Thermistor Protective Layer Structure for Thermal Stress Relief
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Solution Overview
Problem
Temperature sensors using thermistors face challenges in maintaining accurate electrical resistance values in high-temperature reducing atmospheres due to thermal stress from mismatched linear expansion coefficients between the thermistor, lead wires, and protective layers, leading to incorrect temperature measurements.
Innovation Solution
A temperature sensor element with a protective layer comprising an inner layer made of chemically stable non-metallic particles and an outer layer that seals and supports the inner layer, designed to relieve thermal stress and maintain stable electrical resistance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protective layer made of glass is provided to protect the thermistor, then the thermistor is protected from the environment, but thermal stress occurs due to mismatched coefficient of linear expansion between the glass and the thermistor, causing the electrical characteristics of the thermistor to change
Solution Approach 1:
An inner protective layer is introduced as an intermediary between the thermistor and the outer protective layer. This inner layer has a coefficient of linear expansion that matches the thermistor, thereby eliminating thermal stress at the thermistor interface while the outer layer provides environmental protection. The intermediary layer acts as a buffer that prevents direct stress transmission to the thermistor.
Solution Approach 2:
The protective structure is designed as a composite system with two distinct layers: an inner layer made of material matching the thermistor's expansion characteristics, and an outer layer made of glass or other protective material. This composite structure allows each layer to perform its specific function - the inner layer prevents thermal stress while the outer layer provides environmental protection.
2Strength
If the filling rate of particles in the inner protective layer is increased, then the density and mechanical strength are improved, but the ability to relieve thermal stress is reduced
Solution Approach 1:
The filling rate of particles in the inner protective layer is optimized to a specific range (50-80%) rather than maximizing it. This parameter optimization balances two competing requirements: sufficient particle density to provide mechanical strength and structural integrity, while maintaining enough void space to allow for thermal expansion and stress relief. The specific filling rate range represents an optimal compromise between these conflicting demands.
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 provides a temperature sensor element that maintains stable electrical resistance values even in harsh high-temperature environments, ensuring accurate temperature measurements over time.
Implementation Method 1
If there is a difference in the coefficient of linear expansion between the thermistor and the lead wires, the coefficient of linear expansion of the glass forming the protective layer cannot be matched with both the coefficient of linear expansion of the thermistor and the coefficient of linear expansion of the lead wires. If there is a difference between the coefficient of linear expansion of the glass and the coefficient of linear expansion of the thermistor, the thermistor is subjected to thermal stress
Implementation Method 2
The characteristics of a thermistor are generally represented by the resistance value and a temperature coefficient of resistance (dependence of the resistance value on temperature)
Data Source
AI summary
A temperature sensor element includes: an element main body including a heat sensitive body including a thermistor sintered body of which the electrical characteristics change with temperature, and a pair of lead wires that is connected to the heat sensitive body through electrodes; and a protective layer that protects the heat sensitive body. The protective layer has an inner protective layer covering the heat sensitive body and an outer protective layer covering the outer side of the inner protective layer. The inner protective layer is formed of an aggregate of particles that are chemically stable with respect to the thermistor sintered body and made of non-metal.