Gas Sensor Protective Layer Porosity Design
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Solution Overview
Problem
Gas sensors with protective layers face issues of poor rapid heat-up performance and separation due to water evaporation, which affects their thermal shock resistance and adhesion.
Innovation Solution
A three-layer porous protective layer structure is implemented around the sensor element, with the first layer having a porosity of 40% or more, the second layer having a higher porosity, and the third layer having a lower porosity, ensuring secure adhesion and suppressing water-induced separation during heating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protective layer is provided to improve thermal shock resistance, then water resistance is improved, but heat capacity increases and rapid heat-up performance deteriorates
Solution Approach 1:
The protective layer is designed with a porous structure having a porosity of 30% or more. This porous structure reduces the heat capacity of the protective layer while maintaining its protective function, thereby improving rapid heat-up performance without sacrificing thermal shock resistance.
Solution Approach 2:
The protective layer is formed as a composite structure combining a porous body with a binder material. This composite approach allows the porous structure to provide low heat capacity while the binder ensures adequate adhesion and mechanical strength, resolving the contradiction between protection and heat-up speed.
2Reliability
If a protective layer is provided to improve thermal shock resistance, then water resistance is improved, but force of constraint increases and adhesion deteriorates
Solution Approach 1:
The porous structure of the protective layer reduces its density and constraint force on the sensor element, thereby improving adhesion while maintaining thermal shock resistance. The porosity of 30% or more ensures reduced constraint without compromising protective functionality.
Solution Approach 2:
The porosity of the protective layer is controlled to be 30% or more, which changes the physical parameters of the protective layer to reduce constraint force while maintaining adhesion. This parameter optimization resolves the contradiction between protection and bonding strength.
3Reliability
If water remains in the protective layer, then water resistance is maintained, but volume expansion occurs during evaporation and separation is caused
Solution Approach 1:
The porous structure with 30% or more porosity provides capillary channels that facilitate rapid evaporation of condensed water. This allows water to be quickly removed from the protective layer during heating, preventing volume expansion and separation while maintaining water resistance during normal operation.
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
The solution effectively suppresses separation-at-evaporation of the protective layer, maintaining thermal shock resistance and rapid heat-up performance, thereby enhancing the sensor element's operational stability and reliability.
Implementation Method 1
a leading-end protective layer being a porous layer disposed around an outer periphery of the element base
Implementation Method 2
to prevent water-induced cracking of the sensor element under the action of thermal shock caused by heat (cold) from water droplets adhering to the surface of the sensor element
Data Source
AI summary
A sensor element includes a porous leading-end protective layer disposed around an outer periphery of an element base in a predetermined range from an end portion on a side where a sensing part is disposed, wherein the protective layer includes: a first layer disposed on two main surfaces of the element base; a second layer disposed to cover the end portion and four side surfaces of the element base including the two main surfaces on which the first layer is disposed; and a third layer disposed to cover the second layer, and having a lower porosity than the second layer, the first layer has a porosity of 40% or more, and L1≥L2 and L1≥L3 where L1, L2, and L3 are extension lengths of the first layer, the second layer, and the third layer, respectively, from an end surface of the element base in a longitudinal direction of the element base.