Siderophore production and the evolution of investment in a public good:An adaptive dynamics approach to kin selection

International audienc

A Rapid Survey of the Compatibility of Selected Seal Materials with Conventional and Semi-Synthetic JP-8

Since the synthesis of a liquid hydrocarbon fuel from coal by Franz Fischer and Hans Tropsch in 1923, there has been cyclic interest in developing this fuel for military and commercial applications. In recent years the U.S. Department of Defense has taken interest in producing a unified battlespace fuel using the Fischer Tropsch (FT) process for a variety of reasons including cost, quality, and logistics. In the past year there has been a particular emphasis on moving quickly to demonstrate that an FT fuel can be used in the form of a blend with conventional petroleum-derived jet fuel. The initial objective is to employ this semi-synthetic fuel with blend ratios as high as 50 percent FT with longer range goals to use even high blend ratios and ultimately a fully synthetic jet fuel. A significant concern associated with the use of a semi-synthetic jet fuel with high FT blend ratios is the effect these low aromatic fuels will have on fuel-wetted polymeric materials, most notably seals and sealants. These materials typically swell and soften to some degree when exposed to jet fuel and the aromatic content of these fuels contribute to this effect. Semi-synthetic jet fuels with very low aromatic contents may cause seals and sealants to shrink and harden leading to acute or chronic failure. Unfortunately, most of the material qualification tests are more concerned with excessive swelling than shrinkage and there is little guidance offered as to an acceptable level of shrinkage or other changes in physical properties related to low aromatic content. Given the pressing need for guidance data, a program was developed to rapidly survey the volume swell of selected fuel-wetted materials in a range of conventional and semi-synthetic jet fuels and through a statistical analysis to make a determination as to whether there was a basis to be concerned about using fuels with FT blend ratios as high as 50 percent. Concurrent with this analysis data was obtained as to the composition of the fuel absorbed in fuel-wetted materials through the use of GC-MS analysis of swollen samples as well as other supporting data. In this presentation the authors will present a summary of the results of the volume swell and fuel absorbed by selected O-rings and sealants as well as a description of the measurement protocols developed for this program

In pursuit of blending polymers with carbon nanotubes

A liquid-solid phase separation method has been developed here to separate polymer/CNT blended phases with specific bundle size distribution from master heterogeneous polymer/CNT dispersions. This liquid-solid phase separation is triggered through addition of a non-solvent in the system. The fundamental issue of dispersing carbon nanotubes (CNTs) dispersion within a polymer matrix is also addressed in this work by studying a non-solvent induced liquid-solid phase separation process in polyacrylonitrile/CNT composite systems. To visualize the effect of phase separation, hybrid polymer/CNT buckypapers were formed through filtration. The hybrid film morphology is graded showing a distinct CNT-rich and polymer-rich layer. Examination of this layered structure reveals the separation of CNTs with specific bundle size. CNTs were uniformly dispersed within the polymer-rich layer due to a preferred polymer-CNT interaction during phase separation. Experimental, theoretical, and molecular dynamics studies were performed to show the fundamental mechanism behind layer formation in the composites and to understand the specificity of preferential polymer-CNT interactions. To this end, a geometric dependence described by a ‘cylinder-in-sphere’ model was established and shows a link between the critical CNT bundle size and polymer radius of gyration (Rg), which is dictates preferential polymer-CNT interactions. This model represents the geometric relationship required to form a blended polymer-CNT phase in the system under the phase separation conditions used. Understanding the use of phase separation as well as this geometrical dependence between filler and polymer is important to pinpoint nano-filler dispersion limits. Identifying these limits is critical toward the processing of superior polymer-based composites which fully utilizes the nano-filler reinforcement. Please click Additional Files below to see the full abstract

Transmission-virulence trade-offs in vector-borne diseases

International audienceThough it is commonly supposed that there is a trade-off between virulence and transmission, there is little data and little insight into what it should look like. Here, we consider the specific case of vector-borne parasites (inspired by human malaria) and analyse an embedded model to understand how specific life-cycle aspects may affect this trade-off. First, we find that, for such parasites, the transmission function may have an S-shape. Second, we find that the trade-off obtained for vector-borne parasites is less sensitive to parameter variations than the trade-off obtained for directly transmitted parasites. Third, we find that other parasite traits, such as the conversion from replicative to infective stages, could have important epidemiological implications. Finally, we compare the effect of treatments targeting either the asexual or the sexual parasite life-stage

Thermal properties of metal matrix composites with planar distribution of carbon fibres

High thermal conductivity (TC) and a tunable coefficient of thermal expansion are essential properties for heat management materials operating in a wide temperature range. We combine both properties in a composite with a low‐density metal matrix reinforced with pitch‐based carbon fibres. The thermal conductivity of the metal matrix was increased by 50%, the thermal expansion coefficient was reduced by a factor of five. The samples were produced by powder metallurgy and have a planar random distribution of fibres, leading to high performance in two dimensions

The Impact of Teacher and Student Racial and Ethnic Matching on Student Outcomes: A Quantitative Study

This study aimed to investigate the predictive relationship of teacher race on student outcomes in New Jersey public schools, as measured by the New Jersey Student Learning Assessment (NJSLA), chronic absenteeism, and graduation rates. Specifically, the relationship between teacher and student race matching with Black students, the schoolwide population of students, and a subset of urban schools was examined. The study investigated the potential of race matching to address the problem of racial and ethnic student achievement gaps. Studies indicate that pairing students with teachers with whom they share the same race and ethnicity can lead to increased cultural understanding and stronger teacher–student relationships, which can positively impact school outcome measures (Redding, 2019). Race matching is defined as the congruence between the percentages of Black teachers and Black students in a particular school. Schoolwide data were used to create a race matching variable, which served as the main independent variable. A sample of 2,510 schools was used. The findings indicated that race matching had a statistically significant predictive relationship with student academic growth as measured using the NJSLA, chronic absenteeism and graduation rates for the schoolwide group of students, and graduation rates for the Black student group. The study also found that race matching had a statistically significant predictive relationship with student academic growth and chronic absenteeism in urban schools. The study\u27s findings are used to provide valuable hiring and policy recommendations intended to address racial and ethnic achievement gaps

Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite