| Ag/α-Al2O3 composites are widely used in high temperature and external diffusion catalytic reaction,which can effectively promote the transformation of C2H4,C3H6 to C2H4O,C3H6O respectively.To date,the Ag/α-Al2O3 composite is the only catalytic material which is widely used in ethylene industry.Meanwhile,it is also used as inorganic antibacterial materials for its ordinary antibacterial performance.In this paper,the in-situ reaction synthesis method and theories(the First Principle thermodynamic calculation,Molecular Dynamic simulation and oxidation kinetics analysis)are applied to obtain the thermodynamics and kinetic datum,the core of the synthesis mechanism.The nano-Ag/α-Al2O3 composites are prepared by ball milling,oxidation reaction of Ag-Al alloy solid solution and acid etching.The experimental analysis and theoretical study on the relationship of microstructure and heat treatment temperature,phase composition and performance were carried out.The specific research contents are as follows:1.The calculation of phase transformation and phase stability of nano-Ag/α-Al2O3 composites by First principle calculationThe First Principle thermodynamic calculation combined with thermodynamics manuals were used to judge the process of the reaction,phase transformation and the tendancy of the react,which provided reference data for the reaction temperature and the ratio of the raw materials.The calculation results of CASTEP module in MS software were consistent well with the calculated results of thermodynamics manuals,which verified the accuracy of the simulation.The stability of the oxide phases is:Al2O3>AgAlO2>Ag2O,and that of intermediate phases and solid solution is:Ag2Al>Ag3Al>Al solid solution bearing Ag.AgAlO,AgO and Ag3Al were not formed above a certain temperature.The Al solid solution bearing Ag was oxidized to form Ag and Al2O3 directly,which was related to the solid solubility of Ag in the alloy,and the Ag2Al was appeared at the region of greater solid solubility.The solid solution has been oxidized before Ag2Al,in addition,Ag2Al intermediate phase could transform into Ag during the process of the reaction.2.The Molecular Dynamic analysis of nano-Ag/α-Al2O3 composites preparing by reaction synthesisThe detailed analysis and demonstration of the reaction process together with reaction temperature were given by the Molecular Dynamic software.Through the oxygen dynamic analysis,the reaction rate,the type of oxygen layer and the temperature of phase transformation were confirmed.Al2O3 was produced by the reaction of Al and O during the oxidation reaction process,at the same time,Ag was precipitated from the supersaturated solution.While influenced by the diffusion depth of O,inner layer Ag atoms couldn’t separate out.Appropriate diffusivity temperature of Ag and O could control the precipitation of Ag which ensured the size stability of the nano-material.And the oxidation dynamic was obtained,which followed the 3rd powder regularity by comparing the different growth rate of alloy in oxygen atmosphere at different content of Ag.The precipitation of Ag could be controlled by the oxidation rate of alloy and the broken oxide film.3.The in-situ reaction synthesis procedure of nano-Ag/α-Al2O3 compositesBased on the theoretical analysis of thermodynamics and dynamics,the ratio of alloy,the maximum temperature of growth rate,the maximum reaction temperature and holding time were determined.And the nano-Ag/α-Al2O3 composites were prepared by powder-sprayed,oxidation reaction synthesis and acid etching.In the fabrication process,the high energy ball milling at low temperature can efficiently fined the powder and avoided the welding rate between the alloy powder and ball mill or grinding-ball,simultaneously increased new interfaces of the powder and promoted the formation of supersaturated Ag and α-Al2O3.Ag(10wt%)Al alloy has the largest Ag precipitation when oxidized at 600℃,which made co-occurrence of Al2O3,Ag2Al,and the internal alloy was melt and couldn’t be oxidized by the external oxygen.The phases distribution of the powder particle after reaction was divided into four layers from center to edge:the original alloy layer,the alloy layer where rich in Ag,Ag layer and the Al2O3 film layer.The Al2O3 film layer could be removed by hydrochloric,meanwhile,the intermediate alloy and the internal unreacted alloy around Ag could be eliminated by hydrochloric.After the corrosion,Ag with about 50nm size was obtained,and the specific surface area of the powder was increased significantly.In addition,the interfacial bonding between precipitated Ag and Al2O3 was combined closely.4.The oxygen absorption property and chemistry stability of nano-Ag/α-Al2O3 compositesO was absorbed on the surface of Ag,and the absorption capacity and type of O were closely related the Ag2AI interphase.Ag2Al absorbed amounts of nucleophilic O thus result high desorption temperature,which had harmful effect on the epoxidation of ethylene and antibacterial performances.The nano-Ag/α-Al2O3 composites have the performance of antibacterial,and Ag could absorb lots of O at the binding energy of 375.5eV,meanwhile,the performance was associated with the valence state of the absorption oxygen.Due to the presence of Ag2Al,the kind of valence state of O have changed,which was disadvantageous to the antibacterial performance.The epoxidation of ethylene experiment showed that the Ag/α-Al2O3 exhibited an excellent catalytic effect on ethylene.And the desorption temperature of O is above 400℃ because of the existence of the interphase of Ag2Al.Contrastively,the desorption temperature of O reduce obviously due to the absence of Ag2Al and the desorption temperature was 220℃ and 315℃.The particle size of nano-Ag was almost remained during the experiment process of ethylene epoxidation at 300℃,which reflected the good high temperature stability of the Ag/α-Al2O3 material prepared by reaction synthesis.This paper has clarified the transformation of thermodynamics,the kinetics process,and the relationship between properties and microstructures of nano-Ag/α-Al2O3 composites,enriched and improved the basic theory of metal matrix composites,and established a foundation for the development and application of this material and other related materials. |
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