GB1572000A

GB1572000A - Getter device

Info

Publication number: GB1572000A
Application number: GB5170076A
Authority: GB
Original assignee: SAES Getters SpA
Current assignee: SAES Getters SpA
Priority date: 1975-12-12
Filing date: 1976-12-10
Publication date: 1980-07-23
Also published as: DE2656100A1; JPS6046775B2; FR2335033A1; BE853855A; DE2656100B2; FR2335033B1; JPS5284960A; NL7613694A; NL168645C; DE2656100C3; IT1050069B; NL168645B

Description

(54) GETTER DEVICE (71) We, S.A.E.S. GETTERS S.p.A., an Italian Company, of Via Gallarate, 215, Milano, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to getter devices.

Exothermic getter devices which release an evaporable getter metal such as barium in a vacuum vessel are well known. Such getter devices usually comprise a pulverulent barium-aluminum alloy of approximate composition BaAl4 in admixture with from about 20 to 80% and preferably about 40 to 60% and more preferably about 50% by weight of an additional metal capable of reacting exothermically with the barium-aluminum alloy. This corresponds to weight ratios of 4:1 to 1:4. Such co-reactant may be Ni, Ti, Fe, Mo or alloys such as intermetallic compounds of nickel with titanium etc.However, nickel is the most preferred additional metal due to its availability at low cost, its ease of handling, its relative stability in ambient conditions and its ability to react exothermically with bariumaluminum alloys with the required degree of exo-thermicity to control the rate of barium evaporation and hence the getter device flashing characteristics. The use of intermetallic compounds to promote the exothermic reaction is relatively expensive as it involves an additional production stage in the manufacture of the exothermic getter device.

During the manufacture of electric discharge tubes such as colour display tubes the getter device is subjected to a wide variety of manufacturing and environmental conditions. Exposure of the getter device to excessive atmospheric humidity or to water during the washing of the electrode system of which the getter device forms a part can cause deterioration of the materials comprising the getter device. The water or water vapour may react with the barium-aluminium alloy to produce barium hydroxide which may in its turn react with other constituents of the atmosphere such as CO2 to produce further chemical compounds.These compounds decompose during evaporation of the getter metal in the finished sealed electron tube with the release of noxious gases which may influence in an undesirable way the composition of the residual gas atmosphere within the tube resulting in poor quality tubes having only a short working life.

Frequently it is desirable to have the getter device already mounted within the glass envelope of a colour display tube during the process whereby the phosphor bearing face-plate or screen is hermetically sealed onto the glass cone. This process generally takes place in air, rather than vacuum, at atemperature of about 400"C or more. This high temperature air exposure can again cause deterioration presumably by oxidation of one or more of the materials comprising the getter device to such an extent that the exothermicity of the barium release reaction becomes so high that this reaction is almost explosive in nature, releasing barium in the form of molten particles rather than as a vapour as well as particles of getter alloy mixture.Such uncontrolled release of barium and other particles is detrimental as the particles can later fall onto the electrode structure in the electron tube causing electrical faults. In a colour display tube the particles may also block the small apertures in the shadow mask, causing areas of the phosphors to be prevented from being excited by the electron beam with a consequent reduction in quality of the displayed image.

British Patent Specification No. 1,372,823 proposes to use organic materials such as nitrocellulose, methyl acrylate resins and polyimide resins dissolved in an organic solvent as impregnating agents to form a mechanical protective coating on the getter devices. Polyurethane coatings have also been suggested. However, it is well known that thin mechanical coatings are very prone to the formation of pinholes and therefore afford less protection than is desirable. Thicker, pin-hole free coatings both undesirably increase the weight of the getter device and make it difficult completely to eliminate the solvent. During evaporation of the getter metal charred remains of the decomposed organic material layer are caused to detach from the getter device surface, causing the same disadvantages as with the evaporation of molten barium particles.

Most organic materials decompose at the temperatures required to seal the face plate of a colour display tube to the cone and thus provide very little protection during this process. In this case it is usually necessary to replace the nickel of the getter alloy mixture with other materials such as the titanium-nickel intermetallic compounds to give a less exothermic reaction and to avoid explosive evaporation of barium.

Furthermore, large amounts of impregnating material lead to large quantities of decomposition product gases which can be harmful to cathode activity and tube life.

