Incorporation of surface-modified UHMWPE powders and fibers in tough polyurethane composites

Ultrahigh molecular weight polyethylene (UHMWPE) powder (60 micron diameter) and chopped fiber (6.5 mm length) were surface treated with a blend of reactive gases (fluorine and oxygen.) These powders and fibers were then compounded into a thermoset polyurethane matrix. Surface-treated and control reinforcements are compared at a 10 vol.% loading level. Tensile testing indicates improved stress transfer with surface treatment. Stress transfer is also improved with fibers versus powders. Fracture toughness is characterized by measuring the essential work of fracture, w_e. Surface-treated fiber composites exhibit work of fracture values almost three times the unfilled polyurethane values. Abrasion resistance was also measured. Surface treatment was found to significantly lower wear rates of powder composites. Fiber composites had lower wear rates than powder composites, although surface treatment had little effect. An approximate inverse relationship was found between wear rate and essential work of fracture.     

Trapping Brownian ensemble optimally using Broadcast Stochastic Receding Horizon Control

Collision of suspended entities with surrounding molecules in a fluid environment leads to random movements of these entities, known as Brownian motion. Suppression of this motion in a Brownian ensemble has recently become essential for facilitating emerging applications in biology and in micro and nano scale self-assembled systems. How optimally this suppression can be performed remains an open question of great interest to both the natural science and the control engineering communities. In this paper, we address this question theoretically by introducing a novel “Broadcast Stochastic Receding Horizon Control” strategy for trapping an ensemble of non-interacting Brownian particles. The strategy designs a control input, independent of the number of particles, using measurements from a single particle as the only available feedback information and broadcasts it to all particles in the ensemble. We show the existence of a minimum region in which all particles can be driven and trapped indefinitely using the proposed control action. Under specific conditions, we guarantee the trapping of all particles in this region with probability 1. Finally, we demonstrate the efficacy of our control design in a simulation environment by trapping 100 Brownian particles in one, two and three dimensional homogeneous medium.     

Effect of test frequency and water content on localized crack-tip heating in nylon-6,6

Fatigue crack propagation (FCP) tests were conducted at various frequencies on nylon-6,6 specimens equilibrated over a range of moisture levels to determine the crack growth rates and the crack-tip temperatures as a function of water content. Frequency-sensitivity was correlated with the amount of crack-tip heating taking place, with crack-tip temperature being found to depend strongly on the estimated loss compliance, D″, of the material. The frequency-sensitivity of FCP in nylon was seen also to be affected by mean stress, suggesting that creep processes are often significant in FCP of nylon.     

AMMONIA CATALYZED ORGANOSOLV DELIGNIFICATION OF POPLAR

A parametric investigation of NH₄OH catalyzed solvent delignification of poplar was conducted to define pretreatment conditions which would yield an optimal separation of the biomass components and an enzymatic susceptible solid carbohydrate phase. Delignification parameters of interest included concentration of NH₄OH, time and temperature of the reaction, and type of solvent. The addition of 0.82 M NH₄OH to the delignification liquor increased lignin removal and decreased carbohydrate degradation, but increasing NH₄OH concentration had no additional effect. At lower reaction temperatures, the extent of delignification increased with reaction time; at higher temperatures, a “relignification” of the pretreated wood was observed. The delignification and hemicellulose solubilization were modelled and rate constants reported. No major difference between three potential pulping solvents—ethanol, butanol, phenol—was observed. The enzymatic susceptibility of pretreated wood samples was approximately 6-fold greater than that of the untreated poplar. UV absorbance was used to qualitatively characterize the soiubilized lignins.     

Effect of Slow Crack Growth on the Thermal-Stress Resistance of an Na2O-CaO-SiO2 Glass

