Chemical Engineering & Materials Science

Abstract Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores.

Crystallization of sodalite was studied in an ethanol–Na2O–Al2O3–SiO2–H2O system. The addition of ethanol was observed to significantly affect the crystallization process and final crystal sizes and morphologies. Micron-sized sodalite particles with disc and thread-ball-like shapes were produced at low ethanol contents whereas sodalite particles with core-shell nanostructures were dominant at high ethanol contents. Prolongation of the reaction time led to hollow architectures with polycrystalline shells.

The geometry and internal packing of twisted ropes composed of carbon nanotubes (CNTs) are considered, and a numerical solution in the context of the mesoscopic distinct element method (MDEM) is proposed. Compared to the state of the art, MDEM accounts in a computationally tractable manner for both the deformation of the fiber and the distributed van der Waals cohesive energy between fibers. These features enable us to investigate the torsional response in a new regime where the twisted rope develops packing rearrangements and aspect-ratio-dependent geometric nonlinearities. MDEM emerges as a robust simulation method for studying twisted agglomerates comprising semiflexible nanofibers. ■ INTRODUCTION Modern technology offers a variety of synthetic fibers for rope fabrication. In addition to nylon and polyester fibers, macroscale yarns composed of carbon nanotubes (CNTs) 1 have begun to be manufactured by twist-spinning. 2−4 It is well known that the emergent mechanical behavior of laid ropes depends on both the strength of the fibers and on the twisted geometry. 7 CNTs are the strongest and stiffest synthesized nanofibers, with a Young's modulus (E) of 1030 GPa. 5 Their shear modulus (G) of about 0.5 TPa 6 positions them as the stiffest torsional nanofibers. To manufacture advanced rope structures 8,9 with such outstanding nanofibers, it is crucial to understand the relationship among geometry, internal packing, and the individual nanofiber mechanics. The ability to quantify total energy as a function of twist is an important step in this direction. All-atom simulations uncovered remarkable insights into the torsional mechanics of CNT systems. 6,10−12 Unfortunately, these methods are impractical for simulating twisted CNT assemblies approaching realistic dimensions. To reduce the computational complexity and hence the computational cost, mesoscopic modeling simulations are needed. However, the application of the current models to the problem at hand encounters significant challenges. Drawing from the bead-and-spring modeling approaches developed for semiflexible polymers, 13,14 discretization schemes of CNTs into a sequence of point masses have been pursued. 15 Due to the limited sliding friction, 16−18 there is a poor load transfer between CNTs. When a crystalline close-packed aligned CNT rope is twisted, it is expected that the nanofiber constituents will acquire a variety of deformations, including geometric and physical twists. (Physical twist is the relative rotation of elements along the axis of the rope; geometric twist is bending in the direction of the binormal vector.) Unfortunately, the coarse graining of a group of atoms into a point mass cannot be trained to describe the torsional deformations of the individual CNTs. In addition, theories and simulations of defects in twisted ropes 19−21 evidenced the relationship between packing defects and rope twist. The development of such defects cannot be described with large-diameter bead-and-spring models of the CNTs. These models introduce interfacial corrugated interactions, which prohibit smooth sliding 15,22 and the CNTs' relaxation into the lowest-energy state. We propose to simulate CNT rope twisting with the recently developed mesoscopic distinct element method (MDEM), 22,23 which is the mesoscopic version of a genuine solid mechanics method called the distinct element method. 24 MDEM (i) efficiently simulates large ensembles of CNTs, (ii) captures the CNT linear elasticity in elongation, bending, and twisting deformation modes, and (iii) accounts for the distributed van der Waals (vdW) interactions in a manner that eliminates corrugation artifacts entirely. These features are essential for the problem at hand. In our prior work, we have informed the method from atomistic data and verified it against molecular dynamics (MD) simulations and continuum models for ring aggregates formed by CNTs. 22,25 Here we use MDEM to access the deformation regime needed to investigate packing in twisted CNT ropes. We achieve packing rearrangement

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.

