Microstructure And Defects Of Nano-composites Investigated By Positron Annihilation Spectroscopy

Nano-ceramics have high hardness,corrosion resistance and good stability at high temperature.Many studies have been carried out on single nano-Zr O₂and Al₂O₃both at home and abroad.Mixing them has better performance.Composite nano-structured materials based on Al₂O₃and Zr O₂have potential applications in refractory engineering materials,which is one of the hotspots concerned at home and abroad.In addition,with the urgent need for nuclear energy,the safety of nuclear power materials in nuclear power plants is particularly important under the severe environment of long-term high temperature irradiation.Magnesium oxide has good thermal conductivity and conductivity,which makes up for the shortcomings of poor thermal conductivity of Zr O₂nano-composites.Therefore,the study of Mg O/Zr O₂nano-composites is carried out.The microstructures and defects of nano-composites are very important,which is conducive to the understanding of their radiation resistance.In this thesis,positron annihilation lifetime spectroscopy(PALS),X-ray diffraction(XRD)and transmission electron microscopy(TEM)were used to study the microstructures and defects of Al₂O₃,Zr O₂and Al₂O₃/Zr O₂nano-composites at different annealing temperatures from room temperature to 1000?.In addition,SRIM simulation and slow positron beam Doppler broadening were used to investigate the microstructural changes of Mg O/Zr O₂nano-composites after xenon ion irradiation.The main conclusions are as follows:1.When annealing temperature is lower than 500?the grain sizes of three samples,namely,Al₂O₃nano-ceramics,Zr O₂nano-ceramics and Al₂O₃/Zr O₂nano-composites,remain unchanged.With the increase of annealing temperature,the grain sizes of Al₂O₃nano-ceramics and Al₂O₃/Zr O₂nano-composites increase.The grain growth of Al₂O₃/Zr O₂nano-composites is slower than that of Al₂O₃nano-composites.TEM results show that when annealing temperature changes,the size of Al₂O₃nanocrystalline ceramics remains basically unchanged,while Al₂O₃/Zr O₂nano-composites grow slowly only when annealing temperature is higher than 500?,and agglomeration occurs,which more intuitively confirms the experimental results of XRD and PALS.2.There are mainly three kinds of defects in the samples of Al₂O₃nano-ceramics,Zr O₂nano-ceramics and Al₂O₃/Zr O₂nano-composites:vacancies,vacancy clusters and microvoids,and the locations of the defects are mainly concentrated at grain boundaries.Compared with the defects in Zr O₂nano-ceramics,the size of vacancy clusters or micro-voids in Al₂O₃nano-ceramics and Al₂O₃/Zr O₂nano-composites is larger and the relative number is smaller.The total defect density of Zr O₂nano-ceramics,Al₂O₃nano-ceramics and Al₂O₃/Zr O₂nano-composites remained unchanged when annealed at less than 500?.The defects began to recover due to grain growth at higher than 500?.The change of Zr O₂nano-ceramics was more obvious,while the stability of Al₂O₃/Zr O₂nano-composites was higher because of the mutual inhibition3.Xe²⁰⁺ with energy of 5.2 MeV simulated by SRIM program was implanted into nano-composites Mg O/Zr O₂.The maximum damage occurred at the depth of2.26?m from the surface,while the whole radiation damage layer was about 3.3?m.The vacancies produced by irradiation are distributed in the range of 0?2.3?m.When Xe²⁰⁺with energy of 5.2 Me V is implanted into Mg O/Zr O₂nano-composites,the damage layer caused by Xe²⁰⁺implantation has exceeded the detection depth of positron.The initial S parameter increases with the irradiation dose.When the irradiation dose of Xe²⁰⁺is about 2×10¹⁵ion/cm²,the defect concentration of Mg O/Zr O₂nano-composites has reached saturation,and the S-W data are distributed in a straight line.On-line,it is shown that when Xe²⁰⁺with energy of 5.2 Me V is implanted into Mg O/Zr O₂nano-composites,the defect type of vacancy remains unchanged.