.The coating thickness is only 1.9mm,and the effective attenuation bandwidth is 5.26GHz.