Friday, July 31, 2015

Allotropy: Why Winter Spells Trouble for the Tin Man

Tin is a commonplace metal that’s used industrially in a thousand different ways. From the solder that holds your computer’s motherboard together to the PVC plumbing under your sink, tin compounds are everywhere. In spite of its versatility, tin possesses an interesting physical property which is responsible for its tendency to wear down over time outdoors. This phenomenon, known as “tin pest”, is certainly not due a biological organism, but is widely mistaken for an oxidation reaction. Instead, tin pest happens thanks to something called allotropy—the metal’s atomic lattice can take on multiple different shapes, depending on the temperature it’s kept at. 

At room temperature, tin exists in its “beta” form. This is the everyday, shiny type that was long used in metal smithing and plating, and to create the rust-resistant coating on steel cans of food. But when the temperature drops below 55.8°F, pure tin transitions to its “alpha” form, a dull gray material which is brittle and non-conductive. 
Ordinarily, tin’s crystal lattice is tetragonal, with two square faces and four rectangular ones. Cooling the tin down reduces the amplitude of the atoms’ vibrations, and they can pack together into a more complex and space-efficient shape, shown below. 

This changes the way the electrons flow from atom to atom, rendering the tin useless as a conductor. Given that most modern lead-free solders are made of tin, you might be surprised that this happens at just below room temperature. However, this property is pretty unique to tin in its purest form; doping with other metals will keep the beta-alpha transition from happening until a much lower temperature than 55.8°F. So don’t worry—your laptop won’t necessarily break down if you leave it in the car overnight this winter. (Watch out for condensation when you bring it inside, though!)
You’ve learned your physical chemistry for the day, now for the cool part: watching it happen! Enjoy the video of tin’s beta-alpha transition, filmed as a time-lapse over about 20 hours.

No comments:

Post a Comment