WO2009002569A2

WO2009002569A2 - Electromagnetic nanocomposites and methods of manufacture

Info

Publication number: WO2009002569A2
Application number: PCT/US2008/051725
Authority: WO
Inventors: Zhanhu Guo; Sung Park; Thomas H. Hahn
Original assignee: The Regents Of The University Of California
Priority date: 2007-01-22
Filing date: 2008-01-22
Publication date: 2008-12-31
Also published as: WO2009002569A3

Abstract

Nanocomposites of magnetic nanoparticles in a polymer matrix which has been heat treated is shown to have good giant magnetoresistance characteristics and structural integrity. The flexible nanocomposites were fabricated using a surface-initiated-polymerization (SIP) method. The uniformly distributed nanoparticles within the polymer matrix favor a continuous carbon matrix formation after heat treatment, rendering the transition from insulating to conductive composites. The coercive forces reflect strong particle loading and matrix dependent magnetic properties. One obtained nanocomposites possess fairly good giant magnetoresistance (MR), with a MR of 7.3 % at room temperature and 14 % at 130 K. Furthermore, the formed carbon matrix had a 7 wt.% argon adsorption potential for fuel cell applications. As an example, the method utilizing physicochemical adsorption of an initiator onto the iron-oxide (Fe2O3) nanoparticle surface was used for urethane polymerization in a tetrahydrofuran (THF) solution.

Description

ELECTROMAGNETIC NANOCOMPOSITES AND METHODS OF MANUFACTURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. provisional application serial number 60/886,066 filed on January 22, 2007, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

OR DEVELOPMENT [0002] This invention was made with Government support under Grant No.

FA9SS0-0S-1-0138 awarded by the Air Force AFOSR. The Government has certain rights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

[0003] Not Applicable

BACKGROUND OF THE INVENTION 1. Field of the Invention [0004] This invention pertains generally to ferromagnetic thin-film structures exhibiting giant magnetoresistive characteristics and, more particularly, to high loading iron/carbonized polyurethane nanocomposites for use in a wide variety of computing, storage and magnetic field sensing devices and methods of manufacturing such composites. 2. Description of Related Art

[0005] The change observed in the electrical resistance of a conductor in response to an applied external magnetic field is called magnetoresistance (MR) and is typically expressed as a percentage. This phenomenon was first identified by Lord Kelvin in 1856 when it was observed that the electrical resistance in an iron bar changed depending on its orientation in an applied magnetic field. The change in resistance is a consequence of deflections in the trajectories of the electrons from Lorentz type forces. Magnetoresistance values of common conductors are normally less than one percent. [0006] In 1988, it was discovered that magnetoresistance values that are substantially greater than one percent could be achieved with laminate structures of alternating layers of magnetic and non-magnetic materials.

Magnetoresistance values of as high as 80% have been achieved with multiple magnetic layers. This phenomenon was called "giant magnetoresistance" (GMR), because of the substantially large magnetoresistance values that are produced with the layered structures. [0007] The significant decrease in electronic resistance of a material in a magnetic field with GMR is due to the fact that electrons have a magnetic moment called spin. A magnetic field is created in the opposite direction of the spin of a charged electron. A material can become magnetic when the constituent electrons spin in the same direction. Electrons spinning in opposite directions will have magnetic fields that cancel each other out.

[0008] Conduction electrons encounter scattering resistance in a magnetic material that can change depending on whether their spin is oriented parallel or anti-parallel with respect to the magnetization of the material. Electrons that have a spin that are parallel to the magnetization normally undergo less scattering than those with anti-parallel spin and therefore experience lower resistance.

[0009] The magnetic moments of the magnetic layers spinning in different directions can be come aligned in parallel when an external magnetic field is applied. Conduction electrons that have spins parallel to the aligned magnetic moments they pass move freely through the material and the resistance is low. Therefore, the GMR effect depends on the ability of the magnetic field that is applied to change the alignment of the magnetic moments back and forth between the parallel and anti-parallel orientations. [0010] The giant magnetoresistance effect can be illustrated with a simple structure known as a spin valve. The spin valve has a nonmagnetic layer such as copper or chromium that is sandwiched between two ferromagnetic layers such as iron or cobalt. The magnetic layers of the spin valve will have different resistances depending on whether the magnetizations of the two layers are parallel or anti-parallel to each other. Resistance of the structure is greatest when the magnetic layers have magnetizations that are in anti- parallel alignment. The resistance of the spin valve can be changed with exposure to a magnetic field since the magnetic alignment of the consecutive magnetic layers is sensitive to the applied magnetic field.

[0011] A variety of electronic systems and devices make use of the dramatic resistance changes in alternating magnetic and non-magnetic materials within an applied magnetic field. For example, devices such as magnetometers that provide an output signal referencing the magnetic fields that are "sensed" by the devices are able to be smaller and more sensitive with GMR than seen with earlier devices. Other applications include magnetic disk memory and storage systems, automotive sensors, current transformers, motion detectors, transceivers, gradiometers, swipe-card readers, Magnetoresistive Random

Access Memory (MRAM), and biological detection or drug delivery etc. [0012] Thin films of magnetic and non-magnetic materials can be fabricated on different surfaces including integrated circuit surfaces that provide easy access with electrical connections and circuitry. The use of thin layers of magnetic and non-magnetic materials allows for the easy modulation of magnetic spins in the materials magnetically with relatively small magnetic field sources.

