Synthesis of silicon doped SrMO3 (M = Mn, Co): stabilization of the cubic perovskite and enhancement in conductivity

In this paper we report the successful incorporation of silicon into SrMO(3) (M = Co, Mn) leading to a structural change from a hexagonal to a cubic perovskite. For M = Co, the cubic phase was observed for low doping levels (3%), and these doped phases showed very high conductivities (up to ≈350 Scm(-1) at room temperature). However, annealing studies at intermediate temperatures (700-800 °C), indicated that the cubic phase was metastable with a gradual transformation to a hexagonal cell on annealing. Further work showed that co-doping with Fe resulted in improved stability of the cubic phase; a composition SrCo(0.85)Fe(0.1)Si(0.05)O(3-y) displayed good stability at intermediate temperatures and a high conductivity (≈150 Scm(-1) at room temperature). For M = Mn, the work showed that higher substitution levels were required to form the cubic perovskite (≈15% Si doping), although in these cases the phases were shown to be stable to annealing at intermediate temperatures. Conductivity measurements again showed an enhancement in the conductivity on Si doping, although the conductivities were lower (≈0.3-14 Scm(-1) in the range 20-800 °C) than the cobalt containing systems. The conductivities of both systems suggest potential for use as cathode materials in solid oxide fuel cells.     

The diffusion coefficient of a polymer in an array of obstacles is a non-monotonic function of the degree of disorder in the medium

Using Monte Carlo simulations, we have found that there is a surprising non-monotonic dependence of a polymer's diffusion coefficient upon the degree of disorder of the surrounding environment. Starting with a two-dimensional periodic lattice of obstacles, we randomly displace obstacles to create a quenched gel system with a tunable degree of disorder. Very small displacements increase the diffusion coefficient of polymers since they increase the width of the tube through which the polymer chains reptate. As we displace the obstacles further, however, entropic trapping is observed and the diffusion coefficient of the polymer decreases dramatically. This is a striking example of the delicate balance between entropic and frictional effects for a polymer diffusing in a dense system.     

Convex labelings of trees

A convex labeling of a tree T of order n is a one-to-one function f from the vertex set of T into the nonnegative integers, so that f(y) ? (f(x) + f(z))/2 for every path x, y, z of length 2 in T. If, in addition, f(v) ? n ? 1 for every vertex v of T, then f is a perfect convex labeling and T is called a perfectly convex tree. Jamison introduced this concept and conjectured that every tree is perfectly convex. We show that there exists an infinite class of trees, none of which is perfectly convex, and in fact prove that for every n there exists a tree of order n which requires a convex labeling with maximum value at least 6n/5 – 22. We also prove that every tree of order n admits a convex labeling with maximum label no more than n²/8 + 2. In addition, we present some constructive methods for obtaining perfect convex labelings of large classes of trees.     

XYZH systems as potential 1,3-dipoles. Part 62: 1,3-Dipolar cycloaddition reactions of metallo-azomethine ylides derived from α-iminophosphonates

Metallo-azomethine ylides, generated from iminophosphonates in combination with LiBr or AgOAc and bases Et₃N, DBU, t-butyl tetramethylguanidine(BTMG) undergo cycloaddition to give dialkyl pyrrolidine-2-phosphonates along with the corresponding Michael adduct in some cases. Cycloadditions with the chiral dipolarophile 5R-(1′R,2′S,5′R-menthyloxy)-2-(5H)-furanone (MOF) afforded enantiopure cycloadducts.     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Enhanced cellulase production byAspergillus fumigatus fresenius

