共聚甲醛的自成核结晶行为

采用自成核的方法研究了共聚甲醛（POM）的结晶过程、结晶行为及其形态结构。结果表明,POM的有效自成核温度（Ts）区间为165.5～167 ℃;POM经自成核处理后可在144～148 ℃内迅速结晶,相对其在210 ℃熔融后的结晶温度（Tc）最大提高了5 ℃;在150～154 ℃内自成核处理后的样品随着等温Tc的升高,结晶速率（G）变慢;根据Arrhenius方程和Lauritzen Hoffman方程可获得样品的各项动力学参数,通过对比发现,自成核处理可有效提高POM的Tc和G;POM经自成核处理后晶型未发生变化,且微晶尺寸细化,球晶尺寸变小;自成核处理可有效促进POM的结晶。     

黄磷炉渣浸出液制备硫酸钙结晶动力学

采用间歇动态法测定了黄磷炉渣HNO3浸出液中硫酸沉钙的硫酸钙结晶动力学参数。通过结晶生长速率方程和成核速率方程的计算回归,得到了一定搅拌速度下的结晶生长速率和成核速率方程。研究表明:温度为20℃、搅拌速度250 r/min时生长速率常数和成核速率常数分别为1.440×10-4和5.530×1011;温度为20℃、搅拌速度350 r/min时生长速率常数和成核速率常数分别为5.526×10-5和1.085×109;温度为20℃、搅拌速度450 r/min时生长速率常数和成核速率常数分别为2.512×10-5和7.114×1010。说明增加搅拌速度对硝酸浸出液体系中硫酸钙结晶动力学有较明显的影响。     

丙烯/1-丁烯无规共聚树脂等温结晶动力学研究

采用本体聚合方法合成了丙烯/1-丁烯无规共聚树脂,通过DSC研究了丙烯/1-丁烯无规共聚物的等温结晶动力学。根据Avrami方程求出了各个结晶温度下的结晶动力学参数K(T)、 n、t1/2,以及样品的结晶活化能。结果表明,随着结晶温度的升高,同一样品的结晶速率逐渐下降,说明样品的结晶是依热成核控制为主;Avrami指数 在3~4之间,表明共聚物晶体的生长方式为三维球状生长。在同一结晶温度条件下,随着共聚物中1-丁烯单元含量的增加,晶体的成核和结晶速率均下降,结晶活化能增加,共聚物中1-丁烯单元含量对结晶速率的影响很大。     

碱式碳酸镁纳米花的干燥动力学研究

以六水氯化镁和尿素为原料,采用均匀沉淀法制备出碱式碳酸镁纳米花。通过干燥动力学实验得到碱式碳酸镁纳米花的干燥曲线和干燥速率曲线。研究结果表明：在一定温度下碱式碳酸镁纳米花干燥速率曲线呈现明显的升速、恒速和降速三个干燥阶段;随着干燥介质温度的升高,干燥速率增大．干燥时间缩短。通过比较得出的干燥方程符合Page模型。     

氯化铵结晶过程实验研究

在混合悬浮结晶器中研究了氯化铵结晶热力学和动力学特性。以硝酸锰为添加剂,配制了质量分数为0.23%的硝酸锰水溶液,测定了氯化铵在该溶液中的溶解度及超溶解度。研究了硝酸锰对氯化铵晶体粒度分布的影响,得到了晶体变异系数。考察了过饱和度对氯化铵结晶线性生长速率和成核速率的影响,得到了成核-生长动力学方程。结果表明：介稳区宽度随温度升高而减小;添加晶种,介稳区宽度变窄。随硝酸锰溶液浓度增大,晶体变异系数逐渐降低并趋于平缓。当硝酸锰的质量分数逐渐增大到0.307%时,晶体变异系数几乎不再降低。线性生长速率和成核速率随溶液过饱和度的增大而增加,得到的动力学方程与实验数据吻合较好,有一定的工业参考价值。     

Ozawa方程处理PET非等温结晶动力学过程研究

采用Monte Carlo法计算机模拟在预先成核条件和不同降温速率下PET非等温结晶曲线,将模拟的结果用Ozawa方程及改进的Ozawa方程进行处理。结果表明,用Ozawa方程解析的Avrami指数与理论值相近,用改进的Ozawa方程可直接得到结晶速率常数,求出的线生长速率与理论值基本吻合。     

酒石酸钾钠对HEDP镀铜形核的影响

研究了酒石酸钾钠对羟基乙叉二膦酸(HEDP)镀铜形核的影响。通过线性扫描伏安、交流阻抗和循环伏安曲线研究铜沉积的电化学行为。随着镀铜液中酒石酸钾钠含量的提高,阴极极化曲线负移,X-射线衍射结果表明,晶粒由44 nm减小到40 nm;酒石酸钾钠的加入使循环伏安电流环消失;在-1.44、-1.45和-1.46 V的电位下,酒石酸钾钠的质量浓度在0~21 g/L内,镀液中铜离子在玻碳电极上的形核方式都为三维瞬时形核,不改变铜的形核方式;酒石酸钾钠的质量浓度增加到21 g/L,成核数密度和晶核垂直生长速率较基础液都有所增大;在电位-1.00~-1.20 V之间,铜络离子还原的表观反应活化能随着电位负移而减小,表观反应活化能由14 k J/mol增大到25 k J/mol,电极反应由扩散控制转向扩散过程和电极反应过程联合控制。     

