Browsing by Subject "DFT"

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Application of quantum force computations for Raman spectroscopy and molecular dynamics
(2019-08-16) Neitzel, Joshua Clark; Chelikowsky, James R.
Electronic structure calculations have undergone incredible advancement in the past century. Using modern methods and supercomputing infrastructure we are now able to compute precise electron behavior in a variety of large and complex systems. However, these computations are only as good as their applications. To further these computations we consider two applications of efficient force calculations using first principles density functional theory. We compute the vibrational and Raman spectra for B-doped, P-doped, and B-P codoped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. An experimental peak in the Raman spectra near 650 cm⁻¹ observed in codoped nanocrystals can be best explained by the presence of B-P bonds, which are located near the surface of the nanocrystal. We propose that the spectral details of this peak are related to quantum confinement and the breaking of local symmetry associated with the phonon modes involving dopant bonds. We also illustrate an improved method for calculation of nonlocal contributions to interatomic forces is used to perform molecular dynamics simulations. This method results from the real space density functional theory Hamiltonian utilizing a high order Gaussian integration scheme in real space. The efficacy of this method is demonstrated through molecular dynamics simulations of an O₂ molecule and a benzene molecule. Our method improves convergence of dynamic variables including stability and vibrational frequency
Atomistic simulation of the early stages of solid electrolyte interphase formation in lithium ion batteries
(2019-09-19) Boyer, Mathew J.; Hwang, Gyeong S.; Freeman, Benny D; Manthiram, Arumugam; Ren, Pengyu
Lithium ion batteries have fueled a technological revolution in consumer electronics, power tools, and electric vehicles. Further advancements of this technology to improve charge times and capacity while maintaining safe operability, however, require a deeper fundamental understanding of electrode and electrolyte materials as well as their interfaces. In particular, interfacial stability between the high energy anode and the electrolyte represents one of the greatest hurdles to improving current-generation batteries as well as moving onto next-generation technologies like lithium metal or silicon. Despite the commercial availability of lithium ion batteries for more than a decade, there is no intrinsically stable electrolyte which is able to satisfy the design requirements of a commercial device. Instead, a protective layer formed during the first charge cycle known as the solid electrolyte interphase (SEI) is relied upon to ensure stable operation over subsequent charge/discharge cycles. Despite being critical to battery operability, the SEI and the process by which it forms remains poorly understood. As the SEI is only several to tens of nm thick and decomposes in ambient conditions, its study through experiments presents many challenges. However, computational tools can easily access the size- and time-scales required to elucidate the processes which govern the formation of the SEI. This dissertation presents a computational framework by which reductive decomposition of the electrolyte during the early stages of SEI formation may be studied through atomistic simulations including classical molecular dynamics and density functional theory. Additionally, fundamental descriptions of several reaction and diffusion processes involved in the formation of the SEI from a conventional electrolyte on a graphite electrode are presented. This methodology may be later applied to more complex electrolytes or other electrodes like silicon, but also lays the groundwork for exploring later stages of the SEI formation and growth.
Atomistic simulations of 2D materials and van der Waal’s heterostructures for beyond-Si-CMOS devices
(2017-08) Valsaraj, Amithraj; Register, Leonard F.; Banerjee , Sanjay K.; Tutuc, Emanuel; Yu, Edward T.; MacDonald, Allan H.
