The effects of fiber surface coatings on the mechanical behavior and damage mechanisms of SCS-6 fiber-reinforced titanium aluminide matrix composites have been studied. Two different coating layers are used as model material: a brittle TiB2 and a ductile Ag/Ta duplex layer. The role of the coating layer on the interfacial reaction, interfacial properties, and mechanical behavior of the composites was characterized. Results indicate that both TiB2 and Ag/Ta are effective diffusion barriers in preventing fiber/matrix interfacial reactions during composite consolidation. However, the deformation mechanisms and crack propagation characteristics in these two coated composites are quite different. The criteria for selecting an improved interlayer to tailor a strong and tough fiber-reinforced titanium aluminide matrix composite are also discussed.

Carbon nanofibers (sometimes known as carbon filaments) can be produced in a relative large scale by the catalytic decomposition of certain hydrocarbons on small metal particles. The diameter of the nanofibers is governed by that of the catalyst particles responsible for their growth. By careful manipulation of various parameters it is possible to generate carbon nanofibers in assorted conformations and at the same time also control the degree of their crystalline order. This paper is a review of the recent advances made in the development of these nanostructures, with emphasis both on the fundamental aspects surrounding the growth of the material and a discussion of the key factors which enable one to control their chemical and physical properties. Attention is also given to some of the possible applications of the nanostructures which center around the unique blend of properties exhibited by the material.

A new family of organic-inorganic hybrid materials has been prepared by incorporating polystyrene structure units covalently into the SiO2 glass network via the sol-gel approach. The polymer precursors were synthesized by free-radical copolymerization of styrene with 3-(trimethoxysilyl)propyl methacrylate (MSMA) at various feeds. These copolymers were then hydrolyzed and co-condensed with tetraethyl orthosilicate in tetrahydrofuran at room temperature to afford monolithic polystyrene-SiO2 hybrid sol-gel materials having SiO2 contents of 15 to 84% by weight. The hybrid materials derived from the copolymers with MSMA contents greater than 22 mol% have excellent optical transparency. In these transparent hybrid materials, the polymer chains should be uniformly distributed in and covalently bonded to the amorphous SiO2 matrices. The bulk properties of these materials including density, refractive index, and hardness were found to be related to their molecular compositions and can be tailored by varying the polymer contents.

Sol-gel methods were used to prepare chemically modified lead titanate (PT) powders. A PT alkoxide solution was synthesized and doped with Si (2–12 mole%) or with equimolar amounts (2–12 mole%) of Si and Pb through the addition of Si and Pb–Si alkoxide solutions, respectively. PT alkoxide solutions were also prepared with excess Pb and Ti (7 and 10 mole%). Gels were prepared through controlled additions of water. Crystalline phase development of gel-derived powders with heat treatment (400–700 °C) was studied using x-ray diffraction (XRD) and differential thermal analysis (DTA). While PT powders without added Si crystallized directly into a perovskite phase, Si modified materials crystallized first into a pyrochlore phase and at a higher temperature transformed into perovskite. The pyrochlore lattice parameter and the temperature for the transformation to perovskite increased with Si content. In all cases, the crystal structure of the final perovskite phase was not affected by the Si addition. The effect of Si on phase development and mechanisms of transformation is discussed.

A simple atomistic approach to the mechanical strength and the fracture toughness of brittle materials is made by the use of a universal expression for binding potential energy versus atomic separation curves. The scaling factors for the atomic separation and for the energy amplitude successfully apply to describing the intrinsic fracture toughness K* in a scaled dimensionless form. It is demonstrated that the intrinsic fracture toughness combined with a stress shielding coefficient (SSC) yields the fracture toughness of real materials. Microfracture mechanisms for crack-tip stress-shielding processes, as well as the interrelationship between the stress intensity- and the potential energy-derived fracture toughness, are addressed.

The pest of monolithic poly- and single-crystalline molybdenum disilicide, as well as its composites, has been investigated by low temperature oxidation in air at temperatures ranging from 350 to 700 °C. Pest phenomenon (i.e., disintegration from bulk into powders) was consistently observed from samples oxidized at temperatures between 375 and 500 °C. In contrast, samples oxidized at 550 °C exhibited only severe cracking, and samples oxidized at 350 °C or at/above 600 °C were intact. The pested samples resulted in powdery products consisting of MoO3 whiskers, SiO2 clusters, and residual MoSi2. The MoO3 whiskers exhibited protruding characteristics, and were highly concentrated at microstructural heterogeneous sites, such as interparticle boundaries, grain boundaries, and cracks. Blisters were observed and found to be formed predominantly on the grain boundary and interparticle boundary interfaces of tested samples. These blisters were often found to be erupted, indicating that significant vapor pressure was built up underneath the sample surface. Interrupted oxidation tests at 500 °C revealed that a substantial volume change also occurred. The development of the pest of MoSi2 was found, macroscopically, to consist of nucleation and growth. In most cases, it initiated from some local microstructural inhomogeneities, and propagated throughout the samples. Based on the morphologies of disintegrated powders, two different kinetic processes were identified to be responsible for the pest reaction. According to the results from single crystals, the pest of MoSi2 appeared to be kinetically controlled by the formation and volatilization of MoO3. The mechanism of MoSi2 pest is proposed and discussed. Thermodynamic analyses are also presented to substantiate the experimental observations.

