bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterization of Single-walled Carbon Nanotube Fibers and Correlation with Stretch Alignment
Authors: Chen Michelle , Guthy Csaba , Vavro Juraj , Fischer John E. , Badaire Stéphane , Zakri Cécile , Poulin Philippe , Pichot Vincent , Launois Pascale 
Published: April 2011

AbstractStructural, electrical and thermal methods are applied to characterize single-walled carbon nanotube (SWNT) fibers with post-extrusion stretching as the independent variable. HiPco SWNTs are dispersed in water using sodium dodecyl sulfate (SDS), and then co-extruded with polyvinyl alcohol (PVA)/water through a long syringe into a rotating water/PVA coagulation bath. Partial axial alignment is thereby achieved, and further enhanced by applying tension to the flexible green fibers in the coagulation bath. Our findings include: (1) X-ray diffraction shows that the full width at half maximum (FWHM) of the Bragg peaks decreases from 55 (as-extruded) to less than 30 degrees by 80% elongation. That is, SWNT alignment increases linearly with stretch (up to 80%). (2) In resistivity at room temperature vs. stretch ratio, result shows an initial rapid decrease followed by saturation; essentially all the improvement in electronic transport is obtained once alignment reached 40° FWHM. (3) Annealing in hydrogen at 1000°C is performed to drive out PVA, to improve inter-tube and inter-bundle contacts, and to heal damage on the tube walls. Such annealing is found to increase conductivity by at least 4 orders of magnitude. (4) Below 25 K, resistivity vs. temperature of the annealed fiber is well-represented by Coulomb gap variable range hopping (CG-VRH). It is rationalized that the Coulomb interactions in disordered systems open a gap at the Fermi energy. Above 25 K, the thermal energy is greater than the Coulomb gap, so thermal activation is more probable than correlated electron hops. (5) Finally, a measurable thermal conductivity is observed as stretch alignment increases.
Mechanical Properties of CVD Grown Multi-walled Carbon Nanotubes (MWNTs)
Authors: Gaillard J. , Skove M. , Rao A. M. 
Published: April 2011

ABSTRACTWe have measured the bending modulus of several different CVD (chemical vapor deposition) grown MWNTs using a vibrating reed technique. The MWNTs are produced from a thermal decomposition of three different precursors: (i) xylene/ferrocene, (ii) xylene/ferrocene/melamine (nitrogen-doped), and (iii) trimethylamine (TMA)/ferrocene. The first two precursors are used to compare the mechanical properties of typical CVD-grown to bamboo-type MWNTs. Nanotubes prepared using the third precursor shows relatively fewer walls (∼ 4–20 compared to ∼15–40)) and defects compared to those prepared from the xylene/ferrocene mixture. The resonant frequencies of these nanotubes were measured optically in air using a dark field light microscope. The diameters of these nanotubes range from 50 – 160 nm as determined from TEM and the average length is ∼ 10 microns. For the xylene/ferrocene and trimethylamine/ferrocene tubes, the average bending modulus is estimated to be 0.1 and 0.3 TPa, respectively. However, the bending modulus for the nitrogen-doped tubes is ∼9 GPa which is significantly lower compared to regular MWNTs implying that the bending modulus decreases with an increase in wall defects. These findings are in reasonable agreement with those reported previously by Wang et al.
Carbon nanotube surface chemistry and its effects on interfacial nanomechanics
Authors: Barber Asa H. , Cohen Sidney R. , Daniel H. 
Published: April 2011

ABSTRACTIndividual multi-walled carbon nanotube pullout experiments were used to measure the adhesion strength at a nanotube-epoxy polymer interface. The interfacial strength was found, as expected, to increase when the nanotubes were chemically treated to induce strong bonding with the polymer matrix. At long nanotube embedment lengths within the polymer, the nanotubes were seen to fracture in preference to failure at their interface with the polymer. Interfacial mechanics models are applied to the data to describe interfacial adhesion at the nano-level.
Kohn Anomalies in Graphite and Nanotubes
Authors: Piscanec S. , Lazzeri M. , Ferrari A. C. , Mauri F. , Robertson J. 
Published: April 2011

