A Piezoelectric Flexible Sensor Based on Mild Electric Contact Poling Cellulose Nanocrystals Film Material

Nanocellulose has been used as a promising material in electronic devices for its great advantages of low cost, wide source, renewability, and biodegradability. However, the low piezoelectric coefficient of cellulose material (only the shear piezoelectric coefficient <inline-formula> <tex-math notation="LaTeX">${d} _{{14}}$ </tex-math></inline-formula> = 0.1 pC/N, lack of longitudinal piezoelectric coefficient <inline-formula> <tex-math notation="LaTeX">${d} _{{33}}$ </tex-math></inline-formula>) limits its application in piezoelectric devices. In this article, a piezoelectric flexible sensor is made based on 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose nanocrystals (TOCNCs) film with excellent tensile strength and ease of manufacture. First, TOCNC films are generated from coniferous wood paddles using the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation process. Then, a mild electric contact poling method is used to polarize the TOCNC films to longitudinal piezoelectric coefficient <inline-formula> <tex-math notation="LaTeX">${d} _{{33}}$ </tex-math></inline-formula>. The TOCNC films are converted into piezoelectric sensors by a straightforward manufacturing procedure. Also, a lightweight charge preamplifier is used to increase the output capability of the TOCNC sensors. The tensile strength of TOCNC films measured by mechanical tensile test is 127 MPa, which is 141.9% more than that of polyvinylidene fluoride/cellulose nanocrystals (PVDF) films. The <inline-formula> <tex-math notation="LaTeX">${d} _{{33}}$ </tex-math></inline-formula> sensitivity of TOCNC films after the mild electric contact poling is measured to be 2.08 ± 0.45 pC/N. Finally, combined with the lightweight charge preamplifier, the experimental platforms are built to test the voltage output of the sensor subjected to static forces and vibration. The experimental results show that the TOCNC sensor has good detection capability and stability.