Preparation Of Nano-ZrO₂ And Zr-based Composite Nanomaterials By Continuous Hydrothermal Flow Synthesis

Zirconium dioxide is an important inorganic functional material with excellent physical and chemical properties.In terms of the physical properties,zirconia has high temperature resistance,corrosion resistance,wear resistance and good thermal stability.In terms of the chemical properties,zirconia has both weak acidic and weak basic properties,as well as oxidation and reducibility.Zirconia is also chemically stable and insoluble in organic or inorganic reagents except hot concentrated sulfuric acid and hydrofluoric acid.Various forms of zirconia materials have greatly broadened the application fields of zirconia.Zirconia-based materials are the research hotspots in the fields of oxygen storage materials,catalysts,fuel cells,sensors and heat insulation materials.Traditional preparation methods of zirconia nanomaterials,including precipitation method,sol-gel method,microemulsion method,hydrothermal solvothermal method and etc.,are various and the technology is relatively mature.However,the traditional preparation methods generally have problems such as long reaction time and complex post-treatment,especially the high temperature post-treatment process is easy to cause grain growth and specific surface area decrease.The method of Continuous Hydrothermal Flow Synthesis（CHFS）for the preparation of nanomaterials has attracted much attention due to its potential in the production of controlled nanoparticles.The CHFS method mainly uses water as the solvent,which can be operated continuously and stably.The operating conditions are flexible and adjustable,and it is environmentally friendly.An effective method to improve the degree of solution mixing in CHFS system is to design a suitable reactor.In this investigation,a CHFS system with Confined Jet Mixer（CJM）reactor had been adopted to prepare Nano-ZrO₂ and Zr-based composite oxide nanomaterials.The prepared nanomaterials were characterized and analyzed.CFD-PBM coupling model was adopted to simulate the flow,mixing,nucleation generation and particle growth in the reactor.The reaction conditions were optimized.And the particle size and Particle Size Distribution（PSD）of the product were predicted.The research findings were shown below:（1）Nano-ZrO₂was prepared by the CHFS method.In order to select the preparation conditions,the effects of various precursors,concentrations,temperatures,pressures,feed flow rates and other factors on the morphology and properties of the samples were investigated.The analysis technology of Scanning Electron Microscope Energy Dispersive Spectrometer（SEM-EDS）verified that the composition of the ZrO₂nanoparticles was consistent with the nominal composition.The results of X-ray Diffraction（XRD）,Fourier Transform Infrared Spectroscopy（FTIR）,Raman Spectroscopy and TGA-DSC showed that all samples were mixture of tetragonal（t-ZrO₂）and monoclinic（m-ZrO₂）phases.Rietveld refinement analysis was employed to further analyze the structural information of the obtained ZrO₂ nanoparticles under different synthesis conditions.The mean crystalline sizes calculated by the Rietveld analysis were approximately 4 nm.The d-spacing values measured by the HR-TEM were in good agreements with the calculated results obtained by the XRD Rietveld analysis.DLS analysis showed that the average particle sizes of the obtained samples were about 6-8 nm.BET test results showed that the specific surface areas of the obtained samples were between 106 and204 m²/g.（2）3YSZ,6YSZ,8YSZ and 9YSZ composite oxides with different Y/Zr ratios were prepared by CHFS method.The chemical composition and metal element content of the prepared samples were tested and analyzed by SEM-EDS and ICP-AES.No impurities were found in the four prepared YSZ samples.The Y/Zr ratios of the YSZ samples obtained in this investigation were in good agreements with the initial stoichiometric ratios.By combining XRD,FTIR and Raman characterization methods,all the prepared YSZ nanoparticles showed a cubic crystal structure.TEM images showed that all YSZ samples had high crystallinities,uniform particle shapes and narrow particle size distributions.The BET results showed that the prepared YSZ samples had large specific surface areas,and they tended to gradually increase as the amounts of yttrium increased.The ionic conductivities of the 3YSZ and 8YSZ samples was measured by an alternating current impedance method.The alternating current impedance profiles and corresponding equivalent circuit diagrams of the two samples were obtained at 500-1000℃.The values of the grain boundary resistance,grain resistance and conductivity of the samples at different temperatures were obtained by the Zsimpwin software.It was found that as the increase of temperature,both grain resistance and grain boundary resistance gradually decreased,while the total resistance decreased,and the conductivity increased.The logarithm of the conductivity had a linear relationship with the temperature,which matched the Arrhenius relationship.