EP1996887A2 - Tuyau calorifique avec matériau de mèche nanostructuré - Google Patents

Tuyau calorifique avec matériau de mèche nanostructuré

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Publication number
EP1996887A2
EP1996887A2 EP07840139A EP07840139A EP1996887A2 EP 1996887 A2 EP1996887 A2 EP 1996887A2 EP 07840139 A EP07840139 A EP 07840139A EP 07840139 A EP07840139 A EP 07840139A EP 1996887 A2 EP1996887 A2 EP 1996887A2
Authority
EP
European Patent Office
Prior art keywords
approximately
heat pipe
nanowires
center
wicking material
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07840139A
Other languages
German (de)
English (en)
Inventor
Youssef M Habib
Lyman H. Rickard
John G. Bryan
John W. Steinbeck
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Illuminex Corp
Original Assignee
Illuminex Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Illuminex Corp filed Critical Illuminex Corp
Publication of EP1996887A2 publication Critical patent/EP1996887A2/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D15/00—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
    • F28D15/02—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes
    • F28D15/04—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with tubes having a capillary structure
    • F28D15/046—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with tubes having a capillary structure characterised by the material or the construction of the capillary structure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
    • F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
    • F28F13/18—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by applying coatings, e.g. radiation-absorbing, radiation-reflecting; by surface treatment, e.g. polishing
    • F28F13/185—Heat-exchange surfaces provided with microstructures or with porous coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
    • H01L23/42—Fillings or auxiliary members in containers or encapsulations selected or arranged to facilitate heating or cooling
    • H01L23/427—Cooling by change of state, e.g. use of heat pipes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/4935—Heat exchanger or boiler making
    • Y10T29/49353—Heat pipe device making

Definitions

  • ASSIGNEE Illuminex Corp. Post Office Address 627 Conestoga Blvd. Lancaster PA 17602
  • Heat pipes are passive, closed loop devices used for temperature regulation in electronic and optical systems. Heat pipes are used for transporting heat from electronic device components and packages to a heat sink. For example, in a lap top computer a heat pipe is used to transfer heat from the CPU to the case of the computer to keep the CPU from overheating. In general, the heat transfer can take place between any source (hot) to a sink (cold). With the increasing power demands of integrated electronic, optical, and mechanical systems, localized thermal management is a critical issue for proper device operation.
  • the typical heat pipe is an enclosed tube-like structure or other enclosed package that is typically made of metal where the interior surface of the enclosure has a wicking structure.
  • the wick acts on a coolant (working fluid) in a liquid phase to move the working fluid from the sink (condenser) to the source (evaporator) by means of capillary action.
  • the center of the heat pipe enclosure is open and free of obstruction.
  • the working fluid moves through the wick to the evaporator, in the opposite direction of the gas phase that moves in the open space to the condensor.
  • a phase change from liquid into vapor occurs, while at the cool side, the phase change from vapor into liquid occurs.
  • Heat transport is accomplished by the removal of heat through the latent heat of evaporation and cooling through the latent heat of condensation.
  • the wicking material passively transports the liquid thus making a cycle that continuously cools the heat generating element.
  • the center of the heat pipe continuously transports the vapor via a pressure differential between the hot and cold ends of the heat pipe
  • the invention is a heat pipe where the wicking structure is nanostructured, meaning that the cross sectional dimension of the elementary structures comprising the wicking material are on the order of approximately 10 to 400 nanometers with spacings between the elements between approximately 20 to 600 nanometers, center to center that residing within the interior surface of the enclosed space (package) of the heat pipe and a method of making the nanostructures.
  • the nanostructures are bristles or a plurality of nano wires that are attached on one end to the interior surface of the heatpipe.
  • nanowire it is meant a wire whose cross sectional dimension is between approximately 10 nanometers and approximately 400 nanometers.
  • the nanowires are also substantially free-standing, that is, they are surrounded on the sides by the working fluid in either the liquid phase or gas phase, not a substrate other than the substrate they are grown on, template or other support material. Groups of nanowires are also referred to herein as bristles or nanobristles.
