US3658713A

US3658713A - Alkali metal generating agents

Info

Publication number: US3658713A
Application number: US875064A
Authority: US
Inventors: Yorikatsu Irisaka; Haruo Takashio
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1968-11-12
Filing date: 1969-11-10
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25
Also published as: NL6916993A; GB1237673A

Abstract

ALKALI METAL GENERATING AGENTS GENERALLY USED IN FORMING A PHOTOSENSITIVE LAYER, CHARACTERIZED IN THAT THEY CONSIST OF A MIXTURE OF ALKALI METAL SALTS AND NIOBIUM. THE MIXTURE IS PUT IN A VESSEL MADE OF ELECTRIC CONDUCTIVE MATERIAL TO FORM A GENERATOR. WHEN THE GENERATOR IS HEATED BOTH MATERIALS REACT WITH EACH OTHER TO ALLOW THE ALKALI METAL TO COME OUT OF THE VESSEL AND FORM THE AFOREMENTIONED LAYER.

Description

April 1 2 YORIKATSU IRISAKA ETA}. 3,658,713

ALKALI METAL GENERATING AGENTS Filed Nov. 10, 1969 9 Sheets-Sheet l Nb+Cs2CrO4 Zr+Cs2CrO4 3 Si+Cs2CrO4 '5 s 1 i ii LL O z 3 O 2 4 TEMPERATURE(C) April 25, 1972 YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS Filed Nov. 10, 1969 9 Sheets-Sheet 2 TEMPERATURE (c) Ap 25, 2 YORIKATSU IRISAKA ETAL 3,658,713

ALKALI METAL GENERATING AGENTS Filed Nov. 10, 1969 9 Sheets-Sheet a FIG. 2A

8 X160)% muoowa 00000 OF GOOD PRODUCTS ST REPRODUCIBILITY N0.

Nb/K2Cr04 RATIO BY WEIGHT FIG. 20

NO. OF TESTS REPIRODUCIBILITY (NO- OF 6000 PRODUCTS Nb/C s2CrO4 RATIO BY WETGHT April 25, 1972 YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS Flled Nov. 10, 1969 9 Sheets-Sheet 4 060000 CQIDQ'YON- 001x SiSELL 50 ON AlI'IIQIOHGOHdBH SiOfiCIQHfi April 25, 1972 Filed NOV. 10, 1969 K AMQUNT OF GENERATION (mg) Na AMOUNT OF GENERATIOMmg) YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS 9 Sheets-Sheet 5 6 F|G.3A

TEMPERATURE(C) FIG,3B

TEMPERATURE(C) April 1972 YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS Filed Nov. 10, 1969 9 Sheets-Sheet 6 TEMPERATURE OF GEMRATIONPC) WNW/Nb 0F MIXTURE L FLIG.5A

Cs AMOUNT OF GENERATIOMmg) TEMPERATUREPC) April 25, 1972 Y-ORIKATSU IRISAKA ET AL 3,653,713 I ALKALI METAL GEZ ERATING AGENTS Filed Nov. 10. 1969 9 Sheets-Sheet 7 T FIG. 5

A 5 5 Q E 2 0.3 m o 5 0.2-

.01 o E 4 V v x I I V .L I

- TEMPERATURE(C) FIG. 6

TEMPERATURE OF GENERAT|ON(C) April 25, 1972 Filed NOV. 10, 1969 Cs AMOUNT OF GENERATION 1111 No AMOUNT OF GENERATIOMmg) YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS 9 Sheets-Sheet 8 FIG. 7A

TEMPERATURE (C) TEMPERATUREPC) A ril 25, 1972 Filed Nov. 10, 1969 TEMPERATURE (c) YORIKATSU IRISAKA ET AL 3,658,713

ALKALI METAL GENERATING AGENTS 9 Sheets-Sheet 9 MIXTURE 0.01 0.1 10 mom. RATIO ((EfiNQ/Nb 0F United States Patent 01 ice 3,658,713 Patented Apr. 25, 1972 US. Cl. 252-181.4 Claims ABSTRACT OF THE DISCLOSURE Alkali metal generating agents generally used in forming a photosensitive layer, characterized in that they consist of a mixture of alkali metal salts and niobium. The mixture is put in a vessel made of electric conductive material to form a generator. When the generator is heated both materials react with each other to allow the alkali metal to come out of the vessel and form the aforementioned layer.

