Damage Mechanisms and the Mechanical Properties of a Laminated 0/90 Ceramic/Matrix Composite

The tensile properties of a 0/90 laminated CAS matrix composite reinforced with Nicalon fibers have been measured. Some effects of notches have also been explored. Changes in modulus and permanent strain caused by matrix cracking have been measured and compared with available models. For this comparison, independent measurements have been made of the constituent properties and the residual stress. The ultimate tensile strength has also been measured and compared with a global load-sharing model. It is concluded that lower-bound matrix cracking models provide good predictability of the stresses at which various matrix cracking mechanisms first operate. Also, the ultimate tensile strength is found to be consistent with a global load-sharing model, based on the in situ strength properties of the fibers. Conversely, the evolution of matrix cracks at stresses above the lower bound has yet to be adequately modeled. In addition, a need is identified for improved models relating elastic properties and permanent strains to matrix crack spacing.     

Oxidation and reforming reactions of CH4 on a stepped Pt(5 5 7) single crystal

The oxidation and reforming kinetics of methane by O₂, CO₂ and H₂O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO₂ and H₂O reforming. Total oxidation of methane to CO₂ and H₂O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO₂ or H₂O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO₂ and H₂O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes.     

LOCAL EQUILIBRIUM THEORY FOR THE DYNAMICS OF NONISOTHERMAL MIXED GAS ADSORPTION OF HIGH-CONCENTRATION COMPONENTS

A local equilibrium theory for nonisothermal gas adsorption is developed which accounts for variations in interstitial gas velocity induced by adsorption. Also presented are expressions describing the entropy admissibility criterion. A solution is developed for a Riemann's problem involving adiabatic gas-phase adsorption where there is no inert diluent and where Langmuir isotherms apply.     

Context dependence of phenotype prediction and diversity in combinatorial mutagenesis

Two different combinatorial mutagenesis experiments on the light-harvestingII (LH2) protein of Rhodobacter capsulatus indicate that heuristicrules relating sequence directly to phenotype are dependenton which sets or groups of residues are mutated simultaneously.Previously reported combinatorial mutagenesis of this chromogenicprotein (based on both phylogenetic and structural models) showedthat substituting amino acids with large molar volumes at Gly^ß31caused the mutated protein to have a spectrum characteristicof light-harvesting I (LH1). The six residues that underwentcombinatorial mutagenesis were modeled to lie on one side ofa transmembrane -helix that binds bacteriochlorophyll. In asecond experiment described here, we have not used structuralmodels or phylogeny in choosing mutagenesis sites. Instead,a set of six contiguous residues was selected for combinatorialmutagenesis. In this latter experiment, the residue substitutedat Gly^ß31 was not a determining factor in whetherLH2 or LH1 spectra were obtained; therefore, we conclude thatthe heuristic rules for phenotype prediction are context dependent.While phenotype prediction is context dependent, the abilityto identify elements of primary structure causing phenotypediversity appears not to be. This strengthens the argument forperforming combinatorial mutagenesis with an arbitrary groupingof residues if structural models are unavailable.     

The influence of tillage and cropping-intensity on cereal response to nitrogen,sulfur, and phosphorus

Efficient fertilizer use is a prerequisite for achieving optimum crop yield while avoiding environmental contamination. Cereal response to nitrogen (N), sulfur (S), and phosphorus (P) were determined for 6 years under differing tillage [conventional-till (CT) vs. no-till (NT)] and intensity of cropping (cereal/fallow vs. cereal/cereal). Semidwarf white winter wheat (Triticum aestivum L.) alternated yearly with either fallow or spring cereal [barley (Hordeum vulgare L.) or spring wheat] on a Typic Haploxeroll soil in a 415 mm rainfall zone. Fertilizer treatments were no fertilizer (None), N only (N), N plus S (NS), and N plus S plus P (NSP). Average application rate, when applied, was 109 kg N, 18 kg S, and 11 kg P ha^–1. Average cereal yield without fertilizer was 1.82 t ha^–1. Nitrogen increased grain yield in 6 of 6, S in 4 of 6, and P in 3 of 6 years, with P and S response significant the remaining years at the 10% probability level. Average yield increases were 1.11 t ha^–1 for N, 0.93 t ha^–1 for S, and 0.47 t ha^–1 for P. The NT/CT yield ratio was 0.60, 0.75, 0.93, and 0.95 with None, N, NS, and NSP addition, respectively, indicating that N and S deficiency were more severe in no-till. Limited increase in the NT/CT ratio with P addition indicated that P deficiency was less affected by tillage. Winter wheat always yielded less under NT than CT regardless of fertility, whereas spring cereals reached equality when fertilized with NSP. Annually-cropped wheat yielded 52, 67, 89, and 90% of wheat after fallow with None, N, NS, and NSP, respectively. Thus N and S, but not P, deficiency was more intense with increased frequency of cropping. Adequate fertility was a prime prerequisite for efficient yield in all systems.     

