Patent Application
20080179074
1. Technical Field
The present disclosure relates to a copper conductor with an anodized aluminum dielectric layer and a method for making the same.
2. Background Art
The general idea of creating an electrically insulating coating layer on a conducting material is well known. For example, organic wire coatings of polyesters, polyimides, thermoset epoxies, silicone rubbers, and many others have been used in a variety of applications for many years. These types of materials have very good dielectric properties and are able to withstand high voltages. However, they typically are limited to applications with operating/environmental temperatures below about 200-220° C. and are not suitable for high current density or severe environment applications. In addition, polymeric coatings are excellent thermal insulators, which is undesirable for dissipation of ohmic or resistance heating in coil windings. Inorganic wire coverings or coatings, such as glass-fiber sheaths, glass encapsulation, mica, or ceramic materials, may be used to tolerate higher temperatures, but tend to be relatively thick, brittle, and have low radial dimensional control so that they are not amenable to forming processes common in manufacturing electrical machines.
Anodizing electrically conductive materials such as aluminum or copper has been done for nearly a century. Many overhead transmission lines are implemented by aluminum conductors with a thin (about 1 micron) outer layer of aluminum oxide formed by anodization to resist corrosion. However, this layer or skin is too thin to electrically insulate the conductor, so that other measures are required. While suitable for some overhead transmission line applications, the bulk resistance of aluminum wire is generally too high for electromagnetic coil and electrical machine applications.
Copper is generally preferred for conductors used in electromagnetic machines due to its high electrical conductivity. Electroplating aluminum on copper has been attempted, but the aluminum tends to oxidize before it chemically attaches to the copper so that a poor bond is formed and the aluminum layer flakes off of the copper core. Copper can be plated onto an aluminum conductor core, but does not provide the desired electrical characteristics as described above. Copper can also be anodized as disclosed in U.S. Pat. Nos. 5,078,844 and 5,401,382. However, the direct anodization of copper as described in these patents is subject to high strain and cracking as shown by the dielectric strength drop described in U.S. Pat. No. 5,501,382, and the coatings of copper are porous, which makes it difficult or impossible to halt the oxidation process, eventually resulting in an electrical short or breakdown of the wire.
An electrically insulated wire having a copper or copper alloy core conductor with an aluminum oxide layer used to improve adhesion between the conductor core and an outer oxide film insulating layer is disclosed in U.S. Pat. No. 5,091,609. As described in the '609 patent, a thin aluminum or aluminum alloy layer is anodized to form an anodic oxide film having a thickness of only about 10-15 microns thick, which is porous and has a large number of holes passing from its surface toward the base material so that it is generally impossible to obtain an insulating strength which is proportional to the film thickness of the oxide film. This problem is solved using a sol-gel process or acid salt pyrolytic process to fill the holes with an additional oxide insulating layer having a smooth outer surface that decreases gas adsorption and provides electrical insulation proportional to the film thickness.
An insulated electric conductor for carrying signals or current includes a solid or stranded copper core of various geometries with a single thermally conductive dielectric layer of anodized aluminum (aluminum oxide). The device is made by forming a uniform thickness sheet or foil of aluminum to envelop the solid or stranded copper core. The aluminum has its outer surface partially anodized in an electrolytic process to form a single electrically insulating or dielectric layer of aluminum oxide. The anodization process may be performed either before or after forming of the aluminum sheet to the copper core.
In one embodiment, a method for forming an insulated electric conductor includes enveloping a copper core with a uniform thickness sheet of aluminum having a thickness of between about 3-15 thousandths inch thick and anodizing the outer surface of the aluminum to form a single dielectric layer of aluminum oxide to electrically insulate the copper core. Anodizing the outer layer of the uniform thickness thin sheet or foil of aluminum may be performed before or after mechanically forming the aluminum to the copper core depending upon the particular application and implementation. The anodizing process may be halted using a suitable rinse to remove the electrolytic agent from the aluminum so that the aluminum sheet is only partially anodized. Controlling the thickness of the aluminum sheet and the anodizing process results in a substantially smooth outer dielectric layer without holes or voids with dielectric/insulating properties proportional to the layer thickness. The method may also include annealing the composite conductor after forming to reduce or eliminate any internal stresses in the materials.
