Microstructural Development of ZnO Pellets Doped with Different Vanadium Oxides (V2O5 and V2O3)

This work presents the effect of zinc oxide (ZnO) ceramics, doped with different percentages of vanadium trioxide (V₂O₃) and vanadium pentoxide (V₂O₅). Samples were analyzed by X-ray diffraction and scanning electron microscopy. The addition of V₂O₃ or V₂O₅ produced changes in the composition and morphology of the pellets. In both cases, different zinc vanadates were detected as secondary phases: α-Zn₃(VO₄)₂ and Zn₄V₂O₉. Furthermore, when the vanadium concentration was equal to or higher than 3 wt%, the presence of filaments was detected on the surface of the pellets. These filaments were produced due to vanadium segregation. However, this effect was only observed at the surface of the pellets. On the bulk, the filaments were not observed. Instead, vanadium was found at the interfaces between the ZnO grains and at triple points, as it could be expected.     

Color and Vanadium Valency in V-Doped ZrO2

The distribution and chemical states of vanadium in V-doped ZrO₂ were studied to clarify the origin of the color of vanadium-zirconium yellow pigment in comparison with vanadium-tin yellow pigment. ESCA data and measurements of lattice constants of V-doped ZrO₂ revealed that vanadium was dissolved mainly as V⁴⁺ substituting for Zr in ZrO₂ lattice, and its solubility limit was 0.5 wt% as V₂O₅. It was found that the yellow color of vanadium-zirconium yellow was produced predominantly by the dissolved vanadium and that the contribution of vanadium oxide on ZrO₂ grains to the yellow color was about 1/30 of that of the dissolved vanadium when compared on the basis of equimolar quantity of vanadium. Most of the undissolved vanadium oxide was in an amorphous or a poorly crystallized state.     

New Yellow Ceramic Pigment Based on Codoping Pyrochlore-type Y2Ti2O7 with V5+ and Ca2+

A new yellow pigment with the pyrochlore structure Ca_xY_{2− x} V _x Ti_{2− x} O₇ was prepared as a substitute for the decreasing variety of available yellow ceramic pigments due to the severe regulation of toxic lead and cadmium. The solubility limit of vanadium in this pigment was found to be 1.5 wt% as V₂O₅ or 0.13 as x in the above formula expression. Characterization of vanadium in the vanadium pyrochlore yellow pigment by electron spectroscopy for chemical analysis and electron spin resonance showed that the oxidation state of vanadium was V⁵⁺ and its yellow color mostly originated from V⁵⁺ substituted for Ti⁴⁺. Comparison of color characteristics of Ca_xY_{2− x} V _x Ti_{2− x} O₇ with those of commercial V–SnO₂ and V–ZrO₂ revealed that Ca _x Y_{2− x} V _x Ti₂₋O₇ had better color strength and brightness than the commercial pigments.     

Chemical State of Vanadium in Tin-Based Yellow Pigment

Physicochemical states of vanadium in V-doped SnO₂ were studied to clarify the origin of the color of vanadium-tin yellow pigment and its color instability when fired with glaze material. Precision measurements of lattice parameters of V-doped SnO₂ revealed that vanadium was dissolved as V⁴⁺ and its solubility limit was 0.9 wt% as V₂O₅. It was found that the color of vanadium–tin yellow was produced by two types of undissolved vanadium on SnO₂ grains. One is poorly crystallized vandium oxide (v), (V₂O₅)', having a yellow color, and the other is orange-colored crystalline V₂O₅. The structure of (V₂O₅)' was discussed in connection with its color.     

