Volatile Metal Oxide Evaporation during Aerosol Decomposition

The role of particle evaporation during synthesis of volatile metal oxide powders (Bi₂O₃, MoO₃, PbO, and V₂O₅) by aerosol decomposition (spray pyrolysis) in a heated flow reactor was investigated. Solid particles (0.1–0.6 (Am) of predominantly β-Bi₂O₃ were formed with smooth spherical shape at all reactor temperatures (673–1173 K) employed. Solid MoO₃ particles (0.1–1.2 μm) produced at low temperatures (673–773 K) had a roughly spherical or faceted morphology and at high temperatures (873 K) had a platelike or layered structure. Solid V₂O₅ particles produced at low temperatures (573–673 K) were spherical and at high temperatures (973–1073 K) were platelike. The PbO particles were solid and spherical for all temperatures studied (773°–1073°C). Evaporative losses of up to 100% to the reactor walls were observed for all the metal oxides, due to their substantial vapor pressures. The evaporative losses were modeled by considering simultaneous particle evaporation and mass transfer of the metal oxide vapor to the reactor walls. The calculations indicated that, for most of these volatile metal oxides, evaporative losses were limited by diffusional transport of the vapor to the reactor walls and that evaporative losses occur when the vapor pressure of the oxides in the reactor is above 10^-5-10^-3 mm Hg.     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

New Glassy and Polymorphic Oxide Phases Using Rapid Quenching Techniques

Splat and other rapid quenching techniques were applied to a systematic study of simple oxides, and four new pure oxide glasses were obtained (V₂O₅, TeO₂, MoO₃, and WO₂). The high-temperature tetragonal form of WO₃ coexists with a metastable short-range-order-only phase. The structure of a new metastable high-temperature phase, δ-Ta₂O₆, resembles δ-Nb₂O₆. Predominant phases found in quenched Al₂O₃ belong to the γ-(spinel type) series.     

Phase Equilibria in the System SrO-CdO-V2O5

Phase equilibria in the system SrO-CdO-V₂O₅ in air were established from data obtained by DTA, quenching, and high-temperature solid-state reaction experiments. The SrO-V₂O₅ boundary system contains 4 compounds at SrO to V₂O₅ molar ratios of 4:1, 3:1, 2:1, and 1:1. A fifth compound with a molar composition of ∼10:3 with the apatite crystal structure was also found; it may, however, be a hydroxyapatite phase. The CdO-V₂O₅ system contains the compounds 3CdO·V₂O₅, 2CdO·V₂O₅, and CdO·V₂O₅. The latter compound exhibits a rapid reversible polymorphic transition at 180°C. Complete solid solubility exists in the SrO-CdO system. The most probable compatibility relations were determined from the data available for the SrO-CdO-V₂O₅ ternary system. Limited solid solubility exists between SrO·V₂O₅ and CdO·V₂O₅, and the high-temperature CdO·V₂O₅ polymorph is stabilized to room temperature by solid solution of SrO·V₂O₅. Evidence for the existence of 2 ternary compounds with limited local solid solubility is also presented.     

Crystallization and Solubility of Zircon and Phenacite in Certain Molten Salts

Single crystals of zircon as large as 0.7 cm and phenacite as large as 1.5 cm were grown from molten alkali metal vanadates and molybdates. The best crystals were obtained by slow cooling melts of Na₂O·3MoO₃ or Li₂O·3MoO₃ saturated with zircon or phenacite from 1400° to 900°C at a rate of 2° per hour. Crystals grown on a seed by a thermal gradient process contained inclusions although rates up to 1 mm per day were obtained. Zircon crystals could be doped with Nd³⁺, Eu³⁺, Pr²⁺, Fe³⁺, Cr³⁺, and Mn²⁺. The rare earths fluoresced at their characteristic frequencies but no fluorescence was observed from the transition metal ions. Zircon crystals had the characteristic habit, being bounded by m (110) and p (111) faces. Phenacite had a rhombohe-dral habit when grown above 1000° and crystallized as hexagonal needles below that temperature. The solubility of zircon in Na₂O·3MoO₃ obeys the Van't Hoff equation with a heat of solution of 5.5 kcal per mole. The heat of solution of zircon and phenacite increased when V₂O₅ was substituted for MoO₃ in the solvent, increased when NazO was substituted for Li₂O, and increased further when K₂O was substituted. The solubility was higher in solvents containing V₂O₅ than in those containing Moo₃. Solvent-solute interactions were strongest when the solvents were strong Lewis acids since zircon and phenacite behave as bases.     

