WO2012111837A1

WO2012111837A1 - High-performance thermal interface films and methods thereof

Info

Publication number: WO2012111837A1
Application number: PCT/JP2012/053898
Authority: WO
Inventors: James Caruso
Original assignee: Jnc Corporation
Priority date: 2011-02-14
Filing date: 2012-02-13
Publication date: 2012-08-23
Also published as: JP5867516B2; JP2014511405A

Abstract

A polymer composite including a polymer body including nanowires, wherein the nanowires can be aligned in a direction and have at least one end which extends from the polymer body, and the nanowires have a net magnetic moment throughout the nanowire or are ferromagnetic and can be aligned in the Z-direction by means of an external magnetic field. A method for producing a polymer composite including combining nanowires with a pre-polymer mixture, wherein the nanowires have a negative or non-negative net magnetic moment throughout a nanowire; applying an alignment magnetic field of sufficient strength to align the nanowires contained in the pre-polymer mixture in a direction of the alignment magnetic field; and polymerizing the pre-polymer mixture to form a polymer body. The polymer composite can be used, for example, as a thermal interface material for dissipating heat in electronic devices.

Description

DESCRIPTION

TITLE OF INVENTION

HIGH-PERFORMANCE THERMAL INTERFACE FILMS AND METHODS THEREOF

TECHNICAL FIELD

The present disclosure relates to a polymer composite, which can be used as a thermal interface material, and in particular, as a thermal interface material for electronic devices.

BACKGROUND ART

High performance electronic devices, such as computer processing units and light emitting diodes generate heat, which needs to be removed at a high rate to maintain the functionality of the device. One of the ways in which heat is removed from high performance electronic devices is by placing a thermal interface material between the thermal transfer surfaces of an electronic device and a heat sink. Typically, these gaps between thermal transfer surfaces are filled with air, which is a very poor heat conductor. Thermal interface materials utilize thermally conductive materials to increase the transfer efficiency of heat away from the micro electronic device by filling the gap between the thermally transfer surfaces with a material more conductive than air. Today, thermal greases, most notably silver filled epoxies, dominate the market share of thermal interface materials. However, silver filled epoxies have low thermal transfer rates due to poor interaction between the silver particles and poor interaction between the silver particles and the thermal transfer surfaces of the electronic device and the heat sink. See, for example, Xu, J. et. al., "Silver nanowire array-polymer composite as thermal interface material," J. Appl. Phys., 106, 124310 (2009).

There have been efforts to produce thermal interface materials containing thermally conducted particles that are vertically oriented in films and composites to increase the heat transfer efficiency of the thermally conductive particles along the direction of alignment. However, these methods require expensive templates to orient the thermally conductive particles, and/or vapor phase deposition or chemical vapor deposition, which makes the thermal interface material cost-ineffective for these applications. See, for example, Sun, L., et al. "Polymer Composites with Oriented Magnetic Nanowire as Fillers," Nano materials, (In publication), and Zhanag, K. et. al "Thermal Interface Material with Aligned CNT and its application in HB-LED Packaging," 2006, Electronic Components and Technology Conference.

Other methods (Taguma, et al, Japanese Patent Laid-open Publication No. 62- 194653; Hida, et al Japanese Patent Laid-open Publication No. 2000-191987) which have been tried also have several flaws. One such flaw is the aligned thermal conductors are not continuous from the top surface to the bottom surface and are essentially buried inside a polymer matrix, in other words, not sticking out of the top and bottom. This results in a final material which still lacks the good connection to the thermal transfer surfaces.

There is a need for a thermal interface material which can minimize poor

interactions between thermally conductive particles within a composite. There is also a need for a thermal interface material which can minimize the poor interactions between the thermally conductive particles and the thermal devices themselves, e.g. the electronic device and/or heat sink. Further, there is a need to align thermally conductive particles in a thermal interface material by a cost-effective manufacturing procedure.

The present disclosure provides a solution to at least one of the above needs.

SUMMARY OF THE INVENTION

The following embodiments are not an extensive overview. The following description is not intended to identify critical elements of the various embodiments, nor is it intended to limit the scope of them.

An embodiment includes a polymer composite comprising: a polymer body comprising nanowires, wherein the nanowires (or wire- like acicular materials) are aligned in a direction and have at least one end which extends from the polymer body, and the nanowires have a net magnetic moment throughout a nanowire. In an embodiment, at least 90% of the nanowires are aligned within ±10° of the direction and at least 90% of the nanowires have at least one end extending from the polymer body. In an embodiment, the polymer body is comprised of an epoxy polymer, an acrylate polymer, a polyurethane, a polyolefin, a polyester, or a mixture thereof; and the nanowires are comprised of cobalt, nickel, iron, gadolinium, neodymium, or alloys or composites thereof. In an embodiment, at least 90% of the nanowires have a cross-section from about 10 μηι to about 200 μιη and a length to cross-section aspect ratio of from 3: 1 to 200:1. In an embodiment, a weight percentage of the nanowires is about 75 wt% to about 98 wt% of the total weight of the polymer composite. In a further embodiment, the nanowires have a diameter of about 10 nm to about 200nm and have a cross-section aspect ratio of from 3: 1 to 200: 1. In an embodiment, the polymer composite is a sheet-like polymer composite, and the nanowires are aligned in a direction of the thickness of the polymer composite.

An embodiment includes a method for producing polymer composites comprising: combining nanowires with a pre-polymer mixture, wherein the pre-polymer mixture at least partially encapsulates the nanowires, wherein the nanowires have a non-negative net magnetic moment throughout a nanowire; applying an alignment magnetic field of sufficient strength to align the nanowires contained in the pre-polymer mixture in a direction of the alignment magnetic field; and polymerizing the pre-polymer mixture to form a polymer body. An embodiment includes removing a least a portion of the polymer body to extend at least one end of the nanowires from the polymer body. An embodiment includes before the combining step, applying a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nanowire. In an embodiment, at least 90% of the nanowires are aligned to within ±10° of the direction of alignment magnetic field. In an embodiment, the pre-polymer mixture comprises a monomer or an oligomer, wherein the monomer or the oligomer contains an epoxy group, an acryl group, an urethane group, vinyl group, or an ester group; and, optionally, at least one of an initiator and a cross-linking agent. In an embodiment, the nanowires are comprised of cobalt, nickel, iron, gadolinium, neodymium, or alloys or composites thereof, and at least 90% of the nanowires have a cross-section from about 10 μπι to about 200 μιη and a length to cross-section aspect ratio of from 3: 1 to 200: 1. In a further embodiment, the nanowires have a diameter of about 10 nm to about 200nm and have a cross-section aspect ratio of from 3:1 to 200: 1. In an embodiment, a weight percentage of the nanowires is about 75 wt% to about 98 wt% of the total weight of the polymer composite. In an embodiment, the magnetic field is applied by a magnetic device capable of generating a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nanowire. In an embodiment, the strength of the aligning magnetic field is from about 5 mT to about 1 T. In an embodiment, the polymerizing step comprises exposing the pre- polymer mixture to ultraviolet radiation, and wherein the initiator comprises a photoinitiator or by pre-polymer can be polymerized by the addition of heat and or a chemical additive and heat. In an embodiment, the removing step comprises contacting a portion of the polymer body with a liquid to expose the end of the nanowires such that the end of the nanowires extends from the polymer body. This liquid can be selected from a group of organic solvents having the ability to infiltrate, swell and eventually dissolve a small surface layer or layers of the polymer but leaving the nanowire essentially unaffected. Examples of such solvents depend on the polymer used but can be, as an example but not limited to cyclohexane, THF, DMSO, trichloroethanol and the like. In an embodiment, the liquid is an acid solution or a basic solution that etches and/or dissolves the polymer body at a faster rate than the nanowires. In another embodiment, the polymer can be etched by means of the use of a plasma. Here, by way of example, the plasma can be an Oxygen plasma or an Oxygen/Tetrafiuoromethane plasma. It is important here that the plasma composition and conditions are chosen such that the polymer is removed selectively or at least at a faster rate than the nanowires.

The length that the nanowires stick up above the polymer body is dependent of the surface roughness of the heat-generating device and the heat sink but generally in the range of several nanometers to several microns. However, it is important that the nanowires stick up above the surface to reduce interfacial thermal resistance by making direct contact with the heat- generating surface and the heat sink and fill the nano-scale air gaps that normally exist in these systems.

An embodiment includes, before the application of the alignment magnetic field, placing the pre-polymer mixture into contact with at least one of a microelectronic device and a heat sink. An embodiment includes, after the polymerizing the pre-polymer mixture, placing the polymer composite onto a heat sink or an electric device. In an embodiment, the microelectronic device is a computer processing unit or a light emitting diode.

An embodiment of the method of producing polymer composites includes depositing the polymer composite, between a microelectronic device and a heat sink. In an

embodiment, the microelectronic device is a computer processing unit or a light emitting diode.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the disclosed compounds, compositions, and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the embodiments disclosed therein, there are depicted in the drawings certain embodiments of the polymer composite. However, the methods and related products are not limited to the precise arrangements and instrumentalities of the embodiments depicted in the drawings.

Figure 1 schematically depicts the application of a magnetic field to a nanowire to imbue the nanowire with a permanent net magnetic moment.

Figure 2 schematically depicts the combination of permanent magnetic nanowires and a pre-polymer mixture, before the application of an alignment magnetic field.

Figure 3 schematically depicts the application of an external magnetic alignment field of sufficient strength to align the permanent nanowires (or non-permanent magnetic nanowires) contained in the pre-polymer mixture in a direction of the alignment magnetic field and the exposure of the polymer composite to UV radiation to polymerize the pre- polymer mixture.

Figure 4 schematically depicts the removal of at least a portion of the polymer body to extend at least one end of the nanowires from the polymer body.

