US3783330A

US3783330A - Direct heated cathode

Info

Publication number: US3783330A
Application number: US00238894A
Authority: US
Inventors: H Nakanishi; T Ushiro; K Sano
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1971-04-02
Filing date: 1972-03-28
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01
Also published as: CA946908A; GB1343012A; DE2215784C3; DE2215784A1; DE2215784B2

Abstract

A direct heating cathode for electronic tube application having excellent electrical characteristics, e.g., thermionic emission characteristics and cut-off characteristics, and excellent mechanical characteristics, e.g., thermal strength, impact strength, is provided which comprises a heating element having a cathode contact surface and having side wall portions supporting said contact surface, a cathode base plate contacting said heating element contact surface, a thermionic emitting layer for emitting electrons by thermal energy being in contact with said cathode base plate, wherein said heating element is formed from a metallic material having an electric resistance of greater than 110 Mu Omega -cm at 800*C., a tensile strength of greater than 15 kg/mm2, a thickness of 20 - 50 Mu and a width of 0.5 - 1.5 mm.

Description

United States Patent [191 Nakanishi et al.

[111] 3,783,330 Jan. 1,1974

[ DIRECT HEATED CATHODE [75] Inventors: Hisao Nakanishi; Takushi Ushiro; Kinjiro Sano, all of Kyoto, Japan [22] Filed: Mar. 28, 1972 [21] App]. No.: 238,894

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Norman F. Oblon et a1.

[ ABSTRACT A direct heating cathode for electronic tube application having excellent electrical characteristics. eg, thermionic emission characteristics and cut-off char acteristics, and excellent mechanical characteristics, e.g., thermal strength, impact strength, is provided which comprises a heating element having a cathode contact surface and having side wall portions supporting said contact surface, a cathode base plate contacting said heating element contact surface, a thermionic emitting layer for emitting electrons by thermal energy being in contact with said cathode: base plate, wherein said heating element is formed from a metallic material having an electric resistance of greater than 1 10nfl-cm at 800C, a tensile strength of greater than 15 kglmm a thickness of 20 50p and a width of 0.5 1.5 mm.

3 Claims, 14 Drawing Figures sum 1 BF 5 |2 0 I40 VOLTAGE OF HEATER (SPECIFIC VALUE) PATENTEUJRN Tkmux ("0) IIOO PATENTED I I974 V 3.783.330

sum

2 or 5 WIDTH 0F PLATE OF HEATER F I G, 6 N

O: /'(T E 2.0 LENGTH OF FLAT PLATE(mrn) BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to a direct heated cathode which is suitable for use in a cathode-ray tube.

2. Description Of The Prior Art Direct heated cathodes wherein a thermionic emission layer is contacted directly with a heat element, are known, and it is known that these types of units have the advantageous characteristic of having relatively short damping times. The electronic characteristics of these units, howevenare often unsuitable for many applications. For instance, it is known that the cut-off voltage of an electronic tube having such a direct heated element is deleteriously affected by displacement of the thermionic emission layer due to thermal expansion.

It has been suggested to compensate for this expansion by supporting the heating element by a suitable resilient means, such as a spring. However, the significant temperatures involved quickly cause deterioration of the resiliency so that it has been difficult to obtain a commercially practical direct heated cathode.

Another disadvantage of some of the prior art direct heated cathodes has been that since the thermionic emission layer is contacted directly with the heating element, there is some degree of heat loss by conduction by the heating element supports, so that the cathode temperature is often undesirably lower than expected.

SUMMARY OF THE INVENTION Accordingly, it is one object of this invention to provide a direct heated cathode for electronic tube application which is characterized by simplicity of structure and excellent electrical and mechanical characteristics.

It is another object of this invention to provide a direct heated cathode which has a relatively low cut-off voltage deviation.

It is still another object of this invention to provide a direct heatedcathode having stable electron emission characteristics.

