Showing papers on "Nanocomposite published in 1997"

Nanocomposite Materials for Optical Applications

Abstract: A substantial amount of work has been carried out in the area of nanocomposite materials for optical applications. Composites are typically constructed by embedding an optically functional phase into a processable, transparent matrix material. By doing so, the optical properties can be utilized in more technologically important forms such as films and fibers. This review covers many areas of optical composite research to date. Composites with second- and third-order nonlinearities and laser amplification properties are discussed with examples from the recent literature. Other composites, including transparent magnets, may be made using similar structures. The principles used to construct these composites may have important technological applications soon and are therefore summarized in this review.

Structural ceramic nanocomposites

Abstract: Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Mechanical percolation in cellulose whisker nanocomposites

Abstract: Nanocomposites obtained by casting a mixture of a latex and an aqueous suspension of cellulose whiskers have been studied. Their mechanical properties (e.g. shear modulus) are found to increase by more than three orders of magnitude in the rubbery state of the polymer matrix, when the whisker content is 6 wt%. This large and unusual effect is discussed on the basis of different types of mechanical models, including semi-phenomenological and numerical finite element calculations. It is concluded that cellulose whiskers form a rigid network linked by hydrogen bonds. The formation of this network is assumed to be governed by a percolation mechanism.

Nanocrystalline carbide/amorphous carbon composites

Abstract: Nanocrystalline TiC/amorphous carbon (a-C) composite films were synthesized at near room temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites encapsulated in a sp3 bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable plasticity (40% in indentation deformation) at loads exceeding their elastic limit. They were also found to have a high scratch toughness in addition to a low (about 0.2) friction coefficient. The combination of hardness and ductility was correlated with film phase composition and structural analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites make them beneficial for surface wear and friction protection.

Thermoplastic Nanocomposites Filled With Wheat Straw Cellulose Whiskers. Part II: Effect of Processing and Modeling

Abstract: The reinforcing effect of cellulose microcrystals, or whiskers, dispersed in a thermoplastic matrix is analyzed. In Part I [Polym. Compos., 17, 604 (1996)], the preparation and the characterization of cellulose whiskers obtained from wheat straw were detailed. Composite films were processed by freeze-drying and molding a mixture of aqueous suspension of these microcrystals and a film forming latex of poly(Styrene-co-ButylAcrylate). Using the same filler and latex, materials were processed by casting and evaporating the mixture of aqueous suspensions. It was found that the reinforcing effect is greater in the latter case. This behavior is ascribed to the sedimentation of the filler during evaporation, evidenced by scanning electron microscopy, wide angle X-ray scattering, and dynamic mechanical analysis, and to the formation of a network of whiskers governed by a percolation mechanism. The behavior of the composite is modeled by subdividing the sample into layers with different whisker contents lying parallel to the film surface.

Processible Optically Transparent Block Copolymer Films Containing Superparamagnetic Iron Oxide Nanoclusters

Abstract: Free-standing block copolymer films containing magnetic iron oxide nanoclusters within the microdomains were produced by static casting. The thickness of the nanocomposite films was easily controlled to make optically transparent thin films. The size distribution of the nanoclusters was relatively narrow and the nanoclusters were uniformly distributed within the films. The spherical nanoclusters, ca. 5 nm in diameter, were identified as γ-Fe2O3 and had a well-defined crystalline structure. Magnetic measurements revealed that the nanocomposite films are superparamagnetic.

Semiconductor nanoclusters-physical, chemical, and catalytic aspects

Abstract: Introduction (P.V. Kamat, D. Meisel). Preparation and characterization of semiconductor nanoparticles (H. Weller, A. Eychmuller). The mechanics of nanoscale suspensions (K.T. Miller, C.F. Zukoski). Sonochemistry in colloidal systems (F. Grieser). Charge transfer in nanoparticles (D. Meisel). Spectroscopy of metal colloids -- some comparisons with semiconductor colloids (P. Mulvaney). Surface characterization of nanostructured systems (M. Tomkiewicz). Quantum well and superlattice electrodes (A.J. Nozik). Pseudopotential theory of nanometer silicon quantum dots (L.--W. Wang, A. Zunger). Luminescent porous silicon: synthesis, chemistry, and applications (M.J. Sailor, J.L. Heinrich, J.M. Lauerhaas). Composite semiconductor nanoclusters (P.V. Kamat). Photoelectron transfer in nanocomposite films, layer by layer self--assembled from polycations and anionic semiconductors (J.J. Fendler). Semiconductor nanocrystals in photoconductive polymers: charge generation and charge transport (Y. Wang). Nanostructure and size quantization in chemical solution deposited semiconductor films (S. Gorer, G. Hodes). Electrodeposition and characterization of nanocrystalline semiconductor films (K. Rajeshwar, N.R. de Tacconi). Nanocrystalline electronic junctions (M. Gratzel). Nanostructures in analytical chemistry (E. Pelizzetti, C. Minero). Semiconductor--mediated photocatalysis for organic synthesis (Y. Li, L. Wang). Application of nanoparticles in the photocatalytic degradation of water pollutants (N. Serpone, R.F. Khairutdinov). Applications in photocatalytic purification of air (X. Fu, W.A. Zeltner, M.A. Anderson). Author index. Subject index.

