Research on Microstructures and Properties of Polysilizane Derived SiCN Ceramics and Their Application in Sensors

Author:Ma Bai Sheng

Supervisor:wang yi guang


Degree Year:2018





Temperature and pressure sensors used in high-temperature,harsh environments like turbine engine and nuclear power station are of great importance.Current sensing materials cannot satisfy the requirements in such environments.Silicon carbonitride(SiCN)ceramics prepared by the polymer derived ceramic(PDC)route possess good thermal properties like high temperature stability,high resistance to oxidation and creep,and unique electrical properties like high-temperature semiconductive property and high-temperature piezoresistive effect.What’s more,the PDC route possesses flexible shaping ability based on starting polymers,making PDC-SiCN a potential candidate for temperature and pressure sensing materials used in high-temperature,harsh environment mentioned above.Currently,research on application of PDC-SiCN in sensors is not sufficient.The microstructures and properties of PDC-SiCN ceramics are closely related to the composition and configuration of the starting precursors and the pyrolysis conditions(temperature,time,atmosphere,etc.).The advantage of this relation gives the possibility to adjust and control the properties of final ceramic product.The disadvantage is that SiCN ceramics derived from different precursors are different,which means that we have to study the basic microstructure and property of a ceramic derived from a particular polymer precursor before using it.In this dissertation,SiCN ceramics derived from a commercially available polysilazane precursor provided by Institute of Chemistry,Chinese Academy of Sciences,were fully studied including basic microstructure and electrical property and their evolution with annealing temperature.The relationship between electrical property and microstructure was established.The key factor determining the electrical property was discussed.Then,the piezoresistive effect of dense SiCN ceramics prepared by precursor infiltration and pyrolysis(PIP)designed for pressure sensor was studied.A new temperature sensor configuration consisting of double PDC layers was proposed and successfully fabricated based on a modified polysilazane.The results greatly promote the application of PDC in thin film temperature sensors.The main contents and results are as follows:(1)Evolution of microstructure of the amorphous SiCN ceramics with annealing temperature in a temperature range from 1000 to 1400 oC was studied.SiCN ceramics contain two basic phases:silicon-based matrix phase and free carbon phase.The silicon-based matrix consists of amorphous SiCxN4-x(x=0,1,2,3,4)tetrahedra.The overall ratio of the Si-C to Si-N bond concentration keeps constant with increasing annealing temperature.However,the atomic pair distribution results indicate that there may exist a phase separation process from mix-bonded SiCxN4-x(x=1,2,3)tetrahedra to SiC4 and SiN4 tetrahedra.The free carbon phase can be regarded as an assembly of many relatively ordered carbon clusters with defects(unpaired electrons).With increase in annealing temperature,the free carbon transits from sp3hybridization to sp2 hybridization with an activation energy of3.14 eV.Meanwhile,the defects concentration of the free carbon decreases,ordering level increases,and the cluster size increases with increase in annealing temperature.The free carbon undergoes a graphitization process from amorphous carbon to nanocrystalline graphite.(2)Evolution of electrical conductive behaviors and energy band structure of the amorphous SiCN ceramics with annealing temperature in a temperature range from 1000 to1400 oC was studied.The relation between the microstructure and property was established accordingly.The electrical conductivity of SiCN ceramics increases by 3 orders of magnitude when annealing temperature increases from 1000oC to 1400oC.The electrical conductivity is found to obey an Arrhenius relationship with annealing temperature with an activation energy very close to that of the sp3-sp2 transition of free carbon,indicating that the conductivity of the ceramics is closely related to free carbon in the ceramics.All the ceramics studied in this dissertation are n-type semiconductors.Their conductivities increase with measuring temperature.The conductive behavior of the ceramics obeys a bandtail hopping mechanism at low temperature range,of which the logarithm of conductivity shows a linear relationship when plotted versus T-1/4.SiCN ceramics show an electrical relaxation behavior in the ac conductivity:conductivity is almost frequency independent at low frequencies;and then becomes frequency dependent at high frequency,following a power-law behavior.The relaxation time of this relaxation process decreases with increasing annealing temperature with an activation energy,again,very close to that of the sp3-sp2 transition of free carbon.(3)The preparation of fully dense SiCN ceramics by direct pyrolysis from dense crosslinked precursors and the mechanical properties of SiCN ceramics were studied.The results show that fully dense SiCN ceramics can be obtained by this method.The Young’s modulus of SiCN ceramics is126 GPa,the flexural strength is250 MPa,and the Vicker’s hardness is15 GPa.The SiCN ceramics studied in this dissertation show good mechanical properties,satisfying the mechanical requirements for application in pressure sensors.However,the dimensions of the SiCN ceramics fabricated by this method are greatly limited and the ceramics will crack when annealed at temperatures higher than 1100oC.These disadvantages limit the feasibility of this method in real applications.(4)Dense SiCN ceramics were prepared by precursor infiltration and pyrolysis(PIP)combined with conventional powder consolidation route.The effect of PIP process on the free carbon characteristics,electrical conductivity and piezoresistive effect of SiCN ceramics was studied to explore the application of PIP SiCN ceramics in pressure sensors.The conductivity of the dense ceramics is 4 orders of magnitude higher than that of the original porous ceramics prepared by conventional powder consolidation route before PIP processes.The increase in conductivity is caused by the higher graphitization level of the free carbon in the ceramics prepared by PIP.This result further demonstrates that the conductivity of SiCN ceramics is closely related with free carbon.The conductivity of the dense SiCN ceramics also increases with annealing temperature like the situation in porous ones.However,the activation energy is much less than that of porous ones,the reason of which is that the dense SiCN ceramics accomplish the sp3-sp2 transition around 1200 oC.The piezoresistive gauge factor of both dense and porous SiCN ceramics is about 45,despite of PIP process.This value is much smaller than other PDC systems reported in literatures because the electrical behavior of SiCN ceramics obtained in this dissertation does not satisfy the tunneling effect.(5)A new thermistor structure containing two layers of PDC ceramics(a layer with a lower resistivity as the sensing element and a layer with a higher resistivity as the substrate)was proposed to solve the problem caused by the shrinkage during pyrolysis process when fabricating SiCN films on traditional substrates.A thermistor protype was fabricated by SiCN ceramics with different conductivities modified by divinylbenzene(DVB).The resistance of the SiCN ceramic decreases by 4 order of magnitude after modification with 20 wt%DVB,accompanied with an increase in free carbon content.The shrinkage rates of the modified and unmodified precursors are similar and the shrinkages occur at similar temperature range.Based on these facts,a double layer SiCN ceramic thermistor with a modified SiCN as sensing element and an unmodified SiCN as substrate was fabricated by UV photo-crosslinking.The thickness of the sensing element is100?m and can be further decreased.The resistance of the obtained thermistor obeys a typical Steinhart-Hart relation.The output voltage versus temperature of the thermistor was measured with a circuit.The thermistor shows good stability and repeatability.The proposed thermistor structure is much more suitable for real application.The result is a big progress in the study of application of SiCN ceramic temperature sensors.