Tungsten Filament Research Articles

Degradation of thin tungsten filaments at high temperature in HWCVD

The degradation of the filaments is usually studied by checking the silicidation or carbonization status of the refractory metal used as catalysts, and their effects on the structural stability of the filaments. In this paper, it will be shown that the catalytic stability of a filament heated at high temperature is much shorter than its structural lifetime. The electrical resistance of a thin tungsten filament and the deposition rate of the deposited thin film have been monitored during the filament aging. It has been found that the deposition rate drops drastically once the quantity of dissolved silicon in the tungsten reaches the solubility limit and the silicides start precipitating. This manuscript concludes that the catalytic stability is only guaranteed for a short time and that for sufficiently thick filaments it does not depend on the filament radius.

Technological solution for the automatic replacement of the catalytic filaments in HWCVD

The degradation of the catalytic filaments is the main factor limiting the industrial implementation of the hot wire chemical vapor deposition (HWCVD) technique. Up to now, no solution has been found to protect the catalytic filaments used in HWCVD without compromising their catalytic activity. Probably, the definitive solution relies on the automatic replacement of the catalytic filaments. In this work, the results of the validation tests of a new apparatus for the automatic replacement of the catalytic filaments are reported. The functionalities of the different parts have been validated using a 0.2mm diameter tungsten filament under μc-Si:H deposition conditions.

Carbon nanotube coated carbon fiber based composite filaments for luminescent bulbs

Incandescent bulb filaments, consisting of carbon nanotube (CNT) coated carbon fiber (CNTCF), were fabricated and their incandescent properties were studied. For comparison, carbon fiber (CF) and tungsten filaments were also studied under similar conditions. CNTCF filament of 10Ω resistance exhibits an illuminance enhanced by a factor of ~400 as compared to tungsten filament of 17Ω resistance for an applied voltage of 18V. At an input power of 70W, CNTCF exhibits an enhancement by a factor of ~3.6 in the illuminance as compared to CF filament.

New fluorinated diamond microelectrodes for localized detection of dissolved oxygen

The present work reports on a novel needle type oxygen microsensor based on boron doped diamond (BDD) with tailored surface aiming in better electrochemical performance. The microelectrodes are produced by growing diamond films using HFCVD (hot filament chemical vapor deposition) on top of electrochemically sharpened tungsten filaments. The diamond is functionalized using CF4 RF-plasma post-treatments resulting in fluorine termination of the surface.The quantitative detection of the dissolved oxygen (DO) is demonstrated and supported with successful fitting to the theoretical values calculated for diffusion limited current on hemicylinder shape electrode. The developed MEs were calibrated and tested as probes for microamperometric mapping of dissolved oxygen in a Zn–Fe wire-electrode model galvanic couple immersed in 50mM NaCl. Modified diamond MEs show a fast and stable response towards oxygen mapping, as well as stability for several days of measurements.

Effect of indium (III) content on photoelectrochemical performance of MoBi(2−x)InxS5 thin films

Highly ordered quaternary semiconducting MoBi(2−x)InxS5 nanocrystalline thin films have been successfully synthesized via a relatively simple and convenient Arrested Precipitation Technique (APT). This solution based technique is being scaled up and may serve as the basis for the next generation of low cost solar cells. Depending on the chemical composition and synthesis conditions, the morphology of the nanocrystalline thin films can be controlled by [In]/[Bi] molar ratio. The structural, morphological, compositional and optical absorption properties of as-synthesized MoBi(2−x)InxS5 thin films are characterized using X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and UV–Vis–NIR spectrophotometry techniques. The photoelectrochemical (PEC) characterization of the films were carried out in 0.05 M I−/I3− redox electrolyte under dark and illuminated conditions. The maximum photocurrent density (411 μA/cm2) is obtained for molar ratio [In]/[Bi] = 1 using 500 W tungsten filament lamp with light intensity 30 mW/cm2.

