Interstitial Nitrogen in Diamond – Optical and EPR Studies

Nitrogen is one of the most common impurities in diamond. On a substitutional site it acts as a deep donor, approximately 1.7 eV below the conduction band. Irradiation of nitrogen containing diamond and subsequent annealing creates the nitrogen vacancy centre, which has recently attracted much attention for quantum information processing application. Another possible product of irradiation and annealing of nitrogen containing diamond is interstitial nitrogen. Presumably, a mobile carbon interstitial migrates to a substitutional nitrogen to produce an interstitial nitrogen complex which may or may not be mobile. The configuration(s) of interstitial nitrogen related defects (e.g. bond centred, [001]-split) are not known. An infra-red (IR) absorption peak at 1450 cm-1 labelled H1a has been associated with an nitrogen interstitial complex. [1] Theoretical modelling suggested that this IR local mode is due to a bond centred nitrogen interstitial [2]. However, more recent modelling [3] suggests that this defect is mobile at temperatures were H1a is stable and instead assign H1a to two nitrogen atoms occupying a single lattice site in a [001]-split configuration. To date no electron paramagnetic resonance (EPR) spectra have been conclusively associated with an interstitial nitrogen defect.

In this study we present data from new EPR and optical absorption studies in combination with uniaxial stress of nitrogen interstitial related defects in electron irradiated and annealed nitrogen doped diamond. These measurements yield symmetry information about the defects allowing us to determine which of the proposed models are possible. EPR spectra of nitrogen interstitial related defects in samples isotopically enriched with 15N are reported and we show that these explain the lack of previous EPR data for these defects. Correlations between the IR absorbance and the integrated intensity of the new EPR defects are studied for varying irradiation doses and annealing temperatures.

Comparative performances of birnessite and cryptomelane MnO as electrode material in neutral aqueous lithium salt for supercapacitor application

Na-doped Birnessite-type manganese oxide (d-MnO) has been synthesized using the chemical method and characterized through X-ray diffraction and SEM, showing the lamellar structure and high crystal structure. A comparative study of the electrochemical performances of this material with those of the commercial Cryptomelane-type MnO has then been undertaken in ten neutral aqueous electrolytes for supercapacitor applications. Aqueous electrolytes, containing a lithium salt, LiX (where X = SO , NO, CHCO , CHSO, ClO , CHCO, TFSI, Beti, BOB, or Lact), have been first prepared under neutral pH conditions to reach the salt concentration, providing the maximum in conductivity. Their transport properties are then investigated through conductivities, viscosities, and self-diffusion coefficient measurements. Second, the thermal behaviors of these electrolytic aqueous solutions are then evaluated by using a differential scanning calorimeter from (213.15 to 473.15) K in order to access their liquid range temperatures. Cyclic voltammograms (CV) in three electrode configurations are thereafter investigated using Na Birnessite and Cryptomelane as working electrode material from (-0.05 to 1.5) V versus Ag/AgCl at various sweep rates from (2 to 100) mV·s. According to anion nature/structure and manganese oxide material type, different CV responses are observed, presenting a pure capacitive profile for Beti or CH CO and an additional pseudocapacitive signal for the smallest anions, such as ClO and NO . The capacitances, energies, and efficiencies are finally calculated. These results indicate clearly that electrolytes based on a mineral lithium salt under neutral pH condition and high salt concentration (up to 5 mol·L) have better electrochemical performances than organic ones, up to 1.4 V with good material stability and capacity retention. The relationship between transport properties, electrostatic and steric hindrance considerations of hydrated ions, and their electrochemical performances is discussed in order to understand further the lithium intercalation-deintercalation processes in the lamellar or tunnel structure of investigated MnO. © 2013 American Chemical Society.

Molecular dynamics simulation model for the quantitative assessment of tool wear during single point diamond turning of cubic silicon carbide

Silicon carbide (SiC) is a material of great technological interest for engineering applications concerning hostile environments where silicon-based components cannot work (beyond 623 K). Single point diamond turning (SPDT) has remained a superior and viable method to harness process efficiency and freeform shapes on this harder material. However, it is extremely difficult to machine this ceramic consistently in the ductile regime due to sudden and rapid tool wear. It thus becomes non trivial to develop an accurate understanding of tool wear mechanism during SPDT of SiC in order to identify measures to suppress wear to minimize operational cost.

In this paper, molecular dynamics (MD) simulation has been deployed with a realistic analytical bond order potential (ABOP) formalism based potential energy function to understand tool wear mechanism during single point diamond turning of SiC. The most significant result was obtained using the radial distribution function which suggests graphitization of diamond tool during the machining process. This phenomenon occurs due to the abrasive processes between these two ultra hard materials. The abrasive action results in locally high temperature which compounds with the massive cutting forces leading to sp3–sp2 order–disorder transition of diamond tool. This represents the root cause of tool wear during SPDT operation of cubic SiC. Further testing led to the development of a novel method for quantitative assessment of the progression of diamond tool wear from MD simulations.

