Prediction of Bubble and Cavity Nucleation in High Damage Rate Irradiation of Simulated Fe-Cr Alloys Using a Hybrid Cluster Dynamics Model

A new generation of safer, more efficient nuclear reactors is perhaps the most viable way to combat climate change by weening the power grid from carbon-laden fossil fuels. However, many of these advanced designs require high temperatures, corrosive environments, and large irradiation doses which degrade structural components. Development of radiation-tolerant materials is crucial, but conventionally requires long, expensive test reactor irradiations which activate the sample, making examination difficult and costly as well. Ion irradiation experiments present an extremely appealing alternative, as they can introduce decades of radiation damage in a matter of days with minimal activation. Unfortunately, these accelerated testing techniques come with a cost of several experimental artefacts which must be accounted for, the most fundamental of which is that damage rate itself has a tremendous impact on its accumulation. Material degradation on an engineering scale is the result of an extraordinary number of individual atomic-scale picosecond damage events generating defects that migrate, coalesce, and dissociate in just nanoseconds continuously over decades of reactor operation. Clever kinetic models that bridge this massive gap in length and time scales are necessary to gain physical insight into radiation damage which will aid in the development of candidate materials designed on a microscopic scale to minimize defect accumulation. This gap is particularly pronounced for the slow, but often life-limiting phenomenon of swelling, where vacant lattice sites eventually accumulate into sizable cavities which uniformly increase the volume of a component beyond its tolerance. The objective of this work is to use cluster dynamics modeling to better understand the temperature, helium generation, and dose rate dependencies of swelling behavior in simulated Ferritic-Martensitic (FM) steels. An initial study using a conventional modeling approach based on reaction rate theory was fundamentally incapable of reproducing several key experimental swelling observations, but most of these were recovered by iteratively adding novel physical mechanisms. Foremost among these discrepancies was an overprediction of swelling rates, a complete lack of incubation periods, and an almost negligible effect of co-generated helium. Introducing a bias toward interstitials to cavities informed by molecular statics calculations, presented a new barrier to cavity nucleation that could only be overcome through the accumulation of stabilizing helium. Since FM steels have low transmutation rates, helium generation rates are relatively low, resulting in long incubation periods and low swelling rates. Second, the temperature shift—an offset in optimal swelling temperature between high and low dose rate conditions—was overpredicted. A heterogeneous nucleation mechanism, promoting helium self-clustering, weakened dose rate dependence by disproportionately facilitating cavity nucleation at low dose rate, decreasing temperature shift. Finally, observations of swelling without helium were rationalized with bias suppression catalyzed by segregation of chemical impurities to the cavity surface. These mechanisms ultimately combine to constitute a more complete, nuanced understanding of cavity nucleation as a function of temperature, dose rate, and helium generation rate. This creates a more complete account of how microstructures evolve under irradiation and strengthens the predictive link between accelerated ion irradiation experiments and the reactor conditions they seek to emulate. Therefore, in addition to contributions to the fundamental science of irradiation damage, this model could inform the design of future experiments to optimize their investigative potential, hastening the qualification of candidate materials.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/162957/1/gvanc_1.pd

Ion-Induced Damage In Si: A Fundamental Study of Basic Mechanisms over a Wide Range of Implantation Conditions

A new understanding of the damage formation mechanisms in Si is developed and investigated over an extended range of ion energy, dose, and irradiation temperature. A simple model for dealing with ion-induced damage is proposed, which is shown to be applicable over the range of implantation conditions. In particular the concept of defect "excesses" will be discussed. An excess exists in the lattice when there is a local surplus of one particular type of defect, such as an interstitial, over its complimentary defect (i.e., a vacancy). Mechanisms for producing such excesses by implantation will be discussed. The basis of this model specifies that accumulation of stable lattice damage during implantation depends upon the excess defects and not the total number of defects. The excess defect model is validated by fundamental damage studies involving ion implantation over a range of conditions. Confirmation of the model is provided by comparing damage profiles after implantation with computer simulation results. It will be shown that transport of ions in matter (TRIM) can be used effectively to model the ion-induced damage profile, i.e. excess defect distributions, by a simple subtraction process in which the spatially correlated defects are removed, thereby simulating recombination. Classic defect studies illuminate defect interactions from concomitant implantation of high- and medium-energy Si+-self ions. Also, the predictive quality of the excess defect model was tested by applying the model to develop several experiments to engineer excess defect concentrations to substantially change the nature and distribution of the defects. Not only are the excess defects shown to play a dominant role in defect-related processing issues, but their manipulation is demonstrated to be a powerful tool in tailoring the implantation process to achieve design goals. Pre-amorphization and dual implantation of different energetic ions are two primary investigative tools used in this work. Various analyses, including XTEM, RBS/channeling, PAS, and SIMS, provided experimental verification of the excess defect model disseminated within this dissertation

