Characterization of Ferroelectric Materials by Photopyroelectric Method

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During last decades, the photothermal techniques have been largely applied to the study of thermal and optical properties of condensed matter. Photothermal techniques are based on the same physical principle: the optical energy, absorbed by given material, is partially converted into heat; depending on the way used to measure the quantity of heat and to follow its propagation through the material, several photothermal techniques have been developed (photoacoustic calorimetry, photothermal radiometry, photothermal deflection, thermal lensing, photopyroelectric method) (Tam, 1986). In this chapter we will focus on the simplest one, the photopyroelectric calorimetry and its applications concerning the investigation of some thermal and electrical properties of ferroelectric materials. The photopyroelectric (PPE) detection was introduced in 1984, as a powerful tool for highresolution measurement of thermal properties of materials (Coufal, 1984; Mandelis, 1984). The pyroelectric effect consists in the induction of spontaneous polarization in a noncentrosymmetric, piezoelectric crystal, as a result of temperature change in the crystal. Single crystals as LiTaO3 and TGS, ceramics as PZT or polymers as PVDF were used as pyroelectric sensors, for the main purpose of measuring temperature variations. In principle, in the PPE method, the temperature variation of a sample exposed to a modulated radiation is measured with a pyroelectric sensor, situated in intimate thermal contact with the sample (Mandelis & Zver, 1985; Chirtoc & Mihailescu, 1989). The main advantages of this technique were found to be its simplicity, high sensitivity, non-destructive character and adaptability to practical restrictions imposed by the experimental requirements. From theoretical point of view, in the most general case, the complex PPE signal depends on all optical and thermal parameters of the different layers of the detection cell. A large effort was dedicated in the last decades to simplify the mathematical expression of the PPE signal. As a final result, several particular cases were obtained, in which the information is contained both in the amplitude and phase of the PPE signal (Mandelis & Zver, 1985; Chirtoc & Mihailescu, 1989); the amplitude and phase depend in these cases on one or, in a simple way, on two of the sample's related thermal parameters. The thermal parameters resulting directly from PPE measurements are usually the thermal diffusivity and effusivity. It is well known that the four thermal parameters, the static volume specific heat, C, and the dynamic thermal diffusivity, α , conductivity, k, and effusivity, e, are