Polymer Pyroelectric Sensors: PVDF/PMMA Blend Thin Film . B.Charlot, S.Gauthier, A.Garraud, P.Combette and A. Giani IES CNRS Université Montpellier II. E-mail : charlot@ies.univ-montp2.fr Abstract This paper will present a complete manufacturing process for obtaining pyroelectric thin film sensors composed of a blend of PVDF and PMMA polymers. These sensors comprise a stack of metallic (Ti/Pt) electrodes around an active pyroelectric layer and are able to detect a temperature variation through the pyroelectric effect. Deposition is achieved with solution using a spin-coating and hot plate drying method. Addition of PMMA is a technique for promoting the crystallization of PVDF in the β phase, with one of the PVDF polymer chain conformations producing a ferroelectric behaviour. Analysis of the role of the solvent evaporation rate has been carried out with FTIR and indicates that low temperature evaporation (below 70°C) leads to the presence of β phase in the material. Polarization curve measurement also indicates the ferroelectric behaviour of deposited layers. Finally a thermal transient response indicates a pyroelectric coefficient of 20 µC.m -2 .K -1 which is close to the bulk material value (27 µC.m -2 .K -1 ). 1. Introduction Ferroelectrics are materials that show a non linearity and a hysteresis of polarisation against the electric field curve. Internal polarisation is the result of an alignment of electric dipoles in the material with the application of an electric field, it therefore appears only in polar materials. Ferroelectric materials thus have the ability to be polarised and to show a permanent polarisation with no electric field. A surface charge density appears at the surface of the material to counterbalance the permanent polarisation. The ferroelectric behaviour leads to piezoelectricity and pyroelectricity. Piezoelectricity is the variation of the permanent polarisation with an applied mechanical strain. Pyroelectricity is the result of the following three effects: - a change of polarisation with temperature variation; - a change in polarisation as a results of the strain in the material caused by its thermal elongation; - a polarisation change due to a spatial temperature gradient. The latter often being negligible. The pyroelectric current can be presented as: p dT I pS dt = (1) Whereby, I p is the pyroelectric current, p the pyroelectric constant of the material, S the active surface and dT/dt the time derivative of the temperature. Typical pyroelectric constants range from 27 µC.m -2 .K -1 for PVDF, to 500 for PZT and 200 for BaTiO 3 . Ferroelectric materials have a major application field in areas such as thermal sensors, infrared sensors, tactile sensors, solid state memories[3], Scanning thermal microscopy or micro power sources[2]. Poly(vinylidene fluoride) (PVDF) is a semi- crystalline linear polymer (CH 2 -CF 2 ) n used for its ferroelectric and pyroelectric[1] properties. It has different crystalline structures according to crystallization conditions. Depending on the polymer conformation there are several phases: orthorhombic α, β and γ phases, and monoclinic δ phase. The α phase is non-polar and is the most frequent in standard deposition techniques. The polar β phase is the one that produces the strongest ferroelectric behaviour. The γ phase has an intermediate conformation between α and β phases and can be obtained by crystallisation at a high temperature. Finally the δ phase is a polar version of the α phase and can be obtained by polarisation of the α phase in a high electric field. PVDF has a glass transition temperature of -35°C and shows no Curie temperature as the melting point temperature is below this value. In order to obtain a pyroelectric effect, a deposited layer of PVDF must also be composed of a β or δ phase. Once the material has cristallized in a given phase, the conformation can be changed either by mechanical stretching or by electrical poling.