Michelle Müller

Junctions between metals and blends of conducting and biodegradable polymers (PLLA-PPy and PCL-PPy)

The junctions between newly developed biodegradable conducting polymers (polylactide-polypyrrole PLLA-PPy and polycaprolactone-polypyrrole PCL-PPy) and metal electrodes (Au, Au/Cu, Ag, Ag/Cu, Cu, Cr/Au/Cu, Pd/Au/Cu, Pt/Au/Cu) were studied. The objective was to determine the composite/metal combination having the lowest possible contact resistance and ohmic characteristics. In a first step, different surface treatments, adhesion and metal layers were tested in order to evaluate the contact resistance. Then the current-voltage (IV) characteristics were measured and both ohmic and rectifying behaviour were observed depending on the polymer/metal junctions investigated. The surface treatments studied included an argon sputtering step and a grinding of the polymer surface with the objective of improving the contact between the metal electrode and the polymer. It was found that the most favourable conditions resulted from a process flow without argon sputtering, without grinding for PLLA-PPy and with a slight grinding for PCL-PPy. Moreover the most favourable metal electrodes for PLLA-PPy were Pd/Au/Cu, while the best compromise for PCL-PPy was to use Au/Cu. For the rectifying polymer/metal junctions, the standard thermionic emission model modified with a series resistance was successfully applied to the measured current-voltage IV characteristics. The saturation current density J0, series resistance R, ideality diode factor n and barrier height φB were investigated. The Chot functions were computed for each rectifying junction and the corresponding threshold voltages were calculated. Finally the conductivity of both composites was evaluated as a function of temperature in the range of 30 °C to 80 °C. For PLLA-PPy a decrease of the resistivity was observed when the temperature was increasing, while no clearly recognisable pattern was identified for PCL-PPy in this temperature range. The electrical conductivity of the PLLA-PPy samples was found to follow the empirical Arrhenius model, and the difference between the Fermi energy EF and the mobility edge EC | EF - EC | as well as the conductivity at the mobility edge σC were evaluated. Moreover the electrical conductivity of the PLLA-PPy samples was found to follow the Mott variable range hopping (VRH) model, and the high temperature limit of conductivity σ1 as well as the Mott characteristic temperature T1 were calculated.

A passive micromechanical broadband amplifier for acoustic emission sensing

A novel mechanical amplification mechanism based on a coupled mass-spring system is presented. The mechanism effectively transduces and amplifies structural vibrations within a broad frequency range into out-of-plane motion and needs no electrical power supply. The concept is verified experimentally on two designs consisting of 4 and 8 coupled masses respectively. An average amplification factor of 17.3 (57.8) is achieved for the four (eight) mass design within a bandwidth of 17.3 (6.5) kHz. The proposed mechanism could find uses in ultra-low power detection of weak acoustic or micro-seismic signals for structural health monitoring (e.g. cliffs, buildings and bridges).

Transfer function tuning of a broadband shoaling mechanical amplifier near the electrostatic instability

We present a tunable broadband shoaling mechanical amplifier and a method to extend its operation near the electrostatic pull-in instability. The model has been verified experimentally on a vacuum encapsulated silicon MEMS device. We show that by adding an appropriate mechanical compensation spring, the amplifier can be operated near the pull-in instability in a quasi-linear fashion. Furthermore, electrostatic band-pass region and amplification tuning is shown.

Off-Resonant Vibration Amplifier With Flattened Band-Pass Characteristic and Improved Axis Selectivity

We present the design, fabrication, and characterization of an in-plane vibration sensor with frequency selective displacement amplification and differential capacitive read-out. The mechanical structure is based on six resonators with decreasing stiffness coupled in-plane. A differential capacitance attached to the last mass serves as electrical read out. Finite element and lumped element models are both presented. The devices were fabricated in a single mask silicon on insulator-based process. The mechanical, as well as the capacitive transfer function and the pressure dependence, have been investigated experimentally and compared with simulations. The measured mean (minimum) amplification was 24 dB (16 dB) over a bandwidth of 10 kHz (3–13 kHz). While the mean amplification is pressure dependent, the minimum amplification and bandwidth show a less than 10% decrease over a wide pressure range from 6.3 to 64 mbar. The pressure dependent measurements also show that the minimum amplification is independent of the Q factor of the modes down to values of Q~10. Both simulation and experiment show that the off-axis modes occur outside the bandwidth of the device. Along with the low cross-sensitivity of the capacitive readout (0.06%), this provides good axis selectivity despite the high number of degrees of freedom. The device can be used for detection of broadband vibration signals, e.g., for structural monitoring of infrastructure such as bridges and pipelines.