Stephen Krause

Morphology and properties of rigid-rod poly(p-phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO) fibres

Abstract The morphology of two new high performance polymer fibres, rigid-rod poly( p -phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO), has been examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Heat treatment of as-spun PBO and ABPBO fibres produces transversely broadened crystallites, which are unlike uniaxially elongated crystallites in heat treated fibres of rigid-rod poly( p -phenylene benzobisthiazole) (PBT) or stiff-chain Kevlar, i.e. poly( p -phenylene terephthalamide) (PPTA). Heat treated fibres of PBO have greater three-dimensional crystallinity and long range crystallite ordering than axially disordered PBT, but much less than well ordered PPTA. Fibres of both as-spun and heat treated PBO have a crystalline c -axis with high orientation and low paracrystalline disorder, but heat treatment of as-spun fibres of ABPBO significantly increases c -axis orientation and decreases c -axis paracrystallinity, due to additional extension of the stiff-chain ABPBO molecules. Eventual improvements in orientation and ordering of PBO fibre through improvements in synthesis and processing may, based upon theoretically predicted values, result in up to triple the strength and modulus of commercial PPTA fibre.

Morphology and mechanical properties of a phase separated and a molecular composite 30% PBT/70% ABPBI triblock copolymer

Abstract The morphology of a triblock copolymer of 30% rigid-rod poly(p-phenylene benzobisthiazole) (PBT) and 70% semi-flexible coil poly(2,5(6)benzimidazole) (ABPBI) was examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Samples that were vacuum cast from a solution formed a microphase separated film with 0.1 μm particles and platelets of well-oriented 10 nm PBT crystallites in a ductile ABPBI matrix. Fibres were dry-jet/wet-spun from an optically homogeneous solution into a water coagulation bath to inhibit large scale phase separation. Heat-treated fibre contained crystallites of PBT and ABPBI with lateral dimensions no larger than 3 nm, demonstrating that PBT molecular segments were well dispersed and that a rigid-rod, molecular level composite had been achieved. The molecular level dispersion and high orientation in the ‘molecular composite’ fibre resulted in excellent mechanical properties with a modulus of 100 GPa and a tensile strength of 1.7 GPa which were about an order of magnitude greater than for the vacuum cast copolymer film.

Development, testing, and application of a chemistry concept inventory

A chemistry concept inventory (CCI) has been created that provides linkages to misconceptions observed in chemistry and subsequent introductory materials engineering courses as revealed by a materials concept inventory (MCI). The CCI topics included were bonding, intermolecular forces, electrochemistry, equilibrium, thermochemistry and acids and bases. Numerous students were interviewed in development of questions in order to ascertain that the questions and responses were interpreted as intended. Questioning students on topics of molecular shape gave helpful insight into how students solve problems. For example, a question might be written to test one aspect of the topic, but students might solve it differently. They might use different reasoning that would lead to a correct answer. The item is therefore testing something other than the intended topic. Interviews led to some unique findings in spatial understanding and misconceptions held by these students. Multiple rounds of testing were then used in ascertaining development of a valid chemistry concept inventory.

Dose dependence of microstructural development of buried oxide in oxygen implanted silicon-on-insulator material

The effect of implantation dose on microstructural development of the buried oxide (BOX) of 200 keV oxygen implanted Si was studied by electron microscopy. A continuous BOX layer with a low density of Si islands was obtained for a dose of 0.45×1018 cm−2, following high temperature annealing. At a lower dose of 0.225×1018 cm−2 a layer did not form, but only disjointed, isolated, oxide precipitates developed. At a higher dose, 0.675×1018 cm−2, a continuous BOX layer with a high density of Si islands formed. Microstructures of intermediate-temperature annealed samples showed the formation of oxide precipitates at preferred depths, the morphology being dose dependent. The final microstructure of the BOX is strongly influenced by the evolution of the oxide precipitates during annealing. A qualitative mechanism is proposed for the dose-dependent behavior of BOX formation during the annealing process.

Tools for assessing conceptual understanding in the engineering sciences

One of the hindrances to reform in science, technology, engineering and mathematics (STEM) education is the absence of good assessment instruments that can measure the value added to student learning by new ways of teaching important material. The well-known Force Concept Inventory (FCI) assessment instrument is a good model of an instrument that can be used to check on students understanding of basic concepts in a discipline. This panel session paper discusses work in progress by the panel members and their co-developers to construct FCI-like Concept Inventories in each of the disciplines of thermodynamics, systems and signals, strength of materials, electromagnetics, circuits, materials, fluid mechanics, and transport processes.

