Production of submicron SiC particles by d.c. thermal plasma: a systematic approach based on injection parameters

Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl₄ feeding rate, C/Si and H₂/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental set-up allows the production of β-SiC powders at a rate of 200 g h⁻¹ with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen (1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl₄ feeding rate and the H₂/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling the particle size.     

Calretinin-immunoreactive neurons in the human thalamus

To identify plant defense components that are important in restricting the growth of virulent pathogens, we screened for Arabidopsis mutants in the accession Columbia (carrying the transgene BGL2-GUS) that display enhanced disease susceptibility to the virulent bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) ES4326. Among six (out of a total of 11 isolated) enhanced disease susceptibility (eds) mutants that were studied in detail, we identified one allele of the previously described npr1/nim1/sai1 mutation, which is affected in mounting a systemic acquired resistance response, one allele of the previously identified EDS5 gene, and four EDS genes that have not been previously described. The six eds mutants studied in detail (npr1-4, eds5-2, eds10-1, eds11-1, eds12-1, and eds13-1) displayed different patterns of enhanced susceptibility to a variety of phytopathogenic bacteria and to the obligate biotrophic fungal pathogen Erysiphe orontii, suggesting that particular EDS genes have pathogen-specific roles in conferring resistance. All six eds mutants retained the ability to mount a hypersensitive response and to restrict the growth of the avirulent strain Psm ES4326/avrRpt2. With the exception of npr1-4, the mutants were able to initiate a systemic acquired resistance (SAR) response, although enhanced growth of Psm ES4326 was still detectable in leaves of SAR-induced plants. The data presented here indicate that eds genes define a variety of components involved in limiting pathogen growth, that many additional EDS genes remain to be discovered, and that direct screens for mutants with altered susceptibility to pathogens are helpful in the dissection of complex pathogen response pathways in plants.     

Time and shear dependent rheology of maleated polyethylene and its nanocomposites

Dipole-dipole and/or hydrogen-bonding interactions between the pendant functional groups within maleated high-density polyethylene (PE-g-MAn) establish a physical polymer network, whose formation kinetics and shear-sensitivity are revealed by dynamic oscillatory testing. The pronounced time and shear dependent viscoelastic properties of PE-g-MAn were not observed for a corresponding imide derivative, PE-g-imide, presumably due to weakened functional group associations in the latter material.The melt compounding of PE-g-MAn with onium-ion exchanged montmorillonite clay (NR₄⁺-MM) resulted in a partially exfoliated hybrid nanocomposite structure, whose viscoelastic behaviour differed significantly from that of the unfilled polymer. The presence of dispersed clay platelets altered the extent of functional group associations, thereby changing the dynamics of network formation.     

Theory of reflectivity of an asymmetric mirror

By using the concept of transfer matrices and Bloch waves, we have derived a set of equations that provide insight into the operation of asymmetric Bragg reflectors that have been demonstrated to be useful in achieving high reflectivities in strained-material systems. These equations will be useful in the design of asymmetric mirrors and can be used to compare the trade-offs between the conventional, symmetric (quarter-wavelength), and asymmetric mirrors.     

Estimating percent impervious cover from Landsat-based land cover with a simple and transferable regression model

Percent impervious cover (PIC) is a widely used metric in ecological and hydrological analyses because it is highly correlated with pollutant and storm water run-off. The moderate-resolution satellite data (e.g. Landsat), that are typically used to calculate PIC, tend to overestimate PIC for all but very rural and very urban landscapes. Existing models for calibrating PIC estimates (e.g. ISAT, ETIS) are limited in that they are applicable only for specific land cover datasets and may also require population data; furthermore, these models have not been tested for performance outside of the geographic locations in which they were developed. The goal of this study was to explore simple but widely applicable regression models as tools for calibrating PIC estimates based on moderate resolution satellite data. The regression models used impervious land cover, from Landsat-based datasets, as the sole predictor of actual PIC. PIC was calculated for analysis units, ranging in size from 2.25 ha to ≥100 ha, for locations in Connecticut, Massachusetts, and Ohio in the United States. Regression models were fit for each size class of analysis unit at each study location; generalized versions of the models were created by fitting a regression to all size classes of analysis units at a given study location. Calibrated PIC estimates had root mean square error (RMSE) values that ranged from 1.5–10.7%; these values were considerably better than RMSE values for uncalibrated PIC estimates which ranged from 10.1–23.3%. For both calibrated and uncalibrated PIC, the accuracy of the estimates improved with the increasing size of the analysis units. Model regression coefficients were similar regardless of the analysis unit size, geographic location, or land cover dataset; model performance declined only slightly when applied outside the area in which it was developed. The simple regression models developed in this study had similar performance to previous calibration models (i.e. ISAT, ETIS) but are easier to apply and more widely applicable.     

Shape transformation by boundary representation interpolation: A recursive approach to establishing face correspondences

The issues relating to the shape transformation problem are discussed and a new algorithm is presented for computing the transformation of one shape into another. In this algorithm, the boundary definitions of the two initial shapes are used and a mapping is established between the vertices and edges of the respective objects. New vertices and edges are introduced into the object definitions when necessary to establish a one-to-one vertex correspondence and to match connectivity relationships between vertices. These can then be used to do a vertex-to-vertex interpolation that maintains valid polyhedral topologies for all of the intermediate shapes. The algorithm establishes a mapping between areas of the object such that adjacency relationships are preserved. These areas are recursively subdivided so that adjacency relationships of subareas are also preserved. During subdivision, vertices and edges are added to the boundaries of subareas so that a one-to-one mapping is established between them. Subdivision continues until each subarea consists of a single face. The algorithm presented works for objects that are topologically equivalent to spheres and can easily be extended to other pairs of objects as long as they are topologically equivalent to each other.