Comparison with Calculation Results

78 The deflection of the cantilever can be estimated from the y -direction deformation from the ANSYS simulation results. The deflection difference between composite and single material structures is significant when the length of the structure increases. These simulation results verify the fact that as the length of a cantilever structure increases, the resonant frequency decreases. Once the resonant frequency is reduced, the cantilever would experience a greater magnitude of deflection at a constant acceleration level. For a cantilever of length 20 mm, a single material structure produces as much as three times the magnitude of deflection produced by a composite structure as shown in Figure 3-32 c. This shows that the electrode layers which are stiffer than PZT play an important role in reducing the deformation of the structure when excited to its resonance. As both of the structures were excited with the same excitation level, the maximum stresses on x -direction for both structures are similar, as shown in Figure 3-32 d. These simulation results show that a material with higher elastic modulus can be added on the outer layer of the composite structure in order to protect the more fragile and brittle piezoelectric material from overstress at the centre of the composite structure, since the stress increases with the distance from the neutral axis to the centroid of the material. From the ANSYS simulation results for a single material structure consists of PZT and a multilayer structure consists of PZT and AgPd electrodes, it can be concluded that the natural frequency and the maximum deflection of a cantilever structure depends on the elasticity of the individual layer. The theoretical calculation results for a composite structure are in a good agreement with the ANSYS simulation results for a composite structure. This verifies that the model developed in section 3.4 is reasonable good to be used to estimate the performance of a free-standing cantilever, therefore will be used in the following chapter. 79 Figure 3-32: Comparison between ANSYS simulations and theoretical calculation results on its natural frequency a, maximum cantilever tip acceleration b, maximum stress c and maximum deformation on the tip of the cantilever d. a 1l 2 mm -2 b c d 80

3.7 Screen Printing Design

A free-standing cantilever structure as shown in Figure 3-33 was designed with various length from 5 mm to 20 mm with a fixed width at 10 mm. The effective length of the free-standing structure is the part where it is printed above the sacrificial layer. The sacrificial layer is deliberately designed to be 1 cm wider peripherally than the actual part of the free-standing structure, in order for it to be dissipated effectively when co- fired at high temperature. The lower and upper electrodes are designed to be 0.5 mm narrower peripherally than the piezoelectric material. This is to give a leeway for printing tolerance, preventing a short connection between upper and lower electrodes. Figure 3-33: A free-standing cantilever structure design layout. Each layer of the composite structure was designed with Autodesk Inventor software www.autodesk.com and converted separately into photo-plotter format eg. Gerber, HPGL, DXF or DWG which would than translate into a patterned thick-film printing screen. The layout of the sandwiched composite free-standing structure is shown in Error Reference source not found. a. In total, five printing screens are needed to fabricate a sandwiched layer composite structure. The lower electrode screen can be reused for printing the upper electrode by rotating the screen through 180°. In this research, an IDE cantilever structure will also be investigated, and only one extra screen with an IDE pattern is needed as shown in Error Reference source not found. b. Potential Free- Standing Structure l w Solder Pad Sacrificial Layer Anchor 81 Figure 3-34: Layouts of a plated electrode a and an IDE cantilever structure b. 1 2 3 4 5 1 2 3 4 5 a b Gold layer Carbon sacrificial layer Electrode Layer Piezoelectric layer