US 3389288 A
Description (OCR text may contain errors)
June 18, 1968 p; DELLA PORTA ET AL 3,389,288
GETTERING DEVICE INCLUDING A GETTER METAL AND A GAS RELEASING MATERIAL Filed Aug. 24, 1965 2 Sheets-Sheet 1 Fig.7
June 18, 1968 DELLA om- ET AL 3,389,288
GETTERING DEVICE INCLUDING A GETTER METAL AND A GAS RELEASING MATERIAL 2 Sheets-Sheet 2 Filed Aug. 24, 1965 1250 Qt (cm-t m) V m w 2- m g 6 II F O w w -i\ w 2 0 I' a O .wvwsw n N am arr 3 a v a ma Allll l l l I I I l i l I I v l l I l T w n I, r
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United States Patent 3,389,288 GETTERING DEVICE INCLUDING A GETTER METAL AND A GAS RELEASING MATERIAL Paolo della Porta, Milan, Tiziano A. Giorgi, Rho, Milan, and Elio Rabusiu, Milan, Italy, assignors to S.A.E.S Getter-s S.p.A., Milan, Italy, a company of Italy Filed Aug. 24, 1965, Ser. No. 482,104 Claims priority, application Italy, Feb. 25, 1965,
1,708/65, Patent 742,042 Claims. (Cl. 313174) ABSTRACT OF THE DISCLOSURE Getter devices comprising an evaporable getter metal such as barium and a material which releases gas such as nitrogen. These getter devices find particular utility in television picture tubes and the like wherein they yield a reduced amount of getter metal on the tube screen.
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 container 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% aluminium alloy and nickel. On heating, as previously mentioned by externally induced radio frequency currents, an exothermic reaction occurs between the barium-aluminum 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 employed 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 its gettering or pumping action throughout the life of the tube.
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 ice 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 center 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 center 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 ex cite 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 aluminum 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 aluminum 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 aluminum its effects are more pronounced due to its higher molecular mass. As previously mentioned the thickness of the barium film is greatest at the center of the screen and it is here that electrons are decelerated 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, it 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 aluminum 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 ofa suitable device and technique for efiiciently 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 vapor by means of various types of deflecting battles fashioned on the getter container. In one particular embodiment of this procedure the bafiles are such as to direct the barium vapors towards the center of the bulb, i.e. on the normal to the center 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 means for the direction of the barium vapors the container of the getter has bafiies which direct the evaporating barium atoms in a direction perpendicular to that of the normal to the center of the screen. Various other solutions between these two extremes have also been suggested and adopted. However,
their major drawback has been their low efiiciency or if eflicient 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 reflected 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 baffles, 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 by introducing 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 predominant 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, while 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 having initiated the exothermic reaction.
In addition to rendering possible a fine control of the distribution of 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 prevents obvious advantages from the point of view of tube life or from an economic standpoint. Such porous films are a result of submicroscopic globule formation in the vapor 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 deflecting action increases with:
(a) increase in molecular mass, (b) increase in pressure, (c) decrease in chemical afiinity 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 be extremely detrimental to tube characteristics as such and also because they tend to produce hydrocarbons, water vapor, 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 face 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 5X10 and 1X10 torr. Pressures above 5x101: torr using up too great a fraction of the barium film and pressures below 10- torr not being sufficient 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 while still on the pump and just before tip-off. 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 set 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.
Non-limiting 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. The principle 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 exercised 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 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 action is still useful as far as regards back evaporation of barium towards the gun of the tube.
A practical illustrative and non-limiting 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% metal 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 12. 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. It is to be noted that the thickness of the film at the centre of the screen is 1400 Angstrom units. In a similar tube, there was then mounted a getter of exactly similar characteristics 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 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, and 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 reduces 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 were, also in this case, measured for a 25 mg. Ba film and the constant CO pressure on the getter was 5 x 10- torr. Curve 5 refers to the characteristics of a conventional getter film while 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 thick ness 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 8;, indicated show the instant when the exothermic reaction begins, while the total times T and T are the times at which the radiofrequency current to the getter container is discontinued. The curve 7 refers to the conventional getter, while curve 8 refers to the getter 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 exercised. 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.
