WO2003029502A2

WO2003029502A2 - Getter alloys for the sorption of hydrogen at high temperatures

Info

Publication number: WO2003029502A2
Application number: PCT/IT2002/000591
Authority: WO
Inventors: Claudio Boffito; Bennie Josephus De Maagt
Original assignee: Saes Getters S.P.A.; Koninklijke Philips Electronics N.V.
Priority date: 2001-09-28
Filing date: 2002-09-17
Publication date: 2003-04-10
Also published as: AU2002339736A1; WO2003029502A3; ITMI20012033A1; ITMI20012033A0

Abstract

Yttrium-based getter alloys are described which are provided with high capacity of sorbing hydrogen and a low equilibrium pressure of said gas at temperatures of about 700 °C, to be used in hydrogen-sensitive high temperature applications. Some getter devices making use of these alloys are also described.

Description

"GETTER ALLOYS FOR THE SORPTION OF HYDROGEN AT HIGH

TEMPERATURES"

The present invention relates to getter alloys for sorbing hydrogen at high temperatures.

A number of industrial or research applications require, for proper working, a hydrogen- free environment in a closed vessel; the inner space of the vessel can either be kept under high vacuum or filled with an atmosphere of a given gas (or mixture of gases). Typical examples of devices for these applications where hydrogen is detrimental are evacuated jackets for thermal insulation, because of the high thermal conductivity of this gas; or some kinds of gas-filled lamps, where the presence of hydrogen in the gas generally leads to the modification of the physical parameters of lamp functioning (e.g., the ignition voltage). The manufacturing processes of these devices comprise a step of evacuating the vessel and, optionally, refilling it with the desired gas, but whenever a high vacuum or a hydrogen-free pure gas are obtained, mechanisms exist that tend to reintroduce hydrogen into the vessel. These mechanism are mainly represented by the outgassing from walls defining the vessel, or permeation of hydrogen through these walls from the external atmosphere into the vessel, thus leading to problems in the proper working of said devices.

These problems are normally dealt with by placing in said devices a getter material, that is, a material having the capability of chemically fixing molecules of hydrogen, as well as of other gases such as water, oxygen or carbon oxides. Getter materials are generally elements in the III, IV and V group of transition metals or their alloys with other metals, generally transition metals or aluminum. The getter materials most widely employed are titanium-based, and particularly, zirconium- based alloys. These materials and their use for sorbing gases from evacuated spaces or inert gases are well known and described in a number of documents, such as US Pat. 3,203,901 (Zr-Al alloys), US Pat. 4,071,335 (Zr-Ni alloys), US Pat. 4,306,887 (Zr-Fe alloys), US Pat. 4,312,669 (Zr-V-Fe alloys), US Pat. 4,668,424 (Zr-Ni-Rare Earths) and US Pal. 5,961,750 (Zr-Co-Rare Earths). Getter alloys show a different sorption behaviour for hydrogen compared to other gases. While for most gases the chemical sorption by these alloys is irreversible, the sorption of hydrogen from getter alloys is an equilibrium process, reversible depending on temperature: hydrogen is efficiently sorbed at relatively low temperatures (below 200-400 °C, depending on the chemical composition of the actually employed alloy), but it is released at relatively high temperatures, generally above 500-600 °C.

