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Fundamental concepts
Published in W. John Rankin, Chemical Thermodynamics, 2019
If a system is not in its most stable state and there are no flows of any kind, it is said to be metastable. For example, diamond is a metastable form of carbon at ambient* temperature and pressure. It can be converted to graphite, the stable form, but only after overcoming an activation energy barrier. Metastability occurs as a result of a local minimum in a system’s energy. A more familiar example is super-heated water, that is, water heated above 100°C at atmospheric pressure but without boiling. Such water is in an unstable state, and something which causes water vapour bubbles to nucleate within the liquid will result in the water starting to boil, even in the absence of additional heat. This is sometimes encountered when water is heated in a container in a microwave oven. When the vessel containing the water is removed or the water is stirred nucleation of water vapour bubbles may occur, and the water begins to boil, sometimes with catastrophic results.
High-resolution photoelectron spectroscopy of the ground and first excited electronic states of MgKr+
Published in Molecular Physics, 2022
C. Kreis, M. Holdener, M. Génévriez, F. Merkt
Typical examples of thermodynamically stable diatomic dications involve metal atoms of the second (A = Be, Mg, Ca, etc.) or third/thirteenth (A = Al, Ga, Sc, Yb, etc.) groups of the periodic table of the elements and halogen (B = F, Cl, Br, etc.) or rare gas atoms (B = Ne, Ar, Kr, etc.). Extensive lists of thermodynamically stable diatomic dications have been reported by Falcinelli et al. [5] and Sabzyan et al. [2]. Such diatomic dications offer the prospect of studying the Rydberg states of molecular cations by high-resolution spectroscopy. Moreover, studying series of cations in which either A or B is varied within a group of the periodic table enables one to tune the binding energy of the ground state of the dications and explore the effects of the transition between thermodynamic stability and metastability of the doubly charged ion core on the structure and dynamics of the Rydberg states.
Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study
Published in Science and Technology of Advanced Materials, 2018
Nicola H. Perry, Jae Jin Kim, Harry L. Tuller
Oxide thin films have been undergoing widespread development for both advancement of device performance and fundamental scientific studies. A number of useful functionalities arise from their well-defined and low-dimensional geometries, compared to their bulk counterparts, including lower (cross-plane) electrical resistance, higher optical transmittance, and surface exchange- rather than bulk diffusion-dominated kinetics. Some of these effects scale with the geometry (trivial size effects) while other nonlinear changes can emerge at the limit of nanoscale and confined thin films (true size effects) [1]. Thin films have therefore enabled a number of novel components and studies: low resistance electrolytes [2] and active functional layers [3] in solid oxide fuel cells (SOFCs), sensitive chemi-resistive gas detectors [4], transparent and/or flexible conducting layers for photovoltaics, electronics, and batteries [5,6], and fundamental insights into SOFC electrode processes using model electrode systems [7], to offer just a few examples. The transport behavior and reactivity enabling these applications depend on the underlying point defect thermodynamics and reaction kinetics in the thin films, and so it is important to be able to measure, understand, and thereby rationally manipulate these properties. Some of the established methods for probing point defect behavior (particularly stoichiometry deviations) in bulk oxides, such as thermogravimetric analysis, iodometric titration, or neutron diffraction (with limited precision), are challenging to apply to thin film samples, given their small volumes, although coulometric titration may be appropriate [8]. Since thin film point defect chemistry can significantly differ from bulk defect chemistry [9] – owing to effects including processing-induced metastability [10], substrate- and/or processing-induced strain [11,12], high interface-to-volume ratios, and interface polarity/ charge-transfer/ space charge effects [13] – development of new thin film point defect evaluation techniques is preferable, rather than reliance upon bulk data. Similarly, when considering measurement of surface exchange kinetics, the established techniques traditionally applied to bulk samples (tracer diffusion, electrical conductivity relaxation) are limited by being either non-continuous, ex situ, or dependent on contact between the surfaces under study and metallic current collectors. To truly understand materials performance in devices, one should characterize their kinetics over time to understand stability, in realistic operating conditions, and on native surfaces unaffected by current collectors, which could otherwise enhance or hinder catalytic activity of the specific surfaces being probed.