U.S. patent application number 11/042420 was filed with the patent office on 2005-07-28 for device and method for producing a calcium-rich getter thin film.
This patent application is currently assigned to SAES Getters S.p.A.. Invention is credited to Carretti, Corrado, Toia, Luca.
Application Number | 20050163930 11/042420 |
Document ID | / |
Family ID | 11448557 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050163930 |
Kind Code |
A1 |
Carretti, Corrado ; et
al. |
July 28, 2005 |
Device and method for producing a calcium-rich getter thin film
Abstract
An improved getter device and method for forming a calcium-rich
getter thin film in an electronic vacuum device is disclosed. The
getter device includes a powder of a Ca--Ba--Al ternary alloy
composed of between 53% and 56.8% by weight of aluminum, from 36%
to 41.7% by weight of calcium and from 1.5% to 11% by weight of
barium. The method allows the formation of a calcium-rich getter
thin film with a substantially reduced amount of released hydrogen
in the vacuum device.
Inventors: |
Carretti, Corrado; (Milano,
IT) ; Toia, Luca; (Carnago (VA), IT) |
Correspondence
Address: |
PERKINS COIE LLP
P.O. BOX 2168
MENLO PARK
CA
94026
US
|
Assignee: |
SAES Getters S.p.A.
|
Family ID: |
11448557 |
Appl. No.: |
11/042420 |
Filed: |
January 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11042420 |
Jan 24, 2005 |
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10820571 |
Apr 8, 2004 |
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10820571 |
Apr 8, 2004 |
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10282715 |
Oct 29, 2002 |
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6793461 |
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Current U.S.
Class: |
427/250 ;
252/181.1 |
Current CPC
Class: |
C22C 1/0416 20130101;
H01J 7/183 20130101; C22C 21/00 20130101 |
Class at
Publication: |
427/250 ;
252/181.1 |
International
Class: |
C23C 016/00; H01K
001/56 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2001 |
IT |
MI 2001A 002273 |
Claims
1-14. (canceled)
15. A method, comprising: providing an alloy comprising calcium,
barium, and aluminum within a cavity of a device; at least
partially evacuating the cavity of the device; hermetically sealing
the evacuated cavity of the device; vaporizing at least some of the
alloy within the evacuated cavity of the device; and depositing a
calcium getter film on an inner surface of the evacuated cavity,
wherein said calcium getter film includes at least some of the
calcium from the vaporized alloy.
16. The method of claim 15, wherein the alloy includes between
about 50% and 60% by weight of aluminum, between about 30% and 45%
by weight of calcium and between about 1.5% and 15% by weight of
barium.
17. The method of claim 15, wherein the alloy includes between
about 53% and 56.8% by weight of aluminum, between about 36% and
41.7% by weight of calcium and between about 1.5% and 11% by weight
of barium.
18. The method of claim 15, wherein the alloy includes between
about 2.5% and 5% by weight of barium.
19. The method of claim 15, wherein said vaporizing includes
heating to at least an evaporating temperature of calcium.
20. The method of claim 15, wherein said vaporizing includes
heating to at least an evaporating temperature of barium.
21. The method of claim 15, wherein the alloy includes Ca--Ba--Al
powder having a granulometry between about 50 and 250 .mu.m.
22. The method of claim 15, further comprising mixing the alloy
with powdered nickel or titanium component, at a weight ratio of
the component to alloy of between about 3:1 and 1:3, wherein the
alloy is powdered.
23. The method of claim 15, further comprising admixing the alloy
with up to about 5% by weight of powdered metal nitride component
selected from the group consisting of iron nitride, germanium
nitride, and a combination of iron nitride and germanium
nitride.
24. A device, comprising: a cathode ray tube with inner walls that
define a vacuum cavity, wherein, when operationally configured, the
vacuum cavity is at least partially evacuated; and a getter film
comprising calcium and barium, supported by the inner walls, within
the vacuum cavity.
25. The device of claim 24, wherein said getter film includes
between about 30% and 97% by weight of calcium and between about
1.5% and 30% by weight of barium.
26. The device of claim 24, wherein said getter film includes
between about 36% and 85% by weight of calcium and between 1.5% and
15% by weight of barium.
27. The device of claim 24, wherein said getter film adheres to the
inner walls.
28. The device of claim 24, wherein said getter film is coupled to
the inner walls.
