US20100078151A1

US20100078151A1 - Ceramic heat pipe with porous ceramic wick

Info

Publication number: US20100078151A1
Application number: US12/242,051
Authority: US
Inventors: William D. Koenigsberg; Adam M. Scotch; David W. Hamby; John H. Selverian; David Wentzel
Original assignee: Osram Sylvania Inc
Current assignee: Osram Sylvania Inc
Priority date: 2008-09-30
Filing date: 2008-09-30
Publication date: 2010-04-01
Also published as: EP2332172A4; CN102171819A; WO2010039358A3; EP2332172A2; WO2010039358A8; WO2010039358A2; JP2012504339A; KR20110063844A; CA2738072A1

Abstract

A heat pipe for transporting heat from light emitting elements includes a sealed body made of a non-porous ceramic, a vapor channel inside the body that extends between two heat transfer locations spaced apart on an exterior surface of the body, a ceramic wick inside the body that extends between the two heat transfer locations, and a working fluid that partially fills the vapor transport channel. In a method of making this heat pipe, the body and wick are desirably formed together as a seamless monolithic structure made of the same ceramic material. Using a ceramic makes the heat pipe corrosion resistant and allows electrical components like LEDs to be mounted directly on the body because the ceramic is a dielectric.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to a heat pipe that transports heat away from heat producing bodies, such as light emitting diodes (LEDs).
LEDs generate heat as well as light and the heat is desirably transported away from the LEDs because elevated LED-junction operating temperatures (e.g., above about 115° C.) adversely affect light output. Heat can be transported away from the LEDs by mounting the LEDs on a substrate (heat sink) of sufficient thermal conductivity and appropriate surface area from which to dissipate the heat. However, conventional metal and ceramic substrates often do not have sufficient thermal conductivity, especially when many LEDs are placed in a small area. Consequently, a supporting substrate with improved thermal conductivity suitable for use with LEDs is desired.
A heat pipe is a heat transfer device that can transport large quantities of heat, significantly more than conventional metal and ceramic heat sinks, from one heat transfer location on the heat pipe to another heat transfer location on the heat pipe. The heat pipe is hollow and sealed closed, and contains a wick and a working fluid. Inside the heat pipe, the working fluid vaporizes at the hotter location and the working fluid vapor condenses at the cooler location. The condensed working fluid is impelled from the cooler location back to the hotter location by the capillary action of the wick.
Heat pipes can take various shapes, with a flat heat pipe being disclosed in U.S. Patent Application Publication 2007/0295494 (Celsia Technologies Korea). This heat pipe includes two spaced-apart flat plates having therebetween a hollow vapor channel between two porous fluid channels that extend between two heat transfer locations. The plates are comprised of a board material which has sufficient rigidity that can protect the inner structure, such as aluminum, titanium, plastic, metalized plastic, graphite or other metal material and plastic combinations; preferably, a copper board having a high thermal conductivity can be used. The capillary wick is formed with a plane sheet type structure, which can be a synthetic fiber having a porous structure or a woven body manufactured by weaving wires. This flat heat pipe has been used to transport heat from LEDs in an LED lamp.
It is desirable to provide a heat pipe that is less susceptible to internal and external corrosion than the metal heat pipes of the prior art and on which LEDs can be mounted directly.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel heat pipe made entirely of ceramic that resists corrosion and on which electrical components such as LEDs can be mounted directly.
A further object of the present invention is to provide a novel heat pipe with a body made of a non-porous ceramic, said body being sealed and having a ceramic wick inside the body that extends between two heat transfer locations spaced apart on an exterior surface of said body, a vapor transport channel inside the body that extends between said two heat transfer locations, and a working fluid that partially fills said vapor transport channel.
A yet further object of the present invention is to provide a heat pipe where the body and wick together are a seamless monolithic structure made of the same ceramic material.
Another object of the present invention is to provide a novel method of making this heat pipe that includes providing a body of a non-porous ceramic, providing a ceramic wick and a vapor transport channel inside said body, said wick and vapor transport channel extending between two heat transfer locations spaced apart on an exterior surface of said body, evacuating said body, providing a working fluid inside said body that partially fills the vapor transport channel, and sealing said body closed.
Yet another object of the present invention is to provide a novel method of making this heat pipe where the body and wick are provided from the same ceramic material and are formed together as a seamless monolithic structure made of the same ceramic material.
These and other objects and advantages of the invention will be apparent to those of skill in the art of the present invention after consideration of the following drawings and description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of an embodiment of the heat pipe of the present invention showing where LEDs may be located.

