The mechanical behavior of intermetallic alloys is related to the mobility of the dislocations found in these compounds. Currently the effect of bonding on dislocation core structure and its influence on deformation behavior is not well understood. However, the unusual properties of these materials, such as the anomalous temperature dependence of flow stress observed in TiAl, are derived in part from the aspects of bonding that determine dislocation mobility. Several recent studies have suggested a particular relationship between directional bonding in TiAl and dislocation mobility. To understand better the flow behavior of high temperature intermetallics, and as a step toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using a layered Korringa Kohn Rostoker (LKKR) method which embeds the fault region between two semi-infinite perfect crystals. Calculated defect energies in stoichiometric TiAl agree reasonably well with other theoretical estimates, though overestimating the experimental (111) anti-phase boundary (APB) energy, found for Ti46Al54. We approximate the energy of the (111) APB for the Al-rich stoichiometry by calculating the energy of Al antisites near that defect plane. The calculated (111)APB energy decreases by 6% in going from stoichiometric TiAl to Ti46Al54. The overall hierarchy of fault energies is found to be associated with the number of crystal bond states that are disrupted by the introduction of the fault plane. However, the hierarchy of fault energies is inconsistent with the traditionally accepted ordering. Changes in bonding taking place in the vicinity of the planar defects are illustrated through the density of states and charge density plots. A three body atomistic model is introduced to parameterize the bonding observed in TiAl. The L10 lattice (c/a = 1.00), within a second nearest neighbor three body model, yields a (111)APB energy which is the sum of the complex and superlattice-intrinsic stacking fault energies.

Nanocrystalline fcc metals have been synthesized by mechanical attrition. The crystal refinement and the development of the microstructure have been investigated in detail by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The deformation process causes a decrease of the grain size of the fcc metals to 6–22 nm for the different elements. The final grain size scales with the melting point and the bulk modulus of the respective metal: the higher the melting point and the bulk modulus, the smaller the final grain size of the powder. Thus, the ultimate grain size achievable by this technique is determined by the competition between the heavy mechanical deformation introduced during milling and the recovery behavior of the metal. X-ray diffraction and thermal analysis of the nanocrystalline powders reveal that the crystal size refinement is accompanied by an increase in atomic-level strain and in the mechanically stored enthalpy in comparison to the undeformed state. The excess stored enthalpies of 10–40% of the heat of fusion exceed by far the values known for conventional deformation processes. The contributions of the atomic-level strain and the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth in the nanocrystalline fcc metals are investigated by thermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis, and estimates of the grain boundary enthalpy are given.

A microtwinned monoclinic phase is observed to occur within the grains of the hard magnetic phase in Nd2Fe14B-based permanent magnet alloys. This phase seems to be the product of a displacive solid state phase transformation of tetragonal Nd2Fe14B, distinguished from Nd2Fe14B by the presence of an ordered substitution into alternating unit cells parallel to the crystallographic c-axis and by a small shear of the type 〈100〉 {011}. The four different orientational variants of this monoclinic phase with respect to the parent tetragonal phase are arranged into two distinct colonies of twin-related plates. The observation that Nd2Fe14B can undergo a displacive solid state transformation has broad implications for the entire field of Nd2Fe14B permanent magnets.

Effects of surface roughness and crystallinity of polycrystalline diamond films on their tribological characteristics, as well as the effects of test environment, have been investigated. Friction and wear characteristics of the diamond films deposited on sintered SiC disks have been examined with a ball-on-disk tester in the absence of any lubricant. The friction coefficients of polished diamond films against SiC and Si3N4 balls were below 0.10 at room temperature while those of as-deposited films were around 0.20. The specific wear of counterparts on the polished film was five orders of magnitude smaller than on the as-deposited film. The friction coefficient between the polished diamond film and a AISI 52100 steel ball was about 0.20. Transfer of a small amount of AISI 52100 material to the diamond film was observed along the wear track of the polished diamond surface. Diamond films of high quality were more resistant to wear than the ones of low quality. On the other hand, the friction coefficients were not affected by the crystallinity of the diamond films in the present study. Tribological characteristics of the diamond films deteriorated with increasing sliding speed and ambient temperature. At 600 °C in dry N2, the friction coefficient of diamond films against a SiC ball was about 0.8, which was about ten times higher than that at room temperature in air.