White bright luminescence at room temperature from TEOS-based thin films via catalytic chemical vapor deposition

Strong white luminescence, visible to naked eye at room temperature, was observed from as grown SiOxCy thin films without the need of annealing. SiOxCy thin films were prepared by using catalytic chemical vapor deposition method (Cat-CVD) employing TEOS solution as the precursor and bubbled with the argon gas to form vapors. TEOS vapors were catalytically decomposed in the deposition chamber with the help of a tungsten filament. Scanning electron microscope (SEM) and transmission electron microscope (TEM) confirmed the formation of nano-crystals in SiOxCy matrix. Inter-atomic spacing shows the transition of Si–SiC nano-crystals in matrix. The intense emission is believed to come up either from the carbon incorporation in the nanocrystalline-structured film and/or quantum confinement effect of the observed nano-crystals in SiOxCy matrix.

Design and characterization of electron beam focusing for X-ray generation in novel medical imaging architecture

A novel electron beam focusing scheme for medical X-ray sources is described in this paper. Most vacuum based medical X-ray sources today employ a tungsten filament operated in temperature limited regime, with electrostatic focusing tabs for limited range beam optics. This paper presents the electron beam optics designed for the first distributed X-ray source in the world for Computed Tomography (CT) applications. This distributed source includes 32 electron beamlets in a common vacuum chamber, with 32 circular dispenser cathodes operated in space charge limited regime, where the initial circular beam is transformed into an elliptical beam before being collected at the anode. The electron beam optics designed and validated here are at the heart of the first Inverse Geometry CT system, with potential benefits in terms of improved image quality and dramatic X-ray dose reduction for the patient.

Tungsten Oxide Coated Copper Oxide Nanowire Arrays for Enhanced Activity and Durability with Photo-Electro-Chemical Water Splitting

: Recently, we showed that the deposition of crystalline WO3 layers on copper oxide nanowire arrays to result in a five-fold improvement of photocurrent density (~ 1.7 mA/cm2 at 0 V 10 vs. RHE) over titania coated copper oxide NW arrays. Most importantly, the deposition of WO3 or CuWO4 reduced CuO phase impurity to Cu2O. Improvement in the phase purity of nanowire arrays, led to the enhancement in photoactivity.1 The origin of degradation in photoelectrochemical performance with time is currently being investigated. Introduction : Photoelectrochemical (PEC) water splitting using sunlight is an attractive approach for producing clean hydrogen towards reducing carbon dioxide emissions and meeting the growing global energy demand. The biggest obstacle with PEC water splitting is the unavailability of a suitable semiconductor with proper band gap and band edge energetics and durability. Cu2O is a promising semiconducting material for photo-electro-chemical water splitting due to its band gap (Eg = 2.0 eV) and its band edge 25 positions straddling the water oxidation and reduction reactions. Major drawbacks with Cu2O include its poor stability in aqueous solutions, short carrier diffusion lengths (20-100 nm) and high absorption depth (10 microns). A semiconductor with a 2 eV band gap is expected to yield a photocurrent density of about 15 mA/cm2. The photoactivity performance of Cu2O has been a major concern, in addition to its rapid dissolution in aqueous solution.One-dimensional nanostructures such as nanowires have the potential to produce high photoactivity due to fast charge transport properties and reduced length scales for minority carrier diffusion expected in single crystalline nanowires. In our previous work, we developed a rapid two-step process for the scalable synthesis of copper oxide nanowire arrays on copper foils.2 This involves a wet chemical oxidation of copper foils followed by oxidation in atmospheric microwave plasma. The nanowire arrays were subsequently coated with titania using atomic layer deposition to improve the aqueous stability. The photocurrents obtained with this method are much lower (<0.4 mA/cm2) but better than the data obtained with polycrystalline thin films to that date. The presence of a mixed phase of Cu2O and CuO has been identified as a possible cause for the low photoactivity. CuO has an indirect band gap of 1.4 eV leading to inefficient photon absorption and also cannot generate sufficient energy required to split water. Hence, phase purity of Cu2O is an important factor that needs to be addressed to improve the photoactivity of the Cu2O NW arrays made through atmospheric plasma oxidation. In this contribution, we investigated the use of n-type crystalline WO3 conformal layers on Cu2O nanowire arrays using hot wire chemical vapor deposition and studied their impact on photoactivity and durability. Experimental: Copper oxide nanowire arrays were synthesized by wet chemical oxidation followed by short exposure to atmospheric plasma. Copper foils were immersed in an aqueous solution of sodium hydroxide and ammonium persulfate. The copper hydroxide nanowires produced were converted into copper oxide nanowires by means of exposure to an atmospheric plasma discharge for 2 minutes. The details of the process are described elsewhere. The deposition of WO3 or CuWO4 was performed in a HWCVD reactor using oxygen flow over hot tungsten and copper filaments used as the sources for tungsten oxide and copper oxide vapors. Results and Discussion: The copper oxide nanowires are coated with titania layer using atomic layer deposition and tungsten oxide layers and copper tungstate layers using hot-wire chemical vapor deposition. The PEC characterization studies show 3-4 times enhancement in photoactivity for copper tungstate coated copper oxide nanowire arrays compared to that titania coated copper oxide nanowire arrays. See Figure 1. Acknowledgements: Financial support from US DOE (DE-FG02-07ER46375) is acknowledged.

