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.     

Negative surface ionisation of hyperthermal halogen atoms

Negative surface ionisation of hyperthermal halogen atoms was studied as a function of their kinetic energy on thoriated tungsten and on niobium wires. The transition from a thermal equilibrium process to direct reflection causes the ionisation to increase drastically above thermal energies: efficiencies up to 40% were found for 30 eV Cl atoms impinging on thoriated tungsten.     

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%.     

Estimation of thorium by organic reagents: recovery from worn-out gas mantles and tungsten filaments by 2 : 4-dichlorophenoxyacetic acid

2 : 4-Di- chlorophenoxyacetic acid can be used as a reagent for the determination and extraction of thorium present in industrial wastes like worn-out gas mantles and tungsten filaments. At a pH of 2.8 to 3 thorium gives a voluminous precipitate with 2 : 4-D, which filters, washes, and ignites readily. The voluminous nature of the precipitate is advantageous in the detection and determination of the very small quantity of thorium present in the filaments. The % recovery of thorium from gas mantles and tungsten filaments is 89.5 and 0.8 respectively. The process can only be utilised for the extraction of thorium from the filaments if a huge quantity of such materials can be procured at a time.     

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.     

Microprobe analysis of reaction products of titanium and niobium with tungsten

The investigation of reaction products produced on the tip of tungsten electrodes during the production of titanium- and titaniumalloy-powder by the Rotating-Electrode-Process showed that titanium and titanium alloys can penetrate into cracks of the tungsten electrodes and produce a solid solution between tungsten and titanium. By this reaction a break-away of tungsten particles is avoided. This penetration process was not observed during the production of powder of niobium so that the amount of powder which can be produced by one tungsten electrode doped with thorium oxide from titanium is higher than that which can be produced from niobium.     

Determination of U and Th in tungsten by radiochemical neutron activation analysis

A radiochemical separation method using an anion exchange resin has been applied to 4N grade tungsten for determining U, Th and 4 other elements. While tungsten remained in the resin, Na, K and As were separated with 0.05M HCl and 1M HF and then U, Th and Cr were eluted with 1M HCl and 1M HF. The separation yield of neptunium (U) was influenced largely by the amount of thorium, but this influence could be neglected as the concentration of the thorium was below 0.5g/ml. The content of these elements were calculated by a single comparator method using monitors, gold and cobalt. The detection limits of U and Th are 4.0 and 1.2 ppb, respectively.     

Determination of silver in environmental samples by tungsten wire preconcentration method-electrothermal atomic absorption spectrometry

A preconcentration method for silver in environmental waters involving adsorption on a tungsten wire, followed by electrothermal atomic absorption spectrometry with a tungsten tube atomizer is described. The optimal immersing time was 90 s. The best pH for the adsorption of silver was 3. Under the optimal conditions, the detection limit for silver by the tungsten wire preconcentration method was 5.0 ng l⁻¹ (3S/N) and the relative standard deviation was 8.2%. The effects of large amounts of concomitants on the preconcentration of silver were evaluated. Even though 10³- to 10⁴-fold excess of matrix elements existed in water, the silver response was not significantly affected by the matrix elements. The method with preconcentration on a tungsten wire was applied to the determination of silver in waters and proved to be sensitive, simple, and convenient. This adsorption method can be utilized in in situ sampling of ultra-trace silver in environmental samples (waters). Furthermore, after sampling it is easy to carry and store the tungsten wire without contamination for a long time.     

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.     

Determination of low-level carbon in tungsten wire by combustion gas chromatography

A combustion gas-chromatographic technique for the determination of trace amounts of carbon in tungsten wire is described. The method involves the oxidation of the tungsten wire in a quartz oven at 1000 degrees . The liberated gases are swept into a cooled sample-loop in a gas-sampling valve. Upon completion of the oxidation process, the contents of the sample loop are introduced into a gas Chromatograph. The use of a 3-ft long column of silica gel allows separation of carbon dioxide and oxygen. The presence of oxygen requires that the hot-wire detector used be equipped with filament-protecting circuitry. Calibration curves are constructed by using organic and tungsten carbide standards. A limit of detection of 0.2 microg carbon can be achieved with a precision of better than 10%.     

Determination of trace amounts of gold in waste water by graphite furnace atomic-absorption spectrophotometry with preconcentration on trioctylphosphine oxide chemically modified tungsten wire matrix

Preconcentration on a trioctylphosphine oxide (TOPO) chemically modified tungsten wire matrix followed by graphite furnace atomic-absorption spectrophotometry measurement is described for the determination of trace gold in waste water. The TOPO modified tungsten wire matrix, after accumulating the gold, is placed in a graphite cup for direct atomization and measurement. Under the selected conditions, the absorbance is proportional to the concentration of gold over the range 0.4-18 ng/ml and the detection limit is 0.2 ng/ml. This method is sensitive and convenient. It has been applied to some waste waters with satisfactory results.     

Complexometric determination of molybdenum(VI)

In acidic medium molybdenum(VI) forms a stable complex on boiling with excess of DCTA and hydroxylamine hydrochloride. Molybdenum can then be determined by back-titration of the excess of DCTA either with zinc chloride at pH 5-5.5 or with thorium nitrate at pH 3-4.5, Xylenol Orange being used as indicator in both cases. A simple method for the determination of molybdenum in the presence of moderate amounts of tungsten is also described.     

