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.
We describe a theory for a new type of colloid behavior whereby particles deposited on a surface by electrophoresis are manipulated to form two-dimensional crystals. Since the particles are equally charged, the clustering is opposite that expected from electrostatic considerations. Such behavior is consistent with migration due to electrohydrodynamic flows associated with polarization layers and ion currents. Provided colloid stability is maintained, the assembly processes take place with both dc and ac fields and may be modulated by adjusting the field strength or frequency. No migration is present at frequencies above 1 MHz. Two-dimensional fluid and crystalline states can be formed on the electrode surface. Experiments with patterned electrodes demonstrate the presence of the electrohydrodynamic now. A mathematical model of the electrohydrodynamics provides insight into the assembly process.
We report inelastic neutron scattering experiments on the doping dependence of the energy and spectral weight of the sharp magnetic resonance peak in YBa2Cu3O6+x. These measurements also shed light on the relationship between the magnetic excitations in the normal and superconducting states.
The lyotropic L-3 phase was used as a template to form nanoporous monolithic silicates with continuously adjustable pore sizes. The monolith was optically isotropic and transparent with a nonperiodic network. The pore size was adjusted by a change in the solvent volume fraction rather than by a change of the surfactant. Unlike other silicates, the bicontinuous pores were water-filled; removal of surfactant was not necessary to access the pores. Measured characteristic dimensions were from six to more than 35 nanometers. For a given solvent fraction, x-ray scattering indicated little variation of pore widths, in marked contrast to the polydisperse pores of aerogels.
Recent work has shown that conventional surfactants form ordered aggregates of well-defined shape and size at solid-liquid interfaces.(1, 2) Here we report interfacial aggregate structures as a function of surfactant geometry by using gemini surfactants with varying tail and spacer lengths. On the anionic cleavage plane of mica, aggregates tend to favor a lower curvature than in solution but follow the same general variation with surfactant geometry (i.e., with larger headgroup areas resulting in greater curvature). These morphologies on mica correlate well with those observed in surfactant-silicate mesophases, where electrostatic binding of headgroups also plays a dominant role. In addition, interfacial sphere-to-rod transitions are induced on mica (as in free solution) by binding with a headgroup-specific counterion. In contrast to mica, the hydrophobic cleavage plane of graphite interacts with surfactant tailgroups, giving rise to interfacial aggregates that are surface-controlled and relatively independent of surfactant geometry. This interaction is used to heterogeneously nucleate a surfactant-silicate mesophase which is interfacially controlled and differs from the bulk phase.
Inelastic neutron scattering has been used to obtain a comprehensive description of the absolute dynamical spin susceptibility chi ''(q, omega) of the underdoped superconducting cuprate YBa2Cu3O6.5 (T-c = 52 K) over a wide range of energies and temperatures (2 meV less than or equal to (h) over bar omega less than or equal to 120 meV and 5 K less than or equal to T less than or equal to 200 K). Spin excitations of two distinct symmetries (even and odd under the exchange of two adjacent CuO2 layers) are observed which exhibit two different gaplike features (rather than a single ''spin pseudogap''). The excitations show dispersive behavior at high energies. [S0163-1829(97)51142-8].
The addition of small amounts (2-10 wt %) of SiO2 to gamma-Al2O3 increases the temperature of heat treatment necessary for transformation to alpha-Al2O3 by similar to 100 K. We have studied this system using high-temperature solution calorimetry in molten 2PbO . B2O3 at 1043 K, Our results indicate that the spinel-type Al2O3-SiO2 solid solutions with 2-10 wt % SiO2 are always energetically metastable by 30-35 kJ.mol(-1) (on a 4 O2- per mole basis) with respect to alpha-Al2O3 and quartz. Calculation of the maximum configurational entropy of the solid solutions allowed determination of the likely most negative value of the Gibbs free energy of the materials, The solid solutions are somewhat entropy stabilized, but still thermodynamically metastable by > 10 kJ.mol(-1) at 1400 K, Therefore, SiO2 addition appears to provide mainly a kinetic hindrance to alpha-Al2O3 formation.
