Showing papers on "Tungsten published in 1978"

Electron field emission from ferromagnetic europium sulfide on tungsten

Abstract: The emission process from W-EuS junction field emitters was studied by combination of several experimental methods: measurement of emission current, spin polarization, energy distribution, and energy-selective spin polarization as function of emitter temperature for different annealing conditions. The results show that with critical annealing a stoichiometric crystalline EuS layer can be obtained; the interface of such a layer on tungsten acts as a spin filter below the EuS Curie temperature. With emitters annealed at higher temperatures, evidence for electron trapping and the formation of spin clusters has been obtained.

Coarsening of tungsten grains in liquid nickel-tungsten matrix

Abstract: In sintered W-Ni alloys with 1,7, and 30 wt pct Ni the tungsten grain growth in liquid matrix at 1540°C was investigated. The observed grain size distributions and growth rate are compared with the theoretical predictions of Wagner, Lifshitz and Slyozov, Lay, and Ardell. In the 70 pct W-30 pct Ni alloy the tungsten particles settled to the bottom of the specimens immediately upon melting of the matrix, but the spherical grain shape is maintained during the initial stage of annealing. In these specimens the linear intercept distribution of the grains agrees with the prediction of the LSW (Lifshitz, Slyozov, and Wagner) theory for the reaction controlled growth mechanism. On the other hand the growth rate appears to follow the t1/3 law predicted for the diffusion controlled mechanism. These results are consistent with Lay and Ardell's theory in which the concentration gradient around grains is inversely proportional to the average grain size in the limit of small matrix fraction. In the alloys with 1 and 7 pct Ni a meaningful comparison of the observed linear intercept distribution of the grains with theoretical predictions is difficult because of grain contact flattening due to densification. The grain growth is larger with less matrix fraction in the specimens and this result provides an evidence for the diffusion controlled grain growth during the liquid phase sintering of this alloy.

Binding and activation of enzymic substrates by metal complexes. 4. Structural evidence for acetylene as a four-electron donor in carbonylacetylenebis(diethyldithiocarbamate)tungsten

Abstract: Eine CH, Cl2-Losung von Bis1[N,N-diethyldithiocarbamato]-dicarbonyl-triphenylphosphinwolfram reagiert in reiner Acetylenatmosphare zu der diamagnetischen Titelverbindung (I), die durch IR-, NMR- und sichtbares Elektronenspektrum charakterisiert wurde.

Sintered hard metal and the method for producing the same

Abstract: This invention relates to a sintered hard metal for cutting tools and having excellent wear resistance, resistance to thermal deformation, mechanical strength and the like. The present invention resides in the fact that the substrate metal consists of a rich content of the B-1 type hard phase and the surface thin layer consists of a rich content of the ordinary tungsten carbide. B-1 type crystal structure contains one or more Group IVa, Va and VIa metals, carbon, nitrogen and, moreover, tungsten. The thin layer structure has the layer of 5 to 200μ thickness on the surface thereof.

Kinetics of Tungsten Deposition by the Reaction of WF 6 and Hydrogen

Abstract: Experimental rate data from a number of investigations on the chemical vapor deposition of tungsten by the hydrogen reduction of were analyzed to determine the reaction mechanisms which controlled the process under experimental conditions in which mass transport limitiation was absent. Rate‐controlling mechanisms were identified by comparing reaction orders obtained from the experimental data with those obtained from rate equations derived for each possible rate‐controlling step. Where the reactant stream was expected to remain streamlined and only slightly heated through contact with the tungsten substrate, the rate‐controlling mechanism was determined to be the dissociation of molecules adsorbed on the substrate. The activation energy for this process step was 67,000 J/mole. The reaction orders with respect to and were zero and one‐half, respectively. With experimental conditions more conducive to gas stream turbulence and stream heating, deposition rate was controlled by homogeneous gas phase reactions also having an activation energy of about 67,000 J/mole. The reaction order for this mechanism was 2 with respect to both reactants.

Tungsten carbide powders produced by the vapor phase reaction of the WCl6-CH4-H2 system

Abstract: Black powders of tungsten carbides (WC, W2C), including tungsten metal, were produced by the vapor phase reaction of the WCl6-CH4-H2 system at 1000–1400 °C. The C/W ratio of the product increased sharply with increasing reaction temperature and increasing methane concentration. At 1400 °C the product consisted of almost pure WC. The particle size of the carbide powders, which was significantly influenced by the mixing temperature of WCl6 and H2, was 0.04–0.05 μm for low mixing temperatures and 0.08–0.11 μm for high mixing temperatures. The formation of tungsten carbide powders in this reaction system was found to occur in two steps, namely the initial formation of W particles and their subsequent carburization. This was confirmed by separate examinations of the hydrogen reduction of WCl6 and the carburization of W powders with methane.

DC electrical conduction in tungsten phosphate glasses

Abstract: The dc conductivity of semiconducting tungsten phosphate glasses of three different compositions has been measured over a temperature range 100–400 K. The data have been analysed in the light of existing models of polaronic hopping conduction. The high-temperature region can be qualitatively explained by small-polaron hopping between nearest neighbours, while at low temperatures (below 150 K), the behaviour can be explained by Mott's variable-range hopping. The value of the electron decay constant α estimated from variable-range hopping is of the order of 2 A −1 , but the estimated disorder energy W D is twice as large as the measured low-temperature activation energy.

Chemistry of compounds containing metal-to-metal triple bonds between molybdenum and tungsten

Abstract: : The preparation, properties, structures, dynamical solution behaviors and reactivities towards small unsaturated molecules (CO2, acetylenes, CO, etc.) are described for a series of molybdenum and tungsten compounds of general formula M2X6 or M2X6-nY, where X Y=R (alkyl), NR2, or, O2CNR2, O2COR or halide, and Cp2M2(CO)4, all of which contain triple bonds between the metal atoms (M=Mo or W). (Author)

Extracting tungsten from ores and concentrates

Abstract: A process for recovering tungsten from tungsten bearing ores comprises producing a concentrate from the ore and leaching the concentrate at atmospheric pressure in a strong mineral acid solution, for example, a solution of sulfuric acid, hydrochloric acid or nitric acid within a temperature range of from 40° to 90 Centigrade for from one to four hours in the presence of an inorganic complexing agent, and adding an ammonium salt compound for precipitating the tungsten. The precipitate is then dissolved in Ammonium Hydroxide and the tungsten recovered as Ammonium Paratungstate.

Design and optimization of directly heated LaB6 cathode assemblies for electron‐beam instruments

Abstract: A description is given of the design and testing of a directly heated, stable, electron source utilizing a single‐crystal lanthanum hexaboride (LaB6) cathode. The emitter mounting fixture consists of an adjustable molybdenum base unit supported on gas‐impervious alumina or machinable glass. Single‐crystal cathode rods are securely clamped and positioned between vitreous carbon jaws that are resistively heated. The complete assembly is designed to be a direct ’’plug‐in’’ substitute for the conventional tungsten thermionic filaments used in electron‐beam instruments. The cathode current density for 〈110〉 axial orientations is found to be ten times higher than that for 〈100〉 orientations under equivalent conditions, a value of 50 A cm−2 being measured at 1500°C with an observed lifetime in excess of 300 h. Optimum vacuum conditions for high lifetime and stable operation are in the range 1×10−6 Torr and lower. Comparison values for the emission at various temperatures from other borides, and tungsten, are als...