michealm890

Slow cooling will increase the crystal size, but to really improve the crystal size temperature control is most important. the best results would be achieved by holding the melt just below the melting temperature. This inhibits the nucleation, so the growth of single crystals is promoted. If a seed crystal is introduced the melt would grow on this seed crystal. If you grew these crystal in an oven, a controlled atmosphere would be advantageous. This could be easily achieved by putting some dry ice in the oven as it is heating up. The Bi metal would be in the oven as well so that you would not have to open the oven until after the metal was melted. Since CO2 is heavier than air, so it should stay in the oven. According to the Ellingham diagram generated in the URL http://www.engr.sjsu.edu/ellingham/page3.php and using 0.21 atm of oxygen and selecting all of the Bi compunds and CO and CO2, CO2 is much more stable than the other compounds of Bi at the solidification temperature of Bismuth. If you are worried a chunk of dry ice could be placed over the oven vent to replace any CO^2 that exited the oven. This would prevent the Bi from forming a slag. I would think that the slag is actually a halide rather than an oxide (unless the slag is an oxide of the impurities.) If the edges of the crucible melting the bismuth are kept above the melting temperature and the center of the top of the melt is cooled, the crystals should form there. This is partially due to the fact that Bi crystals are less dense than liquid Bi liquid (which is rare, like ice crystals floating in liquid water, most solids are more dense than their liquids), so the crystals can float in the center of the pool of metal. The best thing would be to promote nucleation on a desired point, rather than on the surface of the vessel. If an ideal crystal growth environment was designed it would be a cooled single crystal of bismuth that was dropped into a melt of bismuth just above the melting temperature and slowly withdrawn. The whole thing would be shielded by an inert atmosphere. Argon would diffidently work. I think N2 should work (if you could get liquid nitrogen) or SF6 since it is so dense (I can't confirm that a nitride or fluoride of Bi would not form.) This would be similar to the Czochralski process. For the type of growth that the average person would want (faceted), the crystal would just be placed in the melt just above the solidification temperature and left there as the melt was cooled. The cooled crystal could be achieved by taking a previously created crystal, and attaching it to a copper wire (actually silver wire would be better, as it has a higher thermal conductivity) and connecting that wire to a cooled surface. Say welding it to a metal bowl filled with ice water. In a pinch, the CO2 could be produced in a pitcher with vinegar and baking soda and "poured" over the pot used to melt the Bi. As an example Any references are in the post or come from the curriculum of my masters in metallurgy. If I am wrong in any of my facts, let me know. Hopefully this helps.