  • the nanostructured wicking material provides improved capillary action for transporting liquid and a low thermal resistance to vapor evolution for improved evaporation and transport compared to conventional wick geometries. Therefore, nanostructured bristle wicks enable more efficient heat exchange in a heat pipe.
  • the wick structure must accommodate two physical behaviors in the heat pipe. At the evaporator, a low thermal resistance is required, while to transport the liquid from the condenser through the adiabatic region and back to the evaporator, a high capillary pumping pressure is desired. This can lead to heat pipes with hybrid or composite wicks to accommodate these two regions. In the present invention, this can be accomplished by engineering nanowires of different dimensions in the two areas of relevance.
  • the heat pipe wicking material is shown schematically in Figure 2.
  • the device has many of the same attributes of larger heat pipe structures.
  • the device comprises a metallic enclosure with a wick material on its inner walls.
  • the wick comprises an array of substantially vertically aligned copper nanowires extending out from the walls of the enclosure.
  • the wires are between 10 and 400 nm in diameter, spaced at 20 to 600 nm and are up to approximately 250 microns in length.
  • SEM micrographs of copper nanowire arrays that can be used for the wick application are also shown in Figure 6 and 7.
  • the wick performs similarly to conventional wicks in that capillary action is used as the mechanism to pump a working fluid to the evaporator.
  • Heat is extracted from the device to be cooled, typically an electronic device, through evaporation of the working fluid. Vapor condensing on the cold (condenser) side of the heat pipe deposits the heat for further dissipation by a conventional convection heat sink or for transfer to another heat pipe in the thermal bus.
  • Nanobristle arrays (shown schematically in Figure 2) have significant attributes that make them advantageous as the wick material in heat pipes.
  • the tight packing of the bristles in the array provides a high capillary pressure to promote fluid flow through the wick while the aligned array configuration of the bristles provides a clear path for vapor to escape .
  • the nanobristle architecture can significantly decrease the thermal resistance and increase the fluid flow compared to currently used sintered powder copper, screen, or axially grooved wicks.
  • These two attributes of the nanobristle array improves the heat flux capacity of heat pipe devices to greater than 300 W/cm . Heat fluxes of approximately 25 W/cm2 to 125 W/cm2 with a thermal resistance less than 0.06 0 C- cm 2 /W , have been observed.
  • the nanobristle array wick allows the height, or profile, of the entire heat pipe to be reduced to less than 1 mm (0.040") and can be used where conventional heat pipe devices and cooling technologies are inadequate. This is critical for portable electronic devices that utilize ever increasingly powerful (heat generating) processors in smaller and smaller packages.
  • Figure 3 schematically shows a thermal bus architecture utilizing nanobristle heat pipe technology as the key link between the high power circuits and the external thermal bus.
  • the current heat pipe technology typically limits the wicking structure to a minimum thickness of lmm to provide adequate cooling.
  • the heat pipe has to be greater than about 2 mm thick, plus packaging, plus evaporation space.
  • 5 mm in width is a minimum dimension.
  • the wicking structure has to be smaller.
  • the current invention can be used to create wicking structures that are only about 50-100 microns (1Ox improvement) in bristle wick length, thus making it possible for a 300 micron width copper layer sufficient to enclose the heat pipe volume while still providing the same heat transport capacity..
  • the invention permits a heat pipe with a 900 micron cross section or less. This reduces weight and size for the same amount of heat transfer efficiency.
  • the nano-structured wicking material exhibits improved capillary action and lower thermal resistance.
  • the bristle structures are arrayed.
  • the bristles have a uniform size and spacing that can be controlled to optimize the capillary action and thermal resistance for a particular application.
  • a 35% to 50% reduction in thermal resistance than sintered copper powder wicks can be achieved by means of the use of nanostructured wicking materials.
  • Sintered copper powder is the industry preference and currently exhibits the lowest thermal resistance of all currently commercially distributed heat pipe wick structures.
  • nanowire array heat pipe can be built with an extremely thin profile and that the nanowire wick will enable operation at any orientation.