[0013] However, traditional thin film fabrication is an expensive process and requires complex preparation and packaging steps. In addition, the structural integrity of metallic thin film layers is often questionable due to the bhttleness that is seen in the resulting structures. Therefore, further improvement in the performance characteristics of these laminates is required before practical application can be made of them in certain circumstances. [0014] Granular thin films were developed the early 1990's using thin films of nickel in a quartz matrix. Granular GMR films have been produced where the magnetic particles are immiscible with the non-magnetic conductive matrix. The most successful granular GMR alloy has been with magnetic particles of cobalt annealed to a copper or silver metal substrate. However, granular GMR materials have not seen the same performance characteristics and high GMR ratios as produced with metal laminates. High particle loading and flexibility of the composites are also required for certain applications, such as electromagnetic wave absorbers, photovoltaic cells, photo detectors and smart structures. However, high-particle-loading in conventional granular GMR always causes brittleness and rigidity of the nanocomposite, and even introduces artificial defects, such as air voids, which limit its applications. [0015] Accordingly a need exists for an electromagnetic nanocomposite and method of manufacture that provides large ratio GMR that is adaptable to thin film applications and is inexpensive and simple to manufacture, thus enabling near-term practical applications. These needs and others are met within the present invention, which overcomes the deficiencies of previously developed apparatus and methods of fabrication of GMR materials.

BRIEF SUMMARY OF THE INVENTION

[0016] The present invention includes nanocomposites comprising a polymer matrix and magnetic nanoparticles dispersed throughout the polymer matrix for use in many different devices utilizing giant magnetoresistance, and a method of manufacturing such nanocomposites. Compared with metallic GMR structures, polymer nanocomposite GMR structures have the benefit of superior structural integrity and can be involved in sensor applications in harsh environments and in applications such as specific molecular targeting due to the multifunctional groups of the polymer matrix. The granular GMR composites of the invention can also be adapted for use in magnetic data storage (hard disk), biological detection, magnetic recording, and rotational sensing in automotive systems, etc. [0017] The fabrication methods of the present invention are simple and comparatively inexpensive and the fabrication processes can produce GMR composites for a wide array of electromagnetic applications. For example, traditional metallic GMR using thin-film technology requires complex film preparation and packaging steps to produce. Also, the nature of metallic layers severely impacts the structural integrity of the conventional sensor due to its brittleness. The metals of the conventional granular GMR may also be exposed to oxidization. [0018] In contrast, the products fabricated by the present methods are protected either with polymer or with carbon. Polymer and carbon both have functional groups or the ability to attach to external molecules and therefore make biological applications of GMR sensors possible. High loading of nanoparticles in a polymer matrix make these types of granular GMR sensors a suitable replacement for the current magnetic thin film or granular structured

GMR sensors.

[0019] According to one aspect of the invention, a nanocomposite is provided that comprises metal nanoparticles evenly dispersed within a polymer matrix providing GMR characteristics. [0020] Another aspect of the invention is to provide a nanocomposite with a polymer matrix that may be thermosetting or thermoplastic and electrically conductive. Polymers may also be carbonized with heat treatments to increase conductivity. [0021] An aspect of the invention is a composite that is loaded with greater than 50% nanoparticles within the polymer matrix that is easy to apply and adapt to conventional GMR based devices.

[0022] According to another aspect of the invention, a composite is provided that has nanoparticles that have an average diameter of less than approximately 100 nm and less than approximately 20nm. [0023] A further aspect of the invention is to provide a composite with nanoparticles and BaTiU₃ or TiO₂ filler within a polymer matrix. [0024] Another aspect of the invention is to provide a method for manufacturing a nanocomposite by activating the surfaces of a plurality of metal nanoparticles that have a metal core and an oxide shell with a promoting agent to form an activated mixture. At lease one monomer and a solvent are combined with the activated particles and the combined activated mixture, monomer and solvent to form a polymerized nanocomposite are polymerized and dried. In one embodiment, the dried nanocomposite is heated until the composite is carbonized.

[0025] A further aspect of the invention is to provide a method of fabricating a nanocomposite by blending a plurality of metal nanoparticles with at least one monomer in an inert environment using energetic mixing, such as by sonication. A promoter is mixed with the blended nanoparticles and monomer to form a polymerized nanocomposite, which is then dried. [0026] Another aspect of the invention is to provide a method of fabricating a nanocomposite of nanoparticles that are supported by carbon fibers by dissolving at least one polymer in an anhydrous, substantially organic solvent and then mixing metal nanoparticles into the polymer solution using energetic mixing. The solid and liquid phases are separated with a polar solvent and the solid phase is filtered and dried. The resulting solid phase is exposed to a reducing environment to form carbon fibers.

[0027] A still further aspect of the invention is to provide a method of manufacturing a thin film nanocomposite with evenly dispersed nanoparticles within the film by dispersing metal nanoparticles and at least one monomer in an anhydrous, anoxic solvent to provide a dispersed solution and then applying an electrochemical potential to said dispersed solution to form a thin film on an anode electrode structure. [0028] According to another aspect of the invention, nanocomposites and methods of fabrication are provided that can be easily adapted for use with a variety of magnetic particles such as iron, cobalt, nickel, mixtures thereof, alloys thereof, or oxides thereof as well as a variety of monomers and polymers.

