Patent Number: 052805107
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a schematic representation of a system 10 for implementing the present invention. A vacuum chamber 12 is connected to a vacuum pump 14 by which the ambient air can be removed. The tubular component 16 to be coated, is supported within the vacuum chamber. A source material rod 20 is positioned, preferably axially, within the component 16, in spaced relation from the confronting inside surface 18. The rod 20 includes a distribution of field emitter structure having a generally nodular, and preferably pointed shape, which will hereafter be referred to as emitter tips 22. The rod 20 must, of necessity, have a rather small effective outside diameter, especially when used for coating the inside of nuclear fuel rod tubing, which is generally on the order of 0.5 inch. The emitter can be formed as a wire on which the desired source material for the coating, has been flame sprayed. Techniques for fabricating the emitter rod per se, do not form a part of the present invention, but rather can be adapted for use in the present invention, by reference to available literature on conventional field emitter technology. In some instances, the rod 20 can be of a type having gated field emitters (the gates 24 are shown in FIG. 1 as a dashed line). FIG. 2 shows a schematic of a gated field emitter rod 100, wherein the rod core 102 has nodular structure such as 108 formed thereon, preferably including a point 110. An insulating material 104 is layered on the core material 102, and a top layer of conductive material 106 covers the insulating material 104. A basin, or bowl is formed by the absence of insulating and conductor material 104, 106 in a region immediately surrounding the emitter structure 108. As an example, the distance from the point 110 to the insulating layer 106 is on the order of the height or projection of the point 110 from the core 102, such distance typically being in the range of about 1-10 .mu.m. In the conventional utilization of a gated field emitter such as shown in FIG. 2, the emitter tip or nodule 108 is biased negatively with respect to its gate 106. This structure will emit electrons 112 if the electrical field at the emitter point 110 is large enough to induce field emission from the tip material. Above a critical current, a gated field emitter structure 108 will exhibit a failure which will cause the point 110 and then a portion of the rest of the tip 108 to be evaporated. This process has been investigated by J. Browning, N. E. McGruer, S. Meassick, C. Chan, W. Bintz and M. Gilmor, in an article entitled "Gated Field Emitter Failures: Experiment and Theory", to be published in IEEE Transactions, Plasma Science, October, 1992. As shown in FIG. 1, the tips 22 are negatively biased via tip power supply 28 and conductor 26, whereas the gates 24 are positively biased via power supply 34 and conductors 30,36. The component substrate can be negatively biased via conductor 32. With non-gated field emitters, which are preferred for the present invention, the tips 22 are biased negatively via conductor 26 and tip power supply 28, whereas the workpiece, i.e., the tubular component 16, would be positively biased via conductors 30,36 and workpiece bias power supply 34. Above a critical current failure of the emitter tip 22 will occur, causing a portion of the tip to be evaporated as microparticles. Because of the small size of the field emitter structures, i.e., less than 10 .mu.m feature size, arrays of field emitters can be used to deposit material on large work pieces while single emitters can be utilized for deposition of material on small workpieces. The size of the emitter tips (volume of material available in each tip), surface density of emitter tips, and the energy available for each tip failure can be used to adjust the amount of material evaporated and, therefore, the thickness of a deposited coating on a workpiece. Arrays of field emitters can be fabricated on sources of arbitrarily complex shape and used for deposition on complexly shaped work pieces. In particular, emitters can be fabricated on a small wire appropriately sized for use on the inside of fuel rods, instrument thimble tubes, burnable poison rods, control rods, or control rod guide tubes. The rate of failures, energy available for each failure, and the surface density of emitters can be used to adjust the rate of deposition, coating thickness and coating parameters. A large fraction of the material evaporated from the field emitter is ionized, allowing a potential gradient between the emitter tip and work piece to accelerate the ions thereby changing the properties of the deposited coating, increasing adherance, decreasing crystal size. The material of the deposited coating is determined by the composition of the emitter tips. Thus, the composition of the deposited coating can be changed by adjusting the composition of individual emitter tips or by making individual emitter tips of different material. The tips 22 thus need not be of the same material as the carrying core of rod 20 and all tips need not be of the same material. As represented at 38 in FIG. 10, the atmosphere within the vacuum chamber 12 can be backfilled with a reactive gas such as nitrogen or oxygen, or carbon vapor plasmas can be introduced, whereby the source material evaporated from the tips 22 chemically reacts with the nitrogen, oxygen and/or carbon, before adhering to the component surface 18. In any event, the tip material deposits on the component substrate surface 18, either in the form as originated on the emitter, or in a form as modified by reaction with the reactive gas in the chamber. The ability to achieve different coatings along different longitudinal regions of the component, with the ability to layer one component upon another, as described in said copending applications, is another advantage achievable with the present invention. Thus, where desirable, neutron absorber poison material can be coated with and in combination with wear resistant or hydrogen getter material. The following tables list particularly desirable coatings that are achievable with the present invention. Table 1 lists a variety of burnable poison metals that can be deposited in accordance with the present invention (BN is a burnable poison coating that can be formed with the use of nitrogen as the reaction gas introduced via reaction gas source 38): TABLE 1 ______________________________________ Burnable Poison Metals and Metallic Compounds ______________________________________ Gadolinium Erbium Boron ZrB.sub.2 BN TiB.sub.2 ______________________________________ Table 2 is a representative list of burnable poison ceramic materials including glasses that are usable with the present invention: TABLE 2 ______________________________________ Burnable Poison Ceramics and Glasses ______________________________________ 20 Li.sub.2 080B.sub.3 15 Na.sub.2 085B 20 B.sub.4 C ______________________________________ Table 3 is a representative list of getter material that can be deposited in accordance with the present invention: TABLE 3 ______________________________________ Getter Material ______________________________________ Yttrium Zirconium-Nickel alloys Zirconium-Titanium-Nickel alloys ______________________________________ Table 4 is a representative list of wear and/or corrosion resistant metal compounds. TABLE 4 ______________________________________ Metals and Metallic Compounds ______________________________________ ZrN TiN CrN HfN TaAlVN TaN ______________________________________ Table 5 is a representative list of wear and/or corrosion resistant ceramic materials including glasses that are usable with the present invention: TABLE 5 ______________________________________ Ceramics and Glasses ______________________________________ Zr.sub.2 O.sub.3 Al.sub.2 O.sub.3 TiCN TiC CrC ZrC WC Calcium Magnesium aluminosilicate Sodium Borosilicate Calcium Zinc borate ______________________________________