Alpha-quartz 3. Origin of cellular structure of synthetic quartz

Surfaces of optically flawless and untwinned quartz single crystals, both natural and synthetic, were dissolved both at room temperature and at high temperatures in autoclaves using appropriate etchants. The features that develop have been optically studied, and x-ray transmission projection topographs of some of the specimens from the same natural quartz were also taken and studied. A model has been proposed to explain the anomalies detected earlier by other workers regarding the nature of line defects found in as-grown synthetic quartz, and the absence of lateral strain in the polygonised microcracked layer of the specimens of the present series of experiments.     

Crystallography of ordered colloids using optical microscopy. 1. Parallel-beam technique

A technique is described for determining the reciprocal lattice basis vectors of randomly oriented colloidal crystals from optical Fourier transform images using a parallel incident beam. This approach is demonstrated by comparing information gathered using this technique with results from real-space images for a single colloidal crystal grain with a random hexagonal close-packed structure. The reciprocal space and available real-space results agreed to within experimental error. The complete set of reciprocal lattice basis vectors was determined using the proposed technique in contrast to the partial lattice information available from real-space images. This technique appears to be generally capable of measuring lattice parameters to within 1% and orientation to better than 1 degrees .     

Silicon nitride crystal structure and observations of lattice defects

In view of the considerable progress that has been made over the last 40 years on the microstructural design of silicon nitride and related materials of tailored properties for specific applications, a clear review of the current understanding of the crystal structure and crystal chemistry of silicon nitride is timely. The crystal structures, crystal chemistry, and lattice defect nature of silicon nitride are critically reviewed and discussed, with emphasis placed firstly on the structural nature of -silicon nitride (whether it is a pure silicon nitride, or should better be regarded as an oxynitride); and secondly on the space group of -silicon nitride (whether it is P6₃/m or P6₃). In conjunction with recent observations of vacancy clusters in -silicon nitride, a comprehensive view compatible with all the experimental facts with respect to the structural nature of -silicon nitride is tentatively presented.     

Study of crystal defects using ion channelling and channelling radiation

The channelling technique to study crystal defects is described and its applications to various kind of defects to study their atomistic nature have been reviewed. Special emphasis has been placed on the applications to extended defects like dislocations. Finally a related new technique being developed for the last few years, namely the channelling radiation technique has been discussed along with its applications to study the dislocations.     

The role of defects in the crystal/quasicrystal transformation

The transformation between quasicrystals and related crystals, the so-called approximant phases, appears as a major point in the understanding of quasicrystal stability. Structural defects of approximant phases seem to be involved in the mechanism of crystal/quasicrystal transformation. Theoretical works as well as observations are supporting this point of view. We here report observations made by transmission electron microscopy on two systems which provide relevant examples for two types of mechanisms. In both cases, the approximant defects are identified as antiphase boundaries. In the first system (Al-Li-Cu), the transformation is due to a progressive organization of the approximant phase defects. In the second system (Fe-Cr-Mo), the vertex of intersecting defects exhibits key structural feature for the transformation in a quasicrystal.     

Effect of crystal defects on solute atoms segregation during cooling

Different quantity and configurations of crystal defects were obtained in an austenite of Fe-30%Ni alloy and an ultra low carbon bainitic steel by different deformations and annealing times at high temperature. The boron segregation at grain boundaries and subgrain boundaries during air cooling were revealed by means of particle tracking autoradiography technique. It was found that the non-equilibrium segregation was resisted in the deformed grains after recovery and polygonization, the boron depletion was more in the recrystallized grains than in the deformed original grains during the cooling. The subgrain boundaries and polygonized dislocation cells had a significant effect on the boron non-equilibrium segregation during the air cooling, but the quantity of dispersed dislocations had not. The result indicated that during segregation process the interaction of boron atoms with dislocations was sensitive to the dislocations configuration rather than the total number of defects in grain.     

Crystallography of ordered colloids using optical microscopy. 2. Divergent-beam technique

A technique has been developed to extract quantitative crystallographic data from randomly oriented colloidal crystals using a divergent-beam approach. This technique was tested on a series of diverse experimental images of colloidal crystals formed from monodisperse suspensions of sterically stabilized poly-(methyl methacrylate) spheres suspended in organic index-matching solvents. Complete sets of reciprocal lattice basis vectors were extracted in all but one case. When data extraction was successful, results appeared to be accurate to about 1% for lattice parameters and to within approximately 2 degrees for orientation. This approach is easier to implement than a previously developed parallel-beam approach with the drawback that the divergent-beam approach is not as robust in certain situations with random hexagonal close-packed crystals. The two techniques are therefore complimentary to each other, and between them it should be possible to extract quantitative crystallographic data with a conventional optical microscope from any closely index-matched colloidal crystal whose lattice parameters are compatible with visible wavelengths.     

Phase Equilibria and Crystallography of Ceramic Oxides

Research in phase equilibria and crystallography has been a tradition in the Ceramics Division at National Bureau of Standards/National Institute of Standatrds and Technology (NBS/NIST) since the early thirties. In the early years, effort was concentrated in areas of Portland cement, ceramic glazes and glasses, instrument bearings, and battery materials. In the past 40 years, a large portion of the work was related to electronic materials, including ferroelectrics, piezoelectrics, ionic conductors, dielectrics, microwave dielectrics, and high-temperature superconductors. As a result of the phase equilibria studies, many new compounds have been discovered. Some of these discoveries have had a significant impact on US industry. Structure determinations of these new phases have often been carried out as a joint effort among NBS/NIST colleagues and also with outside collaborators using both single crystal and neutron and x-ray powder diffraction techniques. All phase equilibria diagrams were included in Phase Diagrams for Ceramists, which are collaborative publications between The American Ceramic Society (ACerS) and NBS/NIST. All x-ray powder diffraction patterns have been included in the Powder Diffraction File (PDF). This article gives a brief account of the history of the development of the phase equilibria and crystallographic research on ceramic oxides in the Ceramics Division. Represented systems, particularly electronic materials, are highlighted.     

