Effect of nano- and micron-sized K0.5Na0.5NbO3 fillers on the dielectric and piezoelectric properties of PVDF composites
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Polymer nanocrystal composites were fabricated by embedding polyvinylidene fluoride (PVDF) with K0.5Na0.5NbO3 (KNN) nanocrystallites of different volume fraction using the hot-pressing technique. For comparison, PVDF–KNN microcrystal composites of the same compositions were also fabricated which facilitated the studies of the crystallite size (wide range) effect on the dielectric and piezoelectric properties. The structural, morphological, dielectric, and piezoelectric properties of these nano and micro crystal composites were investigated. The incorporation of KNN fillers in PVDF at both nanometer and micron scales above 10 vol% resulted in the formation of polar β-form of PVDF. The room temperature dielectric constant as high as 3273 at 100 Hz was obtained for the PVDF comprising 40 vol% KNN nanocrystallites due to dipole–dipole interactions (as the presence of β-PVDF is prominent), whereas it was only 236 for the PVDF containing the same amount (40 vol%) of micron-sized crystallites of KNN at the same frequency. Various theoretical models were employed to predict the dielectric constants of the PVDF–KNN nano and micro crystal composites. The PVDF comprising 70 vol% micron-sized crystallites of KNN exhibited a d33 value of 35 pC/N, while the nanocrystal composites of PVDF–KNN did not exhibit any piezoelectric response perhaps due to the unrelieved internal stress within each grain, besides the fact that they have less domain walls.
Keywordspolyvinylidene fluoride (PVDF) K0.5Na0.5NbO3 (KNN) composites β-PVDF nanocrystallites
One of the authors, Rajasekhar Bhimireddi, acknowledges the University Grants Commission, Government of India, for the financial support provided through Dr. D. S. Kothari’s post-doctoral fellowship. Also, the authors are thankful to Dr. P. Thomas, Joint Director, CPRI, Bangalore, India, for helping in making dielectric breakdown voltage studies.
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