1. A getter device of the exothermic type with whose barium alloy content is mixed a nitrogen releasing material which although stable in air and to temperatures up to 400 C. dissociates liberating nitrogen before barium begins to evaporate at 800-900" C. due to the exothermic reaction. a
2. A getter device of the endothermic type with whose barium alloy content is mixed a nitrogen releasing material which although stable in air and to heat treatments up to 400 C. dissociates slowly liberating nitrogen during the whole period of evaporation of barium atoms.
3. A getter device for use in closed vessels, said device comprising an evaporable getter metal; a gas releasing material which releases a gas at a temperature below the evaporation temperature of the getter metal and which is stable to temperatures up to 400 C. said gas being readsorbable by the getter metal.
4. A getter device comprising an evaporable getter metal and a gas releasing material, said getter device when mounted within a picture tube having a tube screen and tube walls and heated, evaporates the getter metal in the present of a gas released from the gas releasing material wherein said gas is present in an amount great enough to preferentially deposit the evaporated getter metal on the tube walls and wherein said gas is then sorbed by the getter metal [without deterimental effect on the sorptive capacity of the getter metal with respect to other residual gases present within the tube or evolved during operation to the tube].
5. A getter device especially adapted to produce within a picture tube a getter metal of particularly controlled distribution and of high sorptive capacity said getter device comprising:
A. a getter metal and an exothermic heat releasing material, and B. an amount of Fe N in thermal proximity with the exothermic heat releasing material whereby the Fe N releases nitrogen at a pressure of from 1X10 to 5 10* torr.
6. A getter device for use within a picture tube having walls attached to a screen said device comprising an evaporable getter metal and Fe N means for releasing a gas in an amount sutficient to effect deposition of a major portion of the getter metal on the walls of the tube.
7. A getter device for use within a picture tube which has a substantially planar screen, a conically shaped wall section attached to the screen and a neck attached t0 said wall section, said getter device comprising:
A. barium metal,
B. an exothermic heat releasing means for supplying at least a portion of the heat of vaporization of said barium metal,
C. Fe N prescent in an amount such that the mean free path of the barium atoms within the tube during evaporation is equal to or less than the distance between the getter device and the tube screen.
8. A getter device for use Within a picture tube which has a substantially planar screen, a conically shaped wall section attached to the screen and a neck attached to said wall section, said getter device being mountable in the vicinity of the point attachment of the neck of the tube and the wall section of the tube, and which tube has a given volume, said getter device comprising:
A. a getter metal which is evaporable under the influence of heat in a vacuum,
B. a gas containing material adapted to release its gas at a temperature below the temperature of evaporation of the getter metal, said gas containing material present in an amount such that the gas released is present in an amount such that the mean free path of the getter metal atoms within the tube during evaporation is equal to or less than the distance between the getter device and the tube screen.
9. The getter device of claim 8 wherein the gas produces within the tube a gas pressure of 5X10 to 1X 10* torr.
10. A getter device especially adapted to produce within a picture tube a getter metal of particularly controlled distribution and of high sorptive capacity said getter device comprising:
A. a getter metal which is evaporable under the influence of heat in a Vacuum and B. a hydrogen releasing material which releases its hydrogen at a temperature below the evaporation temperature of the getter metal.
References Cited UNITED STATES PATENTS 1,894,948 1/1933 Espe et al 313179 X 2,540,647 2/1951 Bienfait 31625 X 2,899,257 8/1959 Lederer 3 1625 2,615,140 10/1952 Claessen 313179 FOREIGN PATENTS 496,856 12/1938 Great Britain.
JAMES W. LAWRENCE, Primary Examiner. DAVID J. GALVIN, Examiner.
' S. A. SCHNEEBERGER, P. C. DEMEO,
"on" strike out all to an UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,389,288' June 18, 1968 Paolo della Porta et a1.
It is certified that error appears in the above identified patent and that said. Letters Patent are hereby corrected as shown below:
Column 4, line 7, for "prevents" read presents column 6, line 74, beginning with "[without deterimental effect d including "to the tube]" in column 7,
lfne 2; column 7 line 26 for "prescent" rea Present Signed and sealed this 22nd day of July 1969.
WILLIAM E. SCHUYLER, IR.
Edward M. Fletcher, Jr. Amsting Officcl' Commissioner of Patents