| | | PI develops and manufactures its own piezo ceramic materials at the PI Ceramic factory. The manufacturing process for high-voltage piezoceramic starts with mixing and ball milling of the raw materials. Next, to accelerate reaction of the components, the mixture is heated to 75 % of the sintering temperature, and then milled again. Granulation with the binder is next, to improve processing properties. After shaping and pressing, the green ceramic is heated to about 750 °C to burn out the binder. The next phase is sintering, at temperatures between 1250 °C and 1350 °C. Then the ceramic block is cut, ground, polished, lapped, etc., to the desired shape and tolerance. Electrodes are applied by sputtering or industrial screen printing processes. The last step is the poling process which takes place in a | | heated oil bath at electrical fields up to several kV/mm. Only here does the ceramic take on macroscopic piezoelectric properties. Multilayer piezo actuators require a different manufacturing process. After milling, a slurry is prepared for use in a foil casting process which allows layer thickness down to 20 urn. Next, electrodes are screen printed and the sheets laminated. A compacting process increases the density of the green ceramics and removes air trapped between the layers. The final steps are the binder burnout, sintering (co-firing) at temperatures below 1100 °C , wire lead termination and poling. All processes, especially the heating and sintering cycles, must be controlled to very tight | | tolerances. The smallest deviation will affect the quality and properties of the PZT material. One hundred percent final testing of the piezo material and components at PI Ceramic guarantees the highest possible product quality. | | |
| | | Because of the anisotropic nature of PZT ceramics, piezoelectric effects are dependent on direction. To identify directions, the axes 1, 2, and 3 will be introduced (corresponding to X, Y, Z of the classical right-hand orthogonal axis set). The axes 4, 5 and 6 identify rotations (shear), 0x, ©y, 9z (also known as U, V, W.) The direction of polarization (axis 3) is established during the poling process by a strong electrical field applied between two electrodes. For linear actuator (translator) applications, the piezo properties along the poling axis are the most important (largest deflection). Piezoelectric materials are characterized by several coefficients. | | Examples are: ■ djj: Strain coefficients [m/V] or charge output coefficients [C/N]: Strain developed [m/m] per unit of electric field strength applied [V/m] or (due to the sensor / actuator properties of PZT material) charge density developed [C/m2] per given stress [N/m2]. ■ g^: Voltage coefficients or field output coefficients [Vm/N]: Open-circuit electric field developed [V/m] per applied mechanical stress [N/m2] or (due to the sensor / actuator properties of PZT material) strain developed [m/m] per applied charge density [C/m2]. ■ kjj: Coupling coefficients [dimensionless]. The coefficients are energy ratios | | describing the conversion from mechanical to electrical energy or vice versa. k2 is the ratio of energy stored (mechanical or electrical) to energy (mechanical or electrical) applied. Other important parameters are the Young's modulus Y (describing the elastic properties of the material) and er the relative dielectric coefficients (permittivity). Double subscripts, as in dij, are used to describe the relationships between mechanical and electrical parameters. The first index indicates the direction of the stimulus, the second the direction of the reaction of the system. Example: d33 applies when the electric field is along the polarization axis (direction 3) and | | the strain (deflection) is along the same axis. d31 applies if the electric field is in the same direction as before, but the deflection of interest is that along axis 1 (orthogonal to the polarization axis). In addition the superscripts S, T, E, D can be used to describe an electrical or mechanical boundary condition. Definition: S for strain = constant (mechanically clamped) T for stress = constant (not clamped) E for field = 0 (short circuit) D for charge displacement (current) = 0 (open circuit) The individual piezoelectric coefficients are related to each other by systems of equations that will not be explained here. | | |