Robert Schulze

Novel heat durable electromechanical film: processing for electromechanical and electret applications

New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110degC. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels

Voided cyclo-olefin polymer films: ferroelectrets with high thermal stability

Recently, new ferroelectrets were developed from foams of polyethylene terephthalate and of cyclo-olefin polymers and copolymers. Here, we focus on ferroelectret films based on compounds of cyclo-olefin polymers. Suitable compounding and preparation led to cyclo-olefin polymer ferroelectrets which show an electromechanical activity of around 15 pC/N that is thermally stable at least up to 110/spl deg/C. We discuss the preparation, the electrical charging and the electromechanical activity, the short-and long-term thermal stability as well as the electro-acoustical properties of an eight-layer cyclo-olefin film stack. The results obtained on the cyclo-olefin foams represent a significant improvement of the service temperature of ferroelectret transducer materials.

Integration of piezoelectric polymer transducers into microsystems for sensing applications

A polymer-based sensor for low frequency acceleration detection is fabricated by using microinjection molding technologies. Finite Element simulations and characterization of the sensing functionality are done. Due to an out-of-plane acceleration a force is applied to a seismic mass (length and width each 3.2 mm, thickness 1 mm), which leads to a deformation of a connected plate with dimensions of 1 mm × 1 mm × 50 μm. Thus, charge separation at the electrodes of integrated piezoelectric polyvinylidene fluoride (PVDF) copolymer sheets occur and can be measured as sensor signal. A charge sensitivity of 0.57 pC/g is determined which is in good agreement with the simulation results. A resonance frequency of 660 Hz was measured. Furthermore, the sensor concept as well as preparation technologies to assemble a compound structure containing piezoelectric layers and the system integration by micro injection molding are discussed. In addition, different bonding techniques for the assembly of the functional components are investigated and described.

Pulsed DC magnetron sputtered piezoelectric thin film aluminum nitride – Technology and piezoelectric properties

Pulsed DC magnetron sputtered aluminum nitride (AlN) thin films are prepared on several seed layers and at different sputtering conditions. The piezoelectric c-axis (002) orientation of the AlN is analyzed with X-ray diffraction method. The transverse piezoelectric coefficient d 31 is determined with a Laser-Doppler-Vibrometer at cantilevers and membranes by analytical calculations and finite element method. Additionally, thin film AlN on bulk silicon is used to characterize the longitudinal piezoelectric charge coefficient d 33.

Fiber-reinforced composite structures with embedded piezoelectric sensors

Embedding of piezoelectric materials into fiber-reinforced composites is constituted as promising technology to develop ultra-sensitive structural health monitoring functionalities, e. g. strain measurement or acoustic emission testing. Here, we present latest results of embedding piezoelectric polymer foils into fiber-reinforced composite structures. The applied processing technique allows the implementation of several piezoelectric transducers at different positions within the fiber-reinforced composite. For studying the functionality non-destructive impact testing and three-point bending experiments were carried out to characterize the composite. Exemplarily, the functionality of the embedded sensors is demonstrated by means of triggering an embedded light emitting diode which is switched on after the piezoelectric transducer sensed mechanical strain.

Precise determination of piezoelectric longitudinal charge coefficients for piezoelectric thin films assisted by finite element modeling

A determination method to identify the piezoelectric longitudinal charge coefficient d₃₃ for thin films on substrate is presented. Finite Element (FE) simulations and measurements at PZT on bulk silicon show correlations of the substrate characteristics and electrode size subject to the displacement of an electric actuated piezoelectric material. FE models are used to identify the d₃₃ independent of the substrate and electrode characteristic. Thereby the piezoelectric longitudinal charge coefficient of 1.1 μm PLD deposited Pb(Zr_0.52Ti_0.48)O₃ (PZT) is calculated d₃₃ = 272.5 pm/V and a small standard deviation of σ = 4 pm/V for different sample geometries is analyzed.

Development and test of synthetic jet actuators based on dual transducer concept

Synthetic Jet Actuators (SJA) are micro fluidic devices with low power and high compactness. They are used for different applications that require a directed air flow. These kind of fluidic generators require zero mass input and produces non-zero momentum output. The classic design of such an actuator consists of a membrane located on one wall of a small cavity and an orifice that is typically on the opposite side of the membrane. In the new SJA concept a Helmholtz resonator is equipped with two transducers to increase the performance of the actuator. The piezoelectric membranes generate the volumetric flow symmetrically from both sides of the chamber. A common outlet connects them to the acoustic far field. A network model [1] was used for designing and optimizing the SJA. Based on this, a doublewall actuator (DWSJA) was developed. The simulation results show that using two transducers does not fully redouble the exit velocity [2], but it still shows major improvement in comparison to conventional SJA with single transducers.