The unique electrical and optical properties of two-dimensional (2D) materials has spurred intense research interest towards development of nanoelectronic devices utilizing these novel materials. The atomically thin form of 2D materials translates to excellent electrostatic gate control even at nanoscale channel length dimensions, near-ideal two-dimensional carrier behavior, and perhaps conventional and novel devices applications. Monolayer transition metal dichalcogenides (TMDs) are novel, gapped 2D materials. Toward device applications, I consider MoS₂ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional-theory (DFT) simulations, the effects of near-interface oxygen vacancies in the oxide slab, and Mo or S vacancies in the MoS₂ layer are considered. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS₂-dielectrics system and for isolated dielectric layers for reference. Among the results, I find that with O-vacancies, both the HfO₂-MoS₂ and the Al₂O₃-MoS₂ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS₂, in manner analogous to modulation doping. The n-type doping of monolayer MoS₂ by high-k oxides with O-vacancies is confirmed through collaborative experimental work in which back-gated monolayer MoS₂ FETs encapsulated by oxygen deficient high-k oxides have been characterized. Van der Waal’s heterostructures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, I consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, I simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. Rotational misalignment between the channel layers and the tunnel barrier in this van der Waal’s heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between the channels and the tunnel barrier. This weakened coupling in graphene/hBN/graphene with hBN misalignment may be relevant to all such van der Waal’s heterostructures. TMDs are viable alternatives to graphene and hBN as channel and tunnel barrier layers, respectively, for improved performance in interlayer tunnel FET device structures. In particular, I used DFT simulations to study the bilayer-graphene/WSe₂/bilayer-graphene heterostructure as well as single and multilayer ReS₂-layer systems. Significant roadblocks to the widespread use of TMDs for nanoelectronic devices are the large contact resistance and absence of reliable doping techniques. Hence, I studied substitutional doping of, and evaluated various metal contacts to MoS₂ by computing the density of states for the systems. Metal contacts that pin the Fermi level within the desired band are optimal for device applications. My simulation results suggest that monolayer (ML) MoS₂ can be doped n-type or p-type by substituting for an S atom in the supercell with a group-17 Cl atom or a group-15 P atom, respectively. My simulations also suggest that Sc and Ti would serve as excellent contacts to n-type ML MoS₂ due to the strong bonding and large number of states near the Fermi level. But the theoretical expectations are tempered by the material characteristics, i.e., the extremely reactive nature of Sc and the oxidation prone nature of Ti atoms. I also studied commonly used Ag and Au metal contacts to ML MoS₂, which exhibited medium strength bonding to MoS₂ and an apparent pinning of the Fermi level nearer to the nominal MoS₂ conduction band edge
Computational design of carbon nanotube sensors for gas phase explosives detection
(2021-08-02) Doshi, Manasi; Fahrenthold, Eric P.; Djurdjanovic, Dragan; Sepehrnoori, Kamy; Taleff, Eric; Liu, Yuanyue
Gas phase detection of explosive molecules is a sensing application of wide interest. Light weight, low power sensors are needed for mobility and wide dissemination, however low vapor pressures and the presence of similar functional groups in a variety of explosive molecules make the development of sensitive and selective detection systems difficult. Experimental research has reported some success in the development of carbon nanotube based explosives sensors, however safety considerations and strict controls on the distribution of explosive materials hamper experimental progress. In this dissertation, ab initio computational models are developed for metallic and semiconducting carbon nanotube sensors, in a variety of device configurations. Their chemiresistive sensing performance is investigated in the detection of three common explosives. The effects of doping, lattice defects, and functionalizations on sensing performance are analyzed. Their performance in sensor arrays is also analyzed; array selectivity is improved by capitalizing upon the nonlinear current-voltage characteristics of the CNT sensors. A new ab initio molecular dynamics formulation is developed, for spin polarized systems. It employs a novel nonholonomic Hamiltonian modeling methodology to couple a quantum model of the electronic structure to a molecular model of the nuclear dynamics, and quantifies the modeled nanoscale systems interaction with the external thermal and electromagnetic environment. This theoretical model offers future opportunities for the simulation of finite temperature dynamics in carbon nanotube based sensors, under applied electric and magnetic fields.
Computational investigation of materials for energy storage applications
(2023-04-13) Katyal, Naman; Henkelman, Graeme; Milliron, Delia; Humphrey, Simon; Mitlin, David
The need for sustainable, economical, and high energy density rechargeable batteries are mandatory to develop next generation consumer electronics, transportation, and stationary energy devices to meet the growing energy demands of the world. In order to meet those challenges, the fundamental understanding of thermodynamics and diffusion processes in battery materials can help achieve the current goals. Atomic-scale simulations using density functional theory coupled with experimental characterization techniques have the capabilities to reveal local atomic environment of battery electrodes and electrolytes which is directly related to the ionic conductivity, stability of electrode-electrolyte interphase, electrode potential, energy density, and rate capabilities. Developing models using atomistic simulations are powerful tools because structural features can be directly compared to experimental characterization results and develop deeper insights into battery processes. In this thesis, a number of atomistic models were developed using computational characterization techniques, which were compared with experiments to develop an accurate understanding of next-generation battery materials for high-energy density applications. These atomistic models were used to compare the catalytic processes in zin-air batteries and the intercalation process in different ion intercalation materials for dual-ion batteries for high energy density and economical application. Furthermore, the computational cost of running electronic structure calculations limits the lengthscale and timescale of simulations to study kinetic processes at experimental timescales, which involve rare events on potential energy surfaces. This work developed a generalized interatomic potential for bulk lithium using a machine learning package PyAMFF to replace density functional theory calculations to study the metal deposition process in batteries.
