The determination of carbon in molybdenum and tungsten

Vandecasteele et al. (53)(55)(56) describe the determination of carbon in molybdenum and tungsten. The irradiation conditions are given in Table V-6. 

The molybdenum samples are etched in a mixture of 4 volumes hydrofluoric acid (40 %) and 1 volume nitric acid (14 M) at room temperature, dipped for 20 s in 12 M hydrochloric acid, rinsed successively with water and acetone and dried in a stream of hot air. This sequence is repeated twice. For tungsten the samples are etched twice in a mixture of 1 volume hydrofluoric acid (50 %) and 1 volume nitric acid (14 M) at room temperature, rinsed and dried. 

The annihilation activity is repeatedly measured with a y-y coincidence 

The choice of the incident energy is not critical for the determination of carbon in tungsten. It is however very important for the instrumental determination of carbon in molybdenum. Fig. V-1 gives relative thick target yields for the Mo(d,n) Tc and Mo(d,p) Mo reactions as a function of the energy (55). These were determined by irradiation of a stack of molybdenum foils with 5 MeV deuterons. For comparison, the thick 

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The determination of carbon in molybdenum and tungsten is already since 1969 the subject of studies in the frame of the working group "Sonder-und RefraktSrmetalle" of the Chemistry Committee of GDMB (Gesellschaft Deutscher MetalIhutten- und Bergleute). In the case of molybdenum (91) combustion methods carried out in 8 laboratories yielded the results... 

THE DETERMINATION OF CARBON IN NIOBIUM, TANTALUM, MOLYBDENUM AND TUNGSTEN... 

The determination of carbon in niobium, molybdenum, tantalum and tungsten... 

The equipment for the chemical separation of described in Chapter II, section 3.8.1. was developed to determine carbon in molybdenum and tungsten (79). The samples (1 to 2 g) were irradiated for 40 minutes. To preclude the effects of competing nuclear reactions, the maximum energy of the photon beam was limited to 25 MeV. 

It is shown that the cathodic electrolysis products of melts based on sodium tungstate, in which molybde-num(VI) or tungsten(VI) oxides, lithium or sodium molybdate, tungstate, and carbonate are dissolved, are molybdenum, tungsten, their bronzes and carbides, and carbon. It is found that the phase composition of electrolysis products is determined by the concentration of carbonate in the melt. The conditions are determined for the deposition of galvanic coatings of molybdenum and tungsten carbides on different materials. 

NMR resonances of the alkylidyne carbon atom and selected IR absorptions for new carbyne complexes are given in Table 11. In other studies, the Mo and NMR spectra of molybdenum and tungsten alkylidyne complexes were determined by Enemark and co-workers (23). The metal atoms in the alkylidyne complexes were found to be less shielded than those in nitrido complexes but more shielded than those in compounds containing metal-metal multiple bonds. 

Niobium in Tool Steels. In the matrix method of tool-steel development, the composition of the heat-treated matrix determines the steel s initial composition. Carbide volume-fraction requirements then are calculated, based upon historical data, and the carbon content is adjusted accordingly. This approach has been used to design new steels in which niobium is substituted for all or part of the vanadium present as carbides in the heat-treated material. Niobium provides dispersion hardening and grain refinement, and forms carbides that are as hard as vanadium, tungsten, and molybdenum carbides. 

The C-bound metalla tautomers 2 are typical for the less electropositive metals [4e]. They have been postulated occasionally for zinc [20] and copper [21] but are a rule for mercury [10a]. Carbon-bound enolates of molybdenum, tungsten, manganese, rhenium, iron, rhodium, nickel, iridium, and palladium have been detected and characterized [22], but one has to be aware of the phenomenon that they exist in equilibrium with the O-bound metalla tautomers. The interconversion of the palladium enolates 14 and 15 (Scheme 1.6), whose activation barrier has been determined to amount to approximately 10 kcal mol , may serve... 

For tungsten and molybdenum carbides, an increase in substrate surface temperature leads to an increase of the lattice parameter of the film as a result of the increase in carbon concentration. The concentration seems to reach a maximum close to 50 mol% for 500 K in the Mo-C system and 700 K in the W-C system. At these temperatures, the respective M2C phase appears (Table 14.2). Figure 14.2 shows a plot of the carbon concentration (determined by AES) as a function of the substrate temperature. An opposite behaviour was observed for the nitrogen concentration in Mo-N films. Figure 14.3 shows that the N concentration in the a-MoN, phase (fee) decreased with substrate temperature.11 The phase domains were determined by X-ray and electron diffraction. No nitrogen was detected by AES in the Mo bee phase. No significant effect of temperature was observed on W-N films and Cr-N films. 

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