Deoxidation of the tantalum powder produced by self-propagating high-temperature synthesis

The possibility of removal of oxygen and magnesium from the products of the magnesium reduction of tantalum pentoxide under self-propagating high-temperature synthesis conditions that contain 30 wt % Mg₄Ta₂O₉ is studied. Additional reduction of this material can decrease the magnesium content to <0.01%. The oxygen content in the fabricated tantalum powder does not exceed its amount in the surface oxide (3 × 10^?3 g/m²). The specific surface area of the powder is an order of magnitude higher than that of the initial material, which can result from the formation of a tantalum powder with a specific surface area >30 m²/g during the reduction of Mg₄Ta₂O₉.     

Influence of the precursor composition and the reduction conditions on the characteristics of magnesium-thermic niobium powders

The influence of the composition and the conditions of the reduction of niobium oxide compounds with magnesium vapor on the specific surface area of the produced metallic powder is studied. When magnesium niobate Mg₄Nb₂O₉ is used as a precursor, the specific surface of the powder increases by several times compared to that of an Nb₂O₅ precursor. Niobium powders with the specific surface up to 150 m² g^–1 (calculated particle size 4.7 nm) and a bulk density of 0.8 g cm^–3 are formed by the reduction of Mg₄Nb₂O₉. The powders are characterized by a mesoporous structure, and the most part of the specific surface consists of pores with a diameter less than 4 nm.     

Effect of the oxygen content in a salt solution on the characteristics of sodium-reduced tantalum powders

The characteristics of the tantalum powders produced by sodium thermal reduction from salt melts based on K₂TaF₇ and NaCl with various amounts of added oxycompounds K₃TaOF₆ and K₂Ta₂O₃F₆ are studied. At a molar ratio of oxygen to tantalum of 1.25 in the initial melt, capacitor tantalum powders with a specific surface area more than 3 m²/g are produced. The specific capacitance of the anodes made from these powders reaches 58 mC/g.     

Preparation of tantalum powders by the reduction of complex oxyfluoride compounds with sodium

The substances most suitable for use as oxygen-containing additives for the sodium-thermal fabrication of finely dispersed tantalum powder (K₂Ta₂O₃F₆, K₃TaOF₆, K₂TaOF₅, and KTaOF₄) are selected based on the thermodynamic and experimental investigations of reduction reactions of tantalum compounds with sodium from the melts containing complex oxyfluorides compounds. The application of the mentioned compounds gives the opportunity to fabricate tantalum powders with a specific surface area at a level of 3–5 m²/g, which is 8- to 10-fold higher than for the powders fabricated under the same conditions when reducing K₂TaF₇. It is shown that fabricated tantalum powders can be needed as the initial material for the development of the capacitor powder with a charge of 70000–100000 μFV/g.     

Magnesium-thermic synthesis of molybdenum carbide in ion melts

The physicochemical aspects of the synthesis of the powders of molybdenum carbide by the magnesium-thermic reduction of its oxide in melts of lithium, sodium, and potassium carbonates are considered. The thermodynamic evaluation of reactions based on the synthesis is given. The influence of the melt temperature on the granulometric characteristics of the carbides is revealed. It is shown that the powders with the largest specific surface are formed in the melt of lithium carbonate (Mo₂C of 7.96 m²/g) at 750°C.     

Luminescent performance of rare earths doped CaBi2Ta2O9 phosphor

A series of red-emitting phosphors of CaBi₂Ta₂O₉:Pr³⁺ and CaBi₂Ta₂O₉:Eu³⁺ were synthesized by the solid-state reaction method. The crystal structure and photoluminescence properties were investigated by X-ray diffraction (XRD) and photoluminescence spectra. The emission spectra showed that the red emission peaks were located at 622 nm for Pr³⁺ and 615 nm for Eu³⁺, respectively. The optimal doping concentrations for Ca_1?xBi₂Ta₂O₉:xPr³⁺ and Ca_1?yBi₂Ta₂O₉:yEu³⁺ were x=0.02 and y=0.15, respectively. The effect of fluxes (H₃BO₃, NH₄F, CaCl₂ and CaF₂) and charge compensations (Li₂CO₃, Na₂CO₃ and K₂CO₃) on luminescent properties were investigated in detail. It was found that the relative emission intensity of Ca_0.98Bi₂Ta₂O₉:0.02Pr³⁺ with 10 mol.% H₃BO₃ flux was about 2.9 times higher than that of the sample without flux. The relative emission intensity of Ca_0.7Bi₂Ta₂O₉:0.15Eu³⁺, 0.15K⁺ was about the 2.1 times higher than that of Ca_0.85Bi₂Ta₂O₉:0.15Eu³⁺.     

