Mechanical Response of Hollow Metallic Nanolattices: Combining Structural and Material Size Effects

Ordered cellular solids have higher compressive yield strength and stiffness compared to stochastic foams. The mechanical properties of cellular solids depend on their relative density and follow structural scaling laws. These scaling laws assume the mechanical properties of the constituent materials, like modulus and yield strength, to be constant and dictate that equivalent-density cellular solids made from the same material should have identical mechanical properties. We present the fabrication and mechanical properties of three-dimensional hollow gold nanolattices whose compressive responses demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. All nanolattices had octahedron geometry, a constant relative density, ρ ∼ 5%, a unit cell size of 5–20 μm, and a constant grain size in the Au film of 25–50 nm. Structural effects were explored by increasing the unit cell angle from 30 deg to 60 deg while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200 nm to 635 nm, at a constant relative density and grain size. In situ uniaxial compression experiments revealed an order of magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of three-dimensional architected metamaterials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics

Desarrollo de membranas híbridas dieléctricas de tipo Nafión con óxidos de zirconio, hafnio y lantano

En el presente trabajo se realizó la síntesis de membranas híbridas tipo Nafion/(MxOy)n donde M=Zr, Hf o La y n=1wt%. Las membranas fueron preparadas mediante el proceso Sol-Gel. Tres nuevas membranas fueron obtenidas con espesores de 61, 67 y 300 μm respectivamente. Cada membrana fue caracterizada mediante espectroscopia infrarroja, análisis termogravimétrico y microscoía electrónica de barrido. Obteniendo que las membranas híbridas presentan mayor resistencia térmica que el polimero por si solo adquiriendo 40ºC más, lo que da como resultado una temperatura de trabajo de 120ºC, además en los espectros de infrarrojo se evidencia la presencia del efecto del material inorgánico sobre el incremento en el carácter autohumidificante de la membrana híbrida, finalmente la microscopía electrónica de barrido nos permite identificar la obtención de la membrana híbrida mediante las micrografías así como el espesor de las mismas

Lorentz violating electrodynamics

After summarizing the most interesting results in the calculation of synchrotron radiation in the Myers-Pospelov effective model for Lorentz invariance violating (LIV) electrodynamics, we present a general unified way of describing the radiation regime of LIV electrodynamics which include the following three different models : Gambini-Pullin, Ellis et al. and Myers-Pospelov. Such unification reduces to the standard approach of radiation in a dispersive and absortive (in general) medium with a given index of refraction. The formulation is presented up to second order in the LIV parameter and it is explicitly applied to the synchrotron radiation case.Comment: 11 pages, extended version of the talk given by L.F. Urrutia in the VI Mexican School: Approaches to Quantum Gravity, Playa del Carmen, Mexico, Nov. 2004. Minor chages in the text and added reference

Radiation in Lorentz violating electrodynamics

Synchrotron radiation is analyzed in the classical effective Lorentz invariance violating model of Myers-Pospelov. Within the full far-field approximation we compute the electric and magnetic fields, the angular distribution of the power spectrum and the total emitted power in the m-th harmonic, as well as the polarization. We find the appearance of rather unexpected and large amplifying factors, which go together with the otherwise negligible naive expansion parameter. This opens up the possibility of further exploring Lorentz invariance violations by synchrotron radiation measurements in astrophysical sources where these amplifying factors are important.Comment: Presented at the Second Mexican Meeting on Theoretical and Experimental Physics, El Colegio Nacional, Mexico City, 6-10 September 200

Phenomenological description of quantum gravity inspired modified classical electrodynamics

We discuss a large class of phenomenological models incorporating quantum gravity motivated corrections to electrodynamics. The framework is that of electrodynamics in a birefringent and dispersive medium with non-local constitutive relations, which are considered up to second order in the inverse of the energy characterizing the quantum gravity scale. The energy-momentum tensor, Green functions and frequency dependent refraction indices are obtained, leading to departures from standard physics. The effective character of the theory is also emphasized by introducing a frequency cutoff. The analysis of its effects upon the standard notion of causality is performed, showing that in the radiation regime the expected corrections get further suppressed by highly oscillating terms, thus forbiding causality violations to show up in the corresponding observational effects.Comment: 14 pages, to be published in Obregon Festschrift 2006, Gen. Rel. and Gra

Perturbative Hamiltonian constraints for higher order theories

We present a method for constructing a consistent low energy canonical formalism for higher order time-derivative theories, extending the Dirac method to include perturbative Hamiltonian constraints. We apply it to two paradigmatic examples: the Pais-Uhlenbeck oscillator and the Bernard-Duncan scalar field. We also compare the results, both at the classical and quantum level, with the ones corresponding to a direct perturbative construction applied to the exact higher order theory. This comparison highligths the soundness of the present formalism.Comment: 26 pages, 4 figures; review section shortened and appendices change

Determinación de aflatoxinas en maíz y productos derivados por cromatografía líquida de alta resolución

A study is pressented about the extraction and analysis of aflatoxins in contaminated corn and corn products. The extraction is carried out in a single step grounding the sample, packing it in a glass column and pouring choloform through it. On this way high recoveries are obtained. The quantitation is made by reserve phase H.P.L.C. using fluorescence detection of derivatives of the aflatoxins.Se presenta un estudio sobre la extracción de aflatoxinas de maíz y productos derivados contaminados, así como su análisis por cromatografía líquida de alta resolución. La extracción se lleva a cabo en un sólo paso moliendo la muestra, empacándola en una columna y percolando cloroformo a través de ella. En esta forma se logran unos porcentajes de recuperación mucho más altos que por los métodos tradicionales. El análisis se efectuó por cromatografía líquida en fase inversa. y detección por fluorescencia. con formación pre-via de derivados de las aflatoxinas