Effect Of Pyrolysis Temperature On High Temperature Piezoresistivity Of Polymer-derived SiCN Ceramics

With the rapid development of microelectronics and power engineering technology,there is an increasing demand for long-life high-precision high-temperature pressure sensors which can be used for on-line monitoring of high-temperature systems such as aerospace engines and heavy gas turbines,such sensors are mainly used to monitor the dynamic temperature,pressure,heat flux,and structural stress/strain of the system.the parameters obtained will be used in the system.Feedback control,optimization design and safety monitoring,thus improving the efficiency,safety and reducing pollution of the system.high temperature pressure sensor is the core technology of the next generation engine and gas turbine,which is one of the priority areas of development in the world’s major countries.On the other hand,the high temperature materials usually possess low piezoresistive factor,such as SiC ceramic with a piezoresistive factor of 40 only.Polymer derived ceramics are considered as a potential material for high temperature/pressure sensors because of their excellent high temperature stability and high piezoresistivity.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 piezoresistive and their evolution with annealing temperature.The relationship between electrical property and microstructure was established.The key factor determining the piezoresistive was discussed.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 1100¹400 ℃ was studied.SiCN ceramics contain two basic phases:silicon-based matrix phase and free carbon phase.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 SiC_xN_4-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.With increase in annealing temperature,the free carbon transits from sp³ hybridization to sp² hybridization with an activation energy of³.26 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 1100¹400℃ 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 1100¹400 ℃.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.SiCN ceramics exhibit good piezoresistive properties in the temperature range of 1100¹400 ℃ pyrolysis.The effect of pyrolysis temperature on piezoresistive properties of amorphous polymer-converted ceramics was studied.The piezoresistive curves of ceramics pyrolysis at 1200¹400 ℃ conform to the tunnel model.The variation of piezoresistivity of amorphous SiCN with pyrolysis temperature is due to the change of free carbon structure in SiCN.With the increase of pyrolysis temperature,the size of free carbon clusters increases,which leads to the increase of free carbon spacing,the critical concentration drop in the tunnel model increases,which makes the tunnel effect more likely.As a result,the pyrolysis ceramics at 1400 ℃ have higher piezoresistivity than those at 1200 ℃（3）Spark plasma sintered crystalline SiCN ceramics have excellent piezoresistive properties and contain a large number of C=C bonds.The piezoresistive data are consistent with the tunneling effect model,that is,whether the high-voltage resistance performance of spark plasma sintered ceramics is caused by the tunneling effect.For the samples sintered by hot pressing,the piezoresistive properties of SiC ceramics are shown as piezoresistive properties of SiC materials themselves in the experiment because there is no C=C bond in the ceramics.So the piezoresistive effect produced by the tunneling effect of ceramics is determined by the C=C bond in ceramics.