Crystal chemistry of transuranium tungsten bronzes TuxWO3 (Tu = Np,Pu, Am)

Three transuranium tungsten bronzes have been synthetised for the first time in the systems TuO₂-WO₃-W systems (Tu=Np, Pu, Am). They were identified using X-ray powder diffraction; their structures are of perovskite type. Both plutonium and americium bronzes show a trivalent oxidation state, whereas Mőssbauer spectra reveal clearly a Np^IV ion in Np_xWO₃.     

Le systeme SbWO3 et les bronzes de tungstene — Antimoine

Evidence is obtained for a series of tungsten-antimony bronzes, through reaction of antimony with tungsten trioxide at 900°C. Five phases are characterized: the four first derive from WO₃ with the classical increase of symetry: monoclinic, orthorhombic, tetragonal and cubic; the last one is probably a superstructure of the perovskite.     

Hydrogenation of Zr3Al2 in hydrogen and ammonia

We have identified conditions for the formation of Zr₃Al₂-based intermetallic hydrides through reactions with hydrogen and ammonia at temperatures from 150 to 300°C. The use of ammonia is shown to reduce the onset temperature for the formation of a hydride phase by 100°C compared to hydrogenation with hydrogen. Increasing the ammonia-Zr₃Al₂ reaction temperature to 500°C in the presence of NH₄Cl as an activator leads to the decomposition of the intermetallic compound and the formation of finely dispersed zirconium hydride and zirconium nitride powders.     

The study on electrical behavior of Gd2O3-WO3 complex ceramics at high temperature

Gd₂O₃-WO₃ complex ceramics are fabricated by the conventional solid-state reaction process. The electrical characteristics and dielectric properties of the samples were measured at various ambient temperatures in a low electric field (E < 150 V/mm). As the temperature increases, the dielectric constant and the loss tangent show an obvious change at about 50 °C and 330 °C. When the temperature is above 200 °C, the samples display stable nonlinear electrical properties characterized by semiconductivity, and the nonlinearity increases along with increasing temperature. XRD analysis reveals that Gd₂W₂O₉ is the main phase and Gd₂O₃ is the secondary phase. Based on the phase transition of tungsten trioxide, these electrical properties of Gd₂O₃-WO₃ complex ceramics can be simply explained.     

Growth of whiskered ZrO2 crystals by hydrothermal decomposition of zirconium oxide sulphate pseudo-crystals

Several kinds of metastable compounds, pseudo zirconium oxide sulphates (PZOS) with a chemical composition of Zr₃O₅SO₄·nH₂O, were previously synthesized by the thermal hydrolysis of solutions containing zirconium sulphate at 200 or 240°C. The obtained PZOS samples were again hydrothermally treated in different sulphuric acid solutions (<1.0 mol l^-1) at 240°C, and their hydrothermal decomposition behaviour was investigated by TEM observation. The PZOS samples mostly crystallized to plate-like zirconium oxide sulphate (ZOS) in the concentrated sulphuric acid solution (>0.5 mol l^-1), but long-whiskered monoclinic ZrO₂ crystals grew with decomposition of the PZOS samples obtained from the starting mixtures with added Zr(OH)₄ when rapidly heated to the hydrothermal treatment temperatures. It was found that many ultrafine monoclinic ZrO2 crystals were simultaneously formed during the hydrothermal preparation of the PZOS samples, and during the following hydrothermal decomposition of the PZOS samples, the whiskered crystals of monoclinic ZrO2 grew with the consumption of PZOS from the coexisting ultrafine monoclinic ZrO2 particles which act as seed crystals. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of solid solution state of zirconium and Al3Zr (L12) precipitates on the recrystallization behavior of Al-0.2 wt.%Zr alloy