It is also desirable to have a single type of getter device which can be subjected to either a water washing operation or a high temperature air treatment or both without alteration of its evaporation characteristics so that it is not necessary to provide stocks of different devices each capable of being subjected to only one of the above treatments.

The present invention provides a getter device comprising a mixture of a barium alloy and a material, especially a metal, capable of reacting exothermically therewith, the surfaces of said mixture being coated with an organo-silicon compound.

The present invention also provides a method of manufacturing a getter device, which method comprises the steps of pressing a pulverulent mixture of a barium alloy, preferably a barium-aluminium alloy, and a material, preferably a metal, capable of reacting exothermically therewith into a holder and contacting the surface(s) of the mixture with an organo-silicon compound so as to form a coating thereon.

The present invention further provides an electric discharge tube especially a colour display tube, which has been manufactured by using such a getter device.

The present invention is based on our surprising observation that an improved getter device which is substantially free from one or more of the disadvantages of previously proposed devices can be manufactured by providing a coating of an organo-silicon compound on the surfaces of the getter mixture. In particular, it has been found that the getter devices of the present invention do not appreciably deteriorate on exposure to water or water vapour or on exposure to high temperatures in air. In addition, the devices do not substantially alter the gas atmosphere within an electric discharge tube after evaporation of the getter metal.

Thus the air bakeable, water-proof getter devices of the present invention show many advantages over similar devices in which the getter mixture has not been treated with an organo-silicon compound.

Suitable organo-silicon compounds include aliphatic and/or aromatic materials including, for example, monomeric siloxanes, polysiloxanes, aliphatic silanes, aromatic silanes and substitution products thereof. One preferred group of organo-silicon compounds are low and high molecular weight polysiloxanes having the general formula (I)

wherein R1, R2, R3, R6, R7 and R8 are independently selected from the group consisting of aliphatic, aromatic, aliphatic-aromatic and aromatic-aliphatic radicals, preferably hydrocarbon radicals, and hydrogen atoms, for example alkyl, aryl, aralkyl and alkaryl radicals, with the proviso that at least one is selected from the group consisting of alkyl, aryl, aralkyl and alkaryl; wherein R4 and R5 are independently selected from the group consisting of aliphatic, aromatic, ali-phaticaromatic and aromatic-ali-phatic radicals, preferably hydrocarbon radicals, for example alkyl, aryl, aralkyl and alkaryl radicals, and wherein "n" is generally an integer from 0 to 20,000 inclusive and is preferably an integer from 0 to 1,000 inclusive.

Examples of suitable aliphatic radicals are alkyl and alkenyl radicals, for example, methyl, ethyl, propyl, t-butyl, dodecyl and allyl. Examples of suitable aryl radicals include, amongst others, phenyl and naphthyl. Examples of suitable aralkyl radicals include, amongst others, benzyl and 2-phenylpropyl. Examples of suitable alkaryl radicals include, amongst others, 3-methyl-phenyl and 4-butyl-naphthyl.

Any of the above radicals can be substituted by one or more non-interfering moieties such as halogens, e.g.chlorine or bromine. However, there is a widely held belief in the electronic industry that halogens have a damaging effect on the cathode activity of electronic discharge tubes. For this reason it is preferred to use non-halogenated compounds.

Especially preferred polysiloxanes are those of the following formulae (II) and (III)

(polydimethylsiloxane)

( rri-methyl pentaphenyl tri-siloxane) as well as tetramethyl, tetra-phenyl tri-siloxane.

The getter mixture may be coated with the organo-silicon compound by various methods, for example, in one preferred method the getter device may be dipped into a bath of the organosilicon compound which may be present as a pure liquid or in mixture with a suitable solvent. In another preferred method the getter device may be placed in an evacuated and sealed vessel together with a suitable amount of the organosilicon compound which is then heated to raise its vapour pressure so that the vaporised molecules contact the surfaces of the getter mixture and form a coating thereon.

The getter device is generally held at a temperature higher than that of the bath of the organo-silicon compound so that no liquid condensation may take place on the devices.

Furthermore the temperature of the getter devices should be such that a monomolecular layer of a cross-linked organo-silicon compound forms on the surface of the getter device. The formation of such layers has been described in U.S.