聚酯热熔胶的熔融黏度及其Avrami方程表征

采用Brookfield黏度计对不同聚酯热熔胶样品进行黏度测试,根据Arrhenius方程得到样品相应的黏流活化能参数,进口样品的活化能值(56,76 k J/mol)大于国内样品(49,48 k J/mol),在低温下条件下国外进口样品的流动性好;通过在一定熔融温度下黏度随时间的变化,引入Avrami方程对其进行黏度随时间的变化表征,发现其lg[-ln(1-Xt)]～lg t关系图呈线性(线性度R=99%),并根据计算得到相关的降解速率常数Zt,温度越高速率常数越大;在发生高温凝胶的样品体系中,其黏度的Avrami方程发生线性偏离。     

间歇结晶器中地塞米松磷酸钠溶析结晶动力学的研究

在间歇结晶器中,利用矩量变换法对地塞米松磷酸钠溶析结晶过程动力学进行了理论研究。由实验得到的地塞米松磷酸钠晶体的粒数密度分布数据,采用非线性最小二乘法分别对与粒度相关和与粒度无关线性生长速率模型进行了参数估值,通过由各种模型得到的晶体生长速率的计算与实验值的比较,发现与粒度无关生长速率模型不仅计算过程简单且计算的精度也更好。最后,以与粒度无关生长速率模型为基础,由不同实验条件下晶体的粒数密度分布数据非线性回归出其成核速率方程和生长速率方程。对方程的理论分析表明,低过饱和度、低悬浮密度和较高的结晶温度有利于得到平均粒径更大的地塞米松磷酸钠晶体。     

聚丙烯固相接枝甲基丙烯酸乙酯的非等温结晶动力学研究

利用差示扫描量效热仪(DSC)研完了聚丙烯(PP)及PP固相接枝甲基丙烯酸乙酯(PP-g-EMA)的非等温结晶动力学。研究结果表明：PP-g-EMA的非等温结晶动力学能很好地符合Avrami方程和莫志深改进方程。PP接枝了EMA后．结晶起始温度和结晶峰温均增大。结晶速率增大。归因于PP-g-EMA中EMA支链间的偶极作用所致。但球体生长和成核机制并没有发生变化。     

Influence of the presence of bubbles on the parameters of crystal nucleation in the 26Li<Subscript>2</Subscript>O · 74SiO<Subscript>2</Subscript> glass

The crystal nucleation in the glass of composition (mol %) 26Li₂O · 74SiO₂ has been investigated in the cases of homogeneous and heterogeneous nucleation. Parameters of homogeneous nucleation, such as the stationary nucleation rate I _st, the time of nonstationary nucleation τ, and the crystal growth rate U, have been determined. The temperature dependences of these parameters have been constructed. The surface energy σ at the nucleus-glass melt interface has been determined, and its temperature dependence has been obtained. The surface energy σ has been evaluated using the graphical method for solving the transcendental equation derived by transforming the relationships for the stationary crystal nucleation rate and the time of nonstationary crystal nucleation. The critical nucleus sizes r* and the free energy of formation of the critical nucleus Φ* have been determined. The heterogeneous nucleation on bubbles specially produced in the glass has been studied. It has been demonstrated that the presence of bubbles in the initial glass does not affect the crystal growth rate and substantially changes the nonstationary nucleation rate. The largest contribution to the change in the nucleation rate is made by “active” bubbles (filled by water vapor) formed in the glasses synthesized with the use of hydrated silicon dioxide.     

An Analysis of Nucleation-Rate Type of Curves in Glass as Determined by Differential Thermal Analysis

The nucleation rate ( I ) versus temperature type of curves for a Na₂O2·CaO·3SiO₂ (NC₂S₃) glass doped with 0.1 wt% platinum, 0.5 wt% Ag₂O, and 2.0 wt% P₂O₅ were determined using a previously developed differential thermal analysis (DTA) technique. In this DTA technique, a constant amount of glass sample was nucleated at selected temperatures for a specific time, followed by a DTA scan at a fixed heating rate. The functional dependence of the maximum intensity of the exothermic DTA crystallization peak ((delta T )_p) or the inverse temperature at the DTA peak maximum ( T _p⁻¹) on the nucleation temperature ( T _n) was used to determine the nucleation rate versus temperature type of curves. Calculations for qualitatively assessing the dependence of (delta T )_p on T _n were performed using I and crystal growth rate ( U ) curves for a hypothetical system. Values of (delta T )_p calculated for different degrees of overlap between the I and U curves were compared with those measured experimentally. The (delta T )_p vs T _n curves depended strongly on the overlap of I and U , whereas the T _p⁻¹ vs T _n curves were unaffected by the overlap.     

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

The nucleation and crystallization of the Na₂O · 2CaO · 3SiO₂ (NC₂S₃) glass were studied by differential thermal analysis (DTA), and a (nucleation rate—temperature)-like curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T_p , or the height of the crystallization peak, (δ T ) _p , as a function of nucleation temperature, T_n. The temperature where nucleation can occur for this glass ranges from 550° to 650°C and the temperature for maximum nucleation is 600°± 5°C. Both temperatures are in excellent agreement with those determined by the classical technique of nucleation followed by isothermal crystallization. The activation energy for crystallization, E_c , for this glass is the same for surface and/or bulk crystallization, and is 370 ± 15 kJ/mol. The analysis of the crystallization data with the Kissinger equation yelds the correct value for E_c only when crystal growth occurs on a fixed number of nuclei. When a majority of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate E_c . E_c is also independent of the heating rate when determined using a single-crystallization-peak analysis technique. The single-peak analysis technique is useful for a rapid determination of E_c or when only a small amount of sample is available.     

A new approach for determining the critical cooling rates of nucleation in glass‐forming liquids

Crystallization of a liquid below liquidus temperature is a complex process due to simultaneous nucleation and growth of crystals. Nucleation is the crucial initial step of the crystallization process, and affects the glass‐forming ability, especially when there is a large overlap between nucleation and crystal growth versus temperature curves. From the temperature‐time‐transformation (TTT) diagram, one can estimate the critical cooling rate, , of glass‐formation, however this is time‐consuming. In this paper, we establish a simple approach to determine the using calorimetric and viscometric data. Based on the classical nucleation theory, the correlation between the crystallization onset temperature and cooling rate is described by combining two temperature‐dependent functions. The new approach is applicable to a wide range of glass‐forming systems. This work also gives insight into heterogeneous nucleation and glass formation kinetics.     

Nucleation and Crystallization Kinetics in Fragile Glass-Forming Liquids

Isothermal kinetics of crystallization in the "fragile" Ca(NO₃)₂─KNO₃ melts and in AgI─Ag₂SO₄─Ag₂WO₄ melts of intermediate fragility have been investigated using singlestep and multistep calorimetric techniques. Time-temperature-transformation curves for crystal nucleation and growth have been delineated and the temperature for maximum nucleation rate ( T _NN) identified. The results are compared with the kinetics of nucleation observed in other fragile systems, such as fluoride glass melts, and with classical oxide melts (such as Li₂Si₂O₅) which have "strong" liquid characteristics. Reduced-temperature presentations of nucleation-rate data show qualitative correlations between T _NN/ T _L ( T _L is liquidus temperature) and liquid fragility. These correlations show that strong-liquid glass formers survive much larger supercoolings without nucleation than do fragile liquids.     

Influence of Na2O content on the nucleation kinetics in glasses of compositions close to the Na2O · 2CaO · 3SiO2 stoichiometry

The kinetics of nucleation and growth of Na₂O · 2CaO · 3SiO₂ crystals in glasses with small deviations from the stoichiometric composition is studied. The stationary nucleation rate, induction period, and crystal growth rate as functions of the temperature and Na₂O content in the glass are measured. It is found that relatively small variations in the glass composition significantly affect the crystal nucleation rate. The experimental data are analyzed within the framework of the classical nucleation theory. It is shown that an increase in the Na₂O content in the glass brings about a decrease in the kinetic and thermodynamic barriers of nucleation. Deceased.     

Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

CaO-Al_2O_3-SiO_2系统玻璃晶化时首析晶相及TiO_2的作用机理预测和研究

通过ＣａＯＡｌ２Ｏ３ＳｉＯ２系统玻璃的结构分析预测玻璃晶化时首析晶相是钙长石。选取ＣａＯＡｌ２Ｏ３ＳｉＯ２三元相图成玻璃区内的某点作为基础玻璃的组成，在基础玻璃中加入ＴｉＯ２。用ＤＴＡ，ＸＲＤ和ＳＥＭ方法的研究结果表明，不管玻璃中加ＴｉＯ２与否，晶化时首先析出的晶相都是钙长石，且均为从表面向内部生长，与预测相符。ＴｉＯ２作为晶核剂，效果不太明显，在高温时ＴｉＯ２只能使玻璃网络结构松弛，粘度降低，促进玻璃中晶相的成核和生长。     

Nucleation and Growth of Lead Titanate from a Glass

The temperature dependence of the rate of growth of PbTiO₂ crystals from a PbO-B₂O₃-TiO₂ glass was experimentally determined. The Brown-Ginell equation describing the temperature dependence of the growth rate provided an excellent fit for the data. The activation energy for the growth of PbTiO₃ crystals was 34 kcal per mole. Within the same temperature range the activation energy for viscous flow was 75 kcal per mole. Differential thermal analyses of both powdered and solid samples of glass showed that the exotherm associated with crystal growth occurred at 100° to 150°C below that of the maximum rate of crystal growth and corresponded to a temperature at which the rate of surface nucleation was relatively high although the rate of crystal growth was low.