WO2001023822A1

WO2001023822A1 - Fractal absorber for heat pipes with broad range heat radiation absorptivity

Info

Publication number: WO2001023822A1
Application number: PCT/US2000/026346
Authority: WO
Inventors: Oleg A. Yevin; Thomas H. Nufert; David I. Kreimer
Original assignee: Array Bioscience Corporation
Priority date: 1999-09-27
Filing date: 2000-09-26
Publication date: 2001-04-05
Also published as: WO2001023459A1; EP1222432A1; KR20020071848A; JP2003510607A; AU7714800A; EP1242510A1; KR20020070423A; TW445362B; EP1221047A1; JP2003510552A; JP2003510065A; AU7716500A; WO2001023888A1; AU7616100A; KR20020077334A

Abstract

A heat pipe (105) having nanoparticles (160).

Description

FRACTAL ABSORBER FOR HEAT PIPES WITH

BROAD RANGE HEAT RADIATION ABSORPTIVITY

Related Applications

This application claims priority to United States Provisional Patent Application, titled "Nanoparticle Structures With Receptors for Raman Spectroscopy," inventors: David I. Kreimer, Ph.D., Oleg A. Yevin, Ph.D., Thomas H. Nufert, filing date: September 27, 1999, Serial No: 60/156,195, to United States Provisional Patent Application, titled "Addressable Arrays Using

Morphology Dependent Resonance for Analyte Detection," inventors Oleg A. Yevin, Ph.D., David I. Kreimer, Ph.D., filing date September 27, 1999, Serial No. 60/156,145 and to United States Provisional Patent Application titled "Fractal Absorber for Heat Pipes with Broad Range Heat Radiation Absorptivity", inventors, Oleg Yevin, Thomas H. Nufert and David I. Kreimer,

Serial No. 60/156,471. Each of these Provisional Patent Applications is herein incorporated fully by reference.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to the manufacture of heat pipes with improved heat radiation absorptivity. Specifically, the invention relates to the manufacture of heat pipes with surfaces involved in absorption and radiation of heat via radiation heat transfer. More specifically, the invention relates to the manufacture of heat pipes with surfaces covered with nanoparticle structures for absorption of broad band electromagnetic radiation. Description of Related Art

Many useful mechanical and electrical processes result in dissipation of energy in the form of heat. This heat is often undesirable, because it causes increase in the temperature of a system. To prevent heating up the systems, heat pipes have been applied to transfer heat from one point to another.

Additionally, in the use of solar radiation as a source of energy, heating up of solar-radiation absorption elements is the first step in energy conversion. At the next steps, heat pipes are often used for rapid heat transfer from the solar-radiation absorption elements to other elements in these systems. In general, a heat pipe transfers heat faster than a solid copper rod of the same diameter. Heat pipes transfer heat with little loss, and do not require the input of additional sources of energy, such as compressors. This capacity to transfer heat without the need of energy input supports broad technological applications for heat pipes.

I. Heat Pipes

Figure 1 illustrates the principle of operation of a typical heat pipe 100, which is a hollow metal tube 105 sealed at both ends, with a wick 135 covering the inner surface of the tube. The tube is filled with a volatile fluid 140. When one end of the pipe 110 is heated up, as indicated by Qin, the volatile fluid 140 evaporates at this end of the tube. The vapor expands and moves to the other, cooler end 120 of the tube (curved arrows), where condensation of the volatile fluid occurs, releasing heat out (Qout). The condensate moves back to the hot end of the pipe via the wick path 135 due to capillary forces. The cycle of evaporation at a warm end at which the heat energy is absorbed from the environment and condensation at a cool end, thereby releasing the heat to the environment, is thus the basis for heat transfer in heat pipes. Typically, the external tube is made of materials with high thermal conductivity, e.g., copper. These materials provide absorption and/or emission of heat under conditions of contact and convective heat transfer. However, when a radiative heat transfer mechanism is involved, high heat conductivity, by itself, is insufficient for desired highly efficient heat transfer, and other factors can improve the efficiency of the process. These requirements are: 1) the external surface of the pipe should absorb and/or emit electromagnetic radiation in a broad wavelength range; and 2) the transfer of heat between the bulk of the tube material and the external surface should be efficient. One approach to manipulate absorptive and/or emissive characteristics of surfaces is done by their texturing with micron-sized geometric holes and coating the pipes with materials having a desirable emission spectrum, such as tungsten (Stone, et al., U.S. Patent No: 5,932,029, herein incorporated fully by reference).

SUMMARY OF THE INVENTION

Thus, one object of this invention is the development of a heat pipe with superior capacity to absorb and emit electromagnetic radiation in a broad spectral range by means of a nanoparticle structure covering the external surface of the pipe. Another object of this invention is the manufacture of nanoparticle structures having broad range heat absorptive and/or emissive properties.

Yet another object of this invention is the manufacture of nanoparticle structures having specific range heat absorptive and/or emissive proprieties.

