Study On Preparation And Modification Of Rare Earth Doped SrTiO₃ Ceramics

With the rapid development of microelectronics industry,the design of electronic products has been tended to miniaturization and integration for human needs.It is of great importance to improve the performance of dielectric ceramics,since they are the most key components in electronic products.The increasing of dielectric constant was considered to be a significant approach,due to the reason that it benefits the miniaturization and integration of electronic devices.SrTiO₃?ST?based dielectric ceramic is quantum paraelectrics,which shows typical cubic perovskite structure.ST demonstrates a dielectric constant of 300 at room temperature?25??,with the dielectric loss being lower than 0.01.This ceramic shows good temperature and frequency stability of dielectric properties,making it promising candidate for pulse power application.SrTiO₃ was selected as the research object,and the influence of defects,which were introduced by doping,was discussed through structure and dielectric properties characterization.Firstly,dielectric properties of SrTiO₃ based ceramics were modified by doping of different rare earth elements,i.e.,La,Nd,Sm and Gd.Two different doping mechanism,Sr_0.97Re_0.02TiO₃?SRTO?and Sr_0.98Re_0.02TiO₃?SRT?,were employed.Ceramics were prepared by traditional solid state reaction method,and high dielectric constant obtained,especially for Gd³⁺doping.SGTO ceramics showed high dielectric constant?⁵000?,low dielectric loss?<0.03?,and good temperature and frequency stability is well.Although SGT ceramic demonstrated high dielectric constant?⁶000?,the dielectric loss is much higher?⁰.15?.Due to the unequivalence doping of rare earth element,a great deal of oxygen vacancies were introduced when sintered in low oxygen partial pressure.At the same time,electrons were created due to the ionization of oxygen vacancies,which could combine with Ti⁴⁺and create Ti³⁺.In this way,the dielectric constant of ceramics was enhanced.The dielectric relaxation peaks of SGTO and SGT ceramics were fitted and studied.The dielectric relaxation activation energy of SGTO ceramic is about 0.86eV,which is a little higher than that of the second ionization of oxygen vacancy.A mass of strontium vacancies V'_Sr were created,which could combine with oxygen vacancies and created defect dipoles[V'_Sr-V_O^··].The dielectric relaxation activation energy of SGT ceramic is about 0.30 eV,and the mechanism of the dielectric relaxation is the hopping of electrons between Ti³⁺and Ti⁴⁺.Gd³⁺was chosen as the dopants into SrTiO₃ ceramics since SGTO and SGT show better dielectric properties than other rare earth doped ceramics.SGTO and SGT dielectric ceramics were synthesized the doping contents of Gd being at 0.01,0.02 and 0.04 respectively.The optimum dielectric properties were obtained when the doping content was 0.01,with a high dielectric constant of⁵400 and low dielectric loss of<0.03 for SGTO0.01 and a high dielectric constant of⁶000 and low dielectric loss of<0.03 for SGT0.01.With increasing the doping content,the temperature and frequency stability was deteriotated.In addition,for the same doping content,the dielectric constant of SGT ceramics is higher than SGTO ceramics.The possible explanation is that the concentration of free electrons in SGT ceramics is higher than SGTO,thus the content of Ti³⁺in SGT is higher than those in SGTO ceramics,which contribute to high dielectric constant as a positive factor.The concentration of defects in dielectric ceramics was changed by varing the doping contents.The dielectric relaxation behavior of SGTO ceramics was studied by fitting of the dielectric relaxation activation energy.The dielectric relaxation activation energies of SGTO ceramics were about 0.80 eV,which was resulted from the defect dipoles[V'_Sr-V_O^··].The dielectric relaxation activation energies of SGT ceramics were about 0.30 eV,being ascribed to the hopping of electrons between Ti⁴⁺and Ti³⁺.The dielectric relaxation peaks of SGTO ceramics moved to higher temperature and frequency.Complex impedances of SGTO0.01 and SGT0.01 were fitted and the conduction activation energies were about 0.90 eV,which was attributed to the thermal motion of oxygen vacancies.The dielectric temperature spectra of SGT0.01 ceramics sintered in O₂ showed that the same dielectric relaxation peaks with that sintered in air,indicating that the increase of electrons could depress the dielectric relaxation peaks.