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[EN] We show here for the first time the use of a cold sintering process (CSP) to sinter CoSb-based thermoelectric materials. CSP at 150 °C for 90 min under a uniaxial pressure of 750 MPa yields pieces with a relative density of 86 %, which is increased to around 92 % after a post-annealing at temperatures ≥ 500 °C in Ar atmosphere. The reported CSP produces Te doped-CoSb nanocomposites with similar morphological and structural characteristics to the starting nanopowders obtained by ball milling in air atmosphere. The post-thermal treatment induces grain coalescence and grain growth, crystallite size growth as well as compositional changes in the nanocomposite, decreasing the amount of the main phase, CoSb, and increasing the weight of secondary phase, CoSb, up to a 30 wt% at 600 °C. Remarkably, the average valence for the Co, Sb and Te absorbing atoms is neither transformed by the sintering process nor by the subsequent heat treatment. The functional response of the sintered thermoelectric nanocomposites exhibits a maximum figure of merit of 0.12(3) at room temperature for the nanocomposites sintered by CSP with a subsequent post-annealing at 500 °C. This is mainly due to its low thermal conductivity in comparison with similar powders sintered by other approaches, and it is explained by the morphological and structural properties. These findings represent an attractive alternative for obtaining efficient thermoelectric skutterudites by a scalable and cost-effective route.

Construction and demolition wastes (CDW) face low recycling rates due to the absence of reliable and industrially feasible quantification method for valorization. This study explores quantifying mixed construction materials through their homogenization, a novel proposal, and chemometric modeling. The problem materials were homogenized in known proportions for the acquisition of a statistical number of chemical data. Short-wave infrared and Raman spectroscopy (applied here for the first time) provided data for two independent models. Principal Component Analysis and Improved Kernel Partial Least Square Discriminant Analysis were employed for model construction. These models were successfully applied to a real CDW sample from a Spanish management plant, yielding comparable results. This cost-effective methodology allows the quantification of stony aggregates within CDW, enhancing their recycling potential in the ceramic sector. Addressing the scarcity of reliable CDW quantification procedures, this approach and the use of Raman technique hold promise for advancing CDW valorization and sustainability.

The scientific research output of individuals and institutions is commonly evaluated by the number of publications in journals with high impact factor and their corresponding citations. However, the interdisciplinary character of journals reporting advances in conservation science favors their spread over different categories of the Journal Citation Report (JCR). Here we report an analysis of the evolution of the impact factor, the percentile, and the citations on the JCR indexed conservation journals over the last ten years. Our analysis shows that of the 57 conservation journals indexed in the JCR, only eight of them have an impact factor. These journals are distributed across different categories in the Science Citation Index (SCI) and Social Science Citation Index (SSCI), and these categories are sometimes not suitable and decrease their metric impact. This analysis was complemented by a worldwide survey of conservation professionals on their opinions about publishing in these journals. The survey showed two different perspectives about conservation science journals in relation to the background and work of the responders. Conservators and restorers prefer conservation journals with case studies; while researchers prefer high-impact journals, even if they lie outside of the conservation area, so the overall impact factor of conservation journals remains low (low JIF, low Q values). This dichotomy could be fixed with the creation of a new category for heritage science journals in JCR.

[EN] This article presents an interlaboratory comparison (ILC) on Raman spectroscopy as a technique for relative quantification of the two most common polymorphs of titanium dioxide (TiO2)—anatase and rutile—in binary mixtures. Some standard methods are currently employed internationally for the determination of TiO2 content in samples (ISO 591-1, ASTM D3720-90), but require extensive sample preparation, do not distinguish between the two polymorphs or are accurate only for small fractions of either polymorph. Raman spectroscopy is a well-suited characterization technique for measuring and differentiating TiO2 in a fast, non-invasive way, while requiring no particular reagent or sample preparation. Eleven international participants conducted the study under the framework of Versailles Project on Advanced Materials and Standards. The collected data was analyzed by means of partial least squares regression after spectral preprocessing. The resulting models all show discrepancies of lower than 2% from the nominal values in the quantitative analysis over the concentration range of 5%–95% mixture fractions, with many datasets showing substantial improvement margins on this figure. The results of this ILC provide validation of Raman spectroscopy as a reliable method for quantification of TiO2 phases.