Effect of homogenization annealing on the existence form of zirconium in Al-0.2wt.%Zr alloy and effect of various existence form of zirconium on the recrystallization behavior of Al-0.2wt.%Zr cold-rolled (total deformation is 92.8 %) sheet are studied. The results show that large numbers of nearly spherical Al₃Zr (L1₂) nanoparticles precipitated from aluminum matrix after homogenizing at 475 °C for 24 h. Moreover, due to the precipitation of Al₃Zr particles, the hardness and electrical conductivity of the as-cast Al-0.2wt.%Zr alloy is increased from 25.1±0.5 HV 3 and 54.0±0.2 %IACS to 28.6±0.7 HV 3 and 56.2±0.1 %IACS, respectively. Hence, zirconium exists as solid solution state in the as-cast Al-0.2wt.%Zr alloy and metastable Al₃Zr phase in the homogenized alloy. Moreover, the recrystallization temperature of the pure aluminum without addition of zirconium is 300 °C, while the recrystallization temperature of the Al-0.2wt.%Zr alloy without and with homogenization is about 350 °C and 400 °C, respectively. Obviously, the solid solution state of zirconium has certain effect on retarding the recrystallization of aluminum alloy, while the nanometer Al₃Zr particles can inhibit the recrystallization of aluminum alloy effectively and increase the recrystallization temperature remarkably.     

A study of optical absorption in tellurite and tungsten-tellurite glasses

A series of glass specimens was prepared from TeO₂ glass and from the binary tungsten tellurite glasses (TeO₂-WO₃) and their densities, optical absorption edges and infrared absorption spectra were measured. It was found that the fundamental absorption edge is a function of glass composition, and absorption in this region is due to indirect electronic transitions in k-space. The main infrared absorption bands in the TeO₂-WO₃ glasses are related to those characteristics of the TeO₂ component.     

Processing and sintering of yttrium-doped tungsten oxide nanopowders to tungsten-based composites

Innovative chemical methods are capable of fabricating nanoscale tungsten oxide compounds doped with various rare-earth elements with high purity and homogeneity, which can be processed under hydrogen into nanostructured oxide-dispersed tungsten composite powders having several potential applications. However, hydrogen reduction of doped tungsten oxide compounds is rather complex, affecting the morphology and composition of the final powder. In this study, we have investigated the reduction of tungstic acid in the presence of Y and we provide the experimental evidence that Y₂O₃ can be separated from Y-doped tungstic acid via hydrogen reduction to produce Y₂O₃-W powders. The processed powders were further consolidated by spark plasma sintering at different temperatures and holding times at 75 MPa pressure and characterized. The optimized SPS conditions suggest sintering at 1400 °C for 3 min holding time to achieve higher density composites with an optimum finer grain size (3 µm) and a hardness value up to 420 H _V. Major grain growth takes place at temperatures above 1300 °C during sintering. From the density values obtained, it is recommend to apply higher pressure before 900 °C to obtain maximum density. Oxides inclusions present in the matrix were identified as Y₂O₃·3WO₃ and Y₂O₃·WO₃ during high resolution microscopic investigations.     

Bronzoid phases in the pseudo-binary system AxNbxW1−xO3 with A = K and Cs

The systems A_xNb_xW_1?xO₃, A = K, Cs, have been studied at 1200°C for compositions x < 0.5. Phases with intergrowth tungsten bronze (ITB) structure form for x ～ 0.08 – 0.12 and with hexagonal tungsten bronze (HTB) structure for x ～ 0.20 (with a small homogeneity range). The potassium system contains a tetragonal tungsten bronze (TTB)-type phase at x ～ 0.45 – 0.60 and cesium forms a pyrochlore phase at x～0.35 – 0.40. The alkali contents of the HTB and pyrochlore phases are considerably lower than those of phases prepared at 900°C. The tunnel occupancy is approximately the same, ～ 60 %, in the ITB, HTB and pyrochlore phases prepared at 1200°C. Two types of superstructures have been observed in the HTB-type crystals.     

Tungsten thin-film deposition on a silicon wafer: The formation of silicides at W-Si interface

The interphase boundary formed in the process of tungsten thin-film deposition on a silicon wafer is investigated. These films are produced via (1) a CVD technique relying on hydrogen reduction of tungsten hexafluoride, (2) the same technique supplemented with plasmochemical action, and (3) magnetron deposition used for comparison purposes. It is shown that a nanometer tungsten silicide W₅Si₃ layer is formed at the tungsten-silicon interface only under gas-phase deposition. The effect of annealing on the specimen composition and surface resistance is investigated. It is shown that the formation and growth of a silicide WSi₂ layer commences at 700°C for CVD films and at above 750°C for films obtained with plasmochemical deposition; this results in a drastic increase in their electrical resistance. Under optimal conditions, tungsten films of 8 × 10 ?6 Ω cm resistivity are produced.     

