Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States

The geometric structures and vibration frequencies of \begin{document}$ para $\end{document}-chlorofluorobenzene (\begin{document}$ p $\end{document}-ClFPh) in the first excited state of neutral and ground state of cation were investigated by resonance-enhanced multiphoton ionization and slow electron velocity-map imaging. The infrared spectrum of S\begin{document}$ _0 $\end{document} state and absorption spectrum for S\begin{document}$ _1 $\end{document}\begin{document}$ \leftarrow $\end{document}S\begin{document}$ _0 $\end{document} transition in \begin{document}$ p $\end{document}-ClFPh were also recorded. Based on the one-color resonant two-photon ionization spectrum and two-color resonant two-photon ionization spectrum, we obtained the adiabatic excited-state energy of \begin{document}$ p $\end{document}-ClFPh as 36302\begin{document}$ \pm $\end{document}4 cm\begin{document}$ ^{-1} $\end{document}. In the two-color resonant two-photon ionization slow electron velocity-map imagin spectra, the accurate adiabatic ionization potential of \begin{document}$ p $\end{document}-ClFPh was extrapolated as 72937\begin{document}$ \pm $\end{document}8 cm\begin{document}$ ^{-1} $\end{document} via threshold ionization measurement. In addition, Franck-Condon simulation was performed to help us confidently ascertain the main vibrational modes in the S\begin{document}$ _1 $\end{document} and D\begin{document}$ _0 $\end{document} states. Furthermore, the mixing of vibrational modes between S\begin{document}$ _0 $\end{document}\begin{document}$ \rightarrow $\end{document}S\begin{document}$ _1 $\end{document} and S\begin{document}$ _1 $\end{document}\begin{document}$ \rightarrow $\end{document}D\begin{document}$ _0 $\end{document} has been analyzed.     

Mixing time of a non-Newtonian liquid in an unbaffled agitated vessel with an eccentric propeller

Experimental study of mixing time of non-Newtonian shear-thinning fluids within the transitional regime (3 × 10² < Re < 3 × 10³) of liquid flow is presented. The purpose of this study was to analyze the effect of eccentricity and pumping mode of the impeller as well as of position of the tracer dosage point into the agitated liquid on mixing time. The measurements were conducted in an unbaffled agitated vessel with inner diameter D = 0.7 m equipped with an up-or down-pumping propeller located centrically (e/R = 0) or eccentrically (e/R ≠ 0) in the vessel. Experiments were carried out by means of computer-aided unsteady-state thermal method for three positions of the tracer dosage point. The experimental data show that eccentric position of the propeller in an unbaffled vessel causes a decrease of the mixing time compared to that obtained in a vessel with a centrically located propeller. Mixing time depends also on the pumping mode of the propeller as well as on the position of the tracer dosage point. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

无机SiO2-TiO2前驱体体系合成高性能TS-1分子筛

 在热处理条件下,使Ti(SO4)2在多孔硅胶表面自发分散形成具有类似于钛硅分子筛催化活性中心结构单元的Si-O-Ti键的SiO2 -TiO2前驱体,将该前驱体在模板剂四丙基氢氧化铵的作用下水热晶化得到了钛硅分子筛TS-1. X射线衍射、红外光谱和扫描电镜表征表明, TS-1分子筛晶化完全,钛原子已进入TS-1分子筛骨架. 苯酚羟基化反应测试结果表明,前驱体的热处理温度对合成的TS-1分子筛的催化性能影响很大,处理温度以450 ℃为宜.     

Dielectric and electro-optic properties of two biphenylyl benzoate-based ferroelectric mesogens with tilted hexagonal phases

ABSTRACT

Two partially fluorinated ferroelectric liquid crystals (2F3R, 3F3R) with biphenylyl benzoate core have been investigated by frequency domain dielectric spectroscopy and electro-optic method. Both the compounds exhibit SmC* phase over a wide range of temperature along with tilted hexatic phase SmF* and soft crystal phases SmJ* (in 2F3R) and SmG* (in 3F3R) and one coexistence phase of (SmF*+SmC*). Both BOO phason and tilt phason relaxations are observed in SmF* phase and cooperative relaxation behaviour is also observed in crystal-like smectic phases. Goldstone mode relaxation is observed in SmC* phase and Maxwell Wagner mode is observed in all the phases. In 2F3R soft mode is detected in both SmC* and SmA* phases but not in 3F3R which melts directly to isotropic phase from SmC* phase. Compounds possess moderate values of spontaneous polarisation and exhibit quite sharp electrical response especially in 2F3R. Rotational viscosities, pitch and elastic constant are also measured. Effect of chain length and fluorination on various physical parameters has been discussed.     

