A number of industrial applications require a suitable vacuum level to be kept in a sealed space for a period of some years. For example, this is the case of cathode ray tubes, also known in the field as CRTs, used as television or computer screens. Vacuum is requested in the CRTs in order to prevent the trajectory of the electrons emitted by the cathode from being deflected due to collision with gas particles. In order to prevent this, CRTs are evacuated in the manufacturing stage by means of mechanical pumps and then hermetically closed.

However, it is known that vacuum in the tube tends to decrease with time, above all due to degassing of the internal components of the tube itself. It is therefore necessary to use a getter material inside the tube, capable of binding the gas molecules thus preserving the vacuum degree necessary for the functioning of the cathode ray tube. For this purpose, barium is commonly used. Recently, the applicant has also proposed the use of calcium, which compared to barium has the double advantage of being less toxic (thereby causing less problems in the manufacturing and disposing steps of cathode ray tubes) and of generating a reduced quantity of X-rays, injurious to health, when hit from the electron beam.

Due to the high reactivity of these metals, which would cause all the manufacturing steps to be problematic, some air stable compounds thereof are used, which are introduced into the cathode ray tube before its evacuation. In the case of the barium, the stable compound is BaAl4 ; in the case of calcium, it is possible to use CaAl2 or a ternary alloy Ca-Ba-Al containing 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. These compounds are generally used in mixture with nickel and, in the case of calcium compounds, optionally also with titanium.

In order to introduce these mixtures into cathode ray tubes, use is normally

made of devices, known as evaporable getters, formed of an upperly open metal container and containing powders of the desired mixture. Evaporable getter devices containing barium are for example described in patents US 4,323, 818, 4, 553, 065,4, 642,516, 4,961, 040 and 5,118, 988. Examples of evaporable getter devices containing a calcium compound which can be cited are those described in international patent application WO01/01436 and in Italian application number MI2001A002273 in the name of the same applicant.

Once the evaporable getter device has been introduced into the cathode ray tube, the latter is connected to a vacuum pump and brought to the desired final internal pressure, generally lower than 10-5 hectoPascal (hPa). Finally, the evacuated cathode ray tube is sealed and heated from the outside by radio- frequencies in order to cause metal evaporation from the barium or calcium compound; then, the evaporated metal condenses onto the internal walls of the evacuated tube, thus forming the film active in gas sorption.

However, it is known that metal deposition onto specific areas of the cathode ray tube internal surface can be noxious for the good working of the tube itself or even totally compromise it. In particular, the formation of metal deposits on the screen and on the phosphors has to be reduced as much as possible.

Another area that in any case must remain free from metal deposits is the one between the electron gun (at cathode potential) and the so-called"anode button" ; as a matter of fact, as it is known the presence of ionizable particles between two points at different electric charge would cause a short circuit of the system.

In order to prevent such drawbacks, it is possible to use particular measures such as evaporable getter devices provided with very high lateral walls, suitably formed so as to convey the evaporated metal jet onto some areas of the internal surfaces of the cathode ray tube; a getter device of this kind is described in patent US 4,323, 818. However, this method is not completely satisfactory, since the effect of directing the metal vapors is limited.

Alternatively, it is possible to use getter devices comprising deflectors positioned above the powder mixture of the barium or calcium precursor compound. Getter devices of this kind are described for example in patent US

3, 719, 433. This solution, however, implies an increase of the time and consequently of the costs necessary for manufacturing said devices.

Therefore, object of the present invention is providing a process for evaporating calcium inside systems which operate under vacuum, which is free from said drawbacks. Said object is achieved by a process whose main features are specified in the first claim and other features are specified in the following claims.

An advantage of the process according to the present invention is that it allows to obtain a calcium deposit selectively in some areas of the internal surface of the cathode ray tube, without the need to adopt the above mentioned measures in order to convey the evaporated metal.

Further advantages and features of the process according to the present invention will appear to those skilled in the art from the following detailed description of one embodiment thereof with reference to the accompanying drawings wherein: - Figure 1 shows in a graphical form the variation of the internal pressure of the cathode ray tube as a function of time, during some steps of the process according to the present invention; - Figure 2, similar to figure 1, shows the variation in time of the pressure in the preferred embodiment of the invention.

The process according to the present invention can be applied in order to accomplish calcium evaporation inside any system operating under vacuum, in particular a cathode ray tube. In the known processes wherein barium based evaporable getter devices are used, evaporation is the last step and is carried out after sealing the system. On the contrary, the process of the invention is characterized in that calcium evaporation is carried out during the evacuation or between two different evacuation steps, before sealing the system.

