Study On The Preparation Of Nano TiO₂ Gas Sensors By Flame Synthesis With A Rotating Surface

Semiconducting metal oxides?SMO?gas sensor is quite promising for application in the future.Flame synthesis with a rotating surface?FSRS?is a superior method for the preparation of SMO gas sensor,while the technique is still not mature.Experiment and simulation have been done on the preparation condition effects on the gas sensing properties of the sensors made by FSRS and associated gas sensing performance and mechanisms.The main content and conclusions are as follows.Effect of the moving stagnation surface on the flame synthesis process was quantified.3-D CFD simulations were done under different rotating speed and gas velocity,to get the velocity and temperature distribution in the region near the stagnation surface.Effect of linear velocity of the surface vsurf and flame stretch rate ? on particle deposition time,deposition temperature and deposition oxygen concentration were studied respectively.Karlovitz number?Ka?,as the ratio of characteristic time of stagnation surface and flame stretch,was used for evaluation of the effect of a moving stagnation surface on the particle deposition process.Results show that deviation of particle deposition parameters can be controlled within 10% when Ka> 1.An ethylene global mechanism with integer rate order was developed to simulate the combustion in the fuel lean side,and its accuracy in the flame synthesis application was proved by a skeletal mechanism.Effect of preparation process and conditions on the performance of nano TiO₂ gas sensor prepared by FSRS method was determined.The preparation process and the gas sensing test method were improved.A FSRS system and a gas sensing test system were constructed respectively.Vast amount of experiments has been done to determine the the whole preparation process and critical parameters,including design of the sensor substrate,particle synthesis,film deposition,densification process and sintering process.Characterization of the particles and film was done.Relation between particle size and precursor concentration,as well as relation between film thickness and deposition time,were studied respectively.Results showed that the optimal deposition time of the TiO₂ sensor prepared by FSRS is 10 min under different precursor concentration.The sensors made by optimized process had excellent and stable oxygen sensing performance.The optimal working temperature for oxygen sensing was 250 ?.They also have excellent CO sensing performance.The lowest CO sensing limit could reach 5 ppm,and the optimal working temperature for was about 280 ?.The energythresholdEa was determined according to the relation between the sensing resistance and testing temperature.The results revealed that that there existed different sensing mechanisms in different temperature ranges.When temperature was lower than 200 ?,the value of Ea decreased from 12 kcal/mol to 4 kcal/mol,and thereby the corresponding sensing mechanism changed from chemisorption to physisorption,opposite to the traditional sensing theory.In addition,it was found that the number of deposition particle layers in the film was the dominated factor for the effect of deposition time on the oxygen sensing performance.Molecular dynamics?MD?was used for the simulation of the heating process of a single TiO₂ particle and the sintering process of two TiO₂ particles in anatase phase.A surface atoms identification model and a neck atoms identification model were developed for atoms classification and analysis.Results showed that energy relaxation was faster than structure relaxation.Displacement of surface atoms was larger than interior atoms,and O atom was more active than Ti atom.In the sintering process,displacement of the atoms in the neck was the largest,and movement of the atoms in the lateral of the neck was the main mechanism for neck growth.