Dynamic bonding of metallic nanocontacts: Insights from experiments and atomistic simulations

The conductance across an atomically narrow metallic contact can be measured by using scanning tunneling microscopy. In certain situations, a jump in the conductance is observed right at the point of contact between the tip and the surface, which is known as “jump to contact” (JC). Such behavior provides a way to explore, at a fundamental level, how bonding between metallic atoms occurs dynamically. This phenomenon depends not only on the type of metal but also on the geometry of the two electrodes. For example, while some authors always find JC when approaching two atomically sharp tips of Cu, others find that a smooth transition occurs when approaching a Cu tip to an adatom on a flat surface of Cu. In an attempt to show that all these results are consistent, we make use of atomistic simulations; in particular, classical molecular dynamics together with density functional theory transport calculations to explore a number of possible scenarios. Simulations are performed for two different materials: Cu and Au in a [100] crystal orientation and at a temperature of 4.2 K. These simulations allow us to study the contribution of short- and long-range interactions to the process of bonding between metallic atoms, as well as to compare directly with experimental measurements of conductance, giving a plausible explanation for the different experimental observations. Moreover, we show a correlation between the cohesive energy of the metal, its Young's modulus, and the frequency of occurrence of a jump to contact.

Surface growth for molten silicon infiltration into carbon millimeter-sized channels: Lattice–Boltzmann simulations, experiments and models

The process of liquid silicon infiltration is investigated for channels with radii from 0.25 to 0.75 [mm] drilled in compact carbon preforms. The advantage of this setup is that the study of the phenomenon results to be simplified. For comparison purposes, attempts are made in order to work out a framework for evaluating the accuracy of simulations. The approach relies on dimensionless numbers involving the properties of the surface reaction. It turns out that complex hydrodynamic behavior derived from second Newton law can be made consistent with Lattice-Boltzmann simulations. The experiments give clear evidence that the growth of silicon carbide proceeds in two different stages and basic mechanisms are highlighted. Lattice-Boltzmann simulations prove to be an effective tool for the description of the growing phase. Namely, essential experimental constraints can be implemented. As a result, the existing models are useful to gain more insight on the process of reactive infiltration into porous media in the first stage of penetration, i.e. up to pore closure because of surface growth. A way allowing to implement the resistance from chemical reaction in Darcy law is also proposed.

Cleavage of the C–C Bond in the Ethanol Oxidation Reaction on Platinum. Insight from Experiments and Calculations

Using a combination of experimental and computational methods, mainly FTIR and DFT calculations, new insights are provided here in order to better understand the cleavage of the C–C bond taking place during the complete oxidation of ethanol on platinum stepped surfaces. First, new experimental results pointing out that platinum stepped surfaces having (111) terraces promote the C–C bond breaking are presented. Second, it is computationally shown that the special adsorption properties of the atoms in the step are able to promote the C–C scission, provided that no other adsorbed species are present on the step, which is in agreement with the experimental results. In comparison with the (111) terrace, the cleavage of the C–C bond on the step has a significantly lower activation energy, which would provide an explanation for the observed experimental results. Finally, reactivity differences under acidic and alkaline conditions are discussed using the new experimental and theoretical evidence.

Admittance Control for Robotic Loading: Design and Experiments with a 1-Tonne Loader and a 14-Tonne LHD

This paper describes the design, tuning, and extensive field testing of an admittance-based Autonomous Loading Controller (ALC) for robotic excavation. Several iterations of the ALC were tuned and tested in fragmented rock piles—similar to those found in operating mines—by using both a robotic 1-tonne capacity Kubota R520S diesel-hydraulic surface loader and a 14-tonne capacity Atlas Copco ST14 underground load-haul-dump (LHD) machine. On the R520S loader, the ALC increased payload by 18 % with greater consistency, although with more energy expended and longer dig times when compared with digging at maximum actuator velocity. On the ST14 LHD, the ALC took 61 % less time to load 39 % more payload when compared to a single manual operator. The manual operator made 28 dig attempts by using three different digging strategies, and had one failed dig. The tuned ALC made 26 dig attempts at 10 and 11 MN target force levels. All 10 11 MN digs succeeded while 6 of the 16 10 MN digs failed. The results presented in this paper suggest that the admittance-based ALC is more productive and consistent than manual operators, but that care should be taken when detecting entry into the muck pile