A still further, object of this invention is to provide a direct heated cathode having a high coefficient of thermal conductivity.

A further object of this invention is to provide a direct heated cathode having short damping time.

Another object of this invention is to provide a direct heated cathode for a cathode-ray tube which is characterized by high electron resolution.

These and other objects, as will hereinafter become more apparent, have been attained by the provision of a direct heated cathodehaving a heating element in direct contact with a cathode base plate, and being supported by an insulating base plate and an electron emission material provided on said cathode base plate.

The heating element is formed from flat metallic stock having a thickness of 20 5011., a width of 0.5 1.5 mm., atensile strength of greater than 15 kg/mm and an electrical resistance at 800C. of 110 pcm. The element is formed into a rectangularly shaped arch-like configuration having a flattened cathode contact surface top portion which is aligned essentially perpendicular to adjacent leg or side wall portions. The length of the cathode contact surface portion between the side wall portion is 0.5 2.5 mm. The cathode base plate, however, does not necessarily contact the entire top portion or cathode contact surface of the heating element. The length of each side portion is 0.5 3.0 mm. as measured from the top portion to a bottom support. The thickness of the cathode base plate is 40 u and the ratio of the cathode base plate area to the area of the heating element in contact therewith is 1.0 2.0.

BRIEF DESCRIPTION OF THE DRAWINGS Various objects, features and advantages of this invention will be more fully appreciated as the same becomes better understood from the following detailed description, when considered in connection with the accompanying Drawings, wherein like reference characters designate like or corresponding parts throughout the several views, and in which:

FIG. 1 is a schematic view of a direct heating cathode according to this invention;

FIG. 2 is a graph showing the relation between the temperature of the cathode and the voltage impressed across the terminals of the heating element of the direct heating cathode or the indirect heating cathode;

FIGS. 3 and 4 are schematic views of alternative direct heating cathodes according to this invention;

FIGS. 5, 6, 7, 8, 9, 10 and 11 are graphs showing the characteristic curves of various structures of direct heating cathode with respect to temperature of direct heating cathode or to the damping time;

FIG. 12 is a graph for showing a characteristic voltage-temperature curve of a direct heating cathode of this invention; and

FIGS. 13 and 14 are graphs showing the characteristic curves of elongation of the direct heating cathode by the heat expansion.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now further to the Drawings, FIG. 1 thereof shows a direct heated cathode wherein a thermionic emitting layer 1 made of complex carbonates of barium, strontium and calcium [(Ba, Sr, Ca) CO is situated on the surface of a cathode base plate 2. The cathode base plate 2 is, in turn, in contact with one surface of a heating element 3. The heating element shown has an arch type configuration, wherein each leg of the arch is supported by

supports

4 and 5 which additionally function as the terminals of the heating element. The

supports

4 and 5 are further supported by an insulating support 6.

The complex carbonates used to form the thermionic emitting layer are formed by heat decomposition usually at a temperature of 1,0,00 1,200C., whereby the carbonates are converted into their corresponding complex oxides of barium, strontium and calcium.

The reaction can be illustrated as follows:

(Ba, Sr, Ca) CO (Ba, Sr, Ca) 0 C0 The resulting carbon dioxide gas CO, shown in the formula is evacuated with the air from the cathode tube.

The complex oxide thermionic emitting layer 1 is then aged, and the complex oxides are reacted with a reducible metal, such as magnesium or silicon, contained in the cathode base plate 2,. at the decomposition temperature, to produce the free elements of barium, strontium and calcium and combinations of complex salts. One example of this reaction can be illustrated by the following formula:

(Ba, Sr, Ca) Mg Ba MgO (Sr, Ca)() the free metal functions as a donor in the thermionic emission, usually at an operating temperature of 800 850C. The thermionic emitting layer, therefore, requires a temperature within the decomposition range for the oxides, which is usually around l,0O0C. to l,200C., to function as an electron emitting source.

