Note: Descriptions are shown in the official language in which they were submitted.
Z~ 7~686
"A COMBINATION OF MATERIALS FOR INTEGRATED GETTER AND
MERCURY-DISPENSING DEVICES AND DEVICES THUS OBTAINED"
The present invention relates to a combination of materials for the
production of devices combining the getter and mercury-dispensing
functions, to the devices thus produced and to a process for the
introduction of mercury inside electron tubes.
The use of small amounts of mercury in electron tubes such as, for
example, mercury-arc rectifiers, lasers, various kinds of alphanumeric
displays and, particularly, fluorescent lamps is well known in the art.
A precise dosage of mercury inside these devices is extremely
important for the quality of the devices and most of all for ecological
reasons. In fact, the high toxicity of this element implies serious problems
of environmental pollution upon end-life disposal of the devices containing
it, or in case of accidental break-up of the devices. These problems of
ecological nature impose the use of amounts of mercury as small as
possible, compatibly with the functionality of the tubes. These
considerations have been lately included also in the legislative sphere,
and the trend of the recent international regulations is to establish top
limits for the amount of mercury which can be introduced into the devices:
for example, for standard fluorescent lamps the use of a total amount of
Hg not greater than 10 mg per lamp has been suggested.
Mercury can be introduced into the tubes in liquid form. However, the
use of liquid mercury first of all poses problems concerning the storing and
handling in the plants for the production of tubes, due to its high vapor
pressure also at room temperature. Secondly, a common drawback of the
techniques for the introduction into the tubes of mercury in liquid form is
the difficulty in precisely and reproducibly dosing volumes of mercury in
the order of microliters, which difficulty usually takes to the introduction of
amounts of the element higher than needed.
These drawbacks have taken to the development of various
techniques in alternative to the use of liquid mercury in free form.
The use of liquid mercury contained in capsules is disclosed in
several prior art documents. This method is described, for example, in US
patents nos.4.823.047 and 4.754.193, referring to the use of metallic
capsules, and in US patents nos.4.182.971 and 4.278.908 wherein the
21 7~6~b
mercury container is made of glass. After closing the tube, the mercury is
released by means of a heat treatment which causes the breakage of the
container. These methods generally have some drawbacks. First of all, the
production of the capsules and their mounting inside the tubes may be
5 complicated, especially when they have to be introduced inside small-size
tubes. Secondly, the breakage of the capsule, particularly if it is made of
glass, may produce fragments of material which can jeopardize the tube
quality, so much that US patent no.4.335.326 discloses an assembly
wherein the mercury-containing capsule is in turn located inside a capsule
10 acting as a shield for the fragments. Moreover, the release of the mercury
is often violent, with possible damages to the inner structure of the tube.
Finally, these systems still have the drawback of employing liquid mercury,
and therefore they do not completely solve the problem of the precise and
reproducible dosage of few milligrams of mercury.
US patent no.4.808.136 and the European patent application
EP-568.317 disclose the use of tablets or small spheres of porous material
soaked with mercury which is then released by heating once the lamp is
closed. However, also these methods require complicated operations for
the loading of mercury into the tablets, and the released amount of
20 mercury is difficult to be reproduced.
The use of amalgams of mercury, for example with indium, bismuth or
zinc, is also known. However, these amalgams generally have the
drawback of a low melting temperature and a high mercury vapor pressure
already at temperatures not very high. For example, the zinc amalgams
25 described in the commercial bulletins of the APL Engineered Materials Inc.
have a vapor pressure at 43C which is about 90% of that of liquid
mercury. Consequently, these amalgams badly withstand the thermal
treatments for the production of the lamps in which they are introduced.
These problems are overcome by US patent no.3.657.589 in the
30 name of the Applicant, which discloses the use of intermetallic compounds
of mercury having the general formula Ti,,ZryHgz, wherein x and y may vary
between 0 and 13, the sum (x+y) may vary between 3 and 13 and z may
be 1 or2.
