啮合型同向旋转双螺杆螺纹头数选择及螺杆间隙设计

本文从双螺杆啮合几何学出发,分析了啮合型同向旋转双螺杆各功能段螺纹头数选择准则,探讨了这类双螺杆挤出机中三种典型螺杆间隙的设计方法。这些准则及方法可直接用于指导螺杆结构设计。     

异向双螺杆挤出机啮合原理及计算机辅助设计（CAD）

本文对异向全啮合双螺杆挤出机的几何原理进行了研究，给出了端面曲线，螺杆形状，加工刀型曲线的数学模型，并编制了相应的计算机设计软件。     

非啮合异向旋转波状双螺杆挤出机性能的研究

在本文中,将波状螺杆概念引入非啮合双螺杆挤出机设计,研制出一种新的NCWTE(非啮合异向旋转波状双螺杆挤出机)挤出系统,并建立了描述并列型NCWTE 熔体输送和混合特性的理论模型。借用流体有限元方法对流场进行求解,并对某些计算结果进行了实验验证,两者吻合性较好。为比较 NCWTE 和 NCNTE(非啮合异向旋转非波状双螺杆挤出机)的混合特性,进行了大量混合实验,结果表明 NCWTE 的混合特性优于 NCNTE。     

啮合双螺杆输送及混合性能评价

比较了单螺杆、啮合同向旋转和异向旋转双螺杆的输送和混合性能，为认识啮合双螺杆的工作机理和选用双螺杆提供了参考。     

从W＆P公司挤出机的两个设计指标看啮合同向双螺杆挤出机的发展趋势

从Ｗ＆Ｐ公司生产的双螺杆挤出的两个设计指标，作者讨论了啮合同向双螺杆挤出机的发展趋势，并建议发展浅槽型和深槽型两种同向双螺杆挤出机。     

啮合同向双螺杆挤出机螺纹元件三维流场分析

利用ANSYS有限元分析软件对啮合同向双螺杆挤出机的螺纹元件流场进行了三维等温非牛顿模拟分析,通过速度场求出了流量及回流量,并求出拉伸速率,剪切速率及剪切应力来衡量混合效果,得到了机筒表面和啮合区的压力分析,找出了螺纹的导程,间隙及两端压差等参数对挤出机混合效果的影响。     

啮合型双螺杆输送及混合性能评价

比较了挤出工程中的单螺杆，啮合同向、异向双螺杆的输送和混合性能的差异，为认识啮合双螺杆的工作机理和选用双螺杆提供了帮助和参考。     

混合元件对玻璃纤维增强聚丙烯中玻璃纤维长度的影响

对啮合同向双螺杆挤出机90 °捏合块、六棱柱、变间隙转子3种混合元件流道流场进行了数值模拟,分析3种元件的混合性能。实验研究了使用3种不同元件的螺杆组合对玻璃纤维增强聚丙烯（GFRPP）复合材料中玻璃纤维长度的影响。结果表明,啮合同向双螺杆挤出机中聚合物熔体产生的剪切对玻璃纤维长度影响明显,混合元件的剪切越强,制品中玻璃纤维的平均长度就越短,制品的拉伸强度和冲击强度越低。     

相似放大理论在大型啮合同向旋转双螺杆挤出机设计中的应用

应用"当量直径"的方法,分析了在啮合同向旋转双螺杆挤出机内高聚物融体热传递相似、混合相似和剪切相似(Maddock)的条件,得到了3组主要性能参数相似放大的关联式.用此关联式对两类大型啮合同向旋转双螺杆挤出机系列的主要性能参数进行了预测,预测结果与实际较为吻合.     

混炼工艺技术的最新趋势

粗略回顾了开炼机、密炼机和挤出机的混炼工艺技术的基本概况，介绍了聚合物原料的性能及投料形状对混炼工艺的影响。深入细致地分析了混炼中各种参数的设定。运用流变学理论，指导密炼机转子断面几何形状的设计，提出凸棱功能技术(WFT)和Coflow密炼机。指出使用同向旋转啮合式双螺杆挤出机可使连续加工成为可能。     

Tetrahedral amorphous boron nitride: A hard material

We generate a tetrahedrally coordinated amorphous boron nitride (BN) model by means of first principles molecular dynamics calculations and report its mechanical and electrical properties in detail. The amorphous configuration is almost free from chemical disorder and consists of about 20% coordination defects, similar to tetrahedral (diamond-like) amorphous carbon. Its theoretical band gap energy is about 2.0 eV, less than 4.85 eV estimated for cubic BN. The bulk modulus and Vickers hardness of tetrahedral amorphous BN are computed as 206 GPa and 28-35 GPa, respectively. Based on these findings, we propose that tetrahedral noncrystalline BN can serve as electronic and hard materials as well.     

