Preparation And Dispersion Of Nano-sized Zirconia Using High Gravity Coprecipitation Hydrothermal Method And Its Applications

As an important functional material,nano-sized zirconia(ZrO2)has been widely used in numerous fields,such as refractory materials,biomaterials,catalysis,optics,because of its excellent physical and chemical properties,including high strength,high temperature resistance,corrosion resistance and good biocompatibility.At present,there are a range of methods for preparing nano-sized zirconia,however,the current prepared zirconia nanopowders have some serious problems,like agglomeration,unstable crystal form and wide particle size distribution,which seriously affect its application performances.In view of this,this thesis proposes to prepare zirconia nanopowders with good dispersibility,stable crystal form,uniform chemical composition and narrow particle size distribution using a high gravity coprecipitation technology combined with hydrothermal method.The preliminary studies on its application performances in dental restoration and luminescent materials were also carried out.The main contents and results of this thesis were as follows:(1)Tetragonal zirconia with an average particle size about 10 nm was successfully prepared without adding any dispersant using high gravity reactive coprecipitation in a rotating packed bed(RPB)combined with hydrothermal method.The effects of alkali source,washing sequence,pH,yttrium element doping,hydrothermal temperature and time on the crystal form,particle size and morphology of zirconia were mainly investigated.The optimal preparation process conditions were that the nano-sized zirconia precursor was prepared using NaOH as precipitant and 3 mol%Y2O3 as stabilizer at pH=9.After the impurities were removed by washing with deionized water,hydrothermal treatment was carried out at 200? for 5 h.Compared with the products obtained by high-temperature calcination,the nano-sized zirconia prepared by hydrothermal method had a lower degree of agglomeration.In addition,the zirconia clusters with regular sphericity and high fluidity could be obtained using spray drying technology.Compared with the products prepared in a conventional stirred tank reactor(STR),the zirconia clusters obtained by the combination of the high gravity technology and the spray drying method had the better sphericity and denser surface.(2)On the basis of controllable preparation of zirconia nanopowders,the dispersion process was further studied,achieving nano-sized zirconia which could be stably dispersed in water and ethanol.The effects of dispersant type,dosage,modification temperature,adding sequence and washing sequence on the dispersibility of nano-sized zirconia in water were mainly studied.The optimal dispersion conditions were thus obtained:sodium citrate as dispersant was added immediately after the formation of precursor precipitation,the addition amount was 30 wt.%,the modification temperature was 60?,and no washing was required before hydrothermal treatment.The nano-sized zirconia could be stably dispersed in water and the Zeta potential was-31 mV.Further study showed that the particle size of nano-sized zirconia gradually increased with raising the rotating speed.When the rotating speed was 500 rpm and the feed flow rate was 200 mL/min,the obtained nanoparticles had an average particle size of 15 nm.Compared with the products prepared in STR,the products from RPB had an obviously smaller particle size and narrower particle size distribution.In addition,the effects of direct modification and ball milling modification on the dispersibility of zirconia in ethanol were compared.The results showed that the nano-sized zirconia could not be stably dispersed in ethanol by direct modification,while it could be stably dispersed in ethanol by ball milling modification without sedimentation within 2 weeks.(3)The application properties of nano-sized zirconia as dental resin filler and luminescent matrix material were further studied.The results showed that the dental restoration resin based on ZrO2@SiO2 core-shell structure filler had the better mechanical properties,when the coating amount of SiO2 was 200 wt.%,the amount of filler was 50 wt.%and the amount of modifier was 10 wt.%.The flexural modulus,the flexural strength and the compressive strength reached 5779.0 MPa,121.0 MPa and 327.0 MPa,respectively.In addition,when the concentration of Eu3+was 3 at.%and the amount of sodium citrate was 10 wt.%,the as-prepared ZrO2:Eu3+nanoparticles had the strongest luminescence intensity.Compared with the products prepared in a conventional STR,the luminescence intensity of ZrO2:Eu3+nanoparticles prepared in RPB was increased by 59.6%,the quantum efficiency could reach 74.38%,and the products had a longer fluorescence lifetime.