The supramolecular assembly of surfactant molecules at a solid-liquid interface can produce tubular structures with diameters of around 10 nm (refs 1-4), which can be used for the templated polymerization of mesoporous silica thin films(3-5). The orientation of the tubules depends primarily on the nature of the substrate-surfactant interaction. These nanostructured films hold much promise for applications such as their use as orientated nanowires(6), sensor/actuator arrays(7-9) and optoelectronic devices(10), But a method of patterning the tubules and orientating them into designed arrangements is required for many of these possibilities to be realized. Here we describe a method that allows the direction of growth of these tubules to be guided by infiltrating a reaction fluid into the microcapillaries of a mould in contact with a substrate(11). An electric field applied tangentially to the surface within the capillaries induces electro-osmotic flow, and also enhances the rates of silica polymerization around the tubules by localized Joule heating. After removal of the mould, patterned bundles of orientated nanotubules remain on the surface. This method permits the formation of orientated mesoporous channels on a non-conducting substrate with an arbitrary microscopic pattern.
We have fabricated multilayer electromechanical composites with controlled piezoelectric coefficient distributions using tape casting. Tapes of doped lead zirconate titanate were cut and stacked in accordance with their characteristic electromechanical coupling values and modulus of elasticity. This technique is an extremely versatile method to fabricate displacement actuators to fabricate monolithic ceramic parts with controlled material property gradients. To obtain a quantifiable method to optimize this type of transducer, we have devised a processing model. Given the functional distribution of the electromechanical coupling coefficient, d(31), and the functional distribution of elastic modulus through the thickness of the transducer, the analysis predicts the displacement as a function of loading. The tape casting method coupled with the model provides an actuator that maximizes displacement and generated force for the given material properties.
A triblock copolymer of polystyrene-polybutadiene-polystyrene (Kraton D1102) has been used to pattern barium titanate precursor with nanoscale modulations. The copolymer self-assembles to yield cylindrical polystyrene nanodomains in a polybutadiene matrix. The preorganized thin films of polymer are then selectively OH-functionalized in situ on the unsaturated carbon bonds in the polybutadiene matrix with antistereochemistry. Anchoring the barium titanate precursor onto the hydroxylated polymer thin films is possible only in the trans-1,2 polybutadiol matrix through the condensation between the barium titanium double alkoxides and the hydroxyl groups. The regioselective deposition of the barium titanium double alkoxides on the original polybutadiene matrix of the Kraton thin films was verified by transmission electron microscopy and electron energy loss spectroscopy. The spacing of the coordinated barium titanium double alkoxide pattern was similar to 23 nm, equivalent to the interdomain spacing of the original polybutadiene matrix.
A single crystal of a high-temp. superconductor of the formula MBa2Cu3O7-x, where M = Y, Sm, Eu, Gd, Dy, Ho, Er, or Yb and x = 0.1-1.0, is prepd. by forming a starting powder by combustion spray pyrolysis, then growth of a crystal on a settling powder of the compn. Ba4Cu2PtOx and/or controlled isothermal growth. [on SciFinder(R)]
We have examined the axial displacement, Delta h, and maximum axial pressure, P-max, of flextensional transducers such as the moonies and the rainbows with both scaling and mechanical analyses. For a constant electric field E across the transducer, Delta h/t alpha E/t(2) where t is the thickness of the rainbow or the thickness of the metal end cap of the moonie and Delta h/t, the relative axial displacement. Thus, for a constant voltage V across the transducer, Delta h/t alpha V/t(3). As for the maximum pressure, P-max alpha t(2) for the rainbows and P-max alpha wt for the moonies where t is the thickness of the rainbow or the thickness of the metal end cap of the moonie and w the thickness of the piezoelectric disk of the moonie. These predictions agree well with the experimental results found in the rainbows and the moonies, Our analysis showed that although the rainbows and the moonies differ in design and processing, the underlying physics for the enhancement in the axial displacement are essentially the same: The nonuniform distribution of d(31) through the thickness of the transducer causes the transducer to arch or flatten with an applied electrical field, which leads to the enhancement in the axial displacement, The only difference is that, for the transducer to arch, the applied field is in the opposite direction to the polarization in the rainbows but in the same direction as the polarization in the moonies.
The interfacial self-assembly structures of a series of poly(oxyethylene) n-dodecyl ether (C12En) nonionic surfactants on graphite has been imaged by atomic farce microscopy using only the steric stabilization force as the contrast mechanism. Aggregates are arranged in parallel stripes perpendicular to the underlying graphite symmetry axes for C12E5-C12E10. These are interpreted as hemicylindrical micelles, consistent with previous studies of ionic surfactants adsorbed on graphite. C12E23 shows a featureless layer and C12E3 forms an anchored lamellar phase growing normal to the graphite surface. We relate the interfacial structures to those formed in bulk solution and show that the initially adsorbed molecules template the interfacial aggregates, modifying their self-assembly behavior.
A review is made of recent developments in inelastic neutron scattering experiments on spin excitations and phonons in the high-temperature superconductor YBa2Cu3O6+x and its antiferromagnetic precursor YBa2Cu3O6.2. These experiments include the detection of high-energy ''optical'' spin waves and the determination of the full spin Hamiltonian in YBa2Cu3O6.2, detailed investigations of the 40 meV magnetic resonance peak in the superconducting state of YBa2Cu3O7 and its precursors in underdoped YBa2Cu3O6+x, and experiments on the effect of superconductivity on phonon lifetimes in YBa2Cu3O7.
Polarized and unpolarized neutron scattering have been used to determine the effect of superconductivity on the magnetic excitation spectra of YBa2Cu3O6.5 (T-c = 52 K) and YBa2Cu3O6.7 (T-c = 67 K); Pronounced enhancements of the spectral weight centered around 25 and 33 meV, respectively; are observed below T-c in both crystals, compensated predominantly by a loss of spectral weight at higher energies. The data provide important clues to the origin of the 40 meV magnetic resonance peak in YBa2Cu3O7.
Recent research on the solution-based fabrication of inorganic thin films and organic/inorganic nanolaminates has ranged from fundamental studies of biomineralization to the synthesis of never materials and devices. Highlights include the elucidation of how biogenics and model organic nucleants affect the form of the biomineral; synthesis of mesoscale nanocomposite films by surfactant templating at interlaces; and fabrication of heterostructures with enhanced electronic and mechanical properties.
Several attempted syntheses of Ti-TMS1, a hexagonal mesoporous TiO2 reported by Antonelli and Ying, have resulted in a lamellar structure as determined by two-dimensional powder X-ray diffraction and transmission electron microscopy (TEM). Regions of partially calcined lamellar materials, when observed by TEM can be mistaken for hexagonal material. In no cases are specimens produced that were unambiguously hexagonal. It is concluded that the hexagonal material exists, if at all, only as a minor component of a larger lamellar structure when phosphate surfactants are used. Ti-TMS1 therefore remains elusive.
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.
THE formation of patterned colloidal structures from dispersions of particles has many potential uses in materials processing(1-3). Structures such as chains of particles that form in the presence of electric or magnetic fields are also central to the behaviour of electrorheological fluids(4-6) and ferrofluids(7). Electrohydrodynamic effects in aqueous suspensions have been described by Rhodes et al.(8). Here we show that such effects can be used to create structures within a non-aqueous colloidal dispersion of dielectric particles, When the conductivity of a particle-rich spherical region (bolus) is higher than that of the surrounding fluid, an electric field deforms the bolus into a prolate ellipsoid. If the conductivities are reversed (by adding salt to the surrounding fluid, for example), a disk-like shape results. In this way, we form colloidal columns, disks and more complex structures. Once formed, these could be frozen in place by solidifying the fluid matrix by gelation or polymerization(9).
The effect of grain size on the elimination of an isolated pore was investigated both by the Monte Carlo simulations and by a scaling analysis. The Monte Carlo statistical mechanics model for sintering was constructed by mapping microstructures onto domains of vectors of different orientations as grains and domains of vacancies as pores. The most distinctive feature of the simulations is that we allow the vacancies to move. By incorporating the outer surfaces of the sample in the simulations, sintering takes place via vacancy diffusion from the pores to the outer sample surfaces. The simulations were performed in two dimensions. The results showed that the model is capable of displaying various sintering phenomena such as evaporation and condensation, rounding of a sharp corner, pore coalescence, thermal etching, neck formation, grain growth, and growth of large pores. For the elimination of an isolated pore, the most salient result is that the scaling law of the pore elimination time t(p) with respect to the pore diameter d(p) changes as pore size changes from larger than the grains to smaller than the grains. For example, in sample-size-fixed simulations, t(p) similar to d(p)(3) for d(p) < G and t(p) similar to d(p)(2) for d(p) > G with the crossover pore diameter d(c) increasing linearly with G where G is the average grain diameter. For sample-size-scaled simulations, t(p) similar to d(p)(4) for d(p) < G and t(p) similar to d(p)(3) for d(p) > G. That t(p) has different scaling laws in different grain-size regimes is a result of grain boundaries serving as diffusion channels in a fine-grain microstructure such as those considered in the simulations. A scaling analysis is provided to explain the scaling relationships among t(p), d(p), and G obtained in the simulations. The scaling analysis also shows that these scaling relationships are independent of the dimensionality. Thus, the results of the two-dimensional simulations should also apply in three dimensions.
We describe a new electrohydrodynamic phenomenon observed in inhomogeneous, nonaqueous colloidal dispersions with a spatially varying particle number concentration. In the presence of an external electric field, the dielectric constant and conductivity gradients in these systems engender fluid motion which results in the formation of patterned colloidal structures: columns, disks, and other more complicated structures. Other workers found similar effects in high conductivity systems, where the particles are dispersed in water with dissolved electrolyte. Our experimental results with barium titanate dispersed in low conductivity, apolar liquids indicate that electrical forces due to free charge and dielectric constant variations each play a role in inducing now. This pattern forming phenomenon differs from previously observed field-induced pattern formation in colloidal dispersions (e.g., colloidal string formation in electrorheological and ferrofluids) largely as a result of the induced fluid flow. A mathematical model has been developed which predicts, qualitatively, the initial now patterns encountered in our system. The theory may also help explain the formation of more complicated field-induced particle morphologies which have been reported in aqueous and nonaqueous media as well as the observation of dispersion band broadening during electrophoresis.
The formation of nonlamellar lipid structures in model lipid membranes has been extensively studied in recent years. These hydrated lipid phases include the inverted hexagonal phase and various bicontinuous cubic phases, which occur at selected lipid concentrations, temperatures, and pressures. Cubic phases that are bicontinuous with respect to the polar and nonpolar regions are especially interesting as organic analogs of zeolites. The recently developed methods used to polymerize and stabilize lamellar assemblies offer certain strategies that are applicable to nonlamellar phases. Here we report the successful stabilization of a nonlamellar phase via the polymerization of reactive amphiphiles. A 3:1 molar mixture of polymerizable mono-dienoyl-substituted phosphoethanolamine and bis-dienoyl-substituted phosphocholine were hydrated to yield bilayers. X-ray diffraction of the unpolymerized mixture at 60 degrees C showed the formation of an inverted hexagonal phase which on prolonged incubation changed to a bicontinuous cubic phase of Pn ($) over bar 3m symmetry. Polymerization of the hexagonal phase produced a stabilized hexagonal structure over the range of 20 to 60 degrees C. The same lipids at lower concentration were characterized by P-31-NMR and transmission electron microscopy (TEM) before and after polymerization. The NMR shows the formation of a sample with isotropic symmetry as expected for a cubic phase. The polymerized sample retained a nonlamellar structure after cooling and extended storage at room temperature or near 0 degrees C. The TEMs show a polydomain square lattice with 6 +/- 1 nm diameter aqueous channels. This stabilized nonlamellar phase is the first representative of a new family of materials with interpenetrating water channels with high surface area and potentially bicompatible lipid-water interfaces.
Nanocomposite materials in the form of nanometer-sized second-phase particles dispersed in a ceramic matrix have been shown to display enhanced mechanical properties. In spite of this potential, processing methodologies to produce these nanocomposites are not well established. In this paper, we describe a new method for processing SiC-mullite-Al2O3 nanocomposites by the reaction sintering of green compacts prepared by colloidal consolidation of a mixture of SIC and Al2O3 powders, In this method, the surface of the SIC particles was first oxidized to produce silicon oxide and to reduce the core of the SiC particles to nanometer size. Next, the surface silicon oxide was reacted with alumina to produce mullite. This process results in particles with two kinds of morphologies: nanometer-sized SiC particles that are distributed in the mullite phase and mullite whiskers in the SiC phase. Both particle types are immersed in an Al2O3 matrix.
We have measured the temperature dependence of the peak position and linewidth of the 42.5 meV phonon branch in a twinned single crystal of YBa2Cu3O7 as a function of wave vector q. In the (100)/(010) direction in the Brillouin zone, considerable softening and broadening occur below the superconducting transition temperature T-c at some values of q. We observe an order of magnitude smaller softening and no linewidth broadening for q in the (110)/(1(1) over bar0$) direction. Possible implications of these findings for the symmetry of the superconducting order parameter are discussed.
We have developed a new scattering geometry for magnetic neutron scattering experiments on YBa2Cu3O7 in which the phonon background around q similar to (pi/alpha,pi/alpha), h omega similar to 40meV is significantly reduced. We use this new approach to study the previously detected, sharp magnetic excitation at similar to 40meV in the superconducting state in detail. The excitation does not shift substantially in energy up to at least 75K (similar to 0.8T(c)). Polarized neutron scattering experiments (horizontal minus vertical field) confirm the magnetic origin of the 40meV excitation and put stringent limits on the magnetic scattering intensity in the normal stale.
Crystallization behavior of cordierite-based glass with excess SiO2 and Al2O3 (CM glass) was studied at the initial crystallization stage. Two kinds of glass-in-glass phase separation occurred during the heat treatment of glass powder. The secondary glass-in-glass phase separation, which consists of an SiO2-rich phase and an Al2O3-rich phase significantly affects the crystallization of the glass and the resultant microstructures of the crystallized glass. Mullite nucleated first in the Al2O3-rich amorphous phase which is no longer a stable glass-forming region. Metastable mu-cordierite crystallized in the SiO2-rich amorphous phase and grew rapidly. A fine-grained cordierite-mullite composite with mullite as the grain boundary phase was produced.
We have examined the equilibrium-state density profiles of centrifuged cakes both theoretically and experimentally. The theoretical density profiles were obtained by implementing the experimental pressure-density relationship into the general differential equation for centrifugation with appropriate boundary conditions. With a power-law pressure-density relationship, P = beta phi(n), we show that phi(Z)/phi(max) = (1 - Z/Z(m))(1/(n - 1)) where phi(max) is the density at the bottom of the cake, Z the distance measured from the bottom of the cake, and Z(m) the distance at which the cake density vanishes. Experimentally, the density profiles were examined with gamma-ray densitometry. The predicted density profiles are in good agreement with the experimental ones. We also show that form phi(Z)/phi(max) = (1 - Z/Z(m))(1/(n - 1)) applies to sedimentation cakes as well, provided the pressure-density relationship of sedimentation cakes is also a power-law one.
We report preliminary results of the high resolution inelastic neutron scattering measurements of the 42.5 meV optical phonon branch in YBCO, whose behavior at momentum transfer q=0 has been extensively studied by Raman scattering. The experiment was done on a large (75g) single crystal of YBa2Cu3O7-x (T-c approximate to 90K) with the resolution of 2 meV (full width at half maximum). In cooling from 100K to 50K we observe a small softening of the phonon energy at q=0.25,0.25,0, but no significant linewidth change.
Slamovich, E. B. ; Aksay, I. A.Hydrothermal processing of BaTiO3 polymer-films. In Better Ceramics through Chemistry Vi; Cheetham, A. K. ; Brinker, C. J. ; Mecartney, M. L. ; Sanchez, C., Ed. Materials Research Soc: Pittsburgh, 1994; Vol. 346, pp. 63-68.
Magnetite particles and monosized polystyrene beads were trapped in a silica-gel, which was then dried by using supercritical fluid extraction. When the monolithic dried gel is sintered, the polystyrene beads are pyrolyzed, leaving a porous magnetized piece of ceramic with controlled pore sizes. These ''magnetic gel'' ceramics provide a novel class of materials for use in gel magnetophoresis and other biophysical applications.
A combined experimental and theoretical investigation of the sedimentation of unstable colloidal ceramic suspensions has been performed. Suspensions containing submicron-sized alpha - Al2O3 particles were prepared at various pH values in order to modify suspension stability. Particle volume fraction during sedimentation was determined as a function of position and time by gamma-ray densitometry. A population balance model was developed to account for various coagulation and decoagulation mechanisms that affect sedimentation behavior in flocculating suspensions. Model predictions were then compared with experimental measurements, in order to establish the validity of the theoretical model.
Keckler, S. E. ; Dabbs, D. M. ; Yao, N. ; Aksay, I. A.Silification of wood-cell walls. In Fifty-Second Annual Meeting - Microscopy Society of America/Twenty-Ninth Annual Meeting - Microbeam Analysis Society, Proceedings; Bailey, G. W. ; GarrattReed, A. J., Ed. San Francisco Press Inc: San Francisco, 1994; pp. 428-429.
