Hans Jelitto

Measurement of the Total Energy Release Rate for Cracks in PZT Under Combined Mechanical and Electrical Loading

Four-point-bending V-notched specimens of lead zirconate titanate (PZT) poled parallel to the long axis are fractured under conditions of controlled crack growth in a custom-made device. In addition to the mechanical loading electric fields, up to 500 V/mm are applied parallel and anti-parallel to the poling direction, i.e., perpendicular to the crack surface. To determine the different contributions to the total energy release rate, the mechanical and the piezoelectric compliance, as well as the electrical capacitance of the sample, are recorded continuously using small signal modulation/demodulation techniques. This allows for the calculation of the mechanical, the piezoelectric, and the electrical part of the total energy release rate due to linear processes. The sum of these linear contributions during controlled crack growth is attributed to the intrinsic toughness of the material. The nonlinear part of the total energy release rate is mostly associated to domain switching leading to a switching zone around the crack tip. The measured force-displacement curve, together with the modulation technique, enables us to determine this mechanical nonlinear contribution to the overall toughness of PZT. The intrinsic material toughness is only slightly dependent on the applied electric field (10% effect), which can be explained by screening charges or electrical breakdown in the crack interior. The part of the toughness due to inelastic processes increases from negative to positive electric fields by up to 100%. For the corresponding nonlinear electric energy change during crack growth, only a rough estimate is performed.

Electrical conduction mechanism in bulk ceramic insulators at high voltages until dielectric breakdown

In order to develop and verify a dielectric breakdown model for bulk insulators thicker than 100 μm, the knowledge of the dominating conduction mechanism at high electric fields, or respectively voltages, is necessary. The dielectric breakdown is the electrical failure of an insulator. In some existing breakdown models, ohmic conduction is assumed as dominating conduction mechanism. For verification, the dominating dc conduction mechanism of bulk insulators at room temperature was investigated by applying high voltages up to 70 kV to the insulator until dielectric breakdown occurs. Four conduction models, namely, ohmic, space charge limited, Schottky, and Poole-Frenkel conduction, were employed to identify the dominating conduction mechanism. Comparing the calculated permittivities from the Schottky and Poole-Frenkel coefficients with experimentally measured permittivity, Schottky and Poole-Frenkel conduction can be excluded as dominating conduction mechanism. Based on the current density voltage characte...

Uniaxial compressive behavior of micro-pillars of dental enamel characterized in multiple directions

In this work, the compressive elastic modulus and failure strength values of bovine enamel at the first hierarchical level formed by hydroxyapatite (HA) nanofibers and organic matter are identified in longitudinal, transverse and oblique direction with the uniaxial micro-compression method. The elastic modulus values (∼70 GPa) measured here are within the range of results reported in the literature but these values were found surprisingly uniform in all orientations as opposed to the previous nanoindentation findings revealing anisotropic elastic properties in enamel. Failure strengths were recorded up to ∼1.7 GPa and different failure modes (such as shear, microbuckling, fiber fracture) governed by the orientation of the HA nanofibers were visualized. Structural irregularities leading to mineral contacts between the nanofibers are postulated as the main reason for the high compressive strength and direction-independent elastic behavior on enamels first hierarchical level.

Influence of small cyclic and DC electrical loads on the fracture toughness of ferroelectric ceramics

The present work investigates the influence of smal l AC, unipolar cyclic and DC electric loads on the fracture toughness of a ferroelectric ceramic. Single-edge-notched beams (S ENB) of a poled and unpoled lead zirconate titanate (PZT) ceramic were fractured in a four-point-bending device under cond itions of controlled crack growth. During crack adv nce the different electrical loads were applied perpendicular to the crack faces. The applied nominal electric field amp litudes were less than one third of the coercive field and maximum frequencies of 20 kHz were applied. The measured R-curves show t at even for these low amplitudes, electrical AC loading causes a drop in the critical mechanical load of up to 25 %. The drop increases with increasing amplitude and frequency. Possible mechan isms to explain the results are discussed.

Dielectric breakdown of alumina single crystals

The bulk breakdown behaviour of alumina single crystals with two different crystal orientations,   0 2 11 -plane (single crystal A) and   1 000 -plane (single crystal C), have been studied. Therefor plan-parallel single crystal samples were electrically loaded until dielectric breakdown was achieved. For each crystal orientation, a characteristic breakdown channel direction through the sample could be defined. In C-oriented crystals the breakdown channel originated parallel to the c-axis. For Aoriented crystals however, the breakdown channel crossed the sample in an oblique direction; the angle between crystal surface and breakdown channel was 60°. Here, the breakdown channel crossed the sample along an A-plane. Although the breakdown channel paths of A and C crystals are different, the observed breakdown strength are identical within the scatter range.