[0029] The present invention can provide a number of beneficial aspects which can be implemented either separately or in any desired combination without departing from the present teachings. Each of the foregoing aspects, and related modes, embodiments, variations, and features thereof, is considered of independent value and benefit as an invention presented hereunder, as are the various combinations therebetween as presented throughout this disclosure and otherwise readily apparent to one of ordinary skill having reviewed these disclosed contents.

[0030] Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS

OF THE DRAWING(S) [0031] The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: [0032] FIG. 1 is a block flow diagram of one method for manufacturing a nanocomposite with the use of nanoparticles with a metal core and an oxide shell according to the invention. [0033] FIG. 2 is a block flow diagram of a method for manufacturing a nanocomposite with metal nanoparticles supported by carbon fibers according to the invention. [0034] FIG. 3 is a block flow diagram of a method for forming a thin film of magnetic nanoparticles dispersed on a polymer matrix. [0035] FIG. 4 is a graph of room temperature hysteresis loops of the as- received nanoparticles and nanocomposites produced according to the invention. [0036] FIG. 5 is a graph of resistance as a function of temperature after heat treatment at 450 ⁰C of a nanocomposite with a 65 wt.% loading of nanoparticles.

[0037] FIG. 6 is a graph of magnetoresistance at room temperature and 130 K as functions of applied magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the methods generally shown in FIG. 1 through FIG. 6. It will be appreciated that the composites may vary as to configuration and as to details of the parts, and that the methods may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein.

[0039] Magnetic nanoparticles (NPs) have attracted considerable interest due to their special physicochemical properties, such as enhanced magnetic moment and larger coercivity, which are different from their bulk and atomic counterparts. Incorporation of the inorganic nanoparticles into a polymer matrix will extend the range of particle applications (for example, as high- sensitivity chemical gas sensors). This is primarily due to the advantages of polymeric nanocomposites possessing high homogeneity, flexible processability and tunable physicochemical properties, such as improved mechanical, magnetic and conductive properties.

[0040] The present invention provides a composite with nanoparticles within a polymer matrix that provides strong chemical bonding between the particles and the polymer yet is flexible and functional, even with the high loading of nanoparticles. In addition, good GMR characteristics are seen with the use of magnetic nanoparticles in a composite produced by the fabrication methods. [0041] Turning now to FIG. 1 , one embodiment of the method 10 of fabricating a nanocomposite of the invention is set forth. This embodiment is particularly suited for metal nanoparticles that have a metal core and an oxide shell.

Variations of the method are used for nanoparticles that have minimum to zero oxides on the surface of the particles.

[0042] Initially, nanoparticles are provided with dimensions that are suited to the particular GMR application. In general, the particle size and uniformity can influence the observed magnetoresistance effect. The observed magnetoresistance (GMR) effect appears to be inversely proportional to the average diameter size of the ferromagnetic particles. Accordingly, the smaller the diameter of the magnetic particles the greater the magnitude of GMR that is observed, to a point. Larger magnetic fields are required to produce the effect when the particles are too small due to superparamagnetism.

Accordingly, the dimensions of the nanoparticles can be optimized to provide the greatest GMR effect. Normally, the preferred range of nanoparticle diameters are less than approximately 100 nm in diameter. In another embodiment, the preferred range of diameters is less than approximately 20 nm in diameter. A minimum particle diameter of approximately 6nm is preferred.

[0043] Magnetic nanoparticles that are preferred for use in the nanocomposites include metal nanoparticles of iron, cobalt, nickel, or mixtures thereof, alloys thereof, or oxides thereof. Although these materials are preferred, other types of metal nanoparticles can be used. Furthermore, in other applications copper or aluminum nanoparticles may be used.

[0044] At block 12 of FIG. 1 , the surfaces of metal nanoparticles are reacted with a promoting agent to form an activated nanoparticle mixture. The response of a granular composite material to an applied stress is strongly dependent on the nature of the bonds within its microstructure. Consequently, the interfacial interactions between the nanoparticles and polymer matrix play a crucial role in determining the quality and properties of the nanocomposite. [0045] Poor bonding linkage between the fillers or nanoparticles and the polymer matrix, such as with composites made by simple mixing, can introduce artificial defects. Defects introduced from conventional fabrication schemes can have a deleterious effect on the mechanical properties of the nanocomposites. However, appropriate chemical functionalization of the nanoparticle surface by introducing proper functional groups may improve both the strength and toughness with an improved compatibility between the nanofillers and the polymer matrix, and make the nanocomposites stable in harsh environments as well.

[0046] Therefore surface functionalization of nanoparticles with a surfactant or a coupling agent is important not only to stabilize the nanoparticles during processing but also to render them compatible with the polymer matrix. In other embodiments, however, surface functionalization of the particular metal surface is not necessary and the step may be optional. [0047] At block 14 of FIG. 1 , at least one monomer is blended with the activated nanoparticles and a solvent, preferably using energetic mixing. Ultrasonic blending of the mixture is preferred. In another embodiment, fillers of BaTiU3 or TiO₂ are added and mixed. Fillers can strengthen or stiffen the composite depending on the application.

