The electrical conductivities of boron carbides, B{sub 12+x}C{sub 3{minus}x} with 0.1 < x < 1.7, between 300 and 1200K suggest the hopping of a nearly temperature-independent density of small (bi)polarons. The activation energies of the nobilities are low, {approx} 0.16 eV, and are nearly independent of the composition. At lower temperatures, conductivities have non-Arrhenius temperature dependencies and strong sensitivity to carbon concentration. Percolative aspects of low-temperature hopping are evident in this sensitivity to composition. Boron carbides' Seebeck coefficients are anomalous in that (1) they are much larger than expected from boron carbides' large carrier densities and (2) they depend only weakly on the carrier density. Carrier-induced softening of local vibrations gives contributions to the Seebeck coefficient that mirror the magnitudes and temperature dependencies found in boron carbides.
A technological break through for supporting rotating shafts is the active magnetic bearing (AMB). Active magnetic bearings offer some important advantages over conventional ball, roller or journal bearings such as reduced frictional drag, no physical contact in the bearing, no need for lubricants, compatibility with high vacuum and ultra-clean environments, and ability to control shaft position within the bearing. The disadvantages of the AMB system are the increased cost and complexity, reduced bearing stiffness and the need for a controller. Still, there are certain applications, such as high speed machining, biomedical devices, and gyroscopes, where the additional cost of an AMB system can be justified. The inherent actuator capabilities of the AMB offer the potential for active balancing of spindles and micro-shaping capabilities for machine tools, The work presented in this paper concentrates on an AMB test program that utilizes the actuator capability to dynamically balance a spindle. In this study, an unbalanced AMB spindle system was enhanced with an LMS (Least Mean Squares) algorithm combined with an existing PID (proportional, integral, differential) control. This enhanced controller significantly improved the concentricity of an intentionally unbalanced shaft. The study included dynamic system analysis, test validation, control design and simulation, as well as experimental implementation using a digital LMS controller.
Mention the words ''stress voiding'', and everyone from technology engineer to manager to customer is likely to cringe. This IC failure mechanism elicits fear because it is insidious, capricious, and difficult to identify and arrest. There are reasons to believe that a damascene-copper future might be void-free. Nevertheless, engineers who continue to produce ICs with Al-alloy interconnects, or who assess the reliability of legacy ICs with long service life, need up-to-date insights and techniques to deal with stress voiding problems. Stress voiding need not be fearful. Not always predictable, neither is it inevitable. On the contrary, stress voids are caused by specific, avoidable processing errors. Analytical work, though often painful, can identify these errors when stress voiding occurs, and vigilance in monitoring the improved process can keep it from recurring. In this article, they show that a methodical, forensics approach to failure analysis can solve suspected cases of stress voiding. This approach uses new techniques, and patiently applies familiar ones, to develop evidence meeting strict standards of proof.
The electrical properties were investigated for ruthenium oxide based devitrifiable resistors embedded within low temperature co-fired ceramics. Special attention was given to the processing conditions and their affects on resistance and temperature coefficient of resistance (TCR). Results indicate that the conductance for these buried resistors is limited by tunneling of charge carriers through the thin glass layer between ruthenium oxide particles. A modified version of the tunneling barrier model is proposed to more accurately account for the microstructure ripening observed during thermal processing. The model parameters determined from curve fitting show that charging energy (i.e., the energy required for a charge carrier to tunnel through the glass barrier) is strongly dependent on particle size and particle-particle separation between ruthenium oxide grains. Initial coarsening of ruthenium oxide grains was found to reduce the charging energy and lower the resistance. However, when extended ripening occurs, the increase in particle-particle separation increases the charging energy, reduces the tunneling probability and gives rise to a higher resistance. The trade-off between these two effects results an optimum microstructure with a minimum resistance and TCR. Furthermore, the TCR of these resistors has been shown to be governed by the magnitude of the charging energy. Model parameters determined by our analysis appear to provide quantitative physical interpretations to the microstructural change in the resistor, which in turn, are controlled by the processing conditions.