Operation temperatures below about 500 °C cover the majority of practical uses, but there are some special applications involving very high temperatures where very low levels of hydrogen are required. A possible industrial application of this kind are the X-ray tubes, made up of an evacuated tube where an anode and a cathode are present: the anode gives off X-rays when hit by electrons emitted by the cathode. During the electronic bombardment the anode raises its temperature, thus outgassing hydrogen (among other gases); if not properly absorbed, this can build up in the tube leading to a reduction in the number of electrons reaching the anode. Another example of vacuum devices where very high temperatures are reached are the so-called "power tubes". Under this general definition are grouped different kinds of electronic tubes, having different shapes and dimensions and used for different purposes; these tubes have in common the fact of comprising a vacuum space containing at least a cathode for electron emission, and an anode receiving the electrons and optionally one or more grids for controlling electrons motion, and the fact that currents travelling in the tube are of high intensity, so that the cathode and the anode can undergo intense heating. Power tubes includes for instance klystrons and magnetrons, used in radio-frequencies communications, or tubes for rectifying high intensity alternate currents. Finally, another important industrial application where hydrogen must be sorbed at high temperatures is in some lamps, in particular small dimensions high pressure metal halide lamps. These lamps consist of a tube of quartz or translucent alumina, called "burner", at the inside of which there are two electrodes (generally made of tungsten) and a lamp atmosphere of a gaseous mixture comprised of a rare gas (generally argon at about 80 mbar), few milligrams of mercury and vapors of metal halides, for example sodium halides and halides of Rare Earth metals. In operation, a plasma is formed in the burner causing halides dissociation, with emission of the characteristic wave-lengths of the resulting metallic atoms or ions, and the walls of the burner reach temperatures in the range 800-1000 °C. At these temperatures, the walls of the burner are easily passed through by gases, such as hydrogen. Hydrogen in the burner is detrimental to operation of these lamps already at extremely low concentrations according to various mechanisms: for instance, when present in the burner, hydrogen leads to an increase of the potential needed for triggering the discharge (so-called "ignition voltage") with consequent possible evaporation of the metal of cathodes, its deposition onto the inner walls of the burner and blackening of the lamp. Furthermore, the hydrogen present in a bulb enclosing the burner gives rise in certain types of lamps to a complex sequence of transfer of gas from the bulb to the burner and of sodium from the burner to the bulb, with the consequence of a modified luminous efficacy of the lamp. Finally, hydrogen may be a carrier of oxygen (both in the form of water molecules or OH radicals) in the lamp atmosphere: in the presence of dysprosium bromide (that's often present in the lamp gas filling), oxygen reacts to form dysprosium oxybromide, that's known in the field to cause devitrification of the quartz burner walls, with consequent opacization and reduction of light output (a phenomenon known as the "wall-attack" problem); to the contrary, when no hydrogen is present, oxygen is fixed in the quartz lattice, thus being not available for dysprosium oxybromide formation. To reduce hydrogen migration into the burner, this is inserted in a controlled-atmosphere container, an external glass bulb that can be filled with nitrogen but more usually (about 90% of the manufacture of these lamps) is kept evacuated. However even this measure does not fully solve the problem of hydrogen pressure build-up in the burner, as this gas can be released by the components of the lamp itself (for instance, through high- temperature dissociation of the traces of water introduced in the burner with the hygroscopic halide salts), or in any case can permeate through the external bulb owing to the high temperatures to which the latter is subject during lamp operation. In lamps of medium or large size it is always possible to find a position in the outer bulb away enough from the burner to have a temperature below about 400 °C; a getter of a known kind placed in this "cold" position is effective in hydrogen sorption (both the hydrogen that can possibly enter the bulb from the external atmosphere and the hydrogen coming from inside the burner); most commonly used getter devices for this purpose are based on an alloy of weight percent composition Zr 84% - Al 16%, described in US Pat. No. 3,203,901, produced and sold by SAES Getters of Lainate, Italy, under the name St 101. On the other hand in small-size lamps (also referred to in the field as "compact" lamps), in which the burner has a diameter generally between about 1 and 2 cm and length up to 4-5 cm, while the external bulb has a diameter up to 4 cm and length of less than about 10 cm, in operation no parts of the lamp are at temperatures lower than about 600 °C. As explained above, at these temperatures the traditional zirconium-based getter alloys cannot avoid hydrogen build up in the outer bulb and, consequently, in the burner. Pure yttrium is known, for instance by US Pat. 3,953,755, to have a low equilibrium pressure of hydrogen at high temperatures, but its properties are not sufficient in applications such as small size high pressure metal halide lamps.

Another property required to getter alloys to be used in high-temperature applications is that they have vapor pressure as low as possible: metal vapors coming from the alloy could interfere with the proper working of the device where they are used, for instance in the case of lamps by depositing of the walls of the bulb leading to the "blackening" phenomenon of the lamp.

