Patent Application
20080085403
As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “substantially” may not to be limited to the precise value specified, in some cases. All ranges in the specifications and claims are inclusive of the endpoints and independently combinable. Numerical values in the specifications and claims are not limited to the specified values and may include values that differ from the specified value. Numerical values are understood to be sufficiently imprecise to include values approximating the stated values, allowing for experimental errors due to the measurement techniques known in the art and/or the precision of an instrument used to determine the values.
As used in this specification and the appended claims, the singular forms “a,” “an” and, “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pyrolytic graphite part” or “pyrolytic graphite particle” includes one or more of such parts or particles.
As used herein, the term “part” is used interchangeably with “particle” in referencing PG particles for use as the hyper-conductive media in the heat transfer composite. As used herein, the term hyper-conductive media refers to pyrolytic graphite parts having thermal conductive property ranging from 300-1850 W/m-° K (or theoretical thermal conductivity) in the ab direction
Heat Transfer Composite: As used herein, the term “pyrolytic graphite” may be used interchangeably with “thermal pyrolytic graphite” (“TPG”), “highly oriented pyrolytic graphite” (“HOPG”), or compression annealed pyrolytic graphite (“CAPG”), referring to graphite materials having an in-plane (a-b direction) thermal conductivity ranging from 300 W/m-° K for pyrolytic graphite, to 1800 W/m-° K for TPG, HOPG, or CAPG.
Pyrolytic graphite (PG) is a unique form of graphite manufactured by decomposition of a hydrocarbon gas at very high temperature in a vacuum furnace. The result is an ultra-pure product which is near theoretical density and extremely anisotropic, with an in-plane thermal conductivity of 300 W/m-° K in the ab direction and 3.5 W/m-° K in the c direction. TPG, HOPG, or CAPG refers to a special form of pyrolytic graphite consisting of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers or a high degree of preferred crystallite orientation. In one embodiment, TPG has an in-plane thermal conductivity greater than 1,500 W/m-° K and <20 W/m-° K for the c direction. In another embodiment, TPG has a thermal conductivity of greater than 1,700 W/m-° K for its (a-b) planar surface.
Pyrolytic graphite (“PG”) is commercially available from GE Advanced Ceramics of Strongsville, Ohio. Pyrolytic graphite material is being commercialized in standard or custom sizes and or forms for applications ranging from thermal insulators, rocket nozzles, ion beam grids, etc. In the manufacture of pyrolytic graphite parts, there are bits and pieces of reject PG parts due to dimensional errors and or damages in processing. There are leftover PG parts from machining/drilling. There are also PG parts that are delaminated or in un-useable sizes, etc. The parts are typically discarded, and of random sizes and shapes. As used herein, the typically discarded parts will be generally referred to as “recycled PG parts.” The recycled PG parts have sizes ranging from a few microns to ten inches (in the longest dimension) in random orientation. The recycled parts have shapes ranging from random chunks or pieces to specific geometric shapes of cubic, cylindrical, half-cylindrical, square, ellipsoidal, half-ellipsoidal, wedges, and the like.
In one embodiment, the heat transfer composite of the invention employs recycled PG parts as the hyper-conductive media. In another embodiment, commercially available or “virgin” PG materials can be used as the hyper-conductive media. In a third embodiment, mixtures of recycled and virgin PG materials are used. In one embodiment where recycled parts are used, the parts may be broken into pieces and sorted into suitable size and shape categories, e.g., PG parts of less than 0.5 cm in longest dimension, PG parts of general chunk sizes of at least 1″ in the shortest dimension, PG parts that are of general elongated sizes (as a strip), etc. The sorting/sizing may be done manually, or it can be done using classifiers known in the art. In one embodiment, mixtures of PG parts with different size and shape distributions may be used to maximize the isotropic property of the heat transfer composite.
In one embodiment, the pyrolytic graphite parts are present in an amount greater than about 50% by volume of the heat transfer composite. In some embodiments, the pyrolytic graphite can be present in an amount of from about 30% to about 95% by volume. In yet other embodiments, the pyrolytic graphite can be present in an amount of from about 40% to about 60% by volume.
