Brief Introduction to Thunder Technology :
Thunder actuators and Lightning Generators are based on a piezoelectric composite technology originally patented by NASA. Face’s trademark Thunder is an acronym for THin Layer UNimorph Ferroelectric DrivER and Sensor. Face uses “Thunder” for these devices when they are designed to be used as actuators. The trademark “Lightning” is used when these piezo devices are used as energy harvesters, generators or sensors. A typical Thunder/Lightning device is composed of a thin wafer of piezoceramic such as PZT bonded to an electrically conductive substrate and a superstrate with high performance epoxy. The epoxy used in the fabrication of Thunder/Lightning device is LaRC-SI, a high performance bonding material with a very specific cure cycle that was also developed by NASA.
The manufacturing process is comprised of precise pressure and temperature cycles. The resulting piezoceramic composite has a characteristic curvature which is the result of a mismatch in thermal coefficient of expansion and the Young’s modulus of elasticity of the composite materials. The prestress within the actuator is such that the piezoceramic is in a state of compression and the substrate is in a state of tension. This prestress enhances the large deflection producing capability of Thunder/Lightning elements that is not observed in any other smart material actuators. A normal piezoceramic wafer actuator could not withstand this magnitude of curvature.
The standard Thunder/Lightning elements that Face International Corporation manufactures have a stainless steel substrate, a piezoceramic layer and an aluminum top layer. The aluminum layer serves principally as a means for soldering lead wires.
Operation of Thunder Elements:
A very important feature of Thunder actuators is the versatility of operation in addition to the extraordinary large deflections. Thunder elements can be operated in many different ways pertaining to mounting, stacking configuration and voltage application.
The method of operation can be chosen depending on the type of application with force and displacement being the two major governing physical quantities. The tabs provided with standard Thunder actuators provide a means for mounting. The method of mounting changes the force and displacement characteristics of these actuators. Thunder actuators produce comparatively higher force together with larger displacement compared to other traditional piezoelectric actuators.
In general, actuators producing high displacements are poor on the force generation capability and vice versa. A Thunder actuator can often meet bridge these two requirements. However, if more force or more displacement is required for an application, these performance characteristics can be enhanced by using multiple Thunder elements in different stacking configurations. The two main configurations commonly implemented are the parallel stack configuration and the clamshell configuration. The parallel stack configuration increases the stiffness of the actuator assembly and the clamshell reduces it. The parallel and clamshell stacking are equivalent to parallel and series spring arrangement respectively. Appropriate measures should be taken to avoid short circuit conditions when stacking Thunder actuators. Thin Teflon sheets work well as electrical insulation layers.
In addition to the stacking configurations shown, a stacking configuration comprised of a combination of these two configurations may be used as well.
The maximum voltage that can be applied to a Thunder is limited by the thickness of the piezoceramic and the direction of movement is dependant on the polarity of the driving signal. The direction of movement of the Thunder in response to a periodic driving signal is also dependant on the direction in which the Thunder is poled after it is manufactured. At Face, the Thunder elements are polarized by applying a DC voltage with the top layer always positive with respect to the bottom substrate. With this kind of polarization, the Thunder curvature decreases (attains a flatter structure) when positive voltage is applied to the top layer and the actuator curvature increases (attains a more domed shape) when substrate in subjected to positive voltage with respect to the top layer.