Molecular packing and symmetry of two-dimensional crystals

Periodic arrangements on surfaces resulting from monolayer formation are critical in determining the electronic structure of thin films, the adhesion of surface coatings, the properties of lubricants, and the polymorphic form of heteronucleated crystals. Unlike substrate-directed chemisorption, the process of physisorption is highly responsive to molecular structure and stands out as a controllable method of creating variable surface patterns with periodicities on the low end of the nanoscale. Despite decades of study focused upon such ordered structures, the principles guiding the formation of these two-dimensional crystals have been obscured by the lack of a systematic and critical compilation. Thus, prediction of two-dimensional structure based upon the composition of the individual building blocks remains in its infancy. Here we demonstrate through the compilation and analysis of a database of two-dimensional structures that molecular-scale patterns are dictated by the same factors that determine bulk crystal structure, but these factors give rise to different preferred packing symmetries. In marked contrast to three-dimensional systems, achiral molecules in two-dimensional crystals are likely to adopt chiral structures, and racemic mixtures are expected to produce enantiopure domains. The determination of plane group frequencies allowed experimental verification of Kitaigorodskii's 50-year old theory of close packing as applied to two-dimensional tiling. This fundamental comparison between bulk crystals and physisorbed monolayers provides new tools and directions for future exploration in the engineering of surfaces with prescribed two-dimensional patterns.     

规整填料波纹结构上的二维两相流模拟方法

规整填料是由规则排列的波纹板组成的。液相是沿着波纹板壁面往下流动,而气相则是在波纹板之间形成的通道内与液相呈并流或逆流的状态。因此,气液两相的水力学特性决定着填料塔的操作弹性和质量传递效率。波纹板上的液膜流动状态因在很多应用领域都发挥作用而备受关注。本文从物理建模、自由界面处理方法、表面张力模型以及湍流模型等方面对规整填料波纹板上的二维液膜数值模拟方法作了详细的陈述,并且对该领域的研究现状作了进一步分析和总结。     

Analysis of the packing stage in injection molding

An unconditionally stable explicit finite difference numerical scheme is used to determine the pressure distribution during the packing stage of a rectangular mold cavity. Different initial conditions arising from both an isothermal and nonisothermal mold filling analysis are considered in relation to subsequent packing behavior. The packing stage is of short duration, may be assumed to be isothermal, and gives rise to a more uniform pressure distribution within the mold cavity.     

Analysis of the packing stage of a viscoelastic melt

The packing stage starts at the end of mold filling. During this stage, additional material is forced into the mold to compensate for the shrinkage during subse-quent cooling. Underpacking results in molded parts with dimensional variation. Overpacking causes flash at the parting lines, stick during ejection, and excess residual stresses resulting in warpage. The packing stage is thus extremely important in the determination of the final quality of the product. Despite its importance, analysis of the packing stage has been relatively ignored, particularly the viscoelastic effect. In this work, the analysis of the isothermal packing stage is presented for a Maxwell fluid. A set of governing equations is derived for a two-dimensional mold and solved using the Galerkin finite element method. In addition to the distribution of velocity and pressure, the model predicts the stresses in the planar direction, which could be used for subsequent calculation of the residual stresses.     

Analysis of the densification of a vibrated sand packing

Two techniques have been used to analyze the densification of silica sand by horizontal sinusoidal vibrations of frequency f = 50 Hz for relative accelerations between 0 and 6: the quantitative analysis of motion observed through the transparent wall, and the altitude map of the free top surface of the sample. The first technique was used to analyze the transient regime: during the first 10 s, slight densification occurs at the bottom of the powder bed, while the upper part enters convective motion and the intermediate part reaches densities higher than 66%. The second technique allowed to quantify the evolution of the overall density vs. acceleration Γ during the steady regime (dynamic density) and after the vibrations (relaxed density): a maximum density is observed in both cases for an optimal acceleration which depends on the initial height of the powder bed. These results are analyzed and discussed.     

快递包装袋挥发性气味成分分析

为深入研究快递包装袋的特征气味成分,通过液液萃取法获得快递包装袋挥发性气味成分并采用气相色谱?质谱联用仪（GC?MS）对其进行分离鉴定。结果表明,利用液液萃取法从快递包装袋中共鉴定出了30 余种物质,其中相对质量分数最高的是醇类化合物及其他类化合物;2?（十八烷氧基）?乙醇、乙酰柠檬酸三丁酯、1?氯?七叶苷、2?十六醇、2?溴十八烷、叔十六硫醇相对质量分数均较高,对快递包装袋特殊气味的形成可能具有关键性影响。     

瓶装氢气经济效益分析

介绍瓶装氢气的工艺流程、主要设备及控制指标.以年充装26万钢瓶氢气为例,计算其投资回收期为2.94年,税后利润可达79.28万元/a.     

