The present invention concerns an evaporable getter device with reduced
activation time.
As is known, the evaporable getter materials are mainly employed for the
maintenance of vacuum at the inside of picture tubes for television sets and of
computer screens. The use of evaporable getter materials at the inside of flat
displays, being at present in a developing stage, is also under study.
The getter material being commonly used in picture tubes is metallic barium
which is deposited in the form of a thin film on an inner wall of the tube. To obtain
such a film use is made of devices, known in the field as evaporable getter devices,
which, are introduced in the tube during its manufacturing. These devices are
formed of an open metallic container comprising therein powders of a compound of
barium and aluminum, BaAl4, generally having a particle size of less than about 250
µm, and powders of nickel, generally with a particle size of less than 60 µm, in a
ratio by weight of about 1:1. These devices are well known in the technique;
reference is made in this respect for example to US patent 5.118.988 in the
applicant's name. Barium is caused to evaporate by induction heating the device by
means of a coil at the outside of the picture tube itself, in an activation process also
defined as "flash"; when the temperature of the powders reaches a value in the
range between about 800 and 850°C, the following reaction takes place:
BaAl4 + 4 Ni → Ba + 4 NiAl
This reaction is strongly exothermic and raises the temperature of the
powders to about 1200°C, at which barium is evaporated and deposited onto the
walls of the tube thus forming the metallic film.
The time required to evaporate all the barium contained in the device, being
measured starting from the moment at which the device begins being supplied with
power by means of the coil, is defined in the field with the term "total time", which
will be used in the following portion of the description and in the claims also an its
abridged form TT. For example, to obtain barium films of about 300 mg as required
by colour picture tubes of big size, the required TT with the present getter devices
is of 40-45 seconds. However this time corresponds to the slow step in the present
lines for manufacturing electronic tubes, whereby it is a requirement of the
manufacturers to have devices which can release barium with lower TT values.
To obtain such a result, in principle the power supplied by the coil can be
increased or an increase of the powders reactivity can be obtained by reducing their
particle size.
However with the available getter devices an increase of the coil power is
impossible. In fact, by doing so, the container of the powders raises its temperature
too quickly and there is not time enough for a homogeneous diffusion in the packet
ofpowders of heat, thus giving rise to the container melting.
Also a reduction of the particle size of the powders is impossible, as this
would bring to an excessive and local increase of the reaction speed between BaAl4
and Ni with consequent bulging of the packet of powders and possible ejection of
pieces from the latter.
Object of the present invention is that of providing an evaporable getter
device with reduced activation time that does not show the inconveniences of the
prior art.
Such an object is obtained according to the present invention with an
evaporable getter device having a reduced activation time comprising a metallic
container in which a mixture is present including:
- powder of BaAl4 compound;
- powder of nickel; and
- powder of a third component chosen among aluminum, iron, titanium and
their alloys in a quantity range between about 0,3% and about 5% of the
total weight of the mixture.
The quantity of powder of the third component in the mixture of powders
depends on the actually employed component and generally is in the range between
about 0,3% and 5%. In particular, the percentage of the third component is
preferably comprised between about 0,8% and 2% in case of aluminum, between
about 0,3% and 1,2% in case of iron and between about 0,5% and 5% in case of
titanium. With quantities of the third component being lower than those indicated
the desired effect of reducing the barium evaporation time is not obtained. On the
contrary, when operating with quantities of the third component being higher than
those above indicated, the barium flash becomes of raging nature and hardly
controllable. The ratio by weight between nickel and BaAl4 is the same as in the
prior art devices, generally of about 1:1; in particular, in this field getter devices are
broadly employed having a ratio of 5.3:4.7 between nickel and BaAl4.
For the purposes of the invention the third component is not required to be of
particularly high purity and use can be made of powders of commercial metals or
alloys, generally having a purity of about 98-99%. The particle size of the
powdered third component being useful for the purposes of the invention is less
than about 80 µm and preferably less than about 55 µm.
The powders of nickel and of the compound BaAl4 which are employed in the
getter devices of the invention are the same as those used in the prior art devices;
generally powders with particle size of less than about 60 µm are used for nickel,
while for the compound BaAl4 powders having a particle size of less than about 250
µm are generally employed.
The metallic container can be made from various materials, such as nickel-plated
iron or constantan; preferred is the use of steels AISI 304 or AISI 305 which
show a good resistance to oxidation and thermal treatment as well as a good cold
workability. The metallic container can have any shape and in particular whichever
one of the shapes known and used in the field, such as those of the devices
according to the US Patents 4,127,361 - 4,323,818 - 4,486,686 - 4,504,765 -
4,642,516 - 4,961,040 and 5,118,988.