An additional object of this invention is the development of the methods for attachment of nanoparticle structures to surfaces.

These and other objects are met by design and manufacture of a heat pipe covered with particle structures, which are capable of effective absorption and/or emission of electromagnetic radiation. These heat pipes are herein termed "Radiation Heat Pipes." In certain embodiments of this invention, surfaces are created that are capable of absorption and/or emission of light in desirable spectral ranges via manipulations of the properties of fractal structures by means of photomodification, by varying the size, and distances between the nanoparticles, by varying the shape of particles, the material of particles and/or the material of the thermal conductive layer and/or material of the tube of the heat pipe.

In other embodiments, a sequence of chemical reactions can be performed to form particles having defined spatial relationships with each other, which, in turn, are used to produce particle structures. In a set of chemical reactions, metal particles can be reacted with molecular linkers. These reactions can proceed nearly to completion if the affinities of reactants are sufficiently high and if the ratios of reactants are controlled. The products of these reactions can include various suspensions of particle structures of a characteristic size as predetermined by the lengths of the linkers.

Another aspect of this invention is the use of ridged linkers of various lengths to maintain a characteristic size of the particle structures. Standard linker lengths allow a degree of control of particle structure formation and manipulation of properties of the particle structures. In a first linking reaction, the use of linker molecules of relatively short fixed lengths allows metal particles to be positioned at fixed distances from one another. In subsequent reactions, the lengths of the linkers can be increased, so as to provide a longer length between pairs of particles. By using the set of products described herein, particle structures can be generated in a controllable fashion, for example in the manufacture of heat pipes having either broad band absorbance and/or emission or, if desired, narrow band absorbance and/or emission. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to the particular embodiments thereof. Other objects, features and advantages of the invention will become apparent with reference to the specification and drawings in which: Figure 1 is a drawing depicting a design for a heat pipe of the prior art and a mechanism of heat transfer.

Figures 2a to 2c depict schematically a set chemical reactions for manufacturing particle structures of this invention.

Figure 3 depicts a detailed design of the radiation absorbing end of a heat pipe of this invention.

Figure 4 depicts a heat pipe of this invention designed for collecting electromagnetic radiation in a broad spectral range, converting this energy into heat and transfer this heat to a cooled end of the pipe.

Figure 5 depicts a design for a heat pipe of this invention capable of both heat absorption and heat radiation via radiative heat transfer.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The following words and terms are used herein. The term "fractal" as used herein means a structure comprised of elements, and having a relationship between the scale of observation and the number of elements, i.e., scale-invariant. By way of illustration only, a continuous line is a 1 -dimensional object. A plane is a two-dimensional object and a volume is a three-dimensional object. However, if a line has gaps therein, and is not a continuous line, the dimension is less than one. For example, if Vz of the line is missing, then the fractal dimension is Vi. Similarly, if points on a plane are missing, the fractal dimension of the plane is between one and 2. If Vz of the points on the plane are missing, the fractal dimension is 1.5. Moreover, if Vz of the points of a solid are missing, the fractal dimension is 2.5. In scale invariant structures, the structure of objects appears to be similar, regardless of the size of the area observed. Thus, fractal structures are a type of ordered structures, as distinguished from random structures, which are not ordered.

The term "fractal associate" as used herein, means a structure of limited size, comprising at least about 100 individual particles associated together, and which demonstrates scale invariance within an area of observation limited on the lower bound by the size of the individual particles comprising the fractal associate and on the upper bound by the size of the fractal associate.

The term "fractal dimension" as used herein, means the exponent D of the following equation: N « R ^D, where R is the area of observation, N is the number of particles, and D is the fractal dimension. Thus in a non-fractal solid, if the radius of observation increases by 2-fold, the number of particles observed within the volume increases by 2³. However, in a corresponding fractal, if the radius of observation increases by 2-fold, the number of particles observed increases by less than 2³.

The term "fractal particle associates" as used herein means a large number of particles arranged so that the number of particles per unit volume (the dependent variable) or per surface unit changes non-linearly with the scale of observation (the independent variable).

The term "linker" as used herein means an atom, molecule, moiety or molecular complex having two or more chemical groups capable of binding to a surface and permitting the attachment of particles together to form groups of particles. The simplest linker connects two particles. A branched linker may link together larger numbers of particles. The term "ordered structures" as used herein means structures that are non-random.

The term "particle structures" as used herein means a group of individual particles that are associated with each other in such a fashion as to permit enhancement of electric fields in response to incident electromagnetic radiation.

Examples of particles include metals, metal-coated polymers and fullerenes.

Also included in the meaning of the term "particle structures" are films or composites comprising particles on a dielectric surface or imbedded in a dielectric material. The term "percolation point" as used herein means a point in time on a conductive surface or medium when the surface exhibits an increase in conductance, as measured either via surface or bulk conductance in the medium.

