Abstract:
Technology of the invention enables use of new engineering materials in design of heat pipes with novel properties and improved performance. In particular milled carbon fibers, carbon nanotubes can be used for construction of high performance wicks.

Description:
CITED PUBLICATIONS  
       [0001]     “Ordered Adlayers of a Non-Planar Molecule on a Surface: Misfit Monolayers and Intercalated Bilayers as the Result of a Dialkyl Amino Group”, Gorman, C. B.; Miller, R. L., Touzov, I., Langmuir, 1998,14, 3052-3061  
         [0002]     Tuzov I., Cramer K., Pfannemuller B., Magonov S. N., Whangbo M. H., “Characterization of N-Alkyl-D Gluconamide Adsorbate Structures by Atomic-Force Microscopy. 1. Microcrystals and layered Structures”, NEW JOURNAL OF CHEMISTRY 1996, Vol. 20, Iss 1, pp. 23-36.  
         [0003]     Tuzov I., Cramer K., Pfannemuller B., Magonov S. N., Whangbo M. H., “Characterization of N-Alkyl-D Gluconamide Adsorbate Structures by Atomic-Force Microscopy. 2. Supramolecular Structures”, NEW JOURNAL OF CHEMISTRY 1996, Vol. 20, Iss 1, pp. 37-52.  
         [0004]     Cramer K., Demharter S., Mulhaupt R., Frey H., Magonov S. N., Tuzov I., Whangbo M. H., “Characterization of Supramolecular Assemblies of N (N-Alkyl)-N′-D-Maltosylsemicarbazone Adsorbates by Atomic-Force Microscopy”, NEW JOURNAL OF CHEMISTRY 1996, Vol. 20, Iss 1, pp. 2-11.  
         [0005]     Tuzov I. V., Yaminsky I. V., “Scanning Force Microscopy Visualization of Adsorption from Liquids”, RUSSIAN CHEMICAL BULLETIN 1 995, Vol. 44, Iss 11, pp. 2073-2078.  
         [0006]     Tuzov I., Cramer K., Pfannemuller B., Kreutz W, Magonov S. N., Molecular-Structure of Self-Organizes Layers of N-Octyl-D-Gluconamide”. ADVANCED MATERIALS, 1995, Vol. 7, Iss 7, pp. 656-659.  
         [0007]     Tuzov I. V., Klinov D. V., Demin V. V., “Catalytic Method for modifying the Surface of Pyrolytic-Graphite”, RUSSIAN CHEMICAL BULLRTIN, 1994, Vol. 43, ISS 7, pp. 11 28-1131.  
       FIELD OF INVENTION  
       [0008]     Objective of this invention is to provide solution that improves overall efficiency of heat pipes and allows significant size and weight reduction for thermal management solutions using novel engineering materials.  
       PRIOR ART  
       [0009]     Heat pipes are capable of producing significant heat flux and have low weight characteristics. Their structure has two essential elements a wick and a shell. Maximum transmitted heat is commonly constrained by ability of the wick to transport working liquid from condenser region to evaporating region.  
         [0010]     Efficiency of the transport strongly bound to a surface angle of the liquid on the wick material, effective average capillary diameter, and effective average capillary length throughout the wick structure. To reduce surface angle the material of a wick should be compatible with selected liquid. Current use of metal wicks is very common in conjunction with water.  
         [0011]     Advances of new materials such as polymer, carbon, graphite, and Al/SiC fibers have a potential that could improve some of characteristics of heat pipes. Nevertheless they do not easily merge into new designs of heat pipes. Prior art explored a feasibility of use graphite fiber based structures to passively transfer heat from heat sources (U.S. Pat. Nos. 6,286,591, 5,720,339, 5,269,369, 4,018,269). Unfortunately none of the previous inventions provide sufficient heat conductivity in moderate temperature range. Graphite and carbon wicks show high efficiency when used in conjunction with liquid metals as a heat transfer fluid (NASA). Unfortunately this requires very high temperatures and introduces hazardous materials that restrict the range of product applications.  
         [0012]     Due to hydrophobic nature of graphite and carbon surfaces the use of water as a working fluid is virtually impossible. Objective of this invention is to demonstrate a use of nanoassembly properties to allow employment of a broad variety of wick materials in combinations with nontraditional working liquids.  
                                     TABLE 1                           Traditional liquids and materials for heat pipe selection.                        Measured               Heat Pipe   Heat Pipe   axial(8)   Measured       Temperature   Working   Vessel   heat flux   surface(8) heat       Range (° C.)   Fluid   Material   (kW/cm2)   flux (W/cm2)               −200 to −80    Liquid   Stainless   0.067 @   1.01 @ −163° C.           Nitrogen   Steel    −163° C.       −70 to +60   Liquid   Nickel,   0.295   2.95           Ammonia   Alumi-               num,               Stainless               Steel        −45 to +120   Methanol   Copper,    0.45 @   75.5 @ 100° C.               Nickel,     100° C.               Stainless   (x)               Steel        +5 to +230   Water   Copper,    0.67 @    146 @ 170° C.               Nickel     200° C.       +190 to +550   Mercury*   Stainless    25.1 @    181 @ 750° C.           +0.02%   Steel     360°           Magnes-       C.*           ium           +0.001%       +400 to +800   Potas-   Nickel,    5.6 @    181 @ 750° C.           sium*   Stainless     750° C.               Steel       +500 to +900   Sodium*   Nickel,    9.3 @    224 @ 760° C.               Stainless     850° C.               Steel         +900 to +1,500   Lithium*   Niobium    2.0 @    207 @ 1250° C.               +1%    1250° C.               Zirconium       1,500 + 2,000   Silver*   Tantalum    4.1    413               +5%               Tungsten                  
 
