Joachim Hillenbrand
Technische Universität Darmstadt
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Featured researches published by Joachim Hillenbrand.
Applied Physics Letters | 1999
Gerhard M. Sessler; Joachim Hillenbrand
Charged polypropylene films with a cellular structure show pseudopiezoelectric properties. Measurements of the direct and inverse electromechanical transducer constants of such films, relating to the operation as sensors and as actuators, respectively, yield values of ≈200 pC/N. These values can be explained with a theoretical model by assuming reasonable charge distributions and charge densities. The experimental and the theoretical results show the reciprocity of the transducer constants.
IEEE Transactions on Dielectrics and Electrical Insulation | 2000
Joachim Hillenbrand; Gerhard M. Sessler
Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high piezoelectric constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene (pp). In particular, measurements of the direct and inverse piezoelectric constants in the frequency range 0 to 10 kHz and of the variation of these constants across the surface of the films are discussed. These measurements, performed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelectric d/sub 33/ constant of /spl lsim/ 220 pC/N. Assuming reasonable charge distributions and charge densities, the calculated piezoelectric constants are in good agreement with the measured values. The theoretical model shows the reciprocity of the piezoelectric constants.
Applied Physics Letters | 2000
Gerhard S. Neugschwandtner; Reinhard Schwödiauer; M. Vieytes; Simona Bauer-Gogonea; Siegfried Bauer; Joachim Hillenbrand; R. Kressmann; Gerhard M. Sessler; Mika Paajanen; J. Lekkala
Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric d33 coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric d33 coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.
Journal of Physics D | 2004
Xiaoqing Zhang; Joachim Hillenbrand; Gerhard M. Sessler
Expansion of cellular polypropylene films through an increase in gas pressure and subsequent pressure release at elevated temperatures prior to charging is known to enhance the piezoelectric d33-coefficient of the material. By means of a second pressure expansion the piezoelectric activity can be further increased by more than 40% in comparison with samples subjected to only a single expansion. The effectiveness of the double-expansion process must be attributed to the gain in thickness through the second expansion, following the charging and metallization processes. This thickness change causes a decrease in Youngs modulus and thus an increase in d33. Typical d33-coefficients of 1400 pC N−1 at 0.01 Hz and about 500 pC N−1 at 25 kHz have been achieved.
Applied Physics Letters | 2004
X. Zhang; Joachim Hillenbrand; Gerhard M. Sessler
The piezoelectric activity of charged cellular polypropylene films of originally 40 and 50μm thickness can be significantly increased by thickness expansion due to an exposure to high pressure for time periods of the order of hours. After such a treatment, the d33 coefficient, measured by quasistatic and interferometric methods, is found to be as high as 1200pC∕N at 0.001Hz, decreasing to 350pC∕N just below resonance. At the resonance, which is located in the range of 150–400kHz for differently treated samples, d33 coefficients up to 1400pC∕N are found. The high d33 coefficients result from a decrease of Young’s modulus Y and an increase of the chargeability of the material due to the expansion, while the decrease of d33 with frequency up to resonance is related to a corresponding increase of Y. Static pressures up to 10kPa have little influence on d33, but higher pressures result in a reversible decrease.
Journal of Applied Physics | 2007
Xiaoqing Zhang; Joachim Hillenbrand; Gerhard M. Sessler
Several layers of polytetrafluoroethylene and fluoroethylenepropylene films are fused such that small interfacial gas voids are formed between the layers. After proper charging and annealing, the fused multilayer films show large and thermally stable quasistatic piezoelectric d33 coefficients of about 1000pC∕N which are subject to a minor reduction of 3% per day if exposed to 90°C. Depending on sample processing, the piezoelectric coefficient is relatively independent of applied pressure in the range up to 20kPa and of applied frequency up to about 100kHz. Thermally-stimulated discharge measurements indicate that the decay of the piezoelectric activity at higher temperatures is at least partially due to charge drift along the surfaces of the voids. A microphone built with the cellular ferroelectrets has a sensitivity of 3mV∕Pa at 1kHz.
Journal of the Acoustical Society of America | 2004
Joachim Hillenbrand; Gerhard M. Sessler
Improvements of the sensitivity of piezoelectric microphones based on charged cellular polymer films are reported. The improvements are achieved by (1) an increase of the piezoelectric d33-coefficient of the cellular polypropylene films by pressure expansion and (2) stacking of the films. Microphones consisting of a single film of such material have sensitivities of about 2 mV/Pa at 1 kHz, independent of size, while for a microphone with five stacked films a sensitivity of 10.5 mV/Pa was measured. The equivalent noise level is about 37 dB(A) for the single-film transducer and 26 dB(A) for the stacked version. Advantages of these new piezoelectric transducers include their simple design, low cost, and small weight, as well as a large range of shapes and sizes possible.
IEEE Transactions on Dielectrics and Electrical Insulation | 2004
Joachim Hillenbrand; Gerhard M. Sessler
Piezoelectric properties of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 /spl mu/m are studied. In addition, bilayer or multilayer structures composed of one of these polymers in solid or voided form plus an additional air layer are investigated. In particular, the quasistatic and the dynamic piezoelectric d/sub 33/ coefficients are determined with electro-mechanical, optical, and acoustic methods. The quasistatic coefficients are of the order of 100 to 350 pC/N for the cellular PP and much lower for the porous PTFE. With increasing frequency up to about 50 kHz the coefficient of cellular PP decreases where it starts to rise toward the resonance at approximately 300 kHz. The bilayer or multilayer structures show considerably higher quasistatic coefficients which reach up to 20,000 pC/N for certain combinations. As for the single-layer systems, a decrease toward higher frequencies is observed. The effect is linear as a function of load pressure at relatively low pressures, ranging up to 100 Pa for some air-gap systems and up to several kPa for the cellular PP.
Journal of Applied Physics | 2005
Joachim Hillenbrand; Gerhard M. Sessler; Xiaoqing Zhang
An existing model for the piezoelectric thickness coefficient (d33 coefficient) of cellular polymers is tested with experimental data obtained from two differently manufactured cellular polypropylene (PP) materials. The model assumes the cellular film to consist of plane parallel solid and gaseous layers charged at their interfaces. The cellular PP films are expanded by a pressure treatment. Subsequently, due to viscoelastic relaxation, the thickness of the films decreases, thus causing a change of their Young’s modulus Y with time. The values of Y are obtained from interferometric measurements of the resonance frequency of the films. Together with the measured thickness of the solid layers and air layers in the material, the d33 coefficients can be determined from the model. These values are compared with experimental results for d33 also obtained interferometrically by means of the inverse piezoelectric effect. A very good agreement between the measured and calculated d33 coefficients as a function of f...
Applied Physics Letters | 2014
Perceval Pondrom; Joachim Hillenbrand; Gerhard M. Sessler; Joachim Bös; Tobias Melz
Vibration-based energy harvesters with multi-layer piezoelectrets (ferroelectrets) are presented. Using a simple setup with nine layers and a seismic mass of 8 g, it is possible to generate a power up to 1.3 µW at 140 Hz with an input acceleration of 1g. With better coupling between seismic mass and piezoelectret, and thus reduced damping, the power output of a single-layer system is increased to 5 µW at 700 Hz. Simulations indicate that for such improved setups with 10-layer stacks, utilizing seismic masses of 80 g, power levels of 0.1 to 1 mW can be expected below 100 Hz.