Rubber toughened thermoplastic resin nano composites

Abstract: Toughened nanocomposite materials are prepared based on a blend of one or more thermoplastic engineering resins, e.g., nylon, a functionalized, e.g. brominated, copolymer of a C 4 -C 7 isomonoolefin, e.g. isobutylene, and a para-alkylstyrene, e.g. para-methylstyrene, and further contain a uniformly dispersed exfoliated phyllosilicate layered clay, e.g., montmorillonite. The nanocomposite materials exhibit superior mechanical properties, including enhanced impact strength.

Handbook of nanophase materials

Abstract: Part 1 Configured nanomaterials: effect on properties of reduced size and dimensions etching technology for microelectronic materials sol gel formation and deposition electrodeposition of nanoscale architectures molecular beam epitaxy plasma-assisted deposition polymers in ordered nanocomposites synthesis, characterization and properties of semiconductor nanoclusters synthesis and characterization of ultrathin metal oxide films. Part 2 Materials characterization and analysis: embedded nanocrystal spectroscopy - semiconductor and metal particles in insulators x-ray diffraction characterization of nanophase materials auger and x-ray photoelectron spectroscopy.

Nanocomposite polymer container

Abstract: The container of the present invention is composed of a polymer material (76) integrated with a plurality of nanosize particles of a clay mineral (78) which act to enhance the barrier properties of the container. The polymer material may be PET, COPET or any mixture thereof. The nanocomposite polymer container (74) decreases the permeability of various gases without substantially altering the fabrication method for producing containers composed of PET or COPET material, and without altering the containers themselves. The nanocomposite polymer containers of the present invention are able to accomplish this due to the minimal amount of clay integrated with the polymer material, between 0.1 % and 10 % weight of the container. The small amount of clay provides a substantial barrier due to the high aspect ratios of the clay particles which will vary between 100 and 2000. The nanocomposite polymer container may be produced using in situ polymerization, solution intercalation, or melt exfoliation to integrate the clay mineral (78) with the polymer material matrix (76). The clay mineral may be smectite, vermiculite, halloysite or any synthetic analog thereof, with a preference for the montmorillonite species of smectite clays.

Fabrication of Quantum Dot-Polymer Composites: Semiconductor Nanoclusters in Dual-Function Polymer Matrices with Electron-Transporting and Cluster-Passivating Properties

Abstract: Hybrid inorganic−organic polymer composites have been prepared by a convergent approach in which nearly monodisperse CdSe or (CdSe)ZnS nanoclusters are sequestered within phosphine-containing domains in a charge-transporting matrix. The motivation for these studies is the potential utility of such composites as combined electron-transport and emitter layers in light-emitting devices. Diblock copolymers with electronically passivating and charge-transport capabilities were prepared via ring-opening metathesis polymerization of octylphosphine- and oxadiazole-functionalized norbornenes. Independently prepared CdSe and ZnS-overcoated CdSe nanoclusters, surface-passivated by trioctylphosphine and trioctylphosphine oxide groups, are tethered by polymer-bound phosphine donors, resulting in immediate, sustained increases in fluorescence. Thin films of the CdSe-block copolymer composites, static-cast from dilute solution, exhibit microphase separation, with segregation of nanoclusters within phosphine-rich microdo...

