钼酸铵结晶成核速率研究

用酸碱质子理论解释钼酸铵结晶过程。在讨论钼酸铵成核机理的基础上，研究了不同酸化速度下四钼酸铵（ＡＱＭ）结晶的成核速率。结果表明，ＡＱＭ结晶溶液过饱和具有持续升高的特征。与晶体生长相比，成核速率的过程阶数更大。大量晶核的生成是过饱和消除的主要因素。ｐＨ＜２．５以后，降低加酸速度是控制成核速率、制取粒度粗、分布窄的ＡＱＭ晶体的关键。     

钼酸铵结晶动力学参数与溶液同多酸根组成变化的关系

研究了钼酸铵中和结晶过程成核速率（N）与晶体线生长速率（L）与溶液中同多酸根离子组成的变化关系。结果表明：AQM的N和L随溶液中[MO8O26^4-]的升高以及[MO7O24^6-]和pH的降低而增大。在加入酸量相同的情况下,酸化速度快的溶液由于偏离缩合平衡较大,其[Mo8O26^4-]始终较高,[Mo8O24^6-]较低。AQM的L高峰值出现在[Mo8O26^4-]较低,而[Mo7O24^6-]较高的结晶初期,其主要原因是此阶段首先接触到无机酸的局部溶液中成核数量较少,过饱和持续的时间长。     

仲钨酸铵成核期晶体生长速率研究

采用将最终产品粒度组成与生长质量速率的测试结果结合的方法研究了钟钨酸铵(APT)成核期晶体的线生长速率()和结晶速率()。试验条件下APT成核期的值为1.0～137um/min,值为1.8～16.5g/min.dm~3。提高起始WO_3和NH_4Cl浓度、增大加热强度,将使和大幅度升高。~(max)出现时间提早。过大的搅拌强度虽可减少连生体的生成,但由于面积A增大、过饱和度△C变小、以及晶体破碎加剧,将使和值下降。APT连生体的生成使成核前、后期的值分别升高和降低。破碎晶体存在的生长散布现象导致和的平均值略为下降。与WO-4~(2-)的聚合反应相比,上述因素对相变反应的影响较大。     

仲钨酸铵晶体生长反应历程及限制性环节研究

提出仲钨酸铵（ＡＰＴ）晶体生长过程由非基元反应，结晶介质扩散和界面反应等三个步骤组成。通过研究多种条件下质量生长速率ｍｍａｘ和线生长速率Ｌｍａｘ的变化特征，探讨了生长的限制性环节由相变反应为为聚合以应，以及相变反应区域内过程由晶粒总表面积Ａ转变过饱和度△Ｃ控制的变化规律。结果表明：试验条件下相变反应区域在１．５～４．６ｍｉｎ内由Ａ控制的时间为０．８９～１．７ｍｉｎ，搅拌转速由０πｒ／ｓ扩散控制已转     

Na+和K+对过饱和铝酸盐溶液均相成核过程的影响

采用原位电导跟踪仪分别检测了铝酸钠和铝酸钾溶液的均相成核诱导期,考察了Na+、K+对均相成核过程的影响。结果表明:60℃时,Na+、K+显著影响溶液的均相成核诱导期,相同过饱和度下的铝酸钠溶液成核速率快于铝酸钾溶液;成核反应级数均为4,临界固-液界面能为31.8 mJ/m2,与阳离子无关。这表明阳离子参与了溶液成核过程的铝酸根离子重构和基元生成,但不影响成核的控制步骤。     

生物冶金过程中黄钾铁矾的生成与抑制

研究初始pH梯度对细菌培养过程中黄钾铁矾生成的影响。结果表明,黄钾铁矾的生成受溶液pH、细菌活性和Fe3+浓度共同影响。溶液pH 1.4~2.0时细菌活性较好,溶液中Fe2+转化成Fe3+的速率快,促进Fe3+水解生成黄钾铁矾;溶液pH 1.0~1.4时Fe3+的水解反应受到显著抑制。Fe3+的水解过程是先生成胶体相Fe(OH)3后逐渐形核、生长、结晶出黄钾铁矾。生物冶金过程中反应前期溶液pH应大于1.4,此阶段以促进细菌生长、加快矿石氧化分解为主,当细菌生长进入对数期后,溶液pH应小于1.4,此阶段以控制浸出液中细菌的活性,抑制黄钾铁矾的生成为主。     

氢化钛的动态分解行为与规律

通过DSC/TG的热分析试验,研究氢化钛升温过程中分解的动力学规律,利用Coast-Redfern积分法计算了分解过程的动力学参数。结果表明,氢化钛热分解的开始温度为510℃,分解过程中总质量损耗率达3.15%,其中565～660℃温度范围内的质量损耗率占总质量损失的50%左右,分解过程中生成了比氢化钛热稳定性更高的TiHx(0.7相似文献   

微混效应对草酸铈结晶动力学的影响

通过文献总结和实际研究,阐述了沉淀结晶过程中微混效应的重要作用.微混效应对于结晶动力学的成核速率和生长速率具有很大的影响,成核速率和生长速率是决定所得产物的粒径和粒径分布的关键因素.在此基础上研究了加料位置和搅拌速度对草酸铈结晶动力学的影响.     