A number of fluoro-carbonate solvents were evaluated as electrolytes for Li-ion cells. These solvents are fluorine analogs of the conventional electrolyte solvents such as dimethyl carbonate, ethylene carbonate, diethyl carbonate in Li-ion cells. Conductivity of single and mixed fluoro carbonate electrolytes containing 1 M LiPF{sub 6} was measured at different temperatures. These electrolytes did not freeze at -40 C. We are evaluating currently, the irreversible 1st cycle capacity loss in carbon anode in these electrolytes and the capacity loss will be compared to that in the conventional electrolytes. Voltage stability windows of the electrolytes were measured at room temperature and compared with that of the conventional electrolytes. The fluoro-carbon electrolytes appear to be more stable than the conventional electrolytes near Li voltage. Few preliminary electrochemical data of the fluoro-carbonate solvents in full cells are reported in the literature. For example, some of the fluorocarbonate solvents appear to have a wider voltage window than the conventional electrolyte solvents. For example, methyl 2,2,2 trifluoro ethyl carbonate containing 1 M LiPF{sub 6} electrolyte has a decomposition voltage exceeding 6 V vs. Li compared to <5 V for conventional electrolytes. The solvent also appears to be stable in contact with lithium at room temperature.
The thermal stability of Li-ion cells with intercalating carbon anodes and metal oxide cathodes was measured as a function of state of charge and temperature for two advanced cell chemistries. Cells of the 18650 design with Li{sub x}CoO{sub 2} cathodes (commercial SONY cells) and Li{sub x}Ni{sub 0.8}Co{sub 0.2}O{sub 2} cathodes were measured for thermal reactivity in the open circuit cell condition. Accelerating rate calorimetry (ARC) was used to measure cell thermal runaway as a function of state of charge (SOC). Microcalorimetry was used to measure the time dependence of heat generating side reactions also as a function of SOC. Components of cells were measured using differential scanning calorimetry (DSC) to study the thermal reactivity of the individual electrodes to determine the temperature regimes and conditions of the major thermal reactions. Thermal decomposition of the SEI layer at the anodes was identified as the initiating source for thermal runaway. The cells with Li{sub x}CoO{sub 2} cathodes showed greater sensitivity to SOC and higher accelerating heating rates than seen for the cells with Li{sub x}Ni{sub 0.8}Co{sub 0.2}O{sub 2}cathodes. Lower temperature reactions starting as low as 40 C were also observed that were SOC dependent but not accelerating. These reactions were also measured in the microcalorimeter and observed to decay over time with a power-law dependence and are believed to result in irreversible capacity loss in the cells.
Thermal modeling and simulations were used to analyze the thermal profiles of a polysilicon-metal test structure generated by thermally-induced voltage alteration (TIVA), a new laser-based failure analysis technique to localize shorted interconnects. The results show that variations in TIVA thermal profiles are due mainly to preferential laser absorption in various locations in the test structure. Differences in oxide thickness also affect the local heat conduction and temperature distribution. Modeling results also show that local variation in heat conduction is less important than the absorbed laser power in determining the local temperatures since our test structure has feature sizes that are small compared to the length over which heat spreads.
The spatial arrangement of sodium cations for a series of sodium phosphate glasses, xNa{sub 2}O(100-x)P{sub 2}O{sub 5} (x<55), were investigated using {sup 23}Na spin-echo NMR spectroscopy. The spin-echo decay rate is a function of the Na-Na homonuclear dipolar coupling and is related to the spatial proximity of neighboring Na nuclei. The spin-echo decay rate in these sodium phosphate glasses increases non-linearly with higher sodium number density, and thus provides a measure of the Na-Na extended range order. The results of these {sup 23}Na NMR experiments are discussed within the context of several structural models, including a decimated crystal lattice model, cubic dilation lattice model, a hard sphere (HS) random distribution model and a pair-wise cluster hard sphere model. While the experimental {sup 23}Na spin-echo M{sub 2} are described adequately by both the decimated lattice and the random HS model, it is demonstrated that the slight non-linear behavior of M{sub 2} as a function of sodium number density is more correctly described by the random distribution in the HS model. At low sodium number densities the experimental M{sub 2} is inconsistent with models incorporating Na-Na clustering. The ability to distinguish between Na-Na clusters and non-clustered distributions becomes more difficult at higher sodium concentrations.
