| High dielectric constant(104-105),low loss and excellent temperature/frequency stability were shown by many transition metal oxides,and they had a wide range of applications in the field of electronic components.The influence of co-doping on the crystal structure,defect structure,carrier transport mechanism and dielectric properties of SnO2 was discussed in detail in this paper by co-doping different metal ions.The main research contents were as follows:1.(Nb,Zn)co-doped SnO2 ceramics(NbxZnySn1-x-yO2)were prepared by traditional solid phase reaction method.The ceramics were densified by the co-doping of donor and acceptor.The co-doped ceramics had dielectric constant(ε’≈900-1000)and loss(tan δ≈0.008-0.05)independent of frequency and temperature.Experimental characterizations and DFT calculation showed that Nb4+delocalized electrons and oxygen vacancy were mainly formed when the doping concentration was low(x=y<0.03).The dielectric properties of the sample were optimal and dominated by these electron-pinning defect dipole clusters.The number of oxygen vacancies in the clusters composed of Nb and Zn adjacent to each other decreased,while the concentration of holes increased,with the increase of doping concentration.These holepinning defect dipole clusters were formed.At the same time,some of the holes recombined with the delocalized electrons near Nb4+,resulting in the decrease of carriers concentration and polarization intensity of the dipole cluster.The degree of carrier localization was weakened,the dielectric constant of the sample was decreased,and the loss was increased.2.(Nb,Cu)co-doped SnO2 ceramics(NbxCuySn1-x-yO2)were synthesized by the solid phase method.The dielectric constant of the sample was 600-1200,and the loss was 0.02-0.60 in the frequency range of 40Hz-1MHz.It was shown by conductivity fitting that the carriers were a Variable Range Hopping(VRH)at low temperatures and a Nearest Neighbor Hopping(NNH)at slightly higher temperatures,which was attributed to the unstable valence state of Cu.The existence of delocalization holes was indicated by both experimental and theoretical calculations that showed electron depletion in the lattice oxygen of Cu coordination.Cu was+1 valence,showing good electron localization and forming electron pinning defect dipole clusters at low doping concentration;Cu was+2/+3 valence at high doping concentration.Local holes were had by these clusters composed of Nb and Cu,some of which recombined with delocalized electrons near Nb,resulting in weakened electron localization and decreased polarization intensity of the clusters.The dielectric constant of the sample decreased and the loss increased,and the dielectric property of Nb0.01Cu0.01Sn0.98O2 was the best.3.(Pr,Li)double acceptor co-doped SnO2 ceramics(PrxLiySn1-x-yO2)were synthesized by the solid phase method.The dielectric constant was 9.5-40.7 and dielectric loss was 0.01-1.68 of Pr0.05Li0.05Sn0.90O2 ceramics.Carrier hopping of NNH was shown by samples with low doping concentration(x=y=0.01),while VRH conduction mechanism at low temperature and NNH mechanism at high temperature were shown by samples with high doping concentration(x=y=0.03～0.05).The unstable valence state of Pr could be attributed to the mechanism.Additional delocalized carriers inside the lattice were not introduced by double-accepter main doping,and the concentration of defect clusters within the lattice was very high due to doping.Lower dielectric constant and slightly reduced loss of the sample were led by over-localized carriers,which indirectly indicated that the concentration of carriers and its pinning effect had a decisive influence on the dielectric properties of transition metal oxide materials. |
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