Poling Treatment and Piezoelectric Properties of Potassium Niobate Ferroelectric Single Crystals

Piezoelectric properties of potassium niobate (KNbO 3 ) single crystals were investigated as a function of crystallographic orientations, i.e., [110] c of polar direction and [001] c of nonpolar direction. Prior to the piezoelectric measurements, the optimum conditions for a conventional poling method were investigated, and it was found that the optimum conditions for a conventional poling method were as follows, i.e., (a) high temperature above 100°C, (b) long soaking time over 1 h, (c) slow heating and cooling rates below 1°C/min, and (d) lower poling current below 1 µA. However, fully poled KNbO 3 crystals were not obtained using the conventional poling method. Thus, to achieve fully poled KNbO 3 single crystals, a new 2-step poling method was proposed. This method is composed of 2 poling stages, i.e., the 1st step for non-180° domain switchings at higher temperatures under a low DC bias field, and the 2nd step for 180° domain switchings at lower temperatures under a high DC bias field. Using the 2-step poling method, KNbO 3 crystals were successfully poled, and then, their piezoelectric properties of k 31 modes were measured using a resonance method. The [110] c poled KNbO 3 crystals exhibited electromechanical coupling factor k 31 of 28.9% and piezoelectric constant d 31 of 18.4 pC/N while the [001] c poled KNbO 3 crystals exhibited k 31 of 31.2% and d 31 of 51.7 pC/N. The piezoelectric constant d 31 along nonpolar [001] c direction was 2.8 times higher than that along polar [110] c direction.