Effusion, gaseous

A Back-Pressure Efficiency Factor. Because a gaseous diffusion stage operates with a low-side pressure p which is not negligible with respect to there is also some tendency for the lighter component to effuse preferentiahy back through the barrier. To a first approximation the back-pressure efficiency factor is equal to (1 — r), where ris the pressure ratiopjpj. 

Our discussion concentrates on the uranium-235 isotope. It makes up only about 0.7% of naturally occurring uranium. The more abundant isotope, uranium-238, does not undergo fission. The first process used to separate these isotopes, and until recently the only one available, was that of gaseous effusion (Chapter 5). The volatile compound uranium hexafluoride, UF6, which sublimes at 56°C, is used for this purpose. 

The vapor pressure of a molten metal can be measured with a device called a Knudsen cell. This is a container closed across the top by a thin foil pierced by a small, measured hole. The cell is heated in a vacuum, until the vapor above the melt streams from the small hole (it effuses). The weight of the material escaping per second tells the rate at which gaseous atoms leave. 

Measurement of the very low pressures associated with many of the Pu-intermetallics requires the high sensitivity afforded by target collection. To identify the gas phase species which are in the effusates, a mass spectrometer is needed. The excellent agreement between the mass spectrometric and target collection results confirms the predominance of the Pu gaseous species in the sublimation of these intermetallics. 

The coupling of a GLC column with the sample inlet system of a mass spectrometer is relatively easy, as the effluents are already in gaseous form. The main problem is the relatively high pressure at which these effluents reach the spectrometer and the excess of carrier gas in the stream. Several experimental devices now allow separation of the sample from the carrier gas, either by an effusion process or with the help of a thin, semi-permeable membrane222,353. The use of capillary columns permits direct insertion of the GLC effluent into the ion source without overtaxing the pumping capacity of the mass spectrometer 311 3 5 5 >3 5 6. 

The effusate which condensed on liquid nitrogen cooled copper collection targets was assayed by X-ray fluorescence. The Eu La radiation was determined at the peak maximum (26 = 36.84°, graphite analyzing crystal) by an external standard technique. Previous data (15, 19) have indicated the sticking coefficient of gaseous europium halide on chilled copper is approximately unity. 

Assume that you have a sample of uranium hexafluoride with the natural abundance (0.7%) of 235U, and that you want to use gaseous effusion to separate the isotopes. 

One ramification of Eq. (18) is Graham s law of effusion, which deals with the rate at which gaseous molecules pass through a small hole in the wall of their enclosure (effusion). According to Graham, the rate per unit concentration is proportional to velocity and, thus, directly proportional to the square root of the absolute temperature and inversely proportional to the square root of the molecular mass. 

MAXWELL-BOLTZMANN DISTRIBUTION, COLLISION FREQUENCY, MEAN FREE PATH, AND GASEOUS EFFUSION... 

With tantalum effusion cells, the results of Ackermann et al. (16) are virtually identical with the earlier work of Phipps et al. (19) (1593-2063 K) and yielded total vapor pressure values of plutonium-bearing species which were considerably higher (by approximately a factor of ten) than those obtained with the less reducing tungsten (as well as Re and Ir) cells. Ackermann et al. (16) correlated the vapor pressure results obtained with tantalum effusion cells with the univariant diphasic system consisting of Pu203(s)-PuOi,51(s) as the condensed phases and PuO(g) as the primary gaseous species. 

The gas species over solutions of hydrogen in liquid lithium were detected by mass spectrometric analysis of the saturated vapor effusing from a Knudsen cell. From the measurements of the gaseous equilibria... 

Much theoretical work has been carried out on the lithium hydride molecule, which has become the workbench of the theoretical chemist (J ). Browne ( ), and Fraga and Ransil ( 3) have given the binding energy for the LiH ion by ab initio calculation Com-panion(j4) has applied the diatomic-in-molecule theory to the Li H and LiH. molecules and predicted the stabilities of these molecules. We have intensively studied the Li-H system by means of Knudsen effusion mass spectrometry, and identified all predicted molecules and ions as cited above(5), and reported the thermochemical properties of these gaseous species (, 2, ) ... 

Although the process is called gaseous diffusion, because the chambers are separated by barriers that effectively allow only individual UF6 molecules to pass through, it behaves like an effusion process. Thus we can find the actual ratio of the two types of UFg in chamber 2 from Graham s law ... 

A schematic diagram for the enrichment of by gaseous diffusion of UFe through an effusion barrier is shown in Figure 5, which also illustrates the counter-current flow and cascade principles. The limiting separation factor a is given by the kinetic theory of gases... 

Oxides. Decomposition pressure measurements on the TbO system by Eyring and his collaborators (64) have been supplemented by similar and related studies on the PrO system (46) and on other lanthanide-oxygen systems (43, 44). Extensive and systematic studies of vaporization processes in lanthanide-oxide systems have been undertaken by White, et al. (6, 188,196) using conventional Knudsen effusion measurements of the rates of vaporization of the oxides into high vacuum. Combination of these data with information on the entropies and Gibbs energy functions of reactants and products of the reaction yields enthalpies of reaction. In favorable instances i.e., if spectroscopic data on the gaseous species are available), the enthalpies of formation and the stabilities of previously undetermined individual species are also derived. The rates of vaporization of 17 lanthanide-oxide systems (196) and the vaporization of lanthanum, neodymium, and yttrium oxides at temperatures between 22° and 2700°K. have been reported (188). 

The enthalpy of formation of YF3 was determined by Rudzitis, Feder, and Hubbard 164) using fluorine bomb calorimetry. NdCla was done by solution methods (179), and the enthalpies of formation of LaFs and PrFa were determined by Polyachenok 161) who employed an indirect equilibration technique. A recent torsion-effusion study of the vapor pressure of CeFs 115) yields second and third law values for the enthalpy of sublimation. The thermodynamics of the chlorination of rare earths with gaseous chlorine have also been investigated 144). Gibbs energies of formation were determined for CeClg by solid-state electromotive force techniques 41). 

The vapor pressure data reported by Eisenstadt et al. ( , 1 ) are too low and so not consistent with the others and are not used for evaluation. The transpiration data of Sense and Stone (jU) and Rnudsen-effusion data of Evseev et al. (13) are not considered for analysis because of the complications involved in converting the report apparent pressures to the real total pressures of the gaseous mixtures containing monomer, dimer and trimer. 

CFgCg) + 282(g) by mass spectrometry. The various molecular species were found to be formed as products of the reaction of gaseous SFg with graphite at temperatures in the range 1436-1611 K. This study employed three different effusion cell configurations which were used to optimize the reaction conditions, and ion abundances for each species were measured at 2 eV above their appearance potentials in order to eliminate fragmentation effects. We analyze the reported equilibrium data by the 2nd and 3rd law methods with the results being presented below. 

Nitrogen effused through a pinhole 1.7 times as fast as another gaseous element at the same conditions. Estimate the other element s molar mass. 

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