The present invention relates to alkali metal generating agents and more particularly to said agents put in a vessel made of electric conductive material so as jointly to form an alkali metal generator.

Alkali metal generating agents heretofore used in forming the photoconductive target or surface of a special tube such as an image pickup tube or photoelectric tube consist of a mixture of a single or plurality of alkali metal salts and a single or plurality of reducing agents. The mixture is received in a vessel made of electric conductive material and perforated with a plurality of small bores to form a generator.

There are selectively used various kinds of alkali metal salts according to the type of a photosensitive layer employed. The known types of said salts include chromates, bichromates, tungstates and molybdenates of sodium, potassium, cesium, lithium and rubidium. And the known types of reducing agents are mainlysilicon, zirconium, aluminum and boron.

Such prior art alkali metal generator is previously disposed in the prescribed part of a tube at which there is to be formed a photosensitive layer. Where it is required to form said layer, the metal vessel is heated by introduction of current or high frequency induction for reaction betweenthe alkali metal salts and reducing agents so as to release alkali metals. The alkali metal thus released is deposited on the surface of a photoelectric substrate, for example, an antimony film previously conditioned for this purpose by maintaining a part of the tube at a suitable level of temperature, thereby forming a photosensitive layer or surface through a further prescribed process.

The aforementioned metal vessel is fabricated by rolling a thin electric conductive sheet in the transverse direction, to form a hollow cylindrical body, spot welding the superposed edge portions at a plurality of points arranged in the longitudinal direction at a prescribed interval and finally closing up an opening at both ends of the hollow cylindrical body by pressure. The aforesaid spot welding is intended to allow the released alkali metal to be drawn out of the vessel through the gaps formed between the welds.

With the prior art alkali metal generator having the aforesaid arrangement, it is required to heat the generator up to the temperature at which there occurs the exothermic reaction of the above-mentioned mixture in order to release a relatively large amount of alkali metal from a given amount of alkali metal salt used. However, heating of the generator to such high level of temperature leads to a rapid occurrence of reaction, thuspresepting difliculties in controlling the amount of alkali metal to be released.

To prevent such rapid occurrence of reaction, the prior art adds tungsten as a buffer agent to mixtures of alkali salts and reducing agents. However, even addition of said tungsten failed reliably to control the aforementioned rapid reaction. Accordingly, a photosensitive layer obtained by the prior art could not display a desired degree of photosensitivity, nor was fully satisfactory in other respects.

It is accordingly the object of the present invention to provide an alkali metal generating agent capable of easily controlling the velocity of exothermal reaction which raised a problem with the prior art, by adding niobium as a reducing agent to the alkali metal salt and forming a photosensitive layer or surface having excellent properties and also an alkali metal generator prepared by putting said generating agent in a vessel made of electric conductive metal.

This invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are diagrams comparing the amount of alkali metal released from the prior art alkali metal generating agents corresponding to the associated temperature with that of a similar agent according to an embodiment of the present invention;

FIGS. 2A to 2C illustrate the effect of a kind of alkali metal generating agent according to the invention and the relationship of the yield of alkali metal in forming a photosensitive film versus the ratio in which there are blended the components of said agent;

FIGS. 3A and 3B are similar diagrams to FIG. 1A comparing the alkali metal generating property associated with Example D of the invention with those of the prior art alkali metal generating agents;

FIG. 4 is a diagram indicating the relationship between the blending ratio of an alkali metal salt and niobium constituting the generating agent used in said Example D and the temperature required for said generation;

FIGS. 5A and 5B are similar diagrams to FIGS. 3A and 3B associated with the alkali metal generating agent of Example E;

FIG. 6 is a similar diagram to FIG. 4 associated with Example E;

, FIGS. 7A and 7B are similar diagrams to FIGS. 5A and 5B associated with the alkali metal generating agent of Example F;

FIG. 8 is a similar diagam to FIG. 6 associated with Example F; and

FIG. 9 is a perspective view, with part broken away, of an exampe of an alkali metal generator filled with the generating agent of the invention.

