The mechanical properties of fly ash concrete prepared and cured at high temperatures

Results of an experimental investigation on the effect of fly ash content, water cement (w/c) ratio of concrete prepared and cured at temperatures of ambience, 40°C and 60°C are presented in this paper. The mechanical properties investigated here include its workability, its behaviour under compression and splitting tension. Microscopic investigations were also carried out using SEM micrographic images to gain understanding of concrete at its microstructural level.The results indicate that high temperature increases early compressive strengths of concrete, but has decreasing effects on the strengths at later ages. The use of fly ash in concretes cast and cured at elevated temperatures improves the physical and mechanical properties of fresh as well as those of hardened concrete.Various mathematical models describing the properties of such concretes are considered at the end of this paper. The strength properties of high temperature fly ash concrete were best represented by a simple exponential function of time, while its stress-strain relationship could be best described by an exponential function of strain of a more complicated form.

Molecular Dynamics Study on Structural Relaxation of Metallic Glasses

Structural relaxation process in the Zr-Cu metallic glasses is investigated by using molecular dynamics simulations. The enthalpy change in isothermal annealing of the glassy state cannot be fitted by a simple exponential function but obeys a stretched exponential function, which indicates that the relaxation in glassy phase is not a single Debye type process. A close examination of individual atomic motion reveals that the enthalpy relaxation is related to a string-like cooperative motion of atoms. The analysis of the local symmetry around each atom shows that a network of the icosahedral clusters grows in the glassy phases during annealing and it closely relates to the free-volume annihilation in the structural relaxation.

Moisture Quenching of Solid-Matrix Phosphorescence on Hydrophilic and Partially Hydrophobic Filter Papers

A detailed investigation was undertaken to determine the important parameters that were responsible for the solid-matrix phosphorescence (SMP) moisture quenching of phosphors adsorbed in Whatman No. 1 (hydrophilic) and Whatman 1PS (partially hydrophobic) papers. The three phosphors used were phenanthrene, perdeuterated phenanthrene, and benzo[ e]pyrene. Both SMP lifetime and SMP intensity data were obtained over a wide range of adsorbed moisture. In addition, moisture isotherms were obtained for the No. 1 and 1PS papers. The SMP lifetimes of phenanthrene and benzo[ e]pyrene were independent of the amount of adsorbed moisture on both the No. 1 and 1PS papers. However, the SMP lifetimes of perdeuterated phenanthrene on No. 1 and 1PS papers were dependent on the extent of the adsorbed moisture. The changes in the SMP intensities as a function of adsorbed moisture for phenanthrene, benzo[ e]pyrene, and perdeuterated phenanthrene on No. 1 paper could be modeled by a simple exponential function with phenanthrene and benzo[ e]pyrene giving better correlations compared to perdeuterated phenanthrene. The change in the modulus of filter paper samples with moisture adsorption was a major factor in causing the SMP intensity to decrease. The decrease in the SMP intensities with moisture adsorption for the three phosphors adsorbed on 1PS paper did not correlate with the simple Stern–Volmer model and several other quenching models discussed in the literature. Thus, these data were fit to a relatively simple empirical equation. The results showed that the SMP quenching phenomena for the three phosphors on No. 1 paper and on 1PS paper were considerably different.

Paramagnetic Nitrogen Defects in Silicon Nitride

ABSTRACTThe photocreation mechanisms and properties of nitrogen dangling bonds in amorphous hydrogenated silicon nitride (a-SiNx:H) thin films are investigated. We find that the creation kinetics are strongly dependent on the post-deposition anneal; this thermal process can be described by a simple exponential function which yields an activation energy of 0.8 eV. The compositional dependence of the nitrogen dangling bond center suggests that its energy level lies close to the valence band edge, in agreement with theoretical calculations. This energy level position can explain why a-SiNx:H films often become conducting following a high post-deposition anneal.

Ultralow-load indentation hardness and modulus of diamond films deposited by hot-filament-assisted CVD

Diamond films, ranging in thickness to approximately 35 μm, were grown on Si(100) substrates using hot-filament-assisted CVD. Two different CH4:H2 ratios were employed during deposition, and the effects on the film morphology and ultralow-load indentation hardness and modulus were investigated. Films deposited from a single, linear filament exhibited a nonuniform deposition thickness profile that can be described by a simple exponential function. Films deposited at lower methane concentrations, 0.11% CH4 in H2, had larger crystallite sizes of ∼5–8 μm, an average hardness of 31 GPa, and an average modulus of 541 GPa. A higher CH4 concentration of 0.99% in H2 resulted in the formation of finer crystallites of approximately 0.5 μm, an average hardness of 65 GPa, and an average modulus of 875 GPa. While these values lie on the low end or outside of the range reported for single crystal diamond, this study has demonstrated that CVD diamond films can be synthesized with ultrahigh or near ultrahigh hardness.

Rhodopsin kinetics in the cat retina.

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 min or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all of the conditions studied (fractional bleaches of from 65 to 100%), the regeneration of rhodopsin to its prebleach levels required greater than 60 min in darkness. After the 1-min exposures, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and it was possible to follow the regenerative process over its full time-course. It was not possible, however, to fit these data with the simple exponential function predicted by first-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. Nevertheless, the kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and found to approximate closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1-e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with approximately 7.0 in man.

Studies in dialogue and discourse; an exponential law of successive questioning

ABSTRACTThe structure of natural conversations in first-grade classrooms is the focus of this inquiry. Analyses of a particular type of discourse, namely, connected conversations initiated and sustained by questioning, suggest that the probability that a conversation will be continued may be expressed as a simple exponential function. The formula, pi = ari−1, generates a curve of theoretically-expected rates of successive questions in a series that closely matches observed rates. The formula is based on the application of a constant ratio, that is, the ratio of rates within each pair of adjacent questions is the same throughout the series: p2:p1=p3:p2 = p4:p3. … Thus, it appears that the probability of a ‘next’ question following an exchange that contains a previous question remains constant through the length of the discourse series. In other words, the probability of a question is independent of the temporal location of an utterance in this type of connected conversation. The analyses suggest further that the model of a finite Markov chain, that is, of a particular type of stochastic process, may be applicable to certain features of a discourse. (Conversational analysis, sequencing in exchanges, U.S. English in first-grade classrooms.)

A Nuclear Spin Relaxation Study of the Spin–Rotation Interaction in Symmetric Top Molecules

The spin–lattice relaxation times T1 of 1H, 19F, and 31P nuclei were measured in gaseous samples of BF3, CHF3, CH3F, PH3, and NH3 at room temperature for densities from 0.03 to 10 amagat. In several cases the behavior of T1 at the lowest densities snowed deviations from the linear variation characteristic of the extreme narrowing region. The spin–rotation interaction provides the dominant relaxation mechanism in all cases. The data are analyzed on the basis of the assumption that the collision modulated spin–rotation interaction may be described by a single correlation function which is a simple exponential function of time. Values of an effective spin–rotation constant and a cross section for molecular reorientation are obtained for each gas. The results obtained are compared with those available from other types of experiments. This comparison indicates that the theory for spin–lattice relaxation in dilute gases of symmetric top molecules needs to be carefully reassessed.