  • Devices less than 1 mm thick can be built that can be directly incorporated into high power component packages. This design flexibility can enable the top of the heat pipe to be specifically designed to incorporate a coupling structure so that the device can be efficiently mated to a thermal (heat pipe) bus as well as the device to be cooled.
  • Heat pipes are well known heat transfer devices that are highly useful for heat management in electronic devices and packaging. Heat pipes are disclosed in a number of U.S. patents, including 3,952,798, issued on April 27, 1976, which is incorporated herein by reference for all that it teaches.
  • Figure 2 Diagram of a nanobristle array.
  • Figure 3 Electronic systems thermal management concept utilizing nanobristle wick
  • the inset is an SEM image of copper nanobristles.
  • Figure 4 Schematic of porous anodized aluminum oxide on a copper substrate.
  • Figure 5 Plot of current vs. time for the formation of the alumina nanochannels.
  • the increase in current corresponds to the nanochannels reaching the copper substrate.
  • Figure 9 Reflectivity inspection for missing nanobristles in the nanobristle array.
  • a rastering laser and photodiode can be used to find patches of missing bristles on a
  • Performance data for tested nanowire wicks are shown in Figure 8. The data clearly show that the nanowire materials can sustain heat fluxes in excess of 100 W/cm with a 35% reduction in thermal resistance. Typical computer applications today have heat fluxes of 100-200 W/cm 2 yet the high thermal resistance of conventional heatpipes do not provide the proper temperature (cooling) for optimal performance. Further, the materials, although using a much lower volume ( ⁇ 100 micron thick) than the competing sintered powders ( ⁇ 1 mm thick ), have superior heat flux performance
  • Nano-bristle arrays are grown on a substrate by the use of a template.
  • the template is formed using an anodization process in order that a regular set of pores are formed that are fully open to the substrate surface.
  • a polymer resist layer coats the back side of the substrate.
  • the substrate, with the polymer and template layers attached is then bathed in a solution that deposits a substance in the pores.
  • the template layer and polymer layer are removed.
  • the result is the substrate with substantially vertical bristles comprised of the substance appended on one side.
  • the dimension and spacing of the bristles is determined by the pore dimensions and spacing in the template.
  • the substrate and substance may be the same material, preferably copper.
  • the template is preferably aluminum oxide and the resist a polymer resist coating. The anodization of aluminum metal to form a porous aluminum oxide layer is well established, in the articles listed here, and each is incorporated herein by reference for all that they teach
  • the first method for producing the nanobristle array uses a thin sheet (30-80 ⁇ m thick) aluminum overlaid on a 1 mm thick copper sheet.
  • the copper is first covered with a polymer film (Coscoat 4560 supplied by General Chemical Corporation) to protect the copper during anodization.
  • Anodization of the Aluminum metal is then performed resulting in the growth of porous alumina in which the diameter, depth, and spacing of the nanochannels can be controlled by varying the electrical parameters and chemical concentrations used in the anodization process. Controlling the dimensions of the self- assembled porous template is important for engineering the desired characteristics of the nanobristle wick material.
  • a schematic of anodized Al on Cu is shown in Figure 4.
  • Metallic nanowires can be produced using an aluminum oxide template, are also disclosed in the articles listed in the following list, and each is incorporated herein by reference for all that they teach:
  • the preferred embodiment uses alumina as the template and copper as both the substrate and the bristle.
  • the method of forming is as follows:
  • the starting material is a metallic substrate with copper being the preferred embodiment due to it's high thermal conductivity.
  • the substrate material can be any electrically conducting substrate. The substrate is first cleaned
  • the aluminum layer thickness is such that when anodized, the oxide layer will be as deep or deeper than the desired length of the nanobristles.
  • Other materials may be used as cladding, including titanium, silicon, zinc, zirconium, lanthanum, niobium, tungsten, tin, indium, strontium, vanadium, molybdenum, calcium, or blends of two or more, and their oxides. In fact, any metal that can produce a porous oxide may be used. Aluminum is preferred because it's oxide produces ordered pores and it is relatively inexpensive in order to define pore size and spacing.
  • the formation step has the following sub- steps: a. Clean Aluminum cladding. This can be done using standard laboratory solvents.