[EN] In this study, we have investigated the stress dynamics arising from the Orthorhombic-Tetragonal Polymorphic Phase Boundary (PPB) in lead-free potassium sodium niobate (KNN)-based ceramics and its temperature dependence, as a means to elucidate the characteristics of PPBs in lead-free piezoelectric oxides. The dynamics are dictated by the phase transitions that occur upon cooling from the cubic phase and the coexistence of different crystal structures. Our findings reveal that the growth of Orthorhombic phases is constrained by the high-temperature tetragonal phase distributions. In particular, it is evidenced that two main mechanisms regulate the decrease of stress processes in which structural and microstructural effects are correlated; the first one is associated to a purely microstructural effect in which bimodal grain distribution hinders the formation of non-180° domains. By contrast, the second is mainly governed by ferroelectric domain distribution and the occurrence of pseudo-cubic regions around room temperature associated with local structural heterogeneity (that is, polar nano regions, PNRs). Specifically, these mechanisms generate regions with different stress state and explain the widening of the phase transition due the phase coexistence.

Partículas de frita de hexametafosfato de sodio activadas y molturadas y material compuesto antimicrobiano que comprende dichas partículas de frita de hexametafosfato de sodio activadas y molturadas embebidas en un polímero termoplástico como polietileno de baja densidad (LDPE). La invención también incluye el procedimiento de obtención del material compuesto de la invención y un procedimiento de activación térmica de una sal de hexametafosfato de sodio para dar lugar a partículas de frita de hexametafosfato de sodio activadas y molturadas. El material antimicrobiano de la invención, se utiliza preferentemente en la industria alimentaria.

There is an important number of thermoelectric applications in the range of medium temperatures (between 200 and 600 °C), such as heat recovery in automotive applications, where these temperatures are easily reached in the exhaust or the motor. Among the various materials which exhibit good thermoelectric properties in this range, Skutterudites (with the general formula CoSb) stand out as some of the most used. In recent years, different improvements have been done to decrease their thermal conductivity, from using rattler atoms inside their structure to nanostructuring by sintering nanoparticles and increasing the number of grain boundaries. In this work, we present the subsequent development of the nanopowder ball-milling fabrication method, to obtain a scalable production of n-type and p-type nanostructured Skutterudites. The sintering of these powders into nano-composites was done by different pressing and annealing treatments, which not only could substitute other sintering processes such as Spark Plasma Sintering, giving competitive values of the thermoelectric properties of the final nanostructured material, but can also be used to directly obtain different geometries to be used in certain implementations. This procedure has been tested by fabricating thermoelectric legs of the thermoelectric nanostructured material in the shape of rings, in contrast with the more generally available thermoelectric modules, which have a planar architecture. The ring-shaped architecture was obtained by pressing the nano-powders between copper rings, and from them, cylindrical thermoelectric generators were implemented, which present several of advantages for their use in automotive waste heat recovery from the exhaust, for instance. The optimal parameters, as far as geometry and the materials used in the final device, can be extracted from simulations based on the actual measurements obtained in the fabricated thermoelectric rings. Taking all these into account, in this work w

The present invention relates to activated and ground sodium hexametaphosphate frit particles and antimicrobial composite material comprising said activated and ground sodium hexametaphosphate frit particles embedded in a thermoplastic polymer such as low density polyethylene (LDPE). The invention also relates to the method for obtaining the composite material of the invention and a thermal activation method for the thermal activation of a sodium hexametaphosphate salt in order to generate the activated and ground sodium hexametaphosphate frit particles. The antimicrobial material of the invention is preferably used in the food industry.