The effect of slow crack growth on the thermal-shock resistance of an Na₂O-CaO-SiO₂ glass was studied. An analytic and numerical technique was developed to calculate the critical quenching temperature difference, Δ T_c , of circular rods quenched in water from known crack velocity data. For rods of a specific radius and crack depth, Δ T_c , was calculated to be 147°C, in favorable correspondence with experimentally observed values of 155° to 160°C. In the absence of crack growth, Δ T_c was estimated to be 238°C, well in excess of the observed value and indicative of the significant effect of slow crack growth on thermal-stress resistance. It is also shown that crack growth significantly extends the time-to-failure to a value much greater than the time of maximum thermal stress. A form of subcritical crack instability is predicted at stress intensity factors well below the critical stress intensity factor, at which catastrophic failure becomes inevitable over the duration of the transient thermal stress. It is suggested that, when all other factors are equal, the effect of slow crack growth on thermal-stress resistance can be minimized by maximizing thermal diffusivity. It is also argued that surface-compression strengthening will be more effective than reduction in flaw size in increasing thermal-stress resistance. Recommendations are made for the design and selection of brittle materials subjected to thermal stress in stress-corrosive environments.     

Structure–activity relationships in supported Au catalysts

Au-based catalysts have great potential because of their unique activity and selectivity for a variety of important reactions. The special catalytic properties of supported Au nano-particles depend critically upon the particle morphology, i.e. size, shape and thickness, as well as support effects. This paper reviews the current understanding of CO oxidation on supported Au catalysts. The electronic structure of Au particles at various nucleation sites and on different supports is summarized, and the effect these changes have on catalytic performance is discussed. Recent results from our laboratories have demonstrated the synthesis of well-ordered Au mono- and bi-layer films on a titanium oxide support and show that the active Au structure for CO oxidation is an electron-rich, Au bi-layer. In contrast, the monolayer structure, which may involve the TiO_x support, is significantly less active (by less than an order of magnitude) than the Au bi-layer. The oxidation state of the Au and how this relates to the catalytic activity are also discussed.     

Cation interdiffusion between thick film of YBa2Cu3O7-δ and ceramic substrate

Thick films of superconducting oxides, YBa₂Cu₃O_7-, were successfully made by conventional screen-printing technology on Al₂O₃, MgO, and ZrO₂ substrates. Interdiffusion between the superconductive film and substrate was investigated using analytical electron microscopy. The results indicate that MgO and ZrO₂ are superior to Al₂O₃ for substrate materials.     

Fracture mechanics model for subthreshold indentation flaws

A fracture mechanics model for subthreshold indentation flaws is. described. The model describes the initiation and extension of a microcrack from a discrete deformation-induced shear fault (shear crack) within the contact zone. A stress-intensity factor analysis for the microcrack extension in residual-contact and applied-stress fields is used in conjunction with appropriate fracture conditions, equilibrium in Part I and non-equilibrium in Part II, to determine critical instability configurations.In Part I, the K-field relations are used in conjunction with the Griffith requirements for crack equilibrium in essentially inert environments to determine: (i) the critical indentation size (or load) for spontaneous radial crack pop-in from a critical shear fault under the action of residual stresses alone; (ii) the inert strengths of surfaces with subthreshold or postthreshold flaws. The theory is fitted to literature data for silicate glasses. These fits are used to calibrate dimensionless parameters in the fracture mechanics expressions, for later use in Part II. The universality of the analysis in its facility to predict the main features of crack initiation and propagation in residual and applied fields will be demonstrated. Special emphasis is placed on the capacity to account for the significant increase in strength (and associated scatter) observed on passing from the postthreshold to the subthreshold domain.     

Morphological and microchemical phenomena in the high-temperature oxidation of binary Al-Li alloys

The high-temperature oxidation behavior of binary Al-Li alloys has been characterized by scanning electron microscopy and secondary ion mass spectrometry in order to understand the mechanism of rapid oxidation in these alloys and to correlate the oxide morphology to its microchemistry. The oxide scale developed on polished specimens during short exposures in air at 530°C shows characteristic nodules that usually nucleate at grain boundaries. Examination of the alloy surface after removal of the oxide layer shows that the initial growth of the oxide nodules occurs laterally in addition to thickening normal to the oxide/alloy interface. Microchemical analysis of the oxide film with a scanning ion microprobe reveals a thick Li-oxide layer at the oxide/gas interface indicating preferential oxidation of Li at the free surface; the rest of the oxide film is composed of both Al- and Li-rich oxides, probably Li₂O and LiAlO₂ The presence of trace impurities (K, Na, F, and Cl) in the oxide scale was also detected. A microstructural model for the development of the oxide film in the Al-Li system is presented on the basis of both morphological and microanalytical data obtained in this study; this new model is compared with existing models.