Preconcentration is a critical analytical procedure when designing a microsystem for trace chemical detection, because it can purify a sample mixture and boost the small analyte concentration to a much higher level allowing a better analysis. This paper describes the development of a micro-fabricated planar preconcentrator for the μChemLab™ at Sandia. To guide the design, an analytical model to predict the analyte transport, adsorption and desorption process in the preconcentrator has been developed. Experiments have also been conducted to analyze the adsorption and desorption process and to validate the model. This combined effort of modeling, simulation, and testing has led us to build a reliable, efficient preconcentrator with good performance.
A study of zeolite crystallization from sol-gel precursors using the vapor phase transport synthesis method has been performed. Zeolites (ZSM-5, ZSM-48, zeolite P, and sodalite) were crystallized by contacting vapor phase organic or organic-water mixtures with dried sodium silicate and dried sodium alumino-silicate gels. For each precursor gel, a ternary phase system of vapor phase organic reactant molecules was explored. The vapor phase reactant mixtures ranged from pure ethylene diamine, triethylamine, or water, to an equimolar mixture of each. In addition, a series of gels with varied physical and chemical properties were crystallized using the same vapor phase solvent mixture for each gel. The precursor gels and the crystalline products were analyzed via scanning electron microscopy, electron dispersive spectroscopy, X-ray mapping, powder X-ray diffraction, nitrogen surface area, Fourier transform infrared spectroscopy, and thermal analyses. The product phase and purity as a function of the solvent mixture, precursor gel structure, and precursor gel chemistry is discussed.
The desire to move high-energy Pulsed Power systems from the laboratory to practical field systems requires the development of compact lightweight drivers. This paper concerns an effort to develop such a system based on a plastic laminate strip Blumlein as the final pulse shaping stage for a 600 kV, 50ns, 5-ohm driver. A lifetime and breakdown study conducted with small-area samples identified Kapton sheet impregnated with Propylene Carbonate as the best material combination of those evaluated. The program has successfully demonstrated techniques for folding large area systems into compact geometry's and vacuum impregnating the laminate in the folded systems. The major operational challenges encountered revolve around edge grading and low inductance, low impedance switching. The design iterations and lessons learned will be discussed. A multistage prototype testing program has demonstrated 600kV operation on a short 6ns line. Full-scale prototypes are currently undergoing development and testing.
The accuracy of solar cells calibrated as primary reference cells is directly dependent on the accuracy of the pyrheliometer used to measure the direct beam solar irradiance on the cell. Pyrheliometers are also used in measuring performance of concentrating photovoltaic modules. In order to reduce errors in photovoltaic performance measurements, we have investigated the calibration uncertainties for pyrheliometers from two manufacturers. Our calibration comparisons are relative to an absolute cavity radiometer traceable to the World Radiometric Reference. This paper quantifies the effects of aging, temperature, time-rate-of-change of temperature, wind, solar spectral shifts, linearity, window transmission, and solar tracking on pyrheliometer calibrations. Uncertainty remaining after accounting for these factors is 0.8% at the 2-sigma level.
The objective of this study was to investigate the technology used by Spectrolab Inc. to manufacture photovoltaic modules that have provided twenty years of reliable service at Natural Bridges National Monument in southeastern Utah. A field survey, system performance tests, and a series of module and materials tests have confirmed the durability of the modules in the array. The combination of manufacturing processes, materials, and quality controls used by Spectrolab resulted in modules that have maintained a performance level close to the original specifications for twenty years. Specific contributors to the durability of the modules included polyinyl-butyral (PVB) encapsulant, expanded metal interconnects, silicon oxide anti-reflective coating, and excellent solder/substrate solderability.
A new valve regulated lead-acid (VRLA) gel motive power battery and PV system power center have been tested in the laboratory and at a PV hybrid telecommunication site. The power center provides battery charge control, system remote communications, and data acquisition at the field test site. Extensive laboratory and field-test data were used to improve battery performance by optimizing regulation voltages, finish-charge, and system design. After 1.5-years of service, battery and charge controller performance have met all performance requirements for the remote communications site at Sandia National Laboratories.
