Vapor Pressure and Evaporation Coefficient of Silicon Monoxide over a Mixture of Silicon and Silica

The evaporation coefficient and equilibrium vapor pressure of silicon monoxide over a mixture of silicon and vitreous silica have been studied over the temperature range (1433 to 1608) K. The evaporation coefficient for this temperature range was (0.007 plus or minus 0.002) and is approximately an order of magnitude lower than the evaporation coefficient over amorphous silicon monoxide powder and in general agreement with previous measurements of this quantity. The enthalpy of reaction at 298.15 K for this reaction was calculated via second and third law analyses as (355 plus or minus 25) kJ per mol and (363.6 plus or minus 4.1) kJ per mol respectively. In comparison with previous work with the evaporation of amorphous silicon monoxide powder as well as other experimental measurements of the vapor pressure of silicon monoxide gas over mixtures of silicon and silica, these systems all tend to give similar equilibrium vapor pressures when the evaporation coefficient is correctly taken into account. This provides further evidence that amorphous silicon monoxide is an intimate mixture of small domains of silicon and silica and not strictly a true compound

Formation of TiC-core, Graphitic-mantle Grains from CO Gas

We demonstrate a new formation route for TiC-core, graphitic-mantle spherules that does not require c-atom addition and the very long timescales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can also be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large cage structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously-nucleated TiC and produce TiC-core, graphitic-mantle spherules. New constraints for the formation conditions and the timescale for the formation of TiC-core, graphitic-mantle spherules are suggested by the results of this study. In particular, TiC-core, graphitic-mantle grains found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the pre-solar fullerenes with water within the meteorite matrix

The influence of buoyant convection on the nucleation of n-propanol in thermal diffusion cloud chambers

A two-dimensional numerical model has been applied to three thermal diffusion cloud chamber (TDCC) investigations of n-propanol in helium taken by two different research groups to provide a quantitative example of how the results in these chambers can be affected by buoyant convection. In the first set of TDCC data, corrections for buoyancy resolve an apparent discontinuity in critical supersaturation data and also yield nucleation rate data that tend to agree better with higher rate, expansion-based studies at the same temperature. In the second TDCC study, the nucleation of propanol was studied over an extended pressure range. When the model was applied to these data, the possible variation in supersaturation values due to convection induced by conditions at the chamber sidewall was found to be comparable in magnitude to the experimentally observed range and may be responsible for some of this observed pressure dependence. In the third TDCC study, the combination of an error in a transport property and buoyant convection appear responsible for a perceived pressure effect in the experimental data. After correcting for this transport property and for buoyancy, the results at higher temperatures agree quite closely with the predictions of classical nucleation theory

Experimental studies of the vapor phase nucleation of refractory compounds. VI. The condensation of sodium.

In this paper we discuss the condensation of sodium vapor and the formation of a sodium aerosol as it occurs in a gas evaporation condensation chamber. A one-dimensional model describing the vapor transport to the vapor/aerosol interface was employed to determine the onset supersaturation, in which we assume the observed location of the interface is coincident with a nucleation rate maximum. We then present and discuss the resulting nucleation onset supersaturation data within the context of nucleation theory based on the liquid droplet model. Nucleation results appear to be consistent with a cesium vapor-to-liquid nucleation study performed in a thermal diffusion cloud chamber

Vapor transport within the thermal diffusion cloud chamber

A review of two different, one-dimensional models of the vapor transport within the thermal diffusion cloud chamber (TDCC) is presented. In one case the assumption is made that there are no convective fluxes within the chamber and that heat and mass transport occur by diffusion only. Although in this model there are no restrictions on the transport of the two components within the chamber, the assumption of no velocities within the chamber results in an incorrect flux boundary condition for the background, carrier gas. The second model is based on the typical, stagnant background gas assumption and the equations of this model closely follow those of the classical Stefan tube problem in which there is transport of a volatile species through a noncondensible, carrier gas. Unfortunately, this model of the TDCC also suffers from the same inconsistencies as noted by several researchers for the Stefan tube. When the convective contributions to the flux are low in the stagnant background gas model, the two models give reasonably close results. For more convective situations, the supersaturation results can differ by more than 50%. One interesting feature of the zero velocity model is that it predicts a change in the supersaturation profile with pressure, whereas no pressure dependence is predicted with the stagnant background gas model. Unfortunately, the direction of this pressure change is opposite to that seen in experimental observations

The effect of carrier gas pressure and wall heating on the operation of the thermal diffusion cloud chamber

Experimental observations indicate that the nucleation behavior within the thermal diffusion cloud chamber (TDCC) changes with increasing carrier gas pressure and applied sidewall heating, even though such an effect is not predicted by typical nucleation theories and it is not seen in typical expansion-based nucleation studies. In this work we present a model of the chamber which shows that both of these effects are likely due to buoyancy-induced convection within the TDCC. As the chamber pressure is increased, the calculated critical supersaturation within the chamber decreases. Results from a simple model of the chamber wall heating are also presented. Previously, it was argued that unheated chamber walls result in a significant, radial concentration gradient which lowers the vapor concentration and condensation flux within the chamber center. In contrast, we show that this reduction is due primarily to a convective flow induced by the sidewall concentration gradient. The model has been applied to recent experimental data for n-pentanol. Results indicate that, with respect to buoyancy-induced convection, the typical 1D model should be regarded as an upper limit to the maximum attainable supersaturation within the chamber

Is the 21-micron Feature Observed in Some Post-AGB Stars Caused by the Interaction Between Ti Atoms and Fullerenes?

Recent measurements of fullerenes and Ti atoms recorded in our laboratory have demonstrated the presence of an infrared feature near 21 pm. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-asymptotic giant blanch (AGB) stars. In our experimental system large cage carbon particles, such as large fullerenes, were produced from CO gas by the Boudouard reaction. Large-cage carbon particles intermixed with Ti atoms were produced by the evaporation of a Ti metal wrapped carbon electrode in CO gas. The infrared spectra of large fullerenes interacting with Ti atoms show a characteristic feature at 20.3 micron that closely corresponds to the 20.1 micron feature observed in post-AGB stars. Both the lab- oratory and stellar spectra also show a small but significant peak at 19.0 micron, which is attributed to fullerenes. Here, we propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21-micron feature seen in some post-AGB stars

On the Use of Fourier Transform Infrared (FT-IR) Spectroscopy and Synthetic Calibration Spectra to Quantify Gas Concentrations in a Fischer-Tropsch Catalyst System

One possible origin of prebiotic organic material is that these compounds were formed via Fischer-Tropsch-type (FTT) reactions of carbon monoxide and hydrogen on silicate and oxide grains in the warm, inner-solar nebula. To investigate this possibility, an experimental system has been built in which the catalytic efficiency of different grain-analog materials can be tested. During such runs, the gas phase above these grain analogs is sampled using Fourier transform infrared (FT-IR) spectroscopy. To provide quantitative estimates of the concentration of these gases, a technique in which high-resolution spectra of the gases are calculated using the high-resolution transmission molecular absorption (HITRAN) database is used. Next, these spectra are processed via a method that mimics the processes giving rise to the instrumental line shape of the FT-IR spectrometer, including apodization, self-apodization, and broadening due to the finite resolution. The result is a very close match between the measured and computed spectra. This technique was tested using four major gases found in the FTT reactions: carbon monoxide, methane, carbon dioxide, and water. For the ranges typical of the FTT reactions, the carbon monoxide results were found to be accurate to within 5% and the remaining gases accurate to within 10%. These spectra can then be used to generate synthetic calibration data, allowing the rapid computation of the gas concentrations in the FTT experiments

Gas/Solid Carbon Branching Ratios in Surface Mediated Reactions and the Incorporation of Carbonaceous Material into Planetesimals

We report the ratio of the initial carbon available as CO that forms gas-phase compounds compared to the fraction that deposits as a carbonaceous solid (the gas solid branching ratio) as a function of time and temperature for iron, magnetite, and amorphous iron silicate smoke catalysts during surface-mediated reactions in an excess of hydrogen and in the presence of N2. This fraction varies from more than 99 for an amorphous iron silicate smoke at 673 K to less than 40% for a magnetite catalyst at 873 K. The CO not converted into solids primarily forms methane, ethane, water, and CO2, as well as a very wide range of organic molecules at very low concentration. Carbon deposits do not form continuous coatings on the catalytic surfaces, but instead form extremely high surface area per unit volume filamentous structures. While these structures will likely form more slowly but over much longer times in protostellar nebulae than in our experiments due to the much lower partial pressure of CO, such fluffy coatings on the surfaces of chondrules or calcium aluminum inclusions could promote grain-grain sticking during low-velocity collisions

Fischer-Tropsch-Type Production of Organic Materials in the Solar Nebula: Studies Using Graphite Catalysts and Measuring the Trapping of Noble Gases

The formation of abundant carbonaceous material in meteorites is a long standing problem and an important factor in the debate on the potential for the origin of life in other stellar systems. The Fischer-Tropsch-type (FTT) catalytic reduction of CO by hydrogen was once the preferred model for production of organic materials in the primitive solar nebula. We have demonstrated that many grain surfaces can catalyze both FTT and HB-type reactions, including amorphous iron and magnesium silicates, pure silica smokes as well as several minerals. Graphite is not a particularly good FTT catalyst, especially compared to iron powder or to amorphous iron silicate. However, like other silicates that we have studied, it gets better with exposure to CO. N2 and H2 over time: e.g., after formation of a macromolecular carbonaceous layer on the surfaces of the underlying gains. While amorphous iron silicates required only 1 or 2 experimental runs to achieve steady state reaction rates, graphite only achieved steady state after 6 or more experiments. We will present results showing the catalytic action of graphite grains increasing with increasing number of experiments and will also discuss the nature of the final "graphite" grains aster completion of our experiments