Mickael Anoufa - Academia.edu (original) (raw)

Papers by Mickael Anoufa

Applied Physics Letters, Oct 26, 2015

Most of the theoretical and experimental studies on the electrocaloric effect (ECE) are devoted t... more Most of the theoretical and experimental studies on the electrocaloric effect (ECE) are devoted to thin films, but they can be hardly envisaged for cooling macroscopic systems; moreover, the results obtained cannot be easily transposed for larger systems like multilayered ceramics. Therefore, efforts should also be focused on predicting, synthesizing, and characterizing interesting bulk single crystal or ceramics. In ferroelectric nanoparticles and ceramics, the core-shell structure of grains is of uttermost importance to explain the experimental results at small sizes. Moreover, it can be used to tailor physical properties, such as energy storage, by experimenting with the composition, thickness, and permittivity of the shell. Here, we report the effect of such structures on the electrocaloric effects in a variety of ferroelectric materials. The magnitude of ECE as well as its field and temperature-dependence are obtained for different types of core-shells. The optimal configuration for a maximal ECE is deduced. V

Journal of Applied Physics, Feb 6, 2013

We have made a quantitative evaluation of the possibility to store electrical energy using ferroe... more We have made a quantitative evaluation of the possibility to store electrical energy using ferroelectric supercapacitor built on core-shell nanoceramics. We have first solved the Laplace equation for a single core-shell particle and used afterward an effective medium approximation to get in a self consistent way the equations for the different internal electric field, the permittivity and the polarization of a nanoceramic constituted of such core-shell particles. A numerical calculation was performed to check these results. We could generalize these results in the case of a ceramic made of core-n shells particles, and for different cases of anisotropic shapes of particles. We have introduced afterward the expression of the core-shell polarization into the Landau potential for nanoceramics by Li et al. [J. Appl. Phys. 98, 064101 (2005)] which allows us to get the phases diagrams for different cases of nanoparticles shapes. Numerical minimization of the Landau potential results in curves of polarization versus external electric field, which were numerically integrated to obtain the maximum density of energy that can be stored in the nanoceramic. The influence of several parameters such as the maximum applied electric field or the breakdown field, the permittivity of the shell, and the anisotropy of particles has been considered for different types of dielectric materials. A final table condensates these results, which shows that the possibility of ferro-supercapacitor is fair but one or two order of magnitude lower than those of electrolytic-supercapacitor. However if some limitations like the breakdown field in the ceramics could be overcome, middle-density ferro-supercapacitors could be built and eventually replace current hybrid supercapacitors.

Physical Review B, Oct 18, 2013

In this paper, we want to emphasize the fact that many experimental properties of ceramics can be... more In this paper, we want to emphasize the fact that many experimental properties of ceramics can be explained by the existence of a core-shell structure of the grains, particularly at small sizes. In this framework, we have studied BaTiO 3 (BT) ceramics constituted of core-shell nanoparticles, nanowires, or nanoplanes by using ab initio derived effective Hamiltonian calculations whose application range is for large values of shell thickness and low values of shell permittivity. Many differences and new features compared to the situation of nanodots are induced by the core-shell structure. For instance, phase sequences are different; there is also a coexistence of vortices found by Naumov, Bellaiche, and Fu [I. I. Naumov, L. Bellaiche, and H. Fu, Nature (London) 432, 737 (2004)] in the case of isolated dots with a homogeneous polarization, a transition from cubic paraelectric phase towards nonpolar rhombohedral phase, anomalies in dielectric permittivity associated with the onset of toroidal moments, etc. Afterwards, we compare these results with those obtained by the Landau theory of core-shell ceramics we have recently published. However, the ab initio calculations fail to capture the physics at small shell thickness and/or high shell permittivity, whereas the Landau theory fails to predict the peculiar properties of the phases in which vortices exist. Therefore, in a tentative way to build a global theory, we have constructed a Landau potential using both the polarization and the toroidal moment as competing order parameters, which allows us to propose a phase diagram, whatever the thickness and permittivity of the shell are.

HAL (Le Centre pour la Communication Scientifique Directe), May 22, 2017

Hyper-Raman scattering (HRS) is a non-linear spectroscopy sensitive to all polar excitations, in ... more Hyper-Raman scattering (HRS) is a non-linear spectroscopy sensitive to all polar excitations, in particular the soft modes inactive in Raman. HRS has been applied to nano-ceramics of strontium titanate (SrTiO 3) of controlled grain size. Contrary to infrared absorption which sees an average medium, the vibrational responses of the core and the shell(s) are split in HRS, allowing thereby probing the structural and dielectric properties of the two subsystems. The resulting structural model successfully reproduces the effective dielectric permittivity measured by dielectric experiments [1]. These results confirm the strong, but still under-exploited, potentialities of HRS for the investigation of polar materials [2]. We also demonstrate that a new ground state can be obtained by tailoring the core-shell structure of the particles. High energy X-rays at synchrotron combined to neutron diffraction and HRS revealed a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shift of the critical temperature [3,4]. This new phase is discussed within the Landau theory, and the crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed as well. This shows that controlling the core-shell structure provides an easy way to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.

