Damage production, accumulation and materials performance in radiation environment

Collisions of energetic projectile particles with host atoms produce atomic displacements in the target materials. Subsequently, some of these displacements are transformed into lattice defects and survive in the form of single defects and defect clusters. Depending on the ambient temperature, these defects and their clusters diffuse, interact, annihilate, segregate and accumulate in various forms and are responsible for the evolution of the irradiation-induced microstructure. Naturally, both physical and mechanical properties and thereby the performance and lifetime of target materials are likely to be determined by the nature and the magnitude of the accumulated defects and their spatial dispositions. A multitude of processes covering a variety of temporal and spatial scales contribute to the evolution of the global microstructure. Results of computer simulations as well as theoretical modelling describing some of these processes will be reviewed and discussed. The framework within which the influence of irradiation on void swelling, radiation hardening and loss of ductility can be treated will be discussed. It will be emphasized that the nature of displacement damage production plays an important role in the evolution of the global microstructure. Finally, a brief summary of the current status in the fields of computer simulations and theoretical modelling is presented in the form of concluding remarks. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO2 storage in geological media: Role,means, status and barriers to deployment

Carbon dioxide capture and geological storage is an enabling technology that will allow the continued use well into this century of fossil fuels, mainly coal, for power generation and combustion in industrial processes because they are relatively abundant, cheap, available and globally distributed, thus enhancing the security and stability of energy systems. Geological media suitable for CO₂ storage through various physical and chemical trapping mechanisms must have the necessary capacity and injectivity, and must confine the CO₂ and impede its lateral migration and/or vertical leakage to other strata, shallow potable groundwater, soils and/or atmosphere. Such geological media are mainly oil and gas reservoirs and deep saline aquifers that are found in sedimentary basins. Storage of gases, including CO₂, in these media has been demonstrated on a commercial scale by enhanced oil recovery operations, natural gas storage and acid gas disposal. Some of the risks associated with CO₂ capture and geological storage are similar to, and comparable with, any other industrial activity for which extensive safety and regulatory frameworks are in place. Specific risks associated with CO₂ storage relate to the operational (injection) phase and to the post-operational phase, of which the risks of most concern are those posed by the potential for acute or chronic CO₂ leakage from the storage site. Notwithstanding the global climate effect of CO₂ returning to the atmosphere, the local risks to health and safety, environment and equity need to be properly assessed and managed. Currently there are very few operations in the world where CO₂ is injected and stored in the ground, mostly if not exclusively as a by-product of an operation driven by other considerations than climate change, such as oil production or regulatory requirements regarding H₂S. These operations show that there are no major technological barriers to CO₂ geological storage, and that challenges and barriers lie elsewhere. A major challenge in the implementation of CO₂ geological storage is the high cost of CO₂ capture, particularly for dilute streams like those from power plants and industrial combustion processes. There are concerns that public opinion and public's acceptance or rejection of this technology will likely affect the large-scale implementation of CO₂ geological storage. The current paucity of policy, legislation and a proper regulatory framework in most jurisdictions is presently the most significant barrier. The resolution of these challenges will affect the economics and financial risk of CO₂ geological storage and will accelerate or delay the deployment of this technology for reducing anthropogenic CO₂ emissions into the atmosphere.     

Modeling the performance of a solar pond as a source of thermal energy

The use of solar ponds is becoming more attractive in today's energy scene. A major advantage of solar ponds over other collectors is the ability to store thermal energy for long periods of time. The solar pond comprises a hydraulic system subject to processes of heat and mass transfer. The design of this system and the related equipment requires a thorough knowledge of the pond heating-up process and expected thermohaline structure within the pond. The current study considers that convection currents in the pond are inhibited by the salinity distribution, and applies a finite difference implicit model in order to investigate the interaction among physical variables represented by various dimensionless parameters. Variables which are included in the analysis comprise the solar radiation input and absorption as it passes through the pond; diffusion and dispersion of heat within the pond; absorption of heat at the bottom of the pond; and withdrawal of heat from layers within the pond. The physical variables generate 3 dimensionless variables associated with the pond's heating-up process. A 4 dimensionless variable is associated with the heat utilization. The analysis represented in this paper concerns the interaction between these dimensionless parameters and its implications.     