Particularly interesting is the possibility of obtaining evaporable getter devices
with a reduced time of activation which are also frittable; with this term getter
devices are meant which can withstand to an oxidizing atmosphere at a temperature
of about 450°C for a duration time of up to 2 hours; these being the conditions
which such devices have to undergo in some processes for manufacturing picture
tubes. During the barium evaporation from frittable getter devices a greater heat
quantity is generated than in the common getter devices, with consequent higher
difficulties in keeping the packet of powders in the container. Frittable getter
devices with a quantity of evaporable barium up to about 200 mg have been
manufactured and sold by the applicant since several years. On the contrary frit
sealable getter devices which can evaporate greater quantities of barium and in
particular of about 300 mg require that particular solutions are adopted to take into
account their greater reactivity; the patent application with the title "FRITTABLE
EVAPORABLE GETTER DEVICE HAVING A HIGH YIELD OF BARIUM" in
the applicant's name and having the same filing date of the present application,
discloses the manufacturing of frittable getter devices obtained through the addition
of elements retarding the heat dispersion in a circumferential direction in the packet
of powders and the addition of a discontinuous metallic element, essentially flat, in
the same packet. By adding a third component to frittable getter devices either of
the traditional type or of the high yield type it is possible to obtain frittable getter
devices with comparable characteristics of barium emission, but during a reduced
evaporation time.
The invention will be further illustrated by means of the following examples.
These non-limiting examples show some embodiments designed to teach those
skilled in the art how to practice the invention and to represent the best considered
mode for putting into practice the invention.
EXAMPLE 1
A number of getter devices all equal to each other is prepared by using for
each of them a container made of steel AISI 304 having diameter of 20 mm and
height of 4 mm with the bottom shaped with elevations 1 mm high as described in
US Patent 5,118,988 cited herein. For each sample a homogeneous mixture is
poured into the container, being comprised of 767 mg of powdered BaAl4 having a
particle size of less than 250 µm, 866 mg of powdered nickel having a particle size
of less than 60 µm and 18 mg of powdered iron with a 99% purity having a particle
size of less than 80 µm. The mixture of powders is then compressed at the inside of
the container by a proper punch. The samples are tested by placing them one at a
time into a measuring chamber made of glass, connected to a pumping system,
evacuating the chamber and carrying out a barium evaporation test by following the
method described in the standard ASTM F 111-72; every device is heated by means
of radio frequencies with such a power that the evaporation starts 12 s after the
heating has begun; the tests are different from one another as to the heating Total
Time, which is caused to vary in the various tests in a range between 35 and 45 s.
At the end of each test the evaporated barium quantity is detected. The TT required
for evaporating from the device a barium quantity of 300 mg is reported in Table 1.
EXAMPLE 2
A number of getter devices all identical to each other is prepared by using for
them a steel container as described in Example 1. Within this container there is
positioned a net of steel AISI 304 with meshes of 1,5 mm width, resting on the
bottom elevations. For each sample a homogeneous mixture is poured into the
container, being comprised of 767 mg of powdered BaAl4 having a particle size of
less than 250 µm, 866 mg of powdered nickel having a particle size of less than 60
µm and 18 mg of powdered aluminum, of 99% purity and having a particle size of
less than 50 µm. The mixture of powders is then compressed at the inside of the
container with a punch so shaped as to form at the packet surface four radial
recesses. The samples thus obtained are treated at 450°C during 1 hour in air to
simulate the frit sealing conditions. A barium evaporation test is then carried out on
every sample like according to Example 1. Also in this case each device is heated
by means of radio frequencies with such a power that the evaporation starts 12 s
after the heating has begun, while the heating is maintained during a TT that is
different from sample to sample and varying in a range between 35 and 45 s, then
detecting the TT value required to evaporate from the devices a barium quantity of
300 mg.
The results of the test are reported in Table 1.
(COMPARATIVE) EXAMPLE 3
The test of example 1 is repeated with a series of samples identical to those of
example 1, but without powdered iron, by heating the devices with radio
frequencies at a power level such that the evaporation starts 12 s after the heating
has started and using different TT, variable between 35 and 45 s. The required TT
for evaporating 300 mg of barium from these samples is reported in Table 1.
(COMPARATIVE) EXAMPLE 4
The series of tests of example 2 is repeated by using getter devices identical
to those of Example 1 but without powdered aluminum. The required TT to
evaporate 300 mg of barium from these samples is reported in Table 1.
As it is appreciated from the results in the Table, with the devices according
to the invention it is possible to obtain a yield of 300 mg of barium with TT of 35 s,
while obtaining the same yield with samples of the prior art takes times 5 or 10 s
longer.