杀虫单介稳区宽度的理论计算

从晶体生长成核、动力学等几个方面出发,在Mersmann模型基础上建立了适合高粘度有机体系的介稳区宽度的理论计算模型,确立了计算程序和方法.结合“杀虫单”农药结晶过程的具体参数,计算和预测了工业结晶过程中介稳区的宽度.理论计算值与实验测定值较为接近.该方法可为工业结晶中过饱和度的选择提供参考.     

Insight into the Nucleation Mechanism of p-Methoxybenzoic Acid in Ethanol-Water System from Metastable Zone Width

The metastable zone width (MSZW) of p-methoxybenzoic acid (PMBA) in an ethanol-water system was measured using the polythermal method. The nucleation order m obtained by the Nývlt’s model indicates the nucleation of PMBA following a progressive nucleation mechanism at low saturation temperature (m = 3.18–7.50) and an instantaneous nucleation mechanism at high saturation temperature (m = 1.46–2.55). Then, combined with the metastable zone experiment and the Sangwal model, we found that the MSZW and the interfacial energy reached the maximum when the mass fraction of ethanol was 0.8, which resulted in the smallest crystal product size. Meanwhile, the maximum rcrit and ΔGcrit obtained based on the modified Sangwal model indicating the PMBA needs to overcome a higher nucleation barrier in the ethanol mass fraction of 0.8. Finally, we proposed a preferential strategy for adjusting MSZW by correlating the interfacial energy with the change in ethanol mass fraction, saturation temperature, and cooling rate, respectively.     

激光法测定硝唑尼特结晶介稳区间

用激光法测定了不同温度下硝唑尼特在N,N-二甲基甲酰胺（DMF）、二甲基亚砜（DMSO）、甲醇和丙酮4种溶剂体系中的溶解度与超溶解度,得到硝唑尼特在不同溶剂和不同温度下的介稳区宽度。 结果表明,硝唑尼特在4种溶剂体系中的溶解度、超溶解度和介稳区宽度随温度升高逐渐增大。 在相同温度下,硝唑尼特在DMF、DMSO、甲醇和丙酮中介稳区宽度关系为：丙酮＜DMSO＜甲醇＜DMF。     

Effect of magnetic field on sodium arsenate metastable zone width and crystal nucleation kinetics for crystallization

Sodium arsenate, the main component of arsenic-containing solid waste pollutants, causes serious environmental health threats. Crystallization is one of the effective methods for separating and purifying sodium arsenate from arsenic-alkali residue lixivium. However, the crystallization process is limited for its low observability and the lack of separation and purification data. In this work, a laser detection system with a magnetic field generator was designed, and the solubility, metastable zone width, interfacial tension, interfacial entropy factor, crystal nucleation, and growth rate of sodium arsenate were investigated in a constant composition environment. The results showed that the solubility, metastable zone width, interfacial tension, and interfacial entropy factor decreases with the presence of a magnetic field. The magnetic field shortened the crystallization induction time and changed the nucleation and growth rate of sodium arsenate. Under the magnetic field, the nucleation rate increased from 2.43 × 10¹⁶ to 8.98 × 10¹⁷ (s m³)⁻¹, and the growth rate decreased from 4.94 × 10⁻⁸ to 2.73 × 10⁻⁸ (s m³)⁻¹, the growth mechanism of sodium arsenate as a continuous growth mode was unchanged. In addition, the X-ray diffraction and infrared showed that the crystal structure of sodium arsenate is unaffected by the magnetic field, indicating that the enhancement of the crystallization process of sodium arsenate with the magnetic field could be a feasible method in engineering application.     