Brittle-ductile transition during diamond turning of single crystal silicon carbide

In this experimental study, diamond turning of single crystal 6H-SiC was performed at a cutting speed of 1 m/s on an ultra-precision diamond turning machine (Moore Nanotech 350 UPL) to elucidate the microscopic origin of ductile-regime machining. Distilled water (pH value 7) was used as a preferred coolant during the course of machining in order to improve the tribological performance. A high magnification scanning electron microscope (SEM FIB- FEI Quanta 3D FEG) was used to examine the cutting tool before and after the machining. A surface finish of Ra=9.2 nm, better than any previously reported value on SiC was obtained. Also, tremendously high cutting resistance was offered by SiC resulting in the observation of significant wear marks on the cutting tool just after 1 km of cutting length. It was found out through a DXR Raman microscope that similar to other classical brittle materials (silicon, germanium, etc.) an occurrence of brittle-ductile transition is responsible for the ductile-regime machining of 6H-SiC. It has also been demonstrated that the structural phase transformations associated with the diamond turning of brittle materials which are normally considered as a prerequisite to ductile-regime machining, may not be observed during ductile-regime machining of polycrystalline materials.

Novel electrode structure for DMFC operated with liquid methanol

Novel electrode structures for the direct methanol fuel cell (DMFC) based on Ti mesh are reported. A new anode with a hydrophilic structure prepared by coating Pt-Ru catalyst on Ti mesh using thermal decomposition showed a performance comparable to that of the conventional porous carbon-based structure one in DMFC, whilst a cathode with the same structure showed a poor performance. When a porous structure based on Ti mesh pre-coated with carbon was used as the cathode structure, the performance increased significantly to reach that of conventional carbon paper-based cathode. © 2005 Elsevier B.V. All rights reserved.

Identification of CO adsorbed at Ru and Pt sites on a polycrystalline Pt/Ru electrode and the observation of their oxidation and free interchange under open circuit conditions

The spontaneous oxidation of CO adsorbates on a Pt electrode modified by Ru under open circuit (OC) conditions in perchloric acid solution has been followed, for the first time, using in situ FTIR spectroscopy, and the dynamics of the surface processes taking place have been elucidated. The IR data show that adsorbed CO present on both the Ru and Pt domains and can be oxidized by the oxygen-containing adlayer on the Ru in a chemical process to produce CO under OC conditions. There is a free exchange of CO is between the Ru and Pt sites. Oxidation of CO may take place at the edges of the Ru islands, but CO is transfer, at least on the time scale of these experiments, allows the two different populations to maintain equilibrium. Oxidation is limited in this region by the rate of supply of oxygen to die surface of the catalyst. A mechanism is postulated to explain the observed behavior.

Structure and reactivity of the Ru(0 0 0 1) electrode towards fuel cell electrocatalysis

The reactivity of the Ru(0 0 0 1) electrode towards the adsorption and electrooxidation of CO and methanol has been studied by variable-temperature in situ FTIR spectroscopy in both perchloric acid and sodium hydroxide solution, and the results interpreted in terms of the surface chemistry of the Ru(0 0 0 1) electrode. Both linear (CO) and threefold hollow (CO) binding CO adsorbates (bands at 1970-2040 and 1770-1820 cm, respectively) were observed on the Ru(0 0 0 1) electrode in both 0.1 M HClO and 0.1 M NaOH solutions from the CO adsorption. In the acid solution, CO was detected as the main adsorbed species on Ru(0 0 0 1) surface over all the potential region studied. In contrast, in the alkaline solution, more CO than CO was detected at lower potentials, whilst increasing the potential resulted in the transformation of CO to CO. At higher potentials, the oxidation of the adsorbed CO took place via reaction with the active (1 × 1)-O oxide/hydroxide. It was found that no dissociative adsorption or electrooxidation of methanol took place at the Ru(0 0 0 1) at potentials below 900 mV vs Ag/AgCl in perchloric acid solution at both 20 and 55°C. However, in the alkaline solution, methanol did undergo dissociative adsorption, to form linearly adsorbed CO (CO) with little or no CO adsorbed at threefold hollow sites (CO) at both 20 and 55°C. Increasing the temperature from 20 to 55°C clearly facilitated the methanol dissociative adsorption to CO and also enhanced the electrooxidation of the CO. At the higher potentials, significant oxidation of methanol to CO and methyl formate in acid solution and to bicarbonate and formate in alkaline solution, was observed, which was attributed to the formation of an active RuO phase on the Ru(0 0 0 1) surface, in agreement with our previous studies. © 2003 Elsevier Ltd. All right reserved.

Electro-oxidation of CO at the Ru(0001) single-crystal electrode surface

In situ FTIR spectroscopic and electrochemical data and ex situ (emersion) electron diffraction (LEED) and RHEED) and Auger spectroscopic data are presented on the structure and reactivity, with respect to the electro-oxidation of CO, of the Ru(0001) single-crystal surface in perchloric acid solution. In both the absence and the presence of adsorbed CO, the Ru(0001) electrode shows the potential-dependent formation of well-defined and ordered oxygen-containing adlayers. At low potentials (e.g., from -80 to +200 mV vs Ag/AgCl), a (2 × 2)-O phase, which is unreactive toward CO oxidation, is formed, in agreement with UHV studies. Increasing the potential results in the formation of (3 × 1) and (1 × 1) phases at 410 and 1100 mV, respectively, with a concomitant increase in the reactivity of the surface toward CO oxidation. Both linear (CO ) and three-fold-hollow (CO ) binding CO adsorbates (bands at 2000-2040 and 1770-1800 cm , respectively) were observed on the Ru(0001) electrode. The in situ FTIR data show that the adsorbed CO species remain in compact islands as CO oxidation proceeds, suggesting that the oxidation occurs at the boundaries between the CO and O domains. At low CO coverages, reversible relaxation (at lower potentials) and compression (at higher potentials) of the CO adlayer were observed and rationalized in terms of the reduction and formation of surface O adlayers. The data obtained from the Ru(0001) electrode are in marked contrast to those observed on polycrystalline Ru, where only linear CO is observed.