Direct Liquid Precipitation For Preparation Of Dispersed Nanosized Metal Oxides

This paper adopted direct liquid phase precipitation in order to establish a facile technique to prepare dispersed metal oxides and this technique has the advantages of simple operation, low cost,high efficiency,no pollution and free-agglomeration of the product and is easy to be industrialized.In terms of the traditional precipitation to prepare nano metal oxide,water was usually used as a reaction medium or major medium,unavoidable hard agglomeration occurred.Our research overcame this agglomeration phenomenon by separating function and electrostatic interaction caused by ammonium salt absorbed on the precipitate surface,and isolation of pyrolytic gas to the precipitate.With ZrOCl₂ aqueous solution as the titrant and NH₃·H₂O aqueous solution as the titrate,nano ZrO₂ powder was prepared via precipitation at room temperature followed by a drying-calcining treatment.From the beginning of the drying process up to the calcining process of 400℃,NH₄Cl was absorbed on the precipitate surface and played a role of dispersive action.When at 500℃,NH₄Cl was fully decomposed and had no influence on product purity and the product existed in tetragonal form.Calcined over 600℃,the products coexisted in monoclinic and tetragonal forms and their average sizes were between 13-35 nm.If the precipitate was washed with water,the product agglomerated. Adding suitable amount of surface active agent PEG made the values of surface tension and viscosity of the precipitation solution access to the lowest and the product dispersity was improved via the synergism of NH₄Cl and PEG.Because the hydrogen bond formed via absorbed water on the precipitate surface is the origin to cause hard agglomeration,alcohol was used as reaction medium instead of water to reduce this possibility in this research.Dissolving the reactants in alcohol,we avoided the agglomeration caused by the great amount of water from the beginning of the formation of the precipitate.By using ZrOCl₂ and NH₃·H₂O as raw materials,nano ZrO₂ was prepared via a series processes of ZrO(OH)₂ gel precipitated in ethanol,filtrated,washed with alcohol,dried and calcined.The influences of five experimental parameters upon the mean size,dispersity and yield of the product were investigated via uniform design.These parameters were molar ratio of NH₃·H₂O to ZrOCl₂,C₂H₅OH percentage in reacting and washing solutions,ZrOCl₂ concentration,precipitation temperature and surfactant PEG-800 dosage.The condition experiments demonstrated that the C₂H₅OH percentage was the major factor and the higher the C₂H₅OH percentage,the smaller the size,the better the dispersity and the higher the yield of the product.In the precipitation and washing process,the mother liquor and the filtrate could be distilled to recycle ethanol and gain byproduct NH₄Cl simultaneously.This was a simple green process.In order to prohibit the powder size from getting larger after calcinated at high temperature,we applied the approach in which the gel was calcinated at low temperature beforehand,marinated in HCl(aq),then calcinated at high temperature again or the other method of grading heat treatment.This kind of alcohol precipitation was adopted to prepare Y₂O₃-ZrO₂,MgO-ZrO₂, Al₂O₃-ZrO₂ and CaO-ZrO₂ composite ceramic powder by using ZrOCl₂·8H₂O as the raw material,Y(NO₃)₃·6H₂O,MgCl₂·6H₂O,AlCl₃·6H₂O,and Ca(NO₃)₂·4H₂O as the dopant respectively,and NH₃·H₂O as the precipitant.The green compacts were formed by dry pressing under certain pressure and then sintered.The effects of the reaction media and calcining temperature on the size and dispersity of the powders as well as the property of the powders,the dosage of the dopant and the compacting pressure on the density of the green compacts and sintered bodies were studied.,The experiment findings showed that when ethanol was used as the reaction medium the sizes of the nano-zirconia composite powders were less than 30 nm and the powders had the narrow size distribution,good dispersity and high sintering activity.The form of ZrO₂ was major tetragonal with minor monoclinic.The fractured surfaces of the sintered bodies prepared by the composite ceramic powders were smooth without any cracks.The optimal pressure for Y₂O₃-ZrO₂,MgO-ZrO₂,Al₂O₃-ZrO₂ and CaO-ZrO₂ sintered bodies were 10MPa,7MPa,20MPa and 20MPa,respectively.The relative densities of the ZrO₂ green compacts respectively doped with 3%(mol)Y₂O₃,5%(mol)MgO, 20～60%(mol)Al and 5～20%(mol)CaO were about 50%.The relative densities of the ZrO₂ sintered bodies respectively doped with 3%(mol)Y₂O₃,10%(mol)MgO,20～80%(mol)Al and 5～15%(mol)CaO exceeded 95%and were close to the theoretical value.Nanosizedγ-alumina powder was prepared via precipitation of AlCl₃ and NH₃·H₂O ethanol solution followed by the process of alcohol washing,drying treatment and calcination.The effects of the reactant concentration,the molar ratio of reactants and the calcination temperature upon the yield and the specific surface area of the product were explored.The experimental results showed that the former two factors determined the product yield and the latter determined the size,phase and specific surface area of the product etc.When the calcination temperatures were controlled between 686-1029℃,the highly dispersed nano-sizedγ-alumina particles with a size range of 5-9 nm and specific surface area of 204-102m²/g were obtained.The direct precipitation process in which ethanol was used as the medium could be applied to synthesize other nano oxides such as magnesium oxide,stannic oxide,aluminum oxide and some stable ZrO₂ ceramic powder.In this paper,the heat treatment process of the precursors was analyzed by thermogravimetry differential thermal analysis(TG-DTA) and infrared spectrum(IR),the morphology,the size and distribution of the particles were observed by transmission electron microscopy(TEM),the phase composition of the powders was detected by X-ray diffraction (XRD),the microstructure of sintered bodies was studied by scanning electron microscopy (SEM).