NOx Sensor Element Heat Treatment for Signal Followability
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Solution Overview
Problem
Conventional NOx sensors face challenges in maintaining good responsiveness to changes in NOx concentration, as heat treatment can lead to unburned carbon adherence and initial output defects, affecting signal followability and appearance.
Innovation Solution
Performing heat treatment in a rich atmosphere with a small amount of NO added, maintaining an air-fuel ratio of 0.80 to 0.9999, and heating at 500°C or higher for 15 minutes or longer to enhance NOx signal followability while preventing unburned carbon adherence and initial output defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If heat treatment is performed in a rich atmosphere at 500°C or higher for 15 minutes or longer, then the followability of the NOx signal is improved, but unburned carbon adheres to the sensor element surface causing bad appearance and initial output defects
Solution Approach 1:
The patent changes the chemical composition parameters of the heat treatment atmosphere by adding a small amount of NO (0.1-10%) to the rich atmosphere. This parameter modification enables the atmosphere to oxidize unburned carbon while maintaining the reducing conditions needed to improve sensor followability, thus resolving the contradiction between signal performance and appearance quality
Solution Approach 2:
NO acts as an intermediary substance in the heat treatment atmosphere. It serves dual functions: maintaining the rich atmosphere conditions necessary for improving NOx signal followability, and simultaneously providing oxidation capability to prevent unburned carbon adherence. This intermediary enables both beneficial effects without the harmful side effects
2Speed
If heat treatment is performed to improve NOx signal responsiveness, then the sensor can promptly follow changes in NOx concentration, but the initial output signal is adversely affected
Solution Approach 1:
The patent modifies the atmospheric composition parameters by introducing NO to the rich atmosphere during heat treatment. This parameter change allows the sensor to achieve good followability while the NO simultaneously prevents initial output defects by oxidizing carbon that would otherwise interfere with the initial signal
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method improves the followability of the NOx signal, prevents excessive output variations, and maintains a good appearance by controlling the heat treatment conditions, ensuring stable and accurate NOx concentration measurements.
Implementation Method 1
a sensor element as set forth in claim 1, wherein the sensor element is heated at a temperature of 500°C or higher and for 15 minutes or longer in a rich atmosphere having a small amount of NO added thereto
Implementation Method 2
a NOx sensor in which a sensor element is formed by using an oxygen-ion conductive solid electrolyte layer such as a zirconia (ZrO2) layer
Data Source
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AI summary
Method to improve the followability of a NOx signal of a NOx sensor relative to a variation in a NOx concentration, and to prevent occurrence of a defect caused by this process. A method of processing a sensor element includes the steps of: (a) preparing a gas atmosphere containing hydrocarbon, having an air-fuel ratio of 0.80 to 0.9999, and having a small amount of oxidizing gas added thereto; and (b) subjecting a sensor element to a heat treatment in the gas atmosphere at a temperature of 500°C or higher for 15 minutes or longer. The sensor element includes an electrochemical pumping cell constituted of an oxygen-ion conductive solid electrolyte and an electrode having a NOx reduction ability. A NOx gas in a measurement gas is reduced or decomposed in the electrode. A NOx concentration in the measurement gas is obtained based on a current which flows in the electrochemical pumping cell at a time of the reduction or decomposition.