High thermal conductivity materials with grafted surface functional groups

HIGH THERMAL CONDUCTIVITY MATERIALS WITH GRAFTED SURFACE FUNCTIONAL GROUPS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 11/152,986 (now U.S. Pat. No. 7,781,063), filed Jun. 14, 2005, which is a continuation-in-part of U.S. application Ser. No. 10/618,125 (now U.S. Pat. No. 7,033,670), filed Jul. 11, 2003, each of which is incorporated herein by reference. Parent application U.S. application Ser. No. 11/152,986 (now U.S. Pat. No. 7,781,063) further claims priority to U.S. Provisional Application No. 60/580,023, filed Jun. 15, 2004, which is incorporated herein by reference. This application is further related to U.S. application Ser. No. 11/152,983 (now abandoned), U.S. application Ser. No. 11/152,985, and U.S. application Ser. No. 11/152,984, all filed Jun. 14, 2005 and incorporated herein by reference.

FIELD OF THE INVENTION

The field of the invention relates to high thennal conductivity materials with surface grafted functional groups impregnated into resins.

BACKGROUND OF THE INVENTION

With the use of any form of electrical appliance, there is a need to electrically insulate conductors. With the push to continuously reduce the size and to streamline all electrical and electronic systems there is a corresponding need to find better and more compact insulators and insulation systems.

Various epoxy resin materials have been used extensively in electrical insulation systems due to their practical benefit of being tough and flexible electrical insulation materials that can be easily adhered to surfaces. Traditional electrical insulation materials, such as mica flake and glass fiber, can be surface coated and bonded with these epoxy resins, to produce composite materials with increased mechanical strength, chemical resistance and electrical insulating properties. In many cases epoxy resins have replaced traditional varnishes despite such materials having continued use in some high voltage electrical equipment.

Good electrical insulators, by their very nature, also tend to be good thennal insulators, which is undesirable. Thennal insulating behavior, particularly for air-cooled electrical equipment and components, reduces the efliciency and durability of the components as well as the equipment as a whole. It is desirable to produce electrical insulation systems having maximum electrical insulation and minimal thennal insulation characteristics.

Electrical insulation often appears in the fonn of insulating tapes, which themselves have various layers. Common to these types of tapes is a paper layer that is bonded at an interface to a fiber layer, both layers tending to be impregnated with a resin. A favored type of insulation material is a mica-tape. Improvements to mica tapes include catalyzed mica tapes as taught in U.S. Pat. No. 6,103,882. The micatape may be wound around conductors to provide extremely good electrical insulation. An example of this is shown in FIG. 1. Illustrated here is a coil 13, comprising a plurality of turns of conductors 14, which in the example illustrated here are assembled into a bakelized coil. The turn insulation 15 is prepared from a fibrous material, for example glass or glass and Dacron which is heat treated. Ground insulation for the

coil is provided by wrapping one or more layers of composite mica tape 16 about the bakelized coil 14. Such composite tape may be a paper or felt of small mica flakes combined with a pliable backing sheet 18 of, for example, glass fiber cloth or polyethylene glycol terephthalate mat, the layer of mica 20 being bonded thereto by a liquid resinous binder. Generally, a plurality of layers of the composite tape 16 are wrapped about the coil depending upon voltage requirements. A wrapping of an outer tape 21 of a tough fibrous material, for example, glass fiber, may be applied to the coil.

Generally, multiple layers of the mica tape 16 are wrapped about the coil with sixteen or more layers generally being used for high voltage coils. Resins are then impregnated into the tape layers. Resins can even be used as insulation independently from the insulating tape. Unfortunately this amount of insulation only further adds to the complications of dissipating heat. What is needed is electrical insulation that can conduct heat higher than that of conventional methods, but that does not compromise the electrical insulation and other performance factors including mechanical and thennal capability.

Other difficulties with the prior art also exist, some of which will be apparent upon further reading.