Patent No. 3,901,761, however, this patent gives no indication that such a monomolecular layer provided on an exothermic getter device would be capable of preventing explosive evaporation of barium after water exposure or water vapour exposure or a high temperature air exposure.

Preferably the temperature of the getter device should be kept higher than about 100"C in order that the organo-silicon compound may form a cross-linked monomolecular layer chemically bonded to the surfaces of the getter device. The highest temperature at which the getter device should be maintained is that temperature at which barium commences to evaporate, which is about 700"C to 7500 C.

However, it is convenient to maintain the getter device at a temperature below the flash point of the particular organo-silicon compound being used. This ensures that no explosive reaction takes place between organo-silicon vapours and atmos pheric oxygen in the event of accidental breakage of or leaks in the evacuated vessel.

In addition to the barium alloy and the exothermic co-reactant, the getter mixture may, for example, further comprise gas releasing materials such as Fe4N or TiH2, as well as thermic moderators such as tungsten.

One form of getter device according to the present invention, its manufacture and use in a colour display tube will now be described in more detail, and by way of example only, with reference to the accompanying drawings in which Figure 1 is a cross-section of a getter device; Figure 2 is a diagrammatic cross-section of an apparatus for manufacturing said getter device; and Figure 3 is a diagrammatic partial cross-section of a colour display tube provided with two such getter devices.

Referring now to the drawings and more especially to Figure 1, there is shown an exothermic getter device 10 of the present invention. Exothermic getter device 10 comprises a stainless steel holder 11 in the form of an annular ring having a Ushaped channel cross section. Within the holder 11 is a pulverulent barium aluminum alloy 12 mixed with pulverulent nickel 13. A cross-linked organo-silicon compound coating 14 is thermally bonded to the surface of the powders 12, 13.

Figure 2 shows schematically, an apparatus 20 suitable for performing the preferred process of the present invention. Apparatus 20 comprises a glass vessel 21 evacuated and sealed at one end 22 by normal glass blowing techniques. Within vessel 21 is a wired support structure 23 supporting a number of exothermic getter devices 10', 10", 10"'. Furthermore the glass vessel 21 contains a non-halogenated organo-silicon fluid 24.

Surrounding the portion of vessel 21, which contains the organo-silicon fluid 24, is placed a first heating device 25. A second heating device 26 in the form of a spiral coil of resistance wire is placed around vessel 21 to heat the exothermic getter devices 10', 10", 10"'. A cooling coil 27 is placed around vessel 21 in a zone between the first and second heating devices to ensure that condensation as a liquid of the organo-silicon vapour takes place preferentially in this zone rather than on the exothermic getter devices 10', 10, 10"'.

In operation the getter devices 10', 10", 10"' are heated to about 200"C by means of coil 26. The organo-silicon fluid is heated to about 200"C by means of the heating device 25. Cooling coil 27 maintains the zone between the two heating devices at about 180"C. The getter devices are maintained at 2000C for 1 hour after which the apparatus is allowed to cool to room temperature and the getter devices are removed from the vessel. They are now ready to be used in the fabrication of electric discharge tubes such as a colour display tube.

- Figure 3 shows a partial cross-section of a colour display tube 30 comprising a shadow mask 31, an electron gun assembly 32 and an iron screening element. A first exothermic getter device 10' treated according to the above organo-silicon coating process is assembled onto the electron gun. The getter device and electron gun are previously washed in tepid water, which may also contain a small amount of surface active agent. A second getter device 10", again treated according to the above process, is attached to the iron screen 33. The second getter device 10" does not have to be washed but it may remain exposed to the atmosphere and hence humidity for relatively long times before final tube assembly. Furthermore, the second getter device 10" is already attached to iron screen 33 during sealing of the face plate 34 to the cone 35 by means of the hermetic seal 36.This sealing process takes place at 4000C or more and lasts for 1 hour.

The following examples illustrate the invention, all parts and percentages being by weight unless otherwise indicated.

Example 1.

Six identical exothermic getter devices were manufactured each comprising a stain-less steel holder in the form of a ring of 18mm outside diameter having a Ushaped channel cross-section. In each channel were placed 320mg of a pulverulent mixture, of approximately 50% by weight of an alloy of 50% by weight barium and aluminum, with 50% by weight of nickel. Three of the getter devices were placed on a wire support within a glass vessel. Approximately lOcc of tri-methyl pentaphenyl tri-siloxane was also placed in the glass vessel, out of direct contact with the getter devices. The glass vessel was evacuated by a rotary pump and then sealed.

