Patent Application
20050169766
The present invention is directed to compositions containing non-evaporable getter alloys which, after having lost their functionality as a consequence of an exposure to reactive gases at a first temperature, can then be reactivated by means of a thermal treatment at a second temperature, lower than the first one.
Non-evaporable getter alloys, also known as NEG alloys, can reversibly sorb hydrogen and irreversibly sorb gases such as oxygen, water, carbon oxides and, in the case of some alloys, nitrogen.
These alloys are employed in a number of industrial applications which require the maintenance of vacuum: examples of these applications are particle accelerators, X-ray generating tubes, cathode ray tube displays or CRTs, flat displays of the field-emission type (called FEDs), and evacuated jackets for thermal insulation, such as thermal bottles (thermos), Dewar bottles or pipes for oil extraction and transportation.
NEG alloys can be also employed to remove the above-mentioned gases when traces thereof are present in other gases, generally noble gases. One example is the use in lamps, particularly fluorescent lamps which are filled with noble gases at pressures of some tens of hectoPascal (hPa), where the NEG alloy has the function of removing traces of oxygen, water, hydrogen, and other gases, so to keep a suitable atmosphere for the lamp operation. Another example is the use in plasma displays, where the function of the NEG alloy is substantially similar to that carried out in fluorescent lamps.
These alloys generally have as main components zirconium and/or titanium and comprise one or more additional elements selected among the transition metals, Rare Earths or aluminum.
NEG alloys are the subject-matter of a number of patents. U.S. Pat. No. 3,203,901 discloses Zr—Al alloys, and in particular the alloy having the weight percent composition Zr 84%-Al 16%, manufactured and sold by the applicant (SAES Getters S.p.A.) under the trademark St 101; U.S. Pat. No. 4,071,335 discloses Zr—Ni alloys, and in particular the alloy having the weight percent composition Zr 75.7%-Ni 24.3%, manufactured and sold by the applicant under the trademark St 199; U.S. Pat. No. 4,306,887 discloses Zr—Fe alloys and in particular the alloy having the weight percent composition Zr 76.6%-Fe 23.4%, manufactured and sold by the applicant under the trademark St 198; U.S. Pat. No. 4,312,669 discloses Zr—V—Fe alloys, and in particular the alloy having the weight percent composition Zr 70%-V 24.6%-Fe 5.4%, manufactured and sold by the applicant under the trademark St 707; U.S. Pat. No. 4,668,424 discloses zirconium-nickel-mischmetal alloys with optional addition of one or more other transition metals; U.S. Pat. No. 4,839,085 discloses Zr—V-E alloys, wherein E is an element selected among iron, nickel, manganese and aluminum or a mixture thereof; U.S. Pat. No. 5,180,568 discloses intermetallic compounds Zr1M′1M″1, wherein M′ and M″, being identical or different from one another, are selected among Cr, Mn, Fe, Co and Ni, and in particular the compound Zr1Mn1Fe1, manufactured and sold by the applicant under the trademark St 909; U.S. Pat. No. 5,961,750 discloses Zr—Co-A alloys, wherein A is an element selected among yttrium, lanthanum, Rare Earths or a mixture thereof, and in particular the alloy having the weight percent composition Zr 80.8%-Co 14.2%-A 5%, manufactured and sold by the applicant under the trademark St 787; U.S. Pat. No. 6,521,014 B2 discloses zirconium-vanadium-iron-manganese-mischmetal alloys, and in particular the alloy having the weight percent composition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-MM 3%, manufactured and sold by the applicant under the trademark St 2002 (by MM is meant mischmetal, i.e., a commercial mixture of Rare Earths, for example having the weight percent composition 50% cerium, 30% lanthanum, 15% neodymium, and the balance 5% of other Rare Earths.
