Determination of tho2 in thoriated tungsten wire

Dissolution of a thoriated tungsten wire in a mixture of nitric and hydrofluoric acids followed by a sodium hydroxide separation of tungsten and EDTA titration of the thorium is used as an improved method for the determination of thorium in thoriated tungsten wire     

Characterization of the arc cathode attachment by emission spectroscopy and comparison to theoretical predictions

Emission spectroscopic diagnostics of electron temperature distribution and electron number density distribution in the cathode region have been carried out employing an OMA/spectrometer system. Single color and two color pyrometry of cathode temperature distributions have also been performed. The experimental results are compared with results of a theoretical model formulated previously to study the arc cathode interaction.The results show that the plasma in the cathode region strongly deviates from LTE. Thermionic cooling is the major cooling mechanism of cathodes at high arc currents. The work function of 2% thoriated tungsten cathodes increases during arcing due to fast evaporation of thorium from 2% thoriated tungsten cathodes.     

Characteristics of transferred-arc plasmas at high TiCl4 concentrations

The possibility of reducing low-boiling metal halides to the metal in thermal plasmas is attracting increasing attention. However, instability of the arcs in the presence of even low halide concentrations has so far thwarted all research efforts. An experimental study of the effects of adding TiCl₄ to an argon transferred arc has shown that instabilities and eventual extinction of the arc are due to severe chemical corrosion of the thoriated tungsten cathode in the presence of chloride. The corrosion results in both the loss of cathode material and the deposition of a blanket of titanium metal on the cathode's surface which supresses electron emission. A systematic search has shown that tantalum carbide cathodes will provide stable operation. Additions of TiCI₄ cause a sharp increase in total arc voltage, largely due to an increased cathode /all potential.     

Cathode erosion phenomena in a transferred-arc plasma reactor

Phenomena occurring on file surface of a thoriated tungsten cathode operating in a transferred-arc reactor were investigated. The effects of cathode geometry (pointed-tip vs. flat-tip) and plasma gas composition (argon vs. helium) on the rate and mechanisms of cathode erosion were studied experimentally by examining the morphology of the surface before and after runs of prespecified duration, up to one hour in length. For flat-tip cathodes in argon, the major characteristic was the migration of thoria and its concentration at segregated sites. Both geometries in helium operated at much higher temperatures, around the boiling point of tungsten, giving rise to extensive vaporization of cathode material, followed by apparent redeposition of the ionized species carried by file ionic current, in characteristic ringlike sites on the surface. Erosion rates were low and similar in magnitude, except for pointed-tip cathodes operated in argon, where the formation of a large molten sphere of tungsten and its subsequent release gave rise to a higher rate of erosion.     

Influence of High Voltage Needle Electrode Material on Hydrogen Peroxide Formation and Electrode Erosion in a Hybrid Gas–Liquid Series Electrical Discharge Reactor

Formation rates of hydrogen peroxide and electrode erosion rates for a range of different electrode materials were determined in a gas–liquid pulsed electrical discharge reactor with a high voltage electrode needle in the liquid and a ground electrode suspended in the gas over the liquid. It was found that the H₂O₂ formation rates and efficiencies did not depend on the electrode material. Electrode erosion from lowest to highest followed the series: nickel chromium, thoriated tungsten, diamond-coated tungsten, stainless steel, tungsten carbide and tungsten copper alloy. Smooth crater-like morphology was found for nickel chromium, titanium and tungsten carbide and a much finer surface structure with small protrusions for the tungsten, tungsten copper and the copper. Doubling the electrode diameter lead to an increase in the H₂O₂ formation per eroded length by a factor of three but it also decreased the energy efficiency yield of H₂O₂ by more than 20%.     

Substituent effect on ionisation potential in a series of related molecules: A theoretical study in a molecular orbital framework

The effect of replacing the hydrogen atoms in thioformaldehyde by halogen atoms (F, Cl) on the ionisation potential of the non-bonding electron is analysed by using the Hellman-Feynman theorem, regarding the nuclear charge of the substituent as a parameter in the many-electron Hamiltonian. The trends predicted by our theory nicely agree with the relevant ionisation potentials computed either by applying Koopmans’ theorem or by the ΔE _SCF method. For the carbonyls, avaible experimental data indicate the reliability of our prediction.     

Tungsten atomizer--theory of atomization mechanism of some volatile analytes

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.     

Tungsten atomizer – theory of atomization mechanism of some volatile analytes

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.     

Atomic tungsten for ultrafast hard X-ray generation

High-resolution X-ray absorption measurements (with an accuracy of +/-0.3 eV) of ZnSO(4) (aq) were performed with ultrafast selected energy X-ray absorption spectroscopy (USEXAS) using a laser-driven tungsten target X-ray source. The results were used to determine the absolute spectral positions of characteristic emission lines. By comparing these positions to those predicted for the line emission from tungsten of different oxidation states using the Dirac-Fock formula, the tungsten species responsible for ultrafast hard X-ray generation were found to be tungsten atoms. This finding provides the first direct evidence to support the mechanism of X-ray generation via high-energy electrons interacting with tungsten atoms in the solid target.     

