It is extremely difficult to obtain a sample of silicon powder by sintering because of its high affinity for oxygen. When sintered a sample from silicon powder, its oxide will always be present, which prevents sintering.

It is easier to place the silicon powder in a corundum crucible and heat in a vacuum furnace above the melting point (more than 1415°C). Although melting the silicon does not separate silicon melt from the silicon oxide (silicon dioxide and silicon are equal in density), the sample will still be obtained. The coarser your silicon powder, the less silica the sample will contain after melting.

Vadim Verlotski, thanks a lot for your reply and suggestion.