Nanometer-sized molecular complexes of superexchange-coupled iron ions with iron nuclearity n < 10 exhibit short-range intramolecular magnetic ordering and ‘particlelike’ superparamagnetism in their low temperature Mössbauer spectra. A comparative study of the magnetic properties of ca 10 Å diameter iron-oxo phases obtained through controlled hydrolytic polymerization of iron in non-aqueous solvents is presented. The magnetic anisotropy energy constant, K, which is proportional to the observed blocking temperature, TB, and the sharpness of the superparamagnetic transition depend critically on the iron oxidation state and degree of 3d-electron delocalization. Representative examples of mixed-valent iron complexes containing oxo-bridged, octahedrally coordinated iron with primarily either Fe(III) or Fe(II) character are discussed.

A nanocomposite of clusters of gold atoms in a silica gel matrix has been prepared by the sol-gel technique through reduction of gold chloride. This composite was fabricated to explore novel techniques for materialsengineering new high dielectric strength substances for capacitor applications. The gold islands amidst the silica gel sea are materials-designed to utilize the percolation threshhold phenomenon to create a peak in the dielectric

constant as a function of optimal topology. Experimental results indicate an average dielectric constant of the order of 5000 at 1 kHz, and a maximum versus temperature at ∼ −100°C. Electron microscope and microprobe analysis indicate ellipsoidal Au particles or clusters of a dimension ranging from a hundred angstroms to several thousand angstroms.

Based on the electrokinetic properties of aqueous silica, boehmite, and ZrO2 dispersions, cordierite-ZrO2 composites were fabricated by the multiphasic sol-gel (or nanometer-scale sol mixing) route. The average particle sizes of boehmite and ZrO2 used in this study are 70 nm and 40 nm, respectively. The fabricated composites were characterized by a dense and homogeneous microstructure and by a uniform spatial distribution of submicron-sized tetragonal ZrO2 particles (200 nm) throughout the matrix. The microstructure of the matrix was represented by small-sized grains (< 400 nm) with a highly uniform grain-size distribution. The enhanced fracture toughness with increasing ZrO2 content was partly attributed to the martensitic t → m transformation. The formation of detrimental zircon (ZrSiO4) was avoided by suitably adjusting the heating schedule during sintering.

Hybrid siloxane-oxide systems were prepared by sol-gel techniques. Siloxane precursors are either monomeric species, dimethyldiethoxysilane, or OH-terminated polydimethylsiloxanes. The oxide component is introduced via metallic alkoxides, M(OR)n (M=Si, Ti, Zr). Transparent materials from flexible to brittle gels, can be obtained over a large M/siloxane range. The structure of the various systems was investigated by 29Si MAS-NMR. The signals due to the difunctional Si units, (CH3)2SiO, depend strongly on the nature of M and on the M/siloxane ratio. Structural models are proposed, ranging from single phase systems with highly interconnected siloxane and oxide units, to nanocomposites made of polysiloxane chains crosslinked with oxide-based particles.

Living organisms make use of inorganic materials such as calcium carbonate and phosphate, silica, iron oxide etc. to construct support structures, protective armour, magnetic devices gravity devices and so on. The structures are remarkably adapted for the duties they have to perform, the adaptation being at all levels from the nanometer to the macroscopic. Microscopic detail is controlled in order that the macroscopic structure forms an efficient functional component of a machine and the structure develops in response to signals from the local environment. The study of these biogenic minerals and the mechanism of their formation can inspire new approaches to the synthesis and design of new materials and to the advantageous modification of old materials. Examples are given in this paper.

Hard siliceous coatings on polyethyleneterephthalate (PET) surfaces have been prepared upon hydrolyzing a tetraisocyanatosilane permeant present in biaxially oriented PET film samples. The structure of these films differs from commercial PET film materials in that silica is present both as a surface layer and as discrete submicron particles dispersed throughout the films. In order to accomplish this, it was found that suitable silica precursors should be chemically compatible with the polymer and have calculated Hildebrand solubility parameter values similar to that for PET.

