PEVD system

PEVD has been applied to deposit auxiliary phases (Na COj, NaNOj and Na SO ) for solid potenfiometric gaseous oxide (CO, NO, and SO ) sensors, as well as a yttria stabilized zirconia (YSZ) ceramic phase to form composite anodes for solid oxide fuel cells. In both cases, the theoretically ideal interfacial microstructures were realized. The performances of these solid state ionic devices improved significantly. Eurthermore, in order to set the foundation for future PEVD applications, a well-defined PEVD system has been studied both thermodynamically and kinetically, indicating that PEVD shows promise for a wide range of technological applications. 

A fundamental PEVD system is schematically shown in Figure 3. From left to right, it consists of... 

The charged reactant for the sink electrochemical reaction is supplied by the solid electrochemical cell of a PEVD system. The solid phase (E) is an exclusive ionic conductor for (A +) or (A ), and serves as the solid electrolyte. (C) and (W) are solid electronic conducting phases, and contact (E) from both sides as counter and working electrodes, respectively. They coimect with the external electric circuit, which consists of a dc source and other possible measurement devices. Because the conductivity changes in nature from ionic to electronic at the electrode/electrolyte interfaces, the solid electrochemical cell in a PEVD system effectively separates the transport paths of ionic and electronic charged carriers... 

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... 

This reaction does not have to be a thermodynamically favorable one, since an external dc electric potential is applied via the solid electrochemical cell in the PEVD system to drive the reaction in the desired direction. Thus, the activity of (A) at the reaction site is controlled by the applied dc electric potential. 

Kinetically, this reaction can be monitored by an ammeter attached to the external electric circuit. Further discussion of mass and charge transport in a PEVD system will be given in the following. 

According to the previous discussion, a PEVD process relies on mass and charge transport in two solid state ionic materials of a PEVD system, i.e., the solid electrolyte (E) and the product (D). Since mass and charge transport occur in solid state ionic materials, the conductivity mechanism imposes some restrictions, and fundamental considerations in a PEVD system can be obtained through the local equilibrium approach. In the following, mass and charge transport in both phases will be discussed. 

During mass and charge transport in a PEVD system, the solid electrolyte serves as an ion-pass filter and the external electric circuit as an electron-pass filter. Consequently, two kinds of conducting passes are separated in the system as shown in Figure 3. One is the ionic conduction path from location (I) through the bulk of the solid electrolyte (E) to location (II), then across the bulk of the PEVD deposit (D) to location (III). The other is the electronic conduction path from location (I) through the source electrode (C), the external electric circuit, and the sink electrode (W) to location (II), then across the bulk of the PEVD deposit (D) to location (III). 

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... 

B) is constant. The chemical potential of product phase (D) is equal to its Gibbs free energy of formation. The chemical potential of (A), which is the combination of the electrochemical potential of (A ) and (e ) according to Eqn. 6, is fixed at location (III) at equilibrium. It is further assumed that the chemical potential of (A) at (I) is greater than its chemical potential at (III) in this PEVD system. 

Open Circuit Condition and Equilibrium Potentials of the PEVD System... 

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... 

Fig. 4 Potential profiles in the PEVD system under open circuit conditions.

The difference in chemical potential of (A) between the source and sink side of the PEVD system causes a gradient of the chemical potential of (A) across the solid electrolyte (E) between (I) and (II). In order to have a constant electrochemical potential of (A+) inside (E) to prevent ionic current under equilibrium, an internal electric field is built up inside solid electrolyte (E). This is justified since electronic conductivity in (E) is negligible. The internal electric field causes an electric potential difference between (I) and (II). The value of the internal electric field is the EME of the cell, and can be calculated from the change in chemical potential of (A) across the solid electrolyte (E) according to Nernsf s equation. It can be measured by a high impedance electrometer in the external electric circuit. According to the Stockholm convention, EME... 

Closed Circuit Condition and Steady-State Potentials in a PEVD System... 

Deposition in a PEVD system is accomplished by transporting both (e ) and (A ) across the interfaces at (II), and from (II) to (III) to react with (B) from the sink vapor phase. This is equivalent to the transport of neutral species (A) from (II) to (III) under a chemical potential gradient of (A). Consequently, the chemical... 

