Showing papers on "Nanocomposite published in 1994"

Generalized synthesis of periodic surfactant/inorganic composite materials

Abstract: THE recent synthesis of silica-based mesoporous materials1,2 by the cooperative assembly of periodic inorganic and surfactant-based structures has attracted great interest because it extends the range of molecular-sieve materials into the very-large-pore regime. If the synthetic approach can be generalized to transition-metal oxide mesostructures, the resulting nanocomposite materials might find applications in electrochromic or solid-electrolyte devices3,4, as high-surface-area redox catalysts5 and as substrates for biochemical separations. We have proposed recently6 that the matching of charge density at the surfactant/inorganic interfaces governs the assembly process; such co-organization of organic and inorganic phases is thought to be a key aspect of biomineralization7. Here we report a generalized approach to the synthesis of periodic mesophases of metal oxides and cationic or anionic surfactants under a range of pH conditions. We suggest that the assembly process is controlled by electrostatic complementarity between the inorganic ions in solution, the charged surfactant head groups and—when these charges both have the same sign—inorganic counterions. We identify a number of different general strategies for obtaining a variety of ordered composite materials.

Organization of Organic Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays

Abstract: The organization of cationic or anionic organic and inorganic molecular species to produce three-dimensional periodic biphase arrays is described. The approach uses cooperative nucleation of molecular inorganic solution species with surfactant molecules and their assembly a t low temperatures into liquid-crystal-like arrays. The organic/inorganic interface chemistry makes use of four synthesis routes with @+I-), @-I+), (S+X-I+), and (S-M+I-) direct and mediated combinations of surfactant (cationic S+, anionic S-) and soluble inorganic (cationic I+, anionic I-) molecular species. The concepts can be widely applied to generate inorganic oxide, phosphate or sulfide framework compositions. Distinct lamellar, cubic silica mesophases were synthesized in a concentrated acidic medium (S+X-I+), with the hexagonal and the cubic phases showing good thermal stability. For the hexagonal mesostructured silica materials high BET surface areas (>lo00 m2/g) are found. Hexagonal tungsten(V1) oxide materials were prepared in the presence of quaternary ammonium surfactants in the pH range 4-8. Cubic (Iu3d) and hexagonal antimony(V) oxides were obtained by acidifying (pH = 6-7) homogeneous solutions of soluble Sb(V) anions and quaternary ammonium surfactants a t room temperature @+I-). Using anionic surfactants, hexagonal and lamellar lead oxide mesostructures were found (S-I+). Crystalline zinc phosphate lamellar phases were obtained a t low synthesis temperatures (4 \"C) and lamellar sulfide phases could be also readily generated a t room temperature. The synthesis procedure presented is relevant to the coorganization of organic and inorganic phases in biomineralization processes, and some of the biomimetic implications are discussed.

Relation between metal electronic structure and morphology of metal compounds inside carbon nanotubes

Abstract: SEVERAL attempts have been made to fill carbon nanotubes1 with metals or metallic compounds to obtain nanocomposite materials with potentially interesting properties. Capillary action, predicted2 to be a filling mechanism, has been used3'4 to encapsulate lead and bismuth in open tubes. Compounds of yttrium5, manganese6 and gadolinium7 have also been encapsulated by formation of the nano-tubes in an arc discharge with the metals present in situ. Very recently, Tsang et al.8 showed that oxides of nickel, cobalt, iron and uranium can be encapsulated by opening the tubes and deposit-ing the filling material using wet chemical techniques. Here we report a search for general principles relating to the nature and structure of the filling material, using the arc-discharge method to fill tubes with fifteen metals and/or their compounds: Ti, Cr, Fe, Co, Ni, Cu, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy and Yb. We find that the propensity for forming continuous 'nanowires' throughout the length of the tubes seems to be strongly correlated with the existence of an incomplete electronic shell in the most stable ionic state of the metal. We also find that the interplay between growth of the nanotube and growth of the filling results, in one case, in the formation of an unusual helical filling morphology.

