US3385644A - Process for filling with mercury discharge tubes and for absorbing residual noxious gases - Google Patents

Description

United States Patent PROCESS FOR FILLING WITH MERCURY DIS- CHARGE TUBES AND FOR ABSORBING RE- SIDUAL N OXIOUS GASES Paolo Della Porta and Bruno Kind], Milan, Italy, assignors to S.A.E.S. Getters-S.p.A., Milan, Italy, a company of Italy No Drawing. Filed Jan. 5, 1966, Ser. No. 518,789 Claims priority, application Italy, Jan. 8, 1965,

319/ 65, Patent 748,529 9 Claims. (Cl. 316-16) ABSTRACT OF THE DISCLOSURE A tablet and process for releasing mercury in a discharge container. The tablet comprises a mixture of a powdered mercury compound and a stoichiometric excess of a reducing agent. In one embodiment the reducing agent is a non-vaporative getter metal such as a zirconium-aluminum alloy. In another embodiment the reducing agent has fractions of different particle sizes.

The present invention relates to a process for filling discharge tubes, or discharge'ampoules with mercury and for removing residual obnoxious gases from said tubes or ampoules.

It is known that many a type of tubes, valve and lamps, such as rectifiers, thyratrcns, luminescence and discharge tubes and the like necessitate, for their operation, mercury vapor atmospheres which are obtained by maintaining the inside of the ampoule saturated with said vapors.

Many a method is known for filling said ampoules with mercury, which is introduced within the ampoule as a liquid and is evaporated in amounts which are more or less significant consistently with the available room and the working temperature.

The minimum amounts of mercury to be introduced within the ampoule may vary from a few milligrams to 'a few decigrams.

The satisfactory operation of the tubes .or bulbs under discussion is also dependent on the absence within the ampoule of noxious gases, more particularly oxygenbearing gases. As is known, said noxious gases are continually evolved within the ampoules and for their absorption, so-called chemical pumps or getter are used, said generic name being applied to metals or metal alloys which are placed within the ampoule, either in the form of a thin film, or as a compressed powder tablet. Metals, or alloys, of the kind referred to, which are extremely active as absorbers of noxious gases, are, for example, Ba, Zr-Al, Oe-Th, Zr-Ti and others.

Films which are deposited onto the inner walls of the ampoules by means of evaporable getters, are objectionable in many kinds of electronic tubes on account of the inherent drawbacks, such as disturbing capacitance, discharges, amalgam formation and others, and cannot be employed in discharge lamps: thus, the general trend of the art is to replace the evaporable get-ters by tablets of non-evaporable getters, of the so called flashless type.

None of the hitherto devised and studied methods for the introduction of mercury within discharge tubes has allowed the simultaneous introduction into an ampoule of mercury dispensing units and of a non-evaporable getter for absorbing the noxious gases. Among the most conventional methods used for filling discharge tubes with mercury, the following can be mentioned:

(a) Introduction of the mercury with a closing mechanism. This method is cumbersome and expensive it it 3,385,644 Patented May 28, 1968 is to 'be operated in a vacuum, the amount of mercury introduced into the ampoule being, moreover, exceedingly larger than necessary. Consequently, the ampoule ought to be used, when it is in service, in a certain position so as to collect the excess mercury in a proper location. Experience shows, moreover, that the presence of excess mercury is often dangerous since it is possible to have both a conveyance of metal impurities from the hot spots (electrodes) to the cold ones due to amalgamation, and a darkening of the glass containers due to amalgamation of sodium liberated on the said glass surfaces by reducing gases which are always present in the tube.

(b) Introduction of mercury compounds in the form of powder compressed into tablets which, when suitably heated, are decomposed and set free mercury and at least a fraction of the residue as a gas. Mercury compounds used to this purpose are, for example, mercury sulphide, oxide, and tungstate. Inasmuch as the presence in an ampoule of large amounts of noxious gases such as oxygen, and still more sulphur-containing gases, is detrimental as outlined above, the powder of the mercury compound to be decomposed is usually mixed with a powder reducing chemical, such as iron, nickel, zirconium, magnesium and the like, intended to facilitate the reduction of the mercury compound and to absorb the noxious gases "as they are evolved.