These compounds have a temperature of mercur,v-release start
35 variable according to the specific compound, however they are all stable
up to about 500C both in the atmosphere and in evacuated volumes, thus
21 72686
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resulting compatible with the operations for the assembly of the electron
tubes, during which the mercury-dispensing devices may reach
temperatures of about 600C. After closing the tube, the mercury is
released from the above-cited compounds by an activation operation,
5 which is usually carried out by heating the material between 750C and
900C for about 30 seconds. This heating may be accomplished by laser
radiation, or by induction heating of the metallic support of the
Hg-dispensing compound. The use of the Ti3Hg compound, manufactured
and sold by the Applicant under the trade name St505, results particularly
10 advantageous; in particular, the St505 compound is sold in the form of
compressed powder in a ring-shaped container or of compressed powder
in pills or tablets, under the trademark STAHGSORB~, or in the form of
powders laminated on a metallic strip, under the trademark GEMEDIS~.
These materials offer various advantages with respect to the prior
1 5 art:
- as mentioned above, they avoid the risks of mercury evaporation
during the cycle of production of the tubes, in which temperatures of about
350400C may be reached;
- as described in the cited US patent no.3.657.589, a getter material
20 can be easily added to the mercury-dispensing compound with the
purpose of chemisorption of gases such as CO, C2, 2~ H2 and H2O,
which would interfere with the tube operation; the getter is activated during
the same heat treatment for the release of mercury;
- the released amount of mercury is easily controllable and
25 reproducible.
Despite their good chemical-physical characteristics and their great
ease of use, these materials have the drawback that the contained
mercury is not completely released during the activation treatment. In fact,
the processes for the production of mercury-containing electron tubes
30 include a tube-closing operation performed by glass fusion (e.g. the
sealing of fluorescent lamps) or by frit sealing, i.e. welding two pre-shaped
glass members by means of a paste of low-melting glass. During these
operations, the mercury-dispensing device may undergo an indirect
heating up to about 600C. In this step the device is exposed to gases and
35 vapors emitted by the melted glass and, in almost all industrial processes,
to air. In these conditions, the mercury-dispensing material undergoes a
~1 72686
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surface oxidation, whose final result is a yield lower than about 40% of the
total mercury content during the activation process. In the particular case
of compact circular lamps, during the lamp sealing and bending steps the
mercury-dispensing material undergoes an indirect heating up to about
600C. In this case the mercury yield during the activation process drops
as low as about 20% of the total mercury content of the device.
The mercury not released during the activation operation is then
slowly released during the life of the electron tube.
This characteristic, together with the fact that the tube must obviously
work from the beginning of its life cycle, leads to the necessity of
introducing into the device an amount of mercury which is at least double
than that which would theoretically be necess~ry.
In order to overcome these problems, patent application
EP-A-091.297 suggests the addition of Ni or Cu powders to the Ti3Hg or
Zr3Hg compounds. According to this document, the addition of Ni and Cu
to the mercury-dispensing compounds c~ ~ses the melting of the
combination of materials thus obtained, favouring the release of almost all
the mercury in few seconds. The melting takes place at the eutectic
temperatures of the systems Ni-Ti, Ni-Zr, Cu-Ti and Cu-Zr, ranging from
about 880C for the Cu 66% - Ti 34% composition to 1280C for the
Ni 81% - Ti 19% composition (atomic percent), though the document
erroneously gives a melting temperature of 770C for the Ni 4% - Ti 96%
composition. The document acknowledges that the mercury-containing
compound is altered during the tube working treatments, and it needs a
protection. To this purpose, there is suggested to close the powder
container by means of a steel, copper or nickel sheet which is broken
during the activation by the pressure of the mercury vapor generated
inside the container. This solution is not completely satisfactory: in fact,
same as it happens in the methods employing capsules, mercury bursts
out violently and can cause damages to portions of the tube; the
manufacturing of the container is quite complicated, since it requires the
welding of small-size metallic members. Furthermore, this document does
not contain experimental data to support the stated good mercury-release
characteristics of the combinations indicated. Finally, the devices in this
application, contrary to those illustrated in the cited US patent
no.3.657.589, do not allow to integrate a getter material, whose presence
21 72686
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is necessary for the correct working of the lamps, into a same device.
Therefore, the object of the present invention is to provide an
improved combination of materials for dispensing mercury in the electron
tubes, which allows to overcomes one or more drawbacks of the prior art.
In particular, the object of the present invention is first of all to
provide an improved combination of materials for dispensing mercury
which is capable of releasing amounts of mercury higher than 60% during
the activation step, even after partial oxidation, so as to be able to reduce
the total amount of employed mercury.