Sol-gel optics: processing and applications

Abstract: Part I: Sol-Gel Processing of Optical Materials. 1. Sol-Gel Processing -- General Principles E.M. Rabinovich. 2. Sol-Gel Chemistry for Optical Materials J. Livage, F. Babonneau, C. Sanchez. 3. Sol-Gel Processing of Net Shape Silica Optics L.L. Hench, J.L. Nogues. 4. Large Silica Monoliths S.R. Chaudhuri, A. Sarkar. 5. Sol-Gel Fabrication of Glass Fibers for Optics K. Kamyia. 6. Optical Coating Fabrication I.M. Thomas. 7. A High Temperature Sol-Gel Process for Glass Formation: Aerosol Doping in Modified Chemical Vapor Deposition T.F. Morse, A. Kilian, L. Reinhart. 8. Nonsilicate Optical Coatings D.S. Hagberg, D.A. Payne. 9. Fluoride Optical Materials R.E. Riman. 10. Nanocomposite Fabrication for Transparent Windows L.C. Klein. Part II: Applications of Sol-Gel Optics. 11. Single Layer and Multilayer Colored Coatings on Glass D. Ganguli. 12. Sol-Gel Processing of Ferroelectric Films M. Sayer, G. Yi. 13. Doped Sol-Gel Films for Fiber Optic Chemical Sensors M.R. Shahriari, J.Y. Ding. 14. Sol-Gel Encapsulated Molecules: Optical Probes and Optical Properties J.I. Zink, B. Dunn. 15. Semiconductor-Doped Sol-Gel Optics M. Nogami. 16. Catalyst Doped Sol-Gel Materials T. Lopez, R. Gomez. 17. Gel Derived Gradient Index Optics-Aspects of Leaching and Diffusion T.M. Che, M.A. Banash, P.R. Soskey, P.B. Dorain. 18. Gradient Index (GRIN) Elements by Sol-Gel Interdiffusion M. Yamane. 19. Photonics and Nonlinear Optics with Sol-Gel Processed Inorganic Glass: Organic Polymer Composites R. Burzynski, P.N. Prasad. 20. Inorganic-Organic Composites for Optoelectronics H. Schmidt. 21. Laser Processing of Sol-Gel Coatings B.D. Fabes. 22. Laser Densification of Micro-Optical Arrays T. Chia, L.L. Hench. 23. Organically Doped Sol-Gel Porous Glasses: Chemical Sensors, Enzymatic Sensors, Electro-Optical Materials, Luminescent Materials and Photochromic Materials D. Avnir, S. Braun, O. Lev, D. Levy, M. Ottolenghi. Index.

Structure and properties of nanocrystalline materials

Abstract: The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.

Preparation and characterisation of novel polypyrrole–silica colloidal nanocomposites

Abstract: We describe the preparation and characterisation of colloidal dispersions of polypyrrole–silica nanocomposite particles. These nanocomposites have been characterised in terms of their particle size and chemical composition by a wide range of experimental techniques, including transmission electron microscopy, disc centrifuge photosedimentometry, photon correlation spectroscopy, elemental microanalyses, thermogravimetry, infrared spectroscopy and dc solid-state conductivity measurements. We find that both the particle size and the chemical composition depend markedly on the type of chemical oxidant selected for the in situ polymerisation of the pyrrole monomer. For example, using the FeCl3 oxidant we obtain polypyrrole–silica particles which contain up to 70 m/m% polypyrrole and have particle diameters in the range 160–320 nm, whereas the (NH4)2S2O8 oxidant yields composite particles which contain only 37–55 m/m% polypyrrole and are ca. 110–180 nm in diameter. In most cases the particle size distributions are reasonably narrow, with standard deviations as low as 13% as measured by disc centrifuge photosedimentometry. We have also investigated the mechanism of formation of these nanocomposite particles by studying: (i) the adsorption of pyrrole monomer from aqueous solution onto colloidal silica; (ii) the adsorption of oxidant [either FeCl3 or (NH4)2S2O8] from aqueous solution onto colloidal silica. These adsorption studies indicate that either the monomer or the oxidant is partially adsorbed onto the silica particles prior to polymerisation in all of our syntheses. Thus, surface polymerisation processes may be important in the formation of these nanocomposite particles.

