bluuebunny/crossref_metadata_2025 huggingface dataset:
Blocking anthrax lethal toxin at the protective antigen channel by using structure-inspired drug design
Authors: Karginov Vladimir A. , Nestorovich Ekaterina M. , Moayeri Mahtab , Leppla Stephen H. , Bezrukov Sergey M.
Published: October 2005

Bacillus anthracis
secretes three polypeptides: protective antigen (PA), lethal factor (LF), and edema factor (EF), which interact at the surface of mammalian cells to form toxic complexes. LF and EF are enzymes that target substrates within the cytosol; PA provides a heptameric pore to facilitate LF and EF transport into the cytosol. Other than administration of antibiotics shortly after exposure, there is currently no approved effective treatment for inhalational anthrax. Here we demonstrate an approach to disabling the toxin: high-affinity blockage of the PA pore by a rationally designed low-molecular weight compound that prevents LF and EF entry into cells. Guided by the sevenfold symmetry and predominantly negative charge of the PA pore, we synthesized small cyclic molecules of sevenfold symmetry, β-cyclodextrins chemically modified to add seven positive charges. By channel reconstitution and high-resolution conductance recording, we show that per-6-(3-aminopropylthio)-β-cyclodextrin interacts strongly with the PA pore lumen, blocking PA-induced transport at subnanomolar concentrations (in 0.1 M KCl). The compound protected RAW 264.7 mouse macrophages from cytotoxicity of anthrax lethal toxin (= PA + LF). More importantly, it completely protected the highly susceptible Fischer F344 rats from lethal toxin. We anticipate that this approach will serve as the basis for a structure-directed drug discovery program to find new and effective treatments for anthrax.
Microfluidic gas-flow profiling using remote-detection NMR
Authors: Hilty Christian , McDonnell Erin E. , Granwehr Josef , Pierce Kimberly L. , Han Song-I , Pines Alexander
Published: October 2005

We have used nuclear magnetic resonance (NMR) to obtain spatially and temporally resolved profiles of gas flow in microfluidic devices. Remote detection of the NMR signal both overcomes the sensitivity limitation of NMR and enables time-of-flight measurement in addition to spatially resolved imaging. Thus, detailed insight is gained into the effects of flow, diffusion, and mixing in specific geometries. The ability for noninvasive measurement of microfluidic flow, without the introduction of foreign tracer particles, is unique to this approach and is important for the design and operation of microfluidic devices. Although here we demonstrate an application to gas flow, extension to liquids, which have higher density, is implicit.
A Sociological Institutionalist Approach to the Study of Innovation in Governance Capacity
Authors: González Sara , Healey Patsy
Published: October 2005

This paper draws on institutionalist approaches as developed in the fields of policy analysis and planning, to develop a methodological approach for assessing how the governance capacity for socially innovative action might emerge. After introducing the problematic of the search for governance relations which have the capacity for social innovation, the second and third parts of the paper summarise the emerging social-constructivist 'institutionalist' approach in policy analysis and planning. The fourth part draws on a three-level analytical model of governance dynamics to explore the dynamics and dialectics of urban governance transformation processes, illustrated with a case study of a socially innovative area-based initiative. The final section considers the power dynamics of episodes of socially innovative governance arising from within civil society and their potential to transform wider governance processes and cultures.
A WASp-binding type II phosphatidylinositol 4-kinase required for actin polymerization-driven endosome motility
Authors: Chang Fanny S. , Han Gil-Soo , Carman George M. , Blumer Kendall J.
Published: October 2005

Endosomes in yeast have been hypothesized to move through the cytoplasm by the momentum gained after actin polymerization has driven endosome abscision from the plasma membrane. Alternatively, after abscission, ongoing actin polymerization on endosomes could power transport. Here, we tested these hypotheses by showing that the Arp2/3 complex activation domain (WCA) of Las17 (Wiskott-Aldrich syndrome protein [WASp] homologue) fused to an endocytic cargo protein (Ste2) rescued endosome motility in las17ΔWCA mutants, and that capping actin filament barbed ends inhibited endosome motility but not endocytic internalization. Motility therefore requires continual actin polymerization on endosomes. We also explored how Las17 is regulated. Endosome motility required the Las17-binding protein Lsb6, a type II phosphatidylinositol 4-kinase. Catalytically inactive Lsb6 interacted with Las17 and promoted endosome motility. Lsb6 therefore is a novel regulator of Las17 that mediates endosome motility independent of phosphatidylinositol 4-phosphate synthesis. Mammalian type II phosphatidylinositol 4-kinases may regulate WASp proteins and endosome motility.
Cdc25B cooperates with Cdc25A to induce mitosis but has a unique role in activating cyclin B1–Cdk1 at the centrosome
Authors: Lindqvist Arne , Källström Helena , Lundgren Andreas , Barsoum Emad , Rosenthal Christina Karlsson
Published: October 2005

