Magnetically interacting low dimensional Ni-nanostructures within porous silicon.

Electrodeposition of ferromagnetic metals, a common method to fabricate magnetic nanostructures, is used for the incorporation of Ni structures into the pores of porous silicon templates. The porous silicon is fabricated in various morphologies with average pore-diameters between 40 and 95 nm and concomitant pore-distances between 60 and 40 nm. The metal nanostructures are deposited with different geometries as spheres, ellipsoids or wires influenced by the deposition process parameters. Furthermore small Ni-particles with diameters between 3 and 6 nm can be deposited on the walls of the porous silicon template forming a metal tube. Analysis of this tube-like arrangement by transmission electron microscopy (TEM) shows that the distribution of the Ni-particles is quite narrow, which means that the distance between the particles is smaller than 10 nm. Such a close arrangement of the Ni-particles assures magnetic interactions between them. Due to their size these small Ni-particles are superparamagnetic but dipolar coupling between them results in a ferromagnetic behavior of the whole system. Thus a semiconducting/ferromagnetic hybrid material with a broad range of magnetic properties can be fabricated. Furthermore this composite is an interesting candidate for silicon based applications and the compatibility with today's process technology.

Ultramicrotomy in the ESEM, a versatile method for materials and life sciences.

We here present the results of the first materials science analyses obtained with the prototype of a serial block-face sectioning and imaging tool, 3Viewtrade mark of Gatan, Inc (Pleasanton, CA, U.S.A.). It is a specially designed ultramicrotome operating in situ within an environmental scanning electron microscope originally developed for life science research. The microtome removes thin slices from the sample and the environmental scanning electron microscope images each new block surface of the specimen (serial block-face scanning electron microscopy). The Schottky emitter (FEG) of the microscope delivers high spatial resolution and has the advantage of stable performance and high durability. The slice thickness can typically be selected between 50 and 100 nm. It is possible to cut hundreds of slices and simultaneously acquire images with Digital Micrographtrade mark Model 700 (Gatan, Inc.). This article outlines the set-up and describes the automated process. The preparation of specimens for in situ ultramicrotomy is explained and the parameters for good image quality are discussed. In addition, special operative and analytic features of the controlling software are presented. Three different technical materials and one botanical specimen were analyzed delivering first results of this method for materials science and for botany.

Investigation of a Mesoporous Silicon Based Ferromagnetic Nanocomposite.

A semiconductor/metal nanocomposite is composed of a porosified silicon wafer and embedded ferromagnetic nanostructures. The obtained hybrid system possesses the electronic properties of silicon together with the magnetic properties of the incorporated ferromagnetic metal. On the one hand, a transition metal is electrochemically deposited from a metal salt solution into the nanostructured silicon skeleton, on the other hand magnetic particles of a few nanometres in size, fabricated in solution, are incorporated by immersion. The electrochemically deposited nanostructures can be tuned in size, shape and their spatial distribution by the process parameters, and thus specimens with desired ferromagnetic properties can be fabricated. Using magnetite nanoparticles for infiltration into porous silicon is of interest not only because of the magnetic properties of the composite material due to the possible modification of the ferromagnetic/superparamagnetic transition but also because of the biocompatibility of the system caused by the low toxicity of both materials. Thus, it is a promising candidate for biomedical applications as drug delivery or biomedical targeting.

Synthesis and Magnetic Characterization of Metal-filled Double-sided Porous Silicon Samples.

A magnetic semiconductor/metal nanocomposite with a nanostructured silicon wafer as base material and incorporated metallic nanostructures (Ni, Co, NiCo) is fabricated in two electrochemical steps. First, the silicon template is anodized in an HF-electrolyte to obtain a porous structure with oriented pores grown perpendicular to the surface. This etching procedure is carried out either in forming a sample with a single porous layer on one side or in producing a double-sided specimen with a porous layer on each side. Second, this matrix is used for deposition of transition metals as Ni, Co or an alloy of these. The achieved hybrid material with incorporated Ni- and Co-nanostructures within one sample is investigated magnetically. The obtained results are compared with the ones gained from samples containing a single metal.

Tensile tests of polymers at low temperatures in the environmental scanning electron microscope: an improved cooling platform.

The investigation of the fracture behavior of polymers in the environmental scanning electron microscope (ESEM) can provide information about the correlation between the microstructure of a specimen and the macroscopic stress-strain characteristic. As the mechanical properties of polymers change dramatically at the glass transition temperature, cooling of the specimens during the tensile tests can yield very valuable information about the influence of individual components of polymer blends on the fracture behavior of the material. A serious problem in this connection is the poor heat conductivity of polymers. A commercially available cooling platform, which can be mounted on the tensile stage used for the tests was substantially modified to both enhance the heat transfer between platform and specimen, and to minimize the temperature gradient along the specimen. The first experiments on modified polypropylene specimens already delivered some unexpected results. Fibril-like structures appeared at the crack tip that would not be expected at temperatures below the glass transition temperature of the polymer blend.

Recrystallization behaviour of the nickel-based alloy 80 A during hot forming.

The dynamic recrystallization as well as meta-dynamic and static recrystallization of the nickel-based alloy 80A was investigated by means of electron backscatter diffraction (EBSD). Specimens were hot compressed at a temperature of 1120 degrees C and a strain rate of 0.1/s at varying strain and soak times to describe the recrystallization behaviour. Various approaches were tested in order to differentiate between recrystallized and deformed grains based on EBSD data. The grain orientation spread was clearly found to be the most reliable procedure. A high twinning of the recrystallized grains was observed, and as a consequence the measured grain size was strongly dependent on whether the coherent and incoherent twin boundaries were regarded as genuine boundaries or removed.

Analysis of submicron particles by scanning electron microscopy-energy-dispersive X-ray spectrometry-accuracy of size measurement.

Scanning electron microscopy combined with energy-dispersive x-ray spectrometry (SEM-EDXS) is widely used for particle analysis. In the case of submicron particles, especially for particles that are smaller than 300 nm, the measured particle size is influenced by specimen preparation, SEM operating parameters, the mean atomic number of the particles, and the threshold value used for binarization. The use of uncoated particles on a conductive substrate and image acquisition using an in-lens detector are recommended for precise morphologic results in this size range.

Automated analysis of submicron particles by computer-controlled scanning electron microscopy.

Automated analysis of submicron particles by computer-controlled scanning electron microscopy is generally possible. The minimum diameter of the detectable particles is dependent on the mean atomic number of the particles and the operating parameters of the scanning microscope. The main limitation with regard to particle size is set by the quality of the particle detection system, which generally is the backscatter electron detector. The accuracy of the results of the x-ray analyses is very often strongly affected by specimen damage, omnipresent especially for environmental particles even at low electron energies and probe currents. With the exception for light elements, the detection limit is approximately 1 wt%. Device-related limitations to automated analysis may be specimen drift and an unreliable autofocus function.