[0048] At block 16 of FIG. 1 , the activated nanoparticle monomer solvent solution polymerizes to form a nanocomposite. The embodiment shown in FIG. 1 preferably uses a nanoparticle surface initiated polymerization (SIP) method to fabricate high-loading nanoparticle reinforced polyurethane magnetic nanocomposites. The illustration of the method in Example 1 utilized the physicochemical adsorption of an initiator onto the iron-oxide (Fe₂Os) nanoparticle surface for urethane polymerization in a tetrahydrofuran (THF) solution. In one embodiment, the monomer contains a hydroxyl moiety. In another embodiment, the monomer is a vinyl ester, ester, vinyl alcohol, or urethane.

[0049] The nascent nanocomposite is then dried at block 18 of FIG. 1 to remove any remaining solvent or volatile reactants.

[0050] At block 20 of FIG. 1 , the resulting nanocomposite is optionally heat treated in order to carbonize the matrix. The post polymerization heat treatment has been shown to improve the conductivity of the composite. The preferred heat treatment is at a temperature of approximately 250⁰C to approximately 450⁰C for approximately 2 hours to approximately 4 hours. [0051] When the selected metal nanoparticles do not have an oxide shell or a minimum oxide shell (less than 10 wt.%), a variation of the method can be used as shown in Example 2 herein. In this embodiment, the metal nanoparticles can be mixed with at least one monomer in an inert environment and blended using energetic mixing (e.g. sonication). Monomers with a reactive oxygen moiety or vinyl alcohol or urethane are preferred monomers. Fillers of BaTiθ3 or TiO₂ may also be added as an option. [0052] A promoter is then added to the mixture and the composite is polymerized. The resulting composite is then preferably dried. The dried composite is then carbonized by a heat treatment as above.

[0053] Referring now to FIG. 2, an alternative embodiment of the method of fabricating a nanocomposite is shown where the nanoparticles are supported by carbon fibers. The selected nanoparticles are preferably iron, cobalt, nickel, or mixtures, alloys, or oxides thereof. Although these materials are preferred, other types of metal nanoparticles can be used.

[0054] At block 22 of FIG. 2, polymer is dissolved into an anhydrous, substantially organic solvent. In one embodiment the anhydrous organic solvent is nonpolar. One preferred polymer is polyacrylonithle (PAN). Although PAN is preferred as a polymer, it will be understood that other polymers with similar characteristics can be used.

[0055] Metal nanoparticles are then mixed into the polymer solution using energetic mixing, preferably sonication, at block 24 of FIG. 2.

[0056] At block 26, a polar solvent is added to the mixed solution to separate the phases. The resulting material is preferably filtered and air dried at block

28 of FIG. 2.

[0057] Finally, the filtered and dried mixture is reacted in a reducing environment in order to produce carbon fibers at block 30.

[0058] Turning to FIG. 3, another embodiment of the invention for the preparation of a thin film composite on a substrate is shown. In this embodiment, metal nanoparticles are combined with a monomer and anhydrous, anoxic solvent at block 32. The nanoparticles are preferably iron, cobalt, nickel, or mixtures, alloys, or oxides thereof. However, it will be understood that other metals can be used and composites created for other purposes as these metals are for purposes of illustration. The nanoparticles are preferably less than approximately 100nm in diameter. In another embodiment, the particle diameters are approximately 20 nm. The preferred monomer is pyrrole and the preferred solvent is nonpolar.

[0059] At block, 34 of FIG. 3, an electrochemical potential is applied to the solution, preferably in an inert environment. The preferred electrochemical potential is approximately 0.7 Volts/SCE. [0060] A thin film is formed at the anode electrode at block 36 of FIG. 3. The cathode electrode is preferable made of gold. [0061] The invention may be better understood with reference to the accompanying examples, which are intended for purposes of illustration only and should not be construed as in any sense limiting the scope of the present invention as defined in the claims appended hereto.

Example 1 [0062] To illustrate the processing and characterization of a granular GMR nanocomposite that consists of iron particles dispersed in a carbon matrix, two different nanocomposites were fabricated with two different particle loadings.

A surface initiated polymerization method was used to load the nanoparticles. [0063] In this example, processing begins with the fabrication of a polyurethane matrix composite reinforced with nanoparticles (NPs) having an iron core/iron oxide shell structure. The surface-initiated polymerization (SIP) method allows a high loading, up to 65 weight percent (wt.%) of nanoparticles to be incorporated into the polymer. The iron nanoparticles with an oxide shell had an approximate diameter of 20 nm. [0064] In the SIP method, both the catalyst (a liquid containing aliphatic amine, parachlorobenzothfluohde and methyl propyl ketone) and the accelerator (polyurethane TD-I02, containing organo-titanate) were added into an iron- nanoparticle suspended tetrahydrofuran (THF) solution. The two-part monomers (diisocyanate and diol) were then introduced into this solution to polymerize for 6 hours, and then poured into a mold for final curing. All the operations (stirring and reaction) were carried out with ultrasonication. [0065] Nanocomposites with two different particles loadings of 35 wt.% and 65 wt.%, respectively, were fabricated using the SIP method. The composites were heat treated at 250 ⁰C for 2 hours in hydrogen gas balanced with nitrogen (5%). In order to carbonize the matrix, the nanocomposite with 65 wt.% particle loading was further heat treated at 450 ⁰C for 2 hours in the same environment to obtain an iron/carbon nanocomposite. [0066] Particle structural characterization was performed by transmission electron microscopy (TEM) using a JEOL TEM-2010 microscope with an accelerating voltage of 200 kV. The valence state in the iron nanoparticles was determined by X-ray photo-electron spectroscopy (XPS). The weight percentage of nanoparticles in the nanocomposites was determined by thermogravimethc analysis (TGA) with an argon flow rate of 50 cubic centimeters per minute. The fabricated polyurethane polymer matrix, particle loading and heat treatment effect on the magnetic properties were investigated in a 9-Tesla Physical Properties Measurement System (PPMS) by Quantum Design. The electric conductivity and magnetic field dependent resistance were carried out using a standard four-probe method.

