Volatile oxides

Some of the early reentry vehicles utilized metallic heat sinks of copper [7440-50-8] or beryllium [7440-41-7] to absorb reentry heat. Other metallic materials that have been evaluated for nosetip appHcations include tungsten [7440-33-7] and molybdenum [7439-98-7]. The melt layers of these materials are beHeved to be very thin because of the high rate at which volatile oxide species are formed. 

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

Molybdenum High melting point less dense than tungsten or tantalum moderately ductile at room temperature Extremely high oxidation rate (volatile oxide)... 

Lead The production of lead from lead sulphide minerals, principally galena, PbS, is considerably more complicated than the production of zinc because tire roasting of the sulphide to prepare the oxide for reduction produces PbO which is a relatively volatile oxide, and therefore the temperature of roasting is limited. The products of roasting also contain unoxidized galena as well as die oxide, some lead basic sulphate, and impurities such as zinc, iron, arsenic and antimony. 

The behaviour of irradiated uranium has been studied mainly with respect to the release of fission products during oxidation at high temperatures The fission products most readily released to the gas phase are krypton, xenon, iodine, tellurium and ruthenium. The release can approach 80-100%. For ruthenium it is dependent upon the environment and only significant in the presence of oxygen to form volatile oxides of ruthenium. 

The reaction is highly exothermic and the reactor contains large quantities of volatile oxide. Careful control of temperature is therefore required to avoid a runaway reaction and excessive pressure generation. 

An extension of the reduction-chlorination technique described so far, wherein reduction and chlorination occur simultaneously, is a process in which the oxide is first reduced and then chlorinated. This technique is particularly useful for chlorinating minerals which contain silica. The chlorination of silica (Si02) by chlorine, in the presence of carbon, occurs above about 1200 °C. However, the silica present in the silicate minerals readily undergoes chlorination at 800 °C. This reaction is undesirable because large amounts of chlorine are wasted to remove silica as silicon tetrachloride. Silica is, therefore, removed by other methods, as described below, before chlorination. Zircon, a typical silicate mineral, is heated with carbon in an electric furnace to form crude zirconium carbide or carbonitride. During this treatment, the silicon in the mineral escapes as the volatile oxide, silicon monoxide. This vapor, on contact with air, oxidizes to silica, which collects as a fine powder in the furnace off-gas handling system ... 

Films, for both mechanical and spectroscopy studies, were affixed to the specimen panels of the weatherometer. Upon completion of the UV exposure, which occurred at 37°C 1°C in the presence of air, the films were removed and kept at room temperature in the dark for at least 24 hours in order to remove any volatile oxidation products. 

Also, the spontaneous ignition temperature for liquid or volatile oxidizers can be investigated by testing [157]. Here, a predetermined quantity of sawdust (12 to 50 mesh) is added to a reaction vessel and brought to the desired test temperature. The liquid oxidizer is then cautiously injected with a long hypodermic syringe into the vessel. The extent of reaction is determined from continuous temperature measurements and by visual observations. 

The following hypotheses was tested in the first approximation if the vaporization of volatile oxides, sulfides, and metals of all the considered chemical elements at roasting and/or conversion temperature plays a significant role in the contamination of Karabash atmosphere, their calculated equilibrium pressure over the Cu-concentrate, slag, matte or copper melt (or their chemical composition) should strongly correlate with the detected abundance of these elements in snow samples. If such a significant correlation is detected, the corresponding process exerts primary... 

Osmium - the atomic number is 76 and the chemical symbol is Os. The name derives from the Greek osme for smell because of the sharp odor of the volatile oxide. Both osmium and iridium were discovered simultaneously in a crude platinum ore by the English chemist Smithson Tennant in 1803. 

The main source of S emissions from riceflelds is the burning of crop residues, during which most of the sulfur in the residnes is converted to volatile oxides (Fox... 

Results of the flow parameters calculation for the stage of ignition of the air-dust mixtures are shown in [6], Here, the results are presented for the volatiles oxidizing intensity for the following stages of the process the formation of... 

The results show that the dynamics of the reaction zone in a turbulent flame can be traced by the evolution of the volatiles oxidation intensity field. Ignited in the ball-shaped volume, the turbulent flame expands as a relatively wide spherical layer containing strong nonuniformities of the reaction rate. Similar nonuniformities were also detected in the experiments by means of direct optical registration of the flame-ball dynamics [10]. Thus, the numerical results qualitatively reflect the influence of dust concentration nonuniformities that existed... 

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

Titanium is therefore an important ingredient in fountain compositions. It is characterised as a non-volatile metal with non-volatile oxides. The particles are easily ignited, even in the form of large flitters , and once ignited they grow progressively brighter and finally explode in a spectacular star formation. 

The first mechanism proposes that metal volatilisation causes rupture of molten droplets (as with magnesium), whereas the second considers the production of a volatile oxide such as CO inside materials such as steels that contain an excess of 0.1% carbon. The third mechanism involves the formation of oxy-nitride compounds which decompose at high temperatures, liberating nitrogen (as with titanium). 

C. However, on further heating the metal starts to lose its weight similar to platinum, probably due to loss of its volatile oxide Rh02 dissolved in the metal. The molten metal readily absorbs gaseous oxygen. 

Tennant investigated it carefully m an attempt to alloy lead with it, and concluded that it contained a new metal (17). In the autumn of the same year H.-V. Collet-Descotils, a friend and pupil of N.-L. Vauquelin, found that this powder contains a metal which gives a red color to the precipitate from an ammoniacal platinum solution (18). When Vauquelin treated the powder with alkali he obtained a volatile oxide which he believed to be that of the same metal with which Descotils was dealing (19). 

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

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