High-temperature dielectric ceramics of the tetragonal tungsten-bronze structure and the role of aerosol deposition in modulating permittivity-temperature relationships

This work explores the development of high temperature dielectrics based on an unfilled tungsten
bronze-structured ceramic, Ca0.28Ba0.72Nb2O6 (CBN28), for potential power electronics applications.
CBN28 exhibited ferroelectric P-E loops, a slightly diffuse permittivity peak around TC at 231 °C, and
high losses above 250 °C due to ionic conductivity (tan δ > 0.2). Subsequent A site substitution of Y3+
for Ca2+/Ba2+ with assumed vacancy addition, in the nominal composition (Ca0.28Ba0.72)1–3w/2YwNb2O6 [0 ≤ w ≤ 0.20], increased permittivity-temperature stability at the single-phase limit, w = 0.04 (ɛr = 1143 ±15 % from 71 to 210 ⁰C), and narrowed P-E loops (Pr at 40 kV cm-1 = 1.5 µC cm-2, whereas in CBN28 Pr = 2.4 µC cm-2). Doping with Y3+ and Ta5+, in the nominal composition (Ca0.28Ba0.72)0.94Y0.04Nb1.8Ta0.2O6, induced permittivity-frequency dependence, lowered the temperature of ɛr-Max (TM) to 13 °C at 1 kHz and 46 °C at 1000 kHz, further improved temperature stability of permittivity (ɛr = 954 ±15 % from -58 to 110 ⁰C) and substantially narrowed P-E loops (Pr at 40 kV cm-1 = ~0.5 µC cm-2). Decreasing Ca content, in the formulation (Ca0.22Ba0.78)0.94Y0.04Nb1.8Ta0.2O6, produced ɛr = 972 ±15 % from -9 to 166 ⁰C and removed a secondary phase.
To develop low-temperature deposition of strained ceramic films, a particle aerosol deposition (PAD)
system was commissioned at Leeds. This produced submicron Al2O3 and CBN28 anchor layer films with
evidence of severe ceramic deformation in the layers by electron microscopy. Subsequent PAD (on a
system in Manchester) of a calcined, high-energy milled and granulated CBN28 powder produced a
thick film (10+ µm) with highly supressed polarisation responses (PMax at 40 kV cm-1 = ~0.3 µC cm-2, εr = ~85). Annealing, up to 750 °C, partially recovered permittivity, retained improved temperature
stability, and induced relaxor-like frequency dependence (εr = ~225 ±15 % from ~20 to ~240 °C). XRD
peak broadening analysis of the CBN film pre- and post- annealing at 750 °C suggested microstrain
reduction from 0.14 to 0.11 % strain in combination with crystallite size increases from ~125 to ~270
nm.