Pyroélectricité

Résumé

La pyroélectricité (du grec πῦρ, pyr, feu) est la propriété d'un matériau dans lequel un changement de température entraine une variation de polarisation électrique. Cette variation de polarisation crée une différence de potentiel temporaire, celle-ci disparaissant après le temps de relaxation diélectrique. Cette variation peut générer un courant électrique, ce qui rend ces matériaux utiles pour la détection de radiations ou la production d'électricité. Ils sont tout particulièrement utilisés dans certains détecteurs infrarouge. L'effet pyroélectrique ne doit pas être confondu avec l'effet thermoélectrique, où un gradient de température fixé donne naissance à une tension permanente. Les cristaux pyroélectriques forment un sous ensemble des cristaux piézoélectriques : 10 des cristallines piézoélectriques sont aussi pyroélectriques. Description La pyroélectricité peut être visualisée comme le côté d'un triangle, dont les sommets représentent l'énergie d'un cristal : ciné

Source officielle

https://fr.wikipedia.org/wiki/Pyro%C3%A9lectricit%C3%A9

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Many perovskite materials experience a temperature-driven phase transition at the Curie temperature from a non-centrosymmetric polar ferroelectric phase to a paraelectric phase, where polarization is lost. The paraelectric phase is usually centrosymmetric and therefore non-piezoelectric. However, ferroelectrics still show some forms of electro-mechanical coupling in their paraelectric phase like flexoelectricity, which is not symmetry limited and is practically strong in ferroelectrics. It has been shown that the flexoelectric coefficient of perovskite (Ba0.67,Sr0.33)TiO3 in its centrosymmetric paraelectric state at room temperature is up to three to four orders of magnitude larger than its theoretically predicted values. Such enormous discrepancy demonstrates that either the theory is unable to fully explain the flexoelectric response, or that another electro-mechanical mechanism is playing a role in the apparently large flexoelectric response of the material. It was found that Ba0.60Sr0.40TiO3 (BST6040) samples with apparently large flexoelectric coefficient exhibit symmetry breaking and consequently exhibit forbidden piezoelectric- and pyroelectric-like behavior in its paraelectric phase. In this thesis work, our interest lies in describing and interpreting the conditions where breaking of the symmetry is not associated with the external field nor with the flexoelectricity, but is proper to the material. Origins and mechanism of symmetry breaking were investigated in two different cases: local breaking of centrosymmetry; and macroscopic symmetry breaking. The local breaking of the centrosymmetry may refer to the existence of microscopic polar entities and/or disordered displacement of ions in the unit cell. The global symmetry breaking could be associated with a non-uniform distribution of charged defects and/or the coupling of polar nanoâregions with the strain gradient in the sample. We compared dielectric, elastic, and pyroelectric properties of the Ba1-xSrxTiO3 (BST) family, pure BaTiO3 in perovskite (p-BTO) and hexagonal (h-BTO) forms, and SrTiO3 to study the governing mechanisms of symmetry breaking. In addition, the atomic structure of BST6040 was studied by high resolution scanning transmission electron microscopy for direct evidence of polar regions and to understand their exact nature and origin. According to experimental data, the paraelectric phase of p-BTO shows local and global breaking of nominal centrosymmetricity and possesses microscopic and macroscopic polarizations, which are associated with precursors of the ferroelectric phase ordered by the strain gradient in the sample. On the other hand, h-BTO does not exhibit macroscopic pyroelectric response because the precursors are paraelectric-ferroelastic at room temperature, and therefore non-polar. Studying the material response to dynamic electric field showed that the Rayleigh-like relation, which describes dynamics of domain walls in a random pinning environment, cannot describe the dynamics of polar entities in these materials. Results of our high-resolution atomic studies showed distinct static polar clusters with the average size of few nanometers where A-site, B-site and O ions are displaced from their ideal cubic positions. We believe that all presented experimental data can be consistently explained by the presence of microscopic precursors of the ferroelectric phase and their interaction with the strain gradient in the material.

EPFL2017

Chargement

Breaking of the macroscopic centric symmetry in Ba₁₋xSrxTiO₃ ceramics and single crystals

Alberto Biancoli

Beyond the possibility of finding a lead-free piezoceramics as replacement for PZT, the world-wide research has focused on other mechanisms of electro-mechanical coupling alternative to piezoelectricity, such as electrostriction and, more recently, flexoelectricity.The mechanism of the flexoelectric polarization in bulk ceramics is still far from being fully understood. Above all, the controversy concerns the fact that the values of the flexoelectric coefficients measured in the ferroelectric ceramics exceed by several order of magnitude the theoretical estimations.During this thesis work, the measurement of the flexoelectric properties in the paraelectric phase of unpoled ceramics of the Ba0.67Sr0.33TiO3 composition, which was reported previously to exhibit the largest flexoelectric polarization ever measured, lead to the uncovering of an electro-mechanical coupling which cannot be ascribed to the flexoelectric response. During this thesis work, the measurement of the flexoelectric properties in the paraelectric phase of unpoled ceramics of the Ba0.67Sr0.33TiO3 composition, which was reported previously to exhibit the largest flexoelectric polarization ever measured, lead to the uncovering of an electro-mechanical coupling which cannot be ascribed to the flexoelectric response.This electromechanical coupling, which was accompanied by pyroelectric response in as-prepared samples, suggests breaking of the macroscopic centric symmetry of the ceramics. In the effort to understand the mechanism of the symmetry breaking, the solid solutions Ba1-xSrxTiO3 (x=0, 0.025, 0.33, 0.40, 0.45, 0.50, 0.67, 0.90, 0.975, 1) were investigated and compared with properties of corresponding single crystals and other ceramic compositions. The study was conducted mainly by means of piezoelectric, pyroelectric and thermally stimulated current(TSC) characterization.In this work, it is proposed that the appearance of the polar symmetry in ceramics and single crystals above the Curie temperature is due to asymmetries associated to the high temperature processing of crystals and ceramics The symmetry breaking in Ba1-xSrxTiO3 ceramics and associated electromechanical response that is of the same order of magnitude as the flexoelectric response in the same materials, can at least partly help resolving the controversy existing between the theoretical models of flexoelectricity and the experiments.

EPFL2014

Chargement

Investigation of pyroelectric thin films and their application in micromachined infrared detectors

EPFL1998

Chargement

Breaking of the macroscopic centric symmetry in Ba₁₋xSrxTiO₃ ceramics and single crystals

Alberto Biancoli

EPFL2014

EPFL2017