WO1999051069A2

WO1999051069A2 - Fiber heat sink and fiber heat exchanger

Info

Publication number: WO1999051069A2
Application number: PCT/US1999/004282
Authority: WO
Inventors: Michael M. Ohadi; Serguei V. Dessiatoun; Igor Ivakhnenko
Original assignee: University Of Maryland
Priority date: 1998-03-30
Filing date: 1999-03-30
Publication date: 1999-10-07
Also published as: WO1999051069A3; AU3181999A

Abstract

A fiber heat sink and a fiber heat exchanger utilize the high thermal conductivity and the high surface area achieved by fibers, such as carbon fibers (10), (43), for enhanced cooling of electronic components (12, 13) and other devices. The utilization of fiber technology provides an inexpensive, durable, and effective method of heat transfer and exchange.

Description

FIBER HEAT SINK" AND FIBER HEAT EXCHANGER BACKGROUND OF THE INVENTION: Field of the Invention:

The present invention is directed to cooling of electronic devices and electronic boards, and other components which require effective heat transfer, and also to heat exchange where heat is conducted from a hot medium such as fluid to a cooler medium. Description of the Related Art:

Cooling of electronic components and other elements is typically performed through the use of heat sinks, which are typically blocks of aluminum or other highly conductive metals which are attached to an electronic component, and which provide an increase in surface area through the utilization of "fins" to radiate or convect (or a combination of the two) heat away from the heat source, and allow the heat to be dissipated in a fluid such as air or other appropriate medium in order to keep the component from overheating. In integrated circuits, for example, heat is generated within a silicon body or chip by electric current flowing through internal circuit resistances. The heat spreads throughout the chip, causing thermomechanical expansion, stresses at wire-bond joints, solder joints, and other mechanical joints. Significant temperature change promotes all types of failures, including circuit degradation, oxide breakdown, and leakage current. Present integrated circuit packaging technology is such that heat conductivity between the chip and the package is very low, thereby resulting in ineffective cooling. As power consumption increases due to density increases in integrated circuit technology, heat intensity becomes more and more of a problem. Current packaging technology utilizes silicon oxide (SiO₂) or aluminum nitride (AIN) as an insulating dielectric, and metal-ceramic configuration for removal of heat produced by electronic circuitry in a silicon crystal. However, thermal conductivity of the metal and the particular dielectrics used in integrated circuits is relatively poor and creates numerous limitations with respect to cooling strategies. SUMMARY OF THE INVENTION:

The present invention takes advantage of the very high thermal conductivity of carbon fiber, which can be as high as 2000 W m ¹ K ¹. Silicon carbide (SiC), having a thermal conductivity of approximately 200 W m^"1 K^"1 can be combined with carbon fiber to create a silicon carbide-carbon composite material, in combination with a carbon fiber braid construction, to improve heat dissipation in electronic components. The present invention, therefore, utilizes a substrate or base made of silicon or silicon carbide with the base being either attached to or actually part of an integrated circuit chip. A carbon fiber braid is reaction bonded to this base, with a plurality of carbon fibers extending therefrom. Through a sintering process, a unique carbon fiber heat sink is formed. The invention is also applicable for use with other types of fibers, such as sapphire, or other fibers having high thermal conductivity. Another embodiment of the invention is a heat exchanger utilizing a plurality of carbon fibers configured with a plurality of input channels and output channels to effectively cool heated air entering the input channels. A further embodiment of the heat exchanger utilizes electrodes in combination with the carbon fibers in the heat exchanger in order to take advantage of the agitating effects provided by the electro hydro dynamic (EHD) principle. Through a direct interaction of an electric field and electric charges and dipoles embedded in a fluid or other media, it has been shown that secondary motions are created near the heat transfer surface to result in a fluid pumping which can significantly enhance heat transfer coefficients. Motions created by the EHD principle cause substantial reductions of the thermal boundary layer thickness near the heat exchange surface in single-phase flows, substantial liquid vapor separation and additional turbulence in phase-change processes, give rise to this enhancement of heat transfer.

BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 illustrates an embodiment of the present invention;

Figure 2 illustrates a variation of the first embodiment; Figure 3 illustrates another embodiment of the present invention; Figures 4a and 4b illustrate a heat exchanger according to the present invention; Figure 5 illustrates a variation of the heat exchanger, utilizing EHD electrodes;

Figure 6 illustrates electric field-induced motions in a thermal boundary of an EHD configuration according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Figures 1 - 3 illustrate configurations of an electronic component heat sink wherein fibers 10, which can be carbon, sapphire, or other fibers, are impregnated into a substrate 11 or a substrate - crystal structure 12. For the sake of this discussion, fibers will be referred to as carbon fibers unless otherwise noted. The invention, however, is not limited to the specific materials disclosed herein. Referring to Figure 1 , substrate 11 , having carbon fibers 10 impregnated therein, is attached to a silicon or silicon carbide crystal 13 to effectively transfer heat therefrom. According to Figure 2, carbon fibers 10 are directly impregnated into silicon or silicon carbide crystal 12. The impregnation is performed, in both cases, by forming a unidirectional or multi-directional carbon/carbon fiber braid 10a from carbon fibers 10, and incorporating reaction bonded silicon carbide plus infiltrated free silicon with the carbon fiber braid, by infiltrating the silicon into a pre-formed silicon carbide/carbon powder green body, which is shaped, but not fired, with the carbon fibers in place. The assembly is then fired. The chemical reaction of silicon infiltration during the firing or sintering process allows a high percentage of free silicon over 10% thereof, to fill the pores created by the oxidation of carbon to carbon-monoxide and carbon-dioxide. The carbon fibers in the form of braid 10a will have an orientation in a direction of the heat flux. This method, therefore, allows the development of highly enhanced heat transfer surfaces using incorporated carbon fiber, which could not easily be arranged otherwise. By having the ends of the carbon fibers 10 looped outwards, and extending into a cooling fluid such as a liquid flow, or air/gas flow, provides a highly efficient heat transfer.

It should be noted that instead of carbon fibers for the heat transfer material, sapphire fibers can be used. Sapphire is especially valuable for cryogenic temperatures, since sapphire has a thermal conductivity at cryogenic temperatures which is higher than that of copper. Sapphire has a thermal conductivity of approximately 1350 W/mK at 40K, and copper has a thermal conductivity of approximately 1020 W/mK at 40K. Present invention is especially suitable for cryogenic applications, especially for transporting thermal energy from sensors. Some existing systems include heat pipes, capillary pumped loops (CPL) and heat conductor. The advantages of CPL over heat pipes include improved ground testability, faster diode shutdown, and lower reverse heat leaks, better mechanical isolation due to longer and flexible line, improved heat transfer coefficients and capabilities, reduced pressure drop and reduced vibration. However, the CPL systems have some significant disadvantages, particularly in that such systems require a start up procedure, and will deprime if vapor bubble or non-gases occupy the liquid line inside the evaporator. Furthermore, operating temperatures of a cryogenic bus is typically from 35 K to 60 K. A significant disadvantage which is shared among fluid - based cryogenic systems is that there is no working fluid which can satisfy this wide range of cryogenic temperatures. As a result, the operating conditions of IR sensors will not be optimum. Otherwise, the cryogenic bus must operate at temperatures which are significantly lower than which would otherwise be required. For example, if the sensor must be kept at 50K, and neon is the working fluid, then there must be a controllable resistance located between the sensor and the cryocooler to reduce temperature by 10-15 K. This imposed temperature reduction significantly degenerates the efficiency of the system. The thermal conductivity of sapphire at cryogenic temperatures as discussed above can help improve efficiency. However, a disadvantage of a sapphire cryogenic bus is that total conductance from the pulse-tube cold finger (55 K) to the sensor (58 K) is reduced by a factor of 20 due to several less conductive interfaces. By using sapphire fiber heat conductors in a cryogenic bus, inefficiency of the bus and associated interfaces can be reduced. Since sapphire fibers have parameters which are similar to silicon, silicon carbide, and silicon oxide, sapphire fibers can be incorporated into various types of substrate designs.

The embodiment of Figure 3 is similar to the embodiment of Figure 1 , but illustrates that the ends 10a of the carbon or sapphire fibers extend outwardly but do not loop back inwardly toward the substrate.

Figures 4a and 4b illustrates a heat exchanger which utilizes the heat transfer advantages of carbon fibers, as discussed above. Hot air input channels 40 receive heated air or other fluid which is intended to be cooled; cold air channels 41 receive cold air or fluid which is intended to receive heat from the hot air coming in to channels 40. The hot air and the cold air remain separated by the series of walls 42. Hot air and cold air channels may be further separated into discrete individual channels by walls 42, dependent upon the desired construction of the heat exchanger. Conventional heat exchangers are known to have a series of channels in which flows a first fluid (hot liquid, gas, or mixture) to be cooled, and second series of channels with a cooling fluid therein, with heat exchange occurring between cooled channels and cooling channels by proximity thereof. The present invention, however, utilizes fibers 43, such as carbon or sapphire, to conduct heat from hot air channels 40 into cold air channels 41. The superior heat conductive qualities of these fibers effectively removes heat from channels 40, and enables the cooling fluid in cold channels 41 to effectively remove heat which is conducted by the carbon fibers into cold channels 41. The fibers extend between channels 50 and 51 by being fixedly attached to and communicating through walls 44. The fibers are configured so as to prevent any fluid communication between the channels 50 and 51. The fibers which are used in the heat exchanger of Figures 4 and 4b can be protected against extremely hot fluid, such as that hotter than 900° F, by coating the fibers with, for example, a silicon carbide thin layer. The utilization of carbon or sapphire fibers for conducting heat in a heat exchanger provides highly efficient heat transfer in a light weight and compact device. Preliminary calculations show that a heat exchanger according to the present invention can provide a same level of heat exchange as a conventional metal heat exchanger with a weight reduction of approximately 14 times. It should be noted that the coating of the fibers discussed above with respect to the heat exchanger of Figure 4 can also be used with respect to the embodiments illustrated in Figures 1-3. Figure 5 illustrates a configuration for a heat exchanger which utilizes