Figure 5 schematically depicts a process for producing a thermal conductive sheet with stick Fe powder and polymer resin, using a magnet on one side of the sheet to align the stick Fe powder and UV light to polymerize the resin. (A) Schematic cross-sectional illustration of a combining step of resin 1 and stick Fe powder 2. (B) Schematic cross- sectional illustration of an applying step an alignment magnetic field to align stick Fe powder 2 parallel to the line of magnetic force 3 applied by a magnet 4. (C) Schematic cross- sectional illustration of a polymerizing step of resin 1 to form polymer resin 11 using UV light 5. (D) Schematic cross-sectional illustration of thermal conductive sheet 10 made by the process.

Figure 6 schematically depicts a process for producing a thermal conductive sheet with stick Fe powder and polymer resin, using a pair of magnets on both sides of the sheet to align the stick Fe powder and UV light to polymerize the resin. (A) Schematic cross- sectional illustration of a combining step of resin 1 and stick Fe powder 2. (B) Schematic cross-sectional illustration of an applying step an alignment magnetic field to align stick Fe powder 2 parallel to the line of magnetic force 3 applied by a magnet 4. (C) Schematic cross- sectional illustration of a polymerizing step of resin 1 to form polymer resin 11 using UV light 5. (D) Schematic cross-sectional illustration of thermal conductive sheet 20 made by the process.

Figure 7 schematically depicts a process of incorporation of magnetic nanoparticles to multi- walled carbon nanotubes to make them ferromagnetic or a permanent (weak) magnet. The magnetic nanoparticles may be combined with carbon nanotubes ex situ (e.g., stirred together) or in situ (synthesized with carbon nanotubes).

DESCRIPTION OF EMBODIMENTS

As used herein, each of the following terms has the meaning associated with it in this section, unless otherwise explicitly stated.

The articles "a" and "an" are used herein to refer to one or more than one object of the article. By way of an example, "an element" means one or more than one element.

The term "about" will be understood by persons of ordinary skill in the art to depend on the context in which it is used. As used herein, "about" is meant to encompass variations from ±20%, including ± 10%, ±5%, ± 1 %, and ±0.1 %.

It is understood that any and all whole or partial integers between any ranges set forth herein are included.

The term "polymer composite" refers to a composite containing at least a polymer and a non-polymer material.

The term "polymer" refers to a molecule having a molecular weight of at least 10,000 grams per mole and being formed from the polymerization of at least one monomer molecule.

The term "polymer body" refers to a solid or semi-solid material including at least one polymer.

The term "nanowire" or "nanowires" refer to an object(s) comprising a magnetic material including a ferromagnetic material such as a metal and/or metal oxide and/or a carbon nanotube/ferromagnetic material composite, wherein the nanowire has an length to cross-section aspect ratio of at least 3: 1 and a cross-section of about 10 μηι to about 200 μιη. In some cases, the nanowires can be much smaller in the nanometer- size range, having a diameter of about 10 nm to about 200 nm and have a cross-section aspect ratio of from 3: 1 to 200: 1. Typically, the entire body of the nanowire is capable of being imbued with a permanent magnet moment, or the nanowires need not be permanent magnetic materials but can be aligned by a magnetic field.

The term "pre-polymer" refers to a mixture of monomer or oligomer, which can be polymerized to form a polymer or a polymer body.

The term "initiator" refers to any compound or material capable of starting a polymerization reaction or cross-linking process. The term "cross- linking agent" refers to a compound or material capable of binding at least one polymer, oligomer, or monomer molecules together to form a cross-linked polymer.

The term "alignment magnetic field" refers to the application of an external magnetic field to align the nanowires.

The term "thermal transfer surface" refers to the surface of a device or heat sink capable of transferring heat to or from the polymer composite.

The term "net magnetic moment" refers to the property of an object comprising at least one magnetic material, where in the magnetic domains of the magnetic material have been aligned such that the object has magnetic moment greater than zero in one direction. For example, ferromagnetic materials may have magnetic domains with magnetic moments of various strength and direction, such that the vector sum of the magnetic moment of the object is zero in any direction. However, if a strong enough magnetic field is applied to the object, the magnetic domains can be aligned such that the magnetic moment in at least one direction is greater than zero.

The term "permanent" refers the state of a magnetic material, wherein the net magnetic moment remains greater than zero in a direction when no external magnetic field is applied.

In an embodiment, a polymer composite comprises a polymer body comprising nanowires, wherein the nanowires are aligned in a direction and have at least one end which extends from the polymer body and the nanowires have a net magnetic moment throughout a nanowire. In an embodiment, the polymer composite is a sheet-like polymer composite, and the nanowires are aligned in a direction of the thickness of the polymer composite. In a particular embodiment, more than one nanowires are aligned together to form an acicular structure in the polymer composite (Figs. 5 & 6), and at least one end of the acicular structure extends from the surface of the sheet-like polymer body. It has been found that a polymer body can contain nanowires, wherein the nanowires can conduct heat more efficiently than the polymer of the polymer body. It has also been found that heat can be conducted more efficiently through the polymer body when the thermally conductive particle inside the polymer body are aligned than when the thermally conductive particles are not aligned. In an embodiment, one end or both ends of the nanowires can extend from the polymer body to allow for direct contact between the nanowires and a thermal transfer surface. An example of such a thermal transfer surface can be the bottom surface of a computer-processing unit. Here, the nanowires stick up above the polymer body to make direct contact with the heat-generating device and/or the heat sink. Ideally, the nanowires stick up above the polymer body dependent on the surface roughness of the heat-generating device and the heat sink, typically several nanometers to several microns.