We report the operation of an electrically injected monolithic coupled resonator vertical cavity laser which consists of an active cavity containing In{sub x}Ga{sub 1{minus}x}As quantum wells optically coupled to a passive GaAs cavity. This device demonstrates novel modulation characteristics arising from dynamic changes in the coupling between the active and passive cavities. A composite mode theory is used to model the output modulation of the coupled resonator vertical cavity laser. It is shown that the laser intensity can be modulated by either forward or reverse biasing the passive cavity. Under forward biasing, the modulation is due to carrier induced changes in the refractive index, while for reverse bias operation the modulation is caused by field dependent cavity enhanced absorption.
We compare 2 angular regimes for the measurement of changes in the real refractive index of bulk fluid analytes. The measurements are based on the use of the Kretschmann-Raether configuration to sense a change in reflectivity with index. Specifically, we numerically simulate the relative sensitivities of the total internal reflection (TIR) and surface-plasmon resonance (SPR) regimes. For a fixed-angle apparatus, the method which gives the greatest change in reflectivity varies with metal film thickness. For films thicker than the skin depth, the SPR regime is the most sensitive to index changes. For thinner films, however, the TIR angle is then dominant, with increases in sensitivity on the order of 75% for 10 nm gold or silver media.
We derive a lumped-element, equivalent-circuit model for the thickness shear mode (TSM) resonator with a viscoelastic film. This modified Butterworth-Van Dyke model includes in the motional branch a series LCR resonator, representing the quartz resonance, and a parallel LCR resonator, representing the film resonance. This model is valid in the vicinity of film resonance, which occurs when the acoustic phase shift across the film is an odd multiple of {pi}/2 radians. This model predicts accurately the frequency changes and damping that arise at resonance and is a reasonable approximation away from resonance. The elements of the model are explicitly related to film properties and can be interpreted in terms of elastic energy storage and viscous power dissipation. The model leads to a simple graphical interpretation of the coupling between the quartz and film resonances and facilitates understanding of the resulting responses. These responses are compared with predictions from the transmission-line and the Sauerbrey models.
We are developing a method of constructing compact, three-dimensional photonics systems consisting of optical elements, e.g., lenses and mirrors, photo-detectors, and light sources, e.g., VCSELS or circular-grating lasers. These optical components, both active and passive, are mounted on a lithographically prepared silicon substrate. We refer to the substrate as a micro-optical table (MOT) in analogy with the macroscopic version routinely used in optics laboratories. The MOT is a zero-alignment, microscopic optical-system concept. The position of each optical element relative to other optical elements on the MOT is determined in the layout of the MOT photomask. Each optical element fits into a slot etched in the silicon MOT. The slots are etched using a high-aspect-ratio silicon etching (HARSE) process. Additional positioning features in each slot's cross-section and complementary features on each optical element permit accurate placement of that element's aperture relative to the MOT substrate. In this paper we present the results of the first fabrication and micro-assembly experiments of a silicon-wafer based MOT. Based on these experiments, estimates of position accuracy are reported. We also report on progress in fabrication of lens elements in a hybrid sol-gel material (HSGM). Diffractive optical elements have been patterned in a 13-micron thick HSGM layer on a 150-micron thick soda-lime glass substrate. The measured ms surface roughness was 20 nm. Finally, we describe modeling of MOT systems using non-sequential ray tracing (NSRT).