For the first time, a laser ionization time of flight mass spectrometry (LI-TOFS) characterization has been performed on silicon oxycarbide glasses obtained by pyrolysis of sol-gel precursors. This technique allows to desorb intermediate range mesounits from the network, and to determine their mass and atomic identity precisely. It is therefore complementary to all the other analysis typically performed on SiOC glasses as it leads to collect information on a length scale which is usually difficult to probe: the medium range. The results indicate the presence, in the SiOC glasses, of mixed silicon oxycarbide units together with pure silicate mesounits (SiO 2 and SiO 3 ). †

We report on the growth of thin films of yttrium iron garnet (YIG) on dielectric substrates. Such films have historically been challenging to grow due to either cracking or incomplete crystallization of the films. We have established the proper growth parameters by tuning seed layer thickness to an optimum of 45 nm. These films were then used as seed layers for growth of films of Bi:YIG and Ce:YIG. Bi:YIG films show a Faraday rotation of 1700 =cm, and Ce:YIG films show a Faraday rotation of 3700 =cm.

In this paper, a novel fiber-based magnetooptic (MO) latching circuit using a bismuth-substituted yttrium iron garnet (Bi:YIG) is presented. Experimentation shows that nearly 90° of rotation of the state of polarization of incident light occurs between material latching states upon application of an external magnetic field greater than 500 G. This amount of rotation is enough to cause sufficient routing at the output of an optical interferometer of Sagnac configuration, which is presented in this paper. Due to the high coercivity of the Bi:YIG, the material remains in its magnetized state for very long periods of time and is thus latched. Reversing
the applied magnetic field changes the state of the material, unlatching it. This capability has great importance for nonreciprocal all-optical devices requiring low power operation. In addition, having such control of the state of the nonreciprocal elements can allow for a wider diversification of small-scale and large-scale optical network design. A discussion of the experimental setup, the resulting measurement data, and its implication for future low power applications is presented.

The first experimental TE-mode silicon-on-insulator (SOI) isolators using Faraday Rotation are here realized to fill the 'missing link' in source-integrated near infrared photonic circuits. The isolators are simple 1D 2-element waveguides, where garnet claddings and longitudinal magnetic fields produce nonreciprocal mode conversion, the waveguide equivalent of Faraday Rotation (FR). Quasi-phase matched claddings are used to overcome the limitations of birefringence. Current experimental SOI isolators use nonreciprocal phase shift (NRPS) in interferometers or ring resonators, but to date NRPS requires TM-modes, so the TE-modes normally produced by integrated lasers cannot be isolated without many ancillary polarisation controls. The presented FR isolators are made via lithography and sputter deposition, which allows facile upscaling compared to the pulsed laser deposition or wafer bonding used in the fabrication of NRPS devices. Here, isolation ratios and losses of 11 dB and ...

Monolithic optical isolators that provide modal (transverse electric, TE) and dimensional (500 nm core) matching to on-chip lasers have been realized with "one step" seedlayer-free garnets. To date, seedlayer-free garnet claddings have required thinner (<340 nm) silicon-on-insulator (SOI) cores because mode-cladding interactions were too weak for laser-matched cores. However, laser matching is important because tapers and mode converters between the laser and the isolator can cause detrimental reflections prior to isolation. This paper reports the use of cerium-doped terbium iron garnet (Ce:TbIG) in a quasi-phase matched nonreciprocal mode conversion (NRMC) isolator that operates on both TE and TM modes without an external field. A key innovation presented here is a repeatable process for foundry-friendly sputter deposition of Ce:TbIG, which enables this high Faraday rotation material (−3200°/cm) to be synthesized in any isolator design that would benefit from one-step lithographical manufacturing. A proof-of-feasibility 500 nm SOI NRMC device is demonstrated with seedlayer-free Ce:TbIG that achieves an isolation ratio of 11 dB. With an optimal length, this NRMC design can provide greater than 30 dB isolation.

