Showing papers by "Sandia National Laboratories published in 1997"

Current issues in recrystallization: a review

Abstract: The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

Combination of multiple classifiers using local accuracy estimates

Abstract: This paper presents a method for combining classifiers that uses estimates of each individual classifier's local accuracy in small regions of feature space surrounding an unknown test sample. An empirical evaluation using five real data sets confirms the validity of our approach compared to some other combination of multiple classifiers algorithms. We also suggest a methodology for determining the best mix of individual classifiers.

Dynamical heterogeneities in a supercooled lennard-jones liquid

Abstract: We present the results of a molecular dynamics computer simulation study in which we investigate whether a supercooled Lennard-Jones liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian parameter for the self part of the van Hove correlation function and use it to identify {open_quotes}mobile{close_quotes} particles. We find that these particles form clusters whose sizes grow with decreasing temperature. We also find that the relaxation time of the mobile particles is significantly shorter than that of the average particle, and that this difference increases with decreasing temperature. {copyright} {ital 1997} {ital The American Physical Society}

Review of advances in cubic boron nitride film synthesis

Abstract: Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology.

Micromachined pressure sensors: review and recent developments

Abstract: Since the discovery of piezoresistivity in silicon in the mid 1950s, silicon-based pressure sensors have been widely produced Micromachining technology has greatly benefited from the success of the integrated circuit industry, borrowing materials, processes, and toolsets Because of this, microelectromechanical systems (MEMS) are now poised to capture large segments of existing sensor markets and to catalyse the development of new markets Given the emerging importance of MEMS, it is instructive to review the history of micromachined pressure sensors, and to examine new developments in the field Pressure sensors will be the focus of this paper, starting from metal diaphragm sensors with bonded silicon strain gauges, and moving to present developments of surface-micromachined, optical, resonant, and smart pressure sensors Considerations for diaphragm design will be discussed in detail, as well as additional considerations for capacitive and piezoresistive devices Results from surface-micromachined pressure sensors developed by the authors will be presented Finally, advantages of micromachined sensors will be discussed

Two-dimensional phase unwrapping with minimum weighted discontinuity

Abstract: Given an interferometric phase image of a surface profile, the task of two-dimensional phase unwrapping is to reconstruct the profile by adding multiples of 2π to the image. Discontinuities in the unwrapped phase must be restricted to areas of noise and true discontinuity in the profile. Such areas can often be identified by their low quality. This suggests that the unwrapped phase should be chosen to minimize a weighted sum of discontinuity magnitudes. An algorithm is presented that computes such an unwrapped phase from any initial guess. The elementary operation of the algorithm is to partition the image into two connected regions, then raise the unwrapped phase by 2π in one of the regions, reducing the weighted sum; this is done repeatedly until no suitable partitions exist. The operations are found by creating paths that follow discontinuity curves and extending them to form complete partitions. The algorithm terminates when no path can be extended. The behavior of the algorithm and the benefits of weighting are illustrated with an example.

Bistable microelectromechanical actuator

Abstract: A bistable microelectromechanical (MEM) actuator is formed on a substrate and includes a stressed membrane of generally rectangular shape that upon release assumes a curvilinear cross-sectional shape due to attachment at a midpoint to a resilient member and at opposing edges to a pair of elongate supports. The stressed membrane can be electrostatically switched between a pair of mechanical states having mirror-image symmetry, with the MEM actuator remaining in a quiescent state after a programming voltage is removed. The bistable MEM actuator according to various embodiments of the present invention can be used to form a nonvolatile memory element, an optical modulator (with a pair of mirrors supported above the membrane and moving in synchronism as the membrane is switched), a switchable mirror (with a single mirror supported above the membrane at the midpoint thereof) and a latching relay (with a pair of contacts that open and close as the membrane is switched). Arrays of bistable MEM actuators can be formed for applications including nonvolatile memories, optical displays and optical computing.

