The structural and electronic properties of small tungsten nanoparticles Wn (n=2-16) were investigated by density functional theory (DFT) calculation. For the W10 nanoparticle, ten lowest-energy structures were first obtained by basin-hopping method (BH) and ten by big-bang method (BB) with the tight-binding many-body potential for bulk tungsten material. These fifty structures were further optimized by the DFT calculation in order to find the better parameters of tight-binding potential adquately for W nanoparticles. With these modified parameters of tight-binding potentials, several lowest-energy W nanoparticles of different sizes can be obtained by BH and BB methods and then further refined by DFT calculation. According to the values of binding energy and second-order energy difference, it reveals that the structure W12 has a relatively higher stability than those of other sizes. The vertical ionization potential (VIP), adiabatic electron affinity (AEA) and HOMO-LUMO Gap are also discussed for W nanoparticles of different sizes.

This study aims to design a novel inserted welding tool to friction stir spot weld SS400 low carbon steel sheets of 4mm thickness. In order to enhance the efficiency of frictional heat generation and to enhance the quality of the welding spot, the welding tool based on a cylindrical tungsten carbide and is inserted by a SS400 low carbon cylinder. The welding apparatus composed of a vertical milling machine and a welding platform that can keep the load between tool and workpiece constant. The plunge load is 8kN and there's no inclination angle on the tools. Welding temperature and the tool plunge depth are measured by thermelcouples and a displacement meter. ¡@¡@At the tool rotational speed of spindle of 900rpm and welding for 60 seconds, the temperature rising rate of the tools with 5 mm and 10 mm inserted material are 5.28 times and 6.31 times greater than the one without insert. While they are 1.36 and 1.42 times greater than at 1200rpm.At the tool rotational speed of spindle of 900rpm and welding for 300 seconds, themaximun welding temperature the tools with 5 mm and 10 mm inserted material can reach are 59¢J and 412¢J higher than the one without insert. While they can reach 35.6¢J and 197.6¢J greater than at 1200rpm. According to the tensile test, the shear failure loads of clad steel plates increase 11.3kN and 15.5kN by using tools with 5 mm and 10 mm inserted material at 900rpm for 60 seconds, and increase 7.6kN and 18.3kN by using tools with 5 mm and 10 mm inserted material at 1200rpm.

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Electrochemical hydride generation is an alternative technique to the chemical hydride generation by NaBH4 which is widely used for atomic spectrometric determination of volatile elements such as As, Bi, Ge, Pb, Sb, Se, Sn and Te. The aim of this research has been to develop an analytical technique at the level of ng/L for determination of antimony by using a simple and inexpensive AA spectrometer and the other parts that can be built in any laboratory. Carbon rod and platinum foil were used as cathode and anode materials, respectively, for the generation of antimony hydride. Argon was used as the carrier gas. Zr coated W-coil was used for on-line preconcentration of generated hydrides. A new apparatus independent from quartz T-tube atomizer was constructed and used to contain the atom trap. Zr coated W-trap was heated to the collection temperature for trapping the analyte species generated electrochemically. For the revolatilization of the trapped species, the trap was further heated to the revolatilization temperature. Revolatilized species were transported to a flame-heated quartz tube atomizer where the analytical signal was recorded. Duringcollection and revolatilization steps hydrogen gas was introduced into the system to prevent the oxidation of atom trap. The experimental operation conditions for electrochemical hydride generation and atom trapping were optimized. 3&amp / #963 / limit of detections were found to be 0.012 ng/mL and 0.41 ng/mL with and without using trap, respectively. The trap has provided 34 fold sensitivity improvement as compared with the electrochemical hydride generation alone. The interferences of some hydride forming elements and some transition metals on electrochemical hydride generation with and without employing the trap were investigated. Analysis of standard reference material was performed to check the accuracy of the proposed method.

Inhalative Partikel können gesundheitsschädigende Wirkungen im Respirationstrakt ausüben. Für Hartmetallstäube aus Wolframcarbidcobaltpartikeln wurden in epidemiologischen Studien Zusammenhänge mit dem Auftreten einer chronisch fibrotischen Lungenerkrankung aufgezeigt, der Hard Metal Lung Disease (HMLD). Zur Aufklärung ihrer Pathogenese wurden die biologischen Effekte mikroskaliger Wolframcarbidpartikel erforscht. Seit einigen Jahren werden zunehmend Pulver zur Herstellung von Hartmetall verwendet, deren Partikel Durchmesser im Nanometerbereich aufweisen. In der vorliegenden Arbeit wurden daher in vitro die Effekte nanoskaliger Wolframcarbidpartikel an humanen Zellen untersucht. Dabei wurden Partikelsuspensionen mit unterschiedlichen Partikelgrößen und –zusammensetzungen verglichen. Beurteilt wurden die Aufnahme der Partikel in die Zellen, ihre toxikologische Wirkung und inflammatorische Mediatoren, die die exponierten Zellen als Reaktion auf die Partikel sezernierten. In Bezug zur Exposition durch Inhalation wurden eine Lungenepithelzelllinie, eine Monozytenzelllinie und primäre mononukleäre Zellen aus dem Blut untersucht. Es zeigte sich, dass die beobachteten Effekte sowohl partikelspezifisch als auch zelltypspezifisch variierten. Dabei wurden die Partikel in alle Zelltypen aufgenommen mit den stärksten Internalisierungsraten in humanen primären Monozyten. Die Wolframcarbidcobalt-Partikel wirkten im Allgemeinen am stärksten vitalitätsmindernd. Alle Partikelarten bewirkten in primären Monozyten eine stark erhöhte Produktion von Zytokinen und Chemokinen. Untersuchungen zum Mechanismus der Partikeleffekte wiesen auf die Beteiligung reaktiver Sauerstoffspezies hin. Es konnten in der vorliegenden Arbeit bestehende Erkenntnisse zur Toxizität von Wolframcarbidcobaltpartikeln bestätigt werden und Hinweise auf die Beeinflussung biologischer Effekte durch verschiedene Partikelgrößen und Oberflächeneigenschaften von Nanopartikeln gefunden werden.

