水热生长碱式硫酸镁纳米线结晶动力学研究

以硫酸镁和氢氧化钠为原料,邻苯二甲酸氢钾为络合剂,采用络合-水热法制备了碱式硫酸镁纳米线;采用宏观动力学方法结合微观结构分析,进行碱式硫酸镁结晶机理研究。通过分析Mg浓度与时间的变化关系曲线,建立结晶动力学方程;然后根据晶体缺陷分析碱式硫酸镁纳米线表面成核机制。实验结果表明：碱式硫酸镁纳米线在140和160℃结晶机理为多核控制表面生长;在180和200℃时结晶机理发生了变化,转变为线性控制表面生长;碱式硫酸镁纳米线的晶格中存在着较多的刃型位错与螺型位错,可以促进其表面成核、快速生长。     

无水硫酸镁对氧化镁水热产物的影响

以轻烧菱镁矿获得的氧化镁为原料,在温度为160℃、搅拌速度为400 r/min的条件下反应,研究无水硫酸镁对氧化镁水热产物的影响.当MgO与MgSO4的摩尔比为10:1时,反应6 h的水热产物是纯度高、结晶度好的六方片状氢氧化镁;当MgO与MgSO4的摩尔比为2.5:1时,前3 h水热产物是六方片状氢氧化镁,随后出现碱式硫酸镁晶须并且其生成量越来越多;当MgO与MgSO4的摩尔比为10:7时,前50 min水热产物是六方片状氢氧化镁,然后出现碱式硫酸镁晶须,直至6 h全部生成直径约为300 nm、表面光滑的碱式硫酸镁晶须.在此过程中,小颗粒氢氧化镁出现溶解现象,形成碱式硫酸镁晶核,大颗粒氢氧化镁与溶液中的MgSO4、H2O生成大量MgSO4·5Mg(OH)2·3H2O(153型碱式硫酸镁),在其生长方向上生长基元Mg-O6进入由于螺旋位错形成的二维台阶的凹陷处促使其沿位错方向稳定生长为晶须.无水硫酸镁的浓度越大,生成碱式硫酸镁晶须越多,生成碱式硫酸镁晶须所用时间越短.     

碱式硫酸镁晶须生长机制研究进展

研究碱式硫酸镁晶须的生长机制,对加深了解晶须自身性质和进一步优化晶须制备条件有着重要的意义。针对碱式硫酸镁晶须生长机制的相关研究,将其总结划分为4类:相图指导的机理研究、晶须尖端生长理论机制及其拓展、晶体生长动力学理论机制和溶解-结晶理论机制及其拓展等,并对每一类生长机制的特点、发展及应用进行了综述。     

PUR-T对POM球晶成核与生长过程的影响

以聚酯型热塑性聚氨酯(PUR-T)作为聚甲醛(POM)球晶生长抑制剂,采用热台偏光显微镜,即时在线跟踪POM球晶的成核与生长过程,研究了PUR-T在其不同用量以及不同结晶温度下对POM球晶成核及生长过程的影响。结果表明,PUR-T存在下,POM的成核密度明显增大,并随结晶温度的提高基本呈下降趋势;在结晶温度为155℃及以下时,PUR-T对POM成核时间基本无影响,156℃以上时,使成核时间明显增加,158℃的增加尤为显著。PUR-T使POM球晶生长速率明显下降,且结晶温度越高,生长速率下降幅度越大;PUR-T质量分数为5%～15%时,用量变化对生长速率的影响甚微。同时,PUR-T使POM结晶过程的成核参数和折叠链表面自由能增加,表明PUR-T的加入使POM成核难度增大,球晶生长所需克服的能量增加。     

