Jose Francisco Fernandez

SrAl2O4:Eu(2+), Dy(3+) has been extensively studied for industrial applications in the luminescent materials field, because of its excellent persistent luminescence properties and chemical stability. Traditionally, this strontium aluminate material is synthesized in bulk form and/or fine powder by the classic solid-state method. Here, we report an original synthetic route, a molten salt assisted process, to obtain highly crystalline SrAl2O4 powder with nanometer-scale crystals. The main advantages of salt addition are the increase of the reaction rate and the significant reduction of the synthesis temperature because of much higher mobility of reactants in the liquid medium than in the solid-state method. In particular, the formation mechanism of SrAl2O4, the role of the salt, and the phase's evolution have been explored as a function of temperature and time. Phosphorescent powders based on SrAl2O4:Eu(2+), Dy(3+) with high crystallinity are obtained after 1 h treatment at 900 °C. This work could promote further interest in adopting the molten salt strategy to process high-crystallinity materials with enhanced luminescence to design technologically relevant phosphors.

Until now, lead zirconate titanate (PZT) based ceramics are the most widely used in piezoelectric devices. However, the use of lead is being avoided due to its toxicity and environmental risks. Indeed, the attention in piezoelectric devices has been moved to lead-free ceramics, especially on (K,Na)NbO3-based materials, due to growing environmental concerns. Here we report a systematic evaluation of the effects of the compositional modifications induced by replacement of the B-sites with Sb5+ ions in 0.96[(K0.48Na0.52)0.95Li0.05Nb1-xSbxO3]-0.04[BaZrO3] lead-free piezoceramics. We show that this compositional design is the driving force for the development of the high piezoelectric properties. So, we find that this phenomenon can be explained by the stabilization of a Rhombohedral-Tetragonal (R-T) phase boundary close to room temperature, that facilities polarization process of the system and exhibit a significant high piezoelectric response with d33 value as high as ~ 400 pC/N, which...

Los composites metal-cerámica tipo luna y címbalo consisten en discos piezoeléctricos encapsulados con cápsulas metálicas que poseen una cavidad. Esta cavidad favorece la amplificación de los efectos piezoeléctricos directo e inverso. Estos actuadores llenan el escalón entre actuadores multicapa, con desplazamientos pequeños y fuerzas generativas elevadas, y actuadores bimorfos, con desplazamientos grandes y fuerzas generativas pequeñas. El actuador tipo luna

A route for fabricating structurally perfect cobalt ferrite magnetic nanostructures is demonstrated. Ultrathin islands of up to 100 μm(2) with atomically flat surfaces and free from antiphase boundaries are developed. The extremely low defect concentration leads to a robust magnetic order, even for thicknesses below 1 nm, and exceptionally large magnetic domains. This approach allows the evaluation of the influence of specific extrinsic effects on domain wall pinning.

The surface modification of lead zirconate titanate (PZT) ceramics with phosphate ester leaves a phosphorus residue absorbed onto the particle surface. During the sintering processes, this surface layer reacts with the PZT to form lead-rich compounds, such as Pb3(PO4)2 and Pb4(P2O9). The formation of such a compounds may be the responsible for the grain growth inhibition observed in PZT-modified ceramics,

The phase transition to perovskite-type structure, that occurs in some hexagonal manganites when foreign cations are incor- porated into solid solution, has been studied. Several solid solution series belonging to the Y(Mn, Ni)O3, (Er, Ca)MnO3, (Y, Ca)MnO3, systems have been prepared by solid state reaction between the corresponding oxides. The crystalline structure of the different solid solutions has been established.

