Lead-free piezoelectric materials for medical application
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In this work, K0.5Na0.5NbO3, (KNN) and K0.495Na0.495Ca0.005NbO3 thin films for biomedical applications have been prepared from an aqueous precursor solutions with 2.5 or 5.0 mol% of alkali excess. Ca2+ was introduced as dopant to enhance the hygroscopic properties of KNN. The precursor solutions were deposited on SrTiO3 substrates by spin coating with subsequent pyrolysis at 550 degrees celsius and final annealing in air at 700 degrees celsius. X-ray diffraction and scanning electron microscopy revealed that 5 mol% alkali excess is necessary to obtain phase pure thin films with homogeneous microstructure. Nanoindentation was used to investigate the effect of doping with 0.5 mol% Ca2+, on the hygroscopicity of KNN thin films. This was done by looking at the change in mechanical properties after exposure the films to an aqueous solution at 37 degrees celsius for 0 to 14 days. The mechanical properties showed relatively small changes after soaking, and only marginally improved stability of the mechanical properties was observed with Ca2+ doping. Although both KNN and Ca-KNN thin films displayed weak and leaky ferroelectric hysteresis loops, doping with 0.5 mol% Ca2+ subtly improved the saturation polarisation, and slightly enhanced the piezoelectric coefficient (18.6pC/N) compared to pure KNN thin films (17.44 pC/N). The results from this work imply that further improvements of the ferroelectric and piezoelectric properties are necessary. However, the mechanical measurements are promising, and show that KNN thin films are potential candidates for biomedical applications.