Piezoelectric Questions

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spxdcp
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Piezoelectric Questions

Postby spxdcp » Thu Jan 13, 2011 2:48 am UTC

Hi guys! I have a few questions on piezoelectricity and it would be great if you can help me out.

1) Does a non-centro-symmetric metal exist?

Reason: Since materials which are centro symmetric are unable to produce the piezoelectric effect, i wonder if a non-symmetrical metal exists.. So far, the materials i have experimented with are i)quartz and ii) Rochelle Salt , and both of them do not have free moving electrons. As a result, my current produced is 0.000001mA, all its good for is lighting an LED for less than a second.

2) Would wrapping the crystals around a magnet help increase current output? I have not been able to test this one out myself as I feared the oscilloscope would get damaged by the magnet.

3) This is the one i'm most interested about. Would going into the nano-level help boost efficiency of piezoelectric materials? Would a block the size of an ipod classic with 100 layers of micro crystals perform better than a solid block of crystal? is yes, how much better would it be?

4) If i did many micro layers, would it be better to connect them in series or parallel to get the most out of it?

5) what kind of battery would be best to harvest the generated energy?


Background info : I am currently on a final year project, and we are to create a piezoelectric energy harvester to collect energy from the 70,000+ people walking through the schools front gates. we have built a prototype out of the igniters found inside gas lighters and all its managed to do so far is to blink an LED for every step we take. Another problem is the annoying clicking sound of the striker hitting the crystal. so right now I have 5 weeks to get around the MAJOR problem of generating enough current and voltage to power more than just an LED for a split second.. If i get an A, my gpa would jump from a 1.8 which SUCKS (yea, i lazed through the first 2 and a half years... study hard guys.. or you wont be able to get into a university) to a 2.2.. which still sucks.. so help me out! =D

Technical Ben
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Re: Piezoelectric Questions

Postby Technical Ben » Thu Jan 13, 2011 11:41 am UTC

See this thread for some problems you may face. :P
viewtopic.php?f=18&t=67370


Not sure how to help though. Sorry.
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Tass
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Re: Piezoelectric Questions

Postby Tass » Thu Jan 13, 2011 12:30 pm UTC

spxdcp wrote:1) Does a non-centro-symmetric metal exist?


Not sure. But a piezoelectric metal would short it self out so you wouldn't actually get anything out. You want it to be an insulator so that the displaced charge has to be offset by charge in your attached wires not just by an equal displacement of the crystals conduction band electrons.

spxdcp wrote:2) Would wrapping the crystals around a magnet help increase current output? I have not been able to test this one out myself as I feared the oscilloscope would get damaged by the magnet.


Can't see why it would. Can't see why the magnet would damage an oscilloscope either, although it might offset the reading by deflecting the electron beam, but unless the magnetic field is changing that would be an easily corrected systematic error.

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wbeaty
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Re: Piezoelectric Questions

Postby wbeaty » Wed Jan 19, 2011 8:22 am UTC

spxdcp wrote:3) This is the one i'm most interested about. Would going into the nano-level help boost efficiency of piezoelectric materials? Would a block the size of an ipod classic with 100 layers of micro crystals perform better than a solid block of crystal? is yes, how much better would it be?


It gives the same milliwatts output, but you could parallel the layers for high current and low voltage. Your problem is that piezos put out extremely high voltage at very low current, and most common electrical devices require the opposite. Either collect the high voltage and then build a "buck-type" switching power supply to step it down to something usable. Or, use thin piezo layers (lower voltage) hooked in parallel (higher current.) I think a guy on eBay is selling piezo stacks. Nope, his store steminc is shut down "until Jan 5th."

spxdcp wrote:4) If i did many micro layers, would it be better to connect them in series or parallel to get the most out of it?


Parallel. The act of swapping voltage for current while keeping wattage the same? That's called "impedance matching."

If a piezo bender gives out a milliwatt in the form of 1000v at 10uA, it behaves as if it has a hundred meg-ohm resistor wired in series. 1e3 volts / 1e-5 amps = 1e8 ohms

But an LED might require 1.5v at 1mA for a 1mW drive. The LED behaves as a 1.5/.001 ohms resistor = 15K

For best wattage transfer, the internal resistor of the source should equal the load resistor. In the above example, you need to step down the piezo voltage to match the LED voltage: a factor of 670.

If this was pure AC, you could just use a stepdown transformer with a high-voltage winding to alter the piezo's output.


spxdcp wrote:5) what kind of battery would be best to harvest the generated energy?


a high-volt diode in series with a high-volt capacitor. Figure out a capacitor value that allows you to charge it up to the piezo voltage in a few seconds. Then use this to flash one of those long CCFL fluorescent tubes sold by the "PC case modder" stores such as coolerguys, etc.
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