Study On Dielectric Properties And Defect Chemistry Of BaTiO₃ Ceramics Co-doped With Two Kinds Of Ions With Different Valence States

Barium titanate?BaTiO₃?is a simple compound with the highest room temperature permittivity(?'_RT?1600).In recent years,research on the preparation and properties of barium titanate has been very smooth,and great progress has been made.It is mainly used in electronic devices such as multilayer ceramic capacitors?MLCC?,and is called"the pillar of the electronic ceramic industry".With the development of the electronics industry,the requirements for electronic materials are also increasing.Due to the non-uniformity and high porosity of the microstructure of barium titanate ceramic materials,it is necessary to modify them by doping to obtain a large number of new functional materials.Nowadays,this has become a hot research topic.In this paper,Nd³⁺/Ce⁴⁺,Eu³⁺/Ca²⁺and Gd³⁺/Ca²⁺co-doping and Mg²⁺single-doped barium titanate?BaTiO₃?ceramics were prepared by conventional cold-pressed ceramic preparation techniques.The performance of ceramics were studied systematically using X-ray diffraction?XRD?,scanning electron microscopy?SEM?,dielectric-temperature characteristics?DTC?,electronparamagneticresonance?EPR?,Ramanspectroscopy?RS?,and photoluminescence?PL?.The specific research results are as follows:1.(Ba_1-xNd_x)(Ti_1-y-x/4Ce_y)O₃ ceramics with high dielectric constant and low dielectric loss(Ba_1-xNd_x)(Ti_1-y-x/4Ce_y)O₃?0.03?x,y?0.06?ceramics?BNTC?have pseudo-cubic structures.The dielectric-peak temperature?T_m?decreases rapidly at a rate of-27°C/at.%Nd for BNTC with y=0.05.BN5TC6 achieves X7V ceramics with high dielectric constant(?'_RT=14100)and low loss?tan?=0.014?.The EPR signal of g=2.509 associated with Ce³⁺Kramers ions confirms the presence of a mixed valence of Ce³⁺/Ce⁴⁺.A g=2.151 EPR signal of Nd³⁺can be detected at room temperature after these ceramics were preserved over a long period of time.2.EPR exploration on oxygen vacancies in BaTiO₃-based ceramicsThe EPR study on oxygen vacancies in BaTiO₃-based ceramics provides scientific evidence for reducing the dielectric loss of double-substituted BaTiO₃ ceramics.The solubility limit of Mg²⁺in BMT was determined by XRD to be x=0.05.The unit-cell volume?V₀?of BMT decreases continuously with x??0.05?,which provides sufficient evidence for the predominant occupation of Mg²⁺at the A-site.With the increase of T_s,the crystalline structure of BM1T changes from pseudo-cubic to the tetragonal,finally to the mixed phases of hexagonal.BM1T?i.e.BMT with x=0.015?exhibits a temperature-stable X8S dielectric specification with??_RT=1200.When T_s?1300?,a g=1.956 EPR signal observed at T=-188?is assigned as ionized oxygen vacancies?_OV^??.3.Study onvalencestability,dielectric propertiesanddefectchemistryof(Ba_1-xEu_x)(Ti_-x/2Ca_x/2)O₃ ceramicsThe solubility limit of Eu ions in BETC was determined by XRD to be x=0.24,and the tetragonal-cubic phase transition point occurs at x=0.10.The XPS results confirm that Eu ions exist as Eu³⁺and no valence state change occurs.The EPR results confirm that when x?0.03,A/B vacancies exist simultaneously and the amount of A-vacancy defects gradually decreases with increasing x;when x=0.04,A-vacancy defects vanish,which implies that a small number of Ca²⁺ions entered the Ba-sites,and the excess Ca²⁺occupied the Ba vacancy;no EPR signal associated with oxygen vacancies is present at low temperatures.The x=0.10 sample satisfies the X9V dielectric specification,with a high permittivity of?'_RT=5800 and a low dielectric loss tan?=0.0177.4.Crystal structure and dielectric properties of(Ba_1-xGd_x)(Ti_1-x/2Ca_x/2)O₃ ceramicsThe solubility limit of Gd³⁺in BGTC was determined by XRD to be 0.23,and the phase transition from tetragonal to cubic occurs at x=0.10.Gd³⁺ions in BGTC exhibit an intense and broad EPR signal,which conceals cation vacancy defects-related EPR signals.The maximum permittivity of the x=0.07 sample at 88°C is?'_m=8894 and the sample exhibits a lower room-temperature dielectric loss of tan?=0.031.