Table 2. Piezoelectric characteristics of the samples.

sensor is related to the dynamic deformation (not the static force), as predicted from the intrinsic piezoelectric property.

To find the dynamic sensitivity and repeatability of the sensors, we investigated the properties of the sensors under forces with frequencies of 0.05, 0.1, 1, 5, 15 and 20 Hz (figure 8). The results show that the responding signal curves depended on the frequency of the deformation. At low frequency (figures 8(a)–(c)), two sharp response peaks correspond to the applying and releasing of the squarewavefunctioned input loads. The results show identical response and good repeatability. At higher frequencies (figures 8(d)–(f)), the triangle-wavefunctioned load signals can be detected precisely in responding individual peaks. As the loading frequency increases, the sensing signals (opposite peaks) for the applying and releasing of a load become fused with those of the next cycle; however, a one-to-one relationship and a good repeatability remain. The results imply that the sensors have promising applications in various fields.

The loading-and-releasing cycle was repeated dozens of times for each sensor measured. The peaks were measured at peak voltage and then averaged for comparison with samples prepared under varied conditions. The results are shown in table 2.

As shown in table 2, the sensitivity of a sensor is defined as the strength ratio of input signal to output signal. The highest sensitivity of 42.00 mV N−1 was found with samples B and F (identical samples), consistent with the β- phase crystalline contents as shown in figure 5. This indicates that crystalline morphology plays an important role in the piezoelectric properties of the samples.

4. Conclusions

In summary, the present results demonstrate the possibility of using electrospun PVDF fabric to design flexible force sensors without mechanical and poling treatment. By controlling the crystalline phases, sensors with high sensitivity could be achieved. Several factors, including the starting materials, fiber alignments, electrospinning parameters and materials of the electrodes, should be taken into account in designing and fabricating the force sensors.

Acknowledgments

The authors would like to thank PhD student Qing Guo and Professor Steve Shen of University of Washington, for

their technical support in setting up the force sensor testing system. The authors are also grateful to Professor Kap Jin Kim of Kyung Hee University, South Korea for his helpful discussions. This work was supported by National Natural Science Foundation of China and Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Zhejiang Sci-Tech University.

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