Maeda, Takafumi, Norihito Takiguchi, Takeshi Morita, Mutsuo Ishikawa, and Tobias Hemsel. “(K,Na)NbO3 Lead-Free Piezoelectric Ceramics Synthesized from Hydrothermal Powders.”https://doi.org/10.1016/j.matlet.2009.10.012__Materials Letters 64, no. 2 (2010): 125–28.https://doi.org/10.1016/j.matlet.2009.10.012 .