J. Carda

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.

Electronic absorption spectroscopy and colour of chromium-doped solids

Twenty-one chromium-containing compounds and solid solutions belonging to ten structural types were obtained by either solid-state reactions or the Pechini method and studied from the point of view of the relationship between structure, composition and colour. Formation of single-phase products was assured by heating on several high-temperature stages and monitoring phase transformations by X-ray diffraction. Electronic absorption spectra and colour coordinates of the samples were obtained and analysed paying special attention to the dependence of the colour on the structural characteristics and the presence of chromium in oxidation states other than Cr(III).

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.

Effect of hydrolysis time and type of catalyst on the stability of tetragonal zirconia-silica composites synthesized from alkoxides

ZrO2-transformation-toughened glass-ceramics have been prepared by the sol-gel process from either colloidal gels (CG) or polymeric gels from alkoxides (PG), using, in the latest method, a different catalyst (acid, basic and without catalyst), and different compositions in the ZrO2·SiO2 system (free ZrO2 included). The different methods of synthesis of tetragonal zirconia in SiO2 matrix were compared in order to obtain thermal stability, and a high amount of stabilized tetragonal zirconia. The effect of catalyst, hydrolysis time, chemical composition, critical particle size in ZrO2(t)→ZrO2(m) transformation, the doping with vanadium on the ZrO2(t) stability in SiO2 matrix and Vickers microhardness of samples have also been studied.

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.

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.

Influence of crystallinity on chromatic parameters of enamels coloured with malayaite pink pigments

Chromium-doped malayaite ceramic pigments, CaSnSiO5:Cr were obtained by solid state reaction using CaCO3, SnO2, SiO2 and PbCrO4 as precursors. The synthesis process was studied by means of thermal analysis and mass spectroscopy, the results evidencing the volatilization of carbon dioxide and lead in a large temperature range. The major aim of the study was to evaluate the dependence of the pigment colour and crystallinity from the soaking time at a maximum synthesis temperature fixed at 1400 C. X-ray diffraction data indicate a nearly linear tendency of crystallinity increase with increasing soaking time. The colour of glazes prepared with the pigments, which was monitored by measuring their chromatic coordinates, presents a clear tendency of improvement with increasing crystallinity. This was interpreted in terms of decreasing solubility of the pigments as their crystallite size is increased. SEM and EDX studies confirm that hypothesis, evidencing very small dissolution of the highly crystallized pigments in the glaze. # 2002 Elsevier Science Ltd. All rights reserved.

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.

Synthesis and characterization of V2O5SiO2ZrO2 pigments by sol-gel method

Abstract Polymeric gels in the V2O5 SiO2 ZrO2 system were prepared from tetraethylorthosilicate (TEOS), zirconium(IV)-n-propoxide and vanadium(IV) oxyacetylacetonate, precursors. The effects of prehydrolysis time of TEOS and drying gel method on the stabilization of tetragonal zirconia in the silica amorphous matrix, nucleation and growth of the zircon (ZrSiO4) phase, and stabilization of the blue and green stains of vanadium zircon were studied. Results provide evidence that long time of prehydrolysis and fast drying do not favour zircon crystallization but stabilize the tetragonal zirconia in the amorphous matrix.

Planar defects in a precursor for phosphor materials: SrAl2 −xBxO4(x < 0.2)

The synthesis and structural characterization of a new precursor solid solution SrAl2 − xBxO4 (0 < x < 0.2), for phosphor materials is reported. The synthesis temperature of SrAl2O4 is considerably reduced by B substitution. The new solid solution keeps the tridymite-like structure (P21) characteristic of SrAl2O4. Microstructural characterization, by means of SAED and HREM, did not evidence additional ordering due to B doping. However, damage under the electron beam gives rise to complex defects, suggesting that B substitutes Al at tetrahedral positions in isolated planes along the [011] direction.