One way to measure surface or "sheet" conductance is via electric probes applied to the surface. The term "Raman signal" as used herein means a Raman spectrum or portion of Raman spectrum.

The term "Raman spectral feature" as used herein means a value obtained as a result of analysis of a Raman spectrum produced for an analyte under conditions of detection. Raman spectral features include, but are not limited to, Raman band frequency, Raman band intensity, Raman band width, a ratio of band widths, a ratio of band intensities, and/or combinations the above.

The term "Raman spectroscopy" as used herein means a method for determining the relationship between intensity of scattered electromagnetic radiation as a function of the frequency of that electromagnetic radiation. The term "Raman spectrum" as used herein means the relationship between the intensity of scattered electromagnetic radiation as a function of the frequency of that radiation. The term "random structures" as used herein means structures that are neither ordered nor fractal. Random structures appear uniform regardless of the point and scale of observation, wherein the scale of observation encompasses at least a few particles. The term "resonance" as used herein means an interaction with either incident, scattered and/or emitted electromagnetic radiation and a surface having electrons that can be excited by the electromagnetic radiation and increase the strength of the electric field of the electromagnetic radiation.

The term "resonance domain" as used herein means an area within or in proximity to a particle structure in which an increase in the electric field of incident electromagnetic radiation occurs.

The term "scaling diameter" as used herein means a relationship between particles in a nested structure, wherein there is a ratio (scaling ratio) of particle diameters that is the same, regardless of the size of the particles. The term "surface enhanced Raman spectroscopy" ("SERS") as used herein means an application of Raman spectroscopy in which intensity of Raman scattering is enhanced in the presence of an enhancing surface.

The term "surface enhanced resonance Raman spectroscopy" ("SERRS") as used herein means an application of Raman spectroscopy in which Raman signals of an analyte are enhanced in the presence of an enhancing surface (see

SERS) and when an absorption band of the analyte overlaps with the wavelength of incident electromagnetic radiation.

Embodiments of the Invention This invention includes the use of particle structures on the absorbent and/or emissive portions of heat pipes. The nanoparticle structures can be applied to the exterior surfaces of a heat pipe and can thereby efficiently absorb or emit heat energy. By increasing the efficiency of absorbance by the exterior surface of the heat pipe, heat energy can be more easily transmitted to the interior of the heat pipe, and can therefore increase the rate of evaporation of the volatile fluid in the interior of the heat pipe. The increased evaporation of the volatile fluid can result in increased flow of evaporated fluid, thereby permitting increased heat flow from the absorptive portion of the heat pipe to the emissive portion of the heat pipe. The increased flow of heat to the emissive portion can result in increased heat loss from that portion of the pipe. In other embodiments of the invention, the emissive portion of the heat pipe can also be coated with a particle structure to permit the increased loss of heat from the emissive portion of the heat pipe.

The structures that are desirable for use according to the methods of this invention include structures of small particles in structures, herein termed particle structures, which includes as a subset, fractal associates. Particle structures can be characterized by having physical and chemical structures that enable oscillations of electrons to be in resonance with incident and outgoing electromagnetic radiation.

I. Manufacture of Particle Structures The particle structures desirable for use according to this invention can include any structure in which electromagnetic signals can be absorbed or emitted across a wide range of wavelengths. The following discussion regarding metal particle structures is not intended to be limiting to the scope of the invention, but is for purposes of illustration only. Other structures, including fractal structures can be desirable. A. Manufacture of Metal Particles

To make metal particles for according to some embodiments of this invention, we can generally use methods known in the art. Tarcha et al., U.S. Patent No: 5,567,628, incorporated herein fully by reference. Metal colloids can be composed of noble metals, specifically, elemental gold or silver, copper, platinum, palladium and other metals known to provide broad band absorption and emission in a desirable spectral range. In general, to make a metal colloid, a dilute solution containing the metal salt is chemically reacted with a reducing agent. Reducing agents can include ascorbate, citrate, borohydride, hydrogen gas, and the like. Chemical reduction of the metal salt can produce elemental metal in solution, which combine to form a colloidal solution containing metal particles that are relatively spherical in shape.

Example 1: Manufacture of Gold Colloid and Fractal Structures In one embodiment of this invention, a solution of gold nuclei is made by preparing a 0.01% solution of NaAuCl₄ in water under vigorous stirring. One milliliter ("ml") of a solution of 1% sodium citrate is added. After 1 minute of mixing, 1 ml of a solution containing 0.075 % NaBH₄ and 1% sodium citrate is added under vigorous stirring. The reaction is permitted to proceed for 5 minutes to prepare the gold nuclei having an average diameter of about 2 nm).

