Evaporable getter device with reduced activation time

Abstract: An evaporable getter device is disclosed containing nickel and BaAl 4 compound, having reduced barium evaporation time; this feature is obtained by using a mixture of two nickel powders of different morphologies.

Description: The present invention relates to an evaporable getter device with reduced activation time. As it is known, getter materials are used in all the applications wherein vacuum-keeping is required over long periods. Particularly kinescopes, either of the conventional cathode-ray tube-type or of flat screen-type, contain getter materials the object whereof is to fix gas traces which may remain in the kinescope after its evacuation or arise from the degassing of its composing materials. The getter material most commonly used in kinescopes is metallic barium, coated in form of a thin film on an inner wall of the kinescope. The barium film may be produced only once the kinescope has already been evacuated and hermetically sealed. Therefore devices are used, known in the field as evaporable getters, formed by an open metal container, wherein there are powders of a barium and aluminium compound, BaAl 4 , and powders of nickel, Ni, in a weight ratio of about 1:1. Devices of this type are well known in the art; see in this connection, e.g., US patent No. 5,118,988 assigned to the applicant. Once the kinescope has been evacuated and sealed, the device is induction-heated by means of a coil located outside the same kinescope, in an activation process during which the barium evaporation occurs; the heating takes place especially on the metal container which transfers heat to the powders packet contained therein. When the temperature in the powders reaches the value of about 800°C, this reaction takes place: BaAl 4 + 4 Ni → Ba + 4 NiAl This reaction is strongly exothermic, and causes the temperature of powders to reach about 1200°C, at which there occurs the evaporation of barium which sublimates on the kinescope walls, thereby forming the metal film. In order to obtain a good reactivity in the powder packet, BaAl 4 compound is used in form of a powder with particle size smaller than about 250 µm. Nickel has usually a particle size smaller than 30 µm, though small amounts of powders with larger particle size, up to about 50 µm, are allowed; the morphology of the nickel powder is different among the various manufacturers of getter devices, and sometimes the same manufacturer may use different types of nickel for different getter devices, but every getter device now on the market always contains only one nickel form. The most used morphologies are the one essentially spherical, wherein the particles have a rounded form with flat surface, and a dendritic morphology, characterized by high specific surface (surface area per unit weight). The activation time necessary to evaporate from the device a predetermined amount of barium, measured from the time when the supply of energy to the device by means of the coil is begun, is usually defined in the art as "Total Time", used in the following text and in the claims also in its shortened form "TT". Modem color kinescopes may require for their working up to about 300 mg of barium in the form of a film. In the current state of the art, TT for evaporating such amounts of barium is about 40 seconds. This time involves a slowing down and corresponds to a "bottle-neck" in the modern production processes for kinescopes, whereby it is a demand of the market to have getter devices requiring, for the same amount of evaporated barium, smaller TT than the current devices. In order to achieve this result, one could in principle increase the power supplied by the coil or increase the powders reactivity by decreasing their particle size. However, with the current getter devices, it is not possible to increase the coil power. In fact, by doing this, the powders container is too quickly heated and there is not time for the heat to be transmitted to the powders packet, causing the temperature of the powders directly against the container to be higher than in the rest of the packet. The reaction between BaAl 4 and Ni begins in the powders against the container, and the pressure of barium vapors produced in this area of the powders packet causes its rising; this involves the possible expulsion of fragments, which has instead to be absolutely prevented not to compromise the kinescope working, and anyhow a reduced barium evaporation. The decrease of the powders particle size also causes an excessive and located increase of the rate of the reaction between