Trace/matrix separation of uranium and thorium from molybdenum and tungsten matrix. A batch method for ultratrace bulk analysis

Summary A combined analytical procedure for spectro-photometric determination of uranium and thorium traces in high-purity molybdenum and tungsten matrices using the ion-exchanger cellulose Hyphan (TM of Riedel de Haën AG, FRG) for preconcentration is described. Following their separation (batch mode) uranium and thorium are determined as arsenazo-III complexes. The limits of detection (3) are 3 ng g^–1 U and 20 ng g^–1 Th. The analytical procedures elaborated improve the detection limits for U and Th in molybdenum and tungsten matrices by four and three orders of magnitude, respectively, compared to their determination by ICP-OES or DCP-OES without matrix separation. Accordingly, the procedures are routinely applicable in an industrial laboratory. The capability of this batch method can be validated by measurements of ion beam ratio with the glow discharge mass spectrometer; comparable results are obtained.     

Electric field assisted thermal desorption ionization using an infrared laser

A new technique has been developed for the formation of gas-phase electrons and ions by electric field assisted thermal desorption ionization at atmospheric pressure. Experiments were carried out using a sharp tungsten wire tip coated with a thin solid sample film which was irradiated by a 10.6 μm infrared laser. By applying a strong electric field on the laser-irradiated tungsten wire tip, abundant ions such as alkali ions, halide ions, and also multiply charged negative ions S(2)O(6)(2-) and S(2)O(8)(2-), were formed. Copyright 1999 John Wiley & Sons, Ltd.     

Potentials of Electron Probe Microanalysis in Studying Zircons

Potentials of electron probe microanalysis in determining hafnium, uranium, thorium, lead, niobium, titanium, manganese, iron, chromium, calcium, aluminum, tin, tungsten, yttrium, and lanthanide impurities in zircons have been studied. It was found that the background signal depends on the composition of the zircon crystal, including the concentration of hafnium. Methods have been proposed to improve the measurements of the background intensity and to optimize analytical conditions for attaining the lowest detection limits.     

Simple template-based method to produce bradbury-nielsen gates

A Bradbury-Nielsen gate (BNG) consists of two interleaved and electrically isolated sets of wires and can transmit or deflect charged particles by applying a varying voltage difference across the two wire sets. We present a simple template-based method to fabricate BNGs with wire spacings as small as 50 microm with minimal use of a microscope. The small wire spacing allows modulation rates at tens of megahertz. Using this method, we have fabricated four BNGs with wire spacings of 500, 200, 100, and 50 microm using 10 microm gold-coated tungsten wires. The performance of the four BNGs is characterized using an imaging detector and compared with theoretical predictions.     

The polarography of molybdenum,titanium and niobium in solutions of organic acids

Further work on the polarographic reduction of molybdenum(VI), niobium(V) and titanium(IV) in base electrolytes containing organic acids is reported. A base electrolyte of 0.5 M citric acid-0.025 M sulphuric acid-0.05 M thorium nitrate proved suitable for the determination of molybdenum and titanium in the presence of niobium, tantalum, tungsten and zirconium. A direct polarographic method using this base electrolyte is described for the determination of molybdenum in a niobium base alloy.     

Uranium analysis by K X-rays from geological samples

Uranium analysis at trace levels from geological samples was performed by K X-ray fluorescence using a semiconductor detector and radioisotope excitation. The exciting⁵⁷Co-source was constructed in such a way as to produce a high peak to background ratio. Using 10 minutes counting times the detection limit with a 0.26 GBq source was 24 μg/g improving to about 9 μg/g with a ten times stronger source. The detection limits for tin, cerium, tungsten and thorium were also measured.     

Coated wire thorium ion selective electrode: Part I

Coated wire ion selective electrode for thorium ion selective potentiometry was developed. Thorium ion selective coated wire electrodes were prepared by depositing a membrane comprising of Aliquat-336 loaded with Th(NO₃)₆²⁻ ions and poly vinyl chloride in varying proportion. A linear near-Nernstian response with a slope of −29.5 ± 0.3 mV over thorium concentration range of 1 × 10⁻¹–3 × 10⁻⁵ M in constant total nitrate concentration of 6 M was obtained for the electrodes of almost all the composition studied. In spite of small drift in response potential from composition to composition, day to day as well as from electrode to electrode, the slope of potential response line was constant within experimental error. Moreover, the electrode once prepared could be conveniently used over a period of one and half month.     

Thermometric determination and analytical application of the landolt effect

The thermometric determination of the Landolt effect applied to the hydrogen peroxide—iodide—ascorbic acid reaction is described. The appropriate kinetic equations are used to interpret the thermometric curve and the calibration curve of the indicator reaction catalysed by molybdenum. The effects of the iodide, hydrogen peroxide, and hydrogen ion concentrations on the shape of the thermometric curves are discussed. This thermometric method can be used to determine micro amounts of iron, zirconium, thorium, vanadium, molybdenum and tungsten, with satisfactory accuracy and precision.