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.     

Synthesis of highly crystalline hexagonal cesium tungsten bronze nanoparticles by flame-assisted spray pyrolysis

Highly crystalline and hexagonal single-phase cesium tungsten bronze (Cs_0.32WO₃) nanoparticles were successfully synthesized by a flame-assisted spray pyrolysis followed by annealing under a reducing gas atmosphere. The resulting Cs_0.32WO₃ nanoparticles featured a pure hexagonal Cs_0.32WO₃ phase with a high crystallinity and homogeneous chemical composition. Unlike conventional methods, the proposed process in this paper has several advantages, including a short reaction time and the ability to yield products with high purity and good energy efficiency. Furthermore, the Cs_0.32WO₃ nanoparticles produced in this research showed a remarkable near-infrared shielding ability with a 97.7% cut-off at 1500?nm.     

Vegard relationships in lithium tungsten bronzes

Data in the literature on the lattice parameter-composition relationships for lithium bronzes are inconsistent. These inconsistencies are discussed and additional experimental data are presented to allow the use of lattice parameters to characterise bronzes in multiphase sample residues from vaporisation studies.     

Preparation of hexagonal potassium tungsten bronze thin films by thermal vacuum evaporation

$\text{K}{}_{\mathrm{x}}\text{WO}{}_{\mathrm{<mtext>3+</mtext><mtext>x</mtext><mtext>2</mtext>}}\text{(}\text{x}\text{= 0.3)}$ thin films have been prepared by the thermal vacuum evaporation technique. Depending on the substrate temperature during deposition, either amorphous or crystalline films could be obtained. X-ray diffraction patterns and pole-figure information revealed that fresh crystalline films deposited on a hot (about 400°C) substrate of indium-tin oxide on glass microscope slides had a preferred orientation, with the c-axis perpendicular to the surface of the film. SIMS depth profiles and RBS experiments were used to confirm the stoichiometry and uniformity of the chemical components in the thermally deposited films.     

Pressure dependence of the superconducting transition temperature in alkali tungsten bronzes

The superconducting transition temperatures have been measured for the tetragonal I phase of Na_0.23WO₃, the hexagonal phase of K_0.33 WO₃, and the hexagonal phase of Rb_0.33WO₃ for pressures up to 20,000 bar. The slope of the curves is + 1.7 × 10^–5 K/bar for the sodium bronze, –5.8 × 10^–5 K /bar for the potassium bronze, and –1.5 × 10^–5 K/bar for the rubidium bronze. A sharp change in slope of the transition temperature vs. pressure curve for the potassium bronze at 4000 bar may indicate a phase change.Prepared for the U.S. Energy Research and Development Administration under Contract No. W-7405-eng-82.     

Internal friction in hexagonal metalś

An overview of the studies of internal friction in hexagonal metals and alloys is presented. An outline of the experimental techniques of measurement and the atomistic mechanisms causing internal friction is also given.     

Improved photochromic stability in less deficient cesium tungsten bronze nanoparticles

Hexagonal cesium tungsten bronze (Cs_0.32WO₃) nanoparticles are well known as near-infrared (NIR) shielding materials. However, one of the critical issues for industrial applications involves a photochromic instability, i.e., a blue color change of this material under ultraviolet (UV) irradiation. This color change is attributed to the formation of H_xWO₃ phase through doping of H⁺ ions into Cs-deficient sites present at the surface of Cs_0.32WO₃ nanoparticles. Therefore, a new approach was adopted to prevent the color change by synthesizing a Cs_0.32WO₃ nanoparticle with less Cs-deficient sites using spray pyrolysis followed by heat treatment and short-time milling. The Cs_0.32WO₃ nanoparticles of spray pyrolysis route exhibited an improved photochromic stability under UV irradiation compared to those synthesized through a solid-state-reaction route. Detailed analysis using atomic resolution transmission electron microscopy revealed that the Cs_0.32WO₃ nanoparticles of spray pyrolysis route retain Cs ions up to a depth of less than 1 nm from the surface. This work demonstrates that the less-Cs-deficient Cs_0.32WO₃ nanoparticles are beneficial to improve the photochromic stability.     

