GB2043701A - Granulatising liquid metals - Google Patents

Abstract

A liquid metal is ionised and dispersed in a vacuum by supporting it on a moving support in the form of a rotating disc 12, 13 and applying an electrical field to the liquid metal for example by an electrode assembly 14, 36, 38 so as to cause the metal to leave the support as ionised particles. The assembly 14, 36, 38 may be segmented in order to direct the particles in selected directions relative to the disc 12, 13. The dispersed particles may be used to coat articles or to form metal powders for example. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the ionised dispersion of liquid metals This invention relates to the ionised dispersion of liquid metals.

Fluids are transformed into particle dispersions (e.g., sprays, mists, fogs, and aerosols) in practical devices by a variety of forces, e.g., mechanical, hydraulic, centrifugal, aerodynamic, electrostatic or ultrasonic. Such devices find application in a wide variety of areas, e.g., medicine, agriculture, pest control and metal powders.

When the particle dispersions are charged electrically, they can be manipulated by electric or magnetic fields, as in ink jet printers and paint and crop sprayers. Charging can be achieved conveniently either by attaching the fluid spray-head to an electrical power supply, or by passing the particle dispersion through a charging ring electrode or corona discharge.

When electrical forces are used to disrupt fluids into particle dispersions, the size and charge (if any) of the resultant particles are considerably influenced by the surface tension and electrical conductivity of the fluid. Liquid metals with a high surface tension require high electric fields, and the dispersion must take place in a vacuum so as to avoid electrical breakdown. As is described in the Journal of Applied Physics, Vol.40, No. 13 (December 1969) page ski 01, in an article entitled Electrohydrodynamic lon Source by Mahoney and others, a fluid spray head of nozzle configuration is disclosed to disperse a liquid metal in a vacuum in the presence of a high electrical field at the tip of the nozzle.When a greaterthrough- put of particles is required, an array of nozzles must be used but these are subject to the disadvantage that it is difficult to get a uniform spray from such an array as the electrical fields tend to differ at each nozzle. This is unacceptable when the spraying device is used for coating components where a uniform coating is required.

According to the invention there is provided a method of forming an ionised dispersion of liquid metals in a vacuum comprising supporting the liquid metal on a moving support and applying an electric field to the metal on the support of such a magnitude as to cause the metal to leave the support as ionised particles.

The movement of the support achieves greater uniformity of particles in the dispersion, since the effect of non-uniform wetting of the support for example is spread over the dispersion by the movement of the support. When the movement of the support is not at a uniform velocity, the metal will tend to be thrown off the support and this tendency can be used to act in concert with the electric field to increase the dispersion of the liquid metal. A centrifugal dispersing device is a good example of a support moving at a non-uniform velocity.

A centrifugal dispersing device used in the absence of a high electrical field will produce a dispersion of particles, but the size of the particles will be much larger, e.g., greater than 10 microns in diameter, and they will not be substantially ionised.

The ionising field draws the liquid metal off the dispersing device in very much finer streams which are broken up into fine particles. The particles so formed range in size down to single atoms.

Centrifugal atomisers have been used for liquid metals, as is disclosed in British Patent Specification 1503635. The apparatus of the specification is used for the formation of metal powders, and any electrical field which is applied to the liquid metal is not of sufficient strength to cause ionisation. The beam of an electron gun is directed at the liquid metal in the atomiser, but this gun is used simply to heat the metal to above its melting temperature.

Centrifugal atomisers have been used in conjunction with the application of an electrical field to produce charged particles, but only in connection with fluids of high electrical resistivity, e.g., oils and paints, where the chance of electrical breakdown is lower and hence there is no need to operate in a vacuum. An example of such apparatus is given in U.S.2893894.

An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic vertical section through an ioniser for liquid metals, Figure 2 is a schematic plan view of the disc and electrode arrangement of the apparatus of Figure 1, Figure 3 is a diametral section through an ioniser, showing components in greater detail, and Figure 4 is a schematic view of an ioniser in a vacuum chamber.

