Fusion with inductive heating
The principle of inductive heating has been discussed briefly. Whereas in the case of metal samples the metal is the coupling medium for the inductive eddy currents and is thus directly heated, in the case of non-conducting materials the crucible must undertake this function.
The most important factors are the frequency and output of the HF generator. Because of the penetration depth a frequency must be selected which produces the necessary field for the geometric body of the crucibles and fusion agents are 500kHz to 1.5MHz.
In order to reach high temperature, the HF generator should deliver sufficient power to compensate for radiation losses, even from an uncovered or uninsulated crucible. The field of the induction coil should be formed to heat the crucible as evenly as possible. The time required to reach the end temperature should be 1-3 min.
Depending on the viscosity of the fusion agent and the sample, which is determined by temperature, it may be necessary to fit a vibration facility to the crucible in order to remove bubbles which may form as a result of air occlusion during melting.
An exhaust facility is necessary in the fusion system since, with the high fusion temperatures, the vapour pressure leads to contamination of the sample space.
If PtAu crucible are used, the temperature is limited to 1500 ℃ because of softening. During the first few min heating should be carried out slowly in order to prevent the fusion agent from overflowing the crucible. The melting process can be observed through a viewing window. Depending on the type of crucible, the fusion process can be carried out in the crucible, with solidification also taking place in the crucible. This has the disadvantage that large crucibles have to be used which, in the case of PtAu, involve high cost. It is more sensible to work with ordinary crucibles and casing trays which can be preheated on the fusion crucible, or on a separate induction coil which can be connected in series to the melting coil. Particularly for fusions which take a long time, eg. chrome ores, high fusion temperatures and high tray temperatures are recommended.
It is not usually necessary to rotate the crucible as the bath movement produced by the electromagnetic field and the heat flow at high temperature are sufficient. At the end of the fusion process the crucible, together with the holder, is removed from the induction coil and the fusion product is poured into the preheated tray or mould. The casting tray can be cooled by normal heat loss, or by cooling from below with compressed air.
A glass disc free of bubbles and streaks is obtained. Careful attention must still be paid to the quality of the casting tray which must have a flat, smooth surface since cracks and roughness may cause interference if the disc is being used direct.
It only remains to mention that PtAu or PtAuZr crucibles, with careful treatment, will withstand 150 to250 fusions. Graphite or glassy carbon exposed to Ar will survive 25 to fusions. Today crucible costs per fusion are about 2 DM, and plant operating costs are about 0.20 DM per fusion.
Here again machines are available equipped with a robot carrying out weight and manipulations automatically, so that even unsupervised night work is possible.
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