Diamond deposits of well-separated particles have been obtained by the hot filament CVD technique on Si(100) wafers. Particle counting in SEM images and determination of their linear dimensions require a separate study of growth rates and of nucleation densities as a function of time, substrate temperature (500 °C–950 °C), gas phase composition (0.5–2% CH4 in H2), and total pressure (15–76 Torr). It is shown that recent models proposed for the growth process can successfully be applied if proper consideration is given to the high catalytic activity of the growing diamond surface for the heterogeneous recombination of gaseous H-atoms. This fast reaction controls the H-atom concentration at the surface and couples growth rates and nucleation densities via the gas phase.

Raman scattering, x-ray diffraction, and BET measurements are used to study the effect of heat treatment on the microstructure of activated carbon fibers (ACFs) and to correlate the structural changes with the metal-insulator transition observed in the electronic transport properties of heat-treated ACFs. A sequence of events is identified, starting with desorption, followed by micropore collapse plus the stacking of basic structural units in the c-direction, and ending up with in-plane crystallization. The graphitization process closely resembles that depicted by Oberlin's model, except that the final material at high-temperature heat treatment remains turbostratic. Because the metal-insulator transition was observed to occur at heat-treatment temperature THT ≃ 1200 °C, which is well below the THT value (2000 °C) for in-plane crystallization, we conclude that this electronic transition is not due to in-plane ordering but rather to the collapse of the micropore structure in the ACFs. Raman scattering also provides strong evidence for the presence of local two-dimensional graphene structures, which is the basis for the transport phenomena observed in heat-treated ACFs.

This fracture study evaluates the role of a fiber/matrix interfacial glass on the toughening of two different carbon/carbon (C/C) composites. Both composites incorporate a two-dimensional layup of 8-harness satin weave continuous fiber fabric, but differ in several aspects, the most significant of which is the presence of an oxidation inhibitor in one of these. The fracture behavior of both materials was determined in three-point flexure at 20 through 1650 °C. Microstructural studies indicate that the nonhomogeneous distribution of the oxidation inhibitor within the fiber bundles controls the fracture behavior. Electron microprobe results indicate a high concentration of the glass oxidation inhibitor associated with the inter-bundle matrix, while the intra-bundle matrix is composed primarily of carbon. Accordingly, debonding along the inter-bundle interfaces characterizes the oxidation inhibited composite, whereas the nonoxidation inhibited samples debond by individual fibers. Both materials exhibit strongly rising R-curves throughout the test temperature range. At the higher test temperatures the oxidation inhibited C/C shows the greatest cumulative toughening component, although at a lower value of the fracture toughness. This is consistent with the observed increase in the percentage of fibers that experience individual pullout at the higher temperatures.

Hydrogenated amorphous carbon (a–C:H) films on stainless steel (AISI430) substrate oxidized in air at 1273 K were prepared from a gas mixture of methane and hydrogen by an rf plasma chemical vapor deposition, and thermally stimulated exoelectron emission (TSEE) was studied for the x-ray irradiated a–C:H films. Glow curves and energy distributions of TSEE from the 80- and 280-nm a–C:H films and from the AISI430 substrate have been measured under ultrahigh vacuum conditions. It was found that the glow curve from the 80-nm a–C:H film was similar to that from the AISI430 substrate, but it was quite different from that from the 280-nm film; the values of the mean energy of exoelectrons at the glow peak temperatures from the 80-nm a–C:H film are almost the same as those from the substrate but are much lower than those of the 280-nm film. The surfaces of 80- and 280-nm a–C:H films are observed with the scanning electron microscope (SEM). Observations by SEM show that the 80-nm film has relatively large-sized clusters of films and the stainless steel substrate still appears in some places, but the surface of the 280-nm film is completely covered by the carbon films. From these results, we propose that TSEE from the 80-nm film originates mainly from the oxide films on the stainless steel substrate and TSEE from the 280-nm film originates from the film itself. Thus, TSEE can be applied to characterize the surface of thin films.

A nucleation model is developed that includes chemical etching of atoms as an additional loss process besides thermal dissociation that competes with the process of atom addition in forming a cluster. The model has the proper qualitative features to describe observations of the evolution of cluster formation on amorphous silicon substrates in the low pressure CVD of a mixture of SiH2Cl2/HCl/H2.