Composite materials may be prepared by bio-mimetic routes where reinforcing particles are grown by chemical reaction in a polymer matrix, by analogy to the mineralization of collagenous bone matrix by hydroxyapatite. Ideally these resulting composites should resemble bone in having sub-micron, elongated particles packed to high densities in the matrix. The resulting material should have good mechanical properties and may also be fired to a dense ceramic.

We have been exploring the control of the precipitation of oxide ceramics, particularly titania, by in situ hydrolysis of metal alkoxides. The size of the precipitate particles can be controlled by variation of the compatibility of polymer and alkoxide. A high level of compatibility leads to particles with sizes below 0.1 microns while less compatible alkoxides give particles up to 1 micron diameter. The kinetics of the hydrolysis also affect the particle size. Elongated and oriented particles can be produced by deformation of the polymer during precipitation. Two-phase polymer blends can be used to provide nanoscale molds within which elongated particles grow. By controlling the scale of phase separation in the blend it is possible to produce rod-shaped particles with diameters in the range from one micron down to a few nanometers.

Truly nanocomposite materials that are stable to about 400 to 700°C can be prepared by intercalating oxides or metal clusters of about 0.4 to 2.0 nm in between ∼1.0 nm layers of smectite clays. Both the chemistry and size of intercalates (pillars) can be varied to introduce unique catalytic, molecular sieving, dehumidifying and adsorption properties in these materials. The intercalated clays also provide opportunities to prepare compositionally and stoichiometrically diverse nanocomposite precursors to high temperature structural and electronic ceramics. Although montmorillonite is the most widely used host, further designing in properties can be achieved by using other members of smectite family having subtle crystal chemical and compositional variations, such as beidellite, nontronite, saponite or hectorite. The sol-gel chemistry involving the preparation of positively charged mono- or multiphasic solution-sol or colloidal-sol particles is a viable approach to introduce chemically diverse oxide particles in the interlayers of smectite. Reduction of transition metal ions or complexes in the interlayers of smectite to zerovalent metal clusters/particles using polar liquids is another novel approach to develop catalytically active, high surface area materials.

Layered inorganic-organic nanocomposites can be prepared by intercalation of organic compounds into various two dimensional inorganic compounds, such as clay minerals and layered polysilicates. Two different roles of organic guest species are described in the application of intercalation compounds to development of functional materials. The first case shows that organic substances with peculiar photochemical properties can be incorporated in the nanometer—scaled interlayer spaces. Luminescence behavior and photochemical hole burning (PHB) of layered inorganic—organic nanocomposites are investigated from the viewpoint of host-guest interactions. The luminescence maxima of intercalated Ru(bpy)32+ incorporated into fluor—tetrasilicic mica cointercalated with poly(vinylpyrrolidone) shifted gradually oward blue with the decrease in the loading of Ru(bpy)32+ Ru(bpy)32+ was effectively isolated to suppress self quenching due to aggregation even at its high concentration loading. 1,4-Dihydroxyanthraquinone which is known to show a PHB reaction was intercalated into a tetramethylammonium pillared clay mineral and persistent spectral zero—phonon hole was observed at liquid helium temperatures. In spite of the high concentration of 1,4-dihydroxyanthraquinone, a narrow hole was obtained with no distinct decrease in burning efficiency if compared with those doped in ordinary polar polymers or organic glasses. In the second case, intercalation of organic substances into a layered polysilicate induces the change of the layered structure to form a three—dimensional silicate network by condensation of silanol groups in adjacent layers. This phenomenon indicates that novel microporous materials with controlled pore sizes can be prepared through inorganic-organic nanocomposites.

