1. Field of the Invention
This invention relates broadly to high voltage resistors and methods for fabricating the same. More particularly, this invention relates to a high voltage resistors which are useful in conjunction with high voltage power supplies such as may be used in conjunction with X-ray tubes, neutron generators, photo-multiplier tubes and the like, although the invention is not limited thereto.
2. State of the Art
High voltage resistors are well known in the art. A high voltage (HV) resistor is a resistor which is typically on the order of 100 mega-ohms (Mohms) or more. HV resistors on the order of giga-ohms (Gohms) are known in the art and are available from companies such as Vishay Intertechnology, Inc. of Malvern, Pa., Ohmcraft-Micropen Technologies Corporation of Honeoye Falls, N.Y., and Caddock Electronics of Riverside, Calif.
A standard high voltage resistor utilizes a ceramic substrate such as alumina on top of which is laid a film of resistive material in a serpentine or patterned fashion. The HV resistor may be arranged in a cylindrical or a planar fashion. Different techniques are known for laying the film down on the substrate. If a sputtering process is used, the resulting resistor is called a “thin film” resistor as the thickness of the film is controllable by the length of the sputtering process. If a screen and stencil printing process is utilized, the resulting resistor is called a “thick film” resistor. Typically, the film is selected from a ceramic-metal (cermet) material such as bismuth iridate (Bi2Ir2O7), ruthenium dioxide (RuO2), iridium dioxide (IrO2), etc. The material of the film, the thickness and width of the film, and the length of the path determine the resistance of the resistor. HV resistors typically operate at a voltage/inch ratio of 10 kV per inch.
HV resistors fail over time due to the effects of high electrical stress, high temperature, physical damage to the film, or a combination of factors. Sometimes surface tracking (charge movement) between loops of the path or the trapping of contaminants between loops of the path causes a short circuit to develop.
In accord with one embodiment of the invention, a high voltage resistor is provided and comprises a ceramic substrate having a surface and defining a groove, and a resistive film deposited in the groove such that the resistive film is recessed relative to said surface of said ceramic substrate.
According to one aspect of the invention, the resistive film is placed in the groove of the ceramic substrate by thick-film micro-pen technology.
According to one embodiment of the invention the ceramic substrate of the HV resistor is planar. According to another embodiment of the invention, the ceramic substrate is cylindrical. According to another embodiment, the ceramic substrate can take any desired shape.
According to a further embodiment of the invention, the ceramic substrate is made from alumina According to yet another embodiment of the invention, the resistive film is made from a flowable ceramic-metal (“cermet”) paste which is sintered or cured in place.
According to another aspect of the invention, where a flat ceramic substrate is utilized, the groove in the ceramic is laid out in a serpentine or winding format.
According to a further aspect of the invention, where a cylindrical substrate is utilized, the groove in the ceramic is laid out helically.
Objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
a is a cross-sectional view through A-A of
b is a cross-sectional view through B-B of
a is a cross-sectional view through A-A of
Turning to
The substrate 110 is preferably made from a relatively non-conductive ceramic material such as alumina. Other ceramic materials such as zirconia may be utilized. The substrate may be etched, subject to a laser cut, or otherwise treated according to well known techniques in order to form the groove.
The resistive film 120 is preferably selected from a ceramic-metal (cermet) material such as bismuth iridate (Bi2Ir2O7), ruthenium -oxide (RuO2), iridium-oxide (IrO2), depending upon the desired resistivity of the resistor, although other materials (cermet or otherwise) can be utilized. According to one embodiment of the invention, the resistive film is laid down as a flowable paste and cured in place, e.g., by sintering.
According to one aspect of the invention, the ceramic substrate 110 may be of any desired thickness. Typical substrate thicknesses are in the range of 0.5 mm to 5 mm. According to another aspect of the invention, the groove is typically at least 200 microns wide, and preferably less than 500 microns wide, although other widths may be utilized depending on the resistive film width. According to a further aspect of the invention, the groove is preferably at least 5 microns deep, and more preferably at least 20 microns deep, although other depths may be utilized. Regardless of depth, the resistive film is preferably recessed at least 5 microns from the top surface 112 of the substrate 110. By recessing the film relative to the top surface 112 of the substrate, it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors where the resistive films are laid on the top surface of the substrate. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations than prior art HV resistors, or may provide the same desired resistance with a smaller footprint than prior art HV resistors.
According to a preferred embodiment of the invention, the resistive film 120 is laid down in the groove of 116 of the substrate 110 utilizing a direct writing technique. One direct writing technique utilizes a micro-pen having a nozzle through which a flowable paste is deposited. (See, “Cai, Zhixiang et al., “Laser sintering of thick-film PTC thermistor paste deposited by micro-pen direct-write technology”: Microelectronic Engineering 86, pages 10-15 (2009). After laying the paste into the groove the paste is cured by subjecting the entire substrate to a high temperature in order to sinter the paste in place. Alternatively, a laser may be used to sinter the paste in the groove.
According to other embodiments of the invention, the resistive film is laid down in the groove using other desired techniques known in the art.
A second embodiment of the invention is seen in
By recessing the film 220 relative to the top surface 212 of the substrate, and by locating the film 220 in the groove 216 but spaced from the side walls 216a, it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations, or may provide the same desired resistance with a smaller footprint than prior art HV resistors.
It is noted that aspects of the HV resistor 200 such as substrate material, film material, groove width and depth, and mechanisms for laying down the film material in the groove may be as described above with respect to HV resistor 100.
A third embodiment of the invention is seen in
By recessing the film 320 relative to the outer surface 312 of the substrate it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations, or may provide the same desired resistance with a smaller footprint than prior art HV resistors.
It is noted that aspects of the HV resistor 300 such as substrate material, film material, groove width and depth, and mechanisms for laying down the film material in the groove may be as described above with respect to HV resistor 100.
The HV resistors 100, 200, 300 may be used in conjunction with high voltage and/or high temperature applications such as high voltage power supplies for X-ray tubes, neutron generators, photo-multiplier tubes and the like, although the invention is not limited thereto.
There have been described and illustrated herein several embodiments of a While particular geometries have been described for the ceramic substrate and groove, it will be appreciated that other geometries could be utilized. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US11/54152 | 1/4/2012 | WO | 00 | 11/19/2014 |