Evaluation of high-Temperature diffusion barriers for the Pt-Mo system

The United States Department of the Interior, Bureau of Mines, has studied the effectiveness of dif-fusion barriers in preventing interdiffusion between a molybdenum alloy (TZM), Mo-0.5Ti-0.07Zr-0.01C, and electrodeposited platinum coatings. Studies were conducted at 1200°, 1300°, and 1400 °C for 10, 5, and 2.5 hours, respectively, in a vacuum furnace. Diffusion barriers tested were TiC, TiN, TaC, TiB₂, TiB₂/TiC, and TiB₂/TiN. Coatings of the diffusion barriers were applied by chemical vapor deposition (CVD) with the exception of the TiB₂ coatings, which were applied by electrodeposition from a molten metaborate electrolyte. The platinum coatings were applied by elec-trodeposition from a molten cyanide electrolyte. Of the barrier materials tested, TaC shows the most promise because it extended the useful lifetime of platinum coated TZM by a factor of at least 2.     

System-Theoretic Representation of Simulation Models

The representation of simulation models, especially those expressed in discrete event languages, by means of system-theoretic formalism is reviewed. The important concepts of decomposition, static and dynamic structure, and state variable selection are explained and their implications for the design of simulation software explored. The system-theoretic approach is compared with other approaches to model representation derived from general software development methodology. Both simulation software design and systems theory may benefit by the challenges each raises for the other.     

Memory for places: a navigational model in support of Marr's theory of hippocampal function

In this report we describe a model that applies Marr's theory of hippocampal function to the problem of map-based navigation. Like many others we attribute a spatial memory function to the hippocampus, but we suggest that the additional functional components required for map-based navigation are located elsewhere in the brain. One of the key functional components in this model is an egocentric map of space, located in the neocortex, that is continuously updated using ideothetic (self-motion) information. The hippocampus stores snapshots of this egocentric map. The modeled activity pattern of head direction cells is used to set the best egocentric map rotation to match the snapshots stored in the hippocampus, resulting in place cells with a nondirectional firing pattern. We describe an evaluation of this model using a mobile robot and demonstrate that with this model the robot can recognize an environment and find a hidden goal. This model is discussed in the context of prior experiments that were designed to discover the map-based spatial processing of animals. We also predict the results of further experiments.     

Amorphous metal-semiconductor contacts for high temperature electronics—I Materials and characterisation

In this and a companion paper, we summarise the achievement of a metal-semiconductor contact whose electrical characteristics degrade only over tens of hours at 500°C. Although initially conceived as a barrier to Au diffusion, the thin amorphous metal film now fails because of out-diffusion of elements of the semiconductor. In this paper we summarise the rationale for using amorphous metal films, the choice of alloys, the fabrication of test structures, and the characterisation of these structures before and after exposure to temperatures as high as 700°C for 24 hr. The companion paper describes the thermal degradation of Schottky barrier characteristics of metal films on both Si and GaAs.     

Brain regional correspondence between Alzheimer's disease histopathology and biomarkers of protein oxidation

Four biomarkers of neuronal protein oxidation [W/S ratio of MAL-6 spin-labeled synaptosomes, phenylhydrazine-reactive protein carbonyl content, glutamine synthetase (GS) activity, creatine kinase (CK) activity] in three brain regions [cerebellum, inferior parietal lobule (IPL), and hippocampus (HIP)] of Alzheimer's disease (AD)-demented and age-matched control subjects were assessed. These endpoints indicate that AD brain protein may be more oxidized than that of control subjects. The W/S ratios of AD hippocampal and inferior parietal synaptosomes are 30 and 46% lower, respectively, than corresponding values of tissue isolated from control brain; however, the difference between the W/S ratios of AD and control cerebellar synaptosomes is not significant. Protein carbonyl content is increased 42 and 37% in the Alzheimer's HIP and IPL regions, respectively, relative to AD cerebellum, whereas carbonyl content in control HIP and IPL is similar to that of control cerebellum. GS activity decreases an average of 27% in the AD brain; CK activity declines by 80%. The brain regional variation of these oxidation-sensitive biomarkers corresponds to established histopathological features of AD (senile plaque and neurofibrillary tangle densities) and is paralleled by an increase in immunoreactive microglia. These data indicate that senile plaque-dense regions of the AD brain may represent environments of elevated oxidative stress.     

Internal carbohydrate complexity of the oligosaccharide chains of recombinant human follicle stimulating hormone (Puregon, Org 32489): a comparison with Metrodin and Metrodin-HP

Glycoforms of recombinant human follicle stimulating hormone (rhFSH) (Org 32489, Puregon) were characterized using concanavalin A lectin affinity chromatography to reveal information about the internal carbohydrate complexity (extent of carbohydrate side-chain branching) of the preparations. The rhFSH glycoforms were measured by radioimmunoassay and a two-site immunoradiometric assay and compared with those in two urinary preparations (Metrodin and Metrodin-HP) used in assisted reproduction programmes and a urinary FSH international standard 70/45 (uFSH IS 70/45). Similar data were obtained with both assays; rhFSH had 6% complex internal carbohydrate structures compared with 22-27% for Metrodin, Metrodin-HP and uFSH. The proportion of simple carbohydrate structures was also different, with rhFSH having 18.5 compared with 4.5-9.3% for Metrodin, Metrodin-HP and uFSH. A linear relationship was observed between the percentage glycoforms with an isoelectric point (pl) < 4 and the log percentage simple forms (logarithmic regression; r = 0.93) indicating a direct relationship between carbohydrate complexity and charge heterogeneity. In summary, rhFSH contains fewer complex forms and an increased proportion of simple carbohydrate structures in comparison with Metrodin, Metrodin-HP and IS 70/45.