A.c. conductivity of a novel large-pore alumina-pillared zirconium phosphate and some lithium ion exchanged samples have been measured by an impedance method. These materials have a conductivity in the range 10-5 to 10-9 Ω-1cm-1 higher than those of alumina-pillared tin phosphate and its lithium derivatives. The electrical behaviour of the pillared zirconium phosphates fits to an equivalent circuit composed by two subcircuits in parallel with a condenser. In a temperature interval (200-500°C), lithium ions are charge carriers and the conductivity increases when heating with activation energies between 0.99 and 1.22 eV.

The cavities of molecular sieves offer an opportunity for synthesis of periodically ordered semiconductor nanoclusters. Ge clusters were synthesized in zeolite Y by thermal decomposition of GeH4 absorbed in the zeolite cages. Upon inclusion of germanium in proton exchanged zeolite Y (HY), the X-ray diffraction (XRD) pattern shows a small change in line positions and considerable change in intensities. No extra reflections were detected in either XRD or transmission electron diffraction (TED) for the samples synthesized at 300°C. These observations are in accordance with the energy dispersive X-ray (EDX) spectroscopy data which reveal the presence of germanium within the HY in concentrations higher than 5x1019 cm-3. At annealing temperatures higher than 500°C, small Ge crystallites (∼0.2μm) form on the exterior surface of the HY, as documented by TED, TEM and absorption spectroscopy. The results from the absorption and photoluminescence spectroscopy of the Ge loaded HY will be discussed.

We have imaged fractured cross-sections of electrodeposited ceramic oxides based on the TI-Pb-O system using a scanning tunneling microscope. The goal of this work is to measure both the modulation wavelength and compositional profile of the superlattices by mapping out the electronic properties in real space on a nanometer scale. Fourier analysis was done on STM images of all superlattices to yield the modulation wavelength. The modulation wavelength from STM was then compared with those obtained, by Faraday calculation and x-ray diffraction. The STM can be used to design “better” superlattices. We have found that the composition profile in superlattices deposited by modulating the potential was more square than in superlattices deposited by modulating the current.

We have studied the structure of W-Cu multilayers with modulation wavelengths between 65 and 110 xsÅ over the temperature range 25-400° C. Using a high temperature diffractometer stage specifically designed for low angle work, thermal expansion coefficients were measured and found to be marginally greater than would be expected from bulk behavior even when interaction with the substrate is taken into account. Upon annealing at temperature as low as 180° C, increased intensity of the low angle superlattice peaks is observed. Heat treatments above 180° C result in an irreversible change in the multilayer associated with the migration of Cu atoms to cracks produced by thermally induced stresses.

The structure, hardness and elastic properties, and oxidation behavior of a nanolayered MoSi2/SiC structure were examined. DC magnetron and rf diode sputtering were used to deposit MoSi2 and SiC, respectively. The total number of sublayers of each kind was 90. The nominal thickness of the MoSi2 sublayers was 10 and that of the SiC 3 nm. The microstructure of the as-deposited samples was amorphous whereas annealing at 500 °C for I h resulted in the transformation of MoSi2 into C40 type hexagonal structure as determined using cross-section transmission electron microscopy (TEM). However, silicon carbide still remained amorphous. Hardness and Young's modulus were determined by using a nanoindentation technique. Hardness of 11.6 and 20.8 GPa were obtained for the asdeposited and annealed structure, respectively. Corresponding Young's moduli were 220 and 290 GPa, which yield values of 232 and 332 GPa for Young's moduli of amorphous and crystalline MoSi2. Oxidation tests were carried out in wet oxidation conditions at 400 and 500 °C. The degree of oxidation was determined by measuring oxygen concentration on the surface using a nuclear reaction 16O(d,p) 17O at a deuterium energy of 950 keV. The MoSi2/SiC coating gave good protection against oxidation for unalloyed steel. As compared to a single MoSi2 coating a slightly higher degree of oxidation and a significantly different mechanism of oxidation was observed. This was also supported by secondary ion mass spectroscopy (SIMS) measurements. Cracking of the coating caused by a thermal mismatch and transformation stresses was worse in the case of the single MoSi2 layer than in the case of the composite coating. Thus cracking of the coating can be significantly reduced by using a nanolayered structure.

