US3388955A - Process for producing within electron tubes,in particular television picture tubes,a thin metallic film capable of sorbing their residual gases - Google Patents

Description

June 18. 1968 P. DELLA PORTA ETAL 3,388,955

PROCESS FOR PRODUCING WITHIN ELECTRON TUBES, IN PARTICULAR TELEVISION PICTURE TUBES, A THIN METALLIC FILM CAPABLE OF SORBING THEIR RESIDUAL GASES Original Filed Aug. 24, 1965 2 Sheets-Sheet 1 INVENTORS PAOLO DELLA PORTA TIZIANO A. moRm EL-lO RABUSIN M, EM

TTORNEYS June 1 IN PARTICULAR TELEVISION PICTURE TUBES A THIN METALLIC FILM Original Filed Aug. 24, 1965 CAPABLE OF SORBING THEIR RESIDUAL GASES 2 Sheets-Sheet 2 V(cm sec") 2 F Qt (cm-K to") 80% fi 7 1 50% I E I i j l 40% [A I I 20% I i I I 5 5', 5' 12 16 T} 20 +2 24 (st-:c.)

INVENTORS PAOLO DELLA DRTA TILIANO A. GIIORGI ELIO RABUSIN United States Patent 3,388,955 PROCESS FOR PRODUCING WITHIN ELECTRON TUBES, IN PARTICULAR TELEVISION PICTURE TUBES, A THIN METALLIC FILM CAPABLE OF SORBING THEIR RESIDUAL GASES Paolo Della Porta, Milan, Tiziano A. Giorgi, Rho, Milan, and Elio Rabnsin, Milan, Italy, assignors to S.A.E.S Getters S.p.A., Milan, Italy, a company of Italy Original application Aug. 24, 1965, Ser. No. 482,104. Divided and this application Aug. 10, 1967, Ser. No. 664,593 7 Claims priority, application Italy, Feb. 25, 1965, I

1,708/65, Patent 742,042 9 Claims. (Cl. 316--25) ABSTRACT OF THE DISCLOSURE A process of producing within a picture tube, having walls and a screen, a thin getter metal film for sorbing the residual gases, comprising evaporating the getter metal in the picture tube in the presence of a gas present in an amount such that the mean free path of the getter metal atoms in the gas is equal to or less than the distance between the metal evaporating source and the tube screen.

This application is a division of application Ser. No. 482,104, filed Aug. 24, 196 5.

This invention relates to a process for producing within electron tubes, and in particular television picture tubes, a thin metallic film capable of sorbing their residual gases and characterized by the fact that its distribution is substantially limited to the desired surfaces of the tube and also by the fact that the film on the screen of the television picture tube is of reduced thickness, when compared to other conventionally produced films, and so presents less impedance to the flow of electrons directed at the phosphors on the screen of the tube.

The present invention also relates to the device to be employed to accomplish the above mentioned process.

It is well known that, in order to maintain the required degree of vacuum in, for example, television picture tubes, as well as in many other types of electron tubes, the generally used technique is to employ an internal chemical pump or getter. The getter consists of a thin metallic film deposited on appropriate surfaces of the device after it has been processed and isolated from the conventional pumps. This thin metallic film is usually barium and is commonly deposited from a getter contained which may be heated by externally induced radio frequency currents. The getter container consists of a non-magnetic stainless steel ring of U-shaped section in which is usually compressed in equal proportions a mixture of powdered 50% barium-50% aluminum alloy and nickel. On heating, as previously mentioned by externally induced radio frequency currents, an exothermic reaction occurs between the barium-aluminium alloy and the nickel when the temperature is about 800 C. causing an instantaneous and spontaneous increase in temperature up to above 1300 C. with the consequent evaporation of 30 to 40% of the barium within the container. Since, however, the radio frequency power is still applied to the getter container the remaining barium also evaporates but at a much decreased rate. Another type of getter is also known where no such reaction occurs due to the absence of the nickel and as a consequence this type of getter is called endothermic. Its main disadvantage is the irreproducibility of yield and as a consequence it is rarely empolyed in manufacturing processes. The barium film obtained from either of these two processes is very active chemically, it reacts with the residual gases present in the tube and efiiciently removes them from the gaseous phase. Furthermore, the action of the barium film is not instantaneous or short lived, but by means of diffusion phenomena it is capable of continuing itsbgettering or pumping action throughout the life of the tu e.

