2005 Vol. 18, No. 4

Letter
Novel GaN nanowires were synthesized by a chemical vapor deposition (CVD) method. The morphology and structure of the nanowires were investigated by SEM, XRD and Raman spectra. Results show that GaN nanowires are formed by aggregated GaN nanocrystals, which is due to the non-uniform precipitation of GaN from catalyst droplet. An asymmetric broadening and shifting to lower frequency of A1(LO) peak are observed in the Raman spectra, which mainly contribute to the Fano interference between scattering from the k=0 optic phonon and electronic continuum scattering from laser-induced electrons.
Catalytic steam reforming of condensable vapors, i.e. bio-oil, derived from pyrolysis of biomass is an important process for hydrogen production, which is expected to form renewable and clean energy. The generation of hydrogen from bio-oil was investigated from 250 to 750 ℃ by a MgO mixed C12A7-O-(C12A7-MgO) catalyst in a fixed-bed micro-reactor. The hydrogen yield on C12A7-MgO was about 44% at 750 ℃. It is found that both the catalytic activity and catalysis life are improved by doping MgO. The XRD results show that the C12A7 structure of the positively charged lattice framework remains in the C12A7-MgO catalyst.
Article
The absorption spectrum of OCS molecules under jet-cooled conditions has been measured between 64150 and 65840 cm-1 with a highly resolved tunable vacuum ultraviolet (VUV) laser (about 0.2 cm-1), generated by two-photon resonant four wave mixing process. During the experiment, the VUV intensities prior and posterior to the OCS absorption were monitored simultaneously in order to increase the sensitivity of absorption measurement, and the absorption sensitivity △I/I was determined to be 1%. The measured spectrum is essentially similar to the S(1S) photofragment excitation spectrum under similar conditions, indicating that the dissociation to produce S(1S) fragment is the main pathway. The difference between these kinds of spectra, however, implies the existence of other dissociation channels in addition to the S(1S) pathway.
The reaction mechanism of CH3 radical with NO was investigated at G2(B3LYP/MP2/CC) level. The equilibrium geometries, harmonic frequencies and energies of various stationary points on the potential energy surfaces have been calculated in the lowest singlet states. Various possible reaction pathways and reaction mechanism were probed in details. Transition-State-RRKM theory calculations for the reaction successfully describe the temperature and pressure dependences. The branching ration is calculated in the range of 300-2500 K. The contribution of stabilization of adducts to the reaction rate constant was discussed. The calculations provide a well consistent description for the previous experimental work.
Both a free volume approach for Helmholtz free energy and a theoretically-based fitted formula for radial distribution function (rdf) of hard sphere solid were employed to describe the Helmholtz free energy of Lennard-Jones (LJ) solid in the framework of first order thermodynamic perturbation theory. Dividing the LJ potential follows from a modified WCA prescription, the specification of equivalent hard sphere diameter was determined by a simple iteration procedure devised originally for liquid state, but extended to solid state in the present study. Two hundred shells were used in the rdf to get an accurate perturbation term. Bulk fluid thermodynamic property of the LJ model is from a recently proposed equation of state. The present approach is very accurate for the description of excess Helmholtz free energy and equation of state of LJ solid, but shows some deviation from the simulation for excess Helmholtz free energy of uniform LJ fluid when the reduced temperature is lower than 0.75. The present approach is satisfactory for description of phase equilibrium of the LJ model. By choosing the appropriate LJ potential parameter, the present approach can describe the melting curve of real molecules accurately.
First a short review on the dependence of growth-rate for crystals on the grown mechanism and initial structure of polymers was presented. Based on the structural model of micro-nucleus and crystal-constituent chains and the feature of statistical dynamics for polymeric crystallization by molecular segregation, a general method for characteristics of the number growth-rate for micro-crystal-constituent chains and the size growth-rate for crystals was proposed. According to the method, a set of quantitative expressions for the correlation of the number growth-rate and the size growth-rate to the four types of growth was obtained (folding, extending and combination of folding and extending). Their dependences of the crystalline temperature and the initial structure of polymers were also derived, then the exponential parameters of H for molecular weight were combined with the fraction of conformation on segments, a new correlation of the exponential parameter to the temperature of crystallization and the flexibility of polymeric chain was theoretically obtained, and the dependence of H on the different types of growth was also studied. Finally the relationship between the growth-rate for crystals and the molecular weight of polymers of solution was verified by a great number of experimental data, and an excellent agreement between the theoretical prediction and experimental data was obtained.