FIG. 2 is a cross-section through line II-II of the embodiment of FIG. 1.

FIG. 3 is a corresponding cross-section of an alternative embodiment of the heat pipe of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference now to FIGS. 1 and 2, one embodiment of a heat pipe 10 of the present invention includes a hollow sealed body 12 made of a non-porous ceramic, a central vapor transport channel 14 that extends between two heat transfer locations 16, 16′ spaced apart on an exterior surface of the body 12, a ceramic wick 18 on the inside wall 7 of the body 12 and surrounding the vapor transport channel 14, the ceramic wick also extending between the two heat transfer locations 16, 16′ and a working fluid 20 inside the body 12 that partially fills the vapor transport channel 14. A thermal load comprising heat-emitting bodies, such as LEDs 22, may be mounted directly on the ceramic body at one of the heat transfer locations 16′ and the other heat transfer location 16 may be exposed to a cooler temperature so that operation of the heat pipe is conventional. The term “non-porous ceramic” as used herein means that the ceramic that forms the body of the heat pipe is sufficiently dense that it is impermeable with respect to the working fluid and vapor contained inside the heat pipe. It does not necessarily mean that the ceramic is 100% dense, i.e., has no pores.
Preferably, the wick 18 is porous and integral with the body 12, and formed in situ. That is, the body 12 and wick 18 together are a seamless monolithic structure made of the same ceramic material with the wick being formed inside the body when the body is formed. Alternatively, the wick can be formed outside the body and inserted into a hollow interior space in the body before it is sealed closed. The wick desirably is made entirely of porous ceramic with plural interconnected pores that generate a capillary action within the wick.
The vapor transport channel 14 extends between the two heat transfer locations 16, 16′ so that in operation the vaporized working fluid (vaporized by the heat from the LEDs 22 at heat transfer location 16′) moves through the vapor channel to heat transfer location 16 where the vapor condenses.
It is essential that a continuous vapor transport channel be maintained through the heat pipe between the thermal load and the condensation zone(s) in order to allow vapor to move freely between the two regions. The pressure gradient inside the heat pipe impels the vapor from the ‘hot spot’ toward other locations where condensation can occur at a slightly lower temperature. The form of the open space is not limited to any particular geometry. Preferred vapor transport channel configurations include the single, central channel 14 as shown in FIG. 2 or a series of smaller channels 25 spaced throughout the porous wick as shown in FIG. 3. Although the vapor transport channels in the embodiments shown in FIGS. 2 and 3 extend linearly through the body, they need not run in a straight line. Curved or meandering channels are permitted provided that the vapor transport function is maintained.
The wick 18 conveys the condensate back to the heat transfer location 16′ by capillary action and the cycle is repeated. The working fluid 20 only partially fills the vapor transport channel(s) inside the heat pipe so there is open space for vapor transport between the heat transfer locations. The interior of the heat pipe preferably is evacuated before the working fluid is introduced in order to maximize the efficiency of the heat transfer as residual gas inside the heat pipe will interfere with the vapor transport within the device. Preferred working fluids include water, alcohols (e.g., methanol), ammonia and freons. The choice of working fluid will depend on the useful temperature range, environmental compatibility, and cost.
In the embodiment shown in FIG. 2, the wick 18 is made entirely of a porous ceramic that is directly on an interior wall 7 of the body 12 and surrounds a single, central vapor transport channel 14. The porous ceramic has plural interconnected pores that extend continuously between the two heat transfer locations to provide a wicking action for moving the working fluid between the heat transfer locations. Alternatively, as shown in FIG. 3 the wick 18′ fills the interior of the heat pipe and a series of open vapor transport channels 25 are spaced throughout the ceramic wick and extend between the heat transfer locations.
The term “interconnected pores” also includes elongated capillaries made after formation of the wick as well as pores in the wick material that appear during formation of the wick. The interconnected pores must sized and sufficiently interconnected such that the working fluid can be transferred by capillary action, i.e., ‘wicked’ from the condensate zone(s) to the region where the thermal load exists. The capillary action in combination with the vapor transport completes the working cycle of the heat pipe, i.e., heat is removed from the thermal load by vaporizing the working fluid, the heat is then removed from the vapor by condensation at a location remote from the thermal load, and the condensed working fluid is re-supplied to the thermal load region by the capillary action of the wick.
