U.S. patent number 3,578,834 [Application Number 04/798,785] was granted by the patent office on 1971-05-18 for generation of alkali metals.
This patent grant is currently assigned to S.A.E.S. Getters S.p.A.. Invention is credited to Paolo Della Porta, Elio Rabusin.
United States Patent |
3,578,834 |
Della Porta , et
al. |
May 18, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
GENERATION OF ALKALI METALS
Abstract
Compositions of matter which release alkali metal vapors
comprising an alkali metal compound and a reducing agent for the
compound wherein the reducing agent is an alloy of 5 to 30 weight
percent aluminum balance zirconium. These compositions are useful
in processes for the release of alkali metal vapors used in the
production of photosensitive surfaces in vacuum tubes.
Inventors: |
Della Porta; Paolo (Milan,
IT), Rabusin; Elio (Milan, IT) |
Assignee: |
S.A.E.S. Getters S.p.A. (Milan,
IT)
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Family
ID: |
26327924 |
Appl.
No.: |
04/798,785 |
Filed: |
February 12, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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690125 |
Dec 13, 1967 |
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Foreign Application Priority Data
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Dec 13, 1966 [IT] |
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3104/66 |
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Current U.S.
Class: |
445/21;
252/181.4; 252/181.6 |
Current CPC
Class: |
H01J
7/18 (20130101); H01J 17/26 (20130101); H01J
5/08 (20130101) |
Current International
Class: |
H01J
7/00 (20060101); H01J 5/08 (20060101); H01J
7/18 (20060101); H01J 17/26 (20060101); H01J
5/02 (20060101); H01J 17/02 (20060101); H01j
009/38 (); H01j 007/12 () |
Field of
Search: |
;252/181.4,181.1,181.3,181.6,181.7 ;316/16 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Levow; Tobias E.
Assistant Examiner: Cooper; J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of parent application
Ser. No. 690,125 filed Dec. 13, 1967 entitled "Metal Vapor
Generators."
Claims
We claim:
1. A composition of matter useful for generating an alkali metal in
a vacuum, said composition of matter consisting of an alkali metal
compound which is reducible to its metal and which is stable up to
400.degree. C and a zirconium-aluminum alloy as a reducing agent
for the alkali metal compound wherein the zirconium-aluminum alloy
contains 5 to 30 weight percent aluminum balance zirconium wherein
the alloy and the alkali metal compound are present in a weight
ratio of 1:1 to 20:1.
2. A composition of claim 1 wherein the compound of the alkali
metal is an alkali metal chromate.
3. A composition of claim 1 wherein the compound of the alkali
metal is selected from the group consisting of cesium chromate,
potassium chromate and sodium chromate.
4. A composition of claim 1 wherein the zirconium-aluminum alloy is
present in excess of stoichiometry.
5. A composition of claim 1 wherein the zirconium-aluminum alloy
contains 16 percent aluminum and 84 percent zirconium.
6. A composition of matter for releasing an alkali metal vapor, in
a vacuum said composition consisting of:
a. an alkali metal chromate and
b. an alloy of 5 to 30 weight percent aluminum balance zirconium
present in excess of stoichiometry.
7. A composition of matter for releasing an alkali metal vapor, in
a vacuum said composition consisting essentially of:
a. an alkali metal chromate and
b. an alloy of 13 to 18 weight percent aluminum, balance zirconium
present in excess of stoichiometry.
8. A composition of matter useful for generating an alkali metal
vapor in a vacuum, said composition consisting essentially of an
alkali metal chromate and an alloy of 16 percent aluminum and 84
percent zirconium wherein the weight ratio of alloy to chromate is
from 3:1 to 10:1 and wherein the alloy and the chromate are both
finely divided such that they pass through a U.S. standard screen
of at least 100 mesh per inch.
