- ItemMaterials modeling by ab initio methods and machine learning interatomic potentials: a critical assessment(2023) Liu, Yunsheng; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atomistic modeling is a crucial research technique in materials science to simulate physical phenomena based on the interactions of atoms. Density functional theory (DFT) calculation has been the standard technique for evaluating atom interactions, but their applications are limited due to high computation cost. Classical interatomic potential, as another widely adopted technique, provides an alternative by enabling large-scale simulations that are computationally inexpensive. However, the employment of classical potentials for cheap atomistic simulations is achieved in exchange of accurately evaluating atom interactions and the transferability to different chemistries. Most recently, machine learning interatomic potential (MLP) emerges as a new computation technique to bridge the gap between first-principles computation and classical potentials.I first utilized the atomistic modeling based on DFT calculations to find novel Li superionic conductor, a key component of the emerging all-solid-state Li-ion battery technology. I performed a systematic study on the Li-ion conduction of lithium chloride materials system and predicted a dozen potential Li superionic conductors. I revealed that the Li-ion migration in the materials is greatly impacted by the Li content, the cation configuration, and the cation concentrations. I further demonstrated tuning these three factors in designing new chloride Li-ion conductors. Then, I studied the atomistic dynamics predicted by MLPs in comparison to DFT calculations to answer the open question whether MLPs can accurately reproduce dynamical phenomena and related physical properties in molecular dynamics simulations. I examined the current state-of-the-art MLPs and uncovered a number of discrepancies related to atomistic dynamics, defects, and rare events compared to DFT methods. I found that testing averaged errors of MLPs are insufficient and developed evaluation metrics that better indicate the accurate prediction of related properties by MLPs in MD simulations. I further demonstrated that the MLPs optimized by the proposed evaluation metrics have improved prediction in multiple properties. I also study the performance of MLPs in assessing the elemental orderings in a large variety of phases across composition range in alloy system. Using the Li-Al alloy system as a case study, I trained MLPs using only a few phases and the trained MLP demonstrated good performances over a number of existing and other hypothetical materials in- and out- of the training data across the Li-Al binary alloy system. We developed several new evaluation metrics on energy rankings to evaluate the elemental ordering, which is critical for studying the phase stabilities of materials. I tested MLP transferability to other phases and the limits of MLP applications on commonly performed simulations. I also studied the effect of diverse training data on MLP performances. With these efforts evaluating MLP performances for a number of properties, metrics, prediction errors, and dynamical phenomena, I constructed a dataset with a large number of MLPs and their performances and performed an empirical analysis to identify the challenging properties to be predicted by the MLPs. Further, I identified pairs of properties that are challenging to predict. This series of works demonstrated that atomistic modeling is an effective computation technique for studying atomistic dynamic mechanisms, evaluating materials thermodynamics, and guiding materials discovery. As an emerging computational technique, MLPs show good performance on many materials and have great potential to enhance the materials research, but the results show that critical assessments are needed to examine their capabilities to accurately reproduce physical phenomena and understand their limits of performing reliable simulations. My thesis evaluates MLP performances on a number of critical issues related to materials simulations and provides guidance to improve MLPs for future studies.
- ItemMagnetic nanoparticle inks for syringe printable inductors(2023) Fedderwitz, Rebecca; Kofinas, Peter; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct Ink Writing (DIW) additive manufacturing (AM) has the transformative potential to construct complex shapes and devices with a single apparatus by exchanging the printable material at the print head. Iron cobalt (FeCo), permalloy (Ni80Fe20), and iron (II,III) oxide (Fe2O3·FeO) nanoparticles with varying magnetic properties were incorporated in resins to explore the influence of particle loading on printability and inductor device performance. It was generally found that increasing particle loading increased ink viscosity, with a loading maximum approaching 29 – 42 vol% depending on the particle type and resin mixtures due to differences in particle shape and size and resin viscosity. With more magnetic content, composites had higher magnetic permeability and inductance. Syringe printable, colloidal, aqueous magnetic inks were made using both stabilized iron oxide and MnZn doped ferrite nanoparticles with added free polymers. MnZn doped ferrite inks are printed into toroids, sintered to improve magnetic permeability and mechanical robustness, and constructed into an inductor device. Inductors with high magnetic permalloy nanoparticle content were also syringe printed into toroids and hand-wound to demonstrate their viability in fabricating three-dimensional inductors. The effect of particle size on stability and printability was observed. The research presented in this thesis investigates various methods for formulating magnetic nanoparticle inks and evaluates the contributions of particle stabilization, free polymer content, solvent composition, and particle loading on the rheological behavior required for syringe printing. Material properties and device performances were characterized using methods such as zeta potential and settling studies to observe particle functionalization and stability, rheology to study viscoelastic flow behavior, and vector network analysis to measure inductance and device efficiency to showcase the viability of this technique to manufacture passive electronic devices.
