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A seven-element Modified Embedded Atom Method (MEAM) potential comprising Fe, Mn, Si, C, Al, Zn, and O is developed by employing a hierarchical multiscale modeling paradigm to simulate low-alloy steels, inhibition layer, and galvanized coatings. Experimental information alongside first-principles calculations based on Density Functional Theory served as calibration data to upscale and develop the MEAM potential. For calibrating the single element potentials, the cohesive energy, lattice parameters, elastic constants, and vacancy and interstitial formation energies are used as target data. The heat of formation and elastic constants of binary compounds along with substitutional and interstitial formation energies serve as binary potential calibration data, while substitutional and interstitial pair binding energies aid in developing the ternary potential. Molecular dynamics simulations employing the developed potentials predict the thermal expansion coefficient, heat capacity, self-diffusion coefficients, thermomechanical stress-strain behavior, and solid-solution strengthening mechanisms for steel alloys comparable to those reported in the literature. Interfacial energies between the steel substrate, inhibition layer, and surface oxides shed light on the interfacial nanostructures observed in the galvanizing process.

Doctor of Philosophy / Startups can and do fail. For an entrepreneur, product developer, or researcher with a physical and capital-intensive product idea, this dissertation can serve as a resource to bridge the gaps between business, engineering, and design and reduce the risk of failure when trying to create a startup. The process described in this dissertation describes how to evaluate the key elements of an idea and conduct a series of interviews with potential customers to find evidence that supports pursing that idea further, challenge the startup team to change some aspect of the idea, or drop it altogether. Once the startup team has found a problem, as well as a solution to that problem, this dissertation describes an approach creating that solution. Then this dissertation describes an approach for critically evaluating the foundational elements of the problem and the solution. The goal for a critical evaluation is to identify additional foundational elements which relate to the product that may increase its value or decrease the risk of product failure.

Thesis advisor: David Broido / Despite its deceptive simplicity and because of its scientific and technological importance, bcc Fe is still the subject of research and debate. We develop an ab initio theoretical framework and apply it to calculate temperature-dependent phonon modes and magnetic interaction parameters in bcc Fe. This framework incorporates realistic thermal disorder in a coupled spin-lattice system. Thermal spin-lattice coupling is found to significantly renormalize the phonon modes and magnetic interaction strength, resulting in significant temperature-dependencies. A method for treating magnetic systems of unknown entropy is developed and applied to calculate phonon modes and investigate the anomalous thermal expansion of the classical invar alloy, Fe0.65Ni0.35. Results over the temperature range 50K to room temperature are consistent with the observed low thermal expansion of this material. Excellent agreement with measured data is achieved for calculated phonon modes in both bcc Fe and the invar alloy. / Thesis (PhD) — Boston College, 2020. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

