Piezoelectric crystals are materials that exhibit the really neat property of producing a voltage when you apply mechanical stress. Here’s the simplest explanation of how it works: normally, a piezoelectric material has a bunch of charges in it, but they all cancel each other out. They are all in exactly the right position to keep a potential difference from showing up on the faces of the material. When you squeeze the material, you move those perfect alignments and can create a charge separation. Another name for a charge separation is a voltage, so if you stick a volt meter on the material in the right spot, you’ll see a voltage. The cool thing is that the process is reversible, so if you apply a voltage to a piezoelectric material, it’ll move. These properties are immensely useful in lots of different applications. Electric lighters often use small pieces of piezoelectric material to generate the spark used to ignite the fuel in the lighter. The advantage is that you never have to change the battery. When you squeeze the trigger, you’re applying stress to the material and creating that potential difference. Going the other way, the speakers in singing birthday cards are made using piezoelectric material, and by applying an audio waveform (which is really just a changing voltage), the material in the speaker will move around and create sound waves.

There are lots of different kinds of materials that have this effect, but the one I grew is called Potassium Sodium Tartrate, aka Rochelle Salt.

K-Na-L-(+)-tartrate

 

It was synthesized through the following mechanism:

KHC4H4O6 +Na2CO3 -> NaKC4H4O6

Luckily, the two reactants are easily available. Potassium bitartrate is cream of tartar, and sodium carbonate is washing soda. If you don’t have washing soda, you can make sodium carbonate through the decomposition of sodium bicarbonate (baking soda). Here are the steps I used (reproduced from here):

What you need:

  • 500 g (1 lb) of baking soda (sodium bicarbonate)[NaHCO3]
  • 200 g (7 oz) of cream of tartar (potassium bitartrate)[KHC4H4O6]
  • Oven
  • Pyrex container
  • Jar with lid
  • 500 mL (2 cup) glass beaker or Pyrex measuring cup
  • Sauce pan with water
  • 2 mL (1/2 tsp) measuring spoon
  • Spoon for stirring
  • Coffee Filter
  • Filter paper or paper towels

Baking soda to sodium carbonate reaction:

  1. Place the contents of a 500 g box of baking soda into a suitable Pyrex container.
  2. Heat in an oven at about 150 deg F (65 deg C) for one hour.
  3. Increase the temperature to 250 deg F (120 deg C) and hold there for about an hour.
  4. Repeat this increase for 350 and 450 deg F (175 and 230 deg C), for an hour each.
  5. Remove the container and allow to cool to room temperature
  6. Place the sodium carbonate into a sealed container until used further.

Sodium carbonate and potassium tartrate to potassium sodium tartrate:

  1. Place a suspension of 200 g (7 oz) (maximum) of cream of tartar in 250 mL (one cup) of water into a beaker of at least 500 mL (2 cups) capacity.
  2. Heat the beaker by placing it into a saucepan containing water.
  3. Heat the saucepan (e.g. on a stove or laboratory hot plate) until the outer water is just simmering.
  4. Add about half a teaspoon (2.5 mL) of sodium carbonate to the beaker and stir the contents. The solution will bubble.
  5. Add more sodium carbonate stepwise until no more bubbles form.
  6. Filter the hot solution by using filter paper of a coffee filter.
  7. Concentrate the solution (by evaporation) to about 400 mL or a little less by heating.
  8. Allow the filtrate to cool and then store in a cool place for several days.
  9. Collect the resulting crystals by decantation (pouring the excess liquid into another container) or by filtration.
  10. Dry the crystals by blotting with clean filter paper or paper towelling.
  11. For a better yield, concentrate again this solution left over after step 9 by heating and repeat steps 7 to 10 above.

Here are some pictures from my preparation

Initially the solution will be white

Initially the solution will be white

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Once the reaction completes, it’ll turn amber.

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I poured it off into a container to cool

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You can see crystallization beginning.

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This is after several days of sitting. I poured off the excess liquid.

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Dumping out the crystals.

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Closeup of the crystals

You’ll notice my results were not ideal. I got a weird, powdery set of very tiny crystals instead of some big ones. I have several theories as to why that happened. I think I cooled the solution too quickly, and didn’t get my solution saturated enough. Another thing you can do it use smaller crystals as a seed crystal and grow them again. So I might have seen more success if I used these crystals to try to grow bigger, single crystals.

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