These alloys are used alone or in a mixture with a second component, generally a metal, capable of providing particular characteristics to a body formed with the alloy, such as a higher mechanical strength. The most commonly used metals for this purpose are zirconium, titanium, nickel, and aluminum. Compositions comprising the cited St 707 alloy and zirconium or titanium are described for example in UK patent GB 2,077,487, while in U.S. Pat. No. 5,976,723 are described compositions containing aluminum and a NEG alloy of the formula Zr1-xTixM′M″, wherein M′ and M″ are metals selected among Cr, Mn, Fe, Co, and Ni, and x is comprised between 0 and 1.
The functioning principle of NEG alloys is the reaction among the metallic atoms on the alloy surface and the absorbed gases, in consequence of which a layer of oxides, nitrides or carbides of the metals is formed on that surface. When surface coverage is complete, the alloy is inactive for further absorption. Its function can be restored by a reactivation treatment, at a temperature which is at least the same and preferably higher than the working temperature.
However, in some cases, it is impossible to treat an alloy for its activation or reactivation at a temperature higher than that at which it has been previously exposed to gases. This is particularly the case for alloys which are used in devices where the space to the kept under vacuum or a controlled atmosphere is defined by walls made of glass, such as CRT-type screens, flat displays which are either field emission displays or plasma display panels, and lamps. The manufacture of these devices generally provides for the getter alloy to be inserted in its final position when the device is still open and its inner space is exposed to the atmosphere. Thereafter, the device is sealed by a so-called “frit-sealing” step, wherein a low-melting glass paste is placed between two glass portions to be welded together, is brought to about 450° C., melts, and thus joins the two portions. The vacuum or the controlled atmosphere can be obtained in the inner space of the device before sealing (in the so-called “in chamber” process, wherein the device assembling steps are carried out in an enclosure under vacuum or controlled atmosphere) or, more commonly, after the frit-sealing, by means of a “tail”, i.e., a small glass tubule admitting to the space and suitable for connection to a pumping system. In the case of devices containing a controlled atmosphere, such as plasma displays and some lamps, the tail is used also for filling with the desired gases. Finally, the device is sealed by closing the tail, usually by heat compression. In any case, during frit-sealing, the NEG alloy is exposed to an atmosphere of reactive gases, namely to the gases released by the low-melting glass paste in the case of “in chamber” processes, and to these same gases plus atmospheric gases in the case of “tail” processes. The contact between the alloy and the reactive gases occurs at a temperature depending on the process. The device can be homogeneously brought to the frit-sealing temperature within a furnace, in which case the NEG alloy will be exposed to the reactive gases at a temperature of about 450° C. Alternatively, it is possible to use a localized heating, e.g., by irradiation, in which case the getter temperature during the operation depends on its distance from the frit-sealing zone. In any case, during these operations the NEG alloy surface reacts with more or less intensity with the gases being present, with consequent at least partial deactivation of the alloy, such that the residual sorption velocity and capacity may result in being insufficient for the foreseen operation in the device. Therefore, there would be required a reactivation treatment at a temperature at least equal to, or preferably higher than, that of frit-sealing, which is however generally impossible, both to prevent a remelting of the frit-sealing paste which would endanger the welding seal, and to avoid impairment of the mechanical stability of the glassy portions forming the walls of the device containing the getter.
It is an object of the present invention to provide compositions containing a non-evaporable getter alloy that, after having lost their functionality in consequence of an exposure to reactive gases at a first temperature, can then be reactivated by a thermal treatment at a second temperature which is lower than the first one.
This object is achieved according to the present invention with getter compositions formed of a mixture of powders of:
For the sake of clarity, in the remainder of the description and in the claims, the elements of the group composed of yttrium, lanthanum, Rare Earths and mixtures thereof will be referred to as “component A”, according to the definition adopted in U.S. Pat. No. 5,961,750. Preferably, as component A there is used mischmetal, namely, commercial mixtures containing either cerium or lanthanum as the main component and a mixture of other Rare Earths as the balance.
The inventors have found that the compositions of the invention, in contrast to the NEG alloys alone and in contrast to the known compositions of a NEG alloy with a metal, can be exposed to reactive gases (such as atmospheric gases) at relatively high temperatures, e.g., about 450° C., required for the welding by frit-sealing of glassy portions, and then can be fully reactivated by a thermal treatment at a lower temperature, so as not to endanger the seal of the glassy welding or the mechanical strength of the glass portions which are near to the composition.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:
The NEG alloys used in the compositions of the invention comprise zirconium, vanadium, iron and at least one further element selected between manganese and component A. Manganese and component A are not necessarily in the alternative and may both be present in the alloys of the invention.
