MXPA01004153A - Cathode-ray tube cathode and alloy therefor - Google Patents

Abstract

A nickel alloy for the manufacture of cathodes (2) for cathode-ray tubes, comprises magnesium and aluminum in proportions chosen so as to allow good adhesion of an emissive oxide layer (12) to the basis metal cap (11) consisting of the alloy. In particular the metal alloy contains magnesium with weight concentration between 0.01%and 0.1%. In addition aluminum may also be present within a determined weight concentration range.

Description

CATHODE FOR CATHODE RAYS TUBE AND ALLOY FOR THE SAME FIELD OF THE INVENTION The invention relates to cathode oxides of a cathode ray tube, used as electron sources emitted by the thermionic effect, and more particularly to the composition of the metal forming the base of the cathode. A conventional oxide cathode consists of a layer of alkaline earth oxides, such as a mixture of barium oxide (BaO), strontium oxide (SrO) and calcium oxide (CaO) or a mixture of BaO and SrO, which is deposited in a base metal made of nickel or a nickel alloy, and comprising one or more reducing elements, such as magnesium (Mg), aluminum (Al), silicon (Si), chromium (Cr), zirconium (Zr) or another element capable of reducing oxides. The mixture of alkaline earth oxide can itself be composed of other oxides, such as, for example, Sc2O3 and Y2O3. A conventional oxide cathode is constructed of a tube made of a Ni alloy (usually Ni-Cr), to which a cap made in the base metal is welded. A layer made of a mixture of carbonates of Ba and Sr or a mixture of carbonates of Ba, Sr and Ca is deposited in the base metal. These carbonates, which are stable in the air, subsequently become oxides in the vacuum inside. of the cathode ray tube.

This oxide layer, heated to a cathode working temperature of about 800 ° C, becomes an electron-emitting layer when a certain amount of BaO is converted to barium metal. The formation of the barium metal is maintained by the following mechanisms: the cathode, during the operation, is heated to a temperature of approximately 800 ° C, which causes the reducing elements to disperse towards the interface between the nickel and the oxides of alkaline earth. These reducing elements, for example, Mg, Al and Si, constantly react with barium oxide and reduce it, in order to form a barium metal in accordance with the reactions: Mg + B --- MgO + Ba 2AI + 4 BaO - BaAI2O4 + 3 Ba Si + 4 BaO Ba2SiO + 2 Ba Therefore, the reducing elements added to the nickel are consumed by the chemical reduction-oxidation reactions with BaO. The lifetime of the cathode is directly related to the consumption of these elements, so that, for each of the selected added reducing elements, a minimum content is desired in order to guarantee a minimum life time. In addition, it is known that some compounds that result from the reduction reactions of Ba, described above, as Ba2SiO or BaAI2O4, are so highly stable that they can accumulate in the interface [A. Eisenstein, H. John et al., J. Appl Phys., T. 24, No. 5, p. 631, 1953] between nickel and alkaline earth oxides. These compounds, due to their high resistance, increase the impedance of the interface, which reduces the current density of the cathode. In addition, they degrade the lifetime of the cathode as they accumulate permanently in the interface during the operation of the cathode. When they accumulate, they limit the diffusion of the reducing elements and, therefore, decrease the reactions between the latter and the BaO, which, in turn, reduces the amount of Ba metal formed, which is necessary for the emission [E.S. Rittner, Philips Res. Rep., T.8, p. 184, 1953]. Another important disadvantage is that the excessive accumulation of these compounds can degrade the adhesion of alkaline earth oxides with nickel. The invention is directed to avoid these disadvantages by suitably selecting a composition of the material forming the base of the cathode, the material comprising a nickel alloy for which the content of reducing elements should be selected within a concentration range by weight, defined from agreement with the elements in question. Each reducing element is added to the nickel in a range of concentration defined by a lower limit and an upper limit, the interval guarantees a long life time as well as an optimal performance in emission and reliability. To achieve these results, the metal alloy according to the invention, which has the purpose of making cathodes for cathode ray tubes, mainly comprises nickel, together with magnesium (Mg), whose concentration by weight C g is between 0.01% and 0.1%. Advantageously, it also includes aluminum, whose concentration of CA? in weight satisfies the following relationship: CA? <; 0.14 x (0.1 - CMg), where: CMg is the concentration of Mg in the nickel expressed as a percentage by weight; AC? is the concentration of Al in the nickel expressed as a percentage by weight.

BACKGROUND OF THE INVENTION BRIEF DESCRIPTION OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS The invention and its various advantages will be more clearly understood with the help of the following description and the drawings in which: Figure 1 illustrates an electron gun for a cathode ray tube; Figure 2 is a longitudinal section through an oxide cathode according to the invention.