SUMMARY OF THE INVENTION

With the foregoing in mind, methods and apparatuses consistent with the present invention, which inter alia facilitates the transport of phonons through a high thennal conductivity (HTC) impregnated medium to reduce the mean distances between the HTC materials below that of the phonon mean free path length. This reduces the phonon scattering and produces a greater net flow or flux of phonons away from the heat source. The resins may then be impregnated into a host matrix medium, such as a multi-layered insulating tape.

High Thermal Conductivity (HTC) organic-inorganic hybrid materials may be formed from discrete two-phase organic-inorganic composites, from organic-inorganic continuous phase materials based on molecular alloys and from discrete organic-dendrimer composites in which the organicinorganic interface is non-discrete within the dendrimer coreshell structure. Continuous phase material structures may be formed which enhance phonon transport and reduce phonon scattering by ensuring the length scales of the structural elements are shorter than or commensurate with the phonon distribution responsible for thennal transport, and/ or that the number of phonon scattering centers are reduced such as by enhancing the overall structural order of the matrix, and/ or by the effective elimination or reduction of interface phonon scattering within the composite. Continuous organic-inorganic hybrids may be fonned by incorporating inorganic, organic or organic-inorganic hybrid nano-particles in linear or cross-linked polymers (including thennoplastics) and thermosetting resins in which nano-particles dimensions are of the order of or less than the polymer or network segmental length (typically 1 to 50 mn or greater). These various types of nano-particles will contain reactive surfaces to form intimate covalently bonded hybrid organic-inorganic homogeneous materials. Similar requirements exist for inorganic-organic dendrimers which may be reacted together or with matrix polymers or reactive resins to form a continuous material. In the case of both discrete and non-discrete organic-inorganic hybrids it is possible to use sol-gel chemistry to form a continuous molecular alloy. The resulting materials will exhibit higher thermal conductivity than conventional electrically insulating materials and may be used as bonding resins in conventional mica-glass tape constructions, when utilized as

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ur1reacted vacuum-pres sure impregnation resins and as stand alone materials to fulfill electrical insulation applications in rotating and static electrical power plant and in both high (approximately over 5 kV) and low voltage (approximately under 5 kV) electrical equipment, components and products.

The formation of engineered electrical insulation materials having prescribed physical properties and performance characteristics, and based on the use of nano-to-micro sized inorganic fillers in the presence of organic host materials, requires the production of particle surfaces which can fonn an intimate interface with the organic host. This may be achieved through the grafting of chemical groups onto the surface of the fillers to make the surface chemically and physically compatible with the host matrix, or the surfaces may contain chemically reactive functional groups that react with the organic host to form covalent bonds between the particle and the host. The use of nano-to-micro sized inorganic fillers in the presence of organic host materials requires the production of particles with defined surface chemistry in addition to bulk dielectric and electrical properties and thennal conductivity. Most inorganic materials do not allow independent selection of structural characteristics such as shape and size and properties to suit different electrical insulation applications or to achieve composites having the right balance of properties and performance. This may be achieved by selecting particles with appropriate bulk properties and shape and size characteristics and then modifying the surface and interfacial properties and other characteristics to achieve the additional control of composite properties and performance required for electrical insulation applications. This may be achieved by appropriate surface coating of the particles which may include the production of metallic and non-metallic inorganic oxides, nitrides, carbides and mixed systems and organic coatings including reactive surface groups capable of reacting with appropriate organic matrices which act as the host material in the electrical insulation system. The resulting hybrid materials and composites in ur1reacted or partially reacted fonn may be used as bonding resins in mica-glass tape constructions, as ur1reacted vacuum-pressure impregnation resins for conventional mica tape constructions, in other glass fiber, carbon fiber and ply-type and textile composites and as stand alone materials to fulfill electrical insulation applications in rotating and static electrical power plant and in both high and low voltage electrical equipment, components and products.