过硼酸钠结晶介稳区的测定

采用激光测定装置测定了过硼酸钠的溶解度和超溶解度,重点研究了过硼酸钠的溶解度、超溶解度以及结晶介稳区的变化规律,获得了过硼酸钠溶液的结晶介稳区,并用Apelblat方程对过硼酸钠的溶解度数据进行了关联。 研究了降温速率、搅拌速率、六偏磷酸钠及聚丙烯酸钠添加量对过硼酸钠介稳区宽度的影响规律。 研究结果表明,过硼酸钠溶解度随温度升高而增大,过硼酸钠的介稳区宽度在高温区较小而在低温区较大;降温速率的增大使过硼酸钠介稳区宽度变大,搅拌速率的增大使介稳区宽度变小;加入0.065%六偏磷酸钠和0.045%聚丙烯酸钠时获得的介稳区宽度最大。     

Molecular Dynamics Studies of the Kinetics of Phase Changes in Clusters IV： Crystal Nucleation from Molten （NaCl）256 and （NaCl）500 Clusters

Molecular dynamics computer simulation based on the Born-Mayer-Huggins potential function has been carried out to study the effects of duster size and temperature on the nucleation rate of sodium chloride dusters in the temperature range of 580 K to 630 K. Clusters with 256 and 500 NaCl molecules have been studied and the results have been compared with those obtained from 108 molecule dusters. The melting point (MP) of the clusters were observed to increase with the size of the clusters and can be well described by a linear equation MP =1107(37)-1229(23)N^-1/3(N is the number of molecules in the duster).The nucleation rate was found to decrease with increasing the duster size or temperature. Various nucleation theories have been used to interpret the nucleation rates obtained from this molecular dynamics simulation. It is possible to use a constant diffuse interface thickness to interpret the nucleation rate from the diffuse interface theory in the temperature range of this study. However, the interfacinl free energy estimated from classical nucleation theory and diffuse interface theory increases too fast with increasing the temperature while that from Gran-Gunton theory does not change with changing temperatures.The sizes of critical nuclei estimated from all the theories are smaller than those estimated from our simulations.     

Molecular Dynamics Studies of the Kinetics of Phase Changes in Clusters II: Crystal Nucleation from Molten (RbCl)256 and (RbCl)500 Clusters

Molecular dynamics computer simulations have been carried out to study the effects of cluster size and temperature on the nucleation rate of rubidium chloride clusters in the temperature range of 500-650 K. Clusters with 256 and 500 RbCl molecules have been studied and the results are compared with those obtained from 108 molecule clusters. The melting point (MP) of the clusters was observed to increase with the size of the clusters and can be described by a linear equation MP=997-405 N^−1/3, where N is the number of molecules in the cluster. The nucleation rate is found to decrease with increasing cluster size or increasing nucleation temperature. Both classical nucleation theory and diffuse interface theory are used to interpret our observed results.     

流场作用对聚丙烯结晶成核过程和晶体生长过程动力学的影响

通过改进基于构象张量构建的流动诱导结晶理论模型,考虑了流场对结晶生长的影响,对比研究了流场作用对聚丙烯(PP)结晶过程中成核与晶体生长两阶段速率的不同影响程度.理论研究表明,虽然相比成核过程,流场诱导作用对PP晶体生长速率的加速程度有限,但要更为精确地描述流动引起的整个结晶动力学过程,特别是在剪切速率较高且熔体结晶温度较低的情况下,考虑流场对结晶生长速率的影响是十分必要的.研究结果还显示,由于分子链的松弛作用,PP在较低温度下的晶体生长速率受剪切流场的影响要大于较高温度下的影响.本文理论预测结果与实验现象和实验数据的很好吻合亦说明模型改进的合理性和可行性.     

过饱和铝酸钠溶液中氢氧化铝自发成核动力学规律的研究

用电导法和吸光光度法首次对苛性比相同的不同浓度过饱和铝酸钠溶液自发分解过程进行了实时跟踪研究,获得铝酸钠溶解氢氧化铝自发成核动力学方程,探索了H₂O和Na⁺对氢氧化铝自发成核过程的影响.结果表明,过饱和铝酸钠溶液分解为氢氧化铝属化学反应控制过程;H₂O参与了溶液分解过程控制步骤的反应;K⁺和Na⁺等阳离子参与了溶液的重构,但对溶液分解的控制步骤影响不大.     

