M. L. Calzada

Piezoelectric properties of Ca-modified PbTiO3 thin films

Low‐field piezoelectric coefficient and strain were investigated in Ca‐modified PbTiO3 thin films by means of optical interferometry. A remanent piezoelectric coefficient of 70 pm/V was observed that agrees well with the values previously reported for the bulk ceramics of the same composition. The efficient poling of the films can be achieved at room temperature under the field of 300 kV/cm. Electrically induced strains as high as 0.8% were observed using a bipolar driving field of 1 MV/cm. Strain response under a unipolar driving field exhibited a good linearity and small hysteresis. These properties combined with the low dielectric constant and high dielectric strength are shown to be attractive for micromechanical applications.

On the effects of stresses in ferroelectric (Pb,Ca)Tio3 thin films

Abstract Grazing angle X-ray diffraction, profilemetry, permittivity vs. temperature and Raman spectrometry measurements have been carried out on Ca-substituted lead titanate thin films prepared by sol–gel and deposited on Pt/TiO 2 /SiO 2 /(100)Si substrates. Films with thicknesses of 150 and 350 nm were obtained. The crystalline (Pb,Ca)TiO 3 films were randomly oriented with a and c parameters slightly different from those of bulk ceramics. Profilemetry measurements indicated that the films were under tensile stresses, developed during the drying of the deposited wet layer and during the thermal treatment of crystallization of the amorphous film. These stresses were confirmed by the shift obtained in the Raman frequencies of these films. As a consequence of these stresses, a positive shift of the maximum of the permittivity vs. temperature is measured.

Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity o...

Sol-gel processing by inroganic route to obtain a TiO2PbO xerogel as ceramic precursor

Abstract The sol-gel process for ceramic preparation has been the focus of interest in the past few years. With this kind of chemical process it is possible to enhance the purity and reactivity of the system and to ensure the control of the microstructure of the obtained ceramics. Pure lead titanate has interesting properties as ferroelectric ceramic, but it has not been used because of its poor sinterability. In this work, a sol-gel process by the inorganic route of the TiO 2 PbO system is presented. With this process a xerogel is obtained which leads to pure PbTiO 3 ceramic materials after firing.

Contributions to the knowledge of calcium-modified lead titanate ceramics

Abstract It will be summarized here the work of the Ferroelectric Materials Group of this Institute on ceramics of calcium modified lead titanate during the past few years.

Chemistry-Crystallization-Microstructure Relations of Sol-Gel Derived Lanthanum Modified Lead Titanate Thin Films

Lanthanum modified lead titanate thin films have been obtained by the deposition of sol-gel solutions onto platinized (100) silicon substrates. Crystallization of perovskite films was achieved by thermal treatments at 650°C with slow or rapid heatings. Lead oxide excesses were used in the precursor solutions to counterbalance the lead losses produced during the thermal treatment. Rapid heatings and large excesses of lead produce a preferred orientation of the films. These films have more homogeneous and denser microstructures than slow heated films without lead excess.

Photo-sensitive sol–gel solutions for the low-temperature UV-assisted processing of PbTiO3 based ferroelectric thin films

Photo-sensitive PbTiO3 solutions were synthesised by a sol–gel process that combines the use of a diol solvent, HO(CH2)3OH, and a titanium alkoxide modified with a β-diketonate, Ti(OC3H7)2(CH3COCHCOCH2)2. O-Nitrobenzyl alcohol (NBA), 1-hydroxy-cyclohexyl-phenyl-ketone (HPK) and acetylacetone (ACACH) were added to the synthesised sols as UV-activators. Solutions with nominal compositions of Pb0.76Ca0.24TiO3 were prepared. These solutions have a maximum of absorption in UV between 200 and 300 nm. Pyrolysis of the solutions was studied by means of thermogravimetry and differential thermal analysis (TGA/DTA). Films were deposited by spin-coating onto TiO2/Pt/TiO2/Ti/SiO2/(100)Si substrates. They were irradiated at 250 °C with light of λ ∼ 225 nm and then treated at 550 or 450 °C for 420 s with Rapid Thermal Processing (RTP) in air, using a heating rate of ∼30 °C s−1. Crystalline phases and structures developed in the films were studied by X-ray diffraction with the Bragg–Brentano geometry and with grazing incidence (XRD and GIXRD). Films showed the formation of a single perovskite phase. These films have spontaneous pyroelectricity with a maximum value of the pyroelectric coefficient of γs ∼ +12 × 10−9 C cm−2 K−1. Non-irradiated films prepared at 550 °C did not show a defined ferroelectric response. The ferroelectric properties measured in the films processed at 550 °C under UV were similar to those reported for films prepared at higher temperatures without irradiation. Values of the remanent polarisation of Pr ∼ 22 µC cm−2 and of the coercive field of Ec ∼ 170 kV cm−1 were obtained in these irradiated films prepared at 550 °C.

Effects of substrate annealing and post-crystallization thermal treatments on the functional properties of preferentially oriented (Pb,Ca)TiO3 thin films

〈111〉 and 〈001〉,〈100〉 preferentially oriented calcium-modified lead titanate thin films have been studied. Effects of the substrate annealing and post-crystallization thermal treatment of the films on the texture and ferro-, piezo-, and pyroelectric properties have been analyzed and discussed. The annealing of the substrate has effect on the texture of 〈001〉,〈100〉-oriented films and, as a consequence, produces an increase in the net polarization in the perpendicular direction to the plane of the film and in the pyroelectric coefficient. The annealing of the substrate does not affect the texture of 〈111〉-oriented films, but the electrical properties at the electrode–substrate interface and, as a consequence, gives place to high internal electric fields leading to the highest spontaneous piezo- (5 pm V−1) and pyroelectric coefficients (25.0×10−9 C cm−2 K−1). The asymmetry that characterizes the functional properties of 〈111〉-oriented films under poling with negative or positive polarity is also consequence ...

Pulsed hysteresis loops on ferroelectric thin films

An alternative method to obtain hysteresis loops for ferroelectric thin films is described that overcomes the drawbacks of the classical methods. In this method, a pulsed signal, obtained by mixing a sinusoidal voltage of low frequency with a square voltage of variable period and width, is applied to a (Pb0.76Ca0.24)TiO3 thin film. Electric field is applied to the sample for a short time and so, contribution of extra charges to the hysteresis loops is very small. The experimental loops measured only need a small compensation.

Stress-induced depolarization of (Pb, La)TiO3 ferroelectric thin films by nanoindentation

Electrical depolarization has been observed in lanthanum-modified lead titanate ferroelectric thin films stressed by nanoindentation. A spherical metallic indenter was used as a top electrode to locally pole the films and then to measure the depolarization current intensity. The current intensity had distinctive maxima at given indentation forces. These are related to the stress thresholds for the depolarization mechanism, which is probably 90° domain wall movements. Knowledge of the depolarization stresses is necessary for the design of microelectromechanical systems that include a ferroelectric layer.