Piezo Elements: Why “co-fired” Piezo stacks are the Leading Technology

 

The basic piezo element is a plate made of piezo material which expand in the presence of an electric field E. Or in other words, if an electrical voltage difference is applied to top and bottom electrodes of the plate it expands. The plate is fully covered on top and bottom faces with conductive material to form electrodes. The phenomenon of expansion in direction of the applied Field E is called "indirect piezoelectric effect". When a force or pressure is applied to the plate an electric voltage difference is generated at the electrodes. This is called "direct piezoelectric effect".  The piezoelectric materials contracts in transverse directions as any structural material when it lengthens when subjected to a force. This is also the case when the elongation is caused by the piezoelectric phenomena - which describes the generation of an internal stress caused by the applied electric field. Piezoelectric elements are made of special ceramics. After sintering the piezoelectric material and processing into a body with electrodes, the piezo element is "polarized" by applying an electric voltage at the electrodes. The piezoelectric material has - like ferroelectric materials - an internal structure of domains of coupled electric dipoles. By common definition, the direction of polarization is indexed with 33. The coefficient of expansion in direction of the polarization field is named d33 and is a positive value. The contraction in transverse direction is negative and named d31 and is a negative value due to the general laws of mechanics.    

Piezo actuation effect

To generate displacement with low voltages, piezo plates are stacked and  connected electrically parallel.

 


Piezo stacks - unique design by applying external insulation structures
Stack design principle - externally applied insulation structures to insulate adjacent layers of opposite voltages

A preferred stack design of a ceramics multilayer device. The electric's layers cover the full surface of the piezo plates. A stack of piezo plates in "green" state, all covered with with electrode paste containing platin and silver are co-fired. There is also two external electrodes necessary to contact the multitude of "internal" electrical electrodes. Insulation is necessary between  All antipole electrodes are alternately insulated on the side faces. The unequal reciprocal internal electrodes and the two + and - external electrode strips, which provide parallel connection of all plots of the stack.  The preferrer method to do this is to realize small external insulation of the external layers and reciprocal internal electrode layers. This method has a fundamental advantage. The field in all the piezo material of a stack is ideally uniform and so there are no bent electrical field lines which would cause non-uniform mechanical stress. The lifetime of those devices is significantly increased.That is why a stack with external insulation of internal electrodes is leading technology.

The above presented stack technology is the benchmark solution for perfect piezo engineering: Full cover electrodes for field homogeneity and on face insulation of the internal electrodes for excellent performance and life. 

Piezo stack with internal insulation of layers with opposite voltages
Piezo stack actuator with conventional inner electrode electrode insulation (green) . The electrodes does not generate homogenous filed.

The above drawing represent a conventional insulation technique applied in many stack types on the market. The conductive electrode surfaces are alternately interrupted in the edge areas in order to insulate the inner electrode (e.g. V-) surfaces from the outer surfaces with opposing voltage (e.g. V+). IThis requires the interruption of the inner electrode surfaces to the outer surfaces of the stack, which are coated with external electrode surfaces. This construction effectively insulates electrode surfaces with opposing voltages. However, it leads to inhomogeneous fields in the edge areas which are associated with inhomogeneous mechanical stress distributions. In the event of prolonged operation, this can possibly lead to the formation of cracks in the piezoceramic and failure of the stack during continuous operation and reduced life.

Thus, an advantageous implementation of a piezo stack with regard to ideal homogeneous deformation and associated mechanical stress distribution is a perfect electrostatic capacitor as sketched in the first drawing, where two parallel electrode plates fully cover the piezo material and generate a homogenous electric field in the piezo material.