| Hollow glass microsphere(HGM)is a common fuel container in inertial confinement fusion(ICF)experiment and is also a good hydrogen storage medium.Both in the two applications,gases are needed to be filled into the shell.In addition,it involves gas transportation and release in hydrogen energy economy.The traditional gas filling and release method depends on thermal diffusion,which is both time-consuming and energy-consuming.Thus,it is important to develop a simple,efficient and safe means for gas filling and release.It has been found that transition metals(iron,cobalt and nickel)doped borosilicate glass shows enhanced gas diffusion velocity under illumination.Titanium,as a transition metal,is a potential photo activator.Moreover,it helps to evaluate the compressed core in ICF experiment.In this work,titanium doped HGM were fabricated by dried gel method and effects of titanium doping on its structure and properties were investigated.At first,titanium doped gel precursor were synthesized and characterized,and the shell-forming process was calculated and analyzed.Moreover,the composition,structure,geometric property,strength,gas retention capacity and photo enhanced gas diffusion effect were investigated and analyzed.The results are as follows.1.In order to avoid phase separation in sol precursor,titanium alkoxide and silicon alkoxide were pre-hydrolyzed separately and acetic acid was added to control the reactivity of titanium alkoxide by chelation.Homogeneous titanium doped sol precursors with mole ratio of Ti to Si ranging from 2%to 20%were synthesized.In addition,the parameters in sol-gel processes were optimized.The test results of gel structure reveal that the content of intermediate production(Ti(OH)4-nOAcn)increases with the increasing titanium concentrations,which decomposes at lower temperature than alkali acetates.Moreover,when mole ratio of Ti to Si reaches 10%,alkali chlorides separate from gels and meanwhile acetic acid evaporates.Thus,the concentration of blowing agent in gels decreases with the increasing titania concentration,leading to the reduced blowing efficiency.2.The structure,composition and compositional uniformity of titanium doped HGMs were determined by the composition of gel particle,including the blowing agent and oxide,and the condition in the falling process(temperature,residence time).The detailed conclusions are as follows.(1)Shell-forming processes have been analyzed and the calculation results of the falling process reveal that smaller gel particle has longer residence time in furnace.Mathematical model of the foaming process has been established,which can be used to estimate the wall thickness and aspect ratio according to gel particle size and the concentration of blowing agent.(2)Titanium doped HGMs with mole ratio of Ti to Si ranging from 2%to 20%were fabricated by dried gel method.When doping level reaches 5%,two classes of spheres appear in the same batch of the sample.One is colorless and the other appears blue.Tests and analysis on the diameter and wall thickness of HGMs show that the blue HGMs are transformed from smaller particles in the same batch of the gel precursors.Thus,the blue HGMs stay longer in furnace than the colorless HGMs,leading to the increased times in encapsulating,foaming,refining and cooling processes.The longer residence time during the encapsulating and foaming processes avails the blowing efficiency of gel particles,leading to large-aspect ratio.The longer residence time during the refining process leads to the volatilization of oxygen atom to generate oxygen vacancy and Ti3+,and the consequent coloration.Therefore,the blue HGMs show intense absorption in the range of 400?2400nm.Meanwhile,the loss of alkali oxides due to volatilization increases,leading to lower concentration in final HGMs.The longer residence time during the cooling process favors the ordering of glass structure,leading to the tendency towards phase separation.Colorless HGMs have thicker shell wall and lower aspect ratio and they are non-crystalline,in which there is no oxygen vacancy and titanium is tetravalent.(2)The loss of alkali oxides in HGMs increases with the increasing refining temperature and residence time in furnace.Salt separation reduces the batch uniformity of gel particles and titanium doped HGMs with more ratio of Ti to Si more than 10%.Si,B and Ti are relatively uniform in glass.Because of the volatilization and movability of the three kinds of alkali metals,they distribute inhomogeneously both in spherical surface and in radial direction.3.The geometric property,morphology,strength and gas retention property of titanium doped HGMs are determined by the composition and structure of glass.The detailed conclusions are as follows.(1)The diameter and wall thickness of HGMs are determined by the blowing efficiency,which is influenced by the content and formation of the blowing agent.The loss of blowing agent and the decrease of the decomposition temperature lead to the reduced blowing efficiency of high doping gels.Thus,with a given initial concentration of blowing agent,the average wall thickness of high doping HGMs is thicker and the average size is smaller.(2)The morphology of HGMs is determined by the refining and homogenization processes of molten sphere,which are influenced by the viscosity and surface tension of glass melt.The two properties of glass melt are determined by the oxide composition.The great loss of alkali oxides,as well as the increase of titania concentration in gels,results in the increased viscosity of glass melts.Therefore,the surface roughness and sphericity deviation of glass shells increase with the increase of titania concentration,as a result of the insufficient refining process of high doped glass.Even so,the percentage of HGMs which satisfy the requirement on sphericity in ICF reaches up to 94%,and that of HGMs which satisfy the requirement on surface finish is 100%.(3)The Young's modulus of HGMs decreases with the increasing titanium concentration as a result of the reduced sphericity.Oxygen vacancy,crystallization and phase separation in blue HGMs serve as stress concentrated center,leading to the lower Young's modulus than colorless HGMs.As the majority of titania takes part in the formation of glass network,the gas retention capacity of HGMs decreases with the increase of titanium concentration.The great loss of alkali oxides in high doped glass also reduces the half-life of gas pressure.Oxygen vacancy in blue HGMs serves as gas diffusion channel,resulting in great gas permeability and poor gas retention capacity.(4)Fortunately,the oxygen vacancy in blue HGMs is a reversible structure defect and it can be eliminated by annealing at 500?600? in air.As a result,the strength and gas retention capacity of HGMs are improved to a certain degree.4.Blue HGMs shows photo enhanced gas discussion velocity by absorption the light with wavelength ranging from 400 nm to 2400 nm,while there is no absorption to light in this wavelength range for colorless HGMs and it does not show this effect."Intense vibration centers" theory has been put forward to interpret the photo enhanced gas release process.When glass doped with activators obtains energy from light source,"intense vibration centers"centering on the activator atoms produce in glass.The "intense vibration centers" drive the neighboring atoms to vibrate intensely to form "intense vibration regions".In these regions,the distance of some atoms decreases and that of other atoms increases.So long as the vibration amplitude of atoms in the "intense vibration regions" is large enough,the adjacent"intense vibration regions" overlap to form connected gas diffusion channel,leading to rapid release of gas. |
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