Size quantization and interfacial effects on a novel γ-Fe2O3/SiO2 magnetic nanocomposite via sol-gel matrix-mediated synthesis

Abstract: Novel isolated magnetic single-domain γ-Fe2O3 nanoclusters have been prepared by coprecipitation of ferrous and ferric salts encapsulated within sol-gel derived silica (SiO2). The nonmagnetic SiO2 coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of the Fe2O3 nanoclusters provides a means for thermally stable dispersion of Fe2O3 clusters. The precipitated particles coated with SiO2 are spherical with 4–5 nm diameters. Surface and strain effects played a critical role in determining the overall magnetic behavior of the spherical single-domain particles. Superparamagnetic behavior was observed by superconducting quantum interference device magnetometry and Mossbauer spectroscopy. Superparamagnetic barrier energies and the low-temperature coercivities were modified through cluster/support interface microstructure manipulation. The optical studies showed the absorption edge of the nanocomposites to be slightly blue shifted in the UV–VIS spectrum range when compared to that ...

Pulsed laser deposition of Cu:Al2O3 nanocrystal thin films with high third-order optical susceptibility

Abstract: Nanocomposite films comprising metal Cu nanocrystals embedded in an Al2O3 matrix were deposited by alternating pulsed laser ablation from metallic Cu and ceramic Al2O3 targets. The films were grown in vacuum on glass substrates held at room temperature. The as-grown films contain 4 nm Cu nanocrystals in an amorphous Al2O3 matrix, with a total thickness of 190 nm. The films show a substantial third-order susceptibility with an electronic nonlinear refractive index of (2.93±1.08)⋅10−10 cm2 W−1 and a nonlinear saturation of −(2.34±0.18)⋅10−5 cm W−1.

Mechanically Attrited Silicon for High Refractive Index Nanocomposites

Abstract: High-energy milling provides an effective and environmentally conscious method for nanosizing silicon. Colloidal suspensions of nanosized silicon are presently demonstrated and utilized for the fabrication of high refractive index nanocomposites. Si nanoparticles with average sizes of 20−40 nm and size distributions of about 25% have been separated from milled powder via sonication and centrifugation. These nanoparticles were analyzed using transmission electron microscopy, dynamic light scattering, X-ray diffraction and UV−vis/FTIR spectroscopy. Formation of stable colloids is in part attributed to a thin surface-oxide layer. The decrease in the average particle size causes a blue shift in their absorption spectrum, thus increasing transparency in the red part of the visible region. These Si nanoparticles were used to fabricate high refractive index nanocomposites, with refractive indexes up to 3.2, when dispersed in gelatin.

Organoclay-polymer composites

Abstract: Nanocomposites are made by melt-blending a melt processable polymer having a high melt processing temperature and an organophosphonium cation modified layered clay. Such modified layered clay is more stable at the higher temperatures encountered during the melt processing of such a melt processable polymer than the modified layered clays used in the prior art, enabling the preparation of the nanocomposite.

Polymer-organoclay-composites and their preparation

Abstract: A polymer composite comprising a hydroxy-phenoxyether or polyester polymer matrix having dispersed therein layers of an inorganic material derived from a multilayered inorganic material having organophilic properties. The dispersion of the multilayered inorganic material in the polymer matrix is such that an increase in the average interlayer spacing of the layered inorganic material occurs. This increase in interlayer spacing occurs to a significant extent resulting in the formation of a nanocomposite. The polymers are preferably derived from a diglycidyl ether or ester or an epihalohydrin and a dinucleophile such as a dicarboxylic acid, a difunctional amine, a bisphenol or a sulfonamide. As the organophilic inorganic material clay minerals modified with organic ammonium compounds are used.

Synthesis and properties of ceramic nanoparticles and nanocomposites

Abstract: This paper describes the possibilities of synthesizing ceramic nanoparticles in a microwave plasma. The properties of these particles can be modified by coating them with a layer of a second material. A two-step process to produce this type of particle is described. The structure of these nanocoated particles depends strongly on the crystallisation behaviour of the phases forming the kernel and the coating. The main applications of these new nanocoated particles may either be seen in the formation of diffusion barriers to avoid grain growth or in the modification of physical properties of the core and the chemical properties of the surface.

Fabrication, properties, and modelling of engineering ceramics reinforced with nanoparticles of silicon carbide

Abstract: The present paper prepared by Professor R. W Davidge, briefly summarises the results obtained from a multinational project aimed at assessing nanocomposite ceramics reinforced with fine (∼ 200 nm) particles of SiC. Various fabrication procedures were developed, e.g. by attritor milling in a water medium, followed by freeze drying and hot pressing. Matrixes were alumina, silicon nitride, mullite, and cordierite. Although modest improvements in properties were obtained for all systems, the most interesting effects were observed for alumina. For a given grain size, strength was increased by ∼ 70% and toughness by ∼ 10% in material with 5 vol.-%SiC, and for the range studied was relatively independent of nanophase content and particle size (10-400 nm). Based on microscopy observations and estimates of the Al 2 O 3 /SiC boundary fracture energy, it is postulated that a key feature of the nanocomposite effect is associated with a strongly increasing R curve behaviour over a small number of grain dimensions. The consequence is that fracture is predominantly transcrystalline, as opposed to intercrystalline for pure material. This gives the nanocomposite a significantly improved wear resistance. The material also showed good strength retention at high temperatures in corrosive atmospheres, and a considerably reduced creep rate.