9. A composition of matter useful for generating an alkali metal
vapor in a vacuum, said composition consisting essentially of
cesium chromate and an alloy of 16 percent aluminum and 84 percent
zirconium wherein the weight ratio of alloy to chromate is from 3:1
to 10:1 and wherein the chromate are both finely divided such that
they pass through a U.S. standard screen of at least 100 mesh per
inch.
10. A composition of matter useful for generating an alkali metal
in a vacuum, said composition of matter comprising an alkali metal
compound which is reducible to its metal and which is stable up to
400.degree. C and a zirconium-aluminum alloy as a reducing agent
for the alkali metal compound wherein the zirconium-aluminum alloy
contains 5 to 30 weight percent aluminum balance zirconium wherein
the alloy and the alkali metal compound are present in a weight
ratio of 1:1 to 20:1.
11. A composition of claim 10 wherein the compound of the alkali
metal is an alkali metal chromate.
12. A composition of claim 10 wherein the zirconium-aluminum alloy
is present in excess of stoichiometry.
13. A composition of claim 10 wherein the zirconium-aluminum alloy
contains 16 percent aluminum and 84 percent zirconium.
14. A composition of matter for releasing an alkali metal vapor, in
a vacuum said composition comprising:
a. an alkali metal chromate and
b. an alloy of 5 to 30 weight percent aluminum balance zirconium
present in excess of stoichiometry.
15. A process for releasing an alkali metal in a vacuum tube
comprising heating to a temperature sufficient to release an alkali
metal, a finely divided mixture consisting essentially of an alkali
metal chromate which is stable up to 400.degree. C. and a
zirconium-aluminum alloy wherein the zirconium-aluminum alloy
contains 5 to 30 weight percent aluminum balance zirconium wherein
the alloy and the alkali metal chromate are present in a weight
ratio of 1:1 to 20:1.
16. The process of claim 15 wherein the zirconium-aluminum alloy is
present in excess of stoichiometry.
17. The process of claim 15 wherein the zirconium-aluminum alloy
contains 13 to 18 weight percent aluminum balance zirconium.
18. The process of claim 17 wherein the zirconium-aluminum alloy
contains 16 percent aluminum and 84 percent zirconium.
19. A process of claim 15 for releasing an alkali metal vapor in a
vacuum tube comprising in sequence the steps of:
I. introducing into the tube a composition of matter of an alkali
metal chromate and an alloy of 13 to 18 weight percent zirconium,
balance aluminum present in excess of stoichiometry;
Ii. closing the tube and connecting it to a vacuum pump;
Iii. heating the tube at 300.degree. to 400.degree. C for 1 to 8
hours while evacuating residual gases; and
Iv. heating the composition of matter at 550.degree. to 900.degree.
C. to release the alkali metal vapor.
20. A process for releasing an alkali metal vapor in a vacuum tube
comprising in sequence the steps of:
I. introducing into the tube a composition of matter of an alkali
metal chromate and an alloy of 5 to 30 weight percent zirconium,
balance aluminum wherein the alloy and the alkali metal chromate
are present in a weight ratio of 1:1 to 20:1;
Ii. closing and evacuating the tube;
Iii. heating the tube to evacuate residual gases; and
Iv. heating the composition of matter to release the alkali metal
vapor.
Description
It is frequently desirable to provide electron tubes containing
alkali metals such as cesium, potassium and/or sodium. For example,
the alkali metals are employed to produce photosensitive surfaces
such as those in television pickup tubes, photo multiplier tubes,
electronic sights, image conversion tubes, and image intensifier
tubes for radiological uses. The alkali metal is commonly
introduced into such tubes by means of an alkali metal vapor
dispenser containing a compound of the desired alkali metal and a
reducing agent for the compound. Such a dispenser employing silicon
as a reducing agent for cesium chromate is described in Eichenbaum
et al. "Cesium Vapor Dispenser," The Review of Scientific
Instruments, Vol. 35, No. 6, June 1964, pp. 691--693. Such
dispensers are also described by Lester U.S. Pat. No. 2,117,735
(1938) who employs a mixture of cesium chromates as the alkali
metal compound and as the reducing agent a mixture of zirconium
powder and aluminum powder in a weight ratio of 8:1. Unfortunately,
the alkali metal vapor releasing compositions of Eichenbaum and
Lester and other prior compositions release an alkali metal vapor
contaminated with noxious gases such as oxygen. These gases react
with the alkali metal and reduce the photosensitivity of the
resulting surface.