[0067] TEM studies showed bright field microstructures around the "as- received iron nanoparticles" due to the oxidation of the iron nanoparticle surface. X-ray photo-electron spectroscopy studies show that the iron oxide was Fe₂O₃ rather than other oxides (FeO and Fe₃O₄). Lattice distances of

0.2035 nm (ring 1 ), 0.1434 nm (ring 3), 0.1 158 nm (ring 4),0.1004 nm (ring 5), and 0.0827 nm (ring 6) of the selected area electron diffraction (SAED) were observed and were assigned to (1 10), (200), (21 1 ), (220) and (222) planes of Fe (Standard XRD file: PDF#06-0696 of iron) respectively. In addition 0.1673 nm (ring 2) arising from the (430) plane of iron oxide (Standard XRD file:

PDF#39-1346 of Fe₂O₃) was observed.

[0068] The clear lattice fringes that were seen indicate a relatively high crystallinity of the nanoparticles. The discontinuous fringes indicate the existence of a small number of defects within the NPs. The calculated fringe spacing of 0.35 nm corresponds to the standard (21 1 ) plane of Fe₂O₃ with a reported d-spacing of 0.341 1 nm (ref: PDF#39-1346), indicating partial oxidation of the iron NPs which was consistent with SAED analysis. [0069] Similarly, TEM scans of the bright field microstructures of the nanocomposite (65 wt.%) after heat treatment at 250 ⁰C for 2 hours and 450 ⁰C for additional 2 hours in hydrogen gas balanced with ultra high purity argon (5 v%) were taken. There is almost no microstructural change observed in the first heat treatment stage (250 ⁰C), and the nanocomposite has little mass loss. However, a large shrinkage is observed in the second stage (450 ⁰C), indicating decomposition of the polymer. The inner ring of the SAED patterns with a d-spacing of 0.34 nm clearly indicated the formation of graphite carbon.

[0070] Clear lattice fringes of high-resolution TEM scans indicate the formation of highly crystalline nanoparticles. The calculated lattice distance of 0.21 nm corresponds to iron nanoparticles, and the surrounding lattice fringe spacing of 0.34 nm corresponds to the (002) plane of graphite carbon. This indicates that iron nanoparticles are embedded in a carbon matrix. No oxides were observed remaining in the nanoparticles indicating that the high temperature heat treatment favors the reduction of iron oxides.

[0071] The particle distribution within the polyurethane matrix before the heat treatment was characterized by a scanning electron microscope (SEM). The samples were prepared by embedding the flexible composite in a cured vinyl ester tab and polishing with 4000 grit sand paper. SEM images of the cross- sectional area of the nanocomposite with a particle loading of 65 wt.% were taken. The uniform particle distribution and no obvious particle agglomeration indicated that the SIP method yields a high-quality nanocomposite, as compared with a direct mixing method which results in a brittle nanocomposite. [0072] Turning now to FIG. 4, the room-temperature hysteresis loops of the as-received nanoparticles and the nanocomposites is shown. Hysteresis loops of as-received nanoparticles (a); nanocomposites with a particle loading of 35 wt.% (b) and 65 wt.% (c); and nanocomposite with 65 wt.% particle loading with heat treatment at 250°C for 2 hours (d) and 450⁰C for additional 2 hours (e) are plotted.

[0073] The saturation magnetization (M_s, 97.6 emu/g, based on the total mass) of the as-received nanoparticles is lower than that of the pure bulk iron (222 emu/g), which is as expected because of the presence of oxide shells. The lower coercive force (coercivity, H₀ , 5 Oe) indicates the superparamagnetic behavior of the as received nanoparticles. Little difference in M_s is observed for the nanoparticles after they are embedded in the polymer matrix. The saturation magnetizations of the nanocomposites, 54 emu/g and 31.6 emu/g for the particle loadings of 65 wt.% and 35 wt.%, respectively, correspond to 84 emu/g and 90.2 emu/g for the nanoparticles. The slightly lower M_s in the nanocomposites than in the "as-received" nanoparticles may be attributed to the further oxidation of the NPs during the nanocomposite fabrication process and the particle-polymer surface interaction effect. [0074] The coercivities of the polyurethane nanocomposites are 685 Oe and 900 Oe for 65 wt% and 35 wt% loading, respectively, which are much larger than that of the as-received NP assembly. Such behavior, however, is typical of magnetic nanocomposites.

[0075] The heat treatment at 250⁰C of the nanocomposite does not show any significant changes in mass, volume, M_s, or H₀, indicating good thermal stability. However, the heat treatment at 450⁰C brings about many changes in the nanocomposite. I was seen that the higher heat treatment carbonizes the matrix and reduces the oxide shells to iron. The mass loss and shrinkage in the matrix essentially increased the particle loading for the composite. All these changes effectively increases M_s while reducing H_c. [0076] Likewise, with the 65 wt.% nanocomposite, the H_c remains practically the same after heat treatment at 250⁰C but decreases to 165 Oe after the additional heat treatment at 450⁰C. This trend is believed to be due to the interparticle dipolar interaction within the nanocomposite with a good dispersion of single-domain NPs, consistent with particle-loading-dependent coercivity in the nanoparticle assembly.