Object to the present invention is that of providing getter alloys for sorbing hydrogen at elevated temperatures, generally in excess of about 600 °C, as well as that of providing getter devices for the use of said alloys.

This object is achieved according to the present invention through the use of yttrium-vanadium alloys comprising from 90%> to 98% by weight of yttrium or yttrium-tin alloys comprising from 80% to 90% by weight of yttrium.

The invention will be described in the following with reference to the drawings in which:

- Figures 1-7 show several possible embodiments of getter devices prepared with the alloys of the invention;

- Figure 8 schematically shows the system for measuring the equilibrium pressure of hydrogen over a getter material;

- Figure 9 shows in a graph the curves of the hydrogen equilibrium pressure for two alloys of the invention and, for comparative puiposes, for pure yttrium.

Alloys containing percentages by weight of yttrium higher than those previously indicated have essentially the same hydrogen sorption features of yttrium, insufficient for some high temperature applications; alloys with yttrium percentage by weight lower than those indicated have, at the foreseen operational temperatures, a higher vapour tension with respect to the alloys of the invention and could give rise to undesired evaporated fractions.

Among the yttrium-vanadium alloys, the preferred one is that of percent composition by weight Y 96%-V 4%, and among the yttrium-tin alloys the preferred one is that of percent composition by weight Y 84%-Sn 16%. The alloys of the invention can be obtained by melting from the pure elements, preferably in powders or pieces, in the desired weight ratios. Melting must be carried out under controlled atmosphere, e.g. under vacuum or inert gas (argon is preferred) to prevent oxidation of the alloy being prepared. Depending on their actual composition, these alloys undergo melting at temperatures comprised between about 1100 and 1450 °C.

The alloys of the invention can be used in form of getter devices made in a single body of alloy. Figs. 1-3 show devices of this type. Figs. 1 and 2 respectively show a small cylinder 10 and a tablet 20 being obtained by shearing a sheet of alloy of suitable thickness. For a practical use, the devices must be placed in a fixed position in the vessel to be kept free of hydrogen. Devices 10 and 20 could be fixed directly onto an inner wall of said vessel, e.g. by spot welding when said wall is made of metal. As an alternative, devices of kind 10 or 20 can be positioned in the vessel by means of suitable supports; mounting on a support can be obtained by welding or mechanical compression. Fig. 3 shows another possible embodiment of getter device, 30, wherein a discrete body of alloy according to the invention is used. In this case the alloy is formed as a strip, from which lengths of desired size are cut; a strip piece 31 is folded in the zone 32 around the support 33 in form of a metal wire; support 33 can be linear but preferably it forms bends 34, 34', 34" which help the positioning of piece 31; the maintenance of the piece shape can be ensured with one or more welding spots (not shown in the drawing) at the overlapping area 35, but even a simple compression during the folding about support 33 can be sufficient, thanks to the plasticity of these alloys.

As an alternative, getter devices can be obtained using powders of the alloys according to the invention. When using powders, these have preferably a particle size of less than 500 μm and, even more preferably, comprised between 40 and 125 μm.

Powder-based devices are represented in Figs. 4-7. Fig. 4 shows a cutaway view of a device 40 having the shape of a pellet 41, in which a support 42 has been inserted; a device of this type can be obtained for example by compression of powders in a mould, having placed the support in the mould before pouring the powders therein. Alternatively the support 42 can be welded onto pellet 41. Fig. 5 shows a device 50 consisting of powders of an alloy 51 of the invention, compressed in a metallic container 52; device 50 can be fixed to a support (not shown in the drawing) for instance by welding the same to container 52. Finally Figs. 6 and 7 show different views of another possible embodiment of a getter device according to the invention. This type of device is formed of a support 60 obtained from a metal sheet 61: a depression 62 is firstly formed in the sheet by pressing in a suitable mould (not shown); then, a portion of the depression bottom is removed by shearing, thus obtaining a hole 63; the support is kept in the compression mould and the depression is filled with powders of the getter alloy which are then compressed therein thus obtaining the device 70 (shown in cross section along line A- A' of Fig. 6) in which a packet of powders 71 has two exposed surfaces 72 and 73 for gas sorption.