Semianalytical solution for CO2 leakage through an abandoned well

Capture and subsequent injection of carbon dioxide into deep geological formations is being considered as a means to reduce anthropogenic emissions of CO2 to the atmosphere. If such a strategy is to be successful, the injected CO2 must remain within the injection formation for long periods of time, at least several hundred years. Because mature continental sedimentary basins have a century-long history of oil and gas exploration and production, they are characterized by large numbers of existing oil and gas wells. For example, more than 1 million such wells have been drilled in the state of Texas in the United States. These existing wells represent potential leakage pathways for injected CO2. To analyze leakage potential, modeling tools are needed that predict leakage rates and patterns in systems with injection and potentially leaky wells. A new semianalytical solution framework allows simple and efficient prediction of leakage rates for the case of injection of supercritical CO2 into a brine-saturated deep aquifer. The solution predicts the extent of the injected CO2 plume, provides leakage rates through an abandoned well located at an arbitrary distance from the injection well, and estimates the CO2 plume extent in the overlying aquifer into which the fluid leaks. Comparison to results from a numerical multiphase flow simulator show excellent agreement. Example calculations show the importance of outer boundary conditions, the influence of both density and viscosity contrasts in the resulting solutions, and the potential importance of local upconing around the leaky well. While several important limiting assumptions are required, the new semianalytical solution provides a simple and efficient procedure for estimation of CO2 leakage for problems involving one injection well, one leaky well, and multiple aquifers separated by impermeable aquitards.     

Synthesis of Graphene–Magnetite Nanoparticle Composite Using Mechanical Milling and Electrochemical Exfoliation

This work describes a two-step methodology for synthesizing a magnetite nanoparticle-graphene composite. In the first step, iron metal powder was ball milled with graphite to produce a mixture that was subsequently compacted to produce a pellet. In the second step, electrochemical exfoliation of this pellet was performed to produce graphene decorated with magnetite nanoparticles. Formation of graphene was established by methods such as x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. The formation of magnetite nanoparticle-graphene composite was established by transmission-electron-microscopy-based imaging and electron diffraction methods. The magnetite nanoparticles on the graphene sheet had an average size of ~12 nm and were nonuniformly distributed.     

Controllable p-Type Doping of 2D WSe2 via Vanadium Substitution

Scalable substitutional doping of 2D transition metal dichalcogenides is a prerequisite to developing next-generation logic and memory devices based on 2D materials. To date, doping efforts are still nascent. Here, scalable growth and vanadium (V) doping of 2D WSe₂ at front-end-of-line and back-end-of-line compatible temperatures of 800 and 400 °C, respectively, is reported. A combination of experimental and theoretical studies confirm that vanadium atoms substitutionally replace tungsten in WSe₂, which results in p-type doping via the introduction of discrete defect levels that lie close to the valence band maxima. The p-type nature of the V dopants is further verified by constructed field-effect transistors, where hole conduction becomes dominant with increasing vanadium concentration. Hence, this study presents a method to precisely control the density of intentionally introduced impurities, which is indispensable in the production of electronic-grade wafer-scale extrinsic 2D semiconductors.     

Sequestration of CO2 in geological media in response to climate change: capacity of deep saline aquifers to sequester CO2 in solution

Geological sequestration is a means of reducing anthropogenic atmospheric emissions of CO₂ that is immediately available and technologically feasible. Among various options, CO₂ can be sequestered in deep aquifers by dissolution in the formation water. The ultimate CO₂ sequestration capacity in solution (UCSCS) of an aquifer is the difference between the total capacity for CO₂ at saturation and the total inorganic carbon currently in solution in that aquifer, and depends on the pressure, temperature and salinity of the formation water. Assuming non-reactive aquifer conditions, the current carbon content is calculated using standard chemical analyses of the formation waters collected by the energy industry on the basis of the concentration of carbonate and bicarbonate ions. Formation water analyses performed at laboratory conditions are brought to in situ conditions using a geochemical speciation model to account for dissolved gasses that are lost from the water sample. To account for the decrease in CO₂ solubility with increasing water salinity, the maximum CO₂ content in formation water is calculated by applying an empirical correction to the CO₂ content at saturation in pure water. The UCSCS in an aquifer is calculated by considering the effect of dissolved CO₂ on the formation water density, the aquifer thickness and porosity to account for the volume of water in the aquifer pore space and for the mass of CO₂ dissolved in the water currently and at saturation. The methodology developed for estimating the ultimate CO₂ sequestration capacity in solution in aquifers has been applied to the Viking aquifer in the Alberta basin in western Canada. Considering only the region where the injected CO₂ would be a dense fluid, the capacity of the Viking aquifer to sequester CO₂ in solution in the formation water is calculated to be 100 Gt. Simple estimates then indicate that the capacity of the Alberta basin to sequester CO₂ dissolved in the formation waters at depths greater than 1000 m is on the order of 4000 Gt CO₂. The results also show that using geochemical models to bring the analyses of the formation waters to in situ conditions is not warranted when the current total inorganic carbon (TIC) in the aquifer water is very small by comparison with the CO₂ solubility at saturation. Furthermore, in such cases, the current TIC may even be neglected.