In an embodiment, the polymer composite has at least 90%, including at least 95%, and including at least 98%, of the nanowires aligned to within plus or minus 10% of a direction, including within plus or minus 5% of a direction. In an embodiment, the direction of alignment can be in the "z" direction, wherein the "z" direction refers the direction along the magnetic moment of an alignment magnetic field. The "z" direction remains the "z" direction even when the magnetic field is no longer applied. The degree of alignment may be calculated by determining the direction of the magnetic moment of the alignment magnetic field. Then, the direction of a sample of at least 20 nanowires can be measured relative to the "z" direction, such that the intersection of the nanowire and the "z" direction defines an angle. In an embodiment, the "z" direction is oriented such that the "z" direction is parallel to the shortest distance between the thermal transfer surfaces of two objects, such as an electronic device and a heat sink. Further, in an embodiment, at least 90% of the nanowires of the composite have at least one end extending from the polymer body. In an embodiment of a sheet-like polymer composite, the "z" direction is parallel (or almost parallel) to the direction of the thickness of the sheet-like polymer composite.

In an embodiment, the polymer composite comprises a polymer body comprising nanowires. The choice of material for the polymer body is not particularly limited, so long as the polymer body is comprised of a polymer which can conduct heat, immobilize nanowires, and is not so electrically conductive that it will short circuit an electronic device. In one embodiment, the polymer body is comprised of an epoxy polymer, an acrylate polymer, a polyurethane, a polyolefin, a polyester, or a copolymer, a blend, or a mixture thereof. A function of the polymer body can be to thermally conduct heat away from a device toward a heat sink in a manner that is more efficient than air while immobilizing the alignment of the nanowires.

The material used to form the nanowires is not particularly limited so long as the material for the nanowires can be induced to form a permanent net magnetic moment or be aligned by a magnetic field. For example, the nanowires can be comprised of Iron, Nickel, a ferrite or rare-earth magnetic material. Further, the nanowires can be comprised of cobalt, nickel, iron, gadolinium, neodymium, or alloys or composites thereof. An example of a material for the nanowires includes Nd₂Fej₄B. Here, isotropic Nd₂Fej₄B nanowires can be magnetized in a strong magnetic field to produce a net permanent magnetic dipole

(anisotropic) in the nanowires (as shown in Figure 1).

An example of the nanowire includes an acicular structure of a ferromagnetic material (e.g., Fe, Ni, Co, Gd, Nd, or alloys or composites thereof). The acicular structure may have a length such that at least one end of the acicular structure extends from the surface of the polymer body.

Another example of the nanowire includes an elongate particle of a ferromagnetic material (e.g., Fe, Ni, Co, Gd, Nd, or alloys or composites thereof). In such an embodiment, each of the elongate particles may not necessarily penetrate through the polymer body in a direction. Rather, a plurality of the elongate particles may align together in the polymer body to form an acicular structure (Figs. 5 & 6) such that at least one end of the acicular structure extends from the surface of the polymer body. The elongate particles of ferromagnetic materials can be prepared by, for example, sorting elongate particle-rich powder of the ferromagnetic material (e.g., Fe) with a sieve (e.g., 330 micrometer) and collecting elongate particles.

In a further embodiment, the nanowire may be a carbon nanotube onto which ferromagnetic materials (e.g., Fe particles) are deposited or within which ferromagnetic materials embedded such that the carbon nanotube can be aligned by an external magnetic field (Fig. 7).

For some applications, there can be a need to balance the magnetic strength, the thermal conductivity, and electrical conductivity of the nanowires. Therefore, composites can contain at least one magnetic component, for example a carbon nanotube/ferro magnetic material composite. Such an embodiment of a composite material can tailor the balance between thermal conductivity and electrical resistivity, especially in applications where electrical isolation is required.

In addition, some applications can require that the permanent magnetic moment of the magnetic nanowires be reduced or eliminated after alignment and polymerization of the polymer composite. A benefit of reducing or eliminating magnetic flux does it that the polymer composite will not interfere with the electronic device. In such case, the choice of polymer material for the polymer body can be a polymer that can be heated above the Curie temperature of the nanowires to randomize the electron spins within the magnetic material of the nanowires. This choice of polymer would allow for the formation of a polymer composite containing magnetically aligned nanowires, but where the magnetic moment of the nanowires has been reduced or eliminated. In another embodiment, the nanowires are imbued with a permanent magnetic dipole moment through spontaneous magnetization, which can occur with a decrease in temperature below the Curie temperature.

The dimensions of the nanowires are not particularly limited so long as the length to cross-section aspect ratio is from 3:1 to 200: 1, including from 50: 1 to 150: 1, and the cross- section of the nanowires is from about 10 μηι to about 200 μηι, including from about 50 μιη to about 100 μηι. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a cross-section from about 10 μηι to about 200 μιη and a length to cross- section aspect ratio of from 3: 1 to 200: 1. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a cross-section from about 50 μιη to about 100 μηι. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a length to cross-section aspect ratio of from 50: 1 to 150: 1. An advantage of the dimensions of the nanowires can be that the nanowires are capable of being aligned in such a way as to minimize the amount of polymer material between each nanowire and/or a thermal transfer surface.