Thermodynamic properties and electrical conductivity of Ta3N5 and TaON

TaON and Ta₃N₅ powders and thin films were prepared from Ta₂O₅ powders and tantalum thin films by equilibrating specimens with ammonia-bearing gas mixtures at 1100°K. TaON was found to be nonstoichiometric and to have semiconducting properties. The free energy of formation of TaO1.05N0.95, which is the composition in equilibrium with Ta₂O₅, is -94.1± 5 kcal per mole. The range of electrical conductivity observed at room temperature was 10^-3 to 5 (ohm-cm)^-1. Ta₃N₅ has a free energy of formation of -65 ± 10 kcal per mole, and its electrical conductivity is 1.3 x 10^-4 (ohm-cm)^-1. Formerly with Bell Telephone Laboratories, Murray Hill, N. J.     

Plasmachemical Processing of Rare Pefractory Metals Fluoride Solutions

Fluoride solutions are commonly used for purification and separation of rare refractory metals. Traditional technologies of oxides production from such solutions are rather complicated and ecologically not ideal. Investigations on plasmachemical processing of fluoride solutions containing niobium, tantalum, titanium and zirconium showed the possible oxides (ultrafine powder Nb_O,Ta_O, and Ti_O) and hydrofluoric acid production from these solutions^1.2)

Experiments were carried out using a MW plasmachemical plant. For plasma generation nitrogen, oxygen, and air were used. The hard product was separated on a metal net filter. Vapors condensed in a Teflon heat exchanger. In the case of titanium solution the sequence of transformations was

Drops drying and hard fluoride substance crystallization

Evaporation of fluoride titanium compounds

Hydrolysis of gaseous fluoride compounds with Ti_O production

Ti_O condensation

Gaseous processes account for production of Ti_O Nb_Oj and Ta_O oxides' ultrafine powders. If the initial titanium solutions were pure enough, Ti0₂ had good pigment characteristics. The low volatility of fluoric zirconium compounds is responsible for hard particles hydrolysis and formation of considerably large (20-100 mm) spherical hollow particles of Zr0₂. The sizes of these particles correspond to the sizes of the initial solution drops.     

Manufacture of sodium-reduced tantalum powders with a specific capacity up to 100000 CV/g

The characteristics of the tantalum powders produced by sodium thermal reduction from salt melts based on potassium heptafluotantalate K₂TaF₇ with additions of oxygen-containing tantalum compounds and sodium chloride are studied. The oxygen sources in a melt during sodium thermal reduction are represented by oxyfluoride compounds K₃TaOF₆, KTaOF₄, and K₂Ta₂O₃F₆. The produced tantalum powders can be used as initial materials for the production of capacitor powders with a capacity up to 100 000 μC/g.     

Refractory Oxide Coatings on Titanium for Nitric Acid Applications

Tantalum and Niobium have good corrosion resistance in nitric acid as well as in molten chloride salt medium encountered in spent fuel nuclear reprocessing plants. Commercially, pure Ti (Cp-Ti) exhibits good corrosion resistance in nitric acid medium; however, in vapor condensates of nitric acid, significant corrosion was observed. In the present study, a thermochemical diffusion method was pursued to coat Ta₂O₅, Nb₂O₅, and Ta₂O₅ + Nb₂O₅ on Ti to improve the corrosion resistance and enhance the life of critical components in reprocessing plants. The coated samples were characterized by XRD, SEM, EDX, profilometry, micro-scratch test, and ASTM A262 Practice-C test in 65 pct boiling nitric acid. The SEM micrograph of the coated samples showed that uniform dense coating containing Ta₂O₅ and/or Nb₂O₅ was formed. XRD patterns indicated the formation of TiO₂, Ta₂O₅/Nb₂O₅, and mixed oxide/solid solution phase on coated Ti samples. ASTM A262 Practice-C test revealed reproducible outstanding corrosion resistance of Ta₂O₅-coated sample in comparison to Nb₂O₅- and Ta₂O₅ + Nb₂O₅-coated sample. The hardness of the Ta₂O₅-coated Cp-Ti sample was found to be twice that of uncoated Cp-Ti. The SEM and XRD results confirmed the presence of protective oxide layer (Ta₂O₅, rutile TiO₂, and mixed phase) on coated sample which improved the corrosion resistance remarkably in boiling liquid phase of nitric acid compared to uncoated Cp-Ti and Ti-5Ta-1.8Nb alloy. Three phase corrosion test conducted on Ta₂O₅-coated samples in boiling 11.5 M nitric acid showed poor corrosion resistance in vapor and condensate phases of nitric acid due to poor adhesion of the coating. The adhesive strength of the coated samples needs to be optimized in order to improve the corrosion resistance in vapor and condensate phases of nitric acid.     