Se ha realizado un estudio comparativo de la influencia de los parámetros reológicos y de procesamiento sobre la microes- tructura y consistencia de chips multicapa de PZT obtenidos mediante un proceso de colado en cinta. Para ello se han reali- zado medidas de viscosidad de distintas barbotinas con diferente contenido en sólidos y su relación con el espesor y densi-

Lead-free KNN-modified piezoceramics of the system (Li,Na,K)(Nb,Ta,Sb)O3 were prepared by conventional solid-state sintering. The X-ray diffraction patterns revealed a perovskite phase, together with some minor secondary phase, which was assigned to K3LiNb6O17, tetragonal tungsten–bronze (TTB). A structural evolution toward a pure tetragonal structure with the increasing sintering time was observed, associated with the decrease of TTB phase. A correlation between

Solid solution formation between the ferroelectric hexagonal compound YMnO3 and the semiconducting perovskite compound CaMnO3 has been studied in the Y-rich region of the pseudobinary system. The materials were prepared by reaction of the corresponding oxides and the calcium carbonate for obtaining the corresponding phases and subsequent sintering of the pressed powders in the 1400–1525 °C-temperature range. Their ceramic characteristics

Nanosized yttria-doped tetragonal polycrystalline zirconia (Y-TZP) powder was prepared by the coprecipitation method using isopropanol solutions of organic and inorganic precursors. A isopropanol-washing step led to a powder mainly constituted by very soft agglomerates with 6 nm crystallite size. Such a weakly agglomerated powder compacted uniformly (average diameter pore size = 6 nm) with a homogeneous shrinkage to full density.

Thick film varistors based on the system ZnO–Bi2O3–Sb2O3 have been prepared by screen printing technology on dense alumina substrates. Different processing strategies have been designed in order to control the excessive volatilization of Bi2O3 in varistor films during ...

A number of piezoelectric devices with wide applicability in industrial and technological areas, i.e. ultrasonic motors and piezoelectric transformers, are based on piezoelectric resonators. Piezoelectric resonators are electromechanical structures for which the operation point is close to resonance. This operation point must be kept close to resonance regardless of external perturbations. This paper introduces the mechatronic design of control electronic

Two different stoichiometric (K 0.44 Na 0.52 Li 0.04 )(Nb 0.86 Ta 0.10 Sb 0.04 )O 3 and non-stoichiometric (K 0.38 Na 0.52 Li 0.04 )(Nb 0.86 Ta 0.10 Sb 0.04 ) O 2.97 compositions were prepared by the conventional mixed oxide and carbonate route. Low temperature synthesis process at ...

Thick films of ZnO-based varistors have been prepared by screen printing and tape casting techniques. The manufacture of varistor thick films by screen printing shows two critical problems; the excessive volatilization of Bi 2 O 3 during the sintering step, due to the ...

Yttria-doped tetragonal zirconia (Y-TZP, 3 mol% Y2O3) amorphous precursors seeded with nanometric Y-TZP particles (10 wt%) were prepared by the sol-precipitation method. Solid-state reactions occurring during heating were studied by differential thermal analysis and X-ray diffraction. It was found that the seeding of Y-TZP amorphous precursors with 10 wt% crystalline Y-TZP nanometric particles considerably lowered the onset of the Y-TZP

Ceramic materials based on the ferroelectric compound YMnO3 have been prepared by solid state reaction of the corresponding oxides to obtain sinterable powders, which were isostatically pressed and sintered at temperatures ranging from 1350 to 1525°C. The samples were characterised by X-ray diffraction, (XRD), scanning electron microscopy, (SEM), apparent density measurements, and their electrical properties were established at several temperatures. The ceramic materials showed a semiconducting behaviour when they were subject to low electrical fields. The activation energy of conductivity was calculated for a narrow range of temperatures. Applying higher electrical fields the ceramics showed non-ohmic behaviour and switching phenomena from low-to-high conductivity state with a threshold voltage which depended on the testing temperature. Values of current density as high as 80 A/cm2 were measured in the high conductivity state. The process is reversible and the ceramics returned to low conductivity mode lowering or leaving the applied field. Analysis of the I–characteristic curves have allowed to advance a model based on the Poole–Frenkel effect.