Restoring observed classical behavior of the carbon nanotube field emission enhancement factor from the electronic structure

Experimental Fowler-Nordheim plots taken from orthodoxly behaving carbon nanotube (CNT) field electron emitters are known to be linear. This shows that, for such emitters, there exists a characteristic field enhancement factor (FEF) that is constant for a range of applied voltages and applied macroscopic fields $F_\text{M}$. A constant FEF of this kind can be evaluated for classical CNT emitter models by finite-element and other methods, but (apparently contrary to experiment) several past quantum-mechanical (QM) CNT calculations find FEF-values that vary with $F_\text{M}$. A common feature of most such calculations is that they focus only on deriving the CNT real-charge distributions. Here we report on calculations that use density functional theory (DFT) to derive real-charge distributions, and then use these to generate the related induced-charge distributions and related fields and FEFs. We have analysed three carbon nanostructures involving CNT-like nanoprotrusions of various lengths, and have also simulated geometrically equivalent classical emitter models, using finite-element methods. We find that when the DFT-generated local induced FEFs (LIFEFs) are used, the resulting values are effectively independent of macroscopic field, and behave in the same qualitative manner as the classical FEF-values. Further, there is fair to good quantitative agreement between a characteristic FEF determined classically and the equivalent characteristic LIFEF generated via DFT approaches. Although many issues of detail remain to be explored, this appears to be a significant step forwards in linking classical and QM theories of CNT electrostatics. It also shows clearly that, for ideal CNTs, the known experimental constancy of the FEF value for a range of macroscopic fields can also be found in appropriately developed QM theory.Comment: A slightly revised version has been published - citation below - under a title different from that originally used. The new title is: "Restoring observed classical behavior of the carbon nanotube field emission enhancement factor from the electronic structure

Modeling the Field Emission Enhancement Factor for Capped Carbon Nanotubes using the Induced Electron Density

In many field electron emission experiments on single-walled carbon nanotubes (SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates, with a macroscopic field FM applied between them. For any given location "L" on the SWCNT surface, a field enhancement factor (FEF) is defined as $F_{\rm{L}}$/$F_{\rm{M}}$, where $F_{\rm{L}}$ is a local field defined at "L". The best emission measurements from small-radii capped SWCNTs exhibit characteristic FEFs that are constant (i.e., independent of $F_{\rm{M}}$). This paper discusses how to retrieve this result in quantum-mechanical (as opposed to classical electrostatic) calculations. Density functional theory (DFT) is used to analyze the properties of two short, floating SWCNTS, capped at both ends, namely a (6,6) and a (10,0) structure. Both have effectively the same height ($\sim 5.46$ nm) and radius ($\sim 0.42$ nm). It is found that apex values of local induced FEF are similar for the two SWCNTs, are independent of $F_{\rm{M}}$, and are similar to FEF-values found from classical conductor models. It is suggested that these induced-FEF values relate to the SWCNT longitudinal system polarizabilities, which are presumed similar. The DFT calculations also generate "real", as opposed to ``induced", potential-energy (PE) barriers for the two SWCNTs, for FM-values from 3 V/$\mu$m to 2 V/nm. PE profiles along the SWCNT axis and along a parallel ``observation line" through one of the topmost atoms are similar. At low macroscopic fields the details of barrier shape differ for the two SWCNT types. Even for $F_{\rm{M}}=0$, there are distinct PE structures present at the emitter apex (different for the two SWCNTs); this suggests the presence of structure-specific chemically induced charge transfers and related patch-field distributions

On the quantum mechanics of how an ideal carbon nanotube field emitter can exhibit a constant field enhancement factor

Measurements of current-voltage characteristics from ideal carbon nanotube (CNT) field electron emitters of small apex radius have shown that these emitters can exhibit a linear Fowler-Nordheim (FN) plot [e.g., Dean and Chalamala, Appl. Phys. Lett., 76, 375, 2000]. From such a plot, a constant (voltage-independent) characteristic field enhancement factor (FEF) can be deduced. Over fifteen years later, this experimental result has not yet been convincingly retrieved from first-principles electronic structure calculations, or more generally from quantum mechanics (QM). On the contrary, several QM calculations have deduced that the characteristic FEF should be a function of the macroscopic field applied to the CNT. This apparent contradiction between experiment and QM theory has been an unexplained feature of CNT emission science, and has raised doubts about the ability of existing QM models to satisfactorily describe experimental CNT emission behavior. In this work we demonstrate, by means of a density functional theory analysis of single-walled CNTs "floating" in an applied macroscopic field, the following significant result. This is that agreement between experiment, classical-conductor CNT models and QM calculations can be achieved if the latter are used to calculate (from the "real" total-charge-density distributions initially obtained) the distributions of $\textit{induced}$ charge-density, induced local fields and induced local FEFs. The present work confirms, more reliably and in significantly greater detail than in earlier work on a different system, that this finding applies to the common "post-on-a-conducing plane" situation of CNT field electron emission. This finding also brings out various further theoretical questions that need to be explored

Experimentos de contrôle do Phytophthora infestans nas condições de Pelotas e São Lourenço do Sul, no Rio Grande do Sul

