EML at IFW Dresden
The key idea of any levitation process is to compensate the gravity and allow the sample to be freely suspended in the vacuum or a definite gas atmosphere. Despite the big variety of the levitation techniques the electromagnetic levitation (EML) is the most suitable method to levitate metallic samples.
Basic principleSchematically the EML system can be represented by metallic sample (up to 1 cm in diameter) placed in an alternating electromagnetic field of levitation coil (one copper winding). When the alternating current is flowing through the coil, the alternating electromagnetic field induces eddy currents in an electrically conducting system. A magnetic moment produced by the eddy currents causes a repulsion force FL that counteracts the gravitational force FG, which leads to levitation if FL = FG. Simultaneously, the eddy currents heat up the sample but also produce strong electromagnetically induced stirring. |
EML in detailsReal levitation coil systems consist of 5-8 windings and 1-2 counterwindings at the top, to stabilize the position of the sample, and are powered by a high-frequency generator. The levitation proceeds in a high-purity environment of an UHV chamber backfilled with 6N He (or He/H2) gas. It is possible to control the temperature of the sample by varying its position along the symmetry axis of the coils (limited range) and by using a cooling gas system which introduces the forced convection. |
Temperature measurementsDuring the levitation the sample temperature is measured from the top by means of a two-color infra-red pyrometer at a sampling rate of 50 Hz with an accuracy of ±5 K. However, the fast temperature changes during rapid solidification cannot be resolved with such a frequency. Therefore, fast responding Si photodiode with sampling rate up to 10 MHz is used to monitor the rapid temperature events. |
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Solidification establishingThe solidification of the undercooled melt can spontanously occur at some undercooling level. However, it can be also externally initiated at a given undercooling level by touching the bottom of the sample with a trigger needle. |
QuenchingTo determine the effect of higher cooling rates on the undercooled samples it is possible to quench samples onto a chill substrate. In that case solidification starts at the touching point of the sample with the substrate. |
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- see animation of melt undercooling experiments
- see video of solidification from undercooled melt