Knudsen Effusion Mass Spectrometry for Study of Vaporization Chemistry

Abstract

The Knudsen effusion mass spectrometry (KEMS) made it possible to unravel the mystery of the vapor phases of various systems -by identification and quantification of the vapor species in equilibrium with the condense phases of interest. KEMS also enabled deduction of many thermodynamic quantities pertinent to reactions amongst the constituents of the vapor and/or the condensed phases. Since the advent of KEMS, thousands of vaporization studies have been conducted. Many studies are still being performed mostly to obtain better results or for some specific industrial applications. The basic steps of a KEMS investigation might appear routine, but the vaporization chemistry associated with each system makes every study interesting, especially those studies dealing with condensed phase transitions. We describe here a few KEMS studies which reported the effects attributed to phase transformations in congruently as well as incongruently vaporizing systems. IntroductionIt is a common knowledge that any substance will vaporize if heated to temperatures high enough. However, the nature and composition of the equilibrium vapor phase of different systems became known better only after the advent of Knudsen effusion mass spectrometry, approximately six decades ago. The technique essentially consists in effecting evaporation of the condensed phase of interest inside a container called a Knudsen cell (under high vacuum) and letting the vapor effuse out through a small orifice on top of the Knudsen cell into a mass spectrometer. The nature of the vapor phase is deduced from the analysis of the ions formed and the composition of the vapor is determined by appropriate pressure calibration. While Gibbs free energy changes for reactions (homogeneous as well as heterogeneous) can be calculated directly from the measured partial pressures, enthalpy changes can be deduced by application of secondand third-law of thermodynamics.Usually, a KEMS study has the following steps: 1) choose a Knudsen cell which is not likely to react with the condensed phase or even the vapor phase; 2) heat the sample to a temperature where the total pressure will be less than 10 Pa; 3) identify the ions by their mass and comparison of isotopic abundance with calculated values and verify whether they exhibit shutter effect; 4) measure the ion intensities as a function of temperature in a reasonably large temperature range (at constant electron energy) and as a function of electron energy (at a constant temperature); 5) perform pressure calibration using silver or an appropriate substance; and 6) deduce partial pressures of the gaseous species identified and thermodynamic quantities for pertinent vaporization reactions. 10.1149/04601.0153ecst ©The Electrochemical Society ECS Transactions, 46 (1) 153-171 (2013) 153 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 137.99.31.134 Downloaded on 2015-05-26 to IPThat it is not always so simple a task as spelt out above is clear from the fact...

Knudsen Effusion Mass Spectrometric Study of Some Congruently and Incongruently Vaporizing Systems

“…With no Mn-bearing species in the vapor phase, it is actually the O-to-Te ratio in the effusate which was deduced. The transition from 4PEQ4 to the 3PEQ1 (upon disappearance of the Mn 2 Te 3 O 8 phase) triggered an anomalous turnaround in p(Te 2 ), which is described in the companion paper (17) of the workshop. The 3PEQ2 involved loss of TeO 2 from Mn 3 TeO 6 .…”

“…During our investigations on Mn-Te (6,16) and Mn-Te-O systems (11), some interesting vaporization behavior -associated with condensed phase transitions were observed. They are discussed in another paper presented in this workshop (17).…”

“…The monotonic variation in the CEC values and the change in the CEC caused by the phase transition  were clearly distinguishable. These aspects are presented in the companion paper (17). The vapor phase of TeO 2 (s) ( 5) is very complex with species, TeO 2 (g), TeO(g), O 2 (g), Te 2 (g), (TeO 2 ) 2 (g), and (TeO) 2 (g).…”

“…This study led us to 1) deduce the phase diagram corresponding to regions on and around the MnO-TeO 2 binary line (12); 2) determine the thermodynamic data for the vaporization reactions corresponding to some three phase regions adjacent to MnTeO 3 , the 1:1 MnO-TeO 2 phase on the MnO-TeO 2 binary line (13); and 3) observe interesting vaporization behavior triggered by the onset of quasi congruent vaporization along the MnTeO 3 -Mn 3 TeO 6 binary line (11). The last aspect is dealt with greater detail in the companion paper (17), and in this paper, we will discuss briefly the first two.…”