| Compared with the traditional ceramic crafts,Polymer derived ceramics(PDCs)are a new class of revolutionary process to synthesize ceramics,which contain advantages such as designed molecular,good manufacturability,low transition temperature to ceramics and great machinability of ceramics materials.While researchers have worked to study the structural evolution from amorphous structure to crystalline structure during the pyrolysis process,there are still some problems about the strucute change need to be researched,such as structural stability change of Si-contained matrix phase and free carbon,the reasons of formation and evolustion of free carbon in PDCs,the relationship between the properties and structure of PDCs.In order to solve these problems,energy transformation of different phase during structural evolutions with thermodynamic analysis,the long-term isothermal stability of PDCs,and the relationship between structure and electrical properties of PDCs will be discussed in our thesis.Polycarbosilane(PCS)is considered suitable to be the first research subject because its simple structure.After systematic characterized and understanding the structure evolutions and electrical properties of PCS derived SiCO ceramics,selecting SiBCN ceramics,which present the better high temperature stability than other PDCs systems,as the second research object in order to study the impact of boron on the structural changes in PDCs and research the relationship between structure and electrical properties of SiBCN ceramics.The process for the formation of SiCO and SiBCN ceramics consists of cross-linking polymer into preceramic networks,gringding the resultant solid to fine powder,powder shaping through pressure,and transforming shaped samples into inorganic mater ials by a thermal heat treatment at different temperatures.In this thesis,the structural evolution of the material was investigated by combining X-ray diffractometer(XRD),X-ray photoelectron spectroscopy(XPS),Raman microscope,Nuclear magnetic resonance(NMR),and Electron paramagnetic resonance(EPR).The electric properties of the material were obtained by measuring complex impedance spectra,temperature-dependent conductivity and electronic structure.The main contents and results are summarized as follows:(1)Based on detailed research the structure information of Si CO ceramics at different pyrolysis temperatures,discussing the structure evolution of Si-containing area and free-carbon area.It was revealed that the SiCO ceramics contain two areas: Si-containing area and free-carbon area.The Si-containing area consists of SiC4 tetrahedra,SiCxOx-4 tetrahedra,and Si-C-C-Si/Si-C-H.With increasing pyrolysis temperature,major structural changes in Si-area include the transition from SiC3 O to SiC2O2.The free-carbon area consists of nano-sized carbon clusters and C-dangling bonds.With increasing pyrolysis temperature,the structural changes of free-carbon area include transformation from amorphous carbon to nanocrystalline graphite,the change in the size of the carbon nano-clusters,and the decrease in C-dangling bonds.Before 1250 oC,the size of the carbon clusters decreased with pyrolysis temperature,likely due to the structural rearrangement of the free carbon.After 1250 oC,the rearrangement process is tending towards stability.The ordering of free carbon increase may lead to the lateral growth of the graphite by following 2D grain growth model.(2)The electric properties of SiCO ceramics were investigated by combining measurements of their impedance performance,temperature-dependent conductivity and electronic structure.The effects of Si-containing area and free-carbon area on different electric properties were studied.It was inferred that the increase in the conductivity of the SiCO was due to the increase in the conductivity of the free carbon phase.The complex impedance spectrum for the material synthesized at 900°C could be best fitted by an equivalent circuit consisting of one impedance mechanism,which illustrated the impedance property was controlled only by the matrix phase;whereas those for the materials synthesized at high temperatures could be fitted by an equivalent circuit consisting of two impedance mechanisms in series,suggesting that the impedance property was controlled by both the matrix and the free carbon in series.It was inferred that the change in the DC and AC conductivity of the SiCO was due to the structural transfromation of the free carbon phase.With respect to the testing temperature,it was found that electrons transport of SiCO ceramics was followed a band tail hopping mechanismThe electronic structure of polymer-derived amorphous silicon carbide pyrolyzed at different temperatures was investigated by combining measurements of their temperature-dependent conductivity and optical absorption.The electronic structures were that the band-tail and defect level overlap with each other,likely resulting from a relatively large amount of defects and high disordered structure of the materials.This feature explained band-tail hopping process observed in the materials.The bandgap of the material decreases with increasing pyrolysis temperature,which was ascribed to the release of the residual hydrogen within the materials.In addition,the band-tail width of the material also decreased with pyrolysis temperature,which was attributed to the increase in the degree of order of the amorphous network.(3)On the basis of studying the thermodynamic structure and electric properties in detail,the isothermal structural evolutions of carbon phase in SiCO ceramics pyrolyzed at 1100 oC and 1300 oC,were investigated by combining experimental methods and conductivity property testing.At 1100 oC,increasing annealed time led to structural rearrangement of free carbon to form new carbon clusters.The size of the carbon clusters decreased first and then maintained at a steady value,while the C-dangling bond content increased first and then decreased.Dividing the rearrangement process of free carbon into two stages,the change mechanism of free carbon would be as surface diffusion and point spread respectively.The increasing conductivity of amorphous SiCO with elevated annealed time at 1100 oC was mainly related to the decreased distance between free carbon clusters.At 1300 oC,increasing annealed time led to the increased oredering of free carbon.The size of the carbon clusters increased first and then remain unchanged,while the C-dangling bond content decreased continuously and then almost invariant.The ordering process of free carbon also could be divided into two stages,the growth mechanism of free carbon would be following 2D cluster growth model and 1D cluster growth respectively.The increasing conductivity of SiCO with elevated annealed time was mainly related to ordering and growth of free carbon phase.(4)The structure evolutions of SiCxN4-x area,BCN area and free-carbon area in SiBCN ceramics were discussed depend on investigating the thermodynamic structure information of SiBCN ceramics.The SiBCN ceramics contain three areas: SiCxN4-x area,BCN area and free-carbon area.With increasing pyrolysis temperature,the structural changes of free-carbon area included transform from amorphous carbon to nanocrystalline graphite.There was no great effect of boron on the conductivity of the free carbon phase.The Si-containing area consisted of SiC4,SiCN3,and SiN4 tetrahedra.The BCN area consisted of BCN2 groups and BN3 groups.With increasing pyrolysis temperature,the reaction between Si-area and B-area included: SiN4+BCN2?SiC4+BN3 would result in increasing the interface area of amorphous phases.It means that B-C-N phase would impeded the crystallization behavior of SiBCN ceramics.(5)Depend on structural research of SiBCN ceramics,the unique electric properties of Si BCN ceramics were investigated by combining different measurements.The relationship between the structure evolution of different areas(SiCxN4-x area,BCN area and free-carbon area)and electric properties were discussed.The complex impedance spectrum for the material synthesized could be fitted by an equivalent circuit consisting of three impedance mechanisms in series,suggesting that the impedance property was controlled by the matrix,BCN phase,and the free carbon in series.The variation of the DC and AC conductivity of the Si BCN was due to the strucral change of the free carbon phase.The amorphous nature of the matrix and BCN phase made the testing temperature dependence of the AC conductivity followed a band-tail hopping mechanism.The overlap phenomenon between band-tail and defect level in SiBCN ceramics energy band structure would explain the band-tail hopping observed in the materials.The bandgap of the material decreased with increasing pyrolysis temperature was attributed to the release of the residual hydrogen within the materials.The existence of amorphous BCN phase in samples restricted the crystalline of Si-area,which led to the band-tail width of the material remain unchanged. |
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