Dr. Asthana Presentation

1.      What are grains and grain boundaries in a material? I suggest you focus on metals. Explain how material properties are affected by the size of these grains.


Grain-An individual crystal in a polycrystalline metal or ceramic.
Grain boundary-The interface separating two touching grains having different crystallographic directions.

Material properties of a material are affected by their grain size in four areas: yield strength, tensile strength, creep strength, and bolt load retention.

[Source from: http://www.am-technologies.com.au/pdf/publication-AM-SC1-The%20effect%20of%20grain%20size%20on%20the%20mechanical%20properties%20of%20AM-SC1.pdf]


2.      How does one engineer or process materials to reduce the grain size? In particular, I would like for you to explore and then explain how single crystal silicon is produced for the solar industry.

Possible ways on how to reduce grain size: Rapidly cooling a molten metal, rolling or forging the metal, and adding an “inoculants” such as Titanium during the solidification process of a metal. During the production of monocrystalline silicon, there are two types silicon: Czochralski silicon and Float zone silicon.

The steps to creating Czochralski silicon include first melting “high purity polysilicon”. Then a single crystal silicon seed is put in a heated chamber on the surface of the Quartz Crucible and is slowly drawn up while at the same time being rotated. This process of the single silicon crystal being drawn up allows for the molten silicon to solidify. During the whole process, the quartz crucible gradually dissolves, which in turn releases a large quantity of oxygen in the melt. Most of this is turned into a gas, but the rest stays in the melt and is dissolved into the silicon. As the single silicon crystal is pulled from the melt, the impurity concentration in the solid part of the crystal will contain different impurities than the molten part of the crystal.

The steps for creating monocrystalline silicon with using the float zone method begin with placing a “high-purity polycrystalline rod and a monocrystalline seed crystal” vertically placed on top of each other (not touching) in a vacuum or “in an inert gaseous atmosphere”. From there, the ends of the rods closest to each other are partially melted with a radio frequency. Then, the monocrystalline seed is brought up to make contact with the molten part of the polycrystalline rod. Next, “a necking process is carried out to establish a dislocation free crystal for increasing the diameter”. As the polycrystalline rod moves along the “molten zone”, the molten silicon solidifies in which completes the float zone process.

[Source used: http://meroli.web.cern.ch/meroli/Lecture_silicon_floatzone_czochralski.html]