In the particular case of the employment of magnesium as a reducing agent, it has already been proposed to add an excess of magnesium to the mercury compound so as to form, upon heating, a getter film within the ampoule so 'as to rid the tube of noxious gases during its lifetime. Such a film, in practice, is active towards oxygen only. As a general rule, said getter films are inapplicable in discharge lamps and the like, as they involve a number of disturbances and shortcomings in said tubes, more particularly in those operating at high temperatures or voltages.

The use of reducing agents such as iron and nickel does not ensure a sufiiciently quick absorption of the gases which are evolved upon decomposition of mercury compounds, so that the advantages of these materials are limited ones.

More active reducing materials, such as zirconium, lead to explosive reaction and it has been suggested, in order that a delayed reaction may be achieved, that a second reducing agent be added, of a less active type. Conventional mixtures of powdered reducing agents are zirconium-iron, zirconium-nickel and others. It has been practically ascertained; however, that the gases which are evolve-d during the reaction in which mercury is set tree are not completely removed. Furthermore, the gettering activity of the residues (formed, for example, by HgO, Fe, and Zr) has proven to be wholly negligible aft-er evaporation of mercury, mainly due to the presence of the less active reducing agent (e.g. iron) which is alloyed to the very active reducing agent (e.g. zirconium).

The process according to the present invention for filling discharge containers with mercury is also of the kind which employs tablets formed by a powdered mercury compound and a powdered reducing agents. Said tablets are placed upon a suitable receptacle in the con tainer .and after suitably degassing the ampoule the container can be heated so as to prime the reaction leading to mercury evaporation.

An object of the present invention is to improve said conventional methods by permitting a complete removal from within the ampoule of the gases evolved during the reaction leading to mercury release, and by giving merc-ury yields close to the theoretical values.

Another fundamental object of the present invention is to provide tablets which not only are capable of setting mercury free as they are heated to the proper reaction temperature but also of being active thereafter as non-evaporable getters so as to absorb the noxious gases during the' life-time of the lamp or the like. That is, the mercury containing tablets should also contain a reducing agent such as to promptly absorb, during the reaction with the mercury compound, all the reaction products and all the foreign gases which are present in the powdered mercury compound without expending all their physico-chemica-l properties thus being capable of exhibiting, thereafter, satisfactory getter features towards the noxious gases during the whole lifetime of the discharge tube.

According to the present invention it is proposed to employ, as a reducing agent in tablets which contain a mercury compound, a powder of a material which is only slightly active as a reducing agent at the degassing temperature and very active both at the mercury evaporation temperature and after the mercury evaporation and having a grain size such as to comprise, along with a relatively fine fratcion, a relatively coarse fraction. By doing so, a quick evolution of mercury vapors has unexpectedly been obtained, with very satisfactory yields after a moderately exothermic reaction accompanied by the emission of very small amounts of gases which, however, are promptly absorbed by the tablets again along with the gaseous impurities retained by the powders. The diffusion of surface reaction products within the reducing powder granules has been concurrently ascertained, as it is shown by the fact that the surface has proven to be capable, in a subsequent time during the discharge contaiuers lifetime, of absorbing the noxious residual gases, and of displaying a getter activity.

An interpretation of said phenomenon is as follows,

(a) Very presumably, the large granules, due to the large clearances between them, originate a sort of minutely chanelled structure which is a mechanical facility for the rapid discharge of the mercury vapors, thus affording good yields.

(b) In addition, the coarse fraction acts as a thermal retarder, in that each granule exposes a tiny surface to the exothermic reaction which sets mercury free, thus providing a relatively large mass to be heated by the heat of reaction, the consequence being that the vehemence of the reaction is mitigated.

(c) The size of the large granules is such to permit the diffusion of the surface reaction products towards the inside, thus contributing towards the evaporation of mercury without however reaching the end point of their activity. The surface of the large granules is thus in a position to absorb, by displaying a getter action, the noxious gases during the lifetime of the discharge container.

