WO2016106398A1

WO2016106398A1 - Thermally conductive wire enamel and varnish formulations

Info

Publication number: WO2016106398A1
Application number: PCT/US2015/067538
Authority: WO
Inventors: Rohit Bhosale; Chandrashekar Raman
Original assignee: Momentive Performance Materials Inc
Current assignee: Momentive Performance Materials Inc
Priority date: 2014-12-23
Filing date: 2015-12-22
Publication date: 2016-06-30
Anticipated expiration: 2017-06-23

Abstract

The present technology provides an enamel composition that is thermally conductive, electrically insulating, or both. The present technology provides an enamel composition that has high dielectric strength. Additionally, the present invention provides an enamel composition in which the dielectric constant is tuned to meet a certain impedance requirement depending on the thickness of enamel layer and the frequency of the electrical field in the conductor.

Description

TITLE

THERMALLY CONDUCTIVE WIRE ENAMEL AND VARNISH FORMULATIONS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/096, 119, entitled "Thermally Conductive Wire Enamel Formulations" filed on December 23, 2014 and U.S. Provisional Patent Application No. 62/102,777 entitled "Thermally Conductive Wire Enamel Formulations" filed on January 13, 2015, which are hereby incorporated by reference in their entirety.

FIELD

[0002] The present technology relates to novel enamel and/or varnish formulations that are thermally conductive and/or electrically insulating. The present technology also relates to enamel and/or varnish formulations having enhanced dielectric strength and other dielectric properties.

BACKGROUND

[0003] Copper or aluminum wires are extensively used in a wide range of electrical equipment or electronic components such as electric motors, transformers, generators, inductors, electromagnets, etc. In such applications, the conductor wires are often coated with a thin layer of electrical insulation, commonly known as enamel, to prevent short circuits between the conductors. A wide range of resin chemistries are used in enamel depending on the desired operating temperature such as epoxies, polyurethanes, polyesters, polyamides, polyamide-imides, and polyimides. These enamels may be either thermoplastic or thermoset resins. The enamel layer may be a single resin layer, or could consist of multiple, concentric layers.

[0004] Once the enamel coated wires are wound to make a coil, the coil is then impregnated with a resin, usually called the "varnish". The varnish may be typically be an epoxy resin, but may also be selected from other suitable chemistries such as polyurethanes, polyesters, polyamides, polyamide-imides, and polyimides.

[0005] With shrinking dimension of electrical and electronic components, conductors used in such equipment are carrying/conducting more power than ever before, leading to more waste heat that needs to be dissipated during operation. The waste heat needs to be adequately dissipated to maintain an acceptable operating temperature necessary for steady, long term, and reliable performance. However the thermal management problem is exacerbated due to the poor thermal conductivity of the enamel and/or varnish and the shrinking footprints of these devices. There is hence a need for thermally conductive formulations of such enamel and/or varnish materials.

SUMMARY

[0006] The present technology provides, in one aspect, a thermally conductive enamel and varnish formulations. The enamel or varnish formulation may include inorganic additive or filler materials that are electrically insulating. The present technology provides a thermally conductive enamel or varnish formulation that is capable of dissipating heat to maintain acceptable operating temperatures for electrical and electronic components. The formulations disclosed herein may be used as an enamel or a varnish depending on the desired outcome. For the purposes of this invention, the terms enamel and varnish may be used interchangeably.

[0007] In one aspect of the invention, the enamel formulations have enhanced dielectric strength. The dielectric strength of the enamel formulation may be enhanced by the addition of various fillers to the enamel resin.

[0008] In one aspect of the invention, the dielectric constant of the enamel formulation may be tuned to meet a certain impedance requirement depending on the thickness of the enamel layer and the frequency of the electrical field in the conductor.

[0009] In one aspect of the invention, the enamel formulation comprises boron nitride (BN). The enamel formulation may also include an additional metal oxide and/or a mineral additive.