It is well known that the rate of reaction of the residual gases in the tube with the barium film increases proportionally with the latters increase in surface area. It is for this reason that in electron tubes, and in particular in television picture tubes, the greatest possible internal surface area is utilized for film deposition. This naturally results in the complete coverage of the screen of the television tube with barium. In fact due to the position of the getter container, which is mounted on the top of the electron gun which is in turn situated on the normal to the centre of the screen, and as a result of the well known distribution laws of evaporation, a substantial fraction of the barium evaporated is deposited on the screen of the television picture tube. Its distribution is circularly symmetrical and is a maximum at the centre of the screen and a minimum on its periphery.

The image on the screen of a television picture tube is due to the high energy electrons which impinge and excite suitable phosphors, evenly distributed on the internal surface of the screen. To increase the luminescence of these phosphors in the forward direction it is standard practice to cover them, on the side from which the electrons are incident on them, by a thin layer of aluminium which has also other electrical functions connected with the potential distribution within the tube, the protection of the phosphors from ion burn, etc. The presence of this aluminium film causes a loss in energy of the electrons which are directed at the phosphors and so decrease their luminosity. The same effect is also caused by the barium present on the screen and although its thickness is much less than that of the aluminium its effects are more pronounced due to its higher molecular mass. As previously mentioned the thickness of the barium film is greatest at the centre of the screen and it is here that electrons are decelera-ted to a greater extent. This sometimes causes the characteristic darker central portion in the image on the screen and can only be avoided by using higher electron accelerating potentials in the design of the set with consequent increase in production costs.

The advantages which would result in television tube production and usage, and in similar electron devices, if the barium film on the screen could be rendered more uniform and thinner are obvious. In fact if such a solution were possible a very appreciable economic saving would result in the production of television sets and similar devices. If such a saving were not required advantage could be taken from the reduced barium film thickness to improve the quality of the tube. Some cases exist where tubes are produced without aluminium backing of the phosphors and in these cases no barium must be deposited on the phosphors since they would be damaged.

Hence in all the above mentioned cases the use of a suitable device and technique for efficiently controlling the barium deposition on the screen of the tube would be extremely useful for the industry as a whole.

The methods employed heretofore for controlling the barium film thickness on the screen of television picture tubes, and similar electron devices, have relied principally in mechanically directing the issuing barium vapour by means of various types of deflecting bafiies fashioned on the getter container. In one particular embodiment of this procedure the baffies are such as to direct the barium vapours towards the centre of the bulb, i.e., on the normal to the centre of the screen of the tube. The coverage being so adjusted as to cause collisions between the barium atoms before they reach the screen and thus cause a virtual point source of barium atoms from which they may evaporate in all directions. In still another embodiment using mechanical direction of the barium vapours the container of the getter has battles which direct the evaporating barium atoms in a direction perpendicular to that of the normal to the centre of the screen. Various other solutions between these two extremes have also been suggested and adopted. However, their major drawback has been their low efficiency or if efficient the fact that they limit to a very great extent the surface area of the film which they produce and thus render it ineffective for the purposes required. The only virtue of some of these methods of mechanically directing the barium evaporation, however inefiiciently, has been to reduce the quantity of barium refiected towards the electron gun. In fact barium evaporated in this direction can be extremely troublesome since it can give rise to secondary emission effects, short circuits, etc.

Thus, the principal object of the present invention is to provide a means of reducing the quantity of barium reaching the screen of a television picture tube or similar device, if required even to zero, and to redistribute the excess barium on other internal surfaces which have up till now been less utilized, for example the cone area. According to the present invention attempts are not made, as occurs with known conventional methods, to direct the barium atoms by means of bafiles, but steps are taken to prevent large amounts, or even all of the barium, from reaching the screen of television picture tubes or similar devices byintroducing a deviating or retarding mass in its way. As will become evident subsequently the invention relates essentially to exothermic type getters.

The process, according to the invention, consists in providing a getter device which in spite of its inherent high rate of evaporation, and just because of this characteristic, provides within the volume in which it is evaporated a suitable deviating and retarding mass which avoids the otherwise pre-dominant deposition of getter material in the forward direction.

