Simone Herth

Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3

Iron tracer diffusion was studied in soft-magnetic nanocrystalline Fe90Zr7B3 without any influence of porosity, relaxation, or grain growth. The interfacial diffusion characteristics differ substantially from grain boundaries in metals due to the presence of an intergranular amorphous phase. The reduced diffusivity in thin amorphous layers compared to in the initial amorphous phase indicates the effect of confinement. The indication of a second, fast interfacial diffusion path is found and quantitatively analyzed within the framework of a two interface-type model.

Self-diffusion in nanocrystalline Fe and Fe-rich alloys

The present work aims at a comparison of the self-diffusion behaviour of nanocrystalline (n-)Fe produced by cluster condensation and compaction with that of Fe-rich n-alloys made by crystallization of melt-spun amorphous ribbons. In cluster-synthesized Fe (relative density higher than 91 %), a decrease of the (59)Fe tracer diffusivity upon annealing indicates interface relaxation. The diffusion coefficients in the relaxed grain boundaries are similar to those extrapolated from high-temperature data of conventional grain boundaries. Substantially lower interface diffusivities in crystallized n-Fe(90)Zr(10) and n-Fe(90)Zr(7)B(3) presumably arise from residual intergranular amorphous layers. Due to the reduced amorphous fraction, in n-Fe(90)Zr(10) additional fast diffusion paths exist like in conventional grain boundaries.

Magnetic tweezers for manipulation of magnetic particles in single cells

Magnetic tweezers gain increasing interest for applications in biology. Here, a setup of magnetic tweezers is introduced using micropatterned conducting lines on transparent glass slides. Magnetic particles of 1 μm diameter were injected in barley cell vacuoles using a microinject system under microscopic control. Time dependent tracking of the particles after application of a magnetic field was used to determine the viscosity of vacuolar sap in vivo relative to water and isolated vacuolar fluid. The viscosity of vacuolar sap in cells was about 2-fold higher than that of extracted vacuolar fluid and 5 times higher than that of water.

Positrons as chemically sensitive probes in interfaces of multicomponent complex materials: Nanocrystalline Fe90Zr7B3

Abstract The present paper reports on a combined analytical and structural study of nanocrystalline Fe90Zr7B3 by means of positron annihilation, (analytical) high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction. Particular focus is laid on the chemical nature of the intergranular amorphous matrix which occurs between the α-Fe nanocrystallites. Energy-dispersive X-ray measurements (EDX) with an electron nanobeam reveal an increased Zr content at the interface between the nanocrystallites and the intergranular amorphous phase. According to positron lifetime measurements, the intergranular amorphous phase and the interfaces between this phase and the nanocrystallites exhibit structural free volumes of the mean size slightly smaller than a lattice vacancy as in the amorphous precursor material. Coincident Doppler broadening measurements of the positron-electron annihilation photons show that the fraction of Zr in the neighborhood of the structural free volumes is higher in nanocryst...

Intergranular melting of ultrafine grained Nd2Fe14B studied by means of radiotracer diffusion

Grain-boundary (Gb) diffusion was studied in ultrafine grained Nd2Fe14B-based permanent magnets below and above the melting transition of the Nd-enriched intergranular phase using the radiotracer technique with the isotope 59Fe. The product δDGb of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in α-Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour δDGb = 1.53 × 10−11 exp(−1.74 eV/kT) m3 s−1 below the intergranular melting transition. Similar values δDGb are observed for ultrafine grained Nd-Fe-B with reduced Nd excess in the grain boundaries. The diffusion characteristics are compared with the kinetics of the hot-deformation which is of technical relevance for the processing of high-performance permanent magnets.

Thermomicrocapillaries as temperature biosensors in single cells

Temperature is an important physical parameter in biology and its deviation from optimum can cause damage in biosystems. Thermocouples based on the Seebeck effect can be structured on glass microcapillaries to obtain thermomicrocapillaries (TMCs) usable in a micromanipulation setup. The suitability of the setup was proven by monitoring the temperature increase upon illumination of leaves and single cells following insertion of the TMC. The increase was 1.5 K in green tissue and 0.75 K in white leaf sections due to lower absorption. In single cells of trichomes, the increase was 0.5 K due to heat dissipation to the surrounding air.

Diffusion and induced magnetic anisotropy in nanocrystalline Fe73.5Si13.5B9Nb3Cu1

71Ge tracer diffusion was studied to gain insight into the atomistic transport processes underlying the formation of magnetic anisotropy in nanocrystalline soft-magnetic Fe73.5Si13.5B9Nb3Cu1. The interfacial diffusion characteristic was determined by the residual intergranular amorphous phase, which gives rise to a strongly reduced interface diffusivity compared with grain boundaries in metals. Ge diffusion in the nanocrystallites, which is considered to characterize Si self-diffusion, is much slower than Fe diffusion owing to the D03 order of the Fe3Si nanocrystallites. Slow Si diffusion in the nanocrystallites is identified as the rate-controlling process for the generation of the field-induced magnetic anisotropy.

Orientation-defined alignment and immobilization of DNA between specific surfaces

DNA-based single-molecule studies, nanoelectronics and nanocargos require a precise placement of DNA in an orientation-defined manner. Until now, there is a lack of orientation-defined alignment and immobilization of DNA over distances smaller than several micrometers. However, this can be realized by designing bifunctionalized DNA with thiol at one end and (3-aminopropyl) tri-ethoxy silane at the other end, which specifically binds to a gold and SiO₂ layer after and during alignment, respectively. The electrode assembly consists of platinum as the electrode material for applying the AC voltage and islands of gold and silicon dioxide fabricated at a distance of about 500-800 nm by electron-beam lithography. The orientation-defined alignment and covalent binding of pUC19 DNA to specific surfaces are carried out in frequency ranges of 50 Hz-1 kHz and 100 kHz-1 MHz and observed after metallization of DNA by palladium ions by field emission scanning electron microscopy (FESEM). The bifunctionalized 890 nm long DNA was effectively aligned and immobilized between a gap of 500 to 600 nm width.

Positioning and stretching of actin filaments by electric fields

The alignment of biological filaments on surfaces offers a high potential for controllable geometries in lab-on-a-chip-structures and micrototal analysis systems. Actin is a polar filamentous protein with a diameter of 7–8 nm that can be manipulated with strong electric fields. It is demonstrated that with the use of microelectrodes or nanoelectrodes and electric fields of 20 kV/m single actin filaments can be manipulated, stretched, and positioned between gold electrodes.

Self-diffusion behaviour and microstructure of ultrafine-grained Nd2Fe14B with intergranular melting transition

The interface diffusion was studied in ultrafine-grained Nd 2 Fe 1 4 B which shows an intergranular melting transition owing to an excess amount of Nd. Below the intergranular melting transition, the interface diffusion of 5 9 Fe is similar to grain boundary diffusivities in bcc Fe. The microstructure and intergranular melting transition show characteristic variations with the Nd excess which sensitively affects the diffusivity at and above the melting transition. In Nd 2 Fe 1 4 B with high Nd excess, which contains extended Nd-rich triple junctions, the melting transition occurs at ca. 950 K in one step leading to an abrupt increase of the diffusion coefficient. In Nd 2 Fe 1 4 B with low Nd excess, which is free from additional phases at triple junctions, the diffusion coefficient above the melting transition increases gradually. Interface diffusion is found to have no rate-controlling influence on the hot-deformation used for the generation of magnetic anisotropy.