Refractory compounds

Minimizing Spectral Interferences The most important spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames in which the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is... 

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

Sihcon nitride is one of the few nonmetaUic nitrides that is able to form alloys with other refractory compounds. Numerous soHd solutions of P-Si N and AI2O2 have gained technical interest. Many companies have begun to mass produce reaction-sintered and hot-pressed Si N parts. 

The materials deposited by PVD techniques include metals, semiconductors (qv), alloys, intermetaUic compounds, refractory compounds, ie, oxides, carbides, nitrides, borides, etc, and mixtures thereof. The source material must be pure and free of gases and inclusions, otherwise spitting may occur. 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

In direction evaporation, the evaporant is the refractory compound itself, whereas in reactive or activated reactive evaporation (ARE), a metal or a low valency metal compound is evaporated in the presence of a partial pressure of a reactive gas to form a compound deposit, eg, Ti is evaporated in the presence of N2 to form TiN, or Si or SiO is evaporated in the presence of O2 to form Si02. 

The microstmcture and morphology of thick single-phase films have been extensively studied for a wide variety of metals, alloys, and refractory compounds. The stmcture model first proposed is shown in Figure 6 (10). It was subsequently modified as shown in Figure 7 (I0,II). 

Refractory compound coatings of carbides, nitrides, and oxides on cemented carbide cutting tools, mainly by the CVD process, are estimated at 300 X 10 annually worldwide. 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

Yu. I. Krasnokutsky, V.G. Verecshak, Poluchenie tugoplavkikh soedinenii v plazme (Preparation of refractory compounds in plasma) Vischa Schola, Kiev, 1987 (in Russian). 

Hafnium carbide (HfC) is an interstitial carbide which, with tantalum carbide, is the most refractory compound known. Its characteristics and properties are summarized in Table 9.4. 

Zirconium carbide is a highly refractory compound with excellent properties but, unlike titanium carbide, it has found only limited industrial importance except as coating for atomic-fuel particles (thoria and urania) for nuclear-fission power plants.l " ] This lack of applications may be due to its high price and difficulty in obtaining it free of impurities. 

Sputtering is an important thin-film process used extensively in the semiconductor and hard-coating industries and for decorative and jewelry coatings. PlP] Excellent coatings of refractory compounds and metals can be readily produced with good adhesion and composition control without the high temperature requirements of CVD. 

Like reactive evaporation, reactive sputtering is used in the deposition of refractory compounds by providing a small partial pressure of hydrocarbons, nitrogen, or oxygen. A problem is target poisoning caused by the reaction of the target with the reactive gas. 

Coatings of refractory compounds can be deposited reactively in a manner similar to reactive evaporation and sputtering by spraying the pure metal in an atmosphere of either a hydrocarbon or nitrogen. 

Most metals and many refractory compounds can be thermal-sprayed. Applications include coating of gas-turbine components for aircraft and industrial use, components of steam turbines and diesel engines, components for the oil and gas industry, paper and pulp industry, and chemical processing industry. 

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

P. S. Kislyi, M. A. Kuzenkova, "Spekanie tupoplavkikh soedinenii (Sintering of Refractory Compounds), Naukova Dumka, Kiev, 1980. 

Details of some inducible P450 forms that play key roles in the metabolism of xenobiotics are shown in Table 2.4. P450s belonging to family lA are induced by various lipophilic planar compounds including PAHs, coplanar PCBs, TCDD and other dioxins, and beta naphthoflavone (Monod 1997). As noted earlier, such planar compounds are also substrates for P450 lA. In many cases, the compounds induce the enzymes that will catalyze their own metabolism. Exceptions are refractory compounds such as 2,3,7,8-TCDD, which is a powerful inducer for P450 lA but a poor substrate. 

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... 

Alumina, AI2O3, is a very refractory compound. It melts above 1950 °C. and is not very reactive when heated. If we attempt to sinter it at 1730°C., we find the following values to apply ... 

Oxidation can be an effective way of pretreating waste prior to biological treatment either by detoxification or by rendering refractory compounds to be more amenable to biological treatment. 

Certain catalyst manufacturers claims to have optimized the preparation (CoMo catalysts), the formulation or the promotion (aromatic saturation) of their catalysts to achieve an appropriate balance of the hydrogenation function to desulfurize the sterically hindered compounds and yield the 15 ppm S fuel. However, the actual trend is to use NiMo catalyst for the treatment of the more refractory compounds, below 200 ppm S [22],... 

Slower reactions such as those of the refractory compounds require an increase on severity or more active catalysts. The increase in severity may imply either a decrease in space velocity, or higher pressures and temperatures. The approach for decreasing space velocity can be immediately rule out, since the magnitude in which it has to be decreased (probably 100-fold) would be completely impractical, e.g., increase reactor volume by 100 times. 

Kvaerner John Brown developed the Super Treat process addressing the H2S inhibition on HDS and favoring the desulfurization on refractory compounds [92], To begin with, they realized that more than 90% of the total produced H2S takes place in the first reaction stage, from the HDS of the most reactive S-compounds. Kvaerner technology... 

The most refractory compounds in the resids may be the PAHs. Those present in residues consists of fused rings of more than six cycles, with molecular weight above 600. Some of the complex larger PAHs isolated and identified are collected in Table 10. These examples probably correspond to the smallest PAH compounds in a resid cut, but worth to set the lowest limit to the level of complexity, from where the reacting mixture starts. 

Deep desulfurization method of fossil fuels, comprising a first step of HDS and a BDS step for the removal of HDS sulfur refractory compounds, using an effective amount of a biocatalyst. The fuel is incubated in the presence of one or more BDS-active microorganisms, which converts the organic sulfur compounds into water-soluble inorganic sulfur. Then, in a separation stage, the products of the incubation are separated into a deeply desulfurized liquid fossil fuel, and the water-soluble inorganic sulfur. 

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