Co-conception matériaux hyperfréquences : étude et réalisation de couches minces accordables de BST en vue d’une application réseau réflecteur

Nowadays, wireless systems have to be able to adapt to different environments and standards, requiring reconfigurable microwave devices in order to dynamically vary their characteristics. Among all possible technological solutions to achieve this reconfigurability, agile materials, more specifically ferroelectrics, have an increasing interest. The major advantages compared to other technologies are a low bias current and the absence of moving parts.This manuscript presents the optimization and characterization of BaSrTiO3 (BST) ferroelectric thin films elaborated by chemical solution deposition. The complex permittivity was measured as a function of frequency, temperature, DC and AC fields. The fundamental study by impedance spectroscopy allows describing the relations between the structural microscopic properties (doping, grain size, defects, etc.) and the different permittivity contributions, including domain wall motion. This also helps to better understand the origin of the tunability and the dielectric losses in the material.The developed thin films were used for the realization of reconfigurable reflectarray cells, optimized by parametric studies. Two cell versions having increasing complexity and performance have been designed, realized and measured. Cell modeling also helped to develop an equivalent circuit, suitable for fast and efficient optimization of dimensions. The results indicate that the BST is a valuable candidate for this kind of application.Un système sans fil doit dorénavant s’adapter à des environnements et des standards variés, ce qui demande des dispositifs hyperfréquences reconfigurables pour faire varier dynamiquement leurs caractéristiques. Parmi toutes les solutions technologiques possibles pour permettre cette reconfiguration, les matériaux agiles, et plus particulièrement les ferroélectriques, suscitent un intérêt croissant. Leurs avantages majeurs sont un faible courant de polarisation et l'absence de pièces en mouvement.Ce manuscrit présente l'optimisation et la caractérisation de couches minces ferroélectriques de BaSrTiO3 (BST) déposées par voie chimique (procédé sol-gel). La mesure de la permittivité complexe a été effectuée en fonction de la fréquence, de la température et des champs DC et AC. L'étude fondamentale par spectroscopie d'impédance permet de décrire les liens entre les propriétés structurales (dopage, taille de grains, défauts, etc.) et les différentes contributions à la permittivité et notamment les mouvements des parois de domaines. Ceci permet également de mieux comprendre l'origine de l'accordabilité et des pertes diélectriques dans ce matériau. Les couches minces élaborées ont été utilisées pour la réalisation de cellules pour réseau réflecteur reconfigurable, optimisées à l'aide d'une étude paramétrique. Deux versions de cellules, de complexité et de performances croissantes, ont été conçues, réalisées et mesurées. La modélisation des cellules a également permis de mettre au point un schéma équivalent, exploitable pour une optimisation rapide et efficace des dimensions. Les résultats obtenus indiquent que le BST est un candidat sérieux pour ce type d'applications

Effect of thermal annealing on dielectric and ferroelectric properties of aerosol-deposited 0.65Pb(Mg1/3Nb2/3)O3−0.35PbTiO30.65\text{Pb}(\text{Mg}_{1/3}\text{Nb}_{2/3})\text{O}_{3}-0.35\text{PbTiO}_{3} thick films

In this work, the effects of thermal annealing at 500 {\deg}C on aerosol-deposited $0.65\text{Pb}(\text{Mg}_{1/3}\text{Nb}_{2/3})\text{O}_{3}-0.35\text{PbTiO}_{3}$ thick films on stainless-steel substrates are investigated using two complementary methods at high and low applied external electric fields. The first one is Positive Up Negative Down method, which allows us to obtain information about the switching and non-switching contributions to the polarization. It shows that the as-deposited film is ferroelectric before annealing, since it has a switching contribution to the polarization. After annealing, both the switching and non-switching contributions to polarization increased by a factor of 1.6 and 2.33, respectively, indicating stronger ferroelectric behavior. The second method is based on impedance spectroscopy coupled with Rayleigh analysis. The results show that post-deposition thermal annealing increases the reversible domain wall contribution to the dielectric permittivity by a factor 11 while keeping the threshold field similar. This indicates, after annealing, domain wall density is larger while domain wall mobility remains similar. These two complementary characterization methods show that annealing increases the ferroelectric behavior of the thick film by increasing the domain wall density and its influence is visible both on polarization versus electric field loop and dielectric permittivity

A New Lower Bound on the Ergodic Capacity of Optical MIMO Channels

International audienceIn this paper, we present an analytical lower bound on the ergodic capacity of optical multiple-input multiple-output (MIMO) channels. It turns out that the optical MIMO channel matrix which couples the mt inputs (modes/cores) into mr outputs (modes/cores) can be modeled as a sub-matrix of a m × m Haar-distributed unitary matrix where m > mt, mr. Using the fact that the probability density of the eigenvalues of a random matrix from unitary ensemble can be expressed in terms of the Christoffel-Darboux kernel. We provide a new analytical expression of the ergodic capacity as function of signal-to-noise ratio (SNR). Moreover, we derive a closed-form lower-bound expression to the ergodic capacity. In addition, we also derive an approximation to the ergodic capacity in low-SNR regimes. Finally, we present numerical results supporting the expressions derived

Effect of the excitation waveform on the average power and peak power delivered by a piezoelectric generator

International audienceThe theoretical power delivered by piezoelectric generators (PG) is a key parameter for their design optimization and for the determination of the power management circuit which should be designed especially for the targeted device to be supplied. Most often, when the PG is connected to a resistive load, a maximum of average power occurs at a given load value which should be determined by sweeping the resistive load. Although the determination of this optimal load is obvious in the case of a sine signal, as it is matching the internal impedance of the PG, the cases of non-sine signals are more complex. In this paper, we show that the waveform of the mechanical force applied to the PG significantly influences the generated power and the optimal load. We propose a method to predict the average power produced by a PG as a function of the load, even if the waveform is not a sine. The study is focused on a square waveform with variable exponential rising time, corresponding to realistic mechanical excitations, and shows that the force rising time drastically influences the mean power delivered by PGs having a capacitive internal impedance, for example some ZnO nanowires-based PGs. We also show that the peak instantaneous power, often reported as PG performance metric in literature, largely overestimates the truly available power

Influence of topology and diode characteristics of AC-DC converters for low power piezoelectric energy harvesting

International audienceMost often, piezoelectric generators are connected to a AC-DC converter, also called rectifier, to obtain a continuous energy which can be regulated and stored. In the case of low power generators, the choice of the rectifier is critical since the losses into this circuit are not negligible and should be as low as possible. In this paper, different topologies of rectifier using different diode references are studied to rectify the output voltage of a piezoelectric generator. We show that the differences between the fullwave rectifier and a voltage doubler are quite small in terms of output power. The main differences are visible on the final voltage value and the optimal load. On the contrary, the choice of diodes is critical, especially the reverse leakage current which must be limited since it largely affects the output power for low power generators. The internal capacitance of the generator also plays an important role since it changes the capability of the generator to provide current. We show that low threshold voltage diodes are more appropriate for generators presenting a high internal capacitance while low leakage current are preferable for generators presenting a low internal capacitance value

INNOVATIVE SWITCHING TECHNOLOGIES FOR FUTURE TUNABLE WIRELESS COMMUNICATIONS SYSTEMS

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Dielectric losses due to domain walls in ferroelectric materials

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Decomposition of the different contributions to permittivity, losses, and tunability in BaSrTiO3 thin films using the hyperbolic law

International audienceIn this paper, the different contributions to the permittivity of a 1% manganese-doped BaSrTiO3 thin film are presented as a function of the applied DC field. The hyperbolic law has been used to discern the lattice, domain wall vibration, and pinning/unpinning contributions. This decomposition permits us to study the weight of the respective contribution in the total permittivity, the losses, and the tunability. By determining the figure of merit (FoM) of each contribution, the ratio between tun-ability and losses, it is possible to identify the phenomenon which should be limited or enhanced in order to optimize the material's dielectric properties. It is shown that the tunability of the domain wall contribution (approximately 80%) is very important compared to the lattice contribution (41%), the associated dissipation factor, however, is also much larger (0.2 instead of 0.014). Even if the domain wall contribution has been shown to be weak in the investigated thin film (less than 3% in permittivity and tunability), the weight of the losses is not negligible (around 18%). Hence, the domain contribution has to be limited in order to conserve a high FoM for the material. Moreover, it is shown that the AC field used for the material's characterization is important because it governs the weight of the domain wall losses and thus the FoM

Realization and characterization of manganese doped BST thin films for reflectarray applications

International audienceIn the present work, Mn-doped Ba1-xSrxTiO3 (BST) thin films were realized by Chemical Solution Deposition (CSD) on alumina as the foreseen application for a reflect array needs integration of the ferroelectric on an insulating substrate. The optimum dopant rate to be inserted depending on the material's defect density, we have studied BST doping with a manganese content ranging from 0 % to 2 %. The dielectric and electrical characteristics were investigated as a function of the Mn content in the frequency range of 100 Hz to 5 GHz

Filtering slot antenna using coupled line resonator

International audienceThis paper presents the design of a novel slot filter-ing antenna suitable for modern electronic devices integrating various wireless systems that have to coexist. The approach consists in feeding the slot antenna through a filtering microstrip-to-slot-line transition. This transition comprises a coupled line resonator which enables to create two out-of-band transmission zeros and by this way to increase drastically the frequency selectivity of the transition. As proof-of-concept, a slot filtering antenna has been designed for WLAN applications operating in the 2.4GHz band, by using the HFSS TM simulation tool. A prototype has been also fabricated, using a low-cost FR4 substrate, where 2 antennas are printed on a board in order to accommodate the requirements of a 2×2 MIMO system