Fractional order models for the homogenization and wave propagation analysis in periodic elastic beams

Meccanica - The advancement of manufacturing techniques has led to a rapid increase in the design and fabrication of periodic and architectured materials for a variety of dynamics and wave...     

Application of variable- and distributed-order fractional operators to the dynamic analysis of nonlinear oscillators

Nonlinear Dynamics - Many physical processes in nature exhibit complex dynamics that result from a combination of multiscale, nonlinear, non-local, and memory effects. Recent experimental...     

Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C−O Bond Cleavage

The high catalytic reactivity of homoleptic tris(alkyl) lanthanum La{C(SiHMe₂)₃}₃ is highlighted by C?O bond cleavage in the hydroboration of esters and epoxides at room temperature. The catalytic hydroboration tolerates functionality typically susceptible to insertion, reduction, or cleavage reactions. Turnover numbers (TON) up to 10 000 are observed for aliphatic esters. Lanthanum hydrides, generated by reactions with pinacolborane, are competent for reduction of ketones but are inert toward esters. Instead, catalytic reduction of esters requires activation of the lanthanum hydride by pinacolborane.     

Noise Reduction in Non-Premixed Lifted-Jet Flames

The characteristic changes in non-premixed lifted flames when excited by hole tones from a cavity, placed in the flow path of the fuel gas, were studied. A significant reduction of the sound pressure level was observed in the low-frequency noise at the flame base of the lifted flame when the hole tones were induced in the jet. The liftoff height and the mean diameter of the flame base decreased for a given jet Reynolds number. The blow-off velocities also increased suggesting improved flame stability in the presence of the hole tones induced by the cavity. Incorporation of the cavity upstream of a burner nozzle is demonstrated to give a quieter lifted flame with improved stability characteristics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Donor-Acceptor Complexes in Copolymerization. XLIV. Copolymerization of Styrene and α-Methylstyrene with Acrylic and α-Substituted Acrylic Esters in the Presence of Aluminum Compounds

Abstract

The copolymerization of styrene (S) with methyl acrylate (MA) and with methyl methacrylate (MMA) in the presence of AlEt₃ yields equimolar, alternating copolymers while no polymer is formed in α-methylstyrene (MS)-MA and MS-MMA systems. In the presence of AlEt_1.5Cl_l,5 (EASC), S-MA and S-MMA yield alternating copolymers, S-methyl a-chloroacrylate (MCA), MS-MA and MS-MMA yield a mixture of alternating and cationic polymers, and MS-MCA yields cationic polymer only. In the presence of A1C1₃, S-MA and MS-MA yield a mixture of alternating and cationic polymers and S-MMA and MS-MMA yield cationic polymer only. The cotacticity distributions of the alternating S-MA and S-MMA copolymers prepared in the presence of AlEt₃, EASC, and A1C1, are the same; the coisotactic, co-heterotactic, and cosyndiotactic fractions being approximately in the ratio 1:2:1. The cosyndiotactic fractions of the alter-nating copolymers prepared in the presence of EASC are in the order MS-MMA > MS-MA > S-MCA > S-MMA=S-MA.     

Donor-Acceptor Complexes in Copolymerization. XXVI. Effect of Solvent,Temperature, and Complexing Agent on the Polymerization of Comonomer Charge Transfer Complexes

The copolymerization of styrene with methyl methacrylate (S/MMA = 4/1) or acrylonitrile (S/AN = 1/1) in the presence of ethylaluminum sesquichloride (EASC) yields 1/1 copolymer in toluene or chlorobenzene. In chloroform the S-MMA-EASC polymerization yields 60/40 copolymer while the S-AN-EASC polymerization yields 1/1 copolymer. In the presence of EASC, styrene-α-chloroacrylonitrile yields 1/1 copolymer (DMF or DMSO), S-AN yields 1/1 copolymer (DMSO) or radical copolymer (DMF), S-MMA yields radical copolymer (DMF or DMSO), α-methylstyrene-AN yields radical copolymer (DMSO) or traces of copolymer (DMF), and α-MS-methacrylo-nitrile yields traces of copolymer (DMSO) or no copolymer (DMF). When zinc chloride is used as complexing agent in DMF or DMSO, none of the monomer pairs undergoes polymerization. However, radical catalyzed polymerization of isoprene-AN-ZnCl₂ in DMF yields 1/1 alternating copolymer. The copolymerization of S/MMA in the presence of EASC yields 1/1 alternating copolymer up to 100°C, while the copolymerization of S/AN deviates from 1/1 alternating copolymer above 50°C. The copolymerization of S/MMA deviates from 1/1 copolymer at MMA/EASC mole ratios above 20 while the copolymerization of S/AN deviates from 1/1 copolymer at MMA/EASC ratios above 50.     

Kinetics of Polymerization of Methyl Methacrylate initiated by AIBN in the Presence of Acetophenone

The present investigation aims at estimating the retarding effect of acetophenone in the polymerization of methyl methacrylate initiated by α,α' -azobisisobutyronitrile in the temperature range of 50 to 80°C. The results are interpreted in terms of Tüdös kinetic parameter (β). The effects of varying concentrations of substrate, monomer, initiator, and salts have been investigated. A suitable reaction scheme and rate expression have been suggested on the basis of the experimental findings, and another kinetic parameter (K) to represent the reactivity of acetophenones toward the polymer radical has been obtained graphically.     

Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11 nm and nanodiscs of dimensions ∼2.5×15 nm² were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5 nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.     

Magnetoelectric behavior of ferrimagnetic BixCo2−xMnO4 (x=0, 0.1 and 0.3) thin films

Thin films of Bi_xCo_2−xMnO₄ (x=0, 0.1 and 0.3) were grown on quartz, LaAlO₃ (LAO) and YBa₂Cu₃O₇ (YBCO) buffer layer coated LAO substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) and Raman scattering measurements showed that the thin films exhibit single phase polycrystalline cubic spinel structure on all the substrates. Near edge X-ray absorption fine structure (NEXAFS) studies confirmed the octahedral occupancy of the substituted Bi³⁺ ions. Temperature dependent zero-field cooled (ZFC) magnetization measurements show the ferrimagnetic (FM) behavior (T_C∼186 K) and magnetization undergoes a crossover from positive to negative, owing to the opposite contributions of magnetic moments from Co and Mn ions. A weak ferroelectric property exhibited by the films above room temperature was evidenced through the capacitance-voltage (C-V) and dielectric measurements. Magnetoelectric coupling was found to be maximum just below FM-T_C.