Unified modeling technique for high-viscosity,melt-phase,condensation polymer reactors

Mathematical models for chemical reaction and mass transfer occurring in the manufacture of high-viscosity condensation polymers are considered. A preliminary study indicates that several diverse models can be represented using a single formula based upon an effectiveness factor. The effectiveness factor is shown to depend upon a ratio of time scales of mixing and reaction. The formula giving the effectiveness factor in terms of the time scales ratio is shown to depend upon mixing assumptions only. Starting with a mixing-cell model, a new modeling framework is developed and shown to include the previous models as special cases. The framework is free of inherent mixing assumptions and can be applied to a wide variety of situations once the mixing characteristics are specified. © 1993 John Wiley & Sons, Inc.     

Effects of Ring Structure on the Reaction Pathways of Cyclic Esters and Ethers on Pd(111)

A combination of experimental surface science techniques and density functional theory calculations has been employed to understand the adsorption and surface chemistry of a variety of C₄ cyclic oxygenates on the (111) surface of Pd. These C₄ cyclic oxygenates represent important probe molecules for production of chemicals from biomass-derived carbohydrates. The surface level studies of these intermediates reveal that adsorption and reactivity trends are determined by ring size/strain, degree of unsaturation, nature of the oxygenate function, and composition of the metal surface.     

Dissociated \frac{1}{3}\langle 0\bar{1}11\rangle dislocations in Bi2Te3 and their relationship to seven-layer Bi3Te4 defects

We investigate the structure of $ \frac{1}{3}\langle 0\bar{1}11\rangle $ dislocations observed in Bi₂Te₃ nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 nm) with a component normal to the basal planes equal to the thickness of one full Bi₂Te₃ quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of Bi₃Te₄, rather than the normal Bi₂Te₃ quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at $ \frac{1}{3}\langle 0\bar{1}11\rangle $ —type dislocations.     

Challenges and new methods for designing reliability experiments

Abstract

Engineers use reliability experiments to determine the factors that drive product reliability, build robust products, and predict reliability under use conditions. This article uses recent testing of a howitzer to illustrate the challenges in designing reliability experiments for complex, repairable systems. We review research in complex system reliability models, failure-time experiments, and experimental design principles. We highlight the need for reliability experiments that account for various intended uses and environments. We leverage lessons learned from current research and propose methods for designing an experiment for a complex, repairable system.     

Electrodeposition and characterization of nano-crystalline antimony telluride thin films

Electrodeposition is a promising low-cost method to fabricate nanostructured thermoelectric thin films such as Sb₂Te₃. However, electrodeposition of crystalline Sb₂Te₃ without the need for additional processing and with good compositional control has presented a challenge. Here we report on the electrodeposition of crystalline Sb₂Te₃ thin films at room temperature from a tartaric-nitric acid electrolyte using a pulsed, potentiostatic process. The effects of synthesis conditions on the resulting microstructure and compositional homogeneity are investigated using x-ray diffraction, electron diffraction, electron microscopy, and energy dispersive x-ray spectroscopy. The composition of the Sb-Te films was found to be dependent on the interval between pulses, a result that is likely due to the slow kinetics associated with Sb₂Te₃ formation at the surface. We also observed a change in texture and microstructure with varied applied pulse duration: for short pulse durations a lamellar microstructure with a {000?} texture forms, whereas for longer pulse durations a more equiaxed and randomly oriented microstructure forms. The thermal conductivities of the pulsed electrodeposited films are surprisingly low at less than 2 W/K·m and are found to systematically decrease with reduced pulse time.     

Interfacial Step Structure at a (0001) Basal Twin in Bi2Te3

At present, little is known concerning the atomic-scale structure of grain boundaries in Bi₂Te₃ and related thermoelectric materials. As twins are perhaps the simplest possible type of grain boundary, they provide a good starting point for studies of interface structure in this class of materials. Here, we present an electron microscopic study of an interfacial step observed at a (0001) basal twin boundary in Bi₂Te₃. We discuss the crystallography of defects at twins in Bi₂Te₃ by considering the geometric implications for differently terminated step arrangements and comparing the defect configurations anticipated for deformation and annealing/growth twins. Finally, we consider the observed defect in this framework and discuss its relationship to analogous features in face-centered-cubic metals, namely the {112}-type interfaces that are commonly observed terminating {111} growth and annealing twins in these materials. This analysis provides insight concerning the relationships of dislocations to twin formation and morphology in Bi₂Te₃.     

Retrieving for rehearsal: An analysis of active rehearsal in children's memory.

Two experiments with 96 2nd graders and 24 6th graders explored the operation of retrieval processes in Ss' active rehearsal strategies. Both experiments used free-recall tasks, in which Ss were given instructions in active rehearsal as well as supports that might facilitate the retrieval operation and thus enhance both rehearsal and recall performance. In Exp I, 2nd and 6th graders were given visual or auditory access to an 18-item stimulus-word list. Results show that, by providing 2nd graders with an opportunity to view previously presented words, rehearsal activity and recall increased substantially, whereas the performance of the 6th graders was not affected. In Exp II, presentation time and visual access to a similar 18-word list were manipulated for 2nd graders. Results show that the provision of extra time for an item enabled Ss to execute a more active rehearsal strategy. Extra time had only minimal effects on recall, except when it was combined with visual access to the items. Findings suggest that retrieval per se is not necessary for the beneficial effects of active rehearsal, if other procedures can be followed to permit the juxtaposition of several items in rehearsal. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Immunohistochemical analysis of proliferation and differentiation in organotypic cultures of cervical tumor cell lines

Researchers have previously demonstrated that organotypic cultures of cervical tumor cell lines exhibit morphological characteristics similar to the in vivo biopsies from which they were derived (Rader et al., 1990). Both the in vivo biopsy and organotypic culture appeared undifferentiated. We have extended these studies with immunohistochemical analysis using the proliferation and differentiation markers, proliferating cell nuclear antigen (PCNA) and involucrin, respectively, to evaluate in more detail the ability of cervical tumor cell lines to differentiate in organotypic culture. An HPV-immortalized keratinocyte cell line, PE-4, expressed PCNA in the lower half and involucrin in the upper half of the organotypic culture which is consistent with the characteristics of a preneoplastic lesion in vivo. The CC-1 cell line, derived from an invasive squamous cell carcinoma, appeared undifferentiated, but expressed involucrin in the upper half of the organotypic culture. This is the first observation of expression of a differentiation marker in an organotypic culture of a cervical tumor cell line. The other cervical tumor cell lines, SiHa and HeLa, derived from a squamous cell carcinoma, and an adenocarcinoma of the cervix, respectively, did not express detectable levels of involucrin or mucin. All three cervical tumor cell lines, CC-1, SiHa and HeLa, expressed PCNA throughout their entire thickness. The majority of nuclei in SiHa and HeLa cultures were PCNA-positive, while the CC-1 cell line exhibited a lower growth fraction.(ABSTRACT TRUNCATED AT 250 WORDS)