Sandbox experiments on basin inversion: testing the influence of basin orientation and basin fill

Analogue modelling experiments using brittle materials are performed to study the inversion of extensional structures. Asymmetric grabens of two different orientations are first created during a phase of extension and progressively filled. They are subsequently shortened in the same direction. The aim of our experiments is to determine factors affecting the style of deformation during inversion. We specifically investigate variations in thickness and distribution of strong and weak layers constituting the graben fill and in initial basin orientation. The main advantage of our experimental set-up is that we have a complete control on graben location, width, infill and orientation before inversion. The experiments show that shortening results only in limited reactivation of pre-existing normal faults. In general, forward thrusts and backthrusts cut across normal faults into the footwall of the graben. The forward thrusts either propagate parallel to the enveloping surface of faulted blocks or they cut across basin-limiting normal faults at various angles. The graben fill is mechanically extruded by displacement along forward thrusts that accommodate most of the shortening. Both pre-existing faults and weak graben fill act as zones of weakness during inversion and determine the orientation and location of both backthrusts and forward thrusts. The results of our experiments conform well to natural examples of inverted graben structures.

A Comprehensive Discussion of HMBC Pulse Sequences: 4. Establishing Two-Bond Correlations from HMBC and Related Experiments

The utility of the HMBC experiment for structure elucidation is unquestionable, but the nature of the coupling pathways leading to correlations in an HMBC experiment creates the potential for misinterpretation. This misinterpretation potential is intimately linked to the size of the long-range heteronuclear couplings involved, and may become troublesome in those cases of a particularly strong 2JCH correlation that might be mistaken for a 3JCH correlation or a 4JCH correlation of appreciable strength that could be mistaken for a weaker 3JCH correlation. To address these potential avenues of confusion, work from several laboratories has been focused on the development of what might be considered “coupling pathway edited” long-range heteronuclear correlation experiments that are derived from or related to the HMBC experiment. The first example of an effort to address the problems associated with correlation path length was seen in the heteronucleus-detected XCORFE experiment described by Reynolds and co-workers that predated the development of the HMBC experiment. Proton-detected analogs of the HMBC experiment intended to differentiate 2JCH correlations from nJCH correlations where n = 3, 4, include the 2J,3J-HMBC, HMBC-RELAY, H2BC, edited-HMBC, and HAT H2BC experiments. The principles underlying the critical components of each of these experiments are discussed and experimental verification of the results that can be obtained using model compounds are shown. This contribution concludes with a brief discussion of the 1,1-ADEQUATE experiments that provide an alternative means of identifying adjacent protonated and non-protonated carbon correlations by exploiting 1JCC correlations at natural abundance.

DVD-based distance-learning program for university entrance exams -- RCT experiments in rural Bangladesh

In contrast to the remarkable progress in developing countries in improving primary education, access to higher education in many countries remains limited, especially in rural areas where the quality of education is inadequate. We evaluate a DVD-based distance-learning program in rural Bangladesh, targeted at students aiming to take university entrance tests. We conducted two experiments: one to evaluate the effect of the distance-learning program and the second to determine the demand and price sensitivity. Our first experiment shows that the DVD-based distance-learning program has a considerable positive effect on the number of students passing entrance exams. This effect does not depend on cognitive scores, but does depend on non-cognitive attributes, indicating the importance of commitment, which is imposed through our program. In the second experiment, we offered a random subsidy to interested participants. The uptake decision is price-sensitive, although the price sensitivity is not correlated with students' past academic performance or their socio-economic status, suggesting that increasing the price should not disproportionately exclude poor students.