29. A composition of matter for use as a getter source, comprising:
between about 50% and 60% by weight of aluminum; between about 30%
and 45% by weight of calcium; and between about 1.5% and 15% by
weight of barium.
30. The composition of matter of claim 29, wherein said composition
of matter is an ingot.
31. The composition of matter of claim 29, wherein said composition
of matter is a powder.
32. The composition of matter of claim 29, further comprising
nickel, titanium, or a combination of nickel and titanium.
33. The composition of matter of claim 29, wherein said composition
of matter is a powder admixed with powders of nickel or
titanium.
34. The composition of matter of claim 29, further comprising iron
nitride or germanium nitride, or a combination of iron nitride and
germanium nitride.
Description
REFERENCE TO PRIORITY DOCUMENTS
[0001] This application is a Continuation of U.S. application Ser.
No. 10/820,571. filed Apr. 8, 2004, which is a Continuation of U.S.
application Ser. No. 10/282,715, filed Oct. 29, 2002, which issued
as U.S. Pat. No. 6,793,461, on Sep. 21, 2004, which claims priority
under 35 U.S.C. .sctn. 119 to Italian Application MI2001A-002273,
filed Oct. 29, 2001, all of which are incorporated herein in their
entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a device and method for
forming a calcium-rich getter thin film in an electronic vacuum
device.
BACKGROUND
[0003] A number of industrial applications require a suitable
vacuum to be kept in a sealed space for a period of several years.
In particular, electronic vacuum devices such as CRTs (Cathode Ray
Tubes), which are used as screens of television sets or computers,
have this requirement. In CRTs, vacuum is required to avoid
electrons emitted by a cathode from being deflected by collisions
with gas particles. CRTs are evacuated during the manufacturing
step through mechanical pumps and then hermetically sealed.
[0004] The vacuum in the tube tends however to decrease during
time, mainly because of the degassing from internal components of
the tube. It is therefore necessary to use inside the tube a getter
material capable of capturing the gaseous molecules, thus
preserving the vacuum degree necessary for the cathodic tube to
work for the time needed. For this purpose barium is usually used
in the form of a thin film deposited on inner walls of the cathodic
tube. Because of the high reactivity of this metal, which would
make every manufacturing operation troublesome, barium is used in
the form of the air stable compound BaAl.sub.4. To introduce the
compound inside the cathodic tube there are utilized the so-called
"evaporable getter" devices, formed of an open metallic container,
inside which there is a compressed mixture of BaAl.sub.4 and nickel
powders (in a weight ratio of about 1:1); devices of this type are
disclosed for example in patents U.S. Pat. Nos. 2,842,640,
2,907,451, 3,033,354, 3,225,911, 3,381,805, 3,719,433, 4,134,041,
4,486,686, 4,504,765, 4,642,516 and 4,961,040. These patents are
incorporated herein by reference, in particular, for their teaching
of methods of vaporizing BaAl.sub.4 alloys within a sealed vacuum
chamber, and various electronic devices employing such getters.
[0005] The BaAl.sub.4 alloys are introduced inside the cathodic
tube before sealing it, and then are heated from outside through
radio frequencies to cause the evaporation of barium, which then
condenses on the internal walls thus forming the film active in
sorbing gases. Nickel has the function of reducing the energy
required at radio-frequency heating: when the temperature of the
mixture reaches about 850.degree. C., the following exothermal
reaction takes place: BaAl.sub.4+4Ni.fwdarw.Ba+4NiAl. The heat
generated by this reaction raises the temperature of the system up
to about 1200.degree. C., necessary to have barium evaporation;
these devices are defined "exothermal" in the field.
[0006] The use of barium, however, has some drawbacks. First of
all, like all heavy metals, it is a toxic material, so that the
more barium material used, the more precautions that must be taken
in manufacture, and also the greater the problems associated with
disposing of the device to avoid environmental contamination.
Furthermore, inside the cathodic tubes, barium is present also in
areas hit by highly energetic electron beams used to generate the
image inside the kinescope; in these conditions barium, and
consequently the screen of the kinescope, emit X rays (even though
in small quantities) that may be harmful to health.
[0007] In order to avoid the problems caused by the use of barium,
co-owned PCT application WO 01/01436, discloses the use of calcium
as a gas sorbing getter material, and the compound CaAl.sub.2 as a
precursor to be utilized for evaporating calcium. The compound
CaAl.sub.2 is preferably used in mixture with titanium powders.
