The objective of this investigation was to develop a vapor pressure (VP) acquisition system and methodology for performing temperature-dependent VP measurements and predicting the enthalpy of vaporization (ΔHvap) of volatile organic compounds, i.e. VOCs. High quality VP data were acquired for acetone, ethanol, and toluene. VP data were also obtained for water, which served as the system calibration standard. The empirical VP data were in excellent agreement with its reference data confirming the reliability/performance of the system and methodology. The predicted values of ΔHvap for water (43.3 kJ/mol, 1.0%), acetone (31.4 kJ/mol; 3.4%), ethanol (42.0 kJ/mol; 1.0%) and toluene (35.3 kJ/mol; 5.4%) were in excellent agreement with the literature. The computed values of ΔSvap for water (116.0 J/mol•K), acetone (95.2 J/mol•K), ethanol (119.5 J/mol•K) and toluene (92.0.J/mol•K) compared also favorably to the literature.
The vapor pressure (VP) of 87 grade gasoline was measured using an enhanced VP acquisition system over a temperature range of approximately 19.0˚C (292.2 K) and 69.0˚C (342.2 K). The empirical data were used to predict the thermodynamic entities the enthalpy of vaporization (ΔH vap) and the entropy of vaporization (ΔS vap) of gasoline. The results of this investigation yielded a ΔH vap value of 35.1 kJ/mol and ΔS vap of 102.5 J/mol•K. The value of ΔH vap was in excellent agreement with the findings of a prior study (Balabin et al., 2007), which produced a ΔH vap values of 37.3 kJ/mol and 35.4 kJ/mol. The enthalpy and entropy of vaporization of n-heptane (37.2 kJ/mol and 100.1 J/mol•K) and n-octane (39.1 kJ/mol and 98.3 J/mol•K) were also determined after acquiring VP data for the two VOCs. The empirical results for n-heptane and n-octane were also in excellent agreement with the literature. These favorable comparisons strengthen the capacity of our system for acquiring the VP data for pure and volatile multi-component mixtures.
Single crystals of yttrium aluminum garnet (YAG) that were doped with various cations were annealed in air at different temperatures for varying amounts of time. Dopants were chosen to probe the effect of size, charge, and site occupancy on surface segregation. Of the dopants that were chosen for the study (calcium, silicon, neodymium, chromium, and strontium), calcium was the only one that consistently segregated upon annealing in air. Calcium enrichment to the (111) surface was measured using Auger electron spectroscopy, and the segregation enthalpy was determined to be δHseg≈−32 ± 10 kJ/mol. Enrichment occurred according to variations in valence, as opposed to variations in size; therefore, it is suggested that surface segregation is electrostatically driven. The results indicate that aliovalent substituents could be used for interface property tailoring, whereas isovalent dopants would not be useful.
Calculations based on ionic space-charge models of doped yttrium aluminum garnet (YAG) have been compared to experimental measurements of surface segregation in crystals of various compositions. The comparison allows limits for vacancy-formation energies to be set. The range for anion:cation formation-energy ratios has been established to be 0.20-0.23, based on the reasonable assumptions that the formation energy of the yttrium ion is 75% of that of the aluminum ion and the Schottky defect formation energy is 4.2 eV. The model explains the experimental observation of calcium at the surface regardless of net acceptor excess or net donor excess. The relationship between vacancyformation energy and dopant excess has been used to construct segregation maps for YAG, which are useful in materials design strategies.
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