In a particular embodiment the present invention provides for continuous high thennal conductivity resin that comprises a host resin matrix and a high thermal conductivity filler. The high thermal conductivity filler forms a continuous organicinorganic composite with the host resin matrix via surface functional groups that are grafted to the high thermal conductivity filler and fonns covalent linkages with the host resin matrix.

In another particular embodiment the present invention provides for continuous organic-inorganic resin with grafted functional groups bridging the organic-inorganic boundary that comprises a host resin network and inorganic high thermal conductivity fillers evenly dispersed in the host resin network and essentially completely co-reacted with the host resin network. The high thermal conductivity fillers have a length of between 1-1000 mn and aspect ratios of 10-50. The high thennal conductivity fillers are selected from one or more of oxides, nitrides, and carbides and the continuous orgamc-inorgamc resin comprises a maximum of 60% by volume of the high thennal conductivity fillers, and in other embodiments a maximum of 35%. Particularly, the high thermal conductivity fillers have surface functional groups that are grafted to the high thermal conductivity fillers and the

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surface functional groups allow for the essentially complete co-reactivity with the host resin network.

In related embodiments the functional groups comprise one or more of hydroxyl, carboxylic, amine, epoxide, silane and vinyl groups. The one or more of oxides, nitrides, and carbides compriseAl203,AlN, MgO, ZnO, BeO, BN, Si3N4, SiC and SiO2 with mixed stoichiometric and non-stoichiometric combinations. The host resin network includes epoxy, polyimide, polyimide epoxy, liquid crystal epoxy, polybutadiene, polyester and cyanate-ester. The continuous organicinorgamc resin further can also comprises a cross-lir1king agent, and the entire resin can be impregnated into a porous media.

In still another particular embodiment the present invention provides for method of making a high thennal conductivity resin that comprises supplying a host resin matrix and gathering a high thennal conductivity material, which is then surface treated with reactive surface functional groups in a high energy reaction such that the surface functional groups become grafted to the high thennal conductivity materials. Then mixing the treated high thermal conductivity materials with the host resin matrix such that the high thennal conductivity materials are substantially uniformly dispersed within the host resin matrix, and then reacting the surface functional groups that are grafted to the high thennal conductivity materials with the host resin matrix to produce the high thennal conductivity resin. The amount of the high thennal conductivity materials in the high thennal conductivity resin is a maximum of 60% by volume, and the high energy reaction produces bond strength of between approximately 200-500 k]/mol.

Other embodiments of the present invention also exist, which will be apparent upon further reading of the detailed description.

BRIEF DESCRIPTION OF THE FIGURES

The invention is explained in more detail by way of example with reference to the following drawings:

FIG. 1 shows the use of an insulating tape being lapped around a stator coil.

FIG. 2 illustrates phonons traveling through a loaded resin of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

High thennal conductivity (HTC) composites comprise a resinous host network combined with fillers that are two phase organic-inorganic hybrid materials. The organic-inorganic hybrid materials are formed from two phase organicinorgamc composites, from orgamc-inorgamc continuous phase materials that are based on molecular alloys, and from discrete organic-dendrimer composites in which the organicinorgamc interface is non-discrete with the dendrimer coreshell structure. Phonon transport is enhanced and phonon scattering is reduced by ensuring the length scales of the structural elements are shorter than or commensurate with the phonon distribution responsible for thermal transport.

Two phase orgamc-inorgamc hybrids may be formed by incorporating inorganic micro, meso or nano-particles in linear, or cross linked polymers (thennoplastics) and thennosetting resins. Host networks include polymers and other types of resins, definitions of which are given below. In general, the resin that acts as a host network may be any resin that is compatible with the particles and, if required, is able to react with the groups introduced at the surface of the filler. Nanoparticle dimensions are typically of the order of or less than

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the polymer network segmental length. For example 1-30 mn. The inorganic particles contain reactive surfaces to fonn covalently bonded hybrid orgamc-inorgamc homogeneous materials. The particles may be oxides, nitrides, carbides and hybrid stoichiometric and non-stoichiometric mixes of the oxides, nitrides and carbides, more examples of which are given below.

