Patent Application
20100239828
The present invention relates to filaments used for electron emission or production of light.
Filaments are used to produce light and electrons. For example, in an x-ray tube, an alternating current heats a wire filament formed in a coiled cylindrical or helical loop. Due to the high temperature of the filament, and due to a large bias voltage between the filament and an anode, electrons are emitted from the filament and accelerated towards the anode. These electrons form an electron beam. The location where the electron beam impinges on the anode is called the “electron spot”. It is often desirable that this spot be circular with a very small diameter. It is also often desirable that this spot be in the same location on the anode in every x-ray tube that is manufactured.
The shape and placement of the filament in the x-ray tube affects the shape of the spot. Some filaments are very small, especially in portable x-ray tubes. Placing such small filaments, in precisely the same location, in every x-ray tube, is a significant manufacturing challenge. Lack of precision of filament placement during manufacturing results in an electron spot that is in different locations on the anode in different x-ray tubes. Placement of the filament also affects spot size and shape. Lack of precision of filament placement also results in non-circular spots and spots that are larger than desirable.
With a wire filament 130 formed in a coiled cylindrical or helical loop, as shown in
In addition, a filament wire, with a consistent wire diameter, can be hottest at the mid-point 131 along the length of the wire. If there is a consistent wire diameter, the voltage drop or power loss is consistent along the wire, resulting in the same heat generation rate along the wire. The connections at the ends of the wire 132, however, essentially form a heat sink, allowing more heat dissipation, and cooler temperatures, at the each end of the wire. The mid-point of the wire 131 loses less heat by conduction than the wire ends and can be the hottest location on the filament wire. This high heat can result in more rapid deterioration at the wire mid-point 131. As this center section deteriorates, its diameter decreases, resulting in a larger power loss, higher temperatures, and an even greater rate of deterioration at this location. Due to the higher temperatures and more rapid wire deterioration at the center of the filament wire, most failures occur at this location. Such failures result in decreased tube life and decreased tube reliability.
It has been recognized that it would be advantageous to provide a filament which is easier to handle during manufacturing, resulting in more precise and repeatable placement of the filament. Increased precision of filament placement results in less performance variability between devices using these filaments. In addition, it has been recognized that it would be advantageous to provide a filament that maintains its shape during use and which is less susceptible to filament failures. In addition, it has been recognized that it would be advantageous to provide a smaller and more circular electron spot size in an x-ray tube. This smaller and more circular spot size can be in part the result of a filament which is manufactured and placed with high precision. In addition, it has been recognized that it would be advantageous to emit electrons or light from a two dimensional plane, rather than a three dimensional object, such as a coiled cylindrical or helical filament. This allows for increased ease in electron or light beam shaping, where both over-focusing and under-focusing are avoided because the planar filament is spatially constrained to two dimensions.
The present invention is directed to a planar filament device with a layer patterned to form: 1) a pair of spaced-apart bonding pads each configured to receive an electrical connection; and 2) a filament connected between the pair of bonding pads configured to receive an applied electric current through the bonding pads and filament.
In accordance with another aspect of the present invention, the planar filament can be associated with an x-ray tube with a vacuum enclosure; a cathode coupled to the vacuum enclosure; and an anode coupled to the vacuum enclosure and opposing the cathode.
a is a photograph showing the top view of one embodiment of a planar filament;
b is a photograph showing the top view of one embodiment of a planar filament;
Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.
Referring to
The filament 11 can extend non-linearly between the pair of bonding pads 12a and 12b so that the filament has a length (if stretched linearly) longer than a distance between the bonding pads. The filament 11 can have a double spiral-shape oriented parallel with the layer, and which is connected at each end to a bonding pad 12a and 12b. In addition, the filament 11 can have a non-uniform width (taken parallel with the layer and transverse to the length of the filament). Thus, an intermediate portion, such as a middle 15 or center portion, of the filament can be wider while ends 13 of the filament can be narrower near the bonding pads. This results in a wider cross-sectional area for electrical current flow, and thus less electrical power generation and heat generation at the filament center than at other sections of the filament. With this wider cross-sectional area at an intermediate portion of the filament, the temperature in this area can be less than if the filament width were the same across the entire filament. This can result in reduced failures at the filament center and a longer average filament life. The change in filament width can result in more consistent temperatures along the length of the filament. This can also result in more even electron emission along the length of the filament and improved electron spot shape.
