The layered double hydroxides (LDHs) of Li and Al can be synthesized from the four polymorphs of Al(OH)3, namely gibbsite, bayerite, nordstrandite, and doyleite. The crystal structure of this class of compounds depends on the type of the precursor used due to their topotactic reaction mechanism. While the LDHs derived from gibbsite and bayerite yield different crystal structures, the incorporation of Li into nordstrandite was expected to yield new LDH structures different from those derived from gibbsite and bayerite. The structure of nordstrandite derived LDHs were however identical to that derived from the bayerite counterpart. The absence of symmetry in the interlayer of nordstrandite (C1) makes it unsuitable to accommodate the intercalating anions with different molecular symmetries. To make the interlayer gallery suitable for the anions, the metal hydroxide layers of the nordstrandite translate, transforming nordstrandite to bayerite. The bayerite with site symmetries Oh and C2 stabilizes the anions in the interlayer by hydrogen bonding. The transformation of nordstrandite to bayerite, when soaked in lithium salt solution is, therefore, a manifestation of the intercalating anions. 相似文献
Oxidation of the cis isomer of the λ3-cyclotriphosphazane [EtNP(OCH2CF3)]3 with trimethylamine-N-oxide (TMNO) gives the cis isomer of trioxo-λ5-cyclotriphosphazane [EtNP(O)(OCH2CF3)]3; the trans isomer of [EtNP(O)(OCH2CF3)]3 is obtained by the treatment of a cis and trans mixture of [EtNP(OCH2CF3)]3 with aqueous H2O2. The two trioxocyclotriphosphazanes have been characterized by elemental analysis, IR, and NMR (1H, 13C, 19F, and 31P) spectroscopy. The solid state structures of both the isomers have been determined by single crystal X-ray diffraction. The six-membered P3N3 ring in both the isomers exhibits a twist-boat conformation; in the cis isomer, the trifluoroethoxy substituents lie on the same side of the ring, whereas, in the trans isomer, two trifluoroethoxy groups are on one side of the ring and the third on the other side of the ring. Crystal data for cis-[EtNP(O)(OCH2CF3)]3: monoclinic, P 21/ n , a = 13.593(3), b = 9.721(2), c = 17.539(3) Å, β = 99.49(2)°, V = 2286(1) Å3, Z = 4, and Final R = 0.047. Crystal data for trans-[EtNP(O)(OCH2CF3)]3: monoclinic, P 21/ n , a = 11.685(4), b = 15.115(5), c = 13.233(5) Å, - = 102.21(3)°, V = 2284(1) Å3, Z = 4, and Final R = 0.078. 相似文献
A series of sulphate-promoted ZrO2 solid acid catalysts with different contents of SO42− were calcined at 450°C in air for 4 h and tested for the liquid-phase alkylation of catechol to guaiacol in a fixed-bed down-flow
reactor. The 5 wt.% SO42− on ZrO2 showed the best conversion (82%) and selectivity for guaiacol (84%) at 200°C and 1 bar pressure. A smooth correlation was
observed between the catalytical activity and surface acidity of sulphated zirconia. Based on our results, a surface mechanism
is proposed. 相似文献
Despite its significance in polycrystalline materials, there have been few experimental investigations of the activity of grain boundary sliding (GBS) and the relationship between GBS and slip transmission at grain boundaries. The present work addresses this knowledge gap by the characterization of full-field strain and microstructural information in an experimental system of high-purity (99.99%) columnar aluminum subjected to uniaxial tension at 190 °C. High-resolution, full-gage strain fields were characterized on an unloaded specimen by distortion-corrected and stitched scanning electron microscope-enabled digital image correlation (SEM-DIC). Alignment between the lower-resolution electron backscatter diffraction (EBSD) and higher-resolution strain fields was significantly improved by clustering of strain data within an EBSD-defined boundary mantle. Grain boundary sliding was investigated at select boundaries, and it was determined that GBS magnitude profiles can have large gradients along a single boundary and vary significantly between boundaries. Using a geometric compatibility factor (m′) to quantify favorability of slip transmission, the two grain boundaries that exhibited the largest average GBS magnitude experienced contiguous slip on moderately well aligned slip systems, although the exact nature of this slip activity, whether transmission or nucleation, remains under investigation.
Dark currents n+/ν/p+ Hg0.69Cd0.31Te mid wave infrared photodiodes were measured at room temperature. The diodes exhibited negative differential resistance at room-temperature, but with increasing leakage currents as a function of reverse bias. The current–voltage characteristics were simulated and fitted by incorporating trap assisted tunneling via traps and Shockley–Read–Hall generation recombination process due to dislocations in the carrier transport equations. The thermal suppression of carriers was simulated by taking energy level of trap (Et), trap density (Nt) and the doping concentrations of n+ and ν regions as fitting parameters. Values of Et and Nt were 0.78Eg and ~6–9 × 1014 cm?3 respectively for most of the diodes. Variable temperature current voltage measurements on variable area diode array (VADA) structures confirmed the fact that variation in zero bias resistance area product (R0A) is related to g–r processes originating from variation in concentration and kind of defects that intersect a junction area. 相似文献
For , a bounded domain, and for , we improve the Hardy-Sobolev inequality by adding a term with a singular weight of the type . We show that this weight function is optimal in the sense that the inequality fails for any other weight function more singular than this one. Moreover, we show that a series of finite terms can be added to improve the Hardy-Sobolev inequality, which answers a question of Brezis-Vazquez. Finally, we use this result to analyze the behaviour of the first eigenvalue of the operator as increases to for .
Triphenylamine ortho-tricarboxylic acid (1) has been synthesized and the crystal structure reported. This molecule is shown to spontaneously self-assemble into a hydrogen-bonded tetrahedron. Furthermore, Electrospray Ionization Mass Spectroscopy shows evidence for the stability of such aggregates from an ethanol/water solution. 相似文献
We address the problem of designing a general-purpose combinatorial library to screen for pharmaceutical leads. Conventional approaches focus on diversity as the primary factor in designing such libraries. We suggest making screening libraries out of a set of pharmaceutically relevant scaffolds, with multiple analogs per scaffold. The rationale for this rests on the fact that even though the hit-rate in active series is much higher than in the database as a whole, often a large fraction of the compounds in active series are inactive. This is especially true when the series has not been optimized for the target under study. We introduce the concept of hit-rate within a series and use historic screening data to arrive at a crude estimate for it. We then use simple probability arguments to show that 50-100 compounds are required in each series in order to be nearly certain of finding at least one active compound in each true active series for any given target. 相似文献
We present the design and fabrication of a planar structure for coupling light from a multimode feed waveguide into a single-line-defect photonic-crystal waveguide. Finite-difference time-domain calculations predict a coupling efficiency of greater than 90%, and preliminary experimental results indicate successful coupling through a single-line-defect photonic-crystal waveguide. Device design, fabrication, and characterization are presented. 相似文献