Synthesis Of Cadmium Selenide And Barium Titanates Materials By Wet Chemical Methods In Aqueous System

In synthesis or preparation of advanced materials,the whole process from the very original resources to the final products should be well controlled,thus the compositions,structures and even performances of the final materials can be well tailored.In this thesis,cadmium selenide(CdSe) and barium titanates,which were selected as typical materials for non-oxide and multicomponent oxide,were prepared through different aqueous wet chemical synthesis,i.e.,aqueous coprecipitation and aqueous solution combustion.The influences of aqueous wet chemical synthesis on the formation of these materials were investigated.The research results are summarized as:(1)Aqueous coprecipitation was adopted to synthesize CdSe and CdSeS nanocrystals in order to overcome the shortcomings of conventional organic-based methods,such as complex precursors,high reaction temperature,sophisticated equipment and etc.It was found the optimized conditions for CdSe formation were 12 for pH value,80℃for reaction temperature and 5 hours for reaction time.The suitable alkalinity could restrain the fast release of Se^2- and subsequently facilitate CdSe nanocrystal nanocrystal.(2)Organic polymer(PVA) and mercapto compound with two functional groups were used to modify the surface of CdSe nanocrystals respectively.The results showed that the dispersibility and the stability of both modified nanocrystals in aqueous solution were improved,and their photoluminescence was also enhanced.It was found that the amide of the BSA could conjugate to CdSe nanocrystals through carboxyl in mercaptopropionic acid,and the corresponding conjugates exhibited good photoluminescence performance,thus good potential for protein labelling.(3)In synthesizing pure Ba₂Ti₉O₂₀ phase through aqueous wet chemical route,the influences of drying methods on the homogeneity and phase composition of dry product were first investigated.It was found fast plate drying favored the formation of homogeneous dry gel powders,and eventually pure Ba₂Ti₉O(20).Hence,a solution direct combustion method was developed to prepare homogeneous barium titanate precursor powders.It was found that the nitrate/citrate(n/c) ratio in starting solution, greatly affected the phase formation in as-combusted precursor powder,re-calcined powder and the final product.In all the ratios tested,n/c ratio of 6.33 was optimal. More intermediate BaTi₅O₁₁ phase,which is beneficial for final Ba₂Ti₉O₂₀ formation, was found in the re-calcined powder.Then the citrate solution combustion process was also analyzed and discussed.Since Ba₂Ti₉O₂₀ phase was obtained after 900℃calcination and 1200℃re-calcination,the relations between intermediate BaTi₅O₁₁ and the purity of Ba₂Ti₉O₂₀ were investigated.The results showed that the purity of Ba₂Ti₉O₂₀ phase in re-calcined product was affected by the n/c ratio due to different amount of intermediate BaTi₅O₁₁ phase in the calcined products.(4)The influences of processing parameters on intermediate phase content,final Ba₂Ti₉O₂₀ phase formation as well as dielectric performance were studied.Different precursor solutions were designed and prepared to control the combustion.The results showed that the complete combustion facilitated Ba₂Ti₉O₂₀ formation through increasing the content of intermediate BaTi₅O₁₁ in re-calcined product,while incomplete combustion showed opposite effect through segregation of Ti-rich amorphous phase.Also,re-calcination temperature affected the final dielectric performance through a sintering activity manner although the phases were exactly the same.900℃re-calcination favored the densification of final bulk ceramic.As a result, Ba₂Ti₉O₂₀ ceramics from the precursor powders calicined at 900℃for 4 hours had a relative density of 96%(4.26g/cm³) and dielectric constant of 42.(5)Aqueous solution solution combustion method was also adopted to prepare Ba_6.3xSm_8+2xTi₁₈O₅₄ tungsten bronze type dielectric materials.Ba_6.3xSm_8+2xTi₁₈O₅₄ materials with tailorable x value(x=0.5,0.67,0.75) were prepared.It was also found citrate/metal ion(CA/M) molar ratio of the precursor solution is one of the key factor in Ba_6-3xSm_8+2xTi₁₈O₅₄ phase formation.When CA/M value was 1.2,the corresponding Ba_6-3xSm_8+2xTi₁₈O₅₄(x=0.67) ceramics showed a relative density of 97.7%(5.73g/cm³) and a dielectric constant of 82.1.