L’activitat científica del grup es centra en l’estudi de les propietats elèctriques i electròniques de materials i dispositius, de cara a les seves aplicacions. S’incideix en aspectes que van des de la caracterització i millora dels materials actius fins a la optimització de dispositius i sistemes. La recerca esta dotada de un fort contingut interdisciplinari; requereix coneixements de química (síntesi de materials micro i nanoestructurats) i física de estat sòlid (propietats de dielèctrics i ferroelèctrics, processos de conducció en nous materials, etc.), així com de electrònica i de eines matemàtiques complexes. L’objectiu bàsic de l’activitat científica del grup és contribuir al coneixement de la relació processament-estructura-propietats de materials funcionals avançats amb noves i millors propietats, amb potencial ús en aplicacions diverses. Per altra banda, contribuir a la optimització de sistemes, mitjançant una adequada simulació de les seves propietats no lineals.
Rubio-Marcos, F.; Fernández, J.F; Ochoa, D. A.; Garcia, J. E.; Rojas-Hernandez, R. E.; Castro, M.; Ramajo, L. Journal of the european ceramic society Vol. 37, num. 11, p. 3501-3509 DOI: 10.1016/j.jeurceramsoc.2017.04.045 Data de publicació: 2017-09 Article en revista
Lead zirconate titanate (PZT) based ceramics are currently enjoying a wide use in piezoelectric devices despite lead toxicity. Due to growing environmental concerns, the attention on piezoelectric ceramics has been moving to lead-free materials, in particular to (K,Na)NbO3-based ceramics. Here we report a systematic evaluation of the effects of the compositional modifications on [(K0.44Na0.52Li0.04)[(Nb0.86Ta0.10Sb0.04)1-xZr5x/4]O3 lead–free piezoceramics. We show that an interrelationship between the intrinsic and extrinsic factors is the linchpin for the development of good piezoelectric properties. Hence, the stabilization of the tetragonal symmetry on the orthorhombic-tetragonal polymorphic phase boundary facilities the poling process of the system, thereby enhancing the piezoelectric response. Additionally, the microstructure appears to be related to the piezoelectric properties; i.e., the improved piezoelectric properties correlate to the increase in grain size. The results of this work could help to understand the origin of piezoelectricity in potassium–sodium niobate-based ceramics.
This paper deals with the global dynamical analysis
of a self-oscillating resonant inverter which is based on switching
between two symmetrical circuit configurations. The analysis
predicts coexisting steady-state solutions, which are increasingly
relevant for low values of the quality factor of the resonant circuit,
thus likely driving to an improper system operation. A repelling
sliding region is found to be connected with the two unstable limit
cycles that split the phase plane in three basins of attraction. To
avoid the presence of multiple steady-state solutions, a viable
solution consisting of a modification of the control is proposed
and validated by numerical simulations.
Ochoa, D. A.; Levit, R.; Fancher, C.M.; Esteves, G.; Jones, J.L.; Garcia, J. E. Journal of Physics D: Applied Physics (Online) Vol. 50, num. 20, p. 205305-1-205305-14 DOI: 10.1088/1361-6463/aa6b9e Data de publicació: 2017-04-26 Article en revista
Ordinary ferroelectrics exhibit a second order phase transition that is characterized by a sharp peak in the dielectric permittivity at a frequency-independent temperature. Furthermore, these materials show a low temperature dielectric relaxation that appears to be a common behavior of perovskite systems. Tetragonal lead zirconate titanate is used here as a model system in order to explore the origin of such an anomaly, since there is no consensus about the physical phenomenon involved in it. Crystallographic and domain structure studies are performed from temperature dependent synchrotron x-ray diffraction measurement. Results indicate that the dielectric relaxation cannot be associated with crystallographic or domain configuration changes. The relaxation process is then parameterized by using the Vogel–Fulcher–Tammann phenomenological equation. Results allow us to hypothesize that the observed phenomenon is due to changes in the dynamic behavior of the ferroelectric domains related to the fluctuation of the local polarization.
