Air and Oxygen

The kinetics of oxidation of tantalum in pure oxygen have been studied at temperatures up to 1400°C and at pressures ranging from less than 1 to over 40 atm (0.10-4.05 MPa). The reaction is initially parabolic, with a transformation to linear rate after a period of time. Increasing the temperature not only increases the rate of oxidation, but also decreases the time before the reaction changes from parabolic to linear behavior. Above about 500°C and pressures from 10 mm Hg to 600 psi (1333 Pa to 4.13 MPa), the transition occurs almost immediately. From 600 to 800°C, the oxidation shows a pronounced increase in rate with pressure above 0.5 atm (0.05 MPa). At 1300°C and 1 atm oxygen pressure, tantalum oxidizes rapidly and catastrophically, but at 1250°C, the metal oxidizes linearly for a short time, then catastrophically. Unlike tantalum-oxygen reactions, however, tantalum-air reactions do not exhibit catastrophic oxidation at temperatures as high as 1400°C. 

Corrosion rate of tantalum in air as a function of temperature. (From D.F. Taylor. 1956. Tantalum its resistance to corrosion. Presented at the Chicago Section, Electrochemical Society, May 4.) 

Oxygen attack is usually viewed as the primary mechanism for failure of tantalum at low loads and elevated temperature. Consequently, most attempts to protect tantalum against gas corrosion at higji temperature have aimed at imparting resistance to the base metal. Although Ta205 forms thin, adherent, protective films below 500°C, at higher temperatures the film becomes flaky and tends to spall. Two approaches have been used to improve the oxidation resistance of tantalum  

Form a denser, more adherent oxide film by alloy additions to the tantalum to alter and modify the oxide phase. 

Provide a protective coating to inhibit oxygen attack. Coatings include silicides, aluminides, noble metals, and others. 

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In the presence of catalyst, usually platinum, ammonia is oxidised by oxygen (and air) to nitrogen oxide. NO. This reaction, used to obtain nitric acid from ammonia (p. 238), can be demonstrated in the laboratory using the apparatus shown in Figure 9.4 the oxygen rate should be slow. 

Small quantities of ozone are produced when oxygen and air are subjected to an electrical discharge and it is, therefore, found in the neighbourhood of working electrical machines. Probably a small quantity of atomic oxygen is initially produced most of this recombines quickly to give oxygen, Oj, but a few atoms react to form ozone ... 

In general, one day of oven aging at 70°C corresponds to one year of natural or shelf aging (a minimum requirement for mbber products), whereas the oxygen and air bomb methods are more drastic. By varyiag the amounts and types or combiaations of antioxidants the relative effectiveness of these materials against normal oxygen deterioration can be determined. 

Theoretical Oxygen and Air for Combustion The amount of oxidant (oxygen or air) just sufficient to burn the carbon, hydrogen, and sulfur in a fuel to carbon dioxide, water vapor, and sulfur dioxide is the theoretical or stoichiometric oxygen or air requirement. The chemical equation for complete combustion of a fuel is... 

Detonation Limits (Vol %) for Confined and Unconfined Explosions and Flammability Limits (Vol %) in Oxygen and Air... 

Matsui, H., and J. H. S. Lee. 1979. On the measure of relative detonation hazards of gaseous fuel-oxygen and air mixtures. Seventeenth Symposium (International) on Combustion, pp. 1269-1280. Pittsburgh, PA The Combustion Institute. 

Not only do heavy-metal ions catalyze the destruction of peroxide compounds but they also catalyze other chemical processes caused by oxygen and air. Sequestering these metal ions by phosphorus-containing compounds will decrease their undesired effects. 

A method, proposed more recently by FULLER, SCHETTLER and GlDDlNGSf8 . is claimed to give an improved correlation. In this approach the values of the diffusion volume have been modified to give a better correspondence with experimental values, and have then been adjusted arbitrarily to make the coefficient in the equation equal to unity. The method does contain some anomalies, however, particularly in relation to the values of V for nitrogen, oxygen and air. Details of this method are given in Volume 6. 

Coal with a mean particle size of less than 3 mm is slurried with water and then oxidized with oxygen or mixtures of oxygen and air at temperatures ranging from 100° to 300° C, at partial oxygen pressures ranging from 0.1 to 10 MPa and reaction periods ranging from 5 to 600 minutes [425]. In the absence of catalysts, such as alkaline bases, the main products of oxidation are humic acids. 

In oxygen- and air-saturated solutions, 45 released its alcohol moiety and formed products 49 and 48, respectively (Scheme 28). The quantum yields for forming 49 and 48 were 0.17 and 0.14, respectively. ... 

The oxidation of Eu2+ at various HC1 concentrations and at different pressures of hydrogen, oxygen and air has been studied [191]. The rate of oxidation is extremely sensitive to the concentration of oxygen. In the absence of oxygen the oxidation process can be represented as... 

Thus we establish the concept of the concentration limit.6 Investigation of these limits was carried out on a broad scale in connection with the requirements of accident prevention, so that the region of concentrations in which mixtures of gases and vapors with oxygen and air are explosive has been established for a large number of substances. 

Keeping in mind the way sodium and potassium react with oxygen and air, predict the reaction of sodium and potassium with dilute acids Which metals do not react with dilute or concentrated acids ... 

As an extension of the proximate analysis or coal assay, it is worthy of note that new methods continued to be developed. For example, thermogravimetric analysis has been extended to cover determinations of volatile matter (as well as determination of moisture and ash) in coal and coke. These constituents can be measured by pyrolyzing the samples in oxygen and air, and the weight loss at prescribed temperatures was measured by using a thermobalance. 

Fig. 8. Comparison of k a, a and l data measured by pure oxygen and air absorption in pure sulphite solution and with addition of ocenol. Air absorption, filled symbols oxygen absorption, hollow symbols see Table 1.

Plasma. For plasma treatment the mouldings are arranged in a suitable chamber which then is evacuated to the pressure appropriate for the type of plasma being used. A gas or gas mixture is introduced (typically, a mixture of oxygen and air), and plasma formed by energizing the gas for a short time. 

Figure 2. U-tuhe vs. oven test in oxygen and air with 25-mil samples at 140° and 150°C.

Figure 7. Comparison of the thermal oxidative degradation of oxydesulfurized Indiana Minshall Seam (HvCb) coal in oxygen and air...

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