Evolutions on Microstructure and Mechanical Properties of Borosilicate Glass Induced by Ion Beams and γ Rays

Author:Du Zuo

Supervisor:wang tie shan

Database:Doctor

Degree Year:2019

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Pages:124

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Borosilicate glass is one of the candidate materials for High-level radioactive waste(HLW)vitrification.During deep geological disposal,the microstructure and mechanical properties of the glass will be significantly influenced byαdecay,βdecay andγray,which may affect the curing properties.Therefore,it is very necessary to study radiation effects of glasses for HLW vitrification.At present,the experiments and theoretical simulations mainly focus on the modification of the polymerization of the glass network,the variation of[BO3]/[BO4]content,the formation of defects and oxygen bubbles,the change of energy band gap and Urbach energy,the variation of macro-properties.However,the formation mechanism of radiation induced defects and the mechanism of mechanical property changes need more experimental and theoretical studies.Based on the previous research,the mechanism of radiation induced defects,hardness and Young’s modulus variations in the borosilicate glass are studied.P,Kr,Xe ions andγray irradiations are conducted in this work.Many kinds of analytical testing methods:such as Fourier Transform Infrared Spectrum(FT-IR),X-ray Photoelectron Spectrum(XPS),Nano-indentation,Electron Paramagnetic Resonance Spectrum(EPR),UV-visible Absorption Spectroscopy(UV-VIS)are applied to analyze the microstructure and macroscopic properties of the samples before and after irradiation.The mechanisms of the hardness and Young’s modulus variations and the formation mechanism of radiation induced defects are discussed.The two parts of research work are detailed as follows:1.Mechanism of ions andγrays irradiation induced hardness and Young’s modulus variationsTwo types of borosilicate glasses are irradiated by 0.3 MeV P+,4.0 MeV Kr17+,5.0 MeV Xe26+,multi-energy Xe(1.6 MeV,3.2 MeV,5.0 MeV)andγray.FT-IR,XPS and Nano-indentation are utilized to analyze the microstructure and mechanical properties variations of the borosilicate glasses after irradiation.The results show that:(1)Regardless the composition of glasses,the hardness and Young’s modulus of NBS1 and NBS2 glasses dropped exponentially and then saturated(NBS1:(35)H≈36%,(35)M≈18%;NBS2:(35)H≈35%,(35)M≈19%).The Young’s modulus decreased by only 50%of the hardness.(2)The variation trend and value of the hardness and Young’s modulus after different ions irradiation are basically the same,indicating the change of hardness and Young’s modulus is only related to nuclear energy deposition when the electronic energy loss rate is less than 4 keV/nm.(3)It is confirmed for the first time that the increase of NBO contents in the glass is due to the fracture of B-O bond in[BO4]structural unit,instead of correlation with the Si structure.(4)The transformation of Si-O-BIV to Si-O-BIII structure is caused by the nuclear energy deposition.The decrease in the hardness and Young’s modulus can be attributed to the increase of NBO contents and the decrease of glass network polymerization.The reason for the smaller decrease in Young’s modulus is that the reduction in the number of large rings and the densification of the glass structure result in a certain degree of recovery in the Young’s modulus.(5)No obvious variation on hardness and Young’s modulus of the glasses are observed afterγirradiation.It is further explained that the variations on the hardness and Young’s modulus of glass are mainly induced by nuclear energy deposition during ion irradiation.2.Formation mechanism of the defects and microscopic mechanism of energy band gap and Urbach energy variation in the glass afterγirradiationIn this part of the work,Electron Paramagnetic Resonance Spectrum(EPR)is used to analyze the type,concentration and formation mechanism of glass defects after 60Co-γirradiation.The microscopic mechanism of the variation on the energy band gap and Urbach energy are studied by UV-VIS.The results show that:(1)There are no obvious variation on the EPR spectrum shape of NBS1 and NBS2 glass with the increase ofγdose,indicating that the type of defect produced has no dose correlation.(2)The specific defect types existing in NBS1 and NBS2 glasses are analyzed with EPR spectral fitting results in this work.The defect types are more detailed and the fitting degree is better comparing with the previous research results,which is important for the research on the formation mechanism of defects.(3)According to the glass structure analysis and the Imai model,the formation of defects can be mainly attributed to:1.the cleavage and recombination of molecular bonds in the glass structure;2.the conversion of pre-existing defect;3.the conversion of latent defect pair;4.the diffusion of the defect pairs.(4)Theγirradiation induced defects caused the conduction band and valence band of the glass to extend in both low energy and high energy directions,resulting in a decrease in the energy band gap.The reduction of energy band gap is related to the formation process of various defects during irradiation,but the contribution of specific single defects to band gap variations needs further study.(5)This work is based on the formation of defects induced byγirradiation,further clarified the microscopic mechanism of variation on Urbach energy:more localized states are induced by the defects in the glass,which lead to the increase of width of the tail,further cause the increase on the Urbach energy of the glass.