Thus it is a characteristic of the invention that the evaporation of the getter occurs in the presence of a suitable gas having an adequate molecular mass and present at a predetermined pressure so that the mean free path of the barium atoms in it is smaller than the distance between getter container and screen. It is also a characteristic of the invention that the gas used is repumped by the actual barium film, without however, consuming but a very minute fraction of the films gettering capacity, in a very short interval of time. In fact it may be stated that the last barium to leave the getter container does not find in its way any gas and not being hindered can be deposited according to the known distribution laws. However, if no barium is required on the screen the heating of the getter container can be stopped immediately after having initiated the exothermic reaction. Further characteristics of the present invention are that the gas employed be nitrogen, introduced into the tube in the form of a stable compound, which will dissociate only at temperatures immediately below the onset of the exothermic reaction and that the gas be present in the tube before the onset of such exothermic reaction.

Processes are already known whereby continuous evaporation of materials in an inert gas atmosphere is utilized for the production of powders characterized by very fine particle size. However, it must be realized that in television picture tubes and similar electron devices such fine particles must be avoided at all costs and that, therefore conditions leading to such phenomena must be avoided.

The novelty of the invention rests in the fact that, a suitable choice of gas and pressure enables the control of up to 50% of the barium which may be obtained from an exothermic type getter without producing any fine particles and without leaving a high pressure in the tube at the end of the evaporation process. Thus this fraction of the barium is deposited only in the cone and neck zones of the television picture tube, whilst the rest of the barium is evenly distributed on these and the remaining internal surface areas of the tube. Should no barium be required on the screen this result can also be obtained by interrupting the heating of the getter container once being initiated the exothermic reaction In addition to rendering possible a fine control of the distribution if the barium film, the use of getters according to the concepts of the present invention produces barium films, as already known in the art, characterized by a high porosity and so increases appreciably the real surface of the film produced. Practical tests have shown that this increase in specific surface area involves an increase of sorption characteristics by a factor of 2 to 3. This fact presents obvious advantages from the point of view of tube life or from an economic stand point. Such porous films are a result of submicroscopic globule formation in the vapour phase which condense as such on the walls of the vessel.

To put the inventive process into practice any gas may at first sight appear suitable. However, the following points should be borne in mind when trying to decide on the nature of the gas to be employed. Its retarding and defleeting action increases with:

(a) increase in molecular mass,

(b) increase in pressure,

(c) decrease in chemical affinity to barium.

Another point to observe as far as regards (b) is that the pressure used should not be too high since it could cause a serious decrease in gettering capacity of the getter film.

On the basis of the above considerations the rare gases such as argon, krypton, etc. would seem very attractive. However, these gases are not sorbed by the getter material and so if the getter film is deposited, as occurs usually, in the closed tube the rare gases would remain in it rendering its functioning impossible. Therefore such gases may not be utilized for the purposes of the present invention. Other gases such as hydrogen, carbon monoxide, carbon dioxide and oxygen may be considered. However, such gases would result extremely detrimental to tube characteristics as such and also because they tend to produce hydrocarbons, water vapour, etc. which are harmful to cathode activity. Not only, but all these gases would be contrary to one or more of the three points previously indicated. In fact for hydrogen a high pressure would be necessary due to its particularly low mass. For the other three gases a high pressure would be necessary to cope with the extremely high reaction rates which they present with pure barium films.

The gas which to the ends of the present invention has resulted as the most suitable is nitrogen. In fact this gas does not damage the cathode, does not produce undesirable side products, it has a relatively high mass, it is not exceedingly reactive with barium, although it is easily sorbed by it, and as a consequence the pressure necessary to obtain the desired effect is rather low. The nitrogen pressure which has proven to be most satisfactory for the scope of the present invention is of the order of between 5 X 10 and 1X10 torr. Pressures above 5x10- torr using up too great a fraction of the barium film and pressures below 10 torr not being sufiicient to the scope of the present invention.

The introduction of the selected gas into the tube may occur in a number of ways. One of these could be to introduce the required pressure in the tube whilst still on the pump and just before tip-01f. However, such a method would not be technically economical. Hence, we propose to introduce the gas into the tube by utilizing a compound of nitrogen which will be dissociated by heating prior to evaporation of the barium. Such a compound could be in powdered form and be mixed in the correct proportions with the getter alloy in the container, it could also be mounted on a separate support removed from the getter, and it may even be that the actual getter container has been subjected to a nitrogenating action giving rise to suitable compounds. However, the compound employed must be stable to all possible aging and pretreatments such as deionized water wash, drying and vacuum heat treatments up to 400 C. The compound used should, if mounted in the getter container, dissociate at temperatures below the one at which the exothermic reaction sets in. The use of barium azide, for example has been long known, with other finalities of those explained in the present invention, however, barium azide would not be usable since its extreme instability to aging under normal atmosphere conditions or to heating under vacuum are well known.

