Process for producing flat plate illumination devices

Abstract: A multistep process is disclosed which enables hermetically sealed, durable, long-lasting illumination devices which utilize electrical discharges through inert gas and inert gas/mercury vapor mixtures to be produced in an essentially flat-plate configuration without the use of glass tubing to contain the discharge. This process utilizes plates of glass having a particular range of thermal expansion coefficients for the preparation of these display devices through the discovery of a heating/cooling process that enables these thick glass assemblies to be produced rapidly yet without cracking and without significant residual air or water vapor contamination and which includes the heat sealing of a evacuation/backfilling tubulation in the same sequene and also includes the unique feature of the inclusion of finely divided powder in the inert gas chamber which, when subjected to the heating/cooling cycle, acts to getter residual air and water vapor from the inert gas. This process also utilizes a special means for preventing the adhesion of the flat plates to the platens used for their support during the thermal fusing treatment that is required to form the gas discharge channels. An interrupt step, introduced after fusion bonding is complete, enables the usage of a low softening point glass having a thermal expansion coefficient compatible with that of the window glass to produce the evacuation and gas filling tubulation port.

Description: PROCESS FOR PRODUCING FLAT PLATE ILLUMINATION DEVICES The invention concerns a process for producing flat-plate, gas discharge illumination devices. BACKGROUND OF THE INVENTION Luminous devices based upon the use of contained, glowing electrical discharges through inert gases, especially neon, are well known. Neon signs, for example, are commonly seen in everyday use. Neon signs, however, utilize glass tubes bent to form the desired shape and contain electrodes at the ends of the glass tubes. Other devices which utilize gas discharges for producing illumination without using bent glass tubes are known or have been proposed. US-A-1 949 963 for example describes the use of multiple flat plates assembled to produce an inclosed channel which can act as a neon sign. In this case five glass plates are used, including solid top and bottom plates together with three middle plates which contain both channels and perforations between the channels. US-A-1 825 399 utilizes only two glass plates together with the use of either engraved passages or tubular holes angled with respect to the plane of the glass plates to form the continuous gas discharge pathway. US-A-4 584 501 also provides a flat-plate, gas discharge device which can be used in combination with both top and bottom mirrors to produce a device which shows an infinite sequence of signs of ever decreasing intensity. US-A-4 839 555 discloses a process for producing flat-plate gas discharge illumination devices, comprising a middle plate, including channels with interior electrodes, a top plate and a bottom plate. Said platens are assembled by heating their combination to a temperature sufficiently high to soften and to seal said top, middle and bottom plates hermetically together. it is the object of the invention to provide a process for producing flat-plate gas discharge illumination devices in a semi-automated, economical, continuous manner. This object of the invention is solved by a process according to claim 1. OBJECTS OF THE INVENTION It is an advantage of the inventive process that large, essentially flat-plate gas discharge illumination devices can be produced in a semi-automated, economical, continuous manner. This inventive process will determine the ultimate wide spread utility of such illumination devices. It is another advantage that gas discharge illumination devices can be produced without the use of tubes to shape the discharge path. According to the inventive process neon advertising signs can be produced, whereby this process is capable of a substantial degree of automation and does not involve the handwork of artisans for the preparation of these advertising devices. According to the inventive process gas discharge illumination devices can be produced sufficiently economical, so that such devices can be considered for both domestic and public lighting purposes. SUMMARY OF THE INVENTION This invention provides a unique process which enables the semi-automated continuous preparation of hermetically sealed, durable, essentially flat-plate illumination devices to be produced economically and at a high rate of production. This process incorporates features which enable the usage of glass with a particular range of thermal expansion coefficients to produce high intensity illumination devices without cracking during the thermal fusing step. The light from these devices is produced by a gas discharge through inert gas or inert gas/mercury vapor mixtures that are contained in one or more channels cut into the glass and rendered into hermetically sealed passages by the thermal fusing of top and bottom glass plates to a middle glass plate into which the channels have been cut. This cutting process, which can require the removal of a substantial portion of the glass comprising the middle plate, is achieved by the use of an extremely high pressure water jet which carries abrasive grit and whose cutting action is computer controlled so as to make the cutting of highly complex shapes