ABSTRACTAtomic vibrations are partially screened by electrons. In a metal this screening can change rapidly for vibrations associated to certain points of the Brillouin zone, entirely determined by the shape of the Fermi surface. The consequent anomalous behaviour of the phonon dispersion is called Kohn anomaly. Graphite is a semimetal. Nanotubes can be metals or semiconductors. We demonstrate that two Kohn anomalies are present in the phonon dispersion of graphite and that their slope is proportional to the square of the electron-phonon coupling. Metallic nanotubes have much stronger anomalies than graphite, due to their reduced dimensionality. Semiconducting nanotubes have no Kohn anomalies.
Beyond-Mean-Field Treatments of Island Formation during Submonolayer Deposition: Island Size Distributions for Large Critical Sizes
Authors: Li Maozhi , Bartelt Maria C. , Evans J. W. 
Published: April 2011

ABSTRACTKinetic Monte Carlo (KMC) simulation of atomistic models reveals the failure of mean-field treatments of the island size distribution (ISD) for islands formed by homogeneous nucleation during submonolayer deposition on perfect surfaces. KMC also facilitates analysis of scaling properties of the ISD, although here some misperceptions persist which we attempt to clarify. However, KMC becomes inefficient for highly reversible island formation (e.g., for large values of a critical size, i, above which islands are stable) due to the high density of diffusing adatoms on the surface. This reduced efficiency is quantified here with results for CPU time versus i. This feature has motivated development of alternative beyond-mean-field coarse-grained approaches which should be more efficient for large i. We provide results for the ISD for a range of i = 1, 2, 3, and 6 using one such approach, a stochastic geometry-based simulation (GBS) strategy.
Diffusion in Potential Fields: Time-Dependent Capture on Radial and Rectangular Substrates
Authors: Venables J. A. , Yang P. 
Published: April 2011

ABSTRACTIn models of nucleation and growth on surfaces, it is usually assumed that the energy surface of the substrate is flat, that diffusion is isotropic, and that capture numbers can be calculated in the diffusion-controlled limit. We lift these restrictions analytically, and introduce a hybrid discrete FFT method of solving for the 2D time-dependent diffusion field of adparticles on general non-uniform substrates. The method, with periodic boundary conditions, is appropriate, for example, following nucleation on a regular (rectangular) array of defects. The programs, which have been realized in Matlab®6.5, are instructive for visualizing potential and diffusion fields, and for producing illustrative movies of crystal growth under various conditions. Here we demonstrate the time-dependence of capture numbers in the initial stages of annealing at high adparticle concentration in the presence of repulsive adparticle-cluster interactions; however it is clear that the method works in general for deposition, growth and annealing at all times.
Evaluating R&D Progress in the Context of the Safety Case
Authors: Johnson L.H. , Zuidema P. 
Published: April 2011

ABSTRACTThe strategic approach and structuring of safety cases for nuclear waste repositories have seen marked improvements over the past ten to fifteen years as a result of the collective experience of national agencies in developing their safety cases and their efforts, under the auspices of the OECD Nuclear Energy Agency (NEA), to develop a systematic and thorough approach, as well as through systematic reviews by regulators and expert teams. A safety case can be developed at any stage in a program and it provides a remarkably effective method for reassessing design and siting requirements and refocusing R&D needs. The process of developing a safety case for a repository is the critical test of the quality of understanding that exists for a given repository concept, because it forces an integration of the siting and design strategies and the assessment approach. This integration demands that one evaluates all relevant R&D results available and addresses the questions i) is the system good? and ii) is the understanding sufficient, taking the stage of the programme into account?Here we address some questions on the relationship between R&D work and the development of a safety case, including:1) Is the R&D performed in Europe evolving in concert with safety case development for European repositories?2) How much generic work is still needed when robust site-specific repository concepts obviously lead to much more focused R&D?3) How good is good enough and how do we measure this at each successive stage of repository development?There are no easy answers to such questions, but they must increasingly be addressed as national programs progress.
Determining the Transport Properties of Rock Specimens Using an Improved Laboratory Through-Diffusion Technique
Authors: Zhang M. , Takeda M. , Nakajima H. 
Published: April 2011

ABSTRACTThe safe disposal of radioactive nuclear wastes using multi-barrier systems requires a good understanding of their transport properties in geological materials. When groundwater flow is very slow, the most important mechanisms of transport and retardation are diffusion through and sorption onto geological materials, and an evaluation of relevant parameters, particularly the effective diffusion coefficient and rock capacity factor (or sorption coefficient), is of fundamental importance to any safety assessment. Although laboratory diffusion tests can be used to determine these two parameters, conventional through-diffusion testing has some limitations, such as the need for a relatively long test time, cumbersome test procedures and the possibility of errors due to differences between analytical assumptions and actual test conditions. In this paper, we offer a rigorous solution to the through-diffusion test. Boundary conditions are improved by taking into account concentration changes in both the source and the measurement cells. A companion approach for back-calculating the effective diffusion coefficient and rock capacity factor is also proposed. The effectiveness and advantages of this improved technique are demonstrated using experimental data derived from a sedimentary rock sample taken from a research site in Japan.
Coupling of Chemical Processes in the Near Field
Authors: Grambow B. , Giffaut E. 
Published: April 2011

ABSTRACTCoupled modelling has been performed using geochemical/transport codes and radiolysis models to describe the chemical evolution of the waste forms “high-level waste glass” and “spent nuclear fuel” together with its waste package and engineered barrier surroundings. Near field processes considered include container corrosion, hydrogen generation, mass transfer for radionuclides and other waste matrix components in corrosion products and buffer materials, geochemistry of near field solution chemistry, sorption of radionuclides on surface sites in the nano-sized pore space of near field materials and the radiolytic decomposition of pore water. The rate limiting steps in waste form dissolution and secondary phase formation mechanism and the associated radionuclide mobilisation chemistry (solubility, solid solution formation, speciation, redox stability) are strongly influenced by the near field constraints.
Determining Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as a Driving Force for Migration
Authors: André Magnus , Malmström Maria E. , Neretnieks Ivars 
Published: April 2011

ABSTRACTThe transport of radionuclides in indigenous rock is greatly affected by the sorption of cations in the porous rock matrix. For the determination of sorption coefficients, batch experiments have traditionally been used. For these experiments, the rock sample is crushed into fine particles to reduce the experimental time. However, this procedure increases the specific surface area of the sample and the new surfaces created could have different sorption qualities than the naturally occurring surfaces, which may impair the results of sorption coefficient determinations. A new method for determining sorption coefficients in intact rock is being developed, using electromigration as a means to speed up the transport process, thereby allowing for faster equilibration between the rock sample and the tracer solution. Here, we report results from preliminary experiments, using cesium as a sorbing tracer, showing a consistent difference between sorption coefficients obtained using electromigration methods on intact rock samples and traditional batch experiments on crushed samples.
Further Steps Towards Licensing: Underground Characterisation Started for the Spent Fuel Repository in Finland
Authors: Vira Juhani 
Published: April 2011

ABSTRACTPursuant to the Decision-in-Principle of 2001 the Finnish programme for geologic disposal of spent fuel has now moved to the phase of underground characterisation of the repository site. The main objective of this programme phase is to confirm – or refute – the suitability of the Olkiluoto site by investigations conducted underground at the actual depth of the repository. The construction work of the access tunnel to the rock characterisation facility (ONKALO) started in the late summer of 2004. The site research and investigations work aims at the maturity needed for submission of the application for construction license of the actual repository in 2012. This requires, however, that also the technology has reached the maturity needed. The design and technical plans form the necessary platform for the development of the safety case for spent fuel disposal. A plan, “road map”, has been produced for the portfolio of reports that should demonstrate the safety of disposal as required by the criteria set by the government and further detailed by the safety authority, STUK.
Role of ion exchange in the corrosion of nuclear waste glasses
Authors: Ojovan M.I. , Lee W.E. , Hand R.J. 
Published: April 2011