A structural study of the hexagonal potassium tungsten bronze,K0.26WO3

The crystal structure of the hexagonal potassium tungsten bronze, K_XWO₃, has been investigated by single crystal X-ray and powder neutron diffraction techniques. The space group is P6₃22. The new structural feature found in this study is the puckering of the ring of 6 oxygen atoms forming the “windows” in the hexagonal tunnels. The potassium ions appear to be disordered over two sites in the tunnel, towards one of the two sets of window oxygen atoms.     

The yield of cesium atoms in electron-stimulated desorption from germanium-covered tungsten

The yield of cesium (Cs) atoms during electron-stimulated desorption from a cesium layer adsorbed on a tungsten surface covered with a germanium film has been experimentally determined for the first time as a function of the electron energy, the germanium film thickness, the amount of adsorbed cesium, and the tungsten substrate temperature. The results are interpreted on the basis of a model of the Auger stimulated desorption.     

Hardness anisotropy and its dependence on composition in sodium tungsten bronzes and rhenium trioxide single crystals

Hardness anisotropy measurements using a Knoop diamond indenter on {0 0 1} and {0 0 1} surfaces of Na _x WO₃ (0.4<x<0.8) and ReO₃ single crystals with the cubic perovskite structure show that hardness is determined by slip on {1 1 0} 1 1 0. Room temperature slip is produced by Knoop and Vickers microhardness indentations on polished crystals and confirms the active slip system identified from a consideration of anisotropy. The hardness anisotropy is more pronounced as the sodium content of the crystals increases. The data suggests that hardness of Na _x WO₃ is dependent on both plane and direction.     

Thermodynamic properties of cesium and potassium at high pressures and temperatures

Results are presented of PVT experiments of cesium and potassium in the gas phase. An equation of state is derived and tables of the thermodynamic properties of cesium vapor are computed. A survey is given of the PVT properties of cesium in the liquid phase.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 6, pp. 986–992, December, 1980.     

Emission of electron and ion pulses from porous tungsten partially covered with cesium

Cesiated tungsten at about 1700 K provides a low work function (∼ 2 eV) surface for the emission of high current density electron beams as well as for the generation of cesium ions by contact ionization at 1300 K. Tungsten with porosities up to 35% has been investigated, supplying the cesium coverage by diffusion through the body from a heated reservoir. A test diode has been designed by computer simulation, achieving extremely high current densities by a convex emission surface. In dc mode of operation, the obtained ion current density of 8 mA/cm² compares well with data from literature, whereas the electron emission was negligible. In pulsed operation, however, electron densities of up to 512 A/cm² have been obtained for pulses as long as 100 μs. Even more surprising is an increase of the emitted ion current density to 200 mA/cm² for up to 0.8 ms.     

Hydrothermal synthesis and electrochromic properties of potassium tungsten oxide nanorods

Hexagonal K2W4O13 nanorods have been synthesized by hydrothermal treatment of the WO3 x H2O precipitate at 180 degrees C in the presence of K2SO4. The K2W4O13 nanorod single crystals of approximately 12 nm in diameter and tens to few hundreds nm long with the principal axis along the (001) direction were obtained when hydrothermal treatment was conducted in the presence of sufficient amount of K2SO4 while shorter nanorods with the same preferred orientation were obtained when less amount of K2SO4 was employed. On the other hand, square platelets of approximately 100 nm wide were obtained when prepared in the absence of the K2SO4. The important role of K2SO4 on morphological control of the hydrothermal products was explained based on selective adsorption of the sulfate ions on the crystal planes parallel to the (001) direction retarding the growth of these faces. The nanorod film showed satisfactory electrochromic property and can be used as a promising material in electrochromic application.     