In this paper, the dynamics of a dc-ac resonant self-oscillating LC series inverter is analyzed
from the point of view of piecewise smooth dynamical systems. Our system is defined by two
symmetric configurations and its bifurcation analysis can be given in a one dimensional param-
eter space, thus finding a non smooth transition between two strongly different dynamics. The
oscillating regime, which is the one useful for applications and involves a repetitive switching
action between those configurations, is given whenever their open loop equilibrium is a fo-
cus. Otherwise, the only attractors are equilibrium points of node type whose stable manifolds
preclude the appearance of oscillations.
In this paper, the switching dynamics of a dc-ac
resonant self-oscillating inverter is considered. Using bifurcation
analysis coexisting steady-state solutions are predicted, which
are increasingly relevant for low values of the quality factor
of the resonant circuit. A repelling sliding region is found to be
connected with the two unstable limit cycles that split the phase
plane in three basins of attraction. Simulation results obtained
from the switched model confirm the theoretical derivations.
Machado, R.; dos Santos, V.B.; Ochoa, D. A.; Cerdeiras, E.; Mestres, L.; Garcia, J. E. Journal of alloys and compounds Vol. 690, p. 568-574 DOI: 10.1016/j.jallcom.2016.08.116 Data de publicació: 2017-01-05 Article en revista
A systematic study of the functional properties of the (1-x)(Bi0.5Na0.5)TiO3–xBaTiO3 (BNT-xBT) piezoceramic system for 0.05 = x = 0.07 is performed. The samples are obtained through the conventional solid-state route. The expected microstructure for these compounds, with no significant dependence on the composition, is verified by field-emission scanning electron microscopy. The morphotropic phase boundary (MPB) is detected for x = 0.06–0.07 by means of the Rietveld analysis of X-ray diffraction data. The dielectric spectra show a frequency-independent, completely diffuse phase transition with a composition-dependent diffusivity coefficient. The depolarization temperature is effectively evaluated from pyroelectric measurements, the value being strongly dependent on the composition. A significant contribution of the extrinsic effect to elastic, dielectric and electromechanical properties is revealed for MPB BNT-xBT. The Bi3+ substitution by Ba2+ leads to the formation of A-site vacancies, which give rise to the enhancement of domain wall motion, as occurs in other perovskite-type piezoelectrics. Good functional properties are achieved for x = 0.07 (d33 = 180 pC/N), which are similar or even better than those obtained by complex synthesis routes. This system exhibits a remarkable stability in the permittivity that has hitherto not been reported. This fact may open the way for BNT-BT compositions to be used in specific applications in which lead-free piezoceramics have previously been employed with little success, e.g. in high power devices.
Arcos, D.; Gabriel, D.; Dumcenco, D.; kis, A.; Ferrer-Anglada, N. Physica status solidi B. Basic solid state physics Vol. 253, num. 12, p. 2499-2504 DOI: 10.1002/pssb.201600281 Data de publicació: 2016-12-09 Article en revista
In the increasing research field of 2D materials such as graphene, molybdenum disulfide MoS2 has attracted great interest due to the existence of a direct bandgap in monolayer MoS2, which gives the possibility of achieving MoS2 field-effect transistors or optoelectronic devices. We analyzed by THz time-domain spectroscopy (THz-TDS) up to 2 THz and infrared (IR) spectroscopy, CVD-obtained MoS2 using either S or H2S gas as a sulfur precursor, grown on a sapphire substrate. From THz-TDS we obtained the transmittance, conductivity, and attenuation. From IR spectroscopy on the same samples, we deduced the transmittance in the IR frequency range. We observed the coherence of both spectroscopic methods. The advantage of the THz-TDS method is that we can get significant parameters related to the sample quality without the need for depositing any electrical contact or sample preparation. Our results show that at high frequencies MoS2 is even better than graphene as a material for optoelectronic devices.
