| With the trend towards size reduction of many microelectronic devices,high-dielectric-constant oxides have become essential for modern microelectronic devices,such as on-chip capacitors,memory devices,resonators and wave filters.High-performance dielectric materials have been attracted attention for their applications in myriad types of miniaturized devices.The development of high capacity capacitors needs to further enhance the dielectric permittivity(>103),acceptable dielectric loss(under 0.1)and less frequency/temperature dependence.Recently,the co-doped(Nb,In)rutile TiO2,which possesses the extraordinary colossal permittivity(denoted as CP,?'?4×104)with low dielectric loss(below 0.05)over wide frequency(40 Hz-100 kHz)and temperature(80-450 K)ranges,has a wide application prospect in high capacity capacitor and becomes a research focus in the field of giant dielectric.It has been recently found that the origin of CP is also caused by the surface effects of grain boundary and internal barriers layers capacitor.Therefore,such CP behavior can be appropriately described by neither the intrinsic electron-pinned defect-dipoles nor the extrinsic mechanism,and some combined mechanism is needed.How different ions of?A3+ co-doped with Nb5+ influence the dielectric proerpty?How different preparation processes affect the dielectric proerpty?Whether the giant dielectric origin of co-doped TiO2 materials is intrinsic or extrinsic?Those problems are not clear at present,so it is necessary to analyze the dielectric relaxation process and reveal the physical origin of high dielectric.The high dielectric constant TiO2 material with magnetic dielectric response has not reported.Therefore,it is an important subject to study material with high dielectric constant and magnetic dielectric effect.In this study,the giant dielectric TiO2-based(?A1/2Nb1/2)TiO2(?A =In,Y,Sm,Yb,Co and Al)ceramics were prepared by solid state method.The effects of sintering,annealing and doping conditions on the crystal structure,microstructure,dielectric,complex impedance and magnetic dielectric properties of TiO2-based ceramics were studied.The origin of high dielectric constant and magnetic dielectric effect were discussed.1.(In1/2Nb1/2)0.05Ti0.95O2 ceramics are prepared with the average grain size of 10?m.The lattice parameters are a=b=4.5936(7)A and c=2.9622(4)A.The samples show CP properties(?'?103)with low dielectric loss(<0.05)in frequency range of 20 Hz-100 kHz and temperature range of 80-450 K.After annealing process,the dielectric properties of co-doped TiO2 material were improved due to reducing oxygen vacancies.There exist three kinds of dielectric relaxation processes,which occur in the temperature(5 K-300 K)and frequency(20 Hz-100 kHz).Using the universal relaxation law,the experimental results can be suitably explained by the model of electron-pinned defect-dipoles,polaron polarization and interface polarization.A significant contribution of polaron polarization on the origin of co-doped TiO2 high dielectric constant was observed for the first time.2.The(?A1/2Nb1/2)0.05Ti0.95O2(?A=Sm,Y and Yb)ceramics were prepared.At room temperature and 100 kHz,for ?A=Sm,Y and Yb co-doping samples,dielectric constant is 500,2500,4×104,and dielectric loss is 50,0.15,0.08,respectively.Furthremore,the(Yb1/2Nb1/2)xTi1-xO2(0.01?x?0.1)ceramics were investigated.The results reveal that the solid solubility limit of(Yb1/2Nb1/2)xTi1-xO2 ceramics is x=0.07.The average grain sizes decrease from 12 ?m to 6 ?m with increasing x value from 0.01 to 0.1.Only propable doping(0.03?x?0.7)ceramics show CP behavior.At last,the effects of the sintering atmosphere of oxygen partial pressure(10%to 100%)on crystal structures,microstructure and dielectric properties of(Yb1/2Nb1/2)0.05Ti0.95O2 were investigated.The average grain size rises from 5 ?m to 28 ?m with increasing oxygen partial pressure from 10%to 100%.The colossal permittivity of 104 is obtained for all samples.The dielectric constant displays well electric properties at oxygen partial pressure atmosphere higher than 50%.XPS results show that oxygen vacanies concentrations have been restrained by high oxygen partial pressure.The polaron polarization process is observed in the different rare earth ions co-doped Tio2.3.The(Al1/2Nb1/2)0.05Ti0.95O2 ceramics with average grain size of 10 ?m were prepared at 1773 K.The lattice parameters are a=b=4.5637(7)A and c=2.9411(6)A.The samples show CP properties(?'?3.8×104)with low dielectric loss(under 0.08)over wide frequency(20 Hz-100 kHz)and temperature(80-240 K)ranges.Using the model of universal relaxation law and thermal polaron hopping,we conclude that the high dielectric constant of(Al1/2Nb1/2)0.05Ti0.95O2 is due to the thermal activation of electrons,the electron-pinned defect-dipoles,polaron hopping polarization and interfacial polarization.Under the application of magnetic field,it is found that the(Al1/2Nb1/2)0.05Ti0.95O2 ceramic has positive and negative magnetic dielectric effects with a temperature range from 5 K to 300 K:one kind of positive magnetic dielectric effect and the two kinds of negative magnetic dielectric effects.And it also has the magnetic dielectric coefficient with a wide range of change:78%to-55%.The causes of the different magnetic dielectric effects have a close relationship with the different polarization mechanisms.Based on the Jahn-Teller effect and the Pauli principle,the relationship between the magnetic response and the polarization mechanism is explained.4.The crystal structure,magnetic,dielectric,electrical conductivity and magnetic dielectric of(Co1/2Nb1/2)0.05Ti0.95O2 ceramic were investigated.The lattice parameters are a=b=4.5528(3)A and c=2.9311(6)A.XPS results show that Ti valence is influenced by co-doping Co3+ and Nb5+.At room temperature and 100 kHz,dielectric constant is 2100 and dielectric loss is 0.2.Only polaron hopping polarization and interfacial polarization are observed,and the electron-pinned defect-dipoles are disappeared.Thisis because that the balance of localized electrons are broken by co-doping Co3+,which results in more point defects.The cause of different magnetic dielectric effect has a great relationship with polaron hopping polarization and interfacial polarization.5.The PbNb2O6 ceramics were prepared by the two-step sintering method.Through sintering at different second sintering temperatures 1220 and 1180?,the abnormal grain growth of PbNb2O6 ceramic can be restrained.For 0.5 wt%ZrO2-doped ceramics,the sintering temperature was reduced to 1160? and the high temperature orthogonal phase was preserved.The dielectric relaxation,which is caused by oxygen vacancy,was improved.Furthermore,the piezoelectric properties of PbNb2O6 ceramics were enhanced. |
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