Boron | EPFL Graph Search

Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride. Boron is synthesized entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, so it is a low-abundance element in the Solar System and in the Earth's crust. It constitutes about 0.001 percent by weight of Earth's crust. It is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax and kernite. The largest known deposits are in Turkey, the largest producer of boron minerals. Elemental boron is a metalloid that is found

A study of the diffusion brazing process applied to the single crystal superalloy CMSX-4

Alexander Schnell

Diffusion Brazing (DB) of nickel-based superalloys is a standard joining process widely used in gas turbine industry. The DB technique uses a low melting nickel-based filler material, which contains a rapidly diffusing melting point depressant (MPD) such as boron. Upon brazing, the braze filler firstly melts and then solidifies isothermally through the diffusion of boron from the liquid braze alloy into the solid parent material. Fully isothermally solidified joints exhibit a brittle-phase free microstructure and show excellent mechanical properties. The kinetics of the DB process are controlled by the "effective MPD flux" from the liquid into the surrounding bulk material. The effective MPD flux is a function of the boron diffusion rates and thermodynamic parameters of the parent materials such as the MPD solubility. In the case of brazing superalloys, the complex interactions between the braze filler and the parent material are not fully understood with regard to thermodynamic parameters that control the effective MPD flux. In the present work, the DB process applied to a single crystal multi-component superalloy is studied by combining experimental and numerical modelling results. A representative superalloy/braze filler system has been selected, namely CMSX-4/D-15. The main task consisted of studying the thermodynamic parameters that control the effective MPD (boron) flux into the parent superalloy. Boron diffusion into the superalloy material causes the formation of borides due to the low solubility of the matrix phase for boron and the presence of boride forming elements such as Cr, W and Mo. Boron is therefore steadily consumed from the superalloy matrix phase during isothermal solidification by in-situ boride precipitation. As a consequence, the effective boron flux into the parent material is increased. The overall boron content in the parent superalloy is higher than the boron solubility limit would allow. It has been found that there exists an optimum brazing temperature at which the effective boron flux into the CMSX-4 material due to boride formation is highest. With an increase in the brazing temperature the volume fraction of borides that are stable in the parent material steadily decreases. Above the optimum brazing temperature, the effect of boride formation on the effective boron flux into the superalloy material is low. The DB process is then mainly controlled by the low boron solubility in the matrix phase. The results regarding the effective boron flux and an optimum brazing temperature can be applied to most superalloy systems as most superalloys contain boride forming elements. The present work supports generally the selection of appropriate brazing cycles for the diffusion brazing of superalloys.

EPFL2004

Toxicity study of nanostructures

Lenke Horváth

The great achievement of nanotechnology is the controlled synthesis of a large variety of nanometer-size materials like needle-formed nanotubes, nanowires and two-dimensional graphene flakes. Due to their unique physico-chemical properties, these nanostructures are considered to be of great benefit for many applications in engineering, electronics, alternative energies and nanomedicine. Since the expectations for the improvement of our everyday life through engineered nanomaterials are high, there is rapid expansion of their manufacturing, which makes it likely that intentional and unintentional human and environmental exposure will increase in the near future. Consequently, the concern grows, related to their possible health hazards, as some of them strongly resemble asbestos. Motivated by this issue, we have investigated the in vitro acute cellular toxicity associated with four model nanomaterials: carbon nanotubes, boron nitride nanotubes, titanium dioxide nanofilaments and graphene oxide. This study focused on the toxic effect these nanomaterials had on cell types found in the respiratory system, where exposure to these materials is most prominent. These are lung epithelial cells, macrophages, and fibroblasts, but also other cell types like kidney cells were tested. The cytotoxicity was assessed by using MTT, DNA and FMCA assays, which measure different endpoints such as metabolic activity, cell proliferation and viable cell number. The cell death was determined by the Annexin V assay. We employed various microscopic techniques: light microscopy to reveal the morphological alterations associated with the nanomaterial toxicity at the cellular level; scanning electron microscopy to study the cell-nanomaterial interactions on the surface of the cell membrane; transmission electron microscopy to examine the uptake and subsequent localization of the nanomaterials within the cytosol and cell organelles. In addition, the generation of intracellular reactive oxygen species induced by graphene oxide was detected by the DCF assay. Last but not least, the pro-inflammatory potential and the biochemical perturbations in cells exposed to boron nitride nanotubes were investigated by Western blot and Synchrotron Infrared Microspectroscopy (SIRMS), respectively. All these techniques point to the adverse effects of the investigated nanostructures: i) The toxic effect of carbon nanotubes and carbon nanoparticles showed a time- and dose-dependent impairment in the metabolic activity of the cells characteristic for each cell type. Moreover, distinct morphological alterations typical for cell death were particularly apparent in macrophages. ii) The toxic potential of boron nitride nanotubes exhibited more pronounced adverse effects than carbon nanotubes. This was demonstrated by cell viability assays combined with cytopathological and biochemical analyses, showing induction of serious morphological changes, particularly in macrophages and fibroblasts, and biochemical processes characteristic for cell death. A higher acute toxicity was determined for boron nitride nanotubes when compared to crocidolite asbestos and their pro-inflammatory potential was demonstrated by the secretion of mature IL-1β cytokine in macrophages. iii) Titanium dioxide nanofilaments were also shown to impair the metabolic activity of the studied cells and induce morphological changes pointing to cell insult. iv) Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy showed that graphene oxide flakes are taken up via the endocytic pathway. In summary, our findings highlight important physico-chemical parameters, which are important in relation to the toxic effect of nanomaterials. These are: i) the chemical composition; ii) the surface modification including functionalized groups and structural defects; iii) the geometry: length, diameter and tortuosity. In addition to the identified nanomaterial characteristics, we pinpoint that the target cells ́ response depends on their type, which is likely to be linked to their physiological function.

EPFL2012

Evaluation of secondary electron intensities for dopant profiling in ion implanted semiconductors: a correlative study combining SE, SIMS and ECV methods

Franz-Josef Haug, Aïcha Hessler-Wyser, Quentin Thomas Jeangros, Mario Joe Lehmann, Audrey Marie Isabelle Morisset, Philippe Wyss

This study evaluates the secondary electron (SE) dopant contrast in scanning electron microscopy (SEM) and helium ion microscopy (HIM) on boron implanted silicon sample. Complementary techniques like secondary ion mass spectrometry and electrochemical capacitance voltage (ECV) measurements are used to understand the dopant profile and active dopant distribution before and after a thermal firing, a step carried out to remove implantation damage and to electrically activate the implanted boron. Thermal firing resulted in an activation efficiency of 33%. HIM showed higher contrast than SEM having more defined peak with a lower background contribution. Variations in dopant concentration near the peak maximum were observed in ECV measurements, which was not observed in the intensity profiles from both SEM and HIM. This study demonstrates the effectiveness of SE dopant profiling as a quick tool to map the electrically active dopant concentrations even in far-from-equilibrium materials such as ion implanted samples.

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