Under an external magnetic field, the particles in a magnetorheological fluids (MRFs) aggregated into chains of dipoles aligned in the direction of the magnetic field so that the MRFs exhibited solidlike mechanical behavior with a yield shear stress. Based on a microscopic analysis and making use of the statistical approach, a micro-to-macroscopic model is proposed for the constitutive behavior of MRFs and the main influencing factors, such as the intensity of magnetic induction, the size of particles, volume fraction of particles, shear strain rate and saturation magnetization. The effects of the influencing factors on the macroscopic behavior of MRFs are investigated and compared with the experimental data. It is found that the proposed model can efficiently describe the mechanical properties of MRFs and can be easily applied to solve practical engineering problems.
The relationship between the transport properties and thermodynamic properties in glass forming liquids was investigated. The configurational entropy of Adam-Gibbs theory on cooperatively rearranging regions and the theoretic function derived from extremal value model were used to propose a brief that non-exponential stretched exponent in KWW form relaxation function is equal to the relative configurational entropy of cooperatively rearranging region in liquids, and is inversely proportional to the critical number of molecules occurring configurational transformation in a cooperatively rearranging region. Therefore, the new physical significance on glassy configuration is imposed on the stretched exponent, and theoretical developments and empirical correlations between the structural relaxation and configurational entropy are established. Further, an improved expression of β(T) was proposed to eliminate the deviation of the fit by using Vogel-Fulcher-Tammann equation from viscosity data at higher temperatures, which conforms well over 200 K temperature range. The improvement on β(T) is correspondent to the improvement on the difference in thermal capacities between isobaric and isochoric processes.
The reaction for CH3CH2+O(3P) was studied by ab initio method. The geometries of the reactants, intermediates, transition states and products were optimized at MP2/6-311+G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single-point calculations for all the stationary points were carried out at the QCISD(T)/6-311+G(d,p) level using the MP2/6-311+G(d,p) optimized geometries. The results of the theoretical study indicate that the major products are the CH2O+CH3, CH3CHO+H and CH2CH2+OH in the reaction. For the products CH2O+CH3 and CH3CHO+H, the major production channels are A1: (R)→IM1→TS3→(A) and B1: (R)→IM1→TS4→(B), respectively. The majority of the products CH2CH2+OH are formed via the direct abstraction channels C1 and C2: (R)→TS1(TS2)→(C). In addition, the results suggest that the barrier heights to form the CO reaction channels are very high, so the CO is not a major product in the reaction.
The reaction between silyl radicals and nitric oxide was studied by using the B3LYP/6311G and the high-level electron-correlation CCSD(T)/6-311G methods. The geometries for reactants, the transition states and the products were completely optimized. All the transition states are verified by the vibrational analysis and the intrinsic reaction coordinate (IRC) calculations. The results show that the reaction is via multi-channel and multi-step. Five products may be formed via the complex reaction channels, i.e. association, H-shift and dissociation.
The molecular geometries optimization and electronic structures of diphenyl disulfide (DPDS) and dibenzyl disulfide (DBDS) compounds were investigated by density functional theory (DFT) and ab initio method at the 6-31G basis set level. The active atoms and bonds of reaction were provided by frontier molecular orbital theory. The molecular orbital parameters of DPDS and DBDS compounds and iron atom cluster were calculated by using density functional theory. The interaction pattern between the organic disulfide compounds and iron atom cluster was discussed based on the approximate rule of orbital energy. Some parameters characterizing the action strength between the organic disulfide compounds and iron atom cluster, including the bonding strength, reactive strength and static action strength, were analyzed by using frontier electron density, super de-localizability, net atomic charge and the interaction energy of chemical adsorption as criteria. The results indicate that S-S chemical bond and C-S chemical bond of the compounds are inclined to be broken when DPDS and DBDS interact with the metal. The antiwear ability order of DPDS and DBDS compounds is DPDS>DBDS, and the extreme pressure ability order of DPDS and DBDS compounds is DBDS>DPDS, and the prediction results based on quantum chemistry calculations are in good accordance with the friction and wear test results.