For better understanding of the present invention, there will now be described the prior art reducing agents. Reaction of the prior art reducing agents such as silicon, aluminium and zirconium with alkali metal salts is of exothermic type. Among said reducing agents, silicon causes a tremendous evolution of heat. And all these reactions, except between cesium chromate and zirconium, take place very rapidly. Even addition, as heretofore attempted, of powdered tungsten as a buffer agent for suppressing the above-mentioned rapidity of reaction fails to control reaction velocity with respect to other alkali metal generating agents than a mixture of zirconium and potassium chromate. Although a mixture of zirconium and alkali metal salts allows its reaction velocity to be appreciably controlled as described above, there will most likely take place an explosion during reaction, so that said mixture is barred from use. Where the aforementioned both components are slowly mixed dry in the forming of relatively coarse powders, said explosion may be avoided Yield alkali metal (percent) ca. 80 ca. 30 w TABLE 1 Tempera- Generated state Phototure of afiected by Gas evolution gener- Puncture varying prebefore gener- Control of Getter True Bulk lOSSlbll- Mixing tivrty, Generated atron, or breakage heating ation of alkali generated action of density density Addition of lty of l'atlo [IA/[3111.2 state C. by melting conditions metal state reduetant (g./em. (g./em. butter agent explosion 100 Very stable-- ca. 780 Never Little afieeted Very small Easy.. Great"..- 6.42 2.4 Unnecessary None 50 U stable..- ca. 900 Sometimes Notfiieeably Prominent DiIficult Small 2. 745 0.9 Necessary......... -.d a eeted. ca. 900 Quite rare." Affected. Fairly noticeable Fairly easy Great ca. 800 Never Little aileotednl" Very small- Mixture Nb-l-CsrCrO4 to a certain extent. However, such process does not afford uniform mixing, nor does use of coarse powders assist in the reduction of the surface area of zirconium taking part in reaction. Accordingly, if it is desired to generate a 5 prescribed amount of alkali metal, it is necessary to use a large amount of the aforesaid mixture. On the other hand, wet mixing of zirconium and alkali metal salts can indeed prevent the occurrence of an explosion. But the alkali metal salt is dissolved in water to become ueseless. Even where alcohol is used in place of water, minute amounts of water contained in the alcohol denature the alkali metal salt and the segregation of said salt in the subsequent drying process reduces the uniformity of said mixture, thus failing to form a satisfactory photosensitive layer.

The prior art alkali metal generator is generally preheated to minimize the evolution, during reaction, of unnecessary gases from the alkali metal salts, reducing agents and metal vessel used. However, said preheating has the drawback that there are evolved during reaction different amounts of such unnecessary gases from a mixture of an alkali metal salt and silicon or zirconium, depending on the condition in which said preheating is conducted.

Heretofore, there has been further proposed a different type of an alkali metal generator capable of generating appreciable amounts of alkali metal at relatively low temperature, wherein there are added powders of tunsten and boron to a mixture of alkali metal salts and reducing agents. Even this generator, however, presents difiiculties in controlling reaction velocity.

The different properties of the prior art alkali metal generating mixture and the mixture of the present inven tion which typically consists of alkali metal salts and niobium are presented in FIGS. 1A and 1B and Table 1 below. The ordinate of the figures represents the amount of alkali metal generated (the ordinate of FIG. 1A denotes cesium and that of FIG. 1B potassium) and the abscissa indicates heating temperature C.).

Table 1 does not include an alkali metal gencrating agent containing aluminium as a reducing agent. Such generating agent resembles those containing silicon and zirconium in respect of property, but evolves tremendous amounts of gas during reaction, failing to provide a good photosensitive layer. 'On the other, use of scale-like pieces of aluminium can indeed suppress gas evolution to a certain extent, but there is required an extra process of working aluminium itself into such scale-like pieces.

There will now be described the present invention in greater detail. It is preferred that powders of alkali metal salts and those of niobium filled into a vessel made of electric conductive metal from which alkali metal is released be mixed in the ratio by weight ranging from 1:0.1 to 8. The reason is that in the ratio of lzless than 0.1, the content of niobium is too small to display its expected effect, whereas in the ratio of 1:lnore than 8, there likely takes place a rapid exothermic reaction. Further speaking of the effect caused by the content of niobium.

