Preparation Of Core-shell Structure And Study Of Dielectric Properties On Doped Nanoscale BaTiO₃-based Ceramic Materials

The development trend of multilayer ceramic capacitor(MLCC)is in urgent need of dielectric materials with high performance.Currently,BaTiO3 is the most important large-scale commercial MLCC dielectric material due to its relative high dielectric constant and low dielectric loss,but the influence of the synthesis and microstructures on the dielectric properties at nanoscale still need to be further explored and established.Therefore,it is still of great scientific significance and application prospects to prepare the well-defined BaTiO3 nanocrystals and further the high-performance ceramic materials through the structural modifications.In this thesis,BaTiO3 nanoparticles were prepared by solution chemistry strategy,and the effects of different solution chemical conditions on the phase,microstructure and dielectric properties of ceramics were studied through structural modifications on BaTi03 by Fe doping.Furthermore,the core-shell structure was constructed by surface coating of SiO2 to investigate the dielectric properties of the ceramic materials.The following results were obtained:(1)Synthesis of the monodisperse BaTiO3 nanoparticles with showing the surface disorder layer.The tetragonality(c/a ratio)of BaTiO3 nanocrystals can be tuned through adjusting the precursor solution concentration and Ba/Ti ratio.The surfactant oleic acid(OA)can greatly affect the morphology and size of BaTiO3 nanoparticles,e.g.resulting in regular rectangular or pseudo-cubic shapes with the average grain sizes of 20-40 im with adsorbing OA molecules on the surfaces.Microstructure analysis by TEM indicates that a surface disorder layer forms on the surfaces of the as-synthesized BaTiO3 nanocrystals.To identify the nature of the surface layer,sintering the nanoparticles at 8000C and 1100 ? was carried out,which leads to the formation of the new Ti-rich phases of BaTi5O11 and BaTi2O5.Structural refinement gives the Ti/Ba ratio of about 1.50,indicating the non-stoichiometric nature.That is,the as-prepared BaTiO3 is featured as Ti excess,which facilities the visible-light absorption effectively.This work reveals the cation non-stoichiometry in the nanoscale BaTiO3 system for the first time,which is significantly important to guide the synthesis and even the structure-property understanding for the similar oxide materials.(2)The BaTi1-xFexO3(x=0-0.1)nanoparticles were synthesized by utilization of the oleic acid,with achieving the tetragonal structures with regular pseudo-cubic morphology and an average size of 20-40 nm.The unit cell parameters i.e.a and c as well as the cell volume(V)of BaTi1-xFexO3 decrease with the increased doping levels.Fe doping can modify effectively the electronic structure of BaTiO3 as the narrowing optical band gap from 3.25 eV to 2.65 eV was observed.Furthermore,the surface disorder and non-stoichiometry natures of the BaTiO3 nanocrystal system can be tuned by the Fe doping,with the ratio of RBa/R(Ti+Fe)gradually decreases down to be 1 when increasing Fe doping levels.The dielectric properties of BaTi1-xFexO3 ceramics were systematically studied.With increasing the Fe doping levels,the room-temperature dielectric constant decreases from 3000 for x=0 to around 1500 for x=0.1,and simultaneously the Curie temperature decreases but the low-frequency dielectric loss increases,which are due to the soft mode change and introduction of the acceptor oxygen vacancy by Fe doping effect.(3)SiO-2 coated BaTiO3 nanoparticles were prepared to construct the"core-shell"BaTiO3@SiO2 structure.The thickness of SiO2 coating layer increases gradually with the increased TEOS concentration in the solution,which is around 6 nm with the TEOS concentration of 25 wt%.Further increasing the TEOS concentration up to 30 wt%,the average thickness of SiO2 coating layer reaches 14.4 nm.These coated SiO2 layer is in amorphous form.The room-temperature dielectric properties of BaTiO3@SiO2 ceramics were studied,with achieving the slightly improved property when the SiO2 coating amount is 10wt%,while the overall dielectric properties e.g.the dielectric sonstant decreases with the increased Fe doping levels for the BaTi1-xFexO3@SiO2 ceramics.