The direct flexoelectric effect observed in polyvinylidene fluoride films
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Piezoelectricity in Polyvinylidene Fluoride (PVDF) was observed long back and significant development has been made since its discovery. The theory on this property has predominantly revolved about the polymorphism and unique structure of PVDF. Of the four structures PVDF can be fabricated namely α, β, γ and δ, apart from the α phase the other 3 have piezoelectricity property in them. This thesis concentrates on the β phase PVDF as they have the highest piezoelectric effect present due to non cancellation of dipoles. In the past, research in the β phase PVDF was conducted in stretched films. This thesis concentrates on the film properties in the unstretched condition. Flexoelectricty is a property which was first observed in 1969 in crystalline dielectric materials. The extension of this phenomenon in PVDF films is discussed in the thesis. Flexoelectricity is more dominant in the micro and nano scale and it depends on the strain gradients induced in a material thus generating a polarization. Hence this property is present in all dielectric materials when subjected to strain gradients unlike piezoelectricity which corresponds to only a particular class of materials. The films are fabricated by solution polymerization and phase characterization is confirmed by x-ray diffraction. The experimental verification of flexoelectricty and piezoelectricity, and the calculation of coefficients are discussed in unstretched condition of films. The Young’s Modulus for these films is also calculated experimentally. This value is necessary to compute the piezoelectric coefficient. Analyzing the result we notice that the negative value of the flexoelectric coefficient corresponds to the trend seen in paraelectric BST crystals. The hypothesis is that the randomness in the molecular arrangement in unstretched films is synonymous to paraelectric BST crystals. Based on the coefficients computed the flexoelectric coefficient seems to be more dominant in the films. This result seems interesting and encouraging and thus giving us an opportunity to utilize the shape dependent characteristics in sensor applications.