Dielectric loss caused by oxygen vacancies in titania ceramics
Introduction
TiO2 possesses a relative permittivity (ɛ′) which is highly anisotropic in single crystals, ɛ′ = 89 and 173 perpendicular and parallel to the c-axis, respectively,1 but which averages to 100 in randomly oriented polycrystalline samples. Strong temperature dependence of the dielectric constant results in a high temperature coefficient of resonant frequency (τf ∼ 450 ppm K−1) and although microwave filters have been constructed using TiO2,2, 3 it is generally considered to be unsuitable for this reason. Instead, TiO2 is found as a constituent raw material in many temperature stable dielectric resonator (DR) compositions, e.g. Ba–Ti–O, Ba–RE–Ti–O (RE = Rare Earth), Zr–Sn–Ti–O (ZST), in composites with Al2O3, and doped CaTiO3. One problem of utilising TiO2 as a raw material is that the Ti ion can exist in several valence states. This may lead to the presence of O vacancies (VO) which are considered detrimental to the dielectric loss (tan δ), often expressed as a quality factor, Q, where Q = 1/tan δ.
Typically in TiO2-based ceramics such as BaTiO3,4 “coring” is observed; a term used to describe the dark, oxygen deficient interior of a sintered pellet. In order to combat oxygen loss in the capacitor industry, BaTiO3 is modified with acceptor dopants such as Mn which inhibit the formation of Ti3+5 in accordance with the defect equation,
In rutile reduced in H2 to form TiO1.8 (Ti5O9 with <1% Ti6O11/Ti4O7), VO condense onto specific crystallographic planes which ultimately shear to create new structures, generically referred to as Magnelli phases.6, 7 The space charge potential and the spatial distributions of defects in TiO2 have been calculated and shown to vary with donor or acceptor doping.8 In addition, Templeton et al.9 showed that dense, high purity TiO2 had a high dielectric loss, Qf < 6000 GHz (Qf = Q* resonant frequency in GHz). Furthermore, they demonstrated that doping with a range of divalent and trivalent acceptor cations with ionic radii between 0.5 and 0.95 Å greatly decreased dielectric loss, while other cations had little or no effect.9 The low Qf associated with undoped TiO2 was attributed to the presence of Ti3+ and, as a consequence, VO, whereas the improvement in Q in doped TiO2 was considered to arise from a similar charge compensation mechanism to that shown above.
In this paper, the authors report the effect of 16 further cation dopants in TiO2 which are compared with those reported previously.9 The effects of sintering time and annealing were also investigated and the cores of doped and undoped TiO2 were studied using transmission electron microscopy.
Section snippets
Sample preparation
Doped samples were prepared by adding 0.05 mol% of the dopant to TiO2 (PiKem Ltd., UK—purity details given in Section 3) as a dry powder, and then mixing by dry milling for 12 h using a teflon pot containing Y stabilised zirconia (YSZ) balls. The raw materials used for the dopants were either oxides or carbonates in the case of Na, K, Ba, Sr and Ca dopants, with purities of 99.9% or greater with the exception of Ag which was added as a pure metal flake. Cr was added as both Cr3+ (Cr2O3) and Cr6+
Results and discussion
Undoped TiO2 powder (PiKem, UK) had a range of impurities measured at 50 ppm Sn, 10 ppm Si, 3 ppm Ag, 3 ppm B and <1 ppm Mg.9 Sintering at 1500 °C was required to achieve densities >98% theoretical. However by varying sintering temperature between 1400 °C and 1500 °C, the density could be controlled systematically. As density increased to ∼94% of the theoretical value, Qf steadily increased up to ∼25 000 GHz but with a further increase in density to a maximum of 97%, Qf deteriorated to ∼6000 GHz.9 Rutile
Conclusions
In high purity, undoped TiO2 ceramics, sintered at 1400 °C/2 h, high dielectric losses occurred resulting in low Q values, and a dark core was also observed in these samples. Both of these observations were attributed to VO forming at high temperatures, due to the reduction of Ti4+ to Ti3+. The formation of O vacancies may be suppressed by doping with 0.05% of metal ions with either 2+ or 3+ valence state, provided the ionic radius is between 0.5 and 0.95 Å. This upper limit proved to be discrete,
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