Johannes Bernardi

Stability and Photoelectronic Properties of Layered Titanate Nanostructures

Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na(2)Ti(3)O(7) nanowires and scrolled up H(2)Ti(3)O(7) nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na(2)Ti(3)O(7) nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H(2)Ti(3)O(7) nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures T >OR= 670 K. Photoexcitation experiments in O(2) atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O(-) and O(2)(-) species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.

Microstructural analysis of strip cast Nd–Fe–B alloys for high (BH)max magnets

High energy density magnets >400 kJ/m3 are increasingly used in many applications. Conventional casting techniques for sintered magnets reveal the formation of a high quantity of α-Fe and large Nd-rich regions. New techniques, like strip casting, produce homogeneous and fine scaled microstructures and are already used for producing high (BH)max magnets. The fast cooling rate during strip casting suppresses the formation of α-Fe dendrites and of large Nd-rich pockets. Directional solidification causes a formation of columnar grains containing a typical arrangement of hard magnetic Nd2Fe14B regions and Nd-rich regions. The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a periodicity which corresponds to a eutectoidal solidification according to the composition of the liquid and are directed parallel to the temperature gradient during solidification. The lamellae show an average width of 150 nm, a spacing of 3 μm, and a length up to the size of the ha...

Transmission electron microscope characterization of cast and hot‐worked R‐Fe‐B:Cu(R=Nd,Pr) permanent magnets

The hard magnetic properties of hot‐worked R‐Fe‐B permanent magnets are improved by the addition of a minor amount of Cu (up to 2 at. %). Transmission electron microscope investigations show that the reason for the increase of the coercivity is the fact that Cu is mainly dissolved in the intergranular region separating the hard magnetic 2:14:1 grains. The results obtained are similar in Nd‐Fe‐B:Cu and Pr‐Fe‐B:Cu magnets. The intergranular region has been observed to consist of a eutectic consisting of two phases: the Nd‐or Pr‐rich phase and the orthorhombic NdCu‐ or PrCu‐phase. This corresponds to the eutectic decomposition of the liquid intergranular phase during cooling. It is suggested that the decrease of the melting temperature of the intergranular phase leads to a better wetting behavior and separation of the hard magnetic grains, and therefore to an enhancement of the coercivity.

Cation diffusion in La0.6Sr0.4CoO3−δ below 800 °C and its relevance for Sr segregation

Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.

Microstructure and mechanical properties of HVOF sprayed nanocrystalline Cr3C2-25(Ni20Cr) coating

The objectives of this work are to deposit nanocrystalline Cr3C2-25(Ni20Cr) powder by thermal spraying and to compare the performance of this coating with that obtained using conventional powder. Towards that purpose, Cr3C2-25(Ni20Cr) powders with nanocrystalline grain size and with conventional grain size were deposited using OSU-SJS high-velocity oxyfuel (HVOF) system. The microstructural features, such as morphology of the coated surface, thickness of the coating, the interface of the coating with the substrate, distribution of various phases, and grain sizes etc, were characterized with the help of optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM). The amount of oxide phases and pores were determined by means of image analyzer. The presence of various phases was identified by x-ray diffraction (XRD) technique. Hardness, elastic modulus, and indentation toughness were evaluated employing micro indentation technique. The results indicate the presence of three different zones containing only orthorhombic Cr3C2 phase, FCC NiCr phase, and mixture of Cr3C2 and NiCr phases in, both coatings. The grain sizes in the nanocrystalline coating were in the range of 80 to 100 nm. Nanocrystalline coating exhibits 20% increase in hardness, 40% decrease in surface roughness, and comparable fracture toughness and elastic modulus with respect to conventional coating.

Preparation and TEM-study of sintered Nd/sub 18/Fe/sub 74/B/sub 6/Ga/sub 1/Nb/sub 1/ magnets

Ga-doped Nd-Fe-B sintered magnets show an improved wetting behavior of the liquid phase during sintering and the formation of additional Ga-containing intergranular phases. Nb forms precipitates within the hard magnetic grains and enhances the coercivity. In the as-sintered magnets, a new Nd-rich Ga-containing region which contains Nd/sub 3/Ga and Nd/sub 5/Ga/sub 3/ is found. This region is formed from the liquid phase during cooling. In addition, the delta -phase occurs in the annealed magnets. Nb is found in the hexagonal NbFeB intergranular boride and in small Nb-containing precipitates within the hard magnetic phase. There is no Nb detected in the Nd-rich phases. >

Extracellular bone matrix exhibits hardening elastoplasticity and more than double cortical strength: Evidence from homogeneous compression of non-tapered single micron-sized pillars welded to a rigid substrate.

We here report an improved experimental technique for the determination of Young׳s modulus and uniaxial strength of extracellular bone matrix at the single micrometer scale, giving direct access to the (homogeneous) deformation (or strain) states of the tested samples and to the corresponding mechanically recoverable energy, called potential or elastic energy. Therefore, a new protocol for Focused Ion Beam milling of prismatic non-tapered micropillars, and attaching them to a rigid substrate, was developed. Uniaxial strength turns out as at least twice that measured macroscopically, and respective ultimate stresses are preceded by hardening elastoplastic states, already at very low load levels. The unloading portion of quasi-static load-displacement curves revealed Young׳s modulus of 29GPa in bovine extracellular bone matrix. This value is impressively confirmed by the corresponding prediction of a multiscale mechanics model for bone, which has been comprehensively validated at various other observation scales, across tissues from the entire vertebrate animal kingdom.

Solid−Solid Interface Formation in TiO2 Nanoparticle Networks

Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO(2) nanoparticle networks, we subjected entirely different TiO(2-x) precursor structures to vacuum annealing. The transformation of an amorphous TiO(2-x) gel--obtained by sol-gel processing of an ethylene glycol-modified titanium precursor--into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO(2-x) networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.

Microstructural Analysis Of Strip Cast Nd-Fe-B Alloys For High Magnets

High energy density magnets .400 kJ/m are increasingly used in many applications. Conventional casting techniques for sintered magnets reveal the formation of a high quantity of a-Fe and large Nd-rich regions. New techniques, like strip casting, produce homogeneous and fine scaled microstructures and are already used for producing high (BH)max magnets. The fast cooling rate during strip casting suppresses the formation of a-Fe dendrites and of large Nd-rich pockets. Directional solidification causes a formation of columnar grains containing a typical arrangement of hard magnetic Nd2Fe14B regions and Nd-rich regions. The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a periodicity which corresponds to a eutectoidal solidification according to the composition of the liquid and are directed parallel to the temperature gradient during solidification. The lamellae show an average width of 150 nm, a spacing of 3 mm, and a length up to the size of the hard magnetic grains. The fine separation of the hard magnetic and Nd phases is advantageous for the milling of the alloy after hydrogen decripitation and improves sinterability of magnets. Although the microstructure of strip cast alloys is much finer than that of ordinary cast alloys, the alignment of the powder is not deteriorated and Br is not reduced due to a sufficient large interlamellar spacing between the Nd-rich platelets that enables the formation of single crystal powder particles after milling.

Surface modification processes during methane decomposition on Cu-promoted Ni–ZrO2 catalysts

We explored the surface chemistry of methane on Cu-promoted Ni–ZrO2 catalysts and observed a limited stability of the CuNi alloy under relevant reaction conditions.