The absorption length for photon propagation in highly concentrated colloidal dispersions calculated from temporal intensity profiles of 100 femto-second pulses is much longer than the absorption length obtained from the measurements of static light transmission in the pure continuous phase fluid. The difference between these two values is explained on the basis of small interparticle spacing at high particle concentration and hence shorter paths traveled by photons through the absorbing medium relative to the total diffusive path in the dispersion. The two values are in good agreement when the absorption length is rescaled with the interparticle separation.
The bilayer structure of YBa2Cu3O6+x supports spin excitations that are either even or odd under a symmetry operation that exchanges two directly adjacent CuO2 layers. Both types of excitation have been observed in underdoped YBa2Cu3O6+x by inelastic neutron scattering. Even and odd response functions are split in energy. The temperature evolution in the normal state is parallel in both channels, but different behavior is observed in the superconducting state where a resonant enhancement is observed only in the odd channel. The measurements shed new light on the nature of interlayer interactions in the cuprates. (C) 1998 Elsevier Science Ltd. All rights reserved.
Biologically controlled mineralization features an orchestrated balance among various controlling factors such as spatial delineation, template promotion, crystal growth modification and cessation, and so on. Highly ordered calcium carbonate lamellae formed in the nacreous layers of mollusk (aragonite), the foliated calcitic layers of mollusk (calcite), or the semi-nacre of brachiopods (calcite) are excellent examples of the outcome of such synergistic control. Mimicking the concerted interplay of template promotion and growth inhibition as often utilized in biomineralization, we have synthesized macroscopic and continuous calcium carbonate thin films with thickness ranging from 0.4 to 0.6 mu m. The thin films were prepared at air/subphase interfaces by promoting mineral deposition with amphiphilic porphyrin templates, coupled with growth inhibition by the use of poly(acrylic acid) as a soluble inhibitor. Films formed at 22 degrees C were found to have a biphasic structure containing both amorphous and crystalline calcium carbonate. The crystalline regions were identified to be calcite oriented with the (00.1) face parallel to the porphyrin monolayer at the air/subphase interface. Films obtained in the early stage of formation at lower temperature (4 degrees C) displayed characteristics of a single amorphous phase. These observations suggest that films formed through a multistage assembly process, during which an initial amorphous deposition was followed by a phase transformation into the ultimate crystalline phase and the orientation of the crystalline phase was controlled by the porphyrin template during the phase transformation. The results provide new insights into the template-inhibitor-biomineral interaction and a new mechanism for synthesizing ceramic thin film under mild conditions.
We use a topology optimization method to design 1-3 piezocomposites with optimal performance characteristics for hydrophone applications. The performance characteristics we focus on are the hydrostatic charge coefficient d(h)((*)), the hydrophone figure of merit d(h)((*))g(h)((*)), and the electromechanical coupling factor k(h)((*)). The piezocomposite consists of piezoelectric rods embedded in an optimal polymer matrix. We use the topology optimization method to design the optimal (porous) matrix microstructure. When we design for maximum d(h)((*)) and d(h)((*))g(h)((*)) the optimal transversally isotopic matrix material has negative Poisson's ratio in certain directions. When we design for maximum k(h)((*)), the optimal matrix microstructure is layered and simple to build.
Barium titanate can be hydrothermally synthesized by exposing a titanium source to a Ba(OH)(2) solution. In concentrated solutions, time resolved studies indicated that BaTiO3 particles nucleate, grow and agglomerate to form 'raspberry-like' clusters which gradually rearrange to form single crystal BaTiO3 particles. Polymer films containing barium titanate particles can also be produced by a hydrothermal route. Titanium organometallic precursor is dissolved in polymer solutions and spun-cast into films. BaTiO3 is formed in the polymer matrix upon exposure to Ba(OH)(2) solution at temperatures below 100 degrees C. Two modifications to this approach are described which alter the morphology of the BaTiO3 particles utilizing the tendency of the block copolymer matrix to microphase separate. Patterning of the BaTiO3 by redistribution of the precursor in the block copolymer matrix may provide a method of producing 0-3, 1-3 and 2-2 connectivities.
The ceramics have pore diam. approx. 10-100 nm, and are manufd. by templating with a ceramic precursor a lyotropic liq. cryst. L3 phase consisting of a 3-dimensional, random, nonperiodic network packing of a multiconnection continuous membrane. The ceramics are manufd. by prepg. a template of a lyotropic liq. cryst. L3 phase, coating the template with a ceramic precursor, and converting the coated membrane to a ceramic membrane. More specifically, the lyotropic liq. cryst. L3 phase is prepd. by mixing a surfactant with a cosurfactant and HCl, coating the template with a precursor ceramic material by adding to the L3 phase (MeO)4Si or (EtO)4Si, and converting the coated template by removing any liq. The ceramic material, coated with a light- or radiation-sensitive material may be used as optical sensor, and further as filter, data storage device, energy storage device, with TiO2 as ultracapacitor device, in HPLC, with oxides or salts for ceramics and nonoxide ceramics. [on SciFinder(R)]
The intensity temporal profiles of diffusive light propagation in highly concentrated (up to volume fraction phi similar to 0.55) dispersions measured by 100-fs laser pulses showed an increase in transport scattering mean free path above a critical concentration. This observation confirms the previous theoretical predictions of enhanced transmission at high particle concentrations due to correlated scattering. The correlation effects are accounted for by incorporating a hard sphere Percus-Yevick static structure factor into the prediction of transport mean free path. (C) 1998 Optical Society of America.