Graphene terahertz modulators by ionic liquid gating

Graphene based THz modulators are promising due to the conical band structure and high carrier mobility of graphene. Here, we tune the Fermi level of graphene via electrical gating with the help of ionic liquid to control the THz transmittance. It is found that, in the THz range, both the absorbance and reflectance of the device increase proportionately to the available density of states due to intraband transitions. Compact, stable, and repeatable THz transmittance modulation up to 93% (or 99%) for a single (or stacked) device has been demonstrated in a broad frequency range from 0.1 to 2.5 THz, with an applied voltage of only 3 V at room temperature

Broadband lightweight flat lenses for longwave-infrared imaging

We experimentally demonstrate imaging in the longwave-infrared (LWIR) spectral band (8um to 12um) using a single polymer flat lens based upon multi-level diffractive optics. The device thickness is only 10{\mu}m, and chromatic aberrations are corrected over the entire LWIR band with one surface. Due to the drastic reduction in device thickness, we are able to utilize polymers with absorption in the LWIR, allowing for inexpensive manufacturing via imprint lithography. The weight of our lens is less than 100 times those of comparable refractive lenses. We fabricated and characterized two different flat lenses. Even with about 25% absorption losses, experiments show that our flat polymer lenses obtain good imaging with field of view of ~35degrees and angular resolution less than 0.013 degrees. The flat lenses were characterized with two different commercial LWIR image sensors. Finally, we show that by using lossless, higher-refractive-index materials like silicon, focusing efficiencies in excess of 70% can be achieved over the entire LWIR band. Our results firmly establish the potential for lightweight, ultra-thin, broadband lenses for high-quality imaging in the LWIR band

Análise do comportamento ao longo do tempo de peças de concreto armado submetidas a estados planos de tensão, através do método dos elementos finitos

O objetivo deste trabalho é formular um modelo, que permita a simulação do comportamento de peças de concreto armado submetidas a estados planos de tensão. O modelo proposto é baseado no método dos elementos finitos. São analisados estados de carga de curta e longa duração. O concreto é modelado como um material elasto-viscoplástico. Usa-se o conceito de camadas superpostas (overlay models). Deste modo, admite-se, por conveniência matemática, que o concreto é formado de várias camadas superpostas, que apresentam à mesma deformação. Considerando-se diferentes propriedades para cada camada, obtém-se um comportamento comporto que pode simular o funcionamento real do concreto. Para a fissuração do concreto, usa-se um modelo de fissuras distribuídas, que leva em conta a contribuição do concreto entre fissuras. A armadura é introduzida no modelo como uma linha de material mais rígido dentro do elemento de concreto. Os deslocamentos ao longo do elemento de armadura são referidos aos deslocamentos nodais do elemento de concreto. Assim, obtém-se uma matriz de rigidez para armadura com as mesmas dimensões que a matriz de rigidez do elemento de concreto. A matriz de rigidez do elemento de concreto armando é a soma destas duas matrizes de rigidez. Os resultados obtidos com este material computacional são comparados com valores determinados experimentalmente, indicando uma boa aproximação

Simple and rational methodology for the formulation of self-compacting concrete mixes

The increasing use of self-compacting concrete (SCC) in the construction industry should be assured by the development of mix designs adequate to improve their fresh/hardened state properties and their economy. This paper presents a methodology for the formulation of SCC that achieves some of these developmental goals without reliance on extensive laboratory testing and batch trials. Applications, results in the fresh and hardened state, and discussion of the SCC obtained are presented. The proposed method can provide lower costs when compared to a current SCC mix design method and the literature used for comparison.Peer ReviewedPostprint (author's final draft