High-Temperature Stable NO_x Sensor for Exhaust Gas Monitoring in Automobiles

1. INTRODUCTION

The automotive NO_x sensor market is expected to reach US $120 billion by 2021 with a growth rate of more than 10%, and is expected to grow to US $160 billion by 2022. The domestic market for automotive NO_x sensors is expected to reach more than 380 billion won by 2023 because diesel cars, which emit harmful gases, have become the main market for automotive denitrification (DeNO_x) systems [1,2]. Therefore, it is necessary to develop DeNO_x systems to improve fuel efficiency and reduce NO_x emissions in accordance with the European Union’s automotive NO_x emission regulations, and it is crucial to develop high-temperature NO_x sensors that can directly measure NO_x in real time for efficient operation. Most nitrogen oxides (NO_x) are produced when nitrogen and oxygen in air react under high heat during combustion [3]. NO_x is lethal to humans on its own, and it also reacts with various hydrocarbons in the atmosphere to form ozone via a photochemical reaction with sunlight. Despite its usefulness in several industrial applications, ozone contributes to photochemical smog, which significantly reduces air quality [4]. Ozone irritates the mucous membranes of the eyes, causing watery eyes, chest tightness, and development and aggravation of respiratory diseases such as asthma and bronchitis [5]. It is also a strong oxidizing agent that damages plants and their inherent properties, resulting in reduced crop yields. Ozone also oxidizes NO_x and SO₂ in the atmosphere to produce nitric and sulfuric acids, which contribute to acid rain. To reduce NO_x emissions, the emission regulations have been gradually tightened, and strict emission allowance standards have been established to curb emissions from motor vehicles [6]. The emissions that are mainly harmful to human health owing to the increase in number of vehicles in metropolitan areas are unburned hydrocarbons (C_mH_n, 500–1500 ppm), carbon monoxide (CO, 0.5–2.0%), and nitrogen oxides (NO_x, 0.01–0.2%), and ozone (O₃) gas due to the photochemical reactions of NO_x gas in the atmosphere. The health and safety of workers exposed to hazardous gases at industrial sites is of great concern. Therefore, the immediate challenge for human health and safety from these combustion gases is to quickly detect and respond to exposure to hazardous gases. In such systems, the application of highly sensitive and reliable NO_x sensors enables miniaturization, portability, mobility, and low-cost NO_x measurement instruments [8,9]. However, the development of NO_x sensors for high-temperature automotive emissions is currently mostly at the NGK level in Japan [10]. In Korea, certain companies such as Iljin and Sejong Corporation and KAIST are developing them; however, their success has not yet been confirmed [11]. There have been several reports published on NO_x measurement at low temperatures (<300°C) using WO₃ semiconductor oxide [12,13]. In the case of automobiles, the temperature of the exhaust gas is high (>800°C), and NO_x sensors that can be used for this purpose have not been developed in Korea. Therefore, it is essential to develop NO_x sensors with features such as miniaturization, high sensitivity, fast response, and selectivity to save energy by improving fuel efficiency and preventing environmental pollution.

2. EXPERIMENTAL METHOD

The manufacturing process of the sensor elements for the development of automotive nitrogen oxide (NO_x) sensors is shown in Fig. 1.

Fig. 1.

Manufacturing process of NOx sensor element

3. RESULTS AND DISCUSSIONS

3.1 Green Sheet of Ion Conducting Layer and Diffusion Barrier Layer

The two types of green sheets used in this study are solid electrolyte layer for ionic conduction and diffusion barrier layer. Because complete solubility could not be obtained when using a single solvent alone, good solubility results were obtained using mixed solvents. In addition, plasticizers were used appropriately to control viscosity and suppress adhesion to the film. The plasticizers contained in the green sheet decompose during heat treatment to prevent cracking of the green sheet owing to interaction with each other when it burned [14,15]. A 30 cm × 100 cm pilot green sheet without cracks was manufactured by adjusting the type and amount of the binder and organic solvent. Fig. 2 (a) shows the mixing sequence and proportions of the solvent, binder, dispersant, and plasticizer used to prepare the green sheet. Fig. 2 (b) shows a sample of the prepared green sheet and its structure obtained using SEM analysis.