World Wide Web for Crystallography

Some characteristics of the World Wide Web (WWW) and its Virtual Library (W3VL) are described. Aspects of the setting up, maintenance, future development and objectives of the World Wide Web Virtual Library: Crystallography are detailed. An overview of the successful use of WWW in the organisation of two crystallographic conferences and one entirely electronic conference is given. A revolution in scientific publication is under way with the introduction of WWW and CD-ROM technologies and a few of the points important to crystallography are touched upon. An application to distance teaching in crystallography is described. There is no mention of WWW applications to crystallographic databases in this paper as others at the Workshop have adequately described their work.     

Crystallography of mixed Chevrel phases

Chevrel phases containing both cations located at the point of inversion symmetry and cations displaced from the center of the unit cell are investigated crystallographically. We have established the existence of single phase compounds containing both large (Sn) and small (Zn or Fe) cations.     

高分散纳米ZrO2晶体缺陷及电化学性能研究

以ZrOCl2.8H2O为原料,结合溶胶-凝胶法和超临界流体干燥技术制备了高分散纳米ZrO2粉体颗粒,通过X射线衍射仪(XRD)、高分辨透射电镜(HRTEM)、电化学阻抗谱(EIS)和循环伏安(CV)等技术手段研究探讨了ZrO2粉体颗粒的晶体缺陷、粉末晶粒尺寸大小及其电化学性能。成功地制备了粒径为10nm以下的近球形ZrO2三维网络纳米颗粒,实验结果表明,超临界流体干燥处理后的纯ZrO2晶粒基本保持完整,而3Y-ZrO2晶体缺陷较为明显。并首次对纯ZrO2的锂离子电化学性能作了一系列的初步研究,发现超临界流体处理后的ZrO2粉体具有很小的电化学阻抗;即使其作为锂离子电极材料容量可逆容量较低,但其多次循环后稳定性较好,这为锂离子电池材料的开发提供了新的思路。     

Efficient procedures for the optimization of defects in photonic crystal structures

Seven different stochastic binary optimizers--based on the concepts of genetic algorithms and evolutionary strategies--are developed, applied to determine defect locations in several photonic crystal structures that serve as test cases, and compared by extensive statistical analysis. In addition to the stochastic optimizers, a quasi-deterministic optimizer based on an algorithm inspired by hill-climbing algorithms was implemented. The test cases include the prominent 90 degrees photonic crystal waveguide bend and a photonic crystal power divider. The analysis of the results shows that many different photonic crystal structures with high transmission may be found for any operating frequency. All of the eight optimizers outperform standard codes-because they maintain an incomplete fitness table-and find the global optima with a high probability even when the number of fitness evaluations is much smaller than the number of potential solutions contained in the discrete search space. Based on the incomplete fitness table, an algorithm to estimate bit-fitness values is presented. The bit-fitness values are then used to improve the performance of some algorithms. The four best algorithms-an extended microgenetic algorithm, two mutation-based algorithms, and the quasi-deterministic algorithm inspired by hill-climbing algorithms-are considered to be of high value for the optimization of defects in photonic crystals and for similar binary optimization problems.     

Growth parameters on the defects formation in a grown silicon crystal

The growth of macroscopically dislocation-free Czochralski silicon crystals, various defects such as D defects and microdefects causing oxidation-induced stacking faults can form. The effects of growth parameters such as pulling speed or cooling rate of the crystal on the formation of these defects is examined. From an experiment on the continuous cooling of a silicon crystal from 1400 °C, it is found that there is an intermediate cooling rate range in which the nucleation of OISFs is enhanced. The impact of the presence of OISFs on the electrical properties of the silicon crystal is examined with a minority lifetime mapper, and the resistivity is measured with a four-point probe. A higher pulling speed of the crystal from the melt results in a higher density of particles on the polished silicon wafers. This implies that many of the particles present on the polished silicon wafers are related to solidification of the silicon crystal. Slower pulling from the melt followed by controlled cooling thereafter is suggested as a means of lowering these grown-in defects in Czochralski silicon crystals. © 1998 Kluwer Academic Publishers     

An X-ray double crystal topographic assessment of defects in quartz resonators

This paper describes an X-ray double crystal topographic study of defects in eighteen quartz resonators designed to operate at 1.4 MHz. The types of defects found in quartz are described, together with their reported effects on resonator performance. The mode of operation of the bulk resonator and the technique of X-ray double crystal reflection topography are outlined. Topographs reveal the electrode structures and surface features of the resonators together with the presence of growth defects such as dislocation cells, sub-boundaries, growth striations and growth sector boundaries. Spurious flexure modes in two resonators are also shown. It is demonstrated that a correlation exists between the presence of growth striations (and probably sub-boundaries) and a higher equivalent series resistance of the resonators. It is shown that such defects change the contributions to the losses, possibly by changing the nature of the vibration pattern of the resonator.