The present disclosure includes embodiments having various advantages. For example, embodiments of the present disclosure provide an insulated electric conductor that is mechanically tough, chemically resistant, and suitable for operation at extreme operating and/or environmental temperatures hundreds of degrees higher than conventionally insulated wires. The single dielectric/insulating layer is robust against strain-related defects during mechanical forming and economically viable to produce in large quantities and long continuous lengths. In addition, the mechanical toughness facilitates forming conductors of various cross-sectional geometries and gage-diameters. The insulated electric conductor embodiments of the present disclosure have desirable thermal conductivity to dissipate heat and tolerate higher ohmic heating per square while resisting electrical and environmental degradation so the conductor is suitable for use in electromagnetic coil and electric motor applications, for example, and can be wound into volumetric and thermally efficient coils of short total length and improved efficiency. Use of a uniform thickness sheet of aluminum with proper control of the anodizing process results in formation of a single dielectric layer with a substantially smooth outer surface without holes or voids that can be mechanically formed to a solid or stranded copper core.
The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.
As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations.
For the representative process/product illustrated in
Depending upon the particular application and implementation, aluminum sheet 10 may be partially anodized in an electrolytic process as described in greater detail with reference to
Copper or copper alloy core 12 may be solid as represented by copper cores 16, 18, or 20, or may be stranded as represented by core 14, which is made of several discrete strands that may be of the same shape and/or size, or may be of complementary shapes/sizes to improve volumetric efficiency. In one embodiment, core 12 is a solid ribbon-shaped core 18 made of oxygen free high conductivity (OFHC) copper with a nominal width of 0.150 inches (3.81 mm) and nominal thickness of 0.010 inches (0.254 mm).
Composite conductors, represented generally by reference numeral 30, are made by forming thin sheet aluminum 10 to envelop a selected copper or copper alloy core 14. Insulated electric conductor 32 is formed by enveloping stranded copper core 14 with uniform thickness thin sheet of aluminum 10 and partially anodizing the outer surface of sheet 10 to form a dielectric layer 50 of aluminum oxide that electrically insulates copper core 14, but is thermally conductive to dissipate heat. A thin layer 52 of electrically conductive aluminum surrounds core 14 and facilitates adhesion or bonding of dielectric layer 50 to core 14. Representative anodization depths of the aluminum oxide layer may range from about 10% to 80% of the thickness of the uniform thickness thin sheet of aluminum after the anodization process is completed.
A similar process may be used to form electrically insulated conductor 34 using uniform thickness thin sheet 10 enveloping a solid copper or copper alloy core 16. Aluminum sheet 10 is partially anodized either before or after forming to copper core 16 to create a dielectric layer 50 of aluminum oxide. A thin layer of aluminum 52 remains to facilitate adhesion of the dielectric layer 52 to the core 16. In one embodiment, a mechanical cold-forming technique was used to form sheet 10 to a ribbon-shaped core 18 to produce a composite conductor 36 that was fully annealed and subsequently anodized in an electrolytic process to form a dielectric layer 50 of about 0.001 inches (0.0254 mm) thick. The particular forming technique may vary depending upon a number of factors that may include the thickness of sheet 10, the geometry of core 12, and/or the particular ultimate application of the composite conductor and selected implementation of the anodizing process, for example. Other techniques or processes used to form sheet 10 to core 14 may include vacuum welding or radio frequency (RF) bonding, for example. After forming, and/or after anodizing, the composite conductor may be annealed to reduce or eliminate stresses within or between the metals to reduce subsequent separation or delamination of sheet 10 from core 14. Depending upon the particular forming process, either or both of the resulting layers of aluminum 52 and aluminum oxide 50 may not have uniform thickness. For example, in forming a composite oval or ribbon-shaped conductor, two thin sheets of aluminum 10 are used to envelop or “sandwich” a corresponding copper core 18 such that the resulting composite conductor includes overlapping portions or seam areas that are about twice the thickness of thin sheet 10. A similar overlap or seam area may result from various other types of forming processes when using a single uniform thickness aluminum sheet 10 to envelop a copper core.
Composite conductor 38 is formed using a similar process to envelop core 20 with a partially anodized thin sheet of aluminum 10 to form a dielectric or electrically insulating (and thermally conductive) layer 50 with a thin layer of aluminum 52 to facilitate bonding of the dielectric layer 50 to the core 20. As those of ordinary skill in the art will appreciate, core geometries that would otherwise have sharp edges or corners, such as rectangular core 20, may be modified to include radiused or rounded corners to reduce internal stresses in core 20 as well as reducing stresses otherwise created during forming of one or more aluminum sheets 10 to envelop core 20.
Referring now to
Additional guide pulleys 126, 128 may be used to direct wire 120 through an optional rinse 130 having a suitable solution or rinse agent 132, such as deionized water, for example, before being collected by take-up spool 134, which may be driven by an appropriate motor (not-shown). Rinse 130 may be used to remove any residual electrolytic agent 124 from wire 120 to facilitate handling and to further retard or halt the oxidation process. The simplified process illustrated in
A flow chart illustrating a method for making an electrically insulated conductor according to embodiments of the present disclosure is illustrated in
The outer surface of the aluminum is partially anodized using an electrolytic process as represented by block 156 to form a single homogeneous dielectric layer substantially free of holes and voids. Formation of a only a single dielectric layer requires less processing time than multiple layer processes and reduces the resulting size of the electrically insulated composite conductor to achieve improved volumetric efficiency when used in winding applications. In addition, a single dielectric layer is believed to be less susceptible to delamination or flaking of multiple layers during mechanical forming. Some of the aluminum may be removed or etched away during the anodization process. Preferably, the outer layer is only partially anodized so that a thin layer of aluminum remains in contact with the copper/alloy core. Although not specifically illustrated, the anodizing step 156 may be performed before forming the aluminum sheet to envelop the core if desired.
An optional rinsing step, represented by block 158, may be performed to remove the electrolytic solution or agent from the wire to halt the anodization process and/or to facilitate handling of the wire. Preferably, a thin layer of aluminum remains between the copper/alloy core and dielectric layer to facilitate bonding or adhesion of the dielectric layer.
Block 160 represents an optional step of annealing the composite conductor to reduce or eliminate stresses internal to the core, the aluminum/alloy layer, the dielectric aluminum oxide layer, and/or any residual stresses between layers. The annealing step may alternatively or additionally be performed after the forming step 150 and before anodizing as represented by block 156 if desired.
As such, embodiments of the present disclosure provide an electrically insulated conductor that is mechanically tough, chemically resistant, and suitable for operation at extreme operating and/or environmental temperatures. The single dielectric/insulating layer is robust against strain-related defects during mechanical forming and economically viable to produce in large quantities and long continuous lengths. In addition, the mechanical toughness facilitates forming conductors of various cross-sectional geometries and gage-diameters. The embodiments have desirable thermal conductivity to dissipate heat and tolerate higher ohmic heating per square while resisting electrical and environmental degradation so the conductor is suitable for use in electromagnetic coil and electric motor applications, for example, and can be wound into volumetric and thermally efficient coils having improved efficiency. Use of a uniform thickness sheet of aluminum with proper control of the anodizing process results in formation of a single dielectric layer with a substantially smooth outer surface without holes or voids that can be mechanically formed to a solid or stranded copper core.
While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims.