arXiv (Cornell University), Dec 12, 2017

HAL (Le Centre pour la Communication Scientifique Directe), Jun 28, 2015

Ferroelectric nano-objects display a rich variety of shapes, among them nanocubes (bulk or hollow... more Ferroelectric nano-objects display a rich variety of shapes, among them nanocubes (bulk or hollow), nanospheres, nanotori etc with fascinating morphogenesis mechanisms 1. Structural and physical consequences of ferroelectric downsizing to form nanopowders or nanoceramics are very important both from applicative and fundamental point of view (see ref. 2 for a good review). Indeed many interesting effects have already been reported such as diminishing of permittivity and increase of diffusivity 2,3 , cancelation of dielectric relaxation in relaxors 3,4 , lowering of ground states, increase and rotation of polarization in relaxors and morphotropic compounds 3,5 etc. In order to explain these experimental results, one must take into account not only size and microstrains effects but also existence of core-shell structure 2,5-8. Modeling also shows that there is optimal nanostructurations to maximize energy storage 6 and electrocaloric effect 8. Calculations also show the possibility to form new phases in which vortices of polarization compete with homogeneous polarization 8 or in which homogeneous hypertoroidization of nanotori coexists with axial homogeneous toroidization 10. After a survey of these effects, we will give new experimental results observed in SrTiO 3 showing at small size decoupling of oxygen octahedra tiltings and distortion, strong shift of ferroelastic critical temperature 11,12 and lowering of the tetragonal structure towards orthorhombic ground state. Possible connection with a core-shell structure will also be discussed.

International audienceStructural and physical consequences of downsizing to form nanopowders or n... more International audienceStructural and physical consequences of downsizing to form nanopowders or nanoceramics are very important both from applicative and fundamental point of view. We will discuss, using ab‐initio‐based calculation and Landau theory the origins and the consequences on structural properties when synthesizing nano‐objects with BaTiO3 and SrTiO3 , and show how to use nanostructuration to optimize properties such as electrocaloric effects or energy storage. For instance calculations in BaTiO3 show the possibility to form new phases (fig.1) in which vortices of polarization compete with homogeneous polarization or in which (in nanotorus) homogeneous hypertoroidization of nanotori coexists with axialhomogeneous toroidization. In SrTiO3 a new low temperature ground state, evidenced by synchrotron and neutron diffraction is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between partic...

Physical Review B, 2016

A new ground state of textbook compound strontium titanate (SrTiO 3) is obtained by inducing a sp... more A new ground state of textbook compound strontium titanate (SrTiO 3) is obtained by inducing a specific core-shell structure of the particles. Using a combination of high energy synchrotron and neutron diffraction, we demonstrate a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shifts of the critical temperature. This new phase is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed. Inducing a specific core-shell structure is therefore an easy way to tailor structural properties and to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.

Applied Physics Letters, 2015

Http Www Theses Fr, Dec 19, 2012

de thèse Dans le stockage d'énergie, les alternatives aux énergies fossiles sont peu nombreuses e... more de thèse Dans le stockage d'énergie, les alternatives aux énergies fossiles sont peu nombreuses et comportent encore bien des inconvénients. Le stockage d'énergie sous forme électrostatique dans un condensateur, permet d'atteindre de grande puissances électriques, mais pour une densité d'énergie trop faible pour espérer en l'état des applications. La présente thèse à pour objectif la compréhension et l'optimisation des nano-céramiques ferroélectriques afin d'augmenter la densité d'énergie du diélectrique et de se diriger ainsi vers un super-condensateur céramique. Nous avons développé dans un premier temps un modèle par champs effectif moyen du système core-shell, qui permet de rendre compte des mesures expérimentales dans les nano-céramiques. Une fois introduit dans l'énergie libre de Landau, ce modèle permet de faire des prédictions de manière purement phénoménologique et donne une idée de la densité d'énergie que l'on puisse attendre d'un système ferroélectrique donné. Les calculs-sur quelques pérovskites courantes-indiquent que la densité d'énergie reste bien faible en comparaison de systèmes concurrents comme le Li-ion. Néanmoins nos calculs montrent que l'on peu optimiser le stockage d'énergie dans de telles céramiques en enrobant par un diélectrique, des grains de forme allongée comme des disques ou des fils. A noter que ce modèle phénoménologique permet d'expliquer de nombreuses mesures expérimentales sur les céramiques, y compris dans le cas ou s'ajoute des phénomènes de conduction interne comme avec la relaxation de Maxwell-Wagner. Nous nous sommes tournés par la suite vers une modélisation ab-initio, à l'aide d'un modèle d'Hamiltonien effectif paramétré grâce à la méthode de la fonctionnelle densité. La modélisation de nano-système core-shell de BaTiO3 a montré qu'un nouveau paramètre d'ordre pouvait apparaitre : le moment toroïdale. Ce paramètre d'ordre dans le cas d'enrobages conséquents implique un comportement diélectrique différent. En intégrant ce paramètre dans une description phénoménologique de type Landau, on peut montrer qu'il prédit une disparition de la phase orthorhombique dans BaTiO3. La synthèse de nanoparticules de BaTiO3 de formes cubiques homogènes en forme et taille nous a permis de faire une série de mesure. Les mesures MET, RAMAN, RX, diélectriques sur les poudres et les céramiques, suggèrent la présences de transitions de phases générées par la présence d'un paramètre d'ordre similaire à celui observé dans modélisation ab-initio.