Cdc25 phosphatases are essential for the activation of mitotic cyclin–Cdks, but the precise roles of the three mammalian isoforms (A, B, and C) are unclear. Using RNA interference to reduce the expression of each Cdc25 isoform in HeLa and HEK293 cells, we observed that Cdc25A and -B are both needed for mitotic entry, whereas Cdc25C alone cannot induce mitosis. We found that the G2 delay caused by small interfering RNA to Cdc25A or -B was accompanied by reduced activities of both cyclin B1–Cdk1 and cyclin A–Cdk2 complexes and a delayed accumulation of cyclin B1 protein. Further, three-dimensional time-lapse microscopy and quantification of Cdk1 phosphorylation versus cyclin B1 levels in individual cells revealed that Cdc25A and -B exert specific functions in the initiation of mitosis: Cdc25A may play a role in chromatin condensation, whereas Cdc25B specifically activates cyclin B1–Cdk1 on centrosomes.
Granuphilin molecularly docks insulin granules to the fusion machinery
Authors: Gomi Hiroshi , Mizutani Shin , Kasai Kazuo , Itohara Shigeyoshi , Izumi Tetsuro
Published: October 2005

The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis. Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient β cells. Surprisingly, despite the docking defect, the exocytosis of insulin granules in response to a physiological glucose stimulus is significantly augmented, which results in increased glucose tolerance in granuphilin-null mice. The enhanced secretion in mutant β cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a–Munc18-1 complex, with which granuphilin normally interacts. Furthermore, in contrast to wild-type granuphilin, its mutant that is defective in binding to syntaxin-1a fails to restore granule docking or the protein level of syntaxin-1a in granuphilin-null β cells. Thus, granuphilin not only is essential for the docking of insulin granules but simultaneously imposes a fusion constraint on them through an interaction with the syntaxin-1a fusion machinery. These findings provide a novel paradigm for the docking machinery in regulated exocytosis.
Integrin-dependent actomyosin contraction regulates epithelial cell scattering
Authors: de Rooij Johan , Kerstens Andre , Danuser Gaudenz , Schwartz Martin A. , Waterman-Storer Clare M.
Published: October 2005

The scattering of Madin-Darby canine kidney cells in vitro mimics key aspects of epithelial–mesenchymal transitions during development, carcinoma cell invasion, and metastasis. Scattering is induced by hepatocyte growth factor (HGF) and is thought to involve disruption of cadherin-dependent cell–cell junctions. Scattering is enhanced on collagen and fibronectin, as compared with laminin1, suggesting possible cross talk between integrins and cell–cell junctions. We show that HGF does not trigger any detectable decrease in E-cadherin function, but increases integrin-mediated adhesion. Time-lapse imaging suggests that tension on cell–cell junctions may disrupt cell–cell adhesion. Varying the density and type of extracellular matrix proteins shows that scattering correlates with stronger integrin adhesion and increased phosphorylation of the myosin regulatory light chain. To directly test the role of integrin-dependent traction forces, substrate compliance was varied. Rigid substrates that produce high traction forces promoted scattering, in comparison to more compliant substrates. We conclude that integrin-dependent actomyosin traction force mediates the disruption of cell–cell adhesion during epithelial cell scattering.
Domain III from class II fusion proteins functions as a dominant-negative inhibitor of virus membrane fusion
Authors: Liao Maofu , Kielian Margaret
Published: October 2005

Alphaviruses and flaviviruses infect cells through low pH-dependent membrane fusion reactions mediated by their structurally similar viral fusion proteins. During fusion, these class II viral fusion proteins trimerize and refold to form hairpin-like structures, with the domain III and stem regions folded back toward the target membrane-inserted fusion peptides. We demonstrate that exogenous domain III can function as a dominant-negative inhibitor of alphavirus and flavivirus membrane fusion and infection. Domain III binds stably to the fusion protein, thus preventing the foldback reaction and blocking the lipid mixing step of fusion. Our data reveal the existence of a relatively long-lived core trimer intermediate with which domain III interacts to initiate membrane fusion. These novel inhibitors of the class II fusion proteins show cross-inhibition within the virus genus and suggest that the domain III–core trimer interaction can serve as a new target for the development of antiviral reagents.
Arthur Cayley FRS and the four-colour map problem
Authors: Crilly Tony
Published: October 2005

The four-colour map problem (to

prove

that on any map only four colours are needed to separate countries) is celebrated in mathematics. It resisted the attempts of able mathematicians for over a century and when it was successfully proved in 1976 the ‘computer proof’ was controversial: it did not allow scrutiny in the conventional way. At the height of his influence in 1878, Arthur Cayley had drawn attention to the problem at a meeting of the London Mathematical Society and it was duly ‘announced’ in print. He made a short contribution himself and he encouraged the young A. B. Kempe to publish a paper on the subject. Though ultimately unsuccessful, the work of Cayley and Kempe in the late 1870s brought valuable insights. Using previously unpublished historical sources, of letters and manuscripts, this article attempts to piece together Cayley’s contribution against the backcloth of his other deliberations. Francis Galton is revealed as the ‘go-between’ in suggesting Cayley publish his observations in
Proceedings of the Royal Geographical Society
. Of particular interest is that Cayley submitted
two
manuscripts prior to publication. A detailed comparison of these initial and final manuscripts in this article sheds new light on the early history of this great problem.
Representation of space–time variability of soil moisture
Authors: Isham V , Cox D.R , Rodríguez-Iturbe I , Porporato A , Manfreda S
Published: October 2005

