J. Alarcón

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.

Synthesis of V-ZrSiO4 solid solutions

This letter describes the results of the preparation of V-ZrSiO 4 solid solutions from a sol or alkoxide and solute salts. This technique allows a greater degree of product formation to be achieved at lower temperatures, as well as the possibility of studying the characteristics of possible solid solutions

Synthesis and characterization of vanadium-containing ZrO2 solid solutions pigmenting system from gels

A procedure is reported for the synthesis of vanadium-doped zirconia pigmenting system with different vanadium loadings which permitted their complete formation and further characterization. Monoclinic vanadium-zirconia solid solutions were prepared by gelling mixtures of zirconium n-propoxide and vanadyl acetylacetonate and studied over the range of temperature up to 1300 °C. Succesive steps of the reactions leading to the final monoclinic vanadium-zirconia solid solution phase were investigated by X-ray powder diffraction. It was found that the formation of the monoclinic solid solution took place by a phase transformation from a phase with the structure of tetragonal zirconia. The transformation temperature of metastable tetragonal to monoclinic phase was found to be governed by the nominal vanadium amount. Measurements of lattice parameters of monoclinic vanadium-zirconia solid solutions as a function of the nominal vanadium amount revealed that vanadium was dissolved in the zirconia lattice. Energy dispersive X-ray microanalysis and lattice parameters variation indicates that the maximum amount of vanadium into the monoclinic zirconia lattice was about 5 mol % of vanadium (3.7 wt % as V2O5). UV-Vis diffuse reflectance of monoclinic V-ZrO2 solid solutions indicated that vanadium was dissolved as V+4 and that the color of vanadium-zirconia yellow pigments was produced by the dissolved vanadium.

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.

The stoichiometry of blue vanadium doped zircon obtained by sol-gel methods

Abstract In this study, Sol-Gel processes in aqueous solution were used to investigate the stoichiometry of the V/1bZrSiO 4 solid solutions achieved by this method. Compositions with a formal stoichiometric defect in: (a) ZrO 2 ([ZrO 2 ] 1−2x [SiO 2 ] [V 2 O 5 ] x ), and (b) in SiO 2 ([ZrO 2 ] [SiO 2 ] 1−2x [V 2 O 5 ] x ), x=0.02, 0.04, 0.05 and 0.10 have been prepared. X Ray Diffraction analyses in fired samples (1200°/3 days) showed that cristobalite and monoclinic zirconia appear as residual crystalline phase in (a) and (b) compositions respectively, except in sample (a) with x=0.02 in which no residual phase was detected and therefore is considered stoichiometric. This sample and sample (b) with x=0.02, becomes blue and the others green. X Ray Fluorescence analyses carried out in the samples indicate a stoichiometry V 0.004 Zr 0.996 SiO 4 for blue sample. Similarly, amounts of vanadium of 0.02 and 0.03 per mol of zircon were measured in green sample (a) with x=0.05 and in a commercial blue turquoise, obtained adding 0.20 mols of NaF to it and treated by a conventional ceramic method, respectively.

Influence of niobate structures on the formation of rutile solid solutions [MIIINbO4-1bTiO2, where MIII=Al,Fe,Cr]

Abstract In this paper solid solutions with rutile structure in the M III NbO 4 -1b TiO 2 (M III = Al, Cr y Fe) systems have been synthesized and characterized by X-ray diffraction. Structural characterization of these solid solutions was carried out by calculating the unit cell parameters and the interatomic distances. Solid solution formation limits were also established and the structure of the M III NbO 4 compound was observed to affect the extent of the range of compositions in which M III x Ti 1−2x Nb x O 2 solid solutions are formed.

Effect of NiO and/or TiO2 mullite formation and microstructure from gels

Polymeric and colloidal gels with a constant molar ratio of (Al+Ni and/or Ti)/Si=3/1 and various (Al/Ni and/or Ti) ratios (up to 21.42 mol% NiO+TiO2) were prepared and used to study the effect of the precursor chemical homogeneity on mullite formation processes and the resulting microstructure. Both kinds of gel precursors were preheated at 750°C for 3 h in order to obtain appropriate gel-derived glasses for further thermal processing. After annealing for several time periods at temperatures between 750 and 1500°C, differences in crystallization pathways were observed. Polymeric gels crystallized Al–Si and NiAl2O4 spinels from the amorphous form at temperatures in the range between 900 and 1000°C, depending on the amount of aluminium substitution. Mullite formation was initiated at temperatures between 1100 and 1200°C, except for the higher substituted 3:2 mullite in which it was produced at 1000°C. In constrast, γ-Al2O3 and NiAl2O4 spinel were the first crystalline phases identified at 750°C in specimens from colloidal gels, whereas mullite was formed at temperatures higher than 1200°C. In specimens with high substitution, mullite was observed at lower temperatures. Although the sequences of reaction from either kind of gel were rather different, mainly at low temperatures (as could be inferred from the chemical homogeneity attained in both gel-derived glasses), the final set of crystalline phases after long annealing at 1400°C was quite similar. Differences in the microstructure of specimens from either type of gel precursor after annealing at 1400°C concerned the size of mullite particles and the presence of secondary phases in specimens derived from colloidal precursors.

Synthesis and characterization of GaxTi1−2xNbxO2 solid solutions

Abstract In this paper solid solutions in the GaxTi1−2xNbxO2 system have been synthesized and characterized by X-ray diffraction, UV-visible spectroscopy, Infrared spectroscopy and electronic microscopy. Measurements of unit cell parameters and interatomic distances allow two kinds of solid solutions to be considered, one based on the rutile structure when the TiO2 amount is high and the other on the orthorhombic α-PbO2 structure when the TiO2 content is low. The range of solid solution formation has also been established in the two kinds of solid solutions, x ≦ 0.3 for the former and 0.38 ≦ x ≦ 0.45 for the later.

The preparation and thermal evolution of polymeric gels with garnet stoichiometry in the CaOAl2O3SiO2 and CaOCr2O3SiO2 systems

Abstract The synthesis of gels with garnet stoichiometry in the CaOAl 2 O 3 SiO 2 and CaOCr 2 O 3 SiO 2 systems and their evolution in thermal processing have been studied. Dried gels with the stoichiometry 3CaO.Al 2 O 3 .3SiO 2 show a homogeneous aspect without detectable crystalline phase. The thermal evolution of these gels indicates no formation of grossularite at temperatures up to 1000°C after 12 h. In the uvarovite garnet samples, 3CaO.Cr 2 O 3 .3SiO 2 , a small amount of crystalline phase was detected in the dried gels. The homogeneity of the gels was checked by scanning electron microscropy. The infrared study of these polymeric gels confirmed the presence of SiOMe (MeCr 3+ , Ca 2+ ) bonds. This may facilitate the formation of uvarovite at low temperatures.