Shape and size stability of Pt nanoparticles for MeOH electro-oxidation

Pt nanoparticles (NPs) of different shape (cubic and octahedral/tetrahedral) and size (5-10 nm) were synthesized by one-step polyol-based synthetic procedures that permit the concomitant control of the shape and size. The shape and size stability of these particles under repetitive methanol (MeOH) electro-oxidation cycles was investigated by transmission electron microscopy (TEM) as well as cyclic voltammetry (CV). The TEM images of the particles taken before and after being subject to 1000 MeOH electro-oxidation CV cycling and the corresponding hydrogen adsorption/desorption profile in the CVs indicated that, the octahedral/tetrahedral Pt NPs had the highest stability among the samples investigated. Infra-red measurements suggest that this observation may be a result of exceptionally strong interaction of the capping polymer with octa/tetrahedral Pt NPs. These findings indicate that the synthetic procedure employed in this study can be used to synthesize highly stable catalyst particles with tunable shape and size.     

The use of the ring gap sizer for characterization of particle shape in the sieve range

The ring gap sizer sorts particles by their minimum dimension, i.e. thickness. This new instrument has been used to establish particle shape factors and distributions of powders in the sieve range.The powders investigated were quartz and tablet granules, i.e. powders consisting of non-equidimensional particles. It is shown that the combination of results obtained by the ring gap sizer, dry sieving and microscopy gave log-normal distributions of particle flakiness. This was in good agreement with results obtained by a microscopical reference method.For the quartz powder the variation of particle elongation with changing particle size was also determined, using the ring gap sizer to obtain samples of varying particle size, which were then measured on length and breadth by microscopy.Additionally, the two average values of flakiness and elongation have been used to calculate the Heywood surface and volume shape coefficients.     

The influence of cationic surfactant CTAB on optical,dielectric and magnetic properties of cobalt ferrite nanoparticles

In the present work, the influence of cationic surfactant CTAB (cetyltrimethylammonium bromide) on size, shape and coalescence behaviour of cobalt ferrite nanoparticles (CFNPs) synthesized via hydrothermal method is reported. Pure CFNPs show no additional peaks, whereas α-Fe₂O₃ phase is observed in CTAB added CFNPs upon annealing. FT-IR analysis confirms the formation of M − O vibrational bands (metal -oxygen) at tetrahedral A-site and octahedral B-site for both samples. SEM observations reveal less agglomeration and smaller particle size for surfactant added CFNPs. Raman spectral study confirms the formation of cubic spinel structure and Raman active modes of CTAB added CFNPs. UV–Vis spectra indicate a decrease in the energy band gap with annealing. The dielectric constant of surfactant added CFNPs decreases with increasing applied frequencies for both real and imaginary, but ac conductivity increases with increasing frequencies. Two sextet patterns of Fe³⁺ trivalent ions from tetrahedral and octahedral sites are observed in Mössbauer spectra. VSM study indicate the ferrimagnetic nature of CTAB added CFNPs. The electrochemical analysis reveals the pseudocapacitive nature of working electrode prepared by CTAB added CFNPs.     

Size dependence of energy gaps in small carbon clusters: the origin of broadband luminescence

The visible broadband luminescence from carbon-related films has recently been attributed to the band-tail states caused by the variations in the energy gap of individual sp² carbon clusters due to their difference in size and/or shape. In this paper, these band-tail states are classified into two parts: localized and confined. The localized states result from the structural deviation from graphite-like configuration, and the associated luminescence may be described by using the conventional theory for amorphous materials. The confined states are generated due to the existence of stable graphite-like local structures with various sizes and are the main factor for giving efficient, room-temperature luminescence. Our calculations of a series of small hexagonal carbon clusters with first-principle and semi-empirical methods demonstrate that the energy-gap distribution, due to the difference in size, is considerably broad, which may explain the broadband feature of luminescence. Calculations for some tetrahedral clusters were also made for comparison.     

The effects of local structural relaxation on aluminum siting within H-ZSM-5

Semiempirical molecular orbital calculations have been performed to study aluminum siting in H-ZSM-5 zeolites. Local structural rearrangements upon substituting aluminum (with a charge compensating proton) for silicon are found to be important. The T12 site is found to be the most preferred site for aluminum substitution. However, the calculated energetics for substitution show that several tetrahedral sites are energetically comparable with regard to aluminum siting. Results pertaining to the electronic properties of the acidic site upon aluminum substitution at each of the twelve distinct tetrahedral sites are presented. The acidic center is found to be a rather soft species, with the HOMO-LUMO energy gap being roughly 8 eV.     

