The development of unwanted browning in a surface of a glass has long been recognized as resulting from the exposure of the glass to the bombardment of electrons thereupon, with the concomitant generation of x-radiation. The subjection of the glass to x-radiation results in a phenomenon which has been termed x-ray browning. That phenomenon has been defined as a temporary darkening of a glass surface because of damage caused by the exposure thereto of x-radiation. In contrast, the impingement of high voltage electrons on a glass causes a permanent discoloration (browning) in the surface thereof.
The use of ceric oxide (CeO.sub.2) to inhibit discoloration by x-radiation was disclosed in U.S. Pat. No. 2,477,329 (DeGier et al.). Titania (TiO.sub.2) was discovered to be an effective supplement for, but not a total replacement for, CeO.sub.2. Therefore, to minimize the amount of CeO.sub.2 utilized, because of its high cost, a combination of CeO.sub.2 and TiO.sub.2 is customarily employed. Because TiO.sub.2 can impart color to the glass in large amounts, the total thereof will not exceed about 1%.
At least two theories have been proposed to explain the phenomenon of electron browning. The first theory contemplates the reduction of some chemical species to its metallic state. This theory was also disclosed in U.S. Pat. No. 2,477,329, supra, which proposed minimizing the concentrations of readily reducible metal oxides, especially lead oxide, in the glass composition. A more recent theory envisions the creation of damage within the atomic arrangement of the glass structure where the glass composition is free from easily reducible oxides.
In view of the above factors, glasses designed to be used as faceplates for cathode ray tubes, such as television picture tubes and rear projection tubes, have customarily contained CeO.sub.2, with or without TiO.sub.2, and have been relatively free from readily reducible metal oxides such as lead oxide.
As the operating electron voltages of the tubes have been raised, there has been the need to increase the x-radiation absorption capability of the faceplate glass to protect the viewer, and to increase the resistance of the glass to browning by x-radiation and by electron bombardment. As can be appreciated, however, the glass compositions must also satisfy a matrix of chemical and physical characteristics to meet the requirements of the tube manufacturer, as well as the melting and forming properties demanded by the glass manufacturer to shape the glass into desired configuration. Accordingly, the glass will customarily exhibit a linear coefficient of thermal expansion over the temperature range of 25.degree.-300.degree. C. between about 97-100.times.10.sup.-7 /.degree. C., an annealing point not lower than about 475.degree. C., a strain point not lower than about 440.degree. C., and an electrical resistivity expressed in Log R that is greater than 9 at 250.degree. C. and greater than 7 at 350.degree. C. The glass will demonstrate a liquidus temperature below about 1100.degree. C.
Therefore, the principal objective of the present invention was to enhance the resistance of glasses designed for use as cathode ray tube faceplates to browning from exposure to x-radiation and electron bombardment, while maintaining the chemical and physical properties, as well as the melting and forming behaviors, conventionally present in glass compositions designed for use as cathode ray tube faceplates.