Surface chemistry of tribochemical reactions explored in ultrahigh vacuum conditions

The thermal decomposition of model extreme-pressure lubricant additives on clean iron was studied in ultrahigh vacuum conditions using molecular beam strategies. Methylene chloride and chloroform react to deposit a solid film consisting of FeCl₂ and carbon, and evolve only hydrogen into the gas phase. No gas-phase products and less carbon on the surface are detected in the case of carbon tetrachloride. Dimethyl and diethyl disulfide react on clean iron to deposit a saturated sulfur plus carbon layer at low temperatures (∼600 K) and an iron sulfide film onto a Fe + C underlayer at higher temperatures (∼950 K). Methane is the only gas-phase product when dimethyl disulfide reacts with iron. Ethylene and hydrogen are detected when diethyl disulfide is used.     

On the reaction pathway for the formation of benzene from acetylene catalyzed by palladium

The reaction between gas‐phase acetylene and alumina‐supported palladium saturated with ¹³C‐labelled vinylidene is studied using both one‐pulse, ¹³C magic‐angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products to probe the reaction pathway. The presence of vinylidene on alumina‐supported palladium is confirmed by comparing the infrared spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR that a high pressure (∼350 Torr) of gas‐phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene is formed catalytically on the same sample. The resulting benzene incorporates two ¹³C atoms. This indicates that benzene is formed by a slow reaction between gas‐phase (¹²C‐labelled) acetylene and adsorbed vinylidene (¹³CH₂=¹³C=) to form a C₄ intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111), where it was found that benzene synthesis also proceeds via a C₄ intermediate, in this case formed from two adsorbed acetylenes. This revised version was published online in July 2006 with corrections to the Cover Date.     

A molecular beam study of the tribological chemistry of dialkyl disulfides

Molecular beam studies carried out in ultrahigh vacuum show that dimethyl disulfide reacts with initially clean iron to evolve methane. The reaction is proposed to proceed via a methyl thiolate intermediate. Reaction ceases at 600 K, an effect that is proposed to be due to the surface being blocked by an overlayer of sulfur and carbon. Reaction recommences above 950 K as sulfur diffuses into the iron. The activation energy for the film-forming reaction is 52.5±2.1 kcal/mol, in good agreement with the activation energy for the growth of FeS films from dimethyl disulfide at higher pressures measured using a microbalance. A depth profile of the film grown in ultrahigh vacuum shows that the sulfur-containing film grows on a Fe+C underlayer. Similar molecular beam experiments with diethyl disulfide suggest the formation of an intermediate ethyl thiolate species which decomposes via a -hydride elimination reaction to evolve ethylene. The activation energy for film growth, in this case, is 60±2.4 kcal/mol. The results of tribological experiments using a pin and v-block apparatus are consistent with FeS forming the anti-seizure film.     

Application of germanium resistance thermometers below 0.1 K

Several commercially available germanium resistance thermometers have been studied at temperatures below 0.1 K. The major impediment to the removal of heat dissipated by the measuring current is the weak carrier-phonon interaction. Precautions required for the reliable use of these thermometers are dicussed.     

A large haptic device for aircraft engine maintainability

The virtual reality for maintainability (Revima) VR system supports maintainability simulation in aeronautics. Within this project we have developed and integrated a haptic device, the large haptic interface for aeronautic maintainability (LHIfAM). We use this device to track hand movements and provide force feedback within the large geometric models that describe aircraft engines. The user movements are the same as those that occur when testing physical mock-ups. An integrated haptic device and VR system for testing aircraft engines reduces development costs and avoids the necessity of physical mock-ups formaintainability.     