Scaling of Structural Failure

Abstract: This article attempts to review the progress achieved in the understanding of scaling and size ef­ fect in the failure of structures. Particular emphasis is placed on quasi brittle materials for which the size etTect is important and complicated. After reflections on the long history of size effect studies, attention is focused on three main types of size effects, namely the statistical size effect due to randomness of strength, the energy release size effect, and the possible size effect due to fractality of fracture or microcracks. Definitive conclusions on the applicability of these theories are drawn. Subsequently, the article discusses the application of the known size effect law for the measurement of material fracture properties, and the modeling of the size effect by the cohesive crack model, non local finite element models and discrete element models. Extensions to com­ pression failure and to the rate-dependent material behavior are also outlined. The damage con­ stitutive law needed for describing a microcracked material in the fracture process zone is dis­ cussed. Various applications to quasibrittle materials, including concrete, sea ice, fiber compos­ ites, rocks and ceramics are presented. There are 377 references included in this article.

Advances in selective wet oxidation of AlGaAs alloys

Abstract: We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As/sub 2/O/sub 3/ to As The stable oxide is amorphous (Al/sub x/Ga/sub 1-x/)/sub 2/O/sub 3/ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content Oxidation selectivity to thickness is also shown for layer thickness <60 nm Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported

Temperature coefficients for PV modules and arrays: measurement methods, difficulties, and results

Abstract: The term "temperature coefficient" has been applied to several different photovoltaic performance parameters, including voltage, current and power. The procedures for measuring the coefficient(s) for modules and arrays are not yet standardized and systematic influences are common in the test methods used to measure them. There are also misconceptions regarding their application. Yet, temperature coefficients, however obtained, play an important role in PV power system design and sizing, where often the worst case operating condition dictates the array size. This paper: describes effective methods for determining temperature coefficients for cells, modules and arrays; identifies sources of systematic errors in measurements; gives typical measured values for modules; and provides guidance for their application in system engineering.

Linear Elastic Behavior of a Low-Density Kelvin Foam With Open Cells

Abstract: A micromechanical analysis for the linear elastic behavior of a low-density foam with open cells is presented. The foam structure is based on the geometry of Kelvin soap froth with flat faces: 14-sided polyhedral cells contain six squares and eight hexagons. Four struts meet at every joint in the perfectly ordered, spatially periodic, open-cell structure. All of the struts and joints have identical shape. Strut-level force-displacement relations are expressed by compliances for stretching, bending, and twisting. We consider arbitrary homogeneous deformations of the foam and present analytic results for the force, moment, and displacement at each strut midpoint and the rotation at each joint. The effective stress-strain relations for the foam, which has cubic symmetry, are represented by three elastic constants, a bulk modulus, and two shear moduli, that depend on the strut compliances. When these compliances are evaluated for specific strut geometries, the shear moduli are nearly equal and therefore the elastic response is nearly isotropic. The variational results of Hashin and Shtrikman are used to calculate the effective isotropic shear modulus of a polycrystal that contain grains of Kelvin foam.

Fundamentals of technology roadmapping

Abstract: Technology planning is important for many reasons. Globally, companies are facing many competitive problems. Technology roadmapping, a form of technology planning can help deal with this increasingly competitive environment. While it has been used by some companies and industries, the focus has always been on the technology roadmap as a product, not on the process. This report focuses on formalizing the process so that it can be more broadly and easily used. As a DOE national security laboratory with R&D as a major product, Sandia must do effective technology planning to identify and develop the technologies required to meet its national security mission. Once identified, technology enhancements or new technologies may be developed internally or collaboratively with external partners. For either approach, technology roadmapping, as described in this report, is an effective tool for technology planning and coordination, which fits within a broader set of planning activities. This report, the second in a series on technology roadmapping, develops and documents this technology roadmapping process, which can be used by Sandia, other national labs, universities, and industry. The main benefit of technology roadmapping is that it provides information to make better technology investment decisions by identifying critical technologies and technology gaps and identifying ways to leverage R&D investments. It can also be used as a marketing tool. Technology roadmapping is critical when the technology investment decision is not straight forward. This occurs when it is not clear which alternative to pursue, how quickly the technology is needed, or when there is a need to coordinate the development of multiple technologies. The technology roadmapping process consists of three phases - preliminary activity, development of the technology roadmap, and follow-up activity.