The present invention relates to a security marker; a method of preparing same; the use of said security marker; a security article, document, or element comprising said marker; the use of said security article, document, or element; an object of value comprising said marker; a method of preparing said security article, document, or element or said object of value; a method for determining the authenticity of said security article, document, or element or said object of value; and a system for determining the authenticity of said security article, document, or element or said object of value.

Partículas de frita de hexametafosfato de sodio activadas y molturadas y material compuesto antimicrobiano que comprende dichas partículas de frita de hexametafosfato de sodio activadas y molturadas embebidas en un polímero termoplástico como polietileno de baja densidad (LDPE). La invención también incluye el procedimiento de obtención del material compuesto de la invención y un procedimiento de activación térmica de una sal de hexametafosfato de sodio para dar lugar a partículas de frita de hexametafosfato de sodio activadas y molturadas. El material antimicrobiano de la invención, se utiliza preferentemente en la industria alimentaria

[EN] In this work a microstructural qualitative and quantitative study of spatial stress distributions in modified KNN ceramics (KNaLi)Co (NbTaSb)O, according to the polarization state is shown. X-ray diffraction reflects a perovskite crystalline structure with coexistence of Tetragonal and Orthorhombic phases (T/O). Confocal Raman microscopy shows that these crystalline phases are distributed in randomly micrometric regions through the ceramic volume. Tetragonal regions show higher piezoelectric coefficient and exhibit a higher micro-stress that hardens the ferroelectric response. By the contrary, the occurrence of orthorhombic micro-regions softened the ferroelectric behavior and reduced their piezoelectric coefficients. The ferroelectric response of ceramics is studied, where poling is also shown as a factor that affects the spatial micro-stress distributions. Finally, a model that relates the results obtained by Raman characterization with the ferroelectric properties and stress states is proposed.

According to the EU-Directives 2002/95/EC, 2002/96/EC, lead-based piezoceramics must be substituted in the future with more environmentally friendly alternatives, only when a reliable alternative is found. This is why an increasing interest has grown in the research community to find lead free piezoelectric materials that fulfil the requirements for this substitution. Different families of compounds have been shown to be possible candidates for this use, such as bismuth and niobates based perovskites, pyrochlores, etc. However, a material with piezoelectric coefficients similar to those of PZT (lead zirconate titanate, Pb[ZrxTi1-x]O3) has not been yet found. Besides, each of these families has its specific characteristics in terms of remnant polarization, coercive field or application temperature. Thus, the choice of each material should be made according to the specific needs of the application. In this sense, Aurivillius-type structure materials (also known as Bismuth Layered Structure Ferroelectrics, BLSF) can take advantage of their specific properties for uses as Lead Free Piezoelectric systems. Some of them have a high Curie temperature, which make them good candidates to be used as high temperature piezoelectrics; high coercive fields, which facilitates their use as actuators; or a high switching fatigue resistance, which can be useful for future applications as Ferroelectric Random Access Memories (FERAM).

Nanocomposites that combine percolated nano-CoSb3 semiconductors with isolated nano-oxide clusters are shown as an effective approach to decouple electrical and thermal conductivity in thermoelectric applications through the formation of functional interfaces. This type of decoupling is very important to increase the Figure of Merit (zT) of thermoelectrics and it is one of the greatest challenges searched by the community. We carry out an innovative synthesis of Skutterudite/oxide nanocomposites in air by high energy milling and sintering by Spark Plasma Sintering, where functional interfaces are developed in situ. A confocal Raman Microscopy study evidences unequivocally the appearance of a space charge layer at the Skutterudite/oxide interfaces characterized by a crystal lattice softening and a surprising enhanced Raman signal. The functional interfaces act as highly effective phonon scattering and trapping centers that highly reduce the thermal conductivity, while a high electrical conductivity is achieved by selective doping the Skutterudite phase. This approach leads to Figures of Merit ca. 1 for Te-doped CoSb3. This work opens up new possibilities for the facile, large scale synthesis of well performing thermoelectric materials for medium range temperatures applications.

