Vaporization from effusion cell

Interpretation of the decomposition mechanism along the lines of the Knudsen-Langmuir approach is based on the difference between the real vaporization rate of a reactant from a free surface (after Langmuir) and that of the same reactant in practically equilibrium conditions from effusion cells (after Knudsen). The ratio of these rates is called the vaporization coefficient, Oy. Its value turns out in many cases to be smaller (quite frequently, by several orders of magnitude) than unity. As pointed out by Somorjai and Lester [3] All the information on the evaporation mechanism is hidden into a correction factor which is applied to adjust the deviation of the evaporation rate far from the maximum evaporation rate in equilibrium. ... 

For comparison, Battles et al. (15) determined the partial heats of sublimation of Pu02(g) and Pu0(g) above PuOi.33 over the temperature range 1937 to 2342 K by means of mass spectrometric measurements with Iridium effusion cells. The absence of Iridium oxides or Iridium species In the vapor phase Indicated that Iridium was nonreducing toward plutonia. The partial heats of sublimation calculated from the slopes of the temperature dependency data yielded values of 127.1 1.2 and 138.8 1.6 kcal/mol for Pu0(g) and Pu02(g) ... 

Knudsen cells (effusion cells) are exclusively used for vapor pressure measurements (see vapor pressure) in the pressure range from 1 torr to 10-6 torr. In the low temperature range (—20° — +400 °C) pyrex glass cells are applicable. Especially the vapor pressures of dyes, organic compounds can be measured in such cells, because metal cells may sometimes cause catalytic decompositions of the investigated materials. 

The effusion rate Am/At at identical orifice diameter and temperatures was slowest for the cylindrical hole with long effusion channel and fastest for a cylindrical orifice with very short effusion channel. This means that the back scattering of the vapor molecules is strongly reduced if the ratio L/D between length L of the effusion channel and diameter D of the orifice is small, which is in agreement with theoretical calculations by Clausing66,67. There are three contributions to the Knudsen cell current which originate from the cell interior, from the channel wall, and the cell lid. 

J) 10 W cm the species were the most stable cluster ions with the highest intensities in LIMS. The influence of laser power density is illustrated in Figure 9.56 At low laser power density small clusters (C3) are observed to have the highest intensity. The abundance distribution in laser evaporation is similar to that observed from thermal vaporization in Knudsen effusion cells with the following abundance sequence (C3 > C2 > >> C5 > 4)." By increasing the laser power... 

The vaporization of Pr203, Nd203, Sm203, and EU2O3 at temperatures ranging from 1950° to 2350°K. has been studied by Panish 149, 150), who analyzed the species effusing from a Knudsen effusion cell with a time-of-flight mass spectrometer. 

The non-isothermal Knudsen eflusion technique was used to study vapor pressures of tars. The experimental details have been described previously [Oja and Suuberg 1997, 199S]. About 10 mg of dry tar was placed into a hermetic effusion cell with a small orifice, from which the saturated vapor effuses out into the vacuum outside the cell. To overcome effects caused by changes in tar con iosition during effusion, the experiment involved first a continuous cool-down followed by a continuous heat-up of the sample. From the mass loss data (by talcing into account both cool-down and heat-up as a whole cycle) the vapor pressure was calculated using the Knudsen equation. 

The first reliable spectroscopic analysis of saturated sulfur vapor was published by Berkowitz and Marquart [28] who used a combination of a Knud-sen effusion cell with a mass spectrometer and generated the sulfur vapor by evaporating either elemental sulfur (low temperature region) or certain metal sulfides such as HgS which decompose at high temperatures to sulfur and metal vapor. These authors observed ions for all molecules from S2 to Ss and even weak signals for Sg and Sio. From the temperature dependence of the ion intensities the reaction enthalpies for the various equilibria (1) were derived (see Table 1). Berkowitz and Marquart careMly analyzed their data to minimize the influence of fragmentation processes in the ion source of the spectrometer. They also calculated the total pressure of sulfur vapor from their data and compared the results with the vapor pressure measurements by Braune et al. [26]. The agreement is quite satisfactory but it probably... 

Definition The vaporization coefficient, is usually defined as the ratio of the actual flow of gaseous decomposition product J to the flow Jmax coming from an effusion cell, in which, it is assumed, decomposition products are in an ideal equilibrium with the reactant. For many substances, as found from comparative Knudsen-Langmuir TG measurements, -C 1, i.e., their free-surface decomposition proceeds much more slowly than would be expected from effusion observations. It is a common practice to explain this discrepancy by a multistage character of the evaporation process, by surface relief peculiarities or by impurities and defects (imperfections) in the reactant lattice. 

Knudsen collection involves the use of a cell containing a knife edge orifice from which the vapor is allowed to impinge on a suitable target. In order to obtain sufficient sensitivity as the temperature is reduced, the heating time must be increased. This requires that the effusion cell be very inert to the sample and that two-phase systems be studied. While the... 

Experimental values of the vapor pressures (indeed very small mPa to fractions of a Pa) of C mim " NTp2 (n = 2,4, 6, 8) RTILs were obtained by Zaitsau et al. from Knudsen effusion cells [175]. The resulting molar enthalpies of vaporization (at 298 K) are respectively 135, 136, 140, and 150 kJ mol for these four RTILs, The enthalpies of vaporization were then estimated for a variety of l-alkyl-3-methylimidazolium RTILs with Cl , Bp4 , PFg, and NTF2 anions from the surface tensions molar volumes F by the expression A H = + B, Fig. 6.2,... 

Effusion cell A thermal vaporization source that emits vapor through an orifice from a cavity where the vapor pressure is carefully controlled by controlling the temperature. Used in molecular beam epitaxy (MBE) processing. Also called a Knudsen cell. 

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. 

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