2006 Vol. 19, No. 3

Letter
Magnetization of Bi0:9-xGdxLa0:1FeO3 (x=0.3, 0.5, 0.6 and 0.7) is reported. An abnormal negative magnetization appears in the temperature-dependent magnetization curves, and the temperature-dependent coercive field shows a maximum in the vicinity of the compensation temperature where the total magnetization is zero. These results suggest that in the ferrimagnetic-like Bi0:9-xGdxLa0:1FeO3 system the Gd and Fe magnetic sublattices are coupled antiferromagnetically.
Hydrogen production by catalytic steam reforming of the bio-oil, naphtha, and CH4 was investigated over anovel metal-doped catalyst of (Ca24Al28O64)4+¢4O-/Mg (C12A7-Mg). The catalytic steam reforming wasinvestigated from 250 to 850 ±C in the ˉxed-bed continuous °ow reactor. For the reforming of bio-oil, theyield of hydrogen of 80% was obtained at 750 ±C, and the maximum carbon conversion is nearly close to95% under the optimum steam reforming condition. For the reforming of naphtha and CH4, the hydrogenyield and carbon conversion are lower than that of bio-oil at the same temperature. The characteristics ofcatalyst were also investigated by XPS. The catalyst deactivation was mainly caused by the deposition ofcarbon in the catalytic steam reforming process.
Article
The conversion efficiency of stimulated Raman scattering (SRS) in CH4 is studied by using a single longitudinal mode second-harmonic Nd:YAG laser (532 nm, linewidth 0.003 cm-1, pulse-width (FWHM) 6.5 ns).Due to the heat release from vibrationally excited particles, SRS processes often suffer from the thermal defocusing effect (TDE). In view of 6.5 ns laser pulse width is much shorter than the vibrational relaxation time of CH4 molecules, TDE can only affect the SRS processes afterwards. In the cases of low laser repetition, TDE will be not serious, because it will be removed by the thermal diffusion in Raman medium before the next pulse arrives. At the laser repetition rate 2 Hz, CH4 pressure 1.1 MPa and pump laser energy 95 m J, the quantum conversion efficiency of backward first-Stokes (BS1) has attained 73%. This represents the highest first-stokes conversion efficiency in CH4. Furthermore, due to the relaxation oscillation, the BS1pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Its beam quality is also much better and is only slightly affected by TDE. This reason is that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process. Under the same conditions, but pump laser repetition rate as 10 Hz, the conversion efficiency of BS1 goes down to 36% due to TDE. From this study, we expect that a well-behaved 630 nm Raman laser may be designed by using a closed CH4/He circulating-cooling system, which may have some important applications.
Moir patterns on HOPG were studied with scanning tunneling microscopy (STM). The results reveal that the observed Moiré patterms originate from the defects locating several layers below the surface ,which presents the first experimental evidence supporting the prediction that in HOPG the nanoscale electronic waves can propagate through several layers without obvious decay.
An in situ heating system was built for the Auger electron spectroscopy to investigate the thermal effect of Auger lines. A GaN sample was studied in this system. The kinetic energy of Ga LMM and MVV Auger lines were observed to shift negatively with temperature increasing. By using ab initio calculation, the theoretical Ga MVV Auger line shape was fit, which well reflects the inner property of the line. The Auger shift with heating is related with the valence electron rearrangement in the thermal expansion of the local bonds.
Twenty pure elemental metal samples have been studied with a coincidence Doppler broadening system (CDB). The results show the relationship between the CDB spectra and the electronic structure of these samples. The experimental results are compared to simple theoretical predictions, which show that the high-momentum part of the Doppler-broadening spectra can be used to distinguish different elements.