In an embodiment, the weight percentage of nanowires to the whole of the polymer composite is not particularly limited, so long as the amount of polymer is sufficient to immobilize the nanowires and the majority of the nanowires do not directly come into contact with each other. In an embodiment, a weight percentage of the nanowires is from about 75 wt% to about 98 wt%, including from about 80 wt% to about 90 wt%, of the total weight of the polymer composite.

In an embodiment, a method for producing a polymer composite comprises combining nanowires with a pre-polymer mixture, applying an alignment magnetic field of sufficient strength to align the nanowires contained in the pre-polymer mixture in a direction of the alignment magnetic field; and polymerizing the pre-polymer mixture to form a polymer body. In an embodiment, the nanowires have a non-negative net magnetic moment throughout a nanowire.

In an embodiment, the pre-polymer mixture functions to provide a viscous medium, which allows for the alignment of the nanowires by an alignment magnetic field. Once the nanowires are aligned by the alignment magnetic field, the pre-polymer mixture can be polymerized to immobilize the nanowires in their aligned state. The step of combining nanowires with a pre-polymer mixture is not particularly limited as long as the nanowires are distributed in the pre-polymer mixture such that the nanowires are in direct contact with the pre-polymer mixture and the majority of the nanowires do not directly contact each other. The combining step includes injecting, mixing, stirring, milling, shaking, and the like or combinations thereof. The nanowires can be added to the pre-polymer or the pre-polymer may be added to the nanowires to provide the pre-polymer mixture.

The step of applying a magnetic field is not particularly limited so long as the strength of the alignment magnetic field is sufficient to align to the pre-magnetized nanowires contained in the pre-polymer mixture in a direction of the alignment magnetic field. In an embodiment, the alignment magnetic field is oriented in the "z" direction, wherein the "z" direction refers the direction of an alignment magnetic field. A function of the alignment magnetic field can be to align the nanowires in the direction of the magnetic field. Alignment of the nanowires in a direction can have the benefit of preventing or minimizing poor interactions between the magnetic wires. Alignment of the nanowires in a direction can also have the benefit of preventing or minimizing poor interaction between the magnetic nanowires and a thermal transfer surface. Further, a benefit of applying an alignment magnetic field can be the reduction of costs due to the ability to align nanowires without the use of expensive templates or the use of a chemical vapor deposition or a physical vapor disposition. In an embodiment, the strength of the aligning magnetic field is from about 5 mT to about 1 T, including from about 100 mT to about 500 mT. In an embodiment, the alignment magnetic field can be applied by a magnetic material, such a permanent magnet, or a magnetic device, such as an electromagnet or the like. In an embodiment of producing a sheet-like polymer composite, the "z" direction is parallel (or almost parallel) to the "thickness" direction of the sheet-like polymer composite.

The step of polymerizing the pre-polymer mixture is not particularly limited so long as the pre-polymer mixture is polymerized to form a polymer body which is capable of immobilizing the pre-magnetized nanowires contained therein. The polymerizing step can include methods known in the art, such heating the pre-polymer mixture; adding a chemical initiator; or adding a photoinitiator to the pre-polymer mixture, followed by exposing the pre-polymer mixture to electromagnetic radiation, including UV radiation. In an embodiment, the polymerizing step includes exposing the pre-polymer mixture to light having a wavelength of 100 to 450 nm, including 250 to 400 nm. In an embodiment, the polymerizing step comprises exposing the pre-polymer mixture to ultraviolet radiation, and wherein the initiator comprises a photoinitiator

In an embodiment, the method for producing a polymer composite can further comprise removing at least a portion of the polymer body to extend at least one end of the nanowires from the polymer body. The step involves selectively removing a portion of the polymer body at a faster rate than the nanowires. A variety of solvents can be selected depending on the polymer type used or plasma etching, wherein the plasma composition and processing conditions can be varied to remove polymer at a faster rate than the nanowires. In another embodiment, the method for producing a polymer composite can further include contacting a portion of the polymer body with a liquid to expose the end of the nanowires such that the end of the nanowires extends from the polymer body. In an embodiment, the liquid is an acid solution or a basic solution that etches and/or dissolves the polymer body at a faster rate than the nanowires. A function of the removing step can be to expose at least one end of the nanowires to facilitate contact and/or heat transfer from the nanowires of the polymer composite to an adjacent object, such as a thermal transfer surface. The liquid used to remove the portion of the polymer body is not particularly limited so long as the liquid is capable of selectively etching and/or dissolving the polymer body at a faster rate than the nanowires.