Unintentionally doped 3C–SiC (111) films were grown on TiC (111) substrates. The films were characterized by electrical measurements employing Pt Schottky contacts, optical microscopy, and transmission electron microscopy (TEM). The observed current-voltage (I-V) characteristics appear to be dominated by space-charge-limited-current (SCLC) conduction in the films. Analysis of the I-V characteristics has resulted in information pertaining to the electrically active defects in the films. These active defects are believed to be associated with stacking faults and point defects present in the films and contribute to traps at ∼0.656 eV below the conduction band edge. The concentration of traps was found to vary with film thickness and surface morphology.

Some of the problems of hydrogen, nitrogen, and oxygen behavior in boron, which is a promising material for the development of sensitive elements in gas sensors of improved radiation resistance, were studied. Using gas and thermal gravimetric analysis methods the interaction of these gases with boron was investigated, and the influence of temperature and annealing on the sorptivity of boron, the interaction of disperse boron powder with the environment, and thermal gas release from boron and boron-containing materials were studied.

Both natural and synthetic crystals of pyrite, FeS2−x, have been analyzed chemically and examined structurally by transmission electron microscopy and x-ray powder diffraction. Chemical analysis and density measurements have shown the synthetic crystals, grown at 850 K, are frequently deficient in sulfur, with a composition of FeS2−x with x ⋚ 0.15. From a refinement of the pyrite structure using the integral intensities of the x-ray powder pattern, a variation in the sulfur population parameter was obtained ranging from 0.87(2) to 1.03(3). A correlation according to Vegard's rule between the population factor and the lattice parameter a0 has been proven. Transmission electron microscope examination revealed that the crystals did not contain a significant population of disorder defects which may account for this apparent sulfur deficit. Therefore the nonstoichiometry in pyrite has to be interpreted in terms of S vacancies which can be understood as the tendency of the material to reduce the high anion content in the unit cell. The structural nature of nonstoichiometric pyrite is discussed in relationship to other related disulfides.

A structural study using x-ray diffraction and simulation calculations was performed on a–NiPS3, the highly disordered phase of the nickel thiophosphate, synthesized through a soft chemistry process. The structure of the wet gel is shown to be entirely amorphous; its x-ray pattern exhibits diffusion peaks according to the disordered anionic network. For the powdered samples, measurements of the width of the diffraction lines show that there are small crystallized c–NiPS3-type crystallites with a volume V equal to (25 × 25 × 40) Å3 (i.e., 4a × 3b × 6c in terms of volume cell). The simulation calculations of the x-ray diagrams have indicated that the stacking of these crystallites is quite imperfect: the suggested crystallite structures must contain rotated or shifted layers.

An extensive investigation of twin structures in lanthanum aluminate single crystals at room temperature has been undertaken using White Beam Synchrotron X-Ray Topography (WBSXRT). Twin configurations, directly observable via orientation contrast, were found to comprise both large-volume, macroscopic twins and thick twin lamellae. Positional shifts of twin domain images with respect to matrix images, arising from the mutual misorientation of diffraction vectors, were measured on x-ray topographic images recorded from the (220)rhom planes approximately parallel to the specimen surface and compared to calculated values in order to solve the twin laws in this system. The dominant twin operations were found to be (101)rhom, (011)rhom, and (112)rhom mirror reflections. The potential influence of the twins on the growth of epitaxial Ba2YCu3O7−δ thin films is discussed.

In this paper, the techniques of SQUID magnetometry and line broadening in x-ray diffraction are employed for determining an important parameter for catalysts, viz. the particle size distribution. Magnetization versus temperature (5 K–400 K) and magnetization versus field (up to 55 kOe) data are reported for an α–Fe2O3 based catalyst. After determining the region of superparamagnetism, the distribution function f(r) is determined assuming a log normal distribution and Langevin paramagnetism of superparamagnetic particles. The distribution is found to be fairly symmetric with center near 65 Å and range of 35 to 115 Å. From line-broadening of Bragg peaks in x-ray diffraction, particle radii varying between 75 Å and 110 Å are obtained. These results are compared with the reported Mössbauer measurements of Huffman et al. on the same sample.