Machado, R.; Ochoa, D. A.; dos Santos, V.B.; Cerdeiras, E.; Mestres, L.; Garcia, J. E. Materials letters Vol. 183, p. 73-76 DOI: 10.1016/j.matlet.2016.07.045 Data de publicació: 2016-11-15 Article en revista
The (1-x)Bi0.5Na0.5TiO3–xBaTiO3 (BNT-xBT) system with composition at its morphotropic phase boundary (MPB) has received significant attention because of their attractive properties as lead-free piezoceramics. Although the basic properties of this system are well-established, reports about the stability of the functional properties of these piezoelectric materials are still lacking. A study on the dielectric and piezoelectric properties of BNT-xBT close to their MPB, with emphasis on material response under high electric field or mechanical stress, is performed in this work. The results indicate that the BNT-BT system exhibits a high stability of dielectric and piezoelectric properties, making it potentially interesting for specific applications. A direct correlation between piezoelectric properties and nonlinear response is evidenced for a wide number of piezoceramics, which is expected due to the extrinsic nature of the piezoelectric response. Finding compositions that show high electromechanical properties and low nonlinear behavior is a challenge in the search for competitive lead-free piezoceramics.
Vendrell, X.; Garcia, J. E.; Cerdeiras, E.; Ochoa, D. A.; Rubio-Marcos, F.; Fernández, J.F; Mestres, L. Ceramics international Vol. 42, num. 15, p. 17530-17538 DOI: 10.1016/j.ceramint.2016.08.066 Data de publicació: 2016-11-15 Article en revista
Lanthanides-doped K0.5Na0.5NbO3(KNN) lead-free piezoelectric ceramics are prepared by conventional solid-state reaction. The effects of lanthanum concentration and the lanthanide type on the structure, microstructure and ferro-piezoelectric properties are evaluated. Ln3+ doping has a slight effect on the structure, but greatly inhibits the grain growth. Moreover, a decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. Large amounts of lanthanum induce a diffuse phase transition and an increase of the dielectric losses. The piezoelectric properties are greatly improved when doping with small amounts of dopants. As the ionic radii of the lanthanide is reduced, the piezoelectric properties of the ceramics are increased, the Eu3+-doped ceramics show an increase of 29% of the piezoelectric coefficient d33 with respect to pure KNN. The results show that low concentrations of lanthanides improve significantly the functional properties of KNN lead-free piezoceramics. Doping with lanthanides should be taken into consideration in some other compositions based on KNN.
Pérez-Madrigal, Maria M.; Ochoa, D. A.; Garcia, J. E.; Armelin, E.; Aleman, C. Journal of polymer science. Part B, polymer physics Vol. 54, num. 18, p. 1896-1905 DOI: 10.1002/polb.24095 Data de publicació: 2016-09-15 Article en revista
Dielectric elastomer actuators (DEAs) transform electrical energy into mechanical work. However, despite displaying exceptional features, the low permittivity of elastomers restricts their application. Hence, to overcome this limitation, DEAs are fabricated by dispersing poly(3-methylthiophene acetate) (P3TMA), a polarizable conducting polymer, into poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), a thermoplastic elastomer with excellent mechanical properties. Although high-quality SEBS:P3TMA films are obtained for all compositions (between 0.5 and 20 wt % P3TMA), their thickness and surface roughness increase with the nano-sized filler content. Moreover, the conducting particles are well integrated into the SEBS network with no evidence of aggregation or significant change in the mechanical properties of the composites. P3TMA, which forms encapsulated conductive domains within the polymeric matrix, improves the dielectric behavior of SEBS:P3TMA by increasing their dielectric constant with low dielectric losses and no current leakage. Thus, indicating the potential future application of these nanocomposites as elastomer actuators or high energy density capacitors.