[0010] In one embodiment, the formulation comprises at least about 3% BN by weight, or at least about 5% BN by weight, or at least about 10 % BN by weight, or at least about 20% BN by weight, or at least 30% BN by weight, or at least 40% BN by weight, or at least about 50% BN by weight, or even up to about 80% BN by weight.

[0011] In one embodiment, the enamel formulation has an in-plane thermal conductivity of at least 0.5 W/mK, or at least 0.8 W/mK, or at least 1.0 W/mK, or at least 2.0 W/mK, or at least 5 W/mK, or even up to 10 W/mK.

[0012] In one embodiment, the enamel formulation has a through-plane thermal conductivity of at least 0.5 W/mK, or at least 0.8 W/mK, or at least 1.0 W/mK, or at least 2.0 W/mK, or at least 5 W/mK, or even up to 10 W/mK.

[0013] In one embodiment, the enamel formulation has a dielectric strength of at least 10 KV/mm, or at least 20 KV/mm, or at least 30 KV/mm, or at least 50 KV/mm, or even up to 100 KV/mm.

[0014] In one embodiment, the enamel formulation has a dielectric constant of

2. In another embodiment, the dielectric constant is 3, or 4, or 6, or 10, or 50, or even up to 500.

[0015] These and other aspects and embodiments are further understood with reference to the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a cross-sectional view of a wire coated with an enamel formulation according to the invention.

[0017] This drawing is not to scale unless otherwise noted. The drawing is for the purpose of illustrating aspects and embodiments of the present technology and is not intended to limit the technology to those aspects illustrated therein. Aspects and embodiments of the present technology can be further understood with reference to the following detailed description.

DETAILED DESCRIPTION

[0018] The current invention discloses various novel enamel and/or varnish compositions. In one aspect, the enamel and/or varnish compositions are thermally conductive formulations. When present, the varnish may comprise the same formulation as the enamel composition or may comprise a different formulation from the enamel composition.

[0019] The enamel composition comprises a resin matrix and a filler disposed in the resin matrix. Thermally conductive formulations of resin systems may be produced by adding inorganic additives or filler materials to the resin matrix. Because electrical insulation properties may be important, potential additives may also be electrically insulating.

[0020] Depending on the application, it may be desirable to balance the need for higher thermal conductivity with other characteristics of the enamel. Electrical properties such as volume resistivity, surface resistivity, dielectric strength, dielectric constant, dielectric loss properties (loss tangent), etc., physical characteristics such as tensile strength, elongation at break, tensile modulus, flexural strength, flexural modulus, impact strength, heat distortion temperature, etc., adhesive behavior such as adhesion strength, or peel strength all need to be carefully weighed to develop optimized formulations for any given application.

[0021] The present invention also provides an article coated with the present enamel composition. An example of an article that may be provided is an enameled wire. In one embodiment, an enameled wire 10 comprises a wire/conductor 20 and an enamel coating 30 disposed about the wire/conductor 20.

[0022] The resin material is generally not limited and may include any material suitable for an enamel as desired for a particular purpose or intended application. Examples of materials suitable for the enamel resin include, but are not limited to, a polyvinyl formal, a polyester, a polyurethane, a polyamide, a polyimide, a polyamideimide, a polyepoxide, a polyesterimide, and combinations of two or more thereof.

[0023] The enamel composition further comprises a filler. In one embodiment, the enamel composition comprises a thermally conductive filler. The thermally conductive filler may also exhibit good electrical insulation properties, particularly for applications where the enamel composition will be used to coat electrically conductive materials.

[0024] Examples of suitable fillers include, but are not limited to boron nitride, silica, glass fibers, a metal oxide such as, zinc oxide, magnesium oxide, beryllium oxide, titanium oxide, zirconium oxide, yttrium oxide, etc., calcium carbonate, talc, mica, wollastonite, clays, exfoliated clays, aluminum silicate, alumina, aluminum nitride, graphite, metallic powders, e.g., aluminum, copper, bronze, brass, etc., or a combination of two or more thereof.

[0025] In one embodiment, the enamel coating 30 may include a silica, alumina, magnesium oxide, zinc oxide, yttria, zirconia, or a combination of two or more thereof

[0026] In one embodiment, the enamel formulation 30 may include silica.

Along with silica, other standard glasses such as soda-lime glass, borosilicate, E- glass, S-glass may optionally be used as well.

[0027] In one embodiment, more complex materials such as metal oxides with more than one cation such as barium titanate, calcium titanate, lead titanate, strontium titanate, barium strontium titanate (BST), barium zirconate, or materials such silicates, oxynitrides, perovskites, chalcogenides, or a combination of two or more thereof. In addition to these pure materials, doped versions of these materials that demonstrate superior properties may also be considered. For example, BST doped with various rare earth oxides or heavy metal oxides may have very high dielectric constants.

[0028] Because the conductivities of such metal oxides are typically not very high, where higher thermal conductivity is desired, nitride materials such as boron nitride (BN), aluminum nitride, and/or silicon nitride may be used.

[0029] In one embodiment, the enamel formulation may further comprise various mineral fillers such as talc, mica, vermiculite, clay, kaolin, wollastonite, bentonite, montmorillonite, aluminum silicates, or a combination of two or more thereof.

[0030] The materials mentioned above may be used in particulate form or agglomerated form, or as short fibers, or longer fibers, or as flakes of controlled average size and/or controlled size distributions or with controlled aspect ratio for fibers and flakes.

[0031] In one embodiment, the enamel formulation comprises BN. BN has high thermal conductivity (> 300 W/mK), high dielectric strength (> 50 KV/mm), low dielectric constant (approximately 4.1 @ 1 MHz), and very low loss tangent (< 0.0002 @ 1 MHz).

[0032] Once the enamel coating 30 is applied on the conductor 20, the "in- plane" thermal conductivity is defined to be along the length of the conductor, and the "through-plane" thermal conductivity, also known as the through-thickness thermal conductivity, is in the thickness direction of the article. It is possible that the thermal conductivity of such formulations may be anisotropic, i.e., the in-plane and through-plane conductivities may not be equal.

[0033] In one embodiment, the enamel has an in-plane thermal conductivity of at least about 0.5 W/mK, or at least about 0.8 W/mK, or at least about 1.0 W/mK, or at least about 2.0 W/mK, or at least about 5 W/mK, or even up to about 10 W/mK.

[0034] In one embodiment, the enamel has an in-plane thermal conductivity in the range of at least about 0.5 W/mK to at least about 0.8 W/mK; at least about 0.5 W/mK to at least about 1.0 W/mK; at least about 0.5 W/mK to at least about 2.0 W/mK; at least about 0.5 W/mK to at least about 5.0 W/mK; at least about 0.5 W/mK to at least about 10.0 W/mK; or at least about 0.5 W/mK to at least about 20W/mK.

[0035] In one embodiment, the enamel has an in-plane thermal conductivity in the range of at least about at least about 0.8 W/mK to at least about 1.0 W/mK; at least about 0.8 W/mK to at least about 2.0 W/mK; at least about 0.8 W/mK to at least about 5.0 W/mK; at least about 0.8 W/mK to at least about 10.0 W/mK; or at least about 0.8 W/mK to at least about 20 W/mK.

[0036] In one embodiment, the enamel has an in-plane thermal conductivity in the range of at least about 1.0 W/mK to at least about 2.0 W/mK; at least about 1.0 W/mK to at least about 5.0 W/mK; at least about 1.0 W/mK to at least about 10.0 W/mK; or at least about 1.0 W/mK to at least about 20 W/mK.

[0037] In one embodiment, the enamel 10 has a through-plane thermal conductivity of at least about 0.5 W/mK, or at least about 0.8 W/mK, or at least about 1.0 W/mK, or at least about 2.0 W/mK, or at least about 5 W/mK, or even up to about 10 W/mK.