possible in a rapid manner. Hermetic sealing of the top and bottom plates to the middle plate is accomplished by means of a controlled thermal fusing process carried out using a novel, coated carrier platen, and this thermal process also incorporates a special step which enables the evacuation tubulation to be made from a glass of similar thermal properties, especially thermal expansion coefficient, as the glass which comprises the plates themselves. The evacuation of the air from the hermetic channel and the subsequent backfilling of this channel with inert gas or inert gas/mercury vapor is carried out while the hermetically sealed assembly is still hot from thermal sealing. The electrical power is supplied by means of electrodes introduced into the assembly before sealing. The essential critical step is the discovery of a process step which allows the entire assembly to be carried through the thermal fusing treatment without adhesion to the carrier platen which carries the glass while this glass is hot and soft. The result of this novel process is an essentially flat-plate illumination device which is physically robust, has a high illumination intensity and a long life, and which can be made rapidly and in quantity by a semi-automated process with a high production yield. DESCRIPTION OF PREFERRED EMBODIMENTS One preferred embodiment of this invention comprises a high pressure water jet cutting device whose cutting action is augmented by the addition of garnet abrasive to the water jet so that linear cutting rates of up to 254 cm (100 inches) per minute can be achieved in cutting through glass plates between 0,238 and 1,03 cm (3/32 and 13/32 inches) thick to form the basic channel to contain the gas discharge. The cut glass plate thus produced, denoted as the middle plate, is transferred to a glass bottom plate, which itself is between 0,198 and 1,03 cm (5/64 and 13/32 inches) thick, partly to support the fragile pattern produced by the water jet cutting action and partly also to provide a bottom to the channels produced by the cutting action so that fluorescent or other powdered substances can be placed in this channel and retained within it. At this point also the integral, interior eletrodes required to provide electrical power for the gas discharge are also placed within the as yet non-hermetic channel at its end points and connected to the exterior by means of electrical feed-throughs. A top plate is then placed over the assembled middle and bottom plates, the electrical connections to the electrodes passing through holes drilled, by water jet cutting, in the top plate. The entire assembly is then placed on a carrier support, said platen carrier being preferably of a high melting point ceramic material such as an alumina-rich ceramic. It has now been discovered that if said ceramic carrier platen has been coated with a ceramic powder, such as alumina powder, said powder having a sieve size less than 200 mesh and and a softening point substantially in excess of that of the glass plates, said powder having been applied to the platen by spraying, washing, or other suitable means and lightly fired to the surface of said platen so that it is mildly adherent to said platen, then the glass plates will not adhere to the carrier platen, even though the glass plates are thoroughly softened and made sticky at the high temperature to which it is heated during sealing. After placement on the coated platen, glass frit, such as Corning 7075, is placed around the electrode wires. The combined glass plates and platen assembly are then subjected to a sealing step to soften and to seal the plates hermetically. In this step the plates and platen are heated to between 648,9 degrees Celsius and 787,8 degrees Celsius (1200 degrees Fahrenheit and 1450 degrees Fahrenheit) at a rate between - 17,2 and - 3,9 degrees Celsius per minute (1 and 25 degrees Fahrenheit per minute) and then cooled to between 537,8 and 398,9 degrees Celsius (1000 and 750 degrees Fahrenheit) at a rate between - 17,5 and - 9,4 degrees Celsius per minute (one half and 15 degrees Fahrenheit per minute). It is important that the glass plates have thermal expansion coefficients which lie between 165,1 and 279,4 cm per 2,54 cm per degree Celsius (65 and 110 inches per inch per degree Centigrade). At this lower temperature, between 537,8 and 398,9 degrees Celsius (1000 and 750 degrees Fahrenheit), the process is subjected to an interrupt step, during which interrupt step the evacuation tubulation is inserted into a previously drilled hole in the back plate, said hole communicating with the channel that was cut into the middle plate by water jet cutting. Said tubulation can thus have a similar expansion coefficient and a similar softening point as the glas plates, both the expansion coefficient and the softening point being related. That is, the higher the softening point of a glass, the lower will be its expansion coefficient, and conversely the lower the softening point the higher will be its expansion coefficient. This tubulation is encircled during or after placement by a relatively low melting point glass frit that serves to hermetically seal the tubulation to the top plate. After this tubulation has been inserted, the cooling process is continued until a temperature of between 65,5 and 287,8 degrees Celsius (150 and 550 degrees Fahrenheit) has been reached, at which point an air and water gettering metallic material, such as zirconium metal, can be inserted into the tubulation and an