Accordingly, it is an object of the present invention to provide
novel alkali metal vapor releasing compositions which are
substantially free of one or more disadvantages of prior
compositions.
Another object is to provide compositions which release a
substantially pure alkali metal vapor free of noxious gases such as
oxygen.
A further object is to provide novel alkali metal releasing
compounds employing a zirconium-aluminum alloy as the reducing
agent wherein the alloy has advantages not realized by mixtures of
the same metals.
Additional objects and advantages of the present invention will be
apparent by the following detailed description thereof.
In the parent application composite structures are described which
comprise a sintered matrix of a sinterizable material such as iron
and within the matrix and alkali metal chromate and a
zirconium-aluminum alloy as a reducing agent for the chromate.
While such composite structures are effective for the purpose
described, i.e., reduction of undesirable loose particles it has
been discovered that the heating necessary to sinter the
sinterizable material alters the composite structure in some
unknown manner such that when the alkali metal vapor is later
released it is contaminated with large quantities of noxious gases.
In accordance with the present invention it has been discovered
that compositions having no sinterized material and therefore those
which consist essentially of an alkali metal compound and a
zirconium-aluminum alloy release an alkali metal vapor which is
substantially free of noxious gases. Zirconium-aluminum alloys of
differing particle sizes have been employed as described in U.S.
Pat. No. 3,385,644 admixed with mercuric oxide but have not
heretofore been employed with alkali metal compounds as in the
present invention. Ad additional advantage of the compositions of
the present invention is that the zirconium-aluminum alloy need not
be used in differing particle sizes.
The alkali metal compound can be any compound which is reducible to
its metal by a reducing agent and is sufficiently stable alone and
in admixture with its reducing agent to withstand the heat
treatments commonly employed in the outgasing of vacuum tubes. Thus
the preferred alkali metal compounds are those which are stable at
temperatures up to 400.degree. C. Examples of suitable alkali metal
compounds include among others cesium chloride, sodium dichromate
and most preferably cesium chromate, potassium chromate, sodium
chromate, rubidium chromate and lithium chromate. The chromates are
greatly preferred over the dichromates because of the reduced
amount of oxygen which they release.
The reducing agents employed in the present invention are
zirconium-aluminum alloys and generally those of 5 to 30 weight
percent aluminum balance zirconium, preferably those of 13 to 18
weight percent aluminum, balance zirconium and especially those of
16 percent aluminum and 84 percent zirconium produced as described
in U.S. Pat. No. 3,203,901. It will be understood that minor
amounts of other common impurities and compatible metals can be
included in the alloy provided they do not materially affect the
novel characteristics of the composition.
The alkali metal compound and the zirconium-aluminum alloy are both
preferably employed as finely divided particles and generally those
which pass through a screen of 100 mesh per inch and preferably
those which pass through a screen of 270 mesh per inch. The iron or
other sinterizable material when present also has these particle
sizes.
The alkali metal compound and zirconium-aluminum alloy can be mixed
in widely varying ratios but a stoichiometric excess of the
zirconium-aluminum alloy is generally employed in order to
completely reduce the reducible alkali metal compound and generally
in ratios of 1:1 to 20:1 and preferably 3:1 to 10:1 parts by weight
of alloy per part by weight of reducible compound.