(A) Where the alkali metal salt consists of potassium chromate (K CrO it is advisable to mix it with niobium in the ratio by weight of about 1:0.1 to 8 (or in the mol ratio of about 1:0.23 to 18.4), preferably in the ratio by weight of about 1:0.3 to 4 (or in the mol ratio of about 1:0.69 to 9.2). The mol number of alkali metal represents that of salts thereof. Hereinafter, the mol number of alkali salts is expressed by drawing a bar above symbols representing alkali elements, as l\ a, K "Cs. The mol number of for example, Na CrO or K WO is denoted as fia or Ti. This is derived from the fact that where there was manufactured a photoelectric tube using an alkali metal generating agent whose components were mixed in the forming ratio, the yield of alkali metal accounted for about 90 to 100 percent and in the case of the latter mixing ratio approximately 100 percent.

(B) Where the alkali metal salt consists of cesium chromate (Cs CrO it is desired that said salt be mixed 52 1.8 Preierred. Present 7 2.1 Unnecessary 5 0.9 Necessary do 4.44 1.2 Preferred Present. 11.95 3.1 Already contained" None 8.75 2.1 do Present 2:1 60 Stable 2:1 Very stable Easy l.do.

2:1 Unstable ca. 900 Sometimes Nogeeibiy Prominent... Difficult... Sma1l-- a co e 2:1 Stable ca. 900 Quite rare Atteered Fairly noticeable" Fairly easy. Great..

2:1 .do ca. 900 Sometimes do Prominent d0.- Sma1l..

2:1 do ca. 900 Quite rare do do, ...d0.- Great.

with niobium in the ratio by weight of about 1:03 to (or in the mol ratio of about 1:1.21 to 40.49), preferably in the ratio by weight of about 1:05 to 3 (or in the mol ratio of about 1:2.02 to 12.12).

(C) In case sodium chromate is used as an alkali metal salt, the advisable ratio by weight of said salt to niobium is about 1:01 to 10 (or about 1:0.19 to 19 in the mol ratio).

There was formed a photosensitive layer from the aforesaid alkali metal salts, i.e., potassium chromate, cesium chromate and sodium chromate mixed with niobium in varying proportions, the yield of alkali metal being presented in FIGS. 2A, 2B and 2C respectively. Throughout these figures, the ordinate represents the percentage yield of alkali metal or reproducibility (number of good products/num'ber of tests X100) and the abscissa denotes the ratio by weight of niobium to the aforementioned alkali metal salts.

As described above and indicated'in Table l, the alkali metal generator of the presentinvention allows alkali metal to be released at a temperature about 100C. lower than required for the prior art, reduces the evolution of unnecessary gas, eliminates the necessity of -.using any special buffer agent and easily controls reaction velocity because reaction does not occur rapidly. Moreover, the generated state of alkali metal does not Widely vary with changes in the conditions in which preheating is conducted prior to application of heat for reaction, and on this account, too, reaction velocity can be easily controlled. The fact that the photosensitive'layer prepared from the alkali metal generating agent of the present invention is elevated about 70 percent in photosensitivity over the prior art layer evidently proves that said agent is prominently excellent with respect to the aforementioned requirements.

The alkali metal generator of the present invention further offers the undermentioned favourable effect. The alkali metal generating agent of the invention generally has a greater bulk density than the prior art agent (for example, where the alkali-metalsalt consists of cesium chromate and there is added niobium thereto as claimed in the present invention, the entire mass has a bulk density of 2.4 g./cm. whereas the prior art agent containing silicon amounts to 0.9 g./cm. in bulk density and that containing zirconium to 1.8 g./cm. This means that the alkali metal generating agent of the present invention has a bulk density about 25 timesthat of the prior art siliconbearing agent and about 1.3 times that of the prior art zirconium-containing agent). Accordingly, the alkali metal generating agent of the present invention can be reduced in bulk, permittinga vessel of electric conductive metal to be made compact. Also the fact that relatively small amounts of said alkali metal generating agent are used and reaction can proceed at a relatively low temperature naturally decreases the evolution of unnecessary gas during the exothermic reaction. Furthermore, the present alkali metal generating agent enables alkali metal to be released ina prominently elevated yield. Though the reason is not fully understood, it is assumed that the intermediate product probably formed by reaction between niobium and the alkali metal generating agent islessliable to obstruct the release of alkali metal than the intermediate product supposed to occur during the reaction of the prior art alkali metal generating mixture.

For illustration of other. combinations of alkali metal potassium tungstate (K WOQ and. niobium. mixed in different ratios as given below.

