Research on Microstructures and Properties of Modified Polycarbosilane Derived C-SiC

Author:Li Zhen Bao

Supervisor:wang yi guang


Degree Year:2018





Polymer derived ceramics(PDCs)technology is a way to obtain inorganic ceramics from organic precursor,which was prepared by curing,pyrolysis and sintering of the organic polymer step by step.The microstructure of PDCs is unique,mainly including nano free carbon phase with high defects,the coexistence of the amorphous and crystallized phase,the compatibility of multiple nanophase,the high percentage of the interface and so on.Because of the designability of the original polymer molecule and the good processibility of PDCs,the microstructure of PDCs is adjustable.All these entail PDCs excellent property,including good mechanical property,excellent high temperature stability,oxidation-resistance,electrical property and so on.PDCs then,has broad applications on the ceramic fiber,membrane,porous ceramic,ceramic based composites,temperature or stress sensors used at high temperature and so on.The property of PDCs is closely related to its microstructure.It’s necessary to improve or modify the property of PDCs further to satisfy the practical requirements.For this purpose,it’s crucial to understand the relathioship between the microstructure with the property and precisely control the microstructure and the property of the PDCs eventually.Therefore,it’s of great practical significance to understand the controlling ability of the microstructure and the mutual relation between microstructure and the property of PDCs.Based on the typical application of SiC ceramics in nuclear energy、frictional wear and nano-carbon in high electromagnetic shielding,it is aimed to solve the problems related to the thermal conductivity degradation of SiC under irradiation、the wear resistance optimazation of SiC and the high temperature electromagnetic shielding property degradation of nano-carbon.Polycarbosilane(PCS)derived SiC-included ceramics is microstructure-adjustable,and it contains in-situ homogeneous nano-carbon with lubricating function.What’s more,it has good oxidation resistance.By modification of the polymer precursor,the microstructure of PCS derive SiC-included ceramics can be adjusted and the corresponding property could be improved.The research detail and results of the thesis shows as followings:(1)DVB was used to modifty PCS-1.By controlling the content of DVB,the SiC-included ceramics with various microstructure was prepared.The interaction mechanism between the microstructure and thermal conductivity both at room temperature(RT)and high temperature for PCS-1 derived ceramics was studied.The results show:without DVB(marked as SiC10),the prepared ceramics contained oxygen.SiC with average crystal size of15 nm distributed homogeneously in amorphous SiCxOy matrix;with 10%DVB(marked as SiC91),the carbon can consume O.The single SiC ceramics with corresponding crystal size of221 nm and pure interface was obtained;with 20%DVB(marked as SiC82),there are carbon phase remaining.The carbon phase was dispersed in SiC matrix with the crystal size of139 nm.The thermal conductivity order is SiC91>SiC82>SiC10.By using the modified H-J model and based on the feature of the microstructure,the difference betwen theoretical and practical thermal conductivity was compared.The thermal conductivity of the material was mainly controlled by the phase distribution and the crystal size of SiC.The thermal conductivity of SiC91 decreased gradually with temperature increasing,while that of both SiC82 and SiC10 first increased and then decreased;the temperature corresponding to the peak value of the thermal conductivity increased with SiC crystal size decreasing,which mainly controlled by the mean phonon free path and the crystal size of SiC.(2)DVB was used to modifty PCS-1.By controlling the content of DVB,pure SiC(p-SiC)and C-SiC composites(C-SiC-c)was prepared.At RT,with the irradiation fluence of3.70×1014、4.06×1014(C-SiC-c)ions/cm2 under 4 MeV Au2+for p-SiC and C-SiC-c respectively,the irradiation resistance and thermal conductivity of the materials were compared when carbon was added in.The results show:with 10%DVB,the single phase SiC with pure interface was obtained(p-SiC);with 20%DVB,C-SiC was obtained and the carbon phase distributed in SiC matrix(C-SiC-c).After irradiation,by comparing the cross-section TEM,it was found there is still nano-crystallized SiC area in C-SiC-c,whose location is closer to the irradiated surface comparing with p-SiC.At the same time,part of the graphite phase was still crystallized in C-SiC-c.It showed that C-SiC-c had better irradiation resistance.Time-domain thermoreflectance(TDTR)showed that the irradiated membrane in C-SiC-c held a higher thermal conductivity compared with p-SiC membrane.According to this,it was thought that the heterogeneous interface might have the“sinking”effect,which can absorb the defects and entail C-SiC-c a higher irradiation resistance.It was found more that the dominant factors of the thermal conductivity for the material after irradiation included the thermal conductivity of the matrix,the ratio of the thermal conductivity of the dispersed phase to the matrix phase,the interface conductivity and the volume of the dispersed phase.(3)DP was used to modifty PCS-2.By controlling the content of DP(0%,6%and 10%),C-SiC with various composition and microstructure was obtained.The interaction mechanism between the microstructure and the corresponding property of PDCs,including mechanical and wear resistance property was obtained.The results show:with DP increasing,the volume of carbon phase in C-SiC increased from 5%to 16%.The crystal size of SiC first decreased and then increased.The hardness and the elastic modulus decreased gradually.Compared with the single crystal SiC,because of the toughing mechanism of crack deflection and bridging caused by included carbon,all samples showed higher fracture toughness(KIC)and the highest KIC increased84%.The wear mechanism included mechanical wear(micro-crack),abrasive wear,tribochemical reaction and little lubrication of caron.The wear resistance is controlled mainly by the hardness of the material and the crystal size of SiC.Though the hardness of 6%-DP is not the highest,but it showed the highest wear resistance because of the lowest crystal size of SiC.The small SiC size favoured the formation of tribo-oxidation-films and the formation of transgranular fractures,both promoting wear resistance.(4)DVB was used to modifty PCS-3.By controlling the content of DVB,C-SiC nanocomposites with homogeneous distributed nano-carbon and various microstructures was prepared.The electromagnetic shielding mechanism as well as the relation between the electromagnetic shielding performance from RT to 600 ℃ and the microstructure of the samples was obtained.The results show:with DVB increasing,the content,the crystal size,the crystallization degree and the connectivity of carbon phase gradually increased.Therefore,the electrical conductivity of the samples both at RT and high temperature increased correspondingly.The defect degree of the nano-carbon phase and the electrical conductivity influenced the whole shielding effectiveness of the samples mainly,in which the effect of the defect degree of the carbon phase is higher.The higher the defects density was,the higher the shielding effectiveness.With temperature increasing,the reflective shielding effectiveness increased while the absorbing shielding effectiveness decreased because the carbon phase was oxidated a little.The absorbing shielding mechanism is dominant in the whole shielding mechanism,leading to the little decrease of the whole shielding effectiveness with temperature increasing.However,the whole shielding effectiveness of C-SiC can still hold33.8 dB at 600 ℃.