粉状白钨酸的研究(Ⅹ)——粉状白钨酸的离子交换性能

通过粉状白钨酸和黄钨酸的离子交换性能的测定,比较了它们之间化学活性的相对大小。用动态法(交换柱法)测得了它们的离子交换容量,计算了与钨原子相联的羟基数目。并用静态法(振荡平衡法)测定了白钨酸对不同阳离子的交换情况及各种鸽酸的pH滴定曲线。     

Radial particle distributions in a d.c. arc—maximum off-axis

It is shown that for some elements evaporated from the anode of a d.c. arc the position of the maximum concentration of atomic particles (i.e. atoms + ions) occurs not on the axis of the arc column but some distance away from the centre, this distance being largely determined by the inside diameter of the electrode crater. It is shown that the magnitude of the off-axis peak increases with increasing volatility of the element concerned, but decreases with increasing electrical power generated in the plasma as well as with decreasing ionisation potential of the buffer metal. The proposed mechanism for this phenomenon is based on the fact that, due to thermal repulsion, cool vapours do not readily mix with hot gases. Thus volatile sample components evaporating from regions not immediately beneath the anode spot would tend to diffuse laterally around the arc rather than vertically into the hot plasma.     

Electronic structure and catalytic behavior of tungsten carbides

The electronic structure of tungsten carbide was investigated by EHMO semiempirical method. The most stable structure of the carbide was predicted on the basis of binding energy calculations. The W-C bonding is covalent in nature with some charge transfer from the W to the C atoms. The increase in the occupancy of the d shell of the W atoms and the position of the Fermi level may account for the platinum-like catalytic behavior of tungsten carbide.     

Application of pyrolysis-capillary gas chromatography with NPD detection in thermal degradation of polyphosphazenes study

Polyphosphazenes represent a unique class of polymers with a backbone composed of alternating phosphorous and nitrogen atoms. The thermal behaviour and decomposition of a variety of polyphosphazenes depends on the type of side groups present. Especially those that bear aryloxy side groups, possess a high temperature stability as well as excellent flame resistance. Pyrolysis-capillary gas chromatography has been used in a study of three polyphosphazene samples for thermal stability characterisation. Degradation products were detected with three single detectors for flame ionisation (FID), nitrogen-phosphorous sensitivity (NPD) and mass spectrometry (MSD) at different pyrolysis temperatures ranging from 300°C up to 800°C. The NPD responses for phosphorous or nitrogen fragments of polyphosphazenes have been used for the construction of degradation product schemes and the examination of the thermal stability of the polyphosphazene’s backbone. Partial identification of the degradation products present in the gaseous phase was achieved by MSD. The polyphosphazenes thermal degradation conversion rates were at a maximum at 450–500°C. At various pyrolysis temperatures, the calculated N/P peak area ratio is a function of the degree of polyphosphazene-N=P-chain degradation, and reflective of the nitrogen — phosphorous detector sensitivity. NPD proved to be suitable tool for characterization of polyphospazene thermal stability.     

Bond lengths and valences in tungsten oxides

Equations relating bond strength (valence) to bond length have been developed for tungsten-oxygen and phosphorus-oxygen bonds. Bond-valence sums have been carried out for the different tungsten atoms in phosphate tungsten bronzes and other mixed-valence tungsten oxides and for the Mo atoms in TeMo₅O₁₆. Valences intermediate between 5 and 6 are generally found, in agreement with physical measurements that indicate delocalization of d electrons in these materials. Evidence is presented that shows that the degree of distortion of WO₆ octahedra increases with apparent oxidation state of tungsten.     

Energy disposal during desorption of D2 from the surface and subsurface region of Ni(111)

The recombination of surface and subsurface D atoms on Ni(111) has been studied using resonance-enhanced multiphoton ionisation (REMPI) to measure the internal state and translational energy distributions of the desorbing product. By detecting D2 formed during temperature-programmed desorption we were able to examine the reaction between subsurface and surface D atoms, and the recombination of two D atoms chemisorbed on the surface. Translational energy distributions for D2 formed by recombination of surface D are very sensitive to coverage. Desorption from a low coverage surface produced a translational energy release of 2.6 kT, but a thermal rotational distribution, reflecting an entrance channel barrier to dissociative chemisorption on the clean Ni(111) surface. Sticking probabilities predicted from detailed balance are consistent with molecular beam adsorption measurements. Desorption from D coverages above 0.5 ML resulted in a sub-thermal energy release, desorption being mediated by a molecular precursor state with D2 dissociation occurring via a non-activated, trapping-dissociation channel. In contrast, the reaction of subsurface D produces translationally hot D2, with a mean energy approaching 8 kTs at 180 K. This is consistent with the energetics for direct recombination of a chemisorbed D atom with a metastable subsurface D atom, which overcomes an activation barrier to resurface of between 0.35 and 0.47 eV depending on D concentration. The energy release decreases at higher temperature, probably as a result of a reduction in the energy of resurfacing D as the subsurface D concentration drops. This low energy component is attributed to accommodation of resurfacing D which is unable to react directly, followed by slow thermal desorption via the high coverage, surface D recombination channel. No internal rotational or vibration excitation was found in D2 formed by reaction of subsurface D.     

