Scientific publications

Young's modulus, fracture stress, and Poisson's ratio are important mechanical characteristics for micromechanical devices. The Poisson's ratio of a material is a good measure to elucidate its mechanical behavior and generally is the negative ratio of transverse to axial strain. A nanocrystalline (NCD) and an ultrananocrystalline (UNCD) diamond sample with grain boundaries of different chemical and structural constitutions have been investigated by an ultrasonic resonance method. For both samples, the elastic moduli are considerably reduced, compared with the elastic modulus of single crystal diamond (sc-diamond). Depending on the chemical and structural constitution of grain boundaries in nano- and ultrananocrystalline diamond different values for Poisson's ratio and for the fracture strength are observed. We found a Poisson's ratio of 0.201 ± 0.041 for the ultrananocrystalline sample and 0.034 ± 0.017 for the nanocrystalline sample. We discuss these results on the basis of a model for granular media. Higher disorder in the grain boundary leads to lower shear stiffness between the single grains and ultimately results in a decrease of Young's and shear modulus and possibly of the fracture strength of the material.

Size effects, including the occurrence of superparaelectric phases associated with small scale, are a significant research topic for ferroelectrics. Relevant phenomena have been explored in detail, e.g. for homogeneous, thin ferroelectric films, but the related effects associated with nanoparticles are usually only inferred from their structural properties. In contrast, this paper describes all the steps and concepts necessary for the direct characterization and quantitative assessment of the ferroelectric properties of as-synthesized and as-received nanoparticles. The method adopted uses electrical polarization measurements on polymer matrix composites containing ferroelectric nanoparticles. It is applied to ten different BaTiO3 particle types covering a size range from 10 nm to 0.8 μm. The influence of variations of particle characteristics such as tetragonality and dielectric constant is considered based on measurements of these properties. For composites containing different particle types a clearly differing polarization behaviour is found. For decreasing particle size, increasing electric field is required to achieve a given level of polarization. The size dependence of a measure related to the coercive field revealed by this work is qualitatively in line with the state of the knowledge for ferroelectrics having small dimensions. For the first time, such results and size effects are described based on data from experiments on collections of actual nanoparticles.

Thermal signature of supercapacitors are investigated in-situ and ex-situ using commercial supercapacitors. Regarding the in-situ method, four supercapacitors were connected in series, with thermocouples embedded between the supercapacitors. As the applied current was increased, the temperature measured at the intrinsic positions also increased. When cycling at a current density of 0.11 A cm−2 the centre temperature increased by 14 K compared to the stack surface temperature. This is an important figure as literature states that an increase of 10 K leads to a corresponding decrease in the lifetime by a factor of 2. Using the obtained temperature profiles, the effective thermal conductivity of the stack was found to vary between 0.5 W K−1 m−1 and 1.0 W K−1 m−1, depending on the compaction of the stack. For the ex-situ measurements, the thermal conductivity and the thicknesses of the supercapacitor material layers were measured individually in order to determine the corresponding thermal conductivity of the stack. When using this method an effective thermal conductivity of the stack of 0.53 ± 0.06 W K−1 m−1 was obtained. The analysis also demonstrated that the main contributor to the thermal resistivity and conductivity of the supercapacitor construction is the electrodes. This demonstrates that when managing heat from supercapacitors it is important to focus on the thermal conductivity of the components materials.

Sea urchin embryos produce an endoskeleton composed of two symmetric spicules that consist of calcite, containing approximately 5% magnesium. The function of magnesium ions in mineral formation in vivo and the consequence of their incorporation into the mineral on mechanical properties are largely unknown. We investigated the in vivo effects of excess magnesium ion concentrations in the medium on skeletal development of Arbacialixula. Morphological deformations of pluteus larval spicules were observed after cultivation in Mg2+-enriched seawater. Energy dispersive X-ray spectroscopy showed that magnesium ions were homogeneously distributed for complete larvae and spicule cross-sections. Magnesium ion content was quantified by inductively coupled plasma optical emission spectrometry which revealed a considerable increased incorporation of magnesium ions into spicules of larvae from Mg2+-enriched seawater. However, no change in crystal polymorph formation was observed by X-ray diffraction. Mechanical properties of spicule cross-sections were analyzed by nanoindentation and revealed significantly higher stiffness values for spicules from Mg2+-enriched seawater compared to the control, whereas no significant change in hardness values was obtained. This in vivo study shows that increased magnesium ion incorporation into sea urchin larval spicules modifies the mineral properties and supports this model to investigate the effect of minor ions on biomineralization.

Ag nanoparticles with a narrow size distribution down to 3-4 nm have been embedded in an Al2O3 matrix by a single step pulsed laser co-deposition. A two-beam laser setup and a single target which is composed of Ag and Al2O3 are used for achieving a controlled deposition. This approach allows the control of the laser fuency on each target independently. The resulting Ag/Al2O3 nanocomposite thin films exhibit a specific plasmon absorption. This easy-to-apply single step method allows precise particle size and distribution control at room temperature. Therefore the approach can be used for synthesis of various nanocomposite thin films of more complex materials on various substrates.