(d) The fine fraction of the reducing materials, conversely, exhibits a high chemical reactivity both towards the compound which sets free mercury and the gases contained therein.

A grain size mixture of reducing materials, such as provided by the present invention, enables to operate with a single active material which, by reacting in part only, subsequently displays a getter function in the discharge container.

The weight ratio of the reducing active material powder to the powdered mercury compound should be, according to the invention, such that the former be strongly in excess with respect to the minimum amount which is necessary for a complete reaction with the mercury compound.

The properties and physico-chemical specifications of the reducing agent should be such, according to the in vention, that the reducing agent is only slightly active at the degassing temperature (about 360 C.) so as not to react with air or with the mercury-evolving compound either. On the other hand, the reducing agent should be very active at the temperature of mercury evolution reac tion (about 800 C.) so as to immediately react with all the evolved gases and absorb in any case all of those small amounts which may have locally escaped.

The reducing agent, moreover, should allow a rapid diffusion of the products which are formed on the surface of every granule towards the inside thereof so as to maintain said surface reasonably clean until the whole granule mass has been exhausted. Lastly, the reducing agent should undergo, upon its reaction with the mercury compound, modifications such as to cause the formation of strongly active unstable structures for the absorption of the noxious gases.

According to the present invention it has been found that as reducing agents in the mercury dispensers, known non-evaporable alloy powders, such as aluminum-zirconium alloys and others commercially available alloys, can be used with advantage.

So as to confirm the preference to be given to getter alloys over pure metals, the values of oxygen-bearing gas volumes absorbed, all the other conditions being the same, by pure zirconium on the one hand, and by an Al-Zr alloy consisting of 84% zirconium and 16% alluminium On a weight basis (known in the trade with the symbol StlOl), on the other hand, after a 5-hour treatment in water vapor saturated air at C. can be reported.

Volume of CO per second absorbed at C. after an activation heat treatment:

cu.cms. per sec. per sqcm. StlOl 200 Pure Zr 4 These data show that, at a low temperature and in the non-active state, a getter alloy of the kind referred to does not react with air or other oxygen compounds due to the lack of structures which are active as gas-absorbers, whereas, after a heat treatment such as that necessary to set mercury free from tablets containing mercury compounds, said alloy is capable of reacting with said gases much more strongly than a pure metal which has undergone the same treatment.

A further proof of this phenomenon is the amount of oxygen absorbed by such an alloy at high temperatures over that absorbed by pure metal.

Amount of oxygen diffused per second at 800 C. after an activation heat treatment, in the metal mass:

Micrograms per sec. per sq.cm. StlOl 41 Pure Zr 5 These data are an evidence of the high reactivity of the getter alloys over that of pure metals which are nonethe-less known for their fair reducing action towards oxygen compounds. The same data show how quicker is the diffusion of the reaction products with said alloys towards the inside of the metal mass over the diffusion within pure metals.

It should eventually be noted that the powdered mercury compound as contained in the tablets used in the process according to the invention should be of fine grain size so as to obtain a complete reaction with the reducing agent together with an immediate blocking of the gases evolved upon the thermal decomposition of the mercury compound. The lower limit of the grain size is posed by reason of cost, amount of included gases and problems attendant to health and safety of personnel during manufacture.

By way of illustration only, without any limitation, the following is a composition of tablets or rings which, used in the process of the invention have given very satisfactory results.

Aluminium-Zirconium alloy (84% Zr and 16% Al, by

wt.): 30% by weight of granules having a size from to 210 microns,

70% by weight of granules having a size from 0 to 130 microns, HgO granules from 0 to microns.

Weight ratio alloy to HgO variable from 2:1 to 6:1 as a function of the purity of the gases evolved as mercury is evaporated and of the desired delaying action of the exothermic reaction.

Thus, for example, in the case of rings yielding 100 milligrams of mercury, the amount of oxygen emitted at the instant of evaporation of mercury has been reduced from a theoretical value of 4.1 1 ton to 0.1 1 torr only. This oxygen is then absorbed by the still active granules again in a few seconds and thus it completely disappears. The yields of evaporated mercury were within deviations of less than 5% with respect to the theoretical value.