Ochoa, D. A.; Esteves, G.; Iamsasri, T.; Rubio-Marcos, F.; Fernández, J.F; Garcia, J. E.; Jones, J.L. Journal of the european ceramic society Vol. 36, num. 10, p. 2489-2494 DOI: 10.1016/j.jeurceramsoc.2016.03.022 Data de publicació: 2016-08 Article en revista
The origins of high piezoelectric properties in the lead-free (K,Na)NbO3-based tetragonal composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 (KNL-NTS) is investigated by quantifying the intrinsic and extrinsic contributions from high energy X-ray diffraction measurements. The applied methodology, which allows discerning between the intrinsic contribution, related to the field induced lattice distortion, and the extrinsic contributions, related to non-180° domain wall motion, is widely described in this work. The non-180° domain reorientation of the KNL-NTS piezoceramic is quantify from the integrated intensities of the 002 and 200 reflections obtained from line profile, while the shifts in peak position versus the applied electric field is used to obtain the lattice strain contribution. Large non-180° domain wall contribution to the electric field induced macroscopic strain (80% of the macroscopic strain) is verified in KNL-NTS.
Cerdeiras, E.; Ochoa, D. A.; Garcia, J. E.; Mestres, L. Reunión Científico Plenaria de Química Inorgánica y Reunión Científico Plenaria de Química del Estado Sólido Data de presentació: 2016-06 Presentació treball a congrés
Amaral, T. M.; Antonelli, E.; Ochoa, D. A.; Garcia, J. E.; Hernandes, A. Ceramics international Vol. 42, num. 7, p. 8488-8494 DOI: 10.1016/j.ceramint.2016.02.071 Data de publicació: 2016-05-15 Article en revista
The dielectric, pyroelectric and ferroelectric properties of bilayered BaTiO3/BaTi0.8Zr0.2O3 ceramics are described and correlated with their microstructure. Different sintering times are employed to change the microstructure and promote interdiffusion between the layers. The effects of constrained sintering on both compositions are analyzed and their properties are compared to that of single phase BaTiO3 and BaTi0.8Zr0.2O3 ceramics. The results show that, at sintering times until 2 hours, the bilayer properties are predominantly affected by the presence of residual stresses. Only after 4 hours sintering, the properties are predominantly affected by interdiffusion between the layers.
Ochoa, D. A.; Esteves, G.; Jones, J.L.; Rubio-Marcos, F.; Fernández, J.F; Garcia, J. E. Applied physics letters Vol. 108, num. 14, p. 142901-1-142901-5 DOI: 10.1063/1.4945593 Data de publicació: 2016-04-04 Article en revista
Polymorphic phase boundaries (PPBs) in piezoelectric materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity. However, to fully harness their potential as micro-nanoscale functional entities, it is essential to achieve reliable and precise control of their piezoelectric response, which is due to two contributions known as intrinsic and extrinsic. In this work we have used a (K,Na)NbO3-based lead-free piezoceramic as a model system to investigate the evolution of the extrinsic contribution around a PPB. X-ray diffraction measurements are performed over a wide range of temperatures in order to determine the structures and transitions. The relevance of the extrinsic contribution at the PPB region is evaluated by means of nonlinear dielectric response measurements. Though it is widely appreciated that certain intrinsic properties of ferroelectric materials increase as PPBs are approached, our results demonstrate that the extrinsic contribution also maximizes. An enhancement of the extrinsic contribution is therefore also responsible for improving the functional properties at the PPB region. Rayleigh’s law is used to quantitatively analyze the nonlinear response. As a result, an evolution of the domain wall motion dynamics through the PPB region is detected. This work demonstrates that the extrinsic contribution at a PPB may have a dynamic role in lead-free piezoelectric materials, thereby exerting a far greater influence on their functional properties than that considered to date.
