Repository of works by Caltech published authors.
Type = Conference or Workshop Item
Mostrando recursos 1 - 20 de 439
Stresses in Ytterbium Silicate Multilayer Environmental Barrier Coatings - Stolzenburg, F.; Almer, J.; Lee, K. N.; Harder, B. J.; Faber, K. T.
The internal stresses of plasma-sprayed multilayer ytterbium disilicate environmental barrier
coatings were measured using microfocused high-energy X-rays in a transmission geometry.
Stresses were measured for as-sprayed and ex-situ heat-treated ytterbium disilicate topcoats at
room temperature and during in-situ heating and cooling experiments. In-situ loading
experiments were also performed on the topcoat in order to establish its elastic constants. The
ytterbium disilicate was found to have a relatively low coefficient of thermal expansion resulting
in compressive stresses of approximately 100 MPa throughout the topcoat. In-situ heating
experiments revealed a statistically significant stress relaxation in the ytterbium disilicate topcoat
upon thermal cycling to temperatures above 1300°C, indicating the onset of...
Turbulence and Diffusion as Factors in Sediment Transportation - Vanoni, Vito A.; Hsu, En-Yun
The importance of turbulence in the transportation of sediment has long been recognized by hydraulic engineers and research workers in fluid flow. The turbulence contains the cross components of flow that transport or diffuse sediment from the bed into the main body of the stream. Without this turbulent motion no sediment could be picked up from the bed and there would be no muddy
streams. Transport would be limited to the small amount resulting from the rolling and sliding of particles at the bed under the action of viscous shear.
Speed faults in computation by chemical reaction networks - Chen, Ho-Lin; Cummings, Rachel; Doty, David; Soloveichik, David
Chemical reaction networks (CRNs) formally model chemistry in a well-mixed solution. Assuming a fixed molecular population size and bimolecular reactions, CRNs are formally equivalent to population protocols, a model of distributed computing introduced by Angluin, Aspnes,
Diamadi, Fischer, and Peralta (PODC 2004). The challenge of fast computation by CRNs (or population protocols) is to ensure that there is never a bottleneck "slow" reaction that requires two molecules (agent states) to react (communicate), both of which are present in low (O(1))
counts. It is known that CRNs can be fast in expectation by avoiding slow reactions with high probability. However, states may be...
Real Fluid Effects on an Accelerated Sphere Before Boundary-Layer Separation - Mellsen, S. B.; Waugh, J. G.; Ellis, A. T.
Studies were made on the apparent increase in mass on acceleration (added mass) of a sphere accelerated from rest and before boundary-layer separation, in cylinders
of various diameters filled with water or oil. From a comparison of theoretical and experimentally obtained added masses, the following conclusions were drawn: In the
absence of wall effects on the boundary layer, the wall shear stress over elements of the sphere can be approximated by the solution for the flat plate moving parallel to itself and the potential flow over the elements outside the boundary layer. The impulse on the elements is obtained by integration with...
Note on the Application of Cascade Theory to Design of Axial-Flow Pumps - Linhardt, H. D.; Acosta, A. J.
Theoretical and experimental results are presented which reassure the usefulness of two-dimensional cascade theories to the design of axial flow pumps. For this purpose it is necessary to include the effect of the blade thickness upon the impeller flow which has been found to be
responsible for reported discrepancies between predictions of thin airfoil theories and the performance of axial-flow pumps characterized by high stagger angle and low aspect ratio.
Effects of anodic potential and chloride ion on overall reactivity in semiconductor electrochemical reactors - Hoffmann, Michael R.; Cho, Kangwoo
We have investigated electrochem. treatment of real domestic wastewater coupled with simultaneous prodn. of mol. H2 as useful byproduct. The electrolysis cells employ multi-layer semiconductor anodes with electro-active bismuth-doped TiO2 functionalities and stainless steel cathodes. DC-powered Lab.-scale electrolysis expts. were performed under static anodic potentials (+2.2 or +3.0 V NHE) using domestic wastewater samples, with added chloride ion in variable concns. Greater than 95% redns. in COD (COD) and ammonium ion were achieved within 6 h. In addn., we exptl. detd. a decreasing overall reactivity of reactive chlorine species towards COD with an increasing chloride ion concn. under chlorine radicals...
Selective formation of olefins using olefin metathesis - Grubbs, Robert H.
