Ricardo Jiménez

(Bi3TiNbO9)x(SrBi2Nb2O9)1−x aurivillius type structure piezoelectric ceramics obtained from mechanochemically activated oxides

Ferroelectric ceramics of (Bi3TiNbO9)x(SrBi2Nb2O9)1−x compositions are proposed in this work for use as high temperature piezoelectrics. Novel compositions with x>0.40 have been processed and studied. This type of compound grows in lamellar crystals, resembling their layered Aurivillius type structure, with the perovskite c-axis perpendicular to the major surfaces, which tend to pile up with the c-axis parallel to the applied pressure when ceramics are obtained by hot pressing. Such texture does not favor the ferro-piezoelectric properties. Isotropic ceramics with low porosity were processed by natural sintering of a mechanochemically activated oxide mixture. Optical microscopy and computer-aided image analysis and measurements were used in the characterization of the ceramic microstructure. Ceramics with transition temperatures in the range of 420°C≲Tt≲900°C can be obtained. Ferro-piezoelectric characterization shows that ceramics with x=0.60 present Tt=700°C, d33=11 pC N−1 and Kt=9.45%, which make them good candidates to be tested as high temperature piezoelectrics.

Three-dimensional microchanelled electrodes in flow-through configuration for bioanode formation and current generation

Three-dimensional microchannelled nanocomposite electrodes fabricated by ice-segregation induced self-assembly of chitosan-dispersed multiwall carbon nanotubes are shown to provide a scaffold for growth of electroactive bacteria for use as acetate-oxidizing bioanodes in bioelectrochemical systems. The hierarchical structure provides a conductive surface area available for G. sulfurreducens colonization, with a flow through configuration along the electrode providing a substrate for bacterial colonization and bio-electrochemical processes. This configuration, whilst resulting in sub-monolayer biofilm coverage over the three-dimensional surface, is capable of providing acetate oxidation current densities of up to 24.5 A m−2, equating to a volumetric current density of 19 kA m−3, in the flow-through configuration. Such bioanodes, when operated in non-optimized flow-through microbial fuel cell configuration, provide a maximum power density of 2.87 W m−2, which is equivalent to 2.0 kW m−3 volumetric power density.

Deep-Eutectic-Solvent-Assisted Synthesis of Hierarchical Carbon Electrodes Exhibiting Capacitance Retention at High Current Densities

