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2. Experimental
2.1 Thermal history experiments
Three MG-Si samples were collected from a thin cast produced at the Elkem silicon
production plant in Salten (Norway). The dimensions of each sample were of 10x10x
lj
mm, with
lj
varying between 30 and 45 mm and being the thin cast thickness. The first
one was not treated at all, in order to be kept as reference. The second one shared a
surface with the first sample. It was heated up to 600°C in the LHT 04/18 furnace,
available at the department of Materials science and Engineering at NTNU. The
temperature was kept for 17 hours, then the sample was cooled down to room
temperature. These two samples will be respectively called untreated and treated
samples from now on.
The third sample underwent a different thermal history. The annealing consisted of
three steps, as shown in Figure 2. About 250 g of silicon from the original cast were
crushed manually to a lump size of 2-3 cm diameter. They were heated up to 1450°C
at a rate of 30°C/min in a cylindrical carbon mold exposed to air. The carbon crucible
dimensions were respectively 10 cm diameter and 12 cm in height. The bottom of the
mold is spherical-dome-shaped, with height of 5 cm. This temperature was
maintained for 2 hours, in order to melt the material. Afterwards the lid of the furnace
was opened and the sample was cooled down to 1200°C for annealing. The cooling
rate for this step was measured to be 30°C/min. Once the annealing temperature had
been reached steadily in the furnace, the lid was closed again and the temperature was
maintained for 17 hours. In the end, the furnace lid was opened to cool down the mold
as fast as possible to room temperature. A cooling rate of 20°C/min has been reached
in this way. The sample with this thermal history has been called annealed.
Figure 2
:
Schematic representation of the two thermal histories chosen. The continuous and
dashed lines correspond respectively to the annealed and the treated sample.
2.2 XRF analysis
A XRF chemical analysis has been carried on the untreated material and the annealed
sample. The material as cast was analysed by Henning Kjønli and the annealed
material's analysis were carried out by Ingvill Vikan Myhre. Both the analysis have
been performed at Elkem Thamshavn (Norway).
The composition of a piece of material as cast was obtained to represent the
untreated sample. The annealed sample underwent a different analysis. Once a
specimen for metallographic analysis had been extracted, the remaining material was
sliced in the horizontal direction. The cut was performed manually by using a 2 mm
thick diamond blade disc. Five slabs were obtained from this procedure. Each layer
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