Titanium (iv) oxide (amorphous, brookite, anatase, and rutile) fine particles have been synthesized by pyrolysis of an aerosol. As-received materials are constituted by hollow balloons of around 1–2 μm formed by very small crystalline nuclei, whose crystal size can be modified by annealing. By heating at 900 °C for 48 h individual crystals of around 1 μm are obtained. Intermediate sizes can be controlled as a function of the thermal treatment.

Vickers indentation sites in ceramics have been examined after specimen failure from median/radial indentation cracks. Evolution of a new cracking pattern of “ortho-lateral” cracks, originating at the intact corners of the Vickers indentation and running orthogonal to the classic-lateral cracks and parallel to the new fracture surface, has been observed. In some instances postfailure extension of the classic-lateral cracks toward the surface was also observed. Enhanced residual tensile stress from relaxation of constraints on the indentation-plastic cavity by the generation of a fracture surface is postulated to drive the subsidiary cracking. A simple qualitative model to explain this phenomenon is presented. Possible implications of such postfailure subsidiary cracking on residual-stress-driven flaws, postmortem fractography, and wear in ceramics are discussed.

Aged alumina refractories have been investigated by EPR (Electron Paramagnetic Resonance) and EPMA (Electron Probe Micro-Analysis). EPMA shows that Cr is homogeneously distributed inside the corundum grains within spatial resolution (1 μm). Cr3+ concentration at the atomic scale is obtained by EPR by the magnitude of dipolar broadening due to Cr-Cr interactions. This local concentration appears to be systematically higher (up to a factor of 10) than the average chromium content of the corundum crystals determined by EPMA. Cr3+ ions may be preferentially concentrated at the vicinity of extended lattice defects which help in accommodating the lattice strain due to the size difference between Al3+ and Cr3+ cations. A random location of these defects explains the apparent homogeneous chromium distribution found by EPMA in these aged refractories.

Mathematical models emulating DSC/DTA traces of fusion and first-order decomposition were developed. A computer program is described that uses the simplex numerical optimization algorithm to determine the coefficients for the model equations that best fit experimental data, by the criterion of least-squared error. Use of the program permitted deconvolution of various superimposed endotherms.

Tungsta-titania samples with different W loadings up to 15 wt.% and calcined at different temperatures have been prepared and characterized by surface area measurements, mercury porosimetry, x-ray diffraction, microstructural analysis, and laser Raman spectroscopy. It has been found that W inhibits the initial sintering of TiO2 (anatase) and the anatase → rutile transformation. The morphological and structural properties of the samples (surface area, porosity, and phase composition) have been related to the microscopic properties of the materials such as crystallite dimensions and defects concentration. A model for the sintering of TiO2 is discussed. This model is based on the diffusion of surface hydroxyls, formed upon adsorption of water on surface oxygen vacancies. A role for W is proposed in terms of stabilization of both material defects and surface hydroxyls.

Glasses containing semiconductor particles coated with metal were synthesized and studied. Glasses containing CdS and silver were heated to grow the CdS particles first, followed by the growth of metal on them. The solubility of silver in the CdS glass was increased by changing the base glass composition. Optical absorption and transmission electron microscopy results showed that silver coated the CdS particles in these glasses. The CdS particles had to be a critical size before silver grew on them. This critical size was calculated from nucleation theory and was found to be in good agreement with the experimental results. Glasses containing CuCl particles were partially reduced in hydrogen to obtain copper-coated CuCl particles. Absorption spectroscopy and TEM indicated a copper coating on the particles.

The experimental study of wave attenuation in concrete has been achieved in the case of the propagation of plane waves in concrete rods. Different mortars and concretes have been investigated. A transmitter transducer coupled to one of the ends of the concrete rod generates the propagation of a plane wave in the rod. The receiver transducer, similar to the previous one, is coupled to the other end of the rod. The experimental results lead to an analytical expression for wave attenuation as function of the concrete composition, the propagation distance, and the wave frequency.

The Maxwell–Garnett model for isotropic chiral spherical inclusions in free space has been briefly reviewed, and pertinent results for the effective intrinsic and extrinsic properties of the composite medium, along with useful Taylor expansions, have been obtained in the Drude–Born–Fedorov representation. It has been shown that this model does not yield the chirality parameter of the composite independently of the permeability and the permittivity, and treats the permeability and the permittivity as duals of each other. Finally, even if the inclusions are nonmagnetic, the composite medium may not be necessarily so. It is anticipated that the formulae derived here will not only assist in the formulation of more rigorous multiple scattering theories, but will also aid designers of chiral composites.

