Size distribution of BaF2 nanocrystallites in transparent glass ceramics

In glasses with the composition 1.9 Na₂O–15 K₂O–7.5 Al₂O₃–69.6 SiO₂–6 BaF₂ (in mol.%), BaF₂ nanocrystalline precipitates are formed upon heat treatment. Using dark-field and bright-field transmission electron micrographs, crystallite size distributions are obtained for samples crystallized at various temperatures. According to the “tomato-salad problem”, the size distributions are corrected and then compared to various theories of grain growth taking into account coarsening of the crystallites during heat treatment. The experimental crystallite size distributions show for smaller mean crystallite sizes a more symmetric shape in comparison to the theories of Lifshitz–Slyozov–Wagner (LSW) or Brailsford and Wynblatt (B&W). With increasing mean crystallite sizes to about 18 nm at higher heat-treatment temperatures, the full width at half maximum of the observed distributions decreases and becomes even narrower than the LSW function. These findings indicate that in the investigated nano glass ceramics no coarsening by Ostwald ripening or coalescence occurs. This is explained by the formation of a diffusion barrier around each nanocrystallite which limits the size of the crystallites and hence results in such a narrow and uniform crystallite size distribution.     

Solution of the Maxwell field equations in vacuum for arbitrarycharge and current distributions using the methods of matrixalgebra

A new matrix representation of classical electromagnetic theory is presented. The basis of this representation is a space-time, eight-by-eight differential matrix operator. This matrix operator is initially formulated from the differential form of the Maxwell field equations for vacuum. The resulting matrix formulation of Maxwell's equations allows simple and direct derivation of the electromagnetic wave and charge continuity equations, the Lorentz conditions and definition of the electromagnetic potentials, the Lorentz and Coulomb gauges, the electromagnetic potential wave equations, and Poynting's conservation of energy theorem. A four-dimensional Fourier transform of the matrix equations casts them into an eight-dimensional transfer theorem. The transfer function has an inverse, and this allows the equations to be inverted. This expresses the fields directly in terms of the charge and current source distributions     

Effect of chloride ions in Tris buffer solution on bioactive glass apatite mineralization

Degradation and apatite precipitation of Bioglass 45S5 were characterized in two different tris(hydroxymethyl)aminomethane (Tris) buffer solutions. In the conventional one, pH was adjusted to 7.4 using hydrochloric acid (Tris-HCl buffer). In the other one, acetic acid was used instead, thereby obtaining a Tris buffer solution, which was virtually free of chloride ions (Tris-HAc). EDX results showed that chloride ions present in Tris-HCl buffer solution were incorporated into the apatite formed during immersion experiments, forming a (partially) substituted chlorapatite. No chloride ions were detected in apatite precipitated in Tris-HAc. Although no significant differences in the rate of apatite formation were observed during the time points studied here (6 hours to 7 days), the presence of chloride ions may affect very early stages of apatite formation. It may therefore be advantageous to use modified Tris buffer solutions, which do not contain high concentrations of ions involved in apatite formation, when studying early time point release kinetics or apatite precipitation of bioactive glasses.     

Preparation and luminescence properties of glass–ceramics containing Sm3+-doped hexagonal NaGdF4 crystals

Rare earth-doped hexagonal NaGdF₄ nanocrystals were precipitated from a glass. The glass with the mol% composition 70.1SiO₂·4.3Al₂O₃·1.8AlF₃·2.3Na₂O·18.5NaF·3.0Gd₂O₃ doped with 8 × 10¹⁹ Sm³⁺ ions per cm³ was already phase separated after casting. The formed droplets enriched in rare earths and fluoride had sizes in the range mainly from 50 to 90 nm. During annealing at temperatures ≥600 °C, multi core particles of hexagonal NaGdF₄ were formed inside the amorphous droplet phase. The fluorescence spectra show that Sm³⁺ is incorporated into the NaGdF₄ lattice. This is confirmed by electron dispersive X-ray analyzes performed in a transmission electron microscope.     

Molecular cancer disposition diagnosis exemplified by colorectal carcinoma. What is the contribution of pathology?