A method of aerosol introduction for matrix-assisted laser desorption/ionization (MALDI) is described. The aerosol particles containing matrix and analyte enter directly into the aerosol time-of-flight mass spectrometer (ATOFMS) at atmospheric pressure. The scattered light signals from the aerosol particles are collected by a photomultiplier tube (PMT) and are passed on to an external electronic timing circuit, which determines particle size and is used to trigger a 266 nm pulsed Nd:YAG laser. The aerosol MALDI mass spectra and aerodynamic diameter of single particles can be obtained in real-time. Compared with other methods of liquid sample introduction, this method realizes detection of single particles and, more importantly, the sample consumption is lower. The effects of matrix-to-analyte ratio and laser pulse energy on analyte ion yield are examined. The optimal matrix-to-analyte ratio and laser energy are 50-110:1 and 200-400 μJ respectively.
Tl2Ba2Ca2Cu3O10 was reported to be a superconductor with a highest transition temperature of 125 K among the homologous series of Tl2Ba2Can-1CunO2n+4. The direct information on the Cu ion site at the atomic level is important for elucidating the superconductivity mechanism. The local bond properties of Tl2Ba2Ca2Cu3O10 were studied using the average band-gap model. The calculated results show that the covalency of Cu(1)-O bond is 0.561, and the average covalency of Cu(2)-O is 0.296. M(o)ssbauer isomer shifts of 57Fe in Tl2Ba2Ca2Cu3O10 were calculated using the chemical surrounding factor, defined by covalency and electronic polarizability. It is verified that for lower doping, Fe substitute the Cu at the Cu (1) site in forms of Fe3+ and Fe4+; for higher doping, Fe3+ and Fe4+ ion occupies Cu(1) and Cu(2) site respectively.The studies show that the determination of the correspondence between spectrum components and actual copper sites occupied by M(o)ssbauer nucleus was made easier with the aid of the calculation results of the chemical bond parameters.
Two different types of potential oscillations have been identified for the first time during dichromate reduction on a gold electrode in a solution of dilute sulfuric acid. One occurs before hydrogen evolution due to the formation and dissolution of passivating films of low-valence chromium oxides, and the other generates,accompanying periodic hydrogen evolution, from the coupling of electrochemical reactions with diffusive and convective mass transport. More interestingly, these two types of oscillations correspond to two crossing loops in the cyclic voltammogram (CV). Such a relation of oscillations with crossed CVs will provide a new way to find electrochemical oscillatory systems systematically and rapidly.
The structural stabilities and electronic structures of Ga atomic chains are studied by the first-principles plane wave pseudopotential method based on the density functional theory. The present calculations show that gallium can form planar chains in linear-, zigzag- and ladder-form one-dimensional structures. The most stable one among the studied structures is the zigzag chain with a unit cell rather close to equilateral triangles with four nearest neighbors, and all the other structures are metastable. The relative structural stability, the energy bands and the charge densities are discussed based on the ab initio calculations and the Jahn-Teller effect.
The semi-empirical INDO method was used to study the electronic structures and the spectra of all of the 34 possible isomers of C78O based on C78 with group C2v. This calculation can simulate positions of an additional oxygen atom in C78 and predict the spectroscopic characteristics of the isomers. The most stable geometry of C78O is the 73,78-C78O molecule with an epoxide structure. The added 73,78-bond is located between two hexagons (6-6) and is intersected by the shortest C2 axis in C78 with group C2v. Atomic orbitals of the oxygen atom play an important role in lowering HOMO energy of 73,78-C78O. Compared with C78 with group C2v, the blue-shift in the electronic absorption spectrum for 73,78-C78O was observed.The reason of the blue-shift effect was discussed, and the electronic transitions were assigned based on the theoretical calculations.