When the NEG alloy used in a composition of the invention does not comprise component A, the weight percents of the elements can vary in the following ranges:
When the NEG alloy used in a composition of the invention does not comprise manganese, the weight percents of the elements can vary in the following ranges:
Finally, when the NEG alloy used in a composition of the invention comprises both manganese and the component A, the weight percents of the elements can vary in the following ranges:
The above NEG alloys may also contain small percentages, generally lower than 5%, of other transition elements.
These alloys are generally employed in the form of a powder, having a particle size between about 10 and 250 μm, and preferably about 128 μm.
Titanium is generally employed in the compositions of the invention in the form of a powder, having a particle size comprised between about 0 and 40 μm. Alternatively, it is possible to use titanium hydride, TiH2, which during the subsequent thermal treatments releases hydrogen, thus forming titanium “in situ”.
The weight ratio between NEG alloy and titanium can be comprised within broad limits, such as between about 1:4 and 4:1, preferably within about 1:2 and 2:1, even more preferably in a ratio of about 3:2.
In an alternative embodiment of the invention, titanium may be partially replaced by nickel and/or cobalt. It has been observed that the compositions of the invention, during frit-sealing, release hydrogen. This can occur because water sorbed during frit-sealing is decomposed by the material into oxygen and hydrogen (according to a functioning mechanism common to any getter metals and alloys). While oxygen is completely retained by the material, hydrogen sorption is an equilibrium phenomenon, so that this element is partially released. In some applications, hydrogen release is not harmful, and it can even help avoid oxidation of some parts of the final devices during frit-sealing. There are, however, applications where hydrogen release is undesirable and must then at least be minimized. For instance, during tests carried out on flat screens to evaluate the activation and sorption properties of the compositions of the invention, it has been noted that hydrogen thus released leads to non-homogeneity in the screen brightness. The inventors have found that the partial replacement of titanium with powders of nickel and/or cobalt reduces the phenomenon. The powders of these two elements are employed with the same particle sizes previously indicated for titanium. Substitution can reach up to about 50% by weight of the titanium.
Frit-sealing treatments may be different according to the kind of device to be produced and according to the specific working processes adopted by any manufacturer. During these treatments, duration, temperature and atmosphere to which the getter composition are exposed may vary widely. As a consequence, the degree of interaction of the composition with the gases present during frit-sealing may vary in a broad range. That can lead to non-reproducibility of the gas sorption properties of the composition upon subsequent reactivation. In order to avoid this problem, it is possible to subject the compositions of the invention to a pre-oxidation treatment under controlled conditions, generally severe enough. For example, a typical treatment can be carried out at 450° C. for 20 minutes in air, thus obtaining a controlled oxidation of the composition. By pre-oxidizing the composition under conditions of time, temperature and atmosphere which are at least the same as the most severe foreseen for the frit-sealing treatment, it is assured that during the actual frit-sealing the further interaction of the composition of the invention with the surrounding environment will be nil or at least reduced. In this way, a “normalization” of the chemical composition of the composition of the invention, and a consequent higher reproducibility of its gas sorption characteristics after reactivation, is obtained.
The compositions of the invention can be used to produce getter devices of various shapes, with or without a support.
When the getter device is formed of the composition only, it will be generally in the form of pellets obtained by compression, pouring the mixture of powders into a suitable mold and compressing the same by a suitable punch, with values of pressure applied generally higher than 5000 Kg/cm2. Compression may be followed by a sintering step, wherein the pellet undergoes a thermal treatment at temperatures comprised between about 700 and 1000° C. under vacuum or inert atmosphere. While in the case of compression only the getter devices have generally the shape of a pellet, and also when sintering is carried out, which increases the mechanical resistance of the finished body, other shapes can also be obtained, such as relatively thin tablets.
