Graphite Sheet Surface Structure for Lower Contact Thermal Resistance
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Solution Overview
Problem
Graphite sheets used for heat dissipation in electric vehicles face high contact thermal resistance due to surface hardness caused by high-temperature firing, which hinders efficient heat transfer between IGBTs and cooling members.
Innovation Solution
A graphite sheet is manufactured by stacking multiple graphite layers with a second layer exposed through the surface, and a surface portion is removed to achieve a soft enough surface with controlled shear strength, reducing contact thermal resistance and enhancing heat dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the graphite sheet is fired at high temperature (2600°C) to convert polymer film into graphite, then the graphite structure is formed and thermal conductivity is improved, but the surface hardness increases causing high contact thermal resistance
Solution Approach 1:
The invention applies different properties to different parts of the graphite sheet: the inner portion maintains high crystallinity and hardness for structural integrity and thermal conductivity, while the surface portion is treated to have lower hardness and higher deformability for better contact. This is achieved by controlling the firing process to create a gradient structure where the surface has different properties from the bulk material.
Solution Approach 2:
The invention changes the physical parameters of the graphite sheet surface by controlling the firing temperature profile and duration to create a surface layer with different hardness and deformability characteristics from the bulk material. The surface is designed to have lower apparent shear strength to enable better conformal contact while the bulk maintains high thermal conductivity.
2Strength
If the surface of the graphite sheet is made hard through high-temperature firing, then structural strength is improved, but contact thermal resistance increases preventing sufficient heat transfer
Solution Approach 1:
The graphite sheet is designed with spatially varying properties: the bulk material maintains high hardness and strength for structural integrity, while the surface layer is engineered to have lower hardness and higher deformability. This local differentiation allows the surface to conform to mating surfaces for low contact thermal resistance while the bulk provides necessary mechanical strength.
Solution Approach 2:
The graphite sheet is effectively segmented into two functional zones: a bulk region for structural support and thermal conduction, and a surface region for contact optimization. The surface portion is treated differently during firing to create distinct properties, allowing independent optimization of strength and contact characteristics.
3Ease of operation
If a grease is used to transfer heat from IGBT to cooling member, then heat transfer is smoothed, but thermal conductivity is insufficient and grease is gradually pushed out causing deterioration
Solution Approach 1:
The graphite sheet serves as an intermediary thermal conduction layer between the IGBT and cooling member. Unlike grease which is a fluid mediator that can be pushed out, the graphite sheet is a solid material that maintains stable thermal conductivity while providing smooth heat transfer through its deformable surface that conforms to mating surfaces.
4Strength
If the graphite sheet surface is too hard, then structural integrity is maintained, but sufficient contact with IGBT or cooling member cannot be achieved
Solution Approach 1:
The graphite sheet exhibits local quality differentiation where the bulk material maintains high strength and structural integrity, while the surface layer has reduced hardness and enhanced deformability. This allows the surface to deform and conform to the contours of IGBT or cooling member surfaces, ensuring sufficient contact area for effective heat transfer.
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 modified graphite sheet achieves lower contact thermal resistance and improved heat dissipation by allowing closer contact with IGBTs and cooling members, while maintaining sufficient strength and ease of handling.
Implementation Method 1
heat is transferred with a solid thermal conductive sheet such as a graphite sheet sandwiched
Implementation Method 2
turning a polymer film into graphite through pyrolysis. When turned into graphite, the polymer film is fired at as high a temperature as 2600° C.
Data Source
AI summary
The problem to be overcome by the present disclosure is to provide a graphite sheet with the ability to lower the contact thermal resistance at the surface of the graphite sheet. In a graphite sheet, a plurality of graphite layers are stacked one on top of another in a thickness direction. The plurality of graphite layers includes: a first graphite layer that forms an outermost layer of the graphite sheet; and a second graphite layer other than the first graphite layer. At least a part of the second graphite layer is exposed through a surface of the first graphite layer. The graphite sheet has an apparent shear strength equal to or greater than 0.1 MPa and equal to or less than 0.5 MPa as measured by SAICAS method from a depth of 0.5 m to a depth of 19 m.