Directive metamaterial-based subwavelength resonant cavity antennas – Applications for beam steering

This article presents the use of composite resonant metamaterials for the design of highly directive subwavelength cavity antennas. These metamaterials, composed of planar metallic patterns periodically organized on dielectric substrates, exhibit frequency dispersive phase characteristics. Different models of metamaterial-based surfaces (metasurfaces), introducing a zero degree reflection phase shift to incident waves, are firstly studied where the bandwidth and operation frequency are predicted. These surfaces are then applied in a resonant Fabry–Perot type cavity and a ray optics analysis is used to design different models of ultra-compact high-gain microstrip printed antennas. Another surface presenting a variable reflection phase by the use of a non-periodic metamaterial-based metallic strips array is designed for a passive low-profile steering beam antenna application. Finally, the incorporation of active electronic components on the metasurfaces, allowing an electronic control of the phase responses, is applied to an operation frequency reconfigurable cavity and a beam steering cavity. All these cavity antennas operate on subwavelength modes, the smallest cavity thickness being of the order of λ/60. To cite this article: A. Ourir et al., C. R. Physique 10 (2009).     

Lower Bounds for the Eigenvalues of the Dirac Operator: Part II. The Submanifold Dirac Operator

In this paper we generalize the results of Part I to the submanifoldDirac operator. In particular, we give optimal lower bounds for thesubmanifold Dirac operator in terms of the mean curvature and othergeometric invariants as the Yamabe number or the energy-momentum tensor.In the limiting case, we prove that the submanifold is Einstein if thenormal bundle is flat.     

Robust Variable-Step Perturb-and-Observe Sliding Mode Controller for Grid-Connected Wind-Energy-Conversion Systems

In order to extract efficient power generation, a wind turbine (WT) system requires an accurate maximum power point tracking (MPPT) technique. Therefore, a novel robust variable-step perturb-and-observe (RVS-P&O) algorithm was developed for the machine-side converter (MSC). The control strategy was applied on a WT based permanent-magnet synchronous generator (PMSG) to overcome the downsides of the currently published P&O MPPT methods. Particularly, two main points were involved. Firstly, a systematic step-size selection on the basis of power and speed measurement normalization was proposed; secondly, to obtain acceptable robustness for high and long wind-speed variations, a new correction to calculate the power variation was carried out. The grid-side converter (GSC) was controlled using a second-order sliding mode controller (SOSMC) with an adaptive-gain super-twisting algorithm (STA) to realize the high-quality seamless setting of power injected into the grid, a satisfactory power factor correction, a high harmonic performance of the AC source, and removal of the chatter effect compared to the traditional first-order sliding mode controller (FOSMC). Simulation results showed the superiority of the suggested RVS-P&O over the competing based P&O techniques. The RVS-P&O offered the WT an efficiency of 99.35%, which was an increase of 3.82% over the variable-step P&O algorithm. Indeed, the settling time was remarkably enhanced; it was 0.00794 s, which was better than for LS-P&O (0.0841 s), SS-P&O (0.1617 s), and VS-P&O (0.2224 s). Therefore, in terms of energy efficiency, as well as transient and steady-state response performances under various operating conditions, the RVS-P&O algorithm could be an accurate candidate for MPP online operation tracking.     

Molecular structure investigation of Z-3N(2-ethoxyphenyl)-2-N′(2-ethoxyphenyl)-imino-thiazolidin-4-one by <Emphasis Type="Italic">ab initio</Emphasis>, DFT and X-ray diffraction methods

We report here the synthesis of the Z-3N(2-ethoxyphenyl)-2-N′(2-ethoxyphenyl)-imino-thiazolidin-4-one compound. The crystal structure is determined by X-ray diffraction. The compound crystallizes in the monoclinic system with the space group P2_1/n and cell parameters: a = 9.4094(10) Å, b = 9.3066(10) Å, c = 20.960(2) Å, β = 99.0375(10)°, V = 1812.7(3) ų and Z = 4. The structure is refined to final R = 0.05 for 2083 observed reflections. The molecule in the crystal exhibits the intermolecular hydrogen bonds of C–H…O, C–H…N, and C–H…S types. Ab initio calculations are also performed at Hartree–Fock (HF) and density functional theory (DFT) levels. The full HF and DFT geometry optimization is carried out using the 6-31G(d,p) basis set. The observed molecular structure is compared with that calculated by both HF and DFT methods. The optimized geometry of the titled compound is found to be consistent with the structure determined by X-ray diffraction.     

Nickel-catalyzed synthesis of nanoporous organic frameworks and their potential use in gas storage applications

The synthesis of highly nanoporous organic frameworks (NPOFs) has been established using nickel(0)-catalyzed Yamamoto coupling reactions, which has afforded highly porous polymers featuring remarkable chemical and thermal stability. Treatment of 1,3,5-tris(4-bromophenyl)benzene, 1,2,4,5-tetrakis(4-bromophenyl)benzene, or 1,3,5,7-tetrakis(4-iodophenyl)adamantane with Ni(cod)₂ in DMF at 80°C for 48 h afforded the nanoporous organic frameworks, NPOF-1, NPOF-2, and NPOF-3, respectively, as white powders in quantitative yields. All NPOFs are insoluble in common organic solvents such as dimethylformamide, tetrahydrofuran, toluene, dichloromethane, and methanol. The chemical composition and structural aspects of NPOFs were investigated by spectral and analytical methods while porosity was examined by nitrogen porosity measurements. In spite of their amorphous nature, NPOFs exhibit permanent porosity and high Langmuir surface areas (NPOF-1: 2,635 m² g^?1; NPOF-2: 4,227 m² g^?1; NPOF-3: 2,423 m² g^?1), which make them attractive for subsequent use in gas storage and separation applications, among others. The performance of NPOFs in hydrogen storage was evaluated at 1 bar and 77 K and revealed that these highly porous architectures can store up to 1.45 wt% of hydrogen.     

Tetramethylcyclobutadienecobalt(I) complexes containing pyrazolate or tetrazolate ligands with various coordination modes

Treatment of Cb^∗Co(CO)₂I (Cb^∗ = η⁴-C₄Me₄) with one equivalent of potassium 3,5-dimethylpyrazolate (Me₂pzK) or potassium 3,5-bis(trifluoromethyl)pyrazolate ((CF₃)₂pzK) in tetrahydrofuran at 0 °C afforded (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ and Cb^∗Co((CF₃)₂pz)(CO)₂ in 90 and 71% yields, respectively. Treatment of Cb^∗Co(CO)₂I with one equivalent of Me₂pzH followed by the addition of one equivalent Me₂pzK in tetrahydrofuran afforded Cb^∗Co(Me₂pzH)(Me₂pz)(CO) in 74% yield. The reaction of Cb^∗Co(CO)₂I with one equivalent of potassium phenyl tetrazolate (PhtetzK) in tetrahydrofuran at 0 °C afforded [Cb^∗Co(Phtetz)(CO)]₂ in 44% yield. The solid state structure of (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ revealed nucleophilic addition of a pyrazolate nitrogen atom to a Cb^∗ ligand carbon atom to afford a novel tetradentate ligand that bonds to the cobalt ion through an η³-π-allyl interaction and one pyrazolyl nitrogen atom. Two carbonyl ligands are also present. An X-ray crystal structure determination of Cb^∗Co((CF₃)₂pz)(CO)₂ showed η¹-coordination of the (CF₃)₂pz ligand and η⁴-coordination of the Cb^∗ ligand. The solid state structure of Cb^∗Co(Me₂pz)(Me₂pzH)(CO) is monomeric with one η¹-Me₂pz, one η¹-Me₂pzH, two carbonyl, and one η⁴-Cb^∗ ligands. The η¹-Me₂pz and η¹-Me₂pzH ligands are linked by a hydrogen bond involving the uncoordinated nitrogen atoms. The X-ray crystal structure determination of [Cb^∗Co(Phtetz)(CO)]₂ showed a dimeric molecular structure with two μ:η¹,η¹-Phtetz ligands connected to the cobalt ions through the 2- and 3-nitrogen atoms, one η⁴-Cb^∗ ligand, and one carbonyl ligand per cobalt center. (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ is highly volatile and sublimes at 60 °C/0.03 Torr.     

Luminescence enhancement and tuning via multiple cooperative supramolecular interactions in an ion-paired multinuclear complex

[(3,5-(CF(3))(2)Pz)(AgL)(2)](+)[Ag(5)(3,5-(CF(3))(2)Pz)(6)(CH(3)CN)](-) (L = 2-(N,N-diethylanilino-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine) shows bright and tunable emissions influenced by its supramolecular structure. Columnar stacks are assembled via cooperative interactions that include Ag(I)···Ag(I) argentophilic bonding, π···π stacking and Ag(I)···π interactions.     

Photochemistry of neutral isonitrile gold(I) complexes: modulation of photoreactivity by aurophilicity and pi-acceptance ability

This work demonstrates for the first time that aurophilicity and ligand pi-acceptance ability sensitize the photoreactivity of Au(I) complexes. Photolysis of LAu(I)Cl (L = RNC or CO) complexes leads to free L, Au(III), and Au(0) photoproducts. Solutions of (p-tosyl)CH(2)NCAuCl in dichloromethane undergo significant oligomerization leading to dimers and trimers with formation constants of 1.61 x 10(3) and 6.61 x 10(3) M(-1), respectively, representing the highest values reported to date for complexes that exhibit aurophilic association in solution. The photoproduct quantum yield (Phi) varies with the LAu(I)Cl concentration in solution. For (p-tosyl)CH(2)NCAuCl, metallic gold forms with Phi = 0.0065 and 0.032 in 4.0 x 10(-5) and 4.0 x 10(-3) M dichloromethane solutions, respectively. Meanwhile, irradiation of t-BuNCAuCl primarily produces t-BuNCAuCl(3) with Phi = 0.0045 and 0.013 for 5.0 x 10(-5) and 5.0 x 10(-3) M dichloromethane solutions, respectively. For Au(CO)Cl, metallic gold forms with Phi = 0.013 and 0.065 upon irradiation of 8.0 x 10(-5) and 8.0 x 10(-3) M dichloromethane solutions, respectively. Hence, *[LAuX](n) oligomeric species are more photoreactive than monomeric species. The results also demonstrate intuitive control of Phi via modulation of the pi-acceptance ability of L, as both follow CO > (p-tosyl)CH(2)NC > (alkyl)NC in LAuCl, a trend that is also commensurate with the relative long-term photosensitivity of the corresponding solids and solutions. A new method for preparing stable small gold nanoparticles is described based on the fundamental findings above. Thus, photolysis of different concentrations of LAuX in solutions containing a primary amine-terminated dendrimer leads to clear solutions exhibiting tunable visible plasmon absorptions of gold nanoparticles; these solutions maintain their colors and stability indefinitely. TEM measurements for representative samples prepared by photolysis of (p-tosyl)CH(2)NCAuCl solutions give rise to spherical nanoparticles as small as 5 nm.