In the apparatus of Figures 1 and 2, liquid metal is applied from a supply conduit 11 to the upper surface of a disc 12 which in use is rotated at high speed about a supporting shaft 13. Around the periphery of the disc 12 is mounted an electrode assembly 14 which is formed with a gap level with the liquid metal on the surface of the disc to allow liquid droplets to be flung off the surface of the disc through the electrode assembly to a target or collector beyond the electrode assembly. The ionising field is created by applying a high positive potential to the shaft of the atomiser, the electrode assembly being either earthed or connected to a negative potential.The whole apparatus is housed in a vacuum chamber, to prevent collisions of the ions with molecules of ambient gases, electrical breakdown between the electrode assembly and the disc and also any oxidation of liquid metal which might otherwise occur.

The field can of course be created by applying a high potential of the correct potency to the electrode assembly and earthing the shaft, or by applying different potentials to the shaft and electrode assembly.

As the disc is rotated, liquid metal on its upper surface is caused to flow to the periphery. As the liquid flows out to the periphery of the disc, it will tend to form in waves around the rim, and as the rotational speed of the disc increases the liquid will break up into ligaments and form spray extending tangentially from the periphery of the disc. The creation of the high electrical field between the periphery of the disc and the electrode assembly will increase the wave-making effect of the disc rotation, so that the liquid metal will be drawn off the disc in a finer and faster-moving spray and the spray in addition will be ionised.

The electrode assembly can extend uniformly around the disc in which case a uniform distribution of particles is emitted from the disc. However, the electrode may be segmented as shown in Figure 2, so that the field is concentrated in certain directions to provide sector-shaped beams of particles which will be provided through each of the electrodes of the electrode assembly. Thus, when the droplets are required to coat a number of articles arranged around a rotating atomiser, the droplets can be divided into a numberofsector-shaped beams, one for each object, by suitable arrangement of the electrodes.

The high velocity of the charged particles increases the quality of the coating since the smallest particles (e.g., atomic ions) tend to spritter clean the surface to provide a good key for subsequent particles to form the coating.

The use of a rotating disc rather than a series of nozzles has the advantage that in the case of a single electrode extending uniformly around the disc 12 the electric field is uniform around the disc. The rotation of the disc integrates out any non-uniformities in the special distribution of the spray arising from imperfections in wetting of the disc surface or in the fabrication of the rim.

In the absence of an applied electrical field, a critical speed of rotation of the disc 12 would be reached at which centrifugal forces are sufficient to draw the waves which form around the rim into ligaments which subsequently break up into large particles.

When the electric field is applied, the large particles will not ordinarily be formed, since the ligaments will be additionally and coincidentally acted on by the electrical forces to produce small charged particles. The dispersion of particles will be achieved at lower rotational speeds then when no electric field is applied.

The positive polarity of the charge applied to the metal on the disc 12 is preferred, since the presence of positive ions in the metal spray assists the formation of adherent metallic coatings. Electric field emission of positive ions occurs at fields in the region of 10'0 volts per metre. If negative polarity is applied, field emission of electrons can occur at fields in the region of 10e volts per metre, but the spray produced in this arrangement will contain a substantial current of free electrons which may not be desirable for the purpose of producing metallic coatings or metallic powders.

Although the liquid metal has been described above as being fed to the surface of the disc 12 by means of a conduit 11, other means can be provided.

For example, the metal feedstock can be formed by the disc itself, melting being achieved by localised electron or laser beam heating. The surface of the disc 12 can be coated to improve its wetting properties for uniform distribution of the liquid metal, and flow of the liquid metal to the rim can be aided by grooves or channels. If desired, the rim can be toothed or serrated to improve lane separation of the ligaments, especially at high liquid metal feed rates.