BaAl 4 and Ni, resulting in the packet rising. It is an object of the present invention to provide an evaporable getter device with reduced activation time not showing the drawbacks of the known art. Such an object is achieved according to the present invention with an evaporable getter device comprising a metal container wherein there are BaAl 4 powder and nickel powder, characterized in that the nickel powder is constituted by a mixture of particles of two different morphologies, the first essentially spherical and the second dendritic, wherein the weight ratio between the two nickel forms ranges from about 4:1 to 1:2,5. JP-A-03/078 928 discloses an evaporable getter comprising BaAl 4 and a mixture of about 9 parts nickel powder with a grain size of 3-7µm and 1 part rough nicker powder with a particle size of 20 - 25 µm. The invention will be hereinafter described in details with reference to the drawings, wherein: Fig. 1 is a reproduction of a microphotography of a sample of a nickel powder being of essentially spherical morphology; Fig. 2 is a reproduction of a microphotography, with the same enlargement as the reproduction in Fig. 1, of a sample of a nickel powder being of dendritic morphology. It has been found that the use of mixtures of nickel powders of the two mentioned morphologies allows to reduce TT of about 25-30%, for the same amount of evaporated barium, without causing the above-mentioned problems of exceedingly strong reaction. The weight ratio between the nickel particles of essentially spherical morphology and those of dendritic morphology may range from about 4:1 to 1:2.5. It has been found that ratios higher than 4:1 cause problems in getter devices production, because the powders packet further comprising BaAl 4 compound has poor mechanical consistency; on the contrary, ratios smaller than 1:2,5 allow only a small reduction of TT. Preferably, mixtures are used wherein the weight ratio between the two nickel forms is about 1:1. Nickel has particle size smaller than about 50 µm, and preferably smaller than about 20 µm; it has further been found that best results are obtained when nickel of essentially spherical morphology has particle size ranging from about 10 to 18 µm. Nickel of dendritic morphology is available on the market: e.g., company INCO from Sheridan Park, Ontario, Canada, commercializes dendritic nickel of two different particle sizes with the catalogue numbers T-123 and T-128. Nickel of essentially spherical morphology may be found on the market, e.g. from the same above-mentioned company INCO. Alternatively, it may be obtained from nickel of any morphology and particle size slightly larger than that desired, by the technique known as "jet mill". This technique consists in the high-speed introduction of a powder in a grinding chamber, in a flow of a carrier gas; the powder particles are reduced in size, and their surface is rounded, by the collisions with other particles or by means of a hindrance interposed in their trajectory. The particles are subsequently classified to collect the fraction of desired particle size. BaAl 4 compound, useful for the working of the invention, has a particle size smaller than 250 µm. The weight ratio between nickel and BaAl 4 compound generally may range from about 2:1 to 1:2, but a ratio of about 1:1 is generally used. The metal container may be obtained from a variety of materials, such as NiCr or NiCrFe alloys; it is preferred the use of AISI 304 steel, which combines good oxidation resistance and heat treatments strength, with cold mechanical workability. The form of the metal container may be whatever, and particularly any of the forms known and used in the field, such as e.g. the forms of the devices of US patents No. 4,127,361, 4,323,818, 4,486,686, 4,504,765, 4,642,516, 4,961,040 and 5,118,988. The invention will be further illustrated in the following examples. These nonlimiting examples illustrate some embodiments intended to teach those skilled in the art how to work the invention and to represent the best considered way to put the invention into practice. EXAMPLE 1 A series of samples of identical getter devices is prepared, employing for each one an AISI 304 steel container having a diameter of 20 mm and a height of 4 mm and having its bottom shaped with relieves of 1 mm height as disclosed in US patent No. 4,642,516. Each sample is prepared by pouring in the container a homogeneous mixture formed of 660 mg of BaAl 4 powder having particle size smaller than 250 µm, 520 mg of nickel powder of dendritic morphology T-123 from INCO company and 220 mg of nickel powder