[0077] Compared with the 35 wt.% nanocomposite, the smaller coercivity in the 65 wt.% nanocomposite arises from the decreased interparticle distance concomitant with a stronger dipolar interaction. The further decrease in H_c after the 450⁰C heat treatment is for the same reason, i.e. the decreased interparticle distance resulting from shrinkage. In addition, the presence of an oxide shell around the metallic core has been shown to increase the blocking temperature of nanoparticles through an exchange coupling interaction between the ferromagnetic metal core and the anti-ferromagnetic oxide shell. Thus, the loss of the exchange coupling in the heat treated nanocomposites due to the disappearance of anti-ferromagnetic oxide shell also contributes to the smaller coercivity.

[0078] No electrical conductivity is detected in the polyurethane nanocomposites, even at 65 wt.% loading, indicating the particle loading is still lower than the percolation threshold. The conductivity improves considerably after the high-temperature treatment. [0079] FIG. 5 shows the temperature dependent resistance of the 65 wt.% nanocomposite after heat treatment at 450⁰C. The resistance increases dramatically with decreasing temperature, characteristic of a non-metallic behavior. Equipment limitations precluded us from measuring the resistance at temperatures below 80 K. Contrary to the as-prepared nanocomposites, those heat treated at 250⁰C show somewhat improved electric conductivity.

However, the resistance is still about ten times higher than that of the 450 ⁰C heat treated specimen. Also, the lowest possible measurement temperature decreases from 125 K to 80 K as the heat treatment temperature is increased from 250⁰C to 450⁰C. In view of the high conductivity of iron, the high resistance observed in the 450⁰C heat treated specimen is believed to be due to the poor conductivity of the carbon matrix. The observed linear relationship between the logarithmic resistance and the square root of temperature T^"1/2 shown in FIG. 5 indicates a spin-dependent interparticle tunneling/hopping conduction mechanism. [0080] FIG. 6 shows the magnetoresistance (MR) at room temperature of a nanocomposite with 65 wt.% particle loading at 130 K as functions of applied magnetic field. Magnetoresistance as a function of the applied magnetic field H, where MR (%) is defined as:

MR(^o/o) = ^ -^ χ ioo R(O) [0081] The 250⁰C heat treated nanocomposite has a MR of 7 % at liquid nitrogen temperature whereas the 450⁰C heat treated nanocomposite shows a MR of 7.3 % at room temperature and 14 % at liquid nitrogen temperature, all at a fairly high field of 90 kθe. Compared with multilayered GMR materials, a high magnetic field is required to saturate the MR, which is characteristic of the tunneling conduction mechanism. However, a 2 % MR observed at 4000 Oe still indicates that this low-field GMR sensor could be used for biological targeting application. [0082] An initial adsorption test shows that, as expected, the high-temperature heat treated nanocomposite has a fairly high porosity, adsorbing about 7 wt.% of argon. This suggests that this magnetic material can be used for water distillation, as well as for tail gas catalysis or hydrogen storage for fuel cell applications. [0083] Accordingly, a granular GMR nanocomposite can be synthesized using the surface-initiated polymerization to accommodate a high particle loading required and the subsequent heat treatment to induce carbonization in the matrix and reduce oxide shells in the nanoparticles. The final iron/carbon nanocomposite exhibited a room-temperature giant magnetoresistance effect of 7 % at 90 kOe and the interparticle spin-dependent tunneling hopping conduction mechanism. The GMR effect can be further improved with the use of conductive polymer as the matrix for the nanocomposite since it reduces the resistance at the absence of a magnetic field.

Example 2 [0084] In order to further illustrate the methods, a composite was made from bare iron nanoparticles. Fabrication of a cured vinyl ester resin composite reinforced with Fe nanoparticles having an approximate diameter of 20 nm (provided by QuantumSphere Inc.) was undertaken. The bare Fe nanoparticles were very reactive and were easily oxidized. The core-shell structures in the bright-field transmission electron microscope (TEM) images indicated the partial particle oxidation arising from TEM sample preparation.

The nanoparticle production, transfer and composite fabrication are all done in an ultra-high purity nitrogen atmosphere to prevent particle oxidation. [0085] The nanocomposite fabrication began with mixing of an amount of Fe nanoparticles in a 2-neck flask and wetted by the nitrogen degassed vinyl ester resin (30 g) under ultrasonically stirred conditions and kept stirred for another 2 hours until uniform dispersion was obtained. A mixture of the nitrogen-degassed catalyst (2.0 wt%, Trigonox 239-A, organic peroxide, Akzo Nobel Chemicals) and promoter (0.3 wt%, cobalt naphthenate) was introduced quickly. The final solution was poured into silicone molds for 24-hour room temperature curing followed by post curing at 100 ⁰C for 2 hours. Vinyl ester resin in the nanocomposites was found to be fully cured under differential scanning calohmetry (DSC).