A ceramic is defined herein as an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently processed by the action of heat applied to the material, e.g., aluminum oxide, aluminum nitride, and silicon dioxide. Preferably, the ceramic is aluminum oxide (alumina).
Because the ceramic is a dielectric, the surface of the body 12 is not electrically conductive so that LEDs and other electrical components can be mounted directly on the body at a heat transfer location and remain electrically isolated. Further, since the body and wick are ceramic and since there are no metal parts, the heat pipe resists corrosion and galvanic reactions associated with dissimilar metals.
The ceramic heat pipe of the present invention can be manufactured by fabricating the body from a hollow circuit board made of an appropriate ceramic (e.g., glass or alumina). The body may be preformed from a green ceramic using conventional ceramic techniques, such as injection molding, extrusion, dry pressing, or slip casting. The body 12 also may be formed of ceramic parts that are joined together, as shown in FIG. 3, with a suitable adhesive, e.g., a glass frit. The porous wick may be formed in situ inside the body or by inserting the porous wick inside the hollow interior of the body. After evacuating the interior of the heat pipe body and introduction of the working fluid, the body is sealed closed conventionally.
The preferred method of forming the porous wick is an in situ sol-gel process. A sol-gel process uses organic precursors, which are first formed into a gel and then pyrolyzed or decomposed at high temperatures to form a porous ceramic material. In the present invention, the interior walls of the hollow interior of the body are coated with the organic gel precursor that is pyrolyzed to form the porous wick structure. The entire part is then fired to form a monolithic structure made up of the outer ceramic body that is dense and impermeable and the inner ceramic wick that is porous (this is shown in FIG. 2) with interconnected pores that extend between the two heat transfer locations.
Another method is to insert plural ceramic spheres into the hollow interior of the ceramic body to create a packed bed. The spheres are then fused together to the inner walls of the hollow interior of the body by heating to induce viscous sintering. The gaps between the spheres connect to produce the interconnected pores that extend through the wick between the two heat transfer locations.
A still further method is to extrude the entire vessel from one ceramic material so that the final part contains an inner array of open channels as shown in FIG. 3 that extend between the two heat transfer locations. This technology has been used to make autocatalyst support structures (introduced by Corning). A further step generates the interconnected pores in the ceramic wick.
The interconnected pores can also be prepared by introducing fugitive material into a green ceramic that is to form the wick. A polymer (e.g., latex or polystyrene spheres of controlled size), graphite, or other fugitive material can be embedded in the green ceramic in the form of particles, fibers, or continuous foam structures. The body and wick may be made from the same green ceramic material with the fugitive material inserted into the wick part. The fugitive material decomposes at an early part of a sintering cycle, prior to necking of the ceramic particles, consequently evolving gas and thus leaving interconnected pores that extend through the wick between the two heat transfer locations. The pores are too large and too stable to be eliminated during the final sintering step. This a technique that is known for making porous ceramic structures for high temperature or corrosive filtration.
A yet further, and perhaps simpler, method is incomplete sintering. The body is formed from a first green ceramic part having either a first density or first particle size distribution, and the wick is formed by inserting a second green ceramic part into the hollow interior of the first green ceramic part, where the second green ceramic part has either a second density lower than the first density or second particle size distribution larger than the first particle size distribution. The assembly is sintered so that the first green ceramic part is completely sintered and the second green ceramic part is incompletely sintered. This will provide the interconnected pores in the second green ceramic part that extend through the wick between the two heat transfer locations.
While embodiments of the present invention have been described in the foregoing specification and drawings, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawings.

