Molybdenum-silicon system

The molybdenum-silicon and titanium-nitrogen systems also belong to this group. For both, it was shown that U decreases when metal (Mo, Ti) particle size increases (curves 6 and 7, Fig. 48). 

The control of the second critical step, crystallization of the amorphous alloy, is the focus of the following discussion. Iron-silicon and iron-aluminum systems are discussed. The critical lengthscales in the iron-aluminum and iron-silicon systems are much larger than that observed in the molybdenum-selenium systems. By critical lengthscale, we refer to the thickness of the repeat unit in the multilayer below which the multilayer evolves completely into an amorphous material without the nucleation of any crystalline phase. The samples discussed are all layered on a lengthscale which is less than this critical value. That is, they evolve from a layered initial state, through a distinct amorphous intermediate, to a crystalline compound. 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

Cover Illustration Atomic force microscopy image of molybdenum oxide particles on flat, silicon dioxide substrate, which serves as a model system for a supported catalyst. The area shown corresponds to one square micrometer the maximum difference in height is approximately 10 nanometer. The superimposed curve is the secondary ion mass spectrum of the model catalyst, showing the caracteristic isotopic patterns of single molybdenum ions and of molybdenum oxide cluster ions. 

We illustrate the use of RBS with a study on the sulfidation of molybdenum hydrodesulfurization catalysts supported on a thin layer of Si02 on silicon [21], As explained in connection with the SIMS experiments on this model system (Fig. 4.8), the catalyst is sulfided by treating the oxidic Mo03/Si02 precursor in a mixture of H2S and H2. RBS is used to determine the concentrations of Mo and S. 

Vanadium Alloys.—Vanadium alloys readily with many metals, including aluminium, cobalt, copper, iron, manganese, molybdenum, nickel, platinum, and tin, also with silicon. These alloys have hitherto received scant attention, and little is known in most cases of the systems produced. 

To increase the number of catalysts, the binary catalyst system zirconium/ platinum was extended to a ternary system by adding a third component such as molybdenum. The expected quality of such a composition is given in Figure 3.8. First results of such a ternary layer are presented in Figure 3.9 (left). The layer was deposited on a silicon monocrystalline wafer and the layer composition again examined with SNMS. 

The electrochemically active part of the electrode consists of a molybdenum wire or rod that has been oxidised in molten potassium nitrate and soldered to an insulated copper wire [26]. Joints are normally sealed with epoxy or silicone compounds. M0/M0O3 was suggested as a possible reference electrode in highly corrosive alkaline systems as early as 1967 [27], For concrete application, two-year stability is reported [25]. The potential of M0/M0O3 in concrete is approximately -450 mV vs SCE [26]. Field performance documentation is scarce. 

Teflon self-lubricating plastic bearings can be used in situations where a lubricant might contaminate the system. There are also a variety of nonoily lubricants such as silicone lubricant (usually dispensed as a spray) and molybdenum disulfide (suspended in petroleum distillates that evaporate to leave a dry coating). Molybdenum disulfide is an excellent alternative to graphite for lubrication of moving parts in a high-vacuum system. 

In fact Van Wyk ° used something similar to a composite structure for the lubrication of silicon nitride and alumina in plain spherical bearings. He incorporated a 90% molybdenum disulphide/8% molybdenum/2% tantalum compact in holes drilled in the surface of the alumina outer ring, and the details have been described in Chapter 8. The system was very successful, giving a forty times increase in wear life. 

Ingle and Crouch described a diflerential kinetic method for silicate and phosphate based on the faster rate of formation of heteropoly molybdenum blue from the yellow heteropoly acids in the presence of phosphate than in the presence of silicate. They found that silicon in the range of 1 to 10 ppm could be determined with 3% accuracy in the presence of 10 ppm of phosphorus, and phosphorus in the range of 1 to 10 ppm with 1% accuracy in the presence of 50 ppm of silicon. This system was also automated, with the analyses of mixtures being performed in less than 5 min. 

Also. J. B. Pedley, ed., Computer Analysis of Thermochemical Data (CATCH Tables)." Univ. of Sussex, Brighton, 1972. (i) Halogen compounds (1972), (ii) nitrogen compounds (1972), (iii) phosphorus compounds (1972), (iv) silicon compounds (1972), (v) chromium, molybdenum, and tungsten compounds (1974). Booklets (iv), (v) give data on organometallic compounds. The book by Cox and Pilcher (A149) is to be updated under the same system. 

From the physico-chemical and thermodynamic analysis of the molten systems KF-K2M0O4-B2O3 and KF-K2Mo04-Si02, it can be concluded that the formation of heteropolymolybdates containing boron, [BM06O24] , and silicon, [SiMo 12040]" , as a central atom is most probably responsible for an easy molybdenum deposition. Besides, the entry of fluorine atoms into the coordination sphere of molybdenum in the heteropolyanions lowers the symmetry and thus, also the electrochemical stability of such electro-active species. 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

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