Peter Poelt

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, 3View™ 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 Micrograph™ 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.

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°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.

Serial sectioning methods for 3D investigations in materials science

A variety of methods for the investigation and 3D representation of the inner structure of materials has been developed. In this paper, techniques based on slice and view using scanning microscopy for imaging are presented and compared. Three different methods of serial sectioning combined with either scanning electron or scanning ion microscopy or atomic force microscopy (AFM) were placed under scrutiny: serial block-face scanning electron microscopy, which facilitates an ultramicrotome built into the chamber of a variable pressure scanning electron microscope; three-dimensional (3D) AFM, which combines an (cryo-) ultramicrotome with an atomic force microscope, and 3D FIB, which delivers results by slicing with a focused ion beam. These three methods complement one another in many respects, e.g., in the type of materials that can be investigated, the resolution that can be obtained and the information that can be extracted from 3D reconstructions. A detailed review is given about preparation, the slice and view process itself, and the limitations of the methods and possible artifacts. Applications for each technique are also provided.

Magnetic Study of Fe3O4 Nanoparticles Incorporated within Mesoporous Silicon

The fabrication of low dimensional systems as ultrathin layers, nanowires, nanoparticles, and nanodots is a key trend in today’s nanotechnology. Not only the change in the physical properties of low dimensional materials compared to their bulk materials is of interest; their applicability is also a developing subject. Magnetic materials in the nanoscale range are utilized in magnetic data storage, giant magnetoresistance devices; magnetic particles are also employed in biological and medical applications. The fabrication of isolated magnetic nanoparticles is difficult to reach because the large surface areas compared to the volume oxidize easily when using metals and due to the tendency of nanoparticles to agglomerate. A controlled passivation of the particles can be carried out, but this can also lead to interactions between the metal core and the passivating materials. Magnetic iron oxide such as magnetite has the advantage of being more stable. Recently, a new preparation method has been reported based on the decomposition at high temperatures of an organic precursor in the presence of oleic acid, which leads to a monodisperse size distribution of nanopar

Variable blocking temperature of a porous silicon/Fe3O4 composite due to different interactions of the magnetic nanoparticles

In the frame of this work, the aim was to create a superparamagnetic nanocomposite system with a maximized magnetic moment when magnetized by an external field and a blocking temperature far below room temperature. For this purpose, iron oxide nanoparticles of 3.8-, 5- and 8-nm size have been infiltrated into the pores of porous silicon. To fabricate tailored magnetic properties of the system, the particle size and the magnetic interactions among the particles play a crucial role. Different concentrations of the particles dispersed in hexane have been used for the infiltration to vary the blocking temperature TB, which indicates the transition between the superparamagnetic behavior and blocked state. TB is not only dependent on the particle size but also on the magnetic interactions between them, which can be varied by the particle-particle distance. Thus, a modification of the pore loading on the one hand and of the porous silicon morphology on the other hand results in a composite material with a desired blocking temperature. Because both materials, the mesoporous silicon matrices as well as the Fe3O4 nanoparticles, offer low toxicity, the system is a promising candidate for biomedical applications.

Characterization of modified polypropylene by scanning electron microscopy

Characterizing the morphology of modified multiphasic polymer systems, as are often applied for improving the impact strength, is normally a complicated and tedious task. Nevertheless, knowledge about the volume fraction and particle-size distribution of the elastomer phase is important for the specific development of high-impact systems. Direct production in the reactor enables only indirect control of these two quantities. Computer-controlled scanning electron microscopy in combination with image processing allows an automated measurement of both all the necessary particle parameters (size distribution, shape, orientation, etc.) and the elastomer content of the material. Since bulk materials are used for the investigation, additionally, three-dimensional information about the structure of the material can be gained by simply varying the electron energy, without the necessity to resort to multiple slices. This information is especially important in the case of particles with extremely irregular shapes, as obtained, for example, by strong agglomeration of the modifier particles. The mathematical routines used for calculation of the particle-size distributions from the measured profile-size distributions cannot be applied in such cases. The method was tested for several materials with significantly different compositions, both immediately after molding and also after a subsequent thermal relaxation.

Fe3O4-nanoparticles within porous silicon: Magnetic and cytotoxicity characterization

The magnetic properties of porous silicon/Fe3O4 composites are investigated with respect to the adjustability of the blocking temperature along with an evaluation of any size-dependent changes in cytocompatibility. Fe3O4-nanoparticles have been infiltrated within mesoporous silicon, resulting in a system with tunable magnetic properties due to the matrix-morphology, the loading of the nanoparticles, and their size. In order to provide basic information regarding its suitability as a therapeutic platform, the cytotoxicity of these composites have been investigated by a trypan blue exclusion assay with respect to human embryonic kidney 293 cells, and the results compared with cell-only and known cytotoxic controls.

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.

Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates

The magnetic function of a Ni-nanowire/silicon-template system has been explored in corporation with an advanced process. Arrays of nanopores with a mean diameter of 35 nm have been fabricated by anodization of silicon wafers under an external magnetic field (8 T) perpendicular to the substrate. Due to a guided supply of holes from the substrate during the anodization, well controlled straight nanopores have been formed with a high aspect ratio, and then isolated Ni nanowires have been grown along these nanopores by electrodeposition. The fabricated samples show a significantly enhanced magnetic anisotropy with little crosstalk between adjacent pores.

Porous silicon/Ni composites of high coercivity due to magnetic field-assisted etching

Ferromagnetic nanostructures have been electrodeposited within the pores of porous silicon templates with average pore diameters between 25 and 60 nm. In this diameter regime, the pore formation in general is accompanied by dendritic growth resulting in rough pore walls, which involves metal deposits also offering a branched structure. These side branches influence the magnetic properties of the composite system not only due to modified and peculiar stray fields but also because of a reduced interpore spacing by the approaching of adjacent side pores. To improve the morphology of the porous silicon structures, a magnetic field up to 8 T has been applied during the formation process. The magnetic field etching results in smaller pore diameters with less dendritic side pores. Deposition of a ferromagnetic metal within these templates leads to less branched nanostructures and, thus, to an enhancement of the coercivity of the system and also to a significantly increased magnetic anisotropy. So magnetic field-assisted etching is an appropriate tool to improve the structure of the template concerning the decrease of the dendritic pore growth and to advance the magnetic properties of the composite material.