Heat exchange – Heat transmitter
Reexamination Certificate
1999-11-08
2001-11-06
Flanigan, Allen (Department: 3743)
Heat exchange
Heat transmitter
C361S704000
Reexamination Certificate
active
06311769
ABSTRACT:
BACKGROUND
The present invention relates generally to heat conducting materials, and more particularly, to a thermal interface material that uses thermally conductive fiber disposed in a low- or non-outgassing polymer matrix material adapted for use in spacecraft applications.
There are several generally used classes of thermal interface materials. These include loaded organic materials, thermal gasket type materials, and thermal grease. Several thermal interface materials are discussed in the “Satellite Thermal Control handbook”, for example, by David G. Gilmore, at pages 4-52 to 4-58.
Conventional loaded organic thermal interface materials use a highly loaded (organic) carrier with many conductive particles. With such conventional interface materials, the bond line thickness is minimized in order to maximize thermal performance. Unfortunately, in loaded silicone interface materials, for example, a thermal bottleneck exists because of the conductance through the matrix and at (many) particle-to-particle contact points. These conventional interface materials work relatively well when thin bond lines and flat surfaces are present.
To provide the desired thermal performance, the thermal gasket type interface materials must be subjected to high pressure, which impacts structural loads and can cause bowing of mounting panels, for example. Separation (zero pressure) may occur at some distance from bolts that secure the gasket. Consequently, use of such gaskets is generally limited to small scale applications, where bolt-to-bolt spacing is a relatively small distance.
The thermal grease interface materials provide relatively good thermal performance. However, the thermal grease interface materials have been found to be major sources of contamination, and are not generally used in spacecraft applications.
Prior art conducting materials also include Gelvet™, a material manufactured by Johnson Matthey, Inc. and AF Inter Connector material manufactured by Shin Etsu, for example. Both of these products use carbon fibers in a silicone matrix with the carbon fibers disposed normal to the interface surface. the AF material acts as an electrical conductor between two devices, while the Gelvet material is designed to conduct heat between heat sinks and heat generating devices. Both of these solutions are designed for electrical non-space applications and do not use a low- or non-outgassing polymer matrix material. It has also been found that the Gelvet material may also exhibit fiber damage during cutting of the graphitized carbon fiber and may have a less consistent distribution of fiber length.
Accordingly, it is an objective of the present invention to provide for an improved thermal interface material that uses carbon based fiber in a low- or non-outgassing polymer matrix for use in spacecraft applications.
SUMMARY OF THE INVENTION
To accomplish the above and other objectives, the present invention provides for an improved thermal interface material for use in spacecraft heat dissipation applications. The thermal interface material comprises highly thermally conductive (carbon based) fibers contained, embedded, or immersed, in a low- or substantially non-outgassing polymer matrix material or gel encapsulant.
The low- or non-outgassing polymer matrix material comprises high thermal conductivity, small diameter fibers, such as carbon fibers for example, that are disposed substantially normal to interface surfaces, and protrude above at least one of the surfaces of the low- or non-outgassing polymer matrix material. The thermally conductive fibers adjust to and contact surfaces to which they are attached and conform to macroscopic non-planarity of the surfaces. The thermally conductive fibers act as continuous conductors of heat from surfaces to which they are attached.
For some applications, the thermally conductive fibers may be cut in a manner that reduces damage. This is achieved by cutting green state ungraphitized fibers to length, instead of cutting graphitized thermally conductive fibers. Heat treatment of the carbon based fibers may be done after the green state thermally conductive fibers are cut to length.
The thermally conductive fibers, such as carbon fibers, for example, in the green state (ungraphitized) are cut to a desired length and are then graphitized. The graphitized thermally conductive fibers are then embedded in one or more layers of low- or non-outgassing polymer matrix material having a desired thickness. The graphitized thermally conductive fibers protrude from an interface surface of the low- or non-outgassing polymer matrix material by a predetermined amount. The graphitized thermally conductive fibers and the polymer matrix material securing them are then cured for a prescribed time, period and temperature depending on the selected polymer matrix material and the desired end use properties.
The polymer matrix material used in the thermal interface material is chosen to pass stringent spacecraft environmental requirements. For example, the polymer matrix material is substantially non-outgassing which means that it contains volatile condensable material (VCM) of less than 0.1 percent and has a total mass loss (TML) of less than 1.0 percent per an ASTM E
5
95 procedure. The prior art thermal interface materials discussed in the Background section do not meet these criteria. Methods for introducing the graphitized thermally conductive fibers into the polymer matrix material include flocking, mixing, infiltration, electrostatic, preimpregnation, wicking, or vacuum transfer, for example.
By using green state thermally conductive fibers, cost is reduced since the green state fibers are significantly less expensive to purchase and are easier to cut to length with less damage. The graphitized thermally conductive fibers may be heat treated to enhance the properties that provide high thermal conductivity.
The thermal interface material improves heat transfer between components and/or devices that generate heat and an external environment, such as is provided by a radiator panel, or other heat radiating device, for example, employed on a spacecraft. Also, the polymer matrix material used in the thermal interface material exhibits low outgassing, thus enabling the thermal interface material to be used in a space environment.
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Bonneville W. Scott
Cooney John E.
Peck Scott O.
Flanigan Allen
Float Kenneth W.
Space Systems Loral, Inc.
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