[0038] In one embodiment, the enamel has an through-plane thermal conductivity in the range of at least about 0.5 W/mK to at least about 0.8 W/mK; at least about 0.5 W/mK to at least about 1.0 W/mK; at least about 0.5 W/mK to at least about 2.0 W/mK; at least about 0.5 W/mK to at least about 5.0 W/mK; at least about 0.5 W/mK to at least about 10.0 W/mK.

[0039] In one embodiment, the enamel has an through-plane thermal conductivity in the range of at least about at least about 0.8 W/mK to at least about 1.0 W/mK; at least about 0.8 W/mK to at least about 2.0 W/mK; at least about 0.8 W/mK to at least about 5.0 W/mK; at least about 0.8 W/mK to at least about 10.0 W/mK. [0040] In one embodiment, the enamel has an through-plane thermal conductivity in the range of at least about 1.0 W/mK to at least about 2.0 W/mK; at least about 1.0 W/mK to at least about 5.0 W/mK; at least about 1.0 W/mK to at least about 10.0 W/mK.

[0041] In one embodiment, the total weight loading of fillers in the composition is at least about 3% by weight, or at least about 10 % by weight, or at least about 20% by weight, or at least about 50% by weight, or even up to about 90% by weight.

[0042] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 3% by weight to at least about 10% by weight; at least about 3% by weight to at least about 20% by weight; at least about 3% by weight to at least about 50% by weight; or at least about 3% by weight to at least about 90% by weight.

[0043] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 10% by weight to at least about 20% by weight; at least about 10% by weight to at least about 50% by weight; or at least about 10% by weight to at least about 90% by weight.

[0044] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 20% by weight to at least about 50% by weight or at least about 20% by weight to at least about 90% by weight.

[0045] In one embodiment, the enamel formulation comprises boron nitride.

Examples of suitable boron nitride materials include boron nitride platelets, boron nitride agglomerates, or a mixture thereof.

[0046] The average particle size of the boron nitride may be about 3 microns, or about 10 microns, or about 25 microns or about 50 microns, or about 100 microns or about 150 microns, or even up to about 400 microns. The fundamental crystal size of the boron nitride may be about 0.1 microns, or about 0.5 microns, or about 2 microns, or about 10 microns, or even up to about 50 microns. The surface area of the boron nitride may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 5 m²/g, or less than 2 m²/g, or even less than 1 m²/g. The aspect ratio of the fundamental crystals of the boron nitride may be at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000. [0047] In one embodiment, the enamel coating comprises boron nitride having an average particle size in a range of about 1 microns to about 50 microns, about 2 microns to about 30 microns, or about 3 microns to about 25 microns.

[0048] In another embodiment, the varnish comprises boron nitride having an average particle size in a range of about 10 microns to about 150 microns, about 25 microns to about 100 microns, or about 30 microns to about 75 microns. It may be desirable to include a varnish comprising boron nitride having a larger average particle size as compared to the average particle size of the enamel coating.

[0049] In another embodiment the enamel formulation comprises boron nitride and an additional metal oxide or a mineral additive.

[0050] In one embodiment, the formulation comprises at least about 3% BN by weight, or at least about 10 % BN by weight, or at least about 20% BN by weight, or at least about 50% BN by weight, or even up to about 80% BN by weight.

[0051] In one embodiment, the total weight loading of BN in the composition is in the range of at least about 3% BN by weight to at least about 10% BN by weight; at least about 3% BN by weight to at least about 20% BN by weight; at least about 3% BN by weight to at least about 50% BN by weight; or at least about 3% BN by weight to at least about 80% BN by weight.

[0052] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 10% by weight to at least about 20% by weight; at least about 10% by weight to at least about 50% by weight; or at least about 10% by weight to at least about 80% by weight.

[0053] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 20% by weight to at least about 50% by weight or at least about 20% by weight to at least about 80% by weight.

[0054] In one embodiment, the enamel formulation comprises BN flakes or platelets. In one embodiment, the average BN platelet size may be in the range of about 0.1 microns to about 100 microns, about 0.5 microns to about 50 microns, about 1 to about 25 microns, or about 2 to about 20 microns.