Nanocomposite films of polycrystalline nitride superlattices with nanometer grain size were deposited onto tool steel substrates by a one-step process using an opposed-cathode high-rate reactive unbalanced magnetron sputtering system. The superlattices are composed of alternating thin layers of different nitrides such as TiN/NbN and TiN/VN. The thicknesses of two neighboring layers were between 3 and 150 nm and were determined by the rotating speed of the substrate holder and the sputtering rate of the individual layer material. The films exhibited exceptional hardness as high as 5200 kgf/mm2 for TiN/NbN superlattice and 5 100 kgf/mm2 for TiN/VN superlattice. The hardnesses of the superlattice coatings were strongly dependent on several deposition parameters such as the superlattice period, the nitrogen partial pressure, and the substrate bias voltage. One of the possible mechanisms for the hardness enhancement is the effect of nanophase materials, which were created mainly by the influence of the artificially layered-structure and the low energy ion bombardment during film growth.

We describe the use of Jet Vapor Deposition for efficient synthesis of nanoscale multilayer films, ceramic-organic films, cluster embedded films, and colloids.

TiN/Diamond/TiN composite film has been deposited on tool steel substrate by CVD and laser ablation technique. A TiN film of about 0.5µm thick was first deposited as a buffer layer by laser ablation and then diamond was deposited with proper number density by CVD. After this the diamond layer was covered by a second layer of TiN. It was found that the TiN buffer layer was stable during CVD process of diamond and can chemically isolate the depositing surface from steel substrate. This prevents the reaction between the diamond and the substrate and makes it possible to deposit high quality diamond films on steel. The second TiN layer embeds the diamond particles to form a TiN/diamond composite film and therefore greatly enhances adhesion of the deposited diamond. SEM, Raman. adhesion and hardness measurements show that an extremely hard. adherent and thermalshock-resistant diamond/TiN composite film can be deposited on steel.

We have measured the anisotropy of the electron diffusivity in a series of Cu/Al multilayers. The Samples were made by magnetron sputtering and characterized by X-Ray diffraction. The low temperature transverse magnetoresistance of each sample was measured for two different orientations of the magnetic field—parallel and perpendicular to the plane of the sample film. The Weak Localization(WL) contribution to the magnetoresistance is sensitive to the component of the electron diffusivity in a plane perpendicular to the magnetic field. We use this fact to calculate the ratio of different components of the diffusivity from the observed dependence of magnetoresistance on the orientation of magnetic field. For our samples, the ratio of the in and out of plane components of the diffusivity, (Dxy/Dz), is seen to range between 1 and 2.2, and the anisotropy is largest for the sample with the highest conductivity.

The new materials design was established for the structural ceramics to break through the present monolithic and composite ceramics. In this concept, the ceramic-based composites were divided into two groups: micro and nanocomposite. The microcomposite is the so-called composite in which the micro-sized particles, whiskers, platelets and fibers are dispersed at the grain boundaries of matrix grains. The nanocomposites, on the other hand, have three types of structures: intra and intergranular nanocomposites and nano/nano composite. The intra and intergranular nanocomposites were found to show the tremendous improvement of mechanical properties even at elevated temperatures compared with those of monolithic and microcomposite ceramics. The nano/nano composites gave the new functions such as machinability and superplasticity like metals. Our final goal for developing supertough and strong ceramics was achieved by hybridizing the intra and intergranular nanocomposites with the micro-meter sized fibers. This new concept was found to be also applicable for ceramic-metal composite systems.

Displacement reactions between a metal oxide and a more reactive metal can be induced by ball milling. In some cases the reaction progresses gradually and a metal/metal-oxide nanocomposite is formed. Ball milling may also initiate a self propagating combustive reaction. The information available about these processes is reviewed. It is argued that the gradual or combustive nature of the reaction depends on thermodynamic parameters, the microstructure of the reaction mixture, and the way they develop during the milling process.