Niobium oxide

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

Another method of purifying niobium is by distillation of the anhydrous mixed chlorides (29). Niobium and tantalum pentachlorides boil within about 15°C of one another which makes control of the process difficult. Additionally, process materials must withstand the corrosion effects of the chloride. The system must be kept meticulously anhydrous and air-free to avoid plugging resulting from the formation of niobium oxide trichloride, NbOQ. Distillation has been used commercially in the past. 

Niobium Pent chloride. Niobium pentachloride can be prepared in a variety of ways but most easily by direct chlorination of niobium metal. The reaction takes place at 300—350°C. Chlorination of a niobium pentoxide—carbon mixture also yields the pentachloride however, generally the latter is contaminated with niobium oxide trichloride. The pentachloride is a lemon-yeUow crystalline soHd that melts to a red-orange Hquid and hydrolyzes readily to hydrochloric acid and niobic acid. It is soluble in concentrated hydrochloric and sulfuric acids, sulfur monochloride, and many organic solvents. 

Niobium Oxide Trichloride. Niobium oxide trichloride, NbOCl, also can be prepared in a variety of ways, ie, oxidation of the... 

Niobium Oxide Tribromide. Niobium oxide tribromide, NbOBr, is a yeUowbrown soHd which is readily hydrolyzed by moist air. It is prepared by reaction of bromine with a mixture of niobium pentoxide and carbon at 550°C. It decomposes in vacuum to the pentabromide and pentoxide at 320°C. 

Niobium Oxides. The solubihty of oxygen in niobium obeys Henry s law to the solubiHty limit of the first oxide phase of 850—1300°C (123). The amount of oxygen in solution in niobium is 1.3 at. % at 850°C and nearly 2 at. % at 1000°C (124). Only three clearly defined anhydrous oxides of niobium have been obtained in bulk, ie, NbO, Nb02, and Nb20. Niobium monoxide, NbO, is obtained by hydrogen reduction of the pentoxide at... 

Barium sodium niobium oxide [12323-03-4] Ba2NaNb 02, finds appHcation for its dielectric, pie2oelectric, nonlinear crystal and electro-optic properties (35,36). It has been used in conjunction with lasers for second harmonic generation and frequency doubling. The crystalline material can be grown at high temperature, mp ca 1450°C (37). 

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

Odier metals having vety stable oxides can be reduced by the aluminothermic reaction to produce useful feno-alloys. Niobium oxide, NbO, can be reduced to form a feiTO-alloy by the inclusion of iron in die reacting iiiixmre as haematite or magnetite, depending on the niobium content which is requhed in the product. 

The intensive increase in capacitor production has initiated the development of novel processes for the production of tantalum and niobium capacitor-grade powders, and the successful development of a new method, based on the reduction of tantalum or niobium oxide using magnesium vapors, was recently announced [38]. 

Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

Using metallic precursors, HF solutions with higher concentrations of tantalum or niobium can be achieved. It is possible to prepare solutions that have maximum concentrations of about 1000 g/1 tantalum oxide and about 600 g/1 niobium oxide (Me205). 

The stoichiometry of the prepared compounds depends not only on the composition of the initial mixture, but also on the initial oxide s fluorination activity. Unlike tantalum oxide, fluorination of niobium oxide by an ammonium hydrofluoride melt results in the formation of oxyfluoroniobates, but not of fluoroniobates. During the first step of Nb205 fluorination, (NH4)3NbOF6 is formed according to the following interaction [51, 52, 105, 111, 121, 122] ... 

Interaction between niobium oxide and fluorides, chlorides or carbonates of alkali metals in an ammonium hydrofluoride melt, yielded monooxyfluoroniobates with different compositions, MxNbOF3+x, where they were subsequently investigated [123-127]. According to DTA patterns of the Nb205 - 6NFL HF2 - 2MF system, (Fig. 18) a rich variety of endothermic effects result from the formation of ammonium monooxyfluoroniobate, its thermal decomposition and its interaction with alkali metal fluorides. The number of effects decreases and separation of ammonium ceases at lower temperatures and when going from lithium to cesium in the sequence of alkali metal fluorides. 

Nickel and copper containing compounds have been prepared in a similar manner. The phases obtained by the simultaneous fluorination of niobium oxide and other bivalent metal oxides were MHNbOF5, M21,Nb03F3 and M4UNb209, where M11 = Co Ni, Cu [129, 131],... 

Simultaneous fluorination of niobium oxide and oxides of trivalent metals using an ammonium hydrofluoride melt leads only to oxide-type compounds, MinNbC>4 due to low thermal stability of fluoride or oxyfluoride compounds that contain both niobium and trivalent metals. 

An initial solution was prepared by dissolving metallic niobium powder in 40% hydrofluoric acid. The dissolution was performed at elevated temperature with the addition of a small amount of nitric acid, HN03, to accelerate the process. The completeness of niobium oxidation was verified by UV absorption spectroscopy [21]. The prepared solution was evaporated to obtain a small amount of precipitate, which was separated from the solution by filtration. A saturated solution, containing Nb - 7.01 mol/1, HF - 42.63 mol/1, and corresponding to a molar ratio F Nb = 6.08, was prepared by the above method. The density of the solution at ambient temperature was p = 2.0 g/cc. Concentrations needed for the measurements were obtained by diluting the saturated solution with water or hydrofluoric acid. 

In general, the decomposition of potassium metaniobate, forming niobium oxide, can be represented as follows ... 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

The second solution that results from the liquid-liquid extraction process is a high-purity niobium-containing solution. This solution is used in the preparation of niobium oxide, Nb205. The process is similar to the above-described process of tantalum oxide preparation and consists of the precipitation of niobium hydroxide and subsequent thermal treatment to obtain niobium oxide powder. 

It was proposed [445 - 447] that the dissolution of tantalum and niobium oxides in mixtures of hydrofluoric and sulfuric acids takes place through the formation of fluoride-sulfate complexes, at least during the initial steps of the interaction and at relatively low acid concentrations. Nevertheless, it was also assumed that both tantalum and niobium fluoride-sulfate complexes are prone to hydrolysis yielding pure fluoride complexes and sulfuric acid. No data was provided, however, to confirm the formation of fluoride sulfate complexes of tantalum and niobium in the solutions. 

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