Christian Bocker

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.

Nanocrystallization in Oxyfluoride Glasses Controlled by Amorphous Phase Separation

Transparent bulk glass-ceramics containing ZnF2, K2SiF6, and KZnF3 nanocrystals are successfully obtained from xKF-xZnF2-(100 - 2x)SiO2 oxyfluoride glasses for the first time to the best of our knowledge. The glass transition temperatures of heat-treated samples increase with time and approach values that resemble the temperatures chosen for thermal treatment. During nucleation and crystal growth, the residual glass around the crystals is depleted in fluoride which as glass component usually leads to a decrease in viscosity. The crystallization behavior notably depends on the glass composition and changes within a small range from x = 20 to 22.5 mol %. The occurrence of liquid/liquid phase separation in dependence of the composition is responsible for the physicochemical changes. Two different microstructures of droplet and interpenetrating phase separation and their compositional evolution are observed by replica transmission electron microscopy technique in the multicomponent glassy system. This study suggests that the size and crystal phase of precipitated crystallites can be controlled by the initial phase separation.

ASAXS study of CaF2 nanoparticles embedded in a silicate glass matrix

The formation and growth of nanosized CaF2 crystallites by heat treatment of an oxyfluoride glass of composition 7.65Na2O–7.69K2O–10.58CaO–12.5CaF2–5.77Al2O3–55.8SiO2 (wt%) was investigated using anomalous small-angle X-ray scattering (ASAXS). A recently developed vacuum version of the hybrid pixel detector Pilatus 1M was used for the ASAXS measurements below the Ca K-edge of 4038 eV down to 3800 eV. ASAXS investigation allows the determination of structural parameters such as size and size distribution of nanoparticles and characterizes the spatial distribution of the resonant element, Ca. The method reveals quantitatively that the growing CaF2 crystallites are surrounded by a shell of lower electron density. This depletion shell of growing thickness hinders and finally limits the growth of CaF2 crystallites. Moreover, in samples that were annealed for 10 h and more, additional very small heterogeneities (1.6 nm diameter) were found.

Experimental evidence of a diffusion barrier around BaF2 nanocrystals in a silicate glass system by ASAXS

Nanocrystals of BaF2 precipitate during controlled annealing of silicate glasses composed of SiO2, Al2O3, K2O, Na2O, BaF2 and BaO. Structural characteristics such as size, shape, volume fraction and size distribution of nanoparticles in the glass matrix were studied by anomalous small angle x-ray scattering (ASAXS). Fitted ASAXS curves near the Ba absorption edge reveal the formation of a core-shell structure. Furthermore, ASAXS results allowed to determine quantitatively the composition of the nanocrystals, the surrounding region (shell) and the matrix. The evaluated composition gives evidence for an enrichment of Si atoms in a shell, that surrounds the particles and acts as a diffusion barrier. Resonant curve analysis reveals the quantitative information of the Ba atoms used in the formation of BaF2 nanoparticles.

Crystal growth model with stress development and relaxation

During crystallization of glass-forming melts an important amount of stress energy may develop. The reason for this development is the difference in volume of the new system and that in the ambient phase. Therefore, the growth rate decreases with time. Here we develop an algorithm for the process of crystallization using Monte Carlo simulation techniques that takes into account the stress energy. We find that there is a short period of initial fast growth stage followed by a second period of much slower growth, controlled by the relaxation rate. This picture has also been recently observed experimentally. Additionally, we find that during the growth process the shape of the crystal is changing. Although we start from a highly symmetric crystal with flat 10 interfaces, a shape with a large number of facets is soon created.

Broadband near-IR emission from cubic perovskite KZnF(3):Ni(2+) nanocrystals embedded glass-ceramics.

Transparent KF-ZnF(2)-SiO(2) glass-ceramics were prepared with the precipitation of KZnF(3)Ni(2+) nanocrystals. During excitation with a wavelength of 405 nm at room temperature, a broadband near-IR emission centered at 1695 nm with the FWHM of more than 350 nm was observed, which is originated from the T(2g)3(F3)→A(2g)3(F3) transition of octahedral Ni(2+) incorporated in the KZnF(3) crystalline phase. In comparison to oxide glass-ceramics, a redshift of the luminescence is observed, which is due to the low crystal field of these octahedral Ni(2+). The shift and extension of near-IR emission in the KZnF(3):Ni(2+) nanocrystals embedded in a glassy matrix do not only complete the broadband emission in the whole near-IR region for the Ni(2+) ions-based photonics, but also open an easy way to approach the broadband optical amplifier and tunable lasers operating in the wavelength region near 1800 nm, which was up to now achieved by codoping of several types of active ions.