Computational methods for understanding the role of electric fields in quantum confined materials
(2017-12-08) Garrett, Benjamin Fonville; Chelikowsky, James R.; Demkov, Alexander A; Fiete, Gregory A; Henkelman, Graeme A; MacDonald, Allan H
The invention of pseudopotential-density functional theory to solve for the electronic structure of materials is one of the major successes of modern computational physics. A code based on this formalism was used to solve for the electronic structure of systems with limited dimensionality. The code solves for the electronic structure problem on a real-space grid without the use of an explicit basis. This scheme is particularly well suited for studying molecules, clusters, and nanostructures. The code was applied to assess how an applied electric field changes the properties of two different systems: the change of vibrational modes with the field in molecules or clusters and tuning the electronic gap with the field in 2D materials. Three approaches were employed to study the effect of electric fields on the vibrations of small molecules. The approaches used perturbation theory, a finite field method, and an ab initio molecular dynamics approach. This work provides a better understanding of experimental techniques to probe the local electric field in complex materials as in photovoltaics and biomolecules. The second part of this thesis leverages mixed boundary conditions to study the effects of finite electric fields on two-dimensional materials such as phosphorene. These results demonstrate the ability to tune the band gap and drive semiconductor to metallic transitions in novel two-dimensional materials. This property may enable the creation of nanoscale transistors and sensors to power the next generation of electronic devices.
Emerging phenomena in oxide heterostructures
(2010-08) Lee, Jaekwang; Demkov, Alexander A.; Kleinman, Leonard; Chelikowsky, James R.; Macdonald, Allan H.; Hwang, Gyeong S.
Oxide interfaces have attracted considerable attention in recent years due to emerging novel properties that do not exist in the corresponding parent compounds. Furthermore, modern atomic-scale growth and probe techniques enable the formation and study of new artificial interface states distinct from the bulk state. A central issue in controlling the novel behavior in oxide heterostructures is to understand how various physical variables (spin, charge, lattice and/or orbital hybridization) interact with each other. In particular, density function theory (DFT) has provided significant insight into underlying physics of materials at the atomic level, giving quantitative results consistent with experiment. In this dissertation using density functional theory methods, we explore the electronic, magnetic and structural properties developed near the interface in SrTiO3/LaAlO3, EuO/LaAlO3, Fe/PbTiO3/Pt, Fe//BaTiO3/Pt and Cs/SrTiO3 heterostructures. We study the interplay between physical interactions, and quantify parameters that determine physical properties of hetetrostructures. These theoretical studies help understanding how physical variables couple with each other and how they determine new properties at oxide interfaces.
Simulation tools for predicting the atomic configuration of bimetallic catalytic surfaces
(2012-12) Stephens, John Adam; Hwang, Gyeong S.
Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations.
