Molybdenum | EPFL Graph Search

Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin molybdaenum, which is based on Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm. Molybdenum does not occur naturally as a free metal on Earth; it is found only in various oxidation states in minerals. The free element, a silvery metal with a grey cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of the world production of the element (about 80%) is used in steel alloys, including high-strength alloys and superalloys. Most molybdenum compounds have low solu

In-situ Laue Diffraction During Compression of Directionally Solidified Mo Micropillars

Julien René André Zimmermann

With recent developments in micro and nano-technologies, mechanical components such as those used in medicine or electronics tend to be miniaturized, requiring a new set of testing techniques to study their reliability and performance. One of the new methods is micro-pillar compression testing, which allows the study of mechanical properties of confined volumes precisely shaped by focused ion beam (FIB) milling from sometimes complex microstructures. This technique has garnered a great deal of interest in recent years; on one hand because of its apparent simplicity, on the other hand because it revealed an unexpected size dependence in the flow stress of single crystals when the pillar diameter was reduced below a few tenths of a micrometre. The flow stress of FIB milled face centred cubic (fcc) micro-pillars appears to follow a power law as a function of their size. In contrast, non-FIB milled, body centred cubic (bcc) eutectic Molybdenum (Mo) alloy pillars prepared by directional growth exhibit, in their pristine state, whisker-like behaviour without a size effect. Furthermore, after FIB milling or moderate pre-deformation, the whisker character is lost and the typical scatter of the flow stress usually observed for FIB milled pillars is reproduced. After large pre-deformation the scatter vanishes and the pillars exhibit no size effect. These findings show that 1) the deterministic parameter in the size effect can be correlated with the initial microstructure of the pillar, and 2) FIB milling influences this microstructure. This study aims to link the deformation modes occurring in small-scale pillars with their initial microstructure by studying directionally solidified eutectic Mo-alloy pillars with a diameter of 1 micrometre. In-situ Laue diffraction experiments during the compression of such defect-free pillars show that with readily-available dislocation sources from a concave pillar top, the {112}[111] slip system with the highest Schmid factor is initially activated and not the allegedly assumed {110}[111] slip system that is typical for bcc metals. Post-mortem slip trace analysis and TEM observations confirm this finding. Moreover, 3D atom probe measurements and TEM images show that this behaviour can be linked to the presence of both alloying elements and fcc nano-precipitates found in the bcc Mo-alloy. Both can lead to a decrease of the critical temperature and the observation of an fcc-like deformation. Additionally, it is shown that both FIB milling and pre-deformation introduce defects in the initially dislocation-free pillars. In addition to this, pillars prepared under similar conditions yield quite different microstructures. The angle between the focused ion beam and the crystal orientation is found to play an important role in the creation of defects. It can also be shown that pillars with high initial dislocation density deform bulk-like and exhibit strain hardening, while pillars with low initial dislocation content demonstrate a large scatter in the flow and yield stress and exhibit large strain burst at later deformation stages. Furthermore several slip systems are activated before the flow stress is reached for pillars containing initially low dislocation content. Interestingly the first activated slip system can be linked to the initial streaking direction; slip is observed on the {112} plane that contains excess dislocations. For both the as-prepared pillars and the pillars with low initial dislocation content, the yield seems to be source limited, which is not the case for pillars with high initial dislocation content.

EPFL2011

Selective three-phase hydrogenation of aromatic nitro-compounds over β-molybdenum nitride

Fernando Cardenas Lizana, Lioubov Kiwi, Daniel André Samuel Lamey

A tetragonal molybdenum nitride (β-Mo2N) has been prepared by temperature programmed treatment of MoO3 in flowing N2 + H2 and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; PH2=11bar) of a series of para-substituted (-H, -OH, -O-CH3, -CH3, -Cl, -I and -NO2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al2O3, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/ hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. β-Mo2N has been characterized in terms of temperature-programmed reduction (TPR), H2 chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal β-Mo2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O2/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 μmol m-2) associated with β-Mo2N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of β-Mo2N to promote selective nitroarene hydrogenation. © 2011 Elsevier B.V. All rights reserved.

Elsevier2011

2D MoS2 Nanopores: Wafer-scale Fabrication and Monolayer Stability for Long-term Single-Molecule Sensing

Mukeshchand Thakur

Biologically inspired solid-state nanopores are artificial openings or apertures in thin membranes similar to natural protein ion channels in a lipid bilayer of cell membranes. In solid-state nanopores, a thin insulating membrane with single or multiple pores separates two conductive salt solutions. When an electric field is applied across this membrane, electrically charged species such as ions pass through these nanopore(s), generating a nanopore ion current. In essence, nanopores are single-molecule sensors and valuable tools for studying biophysics. For instance, an intrinsically charged biomolecule such as DNA can be electrophoretically threaded through the nanopore, transiently blocking the ionic current characteristic of the molecule passing through the pore.With progress in two-dimensional (2D) materials, the marriage of nanopores with 2D materials - "2D nanopores" have emerged as a new class of ultra-thin membrane solid-state nanopores. Molybdenum disulfide (MoS2) is a 2D material with an atomic thickness (0.7 nm) that approaches the inter-base distance of two DNA bases and is a lucrative 2D material for the DNA sequencing application. However, there are inherent challenges and bottlenecks with using MoS2 due to sensitive fabrication and inherent challenges of the 2D materials leading to low device yield. In this thesis, I will demonstrate ways to improve high-throughput production and the development of more reliable and durable nanopore devices. In the second chapter, I will introduce MoS2 material as a 2D nanopore system and elaborate fabrication of nanopore substrates. I will discuss various problems and issues related to substrate fabrication, transfer, nanopore-creation, and nanopore measurements. Finally, I list a step-by-step protocol and troubleshooting guide for early-stage 2D nanopore researchers. In the third chapter, I specifically focus on "chip-scale" transfer strategies for MoS2 grown using chemical vapor deposition (CVD). I introduce two transfer approaches - direct-transfer and stamp-assisted- with just water as a medium. I will demonstrate these transfer approaches and discuss their advantages and limitations. Furthermore, I discuss hydrocarbon contamination with 2D materials and their implications in nanofluidics. In the fourth chapter, I demonstrate a scalable transfer from chip-scale to a larger "wafer-scale" for batch fabrication of nanopore substrates for single-molecule DNA sensing. With PDMS-based polymer, I will demonstrate 3-inch monolayer MoS2 transfer on nanopore substrates with 128 nanopore devices with high transfer efficiency (>70%). Moreover, the technique is etchant-free, and growth substrates are recyclable after transfer. In the fifth chapter, I study nanopore instability and address those issues. I address the delamination issue by chemically modifying Si-substrates with an organosilicon that increases the adherence of monolayer MoS2 layers. This surface pre-treatment helped reinforce 2D layer attachment to the substrate, increasing the nanofluidic devices' durability. Further, we show that the nanopore enlargement due to dissolved oxygen in an aqueous solution can be considerably reduced in a low dissolved oxygen concentration in solution. These strategies improved MoS2 nanopore stability and enabled long-term DNA sensing. This will pave way toward more durable 2D nanopore sensors.

EPFL2022