碳纳米管改性聚苯硫醚复合材料的等温结晶动力学研究

应用差示扫描量热法(DSC)和Avrami模型分析聚苯硫醚(PPS)/碳纳米管(CNT)复合材料的等温结晶行为,分别考察了PPS和复合材料的结晶动力学参数以及结晶活化能,揭示了PPS的等温结晶特性和少量CNT对PPS结晶行为的作用.结果表明:随着结晶温度的升高,复合材料的结晶速率逐渐下降,说明复合材料的结晶是以依热成核控制为主;少量CNT的加入降低了PPS的结晶活化能,明显提高了PPS的结晶速率,同时使成核方式发生转变;纯PPS的Avrami指数n约为4,结晶方式为均相成核,而复合材料的Avrami指数n约为3,转变为异相成核;成核方式的转变大大的提高了PPS的结晶速率.     

乳酸存在时H_2TiO_3从TiOSO_4溶液中沉积机理的研究

研究硫酸法生产钛白的中间产品偏钛酸(F_2TiC_3)的生长机理对钛白生产具有重要的意义,本文从Nielson理论上推导的结晶生长的积分公式出发,研究了有乳酸(CH_3CHOHCOOH)存在时硫酸氧钛(TiOSO_4)溶液热水解过程中偏钛酸结晶的机理。研究表明,无乳酸存在时,偏钛酸沉积过程是固体表面上多中心生长,每个结晶点上为单分子成核,有乳酸存在时只是减小水解速度而不影响偏钛酸生长的机理。     

硫酸铵、镁、锰复盐结晶动力学研究

在硫酸铵、镁、锰复盐体系中,当Mn2+、Mg2+、NH4+初始浓度分别为28、20、30 g/L,蒸发瓶转速为30 r/min,结晶温度为298、308、318、328 K时,研究了铵、镁、锰复盐体系的悬浮密度、晶体粒度、晶体粒数密度、晶体结晶动力学.结果表明:在复盐结晶过程中,当结晶时间为4500 s,结晶温度为32...     

二钼酸铵蒸发结晶过程控制

对二钼酸铵（ADM）[(NH4)2MoO7]蒸发结晶过程进行实验控制，研究了普通型四钼酸铵溶液配比、pH值对ADM化学成分、纯度的影响；过饱和溶液介稳区的温度、蒸发时间对晶体粒度及分布的影响，并对ADM生产工艺提出建议。     

Amorphous Cr5Si3 thin films— morphology and kinetics of crystallization

The changes in morphology and the crystallization kinetics of amorphous Cr₅Si₃ were studied by means ofin situ resistivity measurements and hot stage transmission electron microscopy (TEM). The crystallization process is controlled by nucleation and growth during the continuous heating as well as in the isothermal annealing. Initially the growth is isotropic. In the later stages it becomes anisotropic due to impingement. The growth characteristics and nucleation rates were deduced from the changes in linear dimension of a growing particle with time and from the number of nucleation points as a function of time. Growth rates were found to remain constant for most of the total transformation time while nucleation rates initially increase, subsequently peak, and then rapidly decrease. The determined nucleation and growth rates were used to calculate the transformed volume fractions. The results were compared with data obtained by assuming a linear relationship between instantaneous resistivity and volume fraction and with data based on measurements of projected crystallized areas in transmission (image analysis). Kinetics studies showed that the isothermal crys-tallization follows a sigmoidal curve. The apparent activation energy was found to be ∼2.4 eV. The transformation mode parameter was found to be ∼3 which, given that the crystallization reaction is interface controlled and the nucleation may be approximated as instantaneous, suggests a three-dimensional mode of crystallization.     

碱度对高速薄板坯保护渣结晶性能的影响

为解决薄板坯连铸在高拉速下坯壳表面纵裂纹较多的问题,通过提高保护渣碱度(B)的方式,增强渣膜的结晶性能,进而降低铸坯向结晶器的传热,促进坯壳均匀生长。研究过程中采用单丝热电偶技术、FactSage热力学软件以及数学模型计算等手段分析了高碱度对薄板坯保护渣结晶行为的影响,并揭示了其影响机理。研究结果表明,当碱度从1.60增至1.80时,保护渣的结晶温度区间增大,临界冷却速率提高,结晶活化能降低,说明提高碱度会促进保护渣结晶。同时,在碱度提高过程中,保护渣中晶体的形核和长大机制存在2种情况,分别是B=1.60时形核速率减小、界面反应控制条件下、以二维长大方式生长,以及B=1.70、1.75和1.80时形核速率减小、界面反应控制条件下、以三维长大方式生长。     

Non-isothermal Crystallization Kinetics of Mold Fluxes for Casting High-Aluminum Steels

This paper investigates the crystallization behavior of CaO-SiO₂- and CaO-Al₂O₃-based mold fluxes for casting high-aluminum steels using single hot thermocouple technology, developed kinetic models, and scanning electron microscope. The results showed that the crystallization ability of the typical CaO-SiO₂-based Flux A (CaO/SiO₂ 0.62, Al₂O₃ 2 mass pct) is weaker than that of CaO-Al₂O₃-based Flux B (CaO/SiO₂ 4.11, Al₂O₃ 31.9 mass pct) because of its higher initial crystallization temperature. The crystallization kinetics of Flux A was “surface nucleation and growth, interface reaction control” in the overall non-isothermal crystallization process, whereas that of Flux B was “constant nucleation rate, 1-dimensional growth, diffusion control, in the primary crystallization stage, and then transformed into constant nucleation rate, 3-dimensional growth, interface reaction control in the secondary crystallization stage.” The energy dispersive spectroscopy results for Flux B suggested that the variations in the crystallization kinetics for Flux B are due to different crystals precipitating in the primary (BaCa₂Al₈O₁₅) and secondary (CaAl₂O₄) crystallization periods during the non-isothermal crystallization process.     

Crystallization Behavior in Fluoride-Free Mold Fluxes Containing TiO2/ZrO2

  The time temperature transformation (TTT) diagrams of fluoride free mold fluxes containing TiO2/ZrO2 were constructed through the confocal scanning laser microscope (CSLM) technique. It was found that the crystallization temperature of F free mold fluxes containing TiO2 or ZrO2 increases, while incubation time decreases with increasing basicity. The crystallization tendency increases with the zirconia addition in slag melt, as it may be thought that the addition solubility is limited in molten slag and the solid particles act as heterogeneous nucleation sites. Three types of crystal morphologies were observed, corresponding to different crystallization mechanisms. CaSiO3 and Ca3Si2O7 precipitate in the slag with low basicity, and Ca2SiO4 was formed with increasing basicity. The addition of TiO2 promotes the precipitation of CaTiO3. The logarithm of crystal growth rate increased with increasing isothermal temperature, suggesting that the crystal/melt interface reaction is the controlling step in the present experiment.     

The Effect of P2O5 on the Crystallization Behaviors of Ti-Bearing Blast Furnace Slags Using Single Hot Thermocouple Technique

The present paper investigates how the P₂O₅ addition influences the crystallization behaviors of Ti-bearing blast furnace (Ti-BF) slags with different basicity using Single Hot Thermocouple Technique. It was found that the basicity showed a significant effect on the crystallization behaviors of the Ti-BF slags, and the trend of formation of the rod-shape crystal decreased while the trend of formation of dendrite crystal increased with increasing basicity. The addition of P₂O₅ was found to promote the formation of rod-shape crystal. The basicity and crystallization temperature that the rod-shape crystal could be formed increased, while the incubation time of formation of the rod-shape crystal decreased with increasing P₂O₅ content. Scanning electron microscope equipped with energy-dispersive X-ray spectroscope and X-ray diffraction were employed to observe the morphology and determine the crystalline phase of the Ti-enriched crystals. The results indicated that the rod-shape crystal was rutile. The kinetics of the formation of rutile was studied, and the mechanism of crystallization and growth was further discussed. The results indicated that the crystallization of rutile was one-dimensional interface-controlled growth, and the nucleation rate varied with the holding time.     

Life circumstances, lifestyles and oral health among older Canadians

Nucleation, growth and aggregation are thought to be the most important crystallization processes in stone formation. Since crystallization properties change with urinary dilution, centrifugation and filtration, crystallization should always be studied in freshly voided and not pretreated urine. Recently we developed an automated method where calcium oxalate crystallization is induced in native urine by an exogenous oxalate load and nucleation and growth are monitored by an ion-selective calcium electrode. The method has now been supplemented with the spectrophotometric measurement of crystal aggregation. Repeated experiments in the same urine with different oxalate loads enable the determination of the critical oxalate additionable to induce crystallization (metastable limit) and the calculation of an oxalate load-independent growth rate constant. Preliminary results obtained in the native urine of healthy controls showed an extremely high limit of metastability and a complete absence of crystal aggregation. These findings may explain why, despite frequent urinary calcium oxalate supersaturation, healthy people do not form stones.