Nanocomposites with complex architectures play an important role in the increase of the Figure of Merit of thermoelectric materials. This is due to the capability of such structures to separate thermal and electrical conductivity, a difficult task in single phase materials. In this work, a nanocomposite with complex architecture that combines a high electric conductor phase (CoSb3) and different scattering points (nano‐oxides) at different scales are prepared by a combination of high energy milling in air, to have nanopowders, and Spark Plasma Sintering to compact it at the nanostructure scale. An important reduction of thermal conductivity and a controlled Figure of Merit with ZT values as high as 1.3 are achieved. This work opens the possibility of the processing of a new concept of nanocomposites as the control of electrical and thermal conductivity by different paths lead to an important improvement of the Figure of Merit.

Skutterudites have attracted great attention for their promising potential on thermoelectric applications, such as harvesting the heat generated in industrial processes and automotive operations. To improve their thermoelectric figure of merit ,emphasis has gone into modifying the band structure through doping to enhance the Power Factor and reducing thermal conductivity through increasing phonon scattering by filling and in-situ nanocomposites formation.Depending on the nature of the nanoinclusions and the proportion in the matrix, it is possible to tailor the thermoelectric properties. In this work, different concentrations of oxides were included in a one-step synthesis mechanism, obtaining the Skutterudite phase with nanoinclusions and therefore, achieving lower thermal conductivities than those reported in literature.

© The Royal Society of Chemistry 2015. The research on skutterudites in the last few years has contributed to a better understanding of the physical processes which play an important role in enhancing their thermoelectric performance and to the discovery of novel filled compounds, with one of the most promising zT values at intermediate temperatures. Skutterudites are still an ongoing field of research, and an improvement of their efficiencies, stabilities, contacts, industrial scalable fabrication processes and other factors are expected in the near future in order to develop viable modules for intermediate temperature range applications, such as in the automobile industry, factories or incinerators. This paper gives a review on the status of research in the field of skutterudites.

Peer Reviewed, Postprint (published version)

The relation between the electrical and the mechanical properties in Sr and Mg doped LaGaO3 ceramics, which can be used as electrolyte for solid oxide fuel cells, was investigated in terms of hardness and ionic conductivity. For this purpose, ceramic materials corresponding to the compositions of La1 xSrxGa1 yMgyO3 d (LSGM), with x ¼ 0.1 and y ¼ 0.2, and x ¼ 0.15 and y ¼ 0.2, were prepared. LSGM powders synthesized by the ethylene glycol complex solution method were shaped into disks by isostatic pressing method. The variation in the microstructure of samples was achieved by varying the sintering temperature between 1300 and 1450 C. While the effect of the different microstructures on the electrical properties of the LSGM electrolytes was determined by impedance spectroscopy, the influence of the hardness was extracted by instrumented indentation technique. The results showed a linear correlation between the hardness and total ionic conductivity within the temperature range of 500-660 C, thus indicating that both properties were strongly influenced on the relative density and purity of the samples. It has a potential practical implication: by measuring the LSGM hardness at room temperature, one can achieve an approach to the ionic conductivity within the studied temperature range., Peer Reviewed, Postprint (published version)

Fe65Co35 alloys are technologically relevant, especially in magnetic storage and composite permanent magnets, due to the fact that they present higher saturation magnetization per volume than any other material. Out of the various approaches undertaken for its production, mechanical ball milling remains the most common and efficient method, especially considering the large industrial scale of the applications. With the development of cost-efficient processing in mind, the influence of performing the synthesis of the FeCo alloys in air instead of the standard argon atmosphere is studied. The structural and magnetic characterization, along with the study of the oxygen content of the samples, proves that synthesizing FeCo alloys in air produce materials with nearly identical magnetic performance as their argon-milled counterpart, with the oxidation extent of the materials consisting almost exclusively of the oxide passivating layer located at the surface. In addition, no aging effect was observed in the saturation magnetization up to 6 months. It is concluded that the use of argon atmospheres, desiccators and/or glove boxes may be entirely removed from the process without affecting the magnetic properties.

