Gas-solid Interface Reaction Of Non-oxide Ceramics At High Temperature

With the progress in science and technology,Non-oxide ceramics(NOCs)are widely applied in metallurgy,chemical engineering and aerospace fields owing to a combination of excellent properties including high strength and hardness,good corrosion and wear resistance,high thermal conductivity and thermal shock resistance,low density as well as excellent mechanic properties,etc.However,under these harsh working conditions,NOCs tend to be confronted with complicated interface reactions with different media,which can cause the failure of NOCs.These interface reactions mainly consist of solid-solid reaction,solid-liquid reaction and gas-solid reaction,in which the gas-solid reaction is the most common one accompanied by the highest reaction rate,and the corresponding principles and methods are also applicable to solid-solid and liquid-solid reactions.Therefore,deep understanding of interface reaction behavior is vital of importance for evaluating the service conditions,predicting the service life and optimizing the performance of these materials.With respect to the gas-solid interface reactions of NOCs and corresponding composites at high temperature,plenty of experimental and theoretical research work has been carried out at home and abroad,but there still remain some unclear issues.For interface reaction behavior,there is lack of the focus on the reaction in corrosion gases such as water vapor compared with that in oxygen.The sole effect of water vapor thus require to be further clarified.For kinetic models,existing kinetic models based on simple mathematical treatment of traditional parabolic and linear equations are implicit function of reaction time and weight change.Therefore,these models can not adequately describe and predict the reaction behavior.For interface structure evolution and reaction mechanism,there is lack of systematic study including simulation of the reaction process,evolution of the atomic structure,calculation of the reaction product and interpretation of the reaction mechanism.Therefore,systematic research has been carried out to solve above issues in this study,and the obtained conclusions are as follows:For interface reaction behavior,the influences of temperature and atmosphere on the phase,structure and morphology evolution of NOCs and their composites with different kinds and dimensions(powder,fiber and bulk)were investiaged.(1)In view of SiC and Si3N4 ceramic powders,in the low-temperature stage(1100-1300 0C),some pores form on the oxide scale when exposed to water vapor,making the reaction interface loose and accelerating the oxidation rate.In the high-temperature stage(above 1400 0C),water vapor intensifies the sintering and densification of reaction product,hindering the further oxidation.As a result,the volatilization reaction of water vapor and oxide scale predominates,leading to a linear weight loss in the later stage.Higher water vapor partial pressure can increase the intensity of above-mentioned volatilization.(2)In terms of SiC and Si3N4 ceramic fibers,ceramic fibers exhibit good oxidation resistance up to 1100 ?.Water vapor accelerates the reaction rate by forming interconnected pores on reaction products.The formation mechansim of these interconnected pores is as follows.When water vapor reacts with ceramic fibers,massive gas products form at the amorphous SiO2 scale/fiber matrix interface.On one hand,the destruction of the SiO2 scale will take place somewhere to form pores when the gas pressure reaches to a critical value.On the other hand,the dissolved gas products have to be released when external amorphous SiO2 start to crystallize.The release of gas product can leave pores in crystalline SiO2.With the time prolonging,the pores will increase and adhere to each other to form interconnected pores,which can provide paths for the entrance of oxidizing gases and release of gas products.Consequently,the reaction of ceramic fibers is accelerated.(3)As for Si3N4/Al2O3 ceramic bulk,water vapor accelerates the reaction rate compared with oxygen.At 900-1100 ?,the interface reaction behavior is controlled by diffusion.At 1300 ?,SiO2 in liquid phase can promote the densification of reaction interface and the morphology of product mullite,improving the oxidation resistance of ceramic bulk.At higher temperature,the reaction rate is accelerated forming more crystalline mullite with a bigger size.Meanwhile,the formation and breakage of protuberance structures further change the reaction behavior of ceramic bulk,leading to obvious fluctuations of kinetic curves.For kinetic models,based on Fick's diffusion law and quasi-steady assumption,double-interfaces model is proposed according to the reaction characteristic.The kinetic expression is given as follows:The formulae of this model are analytic with a form of explicit function,which can make them convenient to discuss the effects of different factors(temperature,time,oxygen/water vapor partial pressure,sample size etc.)on the reaction.In addition,every parameter in this model has respective clear physical meaning.Therefore,double-interfaces model can give a precise description and prediction of simultaneous oxidation and volatilization of NOCs at high temperature containing water vapor.After the appropriate transmission,the kinetic expressions regarding the quantitative relation of reaction friction and different factors together with different demensions of samples(ceramic fibers and ceramic bulks)can be also obtained.For the interface structure evolution and reaction mechanism,the evolution of oxygen adsorption and oxidation of AIN(0001)at the atomic level is simulated via first-principle calculation.With the lowest adsorption energy,the hollow site(H3)of the AIN(0001)surface is the most preferential site for oxygen atoms to be adsored regardless of coverages.Oxygen adsorption becomes difficult at higher oxygen coverage owing to the repulsive interaction among O2-.During the oxidation process,N2 is proved to be the dominant gas product with lowest formation energy.With a N3-being removed,a N vacancy can form,followed by the occupation of nearby O2-at the site of VN.This is the essence of AIN oxidation.The resulting conclusions can provide scientific designing basis for the improvement of material property.