The bulk electrical anisotropy of sedimentary formations is a macroscopic phenomenon whic h can result from the presence of sand/shale laminae and varations in grain size and pore space. Accounting for its effects on induction log response is an ongoing research problem for the w ell-logging communit y since these types of sedimentary stuctures have long been correlated with productive hydrocarbon reservoirs. Presented here is a finite difference method for sim ulatingEM induction in a fully 3D anisotropic medium. This w ork differs from previous modeling efforts in that the electrical conductivity of the formation is represented as a full 3×3 tensor whose elements can vary arbitrarily with position throughout the formation. As an example, we simulate borehole induction tool responses in a crossbedded eolian sandstone to demonstrate the challenge faced by interpreters when electrical anisotropy is neglected.
Photovoltaic (PV) power systems, like other electrical systems, may be subject to unexpected ground faults. Installed PV systems always have invisible elements other than those indicated by their electrical schematics. Stray inductance, capacitance and resistance are distributed throughout the system. Leakage currents associated with the PV modules, the interconnected array, wires, surge protection devices and conduit add up and can become large enough to look like a ground-fault. PV systems are frequently connected to other sources of power or energy storage such as batteries, standby generators, and the utility grid. This complex arrangement of distributed power and energy sources, distributed impedance and proximity to other sources of power requires sensing of ground faults and proper reaction by the ground-fault protection devices. The different dc grounding requirements (country to country) often add more confusion to the situation. This paper discusses the ground-fault issues associated with both the dc and ac side of PV systems and presents test results and operational impacts of backfeeding commercially available ac ground-fault protection devices under various modes of operation. Further, the measured effects of backfeeding the tripped ground-fault devices for periods of time comparable to anti-islanding allowances for utility interconnection of PV inverters in the United States are reported.
Conference Record of the IEEE Photovoltaic Specialists Conference
Aiken, Daniel J.
Triple junction InGaP/GaAs/Ge solar cells are highly current mismatched due to the excess current generating capability of the germanium subcell. This severe current mismatch invites new approaches for increasing performance beyond that of current triple junctions. Presented here are two approaches for improving the efficiency of III-V multi-junctions beyond that of current triple junction technology. Both of these approaches involve the use of thin epitaxial germanium and do not require the development of new ∼1eV photovoltaic materials. The theoretical AM0 efficiency is over 30%. Modeling suggests the potential for over 1.5% absolute efficiency gain with respect to current InGaP/GaAs/Ge triple junction solar cells.
Sub-wavelength periodic texturing (gratings) of crystalline-silicon (c-Si) surfaces for solar cell applications can be designed for maximizing optical absorption in thin c-Si films. We have investigated c-Si grating structures using rigorous modeling, hemispherical reflectance, and internal quantum efficiency measurements. Model calculations predict almost ∼ 100 % energy coupling into obliquely propagating diffraction orders. By fabrication and optical characterization of a wide range of ID & 2D c-Si grating structures, we have achieved broadband, low (∼ 5 %) reflectance without an anti-reflection film. By integrating grating structures into conventional solar cell designs, we have demonstrated short-circuit current density enhancements of 3.4 and 4.1 mA/cm2 for rectangular and triangular 1D grating structures compared to planar controls. The effective path length enhancements due to these gratings were 2.2 and 1.7, respectively. Optimized 2D gratings are expected to have even better performance.
A maskless plasma texturing technique using Reactive Ion Etching for silicon solar cells results in a very low reflectance of 5.4 % before, and 3.9 % after SiN deposition. A detailed study of surface recombination and emitter properties was made, then solar cells were fabricated using the DOSS solar cell process. Different plasma-damage removal treatments are tested to optimize low lifetime solar cell efficiencies. Highest efficiencies are observed for little or no plasma-damage removal etching on mc-Si. Increased Jsc due to the RIE texture proved superior to a single layer anti-reflection coating. This indicates that RIE texturing is a promising texturing technique, especially applicable on lower lifetime (multicrystalline) silicon. The use of non-toxic, non-corrosive SF6 makes this process attractive for mass production.