<p dir="ltr">First-principles method is a powerful approach to study atomic scale physics. With its introduction into thermal transport community, the \textit{ab initio} description of quantized lattice vibrations, phonons, achieved great success in predicting thermal transport properties in the past decade. Though such method is well established, recent theoretical and experimental efforts uncovered new physics and raised new challenges to our community. In particular, high-order phonon anharmonicity, which was assumed to be negligible, shows great impact on thermal transport. Highly nonequilibrium electron and phonon transport occurs in emerging materials with nonuniform temperature field and the equilibrium assumption is no longer valid. Finite temperature effect is found to change the potential landscape even in systems that are quite harmonic, and the previous quasi-harmonic approximation fails. These physical understandings are also closely related to applications that are being extensively studied today: high thermal conductivity materials in thermal management, hot electrons phenomenon in thermal photovoltaic, high temperature radiative properties in thermal barrier coatings, etc.</p><p><br></p><p dir="ltr">In this Dissertation, we seek to establish new physical understanding in thermal transport by studying four-phonon scattering, phonon nonequilibrium behavior, phonon renormalization scheme and their interplay in a wide range of solid state systems. For the benefit of the community, we develop an efficient open-source computational program, \textsc{FourPhonon}, and keep updating its core features to drive sustained scientific innovations. This program is capable of calculating phonon-phonon scattering rates up to the fourth-order and the lattice thermal conductivity of solids ($\kappa$).</p><p><br></p><p dir="ltr">The Raman peak position and linewidth provide insight into phonon anharmonicity and electron-phonon interactions in materials. For monolayer graphene, prior first-principles calculations have yielded decreasing linewidth with increasing temperature, which is opposite to measurement results. Here, we explicitly consider four-phonon anharmonicity, phonon renormalization, and electron-phonon coupling, and find all to be important to successfully explain both the $G$ peak frequency shift and linewidths in suspended graphene sample over a wide temperature range. Four-phonon scattering contributes a prominent linewidth that increases with temperature， while temperature dependence from electron-phonon interactions is found to be reversed above a doping threshold ($\hbar\omega_G/2$, with $\omega_G$ being the frequency of the $G$ phonon).</p><p><br></p><p dir="ltr">While the Raman spectra concerns one particular optical phonon mode, we move to consider $\kappa$ that is determined by full phonon spectrum. The thermal conductivity of monolayer graphene is widely believed to surpass that of diamond even for few-micron size samples and was proposed to diverge with system size. Here, we predict the thermal conductivity from first principles by considering four-phonon scattering, phonon renormalization, an exact solution to phonon Boltzmann transport equation (PBTE), and an unprecedented sampling grid. We show that at room temperature the thermal conductivity saturates at 10~$\rm\upmu m$ size and above and converges to 1300~W/(m$\cdot$K), which is lower than that of diamond. This indicates that four-phonon scattering overall contributes 57\% to the total thermal resistance and becomes the leading phonon scattering mechanism over three-phonon scattering. On the contrary, considering three-phonon scattering only yields higher-than-diamond values and divergence with size due to the momentum-conserving normal processes of flexural phonons.</p><p><br></p><p dir="ltr">Higher-order phonon scattering affects heat conduction and thermal radiation at high temperature to a larger degree than at room temperature. We establish a computational framework to compute temperature-dependent full spectrum optical properties and high temperature $\kappa$ of ceramics materials. From ultraviolet to mid-infrared region, light-matter interaction mechanisms in semiconductors progressively shift from electronic transitions to phononic resonances and are affected by temperature. Here, we present a parallel temperature-dependent treatment of both electrons and phonons entirely from first principles, enabling the prediction of full-spectrum optical responses. At elevated temperatures, \textit{ab initio} molecular dynamics (AIMD) is employed to find thermal perturbations to electronic structures and construct effective force constants describing potential landscape. Four-phonon scattering and phonon renormalization are included in an integrated manner in this approach. As a prototype ceramic material, cerium dioxide (CeO$_2$) is considered. Our first-principles calculated refractive index of CeO$_2$ agrees well with measured data from literature and temperature-dependent ellipsometer experiment.</p><p><br></p><p dir="ltr">The lattice thermal conductivity ($\kappa$) of two ceramic materials, CeO$_2$ and magnesium oxide (MgO), is then computed up to 1500~K using first principles and the PBTE with the same level of physics, and compared to time-domain thermoreflectance (TDTR) measurements up to 800~K. Our calculated thermal conductivities from the PBTE agree well with literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. The temperature scaling law of $\kappa$ is $\sim T^{-1}$ for three-phonon scattering only and remains the same after phonon renormalization. This scaling for three- plus four-phonon scattering is $\sim T^{-1.2}$ but is weakened to $\sim T^{-1}$ by phonon renormalization. This indicates that four-phonon scattering can play an important role in systems where measured $\kappa$ decays with temperature as $\sim T^{-1}$, which was conventionally attributed to three-phonon only. Compared to MgO, we find that CeO$_2$ has weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.</p><p> </p><p dir="ltr">Phonon-phonon scattering, together with electron-phonon coupling, can often show strong selectivity and drive system out of thermal equilibrium. Measurements and a previous multitemperature model (MTM) resolving phonon temperatures at the polarization level have uncovered remarkable nonequilibrium among different phonon polarizations in laser irradiated graphene and metals. Here, we develop a semiconductor-specific MTM (SC-MTM) by including electron-hole pair generation, diffusion, and recombination, and show that a conventional phonon polarization-level model does not yield observable polarization-based nonequilibrium in laser-irradiated molybdenum disulfide (MoS$_2$). In contrast, appreciable nonequilibrium is predicted between zone-center optical phonons and the other modes. The momentum-based nonequilibrium ratio is found to increase with decreasing laser spot size and interaction with a substrate. This finding is relevant to the understanding of the energy relaxation process in two-dimensional optoelectronic devices and Raman measurements of thermal transport. </p><p><br></p><p dir="ltr">In summary, this Dissertation leverages first-principles method to explore thermal transport in emerging materials with a focus on high-order phonon scattering, phonon nonequilibrium behavior, and phonon renormalization. We reveal the importance of these effects in various phenomena including thermal conductivity, optical properties, Raman thermometry and thermal radiation control. </p>

Recent experimental advances for the fabrication of various borophene sheets introduced new structures with a wide range of applications. Borophene is the boron atom analogue of graphene. Borophene exhibits various structural polymorphs all of which are metallic. In this work, we employed first-principles density functional theory calculations to investigate the mechanical properties of five different single-layer borophene sheets. In particular, we analyzed the effect of the loading direction and point vacancy on the mechanical response of borophene. Moreover, we compared the thermal stabilities of the considered borophene systems. Based on the results of our modelling, borophene films depending on the atomic configurations and the loading direction can yield a remarkable elastic modulus in the range of 163–382 GPa nm and a high ultimate tensile strength from 13.5 GPa nm to around 22.8 GPa nm at the corresponding strain from 0.1 to 0.21. Our study reveals the remarkable mechanical characteristics of borophene films.

