Influence of Artificial Porosity on Piezoelectric Behavior of (K,Na)NbO3 (KNN) Bulk Ceramics
MetadataShow full item record
The influence of artificial porosity on the piezoelectric behaviour of KNN was studied for possible biomedical applications. A "piezoelectric effect" observed in bone is expected to trigger bone growth and healing. Therefore, a piezoelectric material with electrical properties similar to bone may enhance bone in-growth and healing after surgery. Stochiometric KNN powder was mixed with a pore forming agent at different weight ratios. The pore former, either rice and corn was burned off in air, giving very fragile samples. The samples were sintered in reducing atmosphere to obtain high densification. Dense samples with artificial porosities was successfully achieved. As the amount of starch was increased, the pores started to merge into interconnected channels. This is beneficial for biomedical applications since large pores enhance the ingrowth of bone. The most porous samples were obtained with corn starch as pore former. Good piezoelectric properties is associated with high density, but the ceramic implant material should be dense with large interconnected pores. The samples with the highest amount of pore former achieved much higher density than predicted. This was attributed to substantial sample shrinkage during sintering caused by elimination of small-sized pores. Since a bone implant material will be subjected to a mechanical load, the piezoelectric coefficients obtained from the direct method were considered the most relevant. The piezoelectric coefficient for all levels of porosity was found to be the range 68 to 89 pC/N, which is comparable to the literature value of dense KNN. Due to a significant conductivity contribution to the polarization loop, it was necessary with heat treatment of all samples at 120 $^\circ$C before piezoelectric measurements. The poor piezoelectric properties observed before heat treatment was believed to arise from the hygroscopic properties of KNN, causing humidity to be absorbed by the samples.