Solid state transported reactant

A source (O) to supply the solid-state transported reactant (A),... 

The role of the source (O) in a PEVD system is to provide a constant supply of the solid-state transported reactant (A) during a PEVD process. Theoretically, it can be either a solid, liquid or vapor phase, as long as it can supply the ionic reactant (A ) or (A ) to the solid electrolyte (E) and the electronic reactant (e) or (h) to the counter electrode (C) via a source side electrochemical reaction. Therefore, the source must be in intimate contact with both solid electrolyte (E) and counter electrode (C) for mass and charge transfer between the source and solid electrochemical cell at location I of Figure 3. Practically, it is preferable to fix the chemical potential at the source. Any gas or solid mixture which does not react with the cell components and establishes a constant chenfical potential of (A) is a suitable source. For instance, elemental (A) provides (A +) or (A ) according to the following reaction... 

In this PEVD system, the source (O) will be a vapor phase, which contains elemental solid-state transported reactant (A), and an anode half-cell reaction... 

At the sink side, once initial formation of (D) has occurred, solid-state transported reactant... 

Based on Eqn. 33, possible process control during PEVD includes many aspects, such as process temperature, the vapor phase at the sink side, the activity of the solid-state transported reactant (A) at both sink and source sides, etc. Further discussion of these factors is subject to the individual process and will be presented later. In this section, PEVD process control is... 

Sodium is selected as the solid state transported reactant in PEVD. This is because not only is Na" a component in the PEVD product phase Na COj, but also the mobile ionic species in the solid electrolyte (Na "-[3"-alumina) and in the auxiliary phase of the sensor. Thus, PEVD can take advantage of the solid electrochemical cell (substrate) of the sensor to transport one reactant (sodium) across the substrate under an electrochemical potential gradient. This gradient... 

The PEVD process takes advantage of the solid electrochemical cell of an SOFC. Oxygen is chosen to be the solid state transported reactant. At the source side (the cathode of the SOFC), oxygen in the source gas phase is reduced to oxygen anions (O ) through a cathodic reaction... 

Step 2 Solid-state transported reactant Na diffusion from location (II) to (III). 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

The second variant is designed for solid state reactants to the exclusion of liquid or gas. This powder variant of Thermostar is described by the Fig. 1.19 (right). The microwave applicator is the same as for the device for liquids heating but the reactant transport is ensured by a metallic screw set within the dielectric pipe. This specific traveling metallic screw crosses all the microwave applicators. The coexistence of this metallic screw with the electric field is ensured by the fact that the major electric field direction is parallel to the major direction and perpendicular to the local curving of the screw. A typical industrial unit for solid or liquid reactants is powered with microwave generators units of 2 or 6 kW for a total microwave power close to 20 or 60 kW. 

Experimental determination of Ay for a reaction requires the rate constant k to be determined at different pressures, k is obtained as a fit parameter by the reproduction of the experimental kinetic data with a suitable model. The data are the concentration of the reactants or of the products, or any other coordinate representing their concentration, as a function of time. The choice of a kinetic model for a solid-state chemical reaction is not trivial because many steps, having comparable rates, may be involved in making the kinetic law the superposition of the kinetics of all the different, and often unknown, processes. The evolution of the reaction should be analyzed considering all the fundamental aspects of condensed phase reactions and, in particular, beside the strictly chemical transformations, also the diffusion (transport of matter to and from the reaction center) and the nucleation processes. 

Heterogeneous solid state reactions occur when two phases, A and B, contact and react to form a different product phase C. A and B may be either chemical elements or compounds. We have already introduced this type of solid state reaction in Section 1.3.4. The rate law is parabolic if the reacting system is in local equilibrium and the growth geometry is linear. The characteristic feature of this type of reaction is the fact that the product C separates the reactants A and B and that growth of the product proceeds by transport of A and/or B through the product layer. 