The inorganic particles are surface treated to introduce a variety of surface functional groups which are capable of participating in reactions with the host network. The surface functional groups include but are not limited to hydroxyl, carboxylic, amine, epoxide, silane and vinyl groups. The groups may be applied using wet chemical methods, nonequilibrium plasma methods, chemical vapor and physical vapor deposition, sputter ion plating and electron and ion beam evaporation methods.

The discrete organic-dendrimer composites may be reacted together or with the resin matrix to fonn a single material. The surface of the dendrimer can contain reactive groups similar to those mentioned above, which will either allow dendrimer-dendrimer or dendrimer-organic matrix reactions to occur. The dendrimer will have an inorganic core and an organic shell containing the reactive groups of interest. It may also be possible to have an organic core with an inorganic shell which also contains reactive groups such as hydroxyl or silane groupings which can participate in inorganic reactions similar to those involved in common sol-gel chemistries.

In regards to the use of non-discrete orgamc-inorgamc hybrids it is possible to use sol-gel chemistry to fonn a continuous molecular alloy. Gel sol-chemistries involving aqueous and non-aqueous reactions may be used. Other compounds for the fonnation of organic-inorganic hybrids include the polyhedral oligomeric silsesquioxanes (POSS), tetraethyl orthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) and related monomeric and oligomeric hybrid compounds which are organic functionalized inorganic compounds. In the example of POSS, molecules are built around a building block of R—SiOi _5 in which the R group is chosen to compatibilize with and/or react with other organic compounds and the host network. The base compounds may be combined to yield larger molecules commensurate with the size of polymer segment and coil structures. POSS may be used to create organic-inorganic hybrids and may be grafted into existing polymers and networks to control properties, including thennal conductivity. The materials may be obtained from suppliers such as AldrichTM Chemical Co., Hybrid PlasticsTM Inc. and GelestTM Inc.

As mentioned, it is important to control the structural fonn of the materials to reduce phonon scattering. This can be further assisted by using nano-particles whose matrices are known to exhibit high thennal conductivity and to ensure that the particles size and its interfacial characteristics with the resin are sufficient to sustain this effect, and also to satisfy the length scale requirement to reduce phonon scattering. A choice of structures that are more highly ordered will also benefit this, including reacted dendrimer lattices having both short and longer range periodicity and ladder or ordered network structures that may be fonned from a host resin, such as liquid crystal epoxies and polybutadienes.

The filled resins may be used as bonding resins in a variety of industries such as circuit boards and insulating tapes. A particular kind of insulating tape is the mica-glass tape used in the electrical generator fields. Resins with these types of tapes can be used as bonding resins, or as impregnating resins as is known in the art. The filled resin may also be used in the

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electrical generator field without the tapes to fulfill electrical insulation applications in the rotating and static electrical equipment components.

The tapes may be impregnated with resin before or after being applied to electrical objects. Resin impregnation techniques include VPI and GVPI, discussed more below. In VPI, once a tape is lapped and impregnated it is compressed. Once in position, the resin in the compressed tape is cured, which effectively locks the position of the HTC materials. In some embodiments the resin is cured in a two stage process, as will be apparent to one of ordinary skill in the art. However, optimal compression of the loaded HTC materials favors a completely uncured resin during the compression stage.

FIG. 2 shows one embodiment of the present invention. Illustrated here are HTC materials 30 loaded into a resinous matrix 32. Phonons 34 traveling through the matrix have a mean path length n, this is the phonon mean free path. This path length can vary depending on the exact composition of the resin matrix, but is generally from 2 to 100 mn, and more typically 5-50 mn, for resins such as epoxy resins. Therefore the mean distance between the loaded HTC materials should be on average less than this distance. Note that the distance between the HTC materials can vary in the thickness versus transverse direction of the tape, and it is generally the thickness direction where the spacing needs to be optimalized.