The sealed vessel was then placed in an oven at 2000C for 1 hour. After removal from the oven the vessel and its contents were allowed to cool to room temp erature and the treated getter devices were removed.

All six getter devices were then heated in air at 4200C for 1 hour to simulate the colour display tube face plate to cone sealing process.

The six getter devices were each placed in separate glass vessels, which were then evacuated by a rotary and a diffusion pump. Each getter device was then caused to evaporate barium by subjecting them to induction heating. The three getter devices which had not been treated with the organosilicon fluid exhibited an explosive evaporation of barium with particles of barium and the getter alloy mixture being violently ejected from stainless steel holder. The three getter devices treated with organo-silicon fluid showed a controlled evaporation of barium vapour with no signs of explosive reaction.

Example 2.

A further six getter devices were prepared as in Example 1, three of which were treated with tri-rnethyl pentaphenyl tri-siloxane exactly as described in Example 1.

The getter devices were then immersed in warm tap water. After about 20 seconds, the untreated getter devices started to evolve bubbles of gas, thus showing attack of the getter metal vapour releasing material by the water. Even after I minute the treated getter devices showed no sign of attack as no gas bubbles were evolved.

Example 3.

Getter devices were prepared as described in Example 1. The getter devices were immersed in di-methyl polysiloxane oil (AK 0.65 manufactured by Wacker Chemie GmbH, Monaco, Germany) by immersion in the oil for 15 minutes. The impregnated getter devices were then air-dried. ,, The getter devices were then heated in air at 4200C for 1 hour and "flashed" in a vacuum as described in Example 1.

The getter devices treated with di-methyl polysiloxane fluid showed a controlled evaporation of barium vapour with no signs of explosive reaction, Example 4.

Getter devices were prepared as in Example 3. After being air-dried, the getter devices were treated thermally in dynamic vacuum at 3000C for 60 minutes. Then the getter devices were heated in air at 4200 for 1 hour and "flashed" in a vacuum as described in Example 3. The results were similar to those of Example 3.

Example 5.

Getter devices were prepared as in Example 3. After being air-dried the getter devices were placed in warm water (about 60"C) and the time from immersion to copious bubble formation was observed visually. Bubbles formed rapidly on the getter devices.

Example 6 Getter devices were prepared as in Example 4. After the thermal treatment in dynamic vacuum at 3000C for 60 minutes the getter devices were washed with warm water as in Example 5. Bubbles formed on the getter devices after 2 1/2 minutes.

Examples 7 to 10 Examples 3 to 6 were repeated using 10% by volume of the dimethyl-poly siloxane oil with 90% by volume of xylene to impregnate the getters. Results were similar to those using 100% oil.

Examples 11 to 14 Examples 7 to 10 were repeated using a dimethyl polysiloxane oil with a chain length having approximately 200 silicon-oxygen groups per molecule. This oil of longer chain length proved preferable to the one used for Examples 7 to 10, as the getter devices showed greater resistance in the "H2O test".

Examples 15 to 18 Examples 7 to 10 were repeated using a dimethylpolysiloxane oil with a chain length having approximately 1400 silicon-oxygen groups per molecule. The results were similar to those found in Examples 11 to 14 except for a failure (rapid bubble formation) in Example 17.

Examples 19 to 22 Examples 11 to 14 were repeated using a methyl phenyl polysiloxane oil with a chain length of approximately 108 silicon-oxygen groups per molecule and approximately a 12% phenyl substitution in the chain. The results were similar to those obtained in examples 11 to 14, though with better results in Example 21 compared with the similar Example 13.

Examples 23 to 24 Examples 1 and 2 were repeated but using the same methyl phenyl polysiloxane used in Examples 19 to 22. The results were similar to those of Examples 1 and 2.

Examples 25 to 28.

These examples were carried out using methyl dichlorosilane (No. 5899 manufactured by Merck and Co., Inc., New Jersey, U.S.A.). The getter devices were exposed to the methyl dichlorosilane vapour in air for 60 seconds and then tested similarly to Examples 7 to 10. The results of the tests were all satisfactory.

Examples 29 to 32.

Examples 25 to 28 were repeated using di-chlorchlormethyl methyl silane (No.

