A Modified SBN System for Pyroelectric Sensors
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A Modified SBN System for Pyroelectric Sensors H. Amorín1, F. Guerrero2, J. Portelles1, M. Venet1, A. Fundora1 and J. M. Siqueiros3 1
Facultad de Física - Instituto de Materiales y Reactivos, Universidad de la Habana, Vedado, La Habana 10400, Cuba. 2 Facultad de Ciencias Naturales, Universidad de Oriente, Santiago de Cuba 90500, Cuba. 3 Centro de Ciencias de la Materia Condensada, UNAM, Apartado Postal 2681, Ensenada, B.C., México, 22800. ABSTRACT The Thermally Stimulated Depolarization Current (TSDC) and pyroelectric properties of the modified SBN ferroelectric ceramic system were studied for different lanthanum and titanium doping concentrations. The TSDC measurements show the pyroelectric peak for all compositions while a second smaller peak at higher temperature, possibly associated to induced vacancy-impurity dipoles, is also observed in all cases. The second peak contribution was experimentally and mathematically eliminated to determine the remanent polarization and pyroelectric coefficient, both associated only to permanent ferroelectric dipoles. The figures of merit for sensor devices are determined for all compositions and compared with those of other pyroelectric systems. The La0.03Sr0.255Ba0.7 Nb1.95Ti0.05O5.975 sample, in particular, has excellent pyroelectric response, making this material very suitable for pyroelectricity-derived applications. INTRODUCTION To select a material for pyroelectric sensors three different criteria have been used in the literature [1-2], each one with an associated figure of merit [3-4]: 1) The voltage response defined as Fv = p
C εo ε
where the pyroelectric voltage response is to be maximized. 2) The detectivity
D=
p C εo ε tanδ
where the
signal to noise ratio of the pyroelectric detector is to be maximized and 3) the current response
FI =
p C
used to characterize detectors in the transverse mode where a fast response and a wide bandwidth are needed. In the latter expressions, p is the pyroelectric coefficient; C is the volume specific heat, ε the dielectric permittivity and tanδ the dielectric losses of the material. Among the ferroelectric materials with outstanding properties, niobate crystals with tungsten-bronze structure, like the SrxBa1-xNb2O6 (SBN) system, stand out [1,5]. Their high pyroelectric and electrooptic coefficients and low dielectric permittivity characterize these materials [5]. SBN solid solutions have been selected for a number of technological applications such as pyroelectric detector [6], and electro-optic [7], photorefractive [8] and acoustic surface wave (SAW) devices [9]. Defects such as vacancies and impurities strongly affect the material property [10]. It is seen, for instance, that the transition temperature takes lower values as the concentration of vacancies and/or dopants increase as opposed to the case where all the crystallographic sites are completely occupied [11]. Rare earth doping of the SBN system will lead to lower Tc and improve dielectric and pyroelectric properties at room temperature [12-13]. Specifically, lanthanum d
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