The solution containing the gold nuclei can be refrigerated at 4° C until needed. This solution can be used as is, or can be used to produce particles of larger size (e.g., up to about 50 nm diameter), by rapidly adding 30 μl of the solution containing gold nuclei and 0.4 ml of a 1% sodium citrate solution to the solution of 1% HAuCl₄3H₂O diluted in 100 ml H₂O, under vigorous stirring. The mixture is boiled for 15 minutes and is then cooled to room temperature. During cooling, the particles in the solution can form fractal structures. The resulting colloid and/or fractal particle structures can be stored in a dark bottle.

Deposition of enhancing particles on dielectric surfaces including glass can generate films that can enhance electromagnetic signals. Such films can be as thin as about 10 nm. In particular, the distribution of electric field enhancement on the surface of such a film can be uneven. Such enhancing areas are resonance domains. Such areas can be particular useful for positioning receptors for analyte binding and detection. For films or particle structures embedded in dielectric materials, one way to manufacture enhancing structures is to treat the surface until "percolation points" appear. Methods for measuring sheet resistance and bulk resistance are well known in the art.

Example 2: Manufacture of Metal Particles and Fractal Structures Using Laser Ablation

In addition to liquid phase synthesis described above, laser ablation is used to make metal particles. A piece of metal foil is placed in a chamber containing a low concentration of a noble gas such as helium, neon, argon, xenon, or krypton. Exposure to the foil to laser light or other heat source causes evaporation of the metal atoms, which, in suspension in the chamber, can spontaneously aggregate to form fractal or other particle structures as a result of random diffusion. These methods are well known in the art.

B. Manufacture of Films Containing Particles To manufacture substrates containing metal colloidal particles of one embodiment of this invention, the colloidal metal particles can be deposited onto quartz slides as described in Examples 1 or 2. Other films can be made that incorporate random structures or non-fractal ordered structures in similar fashions.

Example 3: Manufacture of Quartz Slides Containing Gold Fractal Structures

Quartz slides (2.5 cm x 0.8 cm x 0.1 cm) are cleaned in a mixture of HCl:HNO₃ (3: 1) for several hours. The slides are then rinsed with deionized H₂O (Millipore Corporation) to a resistance of about 18 MΩ and then with CH₃OH. Slides are then immersed for 18 hours in a solution of aminopropyltrimethoxysilane diluted 1 :5 in CH₃OH. The slides are then rinsed extensively with CH₃OH (spectrophotometric grade) and deionized H₂O prior to immersion into colloidal gold solution described above. The slides are then immersed in the gold colloid solution above. During this time, the gold colloid particles can deposit and can become attached to the surface of the quartz slide. After 24 hours, colloid derivatization is complete. Once attached, the binding of colloidal gold nanocomposites to the quartz surfaces is strong and is essentially irreversible. During the procedure, ultraviolet and/or visual light absorbance spectra of such derivatized slides are used to assess the quality and reproducibility of the derivatization procedure. The manufacturing process is monitored using electron microscopy to assess the density of the colloidal coating, the distribution of gold colloid particles on the surface, and the size of the gold colloid particles.

C. Aggregation of Particles to Form Particle Structures According to other embodiments of this invention, several methods can be used to form particle structures. It is known that metal colloids can be deposited onto surfaces, and when aggregated can form fractal structures having a fractal dimension of about 1.8. Safonov et al., Spectral Dependence of Selective Photomodification in Fractal Aggregates of Colloidal Particles, Physical Review Letters 80C5 : 1102-1105 (1998) incorporated herein fully by reference. Figure 1 depicts a particle structure suitable for use with the methods of this invention. The particles are arranged in a scale-invariant fashion, which promotes the formation of resonance domains upon illumination by laser light.

In addition to fractal structures, ordered non-fractal structures and random structures can be generated. These different types of structures can have desirable properties for enhancing signals associated with detection of analytes using electromagnetic radiation.

To make ordered non-fractal structures, one can use, for example, chemical linkers having different lengths sequentially as described in more detail below. In addition, using linkers of the same size, one can generate ordered structures, which can be useful for certain applications. In certain embodiments of this invention, particles can be attached together to form structures having resonance properties. In general, it can be desirable to have the particles being spheres, ellipsoids, or rods. For ellipsoidal particles, it can be desirable for the particles to have a long axis (x), another axis (y) and a third axis (z). In general, it can be desirable to have x be from about 0.05 to about 1 times the wavelength (λ) of the incident electromagnetic radiation to be used. For rods, it can be desirable for x to be less than about 4 λ, alternatively, less than about 3 λ, alternatively less than about 2 λ, in other embodiments, less than about lλ, and in yet other embodiments, less than about Vi λ. The ends of the rods can be either flat, tapered, oblong, or have other shape that can promote resonance.

For two particle structures, it can be desirable for the particle pair to have an x dimension to be less than about 4 λ, alternatively, less than about 3 λ, alternatively less than about 2 λ, in other embodiments, less than about lλ, and in yet other embodiments, less than about Vz λ.

For two-dimensional structures, pairs of particles, rods, rods plus particles together can be used. The arrangement of these elements can be randomly distributed, or can have a distribution density that is dependent upon the scale of observation in a non-linear fashion.