Tomographic Heating Holder forIn SituTEM: Study of Pt/C and PtPd/Al2O3Catalysts as a Function of Temperature

A tomographic heating holder for transmission electron microscopy that can be used to study supported catalysts at temperatures of up to ~1,500°C is described. The specimen is placed in direct thermal contact with a tungsten filament that is oriented perpendicular to the axis of the holder without using a support film, allowing tomographic image acquisition at high specimen tilt angles with minimum optical shadowing. We use the holder to illustrate the evolution of the active phases of Pt nanoparticles on carbon black and PtPd nanoparticles on γ-alumina with temperature. Particle size distributions and changes in active surface area are quantified from tilt series of images acquired after subjecting the specimens to increasing temperatures. The porosity of the alumina support and the sintering mechanisms of the catalysts are shown to depend on distance from the heating filament.

Discharge responses of the optic tract to flash stimuli in Parkinson's disease.

Dopamine has a significant role in retinal processing, and it has been demonstrated that retinal dopamine content is decreased in parkinsonian patients. We measured the latency of the evoked discharges in the optic tract (OT) to flash stimuli during stereotactic pallidal neurosurgery in 25 patients with Parkinson's disease (PD) (13 women and 12 men, age 38–78 years, unified Parkinson's disease rating scale (UPDRS) Motor Score in the Off state 11–54, Hoehn and Yahr stage in the Off state 1.5–5) and investigated the effects of age at surgery, disease duration, levodopa dose, and severity of parkinsonian symptoms on the latency. OT discharges were evoked by monocular flash stimuli delivered from a flashlight with a krypton bulb with a tungsten filament. The luminance at the eye measured ∼4 × 104 cd/m2. The light wavelength of the stimulus was composed of a wide spectrum with its peak at around 800 nm or longer. The latency of OT discharges ranged 49–79 msec, and there was a significant positive correlation between the latencies of evoked activities in the OT to a flashlight and age (r = 0.59, P < 0.001, by Pearson correlation), but no correlation between the latency and the severity of parkinsonian symptoms and between the latency and duration of illness. These results indicate that the delay in visual processing and conduction at the level of the retina and the OT are substantially derived from age-related degenerative changes in the retina and visual pathway which are apparently unrelated to the striatal dopamine deficiency in PD.

A Nonequilibrium Approach to Temperature Calibration for Tungsten Filament Atomic Spectrometry

ABSTRACT A nonequilibrium approach to voltamperometric temperature measurement for tungsten filament atomizers was developed to facilitate the transfer of thermal conditions between instruments that possess different power supply regulation modes and filaments. Large differences in the equilibrium and instantaneous temperatures were found during the pyrolysis and atomization steps of cadmium atomic absorption spectroscopy (AAS) analysis. By using the instantaneous temperature, the pyrolysis and atomization behaviors of cadmium were shown to be equivalent regardless of the power supply regulation mode. The pyrolysis conditions optimized for a 15-V filament were readily and accurately transferred to a 12-V filament by applying the nonequilibrium voltamperometric temperature model.

EBSD Analysis of Tungsten-Filament Carburization During the Hot-Wire CVD of Multi-Walled Carbon Nanotubes

Filament condition during hot-wire chemical vapor deposition conditions of multi-walled carbon nanotubes is a major concern for a stable deposition process. We report on the novel application of electron backscatter diffraction to characterize the carburization of tungsten filaments. During the synthesis, the W-filaments transform to W2C and WC. W-carbide growth followed a parabolic behavior corresponding to the diffusion of C as the rate-determining step. The grain size of W, W2C, and WC increases with longer exposure time and increasing filament temperature. The grain size of the recrystallizing W-core and W2C phase grows from the perimeter inwardly and this phenomenon is enhanced at filament temperatures in excess of 1,400°C. Cracks appear at filament temperatures >1,600°C, accompanied by a reduction in the filament operational lifetime. The increase of the W2C and recrystallized W-core grain size from the perimeter inwardly is ascribed to a thermal gradient within the filament, which in turn influences the hardness measurements and crack formation.

