Auxiliary phases

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. 

Applying PEVD for Auxiliary Phase Deposition at Working Electrodes of Gaseous Oxide Sensors... 

PEVD was developed initially in the course of fabricating type III potentiometric sensors for gaseous oxide (CO, SO, and NO ) detection. Three kinds of PEVD products (NaNOj, Na2C03, and Na SO ) were deposited as the auxiliary phases at the working electrode of NO, CO, and SO sensors, respectively. Because of the underlying similarities, all discussion here will focus on CO gas sensors. Cases of depositing NaNOj and Na SO auxiliary phases for type III NO and SO potentiometric sensors, respectively, can be treated analogously. 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

This sensing model also implies that the geometric properties, instead of solely the physical properties of the auxiliary phase at the working electrode, are critical to kinetic performance. Accordingly, the four most important geometric criteria for an auxiliary phase are as follows ... 

The aspect ratio of the auxiliary phase at the working electrode should be as low as possible to improve gas phase diffusion. 

The auxiliary phase should intimately contact the metallic electrode, and the interface should be as large as possible since transport of electrons is two-dimensional across the interface. 

The auxiliary phase should be as thin as possible to allow readjustment of the Fermi level within the auxiliary phase. 

The auxiliary phase should intimately contact the solid electrolyte to ensure transport of mobile ionic species. This will benefit the performance of sensors under certain conditions. 

The auxiliary phase should cover the entire solid electrolyte surface. 

Secondly, from Wagner s theory, the metallic coatings forming the electrodes are viewed as mere electronic probes for the local electronic distribution in the auxiliary phase. They should be chemically inert. For operation in the lean regime, it is highly desirable that their... 

The auxiliary phase should cover the entire metallic electrode. This also provides maximum interface contact as required in criterion (2). 

In what is the essence of materials engineering, structure and preparation will undoubtedly be of vital important to final properties. In order to deposit better auxiliary phases with close control to meet all six geometric criteria, PEVD has recently been applied to deposit auxiliary phases at the working electrodes of gaseous oxide sensors. ... 

Polarized Electrochemical Vapor Deposition to Deposit Auxiliary Phases at the Working Electrode of Type III Potentiometric CO Sensors... 

Fig. 8 Comparion of geometric structure of an ideal auxiliary phase (a) with those prepared by current techniques, such as in-situ formation (b), mechanically pressed discs (c), physical vapor deposition (d), and melting and quenching (e).

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 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 ... 

NaNOj and Na SO auxiliary phases can be deposited by a similar PEVD method for type III NO2 and SO2 potentiometric sensors, respectively. 

Based on micro structure studies, the geometric structure of an auxiliary phase deposited by the PEVD technique is very similar to the previously discussed ideal auxiliary phase illustrated in Figure 8a. Table 1 compares the geometric properties of PEVD auxiliary phases with other reported techniques using the six proposed criteria. 

The advantages of applying the PEVD process to deposit auxiliary phases are not only based on the mechanism of PEVD product crystal growth, but also arise from close control over the entire process achieved by adjusting the applied dc electric potential and monitoring current. Thus, the superiority of PEVD is obvious since the Na COj auxiliary phase can be deposited in a well-controlled manner at the... 

Table 1. Geometric Properties of Auxiliary Phases Deposited by Various...

Improvement of the geometric structures of the auxiliary phases using the PEVD technique will benefit the performance of gaseous oxide sensors in many ways, e.g., increasing selectivity and stability, shortening response time, and decreasing the influence of gas flow rate. °... 

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... 

Fig. 19 PEVD auxiliary phase deposition and sensor testing procedure.

After each step of auxiliary phase deposition, the PEVD process was stopped for sensor response behavior testing by opening Gate A (Eigure 20b). Under the open circuit condition, the EME of the sensor was indicated by the electrometer. A high flow rate at both sides remained until the equilibrium EME value of the sensor, zero in this case, was reached. 

Before moving forward to the next round of auxiliary phase deposition and subsequent sensor response testing, the sample was taken out of the... 

Figures 21a-e show SEM SE plan-view images of the first sample at five selected PEVD steps, indicated by a to e in Table 2. The auxiliary phase coverage at the working electrode of the sensor increased with PEVD processing time and PEVD flux from a to e. After 14 steps of auxiliary phase deposition and sensor response testing, the final thickness of the product was about 3 pm, which was estimated from an SEM SE image of a cleaved cross-section sample (Eigure 2If).

The general response of the sensor after changing the CO partial pressure at the working electrode is shown in Eigure 22, which is the response curve obtained after seven steps of auxiliary phase deposition for the first sample. When the CO partial pressure decreases at the working electrode, the emf of the cell increases dramatically at first and then slowly reaches an equilibrium value of 76 mV. The time to reach the equilibrium value is the recovery time of the sensor. The same is true when the partial pressure increases at the working electrode of the sensor, and the time to reach the equilibrium value is the response time of the sensor. In practice, researchers commonly use the time for... 

Fig. 21 SEM SE images of the working electrode (a) before PEVD, (b) first stable EMF response, (c) when the response time of the sensor just passed the minimum point, (d) when the recovery time of the sensor just passed the minimum point, (e) final auxiliary phase coverage (plan view), and (f) final auxiliary phase coverage (cross-section). Bars equal to 5 pm.

For sample 1, a stable EMF response from the sensor was obtained after passing about 0.468 C of Na ions. According to the previous discussion, this corresponds to the point where the PEVD auxiliary phase just covers the entire Pt thick film surface. The response times and recovery times of the sensor after each PEVD process step are recorded. Both response and recovery times are plotted against the Na ion flux through the solid electrolyte during the PEVD process in Figure 23. Curve (1) is the response time and curve (2) is the recovery time. 

The sensor response and recovery behavior for various auxiliary phase coverages at the working electrode of a type III potentiometric sensor are revealed for the first time through a combination of Figure 23 and Figures 21a-e. 

Fig. 23 The response (curve 1) and recovery (curve 2) behavior of the type III potentiometric COj sensor (sample 1) with increasing PEVD auxiliary phase at the working electrode.

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