Microstructures And Electromagnetic Properties Of CNWs?CNTs?/Si₃N₄ Composite Ceramics

Carbon/ceramic composites are extremely promising structural electromagnetic?EM?absorbing and shielding materials with low density and good high-temperature stability used in aviation and space flight applications.However,the reflection coefficient of common carbon/ceramic composites is relatively high and hardly less than-10 dB in whole X-band.Additionally,it is also difficult for these composites to get a high total shielding effectiveness?SE?of more than 25 dB with a little reflection when the low carbon loading??5wt%?and small thickness??2.0 mm?are required in advance.Therefore,there is an urgent need for us to exploit new carbon/ceramic absorbing and shielding composites which show broad effective absorption bandwidth,strong EM attenuation,low EM reflection and thin thickness.In this thesis,polycrystalline carbon nanowires?CNWs?and CNWs-CNTs hybrid in-situ grown by CVD are introduced into porous Si₃N₄ ceramic with a thin thickness of 2.0 mm to attain the new carbon/ceramic composites with excellent absorbing and shielding properties.The goal properties of reflection coefficient?RC?lower than-10 dB?90%EM wave is absorbed?and total shielding effectiveness larger than 25 dB?99.7%EM wave is shielded?in whole X band are achieved.The main research contents and conclusions are as follows:1.The effects of CVD time on the microstructures,EM absorption and shielding properties of CNWs/Si₃N₄ composite ceramics are investigated in detail.?1?Carbon nanowires?CNWs?are synthesized at 700 ^oC,and shows the pitted/defected surfaces and highly curved structures with diameter of about 40 nm,thus forming the complex and multi-scale networks in CNWs/Si₃N₄ composite ceramics.When deposition time increases from 10 to 30 min,the mass percent of CNWs increases quickly from 1.34 to 5.15 wt%,and the diameter of some nanowires become thicker.Also the defect density shows an rising tendency.As-received CNWs is solid and polycrystalline,and composed of graphite nanosheets with sizes of 1-2nm,and the?002?planes are rarely perpendicular to the wire axis.?2?When CNWs accounts for 1.84 wt%,CNWs/Si₃N₄ shows favorable electrical conductivity of 2.63 S/m and extremely excellent absorption property.The minimum value of RC is-50.21 dB,meaning that more than 99.999%of EM wave energy can be attenuated,and the effective absorption bandwidth reaches 4.2 GHz,covering the whole X band.The composite ceramic with 5.15 wt%CNWs demonstrates high total SE of 25.01 dB and absorbed SE of 21.29 dB,exhibiting superior shielding property dominated by absorption.Both of which are mainly ascribed to the strong conduction loss,polarization relaxation loss and multi-reflection loss.2.The effects of CVD temperature on the compositions and microstructures of carbon product,and EM properties of C/Si₃N₄ composite ceramics are studied.?1?It is found that amorphous CNWs is in-situ produced at 600-650 ^o C.As CVD temperature increases to 750 ^oC,the grown product consists of mixed CNWs and CNTs?CNWs-CNTs hybrid?,mainly the CNTs.Some of amorphous carbon?a-C?appears at 800-900 ^oC,while much at 1000 ^oC.The growth of CNWs is dominated by uniform interface atomic diffusion at a lower temperature below 700 ^oC,while CNTs is additionally related to the surface atomic diffusion at a higher temperature above750 ^oC.Polycrystalline CNWs can convert itself into CNTs at 1000 ^oC,and the transformation mechanism is a process of carbon atom diffusing,graphite nanosheet rearranging and growing at high temperatures.?2?When CVD temperature is not higher than 850 ^oC,the electrical conductivity of C/Si₃N₄ composite ceramics is less than 6.27 S/m,and both the real and imaginary permittivity are below 10,implying good EM absorbing capability.The composite obtained at 750 ^o C displays a better absorption property with minimum RC of-28.43dB.When CVD is conducted at 900 and 1000 ^oC,the composite ceramics show high electrical conductivity,real and imaginary permittivity which are over 60 S/m,30,and60,respectively,and thus the total SE of C/Si₃N₄ prepared at 900 ^oC reaches 27.37 dB with a small reflection SE of 6.84 dB,meaning excellent EM shielding property better than that of most other carbon/ceramic composites.3.The effects of catalyst concentration on the microstructures and EM properties of CNWs-CNTs/Si₃N₄ composite ceramics are discussed.?1?When the CVD temperature is set as 750^oC and nickel nitrate concentration is 1-3 wt%,the CNWs-CNTs hybrid shows poor crystallinity with very high defect density.When the concentration increases to 5 wt%,the hybrid contains few of CNWs and most CNTs have a small diameter of less than 20 nm.Also the hybrid displays the lowest defect density and best crystal structure.Once the concentration reaches 7 wt%,CNWs-CNTs is grown with quite obvious orientation mainly caused by the enhanced intermolecular force.?2?When the mass percent of CNWs-CNTs hybrid is 3.49 wt%,the electrical conductivity of CNWs-CNTs/Si₃N₄ composite ceramic reaches 36.74S/m and the total SE is 20.85 dB.The composite ceramic with higher carbon loading of 3.91 wt%exhibits more excellent EM shielding property.The total and absorbed SE are 25.40dB and 18.61 dB,respectively,meaning that the main shielding mechanism is absorption.