Effect of Firing Atmosphere on Sintered and Mechanical Properties of Vanadium-Doped Alumina

Vanadium-doped alumina was sintered at 1650°C in both an oxidizing (O) and a reducing (R) atmosphere and the sintered bodies examined. In the O-sintered body, vanadium was present preferentially between the alumina grains, forming a phase (AlVO₄). In the R-sintered body, on the other hand, most of the vanadium was dissolved in alumina as V³⁺, and a small proportion of the vanadium was present as V⁴⁺ in the grain-boundary region. During O sintering, V₂O₅ doping depressed both densification and grain growth, whereas R sintering had no effect on densification but did depress grain growth. The O-sintered, V-doped body exhibited low flexural strength and hardness, whereas the R-sintered body showed comparatively high hardness.     

Structural Evolution and Vanadium Distribution in the Preparation of V4+-ZrSiO4 Solid Solutions from Gels

Vanadium-containing ZrSiO₄-gel precursors with nominal compositions V _x -ZrSiO₄ with x = 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V⁴⁺-ZrSiO₄ solid solution. The structural evolution and the V⁴⁺ location and its homogenous distribution were followed using powder X-ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V⁴⁺-ZrO₂ was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V⁴⁺-ZrO₂ took place. Finally, the monoclinic form reacted with amorphous SiO₂ to form the V⁴⁺-ZrSiO₄ solid solution. A semiquantitative determination of the vanadium solubility in the ZrSiO₄ phase using ESR confirmed previously reported results obtained by lattice parameter variation over the solid-solution series.     

Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors

Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn₃(VO₄)₂ particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn₃(VO₄)₂ has been determined to be monoclinic with space group P 2₁ and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn₃(VO₄)₂ is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn₄V₂O₉ phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.     

Synthesis of Zinc Aluminate Spinel Film through the Solid-Phase Reaction between Zinc Oxide Film and α-Alumina Substrate

We synthesized spinel ZnAl₂O₄ film on α-Al₂O₃ substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al₂O₃ substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl₂O₄ was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl₂O₄ was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl₂O₄ film became disoriented from the single (111) orientation. The ZnAl₂O₄ (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl₂O₄ was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl₂O₄ orientation.     

Synthesis of Nd:YVO4 Thin Films by a Sol-Gel Method

Nd: YVO₄ powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)₃, VO(O i Pr)₃, and Nd(OEt)₃ in 2-methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single-phase YVO₄ at 500°C. Nd:YVO₄ films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neodymium.     

Processing and Properties of Strontium Bismuth Vanadate Niobate Ferroelectric Ceramics

The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi₂(V _x Nb_{1− x} )₂O₉ (SBVN, with 0 ≤ x ≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two-step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi₂Nb₂O₉ (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.     

Destabilization of Zirconia Thermal Barriers in the Presence of V2O5

Impurities, such as vanadium, degrade the operating performance of yttria-stabilized zirconia thermal-barrier coatings. V₂O₅ reacts preferentially with Y₂O₃, forms YVO₄, and leads to the destabilization of zirconia thermal-barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria-stabilized zirconia to V₂O₅ vapor. The growth of YVO₄ from yttria-stabilized zirconia and the destabilization of cubic yttria-stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected-area diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO₄ and cubic zirconia is discussed.     

Thermal, Structural, and Dielectric Properties of Cu-Doped Ge-S Glasses

Glasses of GeS_1.5+y at.% Cu and GeS 1.375 +z at.% Cu were analyzed by DTA. For concentrations y ≊2.5 and z ≊4, new exothermic transformations and an initial small endothermic effect appear. All these transformations occur below the glass transition temperature of the original Ge:S glass, Analysis by X-ray and electron diffraction showed that this devitrification process is connected with crystallization of a Cu₂GeS₃ phase. Electron transmission microscopy showed that this crystalline phase is dispersed in a glassy matrix as disconnected aggregates with irregular symmetry. Near the maximum copper concentration y ≊5, z ≊6) at the boundary of the glass-forming region, there is one additional exothermic transformation in the system GeS_1.5+y at.% Cu and two in the system GeS_1.375+z at.% Cu, one of them corresponding to crystallization of the Cu₃GeS₄ phase. X-ray diffraction patterns of the crystalline phases are shown. The spectrum of ionic thermocurrents in such glassyceramic material shows a new peak at 120 K which is probably related to interfacial polarization associated with the crystalline phase-glassy matrix boundary.     

