Preparation And Grain Growth Characteristics Of Yttrium Aluminum Garnet Ceramic Fibers

Yttrium aluminum garnet(YAG)fibers have excellent properties,with high temperature creep resistance and strength,stable thermal and chemical properties,and low coefficient of tlhermal expansion.It can be used widely as reinforced fiber for ceramics,metal and resin in aerospace and machinery industry.YAG fibers also have excellent optical properties,which has a bright prospect in laser and fluorescence fields.At present,high performance YAG ceramic fibers are not reported at home and abroad.Further researchs are needed,such as stable and continuous spinnable precursor sol,grain size stable at high temperature and refine fiber diameter.In this work,YAG fibers were prepared by sol-gel method.YAG precursor sol was prepared from aluminum chloride sol.The spinnability and stability mechanism of the sol were discussed.The effect of heat-treatment parameters on t:he grain growth of YAG was studied.Cr2O3/YAG and MgO/YAG fibers were prepared respectively,and the mechanism of inhibiting grain growth was revealed.YAG precursor sol was prepared from aluminium nitrate sol system.The structure of the sol was revealed and the fibrillation mechanism of the sol was clarified.The effect of spinning voltage on the diameter of gel fiber was studied,and the movement track of sol in electric field was simulated by Flunt software.The main results are presented as follows.(1)YAG precursor sol was prepared by using alunminum chloride,aluminum powder and yttrium oxide as raw materials.The effects of raw material ratio,stabilizers and spinning additives on the sol spinning properties were studied.The sol has spinnability because the linear particles are formed after concentrating.The optimal molar ratio of aluminum chloride and aluminum powder is 3:1.The stability and spinnability of the sol can be improved because of steric hindrance effect,which acetic acid adsorbed on the surface of colloidal particles.The PVP coordinates with ions and colloidal particles in the sol to form organic-inorganic hybrid molecular chains,which increases the sol spinniability.The effects of different drying humidity and temperature on the stucture of gel fibers were studied.The optimum drying humidity and temperature are 50%and 60?,respectively.YAG phase is observed after the sample fired at 1000? for 2h.The densification of the fibers increases while sintering temperature increase.The crystallization activation energy of YAG fibers is 582.6 KJ/mol by Kissinger equation calculating.The grain growth of the fibers is obvious by holding different time at 1600?,which the grain growth belongs to the mechanism of grain boundary diffusion.The kinetic model of the grain growth accords with Arrhenius formula,which was K=9.08×10-10exp(-22.3 ×103/RT.(2)Chromium oxide/yttrium aluminum garnet(Cr2O3/YAG)fibers were prepared by adding Cr03 into the YAG precursor sol.The spinning properties of YAG sol decrease when the CrO3 is added.When the addition amount of PVP is 22wt%,the length of gel fiber is 350 mm.When the solid content of sol is 16.2wt%,the sol spinning is good,and the viscosity of sol is 1.73 Pa s(shear rate at 60 r/min).The sol presents shear thinning behavior at different temperatures.In the sol prepared process,Cr6+ is reduced to Cr3+ by aluminum powder.YAG phase is observed in the samples fired at 1000°C for 2 h.The crystallization activation energy of the fibers is 436.6 KJ/mol by Kissinger equation calculating.The Cr atom is dissolved in YAG to form(Y3(Al1-xCrx)5O12,and the diffraction peak position of the(420)lattice plane is shifted to a low angle.Grain growth of YAG fibers can be hindered by adding Cr atom.The mechanism of fiber grain growth belongs to the Cr atom solid solution drag mechanism.The kinetic model of grain growth accords with Arrhenius formula,which is K=4.47×10-exp(29.4×10-3/RT).(3)Magnesium Oxide/yttrium aluminum garnet(MgO/YAG)fibers were prepared by adding MgO into the YAG precursor sol.The spinning properties of YAG sol also decrease when the MgO is added.When the addition amount of PVP is 22wt%,the length of gel fiber is 300 mm.The sol also presents a period of shear thinning behavior at different temperatures When the solid content of sol is 19.5wt%,the sol spinning is good,and the viscosity of sol is 0.43 Pa·s(shear rate at 60 r/min).The crystallization of the gel fiber is delayed when the MgO is added.The main YAG crystalline phase is obtained after calcining at 1400 ? for 2 h,and a small amount of magnesium aluminum spinel(MgAl2O4)exists.Mg atom can be dissolved in YAG phase,but the solid solubility of Mg atom decreases with increasing holding time.Grain growth of the fibers is mainly controlled by grain boundary pinning,which is a block mechanism.(4)Using aluminum nitrate as aluminum source and yttrium nitrate as yttrium source,the effects of the addition amount of solvent(isopropanol or ethanol),complexing agent(citric acid,tartaric acid or lactic acid),spinning additives(PVP or polythene butyral(PVB))on the preparation of YAG precursor sol were studied.It is found that the sol spinnability is optimal by using ethanol as solvent,a 20wt%citric acid as complexing agent,and a 5wt%PVB as spinning aid,which the length of gel fiber is 200 mm.With the increase of solid content,the rheological property of sol changes from Newton fluid to non-Newton fluid.The sol has spinnability because organic-inorganic hybrid linear chain colloidal particles(about 4 nm in diameter)are formed.The YAG phase is obtained after gel fibers calcined at 800? for 2 h,and the fibers displais more perfect crystal orientation with the increase of sintering temperature.The crystallization activation energy of YAG gel fibers is 118.9 KJ/mol.(5)Nanoscale YAG fibers were prepared by electrospinning method using the precursor sol of aluminum nitrate as the source.When the spinning voltage is 20-26 kV,the average diameter of the fibers becomes smaller and then increases with the voltage increasing.The amount of organics increase,which is beneficial to electrospinning.The optimal spinning voltage is 24 kV,and the fibers diameter is of 68-300 nm.Each grain is a single crystal in the fibers,and the fibers are formed from the end to end of the single crystal grain.Sol movement track was simulated by Fluent-software in electric field.The movement track is divided into stable and unstable stage in electric field.In the unstable stage,the track is spiral The drip number and the j et amplitude increase when the tangential force is raised.And then,the fibers obviously become thin,the solvent evaporates rapidly,the fibers dry fully,and the nano-fibers with high quality are obtained.