A third type of internal solid state reaction (see later in Fig. 9-12) is characterized by two (solid) reactants A and B which diffuse into a crystal C from opposite sides. C acts as a solvent for A and B. If the reactants form a stable compound AB with each other (but not with the solvent crystal C), an internal solid state reaction eventually takes place. It occurs in the solvent crystal at the location of maximum supersaturation of AB by internal precipitation and subsequent growth of the AB particles. Similar reactions can be observed on a crystal surface which, in this case, plays the role of the solvent matrix C. Surface transport of the reactants leads to a product band precipitated on the surface at some distance from each of the two reactants and completely analogous to the internal reactions described before. In addition, internal reactions have also been observed if (viscous) liquids are chosen as the reaction media (C). 

An endothermic solid-state decomposition proceeds as the product-reactant interface advances into the interior of the sample. The interface can, however, advance only if the necessary heat of reaction is applied to it. Since the source of heat is outside the sample, the rate of heat transfer to the interface may become rate-determining if the inherent rate of reaction exceeds the rate of heat transport. The rate of heat transport depends not only on the properties of the product through which heat must be transported, but also on the general experimental arrangement. It is for this reason that one expects and finds the literature to be conflicting. 

Generally speaking, PEVD is a modified form of chemical vapor deposition (CVD). A comparison between PEVD and conventional CVD is schematically shown in Figure 1 for a product (D) formed from reactants (A) and (B). In a CVD process, both reactants (A) and (B) are supplied through a vapor phase at the same side of a solid substrate (E). They react chemically at the surface of the solid substrate (E), aided by some type of catalytic effect, to form a desired product (D). In a PEVD process, one reactant (A) is transported from one side (source) of a solid substrate (E) to the other side (sink) under well-controlled thermodynamic and kinetic conditions. At the sink side, reaction with (B) occurs to form (D). Further growth of (D) into a continuous thin film with the desired thickness in a PEVD process also relies on (A) transported in the solid state now through (E) and (D) to react with (B). 

The current, I, in a PEVD process can be recorded simultaneously by an ammeter in the external circuit to reveal the kinetics of the PEVD reactions. As discussed in the last section, solid-state reactant (A) needs to be transported as a combination of ionic and electronic species from the source to the sink side through the solid electrochemical cell to participate in a PEVD reaction with vapor phase reactant (B). The PEVD reaction rate, and subsequent product (D) formation rate, v(t), can be expressed as... 

An important way of overcome the diffusion barrier in solid state synthesis is the technique of vapor transport, where an agent is added to the reactants to produce a volatile intermediate in a sealed tube. For example the formation of A12S3 is slow even at 800°C where A1 is liquid and S... 

Over the past twenty-five years the development of efficient catalysts for selective oxidation resulted in a new generation of commercial processes which utilize inexpensive olefinic and paraffinic feeds, replacing more reactive and costly raw materials. The catalysts are complex solid metal oxide systems which selectively activate hydrocarbons. Olefins, in particular, are activated via an allylie intermediate formation. The catalysts contain facile solid state redox couples which allow for efficient electron and lattice oxygen transport between reactant, adsorption and surface active site, and the surface reoxidation site which is then reconstituted by gaseous oxygen. 

Reactions between two solids are analogous to the oxidation of a metal, because the product of the reaction separates the two reactants. Further reaction is dependent on the transport of material across this barrier. As with oxidation, cracking, porosity and volume mismatch can all help in this. In this section, the case when a coherent layer forms between the two reactants will be considered. The mechanism of the reaction may depend on whether electron transport is possible in the intermediate phase, and the rate of reaction will be controlled by the rate of diffusion of the slowest species. To illustrate the problems encountered a typical solid-state reaction, the formation of oxide spinels, is described. 

Physical phase transitions have much in common with solid state reactions. In both cases the bonding between the atoms changes and the heat of reaction is of the same order of magnitude. However, there are some clear differences. In a phase transition the atoms do not have to travel over large distances in the lattice to be brought to or removed from the reaction site. Mass transport limitation slows chemical reaction rates significantly and phase transitions are usually faster than solid state reactions. By mixing the reactant atoms prior to the reaction on an atomic scale chemical solid state reactions start to behave like phase transitions in solids. 

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