A simplified spatial-temporal soil moisture model driven by stochastic spatial rainfall forcing is proposed. The model is mathematically tractable, and allows the spatial and temporal structure of soil moisture fields, induced by the spatial-temporal variability of rainfall and the spatial variability of vegetation, to be explored analytically. The influence of the main model parameters, reflecting the spatial scale of rain cells, the soil storage capacity, the rainfall interception and the soil water loss rate (representing evaporation and deep infiltration) is investigated. The variabilities of the spatially averaged soil moisture process, and that averaged in both space and time, are derived. The present analysis focuses on spatially uniform vegetation conditions; a follow-up paper will incorporate stochastically heterogeneous vegetation.
High lift function of the pteroid bone and forewing of pterosaurs
Authors: Wilkinson Matthew T , Unwin David M , Ellington Charles P
Published: October 2005

The pteroid bone is a rod-like element found only in pterosaurs, the flying reptiles of the Mesozoic. It articulated at the wrist, and supported a membranous forewing in front of the inner part of the wing spar. The function of this bone, particularly its orientation, has been much debated. It is widely believed that it pointed towards the body, and that the forewing was relatively narrow. An alternative hypothesis states that it was directed forwards during flight, resulting in a much broader forewing that acted as a leading edge flap. We tested scale models in a wind tunnel to determine the aerodynamic consequences of these conflicting hypotheses, and found that performance is greatly improved if the pteroid is directed forwards: the lift : drag ratios are superior and the maximum lift is exceptionally high in comparison with conventional aerofoils. This high lift capability may have enabled even the largest pterosaurs to take off and land without difficulty.
Enhanced continua and discrete lattices for modelling granular assemblies
Authors: Suiker Akke S.J , de Borst René
Published: October 2005

This article discusses the derivation of continuum models that can be used for modelling the inhomogeneous mechanical behaviour of granular assemblies. These so-called kinematically enhanced models are of the strain-gradient type and of the strain-gradient micro-polar type, and are derived by means of homogenizing the micro-structural interactions between discrete particles. By analysis of the body wave dispersion curves, the enhanced continuum models are compared to corresponding discrete lattice models. Accordingly, it can be examined up to which deformation level the continuum models are able to accurately describe the discrete particle behaviour. Further, the boundary conditions for the enhanced continuum models are formulated, and their stability is considered. It is demonstrated how to use the body wave dispersion relations for the assessment of stability.
An anisotropic thermomechanical damage model for concrete at transient elevated temperatures
Authors: Baker Graham , Borst René de
Published: October 2005

The behaviour of concrete at elevated temperatures is important for an assessment of integrity (strength and durability) of structures exposed to a high-temperature environment, in applications such as fire exposure, smelting plants and nuclear installations. In modelling terms, a coupled thermomechanical analysis represents a generalization of the computational mechanics of fracture and damage. Here, we develop a fully coupled anisotropic thermomechanical damage model for concrete under high stress and transient temperature, with emphasis on the adherence of the model to the laws of thermodynamics. Specific analytical results are given, deduced from thermodynamics, of a novel interpretation on specific heat, evolution of entropy and the identification of the complete anisotropic, thermomechanical damage surface. The model is also shown to be stable in a computational sense, and to satisfy the laws of thermodynamics.
Isolation, Characterization, and Pericycle-Specific Transcriptome Analyses of the Novel Maize Lateral and Seminal Root Initiation Mutant rum1
Authors: Woll Katrin , Borsuk Lisa A. , Stransky Harald , Nettleton Dan , Schnable Patrick S. , Hochholdinger Frank
Published: October 2005

The monogenic recessive maize (Zea mays) mutant rootless with undetectable meristems 1 (rum1) is deficient in the initiation of the embryonic seminal roots and the postembryonic lateral roots at the primary root. Lateral root initiation at the shoot-borne roots and development of the aerial parts of the mutant rum1 are not affected. The mutant rum1 displays severely reduced auxin transport in the primary root and a delayed gravitropic response. Exogenously applied auxin does not induce lateral roots in the primary root of rum1. Lateral roots are initiated in a specific cell type, the pericycle. Cell-type-specific transcriptome profiling of the primary root pericycle 64 h after germination, thus before lateral root initiation, via a combination of laser capture microdissection and subsequent microarray analyses of 12k maize microarray chips revealed 90 genes preferentially expressed in the wild-type pericycle and 73 genes preferentially expressed in the rum1 pericycle (fold change >2; P-value <0.01; estimated false discovery rate of 13.8%). Among the 51 annotated genes predominately expressed in the wild-type pericycle, 19 genes are involved in signal transduction, transcription, and the cell cycle. This analysis defines an array of genes that is active before lateral root initiation and will contribute to the identification of checkpoints involved in lateral root formation downstream of rum1.
MYB98Is Required for Pollen Tube Guidance and Synergid Cell Differentiation inArabidopsis
Authors: Kasahara Ryushiro D. , Portereiko Michael F. , Sandaklie-Nikolova Linda , Rabiger David S. , Drews Gary N.
Published: October 2005