The nano-crystallization and fluorescence of terbium doped Na2O/K2O/CaO/CaF2/Al2O3/SiO2 glasses

Oxyfluoride glasses with the molar composition 59.3 SiO2·3.7 Al2O3·12.3 CaO·5.3 K2O·9.0 Na2O·10.4 CaF2 were doped with different Tb3+-concentrations (1 × 1019, 5 × 1019 and 1 × 1020 cm−3). The glasses were thermally treated in the temperature range from 550 to 600 °C, which resulted in the crystallization of cubic CaF2. The XRD-patterns showed extremely broadened lines attributed to mean crystallite sizes in the range from 11 to 15 nm. SEM and TEM micrographs showed polycrystalline particles with a size of around 200 nm which are composed of smaller, about 15 nm large, crystallites. Static fluorescence spectra as well as fluorescence decay curves were recorded for samples with different TbF3-concentrations and different thermal treatment. For samples with a low Tb3+ doping concentration notably prolonged fluorescence lifetimes were measured.

Thermal Expansion of Sintered Glass Ceramics in the System BaO-SrO-ZnO-SiO2 and Its Dependence on Particle Size.

The thermal expansion behavior of sintered glass-ceramics containing high concentrations of Ba1-xSrxZn2Si2O7, a phase with very low and highly anisotropic thermal expansion behavior, was investigated. The observed phase has the crystal structure of the high-temperature phase of BaZn2Si2O7, which can be stabilized by the introduction of Sr(2+) into this phase. The high anisotropy leads to microcracking within the volume of the samples, which strongly affects the dilatometric thermal expansion. However, these cracks also have an influence on the nominal thermal expansion of the as-mentioned phase, which decreases if the cracks appear. Below a grain size of approximately 80 μm, the sintered glass-ceramics have almost no cracks and show positive thermal expansion. Hence, coefficients of thermal expansion between -5.6 and 6.5 × 10(-6) K(-1) were measured. In addition to dilatometric studies, the effect of the microstructure on the thermal expansion was also measured using in situ X-ray diffraction at temperatures up to 1000 °C.

Replica Extraction Method on Nanostructured Gold Coatings and Orientation Determination Combining SEM and TEM Techniques

In the field of electron microscopy the replica technique is known as an indirect method and also as an extraction method that is usually applied on metallurgical samples. This contribution describes a fast and simple transmission electron microscopic (TEM) sample preparation by complete removal of nanoparticles from a substrate surface that allows the study of growth mechanisms of nanostructured coatings. The comparison and combination of advanced diffraction techniques in the TEM and scanning electron microscopy (SEM) provide possibilities for operators with access to both facilities. The analysis of TEM-derived diffraction patterns (convergent beam electron diffraction) in the SEM/electron backscatter diffraction software simplifies the application, especially when the patterns are not aligned along a distinct zone axis. The study of the TEM sample directly by SEM and transmission Kikuchi diffraction allows cross-correlation with the TEM results.

Structure and magnetic properties of ultrafine lithium ferrite crystallized from a borate glass

A glass with the mol. % compositions 51.7 B2O3/9.3 K2O/1 P2O5/27.6 Li2O/10.4 Fe2O3 was crystallized at temperatures in the range from 400 to 540 °C for different periods of time (2–12 h). X-ray diffraction showed cubic Li ferrite with a spinel structure, LiFe5O8, with small crystallites with sizes in the range from 3 to 31 nm. While the samples crystallized at 440, 480, and 500 °C reveal a disordered phase, crystallization at 540 °C indicates a phase transformation to the ordered state. Magnetization curves showed that the samples crystallized at 440 °C are superparamagnetic with very low maximum magnetization, while the samples crystallized at 480 and 500 °C show thin clear S-shaped magnetization curves with zero coercivity and hence are also superparamagnetic. By contrast, the sample crystallized at 540 °C shows a coercive field of 40 Oe and thus is ferrimagnetic. The magnetic properties can be tailored by the size of the ferrite crystals and hence by the crystallization conditions.A glass with the mol. % compositions 51.7 B2O3/9.3 K2O/1 P2O5/27.6 Li2O/10.4 Fe2O3 was crystallized at temperatures in the range from 400 to 540 °C for different periods of time (2–12 h). X-ray diffraction showed cubic Li ferrite with a spinel structure, LiFe5O8, with small crystallites with sizes in the range from 3 to 31 nm. While the samples crystallized at 440, 480, and 500 °C reveal a disordered phase, crystallization at 540 °C indicates a phase transformation to the ordered state. Magnetization curves showed that the samples crystallized at 440 °C are superparamagnetic with very low maximum magnetization, while the samples crystallized at 480 and 500 °C show thin clear S-shaped magnetization curves with zero coercivity and hence are also superparamagnetic. By contrast, the sample crystallized at 540 °C shows a coercive field of 40 Oe and thus is ferrimagnetic. The magnetic properties can be tailored by the size of the ferrite crystals and hence by the crystallization conditions.