In an embodiment, the method for producing a polymer composite can include, before the combining step or before the application of the alignment magnetic field step, applying a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nano wire. A function of the step of applying the magnetic field to the nanowires before alignment can be to imbue the nanowires with a permanent net magnetic moment throughout the nanowires. The permanent net magnetic moment can allow for the nanowires to be aligned by the application of an alignment magnetic field in a later step. The strength of the magnetic field during the application of the magnetic field step can depend on factors such as the composition of the materials and dimensions of the nanowires. Generally, the magnetic field should be stronger when the amount of magnetic material in the nanowires is lower and/or aspect ratio of the nanowires is higher. In an embodiment, the strength of the magnetic field can be at least 2 Tesla, including at least 4 Tesla.

A benefit to applying a magnetic field to nanowires is that imbuing the nanowires with a permanent net magnetic moment allows for the alignment of nanowires using a magnetic field having a strength that is compatible with electronic device fabrication methods. In an embodiment, a method of producing a polymer composite includes the application of a magnetic field by a magnetic device capable of generating a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nanowire. Examples of such a magnetic device can include at least one of a discharge capacitor, a Hallbach array, and the like.

In an embodiment, the method for producing a polymer composite can include, aligning at least 90%, including at least 95%, including at least 98%, of the nanowires to within ±10°, including within ±5°, of the direction of the alignment magnetic field.

In an embodiment, the method for producing a polymer composite can include a polymer body. The choice of material for the polymer body is not particularly limited, so long as the polymer body is comprised of a polymer which can conduct heat, immobilize nanowires, and is not so electrically conductive that the polymer composite short circuits electronic devices. In one embodiment, the polymer body is comprised of an epoxy polymer, an acrylate polymer, a polyurethane, a polyolefin, a polyester, or a copolymer, a blend, or a mixture thereof, and optionally, at least one of an initiator and a cross-linking agent. A function of the polymer body can be to thermally conduct heat away from a device toward a heat sink in a manner that is more efficient than air while the alignment of the nanowires.

In an embodiment, the method for producing a polymer composite can include nanowires. The material used to form the nanowires is not particularly limited so long as the material for the nanowires can be induced to form a permanent net magnetic moment. For example, the nanowires can be comprised of a ferromagnetic or a composite magnetic material. Further, the nanowires can be comprised of cobalt, nickel, iron, gadolinium, neodymium, or alloys or composites thereof. An example of a material for the nanowires includes Nd₂Fei₄B. Further, the nanowires can be Nickel or Iron wherein these nanowires can be aligned by an external magnetic field.

In addition, composites can contain at least one magnetic component, for example a carbon nanotube/ferromagnetic material composite. Carbon nanotubes are known to have highest theoretical (axial) thermal conductivity (Xu, J. et al. IEEE Transactions:

Components and Packaging Technology, 29, 261, 2006), and available from various vendors at various qualities and specifications. This embodiment of a composite material can tailor the balance between thermal conductivity and electrical resistivity. In some applications, electrical isolation may be required. Here, nano-particulate Iron can be precipitated onto and/or within carbon nanotubes making them essentially ferromagnetic where they can easily be aligned by an external magnetic field. The method of present invention provides a cost-effective alternative to the known processes (template growth, CVD, etc.).

In an embodiment, the method for producing a polymer composite includes an additional step of heating the polymer composite to above the Curie temperature of the nanowires. In addition, some applications can require that the permanent magnetic moment of the nanowires be reduced or eliminated after the polymerization step. In such case, the choice of polymer material for the polymer body can be a polymer that can be heated above the Curie temperature of the nanowires to randomize the electron spins within the magnetic material of the nanowires. This choice of polymer would allow for the formation of a polymer composite containing magnetically aligned nanowires, but where the magnetic moment of the nanowires has been reduced or eliminated.

In an embodiment, the method for producing a polymer composite also includes nanowires of various dimensions. The dimensions of the nanowires are not particularly limited so long as the length to cross-section aspect ratio is from 3:1 to 200: 1, including from 50: 1 to 150: 1, and the cross-section of the nanowires is from about 10 μπι ίο about 200 μπι, including from about 50 μπι to about 100 μιη. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a cross-section from about 10 μη to about 200 μιη and a length to cross-section aspect ratio of from 3: 1 to 200: 1. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a cross- section from about 50 μιη to about 100 μιη. In an embodiment, at least 90%, including 95%, including 98%, of the nanowires have a length to cross-section aspect ratio of from 50: 1 to 150: 1. An advantage of the dimensions of the nanowires can be that the nanowires are capable of being aligned in such a way as to minimize the amount of polymer material between each nanowire and/or a thermal transfer surface.

In an embodiment, the method for producing a polymer composite, the weight percentage of nanowires to the whole of the polymer composite is not particularly limited, so long as the amount of polymer is sufficient to immobilize the nanowires and the majority of the nanowires do not directly come into contact with one another. In an embodiment, a weight percentage of the nanowires can be from about 75 wt% to about 98 wt%, including from about 80 wt% to about 90 wt%, of the total weight of the polymer composite. In an embodiment, the method for producing a polymer composite further comprises, before the application of the alignment magnetic field, placing the pre-polymer mixture into contact with at least one of a microelectronic device and a heat sink. A benefit of this optional step can be that the pre-polymer mixture can be place in the location where the polymer composite will be used. The steps of applying an alignment magnetic field and polymerizing the pre-polymer mixture can take place at the location where the polymer composite will be used. For example, if the polymer composite will act as a thermal interface material between a microelectronic device and a heat sink, then the method can allow for the pre-polymer mixture to be applied directly to the microelectronic device and/or head sink. Then, the remaining steps for the producing the polymer composite can be incorporated into a microelectronic manufacturing process. In an embodiment, the microelectronic device can be a computer-processing unit or a light emitting diode.