[0055] In one embodiment, the average BN platelet size may be about 0.1 microns, or about 0.5 microns, or about 2 microns, or about 10 microns, or about 50 microns, or even up to about 100 microns. The surface area of the BN platelets may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 6 m²/g, or less than 3 m²/g, or even less than 1 m²/g. The graphitization index, a measure of the extent of crystallinity of the BN crystals, may be less than 8, or less than 4, or less than 2, or less than 1.5, or less than 1.2, or possibly even less than 1.0.

[0056] In one embodiment, the enamel formulation comprises exfoliated or delaminated BN platelets or flakes. The BN flakes or platelets may have an average aspect ratio (defined as ratio of crystal diameter to crystal thickness) of at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000. In one embodiment, the BN flakes or platelets may have an average aspect ratio in the range of about 10 to about 1000; about 10 to about 600; about 10 to about 300; about 20 to about 1000; about 20 to about 600; about 20 to about 300; about 50 to about 1000; about 50 to about 600; about 50 to about 300. The crystal thickness may be estimated by calculations from the surface area and the average aspect ratio may be calculated using the measured average particle size of the BN platelets. The particle size may be measured using suitable methods such as laser light diffraction or sedimentation analysis.

[0057] In one embodiment, the enamel formulation comprises agglomerate grades of BN. Use of BN agglomerates may eliminate the anisotropy in BN crystals. BN agglomerate particles are aggregates of smaller BN crystal platelets. For example, it is possible to make a 60 micron agglomerate of about 6 micron fundamental crystals (e.g., Momentive's PTX60 BN powder grade), or a 150 micron agglomerate of about 8 micron crystals (e.g., Momentive's PT350 BN powder grade).

[0058] The porosity of BN agglomerates is defined as the void fraction within the agglomerate. In one embodiment, the porosity within the BN agglomerate may be less than about 80%, or less than about 50%, or less than about 35%, or less than about 20%, or less than about 10%.

[0059] In one embodiment, the enamel formulation contains an additive to treat the surface of the fillers used in the formulation.

[0060] In one embodiment the additive may be a silane. Examples of suitable silanes include, but are not limited to, an alkacryloxy silane, a vinyl silane, a chloro silane, a mercapto silane, a blocked mercapto silane, or a combination of two or more thereof. The additive may also include a siloxane, polysiloxane, silazane, silicone hydride, polyhedral silsesquioxane or modified polyhedral silsesquioxane, another organometallic such as zirconate, aluminate, titanate, fatty acid derivative such as sorbitol monostearate (sorbitan), non-ionic surfactant, or a combination of two of more thereof.

[0061] In one embodiment, the surface treatment additive may be used at loadings as low as 0.01% by weight, or up to 0.1% by weight, or up to 1% by weight, or up to 5% by weight of the filler. In one embodiment the surface treatment additive may be used at loadings up to about 10% by weight of the filler.

[0062] In one embodiment the surface treatment additive may be used at loadings in the range of about 0.01% by weight to about 0.1% by weight; about 0.01% by weight to about 1% by weight; 0.01% by weight to about 5% by weight, or 0.01% by weight to about 10% by weight.

[0063] In one embodiment the surface treatment additive may be used at loadings in the range of about 0.1% by weight to about 1% by weight or about 0.1% by weight to about 5% by weight; or 0.1% by weight to about 10% by weight.

[0064] In one embodiment, the fillers are treated with the chosen additive prior to addition to the enamel resin. In case the fillers are surface treated prior to addition to the resin, the surface treatment step may include an optional heat treatment or humidity treatment, i.e. exposed to higher temperature and/or higher humidity, after the addition of the surface treatment additive to the filler or blend of fillers. In another embodiment, the chosen additive and the fillers are added simultaneously to the enamel resin formulation.

[0065] In one aspect of the invention, the enamel formulations have enhanced dielectric strength. The dielectric strength of the enamel formulation may be enhanced by the addition of various fillers to the enamel resin.

[0066] In one embodiment, the enamel formulation has a dielectric strength of at least 10 KV/mm, or at least 20 KV/mm, or at least 30 KV/mm, or at least 50 KV/mm, or even up to 100 KV/mm.