The synergid cells of the female gametophyte play a role in many steps of the angiosperm fertilization process, including guidance of pollen tube growth to the female gametophyte. However, the mechanisms by which the synergid cells become specified and develop their unique features during female gametophyte development are not understood. We identified MYB98 in a screen for Arabidopsis thaliana genes expressed in the female gametophyte. MYB98 is a member of the R2R3-MYB gene family, the members of which likely encode transcription factors. In the context of the ovule, MYB98 is expressed exclusively in the synergid cells, and mutations in this gene affect the female gametophyte specifically. myb98 female gametophytes are affected in two unique features of the synergid cell, pollen tube guidance and the filiform apparatus, but are otherwise normal. MYB98 also is expressed in trichomes and endosperm. Homozygous myb98 mutants exhibit no sporophytic defects, including trichome and endosperm defects. Together, these data suggest that MYB98 controls the development of specific features within the synergid cell during female gametophyte development.
Diacidic Motifs Influence the Export of Transmembrane Proteins from the Endoplasmic Reticulum in Plant Cells
Authors: Hanton Sally L. , Renna Luciana , Bortolotti Lauren E. , Chatre Laurent , Stefano Giovanni , Brandizzi Federica
Published: October 2005

In yeast and mammals, amino acid motifs in the cytosolic tails of transmembrane domains play a role in protein trafficking by facilitating export from the endoplasmic reticulum (ER). However, little is known about ER export signals of membrane proteins in plants. Therefore, we investigated the role of diacidic motifs in the ER export of Golgi-localized membrane proteins. We show that diacidic motifs perform a significant function in the export of transmembrane proteins to the Golgi apparatus, as mutations of these signals impede the efficient anterograde transport of multispanning, type II, and type I proteins. Furthermore, we demonstrate that diacidic motifs instigate the export of proteins that reside in the ER due to the lengths of their transmembrane domains. However, not all of the diacidic motifs in the cytosolic tails of the proteins studied were equally important in ER export. Transport of Golgi proteins was disrupted only by mutagenesis of specific diacidic signals, suggesting that the protein environment of these signals affects their function. Our findings indicate that diacidic ER export motifs are present and functional in plant membrane proteins and that they are dominant over transmembrane domain length in determining the export of proteins from the ER in plant cells.
Structure of Ptr ToxA: An RGD-Containing Host-Selective Toxin fromPyrenophora tritici-repentis
Authors: Sarma Ganapathy N. , Manning Viola A. , Ciuffetti Lynda M. , Karplus P. Andrew
Published: October 2005

Tan spot of wheat (Triticum aestivum), caused by the fungus Pyrenophora tritici-repentis, has significant agricultural and economic impact. Ptr ToxA (ToxA), the first discovered proteinaceous host-selective toxin, is produced by certain P. tritici-repentis races and is necessary and sufficient to cause cell death in sensitive wheat cultivars. We present here the high-resolution crystal structure of ToxA in two different crystal forms, providing four independent views of the protein. ToxA adopts a single-domain, β-sandwich fold of novel topology. Mapping of the existing mutation data onto the structure supports the hypothesized importance of an Arg-Gly-Asp (RGD) and surrounding sequence. Its occurrence in a single, solvent-exposed loop in the protein suggests that it is directly involved in recognition events required for ToxA action. Furthermore, the ToxA structure reveals a surprising similarity with the classic mammalian RGD-containing domain, the fibronectin type III (FnIII) domain: the two topologies are related by circular permutation. The similar topologies and the positional conservation of the RGD-containing loop raises the possibility that ToxA is distantly related to mammalian FnIII proteins and that to gain entry it binds to an integrin-like receptor in the plant host.
TheArabidopsisCold-Responsive Transcriptome and Its Regulation by ICE1
Authors: Lee Byeong-ha , Henderson David A. , Zhu Jian-Kang
Published: October 2005

To understand the gene network controlling tolerance to cold stress, we performed an Arabidopsis thaliana genome transcript expression profile using Affymetrix GeneChips that contain ∼24,000 genes. We statistically determined 939 cold-regulated genes with 655 upregulated and 284 downregulated. A large number of early cold-responsive genes encode transcription factors that likely control late-responsive genes, suggesting a multitude of transcriptional cascades. In addition, many genes involved in chromatin level and posttranscriptional regulation were also cold regulated, suggesting their involvement in cold-responsive gene regulation. A number of genes important for the biosynthesis or signaling of plant hormones, such as abscisic acid, gibberellic acid, and auxin, are regulated by cold stress, which is of potential importance in coordinating cold tolerance with growth and development. We compared the cold-responsive transcriptomes of the wild type and inducer of CBF expression 1 (ice1), a mutant defective in an upstream transcription factor required for chilling and freezing tolerance. The transcript levels of many cold-responsive genes were altered in the ice1 mutant not only during cold stress but also before cold treatments. Our study provides a global picture of the Arabidopsis cold-responsive transcriptome and its control by ICE1 and will be valuable for understanding gene regulation under cold stress and the molecular mechanisms of cold tolerance.
The NAC Transcription Factors NST1 and NST2 ofArabidopsisRegulate Secondary Wall Thickenings and Are Required for Anther Dehiscence
Authors: Mitsuda Nobutaka , Seki Motoaki , Shinozaki Kazuo , Ohme-Takagi Masaru
Published: October 2005