An two-electrode molecular bridge model that consists of two benzene rings was presented. The characteristics of electronic transport through the nano-molecular bridge was investigated theoretically by using the tightbinding approach based on the Green’s function with only one π orbital per carbon atom at the site. Electronic transport probabilities through the molecular bridge from the input to the output terminal were obtained. The electronic current distributions inside the molecular bridge were calculated and shown in graphical analogy by the current density method based on Fisher-Lee formula at the energy points E=±0.68 and E=±1.38 where the peaks of transport probabilities appeared, and the maximum bond electronic current was also presented. The reason why the loop current in the benzene ring is induced by the phase difference within the molecular orbits is explained.
Gas-phase metal ion affinities and optimized structures of RNA nucleic acid bases for the Ca+ were determined at a density functional level employing the hybrid B3LYP exchange correlation potential in connection with the 6-311+G(2df,2p) basis set. All the molecular complexes, obtained by the interaction between several low-lying tautomers of RNA nucleic acid and Ca+ on the different binding sites, were considered. For Cytosine, the most stable complex was obtained starting from the most stable tautomer of the free nucleic acid base tautomers. As to thymine, the bond energy of the ion with the most stable tautomer of the free nucleic acid base is the weakest among the three tautomer’s complexes, and that of the ion with least stable tautomer of the free nucleic acid base is the strongest . Uracil is similar to thymine. The two kinds of relation, bond energy and total energy for the complex, are in disagreement, as the metal affinities of RNA bases for the Ca+ depend on binding sites, and total energy of complex (Ca+-RNA base) relies on all atoms and their relative positions in the complex.
The compound Hexanitrohexaazatricyclododecane was designed and calculated by using the density functional theory method at the B3LYP/6-31G level. According to the calculated results, there are two optimized conformational structures, boat (α) and chair (β). It is found that the former is more stable based on the analysis of total energies, frontier molecular orbital energies and Mulliken populations. The IR spectra were obtained and assigned by means of normal-mode analyses. Thermodynamic properties at 200-800 K were provided using the statistical thermodynamics method. Finally, the detonation velocity and detonation pressure were predicted by Kamlet formula based on the calculated theory density and heat of formation. The predicted detonation velocity and detonation pressure of α conformer are 9.46 km/s and 41.74 GPa, while those of β are 9.34 km/s and 40.02 GPa, respectively.
Theoretical studies on the complexes [Ru(bpy)2L]2+, [Ru(phen)2L]2+ (L=pytp,pztp) were carried out by using the density functional theory (DFT) method at B3LYP/LanL2DZ level. The relation between electronic structures and anti-cancer activities of complexes was investigated. The increasing of N in the main ligand can strengthen the interaction of complexes with DNA and anticancer activities of complexes. The calculation results show that for complexes I-IV, their energies of LUMO orbital are in the order of εI>εII, εIII>εIV, the electron cloud components of LUMO come mainly from main ligands and the content distributing is in the order of I
All geometry structures of (CoMn)n (n=1-5) clusters were optimized, and the energy, frequence and magnetism of (CoMn)n (n=1-5) clusters were calculated by using the local spin density approximation and generalized gradient approximation of density functional theory. The same ground state structures of CoMn alloy clusters were confirmed in two methods, and magnetism of CoMn alloy ground state clusters was studied systemically. In order to understand structure and magnetism of CoMn alloy clusters better, Co2n (n=1-5) and Mn2n (n=1-5) clusters were calculated by the same method as alloy clusters, whose ground state structure and magnetism were confirmed. Moreover, the ground state structure and magnetism of clusters with the corresponding CoMn alloy clusters was compared. Results indicated that for (CoMn)n (n=1-4) clusters, geometry structures of CoMn alloy clusters are the same as the corresponding pure clusters still, (CoMn)3 and (CoMn)4 exhibit magnetic bistability, show ferromagnetic and anti-ferromagnetic coupling, local magnetic moment of Co, Mn atoms in CoMn alloy clusters almost preserves magnetism of pure clusters still.
