Showing papers on "Nanocomposite published in 1990"
TL;DR: In this article, three series of cordierite powders were prepared by the sol-gel route: a single phase (monophasic) gel prepared from alkoxides, a nominally triphasic nanocomposite gel made with two nanosized powders and one solution phase, and a truly compositionally triphasic matrix containing three nanosised powders.
Abstract: Three series of cordierite powders were prepared by the sol-gel route: a single phase (monophasic) gel prepared from alkoxides, a nominally triphasic nanocomposite gel made with two nanosized powders and one solution phase, and a truly compositionally triphasic nanocomposite gel prepared from three nanosized powders. Crystalline α-cordierite seeds were also incorporated with the gels and their effectiveness as nucleating agents was investigated and found to lower the crystallization temperature of α-cordierite by 125–150°C. The densification behavior of powder compacts was examined and alterations made to the heat treatment until optimum conditions were found. The truly triphasic compact sintered at 1300°C for 2 h resulted in 100% of theoretical density whereas the nominally triphasic and monophasis pellets densified to 96% and 80%, respectively. The enhanced densification achieved with powder compacct prepared for triphasic nanocomposite gels is due to part to the excess free energy of the three components.
55 citations
TL;DR: A diphasic nanocomposite sol-gel processing has become an excellent way of producing stoichiometric homogeneous powders with many advantages over the solid state reaction method as well as the solution solgel method as mentioned in this paper.
48 citations
TL;DR: In this paper, a glass-metal nanocomposites involving iron, nickel and copper in a silica glass matrix have been prepared as films on glass substrates by the sol-gel route.
Abstract: Glass-metal nanocomposites involving iron, nickel and copper in a silica glass matrix have been prepared as films on glass substrates by the sol-gel route. The metal phase is introduced as chloride and silicon tetraethoxide is used as the glass precursor. DC electrical resistivity of these nanocomposites has been measured over the temperature range 100 to 350 K. Resistivities varying from 10 Omega m to 106 Omega m have been obtained by controlling the volume fraction of the different metal phases within the silica glass. Low-temperature resistivities of the samples are controlled by a simple activation with energies in the range 0.01-0.1 eV. At temperatures above 150 K some of the nanocomposites exhibit another activated mechanism. The latter is believed to arise due to hopping of electrons between the localised states formed by the distributed metal atoms within the silica glass matrix.
40 citations
TL;DR: In this paper, a new method is presented which circumvents the usual thermodynamic limitations (alloying and compound formation) in fabricating phase-separated materials, which opens whole new classes of materials that can be prepared as particulate composites.
Abstract: A new method is presented which circumvents the usual thermodynamic limitations (alloying and compound formation) in fabricating phase‐separated materials. This opens whole new classes of materials that can be prepared as particulate composites.This method utilizes sputtering at high pressures (0.2–0.6 Torr) in a thermal gradient to produce nanoscale (<15 nm diameter) particles, which are then embedded in a matrix produced by normal sputtering. The microstructure and microhardness of 0.5‐μm‐thick composites of molybdenum particles (3–12 nm average particle size) in aluminum are presented as examples. This system cannot be prepared by the conventional phase separation technique of cosputtering.
37 citations
TL;DR: A brief review of the interface chemistry associated with nanophase confinement and packaging and some features of three dimensional surface confinement using molecular sieves and zeolites can be found in this paper.
Abstract: The miniaturization of electronic and optic devices has revolutionized response times, energy loss and transport efficiency. An additional bonus is that as one approaches the nanosize regime the presence or absence of a few atoms and the geometrical disposition of each atom can significantly modify electronic and photonic properties. This control can be further supplemented by “packaging” assemblies of atoms or molecules into thin film or nanocomposite bulk materials to define surface states, cluster environment and geometry, intercluster interactions, and consequently, a wide tunable range of optical and charge carrier responses. The chemist is presented with an intriguing challenge. First the clusters must be unisized with identical geometries. Secondly, the atom or molecular assemblies should ideally have perfect periodicity in order to rigorously define optoelectronic densities and intercluster tunnelling. A third requirement is that the nanocomposite be processable, generally in the form of thin films or single crystals. Numerous approaches are being undertaken in achieve these goals, including molecular beam and atomic layer epitaxy, molecular sieve inclusion chemistry, molecular capping of inorganic clusters, porous glass and aerosol synthesis. This paper presents a brief review of the interface chemistry associated with nanophase confinement and packaging and some features of three dimensional surface confinement using molecular sieves and zeolites.
5 citations
TL;DR: In this article, the magnetic properties of homogeneous gelled composites of iron and vitreous alumina containing 10-40% Fe have been investigated using electron microscopy, x-ray diffraction, and Mossbauer spectroscopy.
