zeolite crystals were attached to the gold electrodes of quartz crystal microbalances (QCM). Monolayers of thiolalkoxysilanes on the gold surface served as interfacial layers for the sukquent adhesion of the zeolite crystals to the QCM. The process of anchoring the zeolite crystals via the thiol-silane monolayers was studied by reflection adsorption infrared (IR) spatrompy, contact angle, and scanning electron microscopy (SEM). The siloxane linkages between the microporous zeolite crystals and the terminal cross-linked polysiloxane groups of the interfacial monolayer play an important role in enhancing the packing density of microporous crystals and the thermal stability (up to at least 350 " C ) of the film on the gold surface. Dynamic sorption isotherms of organic vapors and nitrogen as well as the transient sorption behavior of organic vapor pulses were studied to characterize the zeolitecoated QCMs. The resonance frequency response of zeolite-coated QCMs to vapor pulses could be increased up to 500-fold compared to the bare sensor. The regular micropores (0.34.75 nm) of the QCM-attached zeolite crystals were found to efficiently control molecular access into the coating. Selectivity of the frequency response in excess of 1OO:l toward molecules of different size and/or shape could be demonstrated. The kinetics of vapor desorption from the zeolite layers are strongly dependent on the adsorbate/zeolite combination, thus providing an additional capability for molecular recognition. IntroductionThe design of selective coatings for microsensors such as optical waveguides, chemically sensitive field effect transistors, chemiresistors, and acoustic wave devices has attracted growing attention.'" The goal of these studies is to increase the sensitivity and chemical selectivity of the sensor by controlling the surface interactions and solubilities of analyte vapors to be detected.Our recent development of molecular sieve-based composite films has introduced a novel means for tailoring vapor/surface interactions.1*4-6 A sensor coated with molecular sieve crystals not only detects vapors by the adsorption of target molecules but
Zeolite molecular sieve films (MFI-topology) consisting of one or more layers of pure-silica ZSM-5 crystals have been hydrothermally synthesized on the surfaces of fused silica glass. The resulting films have been characterized by field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray analysis, and Fourier transform infrared spectrometry. At the early stage of reaction a porous gel layer spontaneously condenses on the substrate surface, within which the zeolite crystals are nucleated and grown as mediated by the structure-directing agent. Notable shifts of the C−H stretching vibrations are observed on the tetrapropylammonium cations attached to the gel layer compared to those enclathrated in the crystal frameworks. The zeolite crystals are preferentially oriented with their b axis perpendicular to the substrate surface. The crystal orientation is also strongly dependent on the roughness of the underlayer surface and consequently the thickness of the zeolite film. By control of the synthesis conditions, the individual crystals constituting the zeolite films can be prepared either smaller than 500 nm or as large as 30 μm. Those zeolite films grown in situ on the silica glass also show some unique properties and new potential applications.
Distributed control, as a potential solution to decreasing communication demands in microgrids, has drawn much attention in recent years. Advantages of distributed control have been extensively discussed, while its impacts on microgrid performance and stability, especially in the case of communication latency, have not been explicitly studied or fully understood yet. This paper addresses this gap by proposing a generalized theoretical framework for small-signal stability analysis and performance evaluation for microgrids using distributed control. The proposed framework synthesizes generator and load frequency-domain characteristics, primary and secondary control loops, as well as the communication latency into a frequency-domain representation which is further evaluated by the generalized Nyquist theorem. In addition, various parameters and their impacts on microgrid dynamic performance are investigated and summarized into guidelines to help better design the system. Case studies demonstrate the effectiveness of the proposed approach.
We report on a novel design for chemical sensors, based on the deposition of layered silicate materials ionexchanged with alkylammonium cations ('organoclays") on piezoelectric sensors, and their selectivity toward sorption of various small molecules (Figure 1). A growing interest in selective chemical sensors has prompted efforts to develop thin films with selective sorption capabilities. We have recently explored the design and sorption behavior of zeolite molecular sieve films on quartz crystal microbalances?3 Organoclays are interesting complements to zeolite sorbents because their sorption selectivities can be controlled to a large extent by the choice of layer charge density and structure and chaii length of the alkylammonium ions.4 Different combinations of these fadors will produce very different interlayer environments, containing polar mineral surfaces, hydrophobic organic chains,andparaffin-likelayers. Thesematerials have been studied as selective sorbents in general: in gas chromatography! and for toxic waste removal.' The organoclays can offer preferential sorption of nonpolar and flat molecules.The host chosen in this study was hectorite of approximate compcsition Naos(Lia3Alo.olMg,,)SiOlo(OH)z (with some Ca content), supplied by American Colloid Co., with (TPrA) bromide, and tetrapentylammonium (TPeA) b r e mide (Eastman), respectively, and washing withdeionized water followed by centrifugation. Intercalation of longchain organic cations (3-fold excess) was carried out three times in 30% acetonitrile/water solutions of trimethyloctadecylammonium (TrMOA) bromide, dimethyldioctadecylammonium (DMDOA) bromide, and methyltrioctadecylammonium (MTrOA) bromide (Fluka Chemicals), respectively, followed by washing in water until bromidefree.Formation of the thin f i i coatings on the gold electrodes of the QCM (Fil-Tech. Inc. Boston; resonance frequency 6.0 MHz; a 1-Hz shift corresponds to 12.1 ng/cm2) was achieved by dipping the crystals in the respective suspensions of the intercalated silicates. Depending on the dispersion properties of the organoclay, suspensions of 1 mgin 1 mLofwater,30% acetonitrile/water,orchloroform were used. The films were dried at room temperature and heated at 200 'C in nitrogen overnight. Areal densities of the films were obtained from QCM frequency changes. Sorption isotherms of nitrogen (at liquid nitrogen temperature) and different vapors (at 25 "C) were taken in automated flow systems with computer-controlled adjustment of partial pressures in He, as described previously.* The QCMs were degassed in a helium flow at200°Cfor2 h beforeisothermsweretaken. Equilibrium cation exchange capacity of 8 mequiv/lO g, and charge density of ca. 1 monovalent cation/100 A2.4 Hedorite is a layered material of the smectite family, which can be was assumed when frequency changes were lessthan 1 Hz in 90 s. Transient sorption of vapor pulses was measured bv iniectine 0.4 VL of liauid into a flow svstem at 100 OC. 3a visualized as two tetrahedral silicate sheets fused to an edge-sharing octahed...
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The microporosity of zeolite H-mordenite has been modified by incorporation with silicon modifying groups derived from chemisorption of disilane. The modifying group, occupying more than 110 A3 in the main channel, progressively reduces the pore size of the zeolite with precision that can be less than 1 A. This is indicated by continuous exclusion of gradually smaller molecules @-xylene, propane, ethane, and methane) from the sorption. As the modification degree is further increased, the side pockets of H-mordenite become blocked and are inaccessible to nitrogen and oxygen. Substantial changes in sorption selectivities of oxygen versus nitrogen have also been observed on the modified zeolite after the protons in the main channels have been replaced by the silicon entities. A study of the thermodynamic properties and the contributions of sorption energy has revealed, for the first time, that such changes in molecular selectivities are directly related to the interplay of the pore size and the polarity of the substrate controlled by the modification process.
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