Crucible and method for its use

A crucible particularly intended for use when vaporizing aluminum and comprised of an ultralow cement compound. Subsequent to moulding and drying the crucible, the crucible is baked at a high temperature, which corresponds to the temperature of the crucible when used in a vaporizing process, 1,200.degree. C. for instance. The crucible comprises an outer wall and a bottom in which upstanding projections or corrugations are provided. These upstanding projections or corrugations fulfill the function of enabling gas enclosures generated when mixing the cement compound to rise up and collect in the corrugation peaks without impairing the homogeneity of the material in the actual crucible bottom.

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 made have the 
meanings given for these terms in the relevant edition of the relevant 
American Society For Testing Materials (ASTM) standard, which are 1-2.5 
percent by weight, and 0.2-1.0 percent by weight, respectively, as will be 
understood by those skilled in the art. 
This is achieved in accordance with the invention by adding carbon, for 
instance in the form of graphite, to the smelt and also titanium. 
Practical tests have shown that the aforesaid change in colour of the 
aluminium coating, hereinafter called discolouration, can be totally 
avoided in this way. 
It has been found that some few percentage of titanium (for instance 3-6%) 
and some few per thousand or percentage of graphite will be sufficient for 
this purpose. If too little graphite or too little titanium is added, 
discolouration will occur. On the other hand, no negative effects have 
been observed when greater amounts are added, provided that the 
percentages are so low as not to effect the rate at which the aluminium is 
vapourized. 
The exact manner in which the additions of graphite and titanium clean the 
vapour deposited layer is unknown and even though the aforesaid quantities 
of graphite and titanium have been related to the quantities in which 
aluminium is present, it is possible that instead these quantities are 
ruled by the size of the vapourized surface or the contact surface of the 
crucible with the smelt. 
The word "added" shall be understood in its widest meaning within the scope 
of the inventive concept, i.e. the addition may be made in any selected 
sequence in both solid and molten liquid form. In other words, lose bodies 
of aluminium, titanium and graphite can be placed in the crucible and 
thereafter melted (aluminium) and dissolved (titanium and graphite). 
Alternatively, an aluminium body in which graphite and titanium have been 
alloyed with the aluminium may be used, as can also an aluminium smelt to 
which pieces of graphite and titanium have been added and dissolved 
therein.

DETAILED DESCRIPTION 
The drawing figure is a cross-sectional view of a crucible which includes a 
toroidal, smelt-accommodating space 2. The space 2 is delimited at the 
outer edge by an outer wall 3, and includes a central crucible projection 
4 and a bottom 5. Since the crucible is intended for use with 
induction-heating processes, a short distance between, for instance, an 
induction coil located beneath the crucible and the smelt is desirable. 
The bottom 5 is therefore thin. As will be seen from the drawing, the 
toroidal, smelt-accommodating space 2 is corrugated or serrated at the 
bottom, as shown at 5. This corrugation or serration is suitably in the 
form of concentric circles and enables the bottom 5 to be made thinner 
than would otherwise be possible. Any bubbles present in the crucible 
material will rise upwards in the normal way. This means that the bubbles 
will rise up in the ridges of the corrugated bottom. This reduces the risk 
that the bubbles will end up in the bottom, which is made as thin as 
possible, and therewith also the risk of leakage caused by the presence of 
bubbles in the crucible material is dimensioned. 
The crucible is manufactured by shaping the crucible in a corresponding 
mould, which may consist of silicone rubber for instance. Subsequent to 
the ultralow cement compound solidifying or hardening, which may take from 
some hours to about a full calendar day, the crucible is removed from the 
mould and then baked in a furnace at a temperature of about 1,200.degree. 
C. The crucible is then ready for use and will have, according to tests 
carried out, a high thermal-shock resistance. In other words, when heated, 
the crucible will not crack as a result of the varying temperatures to 
which it is subjected. The crucible is also highly impervious and, when 
used, will not allow vapour jets to pass through the crucible walls, 
despite the fact that the crucible bottom may, for instance, have a 
thinness of from 5-6 mm. In view of these small wall thicknesses, the raw 
material used will preferably not have an excessively large particle or 
grain size. 
When seen from a technical aspect, the crucible is not only much better 
than those crucible variants that are commercially available at present, 
but is also much cheaper to manufacture. 
As a result, the crucible represents a revolutionary decrease in costs and 
in the problems associated with vapourization techniques, despite the 
simplicity of the crucible. 
When a slight yellowish-brown discolouration of the vapour deposited layer 
can be accepted (or is perhaps even desired), the crucible can be used 
without taking any further steps. However, when a more precise "natural" 
coloured result is required, graphite and titanium are added to the smelt. 
This can be effected, for instance, by placing a graphite band around the 
aluminium body placed in the crucible, this aluminium body being smelted 
down to provide the vapourization bath. Satisfactory results have also 
been obtained when placing a graphite disc beneath the aluminium. 
In the case of tests carried out in practice, the titanium was added in the 
form of pieces of titanium wire or titanium plates placed beneath the 
aluminium body to be smelted, and in some instances the titanium was 
placed on top of graphite plates. 
The arrangement of graphite or titanium in a disc beneath the aluminium 
body or in a ring around the aluminium body to be melted, provides an 
electrically stable connection which during the smelting phase counteracts 
the tendency of the aluminium to spit or splash when the aluminium body is 
heated unevenly and when melting the body by induction heating. 
It is conceivable within the scope of the inventive concept to use material 
other than carbon and titanium and still achieve the result desired. For 
instance, a material similar to carbon may be used, for instance boron, 
while some other metal may be used instead of titanium.