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Abstract

Barium zirconate titanate-barium calcium titanate 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) ceramic exhibits a high piezoelectric response in the vicinity of room temperature. It is an environment-friendly alternative to Pb(Zr1-xTix)O3 (PZT), the most used ceramic material in piezoelectric applications. Thin films meet the requirements related to the miniaturization of electronic components such as in energy harvesting or storage applications. Chemical solution deposition (CSD) of thin films steps out among versatile physical or chemical vapour deposition routes due to the easy adaptation of the chemical composition via the solution chemistry.
However, challenges related to CSD of BZT-BCT thin films prepared from the conventional carboxylic-acid-based synthetic route include the instability of the coating solution, fine-grained and porous microstructure, cracks and consequently non-optimal functional properties. To resolve the challenges and establish the microstructure-properties relation in BZT-BCT thin films, we developed an alternative synthetic approach. The new procedure uses ethylene glycol (EG) and ethanol (EtOH) as solvents for alkaline-earth acetates and transition-metal alkoxides, respectively. Barium titanate (BaTiO3, BT), the prototype ferroelectric, was selected as a reference material. EG-EtOH-based BT coating solution was stable for more than one year. Crack-free BT thin films with columnar microstructure and thickness of about 130 nm on platinized silicon substrates (Pt/Si) exhibit dielectric permittivity of 600, measured at room temperature (R.T.) and 1 kHz, maximum polarization (Pmax) of 26 μC∙cm-2 and survive electric fields of 2.4 MV∙cm-1.
The EG-EtOH synthesis was transferred to the BZT-BCT system. Achieving chemically homogeneous BZT-BCT thin films with columnar microstructure and without interaction with Pt/Si substrates required diluting the coating solution to 0.1 M and increasing the annealing temperature to 850 °C.
Manganese doping (1 mole %) effectively reduced the leakage and enabled measurements of macroscopic functional properties. About 120 nm thick BZT-BCT films doped with manganese on Pt/Si substrates exhibit dielectric permittivity of 670 measured at RT and 1 kHz, Pmax of 32 μC∙cm-2 at 1.15 MV∙cm-1, piezoelectric d33 coefficient of 20 pm∙V-1 and strain (S) of 0.18 % measured using a double-beam laser interferometer. The recoverable energy (Urec) and energy storage efficiency (η) of 10 J∙cm-3 and 69 %, respectively, are obtained. The film thickness exceeding ≈120 nm on Pt/Si resulted in the evolution of intergranular cracks due to the thermal expansion mismatch between the film and silicon substrate. Using platinized sapphire (Pt/Sapp) substrates reduced the thermal stress in the films due to the smaller difference in thermal expansion coefficients between BZT-BCT and sapphire. Up to 680 nm thick crack-free BZT-BCT films doped with manganese on Pt/Sapp substrates with columnar microstructure were obtained upon multistep annealing at 850 °C. The 340 nm thick films are characterized by dielectric permittivity of 930 at 1 kHz, d33 of ~40 pm∙V-1, S of ~ 0.77 %, and Pmax ~ 47 μC∙cm-2 at a maximum electric field of about 3.5 MV∙cm-1. The energy storage efficiency of 89 % and Urec ~ 46 J∙cm-3 make BZT-BCT films a promising option for energy storage applications.