Ordered silicon vacancies in the framework structure of the zeolite catalyst SSZ-74

Physico-chemical characterization of the high-silica zeolite catalyst SSZ-74 (ref. 1) suggested that it, like the related materials TNU-9 (ref. 2) and IM-5 (ref. 3), has a multidimensional 10-ring channel system. Such pore systems are ideal for many petrochemical applications, and indeed SSZ-74 has been shown to be a good catalyst for a wide variety of reactions. The elucidation of its framework structure, however, proved to be difficult. Comparable problems were encountered with TNU-9 and IM-5, which were synthesized with related structure-directing agents. Their framework structures, which are the two most complex ones known, both have 24 Si atoms in the asymmetric unit, and were finally solved by combining high-resolution powder diffraction data with information derived from high-resolution electron microscopy images. Therefore, a similar approach, using the powder charge-flipping algorithm to combine the two types of data and molecular modelling to help to locate the structure-directing agent, was applied to SSZ-74. This procedure eventually revealed a most unusual 23-Si-atom framework structure (|(C(16)H(34)N(2))(4)&Si(92)(4)O(184)(OH)(8)]) with ordered Si vacancies.     

Micromechanical compressive response of a zeolite single crystal

In this work the mechanical properties of a widely used zeolite catalyst, ZSM-5, are measured. Single crystals of the ZSM-5, of average size 500 microns, are prepared using an organic template molecule. Testing is performed on the single crystal size scale. An in-house microdeformation tester is designed and manufactured. Loading on the crystal is provided by a high precision stepper motor. Both crystal displacement and applied load are continually monitored through a computer. The entire testing assembly is placed under an optical microscope and a CCD camera is used to obtain real time optical images of the deformation. In this way monotonic and cyclic loading compression stress strain curves are generated. The material is seen to have an average elastic modulus of 4 GPa. It is also seen to be brittle and undergo a gradual stiffness deterioration through a microcracking process. Crystal anisotropy is also investigated by performing two directional tests. However, for the [100] and [010] directions little mechanical anisotropy is observed.     

A型分子筛生长的理论模型

以XRD，TEM，电子衍射对A型分子筛水热晶化过程中晶体生长作了实时观察，分析了晶体生长过程中晶体的聚集行为与演化特征，提出了A型分子筛生长的叠合生长模型，认为，水热体系经过一个晶化诱导阶段后，大量的纳米晶形成，纳米晶作为生长基元，相互迅速了聚集直接叠合演化为微米多晶体，叠合过程不同于邻位面生长所描述的生长基元一个个按次序嵌入晶面，而是许多纳米晶同时相互按一定方向叠合而形成微晶。     

Growth parameters on the defects formation in a grown silicon crystal

The growth of macroscopically dislocation-free Czochralski silicon crystals, various defects such as D defects and microdefects causing oxidation-induced stacking faults can form. The effects of growth parameters such as pulling speed or cooling rate of the crystal on the formation of these defects is examined. From an experiment on the continuous cooling of a silicon crystal from 1400 °C, it is found that there is an intermediate cooling rate range in which the nucleation of OISFs is enhanced. The impact of the presence of OISFs on the electrical properties of the silicon crystal is examined with a minority lifetime mapper, and the resistivity is measured with a four-point probe. A higher pulling speed of the crystal from the melt results in a higher density of particles on the polished silicon wafers. This implies that many of the particles present on the polished silicon wafers are related to solidification of the silicon crystal. Slower pulling from the melt followed by controlled cooling thereafter is suggested as a means of lowering these grown-in defects in Czochralski silicon crystals. © 1998 Kluwer Academic Publishers     

Effect of annealing temperature on the crystal phase fraction in silicon nanoclusters in a silicon oxide matrix

A model of the annealing-induced crystallization of amorphous clusters in a solid matrix is proposed. The calculated dependence of the crystallized phase fraction on the annealing temperature agrees with the available experimental data.     

Polarization-independent Fabry-Perot interferometer in a hole-type silicon photonic crystal

We propose and numerically demonstrate a polarization-independent Fabry-Perot interferometer (PI-FPI) based on the self-collimation effect in a hole-type silicon photonic crystal (PhC). By use of the polarization peak matching method, a resonance frequency of the transverse-electric modes can equal that of the transverse-magnetic modes in the PI-FPI, although the transmission spectra are quite polarization dependent due to birefringence of the PhC. For the operating wavelength of 1550 nm, the PI free spectral range of the PI-FPI is up to 32.3 nm, which nearly covers the whole optical communication C-band window. With its small dimensions, simple structure, and silicon-based material, this PI-FPI may play an important role in photonic integrated circuits.     

On the preparation of a monoclinic (nearly) single crystal of zeolite HZSM-5

HZSM-5 appears to be ferroelastic. Application of a uniaxial mechanical stress during the orthorhombic/monoclinic transition of zeoltie HZSM-5 changes the population of the monoclinic twin domains. The volume fraction of one of the twin mates is changed from 0.5 (before application of mechanical stress) to 0.06 in the crystal studied.     

In situ measurement of crystal growth rate of zeolite

To clarify the growth behavior of zeolite crystals under hydrothermal synthesis conditions, the growth process of ZSM-5 type zeolite crystals was studied using two kinds of in situ observation apparatus. Direct observation of ZSM-5 crystals from amorphous gels during steaming and silicalite crystals from clear aqueous solutions was performed. During the stationary growing stage, a constant rate of crystal growth of zeolites was observed directly for both zeolite crystals. In the case of ZSM-5 zeolites from gels, it was also found that after a certain induction time, crystals grew to 10–20 μm in size, after which no further growth was observed. On the other hand, in the case of silicalite crystals from aqueous solutions, the induction time hardly existed. The size distribution of crystals and the activation energy for crystal growth are also discussed.     