In any of the above ratios, the present invention displayed its claimed effect of easily controlling reaction velocity and realizing the initial generation of alkali metal at a temperature of about 700 C.

A further combination of alkali metal salts and niobium wherein there were mixed sodium molybdenate and niobium in the mol ratio Fa/Nb of 2 to 0.07 exhibited the same effect as described above.

In addition to the aforementioned kinds of alkali metal salts involved in the alkali metal generating agents according to the present invention, there may be used others alone or in combination. Said other salts can still display the same effect as mentioned above. There will now be described the concrete examples where there were used said other salts in combination.

(D) There is used a mixture of sodium and potassium salts as a source of alkali metal. The respective salts are mixed in such a manner that the mol ratio of the total amount of sodium and potassium salts constituting said source to the niobium is about 1:02 to 50 [(Na-HK/Nb=0.02 to 5)] It is also experimentally disclosed that when the mol ratio of sodium and potassium is Na/F=O.1 to 10 the resultant photosensitive layer has good photosensitivity.

(E) There are mixed salts of cesium and potassium to form a source of alkali metal in such a manner that the mol ratio of the total amount of cesium and potassium salts constituting said source to the niobium is about 1:02 to [(6s+K)/Nb=0.0l to 5)]. In this case, the cesium and potassium mixed in the mol ratio of 110.5 to 10' (Es/K=0.l to 20) aifords good results.

(F) There are mixed salts of cesium and sodium as a source of alkali metal in such a manner that the mol ratio of the total amount of cesium and sodium salts constituting said source to the niobium is about 1:0.125 to 100 Cs+ fia)/Nb=0.01 to 8)]. In this case, too, the

mixing of cesium and sodium in the mol ratio of 1:0.03

to 10 (Es/fia=0.l to 30) displays a .good effect.

The aforementioned metal salts used as a source of alkali metal include, for example, chromates, bichromates, tungstates and molybdenates.

The present invention will be more fully understood fromthe examples which follow:

' EXAMPLES A, B AND C There were mixed powders of cesium chromate, potassium chromate and sodium chromate respectively with powders of niobium in the mol ratio of 1:2 to form an alkali metal generating agent. The properties of these mixtures are presented in FIGS. 1A and 1B, FIGS. 2A and 2B, and Tables 1 and 2.

.agent 1..

For comparison with the aforementioned generating agent of the present invention, there was prepared a reference alkali metal generating agent by adding silicon and zirconium respectively in amounts equal to the aforesaid mol ratio of niobium to the above-described mixed powders of sodium chromate and potassium chromate.

The alkali metal generating agents of the present invention and prior art were respectively charged into a perforated vessel made of electric conductive metal to form a photosensitive layer or surface. With respect to the above-mentioned three kinds of mixtures including the alkali metal generating agent of the present invention containing niobium and. the reference generating agents containing silicon and zirconium respectively, there was EXAMPLE E investigated the generated state of sodium and potassium There were mixed powders of cesium chromateand corresponding to the temperature at which said three potassium chromate in such a manner that the cesium and generating agents where heated, the results being given in 1 FIGS. 3A and 3B. There are further presented in Table 2 Potassmm contained therem had 3 mol of CS/K=4- below comparative data on said three mixtures, the con- To said mixture j g added Powders of niobium dition of a layer during formation and its properties after in the mol ratio of (Cs-l-K)/Nb=0.1 to form an alkali formation. metal generating agent according to the present invention.

TABLE 2 Si-l-NazCrOr Zr+Na CrO4 Alkali metal generating agent Nb-I-Na,CrO4+KzCrOz +K2CIO +K1Cl04 Mol radio of components {Na/K=1 Na/K=1 NalK=1 (Na+K)/Nb=0.2 (Na+'K)/Zr (Na+K)/Zr Photosensitivity uA/lm. (nznumber of sample)... 100 (n=5) 70 ('n=5) 80 (11:5). Generated state Very stable Unstable Stable. Temperature of generation, C About 900..... About 900.

Puncture or breakage by melting Never Sometimes. Quite rare. Generated state affected by varying preheating Little afieeted Noticeably Affected.

conditions. afiected. Y Gas evolution before generation of alkali metal... Very small Prominent. Fairly noticeable.