Defects in semiconductors and oxides: where are the gaps in first principles theory?

Defects in semiconductors and oxides have been the subject of some of the most sophisticated approaches to modelling and simulation. The powerful, widely used methods can give the impression that all technologically important materials problems can be addressed reliably. But is this so? This paper looks at some of the gaps in first principles theories and at the situations that still warrant attention. Excited states, non-equilibrium systems and non-adiabatic transitions, the correct handling of different length and time scales and the prediction of characteristically quantum behaviour all present challenges. Whilst the emphasis is on semiconductor and oxide systems, the wider context of materials science points to further issues that should not be overlooked

Effects of helium content of microstructural development in Type 316 stainless steel under neutron irradiation

This work investigated the sensitivity of microstructural evolution, particularly precipitate development, to increased helium content during thermal aging and during neutron irradiation. Helium (110 at. ppM) was cold preinjected into solution annealed (SA) DO-heat type 316 stainess steel (316) via cyclotron irradiation. These specimens were then exposed side by side with uninjected samples. Continuous helium generation was increased considerably relative to EBR-II irradiation by irradiation in HFIR. Data were obtained from quantitative analytical electron microscopy (AEM) in thin foils and on extraction replicas. 480 refs., 86 figs., 19 tabs

Downscaling of 0.35 J.lm to 0.25 J.lm CMOS Transistor by Simulation

Silicon (Si) based integrated circuit (IC) has become the backbone of today's semiconductor world with MOS transistors as its fundamental building blocks. The integrated circuit complexity has moved from the early small-scale integration (SSI) to ultra-large-scale integration (ULSI) that can accommodate millions of transistors on a single chip. This evolution is primarily attributed to the concept of device miniaturization. The resulting scaledown devices do not only improve the packing density but also exhibit enhanced performance in terms of faster switching speed and lower power dissipation. The objective of this work is to perform downscaling of 0.35 Jll11 to 0.25 Jll11 CMOS transistor using Silvaco 2-D ATHENA and ATLAS simulation tool. A "two-step design" approach is proposed in this work to study the feasibility of miniaturization process by scaling method. A scaling factor, K of 1.4 (derived from direct division of 0.35 with 0.25) is adopted for selected parameters. The first design step involves a conversion of the physical data of 0.35 Jll11 CMOS technology to the simulated environment, where process recipe acquired from UC Berkeley Microfabrication Lab serves as the design basis. The electrical data for the simulated structure of 0.35 11m CMOS was extracted with the use of the device simulator. Using the simulated, optimized 0.35 Jll11 structure, downscaling to a smaller geometry of 0.25 Jll11 CMOS transistor was carried out and subsequent electrical characterization was performed in order to evaluate its performance. Parameters that are monitored to evaluate the performance of the designed 0.25 Jll11 CMOS transistor include threshold voltage (VtJJ, saturation current (ldsaJ, off-state leakage current (Ion) and subthreshold swing (SJ. From the simulation, the V1h obtained is of 0.51 V and -0.4 V for NMOS and PMOS respectively, with a difference of 15%-33% as compared to other reported work. However, for results of Idsat. the values obtained which is of 296 ~-tAIJll11 for NMOS and 181 J.lA/Jll11 for PMOS is much lower than other reported work by 28%-50%. This is believed to be due to direct scaling of 0.25 Jll11 transistor from the 0.35 11m geometry without alterations on the existing structure. For Ioffand St. both results show a much better value as compared to other work. I off obtained which is of <1 0 pA/J.lm is about 80%-96% lower than the maximum allowable specification. As for S1, the values obtained which is <90 mY/dec is only within 5% differences as compared to specification. In overall, these results (except for Idsat) accepted values for the particular 0.25 J..Lm technology. From this work, the capability to perform device miniaturization from 0.35 J..Lffi to 0.25 J..Lffi has been developed. This is achieved by acquiring the technical know-how on the important aspects of simulation required for successful simulation of 0.35 J..Lffi technology. Ultimately, the outcome of this work which is a simulated 0.25 J..Lm CMOS transistor can be used as a basis for scaling down to a much smaller device, namely towards 90-nrn geometry