“Chemical vapor infiltration of SiC with microwave heating” [J. Mater. Res. 8, 1057 (1993)]

Fine particles of mixed oxides having the composition MgxFe2−2xO3−2x (x between 0 and 1.0) were prepared via the sol-gel supercritical drying route (aerogels). Samples were then postprocessed by heating in air at 773 K and 1073 K. The structure and chemistry of the samples were deduced through x-ray diffraction, thermal gravimetric and differential thermal analysis, infrared, and specular UV-visible measurements. Surface areas were measured by the BET method. The volume saturation magnetization and particle volume distribution of MgFe2O4 samples were deduced from vibrating sample magnetometer measurements. The samples with x = 0 and x = 0.5 were well-crystallized α–Fe2O3 (hematite) and MgFe2O4 (magnesioferrite), respectively. Samples with 0 < x < 0.5 consisted of disordered lacunar spinels containing an excess of Fe3+ with respect to the spinel stoichiometry and a small amount of Fe2+. Samples heated above 773 K showed progressive segregation of α–Fe2O3. The sample with x = 0.66 was a mixture of a spinel phase and a hydrotalcite-like mixed Fe–Mg hydroxy compound containing methoxy and acetate ions. By heating in air, a monophasic Mg-excess spinel-type structure was obtained. The samples with 0.66 < x < 0.86 were mixtures of a spinel-type phase and another mixed Mg–Fe compound. When heated in air, the samples gave a mixture of MgO (probably containing Fe ions) and MgFe2O4. The saturation magnetization of MgFe2O4 samples is comparable to those previously reported, with the as-produced sample having a mean particle radius of 5.5 nm.

Alumina aerogels were prepared using supercritical drying methods, and their thermal properties were examined. The effects of several additives to the alumina aerogel and supercritical drying methods were examined in order to improve heat resistance. Silica, phosphoric oxide, barium oxide, lanthanum oxide, and SiC whisker were effective for maintaining a high specific surface area of the alumina aerogel at a temperature over 1200 °C. Silica was found to be the most effective among the additives. The addition of 10 mol % silica resulted in an alumina aerogel with the highest specific surface area, 114.3 m2/g at 1200 °C, and increased the transformation temperature to α alumina. Barium oxide and lanthanum oxide formed smaller crystals within the alumina, compared with those of alumina alone. SiC whisker caused many cracks in the alumina aerogel. When supercritically dried, the alumina aerogel was strengthened by treatment at higher temperature and pressure.

High-pressure phase transformation of beryllium gallium oxide (BeGa2O4) has been studied. Applying high pressure at elevated temperatures to the original hexagonal BeGa2O4 (β–Si3N4-type structure), a high-pressure modification with orthorhombic structure (olivine-type structure) was obtained, i.e., o-BeGa2O4. Lattice parameters of the new phase were determined to be a = 0.5698, b = 0.9759, and c = 0.4551 nm. The pressure and temperature ranges where the high-pressure phase was observed are 3.5 to 7.5 GPa and 800 to 1600 °C, respectively. A tentative pressure-temperature phase diagram for BeGa2O4 was proposed. Transformation is not straightforward; decomposition of the original phase into single oxides and their recombination to form o-BeGa2O4 are necessary. This process seems to apply in both ways, formation and decomposition of the high-pressure phase. The stability of the high-pressure phase is explained in terms of the total molar volume for the phase, the result of summing up molar volumes of constituent compounds. This is the first known report on transformation of β–Si3N4-type structure into a denser structure.

A high-temperature polymer electrolyte based on lithium-doped pyromellitic dianhydride + Oxybis(aniline)/2,5-diaminobenzenesulfonic acid (PMDA-ODA/DABSA) copolymer has been synthesized and characterized by ac impedance analysis and differential scanning calorimetry. Its dc conductivity was found to reach 10−8 S/cm near 60 °C at ambient relative humidity and was confirmed to be of ionic nature. Besides the increase of the conductivity achieved by moisture uptake, the doping of lithium was found to yield enhancement of the ionic transport in the absence of water molecules as well.

The interaction of cobalt (Co) and low-pressure chemical-vapor-deposited silicon nitride (LPCVD Si3N4) during anneals from 200 °C−1000 °C in vacuum, Ar, and Ar–H2 ambient (95% Ar and 5% H2) has been studied. After the anneals, reduction of Si3N4 by Co to form cobalt silicide and cobalt nitride phases has been observed. Reduction of Si3N4 initially occurs at 600 °C; however, gross physical damage occurs at temperatures of ∼900 °C in Ar. The addition of hydrogen to the ambient enhances the onset of physical damage to the nitride film by as much as 200 °C. Mechanisms governing the Co/Si3N4 reaction have been proposed.