Agnes Eydam

Evaluation of polarization of embedded piezoelectrics by the thermal wave method

This work demonstrates the benefit of the thermal wave method for the evaluation of the polarization state of embedded piezoelectrics. Two types of samples were investigated: A low-temperature co-fired ceramics (LTCC)/lead zirconate titanate (PZT) sensor-actuator and a macro-fiber composite (MFC) actuator. At modulation frequencies below 10 Hz, the pyroelectric response was governed by thermal losses to the embedding layers. Here, the sample behavior was described by a harmonically heated piezoelectric plate exhibiting heat losses to the environment characterized by a single thermal relaxation time.

Polarization characterization of PZT disks and of embedded PZT plates by thermal wave methods

In this work, the thermal wave method was applied to characterize PZT disks and embedded PZT plates with regard to the polarization magnitude and spatial homogeneity. The samples were exposed to periodic heating by means of a laser beam and the pyroelectric response was determined. Thermal relaxation times (single time constants or distributions of time constants) describe the heat losses of the PZT samples to the environment. The resulting pyroelectric current spectrum was fitted to the superposition of thermal relaxation processes. The pyroelectric coefficient gives insight in the polarization distribution. For PZT disks, the polarization distribution in the surface region showed a characteristic decrease towards the electrodes.

A Laser Intensity Modulation Method for the Evaluation of the Polarization State of Embedded Piezoceramics

In this work we evaluate the polarization state of PZT plates and rods embedded into low temperature co-fired ceramics, polymers and epoxy resin by measuring the pyroelectric current spectrum as a result of temperature oscillations generated by illuminating the sample surface with a square-wave-modulated laser beam. Changes in pyroelectric current amplitude and phase produced by small periodic perturbations around the equilibrium are related to the polarization state of the embedded PZT and to thermal losses governing the return of the system to the equilibrium. The latter allows evaluating the thermal contact of PZT with the embedding material by means of transfer functions H(iω) describing amplitude attenuation and phase lag at each modulation frequency.

Evaluation of polarisation state of light metal embedded piezoelectrics

Abstract Structural components with integrated piezoceramic sensors and actuators manufactured by high pressure die casting were studied using the thermal wave method. Thermal waves were created by irradiating the surface with an intensity modulated laser beam, and the pyroelectric current was monitored in dependence on modulation frequency. Laser modulation frequencies corresponding to specified penetration depths of the thermal wave were determined by a thermal finite element analysis. The current spectrum was fitted to complex relaxation models. The embedded piezoelectric transducer was described as a homogeneously poled, harmonically heated piezoelectric plate exhibiting heat losses to the environment. Nyquist plots were used for physical interpretation as an equivalent circuit.

Reactively sputtered PMN-PT thin films for electrocaloric applications

In this work, lead magnesium niobate-lead titanate (1-x)Pb(Mg<inf>1/3</inf>Nb<inf>2/3</inf>)O<inf>3</inf>-xPbTiO<inf>3</inf>(PMN-PT) thin films with PT-fractions of 6.5 to 20% were fabricated on Pt(111)-coated Si wafers for the first time by multitarget reactive sputtering using four metallic targets (Pb, Mg, Nb, Ti).

Evaluation of the pyroelectric response of embedded piezoelectrics by means of a Nyquist plot

In this work, we evaluate the pyroelectric response of PZT plates and rods embedded in epoxy resin, low temperature cofired ceramics and polyamide by measuring the pyroelectric current spectrum originated by temperature oscillations generated by illuminating the sample surface with a square-wave-modulated laser beam. Transfer functions H(iω) describing amplitude attenuation and phase lag at each modulation frequency were analyzed by means of a Nyquist plot. Changes in pyroelectric current amplitude and phase produced by small periodic perturbations around the equilibrium are related to thermal losses governing the return of the system to the equilibrium. This allows evaluating the thermal contact of PZT with the embedding material.

Evaluation of the Polarization State of Integrated Piezoelectric Sensors and Actuators Using the Thermal Wave Method

In this work, we investigate the polarization state of a Low-Temperature-Cofired-Ceramics (LTCC)/PZT sensor-actuator and a Macro-Fiber Composite (MFC) actuator. An analytical solution for a 1-D thermal problem was derived for an embedded piezoelectric plate. Transient thermal analysis of the more complicated MFC actuator was performed using finite element modelling. At modulation frequencies below 10 Hz both modules are well described by a harmonically heated piezoelectric plate exhibiting heat losses to the environment.

Evaluation of the polarization state of piezofiber composites

In this work, the polarization state of piezofiber composites consisting of lead zirconate titanate (PZT) fibers embedded in epoxy resin was evaluated by means of thermal wave methods. The average pyroelectric coefficient was found to be one order of magnitude less than that of PZT ceramic plates. The pyroelectric current spectrum was recorded during irradiating the composites with an intensity-modulated laser beam. We obtained a homogeneous polarization of the samples and were able to detect differences in the polarization magnitude. Additionally, the thermal conductance at the interface between PZT and epoxy resin was estimated. The thermal diffusivity of the material was determined by three different methods: (i) by means of the frequency dependence of the pyroelectric coefficient, (ii) by the laser flash analysis, and (iii) by calculation based on a parallel connection model. Its value yielded approximately 0.4 mm2/s.

Polarisationsbestimmung integrierter Piezokeramiken mittels Wärmeschwingungen und Wärmeimpulsen

Zusammenfassung In Leichtbaustrukturen wird der Polarisationszustand von integrierten Piezokeramiken mittels thermischer Anregung zerstörungsfrei geprüft. Wärmeschwingungen (Laser-Intensitäts-Modulations-Methode) oder Wärmeimpulse führen zu einem pyroelektrischen Strom im Frequenz- bzw. Zeitbereich. Das Frequenzspektrum wird analytisch mit Hilfe thermischer Relaxationszeiten, die den Wärmeverlust charakterisieren, beschrieben. Die Signale im Zeitbereich werden fourier-transformiert und mit der Übertragungsfunktion der Messstrecke korrigiert. Außerdem wird die mittlere Temperatur der Probe analytisch und numerisch ermittelt, um daraus das Zeitsignal des Stromes abzuleiten. Abstract In lightweight structures, the polarization state of integrated piezoceramics is evaluated nondestructively by thermal excitation. Thermal oscillations (laser intensity modulation method) or thermal pulses lead to a pyroelectric current in frequency or time domain. The frequency spectrum is described by an analytical model with thermal relaxation times which characterize the heat losses. The signals in time domain are Fourier-transformed and corrected by the transfer function of the measurement set-up. Furthermore, the mean temperature of the sample is determined analytically and numerically to derive the current in time domain.

Thermal excitation as a mean for nondestructive evaluation of embedded piezoelectric transducers

In this work, we present analytical and finite element method (FEM) modeling to describe the time dependence of the pyroelectric current of embedded piezoelectrics generated by a laser pulse. Analytical solutions of the one-dimensional heat transfer equation consider a two-layer model, account for the experimentally derived heat pulse shape and the heat loss to the environment. They serve as a proof of the more complex FEM modeling which allows the consideration of three-layer models. It provides an understanding of the heat transfer in structurally complex devices excited by laser pulses.