[0008] The use of calcium-based evaporable getter material has also
some advantages during the manufacture of CRTs, in that the
evaporation of calcium is less violent and more easily controllable
with respect to barium, even after the treatments at relatively
high temperatures (about 450.degree. C.) in oxidizing atmospheres
which occur during some of the manufacturing steps of the
tubes.
[0009] However, the calcium getter material disclosed in the above
WO 01/01436 application has the problem that the CaAl.sub.2 alloy
accumulates a substantial amount of hydrogen during its
manufacture. The hydrogen contained in the alloy is released during
the evaporation of calcium, and can negatively interfere with the
deposition process. Furthermore, it is known that hydrogen can
react with carbon atoms on the surface of metallic films, forming
low molecular weight alkanes, such as methane, which is reabsorbed
only with difficulty and partially by the same film.
SUMMARY OF THE INVENTION
[0010] In one aspect, the invention includes an improved method for
forming a calcium getter film in an electronic vacuum device that
substantially reduces the amount of H.sub.2 released during film
formation. The method includes vaporizing a powder of a Ca--Ba--Al
ternary alloy containing between 50% and 60% by weigh of aluminum,
between 30% and 45% by weight of calcium and between 1.5% and 15%
by weight of barium, and more preferably between 53% and 56.8% by
weight of aluminum, between 36% and 41.7% by weight of calcium and
between 1.5% and 11% by weight of barium. One exemplary alloy
contains between 2.5% and 5% by weight of barium.
[0011] The powder of the ternary alloy has a preferred granularity
between 50 and 250 .mu.m. The powder of the ternary alloy may be
formulated or blended with a powder of nickel or titanium metal,
forming a mixed-powder composition, at a weight ratio of metal to
alloy powders of between 3:1 and 1:3. The metal and alloy powder
composition may also contain up to 5% by weight of a metal nitride
selected from the group consisting of iron nitride, germanium
nitride and combinations of the two nitrides.
[0012] In another aspect, the invention includes a getter device
comprising a container containing a powder of a ternary Ca--Ba--Al
alloy containing between 50% and 60% by weight of aluminum, between
30% and 45% by weight of calcium and between 1.5% and 15% by weight
of barium, and more preferably between 53% and 56.8% by weight of
aluminum, between 36% and 41.7% by weight of calcium and between
1.5% and 11% by weight of barium. One exemplary alloy contains
between 2.5% and 5% by weight of barium.
[0013] The powder of the ternary alloy has a granulometry between
50 and 250 .mu.m. The getter device may further include a nickel or
titanium metal powder, at a weight ratio of metal powder to alloy
powders of between 3:1 and 1:3. The mixed metal and alloy powders
may further include up to 5% by weight of a metal nitride selected
from the group consisting of iron nitride, germanium nitride and
combinations of the two nitrides.
[0014] In still another aspect, the invention includes (i)
providing an electronic vacuum device having a sealed enclosure
under vacuum and having an interior wall surface, and (ii) coating
the wall surface with a thin film composed of between 70% and 97%
by weight calcium and 3% and 30% by weight barium. The film in an
exemplary device is composed of between 85% and 95% weight percent
calcium and 5% and 15% weight percent barium.
[0015] These and other objects and features of the invention will
be more fully apparent when the following detailed description of
the invention is read in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1A shows a ternary diagram wherein the possible
compositions of the alloys according to the present invention are
illustrated;
[0017] FIG. 1B shows the parallelogram in FIG. 1A in enlarged view;
and
[0018] FIG. 2 shows the progress of the amount of hydrogen released
by comparative devices and by the inventive devices as a function
of the quantity of barium present in the alloy utilized in
preparing the device.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The inventors have found that by substituting in compound
CaAl.sub.2 a small fraction of calcium atoms with barium atoms it
is possible to substantially eliminate the problem of the hydrogen
released during the calcium evaporation step.
[0020] The alloys used in the present invention are ternary alloys
Ca--Ba--Al with a content varying between 50% and 60% by weight of
aluminum, between 30% and 45% by weight of calcium and between 1.5%
and 15% by weight of barium, and more preferably 53% and 56.8% by
weight of aluminum, between 36% and 41.7% by weight of calcium and
between 1.5% and 11% by weight of barium.