In an embodiment, the method of using the polymer composite can include dissipating heat in a device comprising: depositing the polymer composite between a microelectronic device and a heat sink. In an embodiment, the microelectronic device is a computer-processing unit or a light emitting diode.

In an embodiment, the method of producing a polymer composite can include, after the polymerizing the pre-polymer mixture, placing the polymer composite onto a heat sink or an electronic device. An advantage of placing the polymer composite onto a heat sink or electronic device after the polymerizing step can be that the polymer composite produced separately from the heat sink, electronic, device etc., and then placed into position in a later step.

In an embodiment, the polymer composite includes a polymer composite comprises the production of vertically aligned or columnar structures inside a composite structure and the method of producing the same. The vertically aligned materials or structures can be applied in a variety of areas including optics, thermal conductivity, barrier films for H₂0 and 0₂, and the like. In many of these applications, a difference in properties associated with the vertical alignment (the Z-direction) of structures in the polymer composite provides an advantage over films containing particles, which are not aligned in the Z-direction. For example, a magnetic ferrite-based nanowire can be aligned in the Z-direction within liquid crystals for various applications. Thus, the polymer composite and method for producing the polymer composite can be a valuable general method for producing polymer composites containing vertically aligned or columnar structures. The term "vertical" as used in this paragraph means that the "z" direction is oriented such that the "z" direction is parallel to the thickness of the polymer composite, wherein the thickness is smaller than the length and width of the polymer composite.

All cited patents and publications referred to in this application are herein incorporated by reference in their entirety for all purposes.

Herein below the present invention will be described in more detail by way of examples. However, these examples should not be construed as limiting the scope of the invention. EXAMPLES

Example 1

A mixture of screened (330 micrometer) Fe powder (6g, Kanto chemical Co., ), Celoxide 2021 (Daicel Chemical Industries, LTD,3.0g), and initiator CPI210S (SAN-APRO Ltd ,0.03g, ) was poured into PP pad (5 cm x 8cm ). The mixture in the pad was placed on the ferrite magnet (unipole plate 10 x 10 cm, 1 lmT) and irradiated UV light with high pressure Hg lamp (1500mJ/cm2). The obtained plate was cooled to ambient condition and measured thermal conductivity by Xenon flash thermal analyzer (Netzsch LFA 447 NanoFlash®). The lcm x 1cm, 1.18mm thickness of the sample shows thermal conductivity 1.02W/m-K.

Comparative example

A mixture of screened (330 micrometer) Fe powder (Kanto chemical Co., 6g), Celoxide 2021 (Daicel Chemical Industries, LTD., 3.0g,), and initiator CPI210S (SAN- APRO Ltd ,0.03g, ) was poured into PP pad (5cm x 8cm ). The mixture in the pad was irradiated UV light ( High pressure Hg lamp with 1500mJ/cm2). The obtained plate was cooled to ambient condition and measured thermal conductivity by Xenon flash thermal analyzer (Netzsch LFA 447 NanoFlash®). The 1 cm x 1cm, 0.83mm thickness of the sample shows thermal conductivity 0.66W/m-K.

The difference between Example 1 and Comparative example is that when the resin was cured a magnet was used in Example 1 while a magnet was not used in Comparative example. The thermal conductivity of Example 1 was significantly higher than that of Comparative example. Example 2

Ni piece (φ0.05πυη x 1mm) was prepared from commercially available Ni wire ((|)0.05mm x 50m). Mixture of Ni piece (150mg), Celoxide 2021 (Daicel Chemical Industries, LTD., lOOmg), and initiator CPI210S (SAN-APRO Ltd., lmg) was poured into metal cell (10 x 10 mm). The mixture in the cell was placed on the ferrite magnet (unipole plate 50mT) and irradiated UV light (High pressure Hg lamp with 1500 mJ/cm2). The obtained plate was cooled to ambient condition and measured thermal conductivity by Xenon flash thermal analyzer. 1.22mm thickness sample showed thermal conductivity 1.84 W/m-K.

As demonstrated in Example 2, an improvement of the thermal conductivity was observed by using shape controlled Ni wires over Fe powder used in Example 1.

INDUSTRIAL APPLICABILITY

The present invention can be used as a thermal interface material, and in particular, as a thermal interface material for electronic devices, and as a process for producing the material.

Claims

1. A polymer composite comprising:

a polymer body comprising nanowires, wherein the nanowires are aligned in a direction and have at least one end which extends from the polymer body, and the nanowires have a net magnetic moment throughout a nanowire.

2. The polymer composite of claim 1 , wherein at least 90% of the nanowires are aligned within ±10° of the direction and at least 90%> of the nanowires have at least one end extending from the polymer body.

