Acoustic characterization of ultrasonic transducer materials: I. Blends of rigid and flexible epoxy resins used in piezocomposites

Five different types of Dow epoxy blends were studied with differential scanning calorimetry. Each epoxy blend consists of a flexible epoxy resin, DER 736 or DER 732, with one of four different rigid epoxies, DER 317, DER 331, DER 332 and DEN 431, crosslinked by 1,4-bis (3-aminopropyl)-piperazine (BAPP). Longitudinal and shear wave acoustic properties of these epoxy resin blends as a function of the composition were measured at room temperature and an ultrasonic frequency range. All moduli and Poisson's ratios were calculated by using longitudinal velocity, shear velocity and density. These systems all have similar behavior. By increasing the amount of flexible epoxy in a mixture of hard and flexible epoxies a critical composition was found (∼50% by weight for DER 736 and ∼30% for DER 732), below which the acoustic properties, the elastic moduli and Poisson's ratios kept an almost constant value, and above which the velocities, impedances and all moduli decreased, whereas attenuations and Poisson's ratios increased gradually. Many epoxy materials ranging from soft and flexible to hard and rigid have been obtained by epoxy blend technology. Some of them have been applied to make 1–3 composites for medical ultrasound transducers.     

Threshold electron excitation of azulene

Intense energy absorption at 2.44 eV is observed in the threshold electron excitation spectrum of azulene vapor. Direct excitation of triplet states or an electron-molecule resonance may account for this striking feature.     

Oxoselenido clusters of the lanthanides: rational introduction of oxo ligands and near-IR emission from Nd(III)

Reactions of Ln(SePh)3 with SeO2 in THF give octanuclear oxoselenido clusters with the general formula (THF)8Ln8O2Se2(SePh)16 (Ln = Ce, Pr, Nd, Sm). In this isomorphous series, the eight Ln(III) ions are connected in the center by a pair of mu3-O2- ligands and mu5-Se2- ligands, with 14 bridging and two terminal selenolate ligands capping the cluster surface. Thermal decomposition at 700 degrees C of the Nd compound in vacuo led to the formation of a phase mixture of NdSe2, Nd2Se3, and Nd2O3. Near-IR emission experiments on the (THF)8Nd8O2Se2(SePh)16 and the fluorinated thiolate compound (DME)2Nd(SC6F5)3 demonstrate that clusters with oxo ligands are not only highly emissive, but also they emit at wavelengths not found in conventional oxides.     

光学特异材料的设计(英文)

In this contribution we review our latest achievements of combined experimental and theoretical studies to tailor the properties of optical metamaterials(MMs) at will. We give three examples of metamaterial designs that have been realized by means of electron-beam lithography and whose spectroscopic characteristics have been comprehensively investigated. In every case, our experiments are complemented by rigorous numerical simulations. Particular emphasis is put on the significance of such tailored effectiv...     

Lanthanide clusters with internal Ln: fragmentation and the formation of dimers with bridging Se(2-) and Se2(2-) ligands

Reactions of "LnI(x)(SePh)(3-x)" (Ln = Dy, Ho) with elemental S/Se give (THF)14Ln10S6(Se2)6I6. The compounds are composed of a Ln6S6 double cubane core, with two twisted "Ln2(SeSe)3" units condensed onto opposing rectangular sides of the Ln6S6 fragment. This deposition of Ln2Se6 totally encapsulates the two central Ln's with chalcogen atoms (four S and four Se atoms), excluding neutral THF donors or iodides from the two primary coordination spheres. Reactions of Ln(10) clusters with a stronger Lewis base result in fragmentation and, in the case of Ln = Er, the isolation of (py)6Er2(Se2)(S0.8Se0.2)I2, with two Ln(III) ions spanned by E2- and (EE)2- ligands. The related homochalcogen dimers (py)6Ln2(Se2)(Se)Br2 (Ln = Ho, Yb) were prepared to establish that such molecules could be prepared rationally, and to confirm the isolability of E2- ligands coordinated to only two sterically unconstrained Ln ions.     

Condensation polymerization of oligomeric polydimethylsiloxanols in aqueous emulsion

Polydimethylsiloxane oligomers with silanol ends condense in aqueous dispersion under mild conditions to form high polymers in the presence of sulfonic acid surfactants. The process follows a second-order rate law in silanol if reversibility is taken into account. The second-order rate constant is proportional to the area at the oil–water interface and is a complex function of surfactant concentration. The principal driving force is the heat of condensation of the water produced in the polymerization. A mechanism paralleling surface catalysis is offered in which a termolecular complex that consists of two surfactant molecules and one silanol end group reacts bimolecularly at the oil–water interface.