Dabbs, D. M. ; Yao, N. ; Aksay, I. A.Surfactant Mediated Deformation of Gold Particles. In Fifty-Second Annual Meeting - Microscopy Society of America/Twenty-Ninth Annual Meeting - Microbeam Analysis Society, Proceedings; Bailey, G. W. ; GarrattReed, A. J., Ed. San Francisco Press Inc: San Francisco, 1994; pp. 446-447.
McGehee, M. D. ; Gruner, S. M. ; Yao, N. ; Chun, C. M. ; Navrotsky, A. ; Aksay, I. A.Synthesis of Mesoscopic Structures by Co-assembly. In Fifty-Second Annual Meeting - Microscopy Society of America/Twenty-Ninth Annual Meeting - Microbeam Analysis Society, Proceedings; Bailey, G. W. ; GarrattReed, A. J., Ed. San Francisco Press Inc: San Francisco, 1994; pp. 448-449.
Mullite fibre was successfully synthesized by pyrolysis of aluminosiloxane formed from ethyl 3-oxobutanoatodiisopropoxyaluminium and di-(sec-butoxy)aluminoxytriethoxysilane. Aluminosiloxane increased in viscosity with increasing coordination number of aluminium, which was analysed by Al-27 NMR spectroscopy. The viscosity of aluminosiloxane was controlled by the amount of added glacial acetic acid as well as the working temperature for the spinning of precursor fibre. Aluminosiloxane gave amorphoUS SiO2-Al2O3 at 500-degrees-C, which began to crystallize to mullite at 930-degrees-C. Single-phase mullite was produced on heating to 1000-degrees-C for 1 h. The polymer fibre spun from aluminosiloxane was pyrolysed yielding crack-free mullite fibre at 1000-degrees-C for 1 h.
Processing of YBCO single crystals was carried out by solidification of semi-liquid YBCO composition using a seeding technique. Microstructural characterization of the pinning centers was investigated by TEM. Characterization of single crystals was carried out, relating grain size and shape to the corresponding flux profiles. Current densities were calculated based on measured trapped fields. Once circulating currents were established, flux pumping and quenching experiments were conducted. These large single crystals will be incorporated into electromagnetic forming devices for use in the military and commercial aircraft manufacturing and service industries.
A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa2Cu3O7 was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBaCu3O7. For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory.
The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparticle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, phi(m), obtained, with phi(m) almost-equal-to 0.54 for the most strongly flocculated suspension to phi(m) almost-equal-to 0.63 for the most weakly flocculated suspension. The compressive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.
The high surface charge of small ceramic particles such as alumina particles prevents them from dispersing evenly in aqueous suspensions and forming high-density compacts. However, suspensions of 400-nm-diameter alumina particles treated with alginate from the bacterium Azotobacter vinelandii were well dispersed. The alginate bound firmly to the particle surface and could not be removed by repeated washing with distilled water (2.82 mg of the bacterial alginate adsorbed to 1 g of the alumina particles). Furthermore, A. vinelandii grew and produced alginate in the presence of up to 15% (vol/vol) alumina particles. These results suggest that an in situ process using this bacterium to coat ceramic particles with alginate might be developed. In in situ processing experiments, the particle-packing densities were significantly increased and the viscosities of 5 and 10% (vol/vol) suspensions were reduced 4- and 60-fold, respectively, over those of controls. The bacteria were readily removed from the alumina particles by washing.
The process comprises forming a consolidated ceramic, e.g., green compact of Al2O3 powder, impregnating the greenware with an inorg. polymer, e.g., polyaluminoxane having general formula [-M(R)n-X(R1)p-]m, (M = trivalent or tetravalent inorg. ion; when M = trivalent, n = 1, when M = tetravalent, n = 2; X is O-2, S-2, or N-3; when X = O-2 or S-2, p = 0; when X is N-3, p = 1; R, R1 = alkyl, alkoxy, acyloxy, Ph, or phenoxy group contg. a chain of ≥3 C atoms; and m = 5-1000) and sintering the greenware, preferably at 1000-1400°. The ceramics are useful for the electronics industry. [on SciFinder(R)]
Aggregation of colloidal particles with a finite attraction energy was investigated with computer simulations and with gold particles coated with a surfactant. Computer simulations were carried out with the Shih-Aksay-Kikuchi (SAK) model, which incorporates a finite nearest-neighbor attraction energy - E into the diffusion-limited-cluster-aggregation (DLCA) model. Both the computer simulations and the experiments showed that (i) with a finite interparticle attraction energy, aggregates can still remain fractal, and (ii) the fractal dimension remains unchanged at large interparticle attraction energies and increases when the interparticle attraction energy is smaller than 4k(B)T, where T is the temperature and k(B) is the Boltzmann constant. The agreement between the simulations and the experimental results suggests that the reversible aggregation process in a colloidal system can be represented by the SAK model.
