| 1. |
- Christensen, Kirsten Elvira, 1977-
(författare)
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Open-Framework Germanates : Crystallography, structures and cluster building units
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focussing on the crystallographic challenges and what knowledge we can gain from studying the different open-framework germanates. Five new open-framework germanates have been synthesized and the structures have been determined by single crystal X-ray diffraction. A thorough description is made of the different problems raised with the different compounds, whether it is choice of crystal system in SU-61, twinning and possible ordering in SU-46, superstructure and variation in elemental content in SU-57, template disorder in JLG-5 or framework disorder in SU-44.Open-framework germanates are often built from one type of cluster, such as the Ge7 [Ge7X19], Ge8 [Ge8X20], Ge9 [Ge9Xn, n =25-26] and Ge10 [Ge10X28], (X =O, OH, F) clusters. The structures built by clusters containing different kinds of polyhedra are discussed, with a focus on the 4-coordinated Ge7 clusters, the larger cluster aggregate found in SU-8 and SU-44 and the structures built by the Ge10 clusters.
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| 2. |
- Ma, Yanhang, 1991-
(författare)
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Structural study of nano-structured materials: electron crystallography approaches
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The structural analysis serves as a bridge to link the structure of materials to their properties. Revealing the structure details allows a better understanding on the growth mechanisms and properties of materials, and a further designed synthesis of functional materials. The widely used methods based on X-ray diffraction have certain limitations for the structural analysis when crystals are small, poorly crystallized or contain many defects. As electrons interact strongly with matter and can be focused by electromagnetic lenses to form an image, electron crystallography (EC) approaches become prime candidates for the structural analysis of a wide range of materials that cannot be done using X-rays, particularly nanomaterials with poor crystallinity.Three-dimensional electron diffraction tomography (3D EDT) is a recently developed method to automatically collect 3D electron diffraction data. By combining mechanical specimen tilt and electronic e-beam tilt, a large volume of reciprocal space can be swept at a fine step size to ensure the completeness and accuracy of the diffraction data with respect to both position and intensity. Effects of the dynamical scattering are enormously reduced as most of the patterns are collected at conditions off the zone axes. In this thesis, 3D EDT has been used for unit cell determination (COF-505), phase identifications and structure solutions (ZnO, Ba-Ta3N5, Zn-Sc, and V4O9), and the study of layer stacking faults (ETS-10 and SAPO-34 nanosheets).High-resolution transmission electron microscope (HRTEM) imaging shows its particular advantages over diffraction by allowing observations of crystal structure projections and the 3D potential map reconstruction. HRTEM imaging has been used to visualize fine structures of different materials (hierarchical zeolites, ETS-10, and SAPO-34). Reconstructed 3D potential maps have been used to locate the positions of metal ions in a woven framework (COF-505) and elucidate the pore shape and connectivity in a silica mesoporous crystal.The last part of this thesis explores the combination with X-ray crystallography to obtain more structure details.
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| 3. |
- Piao, ShuYing, 1965-
(författare)
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Complexity of quasicrystal approximants —The RECd6 and RE13(Zn/Cd)~58 systems
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused on the synthesis and structural determination of a series of quasicrystal approximants in the RE13Zn~58 (RE = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Ce13Cd~58 and RECd6 (RE = Tb, Ho, Er, Tm and Lu) systems. Structural studies were performed by single crystal X-ray diffraction methods.The RECd6 phases are 1/1 cubic approximants to the stable icosahedral quasicrystals. The structure of quasicrystals cannot be solved by conventional crystallography due to the lack of periodicity in three-dimensional space. On the other hand, quasicrystal approximants exhibit long-range order, which makes their structural determination possible by standard methods. Since they are believed to display the same local arrangement as the quasicrystals, the structures of the quasicrystal approximants play a key role in understanding quasicrystals.The RE13Zn~58 and Ce13Cd~58 quasicrystal approximants show measurable compositional variations coupled to subtle structural differences. The crystal structures are generally rather more complex than previously reported, and exhibit a number of different ordering and disordering modes.
