Structural study of a series of ethylene-tetrafluoroethylene copolymers with various ethylene contents, Part 2: Phase transition behavior investigated by temperature dependent measurements of X-ray fiber diagrams

Phase transitional behavior has been investigated for a series of ethylene (E) and tetrafluoroethylene (TFE) two-component copolymers on the basis of the temperature dependent measurement of X-ray fiber diagrams taken for the uniaxially-oriented samples. The usage of such uniaxially-oriented samples has allowed us to clarify the structural changes more definitely than before. So far, the crystalline transition from the low-temperature phase to the high-temperature phase had been assumed to occur continuously, but the detailed analysis of the X-ray reflections at higher scattering angles revealed for the first time that the transition is of the thermodynamically-discontinuous first-order type between the monoclinic and the pseudohexagonal phases. This phase transition temperature shifted toward lower temperature side as the TFE content became higher. The analysis of the X-ray reflection profile allowed us to imagine that low-temperature shift of the transition point is considered to occur due to the easier thermal motion of the planar-zigzag chains around the chain axis because of looser packing of chains containing higher population of bulky CF₂CF₂ groups. From these experimental data a phase diagram has been built up to show the phase transition behaviors of a series of E/TFE copolymers in a systematic manner. The characteristic features of the structural disordering including trans-gauche conformational exchange have been also discussed on the basis of the data of polarized infrared and Raman spectra combined with the X-ray diffuse scatterings.     

Crystallization of TS-1 in the presence of alcohols: influence on Ti incorporation and catalytic activity

TS-1 has been synthesized following various recipes of the literature in the presence of alcohols, i.e., from gels that were not evaporated prior to crystallization, which considerably reduces the preparation time of the zeolites. When samples are prepared following a method adapted from the original patent, extra-framework species are observed even for low Ti contents. In contrast, when hydrolysis of the Si and Ti sources is performed in the presence of isopropyl alcohol, all Ti in the zeolite is incorporated in the lattice. However, incorporation is limited to about 1.9 Ti/u.c., even when catalysts are crystallized from gels with relatively high Ti concentrations. These samples are active in the hydroxylation of phenol with H₂O₂ and their activity is comparable with that of conventional catalysts obtained from alcohol-free gels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural investigation of Li1−xNi0.5Co0.25Mn0.25O2 by in situ XAS and XRD measurements

A combination technique of in situ synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) was employed to study the Li_1−xNi_0.5Co_0.25Mn_0.25O₂ cathode material for Li-ion battery. The Li/Li_1−xNi_0.5Co_0.25Mn_0.25O₂ cell with x = 0.82 charged to 4.5 V showed the first charge capacity of 225 mAh/g. The X-ray absorption near edge structure (XANES) indicated that the initial valences were +2/+3, +3 and +4 for Ni, Co and Mn, respectively. The main redox reaction during delithiation was achieved by Ni via the reaction Ni²⁺ → Ni³⁺ followed by Ni³⁺ → Ni⁴⁺. The oxidation states of Co and Mn remained Co³⁺ and Mn⁴⁺. The bond length of Ni-O decreased drastically, while the Co-O and Mn-O distances exhibited a slight change with the decrease of Li content in the electrode. It was further revealed that all the second shell metal-metal (Ni-M, Co-M and Mn-O) distances decreased due to the oxidation of metal ions. In situ XRD data showed that both a- and c-axes varied with different Li contents in this material system. At the beginning of charge, there was a contraction along the c-axis and a slight expansion along the a-axis. As x reached 0.57, the trend of the variation in c-axis was opposite. The changes of lattice parameters could be explained by the balance between ionic radius and the repulsive force of the layer-structured material.     

Study of the hexagonal to monoclinic celsian phase transition induced by magnetic phase nucleation in barium aluminosilicate glass-ceramics

The inert hexagonal to monoclinic celsian (H-M) phase transition extremely restricts the formation of celsian. In this study, Fe³⁺ dopants were incorporated into nonmagnetic barium aluminosilicate (BAS) glass-ceramics using Fe(NO₃)₃.9 H₂O as raw material for preparing the celsian phase. As the Fe-Ba mole ratio reached 0.238, the celsian phase can be obtained in magnetic BAS glass-ceramics. It implied that some magnetic reaction products facilitated the H-M phase transition. To examine the above inference, magnetic barium ferrite (BFO) additions were externally incorporated into a pure BAS glass matrix. Results suggested that BFO additions can significantly promote the H-M phase transition. When the addition of BFO was increased to 5 wt %, almost all of the hexacelsian phase was converted into the celsian phase. And the reason was that barium ferrite can act as a nucleation agent to induce the distorted hexacelsian phase decreasing the H-M phase transition barrier.     

