Neutral atmospheres

The lanthanum phosphate phosphor is usually prepared by starting with a highly purified coprecipitated oxide of lanthanum, cerium, and terbium blended with a slight excess of the stoichiometric amount of diammonium acid phosphate. Unlike the case of the aluminate phosphor, firing is carried out in an only slightly reducing or a neutral atmosphere of nitrogen at a temperature 1000° C. Also this phosphor is typically made with the addition of a flux,... 

For our experiments, a carbon fiber cloth (Figure 3 a) was prepared by carbonization of viscose under neutral atmosphere for 15 minutes, successively at 400, 700 and 1000°C. The carbon cloth was coated with pyrolytic carbon, using chemical vapor decomposition of propylene (2.5 ml/mn) diluted in nitrogen (100 ml/mn) during 10 minutes at 900°C. The resulting composite carbon material exhibits a very low irreversible capacity and 1.5 times the reversible capacity of graphite9 11. 

Redox initiation, in aqueous dispersion polymerization, 11 197—198 Redox neutral atmosphere, 14 84 Redox potentials, of plutonium in acid,... 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

Attention has been focused recently on sedimentary microbial reduction of sulfate. This process neutralizes atmospheric sulfuric acid deposited into soft-water lakes (8-10) through the production of two equivalents of alkalinity per mole of sulfate reduced (11). 

Thermal degradation experiments in neutral atmosphere to determine if polymer thermolysis has (or has not) to be taken into account in the model. 

Intrinsic thermal stability. This could be judged, for example on the basis of determinations of the rate of substrate consumption d[PH]/dt in a neutral atmosphere. According to this criterion, polyethylene, which begins to decompose at about 450° C, would appear more stable than... 

Drift Budget. Four tests were conducted in atmospheric conditions which ranged from slightly stable through neutral to moderately unstable but with very similar mean wind speeds at 46 m above ground. The results of Crabbe et al. (7) for the airborne fraction of the applied spray are shown in Table II. At 400 m downwind of the swath 31 % of the material is still airborne while under neutral and unstable conditions the drifting fraction decreased to 12% and 9%, respectively. This trend is supported by measurements at 1200 m where under neutral atmospheric conditions 10% of the spray is still drifting while in the unstable case, no airborne droplets were detectable at this distance. 

Much has been written concerning the structures and properties of the neutral atmosphere 2 5, but the details are not relevant here. Only those general features which significantly influence the chemistry of the atmosphere will be discussed and then only very briefly. 

Although the melting point of silicon is about 1410 °C, because of the need to remove the native oxide layer coating the SiC so that the silicon wets the grains, the infiltration is usually carried out at temperatures >2000°C when a-SiC forms. The removal of the native silica layer depends upon the ambient and in neutral atmospheres reaction-bonding can be effected at lower temperatures forming /f-SiC. 

In a neutral atmosphere at red heat, cupric chloride decomposes with formation of cuprous chloride and evolution of chlorine.2... 

Illite -H2O-H2 System. Vaporization of potassium from the highly acidic illite system, in neutral atmospheres, is expected to provide a relatively insignificant source of alkali in most coal combustion systems. However, in the presence of reactive combustion gases, such as H2O and H2, thermodynamic considerations predict a significant KOH partial pressure. In addition, an increase in the K-pressure should result from a reduction in the O2 pressure, in the presence of H2. However, KMS experiments did not indicate formation of KOH or additional K in the presence of H2 gas. Thus, thermodynamic equilibrium does not appear to have been established in this heterogeneous system, even though the temperatures were sufficiently high to have normally ensured a rapid approach to equilibrium. 

Both reactions were accomplished experimentally. It actually turned out (Mel nik, 1966a, c) that in a stream of water vapor the dissociation of siderite to magnetite with liberation of carbon dioxide and hydrogen takes place at a lower temperature (by 60-80°C) than in a stream of nitrogen with liberation of CO2 and CO. The experimental works of other authors (Baykov and Tumarev, 1937 Berg and Buzdov, 1961) have shown that in a neutral atmosphere or in the presence of CO2, siderite dissociates according to reaction (4.18) with the formation of CO. 

Experimental investigations of the thermal decomposition of siderite in equilibrium with graphite under a total pressure up to 2000 bar showed that in more prolonged experiments reaction (4.20) apparently also occurs, accompanying the dissociation reaction (4.18). As a result the C02 CO ratio in the equilibrium gas mixture is determined by the buffer reaction of CO breakdown. In our experiments on thermal decomposition of natural siderite in a neutral atmosphere aX P= bar and T — 380-460°C, the CO2 CO ratio ranged from 6 1 to 3 1 and never reached the 2 1 ratio needed for reaction (4.18). 

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

In addition to the neutral atmospheric flux of particles, a cold, weakly ionized plasma exists in LEO. The ionospheric plasma density varies dramatically with altitude and latitude. Near the equator at an altitude of 300 km, peak plasma densities and temperatures are 10 cm and 2000 K,... 

Fig. 1. Number density of neutral atmospheric constituents versus altitude (from Ref. 1),...

J. T. Visentine, Environmental Definition of the Emth s Neutral Atmosphere, NASA/SDIO Space Environmental Effects on Materials Workshop held in Hampton, VA, 28 June-1 July 1988, NASA Conference Publication 3035, Part 1, Elds. L. A. Teichman and B. A. Stein (NASA, Washington, D.C., 1989) pp. 179-195. 

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