As an alternative, the getter device comprises powders of the composition according to the invention supported on a suitable mechanical substrate, generally of metal. The substrate can be a metallic strip or sheet, in which case the powders of the composition can be deposited by cold rolling or screen-printing, followed by sintering. Cold rolling is a well known technique in the field of powder metallurgy, while the production of deposits of getter material by screen-printing is disclosed in U.S. Pat. No. 5,882,727. The substrate can also be a container of various shapes, provided with at least an open portion through which the composition of the invention can come into contact with the space from which the gaseous impurities must be removed, such as a short cylinder wherein the mixture of powders is poured and in which the mixture is thereafter compressed by a suitable punch. In the case where the composition of the invention is introduced into a container, sintering is generally not required.
The invention will be further illustrated by the following examples. These non-limiting examples show some embodiments designed to teach those skilled in the art how to practice the invention and to represent the best considered mode to carry out the invention. Examples 1 through 10 refer to the gas absorption properties of compositions of the invention and of the prior art, before and after a treatment that simulates the frit-sealing process used in the manufacture of many devices including getter compositions. Example 11 refers to the release of hydrogen from some composition of the invention after frit-sealing.
A pellet having a thickness of 0.5 mm and a diameter of 4 mm is prepared, employing 0.10 g of powdered titanium having a particle size of less than 40 μm and 0.15 g of powdered alloy having a weight percent composition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-MM 3% with a particle size of about 125 μm. The pellet is produced by compression only under 10,000 Kg.
The thus produced pellet is treated in air at 450° C. for 20 minutes to simulate the conditions of a frit-sealing treatment. The pellet is then activated by thermal treatment under vacuum at 350° C. for two hours.
A carbon monoxide (CO) sorption test at room temperature is carried out on the thus-treated pellet, following the procedure described in the standard ASTM F 798-82, by operating with a CO pressure of 4×10−5 hPa. The results of the test are graphically shown as curve 1 in
The test of Example 1 is repeated, but in this case the pellet is subjected, after its formation by compression, to a sintering treatment under inert atmosphere at 870° C. for 40 minutes. A CO sorption test is carried out on the pellet, the results of which are shown in
The test of Example 2 is repeated, employing however a pellet obtained from a composition according to the prior art, formed of 0.10 g of powdered titanium and 0.15 g of powder of an alloy having a weight percent composition Zr 70%-V 24.6%-Fe 5.4%. A CO sorption test is carried out on the pellet, the results of which are shown in
The test of Example 1 is repeated, using however a pellet having a weight of 0.25 g comprised only of powder of the alloy of weight percent composition Zr 70%-V 24.6%-Fe 5.4%, already known in this field. A CO sorption test is carried out on the pellet. The results of this test are not shown in the drawing because this pellet has proved to have a sorption capacity equal to zero in practice, and therefore the relevant absorption data were not detectable.
A pellet having a thickness of 0.5 mm and diameter of 4 mm is prepared, employing 0.10 g of powder of titanium having a particle size of less than 40 μm and 0.15 g of powder of an alloy having a weight percent composition Zr 72.2%-V 15.4%-Fe 3.4%-Mn 9% with a particle size of about 125 μm. The mixture of powders is compressed in a suitable mold under 10,000 Kg, and the pellet is then subjected to a thermal treatment of sintering at 870° C. for 40 minutes under vacuum.
Upon exposure to air (having the effect of passivating the pellet), the thus-produced pellet is activated by thermal treatment under vacuum at a temperature of 350° C. for 2 hours. A carbon monoxide (CO) sorption test at room temperature is carried out on the pellet, as described in Example 1. The results of the test are graphically shown as curve 4 in
The test of Example 5 is repeated with a new pellet, the only difference being that after its preparation, the pellet is treated at 450° C. in air for 20 minutes to simulate the conditions of a frit-sealing treatment. The pellet is then activated by thermal treatment under vacuum at 350° C. for 2 hours. A CO sorption test is carried out on this second pellet under the same conditions of the preceding test. The results of the test are graphically shown as curve 5 in
The procedure of Example 5 is repeated, employing in this case the pellet preparation of 0.15 g of powder of an alloy having the weight percent composition of Zr 76.7%-V 16.4%-Fe 3.6%-MM 3.3%, wherein MM means a mixture of weight percent composition of 50% cerium, 30% lanthanum, 15% neodymium, and the balance 5% of other Rare Earths.
