Ceramic Heater Lead Geometry for Thermal Stress Reduction
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Solution Overview
Problem
Ceramic heaters used in glow plugs face defects such as gap generation at the interface between the heat-generating resistor and the insulating substrate due to thermal stress, caused by differences in thermal expansion coefficients and the structural design, leading to reliability issues during manufacturing and use.
Innovation Solution
A ceramic heater design with an insulating substrate and a heat-generating resistor, where the lead portions are arranged to satisfy specific geometric relationships to minimize thermal stress, including a ≥ 0.15(b + c) and a ≤ D - (b + c) - 0.2 in cross-sectional measurements, and having a sufficient thickness of the insulating substrate to reduce the likelihood of gap formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the tungsten carbide content of the silicon-nitride-tungsten-carbide composite sintered body is increased to lower resistance, then the resistance of the heat-generating resistor is reduced, but the thermal expansion coefficient increases in proportion, causing increased thermal stress and gap generation at the interface
Solution Approach 1:
The patent optimizes the tungsten carbide content within a specific range (30-70 wt%) rather than maximizing it, and controls the average grain size of silicon nitride particles (5-20 μm) to balance electrical conductivity with thermal expansion coefficient, thereby reducing thermal stress while maintaining low resistance
Solution Approach 2:
The patent uses a composite sintered body of silicon nitride and tungsten carbide, where the silicon nitride matrix provides low thermal expansion while tungsten carbide provides conductivity, creating a balanced composite material that mitigates thermal stress
2Speed
If the heat-generating portion is made thin to achieve quick temperature rise, then the temperature rise performance is improved, but high thermal stress is imposed on the large-diameter lead portions, causing gap generation at the interface
Solution Approach 1:
The patent applies different thickness specifications to different portions of the heat-generating resistor: the heat-generating portion has a thickness of 0.5-2.0 mm for quick temperature rise, while the lead portions have a larger diameter of 2.0-5.0 mm for mechanical strength and stress resistance, optimizing both performance and reliability
3Strength
If the overall length of the ceramic heater is increased in an all-ceramic design, then the structural integrity is improved, but the thermal stress imposed on the ceramic heater increases, making gap generation more likely
Solution Approach 1:
The patent transitions from a linear longitudinal structure to a coil-shaped structure, distributing the thermal stress along the coiled path rather than concentrating it in a straight line, thereby maintaining structural integrity while reducing overall thermal stress
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 design effectively reduces thermal stress and prevents gap generation at the interface, enhancing the reliability and durability of the ceramic heater and glow plug by maintaining structural integrity and preventing defects during temperature changes.
Implementation Method 1
a heat-generating resistor formed from a conductive ceramic and embedded in the insulating substrate... generating heat upon energization
Implementation Method 2
the thermal expansion coefficient of the heat-generating resistor formed from the silicon-nitride-tungsten-carbide composite sintered body also increases in proportion to the tungsten carbide content. This increases a difference in thermal expansion coefficient between the heat-generating resistor and an insulating substrate
Data Source
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AI summary
There are provided a ceramic heater in which a defect, such as generation of a gap at the interface between a heat-generating resistor and an insulating substrate, is unlikely to occur in the course of manufacture or use, and a glow plug using the ceramic heater. A ceramic heater 110 includes an insulating substrate 111 extending in the direction of an axis AX and a heat-generating resistor 115, which has a heat-generating portion 116, two lead portions 117, 117 and two lead lead-out portions 118a and 118b. The ceramic heater 110 satisfies an expression at 0.15(b + c) in a section of the ceramic heater perpendicular to the direction of the axis AX, where a represents a minimum gap a between the pair of lead portions 117, 117 on the minimum-gap-associated imaginary straight line, and b and c represent dimensions of the pair of lead portions 117, 117.