Structural and electrocatalytic properties of dendrimer-encapsulated nanoparticles
(2013-12) Yancey, David Francis; Crooks, Richard M. (Richard McConnell)
As computational methods for the prediction of metallic nanoparticle structure and reactivity continue to advance, a need has developed for simple experimental models that can mimic and confirm theoretical predictions. Dendrimer-encapsulated nanoparticles, or DENs, are ideal to fill this role. DENs are synthesized within poly(amido amine) dendrimer templates which allows for the controlled synthesis of monodisperse nanoparticles in the 50-250 atom (1-2 nm) size range. These are small enough to be accessible to high-level theoretical calculations while being large enough to study experimentally. The research reported here consists of several independent but closely related studies. First, the synthesis, structural, and electrochemical properties of Au@Pt (core@shell) DENs are described. These materials are prepared by underpotentially depositing Cu onto Au DENs followed by galvanic exchange of Cu for Pt. Second, Pb UPD onto Au DENs and a detailed experimental and theoretical study of the resulting core@shell particle structures and catalytic activity is discussed. It is found that no matter how much Pt is deposited onto the surface of Au₁₄₇ DENs, a surface reorganization occurs resulting in similar electrocatalytic activity for the oxygen reduction reaction. Third, an in-depth X-ray absorption spectroscopy study of the structural properties of thiol-capped Au₁₄₇ DENs is described. Here, the surfaces of uncapped Au₁₄₇ nanoparticles are titrated with strongly binding thiol ligands to tune the extent of surface disorder. The effect of the increased surface disorder on the standard EXAFS fitting results is discussed from experimental and theoretical perspectives. Lastly, an in-situ electrochemical study of Au₁₄₇ DENs structure is reported. The key result is that the Au lattice expands during electrochemical surface oxidation. This is an important result for understanding electrocatalytic processes on Au nanoparticle
Techniques to increase compaction of output responses with unknown (X) values
(2018-01-24) Bawa, Asad Amin; Touba, Nur A.; Pan, David Z; Swartzlander, Earl; John, Lizy K; Suleman, Muhammad A
Testing requires checking whether the output response of a circuit or system is correct or has an error. Increasingly complex system-on-chip and 3-D integrated circuits require enormous amounts of manufacturing test data. Test compression techniques are widely used to compress the amount of output response data in a way that if an error is present in the uncompacted output response, it will also be present in the compacted output response with only a negligibly small chance of aliasing. Compacting the output response reduces the number of channels needed on the automatic test equipment (ATE) and reduces tester memory requirements. A major challenge for output compaction techniques is dealing with unknown (X) values in the output response which may arise from many sources such as uninitialized memories, analog blocks, tri-states, false paths, etc. While some compactor designs can guarantee observation of errors in the presence of a small number of X's, they may not be sufficient for designs with high X-densities which are becoming increasingly common. This dissertation presents novel advanced techniques to further optimize the handling of X’s and scale existing schemes to handle higher X-densities. New designs and techniques will be presented to reduce the control data required to more efficiently handle X’s and achieve higher compression with experimental results in the respective sections.
Test and security in a System-on-Chip environment
(2017-05) Lee, Yu-Wei; Touba, Nur A.; Pan, David; Swartzlander, Earl; John, Lizy; Huang, Gang
This dissertation outlines new approaches for test and security in a System-on-Chip (SoC) environment. A methodology is proposed for designing a single test access mechanism (TAM) architecture on each die with a "bandwidth adapter" that allows it to be efficiently used for multiple different test data bandwidths in three-dimensional integrated circuits (3D-IC) using through-silicon vias (TSVs). In this way, a single test architecture can be re-used for pre-bond, partial stack, and post-bond testing while minimizing test time across all phases of test. Unlike previous approaches, this methodology does not need multiple TAM architectures or reconfigurable wrappers in order to be efficient when the test data bandwidth changes. In industry, sequential linear decompression is widely used to reduce data and bandwidth requirements. A new scheme using a multiple polynomial linear feedback shift register (LFSR) with rotating polynomial is proposed here to increase encoding flexibility resulting in higher compression ratios. An algorithm is described to assign test cubes to polynomials in a way that enhances encoding efficiency. For hardware security, a new attack strategy against logic obfuscation is described here. It is based on applying brute force iteratively to each logic cone one at a time and is shown to significantly reduce the number of brute force key combinations that need to be tried by an attacker. It is shown that inserting key gates based on MUXes is an effective approach to increase security against this type of attack. In data security for hardware, existing techniques for computing with encrypted operands are either prohibitively expense (e.g., fully homomorphic encryption) or only work for special cases (e.g., linear circuits). A lightweight scheme implemented at the gate-level is proposed for computing with noise-obfuscated data. By carefully selecting internal locations for noise cancellation in arbitrary logic circuits, the overhead can be greatly minimized. One important application of the proposed scheme is for protecting data inside a computing unit obtained from a third party IP provider where a hidden backdoor access mechanism or hardware Trojan could be maliciously inserted.