The Plant Pathology and Horticulture Sections of the IPEAS, in collaboration with the IRI, have carried out eight experiments for comparison of fungicides in the control of Phytophthora infestans (Mont.) De Bary, the agent of the mildew of potato (Solanum tuberosum L.). The tests were made during the period from 1965 to 1967. All of the fungicides tested showed capacity for the control of the disease, even when a small number of applications were made. It was not possible to analyze statistically the combined data of production from all the experiments. Nevertheless, the phytopathological observations made during the course of the work made clear that there was a slight superiority of the fungicides Manzate D, Dithane M-45 and Miltox as compared with the other fungicides. Thus, it was demonstrated that the efficiency of presently available fungicides is adequate for use in chemical prophylactic treatment, however, it is conditioned by other factors that are described in the text and in previous research work.As Seções de Fitopatologia e de Horticultura do Instituto de Pesquisas e Experimentação Agropecuárias do Sul, em colaboração com o IRI, realizaram oito experimentas de competição entre fungicidas, no período de 1965 a 1967, visando controlar o Phytophthora infestans (Mont.) De Bary, agente do míldio da batata (Solanum tuberosum, L.). Todos os fungicidas testados demonstraram capacidade de contrôle da moléstia, mesmo com pequeno número de pulverizações. Não foi possível analisar estatisticamente os dados de produção do conjunto de experimentos. Entretanto, as observações fitopatológicas, efetuadas no transcurso dos trabalhos, deixaram transparecer uma leve superioridade dos fungicidas Manzate D, Dithane M-45 e Miltox, sôbre os demais. Desta forma, ficou demonstrado não constituir problema o tratamento químico profilático do míldio da batata, no que concerne à eficiência de fungicidas. A viabilidade dos tratamentos, entretanto, fica condicionada a outros fatôres, relacionados no texto e em trabalhos anteriores

Prevention of hypertension in patients with pre-hypertension: protocol for the PREVER-prevention trial

<p>Abstract</p> <p>Background</p> <p>Blood pressure (BP) within pre-hypertensive levels confers higher cardiovascular risk and is an intermediate stage for full hypertension, which develops in an annual rate of 7 out of 100 individuals with 40 to 50 years of age. Non-drug interventions to prevent hypertension have had low effectiveness. In individuals with previous cardiovascular disease or diabetes, the use of BP-lowering agents reduces the incidence of major cardiovascular events. In the absence of higher baseline risk, the use of BP agents reduces the incidence of hypertension. The PREVER-prevention trial aims to investigate the efficacy, safety and feasibility of a population-based intervention to prevent the incidence of hypertension and the development of target-organ damage.</p> <p>Methods</p> <p>This is a randomized, double-blind, placebo-controlled clinical trial, with participants aged 30 to 70 years, with pre-hypertension. The trial arms will be chlorthalidone 12.5 mg plus amiloride 2.5 mg or identical placebo. The primary outcomes will be the incidence of hypertension, adverse events and development or worsening of microalbuminuria and of left ventricular hypertrophy in the EKG. The secondary outcomes will be fatal or non-fatal cardiovascular events: myocardial infarction, stroke, heart failure, evidence of new sub-clinical atherosclerosis, and sudden death. The study will last 18 months. The sample size was calculated on the basis of an incidence of hypertension of 14% in the control group, a size effect of 40%, power of 85% and P alpha of 5%, resulting in 625 participants per group. The project was approved by the Ethics committee of each participating institution.</p> <p>Discussion</p> <p>The early use of blood pressure-lowering drugs, particularly diuretics, which act on the main mechanism of blood pressure rising with age, may prevent cardiovascular events and the incidence of hypertension in individuals with hypertension. If this intervention shows to be effective and safe in a population-based perspective, it could be the basis for an innovative public health program to prevent hypertension in Brazil.</p> <p>Trial Registration</p> <p>Clinical Trials <a href="http://www.clinicaltrials.gov/ct2/show/NCT00970931">NCT00970931</a>.</p

Desatando a trama das redes assistenciais sobre drogas: uma revisão narrativa da literatura

Acalorados debates sobre determinados modelos de tratamento para usuários de drogas ocorrem no âmbito da academia, das políticas públicas, além da mídia. A rede assistencial sobre drogas é apresentada neste contexto como um importante mecanismo, mas sua construção torna-se um desafio. Assim, realizou-se uma análise crítica da literatura acadêmica acerca das redes assistenciais sobre drogas, na forma de uma revisão narrativa, visando levantar seus desafios e possibilidades de consolidação. Os resultados encontrados foram: a) uma escassez de estudos específicos sobre a rede assistencial sobre drogas; b) cobertura insuficiente e desintegrada frente à demanda de tratamento; c) necessidade de se repensar o papel dos Centros de Atenção Psicossocial para Álcool e outras Drogas, visando seu fortalecimento, expansão, melhoria estrutural e readequação de práticas; d) ausência de análises críticas sobre o processo de construção dos modelos assistenciais sobre drogas nos serviços públicos; e, e) responsabilidade do Estado em fornecer melhores alternativas ao panorama encontrado, avançando no fortalecimento das ações intersetoriais, articulação do cuidado e no aprimoramento das condições de trabalho