Patent Application
20070258190
The invention relates generally to electrical capacitors, and more particularly to dielectric layers in film capacitors, such as to dielectric layers in metallized film capacitors.
Over the last decade, significant increases in capacitor reliability have been achieved through a combination of advanced manufacturing techniques and new materials. Greatly enhanced performance has been obtained particularly in so-called film capacitors. Film capacitors can be classified into three types based on the manufacturing technology, namely, film and foil capacitors, metallized film capacitors and mixed technology film capacitors.
Generally, metallized film capacitors consist of two metal electrodes separated by a layer of plastic film. The metallized plastic film are constructed by vacuum depositing metal film onto a layer of plastic film. This would offer compact capacitor structure, self-clearing capability, longer lifetime, and higher energy density. Some of the commonly used plastic films are polypropylene and polyetherimide films. The metal film layer is typically extremely thin, in the order of about 200-500 angstroms and is typically aluminum or zinc. Compared to other types of capacitors, metallized film capacitors have advantage in size, simplicity, and cost of manufacturing, and hence been widely used in the power electronics industry.
While significant improvements have been made in metallized film capacitors, certain issues, such as thermal stability and reduced lifetime continue to present challenges to their widespread adoption. For example, metallized film capacitors with polypropylene film as a dielectric layer are not suitable for operation above about 90° C.
Therefore, it would be desirable to design a metallized film capacitor that would address the aforementioned problems and meet the current demands of electronics industry applications.
In accordance with one aspect of the invention, a method for making a capacitor is provided, including plasma treating a surface of a polyetherimide film. The method also includes metallizing the treated surface, followed by disposing an electrode on an opposite surface and finally packaging the capacitor.
In accordance with another aspect of the invention, a method for making a capacitor includes plasma treating a surface of a polyetherimide film in a fluorinated atmosphere. The method also includes metallizing the surface, and then, as before, disposing an electrode on an opposite surface and finally, packaging the capacitor.
In accordance with another aspect of the invention, a capacitor is provided that includes a dielectric layer made of polyetherimide film with at least one fluorinated surface. The capacitor also includes a metallized layer disposed on the fluorinated surface of the polyetherimide film, and an electrode disposed on a side of the polyetherimide film opposite the fluorinated surface.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, embodiments of the present invention include a metallized film capacitor with improved electrical properties and operable at high temperatures. A method of manufacturing such a film and film capacitor are also described. Some of the dielectric properties considered herein are dielectric constant, and breakdown voltage. The “dielectric constant” of a dielectric is a ratio of capacitance of a capacitor, in which the space between and around the electrodes is filled with the dielectric, to the capacitance of the same configuration of electrodes in a vacuum. As used herein, “breakdown voltage” refers to a measure of dielectric breakdown resistance of a dielectric material under an applied AC or DC voltage. The applied voltage prior to breakdown is divided by thickness of the dielectric material to give the breakdown voltage. It is generally measured in units of potential difference over units of length, such as kilovolts per millimeter (kV/mm). As used herein, the term “high temperatures” refers to temperatures above about 100 degrees Celsius (° C.).
A typical metallized film capacitor includes a polymer film interposed between two electrodes on either side. The two electrodes include a layer of a metal such as aluminum, copper or zinc or their combination that is vacuum deposited on the polymer film that acts as a dielectric in the metallized film capacitor. In one embodiment of the present invention, a metallized film capacitor disclosed herein includes an electrode upon which a dielectric layer is disposed. The film capacitor also includes a plasma treated surface of the dielectric layer. Further, the film capacitor includes an electrode, typically made of a metal layer such as aluminum or zinc disposed (e.g., vacuum deposited) upon the plasma treated surface of the dielectric layer.
Turning now to the drawings,
The electrode 20 typically includes metal foils. In one embodiment, the electrode 20 includes at least one of aluminum, copper, or zinc foil. An example of a polyetherimide film used as the dielectric layer 22 may be a film product commercially available from General Electric Plastics under the designation Ultem®. An example of the plasma treated surface includes a surface fluorinated by carbon tetrafluoride. Thickness of the dielectric layer 22 may be in a range between about 0.5 μm to about 50 μm. In another exemplary embodiment, the thickness range of the dielectric layer 22 may vary between about 31 μm to 10 μm. The dielectric layer 22 may operate in a temperature range between about −50° C. to about 250° C. Breakdown voltage of the dielectric layer may be in a range between about 300-700 kV/mm. The typical thickness of the metallized layer 26 varies in the range of about 200 Å to about 500 Å.
In a non-limiting example, the plasma surface treatment may include process of chemical vapor deposition. In another exemplary embodiment, the plasma surface treatment includes plasma treating on the surface of the polyetherimide film for duration of less than approximately 20 seconds. The dielectric material of the present invention may be coated in several ways. Suitable examples of coating processes include spin coating, dip coating, brush painting, solvent casting, and chemical vapor deposition.
The plasma treated surface is then metallized at step 32. The metallizing at step 32 may include process of vapor deposition, sputtering or electrochemical deposition of the polyetetherimide film. In an example, the process of vapor deposition, sputtering or electrochemical deposition may include depositing the surface of a polyetherimide film with aluminum or copper. In another example, the process of vapor deposition, sputtering or electrochemical deposition may include depositing the surface of the polyetherimide film with zinc and aluminum or copper. The method 28 also includes disposing an electrode on the polyetherimide film. Finally, the metallized film capacitor is packaged at step 36. The step 36 of packaging the capacitor will typically include winding and laminating the capacitor, and providing conductors or terminals for applying charge to the wound layers.
The aforementioned embodiments present clear advantages over existing film capacitors and methods for making such capacitors. For example, it has been found the capacitors made by the foregoing techniques offer increased dielectric constant compared to existing film capacitors, increased dielectric breakdown voltage, reduced surface defects, increased thermal stability, increased corona breakdown resistance, and extended life. Polymers containing certain nanoparticles or nanofillers such as aluminum oxide (Al2O3) or silica have been found to show higher breakdown strength and dielectric constant, and may be particularly well suited to the inventive films and capacitors. Particle filled polymers also could offer increased thermal conductivity and may be suitable for use in the invention. The higher glass transition temperature of polyetherimide films such as the commercially available Ultem® films mentioned above also allow a higher operating temperatures of the capacitors.
As mentioned above, surface defects may cause a scattering of breakdown voltages in a dielectric, resulting in varying breakdown voltages at various locations in a capacitor, leading to a lowering of the overall breakdown voltage of the capacitor. Plasma treatment of a surface of the dielectric film as described hereinabove provides greater uniformity in a surface structure thus reducing surface defects. This leads to a narrower breakdown voltage range and consequently, to enhancement and extension of the lifetime of the capacitor. Further, corona resistance, that is a measure of time that a dielectric in a capacitor will withstand a specified level of ionization without resulting in complete breakdown of the dielectric, is increased by such surface treatment. This directly results in an extended lifetime of the capacitor.
The examples that follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.
As seen from the plots 44, 46 and 48, a plasma treated spin coated sample of polyetherimide film provides a higher breakdown voltage than that of a plasma treated nanofilled polyetherimide film and a plasma treated commercial polyetherimide sample. For all the three samples investigated, plasma treatment offers improved dielectric breakdown strength against untreated samples with an optimal treatment duration of 20 s. However, process of plasma treatment appears to be most effective on a spin coated polyetherimide film.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
