Influence of thermal history on primary nucleation and crystal growth rates of isotactic polystyrene

The influence of thermal history on primary nucleation and crystal growth rate of isotactic polystyrene (i-PS) was studied in a wide range of time and temperature. Samples were melted at several temperatures from 230 to 250 °C and then crystallized from those molten states and also crystallized from the glassy state. The primary nucleation rate is strongly influenced by the thermal history but the crystal growth rate is mainly governed by the crystallization temperature. Below a melt temperature of 230 °C, the nucleation density was attributed to the seed nuclei, which result from incomplete melting of the spherulites. Above 250 °C, a limited number of heterogeneous nucleation sites remain, and these sites are activated on the surface of impurities or foreign bodies in the melt. The primary nucleation is controlled heterogeneously both from the molten and the glassy states. The nucleation rate from the glassy state is faster than that from the molten state. A linear relationship between the nucleation rate and the inverse of induction time was found in each experimental condition and their slopes are related to the saturation density of the nuclei. The activation energy for the molecular transport and the primary nucleation energy were smaller than those of the crystal growth. The primary nucleation energy from the glass was lower than that from the melt. These energies are discussed in the present work.     

木糖醇发酵模型液的结晶动力学研究

本研究基于发酵法生产木糖醇的特点配制木糖醇发酵模型液,研究了杂质、温度、过饱和度、晶种投入量及晶种规格等因素对木糖醇结晶过程中晶体生长速率的影响。实验表明,木糖醇晶体生长速率与温度及过饱和度成正比关系。山梨醇、阿东糖醇等共存杂质都会阻碍木糖醇的结晶。而晶种投入量及晶种规格对木糖醇晶体生长速率的影响很小,可忽略不计。根据公式及Arrhenius方程,建立木糖醇晶体生长动力学方程,计算晶体生长活化能的大小,并对木糖醇晶体生长限速机制进行了分析。在木糖醇晶体生长的不同阶段,速率控制机制与表面融合机制都起到了控制结晶的作用。     

Effects of ionic impurities on crystallization of cobalamin

In this paper, the influence of two typical ionic impurities (Na⁺ and Mg²⁺) is investigated with the focused beam reflectance measurement (FBRM) technique. In this system, the on-line FBRM is used as a tool for monitoring the crystallization process of cobalamin by measuring the chord length distribution of particles and the particle counts. It is noted that impurity Mg²⁺ has a more significant effect than Na²⁺ in crystal growth of the whole crystallization process. From the microscopic observation of crystals, Mg²⁺ has an obvious effect on the crystal habit, while Na²⁺ has little effect. In addition, the crystal habit changes can be monitored by particle vision measurement (PVM). Understanding these effects is helpful to aid optimization and improve process control.     

Effect of Homolog Doping on Surface Morphology and Mass‐Loss Rates from PETN Crystals: Studies using Atomic Force Microscope and Thermogravimetric Analysis

Pentaerythritol tetranitrate (PETN) is an important energetic material, whose performance as a secondary explosive depends strongly on the density as well as flow porosity of powdered material, which in turn is governed by the size and surface properties of the PETN crystallite particles. Historically there has been evidence that the surface properties of PETN particles can be strongly influenced by the presence of homolog impurities of PETN, in particular, dipentaerythritol hexanitrate (diPEHN) and tripentaerythritol octanitrate (triPEON), although not many systematic studies characterizing such influence exist. In this work we employ thermogravimetric analysis (TGA) to measure mass‐loss rates at elevated temperatures and show that doping with a small amount of diPEHN and triPEON can reduce the mass‐loss rate from PETN single‐crystal surfaces by as much as 35 % as compared to undoped crystals. Arrhenius plots of mass‐loss rates as a function of temperature suggest that the reduction in evaporation is not due to the change in activation barrier of the molecular evaporation process, but perhaps due to the impedance to the receding motion of the steps by the immobile impurities on the surface. Removal of surface impurities through gentle washing with ethanol leads to enhanced mass‐loss rate relative to pure PETN suggesting a roughened surface morphology. Some surface roughening in doped crystals is supported by Atomic force microscopy (AFM) images of growth layers that show evidences of growth layer stacking and rough edges. We also find that a larger amount of impurity added to the original solution does not necessarily lead to a more highly doped crystal, which could perhaps be interpreted as PETN crystals being able to accommodate only up to a certain weight percent of homolog impurities.     