EHD (electro hydro dynamic) principles. EHD applications are based upon a direct interaction of an electric field with electric charges and dipoles embedded in a fluid or other media. It is well known that a significant problem with heat exchange is thermal resistance of the boundary layer of working fluid or gas/air on a surface of a heat exchanging wall of a heat exchanger. EHD technology uses electric fields interacting with electric charges and dipoles embedded in a fluid to move the fluid, and to create secondary motions to the heat transfer surface, and to influence the shear stresses that contribute to the pressure drop in heat exchanger tubes. Ions and dipoles movement under an electric field causes media movements and stresses. For example, vapor bubbles are pushed out from a boiling liquid or stretched- suppressed by these electrical forces because dipole density inside bubbles are much lower than in a liquid. A Condensing liquid layer on a cold surface is sprayed out by highly non-uniform electrical field forces, freeing space for other vapor to condense. Ions and dipoles movement under an electrical field results in liquid pumping, as was known in the art.

The EHD effect has been demonstrated to result in significantly enhanced heat transfer coefficients. The cause for a problem with heat transfer in many applications is poor thermal conductivity of the near surface layer and vapor bubbles. An electrical field, applied in an appropriate way, produces motions inside liquid and fluid movement destroying the near heat exchange surface thermal boundary layer, and pushes out bubbles resulting in a substantial enhancement of heat transfer. Electric fields for heat transfer and pumping applications can be generated with a variety of electrode configurations, including transverse mesh electrodes, electrical field gradient generating electrodes, longitudinal traveling-wave electrodes, and others. The electrical field pumping effect has been attempted to be applied to practical problems such as the cooling of high-voltage equipment and microchip electronic systems. Collecting and transport-pumping of cryogenic fluids in a sensor-cooling loop can be accomplished by employing EHD technology. EHD has demonstrated potential for pumping with no need for mechanical pumps or moving parts. The combination of pumping and improving heat transfer capabilities makes the use of EHD a viable solution for High- performance electronics cooling applications. Due to its lack of moving parts, this technology is highly reliable. Low cost, low power consumption, and minimal maintenance are other benefits of this technique. Its applicability to heat transfer enhancements of industrially significant substances such as air, refrigerants, and aviation fuels has already been demonstrated. A detailed discussion of the EHD principle can be found in United States Patent No 5,769,155, which is hereby incorporated by reference.

Figures 5 illustrates a configuration wherein EHD technology can enhance heat exchange in an EHD carbon fiber heat exchanger. In a configuration which is similar to Figure 4, but more specifically illustrated in Figure 5, fibers 53 conduct heat from hot fluid flowing in hot channels 50 to a cold fluid flowing in cooling channels 51. The configuration of Figure 5 serves the purpose of providing rapid transfer of heat with a tactical and easy- to-manufacture methodology. The superior heat transfer capabilities of the fibers, as discussed above, results in an extremely effective heat exchanger. Further improvement of heat exchange capability is provided by voltage source V providing voltage to each of fibers 53 through electrical fiber connections 53a. The carbon fibers, therefore, serve as electrodes for implementing the electro hydro dynamic effect. The carbon fibers reduce the thermal resistance between the hot and cold fluids by passing the carbon fibers through the wall separating the hot and cold fluid channels. Since the thermal conductivity of the carbon fibers is approximately eight times greater than the thermal conductivity of aluminum, the carbon fibers provide a minimal thermal resistance between the hot and cold fluid channels. In this embodiment, therefore, alternating carbon fibers are supplied either with voltage from voltage source V, or are grounded. Walls 54, in this embodiment, are formed of an insulating material so as to avoid shorting of the fibers to ground. In an alternative embodiment, however, the carbon fibers 53, fiber connections 53a, and other electrical connections could be provided with insulators so as to appropriately insulate the conducting components. It should be noted that voltage source V can provide a positive or negative voltage ranging from a few volts up to several thousands volts, depending upon the inner space or gap between the electrode pairs, to create an electric field in the fluid flowing in channels 50 and 51.