Another object of the present invention is to provide a combination of
materials whose residue, after the activation operation for releasing
mercury, has a getter activity.
Another object of the present invention is to provide mercury-
dispensing devices containing the combination of materials of the
1 5 invention.
Still another object is to provide a process for introducing mercury by
means of the devices of the invention into the electron tubes which require
said element.
According to the present invention, these and other objects are
achieved by using a mercury-dispensing combination of materials made up
of:
- a mercury-dispensing intermetallic compound A including mercury
and a second metal selected among titanium, zirconium and mixtures
thereof;
- an alloy or an intermetallic compound B including copper, tin and one
or more metals selected among the rare earths.
Further objects and advantages of the present invention will be
apparent from the following detailed description referring to the annexed
drawings wherein:
Fi~.1 is a perspective view of a mercury-dispensing device of the
present invention according to a possible embodiment thereof;
Fi~s.2 and 2a are, respectively, a top plan view and a sectional view
along ll-ll of a device of the invention according to another possible
embodiment;
Fi~s.3, 3a and 3b are, respectively, a top plan view and two sectional
views along lll-lll of a device of the invention according to a further
21 72686
-6 -
embodiment, in two possible variations;
Fi~.4 shows, in a ternary diagram, the alloys of the present invention.
Component A of the combination of the present invention, hereafter
also defined mercury dispenser, is an intermetallic compound
5 corresponding to formula TixZryHgz, as disclosed in the cited US patent
no.3.657.589, to which reference is made for further details. Among the
materials corresponding to said formula, Zr3Hg and, particularly, Ti3Hg are
preferred.
Component B of the combination of the present invention has the
10 function of favouring the release of mercury from component A, and
hereafter will also be defined promoter. This component is a metallic alloy
or an intermetallic compound including copper, tin and a metal selected
among the rare earths or a mixture of rare earths. The use of a mixture of
rare earths is preferred over the use of single elements in that, since these
15 metals have a similar chemistry, the separation of the single elements is a
difficult and expensive operation; on the other hand, by using a mixture of
rare earths it is possible to obtain, in this application, essentially the same
results obtained with the single elements. The mixtures of rare earths are
known in the art by the name Umisch metal"; this denomination, and its
short form MM, will be used hereafter in the rest of the specification and in
the claims.
The weight ratio between copper, tin and MM can vary within a wide
range, but advantageous results have been obtained with compositions
which, in a ternary diagram of percentage compositions on a weight basis
(fig.4), fall within a polygon defined by points:
a) Cu 63% - Sn 36,5% - MM 0,5%
b) Cu 63% - Sn 10 % - MM 27%
c) Cu 30% - Sn 10 % - MM 60%
d) Cu 3% - Sn 37 % - MM 60%
e) Cu 3% -Sn 96,5% -MM 0,5%
With copper percentages higher than 63% the alloy reaches a high
melting point and consequently requires excessive temperatures for its
activation, while on the contrary with copper percentages lower than about
3% the alloy has a melting point too low and this implies the risk of having
a low-viscosity liquid phase at the temperatures, varying about from 600 to
800C, reached during the production of the lamps. With misch metal
21 72686
concentrations higher than 60% by weight the alloy becomes excessively
reactive, and could give rise to violent reactions both during the lamp
production step and during the activation step. Finally, with tin contents
lower than 10% by weight the alloy reaches again a high melting point.
Particularly advantageous results are obtained, within said range of
compositions, by means of compositions which, in a ternary diagram of
percentage compositions on a weight basis (fig.4), fall within a polygon
defined by points:
a) Cu 63% - Sn 36,5% - MM 0,5%
b) Cu 63% - Sn 10 % - MM 27%
c)Cu50%-Sn10 %-MM40%
d) Cu 30% - Sn 30 % - MM 40%
e) Cu 30% - Sn 69,5% - MM 0,5%
A particularly preferred alloy has the percentage composition
Cu 40% - Sn 30% - MM 30%, corresponding to point i) in the composition
- ternary diagram of fig.4.
The weight ratio between components A and B of the combination of
the invention may vary within a wide range, but it is generally included
between 20:1 and 1 :20, and preferably between 10:1 and 1 :5.
