US 2699510 A
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Jan. 11, 1955 J. A. M. SMELT 2,699,519
CATHODE-RAY TUBE Filed Jan. 29, 1952 numei Graphite INVENTOR Jozefus Anromus M riu Selr Agent United States Patent CATHODE-RAY TUBE Application January 29, 1952, Serial No. 268,783
Claims priority, application Netherlands February 24, 1951 2 Claims. (Cl. 313-433) Netherlands, Trust Com- Improvements in cathode-ray tubes.
This invention relates to cathode-ray tubes comprising an iron cone and a fluorescent screen produced on a glass window by the process known as settling. Such tubes have a limitation in that the iron of the cone becomes corroded by the suspension agent of the settling liquid with the result that rust stains may be produced on the fluorescent screen. This may be avoided by coating the iron with a protective layer, for example an enamel layer. Since the cone must be thoroughly cleansed internally by chemical and/or mechanical means, such as etching, scouring and brushing it is apparent that only very persistent, readily adhering substances may be used to constitute the protective layer. It has been found that the usual, readily melting enamel, which is often used for sealing the glass window and the metal cone, is not sufii ciently resistant to the said cleansing means so that only those enamels are suitable for use which melt at a high temperature (from 900 C. to 1100 C.). However, a disadvantage of enamel as a coating of the inner wall of the iron cone is that wall charges are liable to be produced on the insulating enamel layer, with the result that an important advantage of the use of the iron cone becomes lost. As an alternative, a cone made of ferrochrome may be used but such a cone is very expensive.
The object of the invention is to provide an improved cathode-ray tube.
Figure 1 is an outline drawing of a cathode ray tube.
Filgure 2 is a cross-sectional view on lines 2-2 of Figure According to the invention, a cathode ray tube comprising a metal cone and a glass window on which a fluorescent screen is produced by the process known as settling, is characterised in that the cone consists of iron whereas at least the inner surface of the cone is coated with an electrically conductive layer constituted by a mixture of chemically resistant enamel and graphite. Such a layer is found to be resistant to chemical and mechanical cleansing means.
The term chemically resistant enamel is to be understood to mean an enamel which is not corroded by alkalies or acids. As a rule, the said enamel will have a high melting temperature (from 800 C. to 1100 C.). The commercially known acid-resistant ground enamels generally satisfy this requirement.
The composition of the enamel according to the invention is not very critical.
A favourable composition is, for example, 69% by weight SiOz; 2.2% by weight A1203; 18.6% by weight NazO; 8.7% by weight CaO; 0.5% by weight ZnO and 1% by weight CoO, which is admixed for example with from 7 to 9% by weight of the pulverulent graphite. After the cone is entirely or largely coated internally and if desired also externally by spraying or immersion with the said mixture, the cone, after dessication of the mixture, is put in a furnace which has a temperature of about 1100" C. At the end of about 3 minutes it is removed from the furnace and is then found to be coated with a highly persisting, conductive black enamel layer. It is found that due to the addition of graphite, the mechanical and chemical resistance is not decreased to such an extent that damage to or attack on the layer occurs during the cleaning of the cone or the settling process.
It has been suggested to apply conductive layers of glazing to avoid wall charges on the inner surface of a glass cone, the conductivity of the layer being ensured by admixture of pulverulent metals as silver, copper or the like, but since provision was not made of a screen obtained by settling and an iron cone, the above-mentioned thorough cleaning was not required. However, a conductive layer of glazing is unsuited for the present case, since the layer would be wholly or partly removed or damaged by cleaning. In addition, the use of protective layers on metal has been suggested, an enamel being admixed with sugar or carbon. However, the carbon entirely disappears during the heating to about 500 C. and this is partly due to association with the moisture present in the enamel. l t is thus most surprising to find that, during the melting of the enamel, graphite mixed with refractory enamel is burnt to an only slight extent so that a conductive enamel layer can be produced. The use of graphite has the additional advantage that the enamel layer is black and thus does not entail diiiiculty in connection with light reflections. The quantity of graphite is chosen in connection with the desired conductivity of the layer. Satisfactory results are generally given by a quantity of graphite which ranges between 5 and 10% by weight of the mixture. An excessive quantity of graphite adversely affects the mechanical resistance of the layer.
What I claim is:
1. A cathode-ray tube comprising an iron cone, a glass window united with said cone, a fluorescent screen on the inner surface of said glass window, and an electricallyconductive coating on the inner surface of said iron cone, said coating being constituted by a fused mixture of an acidand alkali-resistant enamel having a melting point in excess of 800 C. and between about 5 and 10% by weight of graphite.
2. A cathode-ray tube as claimed in claim 1 in which the enamel coating contains between about 7 and 9% by weight of graphite.
References Cited in the file of this patent UNITED STATES PATENTS 2,151,992 Schwartz Mar. 28, 1939 2,162,391 Schwartz et al June 13, 1939 2,222,197 Engels Nov. 19, 1940 2,223,924 Stephan Dec. 3, 1940 2,254,090 Power Aug. 26, 1941 2,376,196 Scherbatskoy May 15, 1945