Since heat transfer always occurs from hotter to colder, the EHD principle works to improve heat transfer in both channels 50 and 51. Heat exchange is in the direction of the vertical arrows. Referring to Figure 6, carbon fibers can be used to form high density high electrical field needle type electrodes. These needle type electrodes, and other electrodes, destroy the thermal layer or thermal boundary as shown by the ion and electron movement in the Figure.

In one embodiment of the heat exchanger illustrated in Figure 5, the heat exchanger has 133 by 180 carbon fiber of strands, a total of 23,940 strands with an area of 200 x 270 mm². Fabrication of such a heat exchanger requires a high level of complexity and precision, which can be achieved with existing state-of-the-art fiber winding and weaving technologies.

The above embodiments of the invention are intended to be illustrative in nature only. A person of ordinary skill in the art would understand that the principles of the invention can be adapted to other configurations, while remaining within the scope of the invention. The invention is defined, therefore, only by the appended claims.

Claims

Claims:

1. A fiber heat sink, comprising: a substrate having a first surface and a second surface, said first surface configured to conduct heat; fibers attached to said second surface, for conducting and dissipating heat away from said second surface.

2. A heat sink as recited in claim 1 , wherein said fibers are attached to said second surface by impregnation.

3. A heat sink as recited in claim 1 , wherein said fibers are carbon fibers, with an end of each carbon fiber attached to said second surface by impregnation.

4. A heat sink as recited in claim 1 , wherein said fibers are sapphire fibers.

5. A heat sink as recited in claim 1 , wherein said substrate comprises silicon.

6. A heat sink as recited in claim 1 , wherein said substrate comprises a silicon carbide-carbon composite.

7. A heat sink as recited in claim 1 , wherein said fibers are carbon fibers in the form of a carbon fiber braid, with a first end of said carbon fiber braid comprising the fibers attached to the second surface, and second ends of fibers in the carbon fiber braid extend outward to dissipate heat into a cooling fluid.

8. A heat sink as recited in claim 1 , wherein said substrate comprises a microcircuit.

9. A method of manufacturing a fiber heat sink, said method comprising the steps of: forming a silicon carbide/ carbon powder green body; placing ends of a plurality of carbon fibers into a surface of said green body; infiltrating free silicon into the green body; and firing the green body in a sintering process wherein the infiltrated free silicon fills pores created by oxidation of carbon in the green body.

10. A fiber heat exchanger, comprising: a heat exchanger body comprising a plurality of walls forming hot channels and cold channels, said hot channels being separated from said cold channels by walls; a plurality of fibers attached to said walls and thermally conducting between said hot channels and cold channels, whereby said plurality of fibers are configured to conduct heat from said hot channels into said cold channels.

11. A heat exchanger as recited in claim 10, wherein said fibers comprise carbon fibers.

12. A heat exchanger as recited in claim 10, wherein said fibers comprise sapphire fibers.

13. A heat exchanger as recited in claim 10, wherein said heat exchanger body comprises a plurality of coplanar hot channels with a channel flow defined in a first direction, and a plurality of a coplanar cold channels having a channel flow defined in a second direction, and wherein said fibers communicate through a common wall separating planes defined by the plurality of hot channels and plurality of cold channels.

14. A heat exchanger as recited in claim 10, wherein at least one of said hot channels includes an electrode disposed therein, said electrode being electrically isolated from said walls, said electrode being connected to a voltage source; and wherein at least one of said cold channels includes an electrode disposed therein, said electrode being electrically isolated from said walls, and being connected to the voltage source, and wherein said electrodes are configured with respect to said fibers and said walls to utilize an electro hydro dynamic principle for enhanced heat exchange between fluid in respective channels and fibers in the respective channels.

15. A heat exchanger as recited in claim 10, wherein said fibers include a high temperature protective coating thereupon.

16. A heat exchanger as recited in claim 10, wherein said fibers include a silicon carbide thin layer coating thereupon.

17. A heat exchanger as recited in claim 10, wherein at least one fiber of the plurality of fibers is connected to a voltage source, and wherein at least one second fiber, electrically isolated from said first fiber of said plurality of fibers is connected to ground whereby an electric field is created between the at least one first and at least one second fiber, said electric field utilizing an electro hydro dynamic principle for enhanced heat exchange between fluid in respective channels and fibers in the respective channels.

18. A heat exchanger as recited in claim 17, wherein said walls comprise an electrically insulating material.

19. A heat exchanger as recited in claim 17, wherein a plurality of first fibers are electrically connected together, and wherein a plurality of second fibers are electrically connected together.