Evaluation of the finite-element-synthesis model using the 3-D finite-element technique

For the solution of multi group diffusion theory equations a 3-D finite-element (FE) code finerc has been developed. 3-D element shapes with the base orthogonal to the third direction are considered in finerc. For these elements the 3-D element submatrices of the FE formulation are easily computed in terms of the corresponding lower-dimensional element submatrices. The 3-D problems are tackled in finerc with a degree of complexity equivalent to that of 2-D problems.The 3-D FE technique is still somewhat expensive for routine design computations. The method can, however, be used for assessing the accuracy of other faster calculational methods. In this paper we have compared the results of the 3-D FE method with those of the FE-synthesis method which was previously developed by the present author. It is noted that the FE-synthesis method gives, at negligible computational cost, accurate eigenvalue estimates and reasonably good predictions of reactor core power profiles for the 3-D benchmark problems. The FE-synthesis method may be used for a number of survey-type analyses with occasional counter-checking by the 3-D FE technique.     

Flying personal planes: modeling the airport choices of general aviation pilots using stated preference methodology

This study employed stated preference (SP) models to determine why general aviation pilots choose to base and operate their aircraft at some airports and not others. Thirteen decision variables identified in pilot focus groups and in the general aviation literature were incorporated into a series of hypothetical choice tasks or scenarios. The scenarios were offered within a fractional factorial design to establish orthogonality and to preclude dominance in any combination of variables. Data from 113 pilots were analyzed for individual differences across pilots using conditional logit regression with and without controls. The results demonstrate that some airport attributes (e.g., full-range hospitality services, paved parallel taxiway, and specific types of runway lighting and landing aids) increase pilot utility. Heavy airport congestion and airport landing fees, on the other hand, decrease pilot utility. The importance of SP methodology as a vehicle for modeling choice behavior and as an input into the planning and prioritization process is discussed. Actual or potential applications include the development of structured decision-making instruments in the behavioral sciences and in human service programs.     

Multi-scale dynamic failure prediction tool for marine composite structures

A high fidelity assessment of accumulative damage of woven fabric composite structures subjected to aggressive loadings is strongly reliant on the accurate characterization of the inherent multi-scale microstructures and the underlying deformation phenomena. Damage in composite sandwich and joint structures is characterized by the coexistence of discrete (delamination) and continuum damage (matrix cracking and intralaminar damage). A purely fracture mechanics-based or a purely continuum damage mechanics-based tool alone cannot effectively characterize the interaction between the discrete and continuum damage and their compounding effect that leads to the final rupture. In this paper, a hybrid discrete and continuum damage model is developed and numerically implemented within the LS-DYNA environment via a user-defined material model. The continuum damage progression and its associated stiffness degradation are predicted based on the constituent stress/strain and their associated failure criteria while the discrete delamination damage is captured via a cohesive interface model. A multi-scale computational framework is established to bridge the response and failure predictions at constituent, ply, and laminated composite level. The calculated constituent stress and strain are used in a mechanism-driven failure criterion to predict the failure mode, failure sequence, and the synergistic interaction that leads to global stiffness degradation and the final rupture. The use of the cohesive interface model can capture the complicated delamination zone without posing the self-similar crack growth condition. The unified depiction of the continuum and discrete damage via the damage mechanics theory provides a rational way to study the coupling effects between the in-plane and the out-of-plane failure modes. The applicability and accuracy of the damage models used in the hybrid dynamic failure prediction tool are demonstrated via its application to a circular plate and a composite hat stiffener subjected to shock and low velocity impact loading. The synergistic interaction between the continuum and discrete damage is explored via its application to a sandwich beam subjected to a low velocity impact.     

Dynamic state restoration using versioning exceptions

We explore the semantics and analysis of a new kind of control structure called a versioning exception that ensures the state of the program, at the point when an exception handler is invoked, reflects the program state at the point when the handler is installed. Versioning exceptions provide a transaction-like versioning semantics to the code protected by a handler: modifications performed within the dynamic context of the corresponding handler are versioned, and committed to the store only if the computation completes normally. Similar to the role of backtracking in logic programming, this facility allows unwanted effects of computations to be discarded when exceptional or undesirable conditions are detected. We define a novel points-to analysis to efficiently track changes to the store within handler-protected scopes. The role of the analysis is to facilitate optimizations that minimize the number of locations which must be restored when a versioning exception is raised. The analysis is defined by a reachability approximation over locations that indicates which objects have been potentially modified within a handler scope. The analysis is defined for programs which support first-class procedures, locations, and exceptions.     

