Publikationen

The process is a simple chemical method and aims to produce nano-structured calcium phosphate powders from natural sources, for biomedical applications. For this purpose, Atlantic Deer Cowrie (ADC) shells (Cypraea cervus Linnaeus, 1771) were collected from a local gift store in Istanbul. The empty shells were cleaned and crushed then were ball milled and sieved under 100μm. The raw powders were suspended on a hotplate stirrer for a simple chemical agitation. The temperature was kept at 80°C for 15 min. and then appropriate amount of H3PO4 was added by titration into the prepared solution to form calcium phosphate precursors. The solution was stirred on a hotplate for 8 hours then dried at 100°C for 24 hours. Afterwards the resulting dried sediments were collected and heat treated between 400-800°C for 4 hours, dependent on the required specific calcium phosphate phase. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) were carried out for identifying various hydroxyapatite (HA), tricalcium phosphate (TCP) and other calcium phosphate phases. Various particle sizes ranging from nano to micron, are obtained depending on the chemistry used and the processing technique applied during the production. A range of calcium phosphate phases can be obtained from ADC shells, by using a simple and economic conversion method. Proper cleaning methods developed and appropriate preparation techniques will enable us to use these nano calcium phosphate powders in orthopedic and dental applications.

Pulsed laser deposition (PLD) is one of the well-established physical vapor deposition methods used for synthesis of ultra-thin layers. Especially PLD is suitable for the preparation of thin films of complex alloys and ceramics where the conservation of the stoichiometry is critical. Beside several advantages of PLD, inhomogeneity in thickness limits use of PLD in some applications. There are several approaches such as rotation of the substrate or scanning of the laser beam over the target to achieve homogenous layers. On the other hand movement and transition create further complexity in process parameters. Here we present a new approach which we call Matrix Shaped PLD to control the thickness and homogeneity of deposited layers precisely. This new approach is based on shaping of the incoming laser beam by a microlens array and a Fourier lens. The beam is split into much smaller multi-beam array over the target and this leads to a homogenous plasma formation. The uniform intensity distribution over the target yields a very uniform deposit on the substrate. This approach is used to deposit carbide and oxide thin films for biomedical applications. As a case study coating of a stent which has a complex geometry is presented briefly.

Polyetheretherketone (PEEK) is considered as a substitute for metallic implant materials due to its extremely low elastic modulus (3-4 GPa). Despite its good mechanical properties, PEEK exhibits a slow integration with the bone tissue due to its relatively inert surface and low biocompatibility. We introduced a dual modification method, which combines the laser and plasma surface treatments to achieve hierarchically patterned PEEK surfaces. While the plasma treatment leads to nanotopography, the laser treatment induces microstructures over the PEEK surface. On the other hand, plasma and laser treatments induce inhomogeneity in the surface chemistry in addition to the tailored surface topography. Therefore, we coated the structured PEEK surfaces with a thin alumina layer by pulsed laser deposition (PLD) to get identical surface chemistry on each substrate. Such alumina-coated PEEK surfaces are used as a model to investigate the effect of the surface topography on the wetting independent from the surface chemistry. Prepared surfaces bring advantages of enhanced wetting, multiscaled topography, proven biocompatibility (alumina layer), and low elastic modulus (PEEK as substrate), which together may trigger the use of PEEK in bone and other implant applications.

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.