Pyroelectric Properties of Alanine Doped TGS Single Crystalline Thick Films under Constant Electric Stress
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Pyroelectric Properties of Alanine Doped TGS Single Crystalline Thick Films under Constant Electric Stress Lucian Pintilie, Ion Matei, Ioana Pintilie, National Institute of Materials Physics, Atomistilor 105 bis, P.O. Box MG-7, 76900 BucharestMagurele, Romania, Fax: +401- 4930267; E-mail: [email protected] Horia V. Alexandru, Ciceron Berbecaru, Faculty of Physics, University of Bucharest, Romania P.O.Box 74-165, Bucharest, Romania, Phone / Fax: +401 – 7465066; E-mail: horia@ alpha1.inphim.ro ABSTRACT Pyroelectric properties of triglycine sulfate (TGS) thick films, separately doped with L and D alanine were investigated. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilize the polarization in the opposite direction on the ferroelectric axis. Pyroelectric current (under constant stress) was recorded with a computer controlled Keithley 6517 electrometer, crossing up and down the Curie point. A reverse external electric field was applied on doped materials during heating, crossing up the Curie point. It is shown that the pyroelectric coefficient can be increased about four times at room temperature under un optimized DC electric field applied on the pyroelectric wafer. INTRODUCTION Doped triglycine sulfate (TGS) crystal has large applications in pyroelectric detection on a large spectral bandwidth. It has competitive parameters versus other single crystal or PZT type pyroelectric ceramics [1]. Single element detectors, linear and two-dimensional arrays of TGS material have been used for practical applications. Pyroelectric vidicon tubes have also used crystals of the TGS family as target materials [1]. Synthesis of pure TGS material and two fractional recristallysation were performed [2,3]. Middle fraction was used for solution growth in paraelectric phase at 52 oC, using L or D alanine 50% as dopant in solution. Samples of ~1 mm thick and ~0.8 cm2 surface, cleaved from the crystals, were carefully polished on a silk plate using 10 % water in glycerin. Silver paint electrodes were deposited for electric contacts. The P-E dependence was obtained using a standard Sawyer-Tower set-up [3]. Raising the hysteresis loop for different temperatures, the temperature dependence of the coercive field Ec and of the bias field Eb were obtained. These are both presented in fig. 1 for L-alanine doped crystal. Similar results are obtained for D-alanine doped TGS crystals. The pyroelectric measurements were performed after mounting the sample in closed cycle He cryostat that allows an accurate control of the heating rate. The pyroelectric current was computer recorded using a Keithley 6517 electrometer. The pyroelectric coefficient was computed using the well-known formula [4,5]: p=
i p dT −1 A dt
(1)
were ip is the measured pyroelectric current, A is the electrode area and dT/dt is the heating rate (maintained constant around 0,186 K/s). The pyroelectric current was first recorded for as-grown crystals, without any applied voltage on the sample. Some differences between D or L alanine C7.13.1
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