Sylvia Gebhardt

Properties of fine scale piezoelectric PZT fibers with different Zr content

Abstract Smart materials containing piezoelectric fibers have a large potential in sensoric, actuatoric and ultrasonic transducer applications. Fine scale PZT fibers with five systematically adjusted Zr/Ti ratios ranging from x= 58 42 to 48 52 have been prepared using a sol–gel route. The fibers were embedded in a polymer matrix to produce 1-3 composites and the effective piezoelectric and dielectric coefficients were measured. An analytical model was used to extract the corresponding values of the fibers from the effective material properties. The dependence of the coefficients on the Zr/Ti ratio shows a pronounced maximum at x= 53 47 . This correlates with an analysis of the phase content. For x= 53 47 the fibers consist of 31% rhombohedral and 69% tetragonal phase, whereas the content of rhombohedral phase is 100 and 7% for x= 58 42 and 48 52 , respectively. For x= 53 47 maximum values of e33T=1200 and d33=127 pm/V were found. The latter one is about 50% of the value of corresponding bulk materials.

Quasistatic and dynamic properties of 1–3 composites made by soft molding

Abstract The development of the soft mold process allows for the preparation of fine scale 1–3 composites with PZT rods of different size, shape and spacing, which can be used as ultrasonic transducers for frequencies ⩾5 MHz. [Gebhardt, S., Schonecker, A., Steinhausen, R., Hauke, T., Seifert, W. & Beige, H., Fine scale 1-3 composites fabricated by the soft mold process: preparation and modeling. Ferroelectrics , 241 (2000) 67]. By this method, composites with a square, hexagonal and non regular arrangement of PZT rods of different shape have been fabricated and characterized by measuring their quasistatic and dynamic properties. The experimental results were compared with data from finite element method (FEM) modeling and analytical solutions. The vibrational characteristics of the composites were strongly influenced by the rod geometry and the rod arrangement. To evaluate the 1–3 composite performance, modal analysis and modeling of the impedance spectrum were carried out using the FEM package ANSYS.

Acoustic devices for particle and cell manipulation and sensing.

An emerging demand for the precise manipulation of cells and particles for applications in cell biology and analytical chemistry has driven rapid development of ultrasonic manipulation technology. Compared to the other manipulation technologies, such as magnetic tweezing, dielectrophoresis and optical tweezing, ultrasonic manipulation has shown potential in a variety of applications, with its advantages of versatile, inexpensive and easy integration into microfluidic systems, maintenance of cell viability, and generation of sufficient forces to handle particles, cells and their agglomerates. This article briefly reviews current practice and reports our development of various ultrasonic standing wave manipulation devices, including simple devices integrated with high frequency (>20 MHz) ultrasonic transducers for the investigation of biological cells and complex ultrasonic transducer array systems to explore the feasibility of electronically controlled 2-D and 3-D manipulation. Piezoelectric and passive materials, fabrication techniques, characterization methods and possible applications are discussed. The behavior and performance of the devices have been investigated and predicted with computer simulations, and verified experimentally. Issues met during development are highlighted and discussed. To assist long term practical adoption, approaches to low-cost, wafer level batch-production and commercialization potential are also addressed.

Actuators to be integrated in Low Temperature Cofired Ceramics (LTCC) microfluidic systems

LTCC technology has recently been used for microfluidic elements, e.g. channels, cavities and other passive fluidic components. However, microfluidic systems having enhanced functionality, e.g. differential pressure sensors, dosing devices and pumps, require active components that include electrically driven actuators. Up to now, only piezo-cantilevers, electromagnetic and thermo-pneumatic actuators have been engineered for the LTCC integration [1–4]. Due to their specific properties, they are not suitable for all applications that require an actuator. Therefore, a review on actuators capable of being integrated in LTCC is given. On the one hand, the actuators are compared according to their functional properties, e.g. stroke and switching energy, so that actuators can be figured out that fulfil the requirements of a microfluidic system to be designed. On the other hand, the technological challenges to be coped with in the integration of the actuators are listed. Both the functional properties of an actuator and the possibility to integrate it decide on the suitability for a specific application. Using our evaluation method, we introduced actuators for two different microfluidic applications, a piezo-electrically controlled throttle for a DMFC (Direct Methanol Fuel Cell) application and an electrostatic valve for a differential pressure sensor.