- ItemThe Determination of Preferred Orientation in Rolled Electrical Steels Using Single Diffraction of Neutrons(1963) Eugenio, Manuel; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)Preferred orientation in rolled electrical steels has been determined using single diffraction of neutrons from the University of Maryland pool-type nuclear reactor (DMR) operating at 10 KW thermal . X-rays are used extensively to determine preferred orientations in metallic wires and rolled sheets, but X-rays suffer the disadvantage of high absorption and cannot be used effectively on thick samples without chemical or mechanical treatment which ultimately results in the destruction of the samples. The use of reactor neutrons for this purpose is believed to offer particular advantages such as the use of thicker samples and wider beams. To this end, neutrons from the UMR were scattered directly from metallic sheet samples to obtain diffraction patterns from which preferred orientations of the crystallographic axes could be deduced. The neutron diffraction data were obtained in the form of : 1) Maxwellian curves; and 2) rocking curves. To obtain the first type of curve, the sample and neutron detector were rotated at a 1-to-2 angular ratio, respectively, and the diffraction pattern was essentially the Maxwellian neutron energy distribution. From the maximum of the Maxwellian curve, the crystallographic plane mainly responsible for the reflection was calculated; from this, the main orientation was deduced. For the second type of curve, the sample was rocked back and forth, with the neutron detector fixed, and the resulting pattern was used to infer the variation of a given crystallographic direction about its main orientation. The results of this study, particularly on grain-oriented and cube-textured silicon-iron (Si-Fe) alloy sheets demonstrate that single diffraction techniques can be used to determine preferred orientation in highly oriented materials. The results on Si-Fe sheets described as non-oriented indicate the possibility that these techniques may be applicable to ordinary rolled metallic sheets, which are not highly oriented.
- ItemFunctionalized Thin-Film Shape Memory Alloys for Novel MEMS Applications(2023) Curtis , Sabrina M.; Takeuchi, Ichiro; Quandt, Eckhard; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nickel-titanium (NiTi) shape memory alloy (SMA) films are already implemented into microelectromechanical system (MEMS) devices such as sensors, actuators, and implantable medical devices. In this thesis, I used DC magnetron sputter deposition to study the influence of film composition, microstructure, and annealing conditions on the stability of the phase transformation for the NiTi-based SMA thin films TiNiCu, TiNiCuCo, and TiNiHf. SMAs are a type of smart material that can undergo stress or temperature-induced solid-to-solid phase transformation between two different crystalline phases. In NiTi-based SMAs, the two phases are known as martensite with a monoclinic crystalline structure and austenite with a cubic crystal structure. The temperature-induced phase transformations can be used to switch between the martensite and austenite phases, and thus switch between two sets of material properties in the SMA. For example, in NiTi-based SMAs the Young’s modulus, electrical resistivity, and coefficient of thermal expansion of the austenite phase are typically 2X larger than that of the martensite phase. The transformation temperatures, recovery strains, enthalpy of transformation, and fatigue properties of NiTi SMAs can be tuned by alloying NiTi with other elements like copper (Cu), cobalt (Co), and hafnium (Hf). For example, certain compositions of sputtered TiNiCu and TiNiCuCo are known to be ultra-low fatigue SMAs, able to