We present a method to experimentally determine the components of stress tensors within grains of multicrystalline materials by micro-Raman spectroscopy. This method is applied to multicrystalline silicon wafers as they are produced for solar cells. Currently, μ-Raman spectroscopy is intensively used to measure stresses in silicon wafers, structures, and devices of known crystallographic orientations. For these cases, the determination of stresses from Raman peak shifts is straightforward. In multicrystalline silicon, however, arbitrary grain orientations complicate the determination of stress tensor components, which depend on the crystallographic orientations of the particular grains. The Raman intensities depend on the polarization direction of the incident and scattered laser light and again on the crystallographic grain orientations. This intensity dependence is used to determine the crystallographic grain orientations. Once the orientation is determined, the components of the stress tensor (with respect to a fixed reference coordinate system—the sample stage), can be calculated numerically from the Raman peak shifts. As examples, we determine (i) the stress components of a nearly plane stress state around the tip of a microcrack and (ii) the stress components at a grain boundary in a multicrystalline silicon wafer.
A diffusion-driven mineralization approach for the fabrication of calcium carbonate on and in regenerated and functionalized cellulose membranes was investigated. Calcium dichloride was used as the cation source. Ammonium carbonate was applied in a gas-phase diffusion and sodium carbonate in a liquid continuous-flow setup. Calcium carbonate was obtained solely on the membrane surfaces from the gas-diffusion approach, whereas by applying continuous-flow diffusion, crystals were obtained throughout the membranes.[a]
Hydrated europium(III) orthophosphates EuPO 4 × nH 2 O (rhabdophane) of a nanocrystalline particle size ranging from 5 to 40 nm were precipitated from aqueous solution at neutral pH. The hexagonal crystal structure of the synthesized phase remained stable up to a calcination temperature of 600 °C according to X-ray diffraction (XRD) analysis. The complete loss of water at temperatures exceeding 600 °C caused the transformation into monoclinic nonhydrated EuPO 4 isomorphous to monazite. The typical Eu 3+ luminescence emissions excited at 396 nm for hexagonal EuPO 4 × nH 2 O as well as for the monoclinic nonhydrated EuPO 4 were attributed to magneticdipole and vibronic as well as forced electric-dipole 5 D 0 f 7 F J (J ) 1, 2, 3, 4) transitions. If the trivalent europium ion lies on an inversion center, the hypersensitivity is absent. The intensity ratio of the magnetic-dipole 5 D 0 f 7 F 1 transition to the electric-dipole 5 D 0 f 7 F 2 transition decreased with increasing calcination temperature up to 600 °C, indicating the presence of a hypersensitive, forced electric-dipole 5 D 0 f 7 F 2 transition due to the lack of inversion symmetry sites. The loss of water during heating up to 600 °C was considered to be responsible for the variations in the emission characteristics of the EuPO 4 × nH 2 O.
Biomacromolecules control and direct the formation of biominerals and hard tissues in nature. In many cases, polysaccharides are involved during the assembly of the inorganic phase as template. Natural and regenerated polysaccharides exhibit a hierarchical multiscale order as well as self-assembly properties and they appear in a large variety of accessible structures. Therefore, the application of polysaccharide-based structures and composites is a promising approach for the formation of patterned and hierarchically structured inorganic functional and structural materials. The authors report on recent advancements on the biotemplated formation of inorganic functional materials using polysaccharides. Owing to their structural diversity, polysaccharides can be used at various levels from the molecular scale up to complex three-dimensional parts. The versatility of polysaccharide templating is shown on one-dimensional (1D) cellulose nanocrystals for formation of inorganic nanotubes. Two-dimensional (2D) Langmuir–Blodgett films of cellulose and cellulose derivatives are used as reference templates to investigate the mineralization behaviour of inorganic phases from supersaturated solutions. The development of optical and photocatalytic materials from plant tissues (wood and Juncaceae) is presented. In innovative route, phototactic microalgae are used as biotemplates for the mineralization of inorganic phases on their exopolysaccharides, which provide novel pathways for the fabrication of a variety of functional materials.
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