Abstract: Homogeneous gelled composites of iron and vitreous alumina containing 10-40% Fe have been prepared by room temperature polymerization of aqueous aluminum alkoxide solutions containing ferric nitrate and nitric acid at low pH. Scanning electron microscopy, x-ray diffraction, and Mossbauer spectroscopy demonstrated that this bulk material is comprised of nanometer-sized regions of iron compounds embedded in a vitreous alumina gel matrix. Magnetization data showed that in the as-cured condition these nanocomposites are paramagnetic at room temperature and become either superparamagnetic or ferromagnetic on cooling to 10 K. The magnetic susceptibility increased with the Fe content and with decreasing temperature. Analysis of the temperature dependence of the magnetic susceptibility indicated the magnetic moment per Fe atom was 1.87 μB for the 10% Fe nanocomposite and that it increased linearly with composition to 1.96 μB for the 40% Fe material. Mossbauer effect data showed that subsequent treatment of these materials in a gaseous environment of hydrogen at elevated temperatures (T<400 C) changed the form of the iron in the magnetic regions. These results are compared to that observed for similar nanocomposites prepared using a silica gel matrix.
2 citations
TL;DR: In this article, the method of preparing composites of nanosized metal particles dispersed in a glass matrix by the conventional melt-quench technique is reviewed and the versatility of sol-gel method in making nanocomposites is brought out.
Abstract: Nanocrystalline materials are interesting both from the point of view of basic physics as well as technological applications. Preparation of nanocomposites is necessary to facilitate investigation of the effect of small size of a material on various physical properties. The method of preparing composites of nanosized metal particles dispersed in a glass matrix by the conventional melt-quench technique is reviewed. The versatility of sol-gel method in making nanocomposites is brought out. Results obtained on the metal organic route to certain glass-metal nanocomposites are described. The effectiveness of the ion exchange reaction followed by reduction treatment in the synthesis of ultra fine metal particles in a glassy environment is stressed upon. Finally, the glass-in-glass nanocomposites prepared by an ion exchange process and possessing very high electrical conductivity are discussed.
1 citations
TL;DR: In this article, the microstructures and microhardness of nanocomposite coatings of Al matrix reinforced by Mo particles (grain diameter ranging from 7 to 20 nm) are reported.
Abstract: A brief study of the microhardness of nanocomposite coatings fabricated by a new technique is reported. The new fabrication technique utilizes sputtering at high pressures in a thermal gradient to produce nanometer-size particles, which are then embedded in a matrix produced by conventional sputtering. The microstructures and microhardness of nanocomposite coatings of Al matrix reinforced by Mo particles (grain diameter ranging from 7 to 20 nm) are reported.
1 citations
TL;DR: In this paper, the basic features of nanocomposites are low dimensionality of their composition components, widespreadness of the electronic interactions between the components and the great variety of microstructure of nanometrics ranging from high level ordered 3-dimensional periodic structure to stochastically dispersed medium of superfine particle.
Abstract: Fundamental principles of nanocomposites are discussed. The basic features of nanocomposites are low dimensionality of their composition components, widespreadness of the electronic interactions between the components and the great variety of microstructure of nanocomposites ranging from high level ordered 3-dimensional periodic structure to stochastically dispersed medium of superfine particle. All these offer much more potential for tailoring the property of materials than by formation of chemical compound or mixture including microcomposites. Vapour deposition enhanced by ion beam and electrical discharge plasma provides a class of versatile processes both for preparing low dimensional components of nanocomposites and for synthesizing nanocomposite film itself. Elements of designing nanocomposite thin films and their preparing by ion beam and plasma enhanced vapour deposition are also discussed. Examples of nanocomposite films are given.
TL;DR: In this paper, a brief study of the fabrication of granular materials by high-pressure sputtering is presented, which utilizes sputtering at high pressures (p > 100mTorr) in a thermal gradient to produce nanoscale particles, which are then embedded in a matrix by normal sputtering (p ∼ a few mTorr).
Abstract: A brief study of the fabrication of granular materials by high-pressure sputtering is presented. This method utilizes sputtering at high pressures (p > 100 mTorr) in a thermal gradient to produce nanoscale particles, which are then embedded in a matrix by normal sputtering (p ∼ a few mTorr). The shape, size and the degree of aggregation of these nanoscale crystals can be changed by varying such processing parameters as the sputtering gas pressure and the target voltage. Examples are presented of nanocomposite films containing Mo nanocrystals (grain size ranging from 3 to 20 nm ) in an Al matrix.
TL;DR: In this paper, the dispersion is no longer a solid material but rather an element and the matrix is composed of materials (matrix) and elements (dispersion), which are various types of crystal structures, as well as certain preferred orientations, dispersion states, etc.
Abstract: Composites are composed of a matrix and a dispersion. Studies are underway to develop "nanocomposites" which have much smaller dispersion size in the order of nanometers. Here, the dispersion is a solid material. In recent years much research has focused on the development of new composites, termed "fine-composites", prepared by the in-situ CVD technique. In these composites, dispersion is no longer a solid materials but rather an element. "Fine-composites" are composed of materials (matrix) and elements (dispersion). Elements are various types of crystal structures, as well as certain preferred orientations, dispersion states, etc. The Y-Ba-Cu-O superconducting oxide films prepared by CVD are fine-composites composed of matrix of the (001) oriented YBCO and dispersions of the (100) oriented grains and very small Cu-rich flakes. Functionally gradient materials are also introduced as an example of fine-composite.