GB2067007A - A device for generating electron beams - Google Patents

Abstract

A device for hardening a surface or surface layer, especially a viscous, preferably printed surface layer, such as a coat of paint, using electron beams, includes a thin-film field emission cathode (10) having a plurality of electron-emitting cones (16) for generating the beams. The cathode (10) comprises several oblong, flat lamellae units arranged in a line end to end on a ceramic backing with the apexes (20) of the cones (16) in one plane (17), and is housed within an evacuated tubular casing (22) opposite an electron escape window (23) in the casing (22). Two tape-shaped control electrodes (24 and 24') are housed in the interior (21) of the casing (22) as well. The cathode has an extended area and the distribution of electron-emitting cones may vary over different regions thereof (Fig. 4, not shown). As shown in Fig. 3, each Mo cone 16 is provided on Si layer 13 in a cavity 18 in SiO2 layer 14 which is covered by apertured Mo layer 15. <IMAGE>

Description

SPECIFICATION A device for generating electron beams This invention relates to a device for hardening a surface or surface layer, especially a viscous, preferably printed surface layer, such as a coat of paint, said device including an electron-emitting cathode.

The electron or electron beam hardening effect referred to above usually represents polymerisation of a liqud induced by radiation via free radial chain reactions. A liquid to be hardened in this manner nearly always is a synthetic substance,such as an acrylic compound. A layer to be hardened may also be, e.g. a varnish or insulating layer. A layer to be hardened, which would generally be a surface layer but which may also be an intermediate layer, is characterised by its relative thinness.

The work or material to be hardened is normally moved, during radiaton, in a direction transverse to the electron beams.

A device of the kind to which this invention relates has been disclosed by Technical Paper FC75-311 of SME Society of Manufacturing Engineers" 20501 Ford Road, Dearborn, Michigan 48128, U.S.A. This involves the hardening of a surface layer consisting of a viscous liquid, more specifically of a monomer-polymer system, such as urethane. The surface layer to be hardened is produced more especially by printing on a paper web. It is hardened by causing, by electron energy, free radicals in it which trigger or induce the polymerization reaction of the system and propogate polymerisation.

This previously disclosed device is a cylindrical, evacuated high-tension electron tube, and the cathode is a straight metal wire extending along the longitudinal axis of that tube. The electron stream takes the shape of a curtain and is accelerated radially through the vacuum chamber of the tube to a metal aperture of the tube which allows passage of the electrons and through which the stream of electrons enters the ambient air and penetrated through it and into the surface layer to be hardened.

This previously disclosed device has a number of disadvantages. The electrons are released by heating the cathode. The accompanying very high temperatures (up to 2500"K) cause thermal radiation and conduction losses impairing the efficiency with which electrons are generated. The high temperature and heating effect are needed because of the high electron affinity of the wire metal. Also, inhomogeneities of the wire metal may cause problems in the uniformity of electron radiaton (or emission). On the other hand the rather compact design, the modest effort required for shielding, and the fact that the overall height is independent of the width of the work to be hardened are advantageous.

The Technical Paper FC75-311 referred to above also discloses for the same purpose an evacuated accelerating tube including a spotshaped cathode. The electron beams emitted by that cathode are deflected over the width of the surface layer. The advantages of this device are, among others, the relatively small amount of heating effort required and the relatively low temperature of the cathode.

Disadvantages of this device include the fact that it has a large volume to be screened and it is many times higher than the previously described device comprising a metal wire cathode in an electron tube. Also, electronic or electronoptical provisions must be made to keep the electron beams continuously focused on the work or material to be hardened. The great overall height, as well as additional control measures, make the device expensive, bulky and unwieldy. The accelerating voltage is higher than that required for operating the device that includes a metal wire cathode, amounting as it does to several hundred kilovolts.

There is still another practicable method of releasing electrons from metal. This involves the generation of electrons by field emission.

The phenomenon of field emission is tied to high field intensities u/r (u = voltage, r = radius of curvature). High voltages are uneconomical for several technical reasons. As a result, use is made of small radii to achieve high field intensities.

This is the case also with the thin-film field emission cathodes promulgated by the Journal of Applied Physics, Vol. 47, No. 12, Dec 1976, pp. 5248 to 5263, copyright 1976, American Institute of Physics, USA. The entire cathode surface covers, e.g. a square 0.25 mm by 0.25 mm. The thin film field emission cathode has three layers. The bottom most layer (substratum, backing) consists of electronically highly conductive silicon, is several tenths of a millimeter thick, and is oxidized on its upper surface, which produces the intermediate layer consisting of silicon dioxide. This is an insulating, or dielectric layer. It is about 1.5 um (1 um = 0.001 mm) thick. The top layer is about 0.4 um thick and consists of a suitable metal, more especially of molybdenum, which makes it electrically conductive.It incorporates 100 equally spaced holes of approximately equal diameter that were produced by etching. The diameter of each such hole is in the range 1 um to 3 um, depending on the particular version. The intermediate layer is etched, using a different medium, through the holes in the upper layer, to form flaring cavities that reach as far as the upper surface of the bottom layer. By means of electron beam evaporation a cone of a suitable metal, more particularly molybdenum, is produced in each of these cavities and on the upper surface of the bottom layer, the apex of that cone lying in the hole area. The thin film field emission cathode has a great density of such cones which are individual cathodes.Three such thin film field emission cathodes are provided in a vacuum tube, which is a life testing tube, on a small head opposite the face of a rod-shaped copper-type collector electrode.