Investigation of electrochromic properties of nanocrystalline tungsten oxide thin film

Thin films of tungsten oxide were grown by organometallic chemical vapor deposition (OMCVD) using tetra(allyl)tungsten, W(η³-C₃H₅)₄. X-Ray diffraction (XRD) analyses showed amorphous films at substrate temperatures (T_s) <350°C and polycrystalline films at T_s>350°C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed grain sizes in the range 20–40 nm. In situ electrochemical reduction of WO_3.2/ITO (2.0 M HCl) produced a faint blue color in less than 1 s. The maximum coloration efficiency (CE) was found to be 22 cm²/mC at 630 nm. The density of the films decreases from 4.53 to 4.29 g/cm³ after annealing. An optical bandgap (E_g) of ∼3.2 eV was estimated for both as-deposited and annealed films.     

Synthesis of tungsten bronze powder and determination of its composition

Sodium tungsten bronze powders were synthesized by thermal reduction of a gas/melt system at high temperature. Samples having a cubic structure with different compositions were prepared. The initial melt included Na₂WO₄, WO₃ and 10–40% mol. NaCl while the reducing gas was hydrogen at 750 °C. An original mechanism of controlling the powders size and distribution was suggested and discussed. A quantitative novel and simple method to determine the bronze composition based on TGA data was developed. An increase in the NaCl content led to a decrease of the crystals size and improved the powder uniformity. Fine powders, in the 2–5 μm size range, were synthesized from melt with 40% mol of NaCl. The stoichiometry parameter x of the obtained bronzes ranged from 0.8 to 0.92. An excellent agreement between x values determined by the classical XRD route and the proposed TGA method was demonstrated.     

Vaporisation of tungsten bronzes II Sodium and potassium tungsten bronzes

Thermodynamic calculations indicate that the disproportionation of sodium tungsten bronze is expected to occur at a lower temperature than that required for direct decomposition via congruent vaporisation. This expectation has been tested for cubic sodium bronzes and also for cubic europium and cerium bronzes, a tetragonal II potassium bronze and hexagonal bronzes of potassium, indium and thallium. Knudsen weight loss and Knudsen mass spectrometry results confirm that disproportionation is the dominant reaction for cubic and tetragonal bronzes. The hexagonal bronzes are resistant to disproportionation and vaporisation of K_0.22WO₃ occurs by the following K_0.22WO₃(s) → 0.22K(g) + 0.14 O₂(g + 0.056 W₁₈O₄₉(s) with an enthalpy of vaporisation of 405kJmol^?1.     

Influence of the chemical composition of precursors and reduction conditions on the properties of magnesiothermic tungsten powders

We have studied tungsten powders prepared by reducing the tungsten oxide compounds WO₃, MgWO₄, CaWO₄, and Ca₃WO₆ with magnesium vapor in the temperature range 700–800°C. In the case of the reduction of WO₃ and MgWO₄, we observed segregation of reaction products: removal of most of the forming magnesium oxide from the reaction zone. The powders prepared by reducing WO₃, MgWO₄, and CaWO₄ consisted of α-W, whereas the reduction of Ca₃WO₆ at temperatures below 740°C led to the formation of both α-W and β-W. The use of tungsten precursors containing refractory oxides (MgWO₄, CaWO₄, and Ca₃WO₆) allowed us to obtain tungsten powders with specific surface areas at a level of 20 m²/g and mesoporous structures.     

Hydrogenation of the intermetallic compound Zr2Ni

We have determined conditions for the preparation of hydride phases with the composition Zr₂NiH_～5 by reacting the intermetallic compound Zr₂Ni with hydrogen or ammonia and identified the products of the reaction between the intermetallic compound and ammonia in the temperature range 150–500°C in the presence of NH₄Cl as an activator. The results demonstrate that the use of ammonia at 500°C leads to decomposition of the intermetallic compound and formation of zirconium hydride, zirconium nitride, and metallic nickel.     

Iodine-Assisted Solid-State Synthesis and Characterization of Nanocrystalline Zirconium Diboride Nanosheets

A solid-state route was developed to prepare zirconium diboride nanosheets with the dimension of about 500 nm and thickness of about 20 nm from zirconium dioxide, iodine and sodium borohydride at 700°C in an autoclave reactor. The obtained ZrB₂ product was investigated by X-ray diffraction, scanning electron microscope and transmission electron microscopy. The obtained product was also studied by thermogravimetric analysis. It had good thermal stability and oxidation resistance below 400°C in air. Furthermore, the possible formation mechanism of ZrB₂ was also discussed.     