Fine scale 1-3 composites fabricated by the soft mold process: Preparation and modeling

Abstract The “soft mold” process offers the opportunity to fabricate fine scale 1-3 composites with free design of the piezoceramic rods. 1-3 composites with different volume fractions of arrayed PZT rods in a polymer matrix with diameters between 35 μm and 150 μm have been prepared successfully by this method. After poling the dielectric and electromechanical properties were determined. The experimental results were compared to calculated data from analytical approximations and modeling with the Finite Element Method (FEM).

Integrated Piezoelectrics for Smart Microsystems - A Teamwork of Substrate and Piezo

Microelectronic substrates like silicon, alumina and LTCC (Low Temperature Cofired Ceramics) allow for high robustness and reliability, 3D packaging (electrical connection, channels, cavities and membranes) as well as integration and application of electronic components whereas piezoceramic materials offer sensor and actuator operations. To combine the advantages of both, integrated solutions are of great interest. This paper deals with two approaches of monolithic integration, (i) screen printing of piezoceramic thick films on microelectronic substrates and subsequent post firing and (ii) integration of pre-fired piezoceramic components into green LTCC multilayer packages and subsequent sintering. Functionality of smart microsystems not only depends on the outer design and construction but to a great part on interaction of substrate and piezoceramic material properties. A thorough choice of materials as well as the understanding and prevention of chemical reactions are necessary to build effective systems.

A New Method for the Determination of Elastic Properties of Thin Piezoelectric PZT Fibers

For the optimization of thin piezoelectric fibers it is necessary to determine their dielectric, elastic and piezoelectric properties. We developed analytical approximations to calculate the elastic coefficient S E 33,f of the fibers from the effective coefficients C E 33,eff and S E 33,eff of 1-3 composites containing such fibers. The analytic solutions have been checked by modeling with the Finite-Element-Method and compared with experimental results of model structures containing rods of soft PZT ceramic with known material properties. The elastic coefficients of the 1-3 composites were determined using the resonance method with different sample geometries for different vibration modes. The new method was applied to characterize the elastic properties of undoped PZT fibers with a Zr-content from 48 to 58 mol%. The elastic coefficient S E 33 of the fibers shows a maximum value of 15*10 m 12 m 2 /N at a Zr/Ti ratio of 51/49.

Active electrocaloric demonstrator for direct comparison of PMN-PT bulk and multilayer samples

ABSTRACT One potential use of ferroelectrics is as active material in electrocaloric cooling systems. These systems promise a more energy efficient cooling process than vapor compression, thermoelectric or other current cooling systems. Currently different design types of electrocaloric cooling devices are in the focus of research. In this paper, we present an electrocaloric cooling device demonstrator working as “Active Electrocaloric Regenerator” (AER) and employing relaxor ferroelectric elements as active material. The device design is such that it allows the integration of different material systems and regenerator designs as well as a broad variation of operational parameters.

State transition and electrocaloric effect of BaZrxTi1−xO3: Simulation and experiment

We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1−xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1−xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature.

PMN-8PT device structures for electrocaloric cooling applications

ABSTRACT Based on fundamental research carried out on PMN-8PT bulk ceramics, screen-printed thick films and multilayer ceramic (MLC) structures have been developed. The influence of device design on the microstructural, dielectric, ferroelectric and electrocaloric properties has been investigated in detail. MLC structures turned out to be promising candidates for implementation into electrocaloric cooling systems providing large refrigerant volume and low single layer thickness. The latter allows for the application of high electric fields under low operation voltages. An electrocaloric temperature change of ΔT = 0.21 K was measured directly, by application/withdrawal of an electric field of ΔE = 2.1 kV·mm−1.