Prediction of the zeolite crystal size distribution in batchwise hydrothermal synthesis

Several ways of characterizing the crystal size distribution during batchwise hydrothermal synthesis of molecular sieve zeolites are shown. In particular, we show how the crystal size distribution function can be predicted once the transient birth rate and growth rate functions have been determined, and how average size, standard deviation, and skewness can be used to characterize the size distribution.Simulation results are presented which show the effect of manipulating several key parameters in the synthesis system (e.g. initial reagent concentration and nucleation rate). It is shown how one might change the operating policy of the batch crystallizer to obtain larger particle sizes and/or narrower crystal size distribution.     

Holographic-recording improvement in a bismuth silicon oxide crystal by the moving-grating technique

We investigate the enhancement of the diffraction efficiency of dynamic gratings recorded in a bismuth silicon oxide crystal at large modulation by the moving-grating technique. The optimum fringe velocity for maximum diffraction efficiency and the degree of enhancement of the diffraction efficiency at optimum fringe velocity are experimentally found to be dependent on the fringe modulation. We apply this technique to real-time incoherent correlation using bismuth silicon oxide. There are two main advantages in using moving gratings: First, the signal-to-noise ratio can be improved considerably because of the improved diffraction efficiency. Second, the resonant effect reduces the effect of environmental disturbances on the peak-correlation intensity, which is significant when the threshold detection level of the correlator is set.     

The relation between zeolite framework structure and vibrational spectra

Using a general valence force field method, infrared and Raman spectra of pure silica zeolite crystals and molecular substructures of zeolites are calculated. Computed spectra of the substructures are directly compared with computed spectra of the crystal. Also, a comparison of the atomic displacement vectors of the vibrational modes is made. No general theoretical basis for a correlation between the presence of large structural elements and spectral features, sometimes reported, is found to exist. An extensive comparison between experimental and theoretical infrared and Raman spectra of the SiO₂ forms of sodalite and faujasite is made. Other applications of vibrational spectroscopy to zeolite structure research, such as the SiOSi angle distribution and the location of acid sites, are evaluated as well.     

Vertically bent single crystal silicon wafers as a neutron monochromator

A simple technique to construct monochromators for neutron beams is presented. The monochromator consists of a set of Si wafers stacked together and elastically bent to focus in the vertical geometry. The apparatus is described. It is shown that the peak reflectivity of the set is mainly governed by the number of wafers, while symmetry and smoothness of the rocking curve is achieved by the bending process. A prototype monochromator based on the (220) reflection of Si wafers has been tested and has successfully performed as a fixed monochromator selecting a flux of 3 × 10⁶ n/(s cm²) (at 45 meV) at the sample position (with 20′ collimation in the white beam and 10′ after the reflection from the monochromator). We conjecture that such a technique could be applied to any kind of material provided the wafers are cut from a single crystal with the surface parallel to the reflecting planes.     

Preparation and characterization of zeolite framework stabilized cuprous oxide nanoparticles

Zeolite framework stabilized copper(I) oxide nanoparticles (4.8 ± 2.6 nm) were prepared for the first time by using a four step procedure: the ion exchange of Cu²⁺ ions with the extra framework Na⁺ ions in Zeolite-Y, the reduction of the Cu²⁺ ions within the cavities of zeolite with sodium borohydride in aqueous solution, the dehydration of Zeolite-Y with the copper(0) nanoclusters, and the oxidation of intrazeolite copper(0) nanoclusters by O₂ at room temperature. Zeolite stabilized copper(I) oxide nanoparticles were thoroughly characterized by ICP-OES, XRD, HR-TEM, Raman, XPS, UV-vis spectroscopy and N₂ adsorption-desorption technique.     

Modeling deflection of 450-GeV protons in a bent silicon crystal

It is shown that the numerical solution of the kinetic Fokker-Planck equation (proposed by Kitagawa and Ohtsuki in 1973) by means of computer simulation of the trajectories of channeled particles using the PST program describes the experimental data on the deflection of 450-GeV protons in a bent silicon crystal well.     

Time-resolved Mueller matrix analysis of a liquid crystal on silicon display

We present a full polarimetric characterization of a liquid crystal on silicon (LCoS) display, with time resolution measurements below the frame period of the device. This time-resolved analysis shows evidence of temporal fluctuations in the millisecond range in the state of polarization of the beam reflected by the display. We demonstrate that light reflected by the display is maintained fully polarized, but these temporal fluctuations result in an effective depolarization effect when detectors with long time integration intervals are used in the characterization of the display.     

Encapsulation of α-cyano-4-hydroxycinnamic acid into a NaY zeolite

The faujasite zeolite structure was studied to investigate its suitability for development of new drug delivery systems (DDS). The sodium form (NaY) of the zeolite was used for encapsulation of α-cyano-4-hydroxycinnamic acid (CHC), an experimental anticancer drug used in colorectal cancer therapy. The DDS was prepared by diffusion in liquid phase of CHC as a guest in the void space of the host zeolite structure at pH 7.0. The molecular integrity of CHC in the encapsulation process was evaluated by proton nuclear magnetic resonance spectroscopy (¹H NMR) and Ultraviolet–Visible spectroscopy (UV–Vis). The new drug delivery system, CHC@NaY, was characterized by Fourier transform infrared spectroscopy and UV–Vis, chemical analysis, powder X-ray diffraction, and Scanning electron microscopy. Analysis of the data of the drug alone and encapsulated in NaY show that CHC and the zeolite framework preserved their original structure. The effect of the zeolite and DDS on HCT-15 human colon carcinoma cell line viability was evaluated. The encapsulation of CHC significantly increased its potency.