Non-steady-state Photovoltaic Current in (Pb 0.85 La 0.15 )TiO 3 and BaTiO 3 Ceramics
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The effect of poling field on the photocurrent was investigated in (Pb0.85La 0.15)TiO3 and BaTiO3 ceramics. Non-steady-state photovoltaic current and no pyrocurrent on cooling were observed in both poled ceramics. The maximum behaviors were obtained at the poling field of 1.5 kV/mm. They were explained in terms of the photoinduced reversible and irreversible increases in the remanent polarization resulting from the space charge field, which was established by electrons trapped at grain boundary during illumination. No pyrocurrent on cooling was responsible for the reversible increase of the remanent polarization. The photoinduced irreversible increase of the remanent polarization, i.e., photodomain effect, was confirmed using the acoustic emission technique.
I. INTRODUCTION
It is well known that ferroelectric materials possess the photovoltaic effect that produces a greater steadystate voltage than a band gap under illumination in the absence of an external field.1 The photovoltaic effect has been investigated due to its potential usefulness for optical storage,2 photostriction,3 and power conversion,4 and these studies have been mostly carried out on single crystals. The photovoltaic effect has been also observed in ferroelectric ceramics.5 The principal difference is that ceramics have grain boundaries, which act as the trap sites of the nonequilibrium carriers generated by illumination. In single crystals, illumination not only induces the photovoltaic effect but also changes the ferroelectric properties including domain structure,6,7 spontaneous polarization,8 etc., which is defined as the photoferroelectric effect. Although it is expected that ceramics have photoferroelectric properties different from those of a single crystal, the photovoltaic effects on the ferroelectric ceramics, such as the effects of poling field, grain size, and light intensity on the photovoltaic current, are not yet known exactly. In this paper, we investigated the effect of poling field on the photocurrent in (Pb0.85La 0.15)TiO3 and BaTiO3 ceramics. Non-steady-state photovoltaic current and no pyrocurrent on cooling were observed in both poled ceramics. These behaviors were discussed in terms of the photoinduced reversible and irreversible increases in the remanent polarization, resulting from the space charge field which was established by electrons trapped at grain boundary. The acoustic emission (AE) 1354
J. Mater. Res., Vol. 15, No. 6, Jun 2000
technique was used in order to observe the photodomain effect, where illumination leads to a change in domain structure. II. EXPERIMENTAL PROCESS
(Pb0.85La 0.15)TiO3 (PLT) ceramics were prepared by a conventional solid-state sintering, and their relative densities were about 97%. A hot isostatic press (HIP) was used for BaTiO3 (BT) ceramics after presintering to get a relative density of about 99%. Grain sizes were fixed 3 m for lead lanthanum titante (PLT) ceramics and 10 m for bipolar transistor (BT) ceramics to avoid the grain size effect.9 Both ceramics were electroded on 2 × 5 m
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