The strain sensitivity, i.e. the resistivity change due to mechanical strain of thin composite nano-cermet films of Pt–SiO2 was investigated. We prepared films with a thickness of 400 nm by means of co-sputtering processes at substrate temperatures around 400 °C. The specimens respond to uniform strain (ɛ = 0.2‰) with gauge factors up to 18. These gauge factors remained high at least up to 250 °C in air and also after further annealing up to 600 °C in vacuum. Therefore we state these functional films might be suitable for high temperature pressure and force sensors. The films have a relatively high film resistivity of some MΩ/sq and exhibit temperature coefficients of resistance (TCR) in the range of −2000 ppm/K up to −600 ppm/K. X-ray diffraction revealed a single crystalline fcc platinum phase while transmission electron microscopy proved a typical granular structure of the films. Pt-clusters sized 5–10 nm are embedded in an amorphous insulating matrix of silica. The composition of such nano-cermet films displaying high gauge factors is approx. 40 at% Pt, 20 at% Si and 40 at% of O.

The molecular alumosiloxanes (O-SiPh2-O-SiPh2-O)3Al2 and (O-SiPh2-O-SiPh2-O)4[Al(OH)]4 react either with water in diethyl ether, or with 1,4-butane-diol to form the new macro-cyclic compounds Al6(OH)8(O-SiPh2-O-SiPh2-O)4(O-SiPh2-O-SiPh2-OH)2·6H2O (2) or Al6(OH)8(O-SiPh2-O-SiPh2-O)4(O-SiPh2-O-SiPh2-O-CH2–CH2–CH2–CH2–OH)2 (3). As shown by single crystal structure analyses, both compounds 2 and 3 have a center of symmetry, resemble each other structurally and display in their center an [Al2(OH)8]2– unit, which is coordinated in a similar fashion to a 24-membered [Al4(O-SiPh2-O-SiPh2-O)4]2+ cycle branched at two aluminum atoms displaying either two (O-SiPh2-O-SiPh2-OH) (2) or two (O-SiPh2-O-SiPh2-O-CH2–CH2–CH2–CH2–OH) (3) arms. The [Al2(OH)8]2– groups are connected to the cycles through oxygen atoms of their hydroxide groups which link to the aluminum atoms of the ring (of the four aluminum atoms in the ring two have a double contact and two a single). Parallel to this bonding, the aluminum atoms of [Al2(OH)8]2– have either a water molecule in their coordination sphere, which is incorporated in a complex hydrogen bridged network including the silanol function (2), or are bonded to the –CH2-OH group of the siloxane-alcohol arm of the cycle (3). The aluminum atoms of the central part are in both compounds in the centers of two distorted edge sharing oxygen octahedra (mean Al–O = 1.881(7) Å (2), 1.893(7) Å (3)), while the other four aluminum atoms display a tetrahedral oxygen environment (Al–O = 1.752(8) Å (2), 1.754(8) Å (3)). Whereas all hydroxy groups of the [Al2(OH)8]2– unit in 2 are engaged in hydrogen bonding including further water molecules which make part of the network, the [Al2(OH)8]2– unit in 3 shows fewer hydrogen bridges. We have here a rare example of the same chemical species in a more hydrophilic and a less hydrophilic surrounding. This allows a detailed study of the impact of secondary hydrogen bonding on the structure.

The atomic structures of SrTiO3 (STO)/LaNiO3 (LNO)/STO heterostructure interfaces were investigated by spherical aberration-corrected (CS) (scanning) transmission electron microscopy. Atomic displacement and lattice distortion measurements and electron energy loss spectroscopy (EELS) were used to quantitatively analyze the distortion of the interfacial octahedra and the bond length at the interfaces. Combined with high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy analyses, two distinct interfacial atomic terminating layers are unambiguously determined. Ensuing quantitative HRTEM measurements revealed that the Ni-O bond length in the interfacial octahedral is elongated at the bottom interface (-NiO2-SrO-). Atomic displacement shows structural relaxation effects when crossing the interfaces and lattice distortions across the interface is more pronounced in LNO than in STO. The Ti/O atomic ratio, La and Ti relative atomic ratio as derived by EELS quantification indicate non-stoichiometric composition at the interfaces. Distinct fine structures of Ti-L2,3 edge and O-K edge at the bottom and top interfaces are observed. By comparison, we are able to estimate Ti valency at both interfaces. Combining the structural distortions and Ti valency, the polar discontinuity and charge transfer at the interfaces are discussed.

We here present the nucleation and growth of calcium carbonate under the influence of synthetic peptides on topographically patterned poly(dimethylsiloxane)
(PDMS) substrates, which have a controlled density of defects between the wrinkles. Experiments with two lysine-rich peptides derived from the extracellular conserved domain E22 of the mollusc chitin synthase Ar-CS1, AKKKKKAS (AS8) and EEKKKKKES (ES9) on these substrates showed their influence on the calcium carbonate morphology. A transition from polycrystalline composites to single crystalline phases was achieved with the peptide AS8 by changing the pH of the buffer solution. We analyzed three different pH values as previous experiments showed that E22 interacts with aragonite biominerals more strongly at pH 7.75 than at pH 9.0. At any given pH, crystals appeared in characteristic morphologies only on wrinkled substrates, and did not occur on the flat, wrinkle-free PDMS substrate. These results suggest that these wrinkled substrates could be useful for controlling the morphologies of other mineral/peptide and mineral/protein composites. In nature, these templates are formed enzymatically by glycosyltransferases containing pH-sensitive epitopes, similar to the peptides investigated here. Our in vitro test systems may be useful to gain understanding of the formation of distinct 3D morphologies in mollusc shells in response to local pH shifts during the mineralization of organic templates.