Planar Sensor Element Insulating Layer Porosity Design
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Solution Overview
Problem
Conventional sensor elements with Pt heaters experience increased heater resistance over time due to Pt diffusion in porous insulating layers at high temperatures, leading to potential disablement and cracking from thermal expansion differences between solid electrolyte and insulating layers.
Innovation Solution
A planar sensor element with a heater part comprising a Pt heater element, an insulating layer with 90-99.9 wt% alumina having a different thermal expansion coefficient, and a laminated structure of porous and dense portions to prevent Pt diffusion and cracking, where the dense portion covers the heater element and the porous portion surrounds it, with specific thickness ratios and porosities to manage thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a porous insulating layer is used to reduce thermal expansion stress, then cracking is prevented, but Pt diffusion increases causing heater resistance to rise
Solution Approach 1:
The insulating layer is designed with different porosity in different regions: a first region with 20-40% porosity for stress relief and a second region with 0-5% porosity for Pt diffusion prevention. This local differentiation allows each region to perform its specific function optimally without compromising the other.
Solution Approach 2:
The insulating layer is segmented into two distinct regions with different structures: a porous first region adjacent to the solid electrolyte layer for thermal stress management, and a dense second region adjacent to the heater element for Pt containment. This segmentation resolves the contradiction by spatially separating the two conflicting requirements.
2Measurement precision
If the driving temperature is set to high temperature (approximately 850°C) to improve sensor performance, then detection capability increases, but Pt diffusion accelerates causing heater resistance to increase
Solution Approach 1:
The dense second region of the insulating layer acts as an intermediary barrier between the heater element and the porous first region. This dense intermediate layer prevents Pt from diffusing into the porous structure while allowing the sensor to operate at high temperatures for improved detection capability.
3Reliability
If a dense insulating layer is used to prevent Pt diffusion, then heater resistance stability improves, but thermal expansion stress increases causing cracking
Solution Approach 1:
The insulating layer is designed with different porosity in different regions: a first region with 20-40% porosity for stress relief and a second region with 0-5% porosity for Pt diffusion prevention. This local differentiation allows each region to perform its specific function optimally without compromising the other.
4Strength
If only the interface portion of the insulating layer is densified to ensure adhesion, then bonding strength improves, but Pt diffusion prevention is insufficient
Solution Approach 1:
The insulating layer is segmented into two distinct regions with different structures: a porous first region adjacent to the solid electrolyte layer for thermal stress management, and a dense second region adjacent to the heater element for Pt containment. This segmentation resolves the contradiction by spatially separating the two conflicting requirements.
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 prevents Pt diffusion and cracking, extending the sensor element's lifetime by managing thermal expansion differences and maintaining heater resistance within acceptable limits.
Implementation Method 1
a heater element that generates heat by being externally powered
Implementation Method 2
stress is caused on the insulating layer by a difference in coefficient of thermal expansion between the insulating layer and solid electrolyte layers surrounding the insulating layer
Implementation Method 3
A sensor element in which the insulating layer has a porous structure to prevent or reduce the occurrence of cracking in the insulating layer caused by the action of the stress
Data Source
AI summary
A heater part of a planar sensor element includes: a Pt heater element; and an insulating layer including 90-99.9 wt % of an insulating material having a different coefficient of thermal expansion from a solid electrolyte forming a base part of the element. The heater part other than a heater electrode is buried in the base part. The insulating layer includes: a porous portion having a porosity of 20-40%; and a dense portion having a porosity of 4% or less. The heater element is covered with the dense portion. A laminated portion in which the porous portion and the dense portion are laminated has a total thickness of 25-100 μm. The dense portion has a thickness of 5 μm or more. A thickness ratio of the dense portion to the porous portion is 0.05 to 2.0.