Rochelle salt-DIY Piezoelectric Crystals

I was introduced to the principle of Piezoelectricity after watching a video describing how to build a simple contact microphone.

( you can watch it here: DIY Contact Mic – Collin’s Lab (youtube.com) )

Originally invented by ###

When a molecular structure lacks a center of symmetry and gets compressed, the uneven arrangement of atoms causes a charge separation. This results in a net positive charge at one end of the molecule and a net negative charge at the other end. Compression intensifies this effect by altering the spatial arrangement of the atoms.

Rochelle Salt is an easy and safe chemical to make. In it’s crystal is a piezoelectric material.

Ingredients required:

1 cup Distilled water (or Deionized [DI] water).
80g Cream of Tartar.
2 Tablespoons of Sodium Carbonate (Can make by heating baking soda (Sodium Bicarbonate).
Coffee filter (or equivalent)
Optional: Activated charcoal

Full reaction:

KHC₄H₆O₆+NaHCO₃→KNaC₄H₆O₆+H₂O+CO₂ –crystallization–→ KNaC₄H₆O₆·4H₂O

1. Take 1 Cup Distilled or DI water in a Pyrex measuring cup

2. Place the water filled measuring cup in a pot filled with simmering tap water.

3. Mix 80g Cream of Tartar into the water solution

KHC₄H₆O₆

4. Add a tablespoon of Sodium Carbonate. Prepare for mixture to bubble by producing CO₂. After bubbling is finished, add another spoonful, wait for bubbling to subside, repeat until reaction is finished. Solution should become translucent with a yellow hue.

KHC₄H₆O₆+NaHCO₃→KNaC₄H₆O₆+H₂O+CO₂

*Here’s what the solution should look like before after reaction is finished.

KNaC₄H₆O₆+H₂O

5. Clear solution will have some unreacted Sodium Carbonate that will need to be filtered out. Pour the solution into coffee filter or equivalent (optional to have a layer of activated charcoal to make solution more transparent/remove yellow color).

6. Pour filtered solution into a clean plastic or glass evaporation dish. I used a wide plastic Tupperware container. I placed the lid on loosely to slow evaporation a bit to give the crystals more time to form uniformly.

7. Place the container in a mildly cool place overnight (or until you see satisfactory crystal growth). The water will evaporate from the supersaturated solution and the salt will collect together in a lattice structure KNaC₄H₄O₆·4H₂O

KNaC₄H₆O₆+H₂O

* The results I got the next day.

KNaC₄H₆O₆·4H₂O + H₂O

* The small crystals were saved. They can be used as seeds for growing larger crystals in the future if I feel like it.

KNaC₄H₆O₆·4H₂O

* This one looked good for piezoelectric testing.

Using a plastic locking clamp with copper tape on each jaw, I clamped electrical contact to the top and bottom of the salt crystal.

I used alligator clips to connect the top and bottom of the crystal to my oscilloscope probe.

Here’s an example signal from tapping the clamp against my desk. The negative 2V peak appears as the crystal would compress from the blunt force and the positive 1.5V peak appears as the crystal decompressed due to elasticity.

The opposite reaction will occur as supplying a potential across the crystal will cause it to mechanically deform. Connecting the crystal to any diaphragm with an audio frequency signal will make a speaker/buzzer.

Here is a diagram of The Rochelle-Salt Crystal Reproducer from the July 1932 Radio-Craft magazine (page 15):