Determination of the Cerium Oxidation State in Cerium Vanadate

Use of low-quality fuels in gas turbines can lead to vana-dium hot corrosion of stabilized-zirconia turbine blade coatings. The stabilizing oxide reacts with V₂O₅ in the melt, forming a vanadate, thus removing the stabilizer from the zirconia. To better understand the vanadation reaction of one such oxide, CeO₂, we have employed X-ray absorption spectroscopy to determine the oxidation state of the resulting vanadate, CeVO₄. A comparison of the cerium vanadate X-ray absorption spectra with Ce⁴⁺ and Ce³⁺ standards has established that the cerium valence in this compound is 3⁺. This result indicates that cerium is reduced during CeVO₄ formation and suggests that the vanadium reactant remains in the 5⁺ oxidation state.     

Spectroscopic Studies on the Localization of Vanadium(IV) in Vanadium-Doped Zircon Pigments

The origin of the blue color observed for vanadium-doped zircon powders that have been prepared either in the presence or the absence of fluxes (NaF) has been investigated by analyzing the localization and distribution of the vanadium species in the zircon matrix. In both cases, the unit-cell parameters determined from the X-ray diffraction patterns of the samples and the results obtained from X-ray absorption (X-ray absorption near-edge structure and extended X-ray absorption fine structure), infrared, and electron spin resonance spectroscopies indicated that V⁴⁺ cations form a solid solution with the zircon lattice, substituting for both Si⁴⁺ and Zr⁴⁺ cations, although to a greater extent for the former. These vanadium cations are heterogeneously distributed in the zircon matrix, being mainly located in the outer layers of the particles in two different situations: "aggregated" in nearby lattice positions with V⁴⁺-V⁴⁺ distances ( d _V-V) of <8 Å (0.8 nm), and isolated with respect to other V⁴⁺ cations ( d _V-V > 8 Å). The strongest blue color that is observed for samples prepared in the presence of NaF seems to be due to the higher amount of V⁴⁺ cations incorporated to the zircon lattice, because this flux agent does not have any structural role.     

Preparation and Upconversion Properties of Yb3+, Ho3+: Lu2O3 Nanocrystalline Powders

Yb³⁺, Ho³⁺: Lu₂O₃ nanocrystalline powders were synthesized by a reverse-strike co-precipitation method using NH₄HCO₃ and NH₃·H₂O as precipitators. X-ray diffraction analysis and field emission scanning electron microscopy observation showed that the phase composition of the powders was cubic and the particle size was 30–50 nm. Under the excitation of a 980 nm continuous wave diode laser, green and red emissions centered around 548 and 667 nm, respectively, were observed and the green emission dominated the upconversion spectrum. Power studies revealed that a two-photon process was involved in the upconversion emissions and the possible upconversion mechanisms were discussed.     

Magnetic Behavior and Microstructure of Vanadium Phosphate Glasses

The magnetic properties and microstructures of the vanadium phosphate glass system over the composition range 60 to 90 mol% V₂O₅ were investigated to study magnetic ordering in the glass and the effect of microstructure on its magnetic properties. Direct antiferromagnetic coupling between V⁴⁺ ions in the glassy matrix exists, and a transition temperature near - 70°C was observed. As-cast glasses with high V₂O₅ concentrations separated into two glassy phases; this separation increased the ESR line width as a result of inhomogeneity broadening. The separation, which concentrated the vanadium ions in a vanadium-rich phase, caused a hysteresis in the plot of ESR line intensity vs temperature at the transition temperature. Reduction of the vanadium ions by dextrose added to the melt enhanced phase separation and resulted in weak antiferromagnetic transitions at +70° and -120°C, the Neel temperatures of VO₂ and V₂O₃, respectively.     

