CA1104632A

CA1104632A - Cathode-ray tube

Info

Publication number: CA1104632A
Application number: CA308,397A
Authority: CA
Inventors: Akio Ohkoshi; Yoshio Matsumoto; Teruyasu Suzuki
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1977-07-30
Filing date: 1978-07-28
Publication date: 1981-07-07
Also published as: AU522395B2; FR2399123A1; GB2002952B; JPS5426657A; JPS6120990B2; GB2002952A; FR2399123B1; DE2833415A1; NL7808073A; AU3844178A

Abstract

ABSTRACT OF THE DISCLOSURE

A cathode-ray tube having an envelope consisting of a panel portion, a funnel portion and a neck portion with an electron gun mounted in the neck portion, has a phosphor coated on the interior surface of the panel portion to provide a screen and a conductive layer is coated on the interior surface of at least the funnel portion. In this invention, the conductive layer comprises a mixture of graphite, silicon carbide and silicate such as alkali silicate.

Description

11~4632 BAC~GROU~i~ OF TH~ IWVENTION

Field of the Invention The present invention relates generally to a cathode-ray tube, and is directed more particularly to an improved cathode-ray tube having coated on its interior surface thereof an internal conductive layer which comprises a mixture of silicon carbide and graphite.

Description of the Prior Art:
In the prior art, the internal coating layer formed on the interior surface of a cathode-ray tube consists of graphite particles and alkali silicate binder, for example, sodium silicate or potassium silicate etc.. Since graphite is soft, the coating layer is not hard enough. Prior art cathode-ray tubes having a conductive coating layer which consists of the above material, when transported are subjected to vibrations or impacts which causes the coating layer to be rubbed due to con~act with elements such as springs, getter containers and so on and hence conductive particles (graphite particles) forming the conductive coating layer tend to be rubbed off. These rubbed-off graphite particles cause undesired discharges between respective parts of the prior art cathode-ray tube during operation.
In order to avoid the rubbing-off of the graphite particles from the conductive coating layer, if it is desired that the mechanical binding strength of the conductive coating layer be increased, it is sufficient to increase the a~ount o water glass used in the conductive coating layer which serves as binder. ~owever, if the amount of water glass is increased, the amount of gas which discha ges into the cathode-ray tube is also increased and such increase o. gas will cause the vacuum

-2- ~

llG4632 in the cathode-ray tube to be lowered.
In the art it has been proposed to provide a conductive coating layer made of graphite and iron oxide (such as Fe203) which are bound together by alkali silicate. This prior art conductive coating layer, however, shortens the life span of a cathode-ray tube.

SU~RY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a cathode-ray tube having coated on its interior surface an improved conductive coating layer.
It is another object of the invention to provide a cathode-ray tube free from the defects inherent in the prior art cathode-ray tube.
It is a further object of the invention to provide a cathode-ray tube having coated on its interior surface a conductive coating layer which will avoid any increase in the amount of gas in the cathode-ray tube and which will not shorten the life span of the tube.
It is a still further object of the invention to provide a cathode-ray tube having coated on its interior surface an improved conductive coating layer which is not rubbed off by contact with springs, getter containers and so that the cathode-ray tube will operate in a stable condition.
According to an aspect of the present invention, there is provided a cathode-ray tu~e having an envelope consisting of a panel portion, a funnel portion and a neck portion with the panel portion being coated with a phosphor layer on its interior surface, the neck portion being provided with an electron gun, and at least the funnel portion being coated with a conductive layer, characterized in that the conductive coating layer comprises a mixture of graphite, silicon carbide and silicate.

11(~463Z

The other objects, features and advantages of tne present invention will become clear from the following description taken in conjunction with the accompanying drawings.

BRIEF DÆSCRIPTION OF THE DRAWII~GS
~ igure 1 is a partially cross-sectional side view showing an example of the cathode-ray tube according to the present invention; and Figure 2 is a cross-sectional view showing a conductive layer coated on a glass plate in an enlarged scale.

DESCRIPTION OF T~I~ PREFERRE~ EMBO~I~lENTS
- An example of the cathode-ray tube according to the present invention will be hereinafter described with reference to Figures 1 and 2. The cathode-ray tube of the invention can be used in a color television receiver by way of example.
In Figure 1, a cathode-ray tube 1 according to the invention has an envelope le. ~he envelope le has a neck portion ln, a panel portion lp and a funnel portion lf coupling the neck portion ln to the panel portion lp. The funnel portion lf has an electrically conductive layer 2 coated on its interior surface which is supplied with a high voltage through an anode button 10 which extends through the funnel portion lf. A color phosphor screen 3 is coated onto the interior surface of the panel portion lp of the envelope le of the cathode-ray tube 1. A
beam selecting device 4 such as a shadow mask or aperture grill, which will serve to dete~mine the landing position of the electron beam emitted from an electron gun 5 disposed in the neck portion ln of the envelope le on the color phosphor screen 3, is located in the envelope le of the cathode-ray tube 1 in opposed relation to the phosphor screen 3. Thus, the electron beams er,litted from the electron gun 5 and corresponding to, for e~ar~lple, 11~34~3Z

