Optimization of the electro-mechanical behavior of a bimorph piezoelectric actuator for drop-on-demand techniques based on finite element method

The optimization of the electro-mechanical behavior of a bimorph piezoelectric actuator for microdrop generation is presented. The objective of this project is to enlarge the travel of this actuator which is mounted above a fluid filled chamber. Its bending inwards this chamber leads to the reduction of its volume. The generated pressure pulse leads to the ejection of a droplet out of the nozzle. The higher the travel, the higher the pressure pulse. Especially for printing high viscous media high pressure pulses are required. This microdrop generator consists of a piezoelectric transducer with surface electrodes, of a borosilicate glass diaphragm, and of a silicon chip including the fluidic components (nozzle, fluid filled chamber, throttle, and fluid inlet port). The transducer is bonded with a two component adhesive onto the glass diaphragm. Hereby, the bimorphic actuator is formed. Up to now, the electrodes have a width of 1.5 mm and they are electrically separated from each other by ablated areas with a depth of 20 μm. Each electrode belongs to one nozzle. Three nozzles are integrated in one microdrop generator. The advantage is that two other nozzles are working even if one nozzle is clogged. Within this optimization process the depth of the ablated area between the electrodes, the width of the electrodes, and the thickness of the diaphragm, of the adhesive layer as well as of the piezoelectric transducer are investigated. The simulation tool "ANSYS® 14" is used. The results show, the deeper the ablated area between the electrodes, the higher the travel. To ablate this area respectively to cut grooves through the piezoelectric material up to the glass diaphragm lead to a higher travel because the electrodes are not clamped laterally. Here, a solid state hinge characteristic enables the bending. Furthermore, widening the electrodes also leads to a higher travel because the capacitance is enlarged. Moreover, reducing the thickness of the glass diaphragm also leads to the enlargement of the travel up to a thickness of 25 μm. But during this optimization process a strong attention is paid to the manufacturability of all components with the available rapid manufacturing (RM) machines, such as laser system, dicing saw, or anodic bonding device. Glass diaphragms thinner than 100 μm are difficult to handle because the material is very brittle and the risk for damaging them during the manufacturing process of the microdrop generator is too high. For thicker diaphragms the resultant travel decreases due to the enlarged bending stiffness. The result is that a 100 μm thick glass diaphragm is chosen. The result for the adhesive layer thickness is, the thinner this layer, the higher the travel of the actuator. The adhesive has a small Young's modulus. Therefore, the direct transmission of forces is reduced for thick adhesive layers. For production-related reasons a thickness of 20 μm is chosen. All components can be manufactured with the available RM machines.