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首页> 外文学位 >Structural characterization of hard materials by transmission electron microscopy (TEM): Diamond-Silicon Carbide composites and Yttria-stabilized Zirconia.
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Structural characterization of hard materials by transmission electron microscopy (TEM): Diamond-Silicon Carbide composites and Yttria-stabilized Zirconia.

机译：通过透射电子显微镜（TEM）对硬质材料进行结构表征：金刚石-碳化硅复合材料和氧化钇稳定的氧化锆。

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摘要

Diamond-Silicon Carbide (SiC) composites are excellent heat spreaders for high performance microprocessors, owing to the unparalleled thermal conductivity of the former component. Such a combination is obtained by the infiltration of liquid silicon in a synthetic diamond compact, where a rigid SiC matrix forms by the reaction between the raw materials. As well as the outstanding thermal properties, this engineered compound also retains the extreme hardness of the artificial gem. This makes it difficult to perform structural analysis by transmission electron microscopy (TEM), for it is not possible to produce thin foils out of this solid by conventional polishing methods. For the first time, a dual-beam focused ion beam (FIB) instrument successfully allowed site-specific preparation of electron-transparent specimens by the lift-out technique.;Subsequent TEM studies revealed that the highest concentration of structural defects occurs in the vicinity of the diamond-SiC interfaces, which are believed to act as the major barriers to the transport of thermal energy. Diffraction contrast analyses showed that the majority of the defects in diamond are isolated perfect screw or 60° dislocations. On the other hand, SiC grains contain partial dislocations and a variety of imperfections such as microtwins, stacking faults and planar defects that are conjectured to consist of antiphase (or inversion) boundaries. Clusters of nanocrystalline SiC were also observed at the diamond-SiC boundaries, and a specific heteroepitaxial orientation relationship was discovered for all cubic SiC that grows on diamond {111} facets.;Yttria-stabilized Zirconia (YSZ) is the most common electrolyte material for solid oxide fuel cell (SOFC) applications. It is an ionic conductor in which charge transfer is achieved by the transport of oxygen ions (O 2-). Like the diamond composite above, it is hard and brittle, and difficult to make into electron transparent TEM samples. Provided an effective supply of the "fuel" (oxygen and hydrogen gas), the performance of an SOFC device is primarily limited by the Ohmic resistance of the electrolyte and the electrochemical reaction kinetics at the electrode/electrolyte interfaces. While the former constraint may be substantially diminished by reducing the electrolyte's physical dimension into nanoscale thin films, the incorporation of oxygen ions into YSZ from the cathode side remains a relatively sluggish process. In order to study how structural modifications influence the effectiveness of the oxygen transfer at the cathode/YSZ boundary, ion implantation at different energies and doses was performed on the electrolyte, prior to the deposition of platinum (Pt) electrodes.;Xenon ions (Xe+) were used as the implant species, and the irradiation was done on atomic layer deposited (ALD) YSZ films and monocrystalline YSZ (001) substrates. From direct electrochemical measurements on fuel cell structures made on the ALD layers, an improvement by a factor of two was witnessed in the peak power density with relatively low implantation dose (10 13 cm-2) as compared to no irradiation. However the fuel cell properties worsened significantly with elevated dosage. Cross sectional TEM images of xenon implanted YSZ single crystals demonstrated the evidence of considerable defect accumulation (dislocation loops and extended dislocation lines) at 1015 and 1016 cm-2 doses. It is speculated that the bombardment with a relatively low concentration of xenon generates an optimum density of structural defects in the electrolyte that facilitate the incorporation or diffusion of O2- ions, whereas at higher radiation fluences the associated buildup of the imperfections or the implanted elements themselves may act as impediments to the anion transfer and conduction.
机译：由于前者具有无与伦比的导热性，因此金刚石-碳化硅（SiC）复合材料是高性能微处理器的出色散热器。这种组合是通过将液态硅渗透到合成金刚石复合片中而获得的，在合成金刚石复合片中，通过原料之间的反应形成了刚性的SiC基体。除了出色的热性能外，这种工程化合物还保留了人造宝石的极高硬度。这使得难以通过透射电子显微镜（TEM）进行结构分析，因为不可能通过常规抛光方法由该固体产生薄箔。首次，双束聚焦离子束（FIB）仪器成功地通过提升技术成功地进行了电子透明标本的定点制备;随后的TEM研究表明，结构缺陷的浓度最高金刚石-SiC界面中的一部分被认为是热能传输的主要障碍。衍射对比分析表明，金刚石中的大多数缺陷是孤立的完美螺钉或60°位错。另一方面，SiC晶粒包含部分位错和各种缺陷，例如微孪晶，堆垛层错和平面缺陷，这些缺陷被推测为由反相（或反转）边界组成。在金刚石-SiC边界处还观察到纳米晶SiC簇，并且发现了在金刚石{111}面上生长的所有立方SiC的特定异质外延取向关系。氧化钇稳定的氧化锆（YSZ）是最常见的电解质材料固体氧化物燃料电池（SOFC）应用。它是一种离子导体，通过氧离子（O 2-）的传输实现电荷转移。像上面的金刚石复合材料一样，它又硬又脆，并且很难制成电子透明的TEM样品。提供“燃料”（氧气和氢气）的有效供应，SOFC装置的性能主要受到电解质的欧姆电阻和电极/电解质界面处的电化学反应动力学的限制。尽管通过减小电解质的物理尺寸成为纳米级薄膜可以大大减少前一个约束，但是从阴极侧将氧离子引入YSZ仍然是一个相对缓慢的过程。为了研究结构修饰如何影响阴极/ YSZ边界处的氧气转移效果，在沉积铂（Pt）电极之前，先在电解质上以不同的能量和剂量进行离子注入。;氙离子（Xe + ）用作植入物种类，并在原子层沉积（ALD）YSZ膜和单晶YSZ（001）衬底上进行辐照。通过对在ALD层上制作的燃料电池结构进行直接电化学测量，与没有辐照相比，在相对较低的注入剂量（10 13 cm-2）下，峰值功率密度提高了两倍。然而，燃料电池的性能随着剂量的增加而显着恶化。氙注入的YSZ单晶的横截面TEM图像证明了在1015和1016 cm-2剂量下有大量缺陷积累（位错环和扩展的位错线）的证据。据推测，用较低浓度的氙气轰击会在电解液中产生最佳密度的结构缺陷，从而促进O2-离子的掺入或扩散，而在较高的辐射通量下，缺陷或植入元素本身的相关堆积可能会阻碍阴离子的转移和传导。

著录项

  • 作者

    Park, Joon Seok.;

  • 作者单位

    Stanford University.;

  • 授予单位 Stanford University.;
  • 学科 Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 346 p.
  • 总页数 346
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 工程材料学;
  • 关键词

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