Heat pipes with self assembled compositions

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

“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

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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

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.

Claims

1. A heat pipe having a capillary structure commonly known as a wick, and a liquid, wherein there is a chemical substance or substances present on interface of the liquid and the material of the wick, and said chemical substance or substances improve capillary properties of said material with respect to said liquid.

2. A device of claim 1, wherein said material is carbon in any of its forms e.g. graphite, diamond, fullerene, fiber, amorphous.

3. A device of claim 1, wherein said material is natural or synthetic polymer.

4. A device of claim 1, wherein said material is inorganic or ceramic composite.

5. A device of claim 1, wherein said material is metal or alloy.

6. A heat pipe wherein a wick structure composed primarily of carbon fiber or carbon tube based material and a heat transfer liquid of the pipe forms self-assembled layers or domains of self-assembled structures on surface of graphite crystals or forms self-assembled aggregates on edges and pits of crystal plates of graphite.

7. A device of claim 6, wherein said liquid is one of commercially available refrigerants which may include R-13, R-114 and others.

8. A device of claim 6, wherein said wick composed of a felt like material with primary content of milled graphite or carbon fibers or tubes.