Anomalous ferroelectricity in P(VDF₇₀-TrFE₃₀)

ABSTRACT

The ferroelectric phase transition of copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) is well known and related to conformational changes in the polymer chain. Contrary to the expected paraelectric behaviour in the high temperature phase a pyroelectric investigation of the phase transition in the range from 0°C to 130°C combined with X-ray diffraction indicate the copolymer as ferroelectric when prepared and polarised in the high-temperature phase. Based on this finding the orthorhombic space-group Fmm2 is proposed for the polar high-temperature phase. Above the Curie temperature the material exhibits pyroelectricity with inverted sign, whose origin is interpreted as flexoelectricity.

KEYWORDS:

• Pyroelectricity
• ferroelectricity
• phase transition
• copolymer
• vinylidene fluoride
• trifluoroethylene
• X-ray diffraction
• flexoelectricity
• differential scanning calorimetry

Acknowledgments

The authors thank C. Irmer and P. Zill for their supporting the construction of the pyroelectric measurement setup. Part of this work has been performed within the joint research project “CryPhysConcept – Mit Kristallphysik zum Zukunftskonzept elektrochemischer Energiespeicher” (03EK3029A), which was financially supported by the Federal Ministry of Education and Research (BMBF). Part of this work has been done within the PyroConvert junior research group (100109976) that was financially supported by the European Union (European Regional Development Fund) and the Ministry of Science and Art of Saxony (SMWK). Part of this work has been performed in the research project “Nutzung von Abwärme zur Erzeugung von Wasserstoff und Elektrizität mit pyroelektrischen Oxiden” (100245339) that is financially supported by the Sächsische Aufbaubank (SAB). Part of this work has been performed within the HGF-funded Virtual Institute MEMRIOX Memory Effects in Resistive Ion-beam Modifed Oxides (VH-VI-422).

Notes

¹ In the LT phase by heating from room temperature, in the HT phase by cooling from above T_C.

² Additionally a linear offset current is included in the part-wise fit of I_AC to accommodate the shifting background in the vicinity of the phase transition.

³ Otherwise the measured pyroelectric coefficient is an effective coefficient that includes ohmic losses[52].

⁴ Temperatures interpolated, if cited source does not contain data on 70/30 copolymer.

⁵ In the literature typically set as Cm2m.

⁶ Sample to sample variation is similarly large, cycle to cycle variation about half as big.

⁷ See points of inflexion in the p(T) traces in the marked temperature range in Fig. 9 indicating the temperature windows of coexistence of LT and HT phase obtained from XRD.

⁸ Usually set as Cm2m.