In plants, secondary wall thickenings play important roles in various biological processes, although the factors regulating these processes remain to be characterized. We show that expression of chimeric repressors derived from NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2 in Arabidopsis thaliana resulted in an anther dehiscence defect due to loss of secondary wall thickening in anther endothecium. Plants with double, but not single, T-DNA–tagged lines for NST1 and NST2 had the same anther-indehiscent phenotype as transgenic plants that expressed the individual chimeric repressors, indicating that NST1 and NST2 are redundant in regulating secondary wall thickening in anther walls. The activity of the NST2 promoter was particularly strong in anther tissue, while that of the NST1 promoter was detected in various tissues in which lignified secondary walls develop. Ectopic expression of NST1 or NST2 induced ectopic thickening of secondary walls in various aboveground tissues. Epidermal cells with ectopic thickening of secondary walls had structural features similar to those of tracheary elements. However, among genes involved in the differentiation of tracheary elements, only those related to secondary wall synthesis were clearly upregulated. None of the genes involved in programmed cell death were similarly affected. Our results suggest NAC transcription factors as possible regulators of secondary wall thickening in various tissues.
Practical color calibration for dermoscopy, applied to a digital epiluminescence microscope
Authors: Grana C. , Pellacani G. , Seidenari S.
Published: October 2005

Background/purpose: The assessment of colors is essential for melanoma (MM) diagnosis, both for pattern analysis on dermoscopic images, and when using semiquantitative methods. Our aim was to provide a simple, precise characterization and reproducible calibration of the color response for dermoscopic instruments.

Methods: Three processes were used to correct the non‐uniform illumination pattern of the instrument, to easily estimate the camera gamma settings and to describe the color space conversion matrices required to produce standard images, in any color space. A specific color space was also developed to optimize the representation of dermatoscopic colors. The calibration technique was tested both on synthetic reference surfaces and on real images by comparing the difference between the images colors obtained with two different equipments.

Results: The differences between the images acquired by means of the two instruments, calculated on the reference patterns after calibration, were up to 10 times lower then before, while comparison of histograms referring to real images provided an improvement of about seven times on average.

Conclusions: A complete workflow for dermatologic image calibration, which allows the user to continue using his own software and algorithms, but with a much higher informative content, is presented. The technique is simple and may improve cooperation between different research centers, in teleconsulting contexts or for result comparisons.
Issues in the Processing of Cuprate Ceramic Superconductors
Authors: Clarke David R. , Shaw Thomas M. , Dimos Duane
Published: October 2005

A number of the important issues in the processing of dense, superconducting ceramics are addressed and illustrated with examples drawn from our studies of the processing of yttrium barium cuprate. Among the phenomena affecting the attainment of dense, fully superconducting materials are those common to the processing of other ceramics, namely, control of composition, prevention of microcracking, adequate time for the decomposition, and escape of intermediaries prior to surface densification (the so‐called “binder burn‐off” problem). Additional requirements include the necessity to ensure complete oxygenation throughout the microstructure for full volume superconductivity and the control of texture development for maximizing the attainable transport critical current density. On the basis of the processing issues described, it is concluded that the optimum microstructure is one having finescale, interconnected porosity (to ensure escape of carbonate‐like species and subsequent, rapid oxygenation) with a fine grain size (∼1 μm) to avoid microcracking.
Spatially Resolved Raman Spectroscopy Study of Transformed Zones in Magnesia‐Partially‐Stabilized Zirconia
Authors: Dauskardt Reinhold H. , Veirs D. Kirk , Ritchie Robert O.
Published: October 2005

Raman vibrational spectroscopy provides an effective phase characterization technique in materials systems containing particle dispersions of the tetragonal and monoclinic polymorphs of zirconia, each of which yields a unique Raman spectrum. An investigation is reported to assess a novel, spatially resolved Raman spectroscopy system in the study of transformed zones surrounding cracks in partially stabilized MgO–ZrO2 (PSZ). The experimental arrangement uses an imaging (two‐dimensional) photomultiplier tube to produce a one‐dimensional Raman profile of phase compositions along a slitlike laser beam without translation of either the sample or the laser beam and without scanning the spectrometer. For the present optical configuration, the spatial resolution is estimated to be equivalent to the detector resolution of 28 pm and does not appear to be reduced because of secondary scattering events in the PSZ. Results from phase characterization studies of the size, frontal morphology, and extent of transformation of transformation zones surrounding cracks produced under monotonic and cyclic loading conditions are presented. Particularly large zones are observed in the peak‐toughened material, extending 1300 μm ahead of the crack tip with widths of up to 3000 μm. Good agreement is found with similar results determined using optical interference microscopy.
Surface Defects in Polished Silicon Studied by Cross‐Sectional Transmission Electron Microscopy
Authors: Johansson Stefan , Schweitz Jan‐Åke , Peter K. , Lagerlöf O.
Published: October 2005

