Solid-state reaction path

Figure 7, Schematic representation of the 1-TS (solid) and 2-TS (dashed) (where TS = transition state) reaction paths in the reaction Ha + HbHc Ha He + Hb- The H3 potential energy surface is represented using the hyperspherical coordinate system of Kuppermann [54], in which the equilateral-triangle geometry of the Cl is in the center (x), and the linear transition states ( ) are on the perimeter of the circle the hyperradius p = 3.9 a.u. The angle is the internal angular coordinate that describes motion around the CL...

The basic parameters which determine the kinetics of internal oxidation processes are 1) alloy composition (in terms of the mole fraction = (1 NA)), 2) the number and type of compounds or solid solutions (structure, phase field width) which exist in the ternary A-B-0 system, 3) the Gibbs energies of formation and the component chemical potentials of the phases involved, and last but not least, 4) the individual mobilities of the components in both the metal alloy and the product determine the (quasi-steady state) reaction path and thus the kinetics. A complete set of the parameters necessary for the quantitative treatment of internal oxidation kinetics is normally not at hand. Nevertheless, a predictive phenomenological theory will be outlined. 

When zeolites are hydrated shows a notable ionic conductivity [112], Consequently, since all electrode processes depend on the transport of charged species zeolites provide an excellent solid matrix for ionic conduction [172], In 1965 [175], Freeman established the possibility of using zeolites in the development of a functional solid-state electrochemical system, that is, a battery where a zeolite, X, was used as the ionic host for the catholyte, specifically, Cu2, Ag+, or Hg2+, and as the ionic separator in its sodium-exchanged form, that is, Na-X. Pressed pellets of Cu-X and Na-X were sandwiched between a gold current collector and a zinc anode. Then, the half-cell reactions are the oxidation of Zn —> Zn2+ + 2e and the reduction of Cu2+ + 2e —> Cu, with type X providing a solid-state ionic path for cationic transport [175], The electrochemical system obtained can be represented as follows (Au I Cu11 -XI Na-X I Zn). 

The superconducting 1-2-3 phases are known to be chemically sensitive, and in fact are bought to be metastable compounds imder all conditions of temperature and oxygen partial pressure (1). Thus, it is imperative that any material in intimate contact with 1-2-3 phases does not react with the component oxides to form more stable compounds, since such a reaction will destroy the superconducting material. This is especially important for thin fflm applications, since the amount of superconductor is small compared to that of the substrate, and the diffusion path for potential solid state reactions, i.e., the film thickness, is very short. In this paper we will concentrate on searching for materials that should be most stable in contact with the 1-2-3 superconductors, since they appear to be more sensitive to chemical disruption than the more recently discovered Bi- and Tl-based phases. The principles and much of the data presented here can be applied to any oxide superconductor, whether it is presently known or yet to be discovered. 

A small particle size of the reactant powders provides a high contact surface area for initiation of the solid state reaction diffusion paths are shorter, leading to more efficient completion of the reaction. Porosity is easily eliminated if the initial pores are very small. A narrow size... 

Magini, M., Colella, C., lasonna. A., and Padella, F., Power measurements during mechanical milling — II. The case of single path cumulative solid state reaction, Acta Mater., 46 (8), 2841 -2850, 1998. 

Diffusion of reactant atoms determines the overall reaction rate and solid state reactions are diffusion-limited and have low rates. What is called the reaction mechanism in solid state reactions is not like a reaction mechanism in molecular chemistry. In the latter a mechanism for a reaction describes the path of the atoms in the reactant molecules during the conversion to the product molecules. The mechanisms in solid state chemistry are really diffusion mechanisms combined with atomic balances. 

Dislocations are one-dimensional defects. They are largely responsible for the plastic behaviour of solids. Two of their properties are particularly important in connection with solid state reactions 1. They can act as sites of repeatable growth within a crystal. 2. They can serve as fast diffusion paths. They also act as preferential nucleation sites for the formation... 

First studies that involved such methods have shown that previous theoretical ideas about the evolution of the processes in the initial stages of solid state reactions have to be thoroughly reconsidered, since the initial stage of a solid-state reaction is determined by the contact zone morphology, which, in turn, is influenced by diffusion and nucleation. At the same time, in the majority of cases, it is the initial stage during which the further path of the system s evolution is determined. 

All solid state reactions, whether alloying or oxidation/reduction reactions, involve the formation of one or more product phases between the reactants. Without mechanical alloying, the reactant phases become separated and the reaction rate is determined by the contact area and the diffusion path length through the product phases. Diffusion through the product phases is invariably the rate-controlling process, and consequently high temperatures are required for flie reaction to occur at a measurable rate. 

The gas product derived from reduction should be expelled from the reaction region along the same path. Here, the reducing agent contacts with iron oxide, namely, it is adsorbed on the surface of the solid and starts surface reactions. The reduction reaction includes seizure of oxygen from oxides, the formation and growth of crystal nuclei of the product. The continuous growth of product layer is maintained by solid state reaction and the diffusion in solid state. 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

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