In all the devices of the invention the supports, containers and any possible other metallic part not being formed of an alloy of the invention are made of metals with the lowest possible vapor pressures, such as tungsten, tantalum, niobium or molybdenum, to prevent these parts from evaporating in consequence of the high working temperatures said devices are exposed to.

The invention will be further illustrated by the following examples. These non-limiting examples describe some embodiments which are intended to teach to those skilled in the art how put into practice the invention and to show the best considered mode for embodying the invention. EXAMPLE 1

This example relates to the measurement of hydrogen equilibrium pressure over the alloy having the weight percent composition Y 96%-V 4%. For this measurement a system is employed as schematically represented in

Fig. 8, being formed of a hydrogen reservoir S communicating, through a needle valve Ni, with a chamber C, to which a capacity manometer MC is connected. Chamber C communicates, through a liquid nitrogen trap T (having the function of blocking impurities, mainly water, in the gas) and a valve V₂, to the measuring chamber M provided with a heating system (not shown in the drawing). A sample holder P is placed in chamber M and has a temperature that can be measured by means of the thermocouple TC. Chambers C and M are connected, respectively through valves V₃ and V , to a pumping system (not shown in the drawing). The isolated space obtained by closing valves Vi, V₂ and V has a known volume, referred to as V_dos (hydrogen dosing volume), that in the case of the test is of 1.1 liters. Similarly the isolated space obtained by closing valves Ni, V₃ and V while opening valve V has a known volume, referred to as V_tot (total volume of the measuring system), equal to 2.2 liters in the case of the test. 1.2 grams of powder of the alloy having weight percent composition Y 96%-V 4% are placed in the sample holder P. Valve Ni is closed, the other valves of the system are opened and chambers C and M are evacuated by means of the pumping system until reaching a pressure of 10^"5 mbar. Still under pumping the sample is activated by induction heating from the outside of chamber M with a treatment at 800 °C during 10 minutes. Thereafter the sample is brought to the test temperature, in this case being 700 °C. Valves V₂, V₃ and N₄ are closed and valve Ni is opened thus letting hydrogen in chamber C until reaching a pressure of 2.6 mbar, whereupon valve Vi is closed; this pressure is referred to as P_ιn (starting pressure). Valve V₂ is opened thus causing hydrogen to expand in chamber M and to be partially sorbed by the sample. The decrease of pressure in the system is monitored, until it reaches a steady value of 3 x 10^"4 mbar: this is the equilibrium pressure, P_eq, of hydrogen over the sample in the test conditions. From the knowledge of values Ntot. j_os, P,_n, P_eq and from the sample mass, called M_c, the quantity of hydrogen sorbed by the sample, referred to as Q_r, is calculated through the relation:

Qr = [(P,_n V_dos) - (P_eq χ V_tot)]/M_c

The thus obtained value of Q_r gives the first point of the hydrogen equilibrium pressure over the alloy.

The test is repeated three times by placing in the chamber M another sample and letting into the system a hydrogen dose different at each subsequent test.

The values obtained in the four tests are plotted as curve 1 in the graph of Fig. 9 showing the hydrogen equilibrium pressure P (in mbar) as a function of the quantity of gas sorbed per gram of alloy (Q, measured in mbar x 1 / g).

EXAMPLE 2

The test of Example 1 is repeated with four samples of powder of the alloy having the weight percentage composition Y 84%-Sn 16%.

The results of the test are reported as curve 2 in the graph of Fig. 9. EXAMPLE 3 (COMPARATIVE)

The test of Example 1 is repeated with four samples of powder of pure yttrium. The results of the test are reported as curve 3 in the graph of Fig. 9.

EXAMPLE 4

In tins example the working characteristics of lamps with a getter of the invention, lamps with a prior art getter and lamps with no getter are compared.