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| 4. |
- Gao, Chuanbo, 1981-
(författare)
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Formation mechanism of anionic-surfactant-templated mesoporous silica (AMS)
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation is focused on synthesis, characterization and formation mechanism of anionic-surfactant-templated mesoporous silica (AMS). Structural control mechanisms of AMS are investigated. First, different ionization degree of anionic surfactant affected by the acidity or alkalinity of the synthesis system gives rise to different charging density of micelles and therefore determines the organic/inorganic interface curvature, producing mesophases from cage-type to cylindrical, bicontinuous and lamellar. Second, mesocage/mesocage electrostatic repulsive interaction affects the formation of cage-type mesostructure, which is derived from a full-scaled synthesis-field diagram of AMS. The mesocage/mesocage interaction changes with charge density of mesocages and gives rise to their different packing manners. Third, the structural properties of AMS materials could be tuned by molecular features of surfactant and co-structure-directing agent (CSDA). The pore size of AMS is found to be controlled by alkyl chain length, ionization degree of surfactant and the CSDA/surfactant ratio. Alkyl chain length of surfactant determines size of micelles and thus mesopores. Larger ionization degrees of anionic surfactant give rise to smaller pore sizes due to thermodynamic coiling of alkyl chains of surfactant. The hydrophobic interactions between the pendant organic groups of CSDA on the silica wall and the hydrophobic core of the micelles drive a contraction of the mesopores. A mesoporous silica with novel bicontinuous cubic Pn-3m structure has been prepared using a diprotic anionic surfactant. 3d-reconstruction of the structure shows that it is bicontinuous composed of an enantiomeric pair of 3d mesoporous networks that are interwoven with each other, divided by a D surface. Inverse replication suggests the possible presence of ordered complimentary micropores in the material.
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| 5. |
- Han, Lu, 1983-
(författare)
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Synthesis and Characterization of Functionalized Silica Mesoporous Crystals : Cationic Surfactant and Co-structure Directing Agent System
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation has been focused on the synthesis and characterization of novel functionalized silica mesoporous crystals by using cationic surfactant and co–structure directing agents (CSDA), the central concept of the synthesis method is to build proper organic/inorganic interactions by introducing CSDA into the synthesis system. By using cationic surfactant as template and anionic CSDA, carboxylic group functionalized mesoporous silicas were successfully synthesized. Well ordered 2D p6mm, cubic Fm-3m, mixture of CCP (Fm-3m) and HCP (P63/mmc), and cubic Fd-3m with uniform carboxylic group distribution have been obtained. Besides, we have investigated the Fm-3m/Fd-3m type intergrowth and new type defects observed in the Fd-3m structure using transmission electron microscopy (TEM) and proposed a “polyhedron packing” model. New amphoteric, inorganic amino acid with highly ordered mesopores were synthesized. Uniform distribution of acid and base organic groups on the mesopore surfaces were formed by interactions between the counter charged surfactant head groups and ionic parts of CSDAs. It has been demonstrated that organic (–NH2 and –COOH) pairs incorporated in the mesopore walls behave as natural amino acids, collectively exhibiting an isoelectric point of ~6.0. Moreover, we have demonstrated that the inorganic amino acid is an efficient catalyst for the reaction between aldehydes and carbon nucleophiles.
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| 6. |
- Klingstedt, Miia, 1982-
(författare)
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Characterizing cavity containing materials using electron microscopy : A study of metal oxides, mesoporous crystals and porous material containing nanosized metal-particles
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the characterization of novel materials by utilizing electron microscopy techniques. The examined materials contain cavities with certain attributes that enables desired properties for applications such as gas separation, catalysis and fuel cells. The specimens concerned herein belong to the following groups of materials: Metal oxides in the Sb-W-Mo-O system; ordered mesoporous silicas and carbons; hollow spheres containing Au-nanoparticles; zeolite LTA incorporated with mesopores; metal organic frameworks doped with nickel. With scanning electron microscopy (SEM) and transmission electron microscopy (TEM) you get vast possibilities within the field of characterization. This thesis utilizes conventional electron microscopy techniques such as imaging, energy-dispersive spectroscopy and electron diffraction as well as reconstruction techniques, such as exit-wave reconstruction, electron tomography and electron crystallography. Furthermore, the sample preparation technique cross-section polishing has been used in conjunction with low voltage SEM studies. The scientific approach is to gain knowledge of nano-sized cavities in materials, in particular their shape, size and content. The cavities often have irregularities that originates from the synthesis procedure. In order to refine the synthesis and to understand the properties of the material it is required to carefully examine the local variations. Therefore average characterization techniques such as crystallography needs to be combined with local examination techniques such as tomography. However, some of the materials are troublesome to investigate since they to some extent bring limitations to or gets easily damaged by the applied characterization technique. For the development of novel materials it is essential to find means of overcoming also these obstacles.
