Knudsen effusion mass spectrometry: Current and future approaches.

RATIONALE: Knudsen effusion mass spectrometry (KEMS) has been a powerful tool in physical chemistry since 1954. There are many excellent reviews of the basic principles of KEMS in the literature. In this review, we focus on the current status and potential growth areas for this instrumental technique. METHODS: We discuss (1) instrumentation, (2) measurement techniques, and (3) selected novel applications of the technique. Improved heating methods and temperature measurement allow for better control of the Knudsen cell effusive source. Accurate computer models of the effusive beam and its introduction to the ionizer allow optimization of such parameters as sensitivity and removal of background signals. Computer models of the ionizer allow for optimized sensitivity and resolution. Additionally, data acquisition systems specifically tailored to a KEMS system permit improved quantity and quality of data. RESULTS: KEMS is traditionally utilized for thermodynamic measurements of pure compounds and solutions. These measurements can now be strengthened using first principles and model-based computational thermochemistry. First principles can be used to calculate accurate Gibbs energy functions (gefs) for improving third law calculations. Calculated enthalpies of formation and dissociation energies from ab initio methods can be compared to those measured using KEMS. For model-based thermochemistry, solution parameters can be derived from measured thermochemical data on metallic and nonmetallic solutions. Beyond thermodynamic measurements, KEMS has been used for many specific applications. We select examples for discussion: measurements of phase changes, measurement/control of low-oxygen potential systems, thermochemistry of ultrahigh-temperature ceramics, geological applications, nuclear applications, applications to organic and organometallic compounds, and thermochemistry of functional room temperature materials, such as lithium ion batteries. CONCLUSIONS: We present an overview of the current status of KEMS and discuss ideas for improving KEMS instrumentation and measurements. We discuss selected KEMS studies to illustrate future directions of KEMS.

Thermochemistry of SinOx(OH)y Vapor Species from Quantum Chemical Calculations.

The thermochemistry of the Si-O-H system has been extensively studied both experimentally and theoretically due to its importance in chemical processes, degradation of silica-protected materials in combustion, and geological processes. In this paper, we review past studies and use quantum mechanical methods to generate a new data set. Molecular geometries were generated with DFT using the B3LYP functional. Energetics were calculated with RCCSD(T) methods extrapolated to the complete basis set (CBS/45) limit. Particular attention was given to the treatment of the vibrational modes. A rigid rotor model was used, corrections for anharmonicity were applied, and the Pitzer-Gwinn treatment of the hindered rotation of the M-OH groups was applied. The generated enthalpies of formation at 298 K are compared to those of experiments and other calculations. Generally, the agreement is good. A set of thermodynamic data (enthalpy of formation at 298 K, entropy at 298 K, and heat capacity polynomial to 3000 K) is presented for SiOH, SiO(OH), Si(OH)2, SiO(OH)2, Si(OH)3, Si(OH)4, Si2O(OH)6, and Si3O2(OH)8. These can be added to any of the common computational thermodynamics packages. The application of these data to high-temperature corrosion and geological problems is discussed.

Quantum chemical and experimental thermodynamic studies of HfO(g).

Hafnium dioxide vaporizes primarily to HfO(g) in a reducing environment. The thermochemistry of HfO(g) is calculated from quantum methods and measured via Knudsen effusion mass spectrometry. For the computations, all-electron and relativistic effective core potential calculations are used. The calculation of an accurate dissociation energy and an entire potential energy curve is reported. These calculations lead to ΔfH°(298) = 63.19 ± 10 kJ/mol, S°(298) = 235.52 J/mol K, and Cp(298-2500 K) = (2.741 × 10-9)T3 - (9.853 × 10-6)T2 + (1.295 × 10-2)T + 2.761 × 10-1 J/mol K. Experimentally, HfO(g) is generated from the reaction of Hf(s) and HfO2(s) in a specially made Hf Knudsen cell. A third law treatment of the data leads to ΔfH°(298) of 58.4 ± 12.3 kJ/mol, in good agreement with the calculated value.

Computational Chemistry Derivation of Cr, Mn, and La Hydroxide and Oxyhydroxide Thermodynamics.

Thermodynamic quantities are calculated for gaseous hydroxides and oxyhydroxides of Cr, Mn, and La. These would form due to water-vapor-containing environments reacting with Cr-forming alloys or oxide components of potential fuel cell interconnects or anode materials. Structures and vibrational modes for the expected hydroxides and oxyhydroxides are calculated with the B3LYP hybrid functional. Enthalpies of formation from selected reactions for each species are calculated using the CCSD(T)/CBS approach. Results show good agreement with literature estimates, measurements, and calculations. The resultant data is reported as ΔfH°(298), S°(298), and Cp(T) and put into the database for a free-energy minimizer code. Calculations are presented to show the hydroxide and oxyhydroxide vapor pressures above H2O + Cr2O3, Mn3O4, and La2O3, as well as the anode material La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM).

Thermochemistry of Gaseous Ytterbium and Gadolinium Hydroxides and Oxyhydroxides.