Properties and Structure of Sodium-iron Phosphate Glasses

Abstract: Selected properties of phosphate glasses, containing from 14 to 43 mol% Fe2O3 and up to 13 mol% Na2O, have been measured. With increasing Fe2O3 and Na2O content, the density and dilatometric softening temperature increased, whereas, the thermal expansion coefficient and dissolution rate in water or saline at 90°C decreased. Glasses containing more than 25 mol% Fe2O3 had an exceedingly good chemical durability. Their dissolution rate at 90°C in distilled water or in saline solution was up to 100 times lower than that of window glass. Mossbauer and X-ray photoelectron spectroscopy indicate that iron(II) and iron(III) were both present in the glasses and the chemical durability improved with increasing iron(III) concentration. The outstanding chemical durability of these glasses was attributed to the replacement of POP bonds by more chemically resistant POFe(II) and POFe(III) bonds.

Structural design of sealing glasses

Abstract: Requirements for enhanced component performance and reliability have led to the development of novel glass compositions for a variety of hermetic sealing applications. The development of technologically useful glass compositions was based on an understanding of the relationships between the molecular-level glass structure and important physical properties. The properties of the alkaline earth aluminoborate glasses for lithium batteries are sensitive to changes in B- and Al-coordination number, characterized by solid state nuclear magnetic resonance (NMR) spectroscopy. In general, the most useful compositions have structures that are dominated by tetrahedral Band Al-sites. Mixed alkali aluminophosphate glasses were developed for aluminum electrical connectors. The properties of sodium aluminophosphate glasses depend on the O/P ratio and significant property changes (e.g. maxima in T g and refractive index) occur when O/P exceeds the pyrophosphate limit at 3.5. Associated with these property changes is a decrease in the average Al-coordination number, from six to four, at O/P > 3.5. Raman spectroscopy provides additional information about the aluminophosphate network. Finally, zinc borophosphate glasses are developed for seals in flat panel displays. Boron-11 NMR shows that tetrahedral borons are preferred in xB 2 O 3 (1-x)(PO 3 ) 2 and in yB 2 O 3 (1 - y)Zn 2 P 2 O 7 glasses for x < 0.4 and y < 0.2. Raman spectroscopy reveals the concomitant evolution from a phosphate to a borophosphate network with increasing x and y.

Method for integrating microelectromechanical devices with electronic circuitry

Abstract: A method is disclosed for integrating one or more microelectromechanical (MEM) devices with electronic circuitry on a common substrate. The MEM device can be fabricated within a substrate cavity and encapsulated with a sacrificial material. This allows the MEM device to be annealed and the substrate planarized prior to forming electronic circuitry on the substrate using a series of standard processing steps. After fabrication of the electronic circuitry, the electronic circuitry can be protected by a two-ply protection layer of titanium nitride (TiN) and tungsten (W) during an etch release process whereby the MEM device is released for operation by etching away a portion of a sacrificial material (e.g. silicon dioxide or a silicate glass) that encapsulates the MEM device. The etch release process is preferably performed using a mixture of hydrofluoric acid (HF) and hydrochloric acid (HCI) which reduces the time for releasing the MEM device compared to use of a buffered oxide etchant. After release of the MEM device, the TiN:W protection layer can be removed with a peroxide-based etchant without damaging the electronic circuitry.