[0067] In one embodiment, the total weight loading of fillers in the composition is at least about 3% by weight, or at least about 10 % by weight, or at least about 20% by weight, or at least about 50% by weight, or even up to about 90% by weight.

[0068] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 3% by weight to at least about 10% by weight; at least about 3% by weight to at least about 20% by weight; at least about 3% by weight to at least about 50% by weight; or at least about 3% by weight to at least about 90% by weight.

[0069] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 10% by weight to at least about 20% by weight; at least about 10% by weight to at least about 50% by weight; or at least about 10% by weight to at least about 90% by weight.

[0070] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 20% by weight to at least about 50% by weight or at least about 20% by weight to at least about 90% by weight.

[0071] In one embodiment, the enamel formulation comprises boron nitride.

In another embodiment, the enamel formulation comprises boron nitride flakes/platelets, or boron nitride agglomerates, or a combination of both.

[0072] In one embodiment, the boron nitride platelets in the enamel formulation have an aspect ratio of at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000.

[0073] In one embodiment, the formulation comprises at least about 3% BN by weight, or at least about 10 % BN by weight, or at least about 20% BN by weight, or at least about 50% BN by weight, or even up to about 80% BN by weight.

[0074] In one embodiment, the total weight loading of BN in the composition is in the range of at least about 3% BN by weight to at least about 10% BN by weight; at least about 3% BN by weight to at least about 20% BN by weight; at least about 3% BN by weight to at least about 50% BN by weight; or at least about 3%

BN by weight to at least about 80% BN by weight.

[0075] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 10% by weight to at least about 20% by weight; at least about 10% by weight to at least about 50% by weight; or at least about 10% by weight to at least about 80% by weight.

[0076] In one embodiment, the total weight loading of fillers in the composition is in the range of at least about 20% by weight to at least about 50% by weight or at least about 20% by weight to at least about 80% by weight.

[0077] In one embodiment, the enamel formulation comprises BN flakes or platelets. In another embodiment, the formulation comprises exfoliated or delaminated boron nitride. The average BN platelet size may be 0.1 microns, or 0.5 microns, or 2 microns, or 10 microns, or 50 microns, or even up to 100 microns. The surface area of the BN platelets may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 6 m²/g, or less than 3 m²/g, or even less than 1 m²/g.

[0078] In one embodiment, the enamel formulation comprises boron nitride agglomerates. The average particle size of the agglomerate may be 3 microns, or 10 microns, or 50 microns, or 150 microns, or even up to 400 microns. The fundamental crystal size of these agglomerates may be 0.1 microns, or 0.5 microns, or 2 microns, or 10 microns, or even up to 50 microns. The surface area of the BN agglomerates may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 5 m²/g, or less than 2 m²/g, or even less than 1 m²/g. The aspect ratio of the fundamental crystals of these agglomerates may be at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000. In one embodiment, the porosity within the BN agglomerate may be less than 10%, or less than 20%, or less than 35%, or less than 50%, or even up to 80%.

[0079] In one embodiment, the enamel formulation comprises BN and an additional filler with a flake or platelet or platelet-like morphology. In one embodiment, the additional platelet filler may be mica, or talc, or clay, or bentonite, or a combination of two or more thereof. In one embodiment, the additional platelet filler may be exfoliated or delaminated.

[0080] In one aspect of the invention, the dielectric constant of the enamel formulation may be tuned. The dielectric constant of the enamel formulation may need to be tuned to meet a particular application's need. In alternating current (AC) applications, the impedance from enamel layer depends on the dielectric constant of the formulations and the frequency of the electrical field in the conductor. The dielectric constant of the insulation may need to be adjusted to tune the impedance to a desired range for a particular application for optimal performance. Methods for adjusting the dielectric constant include, but are not limited to, adjusting the amount and/or type of filler, adjusting the frequency of the electric field in the conductor, introducing voids of air or nitrogen into the enamel formulation, or a combination of two or more thereof.

