Dependence Of Specific Surface Area Research Articles

The low-cyclic fatigue response and its dependence of specific surface area for open-cell nanoporous Cu

We systematically study the low cycle fatigue behavior and its dependence of specific surface area (ζ) for nanoporous copper (NPC) under ultrahigh strain rate (γ˙≈109 s−1) cyclic shear loading by conducting large-scale molecular dynamic simulation and small-angle x-ray scattering analysis. With an increase in ζ, NPC undergoes a transition from the first excellent anti-fatigue property (ζ<1.24nm−1) to the subsequent easy-to-fatigue capacity (ζ≥1.24nm−1). Two different mechanisms are governing fatigue: (i) smooth nucleation and propagation of dislocations for the former and (ii) nanopore compaction/coalescence for the latter by prohibiting the activities of dislocations. For NPC with ζ=0.42nm−1, fatigue contributes to a surprising superelasticity, prompted by the entanglements and reversed disentanglements of longer dislocations. Surface reconstruction contributes to the fatigue tolerance of NPC by facilitating local surface roughening and the emission of dislocation slips, and it becomes more pronounced with decreasing ζ.

Charge generation during the synthesis of doped lanthanum manganites via combustion of organo-inorganic precursors

Complex oxides on the basis of LaMnO3±y were obtained via combustion of precursors containing nitrates and soluble organic agents (polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyethylene glycol, cellulose; glycine, glycerine and citric acid). The phenomenon of charge generation processes during the synthesis was analysed by measuring the potential difference between the ground and precursor. It was proposed that one of the main factors determining the processes of charge generation is the release of gaseous molecular charged particles into the environment. The maximum value of generated charges was fixed for polyvinyl alcohol containing precursors and the lowest for glycine containing ones. The composition of the initial precursors (used organic component and the ϕ value) effects the maximum temperature achieved during synthesis (300 °C–1100 °C), concentration of released gases (50–600 ppm (CO), 250–2800 ppm (NO)) and the potential difference between the ground and precursor. The dependence of specific surface area (7.5–21 m2/g), the temperature of the beginning of the intense sintering (750 °C–1150 °C) and the maximum achievable shrinkage (5–21%) on the value of measured potential difference in precursors was shown. The established regularities allow expanding ability for the production of complex oxide materials with the specified properties.

Evaluation of Mechanical Properties of Alumina Green Compact during Sintering at Relative Low Temperature

Recently, ceramics was used extensively as structural materials and ceramics components became larger and more complex. Fracture sometimes occurs during firing because of large and complex shape, and this fracture interrupts manufacturing process. The simulation of sintering has been studied to prevent this fracture. However, it was difficult to simulate fracture process because there was little data on strength of green compact. It is necessary to measure strength during sintering in order to perform a useful simulation. In this study, we measured strength of two kind of alumina green compact during sintering. Three point bending test at elevated temperature was performed and strength was estimated at each temperature. A model for strength at relative low temperature was also proposed using the temperature dependence of specific surface area. Furthermore, fracture toughness test was performed and the relationship between strength and fracture toughness was obtained. Strength at relative low temperature increased with temperature. Fracture toughness was proportional to strength at the temperature range where materials demonstrated brittle fracture manner. Strength of each alumina was analyzed using this model.

Dendrite coarsening during directional solidification of Al–Cu–Mn alloys

Steady state directional solidification of Al–3.75%Cu–1.5%Mn and Al–1%Cu–1.5%Mn dendritic monocrystals was interrupted for different lengths of time prior to quenching and dendrite coarsening kinetics were evaluated versus temperature. For a given alloy composition, the isothermal coarsening rate, expressed as the time variation of the normalized specific dendrite surface area, increased with temperature and decreased with time. It also decreased with increasing copper concentration. The secondary dendrite arm spacing increased with time and for shorter times, with decreasing temperature. For Al–3.75%Cu–1.5%Mn alloy this trend was reversed for times exceeding about 1.5 min. For all times the rate of increase of the spacing increased with temperature. Secondary dendrite arm spacing increased with time during solidification and with decreasing growth rate. The dependence of specific surface area on time was predicted reasonably well by combining a coarsening model which is valid for the earlier stages of coarsening with a model which is valid for later stages. Copper diffusion through the liquid was found to be the rate controlling step in dendrite coarsening. Experimental measurements of secondary arm spacing during isothermal coarsening, as well as during solidification agreed reasonably well with model predictions.

The structural evolution of alkoxy-derived gel during drying

The effects of drying temperature on the structural evolution of alkoxy-derived silica gel prepared using various catalysts have been investigated. The dependence of specific surface area, S g, reflecting the structure, on the temperature of drying was remarkable for a non-catalyzed xerogel. The effect of drying temperature on the S g of an ammonia-catalyzed xerogel was also found but was not very large. The S g of xerogels obtained by drying at 60°C was always higher by 50% than the gels dried at 30°C without regard to the aging temperature. The S g of xerogels from HCl-catalyzed solution was of the order of several m 2/g, however, the S g of the aerogel obtained by hypercritical drying of the wet gel from a similar solution was about 800 m 2/g. These phenomena were understood on the basis of SAXS measurements on both wet gels and aerogels.