The Preisach model is a classical method for describing nonlinear behavior in hysteretic systems. According to this model, a hysteretic system contains a collection of simple bistable units which are characterized by an internal field and a coercive field. This set of
bistable units exhibits a statistical distribution that depends on these fields as parameters. Thus, nonlinear response depends on the specific distribution function associated with the material. This model is satisfactorily used in this work to describe the temperature-dependent ferroelectric response in PZT- and KNN-based piezoceramics. A distribution function expanded in Maclaurin series considering
only the first terms in the internal field and the coercive field is proposed. Changes in coefficient relations of a single distribution function allow us to explain the complex temperature dependence of hard piezoceramic behavior. A similar analysis based on the same form of the distribution function shows that the KNL–NTS properties soften
around its orthorhombic to tetragonal phase transition.
Cristiano, R.; Pagano, D.; Benadero, L.; Ponce, E. International journal of bifurcation and chaos Vol. 26, num. 4, p. 1630010-1-1630010-18 DOI: 10.1142/S021812741630010X Data de publicació: 2016-04 Article en revista
Direct current (DC) microgrids (MGs) are an emergent option to satisfy new demands for power quality and integration of renewable resources in electrical distribution systems. This work addresses the large-signal stability analysis of a DC–DC bidirectional converter (DBC) connected to a storage device in an islanding MG. This converter is responsible for controlling the balance of power (load demand and generation) under constant power loads (CPLs). In order to control the DC bus voltage through a DBC, we propose a robust sliding mode control (SMC) based on a washout filter. Dynamical systems techniques are exploited to assess the quality of this switching control strategy. In this sense, a bifurcation analysis is performed to study the nonlinear stability of a reduced model of this system. The appearance of different bifurcations when load parameters and control gains are changed is studied in detail. In the specific case of Teixeira Singularity (TS) bifurcation, some experimental results are provided, confirming the mathematical predictions. Both a deeper insight in the dynamic behavior of the controlled system and valuable design criteria are obtained.
Piezoceramic materials generally exhibit a notable instability of their functional properties when they work under real external conditions. This undesirable effect, known as nonlinear behavior, is mostly associated with the extrinsic contribution to material response. In this article, the role of the ferroelectric domain walls' motion in the nonlinear response in the most workable lead-based and lead-free piezoceramics is reviewed. Initially, the extrinsic origin of the nonlinear response is discussed in terms of the temperature dependence of material response. The influence of the crystallographic phase and of the phase boundaries on the material response are then reviewed. Subsequently, the impact of the defects created by doping in order to control the extrinsic contribution is discussed as a way of tuning material properties. Finally, some aspects related to the grain-size effect on the nonlinear response of piezoceramics are surveyed.
Gabriel, D.; Bernat Sempere; Colominas, C.; Ferrer-Anglada, N. Physica status solidi B. Basic solid state physics Vol. 252, num. 11, p. 2423-2428 DOI: 10.1002/pssb.201552248 Data de publicació: 2015-10-23 Article en revista
Optoelectronic properties of CVD graphene are charac-terized over a wide frequency range: THz, IR, visible and near-UV. We used Raman spectroscopy to characterize the synthesized graphene films. All graphene layers were deposited on various substrates, some ones transparent or flexible, such as polyethylene terephthalate (PET), poly-ethylene naphthalate (PEN), quartz and silicon. Trans-mission Terahertz time-domain spectroscopy (THz-TDS) method, in the range from 100 GHz to 3 THz, is used to analyze the transmittance, sheet conductivity and attenua-tion of graphene and the complex refractive index of sub-strates. From IR, near-UV and visible spectroscopy we obtained the transmittance of the substrate and the sam-ple at those frequency ranges, and we deduced the gra-phene transmittance on each substrate. We found that it is close to 97% in most cases.
Optoelectronic properties of CVD graphene are charac-terized over a wide frequency range: THz, IR, visible and near-UV. We used Raman spectroscopy to characterize the synthesized graphene films. All graphene layers were deposited on various substrates, some ones transparent or flexible, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), quartz and silicon. Transmission Terahertz time-domain spectroscopy (THz-TDS) method, in the range from 100 GHz to 3 THz, is used to analyze the transmittance, sheet conductivity and attenuation of graphene and the complex refractive index of substrates. From IR, near-UV and visible spectroscopy we obtained the transmittance of the substrate and the sample at those frequency ranges, and we deduced the graphene transmittance on each substrate. We found that it is close to 97% in most cases.