The geometry optimizations and the single point energy calculations of iron tetraphenylporphyrin chloride Fe(TPP)Cl and iron tetraphenylporphyrin chloride (Fe(TPP)Cl), iron pentafluorophenylporphyrin chloride (Fe(TPPF20)Cl) were carried out by using the Density Functional Theory (DFT) UB3LYP with STO-3G* and 6-31G* basis sets, respectively. The electronic properties and the structures of high-lying molecular orbitals were analyzed in detail. The results show that partial spin is transferred from the Fe atom to the porphyrin ring and some electron with the spin opposite to the unpaired electron on the Fe atom is transferred from the porphyrin ring to the Fe atom. The π- and σ-type bonding between the Fe atom and the porphyin ring cause the transfer. The fluorination enhances the electron transfer and the chemical stability of the complex. The high stability is important for the complex possessing high catalytic activity. The catalysis mechanism of oxygen molecule activation on the complex surface is also discussed based on the symmetry of the molecular orbitals.
The molecular structures of indazole and 3-halogeno-indazole tautomers were calculated by the B3LYP method at the 6-311G** level, both in the gaseous and aqueous phases, with full geometry optimization.The geometry and electronic structure of the tautomers of indazole, 3-halogeno-indazole and their transition states were obtained. The Onsager solvate theory model was employed for the aqueous solution calculations.The results of the calculation indicated that the N1-H form of the studied molecule is more stable than that of the N2-H form. The influences of the different 3-halogeno and solvent effects on the geometry, energy,charge and activation energy were discussed. The reaction mechanism of the tautomerization of indazole and 3-halogeno-indazole was also studied and a three-membered cyclic transition state of the tautomer reaction has been obtained.
Metal-free indoline dyes for dye-sensitized solar cells were studied by employing quantum chemistry methods.Comparative study of the properties of both ground and excited states of metal-free indoline dyes for dye-sensitized solar cells revealed: (i) as the number of rhodanine rings increases, the energy di?erence betweenHOMO and LUMO decreases and there is a red shift in the absorption spectrum with the binding energyincreased, and the transition dipole moment decreased; (ii) Based on an analysis of charge di?erential density,we observed that the charge and energy are transfered from the phenylethenyl to the indoline and rhodaninerings; (iii) The electron-hole coherences are mainly on the indoline and rhodanine rings, and the exciton sizesare 30 and 40 atoms for indoline dyes with one and two rhodanline rings, respectively. These results serveas a good example of computer-aided design in metal-free indoline dyes for dye-sensitized solar cells.
Quantitative structure-retention relationship (QSRR) model for the estimation of retention indices (RIs)of 39 oxygen-containing compounds containing ketones and esters was established by our newly introduced distance-based atom-type indices DAI. The useful application of the novel DAI indices has been demonstrated by developing accurate predictive equations for gas chromatographic retention indices. The statistical results of the multiple linear regression for the final model are r=0.9973 and s=8.23. Furthermore, an external test set of 10 oxo-containing compounds can be accurately predicted with the final equation giving the following statistical results: r pred=0.9966 and s pred=8.56.
A versatile metal-organic chemical vapor deposition (MOCVD) system was designed and constructed. Copper films were deposited on silicon (100) substrates by chemical vapor deposition (CVD) using Cu(hfac)2 as a precursor. The growth of Cu nucleus on silicon substrates by H2 reduction of Cu(hfac)2 was studied by atomic force microscopy and scanning electron microscopy. The growth mode of Cu nucleus is initially Volmer-Weber mode (island), and then transforms to Stranski-Rastanov mode (layer-by-layer plus island).The mechanism of Cu nucleation on silicon (100) substrates was further investigated by X-ray photoelectron spectroscopy. From Cu2p, O1s, F1s, Si2p patterns, the observed C=O, OH and CF3/CF2 should belong to Cu(hfac) formed by the thermal dissociation of Cu(hfac)2. H2 reacts with hfac on the surface, producing OH. With its accumulation, OH reacts with hfac, forming HO-hfac, and desorbs, meanwhile, the copper oxide is reduced, and thus the redox reaction between Cu(hafc)2 and H2 occurs.
