A one-dimensional Fermi accelerator model with moving wall described by a nonlinear van der Pol oscillator

A modification of the one-dimensional Fermi accelerator model is considered in this work. The dynamics of a classical particle of mass $m$, confined to bounce elastically between two rigid walls where one is described by a non-linear van der Pol type oscillator while the other one is fixed, working as a re-injection mechanism of the particle for a next collision, is carefully made by the use of a two-dimensional non-linear mapping. Two cases are considered: (i) the situation where the particle has mass negligible as compared to the mass of the moving wall and does not affect the motion of it; (ii) the case where collisions of the particle does affect the movement of the moving wall. For case (i) the phase space is of mixed type leading us to observe a scaling of the average velocity as a function of the parameter ($\c{hi}$) controlling the non-linearity of the moving wall. For large $\c{hi}$, a diffusion on the velocity is observed leading us to conclude that Fermi acceleration is taking place. On the other hand for case (ii), the motion of the moving wall is affected by collisions with the particle. However due to the properties of the van der Pol oscillation, the moving wall relaxes again to a limit cycle. Such kind of motion absorbs part of the energy of the particle leading to a suppression of the unlimited energy gain as observed in case (i). The phase space shows a set of attractors of different periods whose basin of attraction has a complicate organization

One-dimensional Silicon and Germanium Nanostructures With No Carbon Analogues

In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that the structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting possible applications in sensor devices. They also present high Young's modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help the identification of these new structures

Thermodynamic Equilibria in Carbon Nitride Photocatalyst Materials and Conditions for the Existence of Graphitic Carbon Nitride g-C3N4

We quantify the thermodynamic equilibrium conditions that govern the formation of crystalline heptazine-based carbon nitride materials, currently of enormous interest for photocatalytic applications including solar hydrogen evolution. Key phases studied include the monomeric phase melem, the 1D polymer melon, and the hypothetical hydrogen free 2D graphitic carbon nitride phase "g-C3N4". Our study is based on. density functional theory including van der Waals dispersion terms with different experimental conditions represented by the chemical potential of NH3. Graphitic carbon nitride is the subject of a vast number of studies, but its existence is still controversial. We show that typical conditions found in experiments pertain to the polymer melon (2D planes of 1D hydrogen-bonded polymer strands). In contrast, equilibrium synthesis of heptazine (h)-based g-h-C3N4 below its experimentally known decomposition temperature requires much less likely conditions, equivalent to low NH3 partial pressures around 1 Pa at 500 degrees C and around 10(3) Pa even at 700 degrees C. A recently reported synthesis of triazine (t)-based g-t-C3N4 in a salt melt is interpreted as a consequence of the altered local chemical environment of the C3N4 nanocrystallites

Author Correction: Text-mined dataset of inorganic materials synthesis recipes.

An amendment to this paper has been published and can be accessed via a link at the top of the paper

Rational design of carbon nitride photocatalysts by identification of cyanamide defects as catalytically relevant sites

The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ molecular heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant 'defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively, compared with the unmodified melon. Computational modelling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the separation of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts

Aceleração de Fermi em bilhares com fronteiras dependentes do tempo descritas por osciladores não lineares: caso conservativo e dissipativo

Neste trabalho estudamos dois bilhares com fronteira móvel cuja perturbação temporal é dada por um oscilador van der Pol. Estudamos um bilhar unidimensional e outro bidimensional na qual uma ou mais partículas clássicas de massa m não interagentes são confnadas ao interior da fronteira que defne o bilhar. Investigando algumas propriedades dinâmicas e estatísticas da partícula em função do parâmetro X que controla o termo não linear e o parâmetro y0 que controla a amplitude do oscilador de van der Pol. O bilhar unidimensional consiste em duas paredes rígidas, em que uma delas é móvel centrada na origem regida pelo oscilador de van der Pol e a outra xa em L. Descrevemos todos os procedimentos para construção do mapeamento que fornece a dinâmica da partícula, assim como as equações necessárias que defnem o movimento da parede móvel. O espaço de fases, o expoente de Lyapunov e a velocidade média são obtidos para diferentes valores de parâmetros de controle. Para o caso em que massa da partícula (mp) é muito menor que a massa da parede móvel (mw), m = mp=mw ' 0, podemos dividir o regime dinâmico em função do parâmetro c em dois tipos: (i) que recupera os resultados do modelo Fermi- Ulam; e (ii) no qual é observado um regime de crescimento da velocidade média nal. Para o caso em que m 6= 0, as colisões da partícula com a parede móvel perturbam o movimento da parede móvel e o sistema se torna dissipativo. Neste caso a dinâmica da partícula tende a pontos xos de forma assintótica passando por um transiente inicial. Para este caso construímos a bacia de atração e a frequência do número de períodos de um conjunto de condições iniciais. Para o bilhar bidimensional, construímos um modelo em que a fronteira é do tipo ovoide, analisamos o caso estático e o móvel regida pelo oscilador de van der Pol...Some dynamical properties for an ensemble of non-interacting particles con ned in a billiard with a time-dependent boundary are studied. The boundary is given by van der Pol oscillator and two cases are considered namely: (i) one-dimensional and (ii) twodimensional dynamics. For the one-dimensional case, we considered the dynamics of classical particle of mass m con ned to bounce between two rigid walls. One of them is xed at a distance L from the average position of the rst that uctuates according to a van der Pol oscillator. We consider the case where the mass of the particle is su ciently small as compared to the mass of the moving wall. Then we investigate some properties of the phase space including the average velocity of the particle. Our results reveal a scaling invariance for the nal average velocity, i.e., when n!¥. We discuss also the case when the mass of the particle is a fraction of the mass of the moving wall therefore showing the system now shows features of dissipative model. This is characterized speci cally by the presence of attractors in the phase space. For the two-dimensional case, we considered the dynamics of a classical particle of mass m where the particle is con ned to bounce inside a billiard whose boundary is of ellipticaloval like shape. First we analyze the static case. Second we consider the case where the boundary moves according to a van der Pol oscillator. We discuss the model in a similar way as made for the 1-D case including very small mass of the particle (m = 0) and m 6= 0. Dynamical properties for the particle were obtained like the behavior of the average velocity therefore demonstrating that unlimited energy gain is in course, as predicted by the LRA conjecture For the case of... (Complete abstract click electronic access below)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP