Preparation Of PDC(SiCNO)/BN Ceramics And Their Application In Wireless Passive Temperature Sensor

In the field of aero-engine,because of its extremely harsh internal environment,such as high temperature(1000-1400℃),high pressure,corrosive gas and so on,how to detect its internal temperature accurately and in real time is an important problem in the current aviation field.Because the traditional wired temperature sensing equipment can’t detect its internal temperature in real time,it is very important to design a new wireless passive temperature sensor for the temperature detection in the harsh environment.For polymer precursor ceramics(PDC),it has excellent high temperature stability,high temperature creep resistance and thermal shock resistance,and its dielectric constant increases monotonously with temperature,which makes it suitable for wireless passive temperature sensor.The highest temperature of PDC(SiCNO)ceramic based temperature sensor reaches 900℃,and it has good high temperature stability and high sensitivity.However,due to the extremely harsh internal environment(>1000℃)in the field of aero-engine,the performance of PDC ceramic based temperature sensor(limit test temperature,wireless transmission distance,etc)still needs to be further improved.As the dielectric loss of the sensor element decreases,the microwave permeability of the material increases,and the energy loss of the microwave signal in the resonance process of the sensor element will also be reduced.At this time,the signal returned from the sensor will be enhanced,so the sensor can receive the corresponding microwave signal at higher temperature and larger wireless transmission distance.For PDC ceramic,the conductivity can increase rapidly with the increase of the temperature.Since the conductivity is proportional to the conductivity loss,which is the main part of the dielectric loss,the dielectric loss increases rapidly with the rise of temperature.When the dielectric loss of the sensor element reaches a critical point at high temperature,,the sensor’s signal is very weak or even disappears.Therefore,it is necessary to reduce the dielectric loss at high temperature by reducing the conductivity of precursor ceramics at high temperatures,so as to improve the high-temperature detection ability of the temperature sensor.Doping h-BN into SiCNO is a feasible method to improve the performance of temperature sensor.h-BN is a kind of crystal with graphite lamellar structure,whose crystal structure is hexagonal honeycomb formed by the compact combination of boron and nitrogen atoms.h-BN is important for its several properties such as high-temperature stability,high melting point(3000℃),and good resistance to oxidation.Hence,it can be known that h-BN can exist stably at high temperature as an inert dopant of SiCNO.h-BN is also an excellent microwave transmitting material due to its low dielectric constant and dielectric loss.The dielectric loss of SiCNO ceramic can be regulated by the integration of BN due to the change of the graphitization degree of SiCNO ceramic.The graphitization degree of free carbon in precursor ceramics is directly related to its conductivity and dielectric loss.Therefore,the addition of h-BN can significantly reduce the dielectric loss of SiCNO ceramic,thereby improving the performance of temperature sensor.SiCNO/BN composite ceramics were synthesized with polyvinyl silazane and hexagonal boron nitride as raw materials.The conductivity and dielectric loss of SiCNO/BN ceramics with different BN content were studied and analyzed.The results show that the electrical conductivity of the SiCNO has significant change after the integration of BN.It is found that the electrical conductivity of the SiCNO/BN composites varies nonmonotonically with the content of BN,in which the conductivity increases from 4.41 × 10-9(Ω cm)-1 to 1.68 Τ 10-8(Ω cm)-1 after the integration of 5 wt%BN,then drops to 8.29 × 10-10(Ω cm)-1 when BN content reaches to 15 wt%.However,the conductivity increases slightly to 1.44 × 10-9(Ωcm)-1 when BN increases to 20 wt%.The SiCNO/BN composites were analyzed by scanning electron microscope and Raman spectroscopy,the results indicate that the variation of free carbon graphitization degree(ID/IG)in composite ceramics is consistent with that of conductivity with BN content,with the increase of graphitization degree of free carbon in composite ceramics,the electrical conductivity will increase.Compared with SiCNO ceramic,the graphitization degree of free carbon of SiCNO/BN composite ceramics with 10-20%BN content is lower,and the conductivity and dielectric loss of the materials are greatly reduced,which are suitable for the fabrication of wireless passive temperature sensors.In order to further analyze the influence of BN content on the performance of composite ceramic based temperature sensor,SiCNO/BN(5%,10%,15%,20%BN)composite ceramic based wireless passive temperature sensor was prepared and tested.The test results show that the maximum measurement temperature of SiCNO/BN composite ceramic based temperature sensor for 15%BN reaches 1300℃,the wireless transmission distance is 32 mm at room temperature,18 mm at 1100℃ and 8 mm at 1300℃,at the same time,the sensor has good high temperature stability.