Selective Stiffening for Enhancing and/or Reversing the Action of Thermoelastic Actuators

We present a thermoelastic micromotor for driving a rigid plate in an out-of-plane motion. The plate is suspended on spiral thermoelastic bimorph arms, made of a nitride layer coated by gold. Heating of the thermoelastic arms is achieved by driving electric current, either through polysilicon resistors that are embedded in the nitride layer, or through the gold layer itself. Heating generates isotropic internal bending moments in the bimorphs. We show that this bending produces only small deflections of the plate. To improve the performance of the actuator, we use selective stiffeners to convert bending into torsional deformation of the spiral arms. In one orientation of the stiffeners, this conversion may be used to achieve a seven-fold increase of the thermoelastic response of the actuator. In a different orientation of the stiffeners, a complete reversal of the enhanced thermoelastic response is achieved.