red, green and blue colors impinge on the color phosphor screen 3 at the corresponding color areas.
Conductive contacts sucn as springs 11, 12, and 13, which are electricall~ connected to the phosphor screen 3, the beam selecting device 4 and the anode of the electron gun 5, respectively, make contact with the conduc~ive coating layer 2, which is supplied with high voltage with the anode button 10 so as to supply a high voltage to tne p~osphor screen 3, the beam selecting device 4 and the anode of the electron gun 5, respectively. The inner end of the electron gun 5 is connected tllrough, for example, a plate or leaf spring 6 to a container 7, which has therein getter ~.aterial for absorbing residual gases in the envelope le of the cathode-ray tube l. The getter container 7 is located by the biasing force of the leaf spring 6 at a position along the interior wall of the funnel portion lf such that the getter container 7 will not obstruct the passage of tlne electron beam emitted from the electron gun 5.
As shown in Figure l, the conductive coa~ing layer 2 is coated on the interior surface of the envelope le of the cathode-ray tube 1, especially on -the interior surface of the funnel portion lf and on at least a portion of the nect portion ln. With the present invention, the conductive coating layer 2 is made of a conductive layer which comprises a mixture of graphite, silicon carbide and silicate such as an alkali silicate, for example, sodium silicate, potassium silicate and lithium silicate.
In this invention, a part o~ the graphite particles used in the prior art conductive coating layer is replaced with silicon carbide particles.
In the present invention, grapl~ite particles, silicon carbide particles and alkali silicate, ~or example, potassium silicate, which serves as a binder, are added with water to 11(~4632 form an aqueous suspension; then the resultant aqueous suspension is coated onto the interior surface of the envelope le i.e. on the interior surfaces of the funnel portion lf and a part of the neck portion ln by means of a brush, spraying or the like as a layer. This thin layer is left for 30 minutes, for example, at room temperature; then the temperature is increased up to about 400C to 450C by increasing the temperature 5C to 15C per minute; and the layer is maintained for 60 minutes at the final temperature to form the conductive coating layer 2.
The composition of the above aqueous suspension, which is used to form the conductive coating layer 2, is selected such that it contains 20 to 300 parts by weight of silicon carbide with 100 parts per weight of graphite and 11 to 77 parts per weight of alkali silicate relative to the total amount of solid components. In other words, by way of example, lOOg of graphite, 20g to 300g of silicon carbide (for the lOOg of graphite), 50g to 400g of potassium silicate and 600g to 900g of deionized water are mixed with a small amount of a dispersion agent.
The reason why the amount of silicon carbide is selected to be 20 to 300 parts per weight for 100 parts per weight of graphite is that in order to increase the strength of the coating layer, it is desired to increase the amount of silicon carbide and in order to provide the desired strength of coating layer, it is desired to select the a~ount of silicon carbide to be more than 20 parts per weight for 100 parts per weight of graphite. However, since silicon carbide is a semi-conductor and has electrical conductivity, if the amount of silicon carbide is selected to be more than 300 parts per weight for 100 parts per weight of graphite, a problem occurs of the conductivity required for the conductive coating layer of a liC1463Z

cathode-ray tube. The amount of potassium silicate is selected to be 11 to 77 parts per weight for the total a~ount of solid components is that if ~he amount of potassium silicate is selected to be less than 11 parts per weight, the strength of the coating layer is insufficient. If the amount of potassium silicate is selected to exceed 77 parts per weight a problem occurs because of the amount of gas produced. The amount of deionized water is selected so as to make the aqueous suspension have the desired viscosity which depends upon the manner in which the aqueous suspension is coated onto the interior surface of the envelope le.
- An example of the conductive coating layer of the invention:
Graphite Particles (the average particle size or diameter thereof is 1.5 to 1.0~ m (micron) and the particle size of more than 90% particles is less than lO~m) ---lOOg Silicon Carbide Particles (the average particle size thereof is 1.5 to 0. 5~m and the particle size of more than 90%
is less than lO~m) ---lOOg Potassium Silicate ---140g Deionized Water ---650g The aqueous suspension having the a~ove composition is painted on a glass plate lg with a brush, and then sintered at 440C for 30 minutes to form an electrically conductive layer 2 having a thickness of 5 ~m as shown in Figure 2. It is ascertained that the specific resistance of the thus formed conductive coating layer is 0.15 ,Rcm. The specific resistance desired as the internal conductive coating layer of a cathode-ray tube is less than O.8J~cm.

