Lower Apparent Values Research Articles

Experimental and Theoretical Comparison of Addition–Fragmentation Pathways of Diseleno- and Dithiocarbamate RAFT Agents

Theoretical studies suggest that identically substituted dithio- and diselenocarbamate reversible addition–fragmentation chain transfer (RAFT) agents will have similar reactivity but that diselenocarbamate RAFT agents are more likely to undergo intermediate radical termination. These results are supported by an experimental study of polymerizations of vinyl acetate mediated by Se-cyanomethyl N,N-dimethyldiselenocarbamate (1Se) and its sulfur analog, S-cyanomethyl N,N-dimethyldithiocarbamate (1S). While the two polymerizations had similar interchain transfer constants (3.2 for 1S and 3.9 for 1Se), significant retardation was observed for 1Se-mediated polymerizations. External chain transfer constants were also measured for 1S (5.4) and 1Se (45). The low apparent value of the chain transfer constant of 1S is attributed to the selective formation of a single monomer adduct in the early stages of the reaction.

Precipitate Stability in Creep Resistant Ferritic Steels-Experimental Investigations and Modelling.

Predictions of long-term microstructure stability of creep resistant ferritic 9-12% Cr steels up to 200 000-300 000 h at temperatures up to 600-650°C are highly interesting for safe power plant operation. At technically interesting creep conditions the microstructure stability is mainly controlled by the stability of precipitate particles. Predictions of precipitate stability have to rely on i) Microstructure characterisation methods to measure volume fractions and mean particle sizes of individual precipitate types, and ii) Microstructure models to predict the evolution of precipitate volume fractions and mean sizes as functions of temperature, time and applied stress. Characterisation methods, which allow on-line particle type discrimination in 9-12 % Cr steels include energy filtered transmission electron microscopy (EFTEM) and scanning electron microscopy (SEM) with atomic number contrast. Modelling of precipitate stability based on thermodynamic equilibrium calculations and multicomponent diffusion databases is demonstrated. A multi-component coarsening model gives accurate predictions of coarsening rates for MX and Laves phase precipitates in steel P92 with fit values for the interfacial energy in the expected range. For M23C6 carbides in steel P92 the model results in unexpectedly low apparent values for the interfacial energy. Modelling of published data for steel P91 indicate much higher coarsening rates for M23C6 carbides, and the fit value for the interfacial energy is as expected. A possible explanation for the low apparent value of the interfacial energy for M23C6 carbides in steel P92 is the content of boron in the steel.

Effects of pressure and surface initiation efficiency on the flowing H2O2+ NO2+ CO + N2 chain reaction system

The yields of CO2 from the H2O2+ NO2+ CO reaction system in flowing carrier gas at 298 K have been measured at total pressures (P) of 13.3, 40.0 and 93.3 kPa. The variations of the yield as functions of [NO2] and [S](S = acetaldehyde or diethyl ether) have been shown to indicate that the efficiency of conversion of H2O2 to OH radicals in the heterogeneous initial step H2O2+ NO2→ OH + HNO3(1) is independent of total pressure for a particular surface activity. The results are consistent with the pressure dependence of k2 for the propagation step CO + OH → CO2+ H (2) indicated by other work. For the rate constant of the termination reaction OH + NO2(+ M)→ HNO3(+ M)(4) low apparent values (k′4) are indicated compared with literature values of k4. At P= 40.0 kPa, for three surfaces of apparent efficiencies of ca. 20%, ca. 53% and ca. 81% in converting H2O2 to OH in reaction (1), k′4/k4 values were ca. 0.45, ca. 0.65 and ca. 0.85, respectively. This trend is interpreted in terms of decreasing activity of these surfaces for adsorption of H2O2, producing decreasingly localized reaction zones: depletion of [NO2] in the reaction zone depresses k′4/k4 while increased surface concentrations of H2O2 decrease the apparent H2O2 to OH conversion efficiency by inducing an additional route for destruction of the active centre involved in reaction (1).

