The Study On Preparation And Properties Of Toughening 3Y-ZrO₂ Using Carbon Nanotubes By Agarose Gel-casting

This work added zirconia powder,agarose and ammonium citrate to deionized water directly for preparing 3Y-ZrO₂ mixed slurry,then the carbon nanotubes（CNTs）mixed3Y-ZrO₂ ceramic green body has been prepared by adding the CNTs which after oxidation,purification and functionalization to 3Y-ZrO₂ the mixed slurry through the agarose gel-casting.A series of CNTs mixed 3Y-ZrO₂ ceramics have been fabricated via the high temperature sintering in the argon protect atmosphere.The surface activity,zeta potential,microstructure and dispersity of CNTs,and the viscosity of 3Y-ZrO₂ slurry,the line shrinkage,microstructure and relative density of 3Y-ZrO₂ green body,the microstructure and density of 3Y-ZrO₂ sintered body were mainly studied.On this basis,the CNTs suspension with a high concentration and the 3Y-ZrO₂ ceramic with near-to-theoretical density have been prepared and the property effects of CNTs addition on 3Y-ZrO₂ was discussed.On the top of that the surface activity and dispersity of CNTs have been researched,the surface activity and zeta potential effects of optical condition on CNTs and the effect of pH on dispersity were studied.The results show that the lower the optical intensity is,the more sufficient the oxidation is,and the higher the surface activity is,the more functional groups the CNTs were gained.When the optical condition changed into the dark condition,few agglomerations of CNTs happened in the TEM images and their dispersity greatly increased.The pH largely affected zeta potential of CNTs,and the higher the pH is,the larger the absolute value of zeta potential is.This means that the electrostatic repulsion of the surface of CNTs became stronger,also,it is good for carbon nanotubes to disperse in the liquid solution.The CNTs suspension with a concentration of 10 wt.%presented the wonderful dispersity when the pH was larger than 4 and suspended until now for almost365 days,whereas when the pH below 3,the CNTs gradually settled in three months.The mechanism is that the CNTs without oxidation are similar to the carbon fiber and both ofthem are incapable to hydrolyze and ionize.They present nominal zeta potential only through absorbing hydrated ions.When CNTs oxidized by mixed acid,their defects and unsaturatedπbonds fractured and obtained various functional groups quickly.These functional groups produced some groups with the negative electrons by the hydrolysis and ionization,and the CNTs disperse by the electrostatic repulsion.These negative electron groups which produced by functional groups present different zeta potential in different environments.A large amount of OH^-produce by alkaline solution,so with the increase of the pH the absolute value of the zeta potential increases,and the electrostatic repulsion of CNTs become large,the dispersibility is great.When the environment change into acid or the pH value is relatively small,the H⁺neutralize the groups with negative electrons and results the decrease to the absolute value of zeta potential.So,the electrostatic repulsion of CNTs decrease and the dispersibility lower.Secondly,the 3Y-ZrO₂ ceramic with a near-to-theoretical density has been prepared by the“Mixed Operation Method”which after the perfect of agarose gel-casting.The effect of milling time on viscosity of 3Y-ZrO₂ mixed slurry,effect of different agarose gel-casting methods on line shrinkage,relative density and microstructure of 3Y-ZrO₂green body and the density of 3Y-ZrO₂ sintered body were discussed.Results show that the viscosity of mixed slurry slowly raised with the increase of milling time and turned to stabilize after 24 h,the“Mixed Operation Method”dramatically reduced the viscosity of mixed slurry and this is beneficial for removing the defects of 3Y-ZrO₂.When milling time was more than 24 h,the surface of zirconia showed many obvious holes and large defects.The line shrinkage and relative density of zirconia ceramic green bodies,which prepared by the“Mixed Operation Method”,are relatively the same,with 14%and 40%respectively,however,due to the high solid loading and high agarose concentration,the lowest line shrinkage of green bodies which prepared by the normal agarose gel-casting is 4.2%,and their relative density reached 56%.After sintering the green bodies,their particles size which prepared by the“Mixed Operation Method”are relatively small and homogeneous,whereas the particles size of which prepared by the normal agarose gel-casting method are relatively large.The largest density of 3Y-ZrO₂ ceramic which prepared by“Mixed Operation Method”is 6.07 g/cm³ and the lowest density of that is 6.01 g/cm³,and their relative density are above 99%,whilst the density of 3Y-ZrO₂ ceramic which prepare by normal agarose gel-casting method is from 5.98 g/cm³ to 5.71 g/cm³,and the relative density of that is between 98%and 93.6%.Finally,the CNTs mixed 3Y-ZrO₂ ceramic has been prepared by adding an amount of CNTs which obtained great dispersity,the effect of 3Y-ZrO2 and CNTs’zeta potential on the molding,the influence of CNTs on viscosity of mixed zirconia3Y-ZrO₂ slurry,the phase,density and property effects of CNTs on 3Y-ZrO₂ ceramic were mainly studied.The results show that the particles of CNTs and 3Y-ZrO₂ presented the strongest electrostatic repulsion and this is suitable for being the stable mixed slurry when the pH reached 10.The more the addition of CNTs（after modification）is,the higher the viscosity of mixed slurry is,also,with the increase of the addition of CNTs,the zeta potential of particle surface raised firstly and then decreased,and the largest absolute value of zeta potential existed in CNTs addition of 2 mL.The addition of CNTs never changed the phase of3Y-ZrO₂ ceramic,but its density and the mechanical properties,when CNTs suspension was added 2 mL to the 3Y-ZrO2 slurry,the largest density of 3Y-ZrO2 ceramic is only 5.98g/cm³,its largest bending strength is 893.346 MPa and its fracture toughness 5.92MPa·m^1/2.Once the addition of CNTs exceeded 2 mL,the CNTs agglomerated inside the3Y-ZrO₂,the density of sintered body gradually decreased and the mechanical properties of that reduced as well.