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Classical kinetic theories of polymer crystallization were applied to isothermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fitting the isothermal DSC crystallization data of previously self‐nucleated samples. Therefore, we demonstrated that by self‐nucleating the material at the ideal self‐nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crystal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p‐dioxanone), poly(ε‐caprolactone), and a high 1,4 model hydrogenated polybutadiene, we have interpreted the differences in primary nucleation energy barriers as arising from differences in nuclei density. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1478–1487, 2008     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002     

分散液膜气/液/固三相线界面区宽度在金属大气腐蚀中的作用

使用接触角测量和电化学技术研究了三相线界面区(TPB)宽度在分散液膜下金属腐蚀过程中的作用.研究发现,阴极极限电流和腐蚀电流密度随TPB宽度增加而线性增大,表明TPB宽度对气/液/固多相体系腐蚀过程具有重要影响.因此,可根据TPB宽度评价分散液膜厚度对大气腐蚀速率之影响.     

Memory effect on the pressure-temperature condition and induction time of gas hydrate nucleation

The focus of this study is to investigate the influence of memory effect and the relation of its existence with the dissociation temperature, using gas hydrate formation and dissociation experiments. This is beneficial because memory effect is considered as an effective approach to promote the thermodynamic and dynamic conditions of gas hydrate nucleation. Seven experimental systems (twenty tests in total) were performed in a 1 L pressure cell. Three types of hydrate morphology, namely massive, whiskery and jelly crystals were present in the experiments. The pressures and temperatures at the time when visual hydrate crystals appeared were measured. Furthermore, the influence of memory effect was quantified in terms of pressure-temperature-time (p-T-t) relations. The results revealed that memory effect could promote the thermodynamic conditions and shorten the induction time when the dissociation temperature was not higher than 25 ℃. In this study, the nucleation superpressure and induction time decrease gradually with time of tests, when the earlier and the later tests are compared. It is assumed that the residual structure of hydrate dissociation, as the source of the memory effect, provides a site for mass transfer between host and guest molecules. Therefore, a driving force is created between the residual structures and its surrounding bulk phase to promote the hydrate nucleation. However, when the dissociation temperature was higher than 25 ℃, the memory effect vanished. These findings provide references for the application of memory effect in hydrate-based technology.     

Separating Growth from Nucleation for Facile Control over the Size and Shape of Palladium Nanocrystals

In order to maximize the performance of nanocrystals in a specific application, it is necessary to control both their size and shape. Here we report a one-pot protocol that allows us to separate growth from nucleation for achieving better control over the size and shape of Pd nanocrystals. The two processes are temporally separated from each other, although the synthesis is carried out in the same reaction container. Size control is achieved by simply varying the ratio between the amounts of precursor allocated to the growth and nucleation processes. With the involvement of seeds at a fixed number, increasing the amount of precursor for growth leads to increasingly larger nanocrystals. Shape control is made possible by varying the capping agent, with bromide leading to a cubic shape and citrate inducing the formation of an octahedral shape. The synthesis can also be scaled up by at least tenfold without compromising the quality.     

Nucleation and Crystal Formation in Lithium Disilicate‐Apatite Glass‐Ceramic from a Combined Use of X‐Ray Diffraction,Solid‐State NMR,and Microscopy

Glass‐ceramics are multi‐phase materials that are comprised of one amorphous phase and at least one crystalline phase. Their versatile performance and properties can be engineered by alterations of the three fundamental steps – formulation and production of the amorphous base glass, nucleation, and crystallization. Efforts have been made on syntheses of glass‐ceramics with different components, yet little is known about the details of nucleation and crystallization processes that are essential for tailoring glass‐ceramic properties. Herein, we investigate the nucleation and crystallization mechanisms of a multi‐component, that is SiO₂‐Al₂O₃‐CaO‐Li₂O‐K₂O‐P₂O₅‐F, glass‐ceramic system by a combined use of powder X‐ray diffraction (pXRD), solid‐state nuclear magnetic resonance (NMR), and electron microscopic (EM) techniques. The role of P₂O₅ in the nucleation and crystallization processes is particularly studied. We show that the formation of lithium silicate crystals being independent of the P₂O₅‐associated crystals, and the separation of P₂O₅ phases into individual growth domains of lithium orthophosphate and fluorapatite. We also observe the non‐uniform distribution of fluorapatite particles that explains the opalescence effect of this glass‐ceramic.     

