Monday, June 20, 2011

Reality - more magical than anticipated.

Piezoelectricity - a concept that is so cool I cannot believe I hadn’t heard of it until two weeks ago! I work with electricity, and figured I knew it all, but then the universe went and surprised me.

It didn’t start well. I was trying to set up a piezoelectric translator, which apparently someone used about 10 years ago, and it’s still kicking around the lab. it’s OLD, like the manual is type-written and comb-bound old. I read the (nearly useless) manual, and tried to set it up. It did not go well. Various parameters and parts did not function as anticipated. I finally diagnosed some of my issues, but my switching time was still much too long. So I thought, “I’m not a man, I’ll just call tech support.” Well, it turns out the thing is SO old that the company that made it doesn’t exist anymore, and the company they sold the manufacturing rights to doesn’t make this device anymore, and their tech support director was in China. But, you’ll be relieved to know that this story has two happy endings: First, I fixed the switching time after looking at pictures in a book about single channel recording (good news, reading is becoming obsolete!), second, I decided to look up PZT.

Throughout the (nearly useless) manual, they kept yapping about the PZT this and PZT that. Never mind that they never spell out this abbreviation anywhere in the entire manual. So I finally looked it up on Wikipedia, and it turns out to stand for “lead zirconium titanate,” and pointed me back to a page on piezoelectricity, and I was hooked.

So, here’s the basic deal – reality is more magical than anticipated. Tension and compression causes charge accumulation (measured as voltage). Turns out, the reverse is true too, application of voltage to said crystals causes changes in length. Piezoelectricity literally means “electricity from pressure.”
Crystals are made of atoms arranged in repeating blocks called “unit cells,” where the atoms in each repeating block are arranged the same way. Piezoelectric crystals have asymmetric arrangements of atoms in each unit cell. At rest, these crystals are electrically neutral, because the charges of the atoms are arranged such that each positive charge is balanced by a negative charge (really this has to do with dipole moments, if you’re into that level of detail). But when you squeeze or stretch the crystal, you upset this delicate balance of cancelling charges as you move atoms out of place, causing the crystal to have net charge on its faces. Same deal if you apply charge: you apply electrical force that makes the atoms rearrange in an attempt to cancel the charge, causing the shape of the crystal to change. This effect can be pretty large: applying 500 foot-pounds of force to a cubic centimeter of quartz can produce 12500 volts!

Not all asymmetric crystals exhibit piezoelectricity. Some naturally occurring crystals do, like cane sugar, quartz and topaz. Later, scientists began to create manmade crystals for commercial use, like my good friend PZT, which is a manmade ceramic.

Piezoelectricity – practically magic – but what is it good for? Turns out it’s everywhere, not just in science-y sounding equipment. In cigarette lighters, pressing the button makes a little hammer hit a piezoelectric crystal, which generates a high enough voltage that current flows across a spark gap to ignite the gas. You can also find piezoelectricity used in loudspeakers, inkjet printers, clocks, MRI machines and anywhere a cool little crystal motor might come in handy.

It makes me wonder how many other things I use operate on near-miraculous science!

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