Organolithium route to poly(arylsilane)s

Various poly(arylsilane)s The poly(carbosilane)s prepared in this way are of higher molecular weight than when prepared by the Wurtz-type condensation route previously reported. TG shows 40–50% overll weight loss attained at 500–600°C.     

The Chemistry of Water Treatment Processes Involving Ozone,Hydrogen Peroxide and Ultraviolet Radiation

Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals in sufficient quantity to affect water purification. The theoretical and (practical yield of OH from O₃ at high pH, 0₃/H₂0₂, O₃/UV and H₂O₂/UV systems is reviewed. New data is presented which illustrates the importance of direct photolysis in the O₃/UV process, the effect of the H₂0₂:0₃ ratio in the O₃/H₂O₂ process, and the impact of the low extinction coefficient of H₂O₂ in the H₂0₂/UV process.     

Acquired Predator Recognition by Fathead Minnows: Influence of Habitat Characteristics on Survival

In this study we conditioned fathead minnows (Pimephales promelas) to recognize the odor of a perch (Perca flavescens) by exposing them to perch odor coupled with minnow alarm cue. We then staged encounters between the predator and prey in order to assess whether the predator odor training had any effect on survival of the prey. We tested for a survival effect in the presence and absence of shelter. Our results indicate that fish trained with alarm signals to recognize predators gained a survival benefit during staged encounters with a predator and that habitat characteristics influenced the survival of conditioned fish.     

Influence of polymer molecular weight on selected thermodynamic properties of polymer/solvent systems and the application of the UNIFAC theory

The intrinsic viscosities of eleven narrow molecular weight distribution polystyrene samples ranging from number-average molecular weight 1820 to 1 292 000 have been determined in pure solvents of tetrahydrofuran and chloroform with a Ubbelohde No. 1 viscometer at 25°C. By combining the original and modified Fox-Flory viscosity treatments, respectively, with the Flory-Huggins Lattice theory, the interaction parameters were calculated. With the aid of original and modified UNIFAC methods the solvent activity, clustering function of the solvent, and the interaction parameter were computed and compared with the experimental data. It was established that the influence imposed by the polymer molecular weight on the activity of the solvent, the clustering function of solvent, and interaction parameter is insignificant. The modified UNIFAC method satisfactorily predicted the solvent activity as illustrated by previous investigators. However, both UNIFAC methods failed to predict the interaction parameter.     

Carbonization and liquid-crystal (mesophase) development. 7. Optical textures of cokes from acenaphthylene and decacyclene

Optical textures of cokes prepared by carbonizing acenaphthylene, decacyclene and mixtures thereof at selected values of heat-treatment temperatures and soak time have been compared. Optical textures are assessed using polished surfaces and reflected-polarized-light microscopy in conjunction with a half-wave plate. The acenaphthylene is chemically more reactive than the decacyclene which is itself formed during the carbonization of acenaphthylene. Products of carbonization of acenaphthylene can influence rates of carbonization of the decacyclene. Similar optical textures in cokes cannot be formed by compensating low heat-treatment temperatures with long soak periods. In addition to chemical rate-controlling processes, the physical properties of the system must be acknowledged, in particular the viscosity. Very large non-coalesced growth units of mesophase (800 μm diameter) have been observed. Pre-alignment of growth units of mesophase may occur prior to coalescence.