THE POTENTIAL NUTRITIVE VALUE OF SOME WEEDS COMMON TO NORTH‐EASTERN ENGLAND

The proximate and ash constituents of twenty weed species have been determined. These weeds are all common to north‐eastern England. Some are characteristic of hedgerow and roadside and a few species of arable land; the majority, however, usually occur in pasture and meadowland, or in other sites where they are freely accessible to stock. Exact locality, approximate stage of growth and date of harvesting are recorded.A comprehensive examination of their ash has been made and four trace elements of recognized importance, viz. iron, manganese, copper and cobalt, were determined.The weeds have been classified on the basis of a marking system, and it was found possible to place them in one or other of three fairly well‐defined groups, viz. high, medium or low apparent value.Reference has been made to earlier assessments of these weeds and probable values, as reflected by chemical analysis, have been discussed. Most of the weeds listed as being of high apparent value are known or could justifiably be assumed to be of good nutritive status; examples of such are dandelion, nettle, creeping thistle and broad‐leaved plantain. Ragwort, although ranked highly on the marking system used, is well‐known to be toxic to animals. Not all members of the group of medium apparent value are likely to be eaten to any appreciable extent and at least two are of doubtful wholesomeness. Among weeds of low apparent value, only bird's‐foot trefoil would appear to have much potential usefulness.

The Recombination of Ions and of Ions and Electrons in Gases

Coefficient of recombination of ions, produced by x-rays, as function of x-ray exposure time, initial ion concentration and time of recombination.---The present work constitutes a study of the recombination of ions and of ions and electrons in gases, by means of a new direct method, using constant radiation from a Coolidge x-ray tube as the ionizing agent, together with a rotating commutator shutter. This method permits wide limits of independent variation of: (1) x-ray exposure (${t}^{\ensuremath{'}}$); (2) initial ion concentration (${n}_{0}$); (3) time of recombination ($t$). Results in air show that $\ensuremath{\alpha}$, previously assumed constant and equal to 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ actually varies as a function of ${t}^{\ensuremath{'}}$ and of $t$. It is high, 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ or more, for small values of the above quantities and drops rapidly to nearly a constant value. $\ensuremath{\alpha}$ is also found to be a function of ${n}_{0}$. These facts can only be explained on the assumption that the ions are not initially uniformly distributed, but are in pairs along the paths of the x-rays. This leads to fictitiously high calculated values of $\ensuremath{\alpha}$ corresponding to low apparent values of ${n}_{0}$ and $n$, erroneously computed on the assumption of random distribution. As ${t}^{\ensuremath{'}}$, and with it ${n}_{0}$ and also $t$, increase, $\ensuremath{\alpha}$ approaches a constant value between 0.8 and 0.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$, which is probably nearly the true value. This result lends strong support to the theory of Thomson that the process of diffusion plays the major role in the initial stages of the mechanism of recombination; i.e. in bringing ions together.Coefficient of recombination in argon: mechanism of recombination, formation of negative ions by electron attachment to impurities.---In argon, where electrons remain free for appreciable intervals, $\ensuremath{\alpha}$ is nearly constant. For the same time intervals it has only about half the value found in argon-oxygen mixtures, where the electrons attach rapidly, or the value found in dry air. On the basis of the kinetic theory $\ensuremath{\alpha}$ would be expected to be the same in argon as in air. The constancy of $\ensuremath{\alpha}$ with $t$ and ${t}^{\ensuremath{'}}$ is due to the rapid achievement of random distribution when free electrons are present. The value of $\ensuremath{\alpha}$ indicates that the coefficient of recombination for electrons and positive ions is less than for positive and negative ions. This conclusion is in agreement with the results of Kenty on recombination between electrons and positive ions in argon. Consequently $\ensuremath{\alpha}$, as observed in pure argon, is not a true measure of the recombination of positive and negative ions, but serves as an indication of the rate of negative ion formation through the attachment of electrons to impurities present.