[0086] Nanocomposites with different particle loadings were fabricated based on the functionality of the vinyl ester monomers. The hydroxyl groups displace with the Fe nanoparticles was verified by x-ray photoelectron spectroscopic and FT-IR spectroscopic studies. The double carbon-carbon bonds on the other side crosslink with the extra vinyl ester resin (styrene for polymer chain growth or vinyl ester monomers for polymer cross-linking growth) for robust nanocomposite formation. [0087] In order to carbonize the matrix, the nanocomposites with different particle loadings were heat treated at 450 ⁰C for 2 hours in hydrogen gas, balanced with ultra-high purity argon (5%). Particle structures were characterized with a transmission electron microscope with an accelerating voltage of 200 kV. Weight percentage of nanoparticles in the nanocomposites was determined by the thermogravimetric analysis (TGA, PerkinElmer) with an argon flow rate of 50 cm³/min. The polymer matrix, particle loading and heat treatment effect on the magnetic properties were investigated in a 9-Tesla Physical Properties Measurement System (PPMS) by Quantum Design. The electric conductivity and magnetic field dependent resistance were carried out using a standard four-probe method. [0088] As indicated, the composite samples were prepared by displacement reaction between Fe nanoparticles and vinyl ester resin in ultrasonic stirring and nitrogen protection conditions, washing with tetrahydrofuran and drying in a vacuum oven. As compared to the reported coercive force (coercivity, H₀) of 5 Oe for the bare superparamagnetic Fe nanoparticles, H₀ is observed to increase to 153 Oe after stabilization with vinyl ester monomers. This is due to the interparticle dipolar interaction within the nanocomposite with a good dispersion of single-domain nanoparticles, consistent with a particle-loading- dependent coercivity in a nanoparticle assembly. Little difference was observed after annealing. However, the saturation magnetization (76 emu/g) of the as-prepared nanocomposites increased to 109 emu/g after the heat treatment, due to the decomposition of vinyl ester monomers. The lower saturation magnetization in both composite samples indicated that the majority of the iron atoms on the nanoparticle surface have become a nonmagnetic salt by the displacement reaction. The as-prepared vinyl ester monomer stabilized Fe nanoparticles exhibited a lower electric resistivity as compared to the one after heat treatment. The resistivity of the monomer stabilized nanoparticles increased slowly with decreasing temperature in the range of room-temperature to 100 ⁰C and then remains constant at temperatures lower than 100 ⁰C. However, the nanocomposites become less conductive after the heat treatment. The resistivity increases slowly from room-temperature to 100 ⁰C and suddenly increased beyond the equipment limitation. The linear relation between logarithmic resistivity and the square root of temperature T^"1/2 indicating a tunneling conductive mechanism for the heat-treated vinyl ester monomer stabilized nanoparticles. Both non-metallic behaviors observed in the as-prepared and heat-treated monomer stabilized Fe nanoparticle samples indicate that vinyl ester monomers and the subsequently carbonized vinyl ester have effectively protected Fe nanoparticles from oxidation. However, there are three regions for the as-prepared vinyl ester monomer stabilized nanoparticles. This behavior could be due to the thermal shrinkage/expansion of the stabilized polymer chain with a change of the temperature. A room temperature GMR of 1 .7% is observed in the heat-treated vinyl ester stabilized Fe nanoparticles. However, only 0.9% MR is observed in the as-prepared vinyl ester monomer stabilized Fe nanoparticles. [0090] The particle distribution within the cured vinyl ester matrix before the heat treatment was characterized by a scanning electron microscope (SEM). The samples were prepared by polishing the cured vinyl-ester samples with

4000 grit sandpaper. The uniform particle distribution and no obvious particle agglomeration indicate that vinyl ester has effectively protected the nanoparticles from agglomeration by displacement reaction with the active Fe nanoparticles. A large shrinkage was observed in the nanocomposites after a 2-hour 450 ⁰C annealing process. There is no further polymer residue after the heat-treatment as evidenced by FT-IR investigation. These indicate the complete decomposition of the cured vinyl ester resin polymer matrix. Room- temperature hysteresis loops of the nanocomposites with a particle loading of 15 and 40 wt% before and after heat-treatment, respectively, were plotted. Larger coercivity (230 Oe) was observed after the Fe nanoparticles are dispersed in the vinyl ester resin nanocomposites, as compared to those of the bare superparamagnetic and vinyl ester monomer stabilized Fe nanoparticles. This indicated a weak interparticle dipolar interaction after the nanoparticles are dispersed into the polymer matrix. However, H₀ is much lower than those (685 and 900 Oe for 65 and 35 wt% loading, respectively) of the iron/polyurethane system. The saturation magnetizations of the nanocomposites are 34 and 72 emu/g for the particle loadings of 15 and 40 wt%, corresponding to 213 emu/g and 183 emu/g for the nanoparticles, respectively. Ms in the vinyl ester system is much larger than that in the polyurethane system, which is due to the particle oxidation in the polyurethane nanocomposite fabrication process and particle-polymer surface interaction effects. The observed smaller Hc after the heat-treatment is due to the decreased interparticle distance concomitant with a stronger dipolar interaction. [0091] No electrical conductivity was detected in the robust vinyl ester nanocomposites, even at 40 wt% loading, indicating the particle loading is still lower than the percolation threshold. The conductivity improves considerably after the heat treatment. [0092] The temperature dependent resistivity of nanocomposites after heat treatment at 450 ⁰C increases significantly with decreasing temperature, characteristic of a non-metallic behavior. In view of the high conductivity of iron, the high resistance observed in the 450 ⁰C heat treated specimen is due to the poor conductivity of the carbon matrix. With decreasing temperature, the resistivity increases much faster in the heat-treated 15 wt% nanocomposites than in the heat-treated 40 wt% nanocomposites. This is obviously attributed to the dominating less-conductive carbon matrix in the 15 wt% nanocomposites, as compared to the dominating more-conductive iron in 40 wt% nanocomposites. The observed linear relationship between the logarithmic resistivity and the square root of temperature T^"1/2 indicates an interparticle tunneling conduction mechanism. The decreased carbon content in the 40 wt% nanocomposites favors electron spin hopping from one particle to another, thus they have a lower resistivity as compared to the nanocomposites with a particle loading of 15 wt%.