  • the aluminum can be electropolished.
  • the initial oxidization should produce a layer of oxide on the cladding of about 20 to about 100 nm in depth.
  • the temperature can range from about 100 to about 450° Celsius and for times from about 1 to about 240 minutes. All references to "degrees" means degrees Celsius, not Fahrenheit.
  • Coat the copper substrate with an insulator e.g. a polymer to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • an insulator e.g. a polymer to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • an insulator e.g. a polymer to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • an insulator e.g. a polymer to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • an insulator e.g. a polymer to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • Coscoat e.g. a material to prevent anodization on the backside of the copper substrate and to protect the surface from corrosion.
  • Anodize the Aluminum This step converts the Aluminum metal cladding to Aluminum Oxide. As oxide forms it creates the
  • the atomic spacing of the Aluminum atoms in bulk is not the same as the spacing between the Al 2 O 3 molecules, so regular voids begin to form along the surface.
  • These voids work down toward the surface of the substrate forming pores in the oxide layer.
  • the pores self assemble into a quasi-hexagonal matrix structure.
  • the pore diameters are a function of the anodization conditions (voltage, electrolyte, cathode geometry), while the depth is a linear function of anodization time.
  • the voids form because the law of thermodynamics makes the anodization uneven and as a result the voids becomes pores. It is possible to use micro-lithography techniques to define the surface of the aluminum before anodization.
  • Anodization bath In the preferred embodiment, about .3 weight % oxalic acid is used, but other bath solutions may be used, including sulfuric acid, phosphoric acid, chromic acid and mixtures of them. By changing electrolyte bath type and concentration, and the voltage, one can adjust size of the pores.
  • the Al layer In a representative anodization process of Al clad onto copper, the Al layer is anodized in a solution of about 0.3-wt % oxalic acid at about 2° C. The anodization is carried out until all the Al metal is consumed and the channels in the Al 2 O 3 penetrate through to the copper. This takes about 12-to about 72 hours depending on the thickness of the Al.
  • Nanobristle arrays using porous Al 2 O 3 as a template have been successfully engineered using electrodeposition of Cd, Fe, Au, Ag, Cu, Ni, and other metals from aqueous solution, as further disclosed in the articles listed in the following and incorporated herein by reference: Y. Peng, H. Zhang, S. Pan, and H. Li, Jour. Appl. Phys. 87, 7405 (2000) and A. Jansson, G. Thornell, and S. Johansson, J.
  • electroplating is used to deposit copper in the pores.
  • the copper substrate is placed in a copper sulfate bath, with an electric current that causes electroplating in the pores.
  • Other metals may be deposited that are different from the substrate.
  • the sub-steps are as follows: a.
  • the electroplating current and voltage are about .75 volts, about 200 milliamps of current up to about about 600 milliamps.
  • the copper plating bath is commercially available from Transene Co. named acid-copper electroplating bath.
  • the Aluminum Oxide is removed from the substrate by bathing the device in Phosphoric acid at about 37 degrees plus or minus 10, degrees, 5% by weight, plus or minus 2%. It is possible to use other etching acids or bases such as NaOH, HCl, H 2 SO 4 , HF, as described above and further including hydrochloric acid. It is also possible to use bases to etch, including sodium hydroxide..
  • a key step in developing an automated production process is the detection of the point where the aluminum metal is completely oxidized and the oxide layer of pores reaches the copper base. If the anodization is continued past the interface, the copper will anodize leaving a material unsuitable for heat pipe applications.
  • Figure 5 is a plot of current vs. time for the anodization process. When the aluminum is completely consumed and the pores reach the copper base the current increases dramatically. For the production of nano-bristle wicks a breaker was included in the circuit which opened when the current increased stopping the anodization process. A computer controlled current monitoring process more appropriate for manufacturing is typically used to stop the anodization at the correct point.
  • the nanobristle wick comprised the base of the heat pipe where a heating element was mounted. That is, the wick can be formed on a substrate that is then attached to the body of the heat pipe, with the bristles pointed into the cavity of the heat pipe. Alternatively, the body of the heat pipe can be treated and the nanowires grown directly on the surface.