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.     

Preparation of Alkoxy-Derived Tantalum Vanadate

Compound formation in the system Ta₂O₅–V₂O₅ has been studied using amorphous materials prepared by the simultaneous hydrolysis of tantalum and vanadyl alkoxides. Three compounds exist in this system: 9Ta₂O₅· V₂O₅, 9Ta₂O₅·2V₂O₅, and Ta₂O₅·V₂O₅ (TaVO₅). Solid solutions of δ-Ta₂O₅ are formed at low temperatures up to 10 mol% V₂O₅. They transform to β-Ta₂O₅ solid solutions at higher temperatures; the transformation temperature falls with increasing V₂O₅ A new compound, 9Ta₂O₅·V₂O₅, 670° to 755°. It has an orthorhombic unit cell with a = 0.7859 nm, b = 1.733 nm, and c = 1.766 nm. Orthorhombic TaVO₅ crystallized at 535° to 560° decomposes into 9Ta₂O₅°V₂O₅ at 1010°.     

Raman Spectra of Molybednum Phosphate Glasses and Some Crystalline Analogues

The Raman spectra of a series of glasses in the system MoO₃-P₂O₅ and the polyerystalline compounds molybdenum metaphosphate (MoO₂(P0₃)₂) and molybdenum pyrophosphate ((MoO₂)₂P₂O₂) are reported. The spectra indicate that traces of P=O and phosphate chains are present only in glasses having less than 50 mol% MoO₃. Comparison of the glass spectra with spectra of crystalline molybdenum phosphates indicates the presence of metaphosphate and orthophosphate structural groups of MoO₃ concentrations of about 50 mol% or less. As the MoO₃ concentration is increased, the bands characteristic of pyrophosphate groups appear in the spectra and become the dominant feature at 76 mol% MoO₃ concentration. Metaphosphate groups are present in the glass even at the highest MoO₃ concentrations measured.     

Humidity-Sensitive Characteristics of Porous La-Ti-V-O Glass-Ceramics

The humidity-sensitive characteristics of La₂O₃–TiO₂–V₂O₅ glass-ceramics were investigated as a function of additive amount of V₂O₅ to the precursor glass and the heating temperature of the glass to iduce phase separation. The microstructure of each glass-ceramic was strongly dependent upon the heating temperature. The specific impedance was lowered by increasing the amount of V₂O₅ additive. Among the elements studied, the LTV 2–2 element, which was prepared from as-cast glass, consisted of Na₂O:B₂O₃:La₂O₃: TiO₂: V₂O₅= 8.3:32.9:7.2:31.6:20.0 (molar ratio) after heating at 450°C for 12 h and subsequent leaching at 85°C for 24 h, and was found to be the most suitable material from the standpoint of humidity sensitivity, measurability, and response time.     

Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Hot Corrosion Mechanism of Composite Alumina/Yttria-Stabilized Zirconia Coating in Molten Sulfate–Vanadate Salt

In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al₂O₃ overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests have been performed on the YSZ coatings with and without an Al₂O₃ overlay in the molten salt mixture (Na₂SO₄+0–15 wt% V₂O₅) at 950°C. The presence of V₂O₅ in the molten salt exacerbates degradation of both the monolithic YSZ coating and the composite YSZ/Al₂O₃ system. The formation of a low-melting Na₂O–V₂O₅–Al₂O₃ liquid phase is responsible for degradation of the Al₂O₃ overlay. The Al₂O₃ overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5 wt% vanadate.     

Microstructure and Current–Voltage Characteristics of Multicomponent Vanadium-Doped Zinc Oxide Varistors

The effects of the oxide additives MnO₂, Co₃O₄, and Sb₂O₃, commonly incorporated in commercial Bi₂O₃-doped ZnO varistors, on the current–voltage characteristics and microstructure of 0.25 mol% V₂O₅-doped ZnO varistors have been studied. MnO₂ is the most significant additive in terms of its effects on varistor performance. Varistor performance can also be improved by increasing the V₂O₅ content to 0.5 mol% in a ZnO ceramic containing 1 mol% MnO₂. Further increases in the V₂O₅ content of 1 mol% MnO₂-doped material cause a deterioration in varistor behavior. The microstructure of the samples consists mainly of ZnO grains with zinc vanadates as the minority secondary phases. Additional spinel phase is formed when Sb₂O₃ is incorporated.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Electronic Conductivity and Related Properties of Amorphous and Crystallized V2O5-Based Glasses