Tensile Creep Behavior of Alumina/Silicon Carbide Nanocomposite

Abstract: Tensile creep and creep rupture behaviors of alumina/17 vol% silicon carbide nanocomposite and monolithic alumina were investigated at 1,200 to 1,300 C and at 50 to 150 MPa. Compared to the monolithic alumina, the nanocomposite exhibited excellent creep resistance. The minimum creep rate of the nanocomposite was about three orders of magnitude lower and the creep life was 10 times longer than those of the monolith. The nanocomposite demonstrated transient creep until failure, while accelerated creep was observed in the monolith. It was revealed that rotating and plunging of intergranular silicon carbide nanoparticles into the alumina matrix increased the creep resistance with grain boundary sliding.

Diamond-like metallic nanocomposites

Abstract: A class of nanocomposite amorphous materials consisting of interpenetrating random networks of predominantly sp3 bonded carbon stabilized by hydrogen, silicon stabilized by oxygen, and, optionally, random networks of metal elements from groups 1-7b and 8b of the periodic table.

Nanocomposite materials: polyaniline-intercalated layered double hydroxides

Abstract: Open lamellar systems such as layered double hydroxides (LDHs) can be used to generate new intercalation compounds. We report the synthesis of nanocomposite materials consisting of organopolymer molecules encapsulated between ultra-thin mixed-metal hydroxide sheets which are propped apart by spacers [terephthalate or hexacyanoferrate(II) ions acting as pillars]. The oxidising host matrixes [Cu(1 –x)2+Crx3+(OH)2]·[(C6H4-1,4-(CO2)2)x/22–·nH2O] and [Cu(1 –x)2+Alx3+(OH)2]·[(Fe(CN)6)x/44–·nH2O] were used as hosts for the interlamellar oxidative polymerisation of aniline. The materials were prepared using chimie douce pathways and characterised by conventional physical techniques (XRD, FTIR, TG, DTA), which indicated the presence of polyaniline molecules and the retention of the host framework after incorporation of the organopolymer.

Method of preparing layered silicate-epoxy nanocomposites

Abstract: An epoxy-silicate nanocomposite is prepared by dispersing an organically modified smectite-type clay in an epoxy resin together with diglycidyl ether of bisphenol-A (DGEBA), and curing in the presence of either nadic methyl anhydride (NMA), and/or benzyldimethyl amine (BDMA), and/or boron trifluoride monoethylamine (BTFA) at 100°-200° C. Molecular dispersion of the layered silicate within the crosslinked epoxy matrix is obtained, with smectite layer spacings of 100 Å or more and good wetting of the silicate surface by the epoxy matrix. The curing reaction involves the functional groups of the alkylammonium ions located in the galleries of the organically modified clay, which participate in the crosslinking reaction and result in direct attachment of the polymer network to the molecularly dispersed silicate layers. The nanocomposite exhibits a broadened T s at slightly higher temperature than the unmodified epoxy. The dynamic storage modulus of the nanocomposite was considerably higher in the glassy region and very much higher in the rubbery plateau region when compared to such modulus in the unmodified epoxy.

Enhancement of luminescence in polymer nanocomposites

Abstract: Solid-state luminescence of polymers is currently of theoretical interest as well as for application in electroluminescent diodes. The role of supramolecular structure and morphology on solid-state luminescence of polymers is explored in rod-coil copolymers containing conjugated rods. The photoluminescence quantum efficiency of the rigid-rod homopolymer poly(p-phenylene benzobishthiazole) (PBZT) and the flexible-coil homopolymer poly(benzobisthiazoledecamethylene) (PBTC10) films is observed to be dramatically enhanced in the nanocomposite region. Results are given for the optical absorption spectra, the spectrum of the conjugated homopolymer PBZT, and the photoluminescence quantum efficiency.