聚氧乙烯薄膜的结晶形态研究

以不同相对分子质量的聚氧乙烯(PEO)为研究对象,采用差示扫描量热仪、偏光显微镜和原子力显微镜研究了影响PEO薄膜结晶的主要因素。结果表明:影响PEO结晶的主要因素是结晶温度,PEO在25℃形成球晶,温度升高时,晶体尺寸亦略微增加,黑十字消光现象明显,且可观察到环形裂纹;结晶温度较低时,片晶生长先由成核生长机理控制,而后由扩散生长机理控制,结晶温度较高时,片晶生长主要由成核机理控制;相对分子质量对晶体尺寸有一定影响,但不影响结晶形态,随着相对分子质量的增加,PEO薄膜的片晶形成由成核机理控制转变成扩散机理控制,PEO晶体从Flat-on结构逐渐转变为Edge-on结构。     

2-氯-4,6-二硝基间苯二酚的结晶动力学研究

采用间歇动态法测定了2-氯-4,6-二硝基间苯二酚(CDNR)的结晶动力学参数,并回归了CDNR的结晶动力学方程,分析了结晶动力学的影响因素。通过回归结晶成核速率方程和生长速率方程的计算,得到了一定温度、一定搅拌速度下的成核、生长速率常数,温度27℃、搅拌速度180 r.min-1时成核速率常数和成长速率常数分别为9.17×106和2.17×10-12,温度33℃、搅拌速度240 r.min-1时成核速率常数和成长速率常数分别为2.02×108和7.10×10-12。     

丙烯酰胺的结晶动力学

采用MSMPR连续结晶器研究了丙烯酰胺的成核 -生长动力学。实验结果表明 ,丙烯酰胺的晶体生长基本符合生长速率与粒度无关的ΔL定律。在小于 1 0 0 μm的晶体粒度范围内 ,存在大量的细小晶粒 ,分析了产生这种现象的 4种可能原因。得到结晶温度分别为 1 0、1 5、2 0℃时 ,丙烯酰胺的成核 -生长动力学模型分别为 :n0 =1 2 6× 1 0 - 5G3 6 6 、n0 =3 92× 1 0 - 5G2 92 、n0 =7 65× 1 0 - 5G3 31 。在 1 0～ 2 0℃内 ,丙烯酰胺的成核 -生长动力学级数为 4 30。动力学级数对温度敏感性较小 ,不能通过调节晶浆密度来控制晶体产品的主粒度。进行了丙烯酰胺结晶的应用研究 ,完成了连续三步式结晶新工艺的概念设计。     

碱式硫酸镁晶须的生长机理分析

为了解碱式硫酸镁晶体结构和晶体生长过程,构建碱式硫酸镁超级晶胞模型,采用Materials Studio 7.0软件中的Morphology程序对碱式硫酸镁2×2×2超级晶胞及其(201)、(202)、(203)、(111)和(114)真空slab计算模型进行平衡态计算,计算了其主要显露面族的Distance值和晶面面积百分比等相关参数。计算结果表明,碱式硫酸镁生长符合螺旋位错生长机制,其形貌会呈现晶须状但其中会掺杂着六棱柱等不规则的结构与实验结果吻合。     

阿莫西林成核与生长动力学

阿莫西林现有结晶工艺获得的晶体粒度过小,直接影响到后续的过滤、干燥以及包装运输,研究其结晶动力学参数以指导结晶工艺,控制结晶过程中晶体的成核速度与生长速度,对提高晶体粒度有着重要意义。文章分别采用激光法和显微摄像的方法对阿莫西林结晶过程中初级成核、二次成核和单晶体生长速度进行了测定,计算出动力学相关数据。通过实验确定阿莫西林初级成核中均相成核与非均相成核的分界过饱和度S=4.5以及各自对应的成核自由能与成核半径,确定了二次成核发生时间与过饱和度间的关系以及阿莫西林晶体的生长速率与过饱和度的关系,并由生长指数m=1.995 5推出阿莫西林的生长机制为螺旋错位生长。通过阿莫西林结晶动力学研究为改进和优化结晶过程提供了理论基础。     