Nature provides a wide range of entities and/or systems with different functions that may serve as a source of bioinspiration for material chemists. Actually, materials exhibiting a 3D porous texture (combining pores at different scales, from macroto mesoup to micropores) mimic the hierarchical structure of different systems found in living organisms (e.g., the blood circulation or the respiratory system in mammalians). Structural organization at different scales is ultimately responsible for the outstanding properties offered by hierarchical materials (not only as stationary phases in separation and catalytic processes, but also as electrodes in fuel cells and capacitors) because they offer not only large surface areas, but also accessibility to such a surface. A number of synthetic routes have been explored by using different carbonaceous precursors and either exo or endo templates to modulate the porous texture of the resulting carbon structures. Recent efforts have also been focused on the preparation of porous carbon composites containing graphitic carbon entities (e.g., carbon nanotubes and nanohorns, or even graphene oxide) the challenge of which is double and resides in 1) the achievement of a homogenous dispersion of these entities throughout the monolith structure and 2) the preservation of high surface areas. Baumann and co-workers have recently performed a quite extensive and stimulating work on single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) synthesized by resorcinol–formaldehyde polycondensation. Particularly interesting in terms of conductivity and surface area were those composites based on DWCNTs, although the authors expressed the convenience of substituting the surfactants used for carbon-nanotube (CNT) dispersion. Ionic liquids (ILs) and deep eutectic solvents (DESs, a new class of ILs obtained by complexion of quaternary ammonium salts with hydrogen-bond donors, such as acids, amines, and alcohols) have lately been the solvent of choice in a number of chemical processes because of special features: for example, they are nonreactive with water, nonvolatile, and biodegradable as well as excellent solvents for a wide variety of solutes, such as different substrates, enzymes, and even microorganisms of catalytic and biocatalytic interest. Of particular interest for the purpose of this work are those processes for which the capability both as solvents for CNT dispersion and structure-directing agents in the synthesis of different materials was demonstrated. Actually, ILs and DESs have been used as solvents for preparation of CNT-based carbon composites and even as carbonaceous precursors of both nontextured and textured carbons. In particular, we have recently described the preparation of DESs based on mixtures of resorcinol and choline chloride, the rupture of which (via resorcinol polycondensation and subsequent segregation of choline chloride) resulted in the formation of bimodal porous carbons. In this case, hierarchy was obtained through a synthetic mechanism that combined aspects from those original works used for synthesis of zeolites (i.e., based on DES rupture and controlled delivery of an organic template to the reaction mixture) and those used for synthesis of macroporous structures (i.e. , based on spinodal-like processes) . It is also worth noting the “green” character of the process as a result of the absence of residues and/or byproducts eventually released after the synthetic process, that is, one of the components forming the DES (e.g., resorcinol) becomes the material itself, whereas the second one (e.g., choline chloride) is fully recovered and can be reused in subsequent reactions. Based on these previous results, we considered that the use of DESs could open an interesting path for the preparation of hierarchical porous CNT composites. Herein, we describe the preparation of hierarchical porous multiwalled CNT (MWCNT) composites exhibiting high surface areas and outstanding conductivities through furfuryl alcohol (FA) condensation catalyzed by a protic DES based on complexes of para-toluene sulfonic acid [a] Dr. M. C. Guti rrez, Dr. D. Carriazo, Dr. R. Jim nez, Dr. J. M. Rojo, Dr. M. L. Ferrer, Dr. F. del Monte Instituto de Ciencia de Materiales de Madrid-ICMM Consejo Superior de Investigaciones Cient ficas-CSIC Campus de Cantoblanco, 28049-Madrid (Spain) E-mail : mcgutierrez@icmm.csic.es delmonte@icmm.csic.es [b] Dr. A. Tamayo Instituto de Ceramica y Vidrio-ICV Consejo Superior de Investigaciones Cient ficas-CSIC. Campus de Cantoblanco, 28049-Madrid (Spain) [c] Dr. F. Pic Centro Nacional de Investigaciones Metalurgicas-CENIM Consejo Superior de Investigaciones Cient ficas-CSIC Av. Gregorio del Amo s/n, 28040-Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101679.

Size effect in morphotropic phase boundary Pb(Mg1∕3Nb2∕3)O3–PbTiO3

Phases, domain configuration, and properties of 0.65Pb(Mg1∕3Nb2∕3)O3–0.35PbTiO3 ceramics with grain sizes of 4 and 0.15μm have been studied. The average phase is monoclinic Pm in coexistence with tetragonal. An evolution from micron-sized lamellar domains towards submicron/nanometer sized crosshatched domains is found with the decrease in size, which results in electrical relaxor type behavior and hindered switching. This is proposed to be associated with the slowing down of the relaxor to ferroelectric transition that causes the long time presence of an intermediate domain configuration. Nevertheless, a high sensitivity piezoelectric submicron-structured material is obtained under tailored poling (d33∼300pCN−1).

Dielectric and mechanoelastic relaxations due to point defects in layered bismuth titanate ceramics

Complex permittivity and Youngs modulus provide relevant information on the role of point defects in the dielectric and mechano-elastic properties of ferroelectric materials. Low-frequency measurements as a function of the temperature performed on Bi4Ti3O12 (BIT) have shown that point and dipole defects are frozen close to domain walls. Low-temperature dipole defect relaxation processes take place with characteristic times (τ0) of the order of 10-11 s and 10-12 s and activation energies (Ea) of 0.70 eV and 0.65 eV for dielectric and mechano-elastic relaxations, respectively. At higher temperatures new dielectric relaxation peaks appear that can be attributed to jumps of de-iced oxygen vacancies (τ010-11 s, Ea = 1.08 eV, T300 °C) and to vacancy migration (τ010-15 s, Ea = 1.90 eV, T450 °C). Elastic relaxation peaks are also present close to 300 °C whose activation energy (1.50 eV) and characteristic time (10-15 s) suggest a vacancy migration process. Close to 500 °C with Ea = 2.30 eV and τ010-17 s another relaxation peak, which should correspond to domain wall viscous motion near the phase transition temperature, is observed. The Youngs modulus has a smooth step at T300 °C that we attribute to a change in the mobility of oxygen vacancies with respect to the domain walls. Below 300 °C the vacancies are frozen in the domain walls and they are de-iced and distributed throughout the material at temperatures above 300 °C. The experimental results show that the material is softer when the vacancies are linked to domain walls than when they are distributed throughout the material. The diffusion of vacancies back to the domain wall traps at room temperature takes a long time (days).