Polymetalates based organic-inorganic nanocomposites

Abstract: New nanocomposites materials have been synthesized They present electrochemical and photochromic properties They are based on a hybrid organic-inorganic network, in which tungsten heteropolyoxometalates (PW12O 40 3− , SiW12O 40 4− , W10O 32 4− , polymeric tungstate) are entrapped High tungsten ratios could be reached and films or bulk materials have been obtained The structure of these materials is described on the basis of multi-spectroscopic investigations (IR, EPR, NMR) Electrochemical redox reactions have been observed in thin films Dark blue reversible coloration of the materials is obtained under UV irradiation The photochromic mechanism has been investigated and shows the reversible formation of carbonyl group

Nanocomposite formation in the Fe3O4‐Zn system by reaction milling

Abstract: Magnetic nanocomposites of small iron particles embedded in nonmagnetic zinc oxide matrix have been prepared by ball milling, with an in situ displacement reaction between a metal oxide (Fe3O4) and a more reactive metal (Zn). The phase composition of the samples has been analyzed by x‐ray diffraction. Metallic zinc disappears during the first 100 min of milling and the magnetization decreases to almost zero, indicating the formation of a nonmagnetic intermediate iron‐zinc oxide phase. This intermediate phase decomposes into iron and ZnO upon further milling. The change in magnetic properties also reflects the decreasing size of the iron particles. The final particle size is about 9 nm, as estimated from x‐ray diffraction linewidth measurements. The final product of the process is a semihard magnetic material with a room‐temperature saturation magnetization of 40 emu/g and coercivity of 400 Oe. A significant fraction of the final Fe particles is superparamagnetic.

Porous nanocomposites as catalyst supports

Abstract: Porous titania-alumina nanocomposites, with alumina contents in the range of 1 to 50 wt.-%, were prepared by mixing the respective sols to study the stabilization of the titania (matrix) phase by the alumina (second) phase. It was noted that the total stabilization effect could not be explained based on the chemical modification of the titania phase by alumina. An additional physical stabilization effect was proposed based on some circumstantial evidences. Pore structure and phase stability of these nanocomposites were studied using thermal analysis. X-ray diffraction, Raman spectroscopy, and nitrogen physisorption techniques. The presence of alumina in the composite improved the thermal stability of the titania phase by retarding the anatase-to-rutile phase transformation and the grain growth of the anatase phase. The anatase-to-rutile phase transformation temperatures for pure titania and the titania phase of the titania-alumina nanocomposite were found to be around 560 and 1000°C, respectively, from the DSC data. The surface area contributed by the titania phase in the composite was calculated to be in the range of 50 to 100 m2/g at 600°C for 8 h, whereas pure titania lost its whole surface area at this temperature and time.

In situ polymerization of tetraethoxysilane in poly(vinyl acetate)

Abstract: The in situ polymerization of tetraethoxysilane (TEOS) in poly(vinyl acetate) (PVAc) yields homogeneous and transparent nanocomposite materials. It was shown that covalent bonds occur between the organic chains and the inorganic clusters, preventing macroscopic phase separation. For an initial amount of TEOS higher than a critical value, gel formation is observed. Up to this critical TEOS content, improved mechanical properties, which persist at high temperatures, are obtained.

A new type of a sol-gel-derived inorganic-organic nanocomposite

Abstract: A new type of sol-gel-based transparent inorganic-organic nano composites has been developed by increasing the inorganic phase dimension to values just below the point, where scattering can be neglected. For this purpose, nanosized boehmite particles ≤ 50 nm are homogeneously incorporated in a sol based on tetraethoxysilane and an epoxysilane. The nano-scale boehmite particles act as catalysts for the polymerization of the epoxy silane to polyethylene oxide, as proved by13C NMR, and are linked to the matrix by Si-O-Al bridges, as proven by27Al-NMR spectroscopy. The synthesized sols can be applied by standard coating techniques on transparent polymers and are cured thermally. The mechanical properties (scratch resistance, hardness) have been substantially improved compared to systems with molecular dimensions of the inorganic phase. The effect is attributed to the special structure of flexibly suspended nano-scale boehmite particles in an inorganic-organic network by a tailored interface.