Benadero, L.; Cristiano, R.; Pagano, D.; Ponce, E. IEEE Journal on Emerging and Selected Topics in Circuits and Systems Vol. 5, num. 3, p. 326-335 DOI: 10.1109/JETCAS.2015.2462017 Data de publicació: 2015-09-01 Article en revista
In this paper the nonlinear dynamics of interconnected power converters in an islanded direct current (DC) microgrid is analyzed. By using a simplified scheme based on two cascaded converters we analyze the dynamical behavior that can arise from the interconnection of these devices on a common DC bus. Furthermore, in order to address the bus voltage control problem, we propose a Sliding Mode Controller for a DC-DC bidirectional power converter to control the DC bus voltage under instantaneous constant power loads. This class of loads introduces a destabilizing nonlinear effect on the converter through an inverse voltage term that can lead to significant oscillations in the DC bus voltage. Simulation results are shown to illustrate the nonlinear analysis.
El Diccionari de física, elaborat conjuntament per la Universitat Politècnica de Catalunya, Enciclopèdia Catalana i el TERMCAT, aplega i defineix més de tres mil termes de l’àmbit de la física. Forma part de la col·lecció de diccionaris científics i tècnics Ciència i Tecnologia. El diccionari ha comptat amb la col·laboració de la Fundació Catalunya-La Pedrera i la Fundació Torrens-Ibern.
Guerra, J.; Betal, S.; Pal, M.; Garcia, J. E.; Oliveira, A.; M'Peko, J.-C.; Hernandes, A.; guo, R.; Bhalla, A. Journal of the American Ceramic Society Vol. 98, num. 5, p. 1542-1547 DOI: 10.1111/jace.13501 Data de publicació: 2015-05-01 Article en revista
The magnetoelectric (ME) properties of PbZr0.65Ti0.35O3-BaFe12O19 (PZT-BaM) multiferroic ceramic composites have been investigated in this work. These materials, containing both the ferroelectric PZT and ferrimagnetic BaM phases, were prepared from the conventional solid-state reaction method, and their main (micro) structural characteristics are provided. The ME measurements were performed over wide frequency and temperature ranges, and the influence of the BaM content was appraised. An enhanced ME coefficient (alpha) was obtained for the applied magnetic field of 0.8 kOe, at 97 kHz. In addition, the results reveal a high thermal stability of alpha in the whole analyzed temperature range, showing that the studied PZT-based ceramic composites have great potential for practical applications.
Recently, Bi0,5Na0,5TiO3 (BNT) based ceramics has been widely studied as one promising material lead-free for piezoelectric applications. The Ba addition in BNT ((Bi0,5Na0,5)1-xBaxTiO3 - BNT-xBT) has attracted considerable attention due to the existence of a rhombohedral-tetragonal morphotropic phase boundary (MPB) near x = 0.06, where it shows maximized piezoelectric properties. In this study, BNT-XBT ceramics near the MPB (0.05 = x = 0.07), were prepared by solid-state reaction method. The effect of Ba doping on structure and microstructure of the BNT-xBT ceramics was investigated. The X-ray diffraction results show MPB for 0.06 = x <0.07. Raman spectroscopy suggests tetragonal symmetry, wich presence of the band at 300 cm-1 for composition x = 0.07, confirming the presence of the MPB region below this composition. Scanning electron microscopy images indicate apparent decrease of the grain size with increasing Ba concentration.
