抄録
The purpose of this paper is to investigate the piezoelectric anisotropy of wood. It had been considered that the cellulose micelle in wood are oriented in the same direction of fiber axis and consequently wood belongs to the symmetry class ∞2. And so d14 and d25 had been measured as the piezoelectric moduli of wood. However, we have found that there is a difference in the absolute value between d14 and d25 determined experimentally; that is, |d14|>|d25|. Therefore, it seems that wood belongs not to the symmetry class ∞2 but to the symmetry class 222. The piezoelectric tensor of the class 222 has three independent piezoelectric moduli d14, d25 and d36.
In this study, we measured the piezoelectric moduli d14, d25 and d36 as the symmetry class 222, and found that the modulus of d36 exists apparently in our measurement, although it is small compared with d14 and d25. Thus, it is reasonable to consider d14, d25 and d36 as the piezoelectric moduli for wood. The piezoelectric moduli along different directions were also measured by rotating measuring axes, and they were expressed by the components of d14, d25 and d36.
The causes for the existence of d36 and the difference between d14 and d25 were also examined. The polarization associated with d36 is considered to arise from the polarization of ray structure, the components of d14 and d25, and d36cell of cellulose crystallites.
The difference between d14 and d25 agrees with that of the moduli of rigidity GLT and GLR, suggesting that the magnitude of piezoelectric polarization depends on that of shearing strain in cellulose crystallites.
A correlation is found between the magnitudes of d36 and |d25/d14|, and, therefore, the degree of anisotropy of wood can be characterized by either the ratio of |d25/d14| or the magnitude of d36 from the viewpoint of piezoelectricity in wood.