Fig. 2.

(a) Schematic of green sheet fabrication method, and (b) fabricated green sheet sample and SEM image

3.2 Detector Electrode Manufacturing

3.2.1 Preparation of Sensing Powder

ZnFe₂O₄ and ZnCr₂O₄ were selected as sensing materials. Using ZnO (Aldrich Chemical Co, Ltd 99.9%), Fe₂O₃ (Aldrich Chemical Co. Ltd., 99.9%), and Cr₂O₃ (Aldrich Chemical Co. Ltd., 99.9%) as starting materials, Zn:Fe or Zn:Cr in a molar ratio of 1:2 was mixed using a wet ball mill for 24 h and then heat treated at 1600°C for 4 h to obtain synthesized ZnFe₂O₄ and ZnCr₂O₄. Fig. 3 (a) shows the XRD patterns of the synthesized ZnFe₂O₄ and ZnCr₂O₄ powders [16]. All synthesized powders exhibited singlephase spinel crystal structures.

Fig. 3.

(a) XRD patterns of synthesized ZnFe2O4 and ZnCr2O4 powders, (b) SEM microstructure and composition of ZnFe2O4 sensing electrode, and (c) SEM microstructure and composition of ZnCr2O4 sensing electrode

3.2.2 Preparation of Sensing Electrode Paste

The sensing electrode powders were mixed with the solvents and binders listed in Table 1, and prepared as a paste in a threeroll mill. The paste thus prepared was applied to alumina by screen printing method and dried at 50°C for 30 min [17,18]. The dried paste was heat treated at 1600°C to form a sensing electrode and analyzed by SEM and EDS to confirm its printability and composition (Figs. 3 (b) and 3 (c)).

Table 1.

Sensing electrode paste preparation (Mixing ratio of solvent, binder, and additives)

3.3 NO_x Sensor Structure Design and Fabrication

The NO_x sensor device for automotive use was designed to improve the oxygen pumping efficiency and detection of NO and NO₂ using a diffusion barrier layer, as shown in Fig. 4 (a) [19,20]. The designed NO_x sensor is a next-generation NO_x sensor with improved sensitivity achieved by controlling the oxidationreduction rate of NO and NO₂ on the oxide sensing electrode to improve the detection of NO and NO₂ by improving the oxygen pumping efficiency and using a diffusion barrier layer, as shown in Fig. 4 (a) [21]. The nitrogen oxide sensor was designed using 15 green sheets. Each layer has a complex structure, as shown in Fig. 4 (b). The top surface of the first layer consists of an external oxygen-pumping electrode, a terminal electrode, and a lead electrode, which are protected by a porous protective layer and an insulating layer. An internal oxygen-pumping electrode and lead electrode were formed underneath the third layer, and the exhaust gas cavity was separated into two parts. The sixth and eighth layers formed the detection and reference electrodes, respectively, and the air duct was the bottom layer. In the third layer, a heater is formed between the insulating layers. The bottom of the fifth layer is composed of terminal electrodes, including heater terminals, which must be precisely controlled during mass production. In addition, to reduce the defect rate of the sensor element, the process was optimized to prevent adhesion between the paste and the green sheet, and short circuit of each pattern. When printing the heater pattern embedded in the sensor element, insulation layers of 15–30 μm thickness were formed above and below the heater to prevent the deterioration of the heater due to the applied voltage and to block the leakage current from the detection cell or pumping cell. In addition, an insulating layer and a porous protective layer were simultaneously formed to protect the electrode and lead electrode responsible for oxygen pumping outside the sensor, and the sensor element was manufactured to significantly improve the sensing characteristics (Fig. 4 (c)).

Fig. 4.