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Porous tungsten is used to manufacture dispenser cathodes due to it refractory properties. Surface porosity is critical to functional performance of dispenser cathodes because it allows for an impregnated ceramic compound to migrate to the emitting surface, lowering its work function. Likewise, surface roughness is important because it is necessary to ensure uniform wetting of the molten impregnate during high temperature service. Current industry practice to achieve surface roughness and surface porosity requirements involves the use of a plastic infiltrant during machining. After machining, the infiltrant is baked and the cathode pellet is impregnated. In this context, cryogenic machining is investigated as a substitutionary process for the current plastic infiltration process. Along with significant reductions in cycle time and resource use, surface quality of cryogenically machined un-infiltrated (as-sintered) porous tungsten has been shown to significantly outperform dry machining. The present study is focused on examining the relationship between machining parameters and cooling condition on the as-machined surface integrity of porous tungsten. The effects of cryogenic pre-cooling, rake angle, cutting speed, depth of cut and feed are all taken into consideration with respect to machining-induced surface morphology. Cermet and Polycrystalline diamond (PCD) cutting tools are used to develop high performance cryogenic machining of porous tungsten. Dry and pre-heated machining were investigated as a means to allow for ductile mode machining, yet severe tool-wear and undesirable smearing limited the feasibility of these approaches. By using modified PCD cutting tools, high speed machining of porous tungsten at cutting speeds up to 400 m/min is achieved for the first time. Beyond a critical speed, brittle fracture and built-up edge are eliminated as the result of a brittle to ductile transition. A model of critical chip thickness (hc) effects based on cutting force, temperature and surface roughness data is developed and used to study the deformation mechanisms of porous tungsten under different machining conditions. It is found that when hmax = hc, ductile mode machining of otherwise highly brittle porous tungsten is possible. The value of hc is approximately the same as the average ligament size of the 80% density porous tungsten workpiece.

No description available.

A significant factor in the overall efficiency of the balance of plant for a future magnetic fusion energy (MFE) reactor is the thermal performance of the divertor. A significant fraction of the reactor power is delivered to the divertor as plasma impurities and fusion products are deposited on its surface. For an advanced MFE device, an average divertor heat load of 10 MW/m² is expected at steady-state operating conditions. Helium cooling of the divertors is one of the most effective ways to accommodate such a heat load. Several helium-cooled divertor designs have been proposed and/or studied during the past decade including the T-Tube divertor, the helium-cooled flat plate (HCFP) divertor, the helium-cooled multi-jet (HEMJ) divertor, the helium-cooled modular divertor with integral fin array (HEMP), and the helium-cooled modular divertor with slot array (HEMS). All of these designs rely on some form of heat transfer enhancement via impinging jets or cooling fins to help improve the heat removal capability of the divertor. For all of these designs very large heat transfer coefficients on the order of 50-60 kW/m²-K have been predicted. As the conditions of a fusion reactor and associated helium flow conditions (600 °C and 10 MPa) are difficult to achieve safely in a controlled laboratory environment, the study of these divertors often relies on computer simulations and experimental modeling at non-prototypical, albeit dynamically similar, conditions. Earlier studies were based on the assumption that, for geometrically similar divertor test modules, dynamic similarity can be achieved by matching only the Reynolds number. Experiments conducted in this investigation using different coolants and test module materials have shown this assumption to be false. Modified correlations for the Nusselt number and loss coefficients for the HEMJ and HEMP-like divertor modules have been developed. These have been used to develop generalized performance curves to predict the divertor performance, i.e. the maximum allowable heat flux and corresponding pumping power fraction, at prototypical conditions. Additionally, a numerical study has been performed to optimize the fin array geometry of the HEMP-like divertor module. The generalized correlations and performance curves developed in this investigation can be incorporated into system design codes, thereby allowing system designers to optimize the divertor geometry and operating conditions.

No description available.