Claims

1. A heat pipe comprising:

a body made of a non-porous ceramic, said body being sealed and having a ceramic wick inside the body that extends between two heat transfer locations spaced apart on an exterior surface of said body;

a vapor transport channel inside the body that extends between said two heat transfer locations; and

a working fluid that partially fills said vapor transport channel.

2. The heat pipe of claim 1, wherein said wick is made of a porous ceramic with plural interconnected pores that extend continuously between said two heat transfer locations.

3. The heat pipe of claim 2, wherein said body and said wick together are a seamless monolithic structure made of a same ceramic material.

4. The heat pipe of claim 1, wherein said wick is made of a porous ceramic that is directly on an interior wall of said body and surrounds said vapor transport channel, said porous ceramic having plural interconnected pores that extend continuously between said two heat transfer locations.

5. The heat pipe of claim 4, wherein said body and said wick together are a seamless monolithic structure made of a same ceramic material.

6. The heat pipe of claim 1, in combination with a light emitting diode that is mounted directly on said body at one of said two heat transfer locations.

7. The heat pipe of claim 1, wherein the body contains multiple vapor transport channels that are spaced throughout the wick and extend between the two heat transfer locations.

8. A heat pipe comprising:

a body made of a non-porous alumina ceramic, said body being sealed and having a ceramic wick inside the body that extends between two heat transfer locations spaced apart on an exterior surface of said body, the ceramic wick being made of a porous alumina ceramic having interconnected pores that extend continuously between said two heat transfer locations;

a working fluid that partially fills said vapor transport channel.

9. The heat pipe of claim 8, wherein the body and wick are integrally formed.

10. The heat pipe of claim 8, wherein the body contains multiple vapor transport channels that are spaced throughout the wick and extend between the two heat transfer locations.

11. The heat pipe of claim 8 wherein the wick surrounds the vapor transport channel.

12. A method of making a heat pipe comprising the steps of:

providing a body of a non-porous ceramic;

providing a ceramic wick and a vapor transport channel inside said body, said wick and vapor transport channel extending between two heat transfer locations spaced apart on an exterior surface of said body;

evacuating said body;

providing a working fluid inside said body that partially fills the vapor transport channel; and

sealing said body closed.

13. The method of claim 12, wherein said body and said wick are provided from a same ceramic material and are formed together as a seamless monolithic structure made of the same ceramic material.

14. The method of claim 12, wherein said ceramic wick is provided by inserting plural ceramic spheres into said hollow interior and heating said spheres to induce viscous sintering that produces interconnected pores that extend through said wick between said two heat transfer locations.

15. The method of claim 12, wherein said body and said wick are provided by extruding said body and said wick together from a same ceramic material, and further comprising the step of creating interconnected pores through said wick that extend between said two heat transfer locations.

16. The method of claim 12, wherein said body and said wick are provided by forming said body and said wick together from a same green ceramic material, inserting fugitive material into said wick, causing said fugitive material to decompose to provide interconnected pores that extend through said wick between said two heat transfer locations, and sintering said green ceramic material.

17. The method of claim 12, wherein said body is provided by preforming a green ceramic body, and said wick is provided by coating interior walls of said body with an organic gel precursor and pyrolyzing the precursor to form a porous structure, and further comprising the step of firing the green ceramic body and precursor to form a monolithic structure of said body and said wick, said wick having interconnected pores that extend between said two heat transfer locations.

18. The method of claim 12, wherein said body is provided by providing a first green ceramic part having one of a first density and first particle size distribution, and wherein said wick is provided by inserting a second green ceramic part into said body, said second green ceramic part having one of a second density lower than the first density and second particle size distribution larger than said first particle size distribution, and further comprising the step of completely sintering the first green ceramic part and incompletely sintering the second green ceramic part so that said second green ceramic part has interconnected pores that extend through said wick between said two heat transfer locations.