Testing the pyroelectric effect

  • #1

Main Question or Discussion Point

Hey all,

I bought some LiTaO3 crystals in order to test the pyroelectric effect. Unfortunately I am not detecting any pyroelectric voltage during heating.

According to the vendor, the Z-axis is lengthwise as shown in the attached diagram. I then put the crystal between two metal electrodes, with both ends going to an oscilloscope. The crystal is then heated with a hot air gun. The oscilloscope is giving a real time output of 0V (apart from background noise).

Any suggestions on what the problem might be? Chemical analysis by our labs confirmed the crystal to be LiTaO3. If it is not producing any voltage during heating, it could be either my setup is wrong, the crystal is polycrystalline or something I haven't thought of.

Is there any difference between LiTaO3 crystals sold for the pyroelectric vs. piezoelectric effect?

Thanks.
 

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  • #3
150
59
Some thoughts:
  • Are you sure you have a circuit? Can you measure a resistance across your probes?
  • Given the specifics of your crystal and the rate that you increase the temperature, how much voltage do you expect to see?
  • If you suspect it is polycrystalline, is the resistance greater than the resistance of a single crystal?
 
  • #4
jrmichler
Science Advisor
1,001
913
In order to read a Piezoelectric transducer, you need to apply your input to the correct crystal axis. Each axis has a different response, ranging from maximum to minimum or zero. Then you need to take the signal from the correct axis. I don't remember which axis that is. And third, a voltmeter will not measure the crystal output because even a 10 Meg input impedance loads it down so much that there is no signal left.

You need a charge amplifier to measure the output of a piezoelectric crystal. I have had good experience with both PCB (www.pcb.com) and Kistler (www.kistler.com) instrumentation.
 
  • #5
Baluncore
Science Advisor
2019 Award
7,313
2,391
As mentioned above, an oscilloscope input will short circuit the electrostatic voltage. You need an electrometer amplifier as a buffer between the crystal and the oscilloscope input.

If you want to build an electrometer buffer amplifier for your oscilloscope you will need to use low leakage circuit techniques, with the chip on it's back, "dead ant" style, insulated by air, not built on an etched PCB.

There are a many op-amps to select from;
1. I would try a TI op-amp LMC6041 which is low-cost and is listed as having typically a 2 fA input current. It runs on 15 uA so it can be battery powered. It is available from digikey.com for about $5.
2. Alternatively, the Analog Devices ADA4530 is purpose built for electrometer amplifiers but requires more power supply current, digikey price is about $50.
3. Also an outside possibility, using more power supply current but being faster, the Linear Technology LTC6268/LTC6269, digikey price is about $15.
 

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