CN1187684A - Evaporable getter device with reduced time of activation - Google Patents

Abstract

Described is an evaporable getter device with a reduced barium evaporation time, applied to traditional TV picture tubes or flat panel displays. The device also includes a third component powder selected from iron, aluminum, titanium or their alloys, beyonds BaAl4 alloy and nickel powder of sample in the known getter device.

Description

Evaporable getter device with short excitation time

The invention relates to an evaporable getter device having a short activation time.

Evaporable getter materials are known mainly for maintaining a vacuum in the picture tubes of television and computer monitors. The use of volatile getter materials in flat panel displays is under development and is also under investigation.

The getter material commonly used in picture tubes is metallic barium deposited as a thin film on the inner wall of the tube. In order to obtain such a film, devices known in the art as evaporable getters are used, which are introduced into the picture tube during the manufacturingprocess. These devices comprise an open-ended metal container containing a barium and aluminium compound BaAl₄A powder having a particle size generally less than about 250 μm, and nickel powder Ni, the particle size of which isInches are typically less than about 60 μm, and the weight ratio is about 1: 1. These devices are well known in the art and for this purpose reference may be made to US5118988 (the same applicant as the present application). In the process of excitation, also known as "flash evaporation", barium is evaporated by induction heating of said device by means of a coil external to the tube itself; when the temperature of the powder reaches a temperature range between about 800 ℃ and 850 ℃, the following reaction takes place:

(1)

the reaction is strongly exothermic, raising the temperature of the powder to about 1200 ℃, at which point evaporation of barium occurs and deposits onto the walls of the picture tube to form a metal film.

The time required for the evaporation of all the barium contained in the device, timed from the moment when the device starts to be powered by means of the coil, is defined in the field by the term "total time", which is used in the description and claims of the following sections, also in its abbreviated form TT. For example, TT of 40-45 seconds is required for the current getter device to obtain about 300mg of barium film required for a large-sized color picture tube. But this time is equivalent to the slow step in the current production line for the manufacture of tubes and it is therefore a requirement of the manufacturer that the device capable of releasing barium has a low TT value.

To achieve such a result, it is theoretically possible to increase the power supplied by the coil or to increase the reactivity of the powder by reducing the particle size of the powder.

However, with currently available getters, increasing the coil power is not possible. In practice, in doing so, the temperature of the powder container rises too quickly and there is insufficient time for the heat in the powder compact to spread evenly, resulting in melting of the container.

A reduction in the particle size of the powder is also not possible, since this leads to BaAl being present₄And Ni excessively increase locally, with the result that the powder compact expands and fragments may be ejected from the powder compact.

It is an object of the present invention to provide an evaporable getter device having a short activation time, which overcomes the drawbacks of the prior art.

This object is achieved according to the invention by an evaporable getter device having a short excitation time, comprising a metal container in which a mixture is contained, wherein the mixture present comprises:

-BaAl₄a compound powder;

-nickel powder;

-a third component powder selected from the group consisting of aluminum, iron, titanium and alloys thereof in an amount ranging from about 0.3% to about 5% by weight of the total weight of said mixture.

The amount of third component powder in the powder mixture depends on the actual components used and is generally in the range of between about 0.3% and 5%. In particular, the percentage of the third component is preferably in the range between about 0.8% and 2% in the case of aluminium, between about 0.3% and 1.2% in the case of iron and between about 0.5% and 5% in the case of titanium. If the amount of the third component is less than the value, the desired effect of reducing the evaporation time of barium cannot be obtained. Conversely, if the third component is used in an amount greater than the stated value, the flash evaporation of barium is very vigorous and can be hardly controlled. Nickel and BaAl₄The weight ratio therebetween is the same as in the prior art devices, typically about 1: 1; in particular, in this field, nickel and BaAl in getter devices are widely used₄The weight ratio of the components is 5.3: 4.7.

For the purposes of the present invention, it is not necessary for the third component to have a particularly high purity, and it is possible to use powders of metals or alloys in the industry, the purity generally being 98-99%. The particle size of the powdered third component for the purposes of the present invention is less than about 80 μm, preferably less than about 55 μm.

Nickel and the compound BaAl used in the getter device of the invention₄The powder of (a) is the same as that used in the prior art; generally, the nickel powders used have a particle size of less than about 60 μm, BaAl₄Particles of powderThe particle size is generally less than about 250 μm.

The metal container may be made of various materials such as nickel-plated iron or constantan; it is preferred to use AISI 304 or AISI 305 steels because they exhibit good oxidation and heat treatment properties as well as good cold workability. The metal container may have various shapes, in particular any shape known and used in the art, such as those in the devices according to U.S. Pat. Nos. 4,127,361-4,323,818-4,486,686-4,504,765-4,642,516-4,961,040 and 5,118,988.

Of particular interest is the possibility of obtaining evaporable getter devices with a short excitation time, while also being melt-sealable; by the term fusion-sealable it is meant that the getter device can withstand an oxidizing atmosphere ata temperature of about 450 ℃ for a maximum of 2 hours; these are the conditions such devices must withstand during certain processes of manufacturing the tube. During evaporation of barium from the hermetically sealable getter device, a greater amount of heat is generated than in conventional getter devices, and it is therefore more difficult to keep the powder compact inside the container. Several years ago, the applicant of the present invention has manufactured and sold a fusible getter device having an amount of barium vaporizable up to about 200 mg. On the contrary, a fusible getter device that can evaporate a greater quantity of barium, in particular about 300mg of barium, requires particular solutions to be adopted, considering its greater reactivity; the applicant's patent application entitled "fusion sealable evaporable getter device with high barium yield", having the same filing date as the present application, proposes a method for manufacturing a fusion sealable getter device by adding a barrier to the thermal diffusion in the circumferential direction in the powder compact and adding a substantially planar, discontinuous metal part to the same powder compact. By adding a third component to a conventional or high-throughput version of the flushable getter device, it is possible to obtain a flushable getter device that maintains comparable barium excitation characteristics while reducing evaporation time.