Control of generated state Easy Ditficult Fairly easy. Getter action of reductant- Small.. Great Bulk density 0.9 1.8. Addition of butter agent Necessary. Preferred. Possibility of explosion.... None.-- Present. Yield of alkali metal High Low Medium.

When there was added to the alkali metal generating agent of the present invention used in this example about percent by weight of tungsten or aluminium oxide on the basis of the total amount of said agent, there was better controlled the generation of alkali metal or reaction velocity.

There was further investigated the temperature of generation using the same materials (mixture of sodium and Next for comparison, there were prepared reference alkali 40 metal generating agents corresponding to the prior art agent, using silicon and zirconium respectively as a reducing agent in place of niobium with all the components mixed in the same mol ratio as is used in the present invention. There was investigated the amount of cesium and potassium generated from said three kinds of generating potassium) as used in this Example D with the mol ratio agents contammg f slhcon i mqmum E of niobium to said mixture varied. The results are pre- Elvely, the data on Slum potasswm g Present sented in FIG. 4 wherein the ordinate denotes the tem- 5A d 5B respectlvely- There'were also tested perature C. required for the generation of alkali metal K0 the P P of photosensitiv? layer P p and the abscissa represents the mol ratio of niobium to 0 these three kinds of generating agents, the results being said mixture. given in Table 3 below.

TABLE 3 Si Zr H +CsgCrOr +Cs,CrOr Alkali metal generating agent Nb+CSzCIOL+K1Cf04 +K CrO4 +KaQrO| M01 ratio of components {CS/Kai CSIK=4 Cs/K=4 (Cs-kK)/Nb=0.1 (Cs+K)/Si= (Cs-i-K)/Z r= 0.1 p 0.1 Photosensitivity uA/lm. (nznumber oisample).... 100(n=5) (n =5) (n=5). Generated state Very stable...-.. Unstable Stable.

Temperature of generation, O About 780... About 900 About 900. Puncture or breakage by melting Never Sometimes- Quite rare. Generated state afleeted by varying preheating Little affected Notieeably Atieeted.

conditions. afiected. Gas evolution before generation of alkali metal.... Very small Prominent... Fairly y, noticeable.

Control of generated state Easy Dilfieult- Fairly easy. Getter action of reduetant Bulk density Addition of buffer agent Necessary Preferred. Possibility of explosion None None..... Present. Yield of alkali metal High Low Medium.

There is further indicated in FIG. 6 the temperature alkali metal salt to the aforesaid alkali metal salts used required for generation of cesium and potassium with rein combination. There will now be described a concrete spect to the mixing ratio of Nb to E s-PK. In this figure, ample Of this case.

the ordinate denotes the temperature required for gen- There were mixed Powders f cesium eliminate eration of said alkali metals and the abscissa represents 2 4), Potassium 'omate (K CrO and sodium h l i of Nb to 6 +K chromate (Na CrO in such a manner that 'Cs, Ti and EXAMPLE F l\ a are contained in the mol ratio of 1: 1:1. To said mixture was further added niobium in the mol ratio of There were mlxed powders of cesium chromate (Cs Cr0 and sodium chromate (Na CrO in the mol ratio of 6s/fia=3. To this mixture was added niobium in to form. an alkali metal generating agent according to the the mol ratio of (Es,+'fia)/ Nb=1 to form an alkali metal present invention. There were comparatively studied the generating agent according to the present invention. There properties of said agent consisting of only one alkali metal were also prepared prior art generating agents containing and niobium, namely, Nb=+K W0 or Nb+ NaMoO the silicon and zirconium respectively in the same manner as results being presented in Table 5 below.

TABLE 6 N b+CSzCrO4 Alkali metal generating agent Nb+K WO4 Nb+NaMoO4 +K2C1'O4-i-N8aCIO4 USzKzNa=h1z1 Mol ratio of components K/Nb0.2 Na/Nb-02 {(6s+K+Na)/Nb=0'2 9.0

Generated state Very stable-.- Stable Stable.

Temperature of generation, C

Puncture or breakage by melting Never Never Never.

Generated state afieeted by varying preheat- Little afiected Little affected- Slightly aflected.

ing condition. Gas evolution before generation of alkali metaL- Small Small Small. Control of generated state Very easy- Easy Easy. Addition of butter agent Unnecessary" Unnecessary Unnecessary.

Possibility of explosion None None None.

Yield of alkali metal-.. High.

Bulk density 2.4 2.4 2.4.

in Example B. Data of comparative study on the alkali As mentioned above, the present invention is characmetal generating agents of the present invention and prior terized in that there is mixed niobium with alkali metal art are given in FIGS. 7A and 7B and Table 4 below salts. However, there may be further added to said mixformed in the same arrangement as used in FIGS. 5A and ture a material, for example, zirconium used as a reduc- 5B and Table 3 respectively. The temperature of generaing agent and/ or gettering material in the prior art. There tion asociated with FIGS. 7A and 7B is presented in FIG. are presented in Table 6 below the properties of a zirco- 8 formed in the same arrangement as in FIG. 6. nium containing alkali metal generating agent in com TABLE 4 s1 2 +CsCr04 +CSzCIO4 Alkali metal generating agent Nb-i-CSgClO4-l-N3.1CIO4 +N82C1Ol +N8QOIOL Mol ratio oi components [Ci/Naps Cs/Nzpg CSINa=3 "l(os+ya /Nb=1 (os+Na) (Cs-l-N'a)! Si=l Zr=1 Photosensitivity A/Im. (nznumber of sample).... 100 (n=6) ('n=5) 80 (n=5). Generated state Very stable Unstable Stable. Temperature of generation, C About 780 About 900 About 900. Puncture or breakage by melting--- Nev Sometimes- Quite rare. Generated state afiected by varying preheating Little afiected Noticeably Afiected.

conditions. affected. Gas evolution before generation of alkali metal.... Very small Prominent... Fairly noticeable.

Control of generated state Easy Diflicult Fairly easy. Getter action of reductant- Bulk density Addition of butter agent Possibility of explosion- To the present invention may be applicable a different parison with those of other agents of the present invenalkali metal generating agent prepared by adding another tion.

There will now be described by reference to FIG. 9 the case where there is actually prepared a photosensitive layer for a special image pickup tube. Numeral 10 is a vessel made of electric conductive metal. The vessel isprepared by rolling, for example, a thin metal sheet once crosswise into a cylindrical body, and spot welding the longitudinal edge portions at a prescribed interval to form a slight gap 12 between the welds 11. The vessel is filled with an alkali metal generating agent according to 10 the present invention. After the agnt is charged, the wal of each end of the vessel is pressed airtight. I

There will now be described the case where there was formed the antimony-cesium photosensitive layer of a head-on type photoelectric tube, using an alkali metal 15 generator two alkali metal generators filled with a generating agent consisting of a mixture of powdered cesium chromate and powdered niobium and a generating agent consisting of a mixture of powdered potassium chromate and powdered niobium respectively. Each mixture weighed 70 mg. and the mol ratio of the alkali metal salt to niobium was chosen to be 1:2. To the curved lateral inner surface of a glass bulb having a face plate at the front part were fitted each alkali metal generator and a known antimony generator by so bending them as to correspond to the curvature of said lateral inner surface of the glass bulb. To both ends of each of these fitted generators are connected lead wires drawn to the outside. The preparatory step for forming the aforesaid layer ended by disposing secondary electron multiplying electrodes known to this particular field and an anode at prescribed locations respectively. First, the glass bulb was fully evacuated and then there was introduced current through said antimony generator containing an antimony generating agent. Said agent was heated to a temperature of 520 to 570 C. to release antimony so as to deposit a thin film thereof on the inner surface of said face plate to a prescribed thickness. Thereafter, the two alkali metal generators of the present invention were preheated 5 minutes at a temperature of 600 C. by introducing current therethrough. The generators were further heated to 900 C. to release potassium and cesium. Thus, there was deposited a thin cesium-potassium film on said antimony substrat to form a photosensitive layer.

The photosensitive layer prepared as described above had a photosensitivity of 100 ,ua./lm., showing that it had excellent properties.

There will now be described the formation of a multialkali photosensitive layer used in an X-ray fluorescent multiplying tube. There were prepared three mixtures comprising powders of cesium chromate, sodium chromate and potassium chromate each blended with powders of niobium in the mol ratio of 1:2 to form alkali metal generating agents. The mixtures cotaining cesium and sodium each weighing 250 mg. and the mixture containing potassium weighing 200 mg. were separately placed in the aforementioned metal vessels to provide alkali metal generators. Each of these generators was put in a known bulb having a fluorescent face formed at one end and a plate of insulating material disposed opposite to said 0 fluorescent face at a close interval. The generators were preheated 5 to 10 minutes at a temperature of 350 C. Then they were heated to 900 C. to release each alkali metal so as to form a multialkali photosensitive layer on said insulation plate. The resultant photosensitive layers 5 had an excellent phot-osensitivity of 100 ,ua./lm. like that of the preceding case.

What we claim is:

1. An alkali metal generating composition comprising a mixture of an alkali metal salt and niobium in -a ratio by weight of salt to niobium of from about 1:0.1 to about 2. A composition according to claim 1 wherein the alkali metal salt is selected from the group consisting of potassium tungstate, potassium chromate, cesium chromate, sodium chromate and sodium molybdate.

3. A composition according to claim 2 wherein the alkali metal salt is potassium chromate, and wherein the weight ratio of said chromate to niobium is 120.1 to 8.

4. A composition according to claim 2 wherein the alkali metal salt is cesium chromate, and wherein the weight ratio of said chromate to niobium is 1:0.3 to 10.

5. A composition according to claim 2 wherein the alkali metal salt is sodium chromate, and wherein the weight ratio of said chromate to niobium is 120.1 to 10.

6. A composition according to claim 1 wherein said alkali metal salt is cesium chromate and which further contains silicon and wherein the mol ratio of Cs/Nb-i-Si is about 0.2.

7. A composition according to claim 1 wherein said alkali metal salt is potassium chromate and which further contains zirconium and wherein the mol ratio of K/Nb+Zr is about 0.2.

8. A composition according to claim 1 wherein said alkali metal salt is cesium chromate and which further contains silicon, zirconium and tungsten and wherein the mol ratio of Cs/Nb+Si+;Zr+W is about 0.2.

9. A composition according to claim 1 wherein said alkali metal salt is a mixture of cesium chromate, potassium chromate and sodium chromate and, wherein the molar ratio of Cs:K:Na is about 1:1:1.

10. A composition according to claim 1 wherein the alkali metal salt is selected from the group consisting of a first mixture of sodium salts and potassium salts, a second mixture of cesium salts and potassium salts and a third mixture of cesium salts and sodium salts.

11. A composition according to claim 10 wherein the mol ratio of the total amount of sodium and potassium salts of said first mixture to niobium is from 1:02 to .50.

12. A composition according to claim 11 wherein the mol ratio of sodium salt to potassium salt of said first mixture is from 0.1 to 10.

13. A composition according to claim 10 wherein the mol ratio of the total amount of cesium salt and potassium salt of said second mixture to niobium is from 1:0.2 to 100.

14. A composition according to claim 13 wherein the mol ratio of cesium salt to potassium salt of said second mixture is from 0.1 to 20.

15. A composition according to claim 10 wherein the mol ratio of the total amount of cesium salt and sodium salt of said third mixture to niobium is from 1:0.125 to 10.

16. A composition according to claim 10 wherein the mol ratio of cesium salt to sodium salt of said third mixture is from 0.1 to 30.

References Cited UNITED STATES PATENTS 2,154,131 4/1939 Ledever 252181.4 X 2,173,258 9/1939 Ledever 252181.4 1,835,118 12/1931 Marden 252--18l.4X 3,096,211 7/1963 Davis 2S2-l8l.4 X 3,468,807 9/1969 Spangenberg 252-181.4

TOBIAS E. LEVOW, Primary Examiner J. COOPER, Assistant Examiner UNITED STATES PATENT CETIHCATE OF ECTIN Patent N 3 658, 713 D d April 25 1972 Inventor s YORIKATSU IRISAKA et al It. is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

column 5, line 63 after "metal" insert "salts and niobium, there were prepared four samples from" Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWAED MQFLETCPEEJRG ROBERT GQTTSCHALK ttssting Gfficer Commissinner of Patents ag UNITED STATES PATENT OFFICE v a CERTIFICATE OF CORRECTION Patent 3,658,713 Dated April' 125, 1972 IHVQHtQI-(S) YoRIKATsU IRISAKA et al It. is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 63 after "metal" insert "salts and niobium, there were prepared four samples from".

Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWARD PLFLETCEFJRJR. Y RQBERT GOITSCHALK Attesting Gfficer Commissioner of Patents