Decomposition pathway of diaquabis(N,N'-dimethyl-1,2-ethanediamine)Ni(II) acesulfamate

Summary Prediction of the thermal decomposition pathway of the metal complexes is very important from the theoretical and experimental point of view to determine the properties and structural differences of complexes. In the prediction of the decomposition pathways of complexes, besides the thermal analysis techniques, some ancillary techniques e.g. mass spectroscopy is also used in recent years. In the light of the molecular structures and fragmentation components, it is believed that the thermal decomposition pathway of most molecules is similar to the ionisation mechanism occurring in the mass spectrometer ionisation process. In this study, the thermal decomposition pathway of [Ni(dmen)₂(H₂O)₂](acs)₂ complex have been predicted by the help of thermal analysis data (TG, DTG and DTA) and mass spectroscopic fragmentation pattern. The complex was decomposed in four stages: a) dehydration between 84-132°C, b) loss of N,N'-dimethylethylenediamine (dmen) ligand, c) decomposition of remained dmen and acesulfamato (acs) by releasing SO₂, d) burning of the organic residue to resulting in NiO. The volatile products observed in the thermal decomposition process were also observed in the mass spectrometer ionisation process except molecular peak and it was concluded that the ionisation and thermal decomposition pathway of the complex resembles each other.     

Structure and Thermal Stability of La0.10WO3+y; A Hexagonal Tungsten Bronze Related Phase Formed at High Pressure

The structure and thermal stability of a hexagonal tungsten bronze (HTB) related compound, La_xWO_3+y with x≈0.10 and y≈0.15, has been studied by X-ray diffraction, thermal analysis, and electron microscopy. The structure was refined by the Rietveld method from X-ray powder diffractometer data of a La_0.10WO₃ sample prepared at T=1250°C and P=25 kbar, which consisted of two tungsten bronze related phases in 1:1 proportion. The unit cell dimensions are as follows: La_0.108WO_3+y (y≈0.16), a=7.40890(5), and c=3.79329(4) Å (HTB-related structure); La_0.091WO₃, a=3.82458(6) Å (cubic perovskite tungsten bronze (PTB) structure). The lanthanum atoms in La_0.108WO_3+y are located on the hexagonal axis and statistically distributed on two sites close to the tungsten atom plane. Thermal stability studies of the La_0.10WO₃ sample in an argon atmosphere under ambient pressure conditions revealed that the HTB-related compound is metastable, decomposing to the stable PTB-type structure and WO₃. It was also found from the TG experiments in argon and oxygen that additional oxygen atoms (y) are present in the structure, thus forming a lanthanum tungsten oxide of the above composition. The electron diffraction and microanalysis studies confirmed that crystals of the HTB- and PTB-type structures were formed, with a lanthanum content of x≈0.1.     

Synthesis and oxidation of CeN film: an in situ investigation by electron spectroscopies

Nearly stoichiometric CeN film is synthesized on a Re(0001) substrate in an ultrahigh vacuum system involving highly active N atoms in the growth process, generated by thermal decomposition of NH3 by use of a hot tungsten filament. The electronic structure of the CeN film as prepared is equivalent to that of a single crystal observed by in situ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). AES and XPS investigations show that CeN film is directly oxidized to CeO2 after exposure to O2 at room temperature. However, CeN changes into Ce2O3 after annealing in approximately 10(-6) mbar of O2 atmosphere at elevated temperature.     

Imaging of atomic layer deposited (ALD) tungsten monolayers on alpha-TiO2(110) by X-ray standing wave Fourier inversion

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.     

Surface testing of a tungsten atomizer

This paper deals with the structure and surface changes of a tungsten atomizer. Atomizers made by Metallwerk Plansee for the Czech equipment WETA 90 were studied. Their effect on the height and shape of the peaks of absorption signals was investigated in dependence on number of atomization firings. The influence of various sample matrices on material changes and the evaporation of tungsten during the atomization cycle were also studied. A relationship was found between sensitivity changes and surface quality in dependence on the number of firings carried out. Good stability of sensitivity is guaranteed if the right thermal conditioning process of the atomizer is used. The influence of 4 mol dm⁻¹ HNO₃, HCl, H₂SO₄ and HC1O₄ and 5% H₂O₂ on the corrosion of the atomizer surface was investigated, and significant corrosion was observed only for H₂SO₄ and H₂O₂. The concentration of free tungsten atoms was measured from the absorbance at the tungsten 255.1 nm line (bandpass 0.1 nm) and was significantly registered over 3000°C. No influence of the concentration of hydrogen in the argon protective atmosphere on this parameter was observed.