Since the mid-20th century, barium titanate has been recognized as the prototype ferroelectric material. The report of Liu and Ren (2009) on exceptional piezoelectric properties of a barium zirconate titanate barium calcium titanate (0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3, BZT-BCT) ceramic has garnered interest in this material as one of the potential environment-friendly replacements for lead-based piezoelectric ceramics. Thin films comply with the general trend of miniaturization of electronic components and devices, and Chemical Solution Deposition (CSD) is a cost-effective method that, in principle, enables control of the film’s chemical composition. However, the functional response of solution-derived films is strongly influenced by different processing-related characteristics, such as the film thickness, microstructure (grain size and shape, porosity), residual stress, etc. In the case of undoped or chemically modified barium titanate, the crystallization from the amorphous phase occurs predominantly via homogenous nucleation, resulting in films consisting of equiaxed grains. A predominantly columnar microstructure may be engineered by repeated deposition and annealing thin, typically a few nm thick, layers until the final thickness is reached, c.f. Hoffmann et al. (1999). In the CSD of BZT-BCT, when compared to barium titanate, the presence of four metal cations influences the stability of the precursor solution, the decomposition of the precursor, crystallization of the target phase and grain growth, which is further reflected in the functional properties of the films. The lecture examines the processing-microstructure-properties triangle for the case of BZT-BCT thin films.

Barium zirconate titanate - barium calcium titanate solid solution 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) is one of the promising lead-free ferroelectric materials for piezoelectric applications at temperatures up to ~80 °C due to its high piezoelectric coefficients. Miniature piezoelectric devices such as microelectromechanical systems contain nanoscale piezoelectric elements in thin-film form. Chemical solution deposition (CSD) enables control of film characteristics such as grain size, phase purity, and chemical homogeneity, which influence the functional properties. However, BZT-BCT thin films prepared by conventional acetic acid-based CSD are often characterized by porous, fine-grained microstructure, cracks and poor functional properties. Our preliminary investigation revealed that BZT-BCT coating acetic acid-containing solutions were stable for only a few days. We aimed to control the stability of the coating solution, prepare the columnar microstructure, and measure the macroscopic functional properties of the films. The modification of the precursor chemistry included the combination of ethylene glycol (EG) and ethanol (EtOH) as solvents for alkaline earth acetates and transition metal alkoxides, respectively, which contributed to increasing the stability of the coating solutions for several months while manganese doping (1 mol%) contributed to reducing the leakage current and enabling the macroscopic electric field-dependent properties measurements. Upon multi-step rapid thermal annealing at 850 °C the Mn-doped BZT-BCT film on a platinized silicon substrate (Pt/Si) with a thickness of 120 nm consisted of columnar grains and exhibited good low- and high-field properties: dielectric permittivity of about 620; maximum polarization of 32 µC/cm2, a maximum strain of 0.18 %, and d33 of 20 pm/V. Increasing the film thickness on Pt/Si resulted in the appearance of intergranular cracks attributed to the thermal expansion mismatch. Mn-doped BZT-BCT film was deposited on a platinized sapphire substrate (Pt/Sapp) to reduce the thermal stress. A 340 nm thick crack-free film with a columnar microstructure was obtained. Its dielectric permittivity, piezoelectric, and energy storage properties were improved compared to those of a 120 nm BZT-BCT film on Pt/Si, the respective values being about 930, 47 µC/cm2, 0.77 %, and 40 pm/V. CSD-derived BZT-BCT films show promising properties for application in microelectromechanical systems.

The use of PZT has continued despite the restriction of hazardous substances (RoHS) directives being passed by the European union (EU) to limit the use of hazardous materials – such as lead (PZT) – in electronic equipment. This is influenced by a lack of available substitutes that can be implemented and scaled in a reasonable time to eliminate the use of PZT (RoHS Annex III).

The lead-free alkali-niobate, potassium sodium niobate (KxNa1-xNbO3, KNN), is a potential replacement for PZT, since it can exhibit equivalent, if not better, piezoelectric properties without compromise at temperatures >120 °C. The challenges in the synthesis of KNN, however, have hindered its use as an alternative. A notable challenge includes difficulty in achieving the 50% K+ and Na+ occupancy required to make stochiometric KNN (i.e., K0.5Na0.5NbO3) and a lack of control in the formation of phase boundaries. Therefore, developing a reproducible method for synthesising stoichiometric KNN, bodes well for the implementing KNN as a commercial piezoceramic.

Most syntheses of KNN continue to be based on procedures which encompass either solid-state, hydrothermal, spray pyrolysis or sol-gel techniques. The solid-state syntheses may include reacting Na2CO3, K2CO3 and Nb2O5 in a molten salt reaction to yield a precursor oxide that can be sintered to obtain KNN. Notably, the use of a molten salt is to overcome the inertness of Nb2O5. The hydrothermal route circumvents the use of molten salt via the use of alkali hydroxides and Nb2O5, to similarly yield precursors to KNN. However, the solid-state and hydrothermal routes still require prolonged heating to yield KNN. Likewise, a molten salt reaction is challenging to control.

Herein a simple and robust method for synthesis of the lead-free piezoceramic material KNN has been developed via an aqueous route. Stochiometric KNN (K0.5Na0.5NbO3) was prepared by combining alkali-nitrate solutions (NaNO3 and KNO3) with the water-soluble niobium precursor hexaniobate ([HxNb6O19]8-x, Nb6), followed by sintering at elevated temperatures for at least one hour. Limitations associated with stochiometric control and selectivity, that are inherent to conventional methods of solid-state, sol-gel and hydrothermal based KNN synthesis, can be obviated with the use of this water-soluble niobium precursor.

Barium titanate based lead-free ferroelectrics are promising materials for a wide range of applications, such as energy storage devices, tuneable microwave capacitors or electro-mechanical transducers. In this work, we investigate the relaxor ferroelectric properties of (1-x)Ba0.7Ca0.3TiO3–xBaZr0.2Ti0.8O3 (BCT-BZT) thin films grown by pulsed laser deposition. As the functional properties of these materials are extremely sensitive to the cationic composition, we use a high throughput combinatorial chemistry approach to fabricate composition gradient films. We focus on compositions with x=0.5 known to provide high piezoelectric coefficients (~ 490 pC/N) and soft ferroelectric behaviours with Curie temperatures near 100°C in ceramics, making them promising as an alternative to PZT. In particular, we show that Ce doping (from 0% to 0.2%) modifies the lattice parameter, reduces the remnant polarisation, and shifts the maximum permittivity temperature (Tmax) value. For compositions with Tmax close to room temperature, we measure an enhanced room-temperature d33 piezoelectric coefficient. We then use dielectric spectroscopy coupled with the Rayleigh analysis to identify the contribution of domain wall motion to the ferroelectric response of the films, as a function of temperature. We observe residual ferroelectricity above Tmax, similar to what has been observed for (Ba,Sr)TiO3. In addition, measurement of the third harmonic dielectric response reveals the evolution from a conventional ferroelectric character to a relaxor one with increasing temperature. A peculiar phase angle of the third harmonic, which consists of −180◦ → −225◦ → +45◦ → 0◦, instead of the −180◦ → −90◦ → 0◦ found for relaxors, is also observed in several samples and tentatively attributed to changes in the correlations between polar nano - regions.