It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.

Cycling at different C-rates, as well as storage aging is performed with five different types of commercial 18650-type Lithium-ion cells. X-ray computed tomography measurements show strong deformations of the inner part of the jelly rolls for three different cell types without center pin after cycling at rates in the range of 3.6-16.6C. For cells cycled at 1C, this deformation is less pronounced, whereas it is totally absent for stored cells. The consequence on capacity loss of the cells was investigated by Post-Mortem analysis with unrolled electrodes, as well as scanning electron microscopy imaging with cross-sections of 18650 cells. In order to investigate the reason for the jelly roll deformation, we conducted in-operando temperature measurements in the middle of the jelly roll and at the cell surface during discharge with 16C for one selected cell type. Finally, the effect of a center pin on jelly roll deformation is tested by X-ray computed tomography imaging for two different cell types after cycling.

The three-dimensional orientation of polarization vectors at the surface of ferroelectric barium–titanate (BaTiO3) ceramics is evaluated using voltage-modulated scanning force microscopy (SFM). By applying an ac voltage to the conductive SFM tip, we measure the relative amount of the three orthogonal components Px, Py, and Pz, of the polarization vector at any surface point. The measured polarization orientation together with the actual domain-wall orientation allows a precise reconstruction of the crystallographic orientation of the investigated grains down to a 40 nm resolution. Excellent agreement is obtained when comparing this orientation with the crystallographic reconstruction revealed by etch patterns from the sample surface topography. We show that the surface topography manifests a domain structure, which was present in the past, while the actual ferroelectric domain configuration is revealed by the modulation technique.

The local elastic properties and the ferroelectric domain configuration of piezoelectric ceramics have been examined by atomic force acoustic microscopy and by ultrasonic piezoelectric force microscopy. The contrast mechanisms of the two techniques are discussed. From the local contact stiffness which is obtained by evaluation of the contact resonance spectra, the elastic constants of the sample surface can be calculated. In the case of anisotropic materials these elastic constants correspond to the indentation moduli. Indentation moduli for barium titanate and for a lead zirconate-titanate ceramics were calculated theoretically and are in reasonable agreement with experiments. The non-linearity of the tip–sample interaction becomes noticeable at large vibration amplitudes or large mechanical tip loads.

Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8-1.5 MPa*m(1/2) at the beginning of crack propagation up to 4.4 MPa*m(1/2) at 500 microm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m(1/2).

Dielectric breakdown decisively determines the reliability of nano-to centimeter-sized electronic devices and components. A systematic investigation of the size-dependent dielectric breakdown strength reveals a thickness-independent intrinsic regime and a thickness-dependent extrinsic regime. Besides that the breakdown strength scales with the inverse square root of the permittivity. Only recently, the intrinsic breakdown strength could be theoretically explained by density functional theory calculations, which confi rmed von Hippel's electron avalanche model. This thickness dependence resembles the difference between an intrinsic mechanical strength and a volume dependent defect size controlled Weibull mechanical strength distribution. Therefore, the hypothesis whether the thickness dependence of dielectric breakdown can be explained by a weakest link concept is discussed. Finally it is shown that the prevailing electrical conduction mechanism at the onset of dielectric breakdown is most probably dominated by space charge injection. A Griffi th type energy release rate breakdown model including space charge conductivity is presented, which allows for the explanation of the empirical results in the extrinsic regime.

Piezoresponse force microscopy (PFM) is applied to image ferroelastic formed c domains in a single crystal ferroelectric barium titanate bulk material. A simple model and an analytical approach are presented, which provides a basis to understand the complex tip-surface interactions responsible for the image contrast in PFM. In particular, the measured amplitude of the piezoresponse out-of-plane surface displacements of a C− domain is compared with theoretical results based upon a three-dimensional Green’s function solution. The electric field distribution in the tip-surface contact is determined using image-charge calculations for a spherical tip separated by a thin water layer from a mechanically isotropic and electrically anisotropic dielectric half plane.

This chapter reviews experimental results that allow one to interpret the essential features of fracture in ferroelectric ceramics under electric and mechanical load. First, crack growth measurements on unpoled and poled ferroelectric ceramics are reviewed, and numerous experiments that demonstrate the existence and relevance of a domain-switching zone are presented. Thereafter the review concentrates on results of fracture experiments with applied electric fields and addresses the controversial outcomes of such experiments. Theoretical fracture mechanical concepts are then introduced. One part of the discussion focuses on electrical boundary conditions used at the crack surface because they decisively determine the predicted energy release rate. The other part of the discussion, which concerns theoretical concepts, discusses the predicted switching zones around cracks and their influence on the stress intensity factor and energy release rate. Finally, this chapter attempts to mirror the tremendous theoretical framework against the existing experimental results and critically review the relevance of the developed fracture mechanics concepts of piezoelectric ceramics. 491 Annu. Rev. Mater. Res. 2007.37:491-538. Downloaded from www.annualreviews.org by California State University -Fresno on 09/17/13. For personal use only.