Infrared spectroscopy of CO adsorbed on hydrocarbon‐covered Mo(100)

The adsorption of CO at high pressures (up to 120 Torr) on a hydrocarbon‐precovered Mo(100) surface is studied by reflection–absorption infrared spectroscopy (RAIRS). It is shown that CO adsorbs reversibly on the Mo(100) surface even in the presence of a thick (~C₈) carbonaceous layer and forms a saturation coverage of about 20% of that on clean Mo(100). Although CO is used as a probe molecule, this result confirms that, under high pressure, the reactants still can adsorb on the catalyst in spite of the presence of a thick carbonaceous layer formed under the reaction conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Tribological Properties of Monolayer KCl Films on Iron in Ultrahigh Vacuum: Modeling the Extreme-Pressure Lubricating Interface

The friction coefficients of thin KCl films deposited onto clean iron in ultrahigh vacuum are measured using a tungsten carbide tip. A rapid decrease is found in the friction coefficient from 2 for clean iron to 0.27 ± 0.03 after the deposition of 40 Å of KCl. Based on previous contact resistance measurements, this was proposed to be due to the completion of the first layer of KCl. The first-layer KCl coverage was measured by adsorbing deuterium onto an iron surface partially covered by KCl, where deuterium selectively adsorbs onto the iron. This revealed that the first monolayer is complete after the deposition of 40 Å of KCl and that the first-layer KCl film coverage _KCl ⁽¹⁾ is given by _KCl ⁽¹⁾ = 1 - exp(-0.39±0.02t), where t is the film thickness. XPS data suggest that heating a KCl film to 550 K causes it to wet the surface. This leads to decreases in the friction coefficients for thin KCl films in accord with the idea that friction is reduced by the first monolayer of KCl on iron. Temperature-programmed desorption data indicate that KCl in the first monolayer is 5 kJ/mol more stable than the multilayer consistent with the wetting behavior. Finally, the kinetic data are analyzed to suggest that the first-layer film is 2.6 Å thick.     

Modeling of Shotcrete Application for Use in a Real‐Time Training Simulator

Simulators provide significant advantages in training operators of concrete spraying machinery, such as economic savings, the practical absence of safety risks, and environmental and educational benefits. The main challenge in developing a real‐time training simulator for concrete spraying machinery lies in the modeling of shotcrete application. This article presents a novel method that models and simulates in real time the three main factors influencing shotcrete sprayability: adhesion, cohesion, and rebound. Furthermore, thanks to the addition of an obstacle model, the method makes it possible to spray onto additional supporting elements, which is a typical shotcrete application. The proposed method considers a wet‐mix thick flow spraying process and is based on experiments that were run with a real concrete spraying machine and complemented by expert advice. The method was developed and evaluated using a user‐centered methodology, resulting in realistic shotcrete application modeling that meets the needs for training concrete spraying machinery operators.     

Parallel RRT-based path planning for selective disassembly planning

The planning of disassembly sequences requires the identification of the extraction trajectories of the different parts or assemblies. The failure to find these trajectories can make a planner fail to generate correct sequences or not evaluate potential solutions. In this paper, we analyze the disassembly path-planning problem, its relation to the general path-planning problem and the main differences between both of them, such as the lack of a target configuration. We present a modification of the rapid-growing random tree-based algorithm (RRT) that addresses these differences. RRTs are easily parallelized so we analyze two different parallelization methods using dual-core-based CPUs as well as the impact of the target selection probability of the algorithm in execution time. The method described is applied to several real-world and synthetic examples.     

Iris on the Move: Acquisition of Images for Iris Recognition in Less Constrained Environments

Iris recognition is one of the most powerful techniques for biometric identification ever developed. Commercial systems based on the algorithms developed by John Daugman have been available since 1995 and have been used in a variety of practical applications. However, all currently available systems impose substantial constraints on subject position and motion during the recognition process. These constraints are largely driven by the image acquisition process, rather than the particular pattern-matching algorithm used for the recognition process. In this paper we present results of our efforts to substantially reduce constraints on position and motion by means of a new image acquisition system based on high-resolution cameras, video synchronized strobed illumination, and specularity based image segmentation. We discuss the design tradeoffs we made in developing the system and the performance we have been able to achieve when the image acquisition system is combined with a standard iris recognition algorithm. The Iris on the Move (IOM) system is the first system to enable capture of iris images of sufficient quality for iris recognition while the subject is moving at a normal walking pace through a minimally confining portal