Nonlimiting examples of compounds suitable to this end in the present invention are those obtained from nitrogen and any of the following metals or alloys thereof: nickel, iron, molybdenum, manganese, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, tungsten, cobalt, silicon and stainless steel.

'As previously mentioned the nitrogen bearing compound may be mixed with the exothermic charge in the getter container, it may be physically separated from it and thus the gas it contains may be evolved a priori before heating the getter container or the getter container itself may be nitrogenated. Theprinciple of the invention, although essentially for exothermic getters, cannot be excluded from use in endothermic getters. However, since in this case the evaporation is initially low and. then picks up speed the quantity of gas must be initially low and must slowly increase as the rate of'evaporation rises. In the case of exothermic getters since, as previously mentioned, 30 to 40% of the barium is emitted simultaneously and at the same time, the nitrogen pressure present has full effect on this quantity of barium. The remaining barium which evaporates at a slower rate is less influenced due to the sorption of nitrogen which has taken place. Nevertheless an appreciable influence is also exercized on this second quantity of barium.

The exothermic getter, containing the nitrogenous compound most suitable, and having any desired or suitable deflecting baffles arrangement can be mounted as the normal and more conventional getter on the electron gun of the tube or in any required or desired position. The usefulness of the bafiles is, however, very limited in the getters object of the present invention as far as regards the barium film distribution on the screen and on the cone of the television picture tube. Their cation is still useful as far as regards back evaporation of barium towards the gun of the tube.

A practical illustrative and nonlimlting example of the invention now follows.

An 11 inch television picture tube having a flare angle of 110 is used in conjunction with a conventional getter mounted 1% from the yoke reference line (Y.R.L.) consisting of a powdered mixture of equal proportion of 50% Ba-50% alloy and nickel supported in a stainless steel container of ring shape. The getter is evaporated under good vacuum conditions producing the distribution of barium in the tube indicated in FIG. 1 which shows schematically a section of the tube together with two diagrams (a) and (b). These diagrams refer to the barium thickness on the screen (a) and cone (b) sections of the tube. In each diagram the dashed lines, indicated by 1 and 3, refer to the above mentioned type of getter. To be noted that the thickness of the film at the centre of the screen is 1400 Angstrom units. In a similar tube was then mounted a getter of exactly similar characteristic as above but containing also a predetermined quantity of powdered iron nitride (Fe N) such as to produce in the tube a pressure of about 1 10- torr of nitrogen during the normal flashing of the getter and prior to the onset of the barium evaporation due to the exothermic reaction. In this case the barium film distribution in the tube is shown by the continuous lines in the diagrams (a) and (b) of FIG. 1 indicated by 2 and 4. It will be noted that the 6 thickness of the barium film at the centre of the screen is less than 400 Angstrom units in this case.

The relative quantities of barium on the screen, cone and neck of the tube with conventional getters are respectively 7, 0, 1, 8, 16, 2 mg. and with the getters object of the present invention they are respectively 2, 5, 3, 5 and 19 mg. Such significant reduction of barium on the screen is very useful for electron transparence and results in an increased light output from the tube of up to 30%.

The fact that evaporation of the barium film in a nitrogen atmosphere in no way reduce the characteristics of the barium film produced is illustrated in FIG. 2. In this figure on the ordinates are reported the gettering rates of carbon monoxide whilst on the abscissae are reported the related quantities of carbon monoxide sorbed. The television picture tube and getter types as well as position are the same as those previously mentioned. The gettering characteristics where, also in this case, measured for a 25 mg. Ba film and the constant CO pressure on the getter was 5.10- torr. Curve 5 refers to the characteristics of a conventional greater film whilst curve 6 refers to those of a film obtained according to the present invention. It will be observed that the gettering characteristics of the film obtained by evaporating the barium film in a nitrogen pressure have increased since the amount of barium rendered accessible to the gas has been increased appreciably due to the increase in specific surface area of a film formed in this manner.