Synthesis and characterization of niobium-doped potassium tetragonal tungsten bronzes, KxNbyW1?yO3

Needle-shaped crystals of sizes up to 5 μm × 5 μm × 40 μm of nominal composition K _x Nb _y W_1−y O₃ were synthesized by solid state method at 800 °C using appropriate amount of WO₃, WO₂, Nb₂O₅, and K₂WO₄. The samples were characterized with XRD, SEM, microprobe analysis, optical spectroscopy, and Raman spectroscopy. The XRD patterns of the samples show single phase of tetragonal tungsten bronze (TTB) type (P4/mbm, No. 127) up to y = 0.07. Structure refinements reveal an increase in cell parameter with increasing nominal niobium content within the TTB phase. The elemental compositions of the crystals determined by electron microprobe analysis also show an increase in Nb content with increasing y. With increasing Nb content the reflectivity minimum in the near infrared spectral range shifts towards lower wavenumber indicating the effect of decreasing carrier concentration. Pyrochlore type phase (KNbWO₆) is obtained as a second phase when nominal composition y > 0.07.     

Metallography of ancient bronzes: Study of pre-roman metal technology in the Iberian Peninsula

The metallographic examination of 26 bronze archaelogical objects is discussed together with the routine developed for this. The main aim was to define the bronze technology known by the Vaccei (a Celtiberian culture from Spain). All of the objects studied here come from cremation graves and their chronologies cover the last five centuries BC. The older objects have reflected great skill in forging, which compensated for casting problems. These casting problems decreased gradually, but were still important until the Roman conquest, when a real advance in casting techniques can be recorded. Since we were not authorized to cut or mount the objects in plastic, we had to hold them in our bare hands and polish only their outer surfaces, which caused many scratches and other marks. This report also illustrates some of the technical problems that might arise when studying archaeological materials.     

Low-temperature compressibilities and absolute de Haas-van Alphen frequencies in potassium,rubidium, and cesium

The absolute de Haas-van Alphen (dH-vA) frequencies and the scaling effect of small hydrostatic pressures on extremal cross sections of the Fermi surface have been measured in freely mounted samples of K, Rb, and Cs. The dH-vA frequencies are in good agreement with those derived from currently accepted values for the low-temperature lattice constants of the three metals at (1.824±0.002), (1.603±0.003), and (1.365±0.002)×10⁸ G, respectively. The low-temperature compressibilities derived from the pressure measurements at (2.57₇±0.013), (3.64₄±0.015), and (4.02₀±0.02)×10^–2 kbar^–1, respectively, show discrepancies of –5, –8, and –14% with those measured by more conventional techniques. The characteristic energyB ₀ V ₀ (bulk modulus × atomic volume) is found to be constant at about 1.67×10⁵ J/mole in the three metals.     

Sorption of cesium and strontium radionuclides onto crystalline alkali metal titanosilicates

Sorption of tracer amounts of ¹³⁷Cs and ⁹⁰Sr radionuclides from model solutions of various compositions onto synthetic titanosilicates, framework ivanyukite and layered SL3, both synthesized at the Center for Nanomaterials Science, Kola Scientific Center, Russian Academy of Sciences, was studied. Synthetic ivanyukite and titanosilicate SL3 well compete with Termoxid-25 ferrocyanide sorbent in the ability to take up cesium from neutral NaNO₃ solutions and from a simulated solution of bottom residue from a nuclear power plant with RBMK reactors. The maximal sorption of ¹³⁷Cs onto ivanyukite is observed at pH 6–7. The dependence of the ¹³⁷Cs distribution coefficient (K _d) on ivanyukite on the concentration of sodium and potassium ions in the solution was studied. Potassium ions affect the cesium sorption more strongly than sodium ions do. In the ability to take up ⁹⁰Sr, synthetic ivanyukite well competes with synthetic zeolite of type A and with the sorbent based on modified manganese dioxide. The dependences of K _d of ⁹⁰Sr on the concentrations of the Na⁺ and Ca²⁺ ions in the solution were determined. Calcium ions affect the strontium sorption more strongly than sodium ions do. Ivanyukite and SL3 show promise as sorbents for removing cesium and strontium radionuclides from multicomponent salt solutions.