The effect of low temperature on cytosolic pH regulation and buffering capacity was evaluated in the isolated rat liver. The pH changes were followed by phosphorus-31 nuclear magnetic resonance. Cooling from 37 to 4 degrees C, with Krebs-Heinseleit perfusion at an external pH of 7.35, induced an alkaline shift in cytosolic pH (pHcyt) of 0.13 or 0.75 pH units in the presence of bicarbonate, respectively (dpH cys/dT values were 0.004 and 0.022 unit/degrees C. With 4 degrees C perfusion, in the presence or absence of bicarbonate, acute changes of external pH (from 7.40 to 5.90) did not affect pHcyt. In contrast, intracellular loading with isobutyric acid or NH4Cl induced rapid pHcyt variations. The intrinsic buffering power value (10 to 50 slykes) measured in the absence of bicarbonate depended on pHcyt. The larger value was observed for pHcyt 7.30, a value near the pK value of the imidazole group of intracellular proteins at 4 degrees C. The presence of bicarbonate modified the amplitude of the pHcyt change by increasing the total buffering power. It was demonstrated that during hypothermia, ionic carriers are inactivated and the charged forms of molecules are unable to cross the cell membrane; thus, the pHcyt homeostasis depends essentially on intracellular buffering power.     

The effect of stresses during crystallization on the crystallite size distributions

In some glasses during thermal annealing, nano crystals are formed, which scarcely grow with time and exhibit a very narrow crystal size distribution. In this paper, considerations on the crystallite size distributions are given. A variant of the nucleation theory including the role of an induction period is included in the model. Since non-isochemical systems are considered, the oversaturation is decreasing with time and therefore a model is chosen according to which the nucleation rate decreases. For the crystal growth velocity, a model recently derived was used which takes into account the stresses formed during the course of the crystallization process. It is found, that a model taking into account decreasing oversaturation, an induction period as well as the occurrence of stresses fully explains the crystallite size distributions experimentally observed which might even be narrower than those according to the theory of Lifshitz, Slyozov and Wagner.     

Preparation and Structure of New Oxyfluoride Glass‐Ceramics Containing BaLiF3 Nanocrystal

Through a careful composition design, new oxyfluoride glass‐ceramics (GCs) containing BaLiF₃ nanocrystals with sizes of around 30 nm were prepared. Microstructural characterizations show interpenetrating phase separation in the sample with a composition of 15BaF₂–15ZnF₂–70SiO₂, after thermal treatment at 580°C for 40 h which leads to the nanocrystallization of BaLiF₃. The BaLiF₃ nanocrystals embedded in the glassy matrix could provide Ba or Li sites for the incorporation of optically active rare earth and transition‐metal ions, which provides a possibility to explore novel photonic properties by the codoping of rare earth and transition‐metal ions in GC materials.     

New insights into the crystallization of cordierite from a stoichiometric glass by in situ high-temperature SEM

The crystallization of cordierite from stoichiometric glass is observed in situ in the scanning electron microscope. A heating stage was, therefore, operated at 1000 and 1100 °C. The surface crystallization of well-separated crystals was recorded. Surprisingly, at the late stage of crystallization, the formation of cracks as well as a squeezing out of a low viscous phase took place. To our knowledge, this has not been reported before and gives rise to new considerations of the crystallization mechanism of cordierite from glasses.     

Active damping of drive train oscillations for an electrically driven vehicle

A problem encountered with electrically driven vehicles are resonances in the drive train caused by elasticity and gear play. Disadvantageous effects caused by this are noticeable vibrations and high mechanical stresses due to torque oscillations. The oscillations can be damped using a control structure consisting of a nonlinear observer to estimate the torque in the gear and a controller, which computes a damping torque signal that is added to the driver's demand. The control algorithm was implemented in the existing motor control unit without any additional hardware cost. The controller was successfully tested in a test vehicle. The resonances can essentially be eliminated. The controller copes satisfactorily with the backlash problem.     

High dynamic control of a three-level voltage-source-converter drive for a main strip mill

A high dynamic control system for the Alspa VDM 7000 medium-voltage drive was implemented, which provides fast torque response times of a few milliseconds despite the typically low switching frequency of gate-turn-off thyristors which is necessary to achieve high efficiency. The drive system consists of a three-level voltage-source converter with active front end and a synchronous motor. The drive has most recently been applied to a main strip mill. It provides a maximum of 8.3-MW mechanical power with a rated motor voltage of 3 kV. Besides motor torque as the main control objective, the control system has to comply with a number of additional objectives and constraints like DC-link voltage regulation and balancing, current and torque harmonics, motor flux, and excitation.