The cross‐sectional TEM technique is applied to Si(100) surfaces, which are exposed to mild polishing with diamond paste in the 0.25‐ to 6‐μm‐size range as well as with the commercial polishing medium Syton HT 50. Syton polishing is also used to prepare electron‐transparent foils for TEM studies, and this preparation technique shows several advantages over ion thinning. Mild diamond polishing results in small‐scale abrasive grooving and production of superficial dislocation loops of 1/2〈110〉{111} type. Debris of amorphous material tends to aggregate in surface cavities. These aggregates are formed during polishing, not by ion milling. Syton polishing results in very smooth surfaces, with no signs of abrasive grooving or dislocation density. Two major mechanisms of mass removal are suggested: (1) “smearing” of very superficial material at a nearly atomic level, resulting in amorphous aggregates and (2) strongly localized plastic cutting or shearing. In mild polishing, both mechanisms are suggested to be active, localized plastic cuttinglshearing becoming progressively more dominant for increasing particle size.
Electrostrictive Properties of the lead Zirconate Titanate Solid‐Solution System
Authors: Haun Michael J. , Zhuang Z. Q. , Furman E. , Jang Sei‐Joo , Cross L. Eric
Published: October 2005

Values of the electrostrictive constants for the lead zirconate titanate (PZT) solid‐solution system were required to com plete the development of a thermodynamic phenomenological theory of PZT. The electrostrictive Q̄12 constant was meas ured as a function of composition on polycrystalline ceramic PZT samples. These data were used with additional single–crystal and ceramic data from the literature to approximate the compositional dependence of the electrostrictive constants of the PZT system. Series and parallel equations, analogous to the Voigt and Reuss models for the elastic constants, were used to relate the ceramic and single‐crystal data, and to pre dict the upper and lower bounds of the ceramic electrostric tive constants from the single‐crystal constants.
Fracture Behavior of Open‐Cell Ceramics
Authors: Brezny Rasto , Green David J.
Published: October 2005

Cellular structures are commonly used by nature as a means of optimizing the strength‐to‐weight ratio of a body. Cellular ceramics may have potential in structural applications requiring a high modulus‐to‐weight ratio. The models that are used to predict the mechanical behavior of these materials describe them as an interconnected network of columns and plates and predict a dependence on density and cell size. The fracture toughness and strength of several open‐cell alumina materials were measured and compared to a theoretical model. Image analysis was used to fully characterize the macrostructure of the samples and provided a basis for evaluating the toughness and strength behavior. It was observed that the properties of the bulk material depend on the strength of the individual struts in the structure. The fit of the data to an open‐cell model was found to be dependent on the agreement between the actual macrostructure and that assumed in the model. Both microscopic and macroscopic flaws contribute to the variability of the strength in these materials and it was found that there was a wide distribution of bulk and strut strengths. Both of these flaw populations must be controlled to maximize the mechanical properties of these open‐cell ceramics.
Oxygen Self‐Diffusion in Magnesium‐ or Titanium‐Doped Alumina Single Crystals
Authors: Haneda Hajime , Monty Claude
Published: October 2005

Oxygen Setf‐diffusion coefficients have been measured in single crystals of N2O3 doped with Mg or Ti under an oxygen partial pressure of 20 kPa in the temperature range 1400° to 1700°C. Diffusion coefficients in Mg‐doped crystals obey the equation Do (cm2/s) = 1 × 1011∼ (1×1013) exp[−915 ± 50(kJ/mol)/RT]. The diffusivity of oxygen in Ti‐doped A12O3 is lower than Mg‐doped A12O3. A vacancy mechanism explains these results.
Crack Stability and Strength Distribution of Ceramics That Exhibit Rising Crack‐Growth‐Resistance (R‐Curve) Behavior
Authors: Shetty Dinesh K. , Wang Jr‐Sheng
Published: October 2005

Crack stability and strength distribution are examined for ceramics that exhibit R‐curve behavior. An empirical power‐law description for the R curve leads to simple analytical formulations for the Weibull modulus and the scale parameter; however, the power law is inconsistent with the stability of surface cracks recently observed in MgO‐partially‐stabilized zirconia and coarse‐grained alumina. It is shown that alternate empirical fits to the R curve are compatible with the stable extension of surface cracks, even in the absence of any localized residual stresses.
Fracture Analysis of Chemically Strengthened Glass Disks
Authors: Connolly Deny
Published: October 2005

The increase in fracture strength in chemically strengthened, soda‐lime glass disks compared with nonstrengthened ones is investigated analytically and experimentally. An empirical stress‐relaxation function is proposed to predict the stress profile through the disk as a result of ion exchange, with parameters determined from experimental data. The function accounts for stress increase due to concentration rise and decrease caused by stress relaxation, which depends on concentration, treatment time, and exchange temperature. The surface‐flaw closure distance caused by the compressive stress profile is computed using an existing model. The size of the most severe surface flaw for chemically strengthened disks is computed using fracture mechanics, assuming knowledge of the experimental failure strength. The predicted flaw size is slightly smaller than that calculated for a similar sample of nonstrengthened disks and the strengthening is underpredicted by a factor of 2. Reasons for the discrepancies are discussed.
Characterization of Fracture in Fiber‐Reinforced Ceramic Composites under Shear loading
Authors: Mall Shankar , Vozzola Robert P. , Zawada Larry P.
Published: October 2005

This study developed a test technique for mode II fracture testing of a fiber‐reinforced ceramic composite. This method employed a small, straight‐notched, precracked, end‐notched flexure specimen subjected to three‐point bending. This method was demonstrated by measuring the mode II critical strain energy release rate, GIIc of a fiber‐reinforced glass‐ceramic composite at room temperature.
Mechanisms of Improvement of Fracture Strength in Laser‐Surface‐Modified Ceramics
Authors: Jagannadham Kasichainula , Narayan Jagdish
Published: October 2005