The diagram of FIG. 3 clearly shows the retarding action of the gas introduced on the forward evaporation of barium. Such curves have been obtained by using a quartz crystal thickness monitor. The crystal is mounted on a tube similar to the one above at the centre of the screen. Its shift of frequency as barium is deposited on one of its faces is a direct measure of the quantity of barium condensed on the face itself. On the ordinates of this diagram are reported the percentages of barium thickness referred to the conventional getter at the end of evaporation, on the abscissae is reported the time of evaporation. The external radio frequency power is applied at zero time (not shown). The starter times S and S indicated show the instant when the exothermic reaction begins, whilst the total times T and T are the times at which the radio frequency current to the getter container is discontinued. The curve 7 refers to the conventional getter, whilst curve 8 refers to the getter object of the present invention. It will be clearly observed how the gas present in the getter, and liberated in the tube before the starter time, has its principal action essentially during the first few seconds of evaporation although even in the subsequent stages a certain action is also being exercized. It should however be noted that all the gas introduced after a few minutes has been repumped by the getter as has been revealed by pressure measurements carried out during these tests.

We claim:

1. A process of producing within a picture tube, having walls and a screen, a thin getter metal film for sorbing the residual gases comprising evaporating said getter metal in said picture tube in the presence of a gas wherein said gas is present in an amount such that the means free path of the getter metal atoms in the gas is equal to or less than the distance between the metal evaporating source and the tube screen.

2. A process according to claim 1 characterized in that the gas is introducing as such in the picture tube before the evaporation of the getter metal.

3. A process according to claim 1 characterized in that, along with the metal to be evaporated, admixed therewith or mechanically separated therefrom, a compound capable of evolving the desired gas upon heating is introduced into the picture tube.

4. A process according to claim 3, characterized in that a compound is used which is not appreciably dissociated during the preliminary treatment of the tube or during air exposure.

5. A process according to claim 1 characterized in that the gas is nitrogen produced by the thermal decomposition of Fe N.

6. A process of producing within a picture tube, a thin getter metal film for sorbing the residual gases comprising evaporating said getter metal in said picture tube in the presence of a gas having a pressure of between 5 10 torr and 1x torr, wheerby a major portion of the getter metal is deposited on the internal surfaces of the tube in the vicinity of the getter metal source.

7. In a process for depositing a getter metal on the inside surfaces of a picture tube, said picture tube having a viewing screen connected to conical walls, said getter metal being deposited in the form of a thin getter metal film by evaporation from a getter metal source, the improvement comprising evaporating said getter metal in the tube in the presence of a gas causing collision of the getter metal molecules with the gas molecules with a consequent deposition of a reduced amount of getter metal on the screen and an increased amount of getter metal on the walls of the tube.

8. A process of producing a thin getter metal film within a picture tube having a screen, walls connected to the screen and a neck connected to the wall, comprising the steps of:

(a) placing in said tube in the vicinity of the neck a getter device comprising a getter metal admixed with -a gas releasing material which releases its gas at a temperature below the evaporation temperature of the getter metal,

(b) evacuating said tube,

(c) releasing the gas from the gas releasing material,

(d) evaporating the getter metal in the presence of the gas causing collision of the getter metal molecules with the gas molecules depositing an increased amount of the getter metal on the walls of the tube and a decreased amount of getter metal on the screen of the tube.

9. A process of producing a thin getter metal film within a picture tube having a screen, walls connected to the screen and a neck connected to the walls, comprising steps of:

(a) placing in said tube in the vicinity of the neck a getter device comprising a getter metal admixed with a nitrogen releasing material which, releases nitrogen at a temperature below the evaporation temperature of the getter metal,

(b) evacuating said tube,

(0) releasing the nitrogen from the nitrogen releasing material,

(d) evaporating the getter metal in the presence of the nitrogen causing collision of the getter metal molecules with the nitrogen molecules depositing an increased amount of the getter metal on the walls of the tube and a decreased amount of getter metal on the screen of the tube.

References Cited UNITED STATES PATENTS 3,121,182 2/1964 Hui 316- 3,131,983 5/1964 Harries 316-25 1,861,643 6/1932 Pirani 31625 RICHARD H. EANES, JR., Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,388 ,955 June 18 1968 Paolo Della Porta 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. 1, line 67, for "empolyed" read employed column 4 line 7 for "if" read of column 6" line 21, for "greater" read getter line 60, for "means" read mean column 7, line 8, for "l0 read 10' Signed and sealed this 1st day of July 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR.

Commissioner of Pate fits Attesting Officer

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