All of the possible 21 isomers for C77N+, an isoelectronic molecule of C78, were investigated by the INDO methods based on C78(C2V). It indicates from optimization that the most stable isomer is that the nitrogen atom substitutes C(78) located at the C(78)-C(73) bond passed through by the Y shortest axis and the atoms near the Z longest axis are also easy to substitute, whereas the atoms near the X shortest axis are relatively difficult to substitute. At the same time, C78 was compressed a little and ready to perform the further reaction to form C77 NR at the location of substitution. Electronic spectra of C77N+ were calculated by INDO/SCI method and electronic transition was theoretically assigned. The redshift of absorption peaks for electronic spectra of C77N+ took place compared with that of C78(C2V) because of its narrower LUMOHOMO energy gap. There are great differences in characteristic absorptions among C77N+ isomers, which can be considered as evidence of the formation for each isomer.
Ultrasonic processing on sodium aluminate solutions was proved to have effects on the gibbsite crystallization process from sodium aluminate solution. In order to investigate the mechanism of the ultrasonic intensification, the sonofluorescence spectrum was detected by fluorescence spectrometer in sodium alumianteLuminol solutions, and the spectrum under the conditions of various ultrasonic power and different composition of sodium aluminate solutions were studied. It is found that the sodium aluminateLuminol solution can irradiate fluorescence by ultrasonic processing above the given power; the intensity of sonofluorescence are relatively low in sodium aluminate solutions with high concentration and low αk(mol ratio of Na2O/Al2O3 in sodium aluminate solution). The relationship between the sonofluorenscence and ultrasonic intensification on gibbsite crystallization process from sodium aluminate solutions were also discussed. The conclusion that the ultrasonic effects are influenced by ultrasonic cavitation and intensity of sonofluorescence spectrum is deduced.
Pure complex fluorides of KMnF3 crystal of spherical plate with a diameter of about 300 nm were successfully prepared by a simple aqueous synthesis method at room temperature. The sample was characterized by X-ray diffractometer (XRD), transmission electron microscopy, and selected area electron diffraction. An XRD result proves that the product is KMnF3 of perovskite-structure. By TEM and SAED results, it is found that the spherical plate morphology of sample is formed by selforganization of KMnF3 grains. The sample shows two strong emissions at 416 and 438 nm, some discussion is made about the photoluminescence spectra and the resource of the emissions, at the same time, with the formula: Eem=-16963+164R and the photoluminescence spectrum, the bond distance of Mn-F could be estimated. Based on the obtained data it is easy to know the crystal parameter of samples with the perovskite-structure by results of photoluminescence spectra.
The principle and experimental law of testing sets to test reaction temperature change were introduced. Using “powder sputtering plane thin-film SnO2/CeO2 alcohol sensor”, the reaction temperature change was tested and the law of versus gas concentration was given by the inserting method. The rate equation of surface reaction was deduced and reaction probability of surface molecule was calculated. The general reaction rate equation includes two kinds of experiential formula—hyperbolic and power function formula and the concept of molecule reaction probability is minor, which in general is 10-3-10-5.
Using high resolution capabilities of a time-of-flight instrument and ion trap tandem technique, electron impact mass spectra of 5-acetyl (benzoyl)-4-aryl-3,4-dihydropyrimidin-2 (1H)-ones 1-5 were studied. The molecular ion (M) peaks for 1-3 can be found in the spectra with high abundances, but very weak for 4 and 5,in which strong electron-attracting substituents are attached to the benzene ring. The main fragmentation pathways for 1-5 include the cleavage of (M-Ar) + with high intensity, (M-RCO) + with moderate abundance, (M-H) +with high intensity for the compounds without strong electron-attracting substituent in the aromatic ring, and the pyrimidine ring cleavage (loss of neutral NH=C=X). In addition, a prominent cation (Ph + , m/z 77) can be found in the low mass region of the spectra for all the compounds, which give rise by different pathways between 1- 2 and 3-5. Several additional fragmentations for individual compounds are proposed.
The Langevin paramagnetic theory can’t describe the relation between magnetization of ferrofluids and applied magnetic field. The structuralization of ferrofluids, which is considered the main influence factor of the magnetization, is regarded. The part of magnetization works is deposited when the structure is forming. This action influences the magnetization of ferrofluids directly or indirectly. On the base of the “compressing” model, the Langevin function that usually describes the magnetization of ferrofluid is modified, and a well-fitted curve is obtained. An equation of the relation between the equivalent volume fraction after being “compressed” and the intensity of magnetic field is discovered, which approximately describes the process of magnetization. The relation between the approximate initial susceptibility and the volume fraction can be obtained from modified formula.