The authors describe the design and microfabrication of an extremely compact optical system as a key element in an integrated capillary-channel electrochromatograph with laser induced fluorescence detection. The optical design uses substrate-mode propagation within the fused silica substrate. The optical system includes a vertical cavity surface-emitting laser (VCSEL) array, two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. Two generations of optical subsystems are described. The first generation design is integrated directly onto the capillary channel-containing substrate with a 6 mm separation between the VCSEL and photodetector. The second generation design separates the optical system onto its own module and the source to detector length is further compressed to 3.5 mm. The systems are designed for indirect fluorescence detection using infrared dyes. The first generation design has been tested with a 750 nm VCSEL exciting a 10{sup -4} M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and meets the system sensitivity requirements.
A portable, autonomous, hand-held chemical laboratory ({micro}ChemLab{trademark}) is being developed for trace detection (ppb) of chemical warfare (CW) agents and explosives in real-world environments containing high concentrations of interfering compounds. Microfabrication is utilized to provide miniature, low-power components that are characterized by rapid, sensitive and selective response. Sensitivity and selectivity are enhanced using two parallel analysis channels, each containing the sequential connection of a front-end sample collector/concentrator, a gas chromatographic (GC) separator, and a surface acoustic wave (SAW) detector. Component design and fabrication and system performance are described.
InGaAsN alloys are a promising material for increasing the efficiency of multi-junction solar cells now used for satellite power systems. However, the growth of these dilute N containing alloys has been challenging with further improvements in material quality needed before the solar cell higher efficiencies are realized. Nitrogen/V ratios exceeding 0.981 resulted in lower N incorporation and poor surface morphologies. The growth rate was found to depend on not only the total group III transport for a fixed N/V ratio but also on the N/V ratio. Carbon tetrachloride and dimethylzinc were effective for p-type doping. Disilane was not an effective n-type dopant while SiCl4 did result in n-type material but only a narrow range of electron concentrations (2-5e17cm{sup -3}) were achieved.
High expansion aqueous foam is an aggregation of bubbles that has the appearance of soap suds and is used to isolate individuals both visually and acoustically. It was developed in the 1920's in England to fight coal mine fires and has been widely used since for fire fighting and dust suppression. It was developed at Sandia National Laboratories (SNL) in the 1970's for nuclear safeguards and security applications. In the mid-1990s, the National Institute of Justice (NIJ), the research arm of the Department of Justice, began a project with SNL to determine the applicability of high expansion aqueous foam for correctional applications. NIJ funded the project as part of its search for new and better less-than-lethal weapons for responding to violent and dangerous individuals, where other means of force could lead to serious injuries. The phase one objectives of the project were to select a low-to-no toxicity foam concentrate (foaming agent) with physical characteristics suited for use in a single cell or large prison disturbances, and to determine if the selected foam concentrate could serve as a carrier for Oleoresin Capsicum (OC) irritant. The phase two objectives were to conduct an extensive toxicology review of the selected foam concentrate and OC irritant, and to conduct respiration simulation experiments in the selected high expansion aqueous foam. The phase three objectives were to build a prototype individual cell aqueous foam system and to study the feasibility of aqueous foams for large prison facility disturbances. The phase four and five objectives were to use the prototype system to do large scale foam physical characteristics testing of the selected foam concentrate, and to have the prototype single cell system further evaluated by correctional representatives. Prison rather than street scenarios were evaluated as the first and most likely place for using the aqueous foam since prisons have recurrent incidents where officers and inmates might be seriously injured during violent confrontations. The very low density of the high expansion foam also makes it more suitable for indoor use. This paper summarizes the results of the project.
Previous work showed the possible existence of a total-dose latch effect in fully-depleted SOI transistors that could severely limit the radiation hardness of SOI devices. Other work showed that worst-case bias configuration during irradiation was the transmission gate bias configuration. In this work we further explore the effects of total-dose ionizing irradiation on fully-depleted SOI transistors. Closed-geometry and standard transistors fabricated in two fully-depleted processes were irradiated with 10-keV x rays. Our results show no evidence for a total-dose latch effect as proposed by others. Instead, in absence of parasitic trench sidewall leakage, our data suggests that the increase in radiation-induced leakage current is caused by positive charge trapping in the buried oxide inverting the back-channel interface. At moderate levels of trapped charge, the back-channel interface is slightly inverted causing a small leakage current to flow. This leakage current is amplified to considerably higher levels by impact ionization. Because the back-channel interface is in weak inversion, the top-gate bias can modulate the back-channel interface and turn the leakage current off at large, negative voltage levels. At high levels of trapped charge, the back-channel interface is fully inverted and the gate bias has little effect on leakage current. However, it is likely that this current also is amplified by impact ionization. For these transistors, the worst-case bias configuration was determined to be the ''ON'' bias configuration. These results have important implication on hardness assurance.