When both reactions in Eqns. 5 and 7 proceed to the right, the equilibria at both sides of the cell no longer exist. This will decrease the chemical potential of (A) at (I) and increase it at (III). If the current in the PEVD system is assumed to be very small, the change in chemical potential of (A) at both the source and sink side will not be significant. The steady state potential profiles of the PEVD system are illustrated in Figure 5. 

When the current is limited by the solid transport of (A) in both the solid electrolyte (E) and product (D), the chemical potential of (A) at (II) for the PEVD system is related to the current in both phases. Thus, the chemical potential of (A) at (II) is critical to reveal the current and potential behavior in the PEVD system. 

Only resistive overpotenfial in the bulk of (E) and (D) of the PEVD system is considered. 

Surface leakage current from the source (O) to the sink (S) in the PEVD system is negligible. 

Due to conservation of charges, the total current / in any part of the solid electrochemical cell of the PEVD system is the same. 

The current relation in Eqn. 20 is achieved by adjusting the chemical potential of (A) at (II) under closed circuit conditions. Since the gradient in electrochemical potential is the driving force for the flow of charged particles in the multiphase PEVD system, the current density carried by (A+) in either the solid electrolyte (E) or deposit (D) can be written as ... 

The most distinguishing feature of PEVD process control is made possible because of the solid electrochemical cell involved in a PEVD system. Compared with other vapor deposition techniques, utilization of solid electrochemical cells is one of the most significant advantages of the PEVD technique, since deposition process control and monitoring are easy to realize. By connecting several measurement devices to the external electrical circuit of a PEVD system, the reaction in a PEVD process is easily monitored and possibly controlled by the electrical current and applied potential, respectively. ... 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

In practice, when current flows in the system, thermodynamic equilibrium conditions no longer exist at the interfaces. The response of the PEVD system is also related to the... 

Fig. 6 Ideal current-applied potential behavior of a PEVD system...

As schematically shown in Figure 7a, initial PEVD reaction and product nucleation occurs at the three-phase boundary of solid electrolyte (E), working electrode (W) and the sink vapor phase (S) which contains vapor phase reactant (B). Only here are all reactants available for the half-cell electrochemical reaction at the sink side of a PEVD system. Although the ionic and electronic species can sometimes surface diffuse at elevated temperature to other sites to react with (B) in the vapor phase, the supply of the reactants continuously along the diffusion route is less feasible and the nuclei are too small to be stabilized under normal PEVD conditions. Only along the three phase boundary line are all the reactants available for further growth to stabilize the nuclei. Consequently, initial deposition in a PEVD process is restricted to certain areas on a substrate where all reactants for the sink electrochemical reaction are available. 

In order to sustain this reaction at the sink side of the PEVD system, a source is required at the other side of the substrate (anode) to supply sodium. Otherwise, depletion of sodium in the Na" -P"-alumina solid electrolyte will lead to an a-alumina phase buildup at the anode that will block the ionic transport path of the PEVD system. The electrolytic properties of the solid electrolyte in this PEVD system will then be lost. Elemental sodium, for instance, could be the source giving the following anodic reaction ... 

The PEVD system for Na CO auxiliary phase formation at the working electrode of a type III potentiometric CO sensor is schematically shown in Eigure 10. The electrochemical cell for this PEVD process can be illustrated as ... 

During the PEVD process, a chemical redox reaction takes place and the whole PEVD system can he viewed as a chemical reactor where the reactants are distributed over both the source and sink sides. According to the previous discussion, the driving force for this PEVD process can be solely provided by a dc electric potential, so that isolation of the source and sink vapor phases is not necessary. Consequently, the PEVD process is equivalent to physically moving a solid phase Na COj through another solid phase (Na+-[3"-alumina) by electric energy. Furthermore, it should be pointed out that the overall cell reaction in this PEVD system is reversible. 

The phases, microstructure and chemical composition at the sink side of the PEVD system were studied before and after the PEVD process by x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy, respectively. 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

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