[EN] Electrolyte-supported cells were made of a La0.8Sr0.2Ga0.85Mg0.15O3 (LSGM2015) electrolyte (200 m thickness) prepared by ethylene glycol complex solution synthesis, isostatic pressing and sintered at 1400 °C, a Ni-SDC anode, a Sm0.2Ce0.8O3 (SDC) buffer-layer between anode and electrolyte, and a La0.5Sr0.5CoO3 -SDC cathode. The cells were tested in single-chamber configuration using methane¿air mixtures. The results of X-ray diffraction and SEM-EDS showed a single-phase in the electrolyte and conductivities (~0.01 S cm¿1 at 650 °C) close to the typical values. Good cell power densities of 215 and 102 mW cm¿2 were achieved under CH4/O2 = 1.4 of at 800 and 650 °C, respectively. However, the cell stability tests indicated that the operating temperature strongly influenced on the cell performance after 100 h. While no significant change in the power density was observed working at 650 °C, a clear performance degradation was evidenced at 800 °C. SEM-EDS revealed an appreciable degradation of the electrolyte and both the electrodes.

We report on the development and capabilities of two new measurement systems developed at Fraunhofer-IPM. The first measurement system is based on an extension of the Van der Pauw method and is suitable for cube-shaped samples. A mapping of the electrical conductivity tensor of a Skutterudite-SPS samples produced at the Instituto de Microelectrónica de Madrid is presented. The second measurement system is a ZTmeter also developed at the Fraunhofer-IPM. It enables the simultaneous measurement of the electrical conductivity, Seebeck coefficient and thermal conductivity up to 900 K of cubes at least 5x5x5 mm3 in size. The capacity of this measurement system for measuring the anisotropy of the transport properties of a (Bi,Sb)2Te3SPS sample produced by KTH is demonstrated by simply rotating the samples.

[EN] Mechanosynthesis in tungsten carbide vessels and planetary mill is an effective method to prepare materials with ErMnO 3 derived compositions and perovskite-type structure. In this work, the effect of the doping with low contents of Co and Ni cations on the stability of such oxides is studied. The obtained perovskites are highly distorted for all compositions. The transition to the more stable hexagonal phase occurs at lower temperatures for Co-doped materials, due to the presence of Co 3+ cations that do not involve any change of oxidation state for Mn 3+ Jahn-Teller cations. Spark Plasma Sintering (SPS) was carried out at 900 °C or 950 °C and 120 MPa to obtain dense ceramics, keeping the perovskite-type structures. The magnetic properties of the obtained materials are also shown. © 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

We study in this work the processing of NaNbO3 ceramics prepared in a single thermal treatment of highly reactive precursors obtained by mechanical activation of different reagents, aiming to determine optimum conditions for piezoelectric ceramics production. Pressure-less sintering at 1200ºC leads to dense ceramics (<5% porosity) with poor mechanical stability, unsuitable for practical uses. Dense hot pressed ceramics were also obtained at lower temperatures (900-1100ºC), all of them in the submicron range of average grain sizes (< 400 nm). Their microstructure was quantitatively characterised and their elastic and electromechanical properties determined by an automatic iterative method from impedance measurements at resonance. A noticeable ensemble of piezoelectric and elastic properties (d33=38 pC.N-1 and Np=3252 kHz.mm) was measured for hot pressed ceramics, from precursors obtained by a combined route of wet-chemistry and mechanical activation, with a microstructure characterized by 0.4% residual porosity and a bimodal lognormal distribution of grain size.

