A model based on Rayleigh–Gans–Debye light‐scattering theory has been developed to describe the light transmission properties of fine‐grained, fully dense, polycrystalline ceramics consisting of birefringent crystals. This model extends light transmission models based on geometrical optics, which are valid only for coarse‐grained microstructures, to smaller crystal sizes. We verify our model by measuring the light transmission properties of fully dense (>99.99%), polycrystalline α‐Al2O3 (PCA) with mean crystal sizes ranging from 60 to 0.3 μm. The remarkable transparency exhibited by PCA samples with small crystal sizes (<2 μm) is well explained by this model.

Preparation and properties are discussed of a novel magnetic dispersion, containing surface-modified silica colloids with a core of single-domain magnetite particles. The underlying idea is to tailor the silica shell thickness and surface properties such that the colloids are stable spheres with isotropic interactions, whereas an external magnetic field produces weak dipolar attractions and consequently reversible anisotropic structures. The influence of the shell thickness is analyzed in terms of a dipolar-sphere potential. Preparation of magnetite cores, silica growth, and surface modifications with an alcohol and a silane are described in detail. Particle properties are investigated, among other things, with SAXS, light scattering, and electron microscopy. The major conclusion is that the preparation route yields stable, nonaggregated magnetic silica particles with a shape and internal structure which is mainly determined by small magnetite clusters in the starting ferrofluid.

Commercial corundum powder and a liquid‐shaping approach are used for manufacturing complex hollow components and large flat windows of sintered and hot isostatically pressed Al2O3 ceramics having grain sizes of 0.4–0.6 μm at relative densities of >99.9%. High macrohardness (HV10 = 20–21 GPa) and four‐point bending strength (600–700 MPa; 750–900 MPa in three‐point bending) are associated with a real in‐line transmission of 55%–65% through polished plates. The submicrometer microstructure and the optical properties can be retained for use at >1100°C using dopants that shift the sintering temperature to high values without additional grain growth.

The isotropic-nematic (I-N) phase transition in dispersions of sterically stabilized rod-like boehmite (A1OOH) colloids is studied. We have examined the influence of the steric stabilizer, the dispersion medium and the presence of non-adsorbing polymer on the phase transition process. Dispersions in cyclohexane show an IN phase separation that proceeds by a slow sedimentation of a pinned structure, shrinking from the meniscus, finally forming the nematic phase after weeks or months, depending on the steric stabilizer used. In toluene the onset of the IN has shifted to higher volume fractions where individual nematic droplets grow and sediment, forming the nematic phase after one week. By adding non-adsorbing polymer to dispersions in cyclohexane the onset of the IN phase separation shifts to lower colloid volume fractions. At polymer concentrations just above the phase boundary the same scenario as in the toluene dispersions without added non-adsorbing polymer is observed. At slightly higher polymer concentrations an abrupt change in scenario occurs. Now an interconnected network is formed which starts to sediment, resembling the process in pure cyclohexane dispersions. Clearly small variations in colloidal interactions engendered by the changes in dispersion characteristics considered in this work, have a strong influence on the IN phase transition process.

The time evolution of the isotropic-nematic phase separation in dispersions of sterically stabilized colloidal rods was studied with polarization microscopy and static small angle light scattering (SALS). The rods aspect ratio is 14. In the biphasic region (between the isotropic-nematic transition volume fraction φ I )12.1% and the nematic-melting volume fraction φN ) 35.1%) a SALS ring develops at a wavevector Kmax which shifts to smaller K values in time. Increasing the concentration from φI to φN, polarization microscopy indicates a crossover from nucleation-and-growth to spinodal decomposition. Nucleation is accompanied by a decrease of the intensity at large K as ∼K -4 , which is typically found when sharp interfaces have developed in the system. For spinodal decomposition a much less pronounced decrease is observed within the experimental time window. The rate of phase separation was studied by monitoring both the change of the turbidity and the shift of Kmax in time. A maximum rate is found just within the spinodal region.

Colloidal boehmite (AlOOH) rods were used as cores for the preparation of rods with a silica shell. Via a three-step coating procedure silica rods with adjustable aspect ratios up to 20 were obtained by varying the amount of the added silica precursor tetraethoxysilane (TES). Silica rods are stable in various solvents such as water, ethanol, propanol, and dimethylformamide (DMF). Moreover, polymer-grafted silica rods can be dispersed in ethanol/toluene mixtures and cyclohexane. Incorporation of fluorescein isothiocyanate (FITC) in the silica shell enables the study of long-time self-diffusion processes in dispersions of rodlike particles with fluorescence recovery after photobleaching (FRAP). Fluorescent silica rods also may be used in direct imaging studies with confocal scanning laser fluorescence microscopy (CSLM).

Long-time self-diffusion in dispersions of rigid colloidal rods with an aspect ratio of 19 is studied with fluorescence recovery after photobleaching ͑FRAP͒ in isotropic and nematic phases. The long-time selfdiffusion coefficient D s L is found to decrease linearly with concentration up to (L/D)ϭ0.12 ͑with L the length and D the diameter of the rods, and the volume fraction͒. In the isotropic phase in coexistence with the nematic phase, D s L remains virtually constant at about 1% of its value at infinite dilution. In the nematic phase long-time self-diffusion is found to be ten times slower than in the coexisting isotropic phase. In addition, by modifying the FRAP geometry we were able to distinguish between sidewise and lengthwise diffusion in aligned nematic phases. ͓S1063-651X͑98͒11712-9͔

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