Knudsen cells vapor source

KNUDSEN CELL VAPOR SOURCES AND MOLECULAR BEAMS... 

Pyrometry When the temperatures of interest are in a suitable range, pyrometry is the best technique for measuring the temperature of the Knudsen cell vapor sources in KEMS instruments. Some of the major advantages are that it is a noncontact technique, with the pyrometer placed outside the furnace and vacuum chamber. Also, one pyrometer can be used to measure temperature at multiple locations, which improves the consistency of calibration. The key advantage is that pyrometry, as stated in the Temperature Measurement section, is based on the Planck radiation law, which in ratio form defines ITS-90 at all temperatures above the Ag fixed point (1234.93 K).Thus,pyrometry is the standard method for realizing thermodynamic temperature through the use of Equation 48.10 ... 

The central KEMS equation can be derived now that the Knudsen cell vapor source and mass spectrometer have been described. This follows directly from the vapor flux in the molecular beam selected from the distribution of material effusing from the Knudsen cell (molecular beam flux equation) and the definition of the ionization cross section (Equation 48.18). However, in accordance with the aim of identifying factors that affect the measured ion intensity and that are unrelated to sample temperature and composition, it useful to rewrite Equation 48.18 in terms of the number of ions produced per second in the elementary volume dv in the region defined by the intersection of the molecular and electron beams, ni(E) [71,80] (this is prior to the formation of the ion beam) ... 

Most KEMS instruments use a single Knudsen cell vapor source. A variety of techniques have been developed for measuring thermodynamic quantities with a single-cell configuration [12-17]. First consider a pure metal. The determination of the heat of vaporization... 

In regard to the Knudsen cell vapor source, improvement in micromachining techniques may give a more uniform orifice and the measured GFR may be closer to 1. An intriguing idea explored by only a few investigators is the electrochemical Knudsen cell [107,108]. In such a case, it should be possible to fix the activity of one component or follow changes in stoichiometry as the contents of the cell vaporize over long periods [16]. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Rather different spectra are obtained when the equilibrium vapor above metallic selenium (160°C) is ionized by a field ion source (2.5-8.0 kV), resulting in the ions Se + (n = 2 and 5-8) neither Se+ nor Se3+ and Se4+ have been observed (these species usually result from fragmentation processes), but occasionally traces of Seg+ have been found. Using a Knudsen cell, Se6+ is the most abundant species, followed by Se8+ and Se7+, which all originate from the corresponding neutral, cyclic molecules (56). Field evaporation (20-100°C) of whiskers of selenium (prepared by field condensation of Se vapor) produces mainly Se5+, but small amounts of molecular ions with one, two, and four positive charges up to Se33 have been identified. However, these species may have chainlike structures (56). 

In this method the sample is vaporized in a micro-oven placed in the ion source or out of a Knudsen-cell. Many metals can be analyzed qualitatively and quantitatively by this technique as metal organic compounds (see Refs. ). The metal chelates have lower volatilities than the metals and in many cases the mass spectra reveal higher sensitivities for these compounds compared with the analysis using direct evaporation of the metal. The latter technique of direct metal analysis by EIMS is only applied if the ionization energies of the metals are too high for thermal ionization mass spectrometry . 

Generally, vaporization source temperatures are very difficult to monitor or control in a precise maimer. Since the vaporization rate is very temperature-dependent, this makes controlling the deposition rate by controlling the source temperature very difficult. In molecular beam epitaxy (MBE), the deposition rate is controlled by careful control of the temperature of a well-shielded Knudsen cell source using embedded thermocouples. 

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. 

In addition to the estimated properties, we measured the thermochemistry of several important vapor species. These measurements were conducted in a Knudsen effusion cell using special line-of-sight vaporization under subambient pressures with flowing O2 and H2O vapor mixtures [4]. The gaseous species over silica [5], manganese oxide [6], lanthana, alumina, and palladium metal were detected and relative partial pressures measured as a function of temperature. These vapor pressure measurements were calibrated by using the known metal atom or binary metal oxide volatility as a calibration source. Oxide species concentrations were measured relative to that of a reference compound, e.g., metal atom. The identification of oxide and hydroxide compounds was facilitated by Ae technique of threshold electron ionization [7]. These data were then evaluated using estimated entropy functions and the third law temperatures. 

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