As phonons 34 travel through the resin 32 they will tend to pass along the embedded HTC materials 30. This will increase the local phonon flux since the raw HTC materials will have a thermal conductivity of between 10-1000 W/mK, as opposed to the resin which is about 0.1-0.5 W/mK. As phonons pass along a loaded HTC material the phonons 36 pass to the next HTC material if the distance between the materials is less than n, therefore the HTC materials fonn an interconnecting network. FIG. 2 illustrates an idealized path. In practice there will be phonon scattering as the phonons pass between the resin and HTC materials, although the shorter the distance between the materials, and the better the match of phonon propagation characteristics between the HTC materials and the resin, the less the scattering.

The amount of HTC materials loaded in the resin could actually be quite low, for example about 10% as illustrated in FIG. 2. The average distances, or length scales, between loaded HTC materials therefore may be slightly greater than n, however, a large percentage will still be less than n and therefore fall within embodiments of the present invention. In particular embodiment, the percentage materials that are less than n distance from the next HTC material is over 50%, with particular embodiment being over 75%. In particular embodiment the average length of the HTC materials is greater than n, which further aids in phonon transport.

The shorter n the greater the concentration of loaded HTC materials, and conversely, the greater the particle size, the less HTC materials needed. Particular embodiment use 5-60% loaded HTC materials by total volume of the resins and fillers, with more particular embodiments at 25 -40%. When the resin is impregnated into the tape, it will fill up the spaces between the tape fibers and substrates. The HTC distribution within the tape at this point, however, is often not optimized, and can even have the mean distance between HTC materials greater than n. Practice of the present invention then compresses the resin impregnated tapes and reduces the distances between the loaded HTC materials.

When a loaded resin is being impregnated into a tape, the fibers or particles of the tape act to block some of the HTC materials, particularly if the resin is 30% or more filler. However, by compressing the tapes, the reverse happens, and more fillers are trapped within the tape as the HTC materials attach

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themselves to non-mobile parts of the overall structure. The HTC fillers even get pirmed to one another. In the embodiments given, it has been implied that the fillers do not react with the resin matrix, however, in some embodiments the fillers do fonn covalent bonds with the resin and fonn more homogeneous matrixes. In a homogenous matrix, the resin molecules that are bound to fillers will be retained better than the unbound resin molecules during compression.

Resins are used in a plurality of industries, and have a large number of uses. Different properties of the resins affect not only their uses, but also the quality and efliciency of the products that they are used with. For example, when resins are used in electrical insulation applications, their characteristics of dielectric strength and voltage endurance needs to be high, as does the thennal stability and thennal endurance. However, often contrary to these objectives, resins usually will also have a low thermal conductivity. The present invention balances the various physical properties of resins and the insulation system they are introduced into to produce a system that has a higher thennal conductivity than conventional electrically insulating materials while maintaining adequate, and even enhancing, key physical properties such as dielectric strength, voltage endurance, thennal stability and thennal endurance, mechanical strength and viscoelastic response. Delamination and microvoid fonnation resulting from stresses caused by thennal, vibration and mechanical cycling effects are reduced or eliminated. As used herein, the tenn resin refers to all resins and epoxy resins, including modified epoxies, polyesters, polyurethanes, polyimides, polyesterimides, polyetherimides, bismaleimides, silicones, polysiloxanes, polybutadienes, cyanate esters, hydrocarbons etc. as well as homogeneous blends of these resins. This definition of resins includes additives such as cross-linking agents, accelerators and other catalysts and processing aids. Certain resins, such as liquid crystal thennosets (LCT) and 1,2 vinyl polybutadiene combine low molecular weights characteristics with good crosslinking properties. The resins can be of an organic matrix, such as hydrocarbons with and without hetero atoms, an inorganic matrix, containing silicate and/or alumino silicate components, and a mixture of an organic and inorganic matrix. Examples of an organic matrix include polymers or reactive themiosetting resins, which if required can react with the reactive groups introduced on inorganic particle surfaces. Cross-linking agents can also be added to the resins to manipulate the structure and segmental length distribution of the final crosslinked network, which can have a positive effect on thennal conductivity. This thermal conductivity enhancement can also be obtained through modifications by other resin additives, such as catalysts, accelerators and other processing aids. Certain resins, such as liquid crystal thennosets (LCT) and 1,2 vinyl polybutadiene combine low molecular weights characteristics with good crosslinking properties. These types of resins tend to conduct heat better because of enhanced micro and macro ordering of their sub-structure which may lead to enhanced conduction of heat as a result of improved phonon transport. The better the phonon transport, the better the heat transfer.

When the high thermal conductivity fillers of the present invention are mixed with resins they form a continuous product, in that there is no interface between the resins and the fillers. In some cases, covalent bonds are formed between the fillers and the resin. However, continuous is somewhat subjective and depends on the scale to which the observer is using. On the macro-scale the product is continuous, but on the nano-scale there can still be distinct phases between the fillers and the resin network. Therefore, when referring high thennal conductivity fillers mixing with the resin, they fonn a

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continuous organic-inorganic composite, on the macro-scale, while on the micro-scale the same mixture can be referred to as a hybrid.

As mentioned, filled resin may be used in the electrical generator field without the tapes to fulfill electrical insulation applications in the rotating and static electrical equipment components. The use of high thennal conductivity materials in a generator is multiple. Within the stator coil there are component materials other than the groundwall which must have high thennal conductivity to optimize the design. Likewise other components associated with the coils to maximize heat removal. Improvements to stator design dictate that improvements be made to rotor design so that generator efliciency can by maximized.

It is important that the interface between the various inorganic and organic components is made to be chemically and physically intimate to ensure a high degree of physical continuity between the different phases and to provide interfaces which are mechanically strong and not prone to failure. This is especially important during the operation of the electrical insulation embodiments discussed, such as the electrical insulation systems forboth high and low voltage applications. An enhanced interface integrity would enable an enhanced power rating, higher voltage stressing, reduced insulation thickness and high heat transfer.

Surface treatments to fillers introduce a variety of surface functional groups that are capable of compatibilizing inorganic surface of the filler with the organic resin matrix. Typical surface treatment is to introduce surface functional groups is to treat a surface physically (e.g. silane solution on metal oxides) to give reactive groups. The interface between the particle surface, such as the HTC filler, surface and the silane layer would only be held by physical bonding, such as polar attraction and H-bonds. Although the silane surface could react with a resin that it is mixed in, there is no true chemical bond formed between the particle surface and the silane, i.e. essentially unreactive coupling. Even if the substrate surface was rich in OH groups, such as hydrated Alumina, that could potentially react with the silane, it is unlikely that significant chemical bonds will form. In the case of the HTC fillers discussed herein, there would be virtually no chemical bond formation.

In order to obtain functional groups that are chemically attached to the HTC material (particle) surface, the present invention uses reactive grafting. Reactive grafting occurs when the functional groups are chemically attached to the nanoparticle surface by a reactive process, such as by chemical reaction. Other processes include those that are plasma and radiation (e.g. UV, gamma, electron, etc.) driven, which require appropriate enviromnents and may be done in a multistage process. In this manner, a strong chemical bond is produced between the nanoparticle surface and the functional group attached (e.g., OH, COOH, NH2 and vinyl); i.e. reactive coupling. This would be the definition of a reactive functional graft, i.e., the chemical attachment of a functional group directly onto the particle surface. These reactive grafting procedures are high energy compared to the physical bonding of the prior art, and use, for example, non-equilibrium plasma methods, chemical vapor and physical vapor deposition, sputter ion plating, laser beams, electron and ion beam evaporation methods to chemically modify the surfaces of the more inert surfaces of the HTC material, producing chemically attached functional species (e.g. OH, COOH, NH2, vinyl) which are then reacted with resin to produce a continuous HTC matrix.

Specific examples of this include treating boron nitride (BN) nanoparticles with an electron beam in the presence of