P. Escribano

Study of solid solutions, with perovskite structure, for application in the field of the ceramic pigments

Abstract There is a current need of different ways to obtain red colored systems. One way could be the intensification of the crystal field of the chromium(III) ion when it substitutes for aluminum(III) in the corundum structure (α-Al 2 O 3 ). In the present work, solid solutions with perovskite structure ABO 3 were prepared in the system Nd 1− x Y x Al 1− y Cr y O 3 using both conventional solid state synthesis and coprecipitation. The synthesized structures were characterized structurally and microstructurally, thus determining the nature and purity of the resulting phases. A discussion is given in terms of crystal chemistry. Finally, the synthesized solid solutions were tested as ceramic pigments in enamels by measuring the chromatic coordinates, CIE-Lab.

Blue emitting hybrid organic-inorganic materials

Abstract Undoped and Eu2+- or Ce3+-doped hybrid organic–inorganic materials were prepared at room temperature from hydrolysis and condensation of organohydrosilanes (HSi(CH3)(OCH2CH3)2, HSi(OCH2CH3)3), alkoxysilanes (Si(CH3)(OCH2CH3)3) and zirconium propoxide in the presence of EuCl3 or Ce(NO3)4, 2NH4NO3. The transition metal alkoxide catalyzed cleavage of the SiH bonds was used to reduce in situ at room temperature the rare earth cations. Depending on the chemical strategy, the resulting hybrid materials can be processed as transparent bulks or coatings, which exhibit a good transparency in the UV–visible domain (cut-off at 250–280 nm). Both the undoped matrices and the rare earth-doped matrices exhibit a strong blue emission. The nature of different emitting species is clearly assigned by using their very different kinetics of fluorescence (τ∼50 ns for Ce3+, τ μs for Eu2+ τ∼3 and 4 μs for the undoped matrices) and through excitation experiments.

Influence of some mineralizers in the synthesis of sphene-pink pigments

Abstract In this paper the sphene-pink pigment with and without different mineralizers has been synthesized in order to know the best additive and proportion in which it has to be added. Characterisation of samples by X-ray diffraction indicates that samples without mineralizers exhibit peaks of small intensities assigned to the sphene structure after firing at 1300 °C/2h. The addition of H 3 BO 3 points out that the synthesis of the above compound takes place at lower temperatures and establishes the best proportion in which Cr 2 O 3 could be added. In order to improve the synthesis of the sphene pigment, LiBO 2 , Li 2 CO 3 and H 3 BO 3 -Li 2 CO 3 have been employed. Results obtained by X-ray diffraction and colour measurements show that LiBO 2 is the best additive tested for sphene-pink synthesis. The influence of quantities of LiBO 2 and Cr(VI), as well as B contents in the washing liquids have also been established.

Electrical properties of Fe-doped BaTiO3

The polymorphism of BaTi1−xFexO3−δ solid solutions: 0 ∼1000 °C. With increasing x, the high temperature hexagonal polymorph is stabilised to increasingly lower temperatures. The temperature of the tetragonal to cubic transition also decreases with increasing x, as shown by a combination of permittivity measurements, DSC and X-ray powder diffraction. The electrical properties of ceramics of tetragonal/cubic solid solutions, determined by impedance spectroscopy, show a ferroelectric bulk phase (for T < Tc) and a Schottky barrier associated with the grain boundaries whose resistance is sensitive to a dc bias voltage. The bulk properties are sensitive to sample heat treatment and in particular, for samples reheated in air ≤700 °C, oxidation occurs, generating Fe4+ and an increase in the level of p-type conductivity.

Synthesis and electrical properties of Nb-doped BaTiO3

Nb-doped BaTiO3 solid solutions of general formula, BaTi1−5xNb4xO3 prepared by two methods, solid state reaction at 1500 °C and sol–gel synthesis from high purity alkoxide precursors with final heat treatment at 900 °C, form over the range 0 ≤ x ≤ 0.055(5). Permittivity measurements demonstrated the difficulty in obtaining high quality, electrically-homogeneous samples by solid state reaction, even though samples were nominally phase-pure by X-ray powder diffraction. A partial phase diagram has been constructed showing the transitions between tetragonal, cubic and hexagonal polymorphs as a function of composition. Pellets derived from sol–gel samples and sintered at 900 °C were highly insulating, with an activation energy of ∼1.5 eV. Under these firing conditions, there was no evidence of electronic compensation, and semiconductivity, on doping BaTiO3 with Nb.