Induction of small E.M.F from A.C ripples obtained from a rectifier

Abstract: A half wave rectifier utilizes only one half cycle of the A.C signal. Since the circuit can not be 100% D.C the output contains ripple. These A.C components when fed to a transformer may give an A.C voltage at the transformer output. But this voltage varies with load at the output i.e. it depends on whether the load is linear (resistor), non linear (diode), capacitive or inductive etc. Also the output voltage varies if a capacitor is connected in parallel to the resistance which is connected in series with the half wave rectifier. An incandescent lamp having a tungsten filament is used to control current. An amplifier constructed using a single transistor was also used to study the relationship between the no load voltage of the transformer and the full load voltage. At 50Hz the ripple components in a half wave rectifier are just enough to give about 6V amplified output when a 10W lamp is used to control D.C current in the circuit (later explained in the diagram). The power consumption in the circuit considering stray losses is about 15W which is fairly small.

Optically pumped spin-exchange polarized-electron source

We describe the operation of a prototype polarized-electron source. Rubidium vapor, contained in a cell, is optically pumped in the presence of a buffer gas. Unpolarized electrons from a tungsten filament are injected into the cell and extracted after undergoing spin exchange with the Rb atoms. We compare the performance of the source when different buffer gases are used. We measure a decrease in electron polarization as their injection energy increases, but find an unexpected regime at higher injection energies yielding increased electron polarization accompanied by a 40-fold increase in current, suggesting the production of slow secondary electrons in the target cell. With ethylene, we have measured electron currents of 4 \ensuremath{\mu}A simultaneously with electron polarizations of 24%. This work offers the promise of a simple, benchtop, ``turnkey'' source of polarized electrons.

Novel diamond microelectrode for pH sensing

A novel diamond based solid-contact pH-selective microelectrode is reported in the paper for the first time. The diamond films were grown on top of sharp tungsten filaments by HFCVD (Hot Filament Chemical Vapor Deposition). The pH sensitivity of the electrode is achieved via oxygen-termination of the diamond surface with RF oxygen plasma. The MEs were calibrated by zero-current chronopotentiometry registering linearity in the pH2–12 range. The performance of the diamond microelectrode was demonstrated mapping the pH distribution over corroding Zn-Fe model galvanic cell.

A current controlled miniaturized non-radioactive electron emitter for atmospheric pressure chemical ionization based on thermionic emission

In this paper, we introduce a simple to manufacture, emission current controlled, miniaturized non-radioactive electron source to provide free electrons at atmospheric pressure. In atmospheric pressure chemical ionization as usually used in ion mobility spectrometry radioactive beta minus sources provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes. Here, classical thermionic emission from a heat controlled tungsten filament in vacuum is used to generate free electrons. These are then accelerated towards an electron transparent 300nm thin Si3N4-membrane through which the electrons are transferred from vacuum to atmospheric pressure. Both, the emitted electron current and the average kinetic electron energy can be controlled. For similar ionization processes as compared to radioactive beta minus sources we operate the electron source with an electron emission current at atmospheric pressure of 50pA – corresponding to an activity of 300MBq – and an average kinetic electron energy of 8.6keV.

Photostability and toxicity of finasteride, diclofenac and naproxen under simulating sunlight exposure: evaluation of the toxicity trend and of the packaging photoprotection