DETAILED DESCRIPTION OF THE INVENTION A cathode ray tube comprises at least one source to create an electron beam, which has the purpose of tracking the screen of the tube, in order to excite the phosphors on it, which serve to create a visible image. As shown in Figure 1, the gun 1 of the tube, therefore, comprises at least one cathode 2 and a succession of electrodes (3, 4, 5, 6, etc.), which are intended to form a ray or 7, 8, 9 electron beams to focus on the screen of the tube. As shown in Figure 2, the cathode 2 is generally in the form of a hollow, approximately cylindrical tube 10 made of nickel or a nickel alloy, for example, nickel-chromium. The tube 10 is closed at one end by a cover 11, which can be a metal part coupled or an integral part of the tube, obtained by extraction. The lid is made of a nickel alloy and serves as a support for the emitting layer 12 of the alkaline earth oxides. This layer 12, heated to a temperature by the filament 13, becomes the source of the electron beam which serves to track the surface of the tube screen. When the nickel of conventional oxide cathodes is heated, compounds can be formed, not only as a result of the reduction of barium oxide BaO by the reducing elements, but also by the direct reaction of the reducing elements, with residual oxygen present in the nickel or with the oxygen present in the atmosphere to which nickel is exposed during the different steps in the production of the cathodes. For example, the production of cathodes usually includes the step of destemming the metal base in hydrogen at a temperature close to 1000 ° C. The water content (H2O) of hydrogen is generally very low, so that the atmosphere is reduced for nickel at the tempering temperature. On the other hand, the content of H2O, still reduced for nickel, may be sufficient to oxidize the reducing elements present in nickel, such as Mg and Al. Magnesia (MgO) and alumina (AI2O3) thus form on the surface of nickel during annealing. Also, more complex compounds resulting from the reaction of two reducing elements with oxygen are observed, for example MgAI2O4 and BaAI2O4. The formation of these compounds was studied together with their persistence during the cathode activation step in the tube cathode rays. During this activation step, the cathode is heated in the vacuum of the cathode ray tube (typically, P <10"6 torr) at a maximum temperature of between 900 ° C and 1100 ° C. The purpose of this operation is, On the one hand, converting the carbonates into oxides and on the other, optimizing the emission of cathode electrons For the nickel of various Mg and Al compositions, the compound MgAI2O4 is formed during the step of annealing by hydrogen at the interface between the metal of base and cover 11 and the coating of the emitting oxides.This compound is a stable compound and is in the form of small crystals that partially cover the nickel surface and have a tendency to accumulate in the interface during the life of the Cathode Since this kind of stable compound is deleterious, its presence at the interface should be as limited as possible, in order to maintain good adhesion of the oxide layer to the base metal. Ad of crystals was determined by an image analysis on the nickel surfaces taken in a scanning electron microscope (SEM). The percentage of the surface covered by the crystals can be measured by the image analysis, since these crystals appear white against the black background of the nickel. This percentage was measured after the activation step in the cathode ray tube, that is, it represents the crystals that persist after activation and are present at the beginning of the cathode life.

A statistical analysis of the experimental measurements of the amount of coverage by crystals present on the surface of the base metal after activation, carried out in several nickel molds has shown that it is important to link the amount of coverage for stable crystals with the magnesium and aluminum concentrations of the base metal. The results of this analysis have guided the equation representing this percentage of the surface coverage, and therefore, the amount of crystals on the surface, as a function of the aluminum content and the magnesium content in the alloy that forms the base metal: Cs = [-2 + (50 x CMg) + (350 x CA,)]% (1) where: -C3 is the percentage of the surface S is the percentage of the nickel surface covered for crystals; - CMg is the concentration of Mg in the nickel expressed as a percentage by weight; - CA? is the concentration of Al in nickel expressed as a percentage by weight.

It is common practice to have a minimum magnesium content in nickel, since magnesium is highly reducing and disperses very quickly. Consequently, magnesium ensures that the cathode is activated rapidly during the activation process described above and ensures the proper emission of electrons during the first hundreds of hours of the cathode's life. Magnesium is preferred as it has this favorable behavior to limit the amount of MgAI2O crystals, to optimize the content of Al rather than limit the magnesium content. The magnesium content can advantageously be adjusted to a value between 0.01% and 0.1%. It is known from experience that the maximum percentage of stable crystals considered to be acceptable, ie, which provides good adhesion of the oxide layer to the base metal, is 3%, the maximum content of Al of the Nickel alloy according to the invention is calculated from the magnesium content, by using the following equation derived from equation (1): CA, < 0.14 x (0.1 -CMg) '(2) The following table shows the variations in the adhesion of the oxide layer according to the different magnesium and aluminum contents in the base metal. Therefore, good adhesion is guaranteed when the inequality (2) is satisfied.

Table: Percentage of nickel surface covered by the crystals with the different magnesium and aluminum contents in the nickel (measured values and values calculated from equation (1)).

Claims (5)

1. An alloy of metal for the manufacture of cathodes for cathode ray tube, which mainly comprises nickel, characterized in that the alloy includes magnesium (Mg), whose concentration by weight of C g is between 0.01% and 0.1%.

2. The metal alloy according to claim 1, characterized in that the alloy also includes aluminum, the concentration by weight of CA? complies with the relationship: CAÍ 10.14 x (0.1 - CMg).

3. The metal alloy according to claim 1, characterized in that the alloy also contains aluminum and because, after the cathode has been activated, the percentage of the surface of the alloy below a cathode emitting layer covered by stable crystals is less than or equal to 3%.

4. A cathode comprising a base metal, which is a metal alloy according to any of the preceding claims, characterized in that the emitting part comprises a layer of alkaline earth oxides.

5. A cathode ray tube comprising at least one cathode, the base metal of which is a metal alloy of according to any of claims 1 to 3.