Neurochemicals measured by 1H-MR spectroscopy: putative vulnerability biomarkers for obsessive compulsive disorder

Object

Obsessive compulsive disorder (OCD) is the fourth most common psychiatric disorder characterized by recurrent, intrusive thoughts and repetitive, ritualistic behaviors that are debilitating to the patient. Despite its high prevalence and the attendant morbidity, the pathophysiology of OCD remains unclear. Magnetic resonance spectroscopy (MRS) provides a noninvasive method to characterize the molecular biochemistry that may contribute to the pathophysiology of OCD. This study aimed to identify alterations in neurochemical measures that are specific to OCD using in vivo proton (¹H) MRS of the caudate nucleus, anterior cingulate cortex, and medial thalamus in these patients, and to identify their role as vulnerability markers by comparing them with the healthy first degree relatives of these patients and healthy controls.

Materials and methods

Appropriate psychometric instruments were applied in the study population followed by ¹H- MRS. The absolute neurochemical measures were quantified using a linear combination model.

Results

Significant differences in neurochemical measures were demonstrated in two of the three candidate regions (except the medial thalamus) between the three study groups.

Conclusions

Our results lend support to the neurodegenerative hypothesis of OCD, and also raise the possibility of exploring these neurochemical measures (as measured by MRS) as putative vulnerability biomarkers in OCD that may aid in early identification and devising early prevention or management strategies for the population vulnerable to OCD.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

Status and Direction of Communication Technologies - SiGe BiCMOS and RFCMOS

We present the status and direction of silicon semiconductor technologies targeted for applications such as wireless, networking, instrumentation, and storage markets. Various technological aspects for multiple branches of RF foundry technologies that are based on the standard foundry compatible CMOS node are discussed - SiGe BiCMOS HP ("high performance") tailored to high-frequency applications, SiGe BiCMOS WL ("cost performance") tailored to wireless/storage applications, and RF-CMOS optimized for low-cost consumer applications. Future opportunities and challenges for advancement in RF technologies are described in light of CMOS and SiGe heterojunction bipolar transistor scaling. In addition, we discuss the maturity of SiGe BiCMOS by looking at the levels of integration and manufacturability.     

Comparison of two surveys of hospitalization: the National Hospital Discharge Survey and the NHANES I Epidemiologic Followup Study

OBJECTIVES: This report compares hospitalization data from the NHANES I Epidemiologic Followup Study (NHEFS) with data from the National Hospital Discharge Survey (NHDS), the benchmark for hospitalization in the United States, for men and women 35 years and older for the period 1971-87. The comparison is intended to help analysts evaluate the validity and generality of analyses based on the NHEFS. METHODS: Hospital stays per 1,000 population and average lengths of stay are compared year by year for each age-sex group and for the entire period. Regression analyses test for differences between the two surveys by age and sex, and for differences in trends over time and the effect of the Medicare program's prospective hospital payment system. RESULTS: Hospital stays per 1,000 population were lower in NHEFS than in NHDS in all age-sex groups at the beginning of the period, but the differences had almost disappeared by 1987. Lengths of stay, although somewhat longer in NHEFS, matched NHDS more closely. Differentials by age and sex were similar in the two surveys for both hospital stays per 1,000 population and length of hospital stay. With its extensive information on baseline risk factors, the NHEFS offers a unique opportunity to study determinants of hospitalization in a representative sample of U.S. adults. The evaluation presented here suggests two points for researchers who want to use the NHEFS. First, including age as a control should largely correct for differences in age distribution between NHEFS and NHDS. Second, a time trend should also be included to capture the effects of several factors that caused the count of stays to be low in the early years of NHEFS followup.     

A 4.2-ps ECL ring-oscillator in a 285-GHz fMAX SiGetechnology

This letter reports on the room temperature operation of a conventional SiGe bipolar ECL ring oscillator with a minimum stage delay of 4.2 ps for ~250 mV single ended voltage swing. To our knowledge, this is the lowest reported delay for a gate fabricated using transistor devices. The circuit uses 0.12 × 2 μm² emitter size SiGe n-p-n transistors with a room temperature f_T of 207 GHz and f_MAX (unilateral gain extrapolation) of 285 GHz. The ring oscillator was studied as a function of various device and circuit parameters and it was found that minimum delay is more dependent on the parasitic resistance and capacitance in the n-p-n device than on pure transit time across the device