The invention will be further illustrated by the following examples. These non-limiting examples represent embodiments designed to teach those skilled in the art how to practice the invention and represent the best mode of putting the invention into practice.

Example 1

Several mutually identical suction units were prepared from AISI 304 steel containers of diameter 20mm, height 4mm, supported at the bottom by 1mm height as described in the U.S. Pat. No. 5,118,988 cited herein. For each sample, a homogeneous mixture comprising 767mg of BaAl having a particle size of less than 250 μm was poured into the container₄Powder, and 866mg nickel powder having a particle size of less than 60 μm, and 18mg iron powder having a purity of 99% having a particle size of less than 80 μm. The powder mixture is then compacted in the container using a suitable punch. By placing them one at a time in a measuring chamber made of glass connected to a pumping system, evacuating said measuring chamber, by following ASTM F111-72 standard, the barium evaporation test was carried out by radio frequency heating each unit with such power that evaporation started 12 seconds after heating started; as for the total time of heating, these tests were different from each other, and the total time of heating was varied within a range of 35 to 45 seconds in each test. At the end of each test, the amount of barium evaporated was measured. Table 1 reports the TT required for evaporation of 300mg of barium from the device.

Example 2

Several mutually identical suction devices were prepared from the steel containers described in example 1. Into this vessel, a mesh of AISI 304 steel of 1.5mm mesh width was placed, placed on a support at the bottom, and for each sample, a homogeneous mixture comprising 767mg of BaAl with a grain size of less than 250 μm was poured into the vessel₄Powders, and 866mg of nickel powder having a particle size of less than 60 μm, and 18mg of 99% pure aluminum powder having a particle size of less than 50 μm. The powder mixture was then compacted in the container using a suitable punch to form 4 radial grooves in the powder compact surface. The thus obtained sample was treated at 450 ℃ for 1 hour in air to simulate the sealing conditions. Each sample was then run according to example 1And (5) carrying out a barium evaporation test. Also in this case, by heating each device by radio frequency, the power used should be such that evaporation starts 12 seconds after the start of heating; while heating was maintained during TT, the TT value was different for each sample and varied between 35 and 45 seconds, and the TT value required for evaporation of 300mg of barium from the device was measured.

Table 1 reports the test results.

(comparative) example 3

The test of example 1 was repeated with a series of samples identical to example 1, but without the iron powder, each device being heated by radio frequency with such a power that evaporation starts 12 seconds after the start of heating; TT values varied between 35 and 45 seconds using different TT values, and Table 1 reports the TT values required to evaporate 300mg of barium from these samples.

(comparative) example 4

The series of tests of example 2 was repeated with the same getter device as in example 1, but without aluminium powder. Table 1 reports the TT values required to evaporate 300mg of barium from these samples.

TABLE 1

Examples	Percentage of third component	Total time (seconds)
			1	1.09(Fe)	35
2	1.09(Al)	35
			3	0	45
4	0	40

As can be seen from the results in the table, it is possible to obtain a yield of 300mg barium with a TT of 35 seconds with the device according to the invention, while it takes 5-10 seconds to obtain the same yield with the samples of the previous technique.

Claims (11)

1. Evaporable getter devices with short excitation times comprising a metal container with a mixture contained therein, said mixture comprising:

-BaAl₄a powder of the compound;

-nickel powder;

-a third component selected from the group consisting of aluminum, iron, titanium and alloys thereof, in an amount between about 0.3% and 5% by weight of the total weight of said mixture.

2. An evaporable getter device according to claim 1, wherein when said third component is aluminum, its percentage by weight in said mixture is comprised between about 0.8% and 2%.

3. An evaporable getter device according to claim 1, wherein when said third component is iron, its percentage by weight in said mixture is comprised between about 0.3% and 1.2%.

4. An evaporable getter device according to claim 1, wherein when said third component is titanium, its percentage by weight in said mixture is comprised between about 0.5% and 5%.

5. An apparatus according to claim 1, wherein the nickel and BaAl are₄In a weight ratio of about 1: 1.

6. An apparatus according to claim 1, wherein the nickel and BaAl are₄The weight ratio of the components is 5.3: 4.7.

7. An apparatus according to claim 1, wherein said third component powder has a particle size of less than about 80 μm.

8. An apparatus according to claim 7, wherein said third component powder has a particle size of less than about 55 μm.

9. A device according to claim 1, wherein the nickel powder particles are less than about 60 μm in size.

10. An apparatus according to claim 1, wherein BaAl₄Has aparticle size of less than 250 μm.

11. An evaporable and sealable getter device having a reduced activation time, comprising:

-an open-topped metal container;

-a powder mixture in the form of a compact in said container with radial grooves formed on its upper surface, wherein said mixture comprises BaAl₄Nickel and a third component, saidBetween about 0.3% and about 5% by weight of the total weight of the mixture.

-a discrete metal part, substantially planar and substantially parallel to the bottom of said container, embedded in said powder compact, at a distance from the bottom of said container such that it is not exposed on the free surface of said compact.