Density of xNa2S.(1 – x)B2S3 (x = 0 to 0.8) Glasses: Correlation with Short-Range Order

The densities of xNa₂S + (1 – x)B₂S₃ glasses have been measured for 0 ≤ x ≤ 0.80. The variation of the density with x is quite strong and is characterized by a sharp maximum at x ∼0.30. The density increases on the low-alkali side of the maximum from 1.7 g/mL for vitreous B₂S₃ up to 2.2 g/mL for the x = 0.3 glass and decreases on the highalkali side to 1.8 g/mL. The increase in the density at low alkali is associated with the increasing fraction of the heavy mass and relatively small volume tetrahedral boron group, Na⁺BS₄^–. The density decrease in the high-alkali range is associated with the decreasing fraction of these groups with the concomitant increase in the fraction of trigonal boron groups with increasing numbers of nonbridging (terminal) sulfurs. To model the density, a weighted fractional mass and volume model was used. The molar volumes of the individual short-range order (SRO) groups were determined from compositions where a particular SRO group was the single group in the glass, by fitting the density data, and by interpolating between groups of known volume. The composition dependence of the fractions of the individual SRO groups was developed by combining spectroscopic evidence with chemically reasonable hypotheses of the SRO of the glasses. In this way, the density was calculated from first principles with one adjustable parameter, that of the volume of the tetrahedral boron group. The calculated densities were found to agree well with the experimental values.     

Theoretical Structure Determination of a Complex Material: κ-Al2O3

The atomic and electronic structures of kappa-Al₂O₃ are determined using theoretical first-principles techniques based on density-functional theory (DFT), plane waves, and pseudopotentials. The obtained structure is confirmed by analysis of powder X-ray diffraction data. The structure is orthorhombic with oxygen ions in close-packed ABAC stacking and aluminum ions occupying both tetrahedral (1/4) and octahedral (3/4) interstitial sites. A growth model for chemical vapor deposition of kappa-Al₂O₃ is proposed based on the atomic structure. Calculated electronic structure and charge density yield a band gap of 5.3 eV and a high ionic character of the bonds. The study shows the applicability of DFT-based methods to complex and metastable materials.     

Three‐dimensional numerical simulation of injection molding filling of optical lens and multiscale geometry using finite element method

This article presents the development, verification, and validation of three‐dimensional (3‐D) numerical simulation for injection molding filling of 3‐D parts and parts with microsurface features. For purpose of verification and comparison, two numerical models, the mixed model and the equal‐order model, were used to solve the Stokes equations with three different tetrahedral elements (Taylor‐Hood, MINI, and equal‐order). The control volume scheme with tetrahedral finite element mesh was used for tracking advancing melt fronts and the operator splitting method was selected to solve the energy equation. A new, simple memory management procedure was introduced to deal with the large sparse matrix system without using a huge amount of storage space. The numerical simulation was validated for mold filling of a 3‐D optical lens. The numerical simulation agreed very well with the experimental results and was useful in suggesting a better processing condition. As a new application area, a two‐step macro–micro filling approach was adopted for the filling analysis of a part with a micro‐surface feature to handle both macro and micro dimensions while avoiding an excessive number of elements. POLYM. ENG. SCI., 46:1263–1274, 2006. © 2006 Society of Plastics Engineers     

Platinum Nanoclusters Exhibit Enhanced Catalytic Activity for Methane Dehydrogenation

Methane utilization, whether by steam reforming or selective oxidation to produce synthesis gas or alcohols, requires the activation and dissociation of at least one carbon?Chydrogen bond. At high temperatures, using platinum nanoparticles as catalysts, this process operates with low activity. However, the catalyst particle shape may be controlled at low temperatures, and faceted particles may catalyze hydrocarbon transformation with increased activity. In this study, we use density functional theory calculations to calculate the thermodynamics of methane dehydrogenation on both (hemi)spherical and tetrahedral platinum nanoclusters. We show all steps of methane dehydrogenation on the hemispherical cluster have high activation barriers (0.4?C1.0?eV), thus requiring high temperatures for this process. However, the energy barriers for methane dehydrogenation on the tetrahedral cluster are lower than the corresponding barriers on the hemispherical cluster, and in particular, the dissociation of the methyl group to form methylene and hydrogen has an activation barrier of only 0.2?eV. Thus, we expect that hydrogen production from methane would proceed at a higher rate and conversion on tetrahedral clusters than on hemispherical clusters. The resulting hydrogen and carbon-containing species may then serve as building blocks for the production of chemicals and fuels. We believe that catalyst shape is vitally important in controlling catalytic activity, and the use of faceted catalyst particles opens up possibilities for low-temperature and energy-efficient hydrocarbon transformations.