Repository of works by Caltech published authors.
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Fracture of Brittle and Quasi-Brittle Engineering Materials - Tandon, S.; Faber, K. T.
The study of fracture of engineering materials involves a number of science and engineering
disciplines. Continuum fracture mechanics is deeply rooted in the problem of fracture
because it treats the relationship between a crack or inhomogeneity and the stress
state in a material. Physics and chemistry are important because they help to explain the
reactions between the environment and the crack tip. Finally, materials science is essential
in understanding the relations among bond rupture, structure, processing and performance
of a material.
Fracture of materials can be divided into two broad categories - ductile and brittle
fracture. Ductile fracture is associated with appreciable plastic deformation. "Cup and
cone" fracture demonstrated...
Real-time earthquake hazard assessment in California; the early post-earthquake damage assessment tool and the Caltech-USGS broadcast of earthquakes - Eguchi, Ronald T.; Goltz, James D.; Seligson, Hope A.; Heaton, Thomas H.
A real-time earthquake monitoring system which provides source parameters
to user groups through a commercial paging service is now in place in California.
A GIS-based system to predict and display near real-time damage and casualty
estimates is currently being developed by EQE International under contract with
the State of California. These new technologies offer immediate tangible benefits
to state and local governments, utilities, lifelines and corporations with facilities
or operations at risk. This paper will outline the development of these new
technologies, identify the contributions they will make to emergency management
and explore some directions these innovative systems may take in the future.
High temperature properties of sintered alpha silicon carbide - Coppola, J. A.; Srinivasan, M.; Faber, K. T.; Smoak, R. H.
Sintered alpha silicon carbides are being developed to meet the requirements
of conventional and advanced heat engine systems. Through pressureless sintering,
low cost, near-net complex shapes requiring little finish grinding can be
fabricated from this form of silicon carbide.
The use of ceramics such as silicon carbide as engine components requires
detailed data collection and understanding of material's performance especially
at high temperatures. Much data have been generated on sintered alpha SiC (1-5)
over the past two years. Specifically, it has been shown that the fast fracture
strength of a sintered alpha silicon carbide tested in air is constant from room
temperature to above 1400° (2) and that an...
Effects of dams on beach sand supply - Brownlie, William R.; Brown, William R., III
In 1975 a regional sediment management study was initiated as a joint applied research project of the Environmental Quality Laboratory, California Institute of Technology, and the Shore Processes Laboratory, Scripps Institution of Oceanography. The project is a broad-based, long-term multidisciplinary effort intended to define the regional
sediment budget for coastal Southern California (Figure 1), and to quantify the effects of various human activities on changes in that budget.
Design and growth of III-V on Si microwire array tandem solar cells - Chen, Christopher T.; Turner-Evans, Daniel B.; Emmer, Hal; Aloni, Shaul; Atwater, Harry A.
Tandem Ga_(1-x)In_xP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Coupled full-field optical and device physics simulations of a Ga_(0.51)In_(0.49)P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is...
Low Thermal Expansion Coatings for Carbon/Carbon Composites - Kerans, R. J.; Faber, K. T.
Two classes of materials are considered for low expansion protective
coatings for carbon/carbon composites. They include composites containing
particles which undergo allotropic phase transformations accompanied by
negative volume changes with increasing temperature and anisotropic oxides
which demonstrate low expansion due to microcracking. Conditions for
failure of coatings by either cracking or spallation due to thermal
mismatch are evaluated.
Spectrum splitting photovoltaics: Light trapping filtered concentrator for ultrahigh photovoltaic efficiency - Kosten, Emily D.; Lloyd, John; Warmann, Emily; Atwater, Harry A.
We investigate a spectrum splitting approach where light is trapped in a textured dielectric slab and coupled into underlying solar cells of different bandgaps through filters. We describe a specific design featuring an SiO_2 textured slab coated with angle restricting incoupling elements and four underlying dual junction subcells, based on light trapping and efficiency considerations. We then design a full set of four omnidirectional filters based on aperiodic dielectric multilayer films to split the light entering each subcell into spectral bands tuned to the cell bandgaps. Based on the calculated filter performance, we estimate a receiver efficiency of 45% with...
Interface stoichiometry control in ZnO/Cu_2O photovoltaic devices - Wilson, Samantha S.; Tolstova, Yulia; Atwater, Harry A.
Cu_2O is a potential earth-abundant alternative to established thin photovoltaic materials (CIGS, CdTe, etc.) because of its low cost, high availability, and inexpensive processing, but Cu_2O has seen limited development as a photovoltaic device material owing to challenges in measurement and control of interface stoichiometry and doping. We report measurements of Cu_2O interface stoichiometry and the effect of interface composition on heterojunction device performance. ZnO/Cu_2O interface stoichiometry was varied by adjusting the ZnO window layer deposition conditions and stoichiometry was measured by X-ray photoelectron spectroscopy. Current-voltage characteristics of ZnO/Cu_2O heterojunctions indicate open circuit voltages of Voc ~ 530 mV for...
Spectrum splitting photovoltaics: Materials and device parameters to achieve ultrahigh system efficiency - Warmann, Emily C.; Eisler, Carissa; Kosten, Emily; Escarra, Matthew; Atwater, Harry A.
We describe an approach for spectrum splitting solar module design using 8 independently connected subcells with optimized band gaps. Modified detailed balance calculations using parameters to account for nonideal absorption and recombination behavior are used to identify the efficiency of spectrum splitting modules with 2 to 20 subcells. Potential optical designs for solar spectrum splitting among subcells are briefly described. Spectrum splitting improves use of the power in the solar spectrum by decreasing thermalization losses. A multijunction design utilizing independently connected subcells employed in a concentrator photovoltaic receiver allows flexibility in subcell selection, fabrication and operating power point optimization when...
Use of models to establish source-receptor relationships and estimate relative source contributions of NO_x to air quality problems - Seinfeld, John H.; McRae, Gregory J.
A basic objective of the Technical Symposium on the Implications of a Low NO_x Vehicle Emission Standard is to examine the need for and technical feasibility of a low NO_x emission standard for future light-duty motor vehicles.
A major component of this assessment is to estimate the changes in air quality that would result from the imposition of a low NO_x vehicle emission standard. Such an assessment requires a method for relating emission to air
quality changes, i.e. a so-called air quality model. In this summary, we focus on those air quality models that may be used to estimate the impact of...
Graded index sol-gel antireflection coatings - Wang, Max; Grandidier, Jonathan; Jones, Steven M.; Atwater, Harry A.
We explore graded index antireflection coatings that can reduce reflection of light at the ambient/cell or ambient/module boundary at all wavelengths and incidence angles. Simulations show that graded index coatings individually enhance absorption in solar cells across nearly the entire wavelength range. Silica sol-gel coatings with varying refractive indices deposited onto glass and silicon substrates are used to form graded index coatings, and the sol-gel layer processes have been adjusted so that the refractive indices are tunable from n=1.1 to n=1.5. Reflectance measurements also show that reflectivity decreases across the entire spectrum with added solgel multilayers.
Spectrum-splitting photovoltaics: Holographic spectrum splitting in eight-junction, ultra-high efficiency module - Escarra, Matthew D.; Darbe, Sunita; Warmann, Emily C.; Atwater, Harry A.
To achieve photovoltaic energy conversion with ultra-high module efficiency, the number of junctions can be increased beyond the 3-5 used in conventional lattice-matched multi-junction photovoltaics. We demonstrate a photovoltaic design that incorporates eight III-V semiconductor junctions arranged laterally as four dual-junction subcells operating electrically and optically in an independent manner. An integrated holographic optical element is used to split the incoming solar spectrum into four different spectral bands, each diffracted onto the appropriate subcell. In addition, two-axis concentration is used to achieve total concentration of 672 suns. A preliminary design for this holographic spectrum splitter composed of twelve simple, commercially...
Spectrum splitting photovoltaics: Polyhedral specular reflector design for ultra-high efficiency modules - Eisler, Carissa N.; Kosten, Emily D.; Warmann, Emily C.; Atwater, Harry A.
A design for ultra-high efficiency solar modules (>50%) using spectrum splitting is proposed. In the polyhedral specular reflector design, seven subcells are arranged around a solid parallelepiped. Incident light enters the parallelepiped and is directed via specular reflection onto each subcell in order from highest to lowest bandgap. We analyze optical losses due to external concentration and parasitic absorption and optimize the design for >50% module efficiency. We find that moderate concentration designs (90-170x) with a high index parallelepiped and perfect shortpass filters meet target efficiencies and demonstrate an initial design.
Absorption enhancement in ultra-thin film Si slabs using novel photonic crystal textures - Callahan, Dennis M.; Whitesell, Kelsey A.; Atwater, Harry A.