having average particle size 18 µm and being of essentially spherical morphology, obtained by grinding INCO T-123 nickel with the "jet mill" technique, and sieving the powders thereby obtained to collect the fraction of desired particle size; the total weight of nickel is 740 mg. The powders mixture is compressed in the container by means of a suitable punch. The samples are tested by inserting them one by one in a glass measure chamber connected to a pump system, by evacuating the chamber and carrying out an evaporation test according to the methodology described in ASTM F 111-72 standard; each sample is heated by radio-frequencies with such a power that evaporation begins 10 seconds after heating has begun; the tests are different from one another in heating time, ranging in the different tests from 20 to 45 seconds. Once each test is ended, the amount of evaporated barium is measured, and from this data series, a curve of barium yield as a function of heating time is drawn. In Table 1, the weight ratio between essentially spherical nickel (in Table indicated as Ni S ) and dendritic nickel (indicated as Ni D ) is reported, as well as the TT value necessary to evaporate from the devices a barium amount of 300 mg. EXAMPLE 2 The tests of Example 1 are repeated with a series of samples of identical getter devices, containing a homogeneous mixture formed of 660 mg of BaAl 4 powder having particle size smaller than 250 µm, 370 mg of nickel powder being of essentially spherical morphology, obtained by "jet mill" as described in Example 1 and 370 mg of INCO T-123 nickel, for a total nickel weight of 740 mg. The weight ratio between the two nickel forms and the time necessary to evaporate 300 mg of barium are reported in Table 1. EXAMPLE 3 The tests of Example 1 are repeated with a series of identical getter devices, containing a homogeneous mixture formed of 660 mg of BaAl 4 powder having particle size smaller than 250 µm, 590 mg of nickel powder being of essentially spherical morphology, obtained by "jet mill" as described in Example 1 and 150 mg of INCO T-123 nickel, for a total nickel weight of 740 mg. The weight ratio between the two nickel forms and the time necessary to evaporate 300 mg of barium are reported in Table 1. EXAMPLE 4 (COMPARATIVE) The tests of Example 1 are repeated with a series of identical getter devices, containing a homogeneous mixture formed of 660 mg of BaAl 4 powder having particle size smaller than 250 µm, and 740 mg of T-123 nickel powder. The time necessary to evaporate 300 mg of barium is reported in Table 1. EXAMPLE Ni S : Ni D Total Time (seconds) 1 1 : 2,36 36 2 1 : 1 30 3 3,93 : 1 29 4 / 40 The results reported in Table 1 confirm that, other conditions being the same, by using mixtures of nickel powders of essentially spherical morphology and of dendritic morphology, a reduction of the Total Time required for the barium evaporation is obtained with respect to the use of nickel of a single morphology; furthermore, these powders mixtures allow to obtain good mechanical properties of the powders packet, allowing an easy production of getter devices.

Claims: Evaporable getter device comprising a metal container wherein there are BaAl 4 powder and nickel powder, characterized in that nickel powder is formed of a mixture of particles of two different morphologies, the first being essentially spherical and the second dendritic, wherein the weight ratio between the two nickel forms ranges from about 4:1 to 1:2,5. Device according to claim 1, wherein the ratio between the two nickel forms is about 1:1. Device according to claim 1, wherein nickel has particle size smaller than 50 µm. Device according to claim 3, wherein nickel has particle size smaller than 20 µm. Device according to claim 3, wherein nickel of essentially spherical morphology has average particle size ranging from 10 to 18 µm. Device according to claim 1, wherein nickel of essentially spherical morphology is obtained by the so-called "jet mill" technique. Device according to claim 1, wherein BaAl 4 compound has particle size smaller than 250 µm. Device according to claim 1, wherein the ratio between nickel and BaAl 4 compound ranges from about 2:1 to 1:2. Device according to claim 1, wherein the ratio between nickel and BaAl 4 compound is about 1:1.

Inventors: CARRETTI CORRADO, LATTUADA RAFFAELLO, MANTOVANI LUISA, MARTELLI DANIELE, URSO GIUSEPPE, CARRETTI, CORRADO, LATTUADA, RAFFAELLO, MANTOVANI, LUISA, MARTELLI, DANIELE, URSO, GIUSEPPE
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1998
Main classification: H01J7
IPC codes: H01J7, H01J19, B22F1, H01J29