M~T curves and M~H curves of the systems La0.67Sr0.33CoO3, La0.67Sr0.33MnO3 (x=0.00, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.67) were studied by experiments. Experimental results show that the magnetic structure of La0.67-xGdxSr0.33CoO3 system exhibits the cluster glass state with doping Gd increasing. The M~T curves of samples x>0.10 exhibit a peculiar phenomenon that M value rises sharply at low temperature range. The magnetic structure of La0.67-xGdxSr0.33MnO3 system changes from the longrang ferromagnetic order to the cluster glass state, and antiferromagnetic state. The M~T curves of samples x≥0.50 decline sharply at low temperature range. The different phenomena shown by the two systems come from different coupling function between Gd and Co, Mn and from spin state transition of Co.
AgCuSe nanorods were prepared at room temperature by a redox reaction. The as-prepared product was characterized by X-ray powder diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.X-ray powder diffraction shows that the as-prepared product is the tetragonal phase of AgCuSe. Transmission electron microscopy shows that the sample consists of nanorods with a diameter varying from 5 nm to 20 nm and a length varying from 200 nm to 600 nm. X-ray photoelectron spectroscopy shows that the purity of the sample is high. The formation mechanism of AgCuSe and the growth mechanism of AgCuSe nanorods were discussed.Thermodynamic calulations show that the final product in the synthetic system is pure AgCuSe. The solvent ethylenediamine did not only acts a didentate ligand to form relatively state Ag + and Cu + complexes, but also dissolved Se and enhanced the reactivity of Se. It played an important role in controlling the nucleation and growth of AgCuSe nanorods.
Using the Sol-Gel method for producing the KTN ultrafine powder and the sintering technique with K2O atmosphere to prepare KTN ceramics as the targets instead of the KTN single crystal, highly oriented KTN thin films were produced on the transparent single crystal quartz (100) by the pulsed laser deposition (PLD). The XRD analysis indicates that the perovskite structure with (h00) orientation is the major phase (>98%) in KTN. The Current-Voltage characteristics were found to be Ohmic at low fields and space-charge-limited at higher fields. This phenomenon is reasonably explained by SCLC theory. The leakage current was lower than 250 μA/mm2 at 0-5 V, which shows the film has good ferroelectric performances. The frequency dependence of dielectric constant results demonstrates that the dispersion of electric capacity is large at low frequency but small at high frequency. The dielectric constant was 12600 at frequency of 10 kHz. The P~E hysteresis loop shows that the remanent polarization and the coercive field are 9.25 μC/cm2 and 7.32 kV/cm, respectively. The SEM results shows that the surfaces are homogeneous, smooth, crack-free and dense. The refractive index was 1.776 at incident wave length 1.2 μm, the thickness 968 nm, and the growth rate 0.027 nm per pulse.
By means of chemical co-deposition method in which added Ferric citrate and Ferrous Gluconate, Fe3O4-Dextran composite nano-particles having average diameter 26 nm, was obtained. The particles had a core of diameter 4.1 nm Fe3O4 with a shell of thickness 11 nm Dextran. In the experiment the critical structure of Fe3O4 inorganic particles abstained without N2 protection had no changes. From the result, it can be proven that organic salt has antioxidation properties and the composite particles have superparamagnetic whose good magnetization is 52 emu/g at 300 K, and 63 emu/g at 5 K. The structure, pattern diameter and magnetic properties of the Fe3O4-Dextran composite particles were examined by means of TEM, XRD, DLS and VSM.
ZnS:Mn2+ polystyrene (PS) core-shell structures and ZnS:Mn2+ hollow spheres were prepared by a sonoehemical deposition approach. Transmission electron micrograph (TEM) studies show that the PS surface is covered by a thin shell consisted of ZnS: Mn2+ nanoparticles with an average size of 9 nm. ZnS: Mn2+ hollow spheres were obtained by heating the core-shell particles in air at 500 ℃ to drive off PS. The photoluminescence spectrum for the emission band of Mn2+ peaked at 540 nm, and a 45 nm blue shift compared to that of corresponding bulk sample, was discussed based on the Mn-O octahedral distortion induced by shell structure.