At its apex, each cone has a very small radius r of curvature of about 500 . Because of this, and because of an accelerating electrode near that apex, the cathode can be operated at low driving potential. For molybdenum, it runs at only about 1 0O volts to 200 volts. The continuous currents of the various cones are high, running between 50 A and 1 50 ym, or to about 1 2 A/cm2 when referred to current densities.

All that has been written above on electron generation by field emission and on thinfilm field emission cathodes has not been known in connection with electron hardening.

An object of this invention is to provide a design of a device for hardening a surface or surface layer, especially a viscous, preferably printed surface layer such as a coat of paint, the device being arranged such that both the heating effort and the cathode temperature, as well as the overall height, are minimized, that other cited disadvantages are eliminated, and that the hardening effect is augmented.

According to the present invention there is provided a device for generating electron beams for hardening a surface or surface layer, having an electron-emitting cathode, wherein the cathode is formed by a thin-film field emission cathode having a plurality of electron-emitting cones.

This, accordingly, involves the use of thinfilm emission cathodes for electron beam hardening, where the cones, or individual cathodes, are arranged over a surface area, or width and length, to suit the hardening effect intended. The present invention provides an excellent means for improving the design, the emission and the hardening effect. The low driving potential at the accelerator electrode and the great density of individual cathodes make for very stable emission. The low driving potential also give a very long life to the thin film emission cathode and low sensitivity to ionization effects from surrounding residual gas. There are no thermal radiation and conduction losses. There is no heating and, therefor, no losses caused by the development of heat, which improves the efficiency of electron generation. The thin film emission cathode is a cold cathode.The device can be designed for little overall height, regardless of the width of the work or material to be hardened, for compactness, and for moderate shielding and control effort. Inhomogeneities of wire metal are here nonexistent. Electron generation is given great uniformity, because all cones or individual electrodes can be made practically the same. The large number of individual cathodes provides sufficient electron yield for hardening. The cathode density will not necessarily have to be as great as in the previously proposed thin film field emission cathode discussed above. For instance, there will be 700 to 2000 cones arranged in a 100 mm2 area.Another consideration is that in the event of failure of individual cones, the great number of individual cones will prevent failure of the whole electron generation effort and idling of the hardening system, as perhaps during the printing process. The desirable improvement to the hardening effect is equally made possible by the large number of individual cathodes, for the distribution of electrons on the work to be hardened can very well be adapted, by arranging the cathodes accordingly, to the reaction kinetics of the material. The incident radiation, e.g. can optionally be made uniform, nonuniform, symmetrical or asymmetrical.

Use of an oblong, thin-film field emission cathode, which is normally used in horizontal arrangement, is especially recommended. It serves to produce an electron beam curtain, especially for hardening work or material of a width approximately equal to that of the curtain. The control effort can be minimized if the apices of the cones of such an oblong thinfilm emission cathode are arranged in one plane. Such an arrangement also makes production of the oblong cathode relatively simple, especially if the oblong cathode is composed of longitudinally successive flat oblong units.

The oblong flat units, which may be arranged as lamella, tapes or the like, are usually strung in a row without a seam or in close proximity. Such a thin-film field emission arrangement which is arranged such that at least a majority of the units have the same length which is in the range 60 mm to 90 mm and that they all have the same width which is in the range 5 mm to 20 mm, is suitable for practical applicaton especially for hardening a layer on a paper web or the like.

Provision of a ceramic backing makes the cathode sturdy and tolerant of abuse. The backing, when viewed in cross section, can be recessed, and the units then inserted into the resultant rail guide. This may also be a backing or a bar of a ceramic material having adhering thereto or thereon, a longitudinally continuous silicon layer on or on top of which is provided two other layers respectively cotn- prising silicon dioxide and molybdenum film and the cones. Variation of the densities of cones or individual electrodes in direction 6f work movement gives, for the beam of rays or the beam curtain, an accordingly different distribution of the electrons in the direction of movement of the work to be hardened. This specifically caters for the reaction kinetics of this work or material. The distribution of electrons can be, e.g. a symmetrical or asymmetrical Gaussian distribution, a rectangular distri bution or a triangular distribution. The difference in cone or individual cathode density can be very pronounced. In the case of the longitudinal cathode, the direction of movement runs transversely to the longitudinal direction of the cathode.Where lateral, tape-like, parallelly-opposed control electrodes are provided in parallel with the oblong thin-film field emission cathode at points between the cathode and an electron escape aperture to prevent or reduce divergence of the electron beams in a direction transverse to the direction of movement of the work to be hardened, the electron beam is given the proper width, or divergence of the electron beam is reduced or prevented to suit the qidth of the electron escape aperture measured in said motional or transverse direction.