Even though the cathode base plate is in contact with the heating element, there is a measurable loss of heat due to non-ideal heat conductance from the heating element to the cathode base plate. Accordingly, in order to reach and maintain the temperature of the thermionic emitting layer at the decomposition temperature, the actual heating element temperature must be higher than the decomposition temperature. For instance, if the decomposition temperature is l,000C. 1,200C., the heating element temperature should be l,200C. 1,400C.

In order to obtain a stable thermionic emission from the cathode, it is essential that the temperature of the thermionic emission charge be maintained as steady as possible. This requires that the voltage impressed across the terminals of the heating element be maintained as constant as possible. This, however, is quite difficult to achieve because of the variable contact resistance at the terminals.

FIG. 2 shows the relation between the voltage impressed across the terminals of the heating element and the cathode temperature. When an operating voltage A is impressed, the temperature of the thermionic emitting layer 1 (hereinafter referring to cathode temperature) A will be provided, such that a thermionic emission will result. When the heater voltage 8 is impressed, the cathode temperature B will result, thereby effecting reactions l and II above. Radiation heat loss from the cathode base plate 2 and the heating element 3 will be less than the heat loss caused by conductivity losses at the lower temperatures, but since radiation loss is proportional to the fourth power of absolute temperature (Ab. Tempf) in accordance with the Stephen and Boltzmann principle, at higher cathode temperatures, the radiation loss will become substantial. This difference in heat losses causes the characteristic curve 7. This curve can be compared with the characteristic curve 8 for conventional indirect heated cathodes.

In the characteristic curve 7, the fluctuation of the cathode temperature, caused by the fluctuation in the heater voltage, is higher than that of the characteristic curve 8. This makes it difficult to maintain a constant thermionic emission with direct heated cathodes, as compared with indirect heated cathodes. Because of this, the heater voltage A for providing the temperature A in FIG. 2 should preferably by within the range of 70 90 percent of the heater voltage B. A direct heated cathode having this type of characteristic can be obtained using the following materials and structure.

The material used in forming the heating element should have a high electrical resistance to enable a rapid rise in temperature by Joules heat. The material should be strong enough to prevent deformation at high temperatures and should have a low specific heat, and a suitable density and heat conductivity sufficient to decrease the damping time. A suitable material for the heating element should have an FM, in the following equation, of at least 50 when measured at 800C:

wherein FM: Figure of merit R: electrical resistance (0 -cm) F: strength (kglmm C: specific heat (cal/gC) p: density (g/cm) The metallic materials having FMs of more than 50 at 800C. include rhenium, titanium, Fe-Ni-Mo type alloys and the like.

It is especially desirable to use a metallic material having an electric resistance of greater than llOpAQ, a tensile strength at 800C. of greater than 15 kglmm Referring now to FIG. 3, the heating element is shaped from a flat plate into an arch-shaped configuration, wherein the top portion 9 of the arch contacts the cathode base plate 2. The heating element has a pair of side walls 10 and 11 having

base portions

12 and 13 which are fixedly attached to

supports

4 and 5.

One modification of this structure is shown in FIG. 4. As shown in that embodiment, the heating element 3 is the same as that shown in FIG. 3. However, the edges of the

side walls

10 and 1 1 are each fixed to the vertical portion of respective elbow-shaped supports 4a, 5a which in turn is supported by an insulating support 6.

As the thickness of the heating element plate increases, its strength against deformation increases. However, its electrical resistance decreases and the contact resistance on the socket terminals, caused by the passage of a current through the heating element, and the current value passing through the heater increases, which is disadvantageous in the design of the operating circuit. Moreover, it is necessary to decrease the heat conductivity losses from the heating element, as discussed above, which requires that the thickness of the plate be decreased. It is desirable, therefore, for the thickness of the heating element plate to be between 20 50 1.4..