The filament 11 and bond pads 12a and 12b can be a thin film material. To avoid handling damage to this thin film material during filament manufacturing and placement, the planar filament can be connected to a type of support structure. A support structure which electrically isolates one bond pad 12a from the other bond pad 12b can be used to allow an electrical current to flow from one bond pad to the other through the filament. The support structure can be situated such that it does not touch the filament. This may be desirable in order to avoid conductive heat transfer from the filament to the support structure.
Referring to
The support structure can be attached to a support base 23 for additional structural strength and to aid in handling and placement of the planar filament. This support base can have high electrical resistance in order to electrically isolate one support structure and thus also one bond pad from the other. The support structures can be mounted onto the support base with an adhesive, by pushing the support structures into holes in the support base, with fasteners such as screws, or other appropriate fastening method.
The filament 11 and bond pad 12a and 12b shapes may be formed by laser machining a layer 14 of material that is suitable for filaments and bond pads. The layer 14, and thus the filament 11 and/or bonding pads 12a and 12b, can be a flat layer with planar top and bottom surfaces that are parallel with respect to one another, and with a constant thickness (orthogonal to the top surface of the substrate). A laser can cut the material out of the layer 14 to create the filament 11 and bond pad 12 shapes. Alternately, the filament and bond pad shapes can be made by photolithography techniques. The layer 14 can be coated with photo-resist, exposed to create the desired pattern, then etched. These methods of making the filament 11 and bond pad 12a and 12b shapes apply to all embodiments of the planar filament discussed in this application. These methods also apply to making the beam shaping pads discussed later. Forming the filament and bond pad structure through laser machining or forming the filament and bond pad structure through photolithography techniques may be referred to herein as “patterned” or “patterning”.
The layer 14 can be laser welded onto the support structures 22a and 22b. The support structures 22a and 22b can hold the layer 14 in place while cutting out the filament 11 and bond pads 12a and 12b as discussed previously. Alternatively, the bond pads 12a and 12b can be laser welded onto the support structures 22a and 22b after the bond pads 12a and 12b and filament 11 have been cut.
Referring now to
A space 33 can be disposed between the filament 11 and the substrate 32 such that a substantial portion of the filament, such as all or a majority of the filament, is suspended above the substrate by the pair of boding pads. The space 33 beneath the filament 11 can be an open area such as a vacuum, air, or other gas. The substrate can be wholly or partially removed beneath the filament forming a recess or cavity 33b bounded by the substrate on the sides (and possibly the bottom) with the filament on top. Alternatively, a hole or opening can be made in the substrate, from the top surface of the substrate to the bottom surface, so that no substrate material is beneath the filament. Alternatively, a channel could also be created in the substrate beneath the filament and then replacing removed substrate with a different material. High filament temperatures are normally needed for electron emission in an x-ray tube. To avoid conductive heat transfer away from the filament, the substrate can be removed beneath most of the filament area.
To make a planar filament with a substrate 32, such as the planar filament 10c shown in
The following discussion will cover different exemplary embodiments with various filament 11 shapes and with beam shaping pads. The figures accompanying the following embodiments show an optional substrate 32. The different filament shapes and beam shaping pads may also be used in an embodiment, such as shown in
Referring to
Referring to
Referring to
Referring to
a and 11b show photographs of a planar filament. The planar filament in these figures was made without a substrate and has a spiral shape, similar to the planar filament 10 of
Referring to
Although the present invention has been described above and illustrated with bonding pads that are large relative to the filament, it will be appreciated that the bonding pads can be smaller, and/or can be configured for any type of electrical connection to the power source.
It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.