In other embodiments, rods can be linked together end-to end to form long structures that can provide enhanced resonance properties.

For three-dimensional structures, one can use regular nested particles, or chemical arrays of particles, associated either by chemical linkers in a fractal structure or in ordered, nested arrays.

In yet other embodiments, of third-order structures, a suspension of particles can be desirable. In certain of these embodiments, the suspended particles can have dimensions in the range of about Vz λ to about 1 millimeter (mm).

Using the strategies of this invention, a researcher or developer can satisfy many needs, including, but not limited to selecting the absorbance of electromagnetic radiation by particle elements, the nature of the surface selected, the number of resonance domains, the resonance properties, the wavelengths of electromagnetic radiation showing resonance enhancement, the porosity of the particle structures, and the overall structure of the particle structures, including, but not limited to the fractal dimensions of the structure(s).

1. Photoaggregation Photoaggregation can be used to generate particle structures that have properties which can be desirable for use in broad band electromagnetic radiation absorbers. Irradiation of fractal metal nanocomposites by a laser pulse with an energy above a certain threshold leads to selective photomodification, a process that can result in the formation of "dichroic holes" in the absorption spectrum near the laser wavelength (Safonov et al., Physical Review Letters 80(5 : 1102- 1105 (1998), incorporated herein fully by reference). Selective photomodification of the geometrical structure can be observed for both silver and gold colloids, polymers doped with metal aggregates, and films produced by laser evaporation of metal targets.

One theory for the formation of selective photomodification is that the localization of optical excitations in fractal structures are prevalent in random nanocomposites. According to this theory, the localization of selective photomodification in fractals can arise because of the scale-invariant distribution of highly polarizable particles (monomers). As a result, small groups of particles having different local configurations can interact with the incident light independently of one another, and can resonate at different frequencies, generating different domains, called herein "optical modes." According to the same theory, optical modes formed by the interactions between monomers in fractal are localized in domains that can be smaller than the optical wavelength of the incident light and smaller than the size of the clusters of particles in the colloid. The frequencies of the optical modes can span a spectral range broader than the absorption bandwidth of the monomers associated with plasmon resonance at the surface. However, other theories may account for the effects of photomodification of fractal structures, and this invention is not limited to any particular theory for operability. Photomodification of silver fractal aggregates can occur within domains as small as about 24 x 24 x 48 nm³ (Safonov et al., Physical Review Letters 80(5): 1102- 1105 (1998), incorporated herein fully by reference). The energy absorbed by the fractal medium can be localized in a progressively smaller number of monomers as the laser wavelength is increased. As the energy absorbed into the resonant domains increases, the temperature at those locations can increase. At a power of 11 mJ/cm², light having a wavelength of 550 nm can produce a temperature of about 600 K (Safonov et al., Physical Review Letters

80(5): 1102-1105 (1998), incorporated herein fully by reference). At this temperature, which is about one-half the melting temperature of silver, sintering of the colloids can occur (Safonov et al., Id.) incorporated herein fully by reference), thereby forming stable fractal nanocomposites. As used in this invention, photoaggregation can be accomplished by exposing a metal colloid on a surface to pulses of incident light having a wavelengths in the range of about 400 nm to about 2000 nm. In alternative embodiments, the wavelength can be in the range of about 450 nm to about 1079 nm. The intensity of the incident light can be in the range of about 5 mJ/cm² to about 20 mJ/cm². In an alternative embodiment, the incident light can have a wavelength of 1079 nm at an intensity of 11 mJ/cm².

Fractal aggregates that are especially useful for the present invention can be made from metal particles having dimensions in the range of about 10 nm to about 100 nm in diameter, and in alternative embodiments, about 50 nm in diameter. A typical fractal structure of this invention is composed of up to about

1000 particles, and an area of the aggregate typically used for large-scale arrays can have a size of about 100 μm x 100 μm.

Figure 2 depicts a particle structure that have been photoaggregated and that are suitable for use with the methods of this invention. Local areas of fusion of the metal particles can be observed (circles). H. Particle Structures

In certain embodiments of this invention, the particle structures of this invention can have certain properties of fractals. Fractals are structures which display a pattern of self-similarity. Self-similarity means that their overall structure is scale-invariant in that the structure appears similar over a wide range of magnifications. Fractal-like structures are widely present in nature, by way of example only, clouds. Fractal objects can be also generated artificially. For example, when the landscape of metal surfaces is arranged in the shape of self-similar triangles or other shapes, such fractal objects can serve as a so called "fractal antenna." Such antennas can allow a broader range of radio waves reception and transmission than antennas having more regular structures.

