Study On The Regulation And Polarization Relaxation Of The Giant Dielectric Properties Of Doped TiO₂ Ceramics

Colossal permittivity(CP,?'>103)materials attracted considerable attentions due to their increasing potential for applications in high performance capacitive devices.Not only the CP,low dielectric loss(tan?<0.05)did the excellent colossal dielectric material should possess,particularly,the excellent dielectric performances should with a good temperature,frequency,or DC bias stability.In recent years,co-doped Ti02 with CP had received widely attention since(In+Nb)co-doped TiO2 ceramics exhibited CP,low dielectric loss,and excellent frequency/temperature stability in a wide temperature range(80 K to 450 K)and a wide frequency range(20 Hz to 2 MHz).However,the origin of CP in co-doped TiO2 ceramics remains unclear.One view is the case that the CP properties originate from defect-dipole polarization between localized electrons in the crystal lattice.Another view is that CP stem from Maxwell-Wagner polarization caused by the inhomogeneous space conductivity in the ceramics.To gain the co-doped TiO2 ceramics with CP and low dielectric loss,figure out the origin of CP is still important subjects in this kind of material.In this thesis,a series of solid-solution and composite TiO2-base ceramics with excellent dielectric properties were prepared by co-doping,mono-doping or introduction of the secondary phase.By investigating the microstructure,dielectric properties and dielectric relaxations,the dependences of dielectric properties on the preparation process,doped-elements and the secondary phases were revealed.The polarization relaxations and the origins of CP are discussed1.A series of La+Nb co-doped TiO2 ceramics were prepared and the influences of the preparation technology on dielectric properties were studied.Through appropriate annealed process,the samples with CP and low dielectric loss were obtained while sintered in N2 atmosphere and annealed in flowing air.The characterization of polarization relaxations and the impedance revealed that the CP mainly stemmed from the EPDD polarization.Low dielectric loss is ascribed to the inhibition of Maxwell Wagner polarization by creating insulating outer surface layer and improvement of grain boundary resistance through the annealed process2.Different trivalent elements with Nb co-doped TiO2 ceramics were prepared.XRD,Raman and SEM,EDS,etc confirmed that the secondary phases were appeared in A = La,Nd,Sm,Gd and Nb co-doped TiO2 ceramics,while A =Tl,Sc,Co,Ni can well replace Ti in co-doped TiO2 to form pure rutile solid-solution.The temperature dielectric spectrum showed that the dielectric properties of(Tl0.5Nb0.5)1.5%Ti98.5%O2 ceramics were affected by different polarizations.The primary contribution to permittivity was EPDD polarization.The free electron/oxygen vacancy hopping,Maxwell-Wagner polarization did not change permittivity in magnitude,but significantly contributed to imaginary part of the permittivity.The impedance-and temperature-dielectric spectrum confirmed that the(A0.5Nb0.5)0.5%Ti99.5%O2 ceramics with the secondary phase possessed higher conductivity activation energy,while the low dielectric loss were obtained for inhibiting the contribution of Maxwell-Wagner polarization;the CP in all the co-doped ceramics were originated from EPDD polarization.4.A series of(In0.5Nb0.5)0.5%(Ti1-xZrx)99.5%O2(0?x?0.80)composite ceramic were prepared by introducing Zr into the In+Nb co-doped TiO2 ceramics.The structure,dielectric performance,impedance characteristic and the breakdown strength(BDS)were investigated.The introduction of Zr elements not only maintain the CP properties,but also effectively reduce the dielectric loss,improve the BDS values and stable the bias dielectric properties.Large BDS values and stable bias dielectric performance in(In0.5Nb0.5)0.5%(Ti0.80Zr0.20)99.5%O2 ceramics are mainly associated with secondary phase ZrTiO4.The presence of the secondary phase can refine the grain size.5.A series of rutile solid-solution(In0.5Nb0.5)0.5%(Ti1-xZrx)99.5%O2 ceramics with ?'>103 and tan?<0.05 were obtained.The results of XPS and temperature-spectrum confirmed that the primary contribution on permittivity is EPDD polarization.The activation energy of EPDD polarization increased with the introduction of Zr,and the distribution of relaxation time r became wider.Defect clusters model is adapted to analyze this phenomenon.In the co-doped TiO2,replacement of Ti by Zr inhabited the correlation between the defect clusters,so the distance of localized electrons hopping was increased,causing the enhancement of activation energy for EPDD polarization;enlargement the distribution of relaxation time r.6.The CP phenomena in mono-doped TiO2 ceramics were studied.The rutile solid-solution with ?'>104 at room temperature were obtained for W6+ or Nb5+mono-doped TiO2 ceramics.NbO.5%Ti99.5%O2 can obtain CP properties at very low temperature(25 K),and the relaxation activation energy was 14.5 meV,while W0.5%Ti0.995%O2 can obtain CP properties above 100 K,and the relaxation activation energy was 620 meV.Density Functional Theory Calculation(DFC)shows that doped Nb in NbO.5%Ti99.5%O2 ceramics tend to form Nb-Nb pairs by adjacent binding,while in W0.5%Ti0.995%O2 ceramics,doped W tends to discrete existence,and there is no correlation between W and W.Therefore,it was speculated that the CP of NbO.5%Ti99.5%O2 ceramics originated from the defect cluster polarization formed by Nb-Nb,while the CP of NbO.5%Ti99.5%O2 is originated from the Maxwell-Wagner polarization.