Synthesis of pure tetragonal zirconium oxide with high surface area

Zirconium oxide (ZrO₂) with high surface area and high content of the tetragonal polymorph was prepared by precipitation from aqueous solutions under basic conditions in the presence of hexadecyltrimethylammonium bromide as surfactant. The surfactant to zirconium molar ratio, pH of precipitation, aging time and zirconium concentration in aqueous solution were optimized by the Taguchi method. The sample, prepared under optimized conditions had a high surface area of 168 m² g⁻¹ after calcination at 600 °C for 10 h. Pellets, prepared by pressing this sample and after calcination at 800 °C for 0.5 h had a surface area of 105 m² g⁻¹. X-ray diffraction analyses showed that both heat treatments gave pure tetragonal zirconium oxide.     

Alkali metal location and tungsten off-center displacement in hexagonal potassium and cesium tungsten bronzes

The crystal structures of hexagonal tungsten bronze phases, $\text{A}$_tWO₃, with $\text{A}\text{=K}$ and Cs and with $\text{t}\text{=0.20}$ and 0.32 have been refined, using the profile refinement procedure, from X-ray powder diffraction data obtained in Guinier-Hägg focussing cameras. The cesium atoms refine nicely in positions at the center of the tunnels, while there are strong indications that the potassium atoms are displaced and probably disordered over many off-center sites in the tunnels. The results for the cesium bronzes combine with those for other simple but distorted structures to give an approximately linear relationship between the amplitude of displacement of the tungsten atoms from the symmetrical sites and the formal valence of tungsten, this line also correlates with the lower compositional limit for the cubic sodium tungsten bronzes. The hexagonal potassium bronzes do not follow this pattern; they are anomalous also in other respects.     

On the structure and properties of wear- and corrosion-resistant nickel-chromium- tungsten-carbon-(silicon) alloys

In some applications, for chemical and physical reasons hard nickel-based alloys have to be used instead of cobalt-based alloys but boron must be avoided. The nickel-chromium-tungsten-carbon system with and without silicon was therefore studied in several concentration ranges at 1050°C with respect to structure, phase, hardness and corrosion and wear resistance. Alloys containing 2% carbon, 10% tungsten and more than 10% chromium are composed of a nickel solid solution and an M₇C₃ carbide in both cast and homogenized (1050°C, 180 h) conditions. On increasing the tungsten content up to 20% the M₂C carbide becomes dominant, and this is associated with a remarkable increase in the hardness of the alloys. Additions of 2% silicon do not change the M₇C₃ and M₂C carbides present. In some cases a carbon-stabilized silicide M₅Si(C) was observed. Silicon additions decrease the liquidus temperature range relatively little, but they affect particle shape and size and the grain size distribution. The relation of various chromium, tungsten and silicon contents to corrosion and wear resistance was studied. The corrosion resistance depends on the chromium content of the nickel solid solution but also on carbide formation (tungsten and carbon content). The silicon content of the nickel solid solution is important too.Because their liquidus temperature is close to 1300°C the alloys cannot be used as self-fluxing and fusing powders for flame spraying but they can be sprayed by plasma torches and they can, of course, be welded.     

Incorporation of cobalt and nickel metal nano-particles in nano-grain zirconia film matrix by solution route

Precursor solutions of cobalt/nickel incorporated nano-grain zirconia films were prepared from aquo-organic solutions of zirconium oxychloride octahydrate and corresponding transition metal nitrate. The films were deposited onto silica glass substrate by the dipping technique. Annealing was made at different temperatures from 450°C to 1200°C ± 5°C in air atmosphere. The range of thickness of the films baked at 450°C was 1800–1870 å. For cobalt system Co₃O₄ was formed initially at 450°C which gradually transformed to alpha cobalt and next to cubic cobalt along with a non-stoichiometric compound (Zr_0.71Co_0.23O_0.06) with increasing annealing temperature. On the other hand, for nickel system nickel metal of nano-size was observed in the nano-grain zirconia film matrix at 450°C. By increasing annealing temperature to 1200°C, a compound, ZrNi₄O, was formed which was found to be stable for ～ 30 days.