Influence of PVP on the Thickness of Ferroelectric (Na,K)NbO3 Film by Sol-Gel

Abstract

(Na, K) <TEX>$NbO_3$</TEX> thick film was successfully achieved using a sol-gel coating process with the addition of polyvinylpyrrolidone (PVP) to a metal alkoxide solution. The transparent coating solution, mixed with Nb:PVP = 1:1 in a molar ration, was synthesized by evaporating the solvent to over 62.5 wt%. Additive PVP increased the viscosity of the solution so that the coating thickness could be enhanced. The thickness of the (Na, K) <TEX>$NbO_3$</TEX> film assisted by PVP was ca. 320 nm at the time of deposition; this value is four times thicker than that of the sample fabricated without PVP. Also, due to PVP binding with the OH groups of the metal alkoxide, the condensation reaction in the film was suppressed. The crystalline size of the (Na, K) <TEX>$NbO_3$</TEX> films assisted by PVP was ca. 15 nm smaller than that of the film fabricated without PVP. After the sintering process at <TEX>$700^{\circ}C$</TEX>, the (Na, K) <TEX>$NbO_3$</TEX> films were mainly composed of randomly oriented (Na, K) <TEX>$NbO_3$</TEX> phase of perovskite crystal structure, including a somewhat secondary phase of <TEX>$K_2Nb_4O_{11}$</TEX>. However, by adding PVP, the content of the secondary phase became quite smaller than that of the sample without PVP. It was thought that the addition of PVP might have the effect of restraining the loss of potassium and that PVP could hold metalloxane by strong hydrogen bonding before complete decomposition. Therefore, the film thickness of the (Na, K) <TEX>$NbO_3$</TEX> films could be considerably advanced and made more crack-free by the addition of PVP.