Three‐dimensional arrays of SiO2 nanometer particles lead to Bragg diffraction effects of visible light—as seen for natural opals, see also this issue's cover—and applications such as photonic bandgap materials. Teh fabrication of the opalline structures is described and details are given of how to obtain ordered compacts. The Figure shows a fracture surface of a sintered sample comprising 390 nm‐diameter silica spheres. magnified image

In this work we propose and demonstrate a solution to the problems which arise when SiO2 monodisperse nanospheres of diameters under 300 nm or over 550 nm are used to obtain opal-based photonic crystals. If the nanospheres are too small, the sedimentation rate is very slow or even may not occur; if they are large enough, no significant order can be achieved because the velocity is too high. This method, based on the electrophoretic phenomenon, allows us to control the sedimentation velocity. Furthermore, other species of importance in this field, such as SiO2 spheres covered with a thick layer of TiO2, do profit from this method.

This 2D WISE NMR study of a polyelectrolyte± surfactant complex reveals that at low temperatures the alkyl chains are conformationally ordered, but not ªcrystalline in a classical senseº owing to motion about the chain axes. Similar behavior occurs for certain stiff macromolecules with alkyl side chains [20b] and likewise describes the dynamic state of the all-trans chains of the CH 3 -terminated alkanethiol monolayers below their respective order/disorder transition temperatures.Materials that present a three-dimensional (3D) periodic modulation of the dielectric constant have attracted much attention from theorists and experimentalists over the last decade.[1] These structures, known as photonic bandgap materials, would present interesting technological applications in photonics and electronics. [2] Such applications rely on the fabrication of materials with photonic crystal properties in the NIR-vis-UV (near infrared-visible-ultraviolet) region of the electromagnetic spectrum. This challenge has prompted many research groups to great effort in order to obtain periodic dielectrics with a lattice parameter of less than one micrometer. [3] Recently, solid colloidal crystals made of submicrometer SiO 2 spheres packed in a face centered cubic (fcc) structure were shown to display photonic crystal properties. [4,5] These can be easily tuned through the sphere diameter, covering the whole visible and NIR region of the spectrum. [6] In order to increase the chances of a photonic gap opening, these structures could be used as matrices in which high dielectric constant materials can be synthesized. [7] In addition, some authors have noticed that the filling factor, defined as the ratio between the volume occupied by the SiO 2 spheres and the total volume of the structure, could be modified by thermal annealing. [4,8] In this communication we show that it is possible to accurately control the optical properties of SiO 2 colloidal photonic crystals through thermal treatment. The optical variation is due to structural and physicochemical modifications of the material caused by thermal annealing that do not imply a loss of order. Optical properties, which are determined by the photonic band structure, were studied by means of light transmission and reflection measurements. The morphology of the samples was characterized by scanning electron microscopy (SEM).[**] We acknowledge M. Planes for his help during SEM characterization.

Changes in crystalline phases resulting from low-temperature ageing of different yttria doped and non-doped zirconia-toughened alumina composites and nanocomposites were investigated under controlled humidity and temperature conditions in autoclave. A classical powder mixing processing route and a new modified colloidal processing route were used to process the composites. Different compositions ranging from 2.5 wt.% zirconia in a matrix of alumina to pure zirconia (3Y-TZP) were studied. It was observed that Al2O3+yttria stabilised ZrO2 composites exhibited significant ageing. However, ageing was much slower than traditionally observed for Y-TZP ceramics, due to the presence of the alumina matrix. Ageing was clearly limited for zirconia content beyond 25 wt.%. On the other side of the spectrum, Al2O3+2.5 wt.% ZrO2 initially presented a monoclinic fraction but did not show any ageing degradation. These composites seem to represent the best choice between slow crack growth and ageing resistance.