The dielectric, piezoelectric and elastic coefficients, as well as the electromechanical coupling factors, of NaNbO3 submicron-structured ceramics have been obtained by an automatic iterative method from impedance measurements at resonance. Poled thin disks were measured from room temperature up to the depoling one, close to 300ºC. Dielectric thermal behaviour was determined also for unpoled ceramics up to the highest phase transition temperature. Ceramics were processed by hot-pressing from mechanically activated precursors. Microstructural effects on the properties are discussed. The suppression of the classical maximum in dielectric permittivity in unpoled ceramics at the phase transition at 370ºC was found when a bimodal distribution of grain sizes, in which a population of average grain size of 110 nm in between much coarser grains is observed. The appearance of a phase transition at 150ºC took place when Na vacancies are minimized. The occurrence of a non-centrosymmetric, ferroelectric phase, in the unpoled ceramic from room temperature to ~300ºC, highly polarisable resulting in high ferro-piezoelectric properties was also observed in the ceramic which presents grain size below 160 nm. Maximum values of kp=14%, d31=-8.7•10-12 C•N-1 and Np=3772 Hz•m at room temperature, and kp=18%, d31=-25.4•10-12 C•N-1 and Np=3722 Hz•m at 295ºC were achieved in the best processing conditions of the ceramics.

[EN] Multiferroic ceramics based on BiFeO3 and Sr-doped BiFeO 3 have been processed by high energetic milling and later thermal treatments at reduced temperatures to synthesize the perovskite structure. Single phase materials are obtained at 800 °C, reducing the temperature needed to complete the reaction in solid state method. Ceramics with densities higher than 94% were obtained at 1000 °C, and up to 98% at 1050 °C. All the ceramics were obtained in a single thermal treatment, where synthesis, grain growth and densification take place. The addition of Sr stabilizes the BiFeO3 perovskite structure, avoiding its decomposition. Ceramics with higher dielectric permittivity and conductivity than non-doped materials are obtained, due to the increase of the amount of oxygen vacancies. It is shown for the first time that the grain boundary conductivity is increased in BiFeO3 by doping with Sr. © 2011 Elsevier B.V.

[EN] High energetic milling of the corresponding oxides and carbonates mixture was employed to obtain highly dense ceramics with Ce0.80Sm 0.20-xCaxO1.9 (x = 0, 0.02) compositions at low sintering temperature. After 17 h of milling, the samarium and calcium have entered in the ceria fluorite-type structure, indicating that the mechanosynthesis of the material has occurred. Densities in the order of 97-98% were obtained at temperatures as low as 1250-1300 °C in a single thermal treatment. Besides, the addition of Ca is shown as a clear aid to sinter high-density ceramics with improved grain size for the increase of the ionic conductivity. The high homogeneity of the mixture allows Ca not only to partially incorporate to the structure, but also to react to the glassy siliceous phase located at the grain boundary. This effective cleaning process, together with the additional grain growth promoted by Ca doping, increases the grain boundary conductivity, and thus, the total conductivity of the ceramics. The influence of the microstructure on the electrical properties of the ceramics is discussed. © 2011 Elsevier B.V. All rights reserved.

A homogeneous dispersion of fine CuO in a gadolinia-doped ceria (CGO) ceramic matrix by the polymeric organic complex solution method has been achieved. Highly sinterable powders were prepared by this method after calcining the precursor at 600 °C and attrition milled. The powders consist of individual particles of few tens of nanometer in size with a low agglomeration state. The isopressed compacts were sintered in air at 1000 °C and reducing in N2 90%-H2 10% atmosphere to form Cu-CGO cermets. The microstructures showed a uniform distribution of porous metallic Cu particles surrounded by microporous spaces. The influence of Cu content in Cu-CGO cermets on the electrode performance has been investigated in order to create the most suitable microstructure. The electrical properties of Cu-CGO cermets have been also studied using impedance spectroscopy, in the temperature range form 150 to about 700 °C in argon atmosphere. These measurements determined a high value of electrical conductivity at 700 °C, similar to that corresponded to pure metallic cupper. © 2009 Elsevier B.V. All rights reserved.

