Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1994-09-09
1996-02-27
Andrews, Melvyn
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75686, 118726, 266242, 427250, C23C 1426
Patent
active
054944996
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
It is known from Swedish Patent Application No. 9100372-3 to metallize or plate individually plastic CD-discs and like articles for instance, of low melting point while using a metal smelt, for instance an aluminium smelt of relatively high temperature which is briefly held very close to the article to be coated. The method enables articles to be quickly coated or plated while minimizing the deposit of metal at wrong positions.
One of the problems that occur when using an aluminium smelt as the vapourization source resides in maintaining the actual crucible intact. The aluminium smelt in a vapourization source is heated to much higher temperatures (for instance temperatures above 1,200.degree. C.) than those applied in casting operations and similar operations (for instance 800.degree. C.). This means that the actual aluminium smelt will be much more aggressive and behave as a corrosive acid. Since, among other things, it is desired when carrying out the aforesaid method to achieve long periods of use, on the order of one-hundred hours or more, before it is necessary to replace the smelt and crucible, the demands placed on the crucible are greater than the crucible is able to manage when applying hitherto known techniques.
One troublesome circumstance is that the crucible is used in vacuo, i.e. any small aluminium leakages that occur are not sealed automatically by oxidation of the aluminium on the outer surface of the crucible. Crucibles which are self-sealing in ambient atmospheric conditions will thus continue to sweat or to leak in vacuum conditions, which is unacceptable.
In other words, there is a need for a crucible that is intended for molten aluminium and its alloys or compounds to manage high temperatures and to remain impervious even in vacuum conditions.
SUMMARY OF THE INVENTION
This need is fulfilled in accordance with the invention by constructing the crucible from a so-called ultralow cement compound, such as for instance from "Ultracast BSR" (84%, Al.sub.2 O.sub.3, 0.5% CaO, 0.9% Fe.sub.2 O.sub.3 and the remainder quartz) from Bohlin & Lofgren AB, or from a corresponding material. The advantage with this type of material is that in induction heating processes, the material will only take-up a small amount of energy, or no energy at all, and the crucible is therefore only heated indirectly by the smelt. The thermal load on the crucible is therefore lower than in earlier cases, while minimizing the heat radiated from the crucible, which is essential when, for instance, carrying out the method taught by Swedish Patent Application No. 9100372-3.
According to a further development of the invention, the crucible produced is further treated by baking the crucible at successively higher temperatures, with the terminal temperature lying generally on the same level as the temperature at which the crucible shall be used, in other words about 1,200.degree. C. Experience has shown that this comprehensive baking process will improve the resistance of the crucible to temperature shocks, therewith reducing greatly the risk of thermal cracks occurring during the heating process and also reducing the risk of leakage from the crucible.
It is believed that the crucible should be baked over a relatively long period of time and at successively higher temperatures. It is also conceived suitable to bake the crucible in a vacuum for as long as possible, under those operational conditions to which the crucible will be subjected in time.
It has been found that the aforedescribed crucible can withstand several days continuous use at high temperatures, for instance 1,200.degree. C. Unfortunately, it has also been found that when vapourizing aluminium, the deposited layer turns a yellowish-brown colour after a time. Consequently, the invention also relates to a further development of a method for preventing such an occurrence when using the crucible.
As used in this document, the terms "low binder" and "ultra low binder", in connection with the cement compound of which the crucible is mad
REFERENCES:
patent: 2665223 (1954-01-01), Clough et al.
patent: 2962538 (1960-11-01), Alexander
patent: 3414251 (1968-12-01), Rall
Patent Abstracts of Japan-vol. 9, No. 309-Aug. 1985.
Patent Abstracts of Japan-vol. 9, No. 228-May, 1985.
Maissel et al: "Handbook of Thin Film Technology" 1970.
Andrews Melvyn
Applied Vacuum Technologies 1 AB
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