  • Nanobristle array wicks gave evaporator thermal resistance values ranging from 0.06 to 0.08°C-cm2/W within the desired heat flux working range of 100-200 W/cm2.
  • the heat pipe evaporator is the primary source of thermal resistance in a heat pipe.
  • the nanobristle array wicks show a significant 25-30% reduction in thermal resistance over the sintered copper powder wicks.
  • Sintered copper powder is currently used as the wicking material in a large percentage of heat pipe applications where improved performance is desired.
  • the heat flux capacities obtained for the prototypes provide the required capacities for the current Hewlett Packard and AMD chip packages (80-100 W/cm2) and new generation microprocessors such as the Intel Pentium 4 Extreme Edition (150 W/cm2).
  • the thermal resistances measured for the nanobristle wicks were 25-30% lower than current sintered copper heat pipes. This improvement in thermal resistance coupled with the low profile ( ⁇ 100 ⁇ m compared to ⁇ 1 mm for sintered copper) is very attractive for the production of new heat pipes.
  • Techniques have been developed for monitoring the uniformity of the nanobristle arrays. In the first method a prototype nano-bristle wick inspection system (shown schematically in Figure 9 measures the reflectivity of the surface at one wavelength as an indication of the array uniformity.
  • the light source can also be a laser or a UV -Visible light source, adding a monochromator and replacing the detector with a CCD array detector to allow for the collection of real time reflectance spectra of the nanobristle array surface as it is scanned.
  • UV- Visible spectra give direct information about the plasmonic features of copper nanobristle arrays.
  • Figure 10 shows a spectrum for a copper nanobristle array. Two bands are observed in the spectrum at -314 and 413nm. These bands correspond to the transverse and longitudinal localized surface plasmon resonances of the nanobristle array for that size of nanowire. Changes in the dimensions of the nanobristles in an array result in changes in the spectrum. This provides a mechanism for monitoring the uniformity of the nanobristle arrays.
  • the advantage over the laser technique is that the entire spectrum can be obtained which will provide more details about the array surface.
  • the nanowire wick has anisotropic flow characteristics.
  • the flow channels through the wick are narrow in the plane of the device, but are long normal to the substrate surface.
  • the capillary pressure should be high as the high surface area provides high surface tension.
  • the flow resistance in the plane of the wick may be low, however, since the net channel cross-section can be as high as 10 micron for 100 micron wires spaced 100 nm apart. This is nearly comparable to the channel cross-section in sintered powder wicks made of 15 micron particles.
  • Flow normal to the plane of the nanowire device will be unimpeded as illustrated in Figure 11. This provides a significant advantage as convection currents in the working fluid will easily circulate to the vapor chamber, potentially minimizing bubble formation.
  • the potential combination of these characteristics in the nanowire wick can result in a device with unprecedented thermal performance.
  • the two primary design parameters for copper/water heat pipes are the heat transport capability and the thermal resistance.
  • the heat transport capability is dependent on the pore radius and permeability of the wick structure.
  • An ideal heat pipe wick would have a small pore radius, providing good capillary pumping, and a high permeability, allowing liquid and vapor to easily pass through the wick.
  • Grooved wicks have a large pore radius and high permeability so that they can transport large heat loads in a horizontal or gravity aided position but do not function well against gravity or other forces of acceleration.
  • Sintered metal powder wicks have small pore radii giving good capillary pumping, leading to the ability to transfer large heat loads against gravity, but with relatively low permeability.
  • the second design consideration for heat pipes is thermal resistance. It is desirable to have a low heat pipe thermal resistance. The thermal resistance is a function of heat pipe geometry, wick structure, condenser length, evaporator length, and working fluid.
  • the working fluid used in a heat pipe, as well as the material the wick and package are made of depend on the relative operating temperature of the device. These can be divided into three general classes: Cryogenic (10-150 0 K) Low Temperature (150- 750 0 K) and High Temperature (750-5000 0 K). Most heat pipe applications for electronics thermal management require working fluids with boiling points between 250- 375°K thus limiting the choice of working fluids to ammonia, acetone,methanol and water or dielectrics such as Freon. Cryogenic application utilize liquid H 2 , O 2 , or N 2 , while high temperature applications typically utilize liquid metals such as Mercury, Potassium, Sodium, Lithium or Silver.