The role of the national laboratories, particularly the defense program laboratories, since the end of the cold war, has been a topic of continuing debate. The relationship of national laboratories to industry spurred debate which ranged from designating the labs as instrumental to maintaining U.S. economic competitiveness to concern over the perception of corporate welfare to questions regarding the industrial globalization and the possibility of U.S. taxpayer dollars supporting foreign entities. Less debated, but equally important, has been the national laboratories' potential competition with academia for federal research dollars and discussions detailing the role of each in the national research enterprise.
Starting from the microscopic light-matter interaction in form of the minimal coupling Hamiltonian, the multipole approximation for the optical response of localized electrons in atomic systems is extended to delocalized electrons in solids. A spatial averaging procedure is used to derive the electromagnetic sources for macroscopic Maxwell's equations as well as the corresponding many particle Hamiltonian on a coarse grained length scale. The results are illustrated for semiconductor bulk material up to quadruple moments for the interband transitions, where gauge invariant equations of motion for the optical response are obtained.
Simulation of chemical vapor deposition (CVD) in submicron features typical of semiconductor devices has been facilitated by extending the EVOLVE thin film etch and deposition simulation code to use thermal reaction mechanisms expressed in the Chemkin format. This allows consistent coupling between EVOLVE and reactor simulation codes that use Chemkin. In an application of a reactor-scale simulation code providing surface fluxes to a feature-scale simulation code, a proposed reaction mechanism for TEOS pyrolysis to deposit SiO{sub 2}, which had been applied successfully to reactor-scale simulation, is seen not to predict the low step coverage over trenches observed under short reactor residence time conditions. An apparent discrepancy between the mechanism and profile-evolution observations is a reduced degree of sensitivity of the deposition rate to the presence of reaction products, i.e., the byproduct inhibition effect is underpredicted. The cause of the proposed mechanism's insensitivity to byproduct inhibition is investigated with the combined reactor and topography simulators first by manipulating the surface to volume ratio of a simulated reactor and second by calibrating parameters in the proposed mechanism such as the calculated free energies of surface molecules. The conclusion is that the byproduct inhibition can not be enhanced to fit profile evolution data without comprising agreement with reactor scale data by simply adjusting mechanism parameters. Thus, additional surface reaction channels seem to be required to reproduce simultaneously experimental reactor-scale growth rates and experimental step coverages.
This study is devoted to providing a mechanistic rationale of coarsening induced failure in solder alloys during thermomechanical fatigue. Micromechanical modeling of cyclic deformation of eutectic tin-lead alloy was undertaken using the finite element method. The models consist of regularly arranged tin-rich and lead-rich phases, simulating the lamellar array and colony structure in a typical eutectic system. A fine structure and a coarse structure, bearing the same phase fraction but different in the aspect ratio of each lead-rich layer and in the number of lead-rich layers in each colony, are utilized for representing the microstructure before and after coarsening, respectively. Both phases are treated as elastic-plastic solids with their respective properties. For simplicity the creep effect is ignored without compromising the main objective of this study. Cyclic loading under pure shear and uniaxial conditions is modeled. It is found that both the fine and coarse structures exhibit essentially the same macroscopic stress-strain response. The coarse structure, however, shows a greater maximum effective plastic strain on a local scale throughout the deformation. The numerical result implies that, in a solder joint, a locally coarsened region may not be mechanically weaker than its surrounding, but it is subject to early damage initiation due to accumulated plasticity. Other implications regarding solder alloy failure and micromechanical modeling of two-phase materials are discussed.