We perform rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) calculations on thin (< 200 nm) slabs of Si with various wavelength scale texturing. Traditional photonic crystal patterning is investigated and compared to a flat Si slab with a double layer anti-reflection coating. These traditional photonic crystal lattices are then modified with superlattice which further improve the absorption. We also investigate the angular spectra of select lattices and show how resonant absorption peaks shift with incident angle. It is concluded that alternate photonic crystal texturing can lead to significantly enhanced absorption over planar slabs with optimized anti-reflection coatings.
Full spectrum ultrahigh efficiency photovoltaics - Atwater, Harry A.; Escarra, Matthew D.; Eisler, Carissa N.; Kosten, Emily D.; Warmann, Emily C.; Darbe, Sunita; Lloyd, John; Flowers, Cristofer
Future photovoltaic systems can be greatly benefited by modules that exhibit simultaneously ultrahigh efficiency (> 50%) and low-cost (< $0.50/Wp) to enable sharp reductions in the levelized cost of electricity. A 'full spectrum' photovoltaic module, which takes advantage of advances in low-cost III-V compound cell fabrication and emerging optical and electronic fabrication/assembly methods, features 6-15 independently connected subcells in a spectrum splitting, concentrating photovoltaic receiver. Module architectures utilizing independently connected single junction and multijunction subcells allow flexibility in subcell selection for optimal energy bandgaps and fabrication, and also reduce the constraints posed by current matching requirements. Several different spectrum-splitting optical...
Experimental measurement of lateral transport in the inversion layer of silicon heterojunction solar cells - Emmer, Hal S.; Deceglie, Michael G.; Holman, Zachary C.; Descoeudres, Antoine; De Wolf, Stefaan; Ballif, Christophe; Atwater, Harry A.
We performed two experiments to measure lateral flow of photoexcited charge carriers near the heterointerface in silicon heterojunction (SHJ) solar cells. Using light beam methods, we probed current extraction differences between areas of varying intrinsic layer thickness and the effective cross section of junction defects. Both measurements demonstrated a strong bias voltage dependence of lateral transport and transport lengths of tens to hundreds of microns as bias approached operating voltages. Lateral carrier flow near the heterointerface is proposed as one of the reasons that SHJ solar cells are extremely sensitive to interfacial defects.
Oxidant and precursor trends in the metropolitan Los Angeles region - Trijonis, John; Peng, Ted; McRae, Gregory; Lees, Lester
This paper describes recent htstorical trends in oxidant and precursors in the Los Angeles region. Control strategies and basin-wide emission trends for nitrogen oxides and reactive hydrocarbons are documented year by year
from 1965 to 1974. Trends in the geographic distribution of emissions are illustrated by computing net percentage emission changes over the decade for individual counties. The changes in emissions are compared to changes in ambient precursor concentrations and oxidant concentrations. It is found that many of the changes in monitored air quality can be explained by trends in total emissions and in the spatial distribution of emissions.
Microcrack Toughening in a SiC-TiB_2 Composite - Cai, H.; Gu, W.-H.; Faber, K. T.
Transmission electron microscopy observations of a fractured SiC-TiB_2
composite have provided conclusive evidence of stress-induced microcracking
in this system. The microcrack density parameter around a steadily growing
crack has been measured as a function of the distance from the crack plane.
Microcrack toughening in this system has been assessed using a discrete
model based on the measured microcrack density parameter. The
computation results indicate that nearly half of the toughening arises from
stress-induced microcracking. Other possible toughening mechanisms are
Search for temporal changes in seismic velocities using large explosions in southern California - Allen, Clarence R.; Helmberger, Donald V.
For many years, large explosions within the
Caltech seismic network have occurred periodically at a
quarry near Corona (75 km southeast of Pasadena) and at
the Eagle Mountain iron mine (240 km east of Pasadena).
Explosions have taken place almost daily since 1948 at
Eagle Mountain. The long-established station at Hayfield,
very close to the mine, permits accurate determinations of
apparent velocities to distant stations such as Barrett,
and this particular path crosses the epicentral region of
the 1968 Borrego Mountain earthquake (M = 6.4). The first
arrival at Barrett, at a distance of 172 km, is a refracted
wave, but a distinct second arrival presumably represents a
crustal phase. There has...
Flow-based load balancing in multipathed layer-2 networks using OpenFlow and multipath-TCP - Bredel, Michael; Bozakov, Zdravko; Barczyk, Artur; Newman, Harvey
In this paper we address the challenge of traffic optimization for big data flows in layer-2 networks. We present an OpenFlow controller implementation that removes the necessity of a Spanning Tree Protocol, allows for the usage of multiple paths, and enables in-network per-flow load balancing. Moreover, we demonstrate how systems deploying Multipath-TCP can benefit from our solution.