The ratio of the heating element voltage at l,O00C. cathode temperature to the heating element voltage at 800C. of the cathode temperature is referred to as TkR. It is preferable that the TkR be percent in an indirect heated cathode. It has been found that the TkR will increase depending upon the width of the plate of the heating element, as shown in FIG. 5. On the other hand, the electric resistance will disadvantageously be decreased upon increase of the plate width. Accordingly, it is preferable that the heating element plate have a width of 0.5 1.5 mm.

The length of the flat plate 9 relates to the heat conductivity to the cathode base plate 2 of the heating element 3 and TkR. FIG. 6 shows graphically that at 1,300C., the difference between the temperature of the heating element 3 and the temperature of the cathode base plate 2 will increase depending upon increases in the length of flat plate 9, while the heat conductivity will decrease while the TkR increases. Tk max refers to heat conductivity.

As the length of the flat plate 9 is decreased, assembly of the cathode becomes difficult. Accordingly, it is preferable that the length be from 0.5 to 2.5 mm.

The thickness of the cathode base plate 2 will depend mainly upon the damping time of the cathode. The relation is shown in FIG. 7. If the thickness is greater than 70g, damping time will not be substantially changed, whereas if the thickness is less than 70;/;, damping time will be decreased. Accordingly, it is advantageous that the thickness of the cathode base plate 2 be as thin as possible to shorten the damping time. On the other hand, when the thickness of the plate is decreased, the function of the reducing agent contained in the cathode base plate which reacts as shown in the formula II is disadvantageously affected and the life of the cathode is decreased. Accordingly, the thickness of the cathode base plate 2 should be within the range of 40 120 [.L.

It has been found thatSp/SH,i.e., the ratio of the area of the cathode base plate 2 Sp to the area of the heating element in contact with the cathode base plate 2 SH depends upon the above-mentioned TkR, Tk max and the damping time, and the mutual relations as shown in FIGS. 8, 9, and 10.

In order to increase TkR, it is preferable to increase Sp/SH; that is, to increase the area of the cathode base plate. In order to increase Tk max, it is desirable to decrease the temperature differential between the heating element 3 and the cathode base plate 2. In order to decrease the damping time, it is preferable to decrease Sp/SH. Good results are attainable if the Sp/SH ratio is between 1.0 and 2.0.

Referring now to FIG. 3, the length of the side walls 10, 11 are referred to as HK. The relationship between TkR and Tk max is shown in FIG. 11. As HK increases, the value of TkR increases. However, the value of Tk max decreases so that the difference between the temperature of the heater 3 and the temperature of the cathode base plate 2 increases and the heat conductivity of the heater decreases. Accordingly, it is preferable that HK be in the range of 1.0 3.0 mm.

In one embodiment of the cathode of this invention, the heating element is formed from a plate having a thickness of 25 o, 1 mm. in width, 2 mm. of length at the top of the arch, and 3 mm. in length each side wall, and the cathode base plate has a thickness of 1 1. and 2.25 mm of area. The heating element was made from F e-Ni-Mo type alloy having an FM of 65.8. The characteristic curve as shown in FIG. 12 was obtained.

The damping time of the cathode was 1 second and the temperature of the cathode base plate was l,l00C., when the temperature of the heater was l,l50C.

In accordance with this invention, a direct heated cathode having excellent characteristics is provided. When the direct heating cathode is used for high resolution electronic tube such as a flying spot tube, the distance between the cathode and the control electrode highly affects resolution. Accordingly, the elongation to the control electrode of the cathode caused by heat expansion, is an important factor. With a heater made of Fe-Ni-Mo type alloy having an electric resistance of 13511.0 -cm, a tensile strength of 40 kg/mm at 800C., and a specific heat of 0.092 cal/g C. which has a thickness of 25 .1., a length of 1.5 mm., and the distance between the flat plate to the contact point of the side wall and the supports being 2 mm. to the relationship of Sp/SH (which is the ratio of the area of the cathode base plate Sp to the area of the heater contacted to the cathode base plate SH) and the elongation of the heater are shown in FIGS. 13 and 14. As shown in FIG. 13, the degree of elongation will decrease depending upon the decrease in the width of the plate. However,

the relation between the width of the plate and TkR as shown in FIG. 5 is preferably in the range of 0.5 1.5 mm. As shown in FIG. 14, elongation will decrease depending upon decrease of Sp/SH. If the Sp/SH is too small, however, the assembly of the cathode will be difficult. Accordingly, it is desirable that Sp/SH be 1.0 2.0.