evacuation coupling is made to this tubulation, following which the air is substantially all removed from the gas discharge passage and the electrodes are separately heated by radio frequency heating or other means to desorb air and water vapor contamination that is adsorbed on them, and the desired inert gas or inert gas/mercury vapor mixture is then backfilled in to this passage. Because the assembly is still hot, it has been discovered that the filling pressure must be between 2,5 and 30 millibar in total pressure in order that the device functions properly at room temperature. The tubulation is then sealed by fusing and pinching or crimping the tubulation shut. The air or water vapor gettering material is then activated by radio frequency heating or other means to remove any residual air or water vapor contamination. After final cooling to room temperature, the desired art-work is applied to the front of the device and the power supply connected to the electrodes to produce the finished illumination device. A multistep process is disclosed which enables hermetically sealed, durable, long-lasting illumination devices which utilize electrical discharges through inert gas and inert gas/mercury vapor mixtures to be produced in an essentially flat-plate configuration without the use of glass tubing to contain the discharge. This process utilizes plates of glass having a particular range of thermal expansion coefficients for the preparation of these display devices through the discovery of a heating/cooling process that enables these thick glass assemblies to be produced rapidly yet without cracking and without significant residual air or water vapor contamination and which includes the heat sealing of a evacuation/backfilling tubulation in the same sequene. This process utilizes a special means for preventing the adhesion of the flat plates to the carrier platens used for their support during the thermal fusing treatment that is required to form the gas discharge channels. An interrupt step, introduced after fusion bonding is complete, enables the usage of a low softening point glass having a thermal expansion coefficient compatible with that of the window glass to produce the evacuation and gas filling tubulation port.

Claims: A process for producing flat-plate, gas discharge, illumination devices which comprises a cutting step utilizing a high pressure water jet, said water jet carrying an abrasive grit, an assembly step comprising the placing of the integral, interior electrodes in the channels cut into a middle plate by said cutting step, together with the assembly of top and bottom plates about said middle plate to form a non-hermetic channel containing eletrodes, said plates being assembled on the surface of a carrier platen, said carrier platen is covered with a ceramic powder to prevent adhesion of said bottom plate to said carrier platen, said ceramic powder having a sieve size less than 200 mesh, a sealing step comprising the heating of the combination of top, middle, bottom plates and said carrier platen to a temperature sufficiently high to soften and to seal said top, middle and bottom plates hermetically together, including the hermetic sealing of electrical feed-throughs to the said electrodes by means of glass frit, a cooling step, comprising the cooling of the tubulated assembly to a temperature low enough to allow evacuation hoses to be connected to said tube, an evacuation and backfilling step comprising the evacuation of air from the said channel and the replacement of this evacuated air by backfilling the said channel with the desired fill gas, a final sealing step comprising the hermetic sealing of the said evacuation and backfilling tube. The process described in claim 1 wherein the said sealing step is carried out by heating at heating rates between - 16,7 and 3,89 degrees Celsius per minute (2 and 25 degrees Fahrenheit per minute) to a final temperature between 648,9 degrees Celsius (1200 degrees Fahrenheit) and 787,8 degrees Celsius (1450 degrees Fahrenheit) followed by cooling at rates between 17,2 degrees Celsius and 9,4 degrees Celsius per minute (1 degree Fahrenheit and 15 degrees Fahrenheit per minute) to a temperature of between 65,5 degrees Celsius and 260 degrees Celsius (150 degrees Fahrenheit and 500 degrees Fahrenheit). The process described in claim 1, which additionally has an interrupt step that is carried out during cooling from the highest temperature reached, said interrupt step occurring when the temperature is between 537,8 degrees Celsius and 398,9 degrees Celsius (1000 degrees Fahrenheit and 750 degrees Fahrenheit), said interrupt step comprising the insertion of a tubulation into a hole in said top plate, said tubulation comprising a glass tube, said glass tube having a thermal expansion coefficient similar to that of the glass which comprises the flat glass plates, said tube being sealed to said plates by means of a low melting point glass frit. The process described in claim 1 wherein the said backfilling step includes the backfilling of the said channel to a pressure of between 2,5 and 30 millibars. The process described in claim 1 which additionally comprises the heating of the electrodes by radio-frequency heating to desorb adsorbed air and water vapor.

Inventors: COCKS FRANKLIN H, FARNER PETER W, COCKS, FRANKLIN H., FARNER, PETER W.
Applicants: COCKS FRANKLIN H, FARNER PETER W, COCKS, FRANKLIN H., FARNER, PETER W.
Application year: 1991
Main classification: H01J9
IPC codes: H01J9