The purpose of the research contained in this thesis is to allow for the prediction of new structures and properties of crystalline structures due to the application of external pressure by using first-principles numerical computations. The body of the thesis is separated into two primary research projects. The properties of cupric oxide (CuO) have been studied at pressures below 70 GPa, and it has been suggested that it may show room-temperature multiferroics at pressure of 20 to 40 GPa. However, at pressures above these ranges, the properties of CuO have yet to be examined thoroughly. The changes in crystal structure of CuO were examined in these high-pressure ranges. It was predicted that the ambient pressure monoclinic structure changes to a rocksalt structure and CsCl structure at high pressure. Changes in the magnetic ordering were also suggested to occur due to superexchange interactions and Jahn-Teller instabilities arising from the d-orbital electrons. Barium chloride (BaCl) has also been observed, which undergoes a similar structural change due to an s – d transition, and whose structural changes can offer further insight into the transitions observed in CuO. Ammonia borane (NH3BH3) is known to have a crystal structure which contains the molecules in staggered conformation at low pressure. The crystalline structure of NH3BH3 was examined at high pressure, which revealed that the staggered configuration transforms to an eclipsed conformation stabilized by homopolar B–Hδ-∙∙∙ δ-H–B dihydrogen bonds. These bonds are shown to be covalent in nature, comparable in bond strength to conventional hydrogen bonds, and may allow for easier molecular hydrogen formation in hydrogen fuel storage.

In this study the elastic properties of a fcc Fe0.87Mn0.13 random alloy are studied by ab initio calculations. Ground state lattice parameters and elastic properties are calculated with Density Functional Theory using the Exact Muffin-Tin Orbital method and the Coherent Potential Approximation. Several magnetic models, approximations and distortion techniques are evaluated for optimized results, which are obtained by a Disordered Local Moment model with the Frozen Core and Generalized Gradient approximations using volume-conserving distortions.  Conclusively the longitudinal sound velocities are calculated from second order elastic stiffness constants and visualized by two different codes. The importance of magnetism for elastic properties is confirmed, as is the usefulness of the optimized computational scheme; all quantities obtained via the scheme is in accord with earlier theoretical and experimental results. Volume-conserving distortions are found to be more precise than volume-altering for calculation of elastic constants but also to be highly dependent on the precision of bulk modulus determination. The two sound-velocity codes are in complete agreement.

