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.
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