Making powders of rapidly crystallized molten YBa2Cu3Ox for a superconducting material

A process is described for making quenched powders from molten YBa₂Cu₃O_x; the material is melted in a solar furnace by the lateral melting method applied to a rotating ceramics, which has a renewable surface. The melt is quenched by shock contact dispersal by rotating blades. With a quenching rate of about 10⁶ deg/sec, the powders are substantially quasiamorphous and contain traces of the phases BaCuO₂, Cu₂O, and Y₂BaCuO₅. After heat treatment, the powders show a superconducting-resistive transition at 87–91 K with a transition region width of less than 1.5 K.     

Preparation of thallium tantalates by the solid-phase sintering of Tl2O3, with Ta2O5

Conclusions Using the methods of differential thermal analysis, and electrical resistivity measurement, a study was made of the solid-phase reaction between Tl₂O₃ and Ta₂O₅, and the phase composition of the binary system of these oxides was determined. Two compounds, TlTa₃O₉ and Tl₄Ta₆O₂₁, were discovered in the system. Measurements were made of the thermal stability of these compounds in air and oxygen, their electrical resistivity in various atmospheres, and several physicochemical properties.Translated from Poroshkovaya Metallurgiya, No. 1 (109), pp. 83–87, January, 1972.     

Effect of the conditions of sintering of sodium-reduced tantalum powders on their characteristics

The effect of the granulation and heat treatment of sodium-reduced tantalum powders with a specific surface area of 2.5–3.6 m²/g on the bulk density, the powder flow time, and the specific surface area of the powders and the specific capacitance of the anodes made of them is studied. It is shown that heat treatment of a granulated powder in vacuum at 1100°C or in a mixture with magnesium at 800°C makes it possible to achieve the required powder flow time.     

Preparing Microcomposite Powders Doped with Antimony Dioxide and Properties of Thick Films Based on Those Powders

The goal of the work reported here was to develop a method of applying glass nanolayers of variable thickness to Sn_0.9Sb_0.1O₂ particles and to investigate how this affects the electrical properties of thick-film resistors. We prepared Sn_0.9Sb_0.1O₂ powders by calcinating coprecipitated tin and antimony hydroxides. Thin layers of aluminum, barium, and boron compounds were then precipitated from aqueous solutions onto powder particles. Nanolayers of glass in the BaO ― Al₂O₃ ― B₂O₃ system were obtained by dissolution followed by heat treatment. Resistive thick films made from such microcomposite powders have a higher resistivity than do those prepared by traditional methods and that resistivity changes little after repeated heat treatments. The thermal coefficient of electrical resistance decreases with increasing thickness of the glassy layer on the surface of the conducting particles.     

Amorphization reaction of Ni-Ta powders during mechanical alloying

This study examined the amorphization behavior of Ni _x Ta_100−x alloy powders synthesized by mechanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni₁₀Ta₉₀ and Ni₈₀Ta₂₀. A supersaturated nickel solid solution formed for Ni₉₀Ta₁₀, as well. X-ray diffraction analysis reveals two different types of amorphization reactions. Through an intermediate solid solution and by direct formation of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition.     

Oxidation of Ni — Ta alloys. III. Mechanism of oxidation

The results of an x-ray diffraction and metallographic study of the kinetics of scale formation during the oxidation of Ni(Ta), Ni₃Ta, and NiTa in air at 600–1000°C are analyzed. The free energies, equilibrium oxygen pressures, and mass balances of the oxidation reactions were calculated, and conditions for the formation of NiO·Ta₂O₅ and NiO on the alloy determined. It is shown that the oxidation process is controlled primarily by the diffusion of oxygen and counter-diffusion of Ni⁺² in the scale, and involves oxidation, reduction, and synthesis reactions. A multilayer scale is formed, consisting of an outer layer containing only oxides (NiO, NiO·Ta₂O₅, Ta₂O₅) and an inner one which additionally contains nickel. The protective ability of the outer scale depends upon the concentrations of NiO and NiO·Ta₂O₅ in it. Preferential oxidation of tantalum is responsible for the appearance of a subscale consisting of Ni(Ta) + Ta₂O₅ on the intermetallic Ni₃Ta, and Ni₃Ta + Ni(Ta) + Ta₂O₅ on NiTa. Differences in molar volumes of phases result in the formation of pores and cracks at interphase boundaries, particularly in the inner scale on Ni₃Ta. A change in the oxidation mechanism occurs at T ≥ 850°C as a result of the p → n transition in Ta₂O₅, which leads to retarded oxygen diffusion and the appearance of Ta⁺⁵ diffusion in the intermetallic. This, as well as the diffusion of Ni⁺², promotes the healing of macrodefects in the scale. However, it also results in enrichment of the outer scale in pentoxide, which decreases its protective ability. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 69–78, May–June, 2000.     

