Poling treatment and piezoelectric properties of potassium niobate ferroelectric single crystals

Piezoelectric properties of potassium niobate (KNbO3) 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 degreesC, (b) long soaking time over 1 h, (c) slow heating and cooling rates below 1 degreesC/min, and d) lower poling current below I muA. However, fully poled KNbO3 crystals were not obtained using the conventional poling method. Thus, to achieve fully poled KNbO3 single crystals, a new 2-step, poling method was proposed. This method is composed of 2 poling stages, i.e., the I st step for non- 180 degrees domain switchings at higher temperatures under a low DC bias field, and the 2nd step for 180 degrees domain switchings at lower temperatures under a high DC bias field. Using the 2-step poling method, KNbO3 crystals were successfully poled, and then, their piezoelectric properties of k(31) modes were measured using a resonance method. The [110](c) poled KNbO3 crystals exhibited electromechanical coupling factor k(31) of 29.9% and piezoelectric constant d(31) of 18.4 pC/N while the [001](c) poled KNbO3 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.