Analogously, when metal particles having diameters of about 1/10 the wavelength of the incident light are arranged on a surface in the form of a fractal-like structure, such fractal surface can display absorbance in a broad optical range (Shalaev V. M., et.al. J. Nonlinear Optical Physics & Materials

70}:131-152 (1998), incorporated herein fully by reference). In this case, fractal-like structure can be due to the fact that the number of particles per a surface area decreases upon increase in the scale of such area. One theory to account for the absorbance of such fractal surfaces is the interference of oscillating dipole moments induced by the electromagnetic waves in individual particles. According to one theory, incident photons induce a field across a particle and thereby can cause mobile electrons in the metal to move with a frequency of the incident electromagnetic field. Such collective movement is referred to herein as a "plasmon wave." According to one theory, the collective oscillation of electrons occurs due to strong dipolar and multipolar interactions of plasmon waves in metal particles within a fractal structure. Because almost any distances and orientations of metal particles can be present in a such fractal system, a large number of possible "resonant cavities" are formed within such structures. By the term "resonant cavity," we mean an arrangement of particles which provides resonance conditions for the wavelengths of incident light. Each of such cavities absorbs and/or emits electromagnetic wave at a specific set of wavelengths,. Because many different resonance cavities can be present in a nanoparticle structure, many different frequencies can be absorbed and/or emitted by the structure. Thus, the collective capacity of all such cavities to resonate within a broad range of wavelengths yields a broad absorption band of such fractal structures. Chemical reduction of metal colloidal solutions, laser ablation of surfaces, surface etching, annealing of films, are, by way of example only, used for generation of fractal-like structures (Kreimer et al., United States Provisional Patent Application, titled "Nanoparticle Structures With Receptors for Raman Spectroscopy," Inventors: David I. Kreimer, Ph.D., Oleg A. Yevin, Ph.D., Thomas H. Nufert, filing Date: September 27, 1999; "Shalaev et. al. (1998), both references incorporated herein fully by reference). Typically fractal-like structures can be obtained by allowing a far-from-equilibrium system to attain a low energy state. In such systems fractal-like structures can form spontaneously. In addition to fractal structures, other types of particle structures can result in the enhanced absorptive and/or emissive properties of this invention.

The manufacture of particle structures that can be applied to the surface of the heat pipe is described herein below.

1. Chemically Directed Synthesis of Particle Structures In certain embodiments of this invention, nanoparticle structures can be made using chemical methods. First, metal particles can be either made according to methods described above, or alternatively can be purchased from commercial suppliers (NanoGram Inc., Fremont, California). Second, the particles can be joined together to form first-order structures, for example, pairs of particles. Then, the first-order structures can be joined together to form second-order structures, for example, pairs of particle pairs. Finally, third-order fractal structures can be made by joining second-order structures together.

In alternative embodiments of this invention, the formation of a fractal array of metal particles can be carried out using chemical methods. Once metal colloid particles have been manufactured, each particle can be attached to a linker molecule via a thiol or other type of suitable chemical bond. The linker molecules then can be attached to one another to link adjacent colloid particles together. The distance between the particles is a function of the total lengths of the linker molecules. It can be desired to select a stoichiometric ratio of particles to linker molecules. If too few linker molecules are used, then the array of particles will be too loose or may not form at all. Conversely, if the ratio of linker molecules to particles is too high, the array may become too tight, and may even tend to form crystalline structures, which are not random, and therefore will not tend to promote Raman resonance.

In general, it can be desirable to perform the linking procedure sequentially, wherein the first step comprises adding linker molecules to individual particles under conditions that do not permit cross-linking of particles together. By way of example only, such a linker can comprise an oligonucleotide having a reactive group at one end only. During this first step, the reactive end of the oligonucleotide can bind with a metal particle, thereby forming a first particle-linker species, and having a free end of the linker. The ratio of linker molecules to particles can be selected, depending on the number of linker molecules are to be attached to the particle. A second linker can be attached to another group of particles in a different reaction chamber, thereby resulting in a second linker-particle species, again with the linker having a free end.

After those reactions have progressed, the different linker-particle species can be mixed together and the linkers can attach together to form "particle pairs" joined by the linker molecules. By way of example, Figure 2a to 2c illustrates methods for manufacturing fractal structures of this invention. In

Figure 2a, metal particles 10 are formed using methods previously described.

Short linkers 20 have chemically active ends that are capable of binding to metal particles 10. For example, linker 20 has sulfhydryl ("SH") groups at each end of the linker 20. When combined, metal particles 10 bind with the SH ends of linkers 20 to form particle pairs 30.

Figure 2b illustrates the steps that can be used to form clusters of particle pairs. Particle pairs 30 are reacted with medium-length linkers 40 to form clusters 50. Figure 2c illustrates the steps that can be used to form nanoscale fractal structures of this invention. Clusters 50 are reacted with long linkers 60 to form nanoscale fractal structure 70.

Linker molecules can be selected to provide any desired length.