Preparing Microcomposite Powders Doped with Antimony Dioxide and Properties of Thick Films Based on Those Powders

The goal of the work reported here was to develop a method of applying glass nanolayers of variable thickness to Sn_0.9Sb_0.1O₂ particles and to investigate how this affects the electrical properties of thick-film resistors. We prepared Sn_0.9Sb_0.1O₂ powders by calcinating coprecipitated tin and antimony hydroxides. Thin layers of aluminum, barium, and boron compounds were then precipitated from aqueous solutions onto powder particles. Nanolayers of glass in the BaO ― Al₂O₃ ― B₂O₃ system were obtained by dissolution followed by heat treatment. Resistive thick films made from such microcomposite powders have a higher resistivity than do those prepared by traditional methods and that resistivity changes little after repeated heat treatments. The thermal coefficient of electrical resistance decreases with increasing thickness of the glassy layer on the surface of the conducting particles.

     

Kinetics of chlorination of tantalum pentoxide in mixture with sucrose carbon by chlorine gas

The mechanism and kinetics of β-Ta₂O₅ chlorination, mixed with sucrose carbon, have been studied by a thermogravimetric technique. The investigated temperature range was 500 °C to 850 °C. The reactants and reaction residues were analyzed by scanning electronic microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller method for surface area (BET). The effect of various experimental parameters was studied, such as carbon percentage, temperature, chlorine partial pressure, and flow, use of the multiple sample method, and carbon previous oxidation. The carbon percentage and previous treatment have an effect on the system reactivity. The temperature has a marked effect on the reaction rate. In the 500 °C to 600 °C temperature interval, the apparent activation energy is 144 kJ/mol of oxide, while at higher temperatures, the activation energy decreases. With high chorine partial pressures, the order of reaction is near zero. The kinetic contractile plate model, X=kt, considering carbon oxidation as the controlling stage, is the one with the best fit to the experimental data. A probable mechanism for the carbochlorination of β-Ta₂O₅ is proposed: (1) activation of chlorine on the carbon surface, (2) chlorination of Ta₂O₅, (3) oxidation of carbon, and (4) recrystallization of β-Ta₂O₅.     

Specific chemical rate constants for the acid dissolution of oxides and silicates

Correlations relating the specific chemical rate for the acid dissolution of MgO, Mg₂SiO₄, ZnO, Zn₂SiO₄ and ZnFe₂O₄ to temperature and hydrogen ion concentration/activity have been produced from a critical analysis of published dissolution data. In determining specific chemical rates it is important that consideration is given to the means of determination of the surface area of the solid and to the likelihood of a diffusional resistance contributing to the dissolution rate. It is demonstrated that at 25°C, the chemical dissolution rates of the silicates are significantly lower than those of the corresponding oxides. This finding is discussed in terms of current theories of the oxide dissolution process.     

Hydrothermal synthesis of nanocrystalline powders in the ZrO2-Y2O3-CeO2 system

The paper examines the properties of the nanocrystalline powder 95 mole% ZrO₂-2 mole% CeO₂-3 mole% Y₂O₃, synthesized using a combination of two methods: coprecipitation and hydrothermal decomposition. It is established that coprecipitation produces an x-ray amorphous gel consisting of hard agglomerates from 5 to 10 μm and having a specific surface area of 120 m²/g and a bottle density of 2.94 g/cm³. Hydrothermal synthesis results in a low-temperature metastable cubic solid solution based on ZrO₂ (F-ZrO₂). Its specific surface area is 101.6 m²/g and bottle density is 4.65 g/cm³. Soft agglomerates (0.5–10 μm) consist of primary particles with sizes to 10 nm. The change in hydrothermal suspension processing steps results in soft agglomerates with branched internal porosity. This method allows synthesizing powders of needed compositions in the ZrO₂-CeO₂-Y₂O₃ system. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 23–30, 2007.