| Temperature measuring and monitoring in extreme-harsh environments,such as turbine engine,nuclear energy reactors,with high temperature and strong oxidation/corrosion species,is a great challenge.Polymer derived ceramics(PDCs)have attracted significant attention due to their excellent high temperature stability,high temperature semiconductor characteristics and outstanding oxidation/corrosion resistance,which are promising candidates for making high temperature and harsh environment temperature sensors.Investigation and evaluation of electrical properties of PDCs is crucial important for designing PDCs sensor.However,the study of electrical properties of PDCs is pretty limited,especially,for polymer-derived SiAlCN systems.In this thesis,the electrical properties of SiAlCN ceramic are studied systematically and the conducting mechanism were explored.Firstly,polymer derived SiAlCN ceramics were directly synthesized by thermal decomposition of polymeric precursors by using liquid-phase polysilazane and aluminum tri-sec-dutoxide as main pre-ceramic precursor and Al sources,respectively.Elemental analysis,X-ray diffraction,Raman and XPS techniques were applied to investigate the composition,crystallization and microstructures.The electrical properties and conducting mechanisms of SiAlCN ceramics were analyzed by DC conductivity,AC conductivity and impedance spectra.Then,a typical high temperature PDC sensor was fabricated and tested.Additionally,the dielectric property of polymer derived SiAlCN ceramics were descried briefly,which can give a solid data base for design wireless PDC sensor in future.The results indicated that the polymer derived SiAl CN ceramics composed of amorphous SiAlCN matrix and free carbon phase with a typical amorphous structure and good high temperature crystallization resistance up to 1400 ~oC.With increasing pyrolysis temperature,Raman spectra showed that the degree of order and size of free carbon were increased generating a homogenous microstructure.XPS results revealed that the C-O bands were almost unchanged of Si AlCN ceramics(neither increase nor decrease).The DC electrical conductivity of Si AlCN ceramics analysis increased gradually with increasing pyrolysis temperature.The conductivity and temperature followed Arrhenius relationship,with an activated energy of 5.15 eV,which is much larger than that of SiCN ceramics.The impedance analysis showed one semicircle at low(1400oC)and high(1000oC)temperatures and two semicircles at moderate temperatures(1100~1300oC),which demonstrated that the interfacial polarization phenomenon is neglectable at low and/or high temperatures but playing significant effects at moderate temperatures.The temperature dependent impedance and electric modulus tests at room temperature to 500 ~oC found that SiAlCN ceramics behaves a typical NTC characteristic and follows one conducting mechanism due to the perfect overlap of normalized AC conductivity and M".This conducting mechanism was demonstrated clearly to be band-tail hopping(BTH)step by step.The dielectric and piezoelectric properties of SiAlCN ceramics were reported briefly by considering the contribution of both conduction loss and dielectric loss with electric modulus and impedance spectra analysis.It's found that polymer derived SiAlCN ceramics showed an obvious piezodielectric effect due to significant interface polarization and space charge polarization,indicating a great potential of making wireless pressure sensor.Finally,a high temperature sensor made of SiAlCN ceramics was designed and prepared successfully.The data acquisition was conducted by a Labview system coupled with a Wheatston bridge circuit.The results shows that the SiAlCN ceramic temperature sensor can work at temperatures as high as 1000oC with excellent repeatability,durability and fast response time. |
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