Dynamic precipitation processes of Al2Cu in Al‐Cu thin films by in situ transmission electron microscopy

Dynamic precipitation processes of the Al2Cu (θ) phase in Al‐1.5 wt % Cu thin films were studied by in situ heat treatments with transmission electron microscopy for films deposited at high temperatures in both the single‐phase and the two‐phase regions. The film deposited at 325 °C, in the two‐phase Al(α)‐Al2Cu region, has large irregular precipitates. Upon heating to 380 °C, the precipitates split and grow, and then, at 450 °C, they dissolve. Upon cooling, θ precipitates form at new locations, namely, at grain boundaries (GBs) and at triple points (TPs). Reheating and cooling does not significantly change their location and morphology. The film deposited at 465 °C, in the single‐phase Al(α) region, has fine precipitates at the GBs and TPs. Heating causes growth and then dissolution of precipitates, but upon cooling, precipitates reform at the original locations. The coarse θ precipitates, in the as‐deposited film at 325 °C, can increase corrosion susceptibility compared to the finer platelike morphology...

Mechanism of defect formation in low-dose oxygen implanted silicon-on-insulator material

The defects and microstructure of low-dose (<0.7 × 1018 cm−2), oxygen-implanted silicon-on-insulator (SIMOX) material were investigated as a function of implant dose and annealing temperature by plan-view and cross-sectional transmission electron microscopy. The threading-dislocations in low-dose (0.2∼0.3×1018 cm−2), annealed SIMOX originate from unfaulting of long (∼10 μm), shallow (0.3 μm), extrinsic stacking faults generated during the ramping stage of annealing. As dose increases, the defect density is reduced and the structure of the buried oxide layer evolves dramatically. It was found that there is a dose window which gives a lower defect density and a continuous buried oxide with a reduced density of Si islands in the buried oxide.

Electrical and structural properties of twin planes in dendritic web silicon

The electrical and structural properties of dendritic silicon have been measured and compared with solar cell efficiencies. The twin planes in the web and their effect on minority carrier diffusion length were of particular interest. The starting material and the cells were always from the same web strips. Cross-sectional electron-beam-induced current (EBIC) analysis was used to identify differences in the electrical behavior of low- and high-efficiency web material, both in the as-grown state and after solar-cell processing. High-efficiency cells exhibit flat EBIC linescans across the web cross sections, high minority-carrier diffusion lengths, few dislocations, and no defect clusters. Low-efficiency cells show EBIC linescans of reduced amplitude near the twin planes, low diffusion lengths, many dislocations, and electrically active defect clusters at the twin planes. Excessive recombination at the twin planes seems to limit the efficiency of these cells. In both high- and low-efficiency material, DLTS (deep-level transient spectroscopy) peaks present in the as-grown material disappear upon cell processing. >

Stacking fault pyramid formation and energetics in silicon‐on‐insulator material formed by multiple cycles of oxygen implantation and annealing

The defect microstructure of silicon‐on‐insulator wafers produced by multiple cycles of oxygen implantation and annealing was studied with transmission electron microscopy. The dominant defects are stacking fault pyramids (SFPs), 30–100 nm wide, located at the upper buried oxide interface at a density of ∼106 cm−2. The defects are produced by the expansion and interaction of narrow stacking fault (NSF) ribbons pinned to residual precipitates in the top silicon layer. Consideration of the energetics of the transformation from a collection of four NSF ribbons to a single SFP indicates that the reaction is energetically favorable below a critical NSF length. Thus small defects are stable as SFPs while large defects are stable as NSF ribbons.

Effect of dose and annealing conditions on the structure of silicon-on-insulator material implanted with oxygen at high temperature and at high current density

Abstract Conventional and high resolution electron microscopy were used to study the structure of silicon-on-insulator material synthesized at higher temperature and higher current density (1 mA cm-2) than are conventionally used. As dose increases from 0.3 to 1.8×1018 cm-2 the buried xide thickensto 0.3 microm and trails of bubbles from at the surface which increase in size to 14 nm and depth to 0.15 microm. The defect structure in the top Si layer, consisting of multiple stacking faults located only near the buried oxide interface, remains constant with dosage. During the early stages of annealing, the bubbles and the multiply faulted defects are eliminated and large (20–30 nm) precipitates with lateral dislocations form near the buried oxide interface. Increasing the temperature from 1250 to 1350°C, causes precipitates to grow and to incorporate into the oxide layer. The pinned dislocations are eliminated simultaneously with the incorporated precipitates. This results in a defect density of only 105 cm-2m which is three to four orders less than material implanted at lower temperatures and medium current density.