11~t4632 A spring of stainless steel (Inconel X), which is formed to be a leaf spring with the thickness of 0.25 mm and which has a contact portion of hemispherical shape with the curvature radius of 2.5 m~, is moved while in contact with the surface of the above conductive coating layer at a speed of 2 cm/sec under a pressure of 200g. A piece of Scotch tape (Trade Name of an adhesive tape made by 3M Co. Ltd.) is stuck to the trace of the conductive coating layer formed by the movement of the stainless steel or contact spring and then the tape is peeled off; and the amount of conductive particles rubbed off by the contact spring on the trace of the conductive coating layer is observed. The same test is conducted on the surface of a prior art conductive coating layer including mainly graphite and the results observed. The amount of coating of the present invention rubbed off is compared with that of the prior art by the naked eye and it is observed that the amount of the conductive particles of the invention removed is less then that of the prior art and is about 1/5 to 1/10 of the particles removed from the prior art coating.
The cathode-ray tube of the invention, which has the above conductive layer coateA on the desired interior surface of its envelope, has improved relative to the prior art discharge characteristics as shown in the following Table in which sample No. 1 shows the case of a prior art coating and sample Nos. 2 to 5 show the cases of the coatings of the present invention.

_~ _ Table 1 Sample SiC C Alkali Ratio of No. (Part/ (Part/ Silicate Discharge Weight) ~eight) (Part/ Initiation Weight)Voltages 1 0 1.0 1.33 1.00 .
2 0.2 1.0 1.33 1.02

3 0.5 1.0 1.33 1.07

4 1.0 1.0 1.33 1.11 3.0 1.0 1.33 1.10 In Table 1, the ratio of discharge initiation voltages is obtained in the following manner. Cathode-rav tubes of 14 inches with the conductive coating layers with the compositions Nos. 1 to 5 in the Table 1 were dropped from a height such that the impact acceleration applied thereto was 45G. Then the electrodes for the lower voltage for the electron gun of the cathode-ray tube were connected in common to the minus side of a voltage source and the electrodes for the higher voltage of the cathode-ray tube were connected by way of the anode button to the plus side of the voltage source. rthen the voltages applied across the electrodes (lower and higher) are increased gradually, the voltages at which discharge occurs or is first initiated in the cathode-ray tube is measured for each cathode-ray tube;
and the ratios between the discharge initiation voltage of the No. 1 example and those of examples Nos. 2 to 5 are calculated.
For each of the Nos. 1 to 5 samples, 10 cathode-ray tubes each having the same composition of the conductive coating layer were subjected to the above test for measuring their ~, ?~63Z

discharge initiation voltages, and the average values were compared to obtain the ratios shown in Table I.
The reason why the discharge initiation voltage of the cathode-ray tube according to the invention is higher than that of the prior art can be explained as follows. The conductive coating layer used in the prior art cathode-ray tube (Sample No. 1) includes only graphite as the conductive material and the graphite itself is relatively brittle, so that the bonding force between the graphite and the alkali silicate as the binder is relatively low. In the present invention silicon carbide is e~ployed in addition to the graphite in the conductive coating layer of the cathode-ray tube and the silicon carbide is high in strength, so that the bonding force to the alkali silicate is high.
The above description is given on the single preferred embodiment of the present invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the invention, so that the scope of the invention should be determined by the appended claims only.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cathode-ray tube comprising an envelope consist-ing of a panel portion, a funnel portion and a neck portion, said panel portion having a phosphor coating on its interior surface, said neck portion being provided with an electron gun, and at least said funnel portion having a conductive layer coat-ing on its interior surface, an electrode for applying a poten-tial to said conductive layer, and at least one conductive contact spring attached to said electron gun and biased against said conductive layer to apply said potential to said gun, characterized in that said conductive layer coating comprises a mixture of graphite, silicon carbide and silicate as binder.

2. A cathode-ray tube as claimed in claim 1, wherein said fixture contains silicon carbide in the range of 20 to 300 parts per weight with respect to 100 parts per weight of graphite.

3. A cathode-ray tube as claimed in claim 1, wherein said mixture contains silicon carbide which has substantially the same parts per weight as said graphite.

4. A cathode-ray tube as claimed in claim 2, wherein said silicate is alkali silicate.

5. A cathode-ray tube as claimed in claim 4, wherein said alkali silicate is one selected from the group consisting of potassium silicate, sodium silicate, lithium silicate or mixtures thereof.

6. A cathode-ray tube as claimed in claim 1, wherein said mixture contains silicate in the range of 11 to 77 parts per weight with respect to the total parts per weight of solid components.