Metal-Polymer Interactions in a Polymer/Metal Nanocomposite

Abstract: A poly(t-butyl acrylate)/gold nanocomposite sandwich was annealed to induce diffusion of the gold particles, which was monitored using Rutherford backscattering spectrometry. Marker motion experiments were also performed to probe particle and polymer mobilities independently. The experiments revealed that particle mobility was decreased by 2 to 3 orders of magnitude compared with the predictions by Stokes-Einstein theory. Diffusion of polymer molecules through a gold particle layer is decreased by a much smaller extent. These results are attributed to bridging between particles arising from slow exchange kinetics of polymer segments at the polymer/metal interface.

Sonochemical Synthesis of Molybdenum Oxide− and Molybdenum Carbide−Silica Nanocomposites

Abstract: A novel sonochemical approach for the preparation of molybdenum oxide (Mo2O5·xH2O) and molybdenum carbide (Mo2C) clusters coated on silica carriers, in which Mo(CO)6 precursor serves as an in situ source for the coating phase of both materials (molybdenum oxide and molybdenum carbide), has been described. Ultrasonic irradiation of a slurry of molybdenum hexacarbonyl, Mo(CO)6, and silica microspheres in decane for 3 h, under ambient air and argon, yields diphasic molybdenum oxide−silica (MOS) and molybdenum carbide−silica (MCS) composites, respectively. Characterization using powder X-ray diffraction and transmission electron microscopy, with selected area electron diffraction, shows the amorphous nature of the nanocomposites. The phase evolution by long-term thermal reduction of MOS under H2 shows that the Mo2O5 undergoes stepwise reduction following the sequence, MoV → MoIV → Moo. The TEM image of MOS and MCS shows that the sonochemical decomposition products of Mo(CO)6 attached on the silica carrier as ...

Polymer sheets with a thin nanocomposite layer acting as a UV filter

Abstract: A surface layer (thickness 1–10 μm) containing colloidal TiO2 or ZnO particles was prepared in EVA (a copolymer of ethylene and vinyl acetate). The inorganic particles were formed in situ by hydrolysis of incorporated titanium tetrachloride or diethyl zinc. The resulting materials were analyzed with UV spectroscopy, electron microscopy, X-ray diffraction, thermogravimetric analysis and atomic emission spectroscopy. The average diameter of the embedded TiO2 particles was 70 nm; these particles absorb UV radiation but also induce opacity in the polymer sheets in the visible wavelengths range. The ZnO particles were smaller (average diameter 15 nm); with a surface layer of embedded ZnO, transparent polymer sheets can be obtained that absorb UV radiation. © 1997 John Wiley & Sons, Ltd.

Electrochemical Characterization of a Polypyrrole/Montmorillonite Nanocomposite

Abstract: The electrochemical activities of films prepared from chemically synthesized polypyrrole and a polypyrrole/montmorillonite nanocomposite are examined using cyclic voltammetry and chronopotentiometry. The nanocomposite exhibits redox chemistry at potentials (V p,c = -1500, V p,a = -1100 mV) approximately 1 V more negative than in native polypyrrole. Chronopotentiometric studies show that the capacities of these two materials, when evaluated in terms of mass of polymer, are similar, and that the charge/discharge plateaus are also shifted by approximately 1 V. Two models are proposed to explain these results, one based on the inductive effect of the encasing anionic sheets, and one on a redox mechanism involving cation insertion.

Diamond-like nanocomposite compositions

Abstract: The invention relates to an improved diamond-like nanocomposite composition comprising networks of a-C:H and a-Si:O wherein the H-concentration is between 40 % and 80 % of the C-concentration and having a coefficient of friction against steel which is smaller than 01 in air with a relative humidity up to 90 %, or in water The invention relates also to a process for depositing the composition on a substrate in a vacuum chamber The composition comprises preferably 30 to 70 at % of C, 20 to 40 at % of H, 5 to 15 at % of Si and 5 to 15 at % of 0 and can be doped with transition metals