纳米粒子相变的分子动力学研究：Cu453的性质、结构与结晶成核

Molecular dynamics （MD） computer simulations have been carried out to study the structures, properties and crystal nucleation of nanoparticles with 453 Cu atoms. Structure information was analyzed from the MD simulations, while properties of nanoparticles of Cu453, such as melting point, freezing temperature, heat capacity and mo- lar volumes, have been estimated. The face center cubic （FCC） phase and icosahedron （Ih） phase were observed during the quenching process, and nucleation rates of crystallization to FCC crystal of Cu453 at temperatures of 650, 700, 750, and 800 K were analyzed. Both classical nucleation theory （CNT） and diffuse interface theory （DIT） were used to interpret our observed nucleation rates. The free energy and diffuse interface thickness between the liquid and the FCC crystal phases were estimated by the CNT and DIT respectively, and the results show that the DIT does not work properly to the system.     

表面能与晶体生长/溶解动力学研究的新动向

界面现象使物质在结晶过程中出现了临界现象.但最近的研究指出在物质溶解过程中,在表面能量的控制下也存在着临界现象以及尺寸效应.实验发现,当晶体自身小到一 定的程度时(通常在纳米尺度上并和临界蚀坑的大小相近),在溶解过程中其速度会自发降 低,反应被抑制乃至停止.尽管在热力学上表面能的因素可以赋予小颗粒晶体较大的溶解度 ,但表面能却也能通过对临界条件的控制而使这些微粒在动力学上不被溶解.这个发现不仅 解决了纳米颗粒在水溶液中稳定性的问题,而且还从动力学的角度解释了生物矿物选择纳米 尺度作为其基本构成单元的原因.由于表面能和晶体生长/溶解的动力学有着密切的关系, 我们可以通过对表面能的调节来修改它们的动力学速度和晶体的形貌.反过来,也可以用动力学的方法来测定表面能及表面吸附/脱附常数等.相对于常规的界面研究手段,通过生 长和溶解动力学途径所得的数据有着很好的可靠性及重复性.我们认为,晶体生长和溶解的 动力学和表面能的研究相结合,不仅为界面研究提供了新的思路和方法,而且也会推动晶体生长和材料科学的发展.     

Nucleation characteristics of the α/β compounded nucleating agents and their influences on crystallization behavior and mechanical properties of isotactic polypropylene

Nucleation characteristics of isotactic polypropylene (iPP) nucleated by the α/β compounded nucleating agents (NAs) were investigated by wide‐angle X‐ray scattering, differential scanning calorimetry and mechanical testing. The results showed that the nucleation effect of the α/β compounded NAs depends on not only nucleation efficiency (NE) of individual β and α NAs and their ratios but also the processing conditions, especially the cooling rates. The nucleating characteristics of the α/β compounded NAs can be illustrated by competitive nucleation. The NA with high NE played a leading role during iPP crystallization even at a low weight ratio and at different cooling rates. The stiffness and toughness of iPP can be simultaneously improved by using suitable compositions at the appropriate ratios. Finally, the nonisothermal crystallization kinetics of iPP nucleated with the α/β compounded NAs was described by Caze method and the crystallization activation energy of nucleated iPP was calculated by Kissinger equation. The result indicated that the crystal growth pattern of nucleated iPP was heterogeneous nucleation followed by three‐dimension spherical growth. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 653–665, 2010     

On the interfacial energy of extended-chain crystals from polyethylene melt by revised Flory equations of fusion

To analyze extended-chain crystalline systems composed of linear polyethylene, Flory's conventional theory of fusion is reconsidered by introducing a new concept of crystallinity. When this new treatment is applied to a melting case of a low molecular weight polyethylene fraction (M_n = 5600) isothermally bulk crystallized, a certain result that very large lamellar thickness was caused by a very small increase in crystallization temperature can satisfactorily be explained by a significant change in interfacial free energy of the crystallite end. Further, it shows 14–17 kJ/mol as a nonequilibrium value range of interfacial free energy for highly crystalline polyethylene fractions of low molecular weight M_n ≦ 5600 by using the previous data presented by other workers. A similar result is also obtained on the M_n = 5600 fraction by analyzing from a standpoint of equilibrium crystallinity. In either case, the estimated range of interfacial free energy is consistent with the conventional range. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1293–1303, 1998