Piezolelectricity

[Elshan Aliyev]

Piezoelectricity effect generated by compression of certain materials. Due to compaction crystalline material releases electrical current.

 

Is there any relationship, dependence between the quantity of electrical charge or current and stress applied to crystalline?

 

I understand that piezoelectricity is the property of some materials. But there should be measure of amount of charge because of pressure applied to material. For example in all compaction pressures quartz mineral generates certain amount of charge or it depends and changes. Which of them is true?

[John Cuthber]

There is a relationship between the force, the strain and the charge (or voltage) , but the maths isn't easy

http://en.wikipedia.org/wiki/Piezoelectricity#Mathematical_description

[Elshan Aliyev]

Yes there is...But it is difficult to visualize the dependence from the source of Wiki...It would be better to see graphical relationship in order to fully understand the process. I never encountered such a research in internet. Have you ever seen like experiment or data?

 

 

[Enthalpy]

It's no that complicated...

 

Pressure, deformation, electric field, and charge are linked together.

 

If there were no piezoelectricity, in an insulator:

- pressure and deformation are linked by the elasticity

- electric field and charge (or rather, polarization) are linked by the permittivity

 

Piezoelectricity adds a coupling between mechanics and electricity, for instance an electric field can induce a pressure.

And since a deformation as well can induce a pressure, and a charge, etc, you get a relation between all four values.

 

In a crystal, this relation uses to be very linear, so proportionality factors suffice.

 

But piezoelectric crystals are not (can't be) isotropic, which is the very reason why we need more factors, which are written as a tensor.

For instance, a shear can induce an electric field - but a compression can as well, and this depends on the direction of your stress relative to the crystal's axis.

 

It also depends on how you chose the mathematical axis, but all people who survived took them in a meaningful way relative to the crystal.

 

Now, without a proper math theory, and if we knew nothing about individual materials, the tensor could have 3^4=27 independant coefficients, but because of symmetries, far fewer coefficients are needed, and use to be re-numbered with simpler subscripts which just tell "compression" or "shear" or "effect parallel to cause".

 

-----

 

Ferroelectric materials, of which PVDF is best known (BaTiO3 would be an other one), are very similar to piezoelectric ones and are often called piezoelectric. They can be isotropic initially, but a first big electric field orders them in one direction, and this persists until the next big field or heat is applied. Once they're formed, they get anisotropic, and behave as piezoelectric materials. Very interesting and useful because, as a plastic, PVDF deforms much more than a ceramic, hence is more efficient. Its acoustic impedance also matches water and human body better.

 

PVDF is a zigzag of CH2 and CF2 alternately - this alternance must be accurate over many atoms to work. The initial polarization puts many macromolecules in the same orientation, say with fluor up and hydrogen down, which makes the plastic polarized and sensitive to fields and deformations.

 

Because field and polarization deform PVDF a lot, it's more important to check if Young's E-modulus is defined in open or short circuit, permittivity at zero force or zero deformation, and so on. The effect is important enough that useful mechanical damping is obtained just by putting a resistor across a part of PVDF - this has been proposed to dampen turbulence on aircraft and boat parts.

 

Some more explanations in Wiki, and if you're lucky, at companies that build components of these materials, like Murata and TDK.

[Elshan Aliyev]

It's no that complicated...

 

Pressure, deformation, electric field, and charge are linked together.

 

If there were no piezoelectricity, in an insulator:

- pressure and deformation are linked by the elasticity

- electric field and charge (or rather, polarization) are linked by the permittivity

 

Piezoelectricity adds a coupling between mechanics and electricity, for instance an electric field can induce a pressure.

And since a deformation as well can induce a pressure, and a charge, etc, you get a relation between all four values.

 

In a crystal, this relation uses to be very linear, so proportionality factors suffice.

 

But piezoelectric crystals are not (can't be) isotropic, which is the very reason why we need more factors, which are written as a tensor.

For instance, a shear can induce an electric field - but a compression can as well, and this depends on the direction of your stress relative to the crystal's axis.

 

It also depends on how you chose the mathematical axis, but all people who survived took them in a meaningful way relative to the crystal.

 

Now, without a proper math theory, and if we knew nothing about individual materials, the tensor could have 3^4=27 independant coefficients, but because of symmetries, far fewer coefficients are needed, and use to be re-numbered with simpler subscripts which just tell "compression" or "shear" or "effect parallel to cause".

 

-----

 

Ferroelectric materials, of which PVDF is best known (BaTiO3 would be an other one), are very similar to piezoelectric ones and are often called piezoelectric. They can be isotropic initially, but a first big electric field orders them in one direction, and this persists until the next big field or heat is applied. Once they're formed, they get anisotropic, and behave as piezoelectric materials. Very interesting and useful because, as a plastic, PVDF deforms much more than a ceramic, hence is more efficient. Its acoustic impedance also matches water and human body better.

 

PVDF is a zigzag of CH2 and CF2 alternately - this alternance must be accurate over many atoms to work. The initial polarization puts many macromolecules in the same orientation, say with fluor up and hydrogen down, which makes the plastic polarized and sensitive to fields and deformations.

 

Because field and polarization deform PVDF a lot, it's more important to check if Young's E-modulus is defined in open or short circuit, permittivity at zero force or zero deformation, and so on. The effect is important enough that useful mechanical damping is obtained just by putting a resistor across a part of PVDF - this has been proposed to dampen turbulence on aircraft and boat parts.

 

Some more explanations in Wiki, and if you're lucky, at companies that build components of these materials, like Murata and TDK.

 

 

thanks for great information.

 

Is it possible to bulid up the graph of stress & electric charge or pressure & voltage in a coordinate sysytem? I am talking about piezoelectric material for instance quartz