Microstructure Tailoring and Property Characterizations of Novel Titanate Microwave Dielectric Ceramics

Author:Lei Shen Hui

Supervisor:fan hui qing


Degree Year:2018





Microwave dielectric ceramics are one of the basic foundation materials closely related to the mobile communication industries and electronic industries.It has many important applications that facilitates greatly our daily lives,such as mobile phones and base stations,satellite navigation and guidance,high definition televisions,WLAN and WiFi.In the present thesis,effects of sintering aids,A/B site substitution,optimization of the electron energy band,regulation of the ordered domains and twin structure,regulation of insulative interfaces in the subgrains are applied to control the structures and modify the microwave dielectric properties of microwave dielectric ceramics(MWDC).In this paper,WMDC with low sintering temperature and excellent microwave dielectric properties were prepared and their dielectric mechanisms were also discussed.The researches were carried out as the following aspects:Firstly,SrxLa(1-x)TixAl(1-x)O3(SLTA,x=0.4-0.7)ceramics with additive of CaO-B2O3 glass(1 wt%–10 wt%)were prepared by a solid-state reaction process,which led to a significant decrease in sintering temperature,from 1550°C to 1100°C.Dielectric properties of ceramics at low frequency were significantly improved by CaO-B2O3 glass addition,especially the obtained dielectric loss6×10-4 at room temperature and 1 MHz.Superlattice reflection was detected in the prepared oxides by X-ray diffraction(XRD),selected area electron diffraction(SAED)and Raman spectra,indicating presence of cation ordering.Meanwhile,lattice images of high-resolution transmission electron microscopy(HRTEM)also certificated the presence of cations ordering.Notably,the detected superlattice reflection in perovskite structure associated with antiparallel A-site cation displacements and anti-phase octahedral tilting.This ordered structure as well as low temperature sintering both affected the microwave dielectric properties of Sr0.7La0.3Ti0.7Al0.3O3ceramics,which exhibited with?r43.3,Q×f10780 GHz andτ?20.8 ppm/°C.The smaller cations doped in A/B site,a larger difference in ionic radius or cation valence of A/B-site cations were considered to primarily facilitate the formation of cation ordering,which promoted microwave dielectric properties of the ceramics.Secondly,xSrTiO3–(1-x)LaAlO3 ceramics with ZnO-B2O3 sintering aid were prepared by solid-state reaction method leading to a significant decrease in sintering temperature from 1550°C to 1050°C.The structure,microwave dielectric properties,and low-temperature sintering behavior were systematically investigated.The results revealed the relationships between ionic size,ionic polarizability and cell volume.With increasing additive,chemical ordering of B-site cations was indicated with selected area electron diffraction(SAED)patterns,HRTEM images and Raman spectrum,which contributed to the greatly enhanced microwave dielectric properties.Particularly,the 0.7Sr0.85 Mg0.15TiO3-0.3LaAlO3 ceramics modified with 10 wt%ZnO-B2O3 can further decrease the sintering temperature down to 950°C without deteriorating its performance.Thermal tests implied ceramics featured good chemical compatibility with Cu/Ag electrode.Thus,they can be cofired with internal Cu/Ag electrodes in special patterns to fulfill different electrical functions for low temperature cofired ceramic(LTCC)application.Moreover,the ZnTiO3 ceramics with pure phase were successfully synthesized for the first time by solid state reaction method.The corresponding synthesis temperature was higher than the phase transition temperature,wherein the ZnO nano particles act as inhibitor to prevent the formation of secondary phase:Zn2TiO4,which was inevitable by conventional preparation methods.As the small nano ZnO regions dispersed in the ceramic grains,the bulk diffusion of Ti ions,formation of the nucleation centers and migration of phase boundaries were largely suppressed,implying that nano ZnO was desirable for stabilizing the ZnTiO3 phase above the phase transition temperature.The R3(No.148)space group of single phase were determined by the X-ray diffraction(XRD)Rietveld analysis.X-ray photoelectron spectroscopy and photoluminescence emission spectrum were also carried out to investigate the electronic microstructure of obtained ZnTiO3 phase.Finally,excellent microwave dielectric properties was achieved for sintering temperature of 900-950 oC:?r31.5,Q×f59,800 GHz andτ?1.2 ppm/°C.Moreover,given its good chemical compatibility with Ag electrode,and merits of easy scale-up,high-efficiency,low-cost and environmental friendliness,ZnTiO3 was a promising candidate for LTCC applications.This work paves a great step towards the practical application.Meanwhile,Zn0.9Mg0.1TiO3-ZnNb2O6(ZMT-ZN)ceramics was synthesized and characterized successfully for the first time,and phase conversion to secondary phases was largely restrained due to the introduction of ZnO nano inhibitors.Excellent microwave dielectric properties and optimal combination were achieved for the ceramics sintered at 1100 oC,i.e.,?r=27.5,Q╳f=75000 GHz,?f=-3.8 ppm/oC.Particularly,the comparatively insulated interlayers were considered as the key mechanism to impede transportation or transfer of defects and surface polarization charges.Considering the merits of facile,low cost and simple process,this series of ZMT-ZN ceramics are promising new candidates for ultra-low microwave devices.At last,with widespread application,engineering of microstructures,domains,twins and antiphase boundaries(APBs)are usually hot frontiers.However,the origin of the domains especially in the paraelectric phase is still not clear,as well as the mechanism of variation in twins or domains and their relationship.Generally,these structures are recognized as one of key origins of intrinsic loss.Our works,however,reveals that the formation of twins is closely related to the asymmetry of crystal structure.With introduction of a lattice blockage-trigonal NdAlO3 phase,the increasing symmetry of CaTiO3 tetragonal phase results in a transformation from(110)-oriented twins to(111)-oriented twins.Then it forms a new ordered structure.By peak-differentiating and imitating of Raman spectra,the A-site in perovskite structure is found to be a dominant factor in lattice energy and performance.By a design of A-site displacement,i.e.,Sr2+or Ba2+substitution into Ca2+,we created a controllable structure of twins by symmetry regulation and APBs by introducing in ferroelectric spontaneous polarization.Via selected area electron diffraction patterns(SAED)and variable temperature electric field piezoresponse force microscopy(PFM)images,we found that 180o and 90o domains could coexist in the grains of xCaTiO3-(1-x)NdAlO3 ceramics.Interestingly,the 90o domains and twin boundaries(TB)play more important roles in the anisotropic resistance to the electrons/holes transfer.Our results prove that defects engineering can realize controllable enhanced dielectric performance by defects regulating within its host lattice.It may pave a possible way for designing the microstructure of defects to achieve better predictable performances of materials.