Matching with don't-cares and a small number of mismatches

In matching with don't-cares and k mismatches we are given a pattern of length m and a text of length n, both of which may contain don't-cares (a symbol that matches all symbols), and the goal is to find all locations in the text that match the pattern with at most k mismatches, where k is a parameter. We present new algorithms that solve this problem using a combination of convolutions and a dynamic programming procedure. We give randomized and deterministic solutions that run in time O(nk²logm) and O(nk³logm), respectively, and are faster than the most efficient extant methods for small values of k. Our deterministic algorithm is the first to obtain an O(polylog(k)⋅nlogm) running time.     

Superfast densification of nanocrystalline oxide powders by spark plasma sintering

Spark plasma sintering (SPS) is a newly discovered old technique which recently has been used for superfast densification of ceramic powders. Simultaneous application of pulsed high dc current densities and load is the necessary condition for rapid and full densification of ceramic powders by SPS. Commercial nanocrystalline magnesium oxide (nc-MgO) and yttrium aluminum garnet (nc-YAG) powders were densified to optical transparency using spark plasma sintering at distinctly different homologous temperatures (0.3 T _m for nc-MgO and 0.7 T _m for nc-YAG). The observed microstructure, density and grain size evolutions versus the SPS temperature were analyzed. The enhanced densification of the nc-MgO powder at the present SPS conditions was related to plastic deformation followed by diffusion processes. Densification of nc-YAG powder was described by the formation of viscous layer at the particle surfaces and corresponding densification by grain rotation and diffusion through the liquid phase. Densification by normal grain growth takes place at higher relative densities, regardless of the material.     

Effect of grain size on elastic modulus and hardness of nanocrystalline ZrO2-3 wt% Y2O3 ceramic

Dense nanocrystalline ZrO₂-3 wt% Y₂O₃ ceramics with grain sizes ranging between 23 to 130 nm were tested by ultrasonic pulse echo and Vickers hardness. The elastic modulus and hardness results were corrected for the residual porosity and the phase content. The corrected elastic moduli exhibited continuous decrease with decrease in the grain size. In contrast, no correlation was found between the corrected hardness and grain size. The percolative composite model was used to describe the changes in the elastic moduli in terms of percolation of the elastic wave through the intercrystalline phase at the percolation threshold. The absence of correlation with the hardness results was explained due to the other energy absorbing mechanisms such as microcracking beneath the indenter.     

Electric field effects during spark plasma sintering of ceramic nanoparticles

The effects of the applied electric field during the spark plasma sintering of ceramic nanoparticles were examined at various stages of the process. It was assumed that local intensification of the electric field arises due to the nanoscale structural features. Enhanced surface conductivity is expected in the nanoparticles during the heating, which otherwise are electrically non-conducting as a bulk. Percolation of the electric current at “optimal” electrical conductivity is obtained by fractal dimension. The defective nanoparticle surfaces experience charging–discharge cycles which lead to local breakdown and to plasma formation due to the ionized surface molecules. High local temperatures which evolved in a nonlinear fashion at the particle surfaces lead to enhanced sintering and densification kinetics, consistent with the flash sintering phenomenon. The contribution of the pondermotive force to the enhancement of the diffusion kinetics is discussed. Temperature windows for enhanced densification kinetics via plastic deformation or plasma-assisted processes are estimated for MgO, Al₂O₃, and YAG.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

The Effect of External Electrostatic Field Orientation on Aerosol Filtration by Granular Filters

The separation of fine aerosol particles by a packed granular-bed filter, enhanced by external electrostatic fields, was studied experimentally and theoretically. The filtration efficiencies of charged and neutralized aerosols were measured for external fields aligned with the air flow, transverse to the flow, and opposite to the flow. Theoretical models of electrostatically enhanced granular-bed filtration of micrometer and submicrometer particles were developed. Experimental results which demonstrate the relative merit of each configuration were presented and compared with the theory. The parallel-field configuration yielded the best filtration efficiency followed by the transverse configuration.     

On the Microstructures Resulting from the Diffusionless Cubic→Tetragonal Transformation in ZrO2-Y2O3 Alloys

A diffusionless cubic (c)→metastable tetragonal (t') phase transformation occurs in certain alloys in the ZrO₂-Y₂O₃ system on quenching from elevated temperatures. Microstructural features due to this phase transformation, principally anti-phase domain boundaries (APB's) and mechanical accommodation twins, have been characterized using transmission electron microscopy. Certain differences between our interpretation and those of other workers are discussed.     

Abnormal Hall–Petch behavior in nanocrystalline MgO ceramic

Pure and dense nanocrystalline MgO with grain size ranging between 25 and 500 nm were prepared by hot-pressing. Vickers microhardness was found to increase with decrease in the grain size down to 130 nm, following the Hall–Petch relation. Further decrease in the grain size was followed by continuous decrease in microhardness. A composite model was used to describe the microhardness behavior in terms of plastic yield of the nanocrystalline grains accompanied by strain accommodation and nanocracking at the grain boundaries (gb’s). Good agreement between the experimental and the calculated values indicates that gb’s may have significant effect on strengthening and ductility of nanocrystalline-MgO ceramics in the nanometer size range. Critical grain size exists below which limited plastic deformation within the grains and nanocracking at gb’s enhance the brittleness of the ceramic.     

Interfacial chemistry on carboxylate-functionalized monolayer assemblies

Deposition of trichlorosilanes with ester groups at their remote termini provides a convenient entry to carboxylic acid-bearing siloxane-anchored self-assembled monolayers. The de-esterification of these esters has been optimized to minimize monolayer damage, and their quantitative re-esterification provides clear evidence for the stability of these systems. Both the structure of the ester-terminated monolayer and its de-esterification/esterification chemistry can be easily monitored by FTIR-ATR measurements. This spectroscopic tool, together with a liquid cell that enables IR spectra to be measured in an aqueous environment, enables a detailed structural analysis of the carboxylic acid-bearing siloxane-anchored self-assembled monolayers and an assessment of their acid/base behavior (by in situ titration). The use of D₂O instead of H₂O for the in situ titration experiments also improves the available IR window. Both monomeric and dimeric/oligomeric acid groups are seen, and the relative ease of deprotonation of these various species can be directly monitored. Monomers of alkyl carboxylic acids that are hydrogen bonded only to surrounding water molecules have a pK_a = 4.9, while the pK_a for the aggregated molecules is 9.3. Similar behavior is seen for surface-bound benzoic acids, where the two pK_a values are 4.7 and 9.0. The influence of temperature on these structures and their chemistry has been explored to a limited extent as well. When the alkylcarboxylic acid system is cooled to 10 °C, the pK_a value for the acid monomers is reduced from 4.9 to 4.5 and increases from 9.3 to 10.3 for the aggregates.     

Electrochemical Chromium(III) Oxide Coatings on Non-oxide Ceramic Substrates

SiC and TiB₂ were electrochemically coated with Cr₂O₃ from a 0.1 M aqueous solution of chromium nitrate hydrate with ethanol additives. On both substrate materials poly-crystalline Cr₂O₃ was formed at current densities from 5 to 50 mA/cm² and deposition durations of 5 to 30 min. The coating weight increased with current density and with deposition time. The as-deposited coatings contained microcracks due to drying shrinkage. Microstructural observations indicate that sintering of the Cr₂O₃ coatings on TiB₂ at 1100°C for 1 n in a reducing atmosphere in a closed graphite crucible causes the densification of the coating via a liquid phase, which forms by oxidation of TiB₂. Under similar conditions, the Cr₂O₃ coatings on SiC may be sintered via an evaporation–condensation mechanism.