[EN] Because of the Jahn-Teller effect and low tolerance factor, the only reported method to obtain ErMnO3 materials with orthorhombic perovskite structure is to apply a high external pressure (on the order of GPa) at high temperature that transforms the stable room-temperature hexagonal structure to the more dense perovskite one. In this work, single-phase ErMnO3 and ErMn0.9Ni0.1O3 compositions with orthorhombic perovskite structure have been obtained for the first time by mechanosynthesis, after 24 h of milling in a tungsten carbide planetary mill. The high energy state achieved after the prolonged milling allows the perovskite structure to be isolated, instead of the habitual hexagonal one, without any external pressure. Subsequent thermal treatments transform the mechanosynthesized powder to the hexagonal structure at temperatures on the order of 1000 °C or higher. With the aim of obtaining ceramics with perovskite structure, spark plasma sintering (SPS) at 900 and 950 °C and 120 MPa was successfully tested. Ceramics processed by this way have densities greater than 90%. © 2010 American Chemical Society.

[EN] Well-dispersed ceria-gadolinia oxide powders were obtained from thoroughly isopropanol-washed coprecipitated hydroxides and oxalates, followed by a controlled drying at low temperature and calcining at 550°C. The characteristics of the calcined powders and the microstructure of the green compacts were found to be of great importance in the sintering behavior. Green bodies with high agglomerate sizes need higher sintering temperatures for attaining a final density >99% Dth, while those having soft agglomerates with lower sizes were almost fully densified at a sintering temperature as low as 1250°C. The densification process was studied by isothermal and constant heating rate dilatometry, and microstructural development by scanning electron microscopy. By controlling the processing variables, it was possible to obtain this low-temperature, nearly fully dense (better than 99%) sample with a homogeneous microstructure. © 2009 The American Ceramic Society.

Ceramics of composition Bi3TiNbO9 (BTN) and perovskite-layered structure (Aurivillius type) [B. Aurivillius, Ark. Kemi 1, 463 (1949)] were processed by natural sintering and hot pressing from amorphous precursors. Precursors were obtained by mechanochemical activation of stoichiometric mixtures of oxides. These materials are in general interesting for their use as high-temperature piezoelectrics. Among them, BTN possesses the highest ferroparaelectric phase-transition temperature (>900 °C). The transition temperature establishes the working limit of the ceramic and the electric properties, especially the dc conductivity, affect on its polarizability. In this work, dielectric studies of BTN ceramics with controlled texture and microstructure have been made at 1, 100 KHz, and 1 MHZ and in the temperature range from 200 °C up to the ferroparaelectric transition temperature. Values of ~250 at 200 °C are achieved in ceramics hot pressed at temperatures as low as 700 °C for 1 h.

[EN] The environmental protection issues are driving forces nowadays in the scientific research on materials. From this point of view, it is a need to find alternatives to the present commercial piezoceramics, based on lead titanates-zirconates. The production of these materials requires handling of big amounts of lead oxide, which is highly toxic. Alkaline niobate piezoceramics are considered a good alternative for the future substitution of commercial piezoceramics by lead-free materials. However, the processing of these ceramics is difficult by conventional ceramic routes. There is a need to search for new routes of processing. The mechanochemical activation was used in this work for the first time to get Na1-xLixNbO3 ceramics in the Na rich region. Stoichiometric mixtures of Na2CO3 or Na2O, Li2CO3 and Nb2O5 were activated in a vibrating mill. A progressive broadening of the X-ray diffraction peaks of the mixtures was observed as the milling time increased. Nevertheless, the precursors did not became fully amorphous. The thermal evolution of the activated precursors was also tested. Ceramics were obtained by sintering or hot pressing and their piezoelectric properties were studied., [ES] La protección medioambiental es uno de los factores que impulsan diversas áreas de la investigación en materiales. Existe la necesidad de la búsqueda de alternativas a las cerámicas ferro-piezoeléctricas comerciales basadas en los titanatos-circonatos de plomo, que requieren el manejo de grandes cantidades de óxido de plomo, muy tóxico, en su procesado. Los niobatos alcalinos se consideran buenos candidatos para esta sustitución. Sin embargo, es difícil conseguir cerámicas con altas densificaciones. De aquí la necesidad de investigar nuevas rutas de procesado. La activación mecanoquímica se ha utilizado por primera vez en este trabajo para obtener cerámicas de Na1-xLixNbO3, en la región rica en Na. Se han activado mecanoquímicamente mezclas estequiométricas de Na2CO3 o Na2O, Li2CO3 y Nb2O5 en un molino vibratorio. Se ha observado el progresivo ensanchamiento de los picos de difracción de rayos X de las mezclas a medida que se aumentaba el tiempo de molienda, pero los precursores no llegan a ser amorfos para la difracción de rayos X. A partir de estos precursores se han obtenido cerámicas mediante sinterización o prensado en caliente y estudiado sus propiedades piezoeléctricas.