Electrical and rheological percolation of PMMA/MWCNT nanocomposites as a function of CNT geometry and functionality

Composites of poly(methyl methacrylate) (PMMA) with multi-walled carbon nanotubes (MWCNT) of varying aspect ratio and carboxylic acid functionality were prepared using melt mixing. The extent of dispersion and distribution of the MWCNTs in the PMMA matrix was investigated using a combination of high-resolution transmission electron microscopy (HRTEM), wide-angle X-ray diffraction (XRD) and Raman spectroscopy. The electrical resistivity and oscillatory shear rheological properties of the composites were measured as a function of MWCNT geometry, functionality, and concentration. The fundamental ballistic conductance of the pristine free-standing MWCNTs was investigated using a mechanically controlled break-junction method. The electrical conductivity of PMMA was enhanced by up to 11 orders of magnitude for MWCNT concentrations below 0.5 wt.%. MWCNTs having higher aspect ratio, above 500, or functionalized with carboxylic acid groups readily formed rheological percolated networks with thresholds, determined from a power law relationship, of 1.52 and 2.06 wt.%, respectively. The onset of pseudo-solid-like behaviour and network formation is observed as G′, η∗, and tan δ⁻¹ are independent of frequency as MWCNT loading increased. Sufficiently long and/or functionalized tubes are required to physically bridge or provide interfacial interactions with PMMA to alter polymer chain dynamics. Carboxylic acid functionalization disrupts the crystalline order of MWCNTs due to a loss of π-conjugation and electron de-localisation of sp² C-C bonds resulting in non-ballistic electron transport in these tubes, irrespective of how highly dispersed they are in the PMMA matrix.     

NdBaFe2O5+w and steric effect of Nd on valence mixing and ordering of Fe

NdBaFe₂O₅ above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Mössbauer spectroscopy and compared with GdBaFe₂O₅ that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Mössbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d_xz orbitals of Fe²⁺ is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe₂O₅. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe²⁺ nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe₂O₅ causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Mössbauer isomer shift. The charge ordering in NdBaFe₂O_5+w is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe^2.5+ is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms.     

Enthalpies of mixing of liquid alloys in the In-Pd-Sn system and the limiting binary systems

The partial and integral enthalpies of mixing of molten binary In-Pd (up to about 29 at.% Pd), In-Sn (entire compositional range) and Pd-Sn (up to about 53 at.% Pd) alloys were determined at 900 °C. A Calvet-type microcalorimeter was used for the measurements employing a drop calorimetric technique. Additionally, five sections in the ternary In-Pd-Sn system (compositions up to about 40 at.% Pd) were investigated at 900 °C. The ternary interaction parameters were fitted using the Redlich-Kister-Muggiano model for substitutional solutions. The isoenthalpy curves for In-Pd-Sn at 900 °C were constructed for the integral molar enthalpy of mixing. Furthermore, the experimental results in the ternary system were compared with calculated values obtained by employing different binary extrapolation models.     

孪生球状碳酸钙的直接混合沉淀法制备及表征

以醋酸钙和碳酸钠为原料, 柠檬酸三钠为晶形控制剂, 利用液相直接混合沉淀法合成了分散性好、粒度约1.5~3.0 μm、长短轴比约2∶1的孪生球形碳酸钙晶体. 利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、傅里叶红外光谱仪(FTIR)、原子力扫描探针显微镜(ASPM)和粒度分析仪等对样品进行了表征. 结果表明, 在不添加柠檬酸三钠的溶液中得到微米级的立方状碳酸钙晶体, 而添加柠檬酸三钠(质量分数30%~40%)后则得到具有不同表面粗糙度的孪生球状碳酸钙晶体. 同时, 用分形生长理论和成核限制聚集(NLA)模型对孪生球状碳酸钙粒子的形成机理进行了分析.     

Calorimetric investigations of the MBr−NdBr3 melts (M=Li,Na, K,Cs)

Present work is a part of thermodynamic research program on the MX?LnX₃ system (M=alkali metal,X=Cl, Br andLn=lanthanide). Molar enthalpies of mixing in the LiBr?NdBr₃, NaBr?NdBr₃ and KBr?NdBr₃ liquid binary systems have been determined at temperature 1063 K by direct calorimetry in the whole range of composition. Investigated systems are generally characterized by negative enthalpies of mixing with minimum atX _NdBr3≈0.3–0.4. These enthalpies decrease with decrease of ionic radii of alkali metals. Molar enthalpies of solid-solid and solid-liquid phase transitions of K₃NdBr₆ and Cs₃NdBr₆ have been also determined by differential scanning calorimetry (DSC). K₃NdBr₆ is formed at 689 K from KBr and K₂NdBr₅ with enthalpy of 44.0 kJ·mol^?1 whereas Cs₃NdBr₆ is stable at ambient temperature and undergoes phase transition in the solid state at 731 K with enthalpy of 8.8 kJ·mol^?1. Enthalpies of melting have been also determined.