The present invention comprises a first known step wherein at least one evaporable getter device comprising an air stable calcium compound is introduced inside the system. Any known device which uses calcium as getter element can be used in the process according to the present invention. For example, evaporable

getter devices described in the above cited international patent application WO01/01436 or Italian patent application MI2001A00273 can be used. As it will appear more clearly from the following, the evaporable getter device must be positioned at about the center of the area wherein the calcium deposit has to be obtained. In the case of a cathode ray tube, the evaporable getter device can be advantageously positioned in the area of the antenna or of the anode button.

As shown in figure 1, the process implies then the evacuation of the system 'with a pump or, more commonly, a pumping group (a system of more pumps of different type). As soon as the pressure indicated in the figure with Pi is reached, higher than the final pressure which has to be reached by evacuation, the heating operation of the getter device (indicated with R in the figure) is carried out in order to cause calcium evaporation; this operation is generally carried out by induction by means of a coil arranged outside the system in a position corresponding to that of the device itself. As well known to those skilled in the art, this step is continued for a predetermined time period, generally between about 30 and 45 seconds. During this step, the gases trapped in the device are released, thus causing the slight pressure increase shown in the figure.

Surprisingly, although none of the known measures for conveying the evaporated metal has been adopted, a diffusion of calcium atoms in all of the internal space of the system does not take place during said evaporation. As a matter of fact, evaporated calcium atoms begin their diffusion inside the system, but they are "reflected"back thanks to the collision with the molecules of the atmospheric gases or those released by the getter device itself during the evaporation. In this way, the presence of gases inside the system has the effect of preventing the deposit of the calcium atoms in the undesired areas, such as the screen area or that between the electrodes in the case of a cathode ray tube. On the contrary, in these conditions calcium atoms are deposited almost exclusively in the area adjacent to that where the evaporable getter device was first arranged, for example, in the case of a cathode ray tube, near the antenna or the anode button.

Pressure P1 must have a higher value than that of the internal pressure P2 at which the system works, but lower than the air pressure which would be sufficient

for causing inactivation of the calcium which will be evaporated in the course of the subsequent heating step. As a matter of fact, it is to be avoided that the particles of atmospheric gases remained in the system may saturate completely the just formed getter deposit, thus making it unavailable for gas sorption in the course of the normal functioning of the system. It has been experimentally verified that pressure Pi is advantageously comprised between about 10-4 and 10-5 hPa.

Evacuation is then continued until the pressure value of P2 is reached, generally comprised between 10-5 and 10-6 hPa, at which the system is sealed (step indicated with S in the figure).

In the preferred embodiment of the process according to the invention, during step R the evacuation is interrupted by isolating the system from the pumping group with suitable valves. With reference to figure 2, the process in this case comprises (besides the introduction of the getter device in the system and the final sealing thereof) three main steps, that is: a first evacuation step El, wherein the pressure is brought to the value Pl ; the heating step R of the getter device for causing calcium evaporation, during which the system is isolated from the pumping group by means of suitable valves; and a second evacuation step, E2, carried out by opening said valves again, and in which the pressure in the system is reduced to the value P2 at which the sealing S is carried out; in this last step, a major part of the gases emitted by degassing during step R is eliminated. This embodiment is preferred because, by interrupting the pumping during step R, there is a pressure increase due. to the degassing of the internal components of the tube, which contributes to the"back scattering"effect of the evaporated calcium atoms. The pressure values Pi and P2 in this embodiment are the same previously indicated.

The residual pressure reduction, to a final pressure value of about 10-7 hPa, necessary for the correct operation of systems such as a cathode ray tube, is to be carried out by the obtained calcium film.

The process of the invention is not applicable in the case of the barium getter devices, because this element has a much larger mass than that of calcium

(more than three times) and barium"back scattering"by the gas molecules would only be possible at much higher pressure values, higher than about 10-2 hPa; in these conditions, the just formed barium film would be soon spent by the sorption of the great gas quantity, thus being ineffective for maintaining the vacuum during the life of the cathode ray tube.

Possible variations and/or additions can be made by those skilled in the art to the described and illustrated embodiment, by remaining within the scope of the invention itself. For example, the evaporable getter material can be introduced in the system by means of any open container that can be arranged in a defined position inside the system itself.