The compost isolate,Aspergillus fumigatus, produces the exoglucanase, endoglucanase, and Β-glucosidase enzymes required for the breakdown of crystalline cellulose. Cellulose breakdown and extracellular enzyme levels in liquid culture can be affected by low pH values attained during fungal growth. During growth ofA. fumigatus on modified Czapeck-Dox medium containing 1% (w/v) Avicel, it was found that Β-glucosidase activity was lost and endoglucanase activity, reduced, when pH values fell below 3. The effect of buffering (0.2M phosphate, pH 6.15) was examined and compared with the unbuffered medium. Beta-glucosidase activity could be detected throughout the incubation period in the buffered medium and endoglucanase activity was approximately tenfold greater. Exoglucanase activity also showed an increase in the buffered system. Concentrations of phosphate buffer ranging from 0.05 to 0.8M were incorporated into the medium and optimum cellulose breakdown and extracellular enzyme production occurred between 0.1 and 0.2M. Reports suggest that increasing substrate concentration does not improve upon the levels of extracellular cellulase produced because of enzyme inactivation resulting from rapid decreases in pH. Using the buffered medium described previously,A. fumigatus was grown on concentrations of Avicel ranging from 0.5 to 10% (w/v). Cellulose breakdown and extracellular enzyme production was compared with that achieved by a similar nonbuffered system. Endoglucanase and Β-glucosidase activity increased with time and with substrate concentration up to 5% (w/v) in the buffered medium. Beta-glucosidase was negligible at all concentrations of Avicel in the unbuffered medium and endoglucanase activity decreased with increasing substrate concentration with maximum levels approximately eightfold lower than in the buffered system. Extracellular exoglucanase activity was lower in the buffered medium and only increased to levels comparable with those achieved by the unbuffered medium towards the end of the incubation period. In the unbuffered system, exoglucanase activity decreased with increasing substrate concentration, but no such effect was observed in the buffered medium. Negligible growth occurred in both media at 10% (w/v) substrate. The percentage weight loss recorded in the Czapeck-Dox medium also decreased with increasing substrate concentration, while in the buffered medium, over 95% weight loss was recorded in up to 5% (w/v) Avicel. It appeared that cellulose breakdown was more rapid in the buffered medium and a time-course carried out to determine the rate of cellulolysis showed 97% cellulose breakdown after 12 d, corresponding to a plateau and a subsequent decrease in extracellular cellulase levels.     

Two Models for K-12 Hypermediated Earth System Science Lessons Based on Internet Resources

An immense collection of daily and archived earth and space science digital images and database information sources are readily accessible to K-12 teachers through the Internet and the World Wide Web. However, these abundant electronic resources are designed for scientists and often need to be modified to benefit students. Advances in hypermedia as an instructional tool for earth system science education provide a mechanism for designing user-friendly and structured classroom units based on this available data. Two models proposed as a foundation for materials development are the Learning Cycle Model and the Investigation/Experimentation Model. A high demand for Internet-based curricular materials development to take advantage of the current technology suggests an educational research agenda to determine which lesson formats are most effective for learners.     

The Properties of a cellulolytic microbial community growing on pure cellulose and lignocellulose

A microbial community capable of degrading pure cellulose has been isolated from soil using a continuous flow chemostat. The component organisms have been identified as Penicillium nigricans, Paecilomyces liliacinus, Fusarium oxysporum (3 strains), Aspergillus fumigatus, Gliocladium roseum, and Penicillium simplicissimum. Extracellular enzyme production (exoglucanase, endoglucanase, and (Β-glucosidase) and cellulose breakdown by the whole community and the component organisms in pure culture was measured during growth on pure cellulose in batch culture. After 30 d, 50–60% degradation was achieved by Penicillium simplicissimum, Aspergillus fumigatus, and the mixed culture. These cultures also produced the highest levels of extracellular endo- and exoglucanase and were able to produce the full enzyme complex necessary for the degradation of crystalline cellulose. The other isolates were capable of 0–20% degradation and produced lower levels of enzyme activity. Most were unable to produce the full enzyme complex. The results indicated that the whole community was no better at degrading pure cellulose than two members in pure culture. Since the full potential of the community may not be expressed on pure cellulose, breakdown and enzyme production was investigated during growth on lignocellulose. A large proportion of lignocellulosic material consists of xylan and enzymes capable of degrading this component were investigated. The highest percentage degradation and enzyme production (with the exception of Β-glucosidase) after 15 d growth on straw was achieved by the same three cultures. Studies of their growth on hay and straw were extended over a 60-d period. The rate and extent of degradation of hay by P. simplicissimum and A. fumigatus was similar to that of the mixed culture, more than 40% degradation occurring in 40 d. However, differences in levels of enzyme activity were observed. The mixed culture produced lower levels of enzyme activity (with the exception of Β-glucosidase) particularly during the initial states of degradation. Differences in enzyme production by A. fumigatus and P. simplicissimum were also observed. A. fumigatus produced high levels of xylanase and endoglucanase while P. simplicissimum produced high levels of exoglucanase. Similar results were observed on straw. The composition of the lignocellulosic material was measured by sequential chemical extraction of the solubles, hemicellulose, cellulose, and lignin. Changes in the composition during degradation were investigated. During the growth of all three cultures on hay, the total percentage of hemicellulose and cellulose decreased from 84% to less than 45% in 40 d. Despite significant differences in the enzyme activity, P. simplicissimum, A. fumigatus and the mixed culture were capable of degrading lignocellulosic material at the same rate. The production of a full cellulase complex appears to be more important than the production of high levels of individual enzymes.