The importance of interfacial processes in materials joining has a long history. A significant amount of work has suggested that processes collateral to wetting can affect the extent of wetting and moderate or retard wetting rate. Even very small additions of a constituent, known to react with the substrate, cause pronounced improvement in wetting and are exploited in braze alloys, especially those used for joining to ceramics. The wide diversity of processes, such as diffusion, chemical reaction, and fluxing, and their possible combinations suggest that various rate laws should be expected for wetting kinetics depending on the controlling processes. These rate laws are expected to differ crucially from the standard fluid controlled wetting models found in the literature. Voitovitch et al. and Mortensen et al. have shown data that suggests diffusion control for some systems and reaction control for others. They also presented a model of wetting kinetics controlled by the diffusion of a constituent contained by the wetting fluid. In the following a model will be constructed for the wetting kinetics of a small droplet of metal containing a constituent that diffuses to the wetting line and chemically reacts with a flat, smooth substrate. The model is similar to that of Voitovitch et al. and Mortensen et al. but incorporates chemical reaction kinetics such that the result contains both diffusion and reaction kinetics. The model is constructed in the circular cylinder coordinate system, satisfies the diffusion equation under conditions of slow flow, and considers diffusion and reaction at the wetting line to be processes in series. This is done by solving the diffusion equation with proper initial and boundary conditions, computing the diffusive flux at the wetting line and equating this to both the convective flux and reaction flux. This procedure is similar to equating the current flowing in components of a series circuit. The wetting rate will be computed versus time for a variety of diffusion and reaction conditions. A transition is observed from nonlinear (diffusive) to linear (reactive) behavior as the control parameters (such as the diffusion coefficient) are modified. This is in agreement with experimental observations. The adequacy of the slow flow condition, used in this type of analysis, is discussed and an amended procedure is suggested.
Making information available and easy to find is the objective of designing a good web site. A company's Intranet typically provides a great deal of information to its employees in an effort to help them better perform their jobs. If the information is available but is difficult to locate, the usefulness of this information is diminished. Sandia National Laboratories performed a redesign of its home page and has obtained a successful design which enables its employees to locate information quickly and efficiently. Three phases of usability testing were conducted to develop and optimize the home page. This paper will discuss the redesign of the Intranet home page and describe how usability studies were used to help ensure a usable design.
Wind turbine blades are often fabricated with composite materials. These composite blades are frequently attached to a metallic structure with an adhesive bond. For the baseline composite-to-steel joint considered in this study, failure typically occurs when the adhesive debonds from the steel adherend. Previous efforts established that the adhesive peel stresses strongly influence the strength of these joints for both single-cycle and fatigue loading. This study focused on reducing the adhesive peel stresses present in these joints by tapering the steel adherends. Several different tapers were evaluated using finite element analysis before arriving at a final design. To confirm that the selected taper was an improvement to the existing design, the baseline joint and the modified joint were tested in both compression and tension. In these axial tests, the compressive strengths of the joints with tapered adherends were greater than those of the baseline joints for both single-cycle and low-cycle fatigue. In addition, only a minor reduction in tensile strength was observed for the joints with tapered adherends when compared to the baseline joints. Thus, the modification would be expected to enhance the overall performance of this joint.
Within the framework of thermodynamic theory of plasticity and specific structural-variables (associated with individual dislocations), a transition has been made to an expression containing one internal variable of the averaging type--the density of glissile dislocations, N{sub g}. This expression should be considered a tensorial generalization of the well-known Orowan's equation and relates it directly to the simplest possible case of normal flow in metallic materials. Since most metals display deviations from normality in the flow rule{sup 7} it also clearly indicates that more rigorous assessment of the relation between plastic strain rate and dislocation populations is required especially for materials displaying plastic instabilities in the form of dislocation patterning, strain-softening and strain-rate softening phenomena. The obtained result could be a useful starting point in establishing such rigorous macroscopic relations from microscopic considerations associated with individual dislocations and to find useful applications in dislocation density-related constitutive modeling of plastic deformation.