A plurality of discs 12 can be provided in any suitable arrangementto increase the production of liquid metal particles.

Figure3showsan ionisersimilartotheapparatus of Figures 1 and 2 in greater detail. The disc 12 has a central recess 31 to hold molten metal. Metal is fed to the recess 31 by a feedstock 32 which also acts as the positive high tension conductor to the disc 12.

The disc 12 is heated by heating wires 33 arranged underthe disc 12, and energised generally to the same potential as the disc.

The disc 12 is supported on the shaft 13, which has water cooled bore 13A, by a ceramic heat insulator 34, and rotates with the shaft 13. Heat shields 35 encompass the upper surface of the disc 12 and the underside of the heating wires 33.

An earthed copper block electrode 36 is arranged around the heat shields 35 to form a small gap aligned with the upper surface of the disc 12 so that the field between the electrode 36 and the disc 12 will cause ionised particles to be drawn out of the gap from the molten metal in the recess 31. The outer surface of the copper block 36 is provided with water cooling tubes 37.

Around the outside of the electrode 36 is a further, biasing, electrode 38, formed with a gap aligned with the gap in the electrode 36. This electrode 38 is biased negatively, in order to exclude any strong electrons from the ioniser. The bias voltage applied to the electrode 38 can be varied to control the stream of ions emanating from the apparatus.

An alternative method of applying the high tension to the disc 12 would be through a slip ring connection from the heating wires 33.

Figure 4 illustrates the apparatus of Figure 3 generally at41 mounted within a vacuum chamber 42 connected to a pump conduit 43. The diameter of the chamber 42 is of the order often times that of the apparatus 41. The chamber 42 has a view-port 44 above the apparatus 41 and electrical supply connec tions to the disc 12 and the heating wires 33 and the mechanical connection to the shaft 13 are made through the bottom of the chamber from apparatus generally indicated at 45.

Arranged around the apparatus 41 and aligned with the upper surface of the disc 12 and the gaps in electrodes 36 and 38 are a plurality of collectors 46.

The chamber 42 is provided with doors 47 to allow access to the collectors 46.

When the apparatus is used to form powder from the ionised metal, the collectors 46 are provided with a liquid film to provide rapid quenching of the molten metal particles. Rapid quenching is a useful technique, as it provides non-crystalline particles, the cooling time being too short four crystal growth.

In general, powder can be formed by quenching the molten metal particles by cooling them in a gas, liquid or solid (e.g., solid CO2) or by locating the collectors 46 at such a large distance from the apparatus 41 thatthe particles have solidified by the time they reach the collectors. The time of travel can be increased by electrostatic retardation.

When the apparatus is used to coat articles with liquid metal, the articles can be supported on or sub stituted for the collectors 46.

The ionising field can be applied between the disc 12 and the collectors 46 or the articles to be coated if desired, in which case the electrodes 14,36 and 38 may be omitted. The illustrated apparatus provides better control and facilitates the use of lower potentials.

Claims (7)

1. A method of forming an ionised dispersion of liquid metals in a vacuum comprising supporting the liquid metal on a moving support and applying an electric field to the metal on the support of such a magnitude as to cause the metal to leave the support as ionised particles.

2. A method as claimed in claim 1 wherein said support is a disc and the method comprises rotating the disc about its axis.

3. A method as claimed in claim 1 or claim 2 comprising applying the electrical field by applying a potential difference between the atomiser and an electrode adjacent the path of particles leaving the support.

4. A method as claimed in claim 3 comprising directing particles from the support in selected directions by locating said electrode relative to the support only in said selected directions.

5. A method of forming an ionised dispersion of liquid metal substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A method of coating articles comprising forming an ionised dispersion as claimed in any one of claims 1 to 5 and causing said electric field to urge the metal particles on to the articles.

7. A method of forming metal powder comprising forming an ionised dispersion as claimed in any one of claims 1 to 5 and cooling said particles.