The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to mate the two surfaces and subsequently applying a tensile load to peel the surfaces apart. The contact region is assumed the superposition of elastic Hertzian pressure and of the attractive surface forces that act only over the contact area. What are the effects of the degree of surface contamination on adhesive forces? Clean aluminum surfaces were coated with hexadecane as a controlled contaminant. The force required to pull an elastomeric hemisphere from a surface was determined by contact mechanics, via the JKR model, using a model siloxane network for the elastomeric contact sphere. Due to the dispersive nature of the elastomer surface, larger forces were required to pull the sphere from a contaminated surface than a clean aluminum oxide surface.
A new class of semiconductor laser is presented that does not require p-n junctions. Spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies are shown for these lasers based on current filaments in bulk GaAs.
The Waste Isolation Pilot Plan requires a dependable shaft seal system to isolate the waste from the biosphere. This paper describes the shaft sealing system, which is designed to limit fluid transport through the four existing shafts. The design approach applies redundancy to functional elements and specifies multiple, common, low-permeability materials to ensure reliable performance. The system comprises 13 elements that completely fill the shafts with engineered materials possessing high density and low permeability. Laboratory and field measurements of component properties and performance provide the basis for the design and related evaluations. Hydrologic, mechanical, thermal, and physical features of the system are evaluated in a series of calculations. These calculations indicate that the design limits transport of fluids within the shafts, thereby limiting transport of hazardous material to regulatory boundaries. Additionally, the use or adaptation of existing technologies for seal construction combined with the use of available common materials assure that the design can be constructed.
The theory for immiscible mixtures by Drumheller and Bedform was compared with the theory of Passman, Nunziato, and Walsh. The conditions under these theories reduce to an equivalent formulation are described, and the differences in their microinertial descriptions are also investigated. Two variables play special roles in both theories. They are the true material density and the volume fraction.
A long-standing conjecture in combinatorial optimization says that the integrality gap of the famous Held-Karp relaxation of the symmetric TSP is precisely 4/3. In this paper, we show that a slight strengthening of this conjecture implies a tight 4/3 integrality gap for a linear programming relaxation of the asymmetric TSP. This is surprising since no constant-factor approximation is known for the latter problem. Our main tools are a new characterization of the integrality gap for linear objective functions over polyhedra, and the isolation of `hard-to-round' solutions of the relaxations.
A capacitated covering integer programs (IP) is an integer program of the form min{cx|Ux≥d, 0≤x≤b, x∈Z+}, where all entries of c, U and d are nonnegative. Given such a formulation, the ratio between the optimal integer solution and the optimal solution to the linear program relaxation can be as bad as ∥d∥, even when U consists of a single row. It is shown that by adding additional inequalities, this ratio can be improved significantly. In the general case, the improved ratio is shown to be bounded by the maximum number of non-zero coefficients in a row of U, and a polynomial-time approximation is proved to achieve this bound.
The decomposition of unconfined rigid polyurethane foam has been modeled by a kinetic bond-breaking scheme describing degradation of a primary polymer and formation of a thermally stable secondary polymer. The bond-breaking scheme is resolved using percolation theory to describe evolving polymer fragments. The polymer fragments vaporize according to individual vapor pressures. Kinetic parameters for the model were obtained from thermal gravimetric analysis (TGA). The chemical structure of the foam was determined from the preparation techniques and ingredients used to synthesize the foam. Scale-up effects were investigated by simulating the response of an incident heat flux of 25 W/cm2 on a partially confined 8.8-cm diameter by 15-cm long right circular cylinder of foam that contained an encapsulated component. Predictions of internal foam and component temperatures, as well as regression of the foam surface, were in agreement with measurements using thermocouples and X-ray imaging.
