Converse Piezoelectric Effect in Cellulose I Revealed by Wide-Angle X-ray Diffraction

The converse piezoelectric effect in cellulose I was studied by exposing thin pine wood slices to an electric field. Macroscopically, a strong extension of wood was observed in its transverse anatomical direction (grain angle 90), perpendicular to the direction of the electric field. The same effect, albeit to a lesser extent, was observed for specimens with a 45 degrees grain angle, whereas no measurable dimensional change was observed for specimens with grain oriented parallel to the testing direction (0 degrees grain angle). The measured extension in the transverse direction was proportional to the intensity of the applied electric field and amounted to 0.0278% on average at a field intensity of 1 MV m(-1) which results in a piezoelectric charge constant of 278 pm V-1. At the nanoscale, changes in the cellulose crystallites due to the applied electric field were studied by means of wide-angle X-ray diffraction using the same specimens as in macroscopic experiments. Significant radial shifts of the scattering intensity peak attributed to the cellulose 200 crystallographic plane toward smaller scattering angles were observed, while the electric field was applied. These peak shifts were attributed to an increase in the spacing of the 200 crystallographic planes of cellulose I. At an electric field intensity of I MV m(-1), a crystallite strain epsilon(perpendicular to 200) normal to the 200 reflection plane of 0.2% was estimated from Bragg's law.