[0093] The particle loading was observed to have a dramatic effect on the MR performance. A room temperature MR of 8.3 % is observed in the heat-treated nanocomposite with an initial 15 wt% particle loading, whereas the heat- treated nanocomposites with an initial particle loading of 20 and 40 wt% show a MR of 6.8% and 6.0%, respectively. All are observed at a fairly high field of 90 kθe. Compared to multilayered GMR materials, a high magnetic field is required to saturate the MR, which is characteristic of the tunneling conduction mechanism. However, a 2.0% MR observed at 4.5 kOe still indicates that the

GMR in these nanocomposites could be used for biological targeting applications. The particle loading dependent MR is attributed to the interparticle distance. In addition, the spacer materials (vinyl ester resin and carbonized vinyl ester resin) play a role in the MR property. The observed field-dependent MR hysteresis loops in the nanocomposite with high particle loadings were also due to the decreased interparticle distance together with a strong interparticle dipolar interaction. Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U. S. C. 1 12, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

CLAIMS What is claimed is:

1 . A method of fabricating a nanocomposite, comprising: activating the surfaces of a plurality of metal nanoparticles having a metal core and an oxide shell with a promoting agent to form an activated mixture; combining said activated mixture with at least one monomer and at least one solvent; polymerizing said combined activated mixture, monomer and solvent to form a polymerized nanocomposite; and drying said polymerized nanocomposite.

2. An apparatus as recited in claim 1 , further comprising: heating said dried nanocomposite until said nanocomposite is carbonized.

3. An apparatus as recited in claim 1 , wherein said monomer contains a hydroxyl moiety.

4. An apparatus as recited in claim 1 , wherein said monomer is a monomer selected from the group of monomers consisting essentially of a vinyl ester, an ester, a vinyl alcohol or a urethane.

5. An apparatus as recited in claim 1 , wherein said metal nanoparticle is a metal selected from the group of metals consisting essentially of cobalt, iron, or nickel.

6. An apparatus as recited in claim 1 , wherein said metal nanoparticle is a metal selected from the group of metals consisting essentially of an oxide of cobalt, iron, or nickel.

7. An apparatus as recited in claim 1 , wherein said metal nanoparticle is a metal selected from the group of metals consisting essentially of an alloy of cobalt, iron, or nickel.

8. An apparatus as recited in claim 1 , wherein the average diameter of said metal nanoparticles is a diameter less than approximately 100 nm.

9. An apparatus as recited in claim 1 , wherein the average diameter of said metal nanoparticles is a diameter less than approximately 20 nm.

10. A method of fabricating a nanocomposite, comprising: blending a plurality of metal nanoparticles with at least one monomer in an inert environment using energetic mixing; mixing a promoter with the blended nanoparticles and monomer to form a polymerized nanocomposite; and drying said nanocomposite.

1 1. A method as recited in claim 10, further comprising: heating said dried nanocomposite until said nanocomposite is carbonized.

12. A method as recited in claim 10, further comprising: blending BaTiθ3 or TiO₂ with said nanoparticles and said monomer prior to polymerization.

13. A method as recited in claim 10, wherein said monomer contains an oxygen moiety.

14. A method as recited in claim 10, wherein said monomer is a monomer selected from the group of monomers consisting essentially of a vinyl ester, an ester, a vinyl alcohol or a urethane.

15. A method of fabricating a nanocomposite, comprising: dissolving at least one polymer in an anhydrous, substantially organic solvent; mixing metal nanoparticles into the polymer solution using energetic mixing; separating solid and liquid phases with a polar solvent; filtering and drying said solid phase; and reacting said solid phase in a reducing environment to form carbon fibers.

16. A method as recited in claim 15, wherein said polymer comprises PAN.

17. A method as recited in claim 15, wherein said anhydrous organic solvent comprises a nonpolar solvent.

18. A method of fabricating a nanocomposite, comprising: dispersing metal nanoparticles and at least one monomer in an anhydrous, anoxic solvent to provide a dispersed solution; applying an electrochemical potential to said dispersed solution; and forming a thin film on an anode electrode structure.

19. A method as recited in claim 18, wherein said polymer comprises pyrrole.

20. A method as recited in claim 18, wherein said anhydrous anoxic solvent comprises a nonpolar solvent.

21 . A method as recited in claim 18, wherein said electrochemical potential is approximately 0.7Volts/SCE.

22. A nanocomposite, comprising: a nonmetallic polymer matrix; and a plurality of magnetic nanoparticles evenly dispersed throughout the polymer matrix.

23. A nanocomposite as recited in claim 22, wherein said polymer matrix is conductive.

24. A nanocomposite as recited in claim 22, wherein said polymer is a thermosetting polymer or a thermoplastic polymer.

25. An apparatus as recited in claim 22, wherein said nanoparticles are smaller than approximately 100nm.