Study On Defect Chemistry Of New-type High-k Barium Titanate Ceramics Multi-doped With La, Ce, Nd, Eu, And Fe

With the development of the electronic industry, the continuous trend of miniaturization inthe field of dielectrics requires higher and higher volumetric efficiencies, which can berealized in two ways:（1） enhancing dielectric permittivity; and （2） fine-grainedmicrostructure. As the most important dielectric material, barium titanate （BaTiO₃） is a simplecompound with the highest room-temperature （RT） dielectric permittivity （ε′RT¹600）. Asmall amount of rare-earth ions merge into the BaTiO₃lattice, which can greatly reduce thedielectric loss and grain size, improve the dielectric properties, and stabilize the dielectricconstant in a wide temperature range, make dielectric response gradually transfer from thefirst-order phase transition （FPT） to diffuse phase transition （DPT）.Rare-earth reserves are rich in China. In compounds, rare earth elements are usuallytrivalent. Ce （atomic number:58） is the most abundant element （44g/t） on earth. Ce is aspecial element. Ce may stably exist at Ba-sites as Ce³⁺（4f¹5s²5p⁶） or at Ti-sites Ce⁴⁺（4f⁰5s²5p⁶） in BaTiO₃. Ce doping can improve the melting point of BaTiO₃and plays a role inenhancing insulation. The application of Ce dopants in BaTiO₃has bright prospects in thedielectric field.In recent years, Fe-doped BaTiO₃ceramics have received intensive attention because oftheir potential applications based on the Multiferroic materials. Fe³⁺ions located at Ti-sites inBaTiO₃act as acceptors, accompanied by the formation of oxygen vacancies （VO）. The VOwill inevitably give rise to the electrical conduction. Nd （atomic number:60） is the2nd mostabundant element （24g/t） on earth. Nd³⁺ions located at Ba-sites in BaTiO₃act as donors. Inorder to suppress the concentration of the VOand reduce dielectric loss, Nd³⁺donor and Fe³⁺accepter were used as codopants in BaTiO₃to form Nd³⁺-Fe³⁺defect complexes and toprepare ceramics with a single-phase structure, low dielectric loss, and high-k Y5V behavior.In this work, the structure, dielectric properties, and defect chemistry of a series ofCeO₂-BaTiO₃-based ceramics were investigated. The main research: the mutual solidsolubility and phase equilibrium of BaTiO₃-BaCeO₃; the evidence of Ba-site Ce³⁺in BaTiO₃;the formation of a complete solid solution of (Ba_1-xCe_x)(Ti_0.95-x/4Ce_0.05)O₃and development ofhigh-k ceramics; the preference of the site occupation of （Ce, Ca） and （Ce, Mg） and dielectricproperties of three types of (Ba_1-xM_x)(Ti_1-x/4-yCe_y)O₃,(Ba_1-xCe_x)（Ti1-x/2Mx/2）O₃, and(Ba_1-3x/2Ce_xM_x/2) Ti_1-x/4O₃（M=Ca, Mg） ceramics; the development of high-k BaTiO₃ceramics co-doped with （Sm, Ce） and （La, Eu, Ce） and new-type high-frequency Ramanspectra. In addition, the defects of Fe-doped BaTiO₃ceramics; structural evolution, evidencefor the formation of defect complexes Nd³⁺-Fe³⁺, development of high-k Y5V ceramics, and Nd³⁺-Fe³⁺complexes associated with structure evolution, and defect chemistry.Prime Novelty of this master’s thesis:（1） The mutual solid solubility and phase equilibrium of （1-x）BaTiO₃-xBaCeO₃（x=0.1-1.0） were investigated. The mutual solid solution limits of BaCeO₃and BaTiO₃are given; Anerror of the history was clarified, i.e., no pure BaCeO₃phase can be separated fromBa(Ti_1-xCe_x)O₃phase;（2） The nominal (Ba_1-xCe_x)Ti_1-x/4O₃（BCT） and Ba(Ti_1-xCe_x)O₃（BTC） ceramics preparedunder the different conditions were investigated by Raman spectroscopy. The evidence ofCe³⁺at Ba sites in the BaTiO₃lattice was provided in BCT at the first time, and Ramanscattering experiments can give the convincing evidence that a small amount of Ce ionsinevitably enter Ba sites as Ce³⁺in BTC;（3） Nominal (Ba_1-xCe_x)(Ti_0.95-x/4Ce_0.05)O₃ceramics were prepared by a solid state reactionmethod. A complete solid solution called BC3TC5was formed only at x=0.03, with asingle-phase perovskite structure. Ti-vacancy defects exist in BC3TC5, which has a feature ofmixed valence of Ba-site Ce³⁺and Ti-site Ce⁴⁺. The amphoteric Ce³⁺/Ce⁴⁺ions can induce anIn-situ diffuse phase transition at the Curie point of BaTiO₃（TC=125°C）. BC3TC5exhibits ahigh-k behavior （ε′m=9470） and lower dielectric loss （tan δ <0.025）;（4） The site-occupation preference and defect chemistry of (Ba_1-xM_x)(Ti_1-x/4-yCe_y)O₃(Ba_1-xCe_x)（Ti1-x/2Mx/2）O₃(Ba_1-3x/2Ce_xM_x/2) Ti_1-x/4O₃（M=Ca, Mg） were investigated byXRD EPR RS dielectric measurements;（5） High-k BaTiO₃ceramics co-doped with （Sm, Ce） and （La, Eu, Ce） were developed. Thesite occupation and valence state of are-earth ions were determined. New-type high-frequencyRaman spectra were discovered and the law of dielectric response was studied;（6） New-type （Ba1-xNdx）（Ti1-yFey）O₃ceremics were developed. The structure evolutionshowed the law that a single-phase ceramic with a cubic or tetragonal structure could beformed when x≥y, which gave the evidence for formation of defect complexes; and themixed phases of cubic and hexagonal were formed when x <y. A high-k Y5V ceramic couldbe realized at x=y=0.05. Defect chemistry associated with structure evolution wasdiscussed.