Mixed oxide powders, e.g., Al2O3−TiO2, SiO2−GeO2, and TiO2−SiO2, are used in industry to produce ceramics, optical fibers, catalysts, and paint opacifiers. The properties of these products depend upon the morphology of the powders. Ceramics and optical fibers are produced using either a uniform mixture of multicomponent particles or a uniform solution. The desired morphology for catalysts is a high surface area and many active sites. TiO2 coated with a layer of SiO2 is the desired structure for use as a paint opacifier. In this paper, TiO2−SiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. TiCl4 and SiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light scattering. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size, morphology, and crystalline form examined using a transmission electron microscope. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The effect of temperature and of Si to Ti concentration ratio on particle morphology was investigated. Strong temperature dependence was observed. At high temperatures, TiO2 particles were covered with discrete SiO2 particles. At low temperatures, the structure changes to TiO2 particles encapsulated by SiO2. TEM diffraction pattern measurements showed that the TiO2 is rutile and the SiO2 is amorphous silica. At high Si to Ti ratios, SiO2-encapsulated TiO2 particles form. At low Si to Ti ratios, one obtains TiO2 particles covered with discrete SiO2 particles.

SiO2−GeO2 and Al2O3−TiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. SiCl4, GeCl4, Al(CH3)3, and TiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light-scattering. Powders were collected using two different methods, a thermophoretic method (particles are collected onto carbon coated TEM grids) and an electrophoretic method (particles are collected onto stainless steel strips). Their size, morphology, and crystalline form were examined using a transmission electron microscope and an x-ray diffractometer. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The formation of the mixed oxides was investigated using Si to Ge and Al to Ti ratios of 3:5 and 1:1, respectively. Heterogeneous nucleation of the SiO2 on the surface of the GeO2 was observed. In Al2O3−TiO2 mixtures, both oxide particles form at the same temperature. X-ray diffraction analysis of particles sampled at temperatures higher than 1553 K showed the presence of rutile, γ–Al2O3, and aluminum titanate. Although the particle formation process for SiO2−GeO2 is very different from that for Al2O3−TiO2, both mixed oxides result in very uniform mixtures.

We report on the microstructural analyses of chemically prepared Pb(Zr0.53Ti0.47)O3 (PZT 53/47) films. Although several techniques were used to analyze films, transmission electron microscopy (TEM) was emphasized. Phase evolution of these films, fabricated using hybrid metallo-organic decomposition (HMD), was determined by processing films at temperatures ranging from 500 °C to 650 °C. Our films, when observed with an optical microscope, appeared to consist of two distinct phases: (1) a featureless matrix and (2) 1–2 μm diameter “rosettes”. PZT films fired at 500 °C consisted of a pyrochlore containing phase (featureless matrix) and contained no perovskite, whereas films fired at 600 °C were ferroelectric and were approximately 90% perovskite (rosettes) by volume. Our TEM analysis showed that the pyrochlore-containing phase consisted of interpenetrating nanocrystalline pyrochlore and amorphous phases, both with dimensions on the order of 5 nm. For PZT films processed at 650 °C, the perovskite phase was observed in two forms: (1) large (≍2 μm) rosette structures containing 30 nm pores and (2) dense equiaxed particles on the order of 100 nm. We propose that phase evolution—with increasing temperature of HMD PZT 53/47 films—consists of the following steps: (1) phase separation, probably occurring in solution, (2) pyrochlore crystallization, (3) heterogeneous nucleation of perovskite PZT, and (4) homogeneous nucleation of perovskite PZT.

Ba(Mg1/3Nb2/3)O3 powders were prepared through an alkoxide-hydroxide route. Their sintering behavior and crystallographic structures were investigated, and microwave properties of the obtained ceramics were also measured. Crystalline powder with approximately 0.5 μm in particle size was obtained by the reaction of Ba(OH)2−8H2O and Mg–Nb double alkoxide, Mg[Nb(OEt)6]2, in ethanol-acetone solution. A powder compact sintered at 1350 °C for 2 h showed >99% of relative density and >0.9 of ordering degree of the B site. The Q value of Ba(Mg1/3Nb2/3)O3 ceramics was sensitive to sintering conditions due to the formation of an impurity phase. The ceramics sintered at 1350 °C for 5 h showed a dielectric constant ∊ of 32 and Q value of > 13 000 at 11.8 GHz. The temperature coefficient of resonant frequency τf was 23.9 ppm/°C at 9.8 GHz.

A high precision gravimetric method was used to investigate the adsorption of nitrogen (N2) and carbon tetrachloride (CCl4), respectively, at 77 and 298 K, onto various industrial minerals. The solids investigated include silica isomorphs, blast furnace slags, an Y-zeolite, and several naturally occurring fibrous minerals. The adsorption isotherms were analyzed to derive surface areas, BET constants (C), and surface fractal parameters (D). The latter were obtained through an approach recently suggested by Avnir and Jaroniