Living organisms construct various forms of laminated nanocomposites through directed nucleation and growth of inorganics at self-assembled organic templates at temperatures below 100 degrees C and in aqueous solutions. Recent research has focused on the use of functionalized organic surfaces to form continuous thin films of single-phase ceramics. Continuous thin films of mesostructured silicates have also been formed on hydrophobic and hydrophilic surfaces through a two-step mechanism. First, under acidic conditions, surfactant micellar structures are self-assembled at the solid/liquid interface, and second, inorganic precursors condense to form an inorganic-organic nanocomposite. Epitaxial coordination of adsorbed surfactant tubules is observed on mica and graphite substrates, whereas a random arrangement is observed on amorphous silica. The ability to process ceramic-organic nanocomposite films by these methods provides new technological opportunities.
We have used high-energy inelastic neutron scattering to detect optical magnons directly in antiferromagnetic YBa2Cu3O6.2. The optical magnon gap is 67+/-5 meV. This implies an intrabilayer superexchange constant perpendicular to the CuO2 layers of J(perpendicular to) = 0.08 J(parallel to) where J(parallel to) is the in-plane nearest-neighbor superexchange constant.
Prudhomme, R. K. ; Aksay, I. A. ; Garg, R.Fabrication of cellular materials; Crowson, A., Ed. Spie - Int Soc Optical Engineering: Bellingham, 1996; Vol. 2716, pp. 331-334.
An electrohydrodynamic methodology has been developed that makes possible the precise assembly of two- and three-dimensional colloidal crystals on electrode surfaces. Electrophoretically deposited colloidal particles were observed to move toward one another over very large distances (greater than five particle diameters) to form two-dimensional colloidal crystals for both micrometer- and nanometer-size particles, This coalescence of particles with the same charge is opposite to what is expected from electrostatic considerations and appears to result from electrohydrodynamic fluid flow arising from an ionic current flowing through the solution. The ability to modulate this ''lateral attraction'' between particles, by adjusting field strength or frequency, facilitates the reversible formation of two-dimensional fluid and crystalline colloidal states on the electrode surface. Further manipulation allows controlled structures to be assembled.
We have examined heteroflocculation in binary colloidal suspensions by Monte Carlo simulations based on a diffusion-limited-cluster-aggregation (DLCA) model modified with finite attraction energies, The simulations were performed in two dimensions, Under heteroflocculation conditions, i.e., attraction between unlike particles and repulsion between like particles, cluster size undergoes a maximum as the concentration of the second species of particles is increased, similar to the experimental results in both binary suspensions and suspensions with adsorbing polymers. The initial increase in cluster size at low concentrations of the second species of particles is due to the mutual attraction between unlike particles, The decrease in cluster size at higher concentrations of the sea,nd species of particles is due to the repulsion between the second species of particles, The distinction between heteroflocculation and particulate depletion flocculation is discussed.
Cars of the future will require ''smart materials'' that integrate sensors and actuators into a seamless unit. We report on three novel fabrication technologies for these materials: (1) multi-layer tape lamination to make large-scale, integrated piezoelectric materials, (2) stereolithography for the production of complex, three-dimensional ceramic materials by laser photo-curing of ceramic dispersions, and (3) microcontact printing to apply and pattern polymeric or ceramic materials onto two-dimensional surfaces. Each of these processing techniques permits miniaturization of the cell subunits and assembly of these sub-units into large scale active materials.
High-yield mullite (3A1(2)O(3)-2SiO(2)) precursors consist of aluminosiloxanes :synthesized from mixtures of aluminum and silicon alkoxides. Atomic level mixing of the aluminum and silicon oxides is demonstrated by the low-temperature conversion (<1000 degrees C) of the aluminosiloxanes to phase-pure mullite. The proper selection of monomeric side groups serves several functions: (i) controlling reactivity of the silicon and aluminum monomers, thereby favoring atomic-level mixing; (ii) maintaining the tractability of the resulting aluminosiloxane; (iii) improving the yield during mullitization of the aluminosiloxane through easy thermolytic removal. The tractability of the aluminosiloxane compounds permits these materials to be used, in fiber spinning, the casting of thin films and monoliths, and as impregnants to powder compacts.
We report an extensive study of magnetic excitations in fully oxygenated YBa2Cu3O7, using neutron scattering with and without spin polarization analysis. By calibrating the measured magnetic intensity against calculated structure factors of optical phonons and against antiferromagnetic spin waves measured in the same crystal after deoxygenation to YBa2Cu3O6.2, we establish an absolute intensity scale for the dynamical spin susceptibility, chi ''(q,omega). The integrated spectral weight of the sharp magnetic resonance at h omega=40 meV and q(parallel to)=(pi/a,pi/a) in the superconducting state is integral d(h omega)chi(res)''(q,w)=(0.52+/-0.1) at low temperatures. The energy and spectral weight of the resonance are measured up to T=0.8T(c). The resonance disappears in the normal state, and a conservative upper limit of 30 states/eV is established for the normal state dynamical susceptibility at q(parallel to)=(pi/a,pi/a) and 10 meV less than or equal to h omega less than or equal to 40 meV. Our results are compared to previous neutron-scattering data on YBa2Cu3O7, theoretical interpretations of NMR data and current models of the 40 meV resonance.
A method for producing a highly loaded, aq. suspension having a pourable viscosity and contg. 20-50 vol.% colloidal ceramic or metal particles is described. A biol. produced polymer dispersant having a high d. of carboxyl functional groups and an av. mol. wt. of ≥1000 is solubilized in H2O in an amt. of <1.0% dry wt. basis of particles. The ceramic or metal particles are then introduced into the soln., and agitated to form a substantially nonagglomerated suspension. The polymer dispersant may be produced by a bacterium grown in situ with the particles. A biol. produced polymer gelling agent that is miscible with the polymer dispersant may be mixed into the suspension, which is then maintained in a nongelled state while being supplied to a mold. The suspension is then exposed to a gel-triggering condition to form a gelled, sinterable article. [on SciFinder(R)]
Aksay, I. A. ; Groves, J. T. ; Gruner, S. M. ; Lee, P. C. Y. ; Prudhomme, R. K. ; Shih, W. H. ; Torquato, S. ; Whitesides, G. M.Smart materials systems through mesoscale patterning; Crowson, A., Ed. Spie - Int Soc Optical Engineering: Bellingham, 1996; Vol. 2716, pp. 280-291.
A three-dimensional digitized image of a porous magnetic gel is determined by x-ray microtomographic techniques. The complex connected pore-space topology is quantitatively characterized by measuring a variety of statistical correlation functions, including the chord-length distribution function, the pore-size distribution function, and the Lineal-path function. This structural information is then employed to estimate transport properties, such as the fluid permeability and trapping rate, of the gel.
Thin films of cubic BaTiO3 were processed hydrothermally at 40 degrees-80 degrees C by reacting thin layers of titanium organometallic liquid precursors in aqueous solutions of either Ba(OH)(2) or a mixture of NaOH and BaCl2. All films (thickness similar to 1 mu m) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions, BaTiO2 formation was facilitated by increasing [OH-], [Ba2+], and the temperature, The film structure was related to the nucleation and growth behavior of the BaTiO3 particles, Films processed at relatively low [OH-], [Ba2+], and temperatures were coarse grain and opaque, but increasing [OH-], [Ba2+], and temperature caused the grain size to decrease, resulting in transparent films.