Effects of ZrO2 precursors on the synthesis of V-ZrSiO4 solid solutions by the sol-gel method

The preparation of V-ZrSiO4 solid solutions starting from different ZrO2 precursors by using sol-gel methods is reported. The starting materials were hydrolysed and the dried gels were fired at a temperature between 500 and 900 °C with soaking times of 12h. The organic character of zirconia precursors was stronger, i.e. the starting material had more carbon atoms, a higher temperature was necessary to make the first crystalline phase appear (ZrO2(tetragonal)) and the temperature range for the whole phase transformation was narrower. In all dried gel samples the presence of infrared bands which might be associated with either Si-O-Zr or Si-O-V was not observed. On the other hand, some bands could be attributed to a silica network and ZrO8 groups. The main steps in V-ZrSiO4 solid solutions were confirmed. ZrO2(tetragonal) is crystallized on heating from an amorphous sample. The ZrO2(tetragonal) → ZrO2(monoclinic) phase transformation then occurs and immediately afterwards the zircon formation begins. Finer textures in samples were obtained from polymeric gels rather than for colloidal gel samples, as seen from the scanning electron micrographs.

Room temperature synthesis of hybrid organic–inorganic nanocomposites containing Eu2+

For the first time, the room temperature synthesis of new Eu2+ doped hybrid materials together with their absorption and emission properties are reported. These hybrids containing Eu2+ and a small amount of Eu3+ are obtained through the hydrolysis and condensation of diethoxymethylsilane, methyltriethoxysilane and zirconium tetrapropoxide precursors in the presence of europium trichloride. These Eu2+ doped materials exhibit a strong host dependent Eu2+ luminescence, the intensity of which does not change upon air storage, confirming that Eu2+ ions are efficiently trapped inside these hybrid matrices.

Long-Lasting Phosphorescent Pigments of the Type SrAl2O4: Eu2+,R3+(R = Dy, Nd) Synthesized by the Sol-Gel Method

Phosphorescent pigments have been investigated for many years, being the SrAl2O4:Eu2+,R3+ with a structure of the stuffed trydimite type the one which exhibits better luminescent properties. The ceramic synthesis of this pigment requires, however, high temperatures during the firing process, reason why it is necessary to use mineralizers that give rise to problems not only in the firing process but also from an environmental point of view. Besides, a mixture of N2-H2 is needed to reduce the Eu (III) to Eu (II) during the heat treatment.This paper reports the synthesis of the compound by means of an alternative method to the ceramic route. In this case the used method is a variant of the Pechinis one because ethilenglycol is not utilized and a resin, difficult to manipulate, is not formed. The use of gaseous currents is not required, since the organic matter of the gel itself generates the reducing atmosphere. Once mixed the different solutions, the gel formation takes place by the addition of citric acid and further elimination of the solvent. The gel is dried, fired and analyzed by X-ray diffraction. Luminescence related to Eu3+ 4f crystal field transitions shows a drastic reduction of trivalent Eu concentration when codoping with Dy or Nd. The characteristic broad phosphorescence band centered at 520 nm is observed under ultraviolet illumination and is enhanced in Dy codoped pigments.

Spinets from gelatine-protected gels

An organic–inorganic method of synthesis based on protected gels of inorganic salts in a gelatine matrix is presented. Spinel (MgAl2O4), franklinite (ZnMn2O4) and hetaerolite (MnFe2O4) spinet crystalline phases have been obtained by a simple process which involves homogeneous gels generated by a gelatine matrix added to the aqueous solution of precursor salts. The resulting gelatine-protected gel route is compared with a classical coprecipitation route including hydroxide, sulfide or oxalate coprecipitation. The amorphous gelatine-protected gels show a similar or slightly greater reactivity and the resulting spinel shows a microstructure with aggregates of particles, whose particle size is larger for gelatine samples. However, no differences are observed in their crystallite size and crystallographic cell parameters. Therefore, the gelatine-protected gels provide an easy route with a reactivity and subsequent microstructure comparable to the classical coprecipitation routes for the synthesis of spinels.

Different kinds of solid solutions in the V2O5-ZrSiO4-NaF system by sol-gel processes and their characterization

Abstract In the V2O5-ZrSiO4-NaF system, three kinds of substances have been identified by sol-gel methods depending on the V2O5 amounts in the composition. Their characterization by X-ray diffraction, measurements of unit cell parameters, UV-visible (UV-V) spectroscopy, differential thermal analysis and lab colour parameter determination allow the consideration of three kinds of solid solution in the zircon lattice: (a) V+5,V+4-ZrSiO4, thermally stable and with a deep green colour; (b) V+4-ZrSiO4, thermally stable and blue coloured, made by the sol-gel method and with V2O5 amounts lower than 0·03 mol per formula weight; and (c) Na+,F−,V+4-ZrSiO4, a solid solution with a deep blue-turquoise colour, made by addition of NaF to the V2O5-ZrO2-SiO2 system. Similarly, the green substance is obtained from the bluish substance synthesized at low temperatures (800°C) by both introduction of the V+5 ion into the zircon lattice and migration of the V+4 ions to the dodecahedral from the tetrahedral sites when the temperature increases.