Testability considerations for implementing an embedded memory subsystem
(2011-12) Seok, Geewhun; Touba, Nur A.; Womack, Baxter; Ambler, Tony; Swartzlander, Earl; Hallock, Gary
There are a number of testability considerations for VLSI design, but test coverage, test time, accuracy of test patterns and correctness of design information for DFD (Design for debug) are the most important ones in design with embedded memories. The goal of DFT (Design-for-Test) is to achieve zero defects. When it comes to the memory subsystem in SOCs (system on chips), many flavors of memory BIST (built-in self test) are able to get high test coverage in a memory, but often, no proper attention is given to the memory interface logic (shadow logic). Functional testing and BIST are the most prevalent tests for this logic, but functional testing is impractical for complicated SOC designs. As a result, industry has widely used at-speed scan testing to detect delay induced defects. Compared with functional testing, scan-based testing for delay faults reduces overall pattern generation complexity and cost by enhancing both controllability and observability of flip-flops. However, without proper modeling of memory, Xs are generated from memories. Also, when the design has chip compression logic, the number of ATPG patterns is increased significantly due to Xs from memories. In this dissertation, a register based testing method and X prevention logic are presented to tackle these problems. An important design stage for scan based testing with memory subsystems is the step to create a gate level model and verify with this model. The flow needs to provide a robust ATPG netlist model. Most industry standard CAD tools used to analyze fault coverage and generate test vectors require gate level models. However, custom embedded memories are typically designed using a transistor-level flow, there is a need for an abstraction step to generate the gate models, which must be equivalent to the actual design (transistor level). The contribution of the research is a framework to verify that the gate level representation of custom designs is equivalent to the transistor-level design. Compared to basic stuck-at fault testing, the number of patterns for at-speed testing is much larger than for basic stuck-at fault testing. So reducing test and data volume are important. In this desertion, a new scan reordering method is introduced to reduce test data with an optimal routing solution. With in depth understanding of embedded memories and flows developed during the study of custom memory DFT, a custom embedded memory Bit Mapping method using a symbolic simulator is presented in the last chapter to achieve high yield for memories.
Theoretical study of correlation between structure and function for nanoparticle catalysts
(2014-12) Zhang, Liang, 1986; Henkelman, Graeme
The science and technology of catalysis is more important today than at any other time in our history due to the grand energy and environment challenges we are facing. With the explosively growth of computation power nowadays, computer simulation can play an increasingly important role in the design of new catalysts, avoiding the costly trail-and-error attempts and facilitating the development cycle. The goal to inverse design of new materials with desired catalytic property was once far off, but now achievable. The major focus of this dissertation is to find the general rules that govern the catalytic performance of a nanoparticle as the function of its structure. Three types of multi-metallic nanoparticles have been investigated in this dissertation, core-shell, random alloy and alloy-core@shell. Significant structural rearrangement was found on Au@Pt and Pd@Pt nanoparticle, which is responsible for a dramatic improvement in catalytic performance. Nonlin- ear binding trends were found and modeled for random alloy nanoparticles, providing a prescription for tuning catalytic activity through alloying. Studies of ORR on Pd/Au random alloy NP and hydrogenation reaction on Rh/Ag random alloy NP revealed that binding on individual ensemble should be in- vestigated when large disparity of adsorbate affinity is presented between two alloying elements. In the alloy-core@shell system, I demostrated a general linear correlations between the adsorbate binding energy to the shell of an alloy-core@shell nanoparticle and the composition of the core. This relation- ship allows for interpolation of the properties of single-core@shell particles and an approach for tuning the catalytic activity of the particle. A series of promising catalysts were then predicted for ORR, HER and CO oxidation. As a first attempt to bridge the material gap, bimetallic nano clus- ter supported on CeO₂(111) was investigated for CO oxidation. A strong support-metal interaction induces a preferential segregation of the more reac- tive element to the NC-CeO₂ perimeter, generating an interface with the Au component. (Au-Cu)/CeO₂ was found to be optimal for catalyzing CO oxida- tion via a bifunctional mechanism. O₂ preferentially binds to the Cu-rich sites whereas CO binds to the Au-rich sites. A method called distributed replica dynamics (DRD) is proposed at last to utilize enormous distributed computing resources for molecular dynamics simulations of rare-event in chemical reac- tions. High efficiency can be achieved with an appropriate choice of N [subscript rep] and t [subscript rep] for long-time MD simulation.