Six strips comprising 35 mg of the alloy Y 96%-V 4% are treated with an extra pre-baking step of 5 hours at 650 °C under vacuum, and subsequently placed in the outer bulb of six compact high-pressure metal halide discharge lamps; the burner of these lamps is made of quartz, and the gas filling comprises mercury, argon and halides of dysprosium, holmium, gadolinium and cesium. The lamps have a nominal power of 575 Watt and a wall-load of at least 45 W/cm². The lamps are operated for 500 hours, during which re-ignition voltage and wall-attack are observed; wall-attack is measured as the percentage of surface of the burner that has become opaque, and its values are estimated by visual inspection. Values of re-ignition voltage and wall-attack for the six lamps are averaged and given in table 1 below.

For comparison purposes, the same tests are run with five lamps containing a known getter, i.e. small pills with a nominal content of 90 mg of the cited St 101 alloy, and with six reference lamps, i.e. lamps of the same kind but without a getter. The averaged data obtained on these lamps are given in table 1.

Table 1.

The results of tests prove the better hydrogen sorption characteristics of the alloys of the invention. The curves in Fig. 9 show that, at 700 °C, an alloy of the invention for sorbed quantities of hydrogen in the range 1-5 mbar x 1 (per gram of alloy) has an equilibrium pressure of hydrogen that is about one order of magnitude lower than that of pure yttrium, thus allowing a better removal of this gas at high temperature. This finding is also the case with the tests in lamps: the results in table 1 clearly show that the lamps of the invention have favourable properties compared to lamps with prior art getters, here St 101 (Zr-Al), or no getter, i.e. a comparatively low value of the re-ignition voltage peak and a retarded degree of the wall-attack, that indirectly confirm a lower amount of hydrogen in the lamp.

The curves of Fig. 9 can also be used for obtaining the quantity of getter material required to ensure in a closed space a pressure of hydrogen lower than a desired value; by fixing for example this value at 10^"3 mbar it is possible to observe from the graph that the alloys of the invention have a sorption capacity that is about three times as much as that of pure yttrium; therefore an alloy of the invention can guarantee, with respect to pure yttrium, the same hydrogen pressure with about one third of weight, thus allowing to considerably reduce the size of the getter device, with clear advantages in particular when the vessel to be kept hydrogen-free has a limited volume.

Claims

1. Getter alloys for sorbing hydrogen at high temperatures, chosen among:

- yttrium-vanadium alloys comprising from 90% to 98% by weight of yttrium; and

- yttrium-tin alloys comprising from 80% to 90% by weight of yttrium.

2. Yttrium-vanadium alloy according to claim 1 having the percentage composition by weight Y 96%-V 4%.

3. Yttrium-tin alloy according to claim 1 having the percentage composition by weight Y 84%-Sn 16%.

4. Getter devices comprising an alloy of claim 1.

5. Getter devices according to claim 4, formed of a discrete piece of alloy.

6. A getter device (10) according to claim 5 of cylindrical shape.

7. A getter device (20) according to claim 5 of parallelepiped shape.

8. A getter device (30) according to claim 5 formed of a piece of strip (31) of alloy of claim 1 folded in a zone (32) about a metallic support (33).

9. A getter device (30) according to claim 8 wherein said support is formed with bends (34, 34', 34") for keeping at the desired position the piece of strip (31).

10. A getter device according to claim 4 made of powders of alloy.

11. A getter device according to claim 10 wherein said powders have a particle size of less than 500 μm.

12. Getter devices according to claim 11 wherein said powders have a particle size between 40 and 125 μm.

13. A getter device (40) according to claim 10, consisting of a pellet of powders (41) wherein a support (42) is inserted.

14. A getter device (50) according to claim 10, consisting of powders (51) compressed in a metallic container (52).

15. A getter device (70) according to claim 10, consisting of: a support (60) formed of a metal sheet (61) with a depression (62) obtained by pressing said sheet and with a hole (63) at the bottom of said depression obtained by cutting said bottom; and a packet of alloy powders (71) in the depression, with two exposed surfaces (72, 73) for gas sorption.

16. A power tube containing a getter alloy of claim 1.

17. A X-ray generating tube containing a getter alloy of claim 1.

18. A high pressure metal halide lamp containing a getter alloy of claim 1.

19. A lamp according to claim 18, characterized in that the alloy is yttrium- vanadium comprising from 90 % to 98 % by weight of yttrium.