Three mechanisms of improvement in fracture strength of laser‐surface‐modied ceramic materials are proposed to explain the experimental observations of more than 50% increase in fracture strength. First, the improvement in fracture strength by about 50% is considered to arise from either a complete or partial closure of crack surfaces at the interface of the laser‐modified overlayer. The second mechanism of improvement in fracture strength is derived from the physical displacement of the crack tip away from the free surface when a laser‐modified layer is introduced. Thus, the critical crack size, defmed as a crack that propagates with decreasing energy, is increased almost 100%. The Wid mechanism is based upon the compressive stresses introduced in the laser‐modified region. The fast cooling rates attained after laser irradiation are responsible for development of regions of compressive internal stresses. These sources of improvement in fracture strength are analyzed and the results of the calculations compared with experimental results. Through the present understanding of the mechanisms of improvement in fracture strength, it has become possible to calculate a critical thickness of the laser‐modifed layer. It is concluded that the possible improvement of fracture strength is achieved when the thickness of the laser‐modified overlayer is equal to or greater than this critical value.
Defect Structure of Cobalt Monoxide: I, The Ideal Defect Model
Authors: Nowotny Janusz , Rekas Mieczyslaw
Published: October 2005

The nonstoichiometry and electrical properties of Co1‐YO are considered in terms of an ideal defect model. Both intrinsic and extrinsic regions are analyzed. Interrelationships between the oxygen partial pressure and the oxide composition are derived. It has been shown that the pO2 exponent resulting from the deviation from stoichiometry (l/nY) differs substantially from the exponent determined by measurements of electrical conductivity (1/nσ) The limitations of the ideal defect approximation are discussed.
Defect Structure of Cobalt Monoxide: II, The Debye‐Hückel Model
Authors: Nowotny Janusz , Rekas Mieczyslaw
Published: October 2005

The defect structure of CoO is considered, using defect activities instead of concentrations. The proposed model, based on the Debye‐Hückel (D‐H) theory, assumes only one type of predominant ionic lattice defect, i.e., doubly ionized cation vacancies. Application of the mass‐action law to the formation of doubly ionized cobalt vacancies results in the following equilibrium constant: K={VnCo}{h˙}2/pO21/2= 3 × 10−4 exp[−120.6 (kJ)/RT]. Using a modified Jonker's formalism, we have determined the equilibrium constant of the intrinsic electronic equilibrium: Ki={e′}{h˙}= 12.8 exp[−3.0 (eV)/kT]. The width of the forbidden gap determined in this work is equal to Eg= 3.0 – 2.2 × 10−4T (eV). Good agreement between the proposed model and experimental data was found.
Defect Structure of Cobalt Monoxide: III, The Cluster Model
Authors: Nowotny Janusz , Rekas Mieczyslaw
Published: October 2005

The 4:1 cluster model has been considered and its application to the available experimental data for the deviation from stoichiometry, electrical conductivity, and the Seebeck coefticient has been analyzed. It has been shown that the degree of ionization of the cluster is −5. The equilibrium constant of formation is K5= [(VCo)Coi]5−[h˙]5pO23/2= 4.6 exp[−167.5 (kJ)/RT]. It has been shown that the applicability of this model in describing CoO properties is limited to higher values of the deviation from the stoichiometric composition (y). At low y, defects in CoO can be considered as isolated. Then doubly ionized cobalt vacancies are assumed to be the predominant lattice defects. Their equilibrium constant of formation, determined from experimental data in the vicinity of the Co/CoO phase boundary by using an ideal approximation, is KV= [VnCo][h˙]2po2−1/2= 3.2 × 10−4 exp[−121.1 (kJ)/RT]. Excellent agreement between the equilibrium constant KV determined for low pO2 and that resulting from the Debye Hückel model within the entire range of y has been found.
Mechanism for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a liquid Phase
Authors: Bateman Charles A. , Bennison Stephen J. , Harmer Martin P.
Published: October 2005

Microstructural morphology in A1203 containing trace amounts (∼0.25 vol%) of a silicate‐based amorphous phase was studied, as a finction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot‐pressing of undoped Al2O3 powder and are characterized by a large aspect ratio (>3) and long basal plane {0001} facets. The facets are completely wet by a thin (∼3 nm) amorphous phase in contrast with grain ends, which appear to be devoid of an amorphous phase. The shape of the grains is believed to result from differences in mobility between clean (i.e., unwet) grain boundaries and intergranular films which are liquid at the firing temperature. Doping with trace additions of MgO (Mg/A1=250 ppm) results in smaller, more uniform grain structures. Many grains in the MgO doped material exhibit all the features of the incipient abnormal grains in the undoped material, with the exception of the large aspect ratio. It is concluded that the role of MgO in such a system is a grain‐growth inhibitor and a microstructural stabilizer. It is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films. It is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid‐solution pinning mechanism.
Preferential X‐ray line Broadening and Thermal Behavior of γ‐Fe2O3
Authors: Morales Julian , Tirado José L. , Valera Concepción
Published: October 2005

