MELTING METHOD AT VACUUM EVAPORATION

Abstract: A method of preventing aluminium from splashing and spitting from a crucible when melting and vapourizing aluminium in vacuo with the aid of induction heating and comprising placing a graphite ring (4) around the aluminium (2) to be melted in the crucible (1). If the aluminium first melts locally at a given point in the toroidal crucible (1), the graphite ring (4), which has a much higher melting point than the aluminium body (2), will constantly ensure that electric contact is achieved around the aluminium body, thereby preventing the occurrence of induction lifting forces that in turn result in aluminium splashes.

Description: Melting Method at Vacuum Evaporation It is known from Swedish Patent Application No. 9100372-3 to use a metal smelt, for instance an alu- minium smelt, of relatively high temperature to metal¬ lize individual plastic CD-discs and articles like of low melting point, said metal smelt being briefly held very close to the article to be metallized. The method is concerned with the ability to coat articles quickly while minimizing the deposit of metal in wrong places. One problem with the coating method taught by the aforesaid Swedish patent application, however, iε that an aluminium smelt cannot be readily produced. Al- though the aforesaid patent application discloses that induction processes can be used, the induction heating of aluminium per se cannot be achieved as readily as, for instance, the induction heating of iron, since in the former case the inductive coupling to non-magnetic material cannot be achieved aε in the latter as read¬ ily. When aluminium iε heated normally under atmos- pheric conditions, the heating proceεs will function relatively satisfactorily, although a tough layer of oxide will form on the surface of the aluminium bath. This oxide layer, however, can be scraped from the surface of the bath, therewith enabling the molten metal to be cast in a mould, for instance. However, when the smelt is to be used for vapour deposition purposes, it is important that no oxide layer is present on the surface of the smelt, since this would then prevent vapourization of the molten metal and the surface would no longer be bright, resulting in high thermal radiation. In other words, the presence of an oxide layer on the surface of the molten bath would render the smelt impossible for use in the production of a metal coating by vapour deposition processes. Consequently, it is not possible to heat an aluminium smelt in an ambient oxygen-gas atmosphere and then use the aluminium smelt for vapour deposition or metal plating purposes. When attempts are made initially to heat the smelt in vacuum conditions, the smelt will behave in a co - pletely different way than it would otherwise do. For instance, because no oxide layer will be formed, the smelt is much more fluid and because the body to be smelted down is not heated and melted with exact homogeneity throughout its entirety and a precisely homogenous induction field cannot always be ensured instead the electric currents generated in the alumi¬ nium by induction and those forces acting on the aluminium will vary. This will result almost invari¬ ably in the aluminium splashing and spitting as it begins to melt and it is possible to lose the electric induction coupling to the aluminium completely in the case of certain crucible constructions which are desirable from other viewpoints. This spitting phenom¬ enon is particularly manifest in the case of toroidal smelts that are desirable, for instance, in the manu¬ facture of CD-discs. The fundamental cause of the difficulties is probably the result of inhomogeneities in the heated aluminium body in its solid state and also because non-uniformity in the induction field causes the aluminium to melt first at a distinct place/location on the aluminium body. When the alumin¬ ium in this distinct location becomes molten or fluid, the liquid aluminium will run immediately down to the bottom of the crucible, where, among other things, it fills out the requisite thermal expansion gap between crucible and aluminium body. When the aluminium runs thus to the bottom of the crucible, the induction currents in the aluminium body result in a higher current density at the location from which " aluminium has run. This, in turn, may result in a lifting force of such magnitude (aluminium is a lightweight metal) as to subject the residual aluminium bridge to a dynamic lifting force at the same time as the alumin¬ ium body melts, resulting in aluminium being splashed or thrown away. This splashing can be repetitive and is, of course, quite unacceptable. When induction-heating aluminium for instance, the heating ability is dependent on the frequency of the induction field. A lower frequency will result in more effective coupling and therewith in quicker heating. The power unit used in the induction heating process is normally less costly at lower frequencies. However, one drawback with the use of a lower frequency is that the dynamic lifting force is greater on that part of the metal which is first heated locally in the cruci¬ ble. Lower frequencies thus result in greater problems with regard to the spitting of metal droplets from the crucible. Gas-bubble inclusions may also be an explanation as to why aluminium spits from the crucible, or may at least be a contributory factor. Naturally, a smelt can be heated in other ways. Howev¬ er, since it iε desired to place the smelt in a very precise manner, and to reduce the heating of the smelt in order to obtain a low total thermal load on both the crucible and the smelt itself and, for instance, on the CD-discs to be coated or plated, induction heating methods are still an attractive solution, although their use necessitates a solution to the problem occasioned by the instable heating of the smelt. An object of the present invention is therefore to provide a method and a smelt arrangement which will enable an aluminium smelt to be heated in vacuo in a controlled and "spitting-free" fashion. The aforesaid problem is solved by preventing the alu¬ minium body from melting excessively at any distinct location thereon. In other words, the metal forming the aluminium body must be prevented from melting at any one particular place on the body but melting shall take place at different points in time, so that there is always an electrical bridge beyond the location or locations where melting of the aluminium body happened to begin, as the aluminium body melts. Accordingly, it is possible in accordance with the present invention to prevent spitting or splashing of the aluminium by giving the aluminium body a height or vertical extension such as to render it impossible for the entire body to be melted down at once. The alumin¬ ium body may also include discs which are arranged in superimposed' layers and mutually joined by short vertical posts or radially connected, mutually concen¬ tric rings. It is also conceivable to use aluminium bodies which comprise an alloyed bottom part and a non-alloyed upper part, therewith ensuring different melting temperatures, such that prior to melting of the upper layer, which constantly provides an electri¬ cal connection, sufficient of the bottom layer will have melted to ensure a circular contact. It is also conceivable to place other electrically conductive material in the crucible, for instance a copper or graphite annulus. This annulus may be placed around, beneath or in the toroidal case inwardly of the alumi- nium body. Further features of the invention and advantages afforded thereby will be evident from the following description made with reference to the accompanying drawing, which illustrates a crucible that contains aluminium. Illustrated in the drawing is a toroidal crucible 1 in which an aluminium annulus 2 is placed with an inter- mediate clearance. The clearance is necessary in order to prevent the hot aluminium from cracking the cruci¬ ble as the aluminium is heated from room temperature to its melting point. The aluminium annulus 2 is encircled by a graphite ring or band 4. In the illus- trated case, an inductive field is generated with the aid of a coil placed beneath the crucible. The induc¬ tive field will be strongest at the bottom of the crucible and the aluminium located nearest the cruci- ble bottom will melt first and if dividing or parting forces are generated in the annulus 2 as the heating progresses, the current circuit in the annulus 2 will remain closed through the graphite ring 4, so that power will still be delivered to the annulus 2 allow- ing the melting of the aluminium ring to continue without being disturbed by variations in current strength and field strength, such variations otherwise resulting in unstable conditions with excesεively powerful dynamic lifting forces. Because the induction coil is placed beneath the crucible, the distance to the εmelt iε short and the inductive energy transfer consequently good. Further¬ more, the inductor is shielded against vapourized aluminium from the crucible itself and also against the danger of aluminium depositε and because subse¬ quent short-circuiting between the turns of the induc¬ tor coil decrease. Furthermore, different crucible configurations, for instance configurations which are in accord with the shape of the articles to be coated or plated, can be used without inductor changes. The distance between inductor and crucible can also be adjusted with the aid of simple means, so as to obtain optimal coupling conditions. As will be understood, it lies within the concept of the invention to replace the graphite ring with a ring made of some other material of good electrical conduc¬ tivity, high melting point and low vapourizing rate up to intended operating temperatures. It also lies within the concept of the invention to use other substances than aluminium which behave in a similar manner when heating to a vapourized state. In this case, it may be necessary to use other materials which take care of the electrical short-circuiting or shunting to prevent and counter movements in the smelt.

