Flattening and solidification of thermally sprayed particles

In this article, molybdenum particles were plasma sprayed on copper, zirconia, and glass substrates. The impact of the molten particles was monitored using a fast two-color optical fiber pyrometer focused on a small spot on the substrate surface. The apparent duration of the flattening process and the cooling speed, both determined from the pyrometer signals, were found to depend on the substrate conditions and to vary with coating thickness. The substrate material and its roughness were also found to influence the texture in the sprayed coatings. Furthermore, a transient thermal flow numerical model was used to compute reliable thermal histories of the impinging particles and the underlying lamellae, the interfacial thermal resistance being determined by comparison of experimental thermograms with computed ones.     

快速凝固铝硅合金粉末加热过程中硅颗粒的粗化行为

利用差示扫描量热仪(DSC)、电子显微镜等设备对Al-12.8%Si合金粉末的原始状态及热处理后的状态进行了分析。发现其显微组织有两个连续的变化过程:在156℃时,固溶在基体Al中的过饱和Si开始析出,并在340℃发生粗化,随着加热温度的升高,粗化现象仍然存在。另外,对Si颗粒的粗化机理进行了研究,发现其粗化符合经典的LSW理论。     

离心铸造梯度功能材料凝固过程中第二相的迁移行为

金属液在离心铸造下将发生第二相质点的径向移动。本文分析了第二相质点的移动规律。     

超音速微粒轰击表面纳米化及其对耐磨性的影响

采用超音速微粒轰击技术对20钢进行表面纳米化处理。研究了表面纳米化工艺对材料流失与形貌变化的影响，并采用往复磨损试验机研究了纳米化表面的磨损性能。结果表明：超音速微粒轰击在表面形成纳米层过程中使材料发生流失和表面粗糙度增大。在干摩擦和油润滑条件下，微粒轰击样品的磨损率分别是未轰击样品的2．77和1．83倍，轰击抛光样品的磨损率则比未轰击样品分别降低了26％和42％。对其影响耐磨性的原因作了初步讨论。     

Chromatographic performance of monodisperse–macroporous particles produced by “modified seeded polymerization.” I: Effect of monomer/seed latex ratio

In this study, the monodisperse–macroporous particles produced by a relatively new polymerization protocol, the so‐called, “modified seeded polymerization,” were used as column‐packing material in the reversed phase chromatography (RPC) of proteins. The particles were synthesized in the form of styrene‐divinylbenzene copolymer approximately 7.5 μm in size. In the first stage of the synthesis, the monodisperse polystyrene particles 4.4 μm in size were obtained by dispersion polymerization and used as the “seed latex.” The seed particles were swollen by a low‐molecular‐weight organic agent and then by a monomer mixture. The monodisperse–macroporous particles were obtained by the polymerization of monomer mixture in the seed particles. In the proposed polymerization protocol, the number of successive swelling stages was reduced with respect to the present techniques by the use of sufficiently large particles with an appropriate average molecular weight as the seed latex. A series of particles with different porosity properties was obtained by varying the monomer/seed latex ratio. The separation behavior of HPLC columns including the produced particles as packing material was investigated in the RPC mode using a protein mixture including albumin, lysozyme, cytochrome c, and ribonuclease A. The chromatograms were obtained with different flow rates under an acetonitrile–water gradient. The theoretical plate number increased and chromatograms with higher resolutions were obtained with the particles produced by using a lower monomer/seed latex ratio. The separation ability of the column could be protected over a wide range of flow rates (i.e., 0.5–3 mL/min) with most of the materials tested. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 607–618, 2004     

