Sensor Element Au Evaporation Suppression
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Solution Overview
Problem
Gas sensors used for measuring nitrogen oxides (NOx) suffer from deteriorating measurement sensitivity due to the evaporation of Au from the inner pump electrode, which affects the accuracy and responsiveness of NOx detection, especially when the sensor is used continuously at high temperatures.
Innovation Solution
The gas sensor design includes an inner pump electrode made of a Pt-Au alloy and zirconia, positioned on the surface farthest from the heater to minimize Au evaporation, with diffusion control parts to regulate oxygen flow and prevent excessive main pump voltage, maintaining NOx sensitivity over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the sensor is heated to high temperature to activate the solid electrolyte, then the oxygen-ion conductivity is improved, but Au evaporates from the inner pump electrode causing deterioration of measurement sensitivity
Solution Approach 1:
The patent extracts the harmful effect of Au evaporation by removing the inner pump electrode from the high-temperature zone. The inner pump electrode is positioned only on the lower surface of the solid electrolyte layer, away from the heater, so that it operates at a lower temperature where Au evaporation is suppressed while still enabling oxygen pumping function.
Solution Approach 2:
The patent introduces an intermediary spatial arrangement where the inner pump electrode is placed in a temperature gradient zone between the heater and the measurement gas flow. This positioning allows the electrode to experience moderate temperatures that prevent Au evaporation while maintaining sufficient ionic conductivity for oxygen pumping.
2Productivity
If the inner pump electrode is positioned close to the heater to improve oxygen pumping efficiency, then the pumping speed is improved, but Au evaporation increases causing measurement accuracy deterioration
Solution Approach 1:
The patent applies local quality by creating different thermal environments for different parts of the sensor. The heater is localized to maintain high temperature in the measurement zone for ionic conductivity, while the inner pump electrode is positioned in a cooler zone to prevent Au evaporation, achieving both pumping efficiency and measurement accuracy.
3Quantity of substance
If the main pump voltage is increased to enhance oxygen removal from the internal space, then the oxygen concentration reduction is improved, but excessive voltage causes harmful effects on the electrode materials
Solution Approach 1:
The patent changes the operational parameters by controlling the main pump voltage to remain below the threshold that causes harmful effects on the electrode materials. By optimizing the voltage level and the positioning of the inner pump electrode, the system achieves effective oxygen removal while maintaining electrode material stability and preventing premature aging.
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 configuration effectively suppresses the deterioration of NOx sensitivity and maintains accurate NOx detection even during continuous use by preventing Au evaporation and controlling the main pump voltage, ensuring reliable NOx concentration measurements.
Implementation Method 1
a heater part (70) buried in the sensor element (101) and heating the sensor element (101)
Implementation Method 2
a base part made of an oxygen-ion conductive solid electrolyte
Implementation Method 3
oxygen in the measurement gas is pumped out by an electrochemical pump cell, for example, referred to as a main pump cell
Implementation Method 4
NOx in the measurement gas is then reduced or decomposed by a measurement electrode (third inner pump electrode in Japanese Patent No. 3050781) functioning as a reduction catalyst
Implementation Method 5
a gas inlet (10) through which the measurement gas is introduced from an external space; a first internal space (20) communicating with the gas inlet (10) under predetermined diffusion resistance
Data Source
AI summary
In a sensor element for a limiting-current type gas sensor measuring concentration of NOx in a measurement gas, an inner pump electrode located to face a first internal space communicating with a gas inlet through which the measurement gas is introduced from an external space under predetermined diffusion resistance is made of a cermet of a Pt—Au alloy and zirconia, and the inner pump electrode is located, from among surfaces defining the first internal space, at least on a surface farthest from a heater part in a thickness direction of the element, and is not located on a surface closest to the heater part in the thickness direction.