First-principles investigation of carbon-based nanomaterials for supercapacitors

Supercapacitors are electrochemical energy storage devices known for their large power densities and long lifetimes yet limited energy densities. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer (EDL), and the low energy to limited electrode surface area (SA). To improve the energy density, the capacitance may be enhanced by using high SA electrode materials such as carbon-based nanomaterials. While promising results have been experimentally reported, capacitances have also been noted to exhibit a highly non-linear relationship with SA. These interesting observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms in the EDL of carbon nanomaterials. Given that EDLs are typically on the order of 1-3 nm thick, theoretical simulations can elucidate these unknown physical insights in order to identify new design principles for future electrode materials. In this dissertation, we explore two broad types of carbon-based nanomaterials, which are separated into two Parts, using a combined density functional theory and classical molecular dynamics computational approach. In Part I, we study the capacitance using various chemically and/or structurally modified graphene (or graphene-derived) materials which is motivated by previous accounts of the limited capacitance using pristine graphene. Our analysis demonstrates the viability of dramatically improving the capacitance using graphene-derived materials owing to enhancements in the quantum capacitance with marginal effects on the double layer capacitance. In Part II, we investigate the capacitance using nanoporous carbon materials which is motivated by experimental observations that relate capacitance to pore width rather than SA. Our findings confirm that promoting ion confinement through pore width control can enhance capacitance, but also identify pore shape dispersity as another important structural feature that facilitates fast ion dynamics during charging/discharging. The work in this dissertation presents an overview of new insights into charge storage mechanisms using low-dimensional carbon-based nanomaterials and future directions for materials development. Moreover, we anticipate that the established methodologies and analyses can be broadly applicable to the study of other applications utilizing electrified interfaces, including capacitive deionization and liquid-gated field effect transistors.Chemical Engineerin

Ion beam modification of metals: Compositional and microstructural changes

Ion implantation has become a highly developed tool for modifying the structure and properties of metals and alloys. In addition to direct implantation, a variety of other ion beam techniques such as ion beam mixing, ion beam assisted deposition and plasma source ion implantation have been used increasingly in recent years. The modifications constitute compositional and microstructural changes in the surface of the metal. This leads to alterations in physical properties (transport, optical, corrosion, oxidation), as well as mechanical properties (strength, hardness, wear resistance, fatigue resistance). The compositional changes brought about by ion bombardment are classified into recoil implantation, cascade mixing, radiation-enhanced diffusion, radiation-induced segregation, Gibbsian adsorption and sputtering which combine to produce an often complicated compositional variation within the implanted layer and often, well beyond. Microstructurally, the phases present are often altered from what is expected from equilibrium thermodynamics giving rise to order-disorder transformations, metastable (crystalline, amorphous or quasicrystalline) phase formation and growth, as well as densification, grain growth, formation of a preferred texture and the formation of a high density dislocation network. All these effects need to be understood before one can determine the effect of ion bombardment on the physical and mechanical properties of metals. This paper reviews the literature in terms of the compositional and microstructural changes induced by ion bombardment, whether by direct implantation, ion beam mixing or other forms of ion irradiation. The topics are introduced as well as reviewed, making this a more pedogogical approach as opposed to one which treats only recent developments. The aim is to provide the tools needed to understand the consequent changes in physical and mechanical properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28153/1/0000605.pd