ON THE DYNAMICS OF THE COMBUSTION OF SINGLE CHAR PARTICLES

Diffusion and reaction models of varying complexity are used to study the stability of the combustion of single char particles. In contrast to past studies which have considered only the reaction of carbon with oxygen, both the heterogeneous reactions of carbon with oxygen and carbon dioxide and the homogeneous oxidation of carbon monoxide are taken into account in the formulation of the mathematical models. Emphasis is placed on the investigation of the feasibility of occurrence of oscillatory combustion. Our results show that high rates of the C-O₂ reaction and high concentration of O₂ in the ambient favor the occurrence of multiple steady states and oscillatory solutions, but the appearance of oscillatory instability is suppressed by the homogeneous reaction in the gas phase and the presence of CO and CO₂ in the ambient. The parametric investigation of the problem reveals, in agreement with the results of past studies, that the heat capacity of the porous solid, the Lewis number, and the thickness of the stagnant film are three key parameters for the occurrence of the oscillatory response of the system. Reasonably large values of solid heat capacity practically eliminate the possibility of oscillatory combustion, but an appropriate combination of large values of stagnant film thickness and small Lewis numbers may offset the effect of solid heat capacity     

Monodisperse‐porous poly(styrene‐co‐divinylbenzene) beads providing high column efficiency in reversed phase HPLC

A multistage polymerization protocol, the so‐called “modified seeded polymerization,” was developed for the production of monodisperse‐porous poly(styrene‐co‐divinylbenzene) providing high column efficiency as a packing material in reversed phase high performance liquid chromatography (RPLC). In the first stage of the multistage production, uniform polystyrene seed particles, produced by dispersion polymerization, were swollen by an organic agent (i.e., the diluent) and then by a monomer mixture containing styrene and divinylbenzene. The final porous particles were obtained in the monodisperse form by the polymerization of monomer mixture in the seed particles. By the use of a small size seed latex with low molecular weight and by the selection of the appropriate diluent, relatively small monodisperse‐porous particles with suitable pore structure could be achieved. In the reversed phase separation of alkylbenzenes, under isocratic conditions, theoretical plate numbers up to 40,000 plates/m were achieved by using 5.2 μm porous particles, obtained by a toluene‐dibutyl phthalate mixture as the diluent. No significant decrease in the resolution power was observed by the fourfold increase in the mobile phase flow rate. The column efficiency and the resolution observed with 5.2 μm monodisperse‐porous particles were significantly higher with respect to the currently available polymer based packing materials used in the reversed phase HPLC. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1430–1438, 2005     

Kinetics of hydrogen evolution on submicron size Co, Ni, Pd and Co–Ni alloy powder electrodes by d.c. polarization and a.c. impedance studies

Submicron size Co, Ni and Co–Ni alloy powders have been synthesized by the polyol method using the corresponding metal malonates and Pd powder by reduction of PdO _x in methanol. The kinetics of the hydrogen evolution reaction (HER) in 6 M KOH electrolyte have been studied on electrodes made from the pressed powders. The d.c. polarization measurements have resulted in a value close to 120 mV decade^–1 for the Tafel slope, suggesting that the HER follows the Volmer–Heyrovsky mechanism. The values of exchange current density (i _o) are in the range 1–10 mA cm^–2 for electrodes fabricated in the study. The a.c. impedance spectra measured at several potentials in the HER region showed a single semicircle in the Nyquist plots. Exchange current density (i _o) and energy transfer coefficient () have been calculated by employing a nonlinear least square-fitting program.     

聚氯乙烯中杂质粒子的产生原因及解决措施

通过分析聚氯乙烯树脂中杂质粒子产生的主要原因，提出了在采购、保管、生产、清理、分析等环节的解决措施。     

PVC树脂中杂质粒子的来源及消除方法

系统地分析了PVC树脂中杂质粒子的来源和产生原因，提出了解决的方法。杂质粒子与所用原材料、设备、工艺、聚合配方组分及氯乙烯单体中的铁化合物有关。在配方中全用聚乙烯醇做悬浮剂时由于反应前后浓度变化大而容易产生自聚物和粘釜物而增加杂质粒子，加入少量纤雏素可稳定悬浮剂浓度。单体含酸含水所产生的铁锈与氯化铁形成有色胶状物是杂质粒子的重要来源，采用在合成工段滴加碱溶液的方法消除酸和氯化铁，避免胶状物的产生。