Olefin metathesis catalysts have become one of the tools for the efficient synthesis of complex mols. Until recently, the catalysts demonstrated poor catalyst controlled selectivity of the product double bond. Over the past several years, complexes based on molybdenum, tungsten and ruthenium have been discovered that will produce olefins good to excellent selectivity for the generation of Z olefins both in cross and in ring closing metathesis. New ligands have been developed that result in different selectivities and open new applications of metathesis in the synthesis of an array of complex mols. These catalysts are developed through design, computation and...
Multi-scale simulation of electrode interfaces - Miller, Thomas F.
The computational modeling of chem. reactions at electrode interfaces presents extraordinary challenges from the perspective of electronic structure theory. Target applications include electrolyte redn. in formation of the solid-electrolyte interphase (SEI) and the nucleation and growth of metal dendrites. These problems combine large system sizes with subtle interactions, multiple dynamical timescales, and electronically non-adiabatic eects. The development of new methods to perform reliable, on-the-fly electronic structure calcns. at a computational cost that makes feasible the simulation of long-timescale dynamics in large systems remains a central theor. challenge. We describe recent progress towards the development of accurate, scalable treatments for describing...
Accurate and systematically improvable density functional theory embedding for correlated wavefunctions - Miller, Thomas F.
We will describe quantum embedding methods for performing accurate and scalable electronic structure theory calcns. in large mol. systems, with application to clusters, liqs., transition metal complexes, and chem. reactions. "Exact non-additive kinetic potentials for embedded d. functional theory. Goodpaster JD, Ananth N, Manby FR, and Miller TF, JCP, 133, 084103 (2010). "A simple, exact d.-functional-theory embedding scheme. Manby FR, Stella M, Goodpaster JD, and Miller TF, JCTC, 8, 2564 (2012). "Accurate and systematically improvable d. functional theory embedding for correlated wavefunctions. Goodpaster JD, Barnes TA, Manby FR, and Miller TF, JCP, 140, 18A507 (2014).
Aryl ether carbon-oxygen bond cleavage by group 9 and 10 metals: Studies of selectivity and mechanism - Agapie, Theodor
The cleavage of aryl ethers is valuable for org. methodol. and biomass conversion. To gain mechanistic insight into aryl-ether bond cleavage in structurally related metal complexes, the reactivity of terphenyl diphosphines bearing aryl, Me or aryl, aryl ether moieties with M(0), M(II) (M = Ni, Pd, Pt) and M(I) (M' = Co, Rh, Ir) centers was investigated. Overall, selective cleavage of the stronger aryl ether C-O was obsd. only with Ni(0) and Rh(I). Pd(0) and Pt(0) can perform aryl ether C-O activation, but if available, they will cleave the weaker O-Me bond. IrI activates both, unselectively. In the presence of...
Scanning impedance probe for high-throughput electrochemical characterization of solid state electrodes - Usiskin, Robert E.; Maruyama, Shingo; Kucharczyk, Chris J.; Zhang, Xiaohang; Takeuchi, Ichiro; Haile, Sossina M.
We have developed a robotic instrument that can measure the electrochem. impedance of hundreds of thin-film microelectrodes in automated fashion. By measuring electrodes with systematically varied area, thickness, surface decoration, and compn., it is possible to probe reaction pathways, decouple bulk and surface properties, and rapidly screen hundreds of chem. compns. to discover trends and identify new high performing catalysts. Here we introduce the capabilities of this new instrument by using it to measure geometrically graded microdot electrodes of the solid-oxide fuel-cell (SOFC) cathode material (La_(0.8)Sr_(0.2))0.95MnO3-δ (LSM). We collect A.C. impedance spectra from several hundred microdots with diams. ranging from...
PV-powered electrochemical wastewater treatment system: Kinetics and mechanisms - Hoffmann, Michael R.
We have developed a transportable prototype designed for the treatment of raw domestic wastewater, human urine, human feces, and synthetic human waste analogs. After several hours of PV-powered electrochem. treatment, the turbid, black-water influent can be clarified with the elimination of the suspended particles along with the redn. or total elimination of the COD (COD), total enteric coliform disinfection via in situ reactive chlorine species generation, and the elimination of measurable protein after 3 to 4 h of PV-powered treatment. Our advanced prototype incorporates addnl. features such as a residual sludge handling unit, a hydrogen purifn. and filter system, and...
Fifty-years of personal reflection on advances in environmental chemistry and sustainability - Hoffmann, Michael R.