The pyrolytic graphite parts are incorporated in a consolidated mass of a matrix comprising a non-carbonaceous isotropic material, e.g., a metal matrix including a variety of metals and alloys, or other materials that can be diffusion bonded. As used herein, diffusion bonded or diffusion bonding means a process by which two interfaces or two materials, e.g., the pyrolytic graphite parts and the matrix material, can be joined at an elevated temperature using an applied pressure for a time ranging from a few minutes to a few hours, thus holding the plurality of pyrolytic graphite parts in a consolidated mass. In one embodiment, the elevated temperature means a temperature of about 50%-90% of the absolute melting point of the matrix material.
In one embodiment, the non-carbonaceous isotropic material comprises a metal matrix containing at least 50% aluminum by volume. In another embodiment, the metal matrix consists essentially of aluminum, which has proven effective for use as a metal matrix due to its excellent ability to wet pyrolytic graphite. As molten aluminum is infiltrated about pyrolytic graphite elements, the aluminum wets the pyrolytic graphite and forms aluminum carbide while chemically bonding with the pyrolytic graphite. As a result, any voids or air pockets within the heat transfer composite will be significantly minimized, if not eliminated altogether. The minimization of air pockets or voids within the heat transfer composite is an important consideration in that the presence of even very small pores within the heat transfer composite can significantly reduce an overall thermal conductivity of the heat transfer composite. Accordingly, in one embodiment, the heat transfer composite of the present invention is substantially free of voids or unfilled interstitial spaces between pyrolytic graphite particles.
Aluminum has a melting point of about 660° C., which generally is low enough to be used in the process to make the heat transfer composite of the invention. In some embodiments, aluminum alloys are used as the matrix of the heat transfer composite to further reduce its melting point. In one embodiment, the metal matrix comprises an aluminum alloy, e.g., an Al—Mg alloy with a melting point of about 450° C. (at the eutectic composition with about 36% wt. Mg). In a second embodiment, the metal matrix comprises an Al—Si alloy with a melting point of about 577° C. (at the eutectic composition with about 12.6% wt. of Si).
In one embodiment, the use of copper in the aluminum binder can also result in increasing the overall thermal conductivity of the heat transfer composite, which can, of course, increase the efficiency of a heat transfer device in removing heat from a heat source. In another embodiment, the matrix comprises an Al—Cu alloy with 32 wt % Cu, for a melting point of about 548° C. Other metals can also be used to increase the overall heat thermal conductivity of the heat transfer composite. For example, a metal matrix of Al—Ag, with Ag at about 26 wt %, melts at about 567° C., and provides an increase in thermal conductivity. Another example is Al—Li, with Li at about 7 wt %, melts at about 598° C.
In addition to utilizing an aluminum alloy with a relatively low melting point, in one embodiment, the metal matrix can also include various elements that reduce an overall melting point of the matrix. Suitable elements for reducing the melting point of the matrix include Mn, Ni, Sn and Zn. In another embodiment, other materials of interest that can be used in the composite of the invention include but are not limited to Fe, Cu, alloys thereof, and the like.
Process for Making the Heat Transfer Composite: In one embodiment as illustrated in
In one process embodiment, a desired amount of pyrolytic graphite parts is placed in a heated mold. In the next step, molten metal (such as aluminum)/alloy (or another suitable non-carbonaceous isotropic material) is applied to the pyrolytic graphite parts and substantially fill voids between the parts, forming a consolidated mass. In yet another embodiment for a variable thermal conductivity gradient in the matrix, the addition of pyrolytic graphite parts and molten aluminum can be done in stages wherein the size, shape, and or amount (concentration) of pyrolytic graphite parts added in each stage are controlled to vary the thermal conductivity in various sections of the heat transfer matrix.
In one embodiment, after a consolidated mass or matrix is formed, the mass is then machined, cut or sliced into desired thicknesses or shapes depending on the final application and the desired thermal conductivity gradient of the starting consolidated mass. In one embodiment, the heat transfer matrix is cut into strips or sheets having a thickness ranging from 0.5 mm to 2 cm. In a second embodiment, sheets are formed from the consolidated heat transfer matrix having a final thickness of 1 mm to 0.5 cm.
In another process embodiment, a heat transfer composite as illustrated in
The number of non-carbonaceous sheets such as aluminum, the thickness of the sheets, or pallets, the amount, the size, shape, and distribution of the pyrolytic graphite parts in between the sheet can be varied depending on the final application—as well as the type of pyrolytic graphite parts available. In one embodiment, the pyrolytic graphite parts are layered between the sheets such that there is a least one pyrolytic graphite part for each layer of aluminum sheet.
In one embodiment, sheets of aluminum foil having a thickness of 10 microns and 2 mm are used. In a second embodiment, aluminum sheets having a thickness of 5-25 mils are used. In a third embodiment, the composite comprising a plurality of layers has a total thickness of at least 10 mils. In a fourth embodiment, an appropriate amount of aluminum sheets are used for a final composite matrix having a final thickness of 1 mm to 0.5 cm. In one embodiment, the aluminum sheets have a nominal thickness ranging from 1/32″ to 5/18″. In a second embodiment, the aluminum sheets are 0.025″ thick.
As illustrated in
In yet another embodiment of a layered matrix of
In one embodiment with a random distribution of pyrolytic graphite parts in a non-carbonaceous isotropic material matrix, the (a-b) planar surface of the pyrolytic graphite parts in the composite is random, i.e., not uniform/parallel as with the prior art heat management solutions employing pyrolytic graphite.
In one embodiment with a random distribution of pyrolytic graphite parts in a non-carbonaceous isotropic material matrix, the heat transfer composite of the invention has a relatively uniform thermal conductivity, ranging from 100-1000 W/m-° K, in any direction of the composite. As used herein, “relatively uniform” means the thermal conductivity between any two spots within the matrix varies less than 25%. In one embodiment, the heat transfer composite has a thermal conductivity variation of less than 10% between any two spots within the matrix.
In one embodiment wherein the pyrolytic graphite makeup (concentration, size, shape, distribution, etc.) is carefully controlled, the thermal conductivity in the composite can be tailored to aid in matching a thermal expansion coefficient of a particular heat source. This can be beneficial in that the heat spreader and the heat source can expand and contract at similar rates to avoid compromising the bond between the heat source and the heat spreader.
Applications of Heat Transfer Matrix: The heat transfer matrix of the invention can be used in connection with a variety of heat sources (none of which are shown in the figures, as examples of such heat sources typified by CPUs are well known to those of ordinary skill in the art). While not so limited, heat spreaders of the present invention can be used to transfer or conduct heat from a variety of appliances where a relatively low-cost heat spreader that can be easily formed into large shapes is desired.
In addition to the applications disclosed herein, the present invention can be used in connection with a cooling system for transferring heat away from a heat source.
Applications of the Heat Transfer Composite: The heat transfer matrix of the invention can be used in any devices, systems, and methods for transferring heat away from a heat source. In one embodiment, the heat transfer matrix is used to form heat spreaders for use in electronic and/or integrated circuit (“IC”) devices such as microprocessors, memory devices, etc.
Examples are provided herein to illustrate the invention but are not intended to limit the scope of the invention.
Pyrolytic graphite (TPG) parts from GE Advanced Ceramics of Strongsville, Ohio, are poured into a steel die sprayed with a boron nitride release agent. Molten Al—Si, with a melting point of about 577° C. is poured into the mold while simultaneously pressed and mixed with the parts by a rotating steel mixer. The molten alloy, which wetted both the pyrolytic graphite parts, filled substantially all voids between parts to create a consolidated mass heat spreader. The measured thermal conductivity of the resultant heat spreader is about 600 W/m-° K. It should be noted that the performance of the board can be designed such a way that the ultimate bulk or local thermal performance can be tailored by varying the hyper-conductive media ratios.
While the invention has been described with reference to a preferred embodiment, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. It is intended that the invention not be limited to the particular embodiment disclosed as the best mode for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. All citations referred herein are expressly incorporated herein by reference.
This application claims the benefits of U.S. Patent Application Ser. No. 60/828647 filed Oct. 10, 2006, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60828647 | Oct 2006 | US |