[0081] The dielectric constant of a material depends on the frequency of the electric field applied. The embodiments mentioned below are based on properties at a frequency of 1 MHz, but other frequencies may be used as well. [0082] In one embodiment, the enamel formulation has a dielectric constant of

[0083] In one embodiment, the enamel formulation comprises boron nitride.

In another embodiment the enamel formulation comprises boron nitride and an additional metal oxide. These may be materials such as, but not limited to, barium titanate, or strontium titanate, or doped versions of these material. BN has a dielectric constant around 4.0 (3.9 - 4.2) at 1 MHz. Under static electric fields, the dielectric constant of BN is approximately 5 - 7. Like thermal conductivity, the dielectric constant of BN is also anisotropic. This is a fairly low dielectric constant compared some other filler materials. For example, alumina has a dielectric constant of 9 - 9.5. In contrast, some titanates have very high dielectric constants such as strontium titanate (approximately 300); or BST doped with lanthanum oxide and/or antimony oxide may have a dielectric constant as high as 6000.

[0084] In one embodiment, the formulation comprises at least 3% BN by weight, or at least 10 % BN by weight, or at least 20% BN by weight, or at least 50% BN by weight, or even up to 80% BN by weight.

[0085] In one embodiment, the total weight loading of fillers in the composition is at least 3% by weight, or at least 10 % by weight, or at least 20% by weight, or at least 50% by weight, or even up to 90% by weight.

[0086] In one embodiment, the enamel formulation comprises BN flakes or platelets. The average BN platelet size may be 0.1 microns, or 0.5 microns, or 2 microns, or 10 microns, or 50 microns, or even up to 100 microns. The surface area of the BN platelets may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 6 m²/g, or less than 3 m²/g, or even less than 1 m²/g. The graphitization index, a measure of the extent of crystallinity of the BN crystals, may be less 8, or less than 4, or less than 2, or less than 1.5, or less than 1.2 or possibly even less than 1.0.

[0087] In one embodiment, the enamel formulation comprises exfoliated or delaminated BN platelets or flakes. The BN flakes or platelets may have an average aspect ratio (defined as ratio of crystal diameter to crystal thickness) of at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000. The aspect ratio may be estimated by calculations from the surface area and the measured particle size of the BN platelets.

[0088] In one embodiment, the enamel formulation comprises agglomerate grades of BN. BN agglomerate particles are aggregates of smaller BN crystal platelets. For example, it is possible to make a 60 micron agglomerate of about 6 micron fundamental crystals (e.g., Momentive's PTX60 BN powder grade), or a 150 micron agglomerate of about 8 micron crystals (e.g., Momentive's PT350 BN powder grade).

[0089] In one embodiment, the enamel formulation comprises boron nitride agglomerates. The average particle size of the agglomerate may be 3 microns, or 10 microns, or 50 microns, or 150 microns, or even up to 400 microns. The fundamental crystal size of these agglomerates may be 0.1 microns, or 0.5 microns, or 2 microns, or 10 microns, or even up to 50 microns. The surface area of the BN agglomerates may be less than 50 m²/g, or less than 20 m²/g, or less than 10 m²/g, or less than 5 m²/g, or less than 2 m²/g, or even less than 1 m²/g. The aspect ratio of the BN agglomerates may be less than 10, or less than 5, or less than 2, or preferably less than 1.5. The aspect ratio of the fundamental crystals that make up these agglomerates may be at least 5, or at least 10, or at least 20, or at least 50, or at least 100, or at least 300, or at least 600, or even up to 1000.

[0090] The porosity of BN agglomerates is defined as the void fraction within the agglomerate. In one embodiment, the porosity within the BN agglomerate may be less than about 50%, or less than about 35%, or less than about 20%, or less than about 10%. In one embodiment, the porosity with the BN agglomerate may be up to about 80%.

[0091] In addition to adding fillers, introducing voids of air or nitrogen may also be used as a strategy to lower the dielectric constant. In one embodiment, the enamel formulation has at least 3% (by volume) porosity. In another embodiment, the porosity may be 10%, or 20% or even up to 50%. However, excessive porosity may reduce dielectric strength.

