Time averaged temperature calculations in pulse electrochemical machining, spectral approach

Simulation of the temperature distribution during the Pulse Electrochemical Machining (PECM) process provides information on system design and guidelines for practical use. The pulses that are applied to the PECM system have to be described on a time scale that can be orders of magnitude smaller than the time scale on which the thermal effects evolve. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the temperature distribution in PECM can become a computationally very expensive procedure. A different approach is used by time averaging the heat sources of the system. Performing this, the time steps used during the calculations are no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfying results can be obtained. In previous work of the authors, the hybrid calculation and the Quasi Steady State ShortCut (QSSSC) were introduced. This method allows to perform simplified calculations while getting satisfactory results. The method introduces errors however, which were quantified using analytical solutions and found to be acceptable. The results applied only to rectangular pulses. In this work, the more general case of arbitrary pulse forms is considered using a spectral approach.     

Alumina/Silicon Carbide Laminated Composites by Spark Plasma Sintering

Ceramic laminates composed of alumina/silicon carbide composite layers were produced by spark plasma sintering (SPS). Monolithic composite disks containing up to 30 vol% of silicon carbide were fabricated by stacking together and cosintering by SPS green layers prepared by tape casting water-based suspensions. An engineered laminate with a specific layer combination that is able to promote the stable growth of surface defects before final failure was also designed and produced. Fully dense materials with an optimum adhesion between the constituting layers and a homogeneous distribution of the two phases were obtained after SPS. Monolithic composites showed an increasing strength with SiC load, and biaxial strength values as high as 700 MPa were observed for a SiC content of 30 vol%. The engineered laminate showed a peculiar crack propagation that is responsible for the high strength value of about 600 MPa and for the evident insensitivity to surface defects.     

Effect of Glu-143 and His-231 substitutions on the catalytic activity and secretion of Bacillus subtilis neutral protease

On the basis of the homology with the Bacillus thermoproteolyticuszinc endopeptidase thermotysin, we hypothesized that Glu-143and His-231 are the key residues for the catalytic activityof the Bacillus subtilis neutral protease. To test this possibilityby site-directed mutagenesis, we substituted these two residueswith Ala, Ser, Trp and Arg, and Leu, Val and Cys respectively.All these substitutions dramatically affected the amount ofsecreted mutant proteins, as determined by immunological methods,and their catalytic activities. No appreciable secretion wasobserved with the three Glu mutants Trp, Ser and Arg, whereasthe Glu–Ala mutant enzyme was secreted at a level of afew hundred micrograms per litre of culture. The His mutantswere all secreted at higher levels (in the order of a few milligramsper litre) and their residual catalytic activity could be determinedusing Z-Ala-Leu-Ala as substrate. Our results confirm the keyrole played by Glu-143 and His-231 in catalysis and moreoversuggest the existence of a relationship between the catalyticactivity of the enzyme and the extent of its secretion. In thiscontext, we present data suggesting an autoproteolytic mechanismof cleavage of the precursor form of the enzyme, analogous tothe one previously reported for the B.subtilis subtilisin.     

Diffusion of organic penetrants through low density polyethylene (LDPE) films: Effect of size and shape of the penetrant molecules

The diffusion coefficient at zero penetrant concentration D₀ of dichloromethane, chloroform, carbon tetrachloride, cyclohexane, benzene, o-xylene, m-xylene, and p-xylene, and n-hexane in LDPE were measured at 25°C, using the desorption method. The D₀ values obtained in this way are correlated with the size, shape, and chemical nature of the penetrant molecules. The temperature dependence of the diffusion coefficients of toluene and n-hexane in LDPE are also reported in the limited temperature range of 25–45°C. It indicates that, in spite of a size larger than that of toluene, n-hexane has a lower activation energy of diffusion.     