Components A and B of the combination of the invention may be
employed in various physical forms, not necess~rily the same for the two
components. For example, component B may be present in the form of a
coating of the metallic support, and component A as a powder adhered to
component B by rolling. However, the best results are obtained when both
components are in the form of a fine powder, having a particle size lower
than 250 ,um and preferably between 10 and 125 ,um.
The present invention, in a second aspect thereof, relates to the
mercury-dispensing devices which use the above-described combinations
of A and B materials.
As previously mentioned, one of the advantages of the combination
of materials of the invention with respect to prior art systems is that they
do not need a mechanical protection from the environment, thus not
posing the limit of a closed container. Consequently, the mercury-
dispensing devices of the present invention can be manufactured with the
most different geometric shapes, and materials A and B of the combination
can be employed without support or on a support, usually metallic.
~1 72686
Some classes of electron tubes for which the mercury dispensers are
intended further require, for their correct operation, the presence of a
getter material which removes traces of gases such as CO, CO2, H2, 2 or
water vapor: it is the case, for example, of fluorescent lamps. An important
advantage offered by the combinations of the present invention is that the
residue remaining after the evaporation of mercury has a getter activity.
The amount of gas which can be absorbed by said residue, and the
absorption velocity, are sufficient to assure an adequate degree of vacuum
for many applications. In order to increase the total gas absorption velocity
and capacity of this device, it is obviously possible to add thereto another
getter material C, according to the manners described in the cited US
patent no.3.657.589. Obviously, in this case the amount of getter material
C is lower than that required in prior art devices used in the same
application. Examples of getter materials include, among the others,
metals such as titanium, zirconium, tantalum, niobium, vanadium and
mixtures thereof, or alloys thereof with other metals such as nickel, iron,
aluminum, like the alloy having a weight percentage composition Zr 84% -
Al 16%, manufactured and sold by the Applicant under the name St101, or
the intermetallic compounds Zr2Fe and Zr2Ni, manufactured and sold by
the Applicant respectively under the name St198 and St199. The getter
material is activated during the same heat treatment by which mercury is
released inside the tube.
The getter material C may be present in various physical forms, but it
is preferably employed in the form of a fine powder, having a particle size
lower than 250 ~m and preferably between 10 and 125 ~m.
The ratio between the overail weight of the A and B materials and
that of the getter material C may generally range from about 10:1 to 1:10,
and preferably between 5:1 and 1:2.
Some possible embodiments of the devices of the invention are
illustrated hereunder with reference to the drawings.
In a first possible embodiment, the devices of the invention can
simply consist of a tablet 10 made up of compressed and unsupported
powders of the A and B (and possibly C) materials, which for ease of
production generally has a cylindrical or parallelepipedal shape; this latter
possibility is shown in fig.1.
In the case of supported materials, the device may have the shape of
21 126~6
a ring 20 as shown in fig.2, which represents a top plan view of the device,
and in fig.2a which represents a cross-section along ll-ll of device 20. In
this case, the device is made up of a support 21 having the shape of a
toroidal channel containing the A and B (and possibly C) materials. The
5 support is generally metallic, and preferably of nickel-plated steel.
Alternatively, the device may be made in the shape of a strip 30 as
shown in fig.3, which represents a top plan view of the device, and in
figs.3a and 3b wherein a section along lll-lll of device 30 is depicted. In
this case, support 31 consists of a strip, preferably made of nickel-plated
10 steel, onto which the A and B (and possibly C) materials are adhered by
cold compression (rolling). In this case, whenever the presence of the
getter material C is required, materials A, B and C may be mixed together
and rolled on one or both faces of the strip (fig.3a), or materials A and B
are rolled on one surface of the strip and material C on the opposite
15 surface, as shown in fig.3b.
The invention, in a further aspect thereof, relates to a method for
introducing mercury into the electron tubes by using the above-described
devices.
