The material systems investigated were BaTiO3 (BT), (Ba,Ca)(Zr,Ti)O3 (BCZT) and (K,Na)NbO3 (KNN). A wide range of microstructures were produced and their impact on crucial parameters such as piezoelectric performance was determined. Biocompatibility and long-term integrity in liquid environments were also investigated.
The final microstructure and the performance of a piezoelectric ceramic depends on the grain size of the starting materials. Different processing routes were chosen to produce ceramic powders with varying grain sizes. Larger grains were achieved with solid state synthesis, while powders with small grains were produced using sol-gel synthesis and spray pyrolysis.
To achieve samples with a dense ceramic matrix and controlled porosity, the sacrificial template method was chosen. Mixtures of ceramic powder and pore former were pressed into pellets. The starch was burned off, leaving a defined pore structure behind. The piezoelectric properties scaled with the degree of porosity showing minimum values for the most porous compositions. The absolute values depend on pore shape and size, which are determined by the pore former used. Small and irregular shaped grains of the pore former lead to lower piezoelectric coefficient, whereas larger, spherical grains lead to better performance. Even though the values of the piezoelectric coefficient decreased for all materials with increasing porosity, the obtained values always exceeded that of cortical bone.
As the materials are intended to be used as implant components, their stability and reliability is important. To simulate in-vivo conditions, the ceramics were soaked in NaCl-solution. Distinct differences were found for the different material classes. While BT-based materials appeared to be very stable during the soaking, the KNN-samples tend to dissolve. This dissolution process varied from a few days up to several weeks. The mechanism leading to this controllable solubility of KNN most likely depends on the exact phase composition. To control the solubility and maintain the piezoelectric output, KNN was doped with calcium, titanium and zirconium, which was found to stabilize the piezoelectric output after soaking compared to undoped samples.
To determine the biocompatibility of the ceramic materials, cell tests were conducted on BT-based materials in collaboration with the University clinic in Erlangen/Germany. Cell proliferation and viability was found to be higher on the ceramic materials compared to a standard. This makes this material very promising for the usage in biomedical applications.