Engineering plastics from lignin. XVI. Starlike macromers with propylene oxide

Starlike macromers were prepared from hydroxypropyl organosolv lignin by reaction with propylene oxide, and they were analyzed by a combination of conventional analysis techniques. The average number of arms per macromer was controlled by partial capping with an alkoxy group; and the average length of arms by the degree of chain extension with propylene oxide. Analysis methods included treatment with hydriodic acid followed by gas chromatographic separation of alkyl iodides (HI/GC), UV spectroscopy, H-NMR spectroscopy, and thermal analysis. The results were consistent with a hypothetical pentameric model structure having between two and six radiating arms, each with a length of between 1 and 4 propylene oxide units. The UV method was best qualified to determine degree of chain extension, while HI/GC was best suited for analyzing average number of arms per macromer fragment. The synthesis and analysis of starlike macromers from lignin is viewed as an important stepping stone for the formulation of lignin-based engineering plastics and multiphase materials.     

Possible role of two phenylalanine residues in the active site of human cytidine deaminase

Site-directed mutagenesis on human cytidine deaminase (CDA)was employed to mutate specifically two highly conserved phenylalanineresidues, F36 and F137, to tryptophan; at the same time, theunique tryptophan residue present in the sequence at position113 was mutated to phenylalanine. These double mutations wereperformed in order to have for each protein a single tryptophansignal for fluorescence studies relative to position 36 or 137.The mutant enzymes thus obtained, W113F, F36W/W113F and F137W/W113F,showed by circular dicroism and thermal stability an overallstructure not greatly affected by the mutations. The titrationof Trp residues by N-bromosuccinimide (NBS) suggested that residueW113 of the wild-type CDA and W36 of mutant F36W/W113F are buriedin the tertiary structure of the enzyme, whereas the residueW137 of mutant F137W/W113F is located near the surface of themolecule. Kinetic experiments and equilibrium experiments withFZEB showed that the residue W113 seems not to be part of theactive site of the enzyme whereas the Phe/Trp substitution inF36W/W113F and F137W/W113F mutant enzymes had a negative effecton substrate binding and catalysis, suggesting that F137 andF36 of the wild-type CDA are involved in a stabilizing interactionbetween ligand and enzyme.     

PBT的固相缩聚

对固相缩聚的机理和影响因素进行了介绍。认为聚对苯二甲酸丁二酯 ( PBT)是一种重要的工程塑料 ,高的分子质量使其性能得到提高。作为生产 PBT高粘切片的有效方法 ,固相缩聚依赖于化学 (可逆化学反应 )和物理 (小分子的扩散 )两个过程     

催化剂颗粒的形状优化Ⅱ有限长圆柱和常见动力学

从颗粒模型的角度对有限长圆柱催化剂颗粒的外形优化设计（异形化）进行了理论研究,针对常见的反应动力学形式,提出了普遍Ｔｈｉｅｌｅ模数的概念和推导方法,简化了颗粒模型求解,对不同动力学形式下,有限长圆柱形催化剂颗粒的形状优化进行了理论分析。     

Diffusion of saturated hydrocarbons in low density polyethylene (LDPE) films

The diffusion coefficients at zero penetrant concentration, D₀, of n-heptane, n-heptane, n-octane, n-decane, and 2,2,4-trimethylpentane (TMP) in LDPE were obtained in the range of 25–50°C, using the desorption method. The dependence of D₀ on the size and shape of the penetrant is reported. It was found that D₀ decreases with increasing penetrant molecule size. The activation energies of diffusion in the temperature range of 25–50°C increase with increasing penetrant molecule size and are independent of temperature. The results are interpreted in terms of the free volume theory and semiquantitative estimates of the free volume parameters are reported.     

Characterization of the isothermal crystallization of poly(ethylene terephthalate) by microhardness measurements

The two-stage isomerization isotherms of poly(ethylene Terephthalate) have been followed through microhardness analysis. A great variation in microhardness values seems to prove the increase of the strength and stiffness of the samples produced by the secondary isomerization. A smaller variation is observed for the primary isomerization.