Formation mechanism and growth of MNbO3, M=K,Na by in situ X‐ray diffraction

Hydrothermal synthesis is a well‐established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead‐free piezoelectric K_0.5Na_0.5NbO₃. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end‐members, KNbO₃ and NaNbO₃, are nucleating and growing. Here, we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO₃ and NaNbO₃ by in situ synchrotron powder X‐ray diffraction. We show that tetragonal KNbO₃ crystallites form from dissolved T‐Nb₂O₅ at 250°C‐300°C and 250 bar while orthorhombic NaNbO₃ forms via several crystalline intermediate phases at 225°C‐325°C and 250 bar. The crystallite size of KNbO₃ is decreasing while the crystallite size of NaNbO₃ is increasing with increasing temperature, demonstrating that the presence of intermediate phases is highly important for the nucleation and growth of the final product. The different crystallization schemes explain the challenge in obtaining stoichiometric K_0.5Na_0.5NbO₃ by hydrothermal synthesis.     

Effect of PVP in dispersion and seeded dispersion polymerizations

The outcome of seeded dispersion polymerizations of n‐butyl acrylate (BA) and styrene (St) in terms of the success of growing the seed particles without nucleating new particles or generating coagulum was found to be dependent on the seed type (poly[n‐butyl acrylate] [ PBA] or polystyrene [PSt]), the second‐stage monomer (BA or St), and the type of polyvinylpyrrolidone (PVP) stabilizer (PVP K30 or PVP K90). All seeds were first cleaned of excess stabilizer by medium replacement before the seeded polymerizations. In general, successful particle growth was achieved when the second‐stage polymerization employed PVP K30 (1 wt%) as the stabilizer. In contrast, nearly all reactions employing PVP K90 (1 wt%) as the second‐stage stabilizer resulted in the nucleation of a second crop of particles. These phenomena were further investigated by carrying out dispersion polymerizations using the supernatant obtained by separating the seeds from the second‐stage media (containing monomer). The results paralleled those in the seeding studies and were explained by the presence of small amounts of grafted PVP created in situ during the preparation of the seeds. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2721–2732, 2002     

晶核剂对MgO－B_2O_3－SiO_2渣系析晶行为的影响

本文运用偏光显微镜、Ｘ射线衍射分析、差热分析和化学分析等手段，研究了用ＴｉＯ＿２和复合氧化物作晶核剂对ＭｇＯ－Ｂ＿２Ｏ＿３－ＳｉＯ＿２渣系析晶行为和硼提取率的影响。结果表明：硼提取率随硼渣结晶程度的提高而增大；ＴｉＯ＿２的加入降低了硼渣粘度，因此提高了总成核速率和晶体生长速率，加快析晶速度，使渣中含硼组分容易以结晶相析出，从而改善了硼提取率。复合氧化物改善硼提取率和提高硼渣结晶程度比ＴｉＯ＿２显著。     

不同引发速率下的苯乙烯乳液聚合

研究了在引发速率相差千倍的情况下，胶乳粒子的成核机理，胶乳粒子和自由基的静电相互作用对乳液聚合动力学以及胶乳粒子的成核和成长的影响。当反应温度为６０℃，引发剂为过硫酸钾／［ＶＯ（Ｈ２Ｏ）５］＾２＋（ＶＯ＾２＋）或叔丁基过氧化氢（ｔ－ＢＨＰ）／ＶＯ＾２＋时，引发速率由精确计量泵的注入量来控制，其最低和最高值分别为５．３×１０＾－７ｍｏｌ／（ｄｍ＾３·ｍｉｎ）和４．６×１０＾－４ｍｏｌ／（ｄｍ＾３·ｍ     

Online monitoring of the evolution of the number of particles in emulsion polymerization by conductivity measurements. I. Model formulation

A conductivity meter is an inexpensive instrument that can easily be installed in polymerization reactors. This instrument can be used to monitor ionic species without time‐consuming calibrations. A probe is inserted into the media, providing in situ measurements of conductivity in real time. For emulsion polymerization reactions, the conductivity meter can respond to changes in the ionic surfactant concentration, allowing the determination of surfactant dynamics in the media. The surfactant concentration can then be related to the changes in the surface area of the polymer particle phase, which can be linked to nucleation or coagulation phenomena. In this study, a conductivity meter was coupled to a calorimetric reactor to provide in situ and online measurements of conductivity during the emulsion polymerization of styrene, with sodium dodecyl sulfate as an anionic surfactant and with potassium persulfate as a free‐radical initiator. A semiempirical model was built to describe the conductivity signal as a function of the latex composition and the reactor temperature. The model was inverted and combined with the available conductivity signal, conversion, and temperature measurements and was able to accurately predict the number of polymer particles in the latex and the surfactant concentrations in the many phases, without online measurements of the particle size. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1213–1226, 2003     