A simplified and schematic form of a device in which this invention is embodied is illustrated in the accompanying drawings, in which: Figure 1 is a sectional view of a central portion of the device, the section being taken in a longitudinal plane; Figure 2 is a section through the device taken in the plane ll-ll shown in Fig. 1; Figure 3 is an inverted fragment in section of part of the device as seen in Fig. 2, drawn to a larger scale than is Fig. 2 and shown in more detail than in Fig. 2; and Figure 4 is a view on arrow A in Fig. 2 of a detail of the device shown in Figs. 1, 2 and 3, drawn to a larger scale than is Fig. 2.

Figs. 1 and 2 show a device which comprises a cylindrical, horizontally-arranged electron beam tube and which is adapted to generate electron beams for electron hardening. The interior 21 of the casing 22 of the tube is evacuated.

The cathode is a straight thin-film field emission cathode 10 extending in parallel with and a little above the longitudinal centerline 25 of the tube and having a rectangular shape in cross section as shown in Fig. 2. Fig.

1 shows the cathode 10 has a ceramic substratum, i.e. a straight, long, ceramic backing 11 about 2.5 mm thick. Fixedly attached to the bottom of the backing 11 are several oblong, flat, equally wide lamellae 1 2 in longitudinal succession, which are closely arranged together at the front.

Fig. 3 shows that the lamellae 1 2 comprise a silicon layer 1 3 about 0.7 mm thick, a silicon dioxide layer 14 about 1.5 ym thick, a molybdenum film 1 5 0.4 ,um thick, and about 900 identical individual cathodes each in the form of a molybdenum cone 1 6. Each lamella 1 2 is 1 5 mm wide, as is the backing 11, and 70 mm long.

The cathode 10 is flat below, i.e. the molybdenum cones 1 6 are arranged in a horizontal plane 1 7 with their tips pointing down.

Fig. 3 shows that each molybdenum cone 1 6 is arranged in a circular chamber formed by a cavity 1 8 in the silicon dioxide layer 14, with its bottom resting on the silicon layer 1 3 and with its apex in a circular opening 1 9 in the molybdenum film 15, where the apex 20 lies more or less in the plane 1 7.

The length of the cathode 10 is about equal to or somewhat smaller than the width of the work to be hardened, which may perhaps be 1 meter.

The casing 22 forms the anode of the tube.

The tube has a metal electron escape aperture (Lenard window) opposite the bottom of the cathode 10, which allows the passage of electrons. Arranged n parallel with the cathode 10 between the cathode and the aperture 23 or the anode are lateral, tape-shaped control electrodes 24 and 24'. They extend in parallel with the longitudinal centerline 25, are arranged in parallel with one another, and are shown in Fig. 2 as extending approximately normal to the plane 1 7. They are arranged symmetrically to the longitudinal centerline 25, their distance one from the other being somewhat greater than the width of the cathode 10. The aperture 23 is a little wider still.

A relatively large negative voltage (- u) can be applied to the control electrodes 24 and 24'. The arrows 26 indicate the electron beams, and the arrow 27 indicates the direction of movement of the work to be hardened.

Fig. 4 illustrates the growing density of cones of individual cathodes in this direction of movement, which makes for a triangular distribution of the electrons.

Claims (9)

1. A device for generatng electron beams for hardening a surface or surface layer, having an electron-emitting cathode, wherein the cathode is formed by a thin-film field emission cathode having a plurality of electron-emitting cones.

2. A device according to Claim 1, wherein the thin-film field emission cathode is oblong in shape.

3. A device according to Claim 2, wherein the apices of the cones of the oblong thin-film emission cathode are arranged in one plane.

4. A device according to Claim 2 or Claim 3, wherein the oblong thin-film field emission cathode is composed of longitudinally successive, oblong, flat units.

5. A device according to Claim 4, wherein at least the majority of said units have the same length which is in the range 60 mm to 90 mm, and all said units have the same width which is in the range 5 mm to 20 mm.

6. A device according to any one of Claims 2 to 5, wherein the oblong thin-film field emission cathode is arranged on or on top of a ceramic backing extending in the longitudinal direction of the cathode.

7. A device according to any one of Claims 1 to 6, wherein the density of the cones varies, in the direction of movement of the work to be hardened relative thereto, with the reaction kinetics of the work.

8. A device according to any one of Claims 2 to 7, wherein lateral, tape-like, paral lelly-opposed control electrodes are provided in parallel with the oblong thin-film field emission cathode at points between the cathode and an electron escape aperture to prevent or reduce divergence of the electron beams in a direction transverse to the direction of movement of the work to be hardened.

9. A device for generating electron beams for hardening a surface or surface layer substantially as described hereinbefore and as illustrated in the accompanying drawings.

1 0. Any other novel feature or combination disclosed hereinbefore or shown in the accompanying drawings.