Room-temperature compound semiconductor radiation detectors

Abstract: A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Three‐dimensional massively parallel electromagnetic inversion—I. Theory

Abstract: Summary An iterative solution to the non-linear 3-D electromagnetic inverse problem is obtained by successive linearized model updates using the method of conjugate gradients. Full wave equation modelling for controlled sources is employed to compute model sensitivities and predicted data in the frequency domain with an efficient 3-D finite-difference algorithm. Necessity dictates that the inverse be underdetermined, since realistic reconstructions require the solution for tens of thousands of parameters. In addition, large-scale 3-D forward modelling is required and this can easily involve the solution of over several million electric field unknowns per solve. A massively parallel computing platform has therefore been utilized to obtain reasonable execution times, and results are given for the 1840-node Intel Paragon. The solution is demonstrated with a synthetic example with added Gaussian noise, where the data were produced from an integral equation forward-modelling code, and is different from the finite difference code embedded in the inversion algorithm

Resonant-cavity apparatus for cytometry or particle analysis

Abstract: A resonant-cavity apparatus for cytometry or particle analysis. The apparatus comprises a resonant optical cavity having an analysis region within the cavity for containing one or more biological cells or dielectric particles to be analyzed. In the presence of a cell or particle, a light beam in the form of spontaneous emission or lasing is generated within the resonant optical cavity and is encoded with information about the cell or particle. An analysis means including a spectrometer and/or a pulse-height analyzer is provided within the apparatus for recovery of the information from the light beam to determine a size, shape, identification or other characteristics about the cells or particles being analyzed. The recovered information can be grouped in a multi-dimensional coordinate space for identification of particular types of cells or particles. In some embodiments of the apparatus, the resonant optical cavity can be formed, at least in part, from a vertical-cavity surface-emitting laser. The apparatus and method are particularly suited to the analysis of biological cells, including blood cells, and can further include processing means for manipulating, sorting, or eradicating cells after analysis thereof.

Solar cell efficiency tables (version 10)

Abstract: By establishingguidelines for the inclusion of results into these tables, this not only provides an authoritativesummary of the current state of the art but also encourages researchers to seek independent confirmationof results and to report results on a standardized basis.Briefly, the main criterion for inclusion of results in these tables in that they be measured at one of thedesignated test centres previously listed

Thin film and surface characterization by specular X-ray reflectivity

Abstract: We review the use of specular X-ray reflectivity (XRR) for the characterization of thin-film and surface structures. Specular X-ray scattering at small scattering vectors allows characterization of electron density profiles perpendicular to the surface on the length scale of 0.1 to 100 nm. This allows measurement of surface morphology, thin films, multilayer structures, and buried interfaces. The technique is nondestructive and can be applied in situ in a variety of processing environments. In the first half of the article, we review the theory and methods of XRR, including analysis of XRR spectra by a multilayer optical approach and a discussion of surface roughness measurements by XRR and other techniques. In the second half, we present a wide range of examples of XRR applications in thin-film structures, dynamic processes, liquid surfaces, and macromolecular structures.

Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers

Abstract: This paper discusses the issues involving the design and fabrication of vertical-cavity surface-emitting lasers (VCSELs). A review of the basic experimental structures is given, with emphasis on recent developments in distributed Bragg reflectors, gain media, as well as current and optical confinement techniques. The paper describes present VCSEL performance, in particular, those involving selective oxidation and visible wavelength operation.

Hall-Effect Sign Anomaly and Small-Polaron Conduction in \(La 1-x Gd x \) 0.67 Ca 0.33 MnO 3

Abstract: The Hall coefficient of Gd-doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ exhibits Arrhenius behavior over a temperature range from ${2T}_{c}$ to ${4T}_{c}$, with an activation energy very close to $\frac{2}{3}$ that of the electrical conductivity. Although both the doping level and thermoelectric coefficient indicate holelike conduction, the Hall coefficient is electronlike. This unusual result provides strong evidence in favor of small-polaronic conduction in the paramagnetic regime of the manganites.