（3）The Ce O₂-ZrO₂ composite oxides with different Ce/Zr ratios（1CZ,2CZ,3CZ,4CZ,C for Ce and Z for Zr）were prepared by the CHFS technique.The chemical compositions and the metal element contents of the prepared CZ samples were tested and analyzed by the SEM-EDS and ICP-AES.There was no impurity found in the four prepared CZ samples.The Ce/Zr ratios of the CZ samples obtained in this investigation were in good agreements with the initial stoichiometric ratios.By combining the XRD,FTIR and Raman characterization results,all the prepared CZ nanoparticles showed a cubic crystal structure.TEM images showed that all CZ samples have high crystallinities,uniform particle shapes and narrow particle size distributions.The BET results showed that the prepared CZ samples had large specific surface areas,while the amount of Ce had no significant effect on the BET surface areas of the CZ samples.The Oxygen Storage Capacity（OSC）of the samples increased from 433.7 to 631.1μmol/g when the doping amount of Ce element increased from 10 to 40%.The H₂ Temperature Programmed Reduction（H₂-TPR）analysis showed that the four CZ samples appeared an obvious reduction peak between 300 and 800℃,which may due to the0)₂→0)₂₃ reduction.（4）Four kinds of Zr-based composite oxides（CYZ1,CYZ2,CYb Z and CYYb Z,C for Ce,Y for Y,Yb for Yb,C for Ce）were prepared by the CHFS technique.The compositions,morphologies,crystal phases,surface areas and properties of the prepared samples were investigated.The chemical compositions and the metal element contents of the prepared samples were tested and analyzed by SEM-EDS and ICP-AES.No metal element impurities were found in the four prepared samples.By combining the XRD,FTIR and Raman spectroscopy results,all the prepared samples showed a cubic phase crystal structure.The TEM images showed that all the samples had high crystallinities,uniform particle shapes and narrow particle size distributions.The BET results showed that the prepared samples had large specific surface areas.CYZ1 appeared the largest surface area as 245.2584 m²/g.The OSCs of the samples were tested by the oxygen pulse adsorption technology.Among the four samples,the OSC of the CYZ2 sample was the largest as 545.946μmol/g.The OSC of the CYZ1,CYb Z and CYYb Z samples were all between 300 and 400μmol/g.Two reduction peaks were found between 300 and 800℃for the four samples by the H₂-TPR test.As the solid fuel cell electrolyte materials,the ionic conductivities of the four samples were measured by the alternating current impedance method.The alternating current impedance profiles and the corresponding equivalent circuit diagrams of the four samples were obtained at 500-1000℃.The values of the grain boundary resistance,grain resistance and conductivity of the samples at different temperatures were analyzed using the Zsimpwin software.It was found that with the increase of temperature,both grain resistance and grain boundary resistance gradually decreased,while the total resistance decreased,and the conductivity increased.The logarithm of the conductivity had a linear relationship with the temperature,which satisfied with the Arrhenius relationship.（5）CFD-PBM coupling model was used to study the fluid flow,heat transfer and mixing behavior in the Confined Jet Mixer（CJM）reactor in the CHFS system.The main factors which affecting the performance of the CHFS reactor were determined for scaling up approach.The velocity,temperature and mixing phenomenon in the reactor were simulated under the various operating conditions including the inlet flow rates of the supercritical water and the precursor solution,the temperature and the pressure.PBM model was used to simulate the process of nucleation and particle formation of ZrO₂.And the free molecule model was chosen to predict the effect of the particle aggregation.The results showed that the CJM reactor ensured the full mixing of the fluid.Moreover,the predicted results of the CFD-PBM model were in good agreement with the experimental results.