Refractory materials synthesis

For many years, research efforts in materials chemistry have focused on the development of new methods for materials synthesis. Traditional areas of interest have included the synthesis of catalytic, electronic, and refractory materials via aqueous methods (sol-gel and impregnation) and high-temperature reactions [1-3]. More recent strategies have focused on the synthesis of materials with tailored properties and structures, including well-defined pores, homogeneously distributed elements, isolated catalytic sites, comphcated stoichiometries, inorganic/organic hybrids, and nanoparticles [4-13]. A feature... 

Gas-phase nucleation Flame synthesis of particles (e.g., carbon black, silica) cluster formation in chemical vapor deposition manufacture of high-purity silicon cluster structure and energetics plasma synthesis of refractory materials and coatings. 

There also exist structural similarities between the MeN clusters and the class of solid-state compounds known as transition metal nitrides. In these refractory compounds the nitrogen exists in either an octahedral or a trigonal prismatic array of transition metals 125). The interest in using transition metal nitrides for superconducting thin films as well as chemically inert coatings (i25) should promote studies in which nitrido clusters could be used as starting materials for the synthesis of new refractory materials. 

Aluminates are refractory materials and their synthesis often simply involves solid-state growth of mixtures of purified oxides. Alternative synthesis routes are also used in specialist applications, for example in production of materials with controlled porosity and these invariably involve sol-gel methods. For glasses, one notable, commercially important method of production is container-less synthesis, which is necessary because of the non-Arrhenius (fragile) viscosity of aluminate liquids. 

The detection of diamino acids in the Murchison meteorite (41) has stirred up the speculation that these compounds are perhaps direct products of interstellar synthesis processes. In particular, laboratory experiments aimed at the ultraviolet (UV)-irradiation of interstellar ice analogs leads to the formation of refractory material which, after hydrolysis with 6 M HCl, has been shown to contain mono-amino acids as well as diamino acids (42). Although it is evident that some organic compounds that were formed in the interstellar medium, either in the gas phase or in interstellar grains, survived exposure to the interstellar... 

The effect of dilution on the synthesis reaction of hard (refractory) materials has been examined experimentally and through modeling. Figure 16 shows the effect of adding TiC as a diluent on the adiabatic temperature and the fraction melted of the product [15]. The adiabatic temperature remains constant with the addition of TiC up to 20 mol%, at this point the product is entirely solid and further addition of the diluent decreases the adiabatic temperature as shown in the figure. The decrease in the combustion wave temperature is expected to result in a decrease in the wave velocity. Experimental confirmation of this is provided in Fig. 17 for the case of... 

It is clear from the above that h-BN materials possess a variety of unique properties and demands on such ceramics consistently increase. However, the production methods are energy and time consuming and thus expensive. Indeed, being extremely refractory material, similar to graphite, with nitride dissociation, it is difficult to sinter this type of compound. Can one suggest a novel approach to produce BN ceramics The author truly believes that direct synthesis of boron nitride net shape articles by combustion-based approach described below is an effective alternative to the conventional technologies. 

RG. Tsantrizos, and S. Grenier, Process for the synthesis of fullerenes, 1995, Pegasus Refractory Materials, Inc. (Montreal, Ganada) US Patent 5395496. 

Metal impurities and anion impurities such as halogenides, sulfates, nitrates, and phosphates are the most important impurities that occur in carbon black and could have an influence on the electrochemical system. Besides, sulfur, inorganic residue of refractory material, coke particles, and organic molecules formed during the carbon black synthesis are possible contaminants in carbon blacks. 

The small amount of PCDD/PCDF detected in 100% paper may be due to the trace amount of chlorine in the paper (from the bleaching process) and from the interaction with the refractory material, which had trace amounts of chlorine. The residual carbon collected in the region of 300-400°C is responsible for the formation of PCDD/PCDF by de nova synthesis. The presence of chlorine and copper in ash, in trace levels, implies the presence of copper chloride, which can play a vital role in the formation of PCDD and PCDF through de nova synthesis [42, 43]. 

Nuclear synthesis became feasible after invention of the cyclotron and the discoveries of neutrons and artificial radioactivity. In early thirties a few artificial radioisotopes of known elements were synthesized. Syntheses of heavier-than-uranium elements were even reported. But physicists just did not dare to take the challenge of the empty boxes at the very heart of the periodic system. It was explained by a variety of reasons but the major one was enormous technical complexity of nuclear synthesis. A chance helped. At the end of 1936 the young Italian physicist E. Segre went for a post-graduate work at Berkley (USA) where one of the first cyclotrons in the world was successfully put into operation. A small component was instrumental in cyclotron operation. It directed a beam of charged accelerated particles to a target. Absorption of a part of the beam led to intense heating of the component so that it had to be made from a refractory material, for instance, molybdenum. 

Table 2 summarizes the synthesis, evaluation, and measurement technologies for refractory materials. The development of new preparative processes such as plasma, laser, particle, and high-speed ion-ray processes would lead to new exotic materials. In cases in which the preparation of single crystals of refractory materials is very difficult, some film growth techniques are powerful tools for clarifying the intrinsic properties of bulk crystals. Physical vapor deposition (PVD) is supercooling process used to realize a quasi-equilibrium state that could... 

Table 2 Synthesis, Evaluation, and Measurement Technologies for Refractory Materials...

CVD utilizes the solidification generated by a decompositive reaction from a gas phase. Its characteristic is the low-temperature synthesis of refractory materials. The main CVD techniques for depositing ceramic films are thermal CVD carried out at normal or lower pressure and plasma CVD utilizing a discharge. 

The exit gas is oxidized in the secondary reformer, a cylindrical vessel lined with a refractory material whose lower part is filled with the catalyst the exothermic reaction results in temperatures on the order of 1,000 °C. Oxidation is necessary in order to reduce the amount of unconverted methane (anywhere between 5-15% of the gas is still unreacted at that point) and to isolate the appropriate amount of N2 for the synthesis gas containing the stoichiometric H2 N2 ratio of 3 1 ... 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

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