Wolfgang Pompe

Functionally graded materials for biomedical applications

Functional gradation is one characteristic feature of living tissue. Bio-inspired materials open new approaches for manufacturing implants for bone replacement. Different routes for new implant materials are presented using the principle of functional gradation. An artificial biomaterial for knee joint replacement has been developed by building a graded structure consisting of ultra-high molecular weight polyethylene (UHMWPE) fibre reinforced high-density polyethylene combined with a surface of UHMWPE. The ingrowth behaviour of titanium implants into hard tissue can be improved by depositing a graded biopolymer coating of fibronectin, collagen types I and III with a gradation, derived from the mechanisms occurring during healing in vivo. Functionally graded porous hydroxyapatite (HAP) ceramics can be produced using alternative routes, e.g. sintering of laminated structures of HAP tapes filled with polymer spheres or combining biodegradable polyesters such as polylactide, polylactide-co-glycolide and polyglycolide, with carbonated nanocrystalline hydroxyapatite. HAP–collagen I scaffolds are an appropriate material for in vitro growth of bone. The scaffold has to be functionally graded in order to create an optimised mechanical behaviour as well as the intended improvement of the cell ingrowth.

Construction of highly conductive nanowires on a DNA template

We present measurements of the electrical conductivity of metallic nanowires which have been fabricated by chemical deposition of a thin continuous palladium film onto single DNA molecules to install electrical functionality. The DNA molecules have been positioned between macroscopic Au electrodes and are metallized afterwards. Low-resistance electrical interfacing was obtained by pinning the nanowires at the electrodes with electron-beam-induced carbon lines. The investigated nanowires exhibit ohmic transport behavior at room temperature. Their specific conductivity is only one order of magnitude below that of bulk palladium, confirming that DNA is an ideal template for the production of electric wires, which can be utilized for the bottom-up construction of miniaturized electrical circuits.

Domain configurations due to multiple misfit relaxation mechanisms in epitaxial ferroelectric thin films. III. Interfacial defects and domain misorientations

Interfacial defect theory is applied to the epitaxial ferroelectric system consisting of a tetragonal ferroelectric such as BaTiO3 or PbTiO3 grown onto a cubic (001) substrate. The interfacial defects that result from the diffusionless paraelectric to ferroelectric (PE→FE) phase transition are treated under the constraint that no misfit dislocations are generated during or as a result of the transition. The domain pattern develops to provide strain relief in the film. The interfacial defects for the ...c/a1/c/a1... domain pattern include coherency edge dislocations and coherency wedge disclinations. Interfacial defects for the ...a1/a2/a1/a2... domain pattern include coherency edge and screw dislocations. Far‐field strain states for both domain patterns can be predicted from the interfacial defect content. From the twinning geometry, general expressions are derived for the far‐field rotations of the crystal axes of individual domains for the ...a1/c/a1/c... and the ...a1/a2/a1/a2... domain pattern. The ge...

Diameter grouping in bulk samples of single-walled carbon nanotubes from optical absorption spectroscopy

The influence of the synthesis parameters on the mean characteristics of single-wall carbon nanotubes in soot produced by the laser vaporization of graphite has been analyzed using optical absorption spectroscopy. The abundance and mean diameter of the nanotubes were found to be most influenced by the furnace temperature and the cobalt/nickel catalyst mixing ratio. Via an analysis of the fine structure in the optical spectra, the existence of preferred nanotube diameters has been established and their related fractional abundance could be determined. The results are consistent with nanotubes located mainly around the armchair axis.

Solid–Liquid–Solid Growth Mechanism of Single-Wall Carbon Nanotubes

Abstract Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid–liquid–solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon–metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.

Modeling of threading dislocation reduction in growing GaN layers

In this work, a model is developed to treat threading dislocation (TD) reduction in (0 0 0 1) wurtzite epitaxial GaN thin films. The model is based on an approach originally proposed for (0 0 1) FCC thin film growth and uses the concepts of mutual TD motion and reactions. We show that the experimentally observed slow TD reduction in GaN can be explained by low TD reaction probabilities due to TD line directions practically normal to the film surface. The behavior of screw dislocations in III-nitride films is considered and is found to strongly impact TD reduction. Dislocation reduction data in hydride vapor phase epitaxy (HVPE) grown GaN are well described by this model. The model provides an explanation for the non-saturating TD density in thick GaN films. r 2001 Elsevier Science B.V. All rights reserved.

Biocers: ceramics with incorporated microorganisms for biocatalytic, biosorptive and functional materials development

Biologically modified ceramics (biocers) are understood as a class of nanocomposites which combine biocomponents with ceramic-like matrices. Biocers containing biocomponents can be prepared as a bulk material or as coatings by sol–gel and freeze-cast techniques from inorganic nanosols or by special CVD methods. By avoiding critical preparation conditions (high temperature, organic solvents) which would lead to denaturation, even bacteria, fungi, and yeast cells can be incorporated while maintaining their viability (‘living ceramics’). In this article the preparation and structure of such biocers and their applicative potential for biocatalytic, biosorptive and structure-forming processes will be discussed.

Scaling laws for the reduction of threading dislocation densities in homogeneous buffer layers

In the heteroepitaxial growth of films with large misfit with the underlying substrate (linear mismatch strains in excess of 1%–2%) the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggest that the TD density in relaxed homogeneous buffer layers can be divided into three regimes: (i) an entanglement region near the film/substrate interface corresponding to TD densities of ∼1010–1012 cm−2; (ii) a falloff in TD density that is inversely proportional to the film thickness h, applicable to densities in the range ∼107–109 cm−2; and (iii) saturation or weak decay of the TD density with further increase in film thickness. Typical saturation densities are on the order of ∼106–107 cm−2. In this article, we show that the TD reduction may be described in terms of effective lateral motion of TDs with increasing film thickness. An analytic model is developed that successfully predicts both the 1/...

Stress development in alumina scales formed upon oxidation of (111) NiAl single crystals

Abstract Stress evolution in the α phase of the alumina scale formed on single crystals of (111) NiAl as a function of oxidation time at 1100 °C is reported. The measurements are made at room temperature using the fluorescence from trace Cr 3+ impurities in the NiAl that are incorporated into the oxide scale during oxidation. After a short transient period, the room-temperature stress is essentially independent of oxidation time. At all oxidation times, there exist large stress gradients through the scale thickness, indicating that either fresh oxide is formed non-uniformly through the thickness of the growing scale or stress relaxation occurs non-uniformly.

Domain pattern formation in epitaxial rhombohedral ferroelectric films. II. Interfacial defects and energetics

The coherency defect technique is developed for the domain pattern energetics in rhombohedral (001) epitaxial ferroelectric films. The coherency defects that are necessary to maintain the epitaxy during the ferroelectric phase transition are considered to be the only sources of elastic strains and stresses (and, correspondingly energy) in the film/substrate system. The coherency defects include: (i) a uniform distribution of edge dislocations which are responsible for the in-plane tension or compression and have Burgers vectors parallel to the interface; and two kinds of mesoscale defects: (ii) Somigliana screw dislocations which are responsible for in-plane shear; and (iii) wedge disclinations which are responsible for the out of plane rotations in neighboring domains. Using this approach, analytical expressions were found for the elastic energy in the film/substrate system for both the {101}-ri/rj and the {100}-ri/rj domain patterns. These two configurations differ by the orientation of domain walls, co...