An assessment of potential nonlinear circuit models for thecharacterization of resonant tunneling diodes

The intrinsically fast process of resonant tunneling through double barrier heterostructures along with the existence of negative differential resistance in the current-voltage characteristic of these structures has led to their implementation as sources for high frequency electromagnetic energy. While sources based upon resonant tunneling diodes (RTDs) have produced frequency of oscillations up to 712 GHz, only microwatt levels of performance has been achieved above 100 GHz. Since stability criteria plays critical role in determining the deliverable power of any oscillator, a physically accurate equivalent-circuit model for the RTD is extremely important for optimizing the dynamics of the device-cavity package. This study identifies a distinctly new equivalent circuit model for characterizing the modes of oscillation in RTD-based sources. Specifically, in order to exhibit the fundamental self-oscillations and the overall I-V characteristics (plateau structure and hysteresis) observed experimentally, an accurate circuit model of the RTD must incorporate: (i) a quantum-well inductance which directly chokes the nonlinear conductance and, (ii) a nonlinear access resistance, associated with the accumulation of charge in the injection region of the double barriers, with a nonlocal dependence on the bias across the double barrier structure     

Abnormal mucosal glycoprotein synthesis in inflammatory bowel diseases is not related to cigarette smoking

Patients with ulcerative colitis are usually non- or ex-smokers in contrast to Crohn's disease where smoking is common. Abnormalities of quantity and quality of intestinal mucus have been postulated in the pathogenesis of these diseases. It is possible that smoking habit may exert its effects via changes in mucus in inflammatory bowel disease. We have therefore studied incorporation of N-acetylglucosamine into synthesized colonic mucin in explants from 85 controls with normal colonoscopic appearances and histology, including 27 smokers and 58 nonsmokers, 36 patients with ulcerative colitis and 19 with ileocolonic Crohn's disease over 24 h in tissue culture. Incorporation of N-acetylglucosamine into normal explants was 31.3 +/- (SD) 7.1 dpm/microgram biopsy protein, incorporation was increased in patients with active Crohn's disease (mean 41.2 +/- (SD) 10.4 dpm/microgram biopsy protein, p = 0.003), decreased in inactive ulcerative colitis (mean 24.1 +/- 7.8 dpm/microgram biopsy protein, p = 0.0006) but normal in active ulcerative colitis (mean 35.0 +/- 13.8 dpm/microgram biopsy protein, p = 0.44). No significant relationship was found between cigarette smoking habits and mucus synthesis in controls with normal mucosa (nonsmokers, n = 58, mean 31.0 +/- (SD) 7.52 dpm/microgram biopsy protein; smokers, n = 27, mean 31.8 +/- (SD) 6.1 dpm/microgram biopsy protein, p = 0.9). This study shows that mucus glycoprotein synthesis is reduced in inactive ulcerative colitis, rising to normal levels in active disease and that synthesis is increased in Crohn's disease. There is no effect of smoking on mucus synthesis by control biopsies suggesting that the differences seen in inflammatory bowel disease are not related to cigarette smoking.     

Reprocessing of Microcrystalline Cellulose Spheres with Low Drug Concentrations

Spherical dosage forms have been reported to be an efficient and effective method for delivering drugs into the body and controlling their dissolution rate. Substantial work has been conducted in these laboratories illustrating the advantages of microcrystalline cellulose-based spheres for these purposes. Through various methodologies, but most routinely the extrusion and marumerization technique, it has been determined by Funck, et al. that not more than 50% drug can be incorporated into a sphere formulation without the addition of other binders.

Because of the nature of the extrusion and marumerization manufacturing process, the type of drug being processed and the resultant particle size requirements of the spheres, the percent of spheres falling outside the desired particle size can range from between 3% and 20%. For this reason and the possibility of operator errors, our objective was to determine the parameters which needed monitoring when reprocessing was necessary.     

On the dynamic nature of response criterion in recognition memory: Effects of base rate, awareness, and feedback.

The authors examined whether participants can shift their criterion for recognition decisions in response to the probability that an item was previously studied. Participants in 3 experiments were given recognition tests in which the probability that an item was studied was correlated with its location during the test. Results from all 3 experiments indicated that participants' response criteria were sensitive to the probability that an item was previously studied and that shifts in criterion were robust. In addition, awareness of the bases for criterion shifts and feedback on performance were key factors contributing to the observed shifts in decision criteria. These data suggest that decision processes can operate in a dynamic fashion, shifting from item to item. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Kinetics of intermetallic compound growth between nickel,electroless, Ni-P,electroless Ni-B and tin at 453 to 493 K

The growth kinetics, crystal structure, and morphology of the intermetallic compounds formed between nickel, electroless Ni-P and electroless Ni-B coatings with tin at 453 to 493 K for times up to 506 h have been determined by microscopical and X-ray diffraction techniques. The compound Ni₃Sn₄ was formed. All kinetics followed as parabolic law with activation energies of 128.0, 130.4, and 122.3 kJ mol^–1 for the Ni/Sn, Ni-P/Sn and Ni-B/Sn systems, respectively. The rate of growth of Ni₃Sn₄ in the Ni/Sn and Ni-P/Sn systems were similar but the rate of growth in the Ni-B/Sn system was five times faster. Pores occurred in the intermetallic compound formed in the Ni-P/Sn system and these are discussed in relation to the density of the phases.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

Performance optimization of multidisciplinary mechanical systems subject to uncertainties

This paper presents a methodology to solve a new class of stochastic optimization problems for multidisciplinary systems (multidisciplinary stochastic optimization or MSO) wherein the objective is to maximize system mechanical performance (e.g. aerodynamic efficiency) while satisfying reliability-based constraints (e.g. structural safety). Multidisciplinary problems require a different solution approach than those solved in earlier research in reliability-based structural optimization (single discipline) wherein the goal is usually to minimize weight (or cost) for a structural configuration subject to a limiting probability of failure or to minimize probability of failure subject to a limiting weight (or cost). For the problems solved herein, the objective is to maximize performance over the range of operating conditions, while satisfying constraints that ensure safe and reliable operation. Because the objective is performance based and because the constraints are reliability based, the random variables used in the objective must model variability in operating conditions, while the random variables used in the constraints must model uncertainty in extreme values (to ensure safety). Thus, the problem must be formulated to treat these two different types of variables at the same time, including the case when the same physical quantity (e.g. a particular load) appears in both the objective function and the constraints. In addition, the problem must be formulated to treat multiple load cases, which can again require modeling the same physical quantity with different random variables. Deterministic multidisciplinary optimization (MDO) problems have advanced to the stage where they are now commonly formulated with multiple load cases and multiple disciplines governing the objective and constraints. This advancement has enabled MDO to solve more realistic problems of much more practical interest. The formulation used herein solves stochastic optimization problems that are posed in this same way, enabling similar practical benefits but, in addition, producing optimum designs that are more robust than the deterministic optimum designs (since uncertainties are accounted for during the optimization process). The methodology has been implemented in the form of a baseline MSO shell that executes on both a massively parallel computer and a network of workstations. The MSO shell is demonstrated herein by a stochastic shape optimization of an axial compressor blade involving fully coupled aero-structural analysis.     

Design of high-power multiplexers with interacting,synchronously tunable channels

A novel circuit configuration is presented for tunable combining multiplexers. Using reciprocal devices, the new circuit enables any channel to be tuned without the adverse effect of frequency pulling on the other channels. Experimental results in the 900 MHz band using dual-mode dielectric resonators are in close agreement with theory.