Gradient Au Content Inner Pump Electrode for NOx Sensor
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Solution Overview
Problem
Existing NOx sensors face accuracy issues due to the evaporation of Au from the inner pump electrode, which leads to premature decomposition of NOx and reduced measurement sensitivity, especially when measuring gases with high oxygen concentrations.
Innovation Solution
The NOx sensor employs a cermet inner pump electrode made of a Pt-Au alloy and zirconia, with a specific Au content distribution and total area configuration to minimize Au evaporation, ensuring stable NOx measurement accuracy 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 pump electrode causing deterioration of measurement accuracy
Solution Approach 1:
The pump electrode is designed with a gradient Au content distribution, where the Au content varies from one end to the other. Specifically, one end has higher Au content (0.5-3.0 wt%) to suppress NOx decomposition, while the other end has lower Au content (0.0-0.6 wt%) to minimize evaporation. This local quality variation resolves the contradiction by having different regions serve different functions within the same electrode.
Solution Approach 2:
The pump electrode uses a composite structure combining Pt-Au alloy with zirconia (YSZ). This composite material approach allows the electrode to maintain structural integrity and functional performance at high temperatures while the controlled Au distribution prevents both excessive evaporation and NOx decomposition, thus maintaining measurement accuracy.
2Measurement precision
If Au is added to the pump electrode to suppress NOx decomposition, then NOx detection accuracy is improved, but Au evaporates during continuous high-temperature operation
Solution Approach 1:
Instead of uniform Au distribution, the electrode implements a gradient where Au content varies spatially. The high-Au region (0.5-3.0 wt%) suppresses NOx decomposition to maintain detection accuracy, while the low-Au region (0.0-0.6 wt%) minimizes Au evaporation losses during continuous operation.
Solution Approach 2:
The Au content parameter is changed across different regions of the electrode rather than maintaining a constant value. This parameter variation allows optimization of both NOx detection accuracy (requiring higher Au) and evaporation resistance (requiring lower Au) in different spatial zones.
3Measurement precision
If the pump electrode has high Au content to prevent NOx decomposition, then measurement accuracy is improved, but the cost and complexity of the electrode increases
Solution Approach 1:
The electrode uses local quality variation with Au content ranging from 0.0-0.6 wt% to 0.5-3.0 wt% across different regions. This gradient structure provides the necessary Au content for suppressing NOx decomposition only where needed, rather than requiring high Au content throughout the entire electrode, thus balancing performance with complexity.
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 Au evaporation and NOx decomposition, maintaining high NOx measurement sensitivity even under continuous high-temperature exposure, as demonstrated by a NOx sensitivity change rate of 20% or less and a determination coefficient of 0.975 or more.
Implementation Method 1
a sensor element containing an oxygen-ion conductive solid electrolyte as a main component
Implementation Method 2
a heater part buried in the sensor element and heating the sensor element
Implementation Method 3
the inner pump electrode is at least made of a cermet of a Pt—Au alloy and zirconia, and includes: a first portion located on a surface farther from the heater part from among surfaces opposing each other in the first internal space; and a second portion located on a surface closer to the heater part from among the surfaces opposing each other in the first internal space, an Au content with respect to the Pt—Au alloy as a whole of the second portion is 0.3 wt % or more smaller than an Au content with respect to the Pt—Au alloy as a whole of the first portion
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
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, under predetermined diffusion resistance, with a gas inlet through which a measurement gas is introduced from an external space is made of a cermet of a Pt—Au alloy and zirconia, and includes a first portion located on a surface farther from a heater part and a second portion located on a surface closer to the heater part from among surfaces opposing each other in the first internal space, an Au content with respect to the Pt—Au alloy as a whole of the second portion is 0.3 wt % or more smaller than that of the first portion, and a total area of the first portion and the second portion is 10 mm2 to 25 mm2.