As another embodiment of the cathode of this invention, wherein the cathode base plate is made of nickel having a thickness of p. and an area of 1.0 mm and a heater made of Fe-Ni-Mo type alloy having an electric resistance of 11) -cm (800C.), a tensile strength of 40 kg/mm at 800C., and a specific heat of 0.092 cal/gC., a thickness of 25p. a plate width of 0.6 mm. and a length of 1.5 mm. The distance between the flat plate and the contact of the side wall and the supports is 2 mm., the elongation of the heater is 0.024 mm. at 800C. and 0056mm. at 1,200C. and the electric power at 800C. of the cathode temperature is about 0.7 Watt. The value of elongation of the heating element is about one-half of the elongation of the cathode of conventional indirect heated cathodes which is 0.05 mm. at 800C. and 0.12 mm. at 1,200C.

Accordingly, it is possible to decrease the distance between the cathode and the control electrode to 0.03 mm. at 800C. in said embodiment as compared with a conventional electronic tube in which the minimum distance between the cathode and the control electrode is 0.06 mm. at 800C. Accordingly, it is possible to provide higher cut-off voltages than possible in conventional tubes and to decrease the beam-cross-over, and to improve resolution.

As the electric power required for the cathode is quite small, very little deformation of the control electrode is found. Accordingly, it is quite effective fo improving the resolution.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto with out departing from the spirit or scope of the present invention.

Accordingly, what is claimed as new and intended to be covered by letters patent is:

l. A direct heated cathode which comprises:

a heating element having a cathode content surface and having side wall portions supporting said contact surface,

a cathode base plate contacting said heating element contact surface,

a thermionic emitting layer for emitting electrons by thermal energy being in contact with said cathode base plate,

wherein the heating element is formed from an ironnickel-molybdenum alloy having an electric resistance of 135 4.!) -cm at 900C, a tensile strength at 800C. of 40 kglmm and a specific heat of 0.092 cal/gC. and the heater has a thickness of 2511., a width of 0.5-1.5 mm., a length of said contact surface of 0.5-2.5 mm., and a side wall length of 0.5-3.0 mm., and the cathode base plate is made of a nickel plate having a thickness of 40-] 20 1,.

2. A direct heated cathode according to claim 1 wherein the cathode base plate has an area of approximately 1.0 mm.

3. A direct heated cathode according to claim 1 wherein the ratio of the area of the cathode base plate to the surface area of the heating element in contact with the cathode base plate is 1.0-2.0.

Claims

1. A direct heated cathode which comprises: a heating element having a cathode content surface and having side wall portions supporting said contact surface, a cathode base plate contacting said heating element contact surface, a thermionic emitting layer for emitting electrons by thermal energy being in contact with said cathode base plate, wherein the heating element is formed from an iron-nickelmolybdenum alloy having an electric resistance of 135 Mu Omega -cm at 900*C, a tensile strength at 800*C. of 40 kg/mm2, and a specific heat of 0.092 cal/g*C. and the heater has a thickness of 25 Mu , a width of 0.5-1.5 mm., a length of said contact surface of 0.5-2.5 mm., and a side wall length of 0.5-3.0 mm., and the cathode base plate is made of a nickel plate having a thickness of 40-120 Mu .

2. A direct heated cathode according to claim 1 wherein the cathode base plate has an area of approximately 1.0 mm2.