Typically, polymers of organic moieties can be useful. For example, linking can be carried out using an aryl di-thiol or di-isonitrile molecules. Alternatively one can use any active moiety that can be used to attach the linker to the metal particle. It can be desirable to use the above types of aryl linkers with nucleic acid or other types of linker molecules. The linker can have a central area having ethylbenzene moieties, where n is a number between 1 and about 10,000. In general, the ratio of length for each subsequent pairs of linkers can be in the range of about 2 to about 20. Alternatively, the ratios of lengths of subsequent pairs of linkers can be in the range of about 3 to about 10, and in other embodiments, about 5.

For example, for a three-order manufacturing process involving linkers 1, 2, and 3 (LI, L2, and L3, respectively), it can be desirable for the ration of LI :L2:L3 to be in the range of about 1 :2:4. Alternatively, the ratio can be about

1:5:25, and in yet other embodiments, the ratio can be about 1:20:400. In other embodiments, the ratio between LI and L2 and from L2 to L3 need not be the same. Thus, in certain embodiments the ration of L1 :L2:L3 can be 1:3:20, or alternatively, 1 :20:40. Under these conditions, one can manufacture structures having any desired porosity. In general, the size of the nanoscale structures should have average dimensions in the range of about 20 nm to about 10,000 nm. In alternative embodiments, the dimensions can be in the range of about 50 nm to about 300 nm, and in other embodiments, in the range of about 100 to about 200 nm, and in yet other embodiments, about 150 nm.

For certain applications of this invention, it can be desirable to use nanoparticles having a variety of different diameters. Thus, one can manufacture a nanoparticle structure starting with particles having diameters of from about 20 nm to about 10,000 nm in the same reaction. Additionally, one can vary the shape of the particles to achieve improved heat transfer. By way of example, rod-shaped particles with a ratio of length to diameter of about 100: 1 to 2: 1, alternatively, from about 50:1 to about 5:1, and in other embodiments, about 20:1 to about 10: 1.

Besides applications in optical spectroscopy (SERS, surface plasmon resonance spectroscopy, fluorescence, surface-enhanced infrared absorption spectroscopy and other spectroscopic techniques), fractal-like structures prepared from metal particles can be used as broad wavelength filters or antennae.

In addition, such systems can be used in a broad range of thermal-exchange methods and devices. For example, materials can be prepared from metal particles arranged in a fractal-like structure, which will have superior thermal absorption - emission properties. These films can be used with heat-pipes for the removal of heat and integrated with materials for use in building construction, engine cooling systems, microchip cooling, and space technology. In addition, a clothing material could use this invention for military purposes to obscure infrared detection (night vision).

2. Manufacture of Nested Particle Associates

A nested particle associates can be made by selecting colloidal solutions of metal gold particles of uniform size, being 10 nm, 40 nm and 240 nm in diameter, respectively. A plurality of 10 nm gold particles having a linker, such as DNA for example, can be attached thereto. A plurality of 40 nm particle is produced, each having a linker, such as DNA complementary to DNA linker of the 10 nm particle. Mixtures of the linker-derivatized 10 and 40 nm particles are placed in solution and interact with each other. The DNA linkers bind to each other to form a first-order nested structure.

A second-order nested particle structure comprising a plurality of first- order particle structures surrounding a larger particle larger than either of the first two particles, such as 240 nm. Heating the mixture of first-order particles or second-order particles to a temperature less than about 100° C and then cooling the mixture can result in better ordering of the nested particles. 3. Manufacture of Surfaces Having Non-Random

Particle Structures

To manufacture heat pipes of this invention, one can deposit either chemically derived or nested particle structures onto a thermally conductive surface. The particle structures can desirably be affixed to the surface using a conductive polymer. When the so-coated surface is exposed to electromagnetic radiation, some of the incident radiation is absorbed by the particle structures and can be transmitted to an interior portion of the heat pipe. The heat inside the pipe can then be transmitted along the heat pipe to another location, where, for example a cooler environment exists, thereby permitting the heat in the heat pipe to leave the heat pipe into the surrounding environment.

In certain other embodiments of this invention, nested or other particle structures in suspension can be used. These structures can be first attached and then a polymer can be used to strengthen the attachment of the particle structures to increase the durability of the surface covering..

HI. Thermally Conductive Polymers

Thermally and environmentally stable polymers such as polyaniline- or polypyrrole-based composites are capable of heat conduction (ref to the patent ofEEONIX). Other composite materials capable of attachment of metal particles to a metal surface are also well known (U.S. Patent 5,925,467, incorporated herein fully by reference).

Example 4: Heat Pipe Embodying Fractal Heat Absorber Figure 3 depicts a portion 110 of an embodiment 300 of this invention in which a heat pipe 105, similar to that shown in Figure 1, is partially covered with a particle structure. A heat absorber portion 110 comprises a heat pipe 105, a wick element 135 having a volatile fluid 130. A portion of volatilized fluid 140 is shown adjacent to wick 135. Surrounding heat pipe 105, a layer of heat conductive polymer 145 having metal particles 150 imbedded therein. Surrounding heat conductive polymer layer 145, is a region 160 comprising nanoparticle structures.

