Technical Difficulties from on Top of the Mountain
2006-10-26
  Deep science
Most people live their lives day in and day out not worrying in the least about grain boundaries.

Sure, the spark plug wires in your car might wear out and have to be replaced (how can a wire wear out?), but for the most part you just figure that all that metal around you is just a bunch of atoms hanging out, mostly holding their shape (steel & aluminum), or pushing electricity around for you (copper, silver, aluminum, tungsten). Well sometimes what you don't know can reach up and bite you when you least expect it, so a little material science might be helpful.

So everyone's seen pictures of atoms all lined up in a row, which is how solids are organized. Sort of. As anybody who's ever broken open a rock, or chunk of aluminum or other metals knows, the surface you get isn't this perfect sheared face. Its chunky. That's because when something starts solidifying, it starts solidifying in multiple places and all these little crystals grow until they bump into each other. Each of these bits are called a grain, and the point where they meet is called the grain boundary.

If you're in to material science, than these boundaries hold all kind of secrets that scientists spend years glued to an electron microscope just to figure out. But that's not where we're going. The boundaries also present a problem to would be travellers trying to move around in there. I'm talking about electrons, trying to move through a piece of wire.

An electron trying to cross that boundary sees a gap. And that makes life a little difficult. In return for getting beat up crossing over, the electron punches back at the metal as it goes by. Just one electron doing that doesn't do much, as electrons are pretty small, but 1 amp of current going through a wire represents 6.25 quintillion electrons (6.25 X 10 to the 18th), so it can start to add up over time. This explains the problem the folks back in the 1970s were having with DIY solar power projects.

Back then, before computers were common (I know, hard to imagine), the fast switching power transistors that the computer uses in its power supply weren't very common either. That meant it wasn't very easy, inexpensive, or efficient to turn DC voltages from solar panels into 110 V AC that all your appliances know and love. Some people went out and stocked up on 12V RV appliances, but others got more creative and pretty much tried every single device out there to see how it would work on DC power somewhere between 90 and 120V.

Light bulbs (being just a piece of wire that heats up), would seem a natural. That is until you learn the physics:

Incandescent lamps can operate on DC voltages. However, two phenomena must be considered.

The first is the fact that light output of an incandescent lamp follows the Stefan-Boltzman Law which implies that light output is proportional to the fourth power of the operating voltage. Thus a 10% decrease in voltage results in a 34% reduction in light output. (not that great when your batteries are a little run down)

The second phenomena is metallurgical in nature. The incandescent tungsten filament in a lamp undergoes a grain boundary modification on DC operation. This results in a reduced life for some lamps when operated on DC. However this effect is very temperature sensitive, so normal lamps operating over 2800°K show little problem, whereas long-life lamps and very low-wattage types (such as night lights) operate at reduced filament temperatures (below 2700°K) which make them susceptible to this effect.

From: the new solar electric home

Curses, foiled by that grain boundary again. Luckily we now have IGBTs and MOSFETs coming out our ears and can take a DC power supply and chop its power into little tiny pulses which we push through a transformer to turn into anything from 110V AC to 480V 3&Phi,; so our lightbulbs will stay happy. Except of course that they're being replaced by CFL lights which are way more efficient, have they're own little inverter stuck in the base, and as thus, ironically, are perfectly happy running on DC.
 
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