松香型成核剂成核聚丙烯的非等温结晶动力学研究

采用 DSC 法研究了聚丙烯(PP)和松香型成核剂成核 PP 在不同的降温速率下的非等温结晶动力学。采用修正的 Avrami 方程对 DSC 的测试结果进行了分析。结果表明,松香型成核透明剂和分散剂能显著提高 PP 的结晶温度,用 Jeziorny 法来处理松香型成核 PP 的非结晶等温结晶行为是较为吻合的。加入松香型成核透明剂和分散剂后,PP 的半结晶时间减少,结晶动力学常数 Z_c 增加,结晶速率增加;同一降温速率,松香型成核透明剂和分散剂成核PP 的 n 值较纯 PP 减少,说明结晶成核方式发生了改变。     

In situ growth, structure characterization, and enhanced photocatalysis of high-quality, single-crystalline ZnTe/ZnO branched nanoheterostructures

Single-crystalline, high-quality branched ZnTe-core/ZnO-branch nanoheterostructures were synthesized by an in situ strategy in an environmental scanning electron microscope. Composition and structure characterization confirmed that ZnO nanowires were perfectly epitaxially grown on ZnTe nanowires as branches. Noticeably, growth temperature plays a crucial role in determining the density and diameter of the ZnO nanobranches on ZnTe nanowires: a higher growth temperature leads to ZnO nanowires with higher density and smaller diameter. It was demonstrated that ZnO nanobranches exhibited a selective nucleation behavior on distinct side facets of ZnTe nanowires. Highly ordered ZnO nanobranches were found epitaxially grown on {211} facet of ZnTe nanowires, while there was no ZnO nanowire growth on {110} facet of ZnTe nanowires. Using first-principles calculation, we found that surface energy of distinct side facets has a strong impact on ZnO nucleation, and confirm that {211} facet of ZnTe nanowires is energetically more favorable for ZnO nanowire growth than {110} facet, which is in good agreement with our experimental findings. Remarkably, such unique ZnTe/ZnO 3D branched nanowire heterostructures exhibited improved photocatalytic abilities, superior to ZnO nanowires and ZnTe nanowires, due to the much enhanced effective surface area of their unique architecture and effective electron-hole separation at the ZnTe/ZnO interfaces.     

ZnO纳米线阵列膜的自组装生长及其金属接触特性

以磁控溅射制备的ZnO纳米晶薄膜作为籽晶层,用水热法在80℃氧化铟锡(indium tin oxide,ITO)玻璃衬底上,实现了大面积ZnO纳米线阵列膜的取向生长,制备了3种金属-半导体-金属(metal-semiconductor-metal,MSM)结构的ZnO半导体纳米线阵列膜样品,测试了薄膜样品的光学特性和I-V特性。结果表明:在相同的生长液浓度下,籽晶层对所生长的纳米线尺度分布有显著影响。所制备的纳米线薄膜在室温下具有显著的紫外带边发射特性。ZnO纳米线/Ag和ZnO纳米线/Al的金属-半导体接触均具有明显的Schottky接触特性,而ZnO纳米线/Au的金属-半导体接触具有明显Ohmic接触特性。     

Plasma effects in semiconducting nanowire growth

Three case studies are presented to show low-temperature plasma-specific effects in the solution of (i) effective control of nucleation and growth; (ii) environmental friendliness; and (iii) energy efficiency critical issues in semiconducting nanowire growth. The first case (related to (i) and (iii)) shows that in catalytic growth of Si nanowires, plasma-specific effects lead to a substantial increase in growth rates, decrease of the minimum nanowire thickness, and much faster nanowire nucleation at the same growth temperatures. For nucleation and growth of nanowires of the same thickness, much lower temperatures are required. In the second example (related to (ii)), we produce Si nanowire networks with controllable nanowire thickness, length, and area density without any catalyst or external supply of Si building material. This case is an environmentally-friendly alternative to the commonly used Si microfabrication based on a highly-toxic silane precursor gas. The third example is related to (iii) and demonstrates that ZnO nanowires can be synthesized in plasma-enhanced CVD at significantly lower process temperatures than in similar neutral gas-based processes and without compromising structural quality and performance of the nanowires. Our results are relevant to the development of next-generation nanoelectronic, optoelectronic, energy conversion and sensing devices based on semiconducting nanowires.     

Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.     

