PMN-PT thin films: Electromechanical behaviour, polarisability and microstructure
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U14.4.1
PMN-PT thin films : Electromechanical behaviour, polarisability and microstructure N.J. Donnelly, G. Catalan, C. Morros, R.M. Bowman, J.M. Gregg, Condensed Matter Physics and Materials Science, Queens University, Belfast BT7 1NN, N. Ireland. Zian Kighelman, Dragan Damjanovic, Nava Setter, Ceramics Laboratory, Materials Department, Swiss Federal Institute of Technology – EPFL, CH-1015 Lausanne, Switzeland ABSTRACT Thin film capacitor structures of Pb(Mg1/3Nb2/3)O3 (PMN) - PbTiO3 (PT) were fabricated using pulsed laser deposition (PLD) on MgO{100} substrates using (La1/2,Sr1/2)CoO3 (LSCO) as a lower electrode. Crystallographic and dielectric characterisation confirmed perovskite relaxor-like behaviour. Measurements of the electrostrictive coefficients by insitu X-ray diffraction, piezo-response atomic force microscopy and three point bending experiments showed both Q11 and Q13 to be comparable to accepted values for single crystals. However, for a given field, the electric field-induced strain in the thin films was much less than that of single crystal. This was clearly intimately linked to poor thin film polarisability. Previous work had shown sol-gel PMN-PT films to have significantly greater permittivities than PLD films, and a TEM investigation was undertaken to see if functional differences could be related to differences in microstructure, and hence if the functional and electromechanical properties of PLD films could be improved by attempting to replicate sol-gel microstructures. INTRODUCTION Relaxor electroceramics have received much recent attention after discovery of giant electrostrictive strains on the order of 1-2%, observed in single crystals of Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) [1]. However, much of the work carried out on thin film relaxors has focused on dielectric characterisation, and only a few papers address electromechanical properties [2-7]. To date the giant strains seen in single crystals remain to be realised in relaxor thin films. Though relaxors are centro-symmetric and consequently exhibit no piezoelectric effect a piezo-response can be induced under an applied d.c. bias field. In this state the effective piezoelectric coefficient d33eff of the relaxor can be expressed in terms of the induced strain S and the applied field E along the [001] axis as: d 33eff ( E ) =
δS δE
and hence
d 33eff = 2Q33ε 332 E
(1)
From this relation it is apparent that electromechanical response depends entirely on the electrostrictive coefficient Q33 and the polarisability / permittivity ε33, seen to be poor in thin films. In this work the authors characterise the functional properties of PMN-PT thin films grown by Pulsed Laser Deposition (PLD), and attempt to improve polarisability, and hence achievable electromechanical strain.
U14.4.2
EXPERIMENT Thin film capacitor structures were produced from ceramic targets using PLD on {100} MgO substrates with La1/2Sr1/2CoO3 (LSCO) as a lower electrode. A KrF excimer laser (λ = 248nm) operating at a laser fluence of 1.7
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