均相成核—水热法制备纳米氧化锌

制备纳米氧化锌的关键是成核与生长控制。采用尿素为均相沉淀剂 ,结合水热处理 ,温度 130℃ ,反应时间为 3～ 5h制备了粒径小 ,分布窄的纳米氧化锌。Zn2 +的均相成核与其浓度、OH- 的浓度有关 ,体系中Zn(OH) 2 的过饱和度越大 ,则成核数量越多 ,所制备的氧化锌颗粒越小 ;与以氢氧化锌胶体为前驱体的共沉淀技术相比 ,均相沉淀技术有利于氧化锌纳米晶粒均匀生长 ;此外 ,添加有机物作分散剂可以减弱微小晶粒间的叠合生长 ,有利于制备均一性较好的纳米氧化锌。     

The influence of some parameters on the surface roughness of thin copper foils using statistical analysis

The influence of nucleation potential, nucleation time, growth potential and substrate roughness on the surface roughness of thin copper foils deposited electrolytically on titanium substrates from a 83 g dm^–3 Cu²⁺ and 140 g dm^–3 H₂SO₄ solution at a temperature of 65 °C, using 2k⁴ factorial design, was studied. A mathematical model to determine the average surface roughness, a quality parameter of thin copper foils, was established. Statistical adjustment of the model enables its use in accurate prediction (error lower than 5%) of the average surface roughness of thin copper foils. The initial steps of copper nucleation and growth are particularly important for the quality of thin (18 m) copper foils, because during its manufacture, a rapid and homogeneous covering of the entire surface of the electrode becomes fundamental in order to obtain foils with a low surface roughness and a low degree of pinholes per unit area. Mechanisms for copper nucleation on titanium, based on the Thirsk and Harrison model for different experimental conditions, were determined with the aid of potentiostatic current transients. It was shown that the roughness of the thin copper foils is a function of not only the growth process, but also the nucleation process and that the lowest surface roughness is related to a pre-nucleation step at –0.590 V for 10 ms on a titanium substrate polished with 600-grit paper.     

Measurement of polarization parameters impacting on electrodeposit morphology I: Theory and development of technique

A newly extended theory is presented on the role of polarization characteristics in determining the morphology of thick, polycrystalline metal electrodeposits. The theory is applicable to any system in which a single metal deposits. A simple galvanodynamic scanning procedure is more favourable than cyclic voltammetry, for predicting deposit morphology. The galvanodynamic technique represents an improved way of measuring accurately the nucleation potential and plating potential. According to the extended theory, these potentials can be readily related to the major metallographic structures of polycrystalline electrodeposits.     

Calcium dicarboxylates nucleation of β‐polypropylene

Seven dicarboxylates of calcium were synthesized. The effect of dicarboxylate on the formation of β‐form polypropylene was investigated by X‐ray diffraction. Calcium pimelate, calcium suberate, calcium phthalate, and calcium terephthalate have been found to be an effective β‐nucleator. The K_x values of the isotatic propylene samples with 0.5 wt % of the nucleators above are 0.95, 0.96, 0.93, and 0.62, respectively. Calcium succinate, calcium adipate, and calcium sebacate behave invalidly on the nucleating of the β‐phase. We conducted an investigation on the affect of particle shape, crystal form, and crystallinity upon the level of the β‐form. The X‐ray diffraction of the effective nucleators reveals a common character that their first reflection locate at the d‐spacing between 10 to 13 Å, indicating structural similarity of the nucleators with β‐polypropylene. The nucleation mechanism is explained by the cooperative effect of the nonpolar and polar part of nucleating agents in the crystallization of polypropylene. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 633–638, 2002     

Electrochemical nucleation of silver on platinum at a controlled amount of the oxygen containing surface species

Experimental studies of the electrochemical nucleation of silver on a platium substrate containing a controlled amount of oxygenated surface species, were carried out by combining cyclic voltammetry with potential pulses. The dependence of the stationary nucleation rate on temperature, overpotential and the amount of oxide indicated that oxide layers affected the number of active sites for nucleation but not their activity with respect to critical nucleaus formation.