The results of the CO sorption test on this pellet are shown in
The procedure of Example 6 is repeated, but using a pellet obtained with the alloy of Example 7.
The results of the CO sorption test on this pellet are shown in
The procedure of Example 5 is repeated, but using 0.15 g of powder of an alloy of the weight percent composition Zr 70%-V 24.6%-Fe 5.4% for obtaining the pellet.
The results of the CO sorption test on this pellet are shown in
The procedure of Example 6 is repeated, but using a pellet prepared with the alloy of Example 9.
The results of the CO sorption test on this pellet are shown in
This example refers to the release of hydrogen from compositions of the invention after frit-sealing.
By using compositions of the invention, a series of specimens in the form of pellets of thickness 0.5 mm and diameter 4 mm are prepared following the procedure of Example 1. Weight percentages of the components and pre-oxidation conditions of the specimens are given in the following table:
The getter alloy employed is always the one of Example 1, that is, the alloy of weight percent composition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-NM 3%. Also, particle sizes of powders of the different components are as given in Example 1 (nickel and cobalt, when present, have the same particle size as titanium).
Hydrogen content analyses are carried out on the thus-produced specimens, both on the fresh specimen and after a 20 minute treatment at 450° C. in the presence of 1.33 hPa of water vapor. This treatment simulates a frit-sealing used by some PDPs manufacturers, which is carried out under vacuum and in which the atmosphere is essentially composed of the water vapor released by the inner components of the screen (particularly, the phosphors). The hydrogen content of the various specimens is measured with a hydrogen analyzer model RH-402 of LECO Corp. of St. Joseph, Mich., USA. The tests results are reported in Table 2. The “Hin” and “Hfin” columns give, respectively, the hydrogen weight percent contained in the specimen before and after frit-sealing; the column “ΔH” gives the value of the difference Hfin-Hin for each specimen; and the column “Δ weight” gives the weight percent increase of the specimen due to water sorption.
Discussion of Results
As appears from comparing the sorption curves shown in FIGS. 1 to 3, the pellets produced with compositions of the invention show good sorption features upon frit-sealing, even better than those shown before frit-sealing.
In particular,
These tests also confirm that, in contrast to known compositions, in the case of compositions according to the invention upon frit-sealing at 450° C., it is sufficient to reactivate at a lower temperature (350° C. in the Examples) to obtain good sorption properties again.
The tests described in Example 11 refer instead to the capability of retaining hydrogen during frit-sealing by different compositions of the invention. In particular, the relevant data are those reported in Table 2. If all of the hydrogen were retained by the specimens, the “ΔH” value for each specimen should by equal to 1/9 of the “A weight” value. In practice, this doesn't happen, because the before part of hydrogen is released. By dividing the value “ΔH” in the column by the value “A weight” in the column, and then multiplying the result by 100, the percentage of hydrogen retained by the specimen compared to that sorbed upon water sorption is obtained. The higher the value in the last column, the better the specimen is from the standpoint of its capability of retaining hydrogen. From the results in Table 2, it is seen that substitution of part of the titanium with nickel and cobalt, and in particular the substitution of 10% by weight of titanium with nickel, allows the hydrogen from the compositions of the invention to be sensibly decreased.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2002A 001950 | Sep 2002 | IT | national |
| MI2002A 002280 | Oct 2002 | IT | national |
| MI2003A 001392 | Jul 2003 | IT | national |
This application is a continuation of International Application No. PCT/IT2003/000522, filed Aug. 28, 2003, which was published in the English language on Mar. 25, 2004, under International Publication No. WO 2004/024965 A2 and the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IT03/00522 | Aug 2003 | US |
| Child | 11057806 | Feb 2005 | US |
