Thomas Höche

Experimental evidence of self-limited growth of nanocrystals in glass.

Growth of nanocrystals precipitated in glasses with specific compositions can be effectively limited by diffusion barriers forming around crystallites. For the first time, we do experimentally prove this concept of self-limited growth on the nanoscale for a SiO(2)/Al(2)O(3)/Na(2)O/K(2)O/BaF(2) glass in which BaF(2) nanocrystals are formed. As shown by advanced analytical transmission electron microscopy techniques, the growth of these BaF(2) crystals, having great potential for photonic applications, is inherently limited by the formation of a ca. 1 nm wide SiO(2) shell.

High-quality m-plane GaN thin films deposited on γ-LiAlO2 by ion-beam-assisted molecular-beam epitaxy

GaN(11¯00) thin films are deposited on γ-LiAlO2(100) by low-energy-ion-beam-assisted molecular-beam epitaxy. Structural properties of the epitaxial GaN films are investigated by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. X-ray diffraction measurements give evidence for a high crystalline quality far better than previously reported in literature. Cross-section transmission electron microscopy and atomic force microscopy show an anisotropy in defect structure and surface topography parallel and perpendicular to the GaN c axis. Optical properties are examined by photoluminescence spectroscopy at various temperatures. The spectra exhibit a strong and sharp near-band-gap transition, as well as a donor-acceptor pair transition.

Ultrathin membranes in x -cut lithium niobate

Ion-beam enhanced etching is used to pattern a bulk lithium niobate crystal with ultrathin membranes. By the implementation of an air gap beneath the membrane, high index contrast is achieved. A buried amorphous layer, created by irradiation with He ions, is removed by means of wet chemical etching in hydro-fluoric acid. Membranes having thicknesses down to 200 nm are fabricated. The etched air gaps and the membranes exhibit a uniform thickness over the entire etched area, and their widths can be purposefully adjusted over a wide range by choosing appropriate ion energies and fluences as well as annealing conditions.

Incommensurate modulations in Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8

Abstract Incommensurate modulations in Ba 2 TiSi 2 O 8 , Sr 2 TiSi 2 O 8 , and Ba 2 TiGe 2 O 8 are compared based on their corresponding electron diffraction patterns. The dependence of the modulations on chemical composition provides a suitable model system for the investigation of incommensurations in framework structures using high-resolution transmission electron microscopy (HRTEM). A widening of quantitative HRTEM utilising the concept of rigid units is proposed allowing for a determination of atomistic displacements responsible for the formation of incommensurately modulated structures.

Biologically Formed Mesoporous Amorphous Silica

Mesoporous crystalline silica has attracted the attention of scientists due to its extraordinary functionalities. In particular, substantial progress has been made in the synthesis of mesoporous crystalline silica using biomimetic approaches under ambient conditions. However, the biomimetic synthesis of mesoporous amorphous silica has not been well studied so far. Here we show that amorphous silica can be synthesized in aqueous solution under ambient conditions via biological catalysis. The high purity amorphous silica is obtained as spicules (average diameter: 15.6 microm) that are cemented through junctions, thereby forming the skeleton of the freshwater sponge Cauxi. We discover that such amorphous spicules themselves contain mesopores. This opens a potential avenue to develop highly durable mesoporous membranes at room temperature. We also describe the macro- and microstructural features, the mechanism of biological precipitation, and the properties of the Cauxi skeleton.

ZrTiO4 crystallisation in nanosized liquid–liquid phase-separation droplets in glass—a quantitative XANES study

The crystallisation of the nucleation agent ZrTiO4 in a low thermal-expansion lithium aluminosilicate glass-ceramics is monitored as a function of time by combining transmission electron microscopy with Ti-L2,3 X-ray absorption near-edge structure spectroscopy. The formation of liquid–liquid phase-separation droplets is shown to precede ZrTiO4 crystallisation within the latter nanosized droplets. Quantitative data on crystalline fractions enable conclusions on the self-limited growth of ZrTiO4 nanocrystals in low thermal-expansion glass-ceramics and based on Avramis equation, the growth is shown to be restricted by a barrier (the outer border of the phase-separation droplet). It is shown that liquid–liquid phase separation and crystallisation are temporally decoupled. The size of ZrTiO4 crystallites is determined by the restricted volume of the phase-separation droplets they crystallise in. The volume of the droplets in turn is restricted by the formation of a diffusion barrier in the surrounding residual glass.