Impurity effects on crystallization of KCL from aqueous solutions in a MSMPR crystallizer

Extensive experimental investigations into the effect of magnesium and sulfate ionic impurities on cooling crystallization of KCl were carried out in a steady-state Mixed Suspension Mixed Product Removal (MSMPR) crystallizer. The impurities selected for investigation were chosen from those naturally associated with KCl in potash industry. Eighty one experiments were performed in a factorial experimental design scheme. The results exhibited that magnesium ion did not have any significant influence upon crystallizations of KCl, while the sulfate ion enhanced the aggregation rate. The nucleation rate was only slightly affected by the presence of impurities. The size enlargement in presence of sulfate ion was ascribed to an increase in aggregation rate. It was demonstrated that the role of the impurities studied upon KCl crystallization is consistent and predictable and is a function of the chemical character of the impurity species. Based on these results two mechanisms for size enlargement were identified, first due to crystal growth by solid deposition and second due to aggregation. Aggregation was found to be the governing mechanism for producing large crystals.     

EFFECT OF MAGNESIUM AND SULFATE IONS ON KCl CRYSTALLIZATION IN A CONTINUOUS COOLING MSMPR CRYSTALLI2:ER

Crystallizations of KCl from water and reagent grade potassium chloride solution with and without magnesium sulfate and magnesium chloride as impurities were conducted in a one liter continuous MSMPR cooling crystallizer. A series of experiments at a constant operating temperature, various feed temperatures and magnesium and sulfate ions concentrations were carried out to investigate the effect of the type and concentration of impurities on the width of the meta-stable zone, crystal size distribution, crystal habit, crystal purity, growth rate, nucleation rate, and aggregation index. The width of the meta-stable zone increased with impurity concentrations of both magnesium and sulfate ions alike. Sulfate ion increased the mean crystal size at all concentrations, whereas the initial increase in mean crystal size at low magnesium concentrations was moderated at higher concentrations. Polycrystals formation was significant with and without impurities. The crystal surface became irregular at higher sulfate ion concentrations. The impurity incorporation within KCl crystals decreased with crystal size and was much higher in the case of magnesium sulfate. Secondary nucleation rate was unaffected and growth rate was altered in the presence of both impurities, and the influence was apparently independent of the effect upon the meta-stable zone width. The effect of impurities was suppressed with increase in suspension density.     

Crystallisation of pentaerythritol

Growth measurements were made of pentaerythritol crystals in both fluidised beds and stirred suspensions. The measurements were made in both the presence and absence of the two by-product impurities known to be normally present in the commercial material. Maximum growth rates of the order of 10^?9m/s were observed with activation energies of between 100 and 200 kJ/mol depending on the sample tested. The growth rate was second-order with respect to super-saturation. The two main impurities were found not to greatly influence the growth rate. There was evidence that a so far undetected third impurity, which was present in only trace amounts, was responsible for the very low growth rates observed.     

Effect of lattice impurities on the thermal conductivity of β-Si3N4

Effect of impurities in the crystal lattice and microstructure on the thermal conductivity of sintered Si₃N₄ was investigated by the use of high-purity β-Si₃N₄ powder. The sintered materials were fabricated by gas pressure sintering at 1900 °C for 8 and 48 h with addition of 8 wt.% Y₂O₃ and 1 wt.% HFO₂. A chemical analysis was performed on the loose Si₃N₄ grains taken from sintered materials after the chemical treatment. Aluminum was not removed from Si₃N₄ grains, which originated from the raw powder of Si₃N₄. The coarse grains had fewer impurities than the fine grains. Oxygen was the major impurity in the grains, and gradually decreased during grain growth. The thermal conductivity increased from 88 Wm⁻¹ K⁻¹ (8 h) to 120 Wm⁻¹ K⁻¹ (48 h) as the impurities in the crystal lattice decreased. Purification by grain growth thus improved the thermal conductivity, but changing grain boundary phases might also influence the thermal conductivity.     

季戊四醇在反应液中的结晶动力学研究

甲醛、乙醛在碱性条件下合成季戊四醇,反应液中含有大量杂质,结晶是实现季戊四醇提纯和关键杂质组分二季戊四醇回收的主要手段。文章用筛析法对工业季戊四醇反应液中的季戊四醇和二季戊四醇结晶动力学特性进行了研究。考察浓缩反应液从80℃降到30℃,结晶时间为8 h,搅拌速度为60 r/min条件下,测得二季戊四醇与季戊四醇的成核速率分别为3.32×106,5.1×105个/(h.L),比值为6.51;季戊四醇与二季戊四醇晶体生长速率分别为0.032 9,0.006 5 mm/h,比值为5.05,以三维体积计为128倍。理论上证明了季戊四醇晶体粒径大、密度小,二季戊四醇晶体粒径小、密度大的原因,与镜检结果一致。为制备高纯季戊四醇工艺提供理论依据,表明可以根据产品的纯度和收率要求,设计相应的工艺条件。     

Relationship between Surface Roughness,Internal Crystal Perfection,and Crystal Growth Rate

Potential mechanisms affecting growth rate dispersion (GRD) are investigated. Previous studies have identified surface roughness and internal lattice perfection as key mechanisms which are both evaluated with respect to GRD. Crystal growth of potassium dihydrogen phosphate was studied in two solvent mixtures, water and water‐ethanol. The surface roughness was analyzed by atomic force microscopy and the internal crystal perfection by X‐ray diffraction using a synchrotron source. The crystals grown at higher supersaturation have more pronounced and more frequent surface irregularities, supporting previous findings on a feedback mechanism between surface roughness and growth rate. No significant relationship was found between internal crystal perfection and growth rate, however, this is likely due to the size of the crystals analyzed herein and not the absence of any such mechanism in small crystals.     

Investigations into the crystallization of cobalamine and the influence of ionic impurities on crystal growth

This paper presents an experimental investigation on the crystallization of cobalamine (vitamin B₁₂) under different operating conditions and crystallization methods. A systematic analysis of the effects of cooling rate, stirring speed and solvent on the metastable zone and quality of crystals is presented. Focused beam reflectance measurement probe is used for in situ particle size analysis. The process of crystallization is observed by FBRM. The influence of four typical ionic impurities (Na⁺, Mg²⁺, Ca²⁺, and Fe³⁺) on cobalamine crystallization is also studied. From the observation of crystals by SEM, it is clear that Fe³⁺ and Mg²⁺ have significant effect on crystal morphology, while Na⁺ and Ca²⁺ have a little.     

A method of characteristics for solving population balance equations (PBE) describing the adsorption of impurities during crystallization processes

Possible hindering effects of impurities on the crystal growth were shown to take place because of the adsorption of impurity species on the crystal surface. Transient features of this adsorption were observed, such that the growth of a given crystal does not depend on supersaturation only, but also on the time a given particle spent in contact with impurities present in the mother liquor. Meanwhile, few kinetic models describe the effect of impurities on the growth of crystals in solution, and published models are usually derived from data obtained, thanks to specific experiments based on the evaluation of the growth rate of single crystals. Such models are obviously questionable because, in the industrial practice, distributed properties of crystals are actually involved. Considering the “time of contamination” of particles as a new internal variable is thus made necessary. This is the reason why a specific PBE resolution algorithm is presented in this paper. The numerical scheme for the resolution of PBEs is based on the method of characteristics and shown to allow fast and accurate simulation of transient features of the crystal size distribution in the particular case when the growth or nucleation rates are assumed to exhibit unsteady-state dynamics. The algorithm is finally used to simulate the isothermal desupersaturation crystallization of citric acid in water.     

Stresses generated by impurities in diamond

Single dissolved nitrogen, boron or hydrogen atoms cause the diamond lattice around them to expand by 40%, 33.7% and 31%, respectively. Such large atomic strains generate short-range stresses around each dissolved impurity atom. If the impurity is dissolved uniformly in a diamond crystal, the atomic strains around each impurity atom will produce an overall uniform expansion that will not generate macro stresses in the diamond crystal. However, if the impurity is not dissolved uniformly, incompatible macrostrains will result that will generate large elastic stresses in the diamond crystal. These stresses may strengthen or weaken a diamond crystal depending on their magnitudes, signs (i.e. compression or tension) and spatial distributions in the crystal.

An inhomogeneous distribution of impurities can be generated either by the appearance of higher order growth facets during crystal growth, which generate differential segregation of impurities between the high- and low-order crystal growth sectors or by temporal changes in growth conditions which generate radial impurity gradients. Since growth conditions control the distribution of impurities in the diamond, diamonds can be strengthened or weakened by different growth conditions.     

Verneuil growth of TiO2 (rutile) crystals of large size and low dislocation density at low gas flow rate

Large (30 mm in diameter) TiO₂ (rutile) single crystals with low dislocation density were grown by the Verneuil method using a single crystal furnace designed and improved by the authors. The structure of the burner was optimized by numerical simulation analysis so that the crystal could be grown at a low gas flow rate. The investigation of the growth process parameters (i.e., growth rate, outer flow rate of O₂, inner flow rate of O₂, and the increment of H₂ flow rate) shows that the inner flow rate of O₂ and the increment of H₂ flow rate have the strongest influence on the crystal growth process. On this basis, other growth parameters (growth rate, outer flow rate of O₂) were also optimized. Compared with the conventional Verneuil method, the crystal can be grown at a considerably low gas flow rate (40–50% lower) with the method in this work, which reduces the melt turbulence impacted by gas flow, enabling a steady and clear solid-liquid interface and improving the crystal quality. The optimum growth conditions are for the growth rate of 6 mm/h, O₂ outer flow rate of 3.5 L/min, O₂ inner flow rate of 5.5 L/min, and increment of H₂ flow rate of 0.1 L/4 min. The etch pit density of the rutile crystals is 3.29 × 10⁴ cm^?2, an order of magnitude lower than that of the crystals grown by the conventional Verneuil method. The optical properties of the crystal are comparable to those grown by the floating zone method. Especially, it is easier to obtain a larger crystal size with lower production costs. Our results provide a possible route for industrializing the Verneuil production of large, high-quality and low-cost rutile single crystals.     

Effects of ionic impurities on the crystal morphology of phosphoric acid hemihydrate

The adsorption of foreign molecules on the crystal surface is generally believed to contribute to modify crystal morphology and the different adsorption energies on different crystal faces may lead to different modifications of crystal morphologies. In order to obtain a deep understanding of the face growth intervening mechanism, phosphoric acid hemihydrate (H₃PO₄·0.5H₂O) is taken as an example to investigate the influence of several typical ionic impurities on its crystal morphology by applying a molecular simulation study using Cerius 2 software. The absolute values of the calculated binding energies for cations are larger than that for anions. Therefore, it is concluded from the simulation that cations inhibit the growth of some crystal faces, while anions has no significant inhibition to a particular crystal surface, which is in good agreement with the experimental observation. It is also noticed that significant differences of binding energies to the different crystal faces will lead to obvious modifications of crystal morphologies and higher charged cations have more obvious effects than lower charged cations on the crystal morphology modification.     

果糖在高黏度水溶液中的生长模型及机理

果糖是一种高附加值的甜味剂，其水溶液的黏度很高，导致晶体生长速率非常缓慢。利用常规的在线和离线测量方法会因为黏度大和成核而造成测量不准确。这导致果糖晶体在纯水中的生长速率目前尚无可靠数据，难以精确实现果糖工业生产过程的设计及优化。本文通过考察黏度、密度与扩散作用研究了高黏度果糖水溶液中的晶体生长速率。首先，利用旋转黏度计测定了果糖水溶液的黏度，考察了温度、浓度对其黏度的影响，使用经验模型对黏度数据进行了关联。随后利用比重瓶法测定了果糖水溶液的密度，考察了温度、浓度对溶液密度的影响。基于黏度和密度的测定结果，利用自由体积模型预测了果糖饱和水溶液的扩散系数，探究了在高黏度果糖水溶液中影响溶质分子传递过程的关键因素。最后，使用扩散控制的生长模型预测了果糖晶体的理论生长速率。采用单晶生长实验测定了果糖晶体的实际生长速率，将理论生长速率与之进行比较，结果吻合良好。此外，基于扩散控制和实际生长形貌，判断果糖晶体的生长机理属于螺旋错位生长。     

Experiment and numerical simulation of the influence mechanism of kinetic factors on rapid growth of KDP crystal

A series of KDP crystals at different rotation speeds ranging from 9 to 120 rpm were grown by rapid growth method. The results fully demonstrate that rotation speed as a key kinetic parameter has a great influence on the whole crystallization process. Moreover, an important characteristic has been proved that the growth rates along X and Z-direction increase with the increase of rotation speed lower than 100 rpm, while the rates of X-direction are always greater than Z-direction. Combined with numerical simulation method, the correlations of solution stability with kinetic factors including rotation speed, crystal size and growth platform are explored in detail. During the crystallization process, the thickness of boundary layer decrease and a higher flow rate is caused with the increase of rotation speed below 100 rpm, and the generation of inclusions at lower speed may be associated with the slower flow rate of growth solution. The stability of solution gradually decreases as the rotation speed ranges from 100 to 120 rpm. Furthermore, the computed results of R_e imply that reducing rotation speed as increasing of crystal size may be an effective method to obtain high-quality and large-scale KDP crystal by rapid growth technique. Comparing to the growing platform with four columns, the platform with two circular connecting columns play an active role in influencing the growth rate of KDP crystal, especially in axial direction.     

氟硅酸钾(钠)在湿法磷酸中的结晶动力学研究

在MSMPR结晶器中系统地研究了氟硅酸钾（钠）在湿法磷酸中的结晶动力学及主要杂质F^-、Mg^2 、Fe^3 、Al^3 的影响。结果表明,在纯硫磷混酸中,Na2SiF6为八面体晶体、K2SiF6为由立方体晶体聚在一起的聚晶;SO4^2-、Mg^2 结氟硅酸钾（钠）晶体生长有促进作用,F^-的影响不明显,Fe^3 、AI^3 能抑制其生长;四种杂质共存的模拟湿法磷酸及磷矿萃取磷本台氟硅酸钾（钠）晶体生长速率、平均粒径均明显减小,形态发生很大变化。     

Effects of ionic impurities on crystallization of cobalamin

In this paper, the influence of two typical ionic impurities (Na⁺ and Mg²⁺) is investigated with the focused beam reflectance measurement (FBRM) technique. In this system, the on-line FBRM is used as a tool for monitoring the crystallization process of cobalamin by measuring the chord length distribution of particles and the particle counts. It is noted that impurity Mg²⁺ has a more significant effect than Na²⁺ in crystal growth of the whole crystallization process. From the microscopic observation of crystals, Mg²⁺ has an obvious effect on the crystal habit, while Na²⁺ has little effect. In addition, the crystal habit changes can be monitored by particle vision measurement (PVM). Understanding these effects is helpful to aid optimization and improve process control.     

Crystallization behavior and density functional theory study of solution combustion synthesized silicon doped calcium phosphates

The influence of heat-treatment temperatures (700 °C, 900°C, 1200 °C) on the phase, physical properties, crystallization rate, and in vitro properties of the solution combustion synthesized silicon-doped calcium phosphates (CaPs) were investigated. The thermodynamic aspects (enthalpy, entropy, and free energy) of the synthesis process and the crystallographic properties of the final samples were first predicted and then confirmed using density functional theory (DFT). Results demonstrated that the crystallization rate was controlled by the fuel(s) type (glycine, citric acid, and urea) and the amounts of Si⁴⁺ ions (0, 0.1, 0.4 mol). The highest calculated crystallization rate values of the un-doped, 0.1, and 0.4 mol Si-doped samples were 64%, 22%, 38%, respectively. The obtained results from the DFT simulation revealed that crystal growth in the direction of c-axis of hydroxyapatite (HAp) structure could change the stability of (001) surface of (HAp). Also, the computational data confirmed the adsorption of Si–OH groups on the (001) surface of HAp during the SCS process with an adsorption energy of 1.53 eV. AFM results in line with DFT simulation showed that the observed change in the surface roughness of Si-doped CaPs from 2 to 8 nm could be related to the doping of Si⁴⁺ ions onto the surface of CaPs. Besides, the theoretical and experimental investigation showed that crystal growth and doping of Si⁴⁺ ions could decrease the activation energy of oxygen reduction reaction (ORR). Furthermore, the results showed that the crystallized HAp structure could have great potential to efficiently reduce oxidative stress in human body.