Dielectric and piezoelectric characteristics of samarium modified lead titanate ceramics

https://doi.org/10.1016/S0921-5107(03)00178-8Get rights and content

Abstract

Dielectric and piezoelectric properties of samarium (Sm) modified lead titanate ceramics (Pb1−xSmxTi1−2yWyFeyO3 with x=0.02, 0.05, 0.10, 0.20 and y=0.02), prepared by high temperature solid state reaction method, have been studied. Variation of dielectric constant and conductivity at room temperature shows frequency dependence and is expressed by Kramer's Kronig relation (ε′(ω)−εα=A1ωs−1) and σac(ω)=A2ωs, respectively. All compositions show phase transition and the temperature of phase transition is found to decrease with increase in Sm substitution. Nature of transition studied by the relation (1/ε′)−(1/εmax)=C(TTc)γ is found to be diffused. A nonlinear increase in the value of d33 is observed with the increase in Sm content.

Introduction

Amongst the many ferroelectric oxides with perovskite structure of general formula ABO3 (A=mono or divalent, B=trivalent or hexavalent ions) some are in the forefront both in areas of research as well as in industrial applications. Lead titanate (PT) is an important member of the family of perovskite-structured materials that exhibits useful dielectric, piezoelectric and electromechanical properties. These PT ceramics are very useful in high temperature and high frequency applications [1], [2]. Hashimoto et al. [3] have developed PT based high performance human information sensing system, which can monitor the human conditions in a room, especially the number of occupants, their location and activities.

Ferroelectric phase transition of PT ceramics occurs at 490 °C. Above this temperature its crystal structure is cubic and below the curie point the cubic phase changes to tetragonal phase. Due to this phase change, a change in cell dimension is also produced and consequently, a strong internal stress may be induced. This stress renders PT ceramics fragile when cooled through curie temperature [4], [5]. Also pure PT ceramics have low resistivity (107–108 Ω cm) which is due to vacancies caused by loss of lead during sintering [6]. As a result of large c/a, high field at high temperature is required for poling [1] which PT ceramics cannot withstand due to low resistivity.

Dense and hard PT ceramics have been prepared by adding suitable dopants [7] or combining with other compounds to form solid solution [8], [9], [10]. By modifying PT ceramics with isovalent (Ca2+, Ba2+, Cd2+, etc.) or off-valency (Sm3+, Gd3+, Y3+, etc.) ions in Pb2+ sites, the lattice anisotropy is reduced [6], [11] resulting in hard and dense samples with high mechanical strength. Suitable doping also reduces sintering temperature. Chu and Chen [12] have reported reduction in sintering temperature by the addition of perovskite dopant in samarium (Sm) modified ceramics without affecting dielectric and other anisotropy properties. Reduction in sintering temperature helps in reduction of loss of lead during sintering. Deb [13] has reported high dielectric constant, high pyroelectric constant and favourable thermal and mechanical characteristics in Sm modified PT ceramics. Doping reduces curie temperature of PT ceramics which for pure PT ceramics is high (490 °C). Dielectric and piezoelectric properties of hydrothermally derived Sm-modified ceramics and effect of post sintering hot isostatic pressing (HIP) on its properties has been reported by Rictoe et al. [14].

In the present study, we performed experiments aiming at substitution of Sm3+ ion in Pb2+ sites and Fe and W in Ti sites partially. Substitution of Sm3+ helps in reducing lattice anisotropy, thereby making it possible to produce hard dense ceramics. Modification of iron is reported to produce ceramics of high mechanical quality factor [15]. Substitution of W6+ ions in Ti4+ sites shall enhance the resistivity by compensation of charge due to off-valency substitution of Sm3+ in Pb2+ sites. As a result, these modified PT ceramics will have high resistivity and high mechanical strength and can be subjected to severe poling conditions [1], [2].

Section snippets

Experimental

The samples having composition Pb1−xSmxTi1−2yWyFeyO3 with x=0.02, 0.05, 0.1, 0.2 and y=0.02 were prepared by solid state reaction method, the details of which have been reported elsewhere [16]. Single phase formation was confirmed by studying X-ray diffraction pattern using Philips X-ray diffractometer with CuKα radiation (λ=1.5405 Å). The samples were mechanically polished to obtain optically finished surfaces. These were then etched in 5% hydrogen fluoride for 30 min at room temperature in

Effect of samarium substitution

X-ray diffraction pattern of the Sm modified ceramics with composition Pb1−xSmxTi0.96Fe0.02W0.02O3 with x=0.02, 0.05, 0.1 and 0.2 is shown in Fig. 1, which confirms single phase formation in all samples. Crystal tetragonality (c/a) is found to decrease with increase in Sm substitution content resulting in hard and dense ceramics [16]. Fig. 2 shows a representative scanning electron micrograph of 20 mol.% Sm modified PT ceramic. Average grain size measured by linear intercept method [17]

Conclusion

Dielectric constant at room temperature of Sm modified PT ceramics increases with increase in Sm substitution. Increase in Sm concentration decreases crystal tetragonality which results in increased polarization and thereby dielectric constant. Increase in grain size also contributes to increase in dielectric constant. Decrease in dielectric constant at room temperature with increase in frequency is attributed to hopping of charge carriers with random distribution of hopping distances and

Acknowledgements

One of the authors (PNK) is thankful to the Council of Scientific & Industrial Research, New Delhi for support under ‘Emeritus Scientist, CSIR’. The author (RT) is thankful to the Principal Govt. M.A.M. College Jammu and Higher Education Department, J&K Govt. for permission to undertake the research work and the University Grants Commission for awarding ‘UGC Teacher Fellowship’.

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