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Chemical Processing and Characterization of Fiber Reinforced Nanocomposite Silica Materials.

机译：纤维增强纳米复合二氧化硅材料的化学加工和表征。

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摘要

Ultrasound techniques, acoustic and electroacoustic spectroscopy, are used to investigate and characterize concentrated fluid phase nanocomposites. In particular, the data obtained from ultrasound methods are used as tools to improve the understanding of the fundamental process chemistry of concentrated, multicomponent, nanomaterial dispersions. Silicon nitride nanofibers embedded in silica are particularly interesting for lightweight nanocomposites, because silicon nitride is isostructural to carbon nitride, a super hard material. However, the major challenge with processing these composites is retarding particle-particle aggregation, to maintain highly dispersed systems. Therefore, a systematic approach was developed to evaluate the affect of process parameters on particle-particle aggregation, and improving the chemical kinetics for gelation.;From the acoustic analysis of the nanofibers, this thesis was able to deduce that changes in aspect ratio affects the ultrasound propagation. In particular, higher aspect ratio fibers attenuate the ultrasound wave greater than lower aspect fibers of the same material. Furthermore, our results confirm that changes in attenuation depend on the hydrodynamical interactions between particles, the aspect ratio, and the morphology of the dispersant. The results indicate that the attenuation is greater for fumed silica due to its elastic nature and its size, when compared to silica Ludox. Namely, the larger the size, the greater the attenuation. This attenuation is mostly the result of scattering loss in the higher frequency range.;In addition, the silica nanofibers exhibit greater attenuation than their nanoparticle counterparts because of their aspect ratio influences their interaction with the ultrasound wave. In addition, this study observed how 3M NH 4 Cl's acoustic properties changes during the gelation process, and during that change, the frequency dependency deviates from the expected squared of the frequency, until the system becomes fully dense and turns into a pure gel. Moreover, our results demonstrated the use of ultrasound to determine the critical coagulation concentration, and a double logarithm plot of the CVI indicated a possible power law dependency for NH4 Cl concentrations.;Lastly, the mechanism of the gelation reaction of colloidal silica, Si(OH) 4(aq) + Si(OH)3(O) --(aq) → Si2O8H5--( aq) + H2O, by an anionic pathway was investigated using density functional theory (DFT). Using transition state theory, the rate constants were obtained by analyzing the potential energy surface at the reactants, saddle point, and the products. In addition, reaction rate constants were investigated in the presence of ammonium chloride (NH 4 Cl) and sodium chloride (NaCl). These salts act as catalysis to induce gelation due to their ability to destabilize the double layer of the colloid. Furthermore, it was observed that ammonium chloride plays an important role by initiating a hydride transfer allowing the reaction to proceed from the second transition state to the final product, and was predicted to be spontaneous for all temperatures.;In summary, this thesis provides a comprehensive approach on examining the parameters required for the chemical processing of nanofiber dispersions, thus improving the understanding of the physio-chemical interactions, the gelation mechanism, and their influence on obtaining highly dispersed fluid phase composite systems.
机译：超声技术，声学和电声光谱学被用于研究和表征浓缩的液相纳米复合材料。特别地，从超声方法获得的数据用作改善对浓缩的多组分纳米材料分散体的基本过程化学的理解的工具。嵌入二氧化硅中的氮化硅纳米纤维对于轻质纳米复合材料尤其令人感兴趣，因为氮化硅与氮化碳（一种超硬材料）同构。然而，加工这些复合材料的主要挑战是阻碍颗粒-颗粒的聚集，以维持高度分散的体系。因此，开发了一种系统的方法来评估工艺参数对颗粒-颗粒聚集的影响，并改善凝胶化的化学动力学。；从纳米纤维的声学分析，本论文能够推断出长径比的变化会影响凝胶的形成。超声传播。特别地，与相同材料的低纵横比纤维相比，高纵横比纤维对超声波的衰减更大。此外，我们的结果证实，衰减的变化取决于颗粒之间的流体动力学相互作用，长径比和分散剂的形态。结果表明，与二氧化硅Ludox相比，气相二氧化硅由于其弹性和大小而引起的衰减更大。即，尺寸越大，衰减越大。这种衰减主要是由于在较高频率范围内的散射损耗引起的。此外，由于纳米二氧化硅的长宽比会影响其与超声波的相互作用，因此其纳米粒子比其纳米粒子表现出更大的衰减。此外，这项研究观察了3M NH 4 Cl的声学特性在胶凝过程中如何变化，并且在该变化过程中，频率依赖性偏离了预期的频率平方，直到系统完全致密并变成纯凝胶。此外，我们的结果表明使用超声波确定临界凝结浓度，CVI的双对数图表明了NH4 Cl浓度可能存在幂律依赖性。;最后，胶体二氧化硅Si（ OH）4（aq）+ Si（OH）3（O）-（aq）→Si2O8H5-（aq）+ H2O，采用密度泛函理论（DFT）通过阴离子途径进行了研究。使用过渡态理论，通过分析反应物，鞍点和产物的势能表面获得速率常数。另外，在氯化铵（NH 4 Cl）和氯化钠（NaCl）存在下研究了反应速率常数。这些盐由于其使胶体双层不稳定的能力而充当诱导胶凝的催化剂。此外，据观察，氯化铵通过引发氢化物转移而起重要作用，从而使反应从第二个过渡态过渡到最终产物，并据预测在所有温度下都是自发的。全面的方法来检查纳米纤维分散体的化学加工所需的参数，从而增进对理化相互作用，凝胶化机理及其对获得高度分散的液相复合体系的影响的理解。

著录项

  • 作者

    Burnett, Steven Shannon.;

  • 作者单位

    Howard University.;

  • 授予单位 Howard University.;
  • 学科 Chemistry.;Nanoscience.;Acoustics.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 129 p.
  • 总页数 129
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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