Proposed Phenomenological PTCR Model and Accompanying Phenomenological PTCR Chart

The positive temperature coefficient of resistivity (PTCR) behavior of semiconductive BaTiO₃ is well explained by the Heywang model, which predicts the resistivity behavior above the Curie point based on the acceptor state density at the grain boundaries, the charge carrier density, and the energy gap, E _s, between the conduction band and the acceptor levels. However, the relationship between these parameters and the production parameters (sintering time, composition, and cooling rate) is not well understood. Recently, the present authors have found that E _s can be increased by thorough oxidation. This increase is attributed to a change in the oxidation state of the acceptor. Based on this finding and results from the literature, a phenomenological PTCR model and an accompanying PTCR chart for acceptor–donor-codoped BaTiO₃ are proposed to clarify this relationship. The PTCR chart clarifies that acceptor dopant concentrations, oxidation time, and oxygen partial pressure during oxidation or cooling can be optimized simultaneously to obtain optical PTCR properties.     

Structure of polyurethane elastomers. X-ray diffraction and conformational analysis of MDI-propandiol and MDI-ethylene glycol hard segments

X-ray diffraction and conformational analysis have been used to investigate the structure of polyurethane hard segments prepared from diphenylmethane 4,4′-diisocyanate (MDI) with propandiol (PDO) and ethylene glycol (EDO) as chain extenders. The results are compared with those obtained previously for MDI-butandiol (BDO) hard segments. In the latter system, the poly(MDI-BDO) chains are fully extended with an all-trans conformation for the O(CH₂)₄O unit, and a monomer repeat of 18.95 Å. The unit cell is triclinic with a tilted base plane such that adjacent chains are staggered along the fibre axis. In contrast both poly(MDI-PDO) and poly(MDI-EDO) adopt unstaggered structures, i.e. the chains are in register and the unit cell base planes are perpendicular to the chain axis. The monomer repeats of 16.2 Å for poly(MDI-PDO) and 15.0 Å for poly(MDI-EDO) are shorter than the predicted repeats for fully-extended chains, indicating that these polymers have contracted conformations containing some gauche CH₂ groups. Conformational analysis shows that the 16.2 Å repeat for poly(MDI-PDO) can be achieved with the O(CH₂)₃O unit in the trans-gauche⁺-gauche⁺-trans or gauche⁺-trans-trans-gauche⁺ conformations. Similarly the 15.0 Å repeat for poly(MDI-EDO) is predicted for the gauche⁺-trans-gauche^? conformation for the O(CH₂)₂O unit. These conformations are of higher energy than the all-trans fully-extended chains. This may explain the higher crystalline perfection of the poly(MDI-BDO) hard segments, for which crystallization in the all-trans form will probably provide a greater driving force for phase separation.     

Experimental Evaluation of the Acceptor-States Compensation in Positive-Temperature-Coefficient-Type Barium Titanate

Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTiO₃ are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with respect to 1/ T , instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 × 10¹⁷ m⁻²) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 10¹⁷ m⁻² (first reported by Jonker) obtained from the spontaneous polarization of BaTiO₃. This shows that the ferroelectric behavior of BaTiO₃ is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker.     

Water flux,DSC, and cytotoxicity characterization of membranes of cellulose acetate produced from sugar cane bagasse,using PEG 600

Summary In this article, cellulose acetate produced through the homogeneous acetylation of sugar cane bagasse cellulose was used to produce membranes, using poly(ethyleneglycol) 600 (PEG 600) as an admixture. The membranes were characterized using water flux measurements (Payne’s cup), differential scanning calorimetry (DSC) and neutral red uptake (cytotoxicity). The results showed that PEG 600 acts as a crystallinity inductor and/or pore former in the cellulose acetate matrix. The induction of crystallinity is important for this system since it had not been reported on the literature yet. The results also demonstrated that the studied membranes present a nontoxic behavior.     

Conformational analysis of poly(MDI-butandiol) hard segment in polyurethane elastomers

As part of our study of the structure of the hard segments of polyurethane elastomers, we have used conformational analysis to predict the conformation of the polyurethane chain formed from diphenylmethane 4,4′-diisocyanate (MDI), with butandiol as the chain extender. X-ray diffraction indicates the chain conformation is highly extended in the solid state, with a monomer repeat of 18.95 Å. The chain conformation can be predicted from the interaction between the large contiguous groups. Semiempirical and CNDO/2 molecular orbital calculations have been used to investigate the phenyl-phenyl, phenyl-urethane, and urethane-butandiol interactions. Minimum energy for the diphenyl-methane section occurs for a CCH₂C bridge angle of θ = 110°. However, the more likely conformation in the polymer is probably that at the subsidiary minimum at θ = 118°, in which the phenyl rings are mutually perpendicular, which is comparable to the conformation seen for model compounds. For the phenyl-urethane interactions the energy minimum is at $\text{ч = ±90°}$, with the combination $\text{(ч}{}_1\text{, ч}{}_2\text{) = (?90°, +90°)}$ leading to the necessary extended conformation. Calculations for the butandiol segment favour an all-trans conformation which is coplanar with the urethane groups. The predicted conformation of the polymer chain has a fibre repeat which matches the observed value, and this cannot be attained if the butandiol unit contains any gauche bonds. Model compounds show large variations for the phenyl-urethane conformation from the predicted minimum at $\text{ч = ±90°}$, indicating the importance of intermolecular stacking forces in determining the polymer conformation. However, provided the adjacent butandiol and urethane groups are coplanar, the fibre repeat is relatively insensitive to variations of ч.     

八碳烯分离工艺的研究

从能量综合集成的角度提出了八碳烯精馏分离工艺 ,对所提流程进行了优化模拟计算及分析 ,为八碳烯分离工艺的施工设计提供了充分的依据。投产情况表明 ,该研究改变了八碳烯的生产现状 ,使得八碳烯生产的经济效益显著提高     

Activities of MFI-Supported Rhenium Catalysts for the Aromatization of Methane: Effect of Cationic Form of the Inorganic Carrier

MFI type inorganic carrier was used in two different cationic forms, hydrogen and calcium respectively. MFI-supported molybdenum and rhenium catalysts were prepared. The activities of the catalysts were compared for the aromatization reaction of methane. Higher activity values were attained with the catalysts supported on HZSM-5. Aromatics were also observed with the catalysts supported on CaZSM-5, despite their deficiency in acid sites. Highly dispersed rhenium is expected to be formed with the use of the inorganic carrier in calcium form. On the other hand, lower reaction rates were observed with rhenium supported on CaZSM-5, in spite of the improved dispersion of the active rhenium species on this catalyst. This was interpreted in terms of the critical role of the acid sites in the conversion of methane to aromatics, compared to the improved dispersion of the active metal.     

合成丁酸异戊酯的新方法

采用超强酸代替浓硫酸作催化剂，对丁酸异戊酯合成反应进行了研究，讨论了影响收率的多种因素，在最佳条件下，酯化反应时间短，催化剂可以重复使用，丁酸异戊酯收率达到９８％。