[EN] To improve the texture and microstructure with the aim of getting proper mechanical stability and electromechanical properties, a combination of hot-pressing and subsequent thermal treatment of recrystallisation at higher temperature is proposed in this work for the processing of Aurivillius-type structure piezoceramics. Ceramics of (SrBi2Nb2O 9)0.35 (Bi3TiNbO9)0.65 were prepared. The use of mechanically activated ceramic precursor powder allows obtaining isotropic ceramics in a single thermal treatment, in which synthesis, grain growth and sintering takes place simultaneously. A comparative study of the microstructure and the mechanical and electromechanical properties of the recrystallised, the parent hot-pressed and the ceramics naturally sintered at temperatures close to those used for the recrystallisation was carried out. Microstructures were quantitatively characterised by computer-assisted analysis of optical and scanning electron microscopy (SEM) micrographs. The influence of porosity and grain size on the mechanical properties is discussed. © 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

[EN] An increasing interest on ceramics with Aurivillius-type structure has developed during recent years due to their high ferro-paraelectric transition temperatures, which make them good candidates for use as high temperature piezoelectrics. However, there are not many works in the literature that study the evolution of their piezoelectric activity with temperature. An automatic iterative procedure has been used in this work to characterise the piezoelectric, mechanical and dielectric properties at resonance of the (SrBi2Nb2O9)1-x (Bi3TiNbO9)x solid solution, with x=0.35 and 0.65, from room temperature up to the ferro-paraelectric phase transition temperatures, at Tc=580 and 790 °C respectively. In order to study the influence of the microstructure on the properties, sintered and re-crystallized after hot pressing ceramics were prepared. Mechanochemically activated precursors were used to prepare the ceramics with moderate processing temperature and time. Re-crystallization was successfully tested to obtain a range of microstructures with low porosity and variable grain size. In general, it is observed that low porosity and large grain size enhances the piezoelectric properties. Ceramics with x =0.65 present piezoelectric activity at temperatures as high as 500 °C, with kp=2.9%. © 2003 Elsevier Ltd. All rights reserved.

[EN] Sodium niobate is an interesting material that is nowadays considered to be used as a lead-free piezoelectric ceramic. However, it is very difficult to process fully dense ceramics of this composition starting from crystalline powdered precursors. It seems a priority to develop a new synthesis route for the processing of high quality ceramics. A new synthesis method based on high-energy milling, not yet explored for this type of materials, has been applied to obtain NaNbO3. Stoichiometric mixtures of analytical grade Na2CO3 and Nb2O5 were mechanically activated in a vibrating-type mill. A progressive broadening of the different X-ray diffraction peaks was observed when the milling time was increased. The starting products did not become amorphous. The thermal behaviour of the milled samples was studied by high temperature in situ X-ray powder diffraction and thermal analysis techniques. Ceramic materials were processed by natural sintering and hot-pressing or a combination of both. Ferro-piezoelectric properties were measured to assess their practical use as piezoceramics. © 2003 Elsevier Ltd. All rights reserved.