[0021] These preferred compositions fall within the dashed area of
the ternary diagram of FIG. 1A, this area having the form of a
parallelogram shown in FIG. 1B, wherein some compositions produced
and tested in the examples are also indicated. At barium weight
percentages lower than 1.5%, there is no significant noteworthy
reduction of the released hydrogen amount with respect to compound
CaAl.sub.2. At barium weight percentages higher than 11%, no
further reduction in hydrogen emission is observed. Ca--Ba--Al
alloys with a higher barium percentage could well be utilized, but
they would have the drawback of increasing the amount of a
potentially toxic element, not compensated by advantages regarding
hydrogen emission. Within this range, alloys with a content of
barium included between 2.5% and 5% by weight are used in a
preferred embodiment of the invention.
[0022] With reference to compound CaAl.sub.2, it is possible to
produce alloys wherein as the barium percentage by weight
increases, only the calcium percentage correspondingly decreases,
while the aluminum percentage remains constant; preferably, however
also, the aluminum content is decreased as the barium percentage
increases.
[0023] The alloys of the invention are simply prepared by smelting
the component metals in a stoichiometric ratio, and in particular
ratios of CaAl.sub.2 and BaAl.sub.4, according to well-known
methods. The melting can be carried out in a furnace of any type,
for example an induction one, and preferably under an inert
atmosphere such as argon.
[0024] In industrial applications, the alloys of the invention can
be utilized in evaporable getter devices, formed of a container
made up of metal, generally steel. The container is open on the
upper part and has generally the shape of a short cylinder (in the
case of the smaller devices) or of an annular channel with an
essentially rectangular cross-section. The shape of the container
can be essentially the same as the shape of containers utilized for
analogous known devices, as referred to in multiple US patents
mentioned in the background section. As defined herein, a getter
device includes the container and the ternary alloy powder, and,
optionally metal or nitride powders (see below), contained
therein.
[0025] These devices can include the so-called "endothermic" type,
wherein the whole heat necessary for the calcium evaporation is to
be provided from outside, generally through induction heating;
devices of this type contain only a compound of the invention. In a
preferred embodiment, devices of "exothermic" type are used, as
described previously with reference to devices for evaporating
barium, containing, apart from an alloy of the invention, nickel,
titanium or mixtures of powders of these two metals. In a preferred
embodiment titanium is used.
[0026] Inside the getter devices the alloy Ca--Ba--Al is preferably
used in the form of powders, generally with a granulometry lower
than about 500 .mu.m, preferably lower than 250 .mu.m, and still
most preferably between 45 and 150 .mu.m.
[0027] In the case of exothermic devices, nickel or titanium is
preferably utilized in the form of powders having a granulometry
lower than about 100 .mu.m and most preferably between 20 and 70
.mu.m.
[0028] The weight ratio between the alloy Ca--Ba--Al and Ni or Ti
in exothermic devices can vary within a wide range: this ratio is
generally between about 1:3 and 3:1 and is approximately 1:1 in a
preferred embodiment.
[0029] Also in the getter device of the present invention it is
possible to include other components, preferably in powder form.
For example, the device can contain percentages up to about 5% by
weight (on the mixture of powders) of a compound chosen among iron
nitride, germanium nitride or mixtures thereof. In these devices,
nitrogen is released just before the evaporation of calcium, which
allows one to obtain a more diffused metal film having a more
homogeneous thickness. Examples of nitrogen-containing devices are
reported in U.S. Pat. Nos. 3,389,288 and 3,669,567, which are
incorporated herein by reference.
[0030] The free surface of the packet of powders in the container,
both in the case of endothermic and exothermic devices, can have
radial depressions (from 2 to 8, normally 4) to moderate the
transfer of heat in the circular sense in the packet, thus reducing
the problem of a possible expulsion of solid particles during
calcium evaporation. For a more detailed explanation of this
problem, and of the solution provided by the radial depressions,
referred to U.S. Pat. No. 5,118,988, which is herein incorporated
by reference.
[0031] Finally, in order to improve the homogeneity of the
inductive heating of the packet of powders, it is possible to add
in the packet a discontinuous metallic element, essentially
parallel to the bottom of the container, as described in U.S. Pat.
No. 3,558,962 and in European patent application EP-A-853328.
[0032] The invention will be further explained by the following
examples. These non-limiting examples illustrate some embodiments
aimed at teaching to those skilled in the art how to put the
invention into practice and to represent the best regarded mode to
realize the invention.
