Spherical particles of several iron oxides have been obtained by pyrolysis of an aerosol generated by ultrahigh frequency spraying of a solution. Two different precursor solutions were used to produce the aerosol: Fe(NO3)3 · 9H2O and FeC6H5O7 · 4H2O. The iron nitrate decomposition leads to the formation of α-Fe2O3, while the organic precursor pyrolysis leads to magnetic particles of either γ–Fe2O3 or Fe3O4. Structural phase, particle size, texture, and homogeneity of the products so obtained can be controlled as a function of some synthesis conditions such as precursor solution, temperature, solution concentration, and surrounding atmosphere.

The structure and mechanical behavior of the fiber/matrix interface in Ti alloy/SCS-6 SiC metal matrix composites were studied. In these composites the interface region consists of a fiber-coating region and a metal reaction zone between the SiC fiber body and the metal matrix. The fiber coating consists of a number of zones or layers which are comprised of cubic SiC particles in a turbostratic carbon matrix. Some ambiguity remains, concerning the number of distinct layers and the size, shape, and density of the SiC particles. The effect of composite fabrication and heat treatment on the coating structure is relatively small. Studies of the metal reaction zone adjacent to the fiber in Ti alloy/SCS-6 SiC MMC's have shown that a number of discrete zones or layers form. Nearest the fiber, a zone of cubic TiC occurs, with increasing grain size with distance from the fiber. Nearest the metal matrix, a zone of Ti5Si3 forms. In high Al content alloys, an intermediate zone forms that consists of Ti2AlC or Ti3AlC. The fiber/matrix interface plays an important role during transverse tensile loading of these composites. The tensile behavior is controlled by debonding at the interface, followed by deformation of the matrix ligaments. Replica observations show that the debonding initiates and propagates within the coating layers, but is not confined to a single layer interface.

GeSe2 can exist in both amorphous and crystalline phases. Although most semiconductor devices are constructed from crystalline materials, the use of amorphous materials in devices has high potential. The study of GeSe2 is especially interesting since it has been established that an amorphous-to-crystalline transition can be induced by laser irradiation. In order to better understand this phenomenon, it is necessary to study the microstructure of GeSe2 glass. Therefore, transmission electron microscopy studies were undertaken to investigate the degree of crystallinity of GeSe2 glass. It was found that small microcrystallites with diameters in the range of 100–300 Å were embedded in a glass matrix. These microcrystallites formed larger clusters in some areas.

MoSi2-based composites have been synthesized through the mechanical alloying (MA) of elemental molybdenum and silicon powders with and without carbon additions. The interplay between the phase formation sequence in the powders and the microstructural evolution in the consolidated samples is described. It is shown that the glassy SiO2 phase characteristic of conventional powder processed MoSi2 can be effectively eliminated by combining mechanical alloying, carbon additions, and an in situ carbothermal reduction reaction. Using this approach, composites consisting of uniformly distributed micron-size SiC in an MoSi2 matrix can be formed. The effect of important processing variables such as the extent of carbon additions, extraneous iron pickup during MA, partial pressures of oxygen, consolidation temperatures, and consolidation atmospheres is discussed based on the evidence obtained from DTA, TGA, TEM, and XRD.

Functionally Gradient Materials (FGM's) are soon to be used in a variety of important commercial applications; joining and thermal barrier coatings are two of the most widely studied. FGM's of the TiC/NiAl and the TiC/Ni3Al systems were fabricated using a one-step, self-propagating high-temperature synthesis (SHS) and densification method. It was observed that ignition of the starting mixture for these two systems was affected by the initial sample temperature and the external pressure that was applied to the sample during the ignition stage. Quality of the final product (e.g., porosity, grain size, cracking and microcracking, etc.) depends on a number of factors during this one-step operation. Reaction temperature control is important and is necessary to minimize residual porosity of the final product. Particle size of reactant powders, as well as applied pressure, also has an effect on the resulting microstructure. If careful reaction temperature control is achieved, along with optimum reactant powder size and applied pressure, an FGM of minimal porosity is obtained without residual macrocracks. Further, this method can easily be used to fabricate an FGM with a highly precise composition and material properties gradient. Finally, this process results in FGM's of similar quality when compared to those prepared by existing fabrication methods at only a fraction of the cost. Most importantly, it is expected that this process can be scaled up with relative ease.

