Microstructure and Dielectric Properties of Polymer-derived SiCNO Ceramics

Author:Li Xu Qin

Supervisor:wang yi guang

Database:Doctor

Degree Year:2018

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Pages:133

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With the rapid development of new energy vehicles,battery energy storage technology continues to innovate.Solid energy storage materials with high energy density,long life,and good safety is urgent to be developed.Polymer-derived ceramics(PDCs)have a high dielectric constant(104-105 order of magnitude)at low frequencies,which is higher than that of traditional dielectric materials such as barium titanate.In addition,PDCs have a unique near-net shape preparation process,which can realize the direct conversion from organic polymers to inorganic ceramics to prepare various complicated shapes of devices.At the stage of polymer precursors,the precursors can be designed to control the microstructure and adjust dielectric properties.It is a new type of dielectric material with great potential.However,current researches focus on the preparation of PDCs ceramics and their dielectric properties.The correlation between microstructure and dielectric properties has not yet been clarified,and the mechanism of dielectric relaxation has not been revealed.In this paper,amorphous SiCNO ceramics were prepared by polymer-derived ceramic method.Firstly,the microstructure and microstructure evolution rules of the amorphous SiCNO ceramics were systematically studied.Then the dielectric properties of the SiCNO ceramics were tested and the influence of temperature and frequency on the dielectric properties was clarified.Based on the mastery of the interface properties of SiCNO ceramics and their evolution rules,the correlation between interface properties and dielectric properties was established,and the corresponding polarization relaxation mechanism was revealed.The main research contents and main conclusions are as follows:(1)The microstructure and structural evolution of SiCNO ceramics are studied.It shows that:(a)Within the pyrolysis temperature range of 10001350°C,chemically stable SiCNO ceramics can be obtained.The amorphous ceramic consists of free carbon and silicon matrix.(b)As pyrolysis temperature increases,free carbon undergoes a graphitization process,and sp3C transitions to sp2C,gradually forming nanocrystalline graphite.Carbon clusters are formed and grows along the 2D direction of graphite,the degree of order increases,and concentration of carbon dangling bonds decreases.(c)With increasing temperature,sp2C gradually released from the Si-C-N-O network and deposited on the surface of the original carbon cluster;phase separation occurs inside the matrix,and SiCN3 transitions to the nanocrystalline units(SiC4 and SiN4),increasing the degree of order.Oxygen bond units(SiN2O2 and SiCO3)and mixed bond units(SiC2N2 and SiCN3)form a metastable amorphous SiCxNyO4-x-y tetrahedral structure.The free carbon and SiCxNyO4-x-y tetrahedral structure constitute the interface domain.(2)The dielectric properties of SiCNO ceramics were studied.The results show that:(a)The dielectric constant of SiCNO ceramics is 2×104 at 0.1 Hz,and its frequency response curve is“S”shape.The main polarization mechanism is the interface charge polarization.The main loss mechanisms are direct current conduction induced loss and interface polarization induced loss;the time constant is widely distributed.(b)As the test temperature increases,the dielectric constant of the SiCNO ceramic increases,and when it is higher than the critical temperature,the dielectric constant increases sharply;the dielectric loss peak moves with the frequency to the high temperature direction.and moving to the high frequency with increasing test temperature,indicating that the interfacial polarization process is a thermal activation process,which is consistent with the T-1/4 relationship and follows a 3D random hopping.(c)As the pyrolysis temperature increases,the dielectric constant of the SiCNO ceramic increases,the dielectric loss peak shifts to a high frequency,and the relaxation time decreases.(3)The polarization relaxation mechanism of SiCNO ceramics is studied.It is shown that:(a)SiCNO ceramics have a Maxwell-Wagner effect under the action of an alternating electric field,causing an interface charge accumulation effect and a large interfacial capacitance.According to the relaxation time range,the semi-arc in the low frequency band of the impedance spectrum is clearly defined as the contribution from the interface,and the semi-arc in the high frequency band is derived from the contribution of free carbon.(b)As the pyrolysis temperature increases,the free carbon undergoes graphitization,the carbon phase resistance decreases,and the capacitance does not change,resulting in enhanced electron relaxation polarization at higher frequencies which controlled by the free carbon resistance;due to the“local electric field enhancement”effect.interfacial resistance decreases as the conductivity of the carbon phase increases,the interfacial capacitance increases due to the accumulation of interface charge,resulting in increased interfacial charge polarization which controlled by the interfacial resistance and capacitance in the low frequency range.(c)The interfacial charge transition polarization mechanism of SiCNO ceramics is dominated by“vertical”transitions,“horizontal”transitions,and double-potential transitions.As the pyrolysis temperature increases,these three transitions increase and the dielectric constant increases.(4)The interface properties of SiCNO ceramics were studied and showed that:(a)ac impedance spectroscopy was superimposed by dc bias to prove that the free carbon resistance and capacitance were not affected by dc bias,and the interface resistance and capacitance decreased with the bias voltage increases.The interface barrier height of SiCNO ceramics obtained at 1300°C is 1.06 eV,the interfacial donor concentration is2.88×1018 cm-3,the interface density is 2.56×1012 cm-2 and the depletion layer width442 nm is obtained by combining the double Schottky barrier model.It is revealed that the main polarization mechanism of SiCNO ceramics is the double Schottky barrier-dominated interfacial charge polarization.(b)Bias suppression the Maxwell-Wagner effect at interface,increases the width of the interfacial depletion layer,decreasing the dielectric constant of the SiCNO ceramic.(c)As the pyrolysis temperature increases,the interface barrier height increases,the interface donor concentration decreases,the interface state density decreases,and the depletion layer width increases.The probability is reduced that charge crossing the barrier of the interface to reach the edge of the mobility and participating in direct current conduction.The charges tend to make a reciprocate transition in the depletion layer,which helps to increase the dielectric constant.