An algorithm known as SMAC (Synthesize Modes And Correlate), based on principles of modal filtering, has been in development for a few years. The new capabilities of the automated version are demonstrated on test data from a complex shell/payload system. Examples of extractions from impact and shaker data are shown. The automated algorithm extracts 30 to 50 modes in the bandwidth from each column of the frequency response function matrix. Examples of the synthesized Mode Indicator Functions (MIFs) compared with the actual MIFs show the accuracy of the technique. A data set for one input and 170 accelerometer outputs can typically be reduced in an hour. Application to a test with some complex modes is also demonstrated.
The spectacular progress made during the last few years in reaching high energy densities in fast implosions of annular current sheaths (fast Z pinches) opens new possibilities for a broad spectrum of experiments, from x-ray generation to controlled thermonuclear fusion and astrophysics. At present Z pinches are the most intense laboratory x-ray sources (1.8 MJ in 5 ns from a volume 2 mm in diameter and 2 cm tall). Powers in excess of 200 TW have been obtained. This warrants summarizing the present knowledge of physics that governs the behavior of radiating, current-carrying plasma in fast Z pinches. This survey covers essentially all aspects of the physics of fast Z pinches: initiation, instabilities of the early stage, magnetic Rayleigh-Taylor instability in the implosion phase, formation of a transient quasiequilibrium near the stagnation point, and rebound. Considerable attention is paid to the analysis of hydrodynamic instabilities governing the implosion symmetry. Possible ways of mitigating these instabilities are discussed. Nonmagnetohydrodynamic effects (anomalous resistivity, generation of particle beams, etc.) are summarized. Various applications of fast Z pinches are briefly described. Scaling laws governing development of more powerful Z pinches are presented.
Important factors in the application of sensing technology to space applications are low mass, small size, and low power. All of these attributes are enabled by the application of MEMS and micro-fabrication technology to micro-sensors. Two types of sensors are utilized in space applications: remotes sensing from orbit around the earth or another planetary body, and point sensing in the spacecraft or external to it. Several Sandia projects that apply microfabrication technologies to the development of new sensing capabilities having the potential for space applications will be briefly described. The Micro-Navigator is a project to develop a MEMS-based device to measure acceleration and rotation in all three axes for local area navigation. The Polychromator project is a joint project with Honeywell and MIT to develop an electrically programmable diffraction grating that can be programmed to synthesize the spectra of molecules. This grating will be used as the reference cell in a gas correlation radiometer to enable remote chemical detection of most chemical species. Another area of research where micro-fabrication is having a large impact is the development of a "lab on a chip." Sandia's efforts to develop the μChemLab™ will be described including the development of microfabricated pre-concentrators, chromatographic columns, and detectors. Smart sensors that allow the spacecraft independent decision making capabilities depend on pattern recognition. Sandia's development of a new pattern recognition methodology that can be used to interpret sensor response as well as for target recognition applications will be described.
Microsystems involve several fabrication technologies, but share the common trait of dimensions and motions measured in microns. Small feature sizes and deflections make the detection of microdevice motion particularly difficult. The rapid operating frequencies of many microactuators compound the detection problem. Effective feedback, control, and performance measurement of microactuators thus become problematic. These measurements are particularly important, however, due to the developmental nature of many microsystem technologies. Wear, lifetime issues, and optimized drive signals, for example, are poorly understood for many actuation devices. As microactuators move out of the development stage and begin to perform work on external assemblies and environments, the various load conditions will also come into account. Since microactuators involve small masses and inertias, effective driving of external loads may require feedback-based control of the microdevice. Optical sensing technologies offer solutions to these problems of sensor motion, microactuator analysis during the development process, and integrated feedback for microactuators driving external loads. Optical methods also end themselves to the effectively 1D nature of many microsystem motions, limiting the required signal analysis to practical levels that support real-time measurement and control. This paper describes several optical techniques for sensing motion, performance, and feedback data, some of which can integrated with the microsystems themselves. For microactuators, experimental results indicate that real-time performance measurements are particularly revealing for understanding device motion and response. For microsensors, experimental result are also presented for interpreting motion using external and integrated optical techniques.