Sulfation: a simple method to enhance the catalytic activity of TS-1 in epoxidation of 1-octene with aqueous hydrogen peroxide

Titanosilicalite (TS-1) has been successfully modified by sulfation to exhibit enhanced catalytic activity in the oxidation of 1-octene with aqueous H₂O₂. A high activity of the sulfated TS-1 was related to modifications of the local environment of Ti active site upon interaction with the .     

High rhombohedral to tetragonal phase transition temperature and electromechanical response in Pb(Yb1/2Nb1/2)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3 ferroelectric system near the morphotropic phase boundary

Relaxor-PT crystals, that is, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) and Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT), have been used in mechanical to electric energy translation devices, such as for high-frequency imaging and underwater detection. However, low temperature usage remains a problem for high temperature applications, which are restricted by rhombohedral to tetragonal phase temperatures (T _R-T). In this work, we synthesized a new xPb(Yb_1/2Nb_1/2)O₃-yPb(Sc_1/2Nb_1/2)O₃-0.37PbTiO₃ (x = 0.2?0.45) (xPYN-yPSN-0.37 P T) ternary piezoelectric polycrystalline with high T _R-T = 225?245 °C. For 0.45PYN-0.18PSN-0.37 P T ceramics, the piezoelectric coefficient d₃₃, mechanical coupling factors k₃₃, k_p, and k_t, and dielectric coefficients ε_r and T _R-T are on the order of 330 pm/V, 68.3%, 54.3%, 49.3%, 930, and 245 °C, respectively. Thermal piezoelectric and ferroelectric behaviors were investigated in 0.45PYN-0.18PSN-0.37 P T polycrystalline, which are good candidates for high temperature piezoelectric application. DSC curves for xPYN-yPSN-0.37 P T systems present two peaks at 1213 and 1291 °C during the cooling process, showing good crystallization behavior and suitability for single crystal growth.     

Characterization of a thermoset by thermal analysis techniques: Criterion to assign the value of the α‐transition temperature by dielectric analysis

A cured thermoset composed of diglycidyl ether of bisphenol A and m‐xylylene diamine as the cure agent was studied with different thermal analysis techniques, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and dielectric analysis (DEA). DSC was used to measure the glass‐transition temperature and to check the absence of the heat of reaction. DMA and DEA were used to show the existence of two transitions in the temperature range of −100 to 240°C. The transition at a low temperature corresponded to the β transition. The second one, at a higher temperature, was associated with an α transition. The β transition followed Arrhenius behavior, whereas the α transition followed Vogel behavior. For an analysis of the α transition, different equations, such as the Havriliak–Negami, Vogel, and Williams–Landel–Ferry equations, were used. Important differences related to the fitting parameters were found that depended on the type of equation and the operation mode used. For this reason, a new method for calculating the α‐transition temperature was examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2027–2037, 2005     

Electronic phase transition in CrN thin films grown by reactive RF magnetron sputtering

Chromium nitride thin films were prepared by reactive radio frequency magnetron sputtering on glass and Si(001) substrates. Thin films were grown at different nitrogen to argon flow rate ratio and their effect on the film composition, band gap and electronic phase transition was studied by different experimental technique. X-ray diffraction analyses establish that CrN films predominantly grow in [111]_CrN and [002]_CrN directions irrespective of the substrates used for the growths. The band gap was found to vary with the composition of the films. All films are semiconducting at room temperature and show discontinuity in their resistivity versus temperature curve indicating electronic transition. But the low temperature electronic phase depends on the growth conditions and composition of the films.     

Microwave irradiation: an effective method for the preparation of low dispersed Pd/Al2O3 catalysts used in the hydrodechlorination of CCl2F2 to CH2F2

Microwave irradiation is found to be a faster method for the preparation of low dispersed Pd/Al₂O₃ catalysts with higher activity and selectivity to CH₂F₂ in the hydrodechlorination of CCl₂F₂. This revised version was published online in July 2006 with corrections to the Cover Date.