Transforming carbon nanotubes by silylation: An ab initio study

We use ab initio density functional calculations to study the chemical functionalization of single-wall carbon nanotubes and graphene monolayers by silyl (SiH3) radicals and hydrogen. We find that silyl radicals form strong covalent bonds with graphene and nanotube walls, causing local structural relaxations that enhance the sp3 character of these graphitic nanostructures. Silylation transforms all carbon nanotubes into semiconductors, independent of their chirality. Calculated vibrational spectra suggest that specific frequency shifts can be used as a signature of successful silylation.Comment: 4 pages, 3 figure

Microscopic mechanism of fullerene fusion

Combining total energy calculations with a search of phase space, we investigate the microscopic fusion mechanism of C60 fullerenes. We find that the (2 + 2) cycloaddition reaction, a necessary precursor for fullerene fusion, may be accelerated inside a nanotube. Fusion occurs along the minimum energy path as a finite sequence of Stone-Wales transformations, determined by a graphical search program. Search of the phase space using the "string method" indicates that Stone-Wales transformations are multistep processes, and provides detailed information about the transition states and activation barriers associated with fusion.open413

Composição da comunidade bacteriana do solo sob sistemas integrados na região norte de Mato Grosso.

Os sistemas integrados de produção agrícola estão sendo estudados como uma alternativa aos monocultivos tradicionais no intuíto de tornar a atividade agrícola sustentável, particulamente no estado do Mato Grosso. Entretanto, ainda não se conhece como essa atividade afeta as propriedades microbiológicas do solo. Assim, o objetivo deste trabalho foi avaliar e comparar os efeitos de sistemas integrados na composição da comunidade bacteriana do solo com diferentes monocultivos tendo como área referência uma mata nativa do bioma de transição Cerrado e Amazônia. O local de estudo está localizado na Embrapa Agrossilvipastoril no município de Sinop/MT e foram avaliados sete tratamentos (1-mata nativa, 2-lavoura, 3-pecuária, 4-eucalipto, 5-integração eucalipto lavoura (iEL), 6-integração eucalipto pecuária (iEP), 7-integração eucalipto lavoura pecuária (iELP)) dispostos em quatro blocos casualizados, os quais foram conduzidos segundo as recomendações agronômicas das espécies cultivadas. As amostras de solo foram coletadas nas épocas de chuva e estiagem em 2012 com o auxílio de trado holandês na profundidade de 0-10 cm, considerando 20 pontos aleatórios com caminhamento zigue-zague para fazer uma composta de cada tratamento. A extração de DNA total do solo foi realizada com o kit MoBio UltraClean? Soil DNA (MoBio Laboratories, Carlsbad, CA., EUA) conforme protocolo descrito pelo fabricante e sequenciado pela tecnologia Illumina (Illumina, Inc., CA, USA) com a plataforma MiSeq na Universidade da Florida (Gainesville, EUA). Por meio de ferramentas de bioinformática 2.957.127 sequencias do gene 16S rRNA com alta qualidade foram obtidas, sendo que 2.172.432 de OTUs (Operational Taxonomic Units) foram classificados para o domínio Bacteria, 3.564 Archaea e 262.994 não foram classificados. A composição bacteriana do solo com abundancia relativa ≥ 10 % a nível de filo apresentou três táxons, porém Firmicutes e Proteobacteria apresentaram padrão de comportamento de maior abundancia na estiagem em relação a chuva para mata nativa e iELP e para a lavoura o padrão foi o oposto . A nível de classe, dentre cinco filos apenas Actinobacteria variou o comportamento para iELP e mata nativa com menor abundância na estiagem e maior na chuva, tendo a lavoura o comportamento inverso e para Clostridia ocorreu o mesmo padrão porém ao contrário para os respectivos tratamentos. Analisando a diferença entre os tratamentos por meio da dissimiliradidade a nível de filo, corte de abundância relativa ≥ 0.05 % (18 filos na estiagem e 21 na chuva), o dendrograma baseado na distância euclidiana agrupou mata nativa e iELP como os mais semelhantes e lavoura o mais dissimilar dentre todos os tratamentos. Podemos concluir, que o sistema integrado eucalipto lavoura pecuária apresentou maior semelhança na sua composição bacteriana do solo com a da mata nativa em comparação com os demais tratamentos avaliados, como também manteve o padrão do comportamento da abundancia relativa parecido na época de estiagem, assim demonstrando indício de sustentabilidade

Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?

Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes. Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments. Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surface

Thermal transport measurements of individual multiwalled nanotubes

The thermal conductivity and thermoelectric power of a single carbon nanotube were measured using a microfabricated suspended device. The observed thermal conductivity is more than 3000 W/K m at room temperature, which is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. The temperature dependence of the thermal conductivity of nanotubes exhibits a peak at 320 K due to the onset of Umklapp phonon scattering. The measured thermoelectric power shows linear temperature dependence with a value of 80 $\mu$V/K at room temperature.Comment: 4 pages, figures include

Modeling Vesicle Traffic Reveals Unexpected Consequences for Cdc42p-Mediated Polarity Establishment

SummaryBackgroundPolarization in yeast has been proposed to involve a positive feedback loop whereby the polarity regulator Cdc42p orients actin cables, which deliver vesicles carrying Cdc42p to the polarization site. Previous mathematical models treating Cdc42p traffic as a membrane-free flux suggested that directed traffic would polarize Cdc42p, but it remained unclear whether Cdc42p would become polarized without the membrane-free simplifying assumption.ResultsWe present mathematical models that explicitly consider stochastic vesicle traffic via exocytosis and endocytosis, providing several new insights. Our findings suggest that endocytic cargo influences the timing of vesicle internalization in yeast. Moreover, our models provide quantitative support for the view that integral membrane cargo proteins would become polarized by directed vesicle traffic given the experimentally determined rates of vesicle traffic and diffusion. However, such traffic cannot effectively polarize the more rapidly diffusing Cdc42p in the model without making additional assumptions that seem implausible and lack experimental support.ConclusionsOur findings suggest that actin-directed vesicle traffic would perturb, rather than reinforce, polarization in yeast

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range - of over five orders of magnitude - from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.Comment: Review Paper; 37 manuscript pages; 4 figures and 2 boxe

Cut-points for waist circumference in Europids and South Asians

There is little strong evidence that currently recommended higher waist circumference cut-points for Europids compared with South Asians are associated with similar risk for type 2 diabetes. This study was designed to provide such evidence. Longitudinal studies over 5 years were conducted among 5,515 Europid and 2,214 ethnically South Asian participants. Age-standardized diabetes incidence at different levels of waist circumference and incidence difference relative to a reference value were calculated. The Youden Index was used to determine waist circumference cut-points. At currently recommended cut-points, estimated annual diabetes incidence for a 50-year-old Europid was &lt;0.6% for both sexes, and for a 50-year-old South Asian, 5.8% for men and 2.1% for women. Annual diabetes incidence of 1% was observed for a 50 year old at a waist circumference 35&ndash;40 cm greater in Europid compared to South Asian men and women. Incidence difference between recommended cut-points and a reference value (80 cm in men, 70 cm in women) was 0.3 and 4.4% per year for Europid and South Asian men, and 0.2 and 0.8% per year for Europid and South Asian women, respectively. Waist circumference cut-points chosen using the Youden Index were shown to be dependent on obesity levels in the population. The much higher observed risk of diabetes in South Asians compared to Europids at the respective recommended waist circumference cut-points suggests that differences between them should be greater. Approaches that use the Youden Index to select waist circumference cut-points are inappropriate and should not be used for this purpose.<br /

Muscle atrophy and metal-on-metal hip implants

Background and purpose — Muscle atrophy is seen in patients with metal-on-metal (MOM) hip implants, probably because of inflammatory destruction of the musculo-tendon junction. However, like pseudotumors, it is unclear when atrophy occurs and whether it progresses with time. Our objective was to determine whether muscle atrophy associated with MOM hip implants progresses with time. Patients and methods — We retrospectively reviewed 74 hips in 56 patients (32 of them women) using serial MRI. Median age was 59 (23–83) years. The median time post-implantation was 83 (35–142) months, and the median interval between scans was 11 months. Hip muscles were scored using the Pfirrmann system. The mean scores for muscle atrophy were compared between the first and second MRI scans. Blood cobalt and chromium concentrations were determined. Results — The median blood cobalt was 6.84 (0.24–90) ppb and median chromium level was 4.42 (0.20–45) ppb. The median Oxford hip score was 34 (5–48). The change in the gluteus minimus mean atrophy score between first and second MRI was 0.12 (p = 0.002). Mean change in the gluteus medius posterior portion (unaffected by surgical approach) was 0.08 (p = 0.01) and mean change in the inferior portion was 0.10 (p = 0.05). Mean pseudotumor grade increased by 0.18 (p = 0.02). Interpretation — Worsening muscle atrophy and worsening pseudotumor grade occur over a 1-year period in a substantial proportion of patients with MOM hip implants. Serial MRI helps to identify those patients who are at risk of developing worsening soft-tissue pathology. These patients should be considered for revision surgery before irreversible muscle destruction occurs