New piezoelectric moduli of wood: d 31 and d 32

This article reports the piezoelectric moduli of wood d 31, d 32, and d 36. The piezoelectric moduli of wood d 31 and d 32 have not been previously reported, although there has been much research on the d 14 and d 25 moduli of wood. The moduli d 31, d 32, and d 36 were measured carefully because their absolute values were considerably smaller than those of d 14 and d 25. For Softwoods, d 36 values were mostly negative, whereas the values for hardwoods had either positive or negative values. The other moduli, d 31 and d 32, were a mixture of positive and negative values in softwoods and hardwoods. The existence of d 31 and d 32 suggests the presence of an electrical polarity of the cellulose crystal in the fiber direction of the wood. The polarities of d 31 and d 32 became clear from wood in the outer part of the trunk, where the crystallinity of cellulose is large and the alignment of the crystals becomes parallel to the fiber direction.