Origin of unusual fluorophlogopite morphology in mica glass-ceramics of the system SiO2-Al2O3-MgO-K2O-Na2O-F2

Extruded mica glasses were exposed to various heat treatments to convert them into glass-ceramics exhibiting anisotropic properties. In dependence on duration and temperature of the treatment, di⁄erent morphologies of the mica crystals were observed. The TEM study proved the crystallisation of fluorophlogopite formed in the glass system used to be a two-stage process. In coexistence with an intermediate phase (norbergite, Mg 3 F 2 SiO 4 ), highly defective and magnesium depleted phlogopite plates are formed in the first step. Upon dissolution of norbergite, epitaxial growth of non-defective and nearly stoichiometric fluorophlogopite along the c*-direction occurs on both sides of the defective slab in the second step. The morphology of the resulting sandwich structure is sensitively dependent on the tempering conditions. The origin of this e⁄ect is discussed based on the analysis of the defects observed in the inner part of the sandwich. ( 1998 Elsevier Science B.V. All rights reserved.

Microstructural characterization of grain-oriented glass-ceramics in the system Ba2 TiSi2O8-SiO2

In polycrystalline fresnoite, advantage of the piezo- and pyroelectric properties can be taken only if texturing along its polar [0 0 1]-axis can be achieved. We report on a novel technique to prepare grain-oriented glass-ceramics in the system fresnoite-SiO2 by electrochemically induced nucleation. Optimum conditions for grain orientation were determined by characterizing the microstructure of glass-ceramics prepared at 1150, 1200 and 1350 °C using electron microscopy and X-ray texture goniometry. At 1150 °C—due to the smallest crystal growth rate and highest nucleation rate—the most distinct orientation is obtained. The solidification of the glass-ceramic consist of three subsequent steps. First, dendritic fresnoite crystallizes most rapidly along its [0 0 1]-axis with [1 1 0] facet planes. Due to growth selection, the polar {c}-axes become oriented. Secondly, within the interdendritic areas, the pseudo-binary eutectic fresnoite-silica solidifies lamellarly. Finally, in the middle of the interspaces, a crystalline barium silicate coexisting with glassy silica is formed at 1200 °C and 1350 °C. At 1150 °C, however, a glass with the composition of the melt solidifies.

Microstructural design through epitaxial growth in extruded mica glass-ceramics

Upon extrusion of SiO[sub 2]-B[sub 2]O[sub 3]-Al[sub 2]O[sub 3]-MgO-Na[sub 2]O-K[sub 2]O-F[sup [minus]] glasses, mullite-aluminum borate solid-solution crystals become aligned with their elongated axes parallel to the extrusion direction. During subsequent heat treatments, fluorophlogopite platelets crystallize in part epitaxially on these solid-solution crystals. As shown by texture goniometry and electron microscopy, an orientation of mica basal planes either parallel or perpendicular to the extrusion direction is indicated. Using selected-area electron diffraction in a transmission electron microscope, epitaxial orientation relationships between the lattices of the fluorophlogopite and the solid-solution crystal are determined. The combination of texture formation and epitaxial growth opens new ways of microstructural design in extruded glass-ceramics.

(3 + 1)-Dimensional structure refinement of the fresnoite framework-structure type compound Ba2TiGe2O8

The incommensurately modulated structure of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8) has been solved using a (3 + 1)-dimensional superspace approach. The structure is orthorhombic and adopts the superspace group Cmm2(0,beta,1/2)s00 with beta approximately 0.635 at room temperature. The refinement was based on neutron powder diffraction data obtained from a powdered single crystal grown by Czochralski pulling. The modulation parameters that were obtained support the idea that frozen-in rigid-unit modes cause the modulation. The modulation is mainly manifested by positional displacements of O atoms. Barium ions are either eightfold, ninefold or tenfold coordinated in the one-dimensional modulated structure. A significant improvement of the bond-valence sum for both barium positions is achieved by the introduction of the positional modulation. This finding strongly suggests that underbonded barium positions are critically involved in provoking the incommensurate modulation in Ba(2)TiGe(2)O(8).