Frank T. Ferguson

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.

Silicates Do Nucleate in Oxygen-rich Circumstellar Outflows: New Vapor Pressure Data for SiO

We have measured the vapor pressure of solid SiO as a function of temperature over the range from 1325 up to 1785 K in vacuo using a modified Thermo-Cahn thermogravimetric system. Although an extrapolation of the current vapor pressure data to 2200 K is close to that predicted from the work of Schick under reducing conditions, the vapor pressures measured at successively lower temperatures diverge significantly from such predictions and are several orders of magnitude lower than predicted at 1200 K. This new vapor pressure data has been inserted into a simple model for the gas expanding from a late-stage star. Using the new vapor pressure curve makes a significant difference in the temperature and stellar radius at which SiO gas becomes supersaturated, although SiO still becomes supersaturated at temperatures that are too low to be consistent with observations. We have therefore also explored the effects of vibrational disequilibrium (as explored by Nuth & Donn) of SiO in the expanding shell on the conditions under which nucleation occurs. These calculations are much more interesting in that supersaturation now occurs at much higher kinetic temperatures. We note, however, that both vibrational disequilibrium and the new vapor pressure curve are required to induce SiO supersaturation in stellar outflows at temperatures above 1000 K.

A Model of Silicate Grain Nucleation and Growth in Circumstellar Outflows

Based on its abundance, high bond energy, and recent measurements of its vapor pressure SiO is a natural candidate for dust nucleation in circumstellar outflows around asymptotic giant branch stars. In this paper, we describe a model of the nucleation and growth of silicate dust in such outflows. The sensitivity of the model to varying choices of poorly constrained chemical parameters is explored, and the merits of using scaled rather than classical nucleation theory are briefly considered. An elaboration of the model that includes magnesium and iron as growth species is then presented and discussed. The composition of the bulk of the grains derived from the model is consistent with olivines and pyroxenes, but somewhat metal-rich grains and very small, nearly pure SiO grains are also produced.

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.

The influence of buoyant convection on the operation of the upward thermal diffusion cloud nucleation chamber

Recently, the stable operation of the upward thermal diffusion cloud chamber with respect to buoyancy-induced convection has become a concern in obtaining reliable nucleation data. During chamber operation, evidence of strong convective currents are clearly visible due to the curved trajectories of entrained droplets. A potential problem exists when these flows are much smaller in magnitude; there is no visible evidence of convection, yet these minute flows may result in systematic errors in the nucleation data calculated via 1D diffusive models of the transport mechanisms within the chamber. To examine whether such flows are possible and the characteristics of these flows we have developed an extension to recent 2D modeling of the nucleation chamber which includes buoyancy-induced, convective motion. We have examined both wet and dry chamber operation with an example case of 1-propanol in helium at a pressure of 1.18 bar. In addition, for the dry wall case we examined the effect of overheating the chambe...

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 μm. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-asymptotic giant branch (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 μm that closely corresponds to the 20.1 μm feature observed in post-AGB stars. Both the laboratory and stellar spectra also show a small but significant peak at 19.0 μm, which is attributed to fullerenes. Here we propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21 μm feature seen in some post-AGB stars.

Application of scaled nucleation theory to metallic vapor condensation

In this paper we report that scaled nucleation theory (SNT) can describe moderately well the observed nucleation behavior of a significant number of refractory materials if a more appropriate value of a quantity commonly referred to as the excess surface entropy is used. With the availability of more reliable critical point and liquid property data, we are better able to calculate this quantity and we find that for refractory materials it can be as small as one half to one third the quantity traditionally used in its approximation. As a result of using more accurate values, we find considerably better agreement between SNT and experiment than what was originally determined. We also explain why using surface tension slope information to determine the excess surface entropy can lead to substantial errors in the SNT supersaturation prediction.

The Formation of Graphite Whiskers in the Primitive Solar Nebula

It has been suggested that carbonaceous grains are efficiently destroyed in the interstellar medium and must either reform in situ at very low pressures and temperatures or in an alternative environment more conducive to grain growth. Graphite whiskers have been discovered associated with high-temperature phases in meteorites such as calcium aluminum inclusions and chondrules, and it has been suggested that the expulsion of such material from protostellar nebulae could significantly affect the optical properties of the average interstellar grain population. We have experimentally studied the potential for Fischer-Tropsch and Haber-Bosch type reactions to produce organic materials in protostellar systems from the abundant H{sub 2}, CO, and N{sub 2} reacting on the surfaces of available silicate grains. When graphite grains are repeatedly exposed to H{sub 2}, CO, and N{sub 2} at 875 K abundant graphite whiskers are observed to form on or from the surfaces of the graphite grains. In a dense, turbulent nebula, such extended whiskers are very likely to be broken off, and fragments could be ejected either in polar jets or by photon pressure after transport to the outer reaches of the nebula.

Fullerenes in Meteorites and the Nature of Planetary Atmospheres

We address the hypothesis that fullerenes are an important carrier phase for noble gases in carbonaceous chondrite meteorites. Unlike other proposed carbon carriers, nanodiamond, SiC, graphite and phase Q, fullerenes are extractable in an organic solvent. It is this unique property, in fact, this may be why fullerene molecules or fullerene-related compounds were overlooked as a carrier phase of noble gases in meteorites. To further evaluate how fullerenes trap noble gases within their closed-cage structure, we compared the natural meteorite fullerenes to synthetic “Graphitic Smokes” soot. High Resolution Transmission Electron Microscopy used to directly image the fullerene extracted residues clearly showed that C60 and higher fullerenes, predominantly C > 100, are indeed the carrier phase of the noble gases measured in the Tagish Lake, Murchison and Allende carbonaceous chondrite meteorites, and synthetic “Graphitic Smokes” material. The implication for the role of fullerenes, which trap noble gases condensed in the atmosphere of carbon-rich stars, is that the true nature of terrestrial planetary atmospheres is presolar in origin. Fullerene, like other carbon carriers, were then transported to the solar nebula, accreted into carbonaceous chondrites and delivered to the terrestrial planets.

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 model...