We have measured the real (dielectric constant) and imaginary (loss factor) components of the complex relative permittivity at 298 K using microwave frequencies (2, 10, and 18–40 GHz) for bulk SiO2-aerogels and for two types of organic aerogels, resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF). Measured dielectric constants are found to vary linearly between values of 1.0 and 2.0 for aerogel densities from 10 to 500 kg/m3. For the same range of densities, the measured loss tangents vary linearly between values of 2 × 10−4 and 7 × 10−2. The observed linearity of the dielectric properties with density in aerogels at microwave frequencies shows that their dielectric behavior is more gas-like than solid-like. The dielectric properties of aerogels are shown to be significantly affected by the adsorbed water internal to the bulk material. For example, water accounts for 70% of the dielectric constant and 70% of the loss at microwave frequencies for silica aerogels. Because of their very high porosity, even with the water content, the aerogels are among the few materials exhibiting such low dielectric properties. Our measurements show that aerogels with greater than 99% porosity have dielectric constants less than 1.03; these are the lowest values ever reported for a bulk solid material.

The fracture strength and toughness of alumina can be increased by lamination with strategically placed nickel layers and by controlling the geometry of the interfaces. This paper describes the interface design of a ceramic/metal bonded system produced by changing the surface topography of the interface between the metal and ceramic layers in order to vary the strength of bonding. The tortuosity of the interface is described quantitatively using fractal geometry. Experiments and models of single ductile layer laminates show that the work of fracture of ductile layers which contribute to the increment of toughness of ceramic/metal laminates is dependent on the tortuosity of the interface. The more tortuous the interface, the stronger the laminate; the smoother the interface, the tougher the laminate. The results are used to design a ceramic/metal multilayer composite. The strength and toughness of the laminates can be controlled by the tortuosity of the interface and characterized using the fractal dimension.

Tefzel, a copolymer of tetrafluoroethylene and ethylene, was implanted simultaneously with 400 keV boron, 700 keV nitrogen, and 600 keV carbon to a dose of 3 × 1015 ions/cm2 for each ion. The implanted layer was examined using transmission electron microscope and compared with the pristine Tefzel for microstructural changes. The microhardness of the implanted and pristine Tefzel was determined using a nanoindentation technique. TEM bright-field images of the implanted layer show a patch-type contrast with distinguishable bright and dark regions. Electron energy loss spectroscopy (EELS) was used to show that the bright regions had a higher carbon concentration, as compared with the dark regions. The carbon-rich regions had an average size of approximately 40 nm. The pristine material showed a fairly featureless contrast with occasional local patchy regions. These were determined to be due to local thickness variations. The triple implantation improved the hardness of pristine Tefzel by over 66 times. The structure of the carbon-rich regions appears to be clusters of sp2 bonded C atoms with sp3 sites present and hydrogen preferentially bonded in the sp3 C configuration. It was speculated that the carbon-rich regions could be harder than the surrounding regions, but this could not be resolved due to the small size of the regions.

In situ formation of chromium carbide in mullite matrix through the reaction of Cr2O3, SiC, and Al2O3 has been studied. Three different chromium compounds, Cr3Si, Cr3C2, Cr7C3, and mullite were formed. The Cr3Si particles formed first and the Cr7C3 phase was only dominant above 1600 °C. The Cr3C2 phase was the main product chromium carbide at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the reaction of SiO2 with Al2O3. SiO2 is the product of the reaction between Cr2O3 and SiC. The composite hot-pressed at 1450 °C in argon gives a flexural strength and fracture toughness up to 407 MPa and 3.8 MPa m1/2, respectively.

The synthesis of ultrafine tungsten carbide powder from WCl6–C2H2-H2 mixtures was investigated. Under most experimental conditions, a mixture of α–WC and β–W2C was produced. The specific surface area and morphology of the product prepared under various reaction conditions were determined. The product particle size increased with temperature up to 1373 K and then decreased, went through a minimum with increasing WCl6 partial pressure, and decreased with increasing H2 flow rate. The specific surface area of the product showed the opposite trend in all cases.

Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of Al4C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.

Conductivity relaxation spectra of sol-gel-derived glasses in the system Li2O (1 – x) SiO2, with x varying from 0.15 to 0.25, have been analyzed using a heterogeneous conductor model. The latter comprises diagonal layers of phases, one of them being rich in alkali ions and the other deficient in the same. Satisfactory agreement between the experimental data and the theoretical curves has been obtained over the frequency range 100 Hz to 1 MHz. The Kohlrausch–Williams–Watts (KWW) exponent, β, has been calculated from the conductivity spectra for all the glasses. The sol-gel-derived glasses have β values smaller than those obtained in the case of the corresponding melt-quenched ones. The β value is found to be correlated to the conductivity fluctuation in the alkali-rich phase.

The effectiveness of several coating systems which were used as a diffusion barrier for the SiC fiber-reinforced titanium aluminide composites was investigated. TaC, TiC, TiB2, B, C/graded TiB2, and Ag/Ta were applied to the SiC fiber via chemical vapor deposition and physical vapor deposition. The interfacial compatibility, interfacial stability, thermal residual stress, interfacial bond strength, and the transverse fracture behavior of the composites with coated fibers were characterized and determined. The results show that none of the above coating systems can satisfy the requirements for a strong, tough, and damage-tolerant SiC fiber-reinforced titanium aluminide composite. Several multilayer, multifunctional coating systems are proposed.

Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 μm for a film containing 45 wt.% PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite materials.

Powder samples of pure anatase were produced using laser-induced pyrolysis of titanium alkoxides, and the catalysts were prepared using conventional wet impregnation methods. The diffraction patterns were interpreted in microstructural terms by Fourier analysis of their peak profiles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500° and 550 °C. For the reflections of the anatase phase, a decrease of their Bragg (2θ) positions was observed up to 550 °C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the highest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The corresponding microstructural parameters indicate that, in all cases, the transformation is accompanied by an increase of the crystallites and/or of the lattice perfection. The evolution of these parameters is influenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfecting by thermal treatments. Only the transition to rutile is capable of overcoming that restraint by allowing crystallite growth at the expense of the smaller and distorted anatase crystallites.

The effect of solid state heat treatment at 913 K on extruded XD Al/TiC metal matrix composite with 0.7 and 4.0 μm particle sizes has been investigated. The interfaces between Al and TiC after extrusion were atomically abrupt, as observed by HRTEM. On holding at 913 K, the composite with submicron particle size showed substantial changes in the phases present due to reaction between Al and TiC at 913 K. The stable reaction products are Al3Ti and Al4C3. A substantial increase in Young's modulus occurs. The room and elevated temperature strength and hardness of the composite with submicron particles also increase significantly with time of heat treatment, but at the expense of ductility. The effect of heat treatment over the time range investigated is limited to the interfaces for the 4.0 μm TiC particle size composite due to longer diffusion paths.

Natural Na-montmorillonite was cation exchanged for the ammonium cations of various ω-amino acids [H3N+(CH2)n−1 COOH, n = 2, 3, 4, 5, 6, 8, 11, 12, and 18]. X-ray diffraction (XRD) results suggested that the chain axes of ω-amino acids with a carbon number of eight or less were parallel to the silicate layers, and that the chain axes of those with a carbon number of 11 or more were slanted to the layers. The cation-exchanged montmorillonites form intercalated compounds with ∊-caprolactam at 25 °C. The montmorillonites intercalated with both ω-amino acid and ∊-caprolactam were studied by XRD measurement at room temperature and 100 °C. We propose a model where amino acid molecules were arranged perpendicular to silicate layers and ∊-caprolactam molecules filled the space between them. When the ∊-caprolactam was melted at 100 °C, the basal spacing for the montmorillonite increased, in which the carbon number exceeds 11. This phenomenon will be applicable to obtaining the nylon 6-clay hybrid, a molecular composite of nylon 6 and montmorillonite.

Vanadium nitride has been synthesized with a surface area of 120 m2 g−1 by temperature programmed nitridation of a foam-like vanadium oxide (35 m2 g−1), precipitated from vanadate solutions. The nitridation reaction was established to be topotactic and pseudomorphous by x-ray powder diffraction and scanning electron microscopy. The crystallographic relationship between the nitride and oxide was {200}//{001}. The effect of precursor geometry on the product size and shape was investigated by employing vanadium oxide solids of different morphologies.

In the ordinary application of the time-lag method to the measurement of the diffusion coefficient of a gas passing through a plane sheet of an inert solid, the gas is pressurized on one side of the sheet and evacuated on the other. After decay of transients, the cumulative amount, Q(t), of gas diffused through the sheet in time, t, assumes the “time-lag” form, Q(t) = A(t – L). Measurements of the slope, A, and the intercept, L, can be used to determine the diffusion coefficient and the solubility of the gas in the solid. We have rederived this law for the case of a solid that is actively evolving this same gas at an arbitrary rate and have used it to predict the rate of outgassing of the solid upon standing. Practical applications of the theory include radioactive decay of minerals, rejection of plasticizers by plastics, and the decomposition of solid rocket propellants.

A detailed study was conducted of the microstructure and particle-matrix interfaces in Al/TiCp metal matrix composites prepared by the XD process and subsequent extrusion. A study of the morphology of the TiC particles showed that the surfaces are low index (111) and (200) planes, the former being more common. Direct contact on an atomic scale is established between Al and TiC, allowing chemical bonds to form. Young's modulus is in the range expected for a composite of Al and TiC with good interfacial bonding and load transfer to the particles. No third element has been detected at the interfaces, showing that they are clean. Both incoherent and semicoherent interfaces are seen. The interface character depends on the size of the particles and their orientation with respect to the neighboring Al grains. “Special” interfaces with evidence for nearly periodic dislocations were observed in both XD Al/TiC and Al/TiB2 composites, indicating the general tendency of in situ composites to lower their interfacial energy by forming such boundaries.