Synthesis of Vanadate and Arsenate Spodiosites and Interpretation of Optical Spectra of Manganese-Doped Sr2VO4C1

New vanadate and arsenate analogs of spodiosite (Sr₂-(VO₄)C1 and Sr₂(AsO₄)C1) were prepared. Isomorphism between chromium and vanadium spodiosites was established by formation of complete solid solutions. The vanadate analogs of calcium and barium, the arsenate analog of calcium, phosphate analogs of strontium and barium, and the fluorine analogs, Ca₃(VO₄)₂·CaF₂ and Sr₃(VO₄)₂·SrF₂, could not be synthesized. The optical absorption spectra of manganese-doped Sr₂(VO₄)Cl showed the presence of Mn⁵⁺ in the vanadium position, the site symmetry being approximately _Td. Three transitions, ³T₂( F ), ³T₁( F ), and ¹A_l( G ) at (12,000 and 13,000 cm^-1), (16,700 cm^-1), and (14,300 cm^-1), respectively, were observed from the ground state, ³A₂( F ). The lower symmetry components were strong enough to split the energy level, ³T₂( F ). The crystal field parameter Dq and the Racah parameters B and C were calculated to be 1200, 470, and 1927 cm^-1, respectively.     

X-ray Photoelectron Spectroscopy Study of High- and Low-Temperature Forms of Zirconium Titanate

X-ray photoelectron spectroscopy (XPS) spectra were measured for the high-temperature forms (quenched from 1500°C) of zirconium titanate and its solid solutions, the low-temperature forms (quenched from 1000°C), and cubic Ba(Zr_1/2Ti_1/2)O₃. Two distinct differences in the XPS spectra were noted between the high- temperature forms. The O 1 s spectrum of the high-temperature form consisted of two peaks of nearly equal intensities with a spacing of 2.2 eV, which was larger than the corresponding spacing of 1.6 eV, for Ba(Zr_1/2Ti_1/2)O₃, while the spectrum of the low-temperature forms was almost a single peak. The high-temperature form had an unassignable spectrum with a binding energy of 25 eV between the peaks of Zr 4 p and O 2 s . The unassignable spectrum could be a molecular orbital which had been hypothesized to form in these compounds.     

Low-Temperature Preparation and Magnetic Properties of V-Doped SnO2 Nanoparticles

A solid solution of vanadium (V)-doped tin oxide (SnO₂) particles of average diameter 2 nm and V content of 19 at.% was prepared at normal pressure and low temperature (100°C) by mixing wet SnO₂ gel SnO₂· x H₂O with a boiling solution of vanadium oxide (V₂O₅). Experimental characterizations using X-ray, electron diffraction, and transmission electron microscope show evidence of a pure single phase. The nanoparticles exhibit a mixed magnetic behavior, namely paramagnetic and ferromagnetic. Their thermal stability is also investigated. At higher temperature, 850°C, some amount of Fe precipitates from the solid solution.     

Electrochemical Synthesis and Characterization of Barium Tungstate Crystallites

Barium tungstate (BaWO₄) crystallites were successfully synthesized at room temperature via an easily-manipulated electrochemical method, using tungsten plates as electrodes and Ba(OH)₂ solution as electrolyte. Effects of electric current value and reaction time on the crystallization and development of BaWO₄ crystallites were initially studied and characterized by X-ray diffraction (XRD), scanning electron microscopy, and photoluminescent (PL) spectra technique, respectively. Experimental results showed that well-crystallized BaWO₄ crystallites with ca. 2 μm in diameter could be obtained at 0.5 A and a short reaction time of 10 min. XRD analysis showed that the as-prepared sample was a pure tetragonal phase of BaWO₄ with scheelite structure. PL spectra revealed that BaWO₄ crystallites displayed a very strong PL peak at 448 nm with the 380 nm excitation wavelength.