Nucleation control for the growth of vertically aligned GaN nanowires

ABSTRACT: Aligned GaN nanowire arrays have high potentials for applications in future electronic and optoelectronic devices. In this study, the growth of GaN nanowire arrays with high degree of vertical alignment was attempted by plasma-enhanced CVD on the c-plane GaN substrate. We found that the lattice matching between the substrate and the nanowire is essential for the growth of vertically aligned GaN nanowires. In addition, the initial nucleation process is also found to play a key role in creating the high-quality homoepitaxy at the nanowire-substrate interface. By controlling the nucleation stage, the growth of highly aligned vertical GaN nanowire arrays can be achieved. The reasons for the observed effects are discussed.     

Control of composition and size for Pd–Ni alloy nanowires electrodeposited on highly oriented pyrolytic graphite

In order to fabricate effective Pd–Ni alloy nanowire arrays with given compositions and size, the process of nucleation and growth and the dependence of alloy composition on deposition potential were investigated. The results reveal that the compositions and sizes of Pd–Ni alloy nanowires can be controlled within a desired range through adjusting suitable nucleation and growth potentials as well as the time. The Ni content in the alloy nanowires was found to vary from 6 to 28% when the deposition potential was changed from −0.3 to −1.9 V. A growth potential of −0.35 to − 0.50 V was applied to fabricate Pd–Ni alloy nanowires with 8–15% Ni content. Continuous and parallel nanowire arrays can be successfully fabricated when nucleation is performed at a potential of −1.2 V for 50 ms with further growth at −0.45 V for 800 s. Pd–Ni crystal phases exist in the alloy structure forms of 〈111〉, 〈200〉, 〈220〉, 〈311〉. The nanowires have an average diameter of 150 nm and a length of 100–450 μm.     

Effects of ultrasonic radiation on induction period and nucleation kinetics of sodium sulfate

The effects of ultrasound on crystal nucleation and particle size distribution of sodium sulfate were investigated via determining the induction period and particle size. Crystal nucleation parameters and equations for primary nucleation were calculated. The experimental results show that the induction time decreases under the ultrasound irradiation, therefore, we can get a shorter induction period at a higher supersaturation level. Based on these observations, the growth mechanism of sodium sulfate is continuous growth because the value of the surface entropy factor f is smaller than 3. The induction period was observed shorter and particle size was smaller when the ultrasonic radiation time increased. Crystal growth improved with the longer crystallization time.     

硫酸镁亚型盐田卤水低温冷冻结晶规律的研究

针对青海大浪滩盐湖区盐田卤水做了低温冷冻结晶规律的研究,该盐湖区为典型的硫酸镁亚型卤水,研究了其在低温冷冻时卤水的物理性质变化及析盐结晶规律。结果表明,该盐田卤水在-16~0 ℃低温冷冻时析盐顺序为MgSO4·7H2O→MgSO4·6H2O→Na2SO4·3K2SO4。通过对不同温度下的液相组分分析和固相X射线衍射分析表明,利用冷冻方式开采此卤水时,最适宜的冷冻温度应控制在-8 ℃左右。此时,该卤水组成中SO42-的质量分数低于2%,可以简化为Na+,K+,Mg2+∥Cl--H2O四元体系,以便达到开采氯化钾的工艺需求,为该盐湖卤水的综合开发利用提供理论依据。     

Stages in the catalyst-free InP nanowire growth on silicon (100) by metal organic chemical vapor deposition

Catalyst-free InP nanowires were grown on Si (100) substrates by low-pressure metal organic chemical vapor deposition. The different stages of nanowire growth were investigated. The scanning electron microscopy images showed that the density of the nanowires increased as the growth continued. Catalyzing indium droplets could still be fabricated in the nanowire growing process. X-ray diffraction showed that the nanowires grown at different stages were single crystalline with <111 > growth direction. The photoluminescence studies carried out at room temperature on InP nanowires reveal that the blueshift of photoluminescence decreased as the growing time accumulates, which is related to the increase in the diameter, rather than the length. Raman spectra for nanowires at different growing stages show that the quality of the nanowire changes. The growth of InP nanowires at different growing stages is demonstrated as a dynamic process.