804/86 Merck and Co., Inc., New Jersey, U.S.A.). The getter devices formed bubbles rapidly during the immersion in water test, but performed successfully in the "Frit" test.

Example 33.

This comparative example records the behaviour of additional untreated control getters as in Example 1.

Example 34.

This records the behaviour of untreated control getters used in each of the water washing tests described in the foregoing Examples.

The above Examples are summarized in Table 1 below, with its appended explanatory footnotes.

TESTS PERFORMED % H2O Frit Example "n" Oil Bulb Bath Oil Bake Test Test Comments No.

1 DC 705 X X OK - no explosion 1 1 DC 705 X X OK - resists more than 2 5 min 0 AK 0.65 X 100 X OK - no explosion 3 0 AK 0.65 X 100 X X OK - no explosion 4 0 AK 0.65 X 100 X fail 5 0 AK 0.65 X 100 X X reists 2-1/2 min. 6 0 AK 0.65 X 10 X OK - no explosion 7 0 AK 0.65 X 10 X X OK - no explosion 8 0 Ak 0.65 X 10 X fail 9 0 AK 0.65 X 10 X X resists 2-1/2 min. 10 200 AK 350 X 10 X OK - no explosion 11 200 AK 350 X 10 X X OK - no expolsion 12 200 AK 350 X 10 X resists 3 min. 13 200 AK 350 X 10 X X OK - resists more than 14 5 min.

1400 AK100,000 X 10 X OK - no explosion 15 1400 AK100,000 X 10 X X OK - no explosion 16 TESTS PERFORMED (Continued) % H2O Frit Example "n" Oil Bulb Bath Oil Bake Test Test Comments No.

1400 AK 100,000 X 10 X fail 17 1400 AK 100,000 X 10 X X OK - resists more than 18 5 min.

108? AS 200 X 10 X OK - no explosion 19 108? AS 200 X 10 X X OK - no explosion 20 108? AS 200 X 10 X OK - resists more than 21 5 min.

108? AS 200 X 10 X X OK - resists more than 22 5 min.

108? AS 200 X X OK - no explosion 23 108? AS 200 X X OK - resists more than 24 5 min.

- 5899 Vapour in Air X OK - no explosion 25 - 5899 " " " X X OK - noplosion 26 - 5899 " " " X Ok - resists more than 27 5 min.

- 5899 " " " X X OK - resists more than 28 5 min.

- 804/86 " " " X OK - no explosion 29 - 804/86 " " " X X OK - no explosion 30 - 804/86 " " " X fail 31 - 804/86 " " " X X fail 32 - 804/86 - - - X fail - explodes 33 - - - - - X fail - resists only 20 secs. 34 In the table "n" is the approximate number of silicon-oxygen groups per molecule of polysiloxane as shown in Formula I above.

In the table "oil" refers to polysiloxane and chlorsilane manufacturers' identification number. The number following the symbols AK and AS identifies the viscosity of the oil in centistokes.

In the table "bulb" identifies the method of impregnating the getter devices using the pure oil in an evacuated glass vessel.

In the table "bath" identifies the method of impregnating the getter devices by immersing them in the oil or oil-solvent mixture for 15 minutes.

In the table "% oil" gives volume Ó oil to xylene composition of the bath.

In the table "bake" refers to the thermal treatment in dynamic vacuum at 300"C for 60 minutes after air-dry.

In the table in the "H2O test" getter devices are placed in warm water (approx.

60"C) and the time until bubble formation is observed visually.

The "Frit test" referred to in the table is an air heat treatment for 1 hour at 420"C and subsequent "flash" in vacuum. "Explosion" is observed by visual observation of vigorous ejection of glowing particles from the getter device.

From the results obtained in the Examples as described above, it can be seen, by comparing Examples 3 to 6 with Examples 7 to 10, that use of a pure oil or an oil solvent mixture gives identical results.

Comparison of Examples 5, 9, 13, 17 show that without the thermal vacuum treatment ("bake") only oils of "n" about 108 give any substantial degree of water treatment protection. Comparison of Examples 6, 10, 14, 18 show that the "bake" treatment improved the water resistance considerably being satisfactory for "n" greater than zero. It can be seen from the results ofExamples 19to 22 that for methylphenyl oils the "bake" treatment may not be necessary.

