Refractory compounds decomposition

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Reductive nitrosylation, transition metal nitrosyl complexes, 34 296-297 ReFejSj cluster, 38 41-43 self-assembly system, 38 41-42 Refining, of actinide metals, see Actinide, metals, purification Refractory compounds heat treatment of solids, 17 105-110 crystal growth, 17 105, 106 decomposition, 17 107,-110 spheroidization, 17 106, 107 preparation of, using radio-frequency plasma, 17 99-102... 

Kusakabe et al. (1990) reported that total organic carbon (TOC) concentrations decrease rapidly during the first 100 min of treatment with ozone, leveling off somewhat after 100 min. Furthermore, the decomposition rate of TOC was accentuated by UV irradiation however, no direct correlation between UV intensity and TOC concentration was found. Low concentrations of TOC were still detected even after a 5-hour sampling period, indicating that the destruction of humic substances produces refractory compounds that are oxidized quite slowly (Kusakabe et al., 1990). 

During the last decades, there have been a number of studies on the degradation of mangrove litter by use of litter-bags (Wafar etal., 1997 Ashton etal., 1999). These have shown rapid initial decomposition followed by low rates and accumulation of refractory compounds. The detritus is often enriched by nitrogen during the decomposition (Wafar etal., 1997 Holmer Olsen, 2002), and recent results also show enrichment with phosphorus (Nielsen Andersen, 2003). 

The second class of refractory compounds is formed by compounds of metals with each other or the so-called nonmetallic refractory compounds. All these compounds, like the metal-like compounds, are characterized by a heterodesmic character of the bond, but with predominance of the covalent bond, and they have semiconductor properties as well as high electrical resistance at room temperature as a rule, these compounds have a structure with layer, chain, or skeleton structural groups or patterns, and either melt with decomposition or decompose before reaching the melting point. 

The element M will be more readily liberated if compound R-X is more stable. Thus, when calcium is to be measured in a matrix rich in phosphate ions or in refractory combinations containing aluminium, then sodium or lanthanum chloride is added. The desired effect is to liberate calcium and to increase the volatility of the matrix in order to ensure more efficient elimination during the decomposition step. 

Most gases do not affect refractory bricks. Under high pressure hydrogen gas can reduce silicon dioxide. At appr. 400-800 °C, carbon monoxide is converted into carbon and carbon dioxide. The carbon is deposited on the brick and this may lead to brick s compression. However, a solution has been found. The decomposition of carbon monoxide appeared to be stimulated by certain iron compounds. By firing the bricks at a sufficiently high temperature, you can convert these iron compounds into iron silicates which do not act as catalysts. 

To estimate the destruction rate of TOC, it is assumed that the decomposition rate of humic substances is proportional to Clx which is the TOC of refractory components produced (Kusakabe et al., 1990). Consequently, the decomposition rate of humic compounds measured in the recirculating system is expressed as the sum of the rate in the bubble column and in the UV reactor (VR + VB) as follows ... 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

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