| Summary: | We show how sintering in different atmospheres affects the structural, microstructural, and functional properties of ~30 μm thick films of K<sub>0.5</sub>Na<sub>0.5</sub>NbO<sub>3</sub> (KNN) modified with 0.38 mol% K<sub>5.4</sub>Cu<sub>1.3</sub>Ta<sub>10</sub>O<sub>29</sub> and 1 mol% CuO. The films were screen printed on platinized alumina substrates and sintered at 1100 °C in oxygen or in air with or without the packing powder (PP). The films have a preferential crystallographic orientation of the monoclinic perovskite phase in the [100] and [−101] directions. Sintering in the presence of PP contributes to obtaining phase-pure films, which is not the case for the films sintered without any PP notwithstanding the sintering atmosphere. The latter group is characterized by a slightly finer grain size, from 0.1 μm to ~2 μm, and lower porosity, ~6% compared with ~13%. Using piezoresponse force microscopy (PFM) and electron backscatter diffraction (EBSD) analysis of oxygen-sintered films, we found that the perovskite grains are composed of multiple domains which are preferentially oriented. Thick films sintered in oxygen exhibit a piezoelectric <i>d</i><sub>33</sub> coefficient of 64 pm/V and an effective thickness coupling coefficient <i>k<sub>t</sub></i> of 43%, as well as very low mechanical losses of less than 0.5%, making them promising candidates for lead-free piezoelectric energy harvesting applications.
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