A theoretical study of oxidation on copper (100) surface
(2015-12) Yang, Sheng-Che; Henkelman, Graeme; Makarov, Dmitrii E.
The further oxidations to subsurface of the 0.5 ML oxygen coverage copper (100) surface and MRR surface were investigated by DFT calculations. The two possible pathways for continuous oxidation on MRR and the c(2x2) phase could be separated into three parts: the adsorption of oxygen molecule, the dissociation of oxygen molecule and the oxygen atoms diffusion into subsurface. The missing row of MRR can be regarded as an active site for the adsorption and dissociation of oxygen molecule. The overall energy barriers of the two pathways of MRR and the c(2x2) phase are 0.75 and 0.9 eV respectively, which indicates that MRR is more likely to be oxidized to the subsurface. The convex hull analysis agrees with the three relatively stable surfaces of early oxidation in the previous experiments and the convex hull analysis further indicated that a special unit in both MRR and MRR-like surface may be the key element to the stability.
Towards a Bayesian framework for fire origin and evolution in fire forensics
(2020-12-04) Cabrera, Jan-Michael; Ezekoye, Ofodike A.; Moser, Robert D; Hall, Matthew J; Kurzawski, Andrew J
Fire scene reconstruction and determining the fire evolution (i.e. item-to-item ignition events) using the post-fire compartment is an extremely difficult task because of the time-integrated nature of the observed damage within the compartment. Adequately quantifying the uncertainty for quantities of interest is crucial for making statistically sound inferences about the pre-fire compartment. The work needed to implement an experimental fire compartment capable of producing repeatable fire-evolution scenarios is presented. Improvements to the University of Texas Fire Research Group (UTFRG) Burn Structure are discussed including increased automation and reduced setup time for compartment scale fire tests as well as an ability to automate fire evolution tests. Instrumented burners are configured in the compartment, each with a simple ignition model. Heat flux sensors are located around the burners to provide temporal incident heat flux measurements. The heat flux sensors are directional flame thermometers (DFTs); robust measurement devices suitable to the harsh environments found in fire scenarios. The typical DFT is large when compared to other standard heat flux measurement devices. To better understand the uncertainties associated with heat flux measurements in these environments, a Bayesian framework is utilized to propagate uncertainties of both known and unknown parameters describing the thermal model of a modified, smaller DFT. Construction of the modified DFT is described along with a derivation of the thermal model used to predict the incident heat flux to its sensing surface. Markov Chain Monte Carlo simulations were used to obtain posterior distributions for the free parameters of the thermal model as well as the modeling uncertainty. A Bayesian inferential framework is developed to test possible ignition scenarios given measurement uncertainties against data taken at the fire scene. The framework is developed for temporal heat flux measurements as well as for the observed damages at surrogate sensors within the compartment. The framework is first exercised on temporal heat flux measurements recorded from compartment tests in the Burn Structure to highlight the importance of error structure for making calibrated predictions. The use of computational models for making inferences on damage measurements taken in the compartment utilizing a Bayesian inferential framework is also discussed.
Underpotential deposition as a synthetic and characterization tool for core@shell dendrimer-encapsulated nanoparticles
(2011-08) Carino, Emily V.; Crooks, Richard M. (Richard McConnell); Bard, Allen J.; Stevenson, Keith J.; Henkelman, Graeme; Mullins, C. Buddie
The synthesis and characterization of Pt core/ Cu shell (Pt@Cu) dendrimer-encapsulated nanoparticles (DENs) having full and partial Cu shells deposited via electrochemical underpotential deposition (UPD) is described. Pt DENs containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the substrate for the UPD of a Cu monolayer. This results in formation of Pt@Cu DENs. Evidence for this conclusion is based on results from the analysis of cyclic voltammograms (CVs) for the UPD and stripping of Cu on Pt DENs, and from experiments showing that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in-situ electrochemical X-ray absorption spectroscopy (XAS) confirm the core@shell structure. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N₂-saturated solution, and no detectable oxidation when the DENs are in contact with H₂. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd²⁺-complexing ligands such as chloride. For the air- and N₂-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy.