Colossal Dielectric Permittivity in (Nb+Al) Codoped Rutile TiO₂ Ceramics: Compositional Gradient and Local Structure

Abstract

Nb+Al) codoped rutile TiO 2 ceramics with nominal composition Ti 4+ 0.995 Nb 5+ 0.005y Al 3+ 0.005z O 2 , z = (4−5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and Ti 4+ 0.90 Nb 5+ 0.05 Al 3+ 0.05 O 2 have been synthesized. The resultant samples in ceramic pellet form exhibit a colossal dielectric permittivity (>∼10 4 ) with an acceptably low dielectric loss (∼10 −1 ) after optimization of the processing conditions. It is found that a conventional surface barrier layer capacitor (SBLC) effect, while it contributes significantly to the observed colossal permittivity, is not the dominant effect. Rather, there exists a subtle chemical compositional gradient inward from the pellet surface, involving the concentration of Ti 3+ cations gradually increasing from zero at the surface without the introduction of any charge compensating oxygen vacancies. Instead, well-defined G r ± 1 / 3 [011]* satellite reflections with the modulation wave-vector q = 1 / 3 [011] r * and sharp diffuse streaking running along the G r ± ε[011]* direction from electron diffraction suggest that the induced additional metal ions appear to be digested by a locally intergrown, intermediate, metal ion rich structure. This gradient in local chemical composition exists on a scale up to ∼ submillimeters, significantly affecting the overall dielectric properties. This work suggests that such a controllable surface compositional gradient is an alternative method to tailor the desired dielectric performance.

Dielectric properties of Y and Nb co-doped TiO2 ceramics

“…Dielectric materials with colossal permittivity (CP) attract vast research interests due to their great potential applications in wide fields such as device miniaturization and energy storage 1–4 . As typical CP materials, the systems of BaTiO 3 , CaCu 3 Ti 4 O 12 (CCTO), and doped-NiO have been widely explored 5–12 .…”

“…Previous researches proposed AB co-doped rutile TiO 2 ceramics (A was introduced as an electron-acceptor, such as In 3+ ; B was introduced as an electron-donor, such as Nb 5+ ) as a new sequence of CP materials 1, 2, 4, 13, 14 . The existence of a giant dielectric constant (ε r ~ 6 × 10 4 ) along with a low dielectric loss (tan δ < 0.02) at room temperature over varied frequency from 10 2 to 10 6 Hz was found in their research 1 .…”

“…There are other mechanisms used to explain the overall dielectric response in TiO 2 -based ceramics, including electron hopping 18 and non-Ohmic sample-electrode contact 3 . In addition, following researches introduced more similar types of ceramics such as Ga+Nb and Al+Nb co-doped TiO 2 2–4, 14 . However, both of the Ga+Nb and Al+Nb co-doped rutile TiO 2 ceramics were also reported to show poor dielectric properties compared with the In+Nb co-doped in TiO 2 ceramics 1, 3, 4, 13, 16, 17, 19–21 .…”

“…In addition, following researches introduced more similar types of ceramics such as Ga+Nb and Al+Nb co-doped TiO 2 2–4, 14 . However, both of the Ga+Nb and Al+Nb co-doped rutile TiO 2 ceramics were also reported to show poor dielectric properties compared with the In+Nb co-doped in TiO 2 ceramics 1, 3, 4, 13, 16, 17, 19–21 . Thus, developing more acceptor/donor for the AB co-doped rutile TiO 2 ceramics is essential for pursuing high quality colossal permittivity ceramic materials.…”

Colossal permittivity behavior and its origin in rutile (Mg1/3Ta2/3)xTi1-xO2

“…CaCu 3 Ti 4 O 12 (CCTO) and La 2x Sr x NiO 4 (LSNO) 5, 6 . Such a new dielectric polarization mechanism inspires new CP candidate materials to be potentially used in capacitors and permit further scaling advances in electronic devices 7, 8 , though other issues are still required to be considered, such as the insulating resistance (conventional ferroelectric/relaxor dielectric capacitors generally have an insulating resistivity over 1 × 10 11 ~1 × 10 12 Ω·cm) 1, 3, 9–12 . Actually, the formation of EPDD in the host material is complicated since the formation of defect states is generally strongly related to the dopant ions, and to the preparation approach and conditions.…”