Preferential X‐ray line broadenings in γ‐Fe2o3 samples prepared from γ‐FeOOH, α‐FeOOH, N2H5Fe(N2H3‐COO)3‐H2O, FeOOCH3, and colloidal Fe3O4 are compared. Isotropic size and small crystallites are the origin of the uniform and enhanced X‐ray line broadening in samples derived from hydrazinate and colloidal Fe3O4. Nonuniform line broadening in ex‐α‐FeOOH and ex‐γ‐FeOOH is due to an elongated crystallite shape and the presence of stacking faults, respectively. The thermal behavior of samples with low crystallite size and uniform line broadening is characterized by an exothermal recrystallization process simultaneous to the phase transformation γ‐Fe203→α‐Fe2O3.
Preparation of Superconductor/Polymer Composite with Resistive Transition Temperature [Tc) above 77 K
Authors: Sarkar Asok K. , Salyer Lval O. , Kumar Binod
Published: October 2005

A technique for preparing Y1, Ba2Cu3O7‐x/polymer composites showing superconducting resistive transition above liquid‐nitrogen temperature has been developed. A sintered Y1Ba2Cu3O7‐x disk was impregnated with a low‐viscosity 2‐ethylhexyl acrylate monomer containing benzoyl peroxide and dicumyl peroxide as polymerization catalysts. The impregnation of sintered disks was carried out under moderate vacuum for 30 min followed by an overnight soaking. The polymerization treatment consisted of a heat treatment at 60°C for 24 h followed by a second heat treatment at 80°C for 8 to 10 h. The superconductorlpo1ymer specimens exhibited superconducting resistive transition at ≃80 K. The scanning electron micrograph reveals excellent welting, infiltration, and polymerization of the monomer into the superconducting material.
Particle Interactions in Zirconia‐Toughened Alumina
Authors: Wilfinger Keith R. , Cannon W. Roger
Published: October 2005

In order to determine the effect of the coarse tail of the ZrO2 size distribution of the monoclinic ZrO2 content of zirconia‐toughened alumina, quantitative hot‐stage X‐ray measurements were made on (1) A12O3‐10% Zr02 (submicrometer with a coarse tail >1 μm), (2) A12O3‐10% ZrO2 with the coarse tail of the ZrO2 carefully removed by centrifugation, and (3) Al2O3‐10% ZrO2 in which 10% of Zr02(1) and 90% of Zr02(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO2 was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.
Silicon Carbide‐Coated Boron Fibers
Authors: Hwan Luchen , Suib Steven , Galasso Francis
Published: October 2005

A study of a silicon carbide‐coated boron fiber showed that it retained its room‐temperature strength after being heated to 1000°C for 24 h in air. This fiber was consolidated successfully in a titanium matrix at 930°C without degradation, although previous results with another silicon carbide‐coated boron fiber indicated that there might be a reaction problem at this temperature.
Ceramic Engine Valves
Authors: Wills Roger R. , Southam Robert E.
Published: October 2005

Current trends in spark ignition engines are toward multivalve engines capable of operating at relatively high speeds to maximize performance and fuel economy at acceptable emissions levels. Structural ceramics, with their low density and good high‐temperature properties, appear to offer potential as an alternative valve material to the steels, superalloys, and titanium alloys currently in use. Progress to date on materials development, component design, component performance, and engine testing indicates that silicon nitride valves will operate satisfactorily.
Tansformation Range Viscosity Measurements Using a Thermomechanical Analyzer
Authors: Jewell John M. , Shelby James E.
Published: October 2005

A commercial thermomechanical analyzer has been adapted to determine the transformation range viscosities of very small samples using a beam‐bending configuration. Three viscosity standards—NBS‐710, NBS‐711, and NBS‐717—have been used to determine the accuracy of this technique over a wide range of temperatures. The method yielded excellent results that were independent of sample size and heating rate. The largest deviation from National Bureau of Standards data was 3.8°C at 1010 Pa's for one sample of NBS‐711; the average deviation at 1010 Pa's was approximately 2°C for all samples studied. In all cases, the activation energy for viscous flow was in excellent agreement with that calculated from National Bureau of Standards data.
Precise Nondestructive Determination of the Density of Porous Ceramics
Authors: Pennings Erik C. M. , Grellner Wolfgang
Published: October 2005

A fast, accurate, inexpensive, and nondestructive method to determine the density of very porous ceramics has been found. This method is an extension of the water‐displacement technique with three weighings and the use of glycerol instead of water. For small (1.13 g), 29% porosity B4C pellets, the precision is 0.002 g/cm3 and the accuracy is better than 0.006 g/cm3 compared with the geometric density.
Effect of Silicon Carbide Additions on the Crystallization Behavior of a Magnesia‐Lithia‐Alumina‐Silica Glass
Authors: Coon Dennis N.
Published: October 2005

Glass in the MgO‐Li2O‐A12O3‐SiO2 system was observed to crystallize readily at temperatures from 700° to 900°C. The primary crystalline phase evolved was Li2Si2O5, and the secondary phase evolved was Li2SiO3. The glass was amorphous after heating in air at 1050°C for 30 min. The addition of 0.5 wt% SiC powder resulted in the crystallization of Li2SiO3 during heating in air at 1050°C for 30 min. It was suggested that the difference in crystallization behavior with Sic addition was due to dissolution of Sic into the oxide glass.