Simulations demonstrate that undoped yttrium iron garnet (YIG) seedlayers cause reduced Faraday rotation in silicon-oninsulator (SOI) waveguides with Ce-doped YIG claddings. Undoped seedlayers are required for the crystallization of the magneto-optical Ce:YIG claddings, but they diminish the interaction of the Ce:YIG with the guided modes. Therefore new magneto-optical garnets, terbium iron garnet (TIG) and bismuth-doped TIG (Bi:TIG), are introduced that can be integrated directly on Si and quartz substrates without seedlayers. The Faraday rotations of TIG and Bi:TIG films at 1550nm were measured to be +500 and-500°/cm, respectively. Simulations show that these new garnets have the potential to significantly mitigate the negative impact of the seedlayers under Ce:YIG claddings. The successful growth of TIG and Bi:TIG on low-index fused quartz inspired novel garnet-core waveguide isolator designs, simulated using finite difference time domain (FDTD) methods. These designs use alternating segments of positive and negative Faraday rotation for push-pull quasi phase matching in order to overcome birefringence in waveguides with rectangular cross-sections.

technical ROM 9 TO2 _!__ 92/2/4 22 16 SUPPLEMEN T ARY NOTATION submitted for publication/published in: Journal of the American Chemical Society 17 COSATI CODES 18 SUBJECT TERMS (Conttnue on reverse if necessary and identify by block number) FIELD GROUP SUB-GROUP microfabricated structures, mass spectrometry 19 ABSTRACT (Continue on reverse if necessary and identify by block number) see attached document DTIC, S1 199 92-03266

An important goal for organic electronics is the synthesis of organic semiconductors with improved performance in thin film transistors (TFTs). 1 This design process should take into account both molecular properties, such as reduction/oxidation potentials, and solid-state crystal packing to achieve materials with enhanced transport properties. Here we report the single-crystal X-ray structure and TFT results for a novel quinodimethane-substituted terthiophene oligomer, compound 1.

The (Bii,2Naii2)TiO3-BaTiO3 system was prepared by the conventional mixed oxide method and sintering technique. The formation of the compositions was investigated and the influences of the preparation conditions on the (Bii,2Naii2)TiO3-BaTiO3 system were discussed. The results show that a pure perovskite phase can be obtained under proper sintering conditions and the sintering conditions must be controlled strictly for getting the specimens with appropriate microstructure.

Hollow-core fibre (HCF) is a powerful technology platform offering breakthrough performance improvements in sensing, communications, higher-power pulse delivery and other applications. Free from the usual constraints on what materials can guide light, it promises qualitatively new and ideal operating regimes: precision signals transmitted free of nonlinearities, sensors that guide light directly in the samples they are meant to probe and so on. However, these fibres have not been widely adopted, largely because uncontrolled coupling between transverse and polarization modes overshadows their benefits. To deliver on their promises, HCFs must retain their unique properties while achieving the modal and polarization control that are essential for their most compelling applications. Here we present the first single-moded, polarization-maintaining HCF with large core size needed for loss scaling. Single modedness is achieved using a novel scheme for resonantly coupling out unwanted modes, whereas birefringence is engineered by fabricating an asymmetrical glass web surrounding the core.

Complex oxide thin films, such as ferroelectric thin films, are widely used in various devices. However, the chemical composition and chemical bonding of the surface layer are usually different from those in the 'bulk' region of the oxide thin films. The existence of the surface layer will certainly affect the properties, such as the dielectric behavior, of the films. In this paper, a model for evaluating the effect of the surface layer on the dielectric behavior of oxide films is proposed. The effective dielectric constant of the thin films is suggested as a function of the dielectric constants of expected and unexpected compounds in the surface layer, the content of unexpected compounds in the surface layer, and the volume fraction of the surface layer in the whole thin film. As an example, the dependence of the effective dielectric behavior of PbTiO thin films on the surface layer 3 has been estimated using this model. The results show that a surface layer deviating from the stoichiometry of the film has a pronounced effect on the effective dielectric constant of thin films. It is obvious that in the design of oxide films and film devices the effect of the surface layer on the dielectric behavior of the films should be taken into account.