Solid state transport

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

Three-dimensional electrode nanoarchitectures exhibit unique structural features, in the guise of amplified surface area and the extensive intermingling of electrode and electrolyte phases over small length scales. The physical consequences of this type of electrode architecture have already been discussed, and the key components include (i) minimized solid-state transport distances (ii) effective mass transport of necessary electroreactants to the large surface-to-volume electrode and (iii) magnified surface—and surface defect—character of the electrochemical behavior. This new terrain demands a more deliberate evaluation of the electrochemical properties inherent therein. 

While electrochemical experiments provide useful information regarding electron transport through these molecular monolayers, construction of real devices requires formation of a top contact so that solid-state transport measurements can be made. The fabrication of contacts to molecular layers has been the major obstacle to the development of molecular electronic devices, whether based on thiol-based SAMs on gold or covalently attached molecules on silicon. The most popular approach to making contacts involves evaporation of metals onto the molecular layer, which is likely to result in at least partial penetration of the monolayer, and may possibly damage the molecules in the layer. 

Solid-State Transport ot Hydrogen and Off-Board Hydrogen Storage... 

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... 

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... 

According to the above equation, the chemical potential of the solid-state transported... 

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... 

Improvement of the geometric structure of the working electrode by a well-controlled PEVD process benefits the performance of a CO sensor in many ways. To optimize kinetic behavior, the response and recovery times of CO potentiometric sensors were studied at various auxiliary phase coverages. This was realized by a unique experimental arrangement to deposit the Na COj auxiliary phase in-situ at the working electrode of type III potentiometric CO sensors by PEVD in a step-wise fashion. Since the current and flux of solid-state transported material in a series of PEVD processes can be easily moiutoredto control the amount of deposit... 

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). 

Vanadium is transferred by interpaiticle solid state transport. The combination of oxygen or air plus steam promotes surface migration and enrichment of vanadia species which are not crystalline V2O5. Interpaiticle contact is a requirement for vanadium transfer from particle to particle. Evidence for a volatile vanadic acid species could not be found. 

These two phenomena involve solid state transport and are very slow unless very high temperatures are reached (above 900°C). 

These more general models appear to have complete validity in the sense that they describe the ideal case, but are negated by any solid state transport processes at the interface and these, as we have seen, occur with almost all metal- semiconductor combinations. 

In corrosion, phenomena other than mass transport in the electrolyte can slow down the establishment of steady state conditions, including adsorption, precipitation or film growth. Especially, solid state transport processes in passive oxide films are generally slow (Chap. 6) and as a consequence the measured current density will depend on the sweep rate, even if from a solution mass transport point of view steady state prevails (t 1). Polarization curves measured under these conditions are sometimes called pseudo-steady-state polarization curves. When reporting such data one should always indicate the sweep rate used. 

The latter reaction mechanism is favored by most researchers. The presence of NO2 with H2S greatly enhances silver sulfidation and when high relative humidity is added, the rate of sulfidation is extremely rapid and is limited by gas-phase diffusion even at high flow rates. Therefore, the solid-state transport of silver atoms from the bulk is faster than the interfacial reaction and does not affect the rate. 

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