     
    
     DETAILED DESCRIPTION  
       [0013]     Preferred embodiment of the invention utilizes water as the work liquid and uses graphite fibers for wick material. The liquid composition contains minuscule amounts of surface active nanostructural additive N-octyl-D-gluconamide. The additive as shown on  FIG. 1  forms stable liquid crystal monolayer on hydrophobic surface of carbon or epitaxial crystalline monolayer of surface of graphite. This nanostructure is self formed and creates stable hydrophilic interface (SAM) between liquid water and the wick.  
         [0014]     For sustained operation of a heat pipe it is important that nanoassembly does not migrates with the flow of the liquid and has ability of self-regeneration. For lower temperature range self-regenerative activity of N-octyl-D-gluconamide crystals diminishes and this component can be substituted with N-heptyl-D-gluconamide which is less efficient at high temperatures.  
         [0015]     Amount of the additive is chosen based on total surface area of the wick. Some excess of the additive will not harm the pipe operation as it will be adsorbed by evaporator surface. Due to natural diffusion a miniscule amounts of the additive will remain distributed through the volume of the liquid in form of nano-particles and individual molecules. Array of nanostructures formed by this compound is shown on  FIG. 2 . These structures absorb free molecules from the solvent. Because of large size they are limited in mobility and easily absorbed by the SAM layer. Occasional damages in the crystalline nanostructure will utilize these nanostructures to repair the damage.  
         [0016]     Other chemicals and nanostructures such as detergent, and liposomes can be used in place of the additive. Examples of such substances are phospholipids, SDS, alcohols, organic acids, organic salts etc. They are well known to chemists and biologists. Some of them are well characterized as well as their self-assembled behaviors. The methods of their synthesis and preparations are well documented in scientific and industrial literature.  
         [0017]     Graphite fibers are hydrophobic and will prevent capillary phenomena for water. Use of proposed additives creates nanoscale SAM (self assembled monolayer structure) covering entire surface area of the fibers and permitting capillary transport for the water. Benefits of using water are obvious. It has low chemical activity, high specific heat of fusion and heat of evaporation.  
         [0018]     Technology disclosed in this invention enables other wick materials such as Kevlar, UHMWPE, glass, rubber, silicone, ceramic, etc. to become usable in commercial heat pipes. Their advantages and peculiarities are well known and do not alter the essence of the invention. The liquid selection does not limit the subject of the invention as there are well known guidelines for selection of working fluid for heat management applications, and virtually any liquid including metals can be employed within boundaries of the technology of the invention. As an example a long chain thiols as the additive component increase oleofilic properties of metal wick thus increase its performance with organic solvents and inorganic oils.  
       EXAMPLE  
       [0019]     Sometime it is possible to select a liquid that is reveal nanoassembly properties on interface with some solid materials. In this case self assembled monolayer of molecules of the liquid or their nanomers will be formed on solid interface. This behavior is well known to occur for some compound on surface of HOPG. As example 5-(N,N-Didecylamino)-2,4-pentadienal forms stable domains visible through STM and SPM techniques. Interestingly enough broad range of organic liquids reveal similar abilities. In particular commercially available refrigerants:  
                                                                                     Freezing                   Boiling point   Point atmos-                   atmospheric   pheric pres-       Refri-       Mole-   pressure 14.7   sure 14.7       gerant       cular   psia, 1 bar   psia, 1 bar       No.   Name   Mass   abs) (° F.)   abs (° F.)                                R-11   Trichlorofluoromethane   137.38   75   −168       R-13   Chlorotrifluoromethane   104.47   −115   −294       R-13B1   Bromotrifluoromethane   148.93   −72   −270       R-14   Tetrafluoromethane   88.01   −198   −299           (Carbon tetrafluoride)       R-22   Chlorodifluoromethane   86.48   −41   −256       R-40   Chloromethane   50.49   −12   −144       R-113   Trichlorotrifluoroethane   187.39   118   −31       R-114   1,2-dichloro-1,1,2,2-   170.94   39   −137           tetrafluoroethane       R-115   Chloropentafluoroethane   154.48   −38   −159       R-123   Dichlorotrifluoroethane   152.93   82   −161       R-134a   Tetrafluoroethane   102.03   −15   −142       R-142b   1-chloro-1,1-difluoro-   100.50   14   −204           ethane       R-290   Propane   44.10   −44   −306       RC-318   Octafluorocyclobutane   200.04   22   −43       R-500   Dichlorodifluorometh-   99.31   −28   −254           ane/Difluoroethane       R-502   Chlorodifluoromethane/   111.63   −50           Chloropentafluoroethane       R-503   Chlorotrifluoromethane/   87.50   −128           Trifluoromethane       R-600   Butane   58.13   31   −217       R-600a   Isobutane   58.13   11   −256       R-611   Methyl formate   60.05   89   −146                  
 
 These chemicals have dynamic viscosity at moderate temperatures nearly an order or magnitude less then water. Use of carbon fibers for wick material makes heat pipe construction 2-4 times lighter than use of traditional metal wicks. In particular a felt like wick can be constructed from low cost milled carbon fiber. In our example R-114 refrigerant is used. At 60° C. it has surface tension of 0.007 N/m and viscosity of 0.000187 Pa*s, which allows for construction of heat pipe with dimensions of traditional water base pipe while having wick weight nearly three times less.