Figure 4 depicts an embodiment of this invention 400 having a particle structure at a heat absorbing end 110 of a heat pipe. Heat pipe 105 has wick elements 135 having a volatile fluid 130 contained therein. The exterior of the heat pipe is shown having a layer of heat conductive polymer 145, which has a region of particle structures 160 applied thereon. Heat (Qin) is absorbed by particle structure 160 at heat absorber portion 110 of the heat pipe and is transmitted to the heat pipe 105 by the polymer layer 145. The heat vaporizes the volatile fluid 130 in wick region 135, creating vaporized fluid 140, which flows to the emissive end 120 of the heat pipe. At the emissive end 120 of the heat pipe, the vaporized fluid 140 condenses, releasing heat. The liquid flows into the wick structure 135 and is drawn back to heat absorber portion 110 of the heat pipe.

Figure 5 depicts an embodiment of this invention 500 having particle structures at both a heat absorbing end 110 and at an emissive end 120 of the heat pipe. The same features described above in Figure 4 apply to Figure 5.

Additionally, at emissive end 120 a layer of heat conductive polymer 146 surrounds heat pipe 105. A layer of particle structures 161 is applied to the exterior of heat conductive polymer layer 146. The types of heat conductive polymer 146 need not be the same as heat conductive polymer 145, and the particle structure 161 need not be the same as particle structures 160. Heat transferred from the absorptive end 110 of the heat pipe 500 can be carried to the emissive end 120. At the emissive end 120, the volatilized fluid 140 can condense, releasing heat "Qout" which can flow through the heat conductive polymer layer 146, to the nanoparticle structure 161, that can thereby be radiated away.

In other embodiments of this invention, particle structures can be attached directly to the surface of a heat pipe without an intervening layer of heat conductive polymer. By way of example only, photolithographic methods can be used to attach the particle structures. Such methods are described in United States Provisional Patent Application, titled "Nanoparticle Structures With Receptors for Raman Spectroscopy." Inventors: David I. Kreimer, Ph.D., Oleg A. Yevin, Ph.D., Thomas H. Nufert. Filing Date: September 27, 1999, incorporated herein fully by reference.

Furthermore, laser ablation can be used to manufacture particle structures directly on a heat pipe. These methods are described in the United States Provisional Patent Application titled "Nanoparticle Structures With Receptors for Raman Spectroscopy." Inventors: David I. Kreimer, Ph.D., Oleg

A. Yevin, Ph.D., Thomas H. Nufert. Filing Date: September 27, 1999, incorporated herein fully by reference.

INDUSTRIAL APPLICABILITY The nanoparticle structures of this invention are useful for the manufacture of improved heat pipes used to transmit heat from one point to another under radiative heat transfer conditions. Improved heat transfer can keep equipment such as mechanical and electrical equipment within desired operating temperature ranges. Thus, the nanoparticle structures of this invention can be used for solar heating devices, and to protect equipment from becoming overheated due to thermal radiation, including solar radiation.

Claims

We claim:

1. A heat pipe comprising: a pipe; and a particle structure thereon having preselected electromagnetic absorption bands.

2. The heat pipe of claim 1, further comprising a thermal conductor between said pipe and said particle structure.

3. The heat pipe of any of claims 1 - 2, further comprising: a wick; and a liquid for transferring heat from one end of said pipe to another end of said pipe.

4. The heat pipe of any of claims 1 - 3, wherein said particle structure is a fractal structure.

5. The heat pipe of any of claims 1 - 5, wherein said particle structure is a nested particle structure.

6. The heat pipe of any of claims 1 - 6, wherein said particle structure comprises particles and chemical linkers.

7. The heat pipe of any of claims 1 - 7, further comprising a plurality of particle structures.

8. The heat pipe of any of claims 1 - 8, wherein said particle structure is attached to said pipe by a polymer.

9. The heat pipe of claim 2, wherein said thermal conductor is a polymer.

10. The heat pipe of any of claims 1 - 8, wherein said particle structure has predetermined electromagnetic emission bands.

11. The heat pipe of claim 4, wherein said fractal structure has predetermined electromagnetic absorption bands.

12. The heat pipe of claim 4, wherein said fractal structure has predetermined electromagnetic emission bands.

13. The heat pipe of claim 4, further comprising a thermal conductor between said pipe and said fractal structure.

14. The heat pipe of claim 4, further comprising: a wick; and a liquid for transferring heat from one end of said pipe to another end of said pipe.

15. A method for transferring heat from a source to a sink, comprising the steps of: providing a pipe having: a plurality of particle structures thereon; a thermally conductive layer between said plurality of particle structures; a wick; and a liquid for transferring heat; exposing a first end of said pipe to said heat source; and exposing a second end of said pipe to said heat sink.