reversibly undergo the phase transformation for 10+ million cycles without degradation in the mechanical or thermal properties. The primary focus of this thesis was the integration of these sputtered NiTi-based SMA thin-films into the following four novel MEMS devices: 1) TiNiCu for magneoelectric sensors, 2) TiNiHf for bistable actuators, 3) TiNiCuCo for stretchable electronics and 4) thin-film SMA stretchable auxetic structures for wearable and implantable medical devices. The shape memory effect was observed in TiNiCu and TiNiHf films when the film thickness and lateral dimensions are downscaled to micro and nano dimensions. In the research publication “Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys.”, I showed the reproducibility of the thermal-induced phase transformation of Ti50Ni35Cu15 is attractive for integration into MEMS devices that require a high cycle lifetime. I showed how the SMA’s phase transformation can be used to tune the resonant of bending cantilever-type sensors like magnetoelectric sensors. I also demonstrated excellent thin-film piezoelectric and shape memory alloy properties for 2 μm AlN/ 5 μm TiNiCu films composites deposited onto silicon substrates. The large work densities and high strength-to-weight ratio offered by SMAs are attractive for the development of micro and nano actuators. The thermal induced phase transformation between martensite and austenite is also used to develop bi-directional micro-actuators with TiNiHf/Si and TiNiHf/SiO2/Si composites. In another research publication, “TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation”, I demonstrated the influence of film thickness and substrate on the phase transformation properties of TiNiHf thin-films. Ti40.4Ni48Hf11.6 films with thicknesses as low as 220 nm on SiO2/Si substrates can undergo the phase transformation with high transformation temperatures (As > 100 °C) and a wide thermal hysteresis (ΔT > 50 °C). In this publication, we explain how the wide hysteresis and high transformation temperature obtained in TiNiHf films can be used to develop micro and nano-scale bistable actuators based on PMMA/TiNiHf/Si composites. Even though thin-film NiTi-based SMAs are known to reversibly recover superelastic strains of up to 8%, surprisingly, they have not yet been exploited in the growing field of stretchable electronics. In the technical article “Thin-Film Superelastic Alloys for Stretchable Electronics” I demonstrate the first experimental and numerical studies of freestanding thin-film superelastic TiNiCuCo structured into a serpentine geometry for use as stretchable electrical interconnects. Fabricated electropolished serpentine structures were demonstrated to have low fatigue after cycling external strains between 30% - 50% for 100,000 cycles. The electrical resistivity of the austenite phase of a Ti53.3Ni30.9Cu12.9Co2.9 thin-film at room temperature was measured to be 5.43 × 10-7 Ω m, which is larger than reported measurements for copper thin-films (1.87 × 10-8 Ω m). Expanding upon this work, in the conference proceedings paper “Auxetic Superelastic TiNiCuCo Sputtered Thin-Films for Stretchable Electronics”, I present a new platform for functionalized wearable electronics and implantable medical devices based on superelastic thin-film SMA substrates structured into novel stretchable auxetic geometries. Since thin-film SMAs are conductive, the structured substrate itself could serve as the current collector for such stretchable and flexible devices, or a more conductive electrode can be deposited on top of the stretchable auxetic SMA substrate. Overall, the results discussed in this doctoral thesis look to the future of harnessing the functional properties of thin-film sputtered SMAs for novel uses in next-generation MEMS devices.