Examples 3, 4, 7, 8, 11, 12, 15, 16 show that the oil-treated getter devices are "frittable" both with and without the "bake" treatment.

Comparison of Examples 19 to 22 with Examples 23 to 24 shows no difference between the efficiency of "bulb" and "bath" treatments.

It can be seen from the results of Examples 25 to 28 compared with those of Examples 29 to 32 that the halogenated silanes are rather selective in giving water protection.

The oils used in Examples 19 to 22 had approximately 10% phenyl substitution while Examples I and 2 had 62.5% phenyl substitution. Comparison shows similar results with both amounts of phenyl in the chain and for both lengths of the chain of siloxane groups (108 for Examples 19 to 22, 3 for Examples 1 and 2).

Various other modifications and variations falling within the scope of the present invention will be apparent to those skilled in the art.

WHAT WE CLAIM IS: 1. A getter device which comprises a mixture of a barium alloy and a material capable of reacting exothermically therewith, the surfaces of said mixture being coated with an organo-silicon compound.

2. A getter device as claimed in claim 1, wherein the organo-silicon compound is a polysiloxane.

3. A getter device as claimed in claim 2, wherein the polysiloxane is one of the formula I

wherein R1, R2, R3, R6, R7 and R8 are independently selected from the group consisting of aliphatic, aromatic, ali-phatic-aromatic and aromatic-aliphatic radicals and hydrogen atoms, with the proviso that at least one is selected from the group consisting of aliphatic, aromatic, aliphatic-aromatic and aromatic-aliphatic radicals, wherein R4 and R5 are independently selected from the group consisting of aliphatic, aromatic, aliphatic-aromatic and aromatic-aliphatic radicals and wherein "n" is an integer from 0 to 20,000 inclusive.

4. A getter device as claimed in claim 3, wherein "n" is an integer from 0 to 1,000 inclusive.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In the table "n" is the approximate number of silicon-oxygen groups per molecule of polysiloxane as shown in Formula I above. In the table "oil" refers to polysiloxane and chlorsilane manufacturers' identification number. The number following the symbols AK and AS identifies the viscosity of the oil in centistokes. In the table "bulb" identifies the method of impregnating the getter devices using the pure oil in an evacuated glass vessel. In the table "bath" identifies the method of impregnating the getter devices by immersing them in the oil or oil-solvent mixture for 15 minutes. In the table "% oil" gives volume Ó oil to xylene composition of the bath. In the table "bake" refers to the thermal treatment in dynamic vacuum at 300"C for 60 minutes after air-dry. In the table in the "H2O test" getter devices are placed in warm water (approx. 60"C) and the time until bubble formation is observed visually. The "Frit test" referred to in the table is an air heat treatment for 1 hour at 420"C and subsequent "flash" in vacuum. "Explosion" is observed by visual observation of vigorous ejection of glowing particles from the getter device. From the results obtained in the Examples as described above, it can be seen, by comparing Examples 3 to 6 with Examples 7 to 10, that use of a pure oil or an oil solvent mixture gives identical results. Comparison of Examples 5, 9, 13, 17 show that without the thermal vacuum treatment ("bake") only oils of "n" about 108 give any substantial degree of water treatment protection. Comparison of Examples 6, 10, 14, 18 show that the "bake" treatment improved the water resistance considerably being satisfactory for "n" greater than zero. It can be seen from the results ofExamples 19to 22 that for methylphenyl oils the "bake" treatment may not be necessary. Examples 3, 4, 7, 8, 11, 12, 15, 16 show that the oil-treated getter devices are "frittable" both with and without the "bake" treatment. Comparison of Examples 19 to 22 with Examples 23 to 24 shows no difference between the efficiency of "bulb" and "bath" treatments. It can be seen from the results of Examples 25 to 28 compared with those of Examples 29 to 32 that the halogenated silanes are rather selective in giving water protection. The oils used in Examples 19 to 22 had approximately 10% phenyl substitution while Examples I and 2 had 62.5% phenyl substitution. Comparison shows similar results with both amounts of phenyl in the chain and for both lengths of the chain of siloxane groups (108 for Examples 19 to 22, 3 for Examples 1 and 2). Various other modifications and variations falling within the scope of the present invention will be apparent to those skilled in the art. WHAT WE CLAIM IS:

1. A getter device which comprises a mixture of a barium alloy and a material capable of reacting exothermically therewith, the surfaces of said mixture being coated with an organo-silicon compound.