ABSTRACTThe ion exchange phase of corrosion of nuclear waste glasses was modelled using Doremus' theory of interdiffusion and numerically analysed for British Magnox waste and Russian K-26 glasses. It is shown that even in non-silica -saturated conditions the ion exchange phase plays a significant role in the overall radionuclide release inventory particularly for short-lived radionuclides.
Migration Behavior of Cesium in Compacted Bentonite Under Reducing Conditions Using Electromigration
Authors: Idemitsu Kazuya , Yamamoto Masaru , Yamasaki Yosuke , Inagaki Yaohiro , Arima Tatsumi 
Published: April 2011

ABSTRACTCarbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The migration of iron corrosion products through the buffer material will affect migration of redox-sensitive radionuclides. Therefore the authors have carried out electromigration experiments with source ofiron ions supplied by anode corrosion of iron coupons in compacted bentonite. However, their migration behavior was complex and difficult to explain. Thus, authors tried to use cesium, whose migration behavior is well known, inthis experimental configuration to obtain knowledge of the migration behavior of cations. The concentrationsof iron and sodium showed nearly complementary distributions. It is expected that iron ion could migrate as ferrous ion through the interlayer of montmorillonite replacing exchangeable sodium ions in the interlayer. On the other hand, cesium profiles seemed to be controlled by ordinary diffusion. Drift of the cesium profile by electric potential gradient could be observed clearly only after 240 h. Apparent dispersion coefficients of cesium were calculated from the profiles and were in reasonable agreement with literature values of apparent diffusion coefficients. Thus this experiment can provide a diffusion field for cations under a reducing condition with ferrous ions in water-saturated bentonite. The effect of electro-osmotic flow on ion migration was negligibly small in this experiment because electro-osmotic flow was compensated by hydraulic pressure caused by the water content gradient developed in the specimen within 24h.
Modeling Corrosion Processes for Alloy 22 Waste Packages
Authors: Dunn D.S. , Pensado O. , Pan Y.-M. , Yang L.T. , He X. 
Published: April 2011

ABSTRACTThe proposed waste package design for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, consists of an outer container made of Alloy 22, a corrosion resistant Ni-Cr-Mo-W alloy, surrounding an inner container made of Type 316 nuclear grade stainless steel. Models to assess the influence of waste package degradation modes on the overall system performanceconsider uniform and localized corrosion processes. Based on measurements of passive anodic current density, the uniform aqueous corrosion rate of Alloy 22 is estimated to be slow, and long waste package lifetimes are projected in the absence of conditions that promote accelerated degradation processes. The initiation of localized corrosion is possible in chloride-containing waters with low concentrations of inhibiting oxyanions such as nitrate. Although propagation rates for localized corrosion are typically orders of magnitude greater than the passive uniform corrosion rates, the maximum penetration depth of localized attack may be limited to depths significantly less than the container thickness as a result of stifling andarrest of localized corrosion.
A Model for Predicting Stress Corrosion Cracking of Copper Containers in A Deep Geologic Repository
Authors: Maak Peter , King Fraser 
Published: April 2011

ABSTRACTA model has been developed to predict the stress corrosion cracking behaviour (SCC) of used-fuel copper containers in a deep geologic repository. By comparison of the predicted evolution of the environment with criteria for the SCC of pure copper, it is concluded that copper containers in a deep geologic repository will not be susceptible to SCC. In particular, the requirement for the simultaneous presence of a thermodynamically stable Cu2O/CuO surface film and a significant concentration of SCC agents is not satisfied at any time during the 106 year simulation period.
Radiation stability of natural britholites
Authors: Yudintseva Tatiana S. 
Published: April 2011