Exfoliative Mg/Al layered double hydroxide (Mg/Al-LDHGly) was obtained via hydrothermal synthesis in the presence of glycine. The product prepared by hydrothermal reaction for 10 h at 120 ℃ possesses high thermal stability and maximal crystallite size in a, c directions. TEM and SEM analyses show that Mg/AlLDHGly was of well-crystallized hexagonal product with stacks of slightly curved layers. Benefiting from mechanism investigation on its gradual delamination in formamide, rapid delamination of Mg/Al-LDHGly at room temperature was realized, which provided fundamental for preparation of (Mg/Al-LDHGly)/polymer nanocomposites by using exfoliation-adsorption method.
The electroless Ni-P-carbon nanotubes composite plating was studied on the copper substrate. Metallurgical microscope, scanning electronic microscope, X-ray diffractometer and micro hardness tester were used to study the structure, constitution and performance of the electroless Ni-P-carbon nanotubes composite deposit. Experiential results show that, with the increment of carbon nanotubes content in electroless plating solution, the grain size on the sample surface decreases whereas the density of grains and the hardness for composite deposit increases. Moreover, adding carbon nanotubes not only improves the degree of crystallization for the composite deposit but also helps their transformation from the amorphous state to the nanocrystal state.
Monte Carlo simulations were used to investigate the compatibilizing behaviors of multi-block copolymers with different architectures in A/B/(block copolymer) ternary blends. The volume fraction of homopolymer A, employed as the dispersed phase, was 19%. The simulations illustrate how a di- or multi-block copolymer aggregates at the interfaces and influences the phase behaviour of such incompatible polymer blends. The di-block copolymer chains tend to "stand" on the interface whereas the multi-block chains lie on the interface.In comparison with the dj-block copolymer, the block copolymers with 4, or 10 blocks can occupy more areas on the interface, and thus the multi-block copolymers have higher efficiency for the retardation of the phase separation.
Three kinds of ultra-fine Ce2O(CO3)2.H2O powders with different morphologies were prepared by adding CTAB, PEG19000 and OP-10 to a solution of Ce2O (NO3)3.6H2O and urea according to the principle and the characteristics of the homogeneous precipitation method. The products were characterized by TEM and XRD. The results showed that the precursor was a single crystal, and that different surfactants had different influences on the morphology of the products. The cationic surfactant CTAB had little effect on crystal morphology merely reducing its size. Nonionic surfactants PEG19000 and OP-10 are both able to change the crystal morphology to a much greater extent. Adding PEG19000 produces an array of rod-like particles with ordered formation and uniform dimension. Meanwhile, in the system of OP-10, a sort of flower-like with different morphologies occured because of the mechanism of formation and grain growth.
Nd(DBM)3Phen-doped (DBM is dibenzoylmethane and Phen is phenanthroline) polymethyl methacrylate (PMMA) is prepared. Optical absorption, excitation and emission spectra were analyzed for Nd3+ in Nd(DBM)3Phen-doped PMMA. Using the Judd-Ofelt theory, the absorption spectrum was analyzed. The Judd-Ofelt(J-O) intensity parameters of Nd(DBM)3Phen-doped polymethyl methacrylate were calculated to be Ω2 = 20.97 × 10-20 cm2, Ω4 = 3.42 × 10-20 cm2, Ω6 = 2.90 × 10-20 cm2. The radiative lifetime (631 μs)of the excited 4F3/2 level is given. The stimulated emission cross-sections and the fluorescence branch ratios for the 4F3/2 →4 IJ/ transitions are also evaluated. Analysis reveals that Nd(DBM)3Phen-doped PMMA is promising for application in polymer optical fibers and planar waveguides.
The electrostatic potential energy model of hydrotalcites was based on the theory of crystallography. The anionic potential energy of MgAl-hydrotalcites, with 20 layers and 2107 anions per layer, was calculated, and the anionic stability of the hydrotalcites was investigated. The charge density of the layer and the distance between the adjacent anions varied with the molar ratio of Al3+/(Mg2+ +Al3+). Anionic potential energy depended on the charge and size of the anions. Calculation results remained consistent with thermal stability and the ion exchange ability reported. This model is able to predict anionic stability of the hydrotalcites.