Structural Micro-Layered Ceramics with Surfaces under Tension and Compression with Increasing Apparent Fracture Toughness

Two different designs of high fracture toughness micro-laminate ceramics were produced containing 50 μm thick Si3N4 layers and 100 μm thick Si3N4 + TiN layers. The first design with external tensile layers had a predicted maximum apparent fracture toughness of 10.5 MPa m1/2. The second design with external compressive layers had a predicted maximum apparent fracture toughness of 18.0 MPa m1/2. The fracture toughness of these micro-laminates was tested by the SEVNB method. A stiff testing machine was used to measure the R-curve behavior by observing crack growth in single notched specimens. A soft testing machine was used to measure the R-curve behavior using several specimens with notches at different depths.

Metal Nanoparticle Growth within Clay–Polymer Nacre-Inspired Materials for Improved Catalysis and Plasmonic Detection in Complex Biofluids

Recent studies have shown that layered silicate clays can be used to form a nacre-like bioinspired layered structure with various polymer fillers, leading to composite films with good material strength, gas-barrier properties, and high loading capacity. We go one step further by in situ growing metal nanoparticles in nacre-like layered films based on layered silicate clays, which can be used for applications in plasmonic sensing and catalysis. The degree of anisotropy of the nanoparticles grown in the film can be controlled by adjusting the ratio of clay to polymer or gold to clay and reducing agent concentration, as well as silver overgrowth, which greatly enhances the surface enhanced Raman scattering activity of the composite. We show the performance of the films for SERS detection of bacterial quorum sensing molecules in culture medium, and catalytic properties are demonstrated through the reduction of 4-nitroaniline. These films serve as the first example of seedless, in situ nanoparticle growth within nacre-mimetic materials, and open the path to basic research on the influence of different building blocks and polymeric mortars on nanoparticle morphology and distribution, as well as applications in catalysis, sensing, and antimicrobial surfaces using such materials.

The role of carbon and tungsten disulphide nanotubes in the fracture of polymer-interlayered ceramic composites: a microscopy study

Multi-walled carbon nanotubes (MWNT) and tungsten disulphide nanotubes (WS2-INT) have been widely used to improve the strength and toughness of composite materials. The mechanisms of such improvements are extensively studied, but it is not often clear what prompts a specific reinforcement mechanism to work. In this work we prepared two similar systems reinforced with different nanofillers (MWNT and WS2-INT). Using in situ optical microscopy and post-fracture electron microscopy, we established that using different nanofillers results in a different type of fracture and a different reinforcement mechanism. When compared to non-reinforced composites both systems showed significant improvements in both strength and fracture toughness.

Experimenteller Aufbau zur Messung der Energiefreisetzungsrate für Risswachstum in PZT unter elektromechanischer Last

Four-point-bending V-notched specimens of PZT poled parallel to the long axis are fractured under conditions of controlled crack growth in a custom-made device. In addition to the mechanical loading an electric field is applied parallel or anti parallel to the poling direction. This is a typical loading condition for the application of piezoand ferroelectric ceramics, respectively. To determine the different contributions to the total energy release rate the mechanical and piezoelectric compliance as well as the electrical capacitance of the sample are acquired continuously with a special technique as a function of the crack length. The derivation of the data with respect to the crack surface allows calculating all parts of the total energy release rate on the basis of a single bending experiment for the first time. The main focus here is the presentation of the experimental method.

What Do We Know About Surface Charges on Cracks in Ferroelectric Ceramics

The present work investigates the static and time dependent electric potential distribution around cracks in a poled ferroelectric ceramic by Kelvin Probe Force Microscopy (KFM). In a first step a Vickers indentation crack in poled lead zirconate titanate (PZT) was subjected to static electric fields of up to 500V/mm in poling direction, and the potential distribution around the crack was measured. In a second step, the polarity of the applied voltage was reversed against the poling direction during the measurement of the potential. Using a simple model, an effective dielectric constant of the crack, as well as the surface charge density on the crack face were calculated as a function of the distance from the crack tip, the applied field and the time. The results are discussed with reference to free charges on the crack surface, electrically induced domain switching at the crack tip and crack bridging.