<p>In recent years, first principles periodic Density Functional Theory (DFT) calculation</p><p>has been used to investigate heterogeneous catalytic reactions and examine catalyst</p><p>structures as well as adsorption properties in a variety of systems. The increasing</p><p>contribution to give detailed understanding of elementary reaction mechanism is critical to</p><p>provide fundamental insights into the catalyst design. It is a link to the fundamental</p><p>knowledge and a bridge to the practical application. DFT calculations is also a powerful</p><p>tool to predict and yield promising catalysts which is time- and cost-saving in the practical</p><p>end.</p><p>Because of the recent boom in natural shale gas deposit, there is an increasing interest</p><p>in developing more efficient ways to transform light alkanes into desired and high-value</p><p>chemicals, such as propylene. Propylene is a valuable raw material in the petrochemical</p><p>application to make value-added commodities, such as plastics, paints, and fibers, etc. The</p><p>conventional cracking, steam cracking (SC) and fluid catalytic cracking (FCC), could not</p><p>meet the growing demand of propylene. Thus, it has motivated extensive research of</p><p>production technologies. On the other hand, the abundance of light alkanes extracted from</p><p>the shale gas makes on-purpose production an appealing method which is economically</p><p>competitive. Non-oxidative dehydrogenation of propane (PDH) is a one of ways to make</p><p>up the supply and solve the issue.</p><p>xiii</p><p>According to the current research and industrial work, platinum (Pt) shows promising</p><p>performance for the PDH. However, it suffered from some major drawbacks, such as</p><p>thermodynamic limitation, rapid deactivation leading to poor catalytic performance and</p><p>frequent regeneration. In addition, it is a relatively high cost noble metal. Consequently,</p><p>many efforts have been devoted to the enhancement of the catalytic performance. It was</p><p>found that the stability and the selectivity of Pt-based catalysts can be improved via</p><p>modifying its properties with transition metals as promoters.</p><p>In this thesis, DFT calculations were performed for propane dehydrogenation over</p><p>two different catalyst systems, bimetallic platinum-zinc alloy and monometallic platinum</p><p>catalysts. The work provides insights into the catalyst crystal structures, the adsorption</p><p>characteristics of diverse adsorbates as well as the energy profiles regarding to the</p><p>selectivity of the propane dehydrogenation. Bulk calculation signifies a stable tetragonal</p><p>configuration of the PtZn catalyst which is in accordance with the experimental result. The</p><p>thermodynamic stability regarding to the stability of bulk and surface alloys are studied</p><p>with the consideration of physical constrains. We have identified the thermodynamic</p><p>stability of several PtZn low-index surface facets, (101), (110), (001), (100) flat surfaces</p><p>and stepped surface (111), at certain chemical potential environmental conditions through</p><p>the surface energy phase diagram. Stoichiometric and symmetric (101) slab is</p><p>thermodynamically stable under the region of high Pt chemical potential, and the offstoichiometric</p><p>and symmetric (100 Zn-rich) slab under the low Pt chemical potential.</p><p>In this work, PtZn(101) is used as a model surface to demonstrate the effect on the</p><p>catalytic performance with zinc promotion of platinum. In comparison with Pt(111) surface,</p><p>an elimination of 3-fold Pt hollow site on PtZn(101) is of important and it leads to the</p><p>xiv</p><p>change of binding site preferences. The divalent groups (1-propenyl, 2-propenyl) change</p><p>from Pt top site on PtZn(101) to 3-fold site on Pt(111), which is because of the lack of Pt</p><p>3-fold site on alloyed surface. As for propylene, it changes from di-σ site on PtZn to 𝜋 site</p><p>on Pt. The surface reaction intermediates are found to bond more weakly on PtZn(101)</p><p>than on the Pt surface. Especially, the binding energy of propylene reduces from -1.09 to -</p><p>0.16 eV. The weaker binding strength facilitates the activity of propylene on alloyed</p><p>surfaces.</p><p>Through a complete and classic reaction network analysis, the introduction of Zn</p><p>shows an increase in the endothermicity and the energy barrier of each elementary reaction</p><p>on the alloy surface. With the consideration of entropy for kinetic under real experimental</p><p>condition, the alloying of Zn is found to lower the energy barrier for the propylene product</p><p>desorption and increases that for propylene dehydrogenation. Meanwhile, the competition</p><p>between desired C-H and undesired C-C cleavages is investigated. It is found that the</p><p>cleavage of C-H is energetically favorable than that of C-C. These positive factors</p><p>potentially lead to a high selectivity toward propylene production on PtZn(101).</p><p>Subsequently, Microkinetic modeling is performed to estimate kinetic parameters</p><p>including the reaction order, rate-determining step to build a possible reaction mechanism.</p><p>Finally, conclusions brought out about the comparison between bimetallic and</p><p>monometallic catalyst, and suggestions for future work are presented.</p>

Thermoelectric materials play an important role in energy conversion as they represent environmentally safe and solid state devices with a great potential towards enhancing their efficiency. The ability to generate electric power in a reliable way without using non-renewable resources motivates many experimentalists as well as computational physicists to search and design new thermoelectric materials. Several classes of materials have been identified as good candidates for high efficient thermoelectrics because of their inherently low thermal conductivity. The complex study of the crystal and electronic structures of such materials helps to reveal hidden properties and give fundamental understanding, necessary for the development of a new generation of thermoelectrics. In the current thesis, ab-initio computational methods along with experimental observations are applied to investigate several material classes suitable for thermoelectric applications. One example are Bi-Sb bismuth rich alloys, for which it is shown how structural anomalies affect the electronic structure and how inclusion of the Spin-Orbit coupling is necessary for this type of materials. Another example are bournonite materials whose low thermal conductivity is attributed to distortions and interactions associated with lone-electron s^2 pair distributions. In addition, it is shown how doping with similar atoms can affects electronic structure of these materials leading to changes in their transport properties. Clathrate materials from the less studied type II Sn class are also investigated with a detailed analysis for their structural stability, electronic properties and phonons. These systems are considered with partially substituted atoms on the framework and different guests inside. The effect upon insertion of Noble gases into the cage network is also investigated. In addition, the newly synthesized As based cationic material is also studied finding novel structure-property relations. Another class of materials, quaternary chalcogenides, have also been studied. Because of their inherently low thermal conductivity and semiconducting nature their transport properties may be optimized in a favorable way for thermoelectricity. The present work provides an in-depth study of structural and electronic properties of several classes of materials, which can be used by experimentalists for input and guidance in the laboratory.