According to another inseparable aspect of the present invention
there is provided a process for releasing an alkali metal vapor in
a vacuum. The composition of alkali metal compound and
zirconium-aluminum alloy is placed in any metallic container
capable of withstanding elevated temperatures. This container is
then placed in a vacuum tube which is then evacuated mechanically
or by any other suitable means while being maintained at
350.degree. to 400.degree. C. for 1 to 8 hours while evacuating
residual gases. The container is then heated to the temperature of
the onset of the reaction between the alloy and the alkali metal
compound and generally between 450 and 1000.degree. C. and
preferably between 550 and 900.degree. C. for a time sufficient to
completely or partially reduce the alkali metal compound releasing
the desired amount of alkali metal vapor.
The invention is further illustrated by the following examples in
which all parts and percentages are by weight unless otherwise
indicated. These nonlimiting examples are illustrative of certain
embodiments and are designed to teach those skilled in the art how
to practice the invention and to represent the best mode presently
known for carrying out the invention.
EXAMPLE 1
This example illustrates the small amount of gas released when
cesium chromate is reduced with an alloy of 16 percent aluminum and
84 percent zirconium.
Into a mortar is placed 500 grams of a zirconium-aluminum alloy
commercially available as St101 from S.A.E.S. Getters S.p.A.
containing 16 percent aluminum and 84 percent zirconium. This alloy
is ground with a pestle until it is a fine powder which passes
through a screen of 270 mesh/inch, whereupon it is mixed with 100
grams of cesium chromate which has been similarly ground. A portion
of this mixture is placed in a container of Nichrome V having a
slit therein. The container is then placed in a vacuum tube and the
entire assembly heated in a vacuum of 10.sup..sup.-6 torr for 8
hours at 375.degree. C. to remove residual gases. The assembly is
cooled and then the container alone is heated to 600.degree. C.
whereupon cesium begins to evaporate. The temperature is increased
in steps of 8.degree. C/minute up to a maximum temperature of
850.degree. C. This temperature is maintained for 1 minute. The
cesium vapor deposits on the cold walls of the tube. During the
period of cesium vapor release the tube is connected to a vacuum
pump via a conduit containing a conductance which acts as a gas
flow restricting orifice. The pressure on each side of the orifice
is continuously recorded during the period of cesium vapor release
and the total quantity of gases released calculated according to
the following formula:
wherein:
Q = the quantity of gas in cc.-torr
c = conductance of the orifice (146 cc./sec.)
P.sub.D = pressure upstream of the conductance in torr
P.sub.P = pressure downstream of the conductance in torr
t.sub.0 = time at onset of cesium vapor release
t.sub.1 = time at completion of cesium vapor release in seconds
dt = incremental time in seconds
The quantity of gas is found to be 0.3 cc.-torr/mg. of cesium
chromate.
EXAMPLE 2
This comparative example illustrates the large amount of gas
released by cesium chromate reduced with a mixture of one part by
weight of aluminum and eight parts by weight of zirconium following
the teaching of Lester supra.
The above procedure of example 1 is repeated employing the same
times, temperatures, conditions and ingredients except that the
zirconium-aluminum alloy is replaced by an equal weight of a
mixture of one part by weight aluminum and eight parts by weight
zirconium. The quantity of gas released is to be found to be 12
cc.-torr/mg. or approximately 40 times the amount of gas released
in example 1 wherein the preferred zirconium-aluminum alloy is
employed as the reducing agent.
EXAMPLE 3
The procedure of example 1 is repeated twice employing the same
times, temperatures, components and conditions the first time with
the exception that the cesium chromate is replaced by a
stoichiometrically equivalent amount of sodium chromate and the
second time with the exception that the cesium chromate is replaced
by a stoichiometrically equivalent amount of potassium chromate.
Similar results are obtained with low quantities of gas being
evolved with the sodium or potassium vapor.
As used herein the term "cc.-torr" refers to the volume of gas in
cubic centimeters at a pressure of one torr (1 mm. Hg. absolute)
and is independent of the pressure. As can be seen by comparing
example 1 and 2 the compositions of Lester release 40 times the
amount of undesirable gas released by the compositions of the
present invention.
Although the invention has been described in considerable detail
with reference to certain preferred embodiments thereof, it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention as described above and as
defined in the appended claims.
* * * * *