Synthesis of macroporous poly(acrylic acid)–carbon nanotube composites by frontal polymerization in deep-eutectic solvents

Deep Eutectic Solvents (DESs) formed between Acrylic Acid (AA) and Choline Chloride (CCl) exhibit certain properties of ionic liquids (e.g. high viscosity) that make them suitable for frontal polymerization (FP). The use of DESs not only as a monomer but also as the solvent prevents the use of additional solvents (i.e. typically of organic nature) and offers a green tool for the synthesis of functional composites. We have recently explored this approach for the preparation of poly(acrylic acid) (PAA) and poly(methacrylic acid). In this work, we have taken advantage of the outstanding capability of DESs to solubilize and/or disperse a number of substances to incorporate – in a homogeneous fashion – carbon nanotubes (in this particular case, N-doped MWCNT – CNxMWCNTs) in the polymerizable DES. Interestingly, CNxMWCNTs also played the role of an inert filler in FP. The resulting PAA–CNxMWCNT composites exhibited some distinct features as compared to previous PAA also obtained via DES-assisted FP. For instance, PAA–CNxMWCNT composites can undergo swelling depending on the pH, as bare PAA. However, the presence of CNxMWCNTs allows the formation of a macroporous structure after submission to a freeze-drying process, the achievement of which was not possible in bare PAA. The combination of structural (e.g. macroporosity) and functional (e.g. stimuli responsive) properties exhibited by these materials besides an eventually high biocompatibility – coming from the green character of the DES-assisted synthesis – should make the resulting macroporous PAA–CNxMWCNT composites excellent candidates for their future application as biomaterials.

Activated Solutions Enabling Low‐Temperature Processing of Functional Ferroelectric Oxides for Flexible Electronics

Functional ferroelectric oxides for flexible electronics are achieved from activated solutions enabling low-temperature processing and large-area deposition directly on polymeric substrates. This processing technology reaches the lower limit temperature of crystallization at 300 °C, using a strategy that combines seeded diphasic precursors and photochemical solution deposition. Properties of these materials are comparable to those of high-temperature-processed counterparts and organic ferroelectrics.

Structural Effects Behind the Low Temperature Nonconventional Relaxor Behavior of the Sr2NaNb5O15 Bronze

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.

Monoclinic morphotropic phase and grain size-induced polarization rotation in Pb(Mg1∕3Nb2∕3)O3–PbTiO3

A detailed Rietveld analysis of x-ray data, collected at room temperature, was done on ceramics with controlled grain size between 100nm and 4μm for (PbMg1∕3Nb2∕3O3)0.80–(PbTiO3)0.20 (PMN-PT20), i.e., a compound at the border of the so-called morphotropic phase boundary. With size reduction the polarization rotates within the monoclinic plane from MB, i.e., Px=Py>Pz to MA, i.e., Px=Py<Pz, and finally reaches a rhombohedral phase, i.e., Px=Py=Pz, below a critical value of ∼200nm without diminishing the amplitude of the polarization. This study provides an easy way to tailor the direction of polarization of these materials.

Pulsed hysteresis loops on ferroelectric thin films

An alternative method to obtain hysteresis loops for ferroelectric thin films is described that overcomes the drawbacks of the classical methods. In this method, a pulsed signal, obtained by mixing a sinusoidal voltage of low frequency with a square voltage of variable period and width, is applied to a (Pb0.76Ca0.24)TiO3 thin film. Electric field is applied to the sample for a short time and so, contribution of extra charges to the hysteresis loops is very small. The experimental loops measured only need a small compensation.