BackgroundDrugs photostability plays two different opposite roles; a real advantage arises considering the longer expiration time of the drugs while the consequent persistence in the environment involves an obvious negative effect bound to their harmfulness.On this basis we tested the photostability and toxicity of three pharmaceutical active principles: Finasteride, Diclofenac and Naproxen. The pure active principles, as well as commercial drugs containing them, were considered; for the last, the protective effect of the packaging was also evaluated. Samples were irradiated according to the ICH Guidelines for photostability testing (The International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use); a simulating sunlight source (a mercury-vapor lamp coupled to a tungsten filament one) was used to cover the wavelength range 300–2000 nm; Temperature, Relative Humidity, Irradiance and Illuminance were maintained constant during the photodegradation. The concentrations of the pharmaceutical active principles during the photodegradation were monitored by HPLC with UV/Vis detector. Toxicity tests were performed by means of an amperometric biosensor based on suspended yeast cells. Since the products obtained by the photodegradation process can result as toxic or more toxic than the original molecules, tests were performed first and after the photodegadation.ResultsAfter 90 hours of exposure the concentration resulted lowered by 42.9%, 88.4% and 91% for Finasteride, Naproxen and Diclofenac respectively. Toxicity of the pure active principles follows the same order of the photostability. After photodegradation a contribute of the reaction products was evidenced.ConclusionsThe simple and cheap analytical procedure here proposed, allowed to obtain not only data on photostability and toxicity of the pure active principles but, even if roughly, also useful information on the reactions kinetic and toxicity of the photodegradation products.

Direct determination of sodium, potassium, chromium and vanadium in biodiesel fuel by tungsten coil atomic emission spectrometry

High levels of sodium and potassium can be present in biodiesel fuel and contribute to corrosion, reduced performance and shorter engine lifetime. On the other hand, trace amounts of chromium and vanadium can increase the emission of pollutants during biodiesel combustion. Sample viscosity, immiscibility with aqueous solutions and high carbon content can compromise biodiesel analyzes. In this work, tungsten filaments extracted from microscope light bulbs are used to successively decompose biodiesel's organic matrix, and atomize and excite the analytes to determine sodium, potassium, chromium and vanadium by tungsten coil atomic emission spectrometry (WCAES). No sample preparation other than simple dilution in methanol or ethanol is required. Direct analysis of 10-μL sample aliquots using heating cycles with less than 150s results in limits of detection (LOD) as low as 20, 70, 70 and 90μgkg−1 for Na, K, Cr and V, respectively. The procedure's accuracy is checked by determining Na and K in a biodiesel reference sample and carrying out spike experiments for Cr and V. No statistically significant differences were observed between reference and determined values for all analytes at a 95% confidence level. The procedure was applied to three different biodiesel samples and concentrations between 6.08 and 95.6mgkg−1 for Na and K, and between 0.22 and 0.43mgkg−1 for V were obtained. The procedure is simple, fast and environmentally friendly. Small volumes of reagents, samples and gases are used and no residues are generated. Powers of detection are comparable to other traditional methods.

Real-time monitoring of the silicidation process of tungsten filaments at high temperature used as catalysers for silane decomposition

The scope of this work is the systematic study of the silicidation process affecting tungsten filaments at high temperature (1900 °C) used for silane decomposition in the hot-wire chemical vapour deposition technique (HWCVD). The correlation between the electrical resistance evolution of the filaments, Rfil(t), and the different stages of the their silicidation process is exposed. Said stages correspond to: the rapid formation of two WSi2 fronts at the cold ends of the filaments and their further propagation towards the middle of the filaments; and, regarding the hot central portion of the filaments: an initial stage of silicon dissolution into the tungsten bulk, with a random duration for as-manufactured filaments, followed by the inhomogeneous nucleation of W5Si3 (which is later replaced by WSi2) and its further growth towards the filaments core. An electrical model is used to obtain real-time information about the current status of the filaments silicidation process by simply monitoring their Rfil(t) evolution during the HWCVD process. It is shown that implementing an annealing pre-treatment to the filaments leads to a clearly repetitive trend in the monitored Rfil(t) signatures. The influence of hydrogen dilution of silane on the filaments silicidation process is also discussed.

Equipment to detect luminescence induced by cluster ion collision

The possibility to analyze on-line the surface region of solid materials using the cluster ion beam luminescence spectroscopy has been examined. At this aim, the cluster ion beam apparatus for the processing of solid materials was modified. The neutral clusters were ionized by the electron impact ionization to obtain an intense cluster ion beam. The tungsten filament used in this ionization method was replaced with an oxide one to reduce the emission of the background light by decreasing the operating temperature of the filament. To further suppress this light, antireflection materials were used to cover the parts inside the vacuum chamber, such as walls and inner surfaces of the einzel lens. The signal to noise ratio was improved more than one order of magnitude. The emission of photons induced by the irradiation of cluster ion beams was detected.