5. A getter device as claimed in claim 3 or claim 4, wherein the polysiloxane

has the formula II

6. A getter device as claimed in claim 3 or claim 4, wherein the polysiloxane has the formula III

7. A getter device as claimed in claim 3 or claim 4, wherein the polysiloxane is tetra-methyl, tetra-phenyl tri-siloxane.

8. A getter device as claimed in claim 1, wherein the organo-silicon compound is a silane.

9. A getter device as claimed in any one of claims I to 8, wherein the barium alloy is a barium-aluminium alloy.

10. A getter device as claimed in claim 9, wherein the barium-aluminium alloy is Baa4.

11. A getter device as claimed in any one of claims 1 to 10, wherein the material capable of reacting exothermically with the barium alloy is a metal selected from the group consisting of Ni, Ti, Fe, Mo, and alloys thereof.

12. A getter device as claimed in claim 11, wherein the metal is Ni.

13. A getter device as claimed in any one of claims I to 12, wherein the weight ratio of the barium alloy to the exothermic coreactant is from 1 to 4 to 4 to 1.

14. A getter device as claimed in claim 1, which comprises A. an annular ring; B. a pulverulent intimate mixture of a barium-aluminium alloy and nickel held by the ring;; C. An organo-silicon coating covering the entire getter device said coating consisting entirely of a vapour deposited organo-silicon compound of formula (II).

15. A getter device as claimed in claim 1, which comprises A. an annular ring; B. a pulverulent intimate mixture of a barium-aluminium alloy and nickel held by the ring; C. an organo-silicon coating covering the entire getter device, said coating consisting entirely of a vapour deposited organo-silicon compound of formula (III).

16. A getter device as claimed in claim 1, which comprises A. an annular ring; B. a pulverulent intimate mixture of a barium-aluminium alloy and nickel held by the ring; C. an organo-silicon coating covering the entire getter device, said coating consisting entirely of a liquid phase deposited organo-silicon compound of formula (it).

17. A getter device as claimed in claim 1, which comprises A. an annular ring; B. a pulverulent intimate mixture of a barium-aluminium alloy and nickel held by the ring; C. An organo-silicon coating covering the entire getter device, said coating consisting entirely of a liquid phase deposited organo-silicon compound or formula (III).

18. A method of manufacturing a getter device, which method comprises the steps of pressing a pulverulent mixture of a barium alloy and a material capable of reacting exothermically therewith into a holder and contacting the surface(s) of the mixture with an organo-silicon compound so as to form a coating thereon.

19. A method as claimed in claim 18, wherein the organo-silicon compound is one as specified in any one of claims 2 to 8.

20. A method as claimed in claim 18 or claim 19, wherein the pulverulent mixture is one as specified in any one of claims 9 to 13.

21. A method as claimed in any one of claims 18 to 20 wherein the mixture is contacted with the organo-silicon compound whilst the latter is in the vapour phase.

22. A method as claimed in any one of claims 18 to 20, wherein the mixture is contacted with the organo-silicon compound whilst the latter is in the liquid phase.

23. A method as claimed in any one of claims 18 to 22, wherein the coating treatment is carried out at a temperature in the range of from 100 to 7000C, but below the flash point of the organo-silicon compound.

24. A method of manufacturing an exothermic getter device comprising the steps of pressing a pulver-ulent mixture comprising a barium-aluminium alloy together with a metal capable of reacting exothermically with the bariumaluminium alloy into a holder, then contacting the pulver-ulent mixture in the holder with an organo-silicon compound.

25. A getter device whenever manufactured by a method as claimed in any one of claims 18 to 24.

26. A getter device as claimed in claim I and substantially as described in any one of Examples I to 32 herein.

27. A getter device as claimed in claim I and substantially as described herein with reference to, and as illustrated in, Figure 1 of the accompanying drawings.

28. A method as claimed in claim 18 carried out substantially as described herein.

29. An electric discharge tube which has been manufactured by using a getter device as claimed in any one of claims 1 to 17 and 25 to 27.

30. A colour display tube which has been manu-factured by using a getter device as claimed in any one of claims I to 17 and 25 to 27.