PZT

Résumé

Les PZT, ou titano-zirconates de plomb, sont des composés chimiques de formule , où . Il s'agit de céramiques de structure pérovskite présentant plusieurs propriétés intéressantes :

ferroélectricité, c'est-à-dire qu'elles ont une polarisation électrique spontanée, qui peut être inversée par l'application d'un champ électrique ;
piézoélectricité, c'est-à-dire qu'elles peuvent changer de forme si on leur applique un champ électrique, ou qu'elles peuvent générer une tension électrique entre deux faces lorsqu'on leur applique une déformation — propriété particulièrement utile pour les capteurs, les transducteurs, les actionneurs au sens large ;
pyroélectricité, c'est-à-dire qu'elles peuvent générer une tension électrique lorsqu'on modifie leur température, d'où des applications dans les sondes de température.

Ces matériaux ont été développés vers 1952 à l'université de technologie de Tokyo. Ils présentent une meilleure sensibilité et des températures d'utilisation plus élevées que

Source officielle

https://fr.wikipedia.org/wiki/PZT

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Micro-electro-mechanical systems (MEMS) technology is revolutionizing industrial and consumer products by replacing bulky devices by miniaturized versions that are suitable for portable applications as for instance in mobile phones. Besides small size, the keys for the success are reproducibility, batch processing, and significantly cheaper prices due to mass production. Whereas standardMEMS processing relied fromits beginning on standard CMOS materials, increasing need in extreme requirements or exploitation of specific phenomena demands for the integration of new materials. The interest is particularly strong in piezoelectric MEMS devices due to their higher efficiency in converting electrical to mechanical energy and vice versa, and thus their ability to provide electro-mechanical transducers. Since most of the actively used piezoelectric materials are ceramic in nature, they must be synthesized at rather high temperatures, which pose challenges to integration routes. The recent years can be considered as the starting phase of product development of piezo-MEMS. As a result, a very high interest is put on further improving the quality of piezoelectric thin films, meaning higher sensitivity in MEMS sensors and larger stroke inmicro-actuators is remaining a key priority. Themost widely used piezoelectric thin filmmaterials forMEMS devices are zinc oxide (ZnO), aluminumnitride (AlN) and lead zirconate titanate (PZT). AlN is used extensively forMEMS resonator structures due to its high quality factor and low losses. It is also investigated for sensors where linearity and stability of the piezoelectric response is mandatory. PZT films are used in actuators and acoustic transducer applications because of their high piezoelectric coefficients and electromechanical coupling coefficients. In this work, energy harvesting is specifically considered as motivation for property evaluation and optimization. In this case, the piezoelectric thin filmis used like a sensor for vibrations andmotions, though with the optimization requirement that the product of current and voltage must be maximal. [...]

EPFL2014

Chargement

Process optimisation of a MEMS based PZT actuated microswitch

Davide Balma

In this work the fabrication of a piezoelectrically actuated microswitch for high current applications is proposed. The device was obtained by assembling two silicon substrates: one containing the signal lines and the other enclosing a silicon nitride (Si3N4) microcantilever. This latter operates as a switch by closing the circuit with a metal electrode on its tip. A layer of lead zirconate titanate (PZT), deposited by sol gel method, constitutes the actuation element of the microcantilever. The bottom and top electrodes of the PZT were respectively made of thin Ta/Pt and Au layers while the signal lines and the contact electrode on the microcantilever tip were made of Ti/Al. A comparison between microcantilevers characterised by different bottom electrodes is presented: (i) Ta/Pt deposited at 300 degrees C and patterned by wet etching, (ii) Ta/Pt deposited at 100 degrees C and patterned by lift-off. The microcantilevers fabricated with the latter metallisation show a considerable symmetry of the piezoelectric response and a wide microcantilever displacement. Thus, in this device the cantilever microstructures are able to close the gap between the signal lines with a resulting contact resistance of about 2 Omega. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier Science Bv2014

Chargement

Experimental and first-principles study of point defects, domain walls, and point-defect/domain-wall interactions in ferroelectric oxides

Anand Chandrasekaran

Ferroelectric oxides, such as lead zirconate titanate, have proved invaluable due to their excellent dielectric and piezoelectric properties. These classes of materials possess a large electric polarization below the Curie temperature. Regions of the crystal lattice having different directions of polarization are separated by nano-scale interfaces known as domain walls. These interfaces, which can be moved by electric and mechanical fields, are not only important for electromechanical properties but they also exhibit unique structural and electronic properties that may be exploited through novel application in nanoelectronics and photovoltaics. The mobility of domain walls is strongly dependent on the defect chemistry of the material due to defect-domain wall interactions. Indeed, aliovalent doping of these materials has been used to tune the properties of ferroelectric oxides for specific applications. In this work, we investigate the origin of the so-called âhardâ and âsoftâ behavior in lead zirconate titanate. Experimental evidence suggests that small concentrations of donor dopants (such as niobium) results in enhanced domain wall motion while doping with acceptor dopants (such as iron) leads to an inhibited domain wall response. The microscopic origin of this phenomena is best investigated using first-principles simulations of the atomistic properties of defects and domain wall in these materials to bring to light novel structural and electronic properties. First, we show that polar defect complexes are likely to exist in both acceptor-doped and undoped PbTiO3 . These defects and defect associates are attracted to 180o domain walls and cause pinning of such interfaces. Donor- doped materials, on the other hand, do not show the presence of polar defect complexes. A closer investigation of the 180o domain wall in PbTiO3 reveals the presence of a âBlochâ component of polarization at the domain wall. In other words, due to polarization rotation, there exists a ferroelectric phase within the domain wall itself. We characterize the strain dependence of this phenomena and calculate the piezoelectric properties of such domain walls. A complete study of domain walls in PbTiO3 also entails looking closer at the properties of the ferroelastic âhead-to-tailâ 90o domain boundary. We show the presence of an asymmetry in the variation of the lattice parameter across the domain wall. This asymmetry is verified using high resolution aberration corrected electron microscopy. We look at the energy landscape of oxygen vacancies in the vicinity of these walls to explain the pinning effect in terms of random-bond and random-field defects. Next we look at the electronic properties of the recently discovered charged domain walls. We characterize the band bending phenomena in 90o head-to-head and tail-to-tail domain wall in PbTiO3 . We then look at the long range effects of clusters of oxygen vacancies in BaTiO3; especially in terms of the formation of 180o tail-to-tail domain wall. In parallel to the first priciples work, we also prepared high quality samples of PZT 50/50 with different concentrations of donor dopant. We characterized the microstructure and hysteresis loops as a function of donor dopant concentration. Based on experimental observations and ab initio calculations we propose ideas on the origins of hardening and softening in PZT.

EPFL2016

Chargement

Experimental and first-principles study of point defects, domain walls, and point-defect/domain-wall interactions in ferroelectric oxides

Anand Chandrasekaran

EPFL2016

EPFL2014