ABSTRACTBritholite, Ca-REE silicate with apatite structure, is an actinide host phase occurred in vitreous borosilicate waste forms. Such glass-ceramics are considered as potential host phases for immobilization of actinide-containing high-level waste. Crystalline phases have to be radiation resistant for this application. Radiation stability of the britholites was mainly studied by either heavy ions irradiation or incorporation of Cm-244 or Pu-238 and 240. A wide range of critical doses (0.15 - 0.6 dpa at 25°C) and temperatures have been obtained depending on the compositions of the samples. Natural analogue study of the waste forms allows to predict the behavior of actinide host phases for long periods after disposal. Britholites with age from 320 to 2600 millions years, ThO2+UO2 content from 1.0 to 12 wt.%, and cumulative doses from 0.6×1019 to 7.7×1019 α-decays/g have been studied. The britholite becomes amorphous at a dose of 1 dpa (0.9×1019 α-decays/g) and higher. Critical doses for natural minerals are higher than those for synthetic samples, most likely due to re-crystallization during annealing.
Creep of the copper canister in the Swedish program
Authors: Bowyer William H 
Published: April 2011

ABSTRACTThe Swedish programme for disposal of high level nuclear waste includes the development of a container which comprises a cast iron load bearing canister contained in a 50 mm thick copper corrosionshield.The temperature of the outside of the canister is likely be up to 100°C, and repository processes may lead to long term loads of up to 50MPa. Creep of the copper is therefore an issue. SKB (The Swedish Nuclear Fuel and Waste Management Co.) have been conducted tests on three materials, OF copper containing 10ppm sulphur (OF1), OF copper containing 6ppm sulphur (OF2) and OF copper containing 6ppm sulphur and 50ppm phosphorus (OFP). In order to improve confidence in extrapolation of test results to practical temperatures and stresses, it is desirable to support the extrapolation procedures with a physical model.Life predictions made using the Frost and Ashby model [5] together with a simplification of the Cocksand Ashby model [6] provide good agreement with published experimental data for OF2 and OFPmaterials. Extrapolation of this data using the model leads to life predictions of 12,000 years for the OF2material and 120,000 years for OFP material. The prediction for OFP depends on an assumption that an observed strengthening mechanism conferred by phosphorus at high stresses and temperatures, is equally as effective under repository conditions.OF1 material fails after a relatively short life and by a different mechanism to OF2 and OFP. The changein mechanism may be explained, using the model [6], for cases where segregating species reduce surface energy of grain boundary voids. It is suggested that in this case the segregating species is sulphur.
Further Studies of the Anaerobic Corrosion of Steel in Bentonite
Authors: Smart Nick R. , Rance Andrew P. , Carlson Liisa , Werme Lars O. 
Published: April 2011

ABSTRACTIn the horizontal emplacement concept (KBS-3H) for the disposal of radioactive waste, which is being developed in Sweden and Finland, copper canisters with cast iron inserts will be surrounded by bentonite buffer and mounted in perforated carbon steel support structures in boreholes within the bedrock. The groundwater will be reducing, leading to anaerobic corrosion of the ferrous material. It is important to understand both the effect of bentonite on the corrosion behaviour of the steel and the effect of the corrosion products on the performance of the bentonite. Previous work on the corrosion of steel in bentonite was extended to investigate a wider range of conditions, including the possible effects of alkaline plumes released from concrete support structures and the effect of chloride concentration and temperature on the corrosion rate of steel in bentonite. Corrosion rates were measured by collecting hydrogen produced by the anaerobic corrosion of iron. In addition, a range of analytical techniques was applied to study the composition and morphology of the corrosion products and the distribution and chemical state of the iron released into the bentonite. Comparison was also made between corrosion in compacted bentonite and artificial bentonite porewater. In the presence of bentonite, the corrosion product layer was relatively thin compared to fully aqueous conditions, probably because the ferrous ions released by corrosion exchanged with the bentonite interlayer or attached to the surface of the bentonite grains, rather than forming a separate iron oxide phase.