Effect of K0.5Na0.5NbO3 on Properties at and off the Morphotropic Phase Boundary in Bi0.5Na0.5TiO3--Bi0.5K0.5TiO3 Ceramics

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Lead-free ($1-y$)(Bi1/2Na1/2TiO3--$x$Bi1/2K1/2TiO3)--$y$K0.5Na0.5NbO3 [($1-y$)(BNT--$x$BKT)--$y$KNN] ($0.1\leq x\leq 0.6$ and $0\leq y\leq 0.05$) ceramics were prepared with compositions both at as well as off the morphotropic phase boundary (MPB) of $x=0.2$ and $y=0$. It was found that KNN addition destabilizes a ferroelectric order in BNT--BKT at zero electric-field, resulting in low remanent polarization, remanent strain and effective small signal $d_{\text{33,eff}}$ [23 pC/N for $(x,y)=(0.2,0.05)$, 60 pC/N for $(0.2, 0.02)$], while it does not affect the maximum polarisation and strain at high fields and thus enhances large signal piezoelectric coefficient, $d_{33}{}^{*}$ [200 pm/V for $(0.2,0.05)$, 217 pm/V for $(0.2,0.02)$]. This effect of KNN was found to be independent from the MPB and thus equally high $d_{33}{}^{*}$ values were obtained for compositions at and far off the MPB. Thus, adding KNN to BNT--BKT removes the restriction to remain at exactly MPB position and provides more flexibility while offering equal or even improved strain.
2011-05-25