- ItemRADIATION CHEMISTRY IN PRESSURIZED WATER NUCLEAR REACTORS: H2 GENERATION BY 10B(n,α)7Li, AND THE REACTION OF BORATE WITH •OH(2023) Guerin, Steven James; Al-Sheikhly, Mohamad I; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nuclear power plants (NPPs) are complex engineering systems, with malfunctions having enormous potential to lead to widespread and extreme impacts on society and the environment as a whole. Their safe operation depends on a multitude of factors such as intelligent planning, proper design, quality components, high-level safety operations, and economic viability. Due to requiring high temperature and high pressure of an NPP’s cooling fluid, one of the main concerns for further developing safe operating conditions and evaluating component lifetimes is improving our understanding on the issue of corrosion in nuclear systems. In the U.S., all commercially operated Pressurized Water nuclear Reactors (PWRs) are light-water reactors wherein their coolant waters can reach temperatures up to 350 °C. According to a report in 2005 in association with the U.S. Federal Highway Administration, an annual cost of $4.2 billion was directly attributed to corrosion in NPPs in 1998, out of a total $6.9 billion in the electrical utilities industry (Koch, et al., 2005). Boron is added into commercial PWR primary water in the form of boric acid as a soluble chemical neutron “shim” in order to compensate for fuel burnup and allow smooth long-term reactivity control. After a boron nucleus captures a thermal neutron and becomes unstable, the energy of the recoil ions resulting from its fission accounts for up to 33 % of the total dose to the primary water. This event is an important source for H2 and corrosive H2O2, so its product yields must be accurately included in models of the cooling water radiation chemistry. H2 produced in water from the 10B(n,α)7Li fission reaction has been measured up to 300 °C to aid in quantification of the corrosive H2O2 from the same reaction. Thermal energy neutrons from the Rhode Island Nuclear Science Center 2 MW reactor interacted with boric acid contained in N2O-saturated water in temperature-controlled high-pressure cells made from tubing of either titanium or zirconium alloy. After exposure for a minimum of one hour, the solution samples were extracted and sparged with argon. The H2 entrained by the sparging gas was sampled with a small mass spectrometer. A small amount of sodium was included in the boric acid solution so that after sparging, samples could be collected for 24Na activation measurements in a gamma spectrometer to determine the neutron exposure and thus the total energy deposited in solution. The G-value (µmol/J) for H2 production was obtained for water at a pressure of 25 MPa, over a temperature range from 20 °C to 300 °C. These results have been complemented with Monte Carlo N-Particle® (MCNP®) simulations in collaboration with the National Institute of Standards and Technology, and have been compared with previous experimental results at room temperature and simulated results up to 350 °C. Additionally, boric acid has thus far been accepted as a chemically nondisruptive additive, as it was confirmed long ago to have extremely low reactivity with the two main reactive species produced in reactor primary water by radiolysis, the solvated aqueous electron and the hydroxyl radical (e(aq)- and •OH). However, at the Electric Power Research Institute standard desired pH of 7.3 and the operational temperature of 350 °C, approximately 22% of the boron added in PWR primary water exists in the chemical form of the conjugate base, borate, not boric acid. Although borate was previously confirmed to have no appreciable reactions with e(aq)-, it was not adequately studied for reactions with •OH prior to this work. We have observed a clearly apparent reaction between borate and •OH. Current chemistry models are completely ignorant on both the existence of the resultant species and its reactions. The chemical reaction of [B(OH)4]- (borate) with •OH along with cross-reactions of the product species have been studied up to 200 °C to determine those reactions’ rate constants and the products’ spectra. The University of Notre Dame Radiation Laboratory’s 8 MeV electron linear accelerator (LINAC) was configured to perform pulse radiolysis with pulse widths between 4ns to 20ns providing doses between 5.5 Gy and 62 Gy. High-energy electrons from the LINAC interact with the borated solution which has been N2O-saturated and is continuously flowed through a 316 stainless-steel optical cell. The cell temperature was adjusted by resistive-heating silicon cartridges, and pressure was controlled by two syringe pumps to prevent boiling. The cell had two fused silica windows for transmitting light from a xenon arc lamp through the solution and out to a multichromatic spectrophotometer system. Time-resolved spectral data was obtained over nano- and micro-second timeframes, for wavelengths ranging from the deep UV and into the infrared spectrum (250 nm to 820 nm). The reaction rates and products’ spectra were then obtained by analyzing the data using computational aids, namely IGOR Pro by Wavemetrics and KinTek Explorer by KinTek Corp. The product species of the reaction between borate and •OH is conjectured to be •[BO(OH)3]-, on the basis of ab initio calculations, which likely reacts with boric acid or borate to form a polymer radical.