A personal retrospective on the nexus of environmental quality, chem., and sustainability will be presented. As a 1964 freshman chem. engineering and chem. student in Chicago, I became painfully aware of several major environmental problems including air and air pollution, lake eutrophication, and ecosystem disruption. The magnitude of the neg. impacts of air and water quality as evident in the stark visible degrdn. of air, soil, and water and its consequences on aquatic life motivated my 45 years of chem. research directed toward atm. chem., water chem., pollution control technologies, advanced oxidn. and redn. chem., photochem., sonochem., pulsed-plasma chem., electrochem.,...
Technical and operational perspective on the DOE Fuels from Sunlight Energy Innovation Hub, the Joint Center for Artificial Photosynthesis - Lewis, Nathan S.
The design of highly efficient, non biol., mol. level energy conversion "machines" that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. Basic research has already provided enormous advances in our understanding of the subtle and complex photochem. behind the natural photosynthetic system, and in the use of inorg. photocatalytic methods to split water or reduce carbon dioxide-key steps in photosynthesis. Yet we still lack sufficient knowledge to design solar fuel generation systems with the required efficiency, scalability, and sustainability...
Pressure and temperature dependences of rate coefficients for the reaction OH + NO_2 + M → products - Liu, Yingdi; Sander, Stanley P.
The OH + NO_2 reaction is a critically important process for radical chain termination in the atm. with a major impact on the ozone budgets of the troposphere and stratosphere. Rate consts. for the reaction of OH + NO_2 + → products have been measured under conditions relevant to the upper troposphere/lower stratosphere with a Pulsed Laser Photolysis - Pulsed Laser Induced Fluorescence technique augmented by in situ optical spectroscopy for quantification of [NO_2]. The expts. are carried out over the temp. range of 230K - 330K and the pressure range 40-800 Torr of air and N_2. The reaction was...
Organic nitrate contribution to the oxidized nitrogen budget at the 2013 Southern Oxidant and Aerosol Study (SOAS) - Teng, Alexander P.; Nguyen, Tran B.; St. Clair, Jason M.; Duffey, Kaitlin; Romer, Paul; Wooldridge, P. Abigail; Olson, Kevin; Gilman, Jessica B.; Lerner, Brian M.; Wild, Robert J.; Ayres, Benjamin; Fry, Juliane L.; Brown, Steven; Allen, H.; de Gouw, Joost; Cohen, Ronald C.; Wennberg, Paul O.
Org. nitrates (RONO_2) that are formed from the OH-initiated or NO_3-initiated oxidn. of biogenic alkenes can be important reservoirs and sinks for NO_x (NO + NO_2). Biogenic org. nitrates represent a significant fraction of the total NO_y budget in forested regions. During the 2013 Southern Oxidant and Aerosol Study (SOAS), speciated org. nitrates produced from the oxidn. of isoprene (C_5H_8) and monoterpenes (C_(10)H_(16)) were measured with high temporal resoln. and sensitivity by a time-of-flight chem. ionization mass spectrometer (ToF-CIMS) from a 20 m tower. Both dark and photochem. mechanisms were obsd. to be important for the prodn. of multifunctional org....
Semiconductor electrochemical treatment of domestic wastewater coupled with the production of molecular hydrogen - Hoffmann, Michael R.; Cid, Clément; Qu, Yan; Cho, Kangwoo
We have developed a wastewater treatment system that incorporates an electrolysis cell for on-site wastewater treatment coupled with hydrogen prodn. for use in a hydrogen fuel cell. Herein, we report on the efficacy of a lab.-scale wastewater electrolysis cell (WEC) using real human waste for the first time with semiconductor electrode utilizing a mixed particle coating of bismuth oxide doped titanium dioxide (BiO_x/TiO_2). A comprehensive environmental anal. has been coupled together with a robust kinetic model under the chem. reaction limited regime to investigate the role of various redox reactions mediated by chloride present in human waste. The oxidative elimination...
Noncovalent immobilization of electrocatalysts for fuel production on carbon electrodes - Blakemore, James D.; Gupta, Ayush; Warren, Jeffrey J.; Brunschwig, Bruce S.; Gray, Harry B.
Assembling systems for the conversion of solar light energy into chem. fuels (i.e., artificial photosynthesis) requires the development of strategies for functionalization of electrode surfaces with catalysts. We have demonstrated that mol. catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. Our general approach relies on a pyrene-appended bipyridine ligand that serves as the linker between the catalysts and the surface. Immobilization of a rhodium proton-redn. catalyst and a rhenium CO_2-redn. catalyst afford electrocatalytically active assemblies. XPS and electrochem. studies confirm catalyst immobilization. Redn. of the rhodium system in the presence of p-toluenesulfonic acid...