Green mixture

A 500-ml three-necked flask fitted with a mechanical stirrer and a nitrogen inlet and outlet is charged with 30 g (approx. 0.055 mole) of hydrated chromium (III) sulfate, 200 ml of distilled water, 7.5 g (0.12 g-atom) of mossy zinc, and 0.4 ml (5.4 g, 0.03 g-atom) of mercury. The flask is flushed with nitrogen for 30 minutes and a nitrogen atmosphere is maintained. The mixture is then heated to about 80° with stirring for 30 minutes to initiate reaction. Then the mixture is stirred at room temperature for an additional 30 hours, by which time the green mixture has been converted to a clear blue solution. Solutions prepared as above are about 0.55 M in chromium (II) and are indefinitely stable if protected from oxygen. 

Tetraphenyltellurophene To a suspension of 1,4-dilithiotetraphenyl-l,3-butadiene in 100 mL of ether, prepared from 10 g (56 mmol) of diphenyl acetylene and excess of lithium, is added over 15 min a solution of TeCl4 (5.3 g, 19.7 mmol). The green mixture is poured into a mixture of CH2CI2 and water, the organic phase is separated, filtered through anhydrous MgS04, filtered and evaporated. The residue is recrystallized from dichloromethane/ethanol, giving tetraphenyltellurophene 5.35 g (56%), m.p. 239°C. 

A. (1R,4S)-(-)-Camphorquinone (Note 1). A 125-mL, three-necked, round-bottomed flask, equipped for mechanical stirring and outfitted with a reflux condenser, is Charged with 20.0 g (0.13 mol) of (+)-camphor (1) (Note 2), 8.0 g (0.07 mol) of selenium dioxide (Caution Selenium dioxide is toxic) (Note 3), and 14.0 mL of reagent grade acetic anhydride (Note 4). The green mixture is stirred at reflux for 1 hr, cooled to ambient temperature, and an additional 8.0 g (0.07 mol) of selenium dioxide is added. The mixture is again heated to reflux, and two further batches of 8.0 g (0.07 mol) of selenium dioxide... 

Sulphon acid green, mixtures of an azo blue and ellow 01 - ... 

Attempts to generate a Cu(i) complex by reacting the sterically hindered potentially tridentate 1,5-diazocine 67 with one phenolate and two amine donors, with CuCl or [Cu(MeCN)4]X (X = C104 or Sbl 6 ) under an inert atmosphere in a variety of solvents only led to green mixtures indicative of disproportionation. However, treatment of 67 with CuxMesx (X = 2 and 5) in THF under stringent anaerobic conditions, followed by precipitation with pentane yielded the complex 68, as a white powder (Equation 2) <1998CC2521>. 

Classical catalysis of the addition by a redox system utilizes iron or copper salts. The usual catalyst is copper(I) chloride which is slowly oxidized by moist air to yield a green mixture of copper(I) and copper(II) salts. The example of the addition of 1,2-dibromo-l-chloro-l,2,2-trifluoroethane to oct-l-eiie to give 5 shows selective carbon-halogen homolytic cleavage." " ... 

In this section, we discuss the laboratory techniques and production technologies used for the combustion synthesis process. The laboratory studies reveal details of the CS process itself, while the technologies may also include other processing, such as densification of the product by external forces. In both cases, it is necessary to control the green mixture characteristics as well as the reaction conditions. For the production technologies, however, optimization of parameters related to external postcombustion treatment is also necessary in order to produce materials with desired properties. 

The main characteristics of the green mixture used to control the CS process include mean reactant particle sizes, size distribution of the reactant particles reactant stoichiometry, j, initial density, po size of the sample, D initial temperature, Tq dilution, b, that is, fraction of the inert diluent in the initial mixture and reactant or inert gas pressure, p. In general, the combustion front propagation velocity, U, and the temperature-time profile of the synthesis process, T(t), depend on all of these parameters. The most commonly used characteristic of the temperature history is the maximum combustion temperature, T -In the case of negligible heat losses and complete conversion of reactants, this temperature equals the thermodynamically determined adiabatic temperature (see also Section V,A). However, heat losses can be significant and the reaction may be incomplete. In these cases, the maximum combustion temperature also depends on the experimental parameters noted earlier. 

Based on their analyses, and incorporation of additional details, we have outlined some general relationships for gasless combustion synthesis of materials from elements (type 1), as shown schematically in Fig. 3. Both characteristic features of the process, the combustion wave propagation velocity and maximum temperature, have maximum values when the composition of the green mixture corresponds to the most exothermic reaction for a given system (Fig. 3a). In gen-... 

In general, methods for the large-scale production of advanced materials by combustion synthesis consist of three main steps (1) preparation of the green mixture, (2) high-temperature synthesis, and (3) postsynthesis treatment. A schematic diagram of these steps is presented in Fig. 4. The first step is similar to... 

The design of a typical commercial reactor for large-scale production of materials is similar to the laboratory setup, except that the capacity of the former is larger, up to 30 liters. Since the synthesis of materials produced commercially is well understood, most reactors are not equipped with optical windows to monitor the process. A schematic diagram of such a reactor is shown in Fig. 5. Typically, it is a thick-walled stainless steel cylinder that can be water cooled (Borovinskaya et al., 1991). The green mixture or pressed compacts are loaded inside the vessel, which is then sealed and evacuated by a vacuum pump. After this, the reactor is filled with inert or reactive gas (Ar, He, Nj, O2, CO, C02). Alternatively, a constant flow of gas can also be supplied at a rate such that it permeates the porous reactant mixture. The inner surface of the reactor is lined with an inert material to... 

As discussed earlier, aluminides have been used as binders for carbide- and boride-based cermets, for example, by adding Ni and Al powders to exothermic mixtures of Ti with C or B. On the other hand, some intermetallic compounds (e.g., NiAl, Ni3Al, TiAl) possess high enough heats of formation so that composites with intermetallic matrices can be produced either in the VCS or SHS regimes. The ceramic components are added either in the green mixture or synthesized in situ during the reaction. 

In an early work by Kottke and Niiler (1988), a cellular model was used to simulate the combustion wave initiation and propagation for the TH-C model system. The interactions between neighboring cells were described by the electrical circuit analogy to heat conduction. At the reaction initiation temperature (i.e., melting point of titanium), the cell is instantly converted to the product, TiC, at the adiabatic combustion temperature. The cell size was chosen to be twice as large as the Ti particles (44 /xm). Experimentally determined values for the green mixture thermal conductivity as a function of density were used in the simulations. As a result, the effects of thermal conductivity of the reactant mixture on combustion wave velocity were determined (see Fig. 21). Advani et al. (1991) used the same model, and also computed the effects of adding TiC as a diluent on the combustion velocity. 

For any reaction system, the chemical and phase composition of the final product depends on the green mixture composition, gas pressure, reactive volume, and initial temperature. As shown in Section I, CS reactions can be represented in the following general form ... 

The thermodynamic calculations for the equilibrium combustion temperature and product compositions for a three-component (Ni-Al-NiO) reduction-type system are shown in Figs. 33a and b, respectively (Filatov et al, 1988). In Fig. 33a, the curves correspond to constant adiabatic combustion temperature at P= 1 atm. The maximum was calculated to be 3140 K for the 2Al+3NiO green mixture... 

As shown by the preceding examples, changes in the initial composition of the green mixture can result in a wide variation of synthesis conditions and products. 

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