  • electronic cooling application are targeted with water most often used as the working fluid. Close packed nanowire arrays are desirable for the evaporator but may not provide the ultimate capillary pumping. Thus, nanowires with larger spacing can be used for the condenser and adiabatic transport sections of the heat pipe in a graded hybrid nanowire structure where the nanowire packing is sparse at the condenser end becoming tightly packed at the evaporator. It is clearly established that the vertical alignment of the nanowires decreases thermal resistance at the evaporator. However, also due to their small size, patterning of the nanowires where they don't coat the entire surface of the inner- wall may also dramatically improve the capillary pumping characteristics.
  • nanowire template selectively, so that the nanowires have grooves, a cross pattern, triangles or other shapes providing either connecting or separated spaces on the heat pipe inner wall where there are no nanowires in between the parts of the wick that do have a nanowire array coating.
  • nanostructures including nanowires can be used at the evaporator, condenser, and return path regions of the heat pipe.
  • different wick materials could be used to feed the working fluid to the nanowires at the evaporator.
  • a conventional wick could be hybridized with the nanowire wick, either selectively or patterned throughout the inner wick structure of the heat pipe.
  • a sintered powder wick can be used in one part of the pipe while the nanostructure wick is used in another.
  • BSR boiling surface area ratio
  • BSR is calculated based on only the outer cylindrical area of the nano wires.
  • Typical sintered metal powder wicks exhibit a BSR of about 35.
  • the wicking material disclosed in U. S Pat. No. 4,015,659 discloses a material with a BSR estimated to be approximately 5 to approximately 50, depending on a range of whisker length of 100 microns to 1 millimeter. Practitioners of ordinary skill will recognize that if the length of the nanowire is too long relative to its diameter, then it is more likely to crack or otherwise fail.
  • the aspect ratio of the nanowires should be less than approximately 2500 to 1. The aspect ratio is defined to be the cross sectional distance of the nanowire, i.e.
  • the wires are intended to be free standing, and are indicated as that schematically, in practice the wires lean against each other somewhat, which improves their ability to support longer nanowires. As a result, they are substantially free standing nanowires, meaning that the trajectory of their growth is as separated wires, although at the end of the process there is some touching among them. Practitioners of ordinary skill will also recognize that the preferred embodiment produces nanowires that are substantially round in cross section. However, other methods may be used to create other cross sectional shapes, depending on crystal structures, pore shape and other parameters.
  • the dimension "diameter” does not limit the dimension to a circular cross section, but rather also applies to non-circular cross sectional dimension.
  • the point of this invention is that the nano wires form a wicking structure whose BSR on a per-wick thickness basis is very high, where in this case, the wick thickness is essentially the height of the nanowires. In other words, there is more boiling surface in a thinner wick.
  • the BSR can be divided by the thickness of the wick in microns, or BSR/t.
  • the typical application of this invention exhibits a BSR/t on a range of approximately 18 to 125, corresponding to cases 1 , 2 and 3 above.
  • the sintered metal powder example assumes a 1 mm thick coating and thus has a BSR/t calculated to be about .035.
  • the metal whisker approach has a higher BSR/t than the sintered powder, calculated to be about .054.
  • These prior art materials exhibit a BSR on the order of 3 to 5 for wick thicknesses of 100 microns.

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Abstract

L'invention concerne un tuyau calorifique avec une mèche nanostructurée, ainsi que le procédé de fabrication de la mèche nanostructurée sur un substrat métallique. Le matériau de mèche est un motif de nanostructures métalliques sous la forme de soies ou fils attachés à un substrat, les soies étant sensiblement librement dressés.
EP07840139A 2006-03-03 2007-03-05 Tuyau calorifique avec matériau de mèche nanostructuré Withdrawn EP1996887A2 (fr)

Applications Claiming Priority (3)

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US77887306P 2006-03-03 2006-03-03
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