YSZ-based sensors

FIGURE 2.3 Schematic presentation of the interaction of the solid electrolyte oxygen sensor with gas environment 1-6 stages of interaction. (From Zhniykov, S., Mathematical model of electrochemical gas sensors with distributed temporal and spatial parameters and its transformation to models of the real YSZ-based sensors. Ionics 12 (2006) 135-148. With kind permission of Springer Science and Business Media.)... 

The next stage of oxygen interaction with the Pt SE of the YSZ-based sensor is the electrochemical reaction of oxygen ionization at the TPB. We assume that the oxygen atom accepts two electrons from the Pt SE at the TPB and as ion transfers... 

One of the main assumptions for the thin-film YSZ-based sensors is the negligible interinfluence of the thin-film layers. This means that the mechanical, physical, chemical, and electrostatic components of their interactions are close to zero [53]. In order to achieve such conditions, the following requirements must be implemented the purity of raw materials for thin films should be ultra-high (99.999%), and raw materials should have compatible coefficients of thermal expansion and parameters of the crystalline structure. They should also be characterized by the minimum value of the contact difference of potentials. 

Gas flows through the SE will be different ( ventilation effect) and will depend on how the oxide SE was applied on the surface of YSZ and how it was heat-treated and sintered afterwards. The vast majority of the modem technologies will allow creating a uniform thickness of SEs (REs). However, the stmctural orientation of an oxide SE and, especially, its surface and bulk porosity will change from one technology to another and from one sintering temperature to another [9]. This fact, in turn, will influence characteristics of the YSZ-based sensor, such as sensitivity, reproducibility of measurements, response and recovery time, and so on. Let s indicate both necessary and sufficient conditions of existence of such a ventilation effect within the SE. 

Based on the fact that the potentiometric gas sensors establish thermodynamic equilibria at the interfaces with gases, let s consider this process for the YSZ-based sensor measuring the NO2 concentration in a humidified atmosphere. The discharge reactions at the TPB (at x > 8) can be shown as follows ... 

As an example, the verification of adequacy of the malhanatical model of the YSZ-based sensor, by using the statistical Fisher criterion, is shown that the hypothesis about the adequacy of the presented model to the real YSZ-based sensor with the NiO-SE sintered at 1300°C for measuranent 200 ppm NOj at T = 800 C is not rejected by the uniformity of dispersions with a significance level ofq = 0.05. 

Zhuiykov, S., Mathematical model of electrochemical gas sensors with distributed temporal and spatial parameters and its transformation to models of the real YSZ-based sensors, Ionics 12 (2006) 135-148. 

These reactions determine the apparent potential of the zirconia-based sensor. Since the raw exhaust gas at automotive and combustion applications constitutes a nonequilibrium gas mixture, thermodynamic equilibrium has to be achieved at the SE surface of the zirconia-based sensor before monitoring the potential. Consequently, such sensors contain catalytically active materials and are operated at temperatures above 600°C, when the average ionic transference number % > 0.99. For less active materials or temperatures below 600°C the apparent em/starts to deviate significantly from the value under equilibrium conditions due to insufficient catalytic activity. Earlier research of the YSZ-based sensors was focused on the electrode materials with high exchange currents and high catalytic activity for the desired electrode reactions. Pt electrodes were found to be the most suitable for this type of application. 

It was hrst reported in 2001 [32] that the evaluation by impedance spectroscopy of a biased zirconia sensor with an attached LaFeOa-SE has shown that only the electrode resistance is a function of NO2 content. Consequently, impedance spectroscopy offers a method for directly probing the electrode reactions that are the basis for mixed-potential-type gas sensors [67]. As a result of further development by using impedance spectroscopy in zirconia-based gas sensors with oxide-SEs, a new type of YSZ-based sensor for detecting total NO and HCs at high temperatures has been proposed recently [2, 14, 21, 62, 74, 96-100]. In this case, the change in the complex impedance of the device attached with a specific oxide-SE was measured as a sensing signal. 

FIGURE 3.17 Complex impedance plots in base air and the sample gas with each of the various concentrations of (a) NO and(b) NO2 at 700°C for the YSZ-based sensor attached with a ZnCr204-SE. (Reprinted from Miura, N., Nakaton M., and Zhuiykov, S., Impedanee-metric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOj at high temperature, Sens. Actuators B, Chem. 93 (2003) 221-228, with permission from Elsevier Science.)... 

Finally, planar, thick-film, YSZ-based sensors for O2, and NO, are expected to reinforce their place in the maiket owing to their rapid response and potential for implementation as multicomponent gas sensors in vehicle exhausts. The performance of recently developed nltra-lean-bum engines and NO, storage catalysts depends significantly on the performance of such sensors. Thus, solid-state electrochemical sensors must reach even higher levels of performance and reliability, so continued development of these sensors is required in order to address more stringent requironents. 

Fig. 1.5 Schematic diagrams of electrochemicai gas sensors (a) amperometric sensor with iiquid eiectroiyte ruid three-eiectrode configurations (b) potentiometric poiymer-based sensor for hydrogen detection (c) mixed-potentiai-type sensor using YSZ-based soiid eiectroiyte and ZnO-Pt electrode (d) chip-type YSZ-based sensor attached with CdO and SnOj eiectrodes (Reprinted with permission from (a, b) Korotcenkov et ai. (2(X)9). Copyright 2009 American Chemicai Society (c) Lu et al. (1996). Copyright 1996 Elsevier and (d) Miura et al. (1998a, b). Copyright 1998 Elsevier)...

Miura and Yamazoe 1998 Martin et al. 2004 Nakatou and Miura 2005). We also need to take into account that metal oxides are considerably cheaper than Pt. Results related to EMF response to and CO of YSZ-based sensors with various metal oxide sensing electrodes are presented in Table 9.2. 

Table 17.5 EMF responses to CO and H2 of YSZ-based sensors incorporating various metal oxides as the sensing electrode at 600 °C...

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