Modification of Non-Hydrolytic Sol-Gel Derived Alumina by Solvent Treatments

The effect of wetting non-hydrolytic derived alumina xerogels with water and organic solvents in the 20–70°C range on the alumina's properties was investigated. Wetting with organic solvents does not affect the alumina. However, contact with water was found to change the sharp crystallization at 800°C to a continuous crystallization starting at 450°C. Water treatment for a day at room temperature (RT) followed by second calcination decreased the surface area by 10%. This decrease in surface area is less pronounced with increasing wetting periods. On the other hand water treatment at 50–70°C followed by a second calcination resulted in a surface area increase of up to 15%. Upon water treatment the total pore volume has decreased from 0.65 (cm³/gr) to 0.48 (cm³/gr) and the average pore size decreased from 6.8 nm to 4.1 nm. The Cl content was found to be uneffected by the water treatment, remaining at 2.5% wt. Wetting with water at elevated temperature (70°C) accelerated the morphological changes, eliminating the crystallization peak at 800°C in one hour. A dissolution-reprecipitation mechanism is suggested to explain the results. In addition, Mass-Spectroscopy of the effluent gas during heat treatment revealed the emission of CO₂ and water upon phase transition into -Al₂O₃, at 1150–1300°C.     

Effect of intramolecular crosslinker properties on the mechanochemical fragmentation of covalently folded polymers

The mechanochemical stability of polymers in solution is enhanced if the chains are covalently folded. Under shear forces, the additional bonds absorb mechanical energy and inhibit unfolding, and as a result, slow down fragmentation. However, not all crosslinkers are equal in terms of their properties (length, strength, etc.). In order to understand the role of these added bonds in the polymers' stability under mechanical stress, a thorough study compares the rate of mechanochemistry on single-chain polymer nanoparticles which have been folded with crosslinkers with different lengths, strengths, positioning, and valencies. The usage of bonds with different mechanical strengths in the crosslinkers was found to be the most powerful way to change the mechanochemical fragmentation rate. In addition, positioning and valency also play significant role in the mechanical stabilization mechanism. © 2020 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2020 © 2020 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2020 , 58, 692–703     

Multiple coupling of surface plasmons in quasiperiodic gratings

Whereas periodic gratings enable us to couple light into a surface plasmon polariton only at a specific angle and wavelength, we show here that quasiperiodic gratings enable the coupling of light at multiple wavelengths and angles. The quasiperiodic grating can be designed in a systematic manner using the dual-grid method, thereby enabling us to control the coupling strength and grating dimensions. We verified the method experimentally by efficiently coupling light into a surface plasmon from several different illumination angles using a single quasiperiodic grating.     

Buzzwords on their way to a tipping‐point: A view from the blogosphere

“Buzzwords” are new words (i.e., neologisms) that enter the language and acquire great popularity as “fashion words.” To date, the dynamic aspect of buzzwords has not been the subject of a rigorous scientific analysis. In this study, we analyzed the appearance of buzzwords in the blogosphere and compared their dynamics to the one of nonpopular neologisms and well‐established words. It was found that the growth rate of buzzwords is exponential and higher than that of the blogosphere. Moreover, we have identified general early warning signals for an approaching tipping point in the dynamics of buzzwords indicating that as fashion words they are on their way to a tipping‐point of decline. The article presents these findings and others and concludes by presenting a model for studying the dynamics of new words entering the language. © 2010 Wiley Periodicals, Inc. Complexity 16: 58‐68, 2011     

Automatic locking of a parametrically resonating,base-excited,nonlinear beam

Nonlinear Dynamics - Described is a closed-loop control scheme capable of stabilizing a parametrically excited nonlinear structure in several vibration modes. By setting the relative phase between...     

Scheduling precedence graphs of bounded height

The existence of a schedule for a partially ordered set of unit length tasks on m identical processors is known to be NP-complete (J. D. Ullman, NP-complete scheduling problems, J. Comput. System Sci., 10 (1975), 384–393). The problem remains NP-complete even if we restrict the precedence graph to be of height bounded by a constant. (J. K. Lenkstra and A. H. G. Rinnooy Kan, Complexity of scheduling under precedence constraints, Operations Res., 26 (1978), 22–35; D. Dolev and M. K. Warmuth, “Scheduling Flat Graphs,” IBM Research Report RJ 3398, 1982). In these NP-completeness proofs the upper bound on the number of available processors varies with the problem instance. We present a polynomial algorithm for the case where the upper bound on the number of available processors and the height of the precedence graph are both constants.     

Linear and nonlinear optical properties of MgO:LiTaO3

We derive new temperature dependent Sellmeier equations for the extra-ordinary and ordinary refractive indices of 0.5% MgO-doped stoichiometric LiTaO₃ crystal. The equations are based on quasi phase-matched frequency conversion measurements, as well as on interferometric measurements of the thermal expansion and thermal dispersion. These equations fit experimental data over wide spectral ranges: 0.35–6 μm for the extra-ordinary wave and 0.375–3.75 μm for the ordinary wave, from room temperature up to 200°C. The nonlinear optical measurements set lower-limit values for the d ₃₃, d ₂₂, and d ₂₄ elements of the second-order susceptibility tensor χ ⁽²⁾ of 12.9, 1.54, and 0.46 pm/V, respectively. The interferometric measurements enable one to determine the linear and quadratic expansion coefficients of 1.45762e?5 1/°C, and 2.68608e?8 (1/°C)², respectively. The Sellmeier equations are in good agreement with previously published data.     

Black Hole Evaporation Entails an Objective Passage of Time

Time's apparent passage has long been debated by philosophers, with no decisive argument for or against its objective existence. In this paper we show that introducing the issue of determinism gives the debate a new, empirical twist. We prove that any theory that states that the basic laws of physics are time-symmetric must be strictly deterministic. It is only determinism that enables time reversal, whether theoretical or experimental, of any entropy-increasing process. A contradiction therefore arises between Hawking's [1] argument that physical law is time-symmetric and his controversial claim [2] that black-hole evaporation introduces a fundamental unpredictability into the physical world. The latter claim forcibly entails an intrinsic time-arrow independent of boundary conditions. A simulation of a simple system under time reversal shows how an intrinsic time arrow re-emerges, destroying the time reversal, when even the slightest failure of determinism occurs. This proof is then extended to the classical behavior of black holes. We conclude with pointing out the affinity between time's arrow and its apparent passage.     

Airy beam laser

A method to design lasers that emit an arbitrary beam profile is studied. In these lasers, output-coupling is performed by a diffraction grating that imposes a phase and amplitude distribution onto the diffracted light. A solid-state laser emitting beams with a two-dimensional Airy intensity profile is demonstrated both theoretically and experimentally. In this case, the diffraction grating adds a transverse cubic phase to the diffracted light. An Airy beam is obtained by performing optical Fourier transform of the out-coupled light. The laser beam profile and power characteristics are shown to agree with theory.