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| 7. |
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| 8. |
- Lund, Kristina, 1976-
(författare)
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Structural and Morphological Studies of Mesoporous Crystals
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mesoporous silica and carbon have been synthesised and characterised by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) and nitrogen adsorption. Surface structure, size and shape (morphology) of particles have been of special interest in this work. These parameters are of importance in understanding the growth mechanism and structural characteristics of mesoporous materials overall, which has been the main aim of this thesis.The novel synthetic system involving preparation of AMS (anionic surfactant-templated silica) crystals, where the non-toxic N-acyl-amino acids are used as surfactant together with a co-structure directing agent, was investigated in two different surfactant systems. Particle size and shape of AMS solids were controlled through the addition of a polymeric dispersant, which slowed down the system dynamics. However, the response for the two synthetic systems investigated, proved to be different. Nanoparticles of cage-type cubic Fd-3m crystals were obtained in one system, whereas facetted particles formed in a near-to-equilibrium growth in the bicontinuous cubic Ia-3d case. The surface fine structure of mesoporous silica was observed by high resolution SEM, leading to an understanding of the pore accessibility at the particle surface of SBA-15 silica. Furthermore, discrepancies in the XRD patterns of mesoporous tube-type carbon CMK-5 with different pore diameters were observed. XRD patterns displaying accidental extinction of the 10 reflection were observed in CMK-5 samples prepared with the widest pore diameter. An analytical approach was formulated in order to simulate the variation of XRD intensity for carbon crystals with p6mm symmetry. TEM observations of inner and outer diameter of the carbon tubes were used to interpret the diminishing of certain reflections and we observed good agreement between experimental observations and our calculations.
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| 9. |
- Muroyama, Norihiro, 1980-
(författare)
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Studies of inorganic crystal structures and gas adsorption process in mesoporous crystals : New approach through analysis of electron charge distribution by synchrotron powder X-ray diffraction
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Using synchrotron powder XRD experiments, I studied (i) structures of silica mesoporous MCM-41 and MCM-48 crystals, (ii) gas adsorption processes of Ar and N2 in the mesopores of the mesoporous crystals, and (iii) bonding features of CaAl2-xZnx crystals through the analysis of electron charge density distributions. In the case of mesoporous crystals, the following two different approaches were taken depending on the number of observed X-ray reflections1. The analytical expression was chosen and further developed to study the size and shape of the mesopores with the plane group of p6mm by powder XRD experiments. For MCM-41, following the determination of the mesopores’ size and shape, and the thickness of the adsorbed gas layer as a function of the gas pressure were successfully observed. In addition, for carbon pipe mesoporous crystals CMK-5, the carbon pipe thickness was determined and the diffraction pattern was discussed quantitatively focusing on the “accidental extinction.” 2. Maximum entropy method (MEM) was used for the structural study of MCM-48 (3D bicontinuous Ia-3d ) and for the gas adsorption process within the mesopores.By adopting the MEM approach, the study of “bonding electrons” and associated atomic displacements from the ideal Kagome net in the Laves phase CaAl2-xZnx were observed. In particular, the two Kagome nets in the C36 structure showed different feature in the electron density distributions between each other.
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| 10. |
- Xiao, Changhong, 1986-
(författare)
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A structural investigation into the complexity of mesoporous silica crystals : From a view of curvature and micellar interaction to quasicrystallinity
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mesoporous silica crystals have a large variety of structures mainly due to the versatility of their structure template. The configuration and the chemical state of the templating micellar surfactants, together with the kinetic process of silica will determine the final outcome of the synthesis. Increasing the understanding of the complex formation processes involved will enable a possibilityto fine tune the material for specific uses, today focused into the fields of photoniccrystals, drug delivery, catalysis and separation technology.In this thesis emphasis is put on (1) increasing the understanding the formation mechanism yielding the different species of mesoporous silica crystals through an in depth study of quasicrystallinity (2) Characterization and description of the structural complexity through various characterization techniquesand also by studying the kinetic structural transformation phenomenon related to the minimal G- and D-surfaces. (3) The structural studies of the versatile surfactant liquid crystals for establishing a thermodynamically stable basis to evaluate the kinetic mesoporous silica growth processes. Furthermorethe thesis both enlightens the possibilities of and contributes to the developmentof electron microscopy characterization techniques.In these studies, electron microscopy is largely employed in the characterization to give a thorough picture of the mesoporous structures. This is combined with the sample preparation techniques cross-section polishing and ionslicing. Low voltage scanning electron microscopy is utilized for studying the surfaces and cross-sections of various materials at the limit of the resolution. Here, a deep understanding of the electron beam-material interaction is used for a better interpretation of the detected signals. Transmission electron microscopyis combined with electron crystallographic reconstruction to yield a three dimensional structural model. For determination of the quasicrystallinity level for a structure of dodecagonal tiling, revealed in the scope of this study,a phason strain analysis was made.
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