填料塔的填料层高度和塔径计算实例

用闲置金属鞍环填料对填料塔进行防腐处理。介绍了利用现有引风机的条件下,填料塔塔径、塔高的计算示例。     

波纹板填料表面液体降膜流动的特性分析

基于降膜流动实验台,结合计算流体力学方法(Computational Fluid Dynamics, CFD)研究了波纹板表面液体的平均液膜厚度和有效润湿面积等定量信息,并通过三维模拟进一步分析了喷淋密度和波纹倾斜角度β对降膜流动特性的影响。结果表明,液体在波纹板表面的流动并非均匀,分为沟流和溪流两种形式;当喷淋密度较小时,液体在波谷内形成沟流,当喷淋密度达到400 m3/(m2?h),液体跨越相邻波纹进行溪流流动;两种形式波纹板整体的润湿性能均较差,且液膜厚度分布不均;波纹倾斜角度对降膜流动特性影响较大,90°时更有利于提高有效润湿面积。     

填料因子变化时散堆填料泛点气速数值算法

泛点气速是填料塔设计的重要参数,Eckert关联图为目前工程上运用较为广泛的泛点气速求取方法。文中基于散堆填料的Eckert关联图,在考虑泛点填料因子f随液体泛点喷淋密度Lf变化的条件下,提出了泛点气速uf的数值算法;推导出泛点气速uf的表达式为一个高度非线性化方程,并给出了该方程的2种求解方法。同时举例计算了一个散堆填料的泛点气速,并和著名的塔器流体力学计算软件(WPTR)计算结果做了对比。结果表明:文中的计算结果和WPTR的结果完全吻合;所使用的方程求解方法 Aitken-Steffensen具有快速、稳定的收敛性,可用于散堆填料的泛点气速计算。     

填料塔中规整填料及相关装置的设计

介绍了颗粒型填料和规整填料,分析了气液再分布装置的设计要求、喷淋密度确定、设计计算、结构设计等,指出溢流槽式再分布器是适应性较好的分布器,特别适用于大流量操作。     

氟里昂包装钢瓶与钢制焊接气瓶的比较分析

氟里昂包装钢瓶的设计,制造和检验与其它钢制焊接气瓶相比具有明显不同:其安装爆破片,以工作压力作为设计压力而不用水压试验压力,屈服应力采用σ_b而不是σ_s．瓶体材料经冷加工冲压成型后不再进行退火热处理、焊缝不做无损探伤检查而只做外观检查．焊接接头系数取1．0,产品只做气压试验且气压试验压力为工作压力的1．1倍。结合实际情况,基本原则和制造经验,找到了氟里昂包装钢瓶设计、制造及检验的理论依据,论证了依据的合理性。     

Supported catalytic packing prepared from ceramic packing for reactive distillation of ethyl acetate

In this work, a strategy of “etching-modification filling-graft copolymerization” was proposed to load the acidic ionic polyionic liquid on the smooth ceramic surface. In this way, commercial ceramic Raschig rings were successfully transformed into the supported catalytic packing for the reactive distillation, and were further evaluated with esterification reaction of ethyl acetate by means of the fully mixed reactor, the ultrasonic destruction, the cyclic catalysis reaction and the lab-scale distillation column experiment. This catalyst coating has good adhesion with the substrate. It can withstand 24 h of ultrasound damage and shows good stability in three cycle catalytic experiments. This kind of coated catalyst has better catalytic activity than the commercial Amberlyst 15 dry. In the lab-scale reaction distillation, the supported catalyst Raschig ring can achieve a higher conversion in comparison with the tea bag catalytic packing of Amberlyst 15 dry under some conditions.     

碳化硅规整填料的性能实验研究

为了研究新型多孔碳化硅规整填料的分离性能,文中对碳化硅规整填料进行了流体力学实验和传质实验。为了进一步提高碳化硅规整填料的传质性能,实验用硅酸钠溶液对碳化硅规整填料进行了表面改性处理,提高了填料表面的润湿性。实验结果表明:碳化硅规整填料具有较低的压降,较小的持液量,其理论板数可达到5块板以上,说明碳化硅规整填料是一种新型高效规整填料。同时获得了该填料的填料压降、泛点气速、持液量等流体力学性能数据,及填料层压降和液泛气速的关联式。     

Vibrovacuum packing of powder masses

Conclusions The positive influence of vibropacking before isostatic pressing is caused by the effects of vibration breaking of the structural arched formations in dispersed media and is revealed in obtaining blanks of uniform and high density. These phenomena are used most rationally in vibropacking powders in vacuum. For this it is necessary to use a design of container vibrating with harmonic oscillations with an inertia-free overload. A method of isostatic pressing with preliminary vibrovacuum packing and a hydraulic over load has been developed. A feature of the method is pouring of the material into the mold with vibration and then repeated vibropacking of the evacuated powder with the use of an overload. The method makes it possible to obtain large high-quality refractory parts of complex configuration.Translated from Ogneupory, No. 7, pp. 9–16, July, 1982.     

Becoming random close packing

The problem of random close packing has been analyzed as a task of self-development with the doubling of the scale at the stage of becoming. It has been shown that the site of a transforming body at this stage can be described by hexagons in the two-dimensional case and by tetrakaidecahedra in the three-dimensional case. When their vertices, equidistant from the center, are distributed so that the smallest distance between them is largest among its all possible values, the fraction of the surrounding Euclidean space, which is occupied by the figures, coincides with the capacity of a random close packing. The capacity takes the value 0.827, in the two-dimensional case, or one of the two values, i.e., 0.6366 and 0.6457, in the three-dimensional case. The result obtained provides insight into the geometric meaning of the isotropic random close packing and serves to accept the existence of its two forms in the three-dimensional case.     

化妆品包装设计的创新

论述了化妆品包装盒、包装容器盖子及封口的创新设计。介绍了化妆品包装纸盒的印刷、装饰和造型的新颖以及新材料的选择。介绍了包装容器盖子以及封口的一些创新设计。盖子和封口与化妆品包装的其他领域一样,能够随着潮流而发生变化,指出了当今化妆品包装容器盖子和封口的发展潮流。