Flowing gas atmosphere

A dynamic (flowing gas) atmosphere within which the reaction takes place usually exerts a force on the reactant container that changes the apparent mass of the suspended reactant. This may be constant or may be determined by comparative calibration under static and dynamic conditions. 

The presence of gas within the reaction zone usually influences the apparent reactant mass, and corrections are often necessary. A flowing gas atmosphere will exert a constant force on the balance pan that may be determined by calibration with and without flow. 

However, to use thermodynamically calculated results for the interpretation requires the consideration of the materials microstructure, specific crystallization behavior and the kinetics of phase formation and phase reaction. This is true especially for the interpretation of dynamic thermal analysis experiments (e.g. DTA, TG). Moreover, heat treatments (e.g. thermolysis) are often carried out in an inert atmosphere and evaporating gaseous species are continuously removed by flowing gas atmospheres (inert or reactive). This effect deeply... 

In a flowing gas atmosphere this adsorption-desorption equilibrium results in a delayed transport of the fission product noble gases compared to that of the carrier gas. The mean delay time tm (i. e. the time span between the entry of a gas volume element into the adsorption bed and the moment at which half of the radioactivity has passed the column exit) can be calculated according to... 

On this basis, we focused on the sol-gel synthesis and characterization of nanocrystalline copper oxide thin films with the aim to achieve a good control over film chemical composition [66]. In the preparation of such systems, stoichiometry and phase composition represent a major concern, and they can be suitably tailored by a proper choice of the molecular precursors and of the annealing conditions. The chemical composition and copper oxidation state were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XE-AES). Copper oxide thin films (thickness = lOOnm) were produced from copper acetate and were subsequently heated between 100 and 900 °C in different flowing gas atmospheres, i.e. air, nitrogen and forming gas (4% H2 in N2). 

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

For adsorption, the potential was held at 0.35 V in the base electrolyte. The methanol containing solution (from 0.01 M to 5 M) was allowed to flow into the cell. After 15 min the electrode was pushed against the window (CaF2). The measurements started after a sufficient purging of the gas atmosphere in the IR box. Spectra were taken at potentials between 0 V and 1.5 V RHE with a delay of 1 min after setting each potential. 

For changing the gas atmosphere, the entire balance housing and the sample chamber must be evacuated down to approx. 10-3 torr. In cases where evacuation cannot be applied, the flowing exchange of the gases is recommended. Depending on the volume of the sample chamber and on the flow rate, this procedure may take around 3 hours for a total volume of 15 liters and a flow rate of 5 1/h. 

In the range below 1 10 mbar the relationship between the ion flow and partial pressure is strictly linear. Between 1 10 mbar and 1 10" mbar there are minor deviations from linear characteristics. Above 1 10 mbar these deviations grow until, ultimately, in a range above 10 mbar the ions for the dense gas atmosphere will no longer be able to reach the ion trap. The emergency shut-down for the cathode (at excessive pressure) is almost always set for 5 10 mbar. Depending on the information required, there will be differing upper limits for use. 

Flowing afterglow (FA) was developed in the early 1960s primarily to collect data on atmospheric ion chemistry (Ferguson et al 1969). The instrumentation (Fig. 2) consists basically of a plasma created in a long tube (usually 1 m long) which is carried by a fast flowing gas like helium (usually around... 

The following, well-acceptable assumptions are applied in the presented models of automobile exhaust gas converters Ideal gas behavior and constant pressure are considered (system open to ambient atmosphere, very low pressure drop). Relatively low concentration of key reactants enables to approximate diffusion processes by the Fick s law and to assume negligible change in the number of moles caused by the reactions. Axial dispersion and heat conduction effects in the flowing gas can be neglected due to short residence times ( 0.1 s). The description of heat and mass transfer between bulk of flowing gas and catalytic washcoat is approximated by distributed transfer coefficients, calculated from suitable correlations (cf. Section III.C). All physical properties of gas (cp, p, p, X, Z>k) and solid phase heat capacity are evaluated in dependence on temperature. Effective heat conductivity, density and heat capacity are used for the entire solid phase, which consists of catalytic washcoat layer and monolith substrate (wall). 

Positive ions from the aerosol are attracted toward the glass capillary leading into the mass spectrometer by an even more negative potential of —4 500 V. Gas flowing from atmospheric pressure in the spray chamber transports ions to the right through the capillary to its exit, where the pressure is reduced to —3 mbar by a vacuum pump. 

Eventually, for time-resolved characterizations of solid—solid or solid gas reactions in catalysis, a lower limit for the required time resolution exists, because of the required mass transport of reactants to the catalyst surface. In flow reactors, the characteristic time for gas-phase transport to the catalyst surface, after a rapid change in the gas atmosphere, usually amounts to several seconds. Flence, a time resolution in the range of 100 ms should be sufficient to resolve the changes in the bulk structure induced by the variation in gas composition or reaction temperature. 

When experiments are performed in a static neutral gas atmosphere, deoxidation depends on diffusion of volatile species into the gas. For the same temperature and P02 values, deoxidation takes longer in such atmospheres than in a high vacuum but some acceleration can be achieved by using dynamic conditions i.e., gas flow (Ricci et al. 1994). 

Advantages of this procedure are (a) rapidity (heat exchanges are favoured by the gas mixture flowing at atmospheric pressure) (b) sensitivity (surface areas as low as 0.5 m2 can be measured) and (c) simplicity (it does not require vacuum or dead volume calibration). The main interest of this procedure is to provide one experimental point for the application of the single point BET determination (cf. Chapter 6). Thus, one adsorptive-carrier gas mixture (for example 10% N2, 90% He) can be stored and used as required. Measurement of equilibrium pressure is not required, but the atmospheric pressure, p, should be known. In the above example, the equilibrium pressure is p/9. If possible, the mixture is chosen so that the determination is made within the expected linear range of the BET plot. 

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