We claim:

1. A process for filling with mercury discharge containers and for removing noxious gases by employing tablets or the like consisting of a powdered mercury compound and a powdered reducing agent comprising a non-evaporable getter alloy such as a zirconium-aluminum, a zirconium-titanium, or a cerium-thorium alloy or the like, which tablets or the like are placed on a supporting member within said container and are activated by a localized heating, after a degassing treatment of the container so as to induce a mercury evolving reaction, characterized in that as a reducing agent is employed a powdered material which is only slightly active at the degassing temperature and has a grain size such as to comprise, along with a fine grain size fraction, fraction of relatively coarse grain size.

2. A process according to claim 1, characterized in that the mercury compound is mercury oxide and the reducing agent is a zirconium-aluminurn alloy (84% Zr and 16% Al, by weight) present in a weight ratio from 1:2 to 1:6.

3. A process for filling a discharge container with mercury and removing noxious gases, comprising the steps of:

(A) introducing into the container a mixture of "(1) a powdered mercury compound and '(2) a stoichiometric excess of a granular reducing agent for the mercury compound, said reducing agent consisting essentially of large and small granules,

(a) said small granules reacting more rapidly with the mercury compound than said large granules,

(b) said large granules acting as a heat sink to absorb the heat generated by the reaction between said small granules and said mercury compound;

(B) at least partially degassing said container;

(C) heating said mixture so as to reduce said mercury compound, evolve mercury and sorb noxious gases.

4. A process for filling a discharge container with mercury and removing noxious gases, comprising the steps of:

'(A) introducing into the container a mixture of (1) a powdered mercury compound and (2) a stoichiometric excess of a granular reducing reducing agent for the mercury compound, said reducing agent having a major portion of its granules smaller than 130 microns and a minor portion of its granules larger than 130 microns; 60

(B) at least partially degassing said container;

(C) heating said mixture so as to reduce said mercury compound, evolve mercury and sorb noxious gases. 5. A process for filling a discharge container with mercury and removing noxious gases, comprising the steps of:

(A) introducing into the container a mixture of (l) a powdered mercury compound and (2) a stoichiometric excess of a reducing agent for the mercury compound, said reducing agent comprising a metal alloy capable of (a) undergoing a very slight reaction, with said mercury compound during degassing and prior to mercury release,

(b) undergoing structural changes during heating for mercury release so that all the gases released are readily gettered by said alloy and so that its gettering characteristics are so large that a residual capacity remains in the alloy for sorbing the gases released during subsequent operation of the discharge container.

(B) at least partially degassing said container; (C) heating said mixture so as to reduce said mercury compound, evolve mercury and sorb noxious gases. 5. The process of claim 5 wherein said alloy comprises the metals zirconium, cerium, aluminum, titanium, and thorium.

7. The process of claim 5 wherein said alloy has a sorbtive capacity of greater than 200 cm. of carbon monoxide per second per square centimeter of expose alloy surface.

8. A mercury-releasing tablet capable of sorbing noxious gases, said tablet comprising a mixture of:

(A) a powdered mercury compound and (B) a stoichiometric excess of a granular reducing agent for the mercury compound, said reducing agent consisting essentially of large and small granules,

(1) said small granules reacting more rapidly with the mercury compound than said large granules,

(2) said large granules acting as a heat sink to absorb the heat generated by the reaction between said small granules and said mercury compound.

'9. A mercury-releasing tablet capable of sorbing noxious gases, said tablet comprising a mixture of:

(A) a powdered mercury compound and (B) a 'stoichiometric excess of a reducing agent comprising a metal alloy capable of:

(a) undergoing a very slight reaction, with said mercury compound during degassing and prior to mercury release,

(b) undergoing structural changes during heating for mercury release so that all the gases re leased are readily gettered by said alloy and so that its gettering characteristics are so large that a residual capacity remains in the alloy for sorbing subsequently released gasses.

References Cited UNITED STATES PATENTS 3,279,877 10/1966 Smith 316-16 3,230,027 1/ 1966 Mayer 316l6 RICHARD H. EANES, 111., Primary Examiner.