Amaral, T. M.; Antonelli, E.; Ochoa, D. A.; Garcia, J. E.; Hernandes, A. Journal of the American Ceramic Society Vol. 98, num. 4, p. 1169-1174 DOI: 10.1111/jace.13417 Data de publicació: 2015-04-01 Article en revista
Validity of mixture rule for dielectrics in series configuration and the correlation between microstructure and electrical properties in bilayered BaTiO3/BaTi1-xZrxO3 ceramics were studied. Samples were obtained from BaTi1-xZrxO3 (BTZx) nanopowder synthesized by the polymeric precursor technique and had their microstructure, dielectric, piezoelectric, and ferroelectric properties investigated. These bilayered ceramics' properties were compared to the properties of homogeneous BTZx samples. And, also, the formers' electrical permittivities were compared with the predictions of the simple mixture rule. According to the results, the microstructures of the layers do not differ from the microstructure of the corresponding homogeneous BTZx ceramic. And pyroelectric coefficient measurements show that the electrical properties of the interface do not contribute to the functional properties of the bilayered samples. Nevertheless, on increasing Zr4+, the agreement between the experimental and the predicted permittivity of the bilayered ceramics is gradually reduced, mainly at temperatures where the permittivity is governed by the response of the layer containing Zr4+. As a mechanical joint between the layers, the interface induces stresses during sintering due to thermal mismatch between compositions, thereby affecting the bilayers' electrical properties. Our results show that interface's mechanical effects compromise the functional properties of layered ferroelectric ceramics.
Ferrer-Anglada, N.; Gabriel, D.; Iqbal, M.; Eom, J. International Winterschool on the Electronic Properties of Novel Materials-Molecular Nanostructures p. 57 Data de presentació: 2015-03-09 Presentació treball a congrés
Optoelectronic properties of CVD graphene are charac-terized over a wide frequency range: THz, IR, visible and near-UV. We used Raman spectroscopy to characterize the synthesized graphene films. All graphene layers were deposited on various substrates, some ones transparent or flexible, such as polyethylene terephthalate (PET), poly-ethylene naphthalate (PEN), quartz and silicon. Trans-mission Terahertz time-domain spectroscopy (THz-TDS) method, in the range from 100 GHz to 3 THz, is used to analyze the transmittance, sheet conductivity and attenua-tion of graphene and the complex refractive index of substrates. From IR, near-UV and visible spectroscopy we obtained the transmittance of the substrate and the sample at those frequency ranges, and we deduced the graphene transmittance on each substrate. We found that it is close to 97% in most cases.
Lead-free (K,Na)NbO3 (KNN) and La doped (K,Na)NbO3 (KNN-La) thin films are grown on SrTiO3 substrates using the chemical solution deposition method. The effect of adding different amounts of Na and K excess (0-20 mol%) is investigated. The results confirm the necessity of adding 20 mol% excess amounts of Na and K precursor solutions in order to avoid the formation of the secondary phase, K4Nb6O17, as confirmed by X-ray diffraction and Raman spectroscopy. Moreover, when adding a 20 mol% of alkaline metal excess, the thin films are highly textured with out-of-plane preferential orientation in the  direction of the  orientation of the substrate. Doping with lanthanum results in a decrease of the leakage current density at low electric field, and an increase in the dielectric permittivity across the whole temperature range (80-380 K). Although the (100)-oriented KNN and KNN-La films exhibited rounded hysteresis loops, at low temperatures the films show the typical ferroelectric hysteresis loops. (C) 2015 Elsevier B.V. All rights reserved.
Vendrell, X.; Garcia, J. E.; Bril, X.; Ochoa, D. A.; Mestres, L.; Dezanneau, G. Journal of the european ceramic society Vol. 35, num. 1, p. 125-130 DOI: 10.1016/j.jeurceramsoc.2014.08.033 Data de publicació: 2015-01-01 Article en revista
ZrO2 and TiO2 modified lead-free (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect of acceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field E-c and remanent polarization P-r is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d(33) = 134 pC/N and electromechanical coupling factor k(p) = 35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system. (C) 2014 Elsevier Ltd. All rights reserved.