EXAMPLE 1
Comparative
[0033] 100 g of compound CaAl.sub.2 are prepared by smelting in a
refractory crucible (made of mixed oxides of aluminum and
magnesium) 42.6 g of calcium in the form of chips and 57.4 g of
aluminum in the form of drops. In the portion of the ternary
diagram 10 of FIG. 1B, this composition is represented by an empty
circle 12. The melting is carried out in an induction furnace under
argon. After the solidification of the melt product, the ingot is
ground and the powders are sifted, recovering the fraction with
granulometry included between 45 and 150.mu.; 49.5 g of this powder
are mixed with 50.5 g of titanium powder having a mean granulometry
of 40 .mu.m. With this mixture five devices for evaporating calcium
are prepared, by using for each one a steel container shaped as an
annular channel, with an outer diameter of 20 mm and channel width
of 6 mm; each container is filled up with 1 g of mixture,
compressing the powders with a shaped punch to which a pressure of
about 6500 Kg/cm.sup.2 is applied.
EXAMPLE 2
Comparative
[0034] Using the same procedure of example 1, 100 g of a ternary
alloy with a percent composition by weight Ca 42.3%--Ba 0.5%--Al
57.2% is used. This composition corresponds to an empty circle 14
in FIG. 1B. The ingot is ground recovering the fraction having a
granulometry included between 45 and 150 .mu.m; 45 g of powder so
obtained are mixed with 55 g of titanium powder having a mean
granulometry of 40 .mu.m, and with this mixture five devices for
evaporating calcium are prepared.
EXAMPLE 3
[0035] Five getter devices for evaporating calcium are manufactured
following the procedure of example 2, by using, however, an alloy
with a percent composition by weight Ca 41.7%--Ba 1.5%--A 56.8%.
This composition corresponds to point A, represented with a filled
square 16 in FIG. 1B.
EXAMPLE 4
[0036] Five getter devices for evaporating calcium are manufactured
following the procedure of example 2, by using, however, an alloy
with a percent composition by weight Ca 41.1%--Ba 2.5%--Al 56.4%.
This composition corresponds to point B, with a filled square 18 in
FIG. 1B.
EXAMPLE 5
[0037] Five getter devices for evaporating calcium are manufactured
following the procedure of example 2, by using, however, an alloy
with a percent composition by weight Ca 39.5%--Ba 5%--Al 55.5%.
This composition corresponds to point C, represented by a filled
square 20 in FIG. 1B.
EXAMPLE 6
[0038] Five getter devices for evaporating calcium are manufactured
following the procedure of example 2, by using, however, an alloy
with a percent composition by weight Ca 36%--Ba 11%--Al 53%. This
composition corresponds to point D, represented with a filled
square 22 in FIG. 1B.
EXAMPLE 7
[0039] The series of five evaporable getter devices produced in
each of the examples from 1 to 6 (totally 30 devices) are subjected
to evaporation tests. The samples are introduced one at a time in a
glass flask with a volume of 6 liters, vacuum is made in the flask
(with a pressure lower than 10.sup.-8 mbar) and the getter device
is heated from outside by induction through radio-frequency. The
flask is connected to a mass spectrometer, which records the
development of the hydrogen pressure in the flask during time. This
pressure has a maximum value corresponding to the evaporation and
then decreases due to the reabsorption by the calcium film produced
on the inner walls of the flask. According to a usual procedure in
the field of evaporable getters, the evaluation of the hydrogen
pressure is effected 15 minutes after the evaporation. It is made
an average of the results from the five tests carried out for each
composition. The average values so obtained are shown in the
semilogarithmic graph of FIG. 2, wherein the common logarithm of
the hydrogen pressure value (in mbar) 15 minutes after the
evaporation is reported as a function of the percentage by weight
of barium in the sample; the values corresponding to the
comparative samples are represented with empty circles 12 and 14,
and closed squares A, B, C and D as in FIG. 1A.
[0040] As it is noted from the examination of FIG. 2, devices
prepared with alloys of the invention present, shortly after the
evaporation of calcium, a low hydrogen release of about 10.sup.-5
mbar or less, which is compatible with the expected applications in
the manufacturing of CRTs for television sets and computer
screens.
[0041] Although the invention has been described with respect to
specific embodiments and applications, it will be appreciated that
various changes and modifications may be made without departing
from the invention.
* * * * *