Crystallization of V₂O₃ from V₂O₃P₂O₃, glasses containing 0 to 9 mol% B₂O₃, during heat treatment in the range 220° to 410°C, caused progressive micro structural changes which dramatically affected the electronic conductivity (γ), the activation energy for conduction ( W ), and the resistance to chemical attack. All compositions were ≊83% crystalline after heating to 410°C. As a result, the values of γ and W were almost identical to those observed for pure polycrystalline V₂O₅.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Microstructure and Nonlinear Properties of Microwave-Sintered ZnO-V2O5 Varistors: I, Effect of V2O5 Doping

The modification of the densification behavior and the grain-growth characteristics of the microwave-sintered ZnO materials, caused by the incorporation of V₂O₅ additives, have been systematically studied. Generally, the addition of V₂O₅ markedly enhances the densification rate, such that a density as high as 97.9% of the theoretical density and a grain size as large as 10 µm can be attained for a sintering temperature as low as 800°C and a soaking time as short as 10 min. Increasing the sintering temperature or soaking time does not significantly change the sintered density of the ZnO-V₂O₅ materials but it does monotonously increase their grain size. Varying the proportion of V₂O₅ in the range of 0.2-1.0 mol% does not pronouncedly modify such behavior. The leakage current density ( J _L) of these high-density and uniform-granular-structure samples is still large, which is amended by the incorporation of 0.3 mol% of Mn₃O₄ in the ZnO materials, in addition to 0.5 mol% of the V₂O₅ additives. Samples that are obtained using such a method possess good nonohmic characteristics (α= 23.5) and a low leakage current density ( J _L= 2.4 10^-6 A/cm²).     

Low-Temperature Densification of Lead Zirconate-Titanate with Vanadium Pentoxide Additive

Additions of 0.1 to 6.0 wt% V₂O, to lead zirconate titanate (PZT) ceramics promoted rapid densification below 975°C, thereby eliminating the need for PbO atmosphere control The base PZT, Pb(Zr_0.53Ti_0.47)O₃, was prepared by coprecipitation from mixed oxides and butoxides. The V₂O₅ was incorporated as a batch addition during the PZT coprecipitation process, as mill additions to the calcined precipitated powder, and to a commercial PZT powder. Densification rates were enhanced by the addition of V₂O₅ (>98% of theoretical density was obtained in ∼15 min at 960°C by the addition of 0.1 to 1.0 wt% V₂O₅, compared to 4 h at 1280°C for the base PZT). Dielectric properties and piezoelectric coefficients varied slightly within the optimum range of 0.25 to 1.0 wt% V₂O₅ addition but were at least comparable to the base PZT. Indications are that V₂O₅ becomes incorporated into the surface layers of the oxide powders during mixing (or in the coprecipitation process) and that the accelerated densification is due to enhanced surface activation and liquid-phase sintering.     

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part III, SrBi2Nb2O9 and c-Oriented Bi4Ti3O12 Template

Randomly and c-oriented SrBi₂Nb₂O₉ thin films have been obtained by a metalorganic decomposition (MOD) method using a highly c-oriented Bi₄Ti₃O₁₂ layer as a template. This templating procedure is generally applicable to other layered perovskites, and it has the advantage of lowering the temperature for crystallization and texture selection for these thin films. Moreover, the c-oriented Bi₄Ti₃O₁₂ template can serve as a bottom electrode at the same time. Ferroelectric and dielectric properties measured for SrBi₂Nb₂O₉ films of different orientations reveal strong anisotropy following the general trend known for other layered perovskites. Systematic differences in polarization and coercive field can be consistently explained in terms of the different m values and the lack of lone-pair electrons. The latter contribute to a higher polarization and a higher coercive field for SrBi₂Nb₂O₉ thin films.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Free Volume of Network-Forming Oxide Glasses in the Systems P2O5-(GeO2,TeO2,Sb2O3,V2O5)

The free-volume fraction (V_f) defined by Simha and Boyer was measured for network-forming oxide glasses in the systems P₂O₅-(GeO₂, TeO₂,Sb₂O₃.V₂O₅). The V_f values varied from 0.06 to 0.25. The systems P₂O₅-TeO₂: and P₂O₅-Sb₂O₃ have V_f∼0.1, which is near the magnitude of the free-volume fraction for normal metaphosphate glasses and many organic high polymers.