The present invention relates in the manufacture of electronic vacuum devices to a method of treating fused glass structures, particularly fused glass fiber elements intended for use as a component element of an electronic vacuum device, such as a cathode ray tube or image intensifier tube to provide enhanced physical characteristics. More particularly, my invention relates to the treatment of fiberoptic type faceplates and microchannel plates formed from fused-together fiberoptic glass materials to enhance performance and reliability of the electronic vacuum devices into which they are assembled.
A fiberoptic faceplate is manufactured form a plurality of individual glass fibers which are stacked together, pressed and heated under pressure to form a uniform structure with a plurality of light transmitting passages so as to be optically transparent between front and back surfaces. Typically, in order to minimize optical "cross-talk" between the individual glass fibers in the structure, ensuring that the light entering one end of a fiber exits the opposed end and is not diffused or substantially coupled to other adjacent fibers, a coating of light absorbing material is placed on or adjacent the individual fibers or bundle of fibres prior to fusing them together. This coating usually contains material, such as iron oxide particles and carbon particles, dispersed in an adherent suspension or matrix. the aforedescribed structure is known and is typical of the kinds of faceplates employed as the output display in electronic vacuum tube devices, such as those commonly termed "night vision devices" or "image intensifiers". As the output display a phosphor coating is applied to one surface of the faceplate, much like the phosphor coated faceplate of a cathode ray tube found in television sets, so as to cause fluorescence under electron bombardment.
A problem occurs in such a fused fiberoptic structure whenever water vapor or other gases are present in the light absorbing material or are captured in the minute interstices between individual fibers in the plate. Those undesired impurity ingredients create an electrical leakage path between the front and back surfaces of the fiberoptic faceplate, a path through which direct electrical current can flow and a high leakage is undesirable. For example, a low-voltage battery powered power supply is provided in a night vision device to provide the voltages and currents necessary to the operation of the device, the exact circuit details of which are not pertinent to the present invention. Hence, any leakage path between the front and back surfaces of the faceplate is a factor which must be considered by the designer of the power supply for the device since the leakage path poses a power drain on the supply. Not only is battery power in this way wasted, but more importantly it has been found that the leakage resistance may change with time, such as to decrease and drain more current and this detracts from the function of the other elements of the circuit by changing voltages inasmuch as the power supply is intended to supply very minute currents on the order of 200 milliamps at high voltages into a high resistance load. A secondary problem occurs when the extraneous water vapor and active gas molecules migrate from the interstices of the faceplate into the vacuum region of the vacuum device, creating some amount of adverse chemical reaction, termed "poisoning," of electron emitting surfaces, employed as an element therein as is known, thereby reducing device life. From this standpoint the elimination of water vapor and gases from the faceplate prior to assembly into the night vision device is an objective which previously has been sought to be accomplished by the application of high voltage across the plates and under such conditions "aging" the plate, the results of which are not to my satisfaction.
One object of my invention therefore is to remove water vapor and other gases from the interstices between glass fibers in a fiberoptic plate, as well as to stabilize the leakage resistance of the plate.
A related component element of a night vision device is the microchannel plate, a known electron "multiplying" element which is formed of a fused bundle of hollow glass tubes arranged in a uniform structure defining a surface having on the order of one million minute passages per square inch of surface area. The mircochannel plate is formed originally from a fused bundle of fiberoptic fibers, as was the aforedescribed faceplate. The glass composition of the microchannel plate is such that when the passage walls are chemically treated by prior art processing techniques, including firing in a wet hydrogen atmosphere at high temperatures, the passage walls attain an electron secondary emission ratio greater than unity and a subcutaneous semiconductive layer, which structure is known in the prior art. An unfortunate undesired side-effect of such processing to obtain the desired secondary emission characteristics rendering it useful as an electron multiplying element is that the process creates a diffusion of hydrogen gas molecules into the glass and by-products of water vapor, hydroxyls, carbon monoxide, nitrogen and free hydrogen, all of which are trapped within the bulk of the glass, and are extremely difficult to remove by normal thermal outgassing techniques, such as heating. As a result, the ultimate life of the vacuum device containing the microchannel plate is in part determined by the diffusion or permeation of such trapped gasses to the active photocathode surface causing localized poisoning of the photocathode material.
Accordingly, another object of my invention is to eliminate extraneous gas and water vapor from a microchannel plate structure.