Synthetic piezoelectric materials are polarized by applying a high voltage to them and heating them above their Curie point then cooling them. When a voltage of the same polarity is applied they get slightly thicker, and vice versa. Piezoelectric buzzer discs change diameter to bend the diaphragm like a bimetal disc. I have not been able to find a suitable answer as to how they polarize the ceramic disc so it changes diameter when voltage is applied between the diaphragm and back electrode rather than changing thickness. When thickness changes, the diameter changes slightly the other way. The only thing I can think of is that they are using that to make it change diameter. I read on one site that they are radially polarized, but that doesn't make sense. That would require a voltage between the middle and outside. Could someone please confirm my theory if correct, or tell me how they do it?
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\$\begingroup\$ does the thickness change without affecting the diameter? \$\endgroup\$– jsotolaCommented Jun 20, 2023 at 16:24
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\$\begingroup\$ @jsotola The crystal structure gets stretched out a bit when polarized, making it a bit thinner. When the same polarity voltage is applied, it stretches a bit more, making it slightly thinner. And, if the polarity is opposite, the structure is squeezed back a bit, making it slightly fatter like compressing an oval shaped bladder or flexible frame. That's the only way I could think of to make it change diameter when polarized face to face. I originally assumed it was unpolarized and bent the diaphragm by bending itself, but read that it changes diameter and works the way a bimetal strip works. \$\endgroup\$– Peter R. McMahonCommented Jun 27, 2023 at 23:13
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From what I am aware, polarization has two modes:
Thickness Mode: If the electric field is applied across the thickness of the ceramic disc, the polarization will cause the disc to expand and contract in thickness when an alternating electric field is applied during use. This is commonly used in applications where thickness vibration is needed.
Diameter Mode: For changing diameter, the electric field during polarization is applied parallel to the face of the disc. This means that upon application of an alternating electric field during operation, the disc expands and contracts in its diameter rather than its thickness.
I hope that helps