The (14-x)SrCO3xCaCO324CuO system was investigated by means of differential thermal analysis (DTA) technique combined with the powder Xray diffraction (XRD) technique, and the relationship of its thermal behavior with the synthesis of spin-ladder compound Sr14-xCaxCu24O41 was first analyzed in detail. It is found that the DTA curve (x≤5.6) exhibits two endothermic peaks and could be divided into three reaction stages, corresponding to the solid state reaction to synthesize single-phase Sr14-xCaxCu24O41. The DTA curve (x≥8.4) shows three big endothermic peaks below 1000 ℃. The intensity and the position of the first endothermic peak around 800 ℃ is depending on the content of CaCO3 in the system, while the third endothermic peak is the crucial factor of the synthesis of single-phase compound.
Fourier-transform infrared emission spectroscopy was used to study the dehydroxylation behavior of the thermal decomposition of dickite from Chenxi, Hunan Province, China. Dehydroxylation of dickite was followed by a loss of intensity of the 3620.73, 3695.34 cm-1 OH-stretching bands and 916.06, 1009.33 cm-1 OHbending bands. The thermal decomposition was investigated by thermogravimetric analysis (TGA). A good agreement is found with TG curves of dickite and the mass loss is 13.7% (close to the theoretical value). The non-isothermal kinetics of the thermal decomposition of dickite was studied in TG-DTG curves over the temperature range from 298 K to 1123 K by thermogravimetry and differential thermal analysis in air. Mathematical analysis of TG-DTG data using the integral methods (Coats-Redfern equation, HM equation, MKN equation) and differential method (Achar equation) shows that the thermal decomposition of dickite accords F2 mechanism. The kinetic parameters such as the activation energy (E=131.62 kJ/mol), pre-exponential factor (A=108.30 s-1) and reaction order (n=2.1) are reported. The Ozawa method was used to analyse the activation energy of the same sample at different heating rate and gave 133.07 kJ/mol. The kinetic parameters calculated from different equation are rather close to each other.
A type of polymer-in-salt electrolyte composed of poly( vinyl alcohol), KOH and water was prepared by a solution casting method. X-ray diffraction proves that the high concentration of KOH in the electrolyte is in an amorphous state. The ionic conductivities of the PVA-KOH-H2O electrolytes increased as the concentration of KOH increased, and the alkaline electrolyte with PVA/KOH 1/3 (mass ratio) exhibited the highest ionic conductivity of 0. 15 S/cm at room temperature, as measured by electrochemical impedance spectroscopy. The temperature dependence of the conductivity is found to be in agreement with the Arrhenius equation. The potential stability window at the metal/electrolyte interface was of 1.4 V for the nickel electrode determined by cyclic voltammetry.
Two crystal forms (α and δ′ form) of nylon 11 were prepared by melting, icewater bath quenching and annealing. The characteristic of chain movement of two forms was investigated using dielectric relaxation spectroscopy in the frequency range from 42 Hz to 5 MHz. The dielectric temperature spectra at different frequencies show that the primary αrelaxation corresponding to the chain segment movement is located at higher temperature in α form, indicating the chain segment movement is restricted after δ′→α crystal transition. The activation energy of secondary βrelaxation remained almost unchanged for two forms, but the relaxation time was longer and the relaxation strength was weaker for α form, confirming that the local relaxation was also restricted to some extent after δ′→α crystal transition.
The performance of a newly designed, polyaniline activated carbon, hybrid electrochemical capacitor was evaluated. The polyaniline was prepared by the chemical oxidation/polymerization process. The capacitor was assembled by using polyaniline as a positive electrode and an activated carbon as a negative electrode respectively. From a cyclic voltammograms test, a specific capacitance of 420 F/g was obtained for polyaniline electrode. The cycle life of the cell is proved to be more than 1000 times by the Galvanostatic charge and discharge test. Values for the specific energy and real specific power of 15.5 Wh/kg and 2.8 W/g, respectively, are demonstrated for a cell voltage between 0.0 and 1.4 V. The max specific power for the hybrid capacitor amounts to 20.4 W/g.