The method includes the step of introducing inside the tube the
20 above-described mercury~ispensing combination of materials and
preferably in one of the above-described devices 10, 20 or 30, and then
the combination heating step to get mercury free. The heating step may be
carried out with any suitable means such as, for example, by radiation, by
high-frequency induction heating or by having a current flow through the
25 support when the latter is made of a material having a high electric
resistivity. The heating is effected at a temperature which ~lses the
release of mercury from the mercury-dispensing combination, comprised
between 600 and 900C for a time of about 10 seconds to one minute. At
temperatures lower than 600C mercury is almost not dispensed at all,
30 whereas at temperatures higher than 900C there is the danger of the
development of noxious gases by outgassing from the portions of the
electron tube adjacent to the device or of the formation of metal vapors.
The invention will be further illustrated by the following examples.
These non-limiting examples illustrate some embodiments intended to
35 teach to those skilled in the art how to put in practice the invention and to show the accomplishment of the invention which is considered the best.
21 72686
- 10-
Examples 1 and 9 concern the preparation of the mercury-dispensing and
promoting materials, while examples 3 to 6 concern the tests for the
mercury release after the heat treatment simulating the sealing operation.
All the metals used for the preparation of alloys and compounds for the
following tests have a minimum pureness of 99,5%. In the compositions of
the examples all percentages are on a weight basis if not differently
specified.
EXAMPLE 1
This example illustrates the synthesis of the mercury-dispensing
material Ti3Hg.
143,7 9 of titanium are placed in a steel cradle and degassed by a
furnace treatment at a temperature of about 700C and a pressure of 10~
mbar for 30 minutes. After cooling the titanium powder in an inert
atmosphere, 200,6 9 of mercury are introduced in the cradle by means of a
quartz tube. The cradle is then closed and heated at about 750C for 3
hours. After cooling, the product is ground until a powder passing through
a 120,um mesh-size standard sieve is obtained.
The resulting material essentially consists of Ti3Hg, as confirmed by
a diffractometric test carried out on the powder.
EXAMPLE 2
This example concerns the preparation of a promoting alloy which
makes part of the combinations of the invention.
40 9 of Cu, 30 9 of Sn and 30 9 of MM in powder form, are placed
into an alumina cradle and then introduced in a vacuum induction furnace.
The misch metal used contains about 50% by weight of cerium, 30% of
lanthanum, 15% of neodymium and the rest are other rare earths.
The mixture is heated at a temperature of about 900C, kept at that
temperature for 5 minutes to encourage the homogeneity thereof, and
finally cast into a steel ingot-mould. Each ingot is ground in a blade mill
and the powder is sieved like in example 1. The composition of the
obtained alloy is Cu 40% - Sn 30% - MM 30%, and corresponds to point i)
in the diagram of fig.4.
EXAMPLE 3
This example concerns the preparation of a promoting alloy which
makes part of the combinations of the invention. The procedure of
example 2 is repeated using 60 9 of Cu, 30 9 of Sn and 10 9 of MM in
21 726~6
- 1 1 -
powder form. The composition of the obtained alloy is Cu 60% - Sn 30% -
MM 10%, and corresponds to point 1) in the diagram of fig.4.
EXAMPLES 4-9
Examples 4 to 9 concem the tests for the mercury release after a
5 heat treatment in air which simulates the frit conditions to which the device
is subjected during the tube closing (hereafter generally referred to as
sealing). Examples 4 to 7 are comparative examples which show the
release after frit sealing respectively by the dispensing component alone
(ex.4) and by the same mixed only with copper, tin and the above-cited
getter alloy St101 (ex.5-7); a similar comparative test on a mixture of Ti3Hg
and MM powders was not possible due to the excessive reactivity of this
mixture.
For the simulation of the sealing, 150 mg of each powder mixture
have been loaded in a ring-shaped container like in fig.1 or on a strip like
15 in fig.3, and have been subjected to the following thermal cycle in air:
- - heating from room temperature to 450C in about 5 seconds;
- isotherm at 450C for 60 seconds;
- cooling from 450C to 350C, requiring about 2 seconds;
- isotherm at 350C for 30 seconds;
20 - spontaneous cooling to room temperature, requiring about 2 minutes.
Thereafter, the mercury release tests have been carried out on the
thus treated samples by induction heating thereof at 850C for 30 seconds
inside a vacuum chamber and by measuring the mercury remained in the
dispensing device through the method of the complexometric titration
25 according to Volhart.