(a) Structural schematic of the proposed NOx sensor, (b) Detailed schematic of each layer of the sensor element, and (c) Images of the formation of electrodes and insulation layers used in the sensor element

3.4 Analysis of the Structure and Composition of the Sensor Element After Heat Treatment

After the heat-treated NO_x sensor specimens were cut using a cutting machine, the solid electrolytic zirconia sheet, internal and external pumped Pt electrodes, and external lead electrodes were analyzed using an electron microscope (Figs. 5 (a-c)), and the external morphology of the NO_x sensor after firing was observed using an optical microscope (Fig. 5 (d)). The results showed that the interior of the green sheet had a fairly good filling of organic additives and ceramic powder, with few cracks and residual organic matter. This indicates that the selection of the organic additives according to the ceramic powder is appropriate, and that the conditions of the green sheet are satisfied in that it is possible to form an electrode of a certain thickness in the green sheet state and that no burn-out, bending, or cracking occurs during the manufacture of the sensor element. In addition, the reference, pumping, and detection electrodes showed little volatilization or diffusion of the electrode, even after heat treatment.

Fig. 5.

Microstructure and composition analysis images of (a) solid electrolytic zirconia sheet, (b) internally pumped Pt electrode, (c) externally pumped lead electrode, (d) externally pumped Pt electrode, and (e) optical microscopy image of the external shape of the sensor after firing

A test of the heater integrated into the nitrogen oxide sensor element was performed using an infrared thermal imaging camera. The temperature distribution of the entire sensor element and the temperature distribution of the heating area, where the oxygen-pumping, detection and reference electrodes were located, were investigated while maintaining 12 V. Fig. 6 (a) shows the infrared thermal imaging camera measurements, and Fig. 6 (b) shows the thermal test results of the fabricated nitrogen oxide sensor. Nitrogen oxide sensors directly measure the combustion gases in automotive exhaust, and hence, they should be able to operate at high temperatures of 700–800°C. As shown in Fig. 6 (b), the heater in the device is maintained at a temperature >800°C, which is sufficient for high temperature operation. Fig. 6 (c) shows the sensing signal as the NO_x concentration changed under the conditions of 13–14% oxygen and 3% moisture. The results showed that a wide range of NO_x concentrations can be detected and that the sensing signal changed stably with gas in/out. The time required for a sensor to detect a certain concentration of a test gas is called the response time; the faster the response time, the better the sensor. Figs. 6 (d-f) shows the response characteristics of the nitrogen oxide sensor at gas temperatures above 800°C as a function of time. The response time of the sensor obtained in this study was ON (at NO_x 300 ppm): 2.29 s, ON (at NO_x 1350–1500 ppm): 1.2 s, OFF (at NO_x 0 ppm): 1.3 s, which is higher than that of the world's best sensor (NGK, 0.75 s); this is a very good result compared to the response time of most of the solid electrolyte sensors studied previously, which was more than 3 s.

Fig. 6.

(a) Infrared thermal imaging camera measurement of sensor element, (b) heater heating test result of NOx sensor, (c) analysis of sensing signal according to NOx concentration change, (d) response characteristics of NOx sensor (ON, at NOx 300 ppm), (e) response characteristics of NOx sensor (ON, at NOx 1350–1500 ppm), and (f) response characteristics of NOx sensor (OFF, at NOx 0 ppm)

4. CONCLUSIONS

In this study, an automotive NO_x sensor was developed by fabricating a solid electrolyte green sheet with ion conducting and diffusion barrier layers, which served as the foundation for producing the sensor component and assessing its hightemperature NO and NO₂ sensing performance. The automotive NO_x sensor structure was designed using a mixed-potential mechanism, which enables regulation of NO and NO₂ redox reactions, while incorporating an internal heater, porous protective layers, and an insulation to prevent degradation and leakage current. The post-sintering analysis confirmed that electrode diffusion and cracking were effectively minimized, resulting in a uniform structure. Infrared thermography further verified that the heater maintained stable heat distribution at temperatures exceeding 800°C. In addition, NO_x response characteristics were evaluated at gas temperatures above 800°C, and the sensor demonstrated a response time of 1.2–2.29 s, showing significant improvement in speed and stability compared to conventional solid electrolyte sensors. These results indicated that the developed NO_x sensor could reliably detect NO_x with high accuracy and speed under high-temperature conditions, indicating its practical applicability in automotive exhaust gas monitoring.

Acknowledgments

This research was supported by a grant of the Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Korean government (MOTIE) (RS-2023-00257663), (RS-2023-00235283) and the Korea Institute of Industrial Technology (KITECH, Project no. UR240004).

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