Polypyrrole-tin (IV) oxide colloidal nanocomposites

Abstract: We have chemically polymerised pyrrole in the presence of ultra-fine tin (IV) oxide particles which act as a particulate dispersant for polypyrrole. The resulting polypyrrole-tin (IV) oxide nanocomposite colloids have been investigated and compared to the polypyrrole-silica nanocomposite colloids previously reported. We find that essentially the same minimum substrate surface area is required for both the tin (IV) oxide and silica sols to ensure successful colloid formation. Our BET surface area measurements confirm that these nanocomposite particles have significant micro-porosity. The polypyrrole-tin (IV) oxide nanocomposite colloids are more polydisperse than polypyrrole-silica nanocomposite colloids but their solid-state conductivities are somewhat higher (up to 4 S cm −1 ).

Characteristics of Al2O3-SiC Nanocomposite Prepared by Sol-Gel Processing

Abstract: Al2O3-SiC nanocomposite powders containing small crystalline SiC powders have been prepared by sol-gel processing. Less than 50nm ultrasonic dispersion and sedimentation techniques were used in the presence of an organic solvent to remove the agglomerates of SiC. Homogeneous SiC powders with narrow size distributions were mixed with an Al2O3 sol. The characteristics of the nanocomposite powder as well as microstructure of the sintered Al2O3-SiC nanocomposite were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Sol-gel processing was shown to produce Al2O3-SiC nanocomposites with homogeneous distribution of SiC in the Al2O3 matrix.

Clay-Polymer Nanocomposites: Polyether and Polyimide Systems

Abstract: A new type of clay-polyether nanocomposite has been prepared by the self-polymerization of diglycidyl ether of bisphenol A in the galleries of acidic alkylammonium ion exchanged forms of montmorillonite. The acid catalyzed intragallery polymerization process leads to the spontaneous exfoliation to the 10A-thick clay layers. Intra- and extragallery polymerization processes are distinguishable by DSC. Clay-polyimide hybrid composites have also been prepared by the intercalation of polyamic acid in montmorillonites and subsequent thermal conversion to polyimide. In contrast to the completely exfoliated clay-polyether system, the polyimide system contain regularly intercalated clay aggregates in the polymer matrix. Although regular face-face clay layer aggregation is extensive, the clay-polyimide hybrid composite films exhibit greatly improved CO2 barrier properties.

Nano/nanocomposite systems: in situ growth of particles and clusters of semiconductor metal sulfides in porous silica-pillared layered phosphates

Abstract: Silica-pillared metal(IV) hydrogen phosphates formed by the intercalation of octameric aminoalkyl or aryl species from solution into layered zirconium or tin phosphates, followed by thermal degradation of the organic moieties, are crosslinked microporous, nanocomposite materials with surface areas up to 230 m–2 g–1. Their use as templates for size-quantization of metal sulfide particles is described via, the sorption of Zn–2+ and Cd–2+ ions from aqueous solution, followed by reaction with H2S in acetone. Band gaps obtained from optical absorption spectra for occluded ZnS and CdS are significantly increased from those of the corresponding bulk-metal sulfides. These data, and those of X-ray absorption spectroscopic measurements at the zinc and cadmium K edges, allow the metal sulfide aggregates to be described in terms of ‘superclusters’, where discrete ‘molecule-like’ Zn(Cd)S semiconductors confined in the micropores interconnect through windows in the pillar network to form more extended structures of diameter ca. 25 A.

Superparamagnetic Nanocomposite of Silver/Iron-Oxide by Inert Gas Condensation

Abstract: A superparamagnetic nanocomposite of silver and iron oxide was synthesized by gas condensation. The procedure involved (1) coevaporation of silver and iron, (2) in situ oxidation of iron particles, (3) in situ compaction, and (4) post-annealing in an inert or an oxidizing atmosphere. The magnetization plots against H/T fell on a single curve from room temperature to 160 K, thereby providing evidence of superparamagnetism. Annealing treatment modifies the effective magnetic moment size and saturation value of magnetization. The present process is a potential synthesis route for magnetic nanocomposites useful for applications such as magnetic refrigeration, recording and permanent magnets.