The results of the tests are summarized in Table 1, which shows the
mercury-dispensing compound A, the promoting material B (letters (i) or (I)
in examples 8 and 9 refer to the composition of the Cu-Sn-MM alloy as
shown in the diagram of fig.4), the weight ratio between components A and
30 B and the mercury yield as a percentage of released mercury on the total
content of the device.
The comparative examples are marked by a star.
Table 1
EXAMPLEN. A B A/B Hgyield (%)
21 72686
- 12-
4 Ti3Hg ~ - 35,2
5* Ti3Hg Cu 7/3 34,0
6* Ti3Hg Sn 5/1 25,0
7* Ti3Hg St 101 1/1 22,4
8 Ti3HgCu-Sn-MM (i) 2/1 80,0
9 Ti3HgCu-Sn-MM (I) 2/1 87,0
EXAMPLES 10-14
Examples 10 to 14 concern the tests for the functionality as getter
materials of the residues remaining after the mercury release by the
combinations of the invention and by some comparative combinations.
These tests have been carried out by simulating the frit conditions to
which the materials are subjected during the bending and sealing
operations of the compact fluorescent circular lamps, which conditions, as
mentioned above, are harder that those reached for straight lamps. In
particular, the combinations of the examples have been subjected to the
following thermal cycle in air:
- heating from room temperature to 600C in about 10 seconds;
- isotherm at 600C for 15 seconds;
- spontaneous cooling to room temperature, requiring about 2 minutes.
The mercury release tests (activation) have been carried out after
simulation of the frit sealing on the samples. The fritted samples have
been introduced inside a vacuum chamber having a volume of 1 liter, and
heated under vacuum at 850C for 10 seconds and kept at said
temperature for 20 seconds.
The capacity of the residue to work as a getter is measured after the
activation; this measurement is performed by introducing in the chamber
an amount of hydrogen such as to bring the pressure to 0.1 mbar at a
temperature of 30C, and by measuring the time required for the pressure
in the chamber to decrease to 0.01 mbar. The measure of the pressure is
taken by means of a capacitive manometer. The results of these tests are
summarized in Table 2, which shows the composition of the sample and
the hydrogen absorption velocity at 30C. The USAMPLE COMPOSITION"
column gives the weight percentages of the component materials. The
comparative combinations are marked by a star.
21 726~6
- 13-
Table 2
EXAMPLE N. SAMPLE COMPOSITIONH2 absorption
velocity (cc/s)
10* Ti3Hg notmeasurable
11* Ti3Hg: 50% 7,2
St 101: 50%
12 Ti3Hg: 60% 6,9
Cu-Sn-MM (i): 40%
13 Ti3Hg: 60% 3,5
Cu-Sn-MM (I): 40%
14 Ti3Hg: 30% 15,3
Cu-Sn-MM (i): 20%
St 101: 50%
It may be noted from the data of Table 1 that the combinations with
5 promoter of the invention allow mercury yields higher than 80% during the
activation step even after frit sealing in air at 450C, thus permitting the
reduction of the overall mercury amount introduced in the electron tubes.
Furthermore, as shown by the data in Table 2, the residue remaining
after the mercury release has a getter activity: in fact, while the residue
10 remaining after the mercury release by the Ti3Hg compound alone has no
getter activity, the sample of example 13 to which no getter has been
added exhibits a significant hydrogen absorption velocity. Moreover,
sample 12 has a hydrogen absorption velocity comparable to that of the
sample of example 11, which is a combination of a mercury dispenser with
15 a getter, widely used by lamp manufacturers.
When a getter material is added to the combination of example 12,
the hydrogen absorption velocity becomes nearly twice that of example 11,
with the same percentage of getter. These properties of the combination of
the invention make it possible to use very small amounts of additional
20 getter material or even none, while retaining the functionality of the
devices in which it is used.
The combinations with promoter of the present invention offer
another important advantage, consisting in the possibility of carrying out
the activation operation at temperatures or with times lower than those
21 726~6
- 1 4 -
allowed by prior art materials. In fact, in order to have industrially
acceptable activation times, Ti3Hg alone requires an activation
temperature of about 900C, whereas the present combinations allow the
reduction of the operation time and of the size of the lines for the
5 production of the lamps; in both cases a double advantage is achieved of
causing less pollution inside the tube due to the outgassing of all the
materials present therein and of reducing the amount of energy required
for the activation.
