Thermal treatment of aerosol-deposited barium titanate films for piezoelectric applications

Aerosol deposition is a unique method that enables the rapid deposition of thick dense ceramic films at room temperature. There is a vast potential for various applications due to the possibility of combining piezoelectric ceramics with temperature-sensitive substrates such as oxidizing metals, polymers, or biocompatible materials. The deposition at room temperature is enabled by the unique deposition mechanism based on the kinetic impact of accelerated particles. However, this mechanism also leads to the occurrence of defects, residual compressive stress, and a nanocrystalline microstructure. In this work, it is discussed how thermal annealing can impact deposition-induced properties and how the electromechanical response of a material is influenced. As-deposited films exhibit an average grain size of less than 100 nm. In this context, the impact of grain size is investigated separately through a comparative study on polycrystalline Ba~0.85~Ca~0.15~Zr~0.1~Ti~0.9~O~3~, which is a promising lead-free piezoelectric material. Samples were prepared using a combination of hydrothermal synthesis and spark plasma sintering to reach a grain size range of 100 nm to 10 µm. Macroscopic ferroelectric measurements reveal a transition from a nearly linear dielectric to a ferroelectric response with increasing grain size. Furthermore, in situ electric field-dependent synchrotron X-ray diffraction shows an increase of extrinsic as well as intrinsic contributions to the piezoelectric response. The properties of Ba~0.85~Ca~0.15~Zr~0.1~Ti~0.9~O~3~ proved to be especially reliant on grain size, while appearing crystallographic phases are difficult to define; therefore, following work focused on basic research with BaTiO~3~ films on stainless steel substrates as a well-known model material system. To fully recover the ferroelectric behavior of aerosol-deposited films, grain growth has to be enabled, i.e., for BaTiO~3~ films, post-deposition thermal treatment at temperatures above 750 °C is required. However, even annealing at temperatures below the anticipated onset of grain growth improved the electromechanical response of as-deposited films from linear dielectric closer to ferroelectric behavior. In situ temperature-dependent dielectric, optical, and microstructural measurements reveal the mechanism behind the shift during annealing, even without significant grain growth. Above approximately 250 °C, the deposition-induced residual compressive stress transitions to tensile stress due to a local recombination of defects. The decrease in defects is also indirectly observed with transmission electron microscopy images taken before and after annealing, as well as in the shrinkage of free-standing films during annealing. The shrinkage or volume reduction is proposed to be directly caused by the observed defect recombination at grain boundaries. With increasing temperature, defect recombination saturates, as evidenced by a decelerating decrease in microstrain during in situ temperature-dependent X-ray diffraction experiments up to 700 °C. In contrast, the crystallite size, which is correlated to the grain size, increases more rapidly with higher temperatures, further indicating that significant grain growth occurs at higher annealing temperatures. Additionally, higher annealing temperatures lead to a proposed adjustment of the film to the substrate's strain field, introducing additional thermal stress due to the mismatch in the coefficient of thermal expansion between the film and the substrate. Therefore, higher annealing temperatures do not necessarily result in the lowest compressive residual stress. The high-temperature annealing required to enable grain growth and fully recover electromechanical properties contradicts the primary advantage of aerosol deposition, which is the ability to deposit at room temperature. To retain the ability to use temperature-sensitive substrates, flash lamp annealing is proposed as a selective annealing method that heats the film surface to a higher temperature than the substrate. However, while X-ray diffraction indicates that temperatures of approximately 750 °C are reached at the film surface, the thermal gradient and the rapid annealing speed can, depending on the film thickness, lead to surface cracks. These cracks result in high leakage currents, and further research is required to circumvent film cracking. It is proposed that pre-annealing can help to reduce the appearance of tensile stress and, thereby, cracks, as the film shrinkage described above no longer occurs during the flash lamp annealing process. Overall, aerosol deposition remains a promising technique if the aforementioned challenges are thoroughly addressed.