Claims: C l a i m s 1. A method for melting and heating metal, for in¬ stance aluminium, in vacuo. particularly in shallow and/or toroidal crucibles, with the aid of an inductor to obtain a vapourization source, c h r a c ¬ t e r i z e d by taking measures to prevent the aluminium body from melting excessively at any dis¬ tinct location, so as to prevent the electrical con- nection from being impaired or broken. 2. A method according to Claim 1, c h a r a c ¬ t e r i z e d by using metals or alloys of mutually different melting pointε. 3. A method according to Claim 1 or 2, c h a r a c ¬ t e r i z e d by giving the metal body to be melted a configuration such that said body will melt succes¬ sively in an axial and/or radial direction, for in- stance from beneath. 4. A method according to Claim 1, c h a r a c ¬ t e r i z e d by placing a ring or disc of electri¬ cally conductive material around or below the body to be melted, or in the case when the crucible is a toroidal crucible inwardly of said body. 5. A method according to Claim 4, c h a r a c ¬ t e r i z e d in that the ring or the disc is made of graphite or copper.

Inventors: NORDLANDER JOHAN, NORDLANDER, JOHAN
Applicants: APPLIED VACUUM TECH, APPLIED VACUUM TECHNOLOGIES 1 AB
Application year: 1992
Main classification: C23C14
IPC codes: C23C14