Study Of The Electric Conductivity And Structure-property Relationship Of Polymer-derived SiCN Ceramics

Polymer derived ceramic(PDC)is a new type of ceramic material,possessing very unique structure and excellent properties,which can be used in high-temperature micro sensor,micro-electro-mechanical systems(MEMS),energy storage material,etc.The electric conductive property is the basic sensing principle for design and fabricate of temperature and pressure sensor of polymer derived ceramics.Therefore,to know how to tune and control the electric property of polymer-derived ceramics,to know the structure-property relationship and the fundamental conductive mechanisms of polymer-derived ceramics are crucial.In this thesis,polymer-derived silicon carbonitride ceramics are prepared using polysilazane as pre-ceramic precursor.The influence of compositions,heat treatment temperatures and heat treatment methods on the electric conductivity and structure-property evolution of SiCN ceramics were investigated.In the first part,the effects of DVB concentration on the electric conductivity of polymer derived Si CN ceramics was investigated,with the DVB contents of 0,10%,20%,30% and 50% by weight percentage.In the second part,the effect of heat treatment temperatures on the electric conductivity(including piezoresistivity)and microstructure of the polymer derived Si CN ceramicswas studied.The detailed structure and property evolution of 20 wt % DVB samples were characterizedand analyzed.In the third part,the effect of microwave-treatment on the electric conductivity and microstructure of polymer derived SiCN ceramics was considered by counting on the special features of microwave heating method of selective heating and real-time heatingand the unique microstructure of PDC.The crystallization behavior,composition and structure characterization were performed by XRD,elemental analysis,Raman spectra and X-ray photoelectron spectrum analysis.Theproperty tests includ electric conductivity at room temperature and the piezoresistivity.Basing on the composition,microstructure and property test and analysis,the sturture-property relationship of SiCN ceramics was proposed.The results show that the addition of DVB can effectively tune electric conductivity of polymer derived SiCN ceramics.With the increase of DVB content and heat treatment temperature,the electric conductivity of SiCN ceramic is gradually increased.XRD shows that there is no obvious crystallization of SiCN ceramics heat treated at 1500 oC,which suggests that DVB modified SiCN ceramicspossess good crystallization resistance.Raman spectrum shows two typical peaks of D and G peaks indicating the existence of free carbon.The Raman results show that adding DVB has significant effects on Raman peaks compare to that of no DVB samples.At the same time,with the increasing of DVB content,the free carbon content increased.However,the content of DVB has a weak effect on the degree of order and size of free carbon.Pressure-resistance testing shows that the piezoresistivity of polymer-derived ceramics enhanced remarkably by the increasing of DVB content.The piezoresistivity results and Raman analysisdemonstrate that the DVB content has an effect on the electric conductivity of polymer derived SiCN ceramics.The influence of the conductivity of the polymer derived ceramics is mainly caused bythe enhancing of internal electric field due to the decrease of distance between the free carbon clusters.The electric conductivity of 20% DVB modified SiCN polymer derived ceramicsimprove a lot with the increasing heat treatment temperatures and the electric conductivity increased nearly four orders of magnitude when the heat treatment temperature is 1500 oC.The reason is increasing heat treatment temperature not only increased the degree of order of free carbons,but also decreased the internal distance between free carbon clusters,whichresulting the enhancement of the internal electric field.With the the same heat treatment temperature and dwell time,the electric conductivity of SiCN polymer derived ceramic is significantly enhancedby microwave-treatment compared to that of traditional heat treatment.The electric conductivity of microwave posttreated SiCN ceramicis nearly 40 times higher than that of conventional post treated SiCN ceramic with thesame heat-treated temperature and dwelling time.Further Raman and XPS analysis reveals that the increasingof electric conductivity of SiCN ceramic after microwave post treated is mainly due to strong interaction between free carbon and microwave,which induced aobvious sp3-to-sp2 transition of free carbon andimprove the electric conductivity of SiCN ceramics remarkably..In summary,adding DVB,changing heat treatment temperature and using microwave heat treatment can effectively tune the electric conductivity of polymer derivedSiCN ceramics.The structure and relationship analysis reveals thatthe increase of electric conductivity of SiCN ceramics by adding DVB is due to the increase of free carbon content,which leads to decrease of the distance of free carbon clusters and strengthens the internal electric field.The increasing of electric conductivity of polymer derived SiCN ceramics by increasing heat treatment temperatures is due to the comprehensive effect of increasing the degree of order of free carbon and decreasing of the distanceof free carbon clusters.The increasing of electric conductivity of polymer derived SiCN ceramics by microwave heat treatment is due to the strong interaction between free carbons of polymer derived SiCN ceramics and microwaveirradiation,which can result in a significant sp3-to-sp2 transition of free carbon.In other words,the microwave irradiation significantly increased the degree of order of free carbon.