How do you make a pure sample of germanium?

Answer 1

This depends on how pure you want it. For most purposes chemical reagent grade germanium is adequate and it can be purchased from most chemical supply houses quite inexpensively. For electronics purposes however you need the much purer semiconductor grade germanium which can also be purchased easily but at a higher price. There is no real reason to want to go to the trouble of making either.

However I will explain the method by which reagent grade germanium is purified to semiconductor grade germanium. This requires several hundred cycles of a process called zone melting which is a form of fractional crystallization. This is performed by placing the germanium in a special elongated crucible called a graphite boat, because it is made of highly pure boron free graphite (similar to that used as the moderator in graphite moderated nuclear reactors) so that the crucible does not itself recontaminate the germanium and its long narrow shape resembling a boat. The germanium is then melted by sliding the boat through a ring shaped electrically heated furnace that melts only a narrow zone across the width of the boat, which recrystallizes as it leaves the furnace. As the boat passes through the furnace the impurities concentrate in the melted zone and when the boat is removed from the furnace leaves the impurities concentrated in that tail end of the boat that was last to leave the furnace. After roughly every 20 cycles through the furnace the graphite boat is broken up and the germanium crystal removed and the tail end is cut off and recycled as scrap because it contains too high a concentration of impurities. The germanium is now placed in a fresh graphite boat along with germanium of similar purity from other boats and processing continues until the desired semiconductor grade purity is reached.