3. The polymer composite of claim 1, wherein

the polymer body is comprised of an epoxy polymer, an acrylate polymer, a polyurethane, a polyolefin, a polyester, or a mixture thereof; and

the nanowires are comprised of:

(i) a ferromagnetic material, or

(ii) a carbon nanotube/ferro magnetic material composite;

wherein said ferromagnetic material is selected from the group consisting of cobalt, nickel, iron, gadolinium, neodymium, alloys and composites thereof.

4. The polymer composite of claim 1, wherein

at least 90% of the nanowires have a cross-section from about 10 μη to about 200 μιη and a length to cross-section aspect ratio of from 3: 1 to 200: 1.

5. The polymer composite of claim 1, wherein a weight percentage of the nanowires is about 75 wt% to about 98 wt% of the total weight of the polymer composite.

6. A method for producing a polymer composite comprising:

combining nanowires with a pre-polymer mixture, wherein the pre-polymer mixture at least partially encapsulates the nanowires, wherein the nanowires have a non-negative net magnetic moment throughout a nanowire;

applying an alignment magnetic field of sufficient strength to align the nanowires contained in the pre-polymer mixture in a direction of the alignment magnetic field; and polymerizing the pre-polymer mixture to form a polymer body.

7. The method for producing a polymer composite of claim 6, comprising removing a least a portion of the polymer body to extend at least one end of the nanowires from the polymer body.

8. The method for producing a polymer composite of claim 6, further comprising, before the combining step,

applying a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nanowire.

9. The method for producing a polymer composite of claim 6, wherein at least 90% of the nanowires are aligned to within ±10° of the direction of alignment magnetic field.

10. The method for producing a polymer composite of claim 6,

wherein the pre-polymer mixture comprises a monomer or an oligomer, wherein the monomer or the oligomer contains an epoxy group, an acryl group, an urethane group, vinyl group, or an ester group; and, optionally, at least one of an initiator and a cross-linking agent.

11. The method for producing a polymer composite of claim 6,

wherein the nanowires are comprised of:

(i) a ferromagnetic material, or

(ii) a carbon nanotube/ferro magnetic material composite;

wherein said ferromagnetic material is selected from the group consisting of cobalt, nickel, iron, gadolinium, neodymium, alloys and composites thereof, and

at least 90% of the nanowires have a cross-section from about 10 μιη to about 200 μπι and a length to cross-section aspect ratio of from 3 : 1 to 200: 1.

12. The method for producing a polymer composite of claim 6,

wherein a weight percentage of the nanowires is about 75 wt% to about 98 wt% of the total weight of the polymer composite.

13. The method for producing a polymer composite of claim 8, wherein the magnetic field is applied by a magnetic device capable of generating a magnetic field strong enough to imbue the nanowires with a permanent net magnetic moment throughout the nanowire.

14. The method for producing a polymer composite of claim 6,

wherein the strength of the aligning magnetic field is from about 5 mT to about 1 T.

15. The method for producing a polymer composite of claim 10,

wherein the polymerizing step comprises exposing the pre-polymer mixture to ultraviolet radiation, and wherein the initiator comprises a photoinitiator.

16. The method for producing a polymer composite of claim 7,

wherein the removing step comprises contacting a portion of the polymer body with a liquid or a plasma to expose the end of the nanowires such that the end of the nanowires extends from the polymer body.

17. The method for producing a polymer composite of claim 16,

wherein the liquid is (i) an acid solution or a basic solution that etches and/or dissolves the polymer body at a faster rate than the nanowires, or (ii) cyclohexane, THF, DMSO, or trichloroethanol, and

wherein the plasma is an Oxygen plasma or an Oxygen/Tetrafluoromethane plasma.

18. The method of claim 6, further comprising,

before the application of the alignment magnetic field,

placing the pre-polymer mixture into contact with at least one of a microelectronic device and a heat sink.

19. The method of claim 18, wherein the microelectronic device is a computer processing unit or a light emitting diode.

20. A method of dissipating heat in a device comprising: depositing the polymer composite of claim 1, between a microelectronic device and a heat sink.

21. The method of claim 20, wherein the microelectronic device is a computer processing unit or a light emitting diode.

22. The method of claim 6, further comprising:

after the polymerizing the pre-polymer mixture,

placing the polymer composite onto a heat sink or an electric device.

23. The method of claim 18 or 20, wherein

the polymer composite is a sheet-like polymer composite and said direction of the alignment magnetic field is parallel to the thickness of the polymer composite, and

the exposed end of the nanowire is direct contact with a heat-generating surface of the microelectronic device and the heat sink to fill nano-scale air gaps between the surfaces.

24. The polymer composite of claim 1, wherein

the polymer composite is a sheet-like polymer composite and is usable for dissipating heat in a microelectronic device,

the nanowires are aligned in the direction of the thickness of the polymer composite, and

said at least one end of the nanowires from the surface of the polymer body has been made exposed from the polymer body such that the end is direct contact with a heat- generating surface of the microelectronic device to fill nano-scale air gaps between the surfaces.