[0092] Embodiments of the technology have been described above and modifications and alterations may occur to others upon the reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.

Claims

CLAIMS What is claimed is:

1. An insulated wire comprising:

(a) a conductor material; and

(b) a coating disposed about the surface of the conductor material, the coating comprising a thermally conductive enamel composition comprising an enamel resin material and boron nitride.

2. The insulated wire of claim 1, wherein the boron nitride is chosen from boron nitride platelets, boron nitride agglomerates, or a mixture thereof.

3. The insulated wire of claim 2, wherein the boron nitride comprises boron nitride platelets having an average particle size of about 1 micron to about 25 microns.

4. The insulated wire of claims 2 or 3, wherein the boron nitride comprises boron nitride platelets having an aspect ratio of from about 10 to about 300.

5. The insulated wire of any of claims 2-4 wherein the boron nitride comprises boron nitride platelets having an average particle surface area of less than 50 m²/g.

6. The insulated wire of claim 2, wherein the boron nitride comprises boron nitride agglomerates, wherein the average particle size of the agglomerate is less than about 400 microns

7. The insulated wire of any of claims 1, 2, or 6, wherein the boron nitride comprises boron nitride agglomerates, wherein the porosity within the boron nitride agglomerate is less than about 80%.

8. The insulated wire of claim 7, wherein the porosity within the boron nitride agglomerate is less than about 50%.

9. The insulated wire of claim 8, wherein the porosity within the boron nitride agglomerate is less than about 35%,

10. The insulated wire of any of claims 1-9, wherein the boron nitride is present in an amount of about 3% by weight to about 80% by weight.

11. The insulated wire of any of claims 1-9 wherein the boron nitride is present in an amount of about 10% by weight to about 50% by weight.

12. The insulated wire of any of claims 1-11, wherein the enamel composition further comprises a filler selected from silica, glass fibers, a metal oxide, calcium carbonate, talc, mica, wollastonite, a clay, an exfoliated clay, aluminum silicate, alumina, aluminum nitride, graphite, a metallic powders, or a combination of two or more thereof.

13. The insulated wire of claim 12 wherein the filler is selected from silica, alumina, magnesium oxide, zinc oxide, yttria, zirconia, or a combination of two or more thereof.

14. The insulated wire of any of claims 1-13 further comprising a varnish.

15. The insulated wire of claim 14, wherein the varnish has the same formulation as the enamel composition.

16. The insulated wire of claim 14, wherein the varnish has a different formulation than the enamel composition.

17. The insulated wire of any of claims 14-16, wherein the varnish comprises boron nitride having an average particle size that is larger than the average particle size of the boron nitride of the enamel formulation.

18. An enamel composition comprising a thermally conductive enamel resin material and boron nitride chosen from boron nitride platelets, boron nitride agglomerates, or a mixture thereof.

19. The enamel composition of claim 18, wherein the boron nitride comprises boron nitride platelets having an average particle size of about 1 micron to about 25 microns.

20. The enamel composition of claims 18 or 19, wherein the boron nitride comprises boron nitride platelets having an aspect ratio of from about 10 to about 300.

21. The enamel composition of any of claims 18-20, wherein the boron nitride is present in an amount of about 10% by weight to about 50% by weight.

22. The enamel composition of any of claims 18-21, wherein the enamel composition further comprises a filler selected from silica, glass fibers, a metal oxide, calcium carbonate, talc, mica, wollastonite, a clay, an exfoliated clay, aluminum silicate, alumina, aluminum nitride, graphite, a metallic powders, or a combination of two or more thereof.

23. The enamel composition of any of claims 18-22 wherein the enamel formulation has an in-plane thermal conductivity of about 0.5 W/mK to about 10.0 W/mK.

24. The enamel composition of any of claims 18-23 wherein the enamel composition has a through-plane conductivity of about 0.5 W/mK to about 10.0 W/mK.