Ceramic precursor mixt. comprising a metal cation capable of being converted to a metal oxide by thermal energy, a carbohydrate, and an anion capable of participating in an anionic oxidn.-redn. reaction with the carbohydrate is converted to ceramic powder by forming droplets of the precursor mixt., removing all the solvent from the droplets, thermally initiating an anionic oxidn.-redn. reaction between the anion and carbohydrate to form a multiphase ceramic material consisting of carbonates, hydroxides, and oxides of metal cation, and heating the particles to convert the multiphase ceramic into a single-phase ceramic. This process is used for the manuf. of superconductive and nonsuperconductive ceramic powders.
During the last decade, significant advances have been made in the processing of ceramics by a combination of techniques utilizing molecular precursors and colloids for powder consolidation. Powder consolidation methods have mainly dealt with the formation of unagglomerated powders in the size range of 0.1-1-mu-m, the preparation of colloidal suspensions that are suitable for the formation of high density compacts by filtration and/or plastic forming techniques, the removal of the processing aids, and the role of consolidation methods on microstructural evolution. In contrast, the molecular and/or sol-gel techniques dealt with processing at a finer dimensional scale of 10-1000 angstrom with either molecularly homogeneous precursors or nanometer-sized particulates that are used in the preparation of gels that display linear viscoelastic behavior. Similar to green compacts of micron-sized powders, these gels are then converted to dense ceramics by heat treatment. This review summarizes the concepts that are common to both of these regimes and points to the synergistic benefits of coupling molecular precursors with colloids in a process path. The emphasis is on the control of the structure of a final product at scale lengths ranging from molecular to micro- and macroscopic dimensions.
Mullite (3Al2O3.2SiO2) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mol% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for low-temperature (< 1000-degrees-C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for low-temperature (1250-degrees to 1500-degrees-C) densification through viscous deformation. Flexural strength and creep resistance at elevated temperatures are significantly affected by the presence of glassy boundary inclusions; in the absence of glassy inclusions, polycrystalline mullite retains > 90% of its room-temperature strength to 1500-degrees-C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high-performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid-infrared range.
Recent studies have shown that the mullitization of diphasic aluminosilicate matrices comprising transitional alumina and amorphous silica occurs via a nucleation and growth process. Nucleation is preceded by a temperature-dependent incubation period. Following this incubation period, rapid nucleation of mullite occurs, producing about 1.8 x 10(11) nuclei/cm3, which remains constant throughout the rest of the transformation. Both incubation and mullite growth are thermally activated processes with apparent activation energies of 987 +/- 63 and 1070 +/- 200 kJ/mol, respectively. The growth rate of mullite grains under isothermal conditions is time dependent. An interpretation of these results is proposed on the basis of the nucleation and growth concepts of LaMer and Dinegar which supports the concept that the growth rate of mullite grains is controlled by the dissolution of transitional alumina into the amorphous matrix.
Shih, W. H. ; Liu, J. ; Shih, W. Y. ; Kim, S. I. ; Sarikaya, M. ; Aksay, I. A.MECHANICAL-PROPERTIES OF COLLOIDAL GELS; Aksay, I. A. ; McVay, G. L. ; Ulrich, D. R., Ed. Materials Research Soc: Pittsburgh, 1989; Vol. 155, pp. 83-92.
Qian, M. ; Stern, E. A. ; Ma, Y. ; Ingalls, R. ; Sarikaya, M. ; Thiel, D. ; Kurosky, R. ; Han, C. ; Hutter, L. ; Aksay, I.NICKEL IMPURITIES IN THE 90K SUPERCONDUCTOR. Physica B-Condensed MatterPhysica B-Condensed Matter1989, 158, 477-479.