Gd2O3 and Yb2O3 are the proposed constituents of advanced coating systems in combustion environments. In such environments, they are exposed to high-temperature water vapor, which would lead to gaseous hydroxide formation. Thermodynamic parameters are reported for YbOn(OH)m and GdOn(OH)m species. We first study the MH, MO, MF, and MCl (M = Yb and Gd) species, where some experimental data exist. Structures and spectroscopic constants were calculated at the B3LYP level. For YbOn(OH)m, the B3LYP approach is used in conjunction with an effective core potential, while for GdOn(OH)m, it was necessary to use all-electron basis sets. Enthalpies of formation were calculated with a BD(T) approach. The enthalpies of formation, entropies, and heat capacities were added to a thermochemical database. Hydroxide and oxyhydroxide vapor pressures are calculated above pure Yb2O3, Gd2O3, and Y2O3 in 50% H2O/Ar from 1000 to 3000 K. Hydroxide and oxyhydroxide vapor pressures are also calculated for potential coating compositions.

Monte Carlo simulation of a Knudsen effusion mass spectrometer sampling system.

RATIONALE: Knudsen effusion mass spectrometry (KEMS) shows improved performance with the "restricted collimation" method of Chatillon and colleagues, which consists of two apertures between the Knudsen cell orifice and the ionizer. These apertures define the shape and position of the molecular beam independently of the sample and effusion orifice and as a result reduce background and improve sampling from the Knudsen cell. Modeling of the molecular beam in restricted collimation allows optimization of the apertures' diameters and spacing. METHODS: Knudsen flow is easily simulated with a Monte Carlo method. In this study a Visual Basic for Excel (VBA) code is developed to simulate the molecular beam originating from a vaporizing condensed phase in a Knudsen cell and passing through the cell orifice and the two apertures. RESULTS: The code is able to calculate the transmission coefficient through the cell orifice, through the cell orifice and the first aperture, and through the cell orifice and first and second apertures. Also calculated are the angular distributions of the effusate density emerging from the cell and average number of collisions with the orifice walls. CONCLUSIONS: This code allows the geometry (aperture spacing and diameters) of the sampling system to be optimized for maximum transmission. The calculated effusate distributions and low average number of orifice wall collisions illustrated the advantages of restricted collimation. Calculated transmission factors are also compared to literature values calculated via the analytical method of Chatillon and colleagues. Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.

High-temperature vaporization of B2O3(l) under reducing conditions.

The vaporization of B(2)O(3) in a reducing environment leads to the formation of both B(2)O(3)(g) and B(2)O(2)(g). Whereas the formation of B(2)O(3)(g) is well understood, many questions about the formation of B(2)O(2)(g) remain. Previous studies using B(s) + B(2)O(3)(l) have led to inconsistent thermodynamic data. In this study, it was found that, after heating, B(s) and B(2)O(3)(l) appeared to separate and variations in contact area likely led to the inconsistent vapor pressures of B(2)O(2)(g). To circumvent this problem, the activity of boron was fixed with a two-phase mixture of FeB and Fe(2)B. Both second- and third-law enthalpies of formation were measured for B(2)O(2)(g) and B(2)O(3)(g). From these values, the enthalpies of formation at 298.15 K were calculated to be -479.9 ± 25.7 kJ/mol for B(2)O(2)(g) and -833.4 ± 13.1 kJ/mol for B(2)O(3)(g). Ab initio calculations to determine the enthalpies of formation of B(2)O(2)(g) and B(2)O(3)(g) were conducted using the W1BD composite method and showed good agreement with the experimental values.

Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g).

In this paper, we report the results of equilibrium pressure measurements designed to identify the volatile species in the Cr-O-H system and to resolve some of the discrepancies in existing experimental data. In addition, ab initio calculations were performed to lend confidence to a theoretical approach for predicting the thermochemistry of chromium-containing compounds. Equilibrium pressure data for CrO2(OH)2 were measured by the transpiration technique for the reaction 0.5Cr2O3(s) + 0.75O2(g) + H2O(g) = CrO2(OH)2(g) over a temperature range of 573 to 1173 K at 1 bar total pressure. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the condensate in order to quantify the concentration of Cr-containing volatile species. The resulting experimentally measured thermodynamic functions are compared to those computed using B3LYP density functional theory and the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)].

The influence of tungsten on the chemical composition of a temporally evolving nanostructure of a model Ni-Al-Cr superalloy.

The influence of W on the temporal evolution of gamma' precipitation toward equilibrium in a model Ni-Al-Cr alloy is investigated by three-dimensional atom-probe (3DAP) microscopy and transmission electron microscopy (TEM). We report on the alloys Ni-10 Al-8.5 Cr (at.%) and Ni-10 Al-8.5 Cr-2 W (at.%), which were aged isothermally in the gamma + gamma' two-phase field at 1073 K, for times ranging from 0.25 to 264 h. Spheroidal-shaped gamma' precipitates, 5-15 nm diameter, form during quenching from above the solvus temperature in both alloys at a high number density (approximately 1023 m-3). As gamma' precipitates grow with aging at 1073 K, a transition from spheroidal- to cuboidal-shaped precipitates is observed in both alloys. The elemental partitioning and spatially resolved concentration profiles across the gamma' precipitates are obtained as a function of aging time from three-dimensional atom-by-atom reconstructions. Proximity histogram concentration profiles (Hellman et al., 2000) of the quaternary alloy demonstrate that W concentration gradients exist in gamma' precipitates in the as-quenched and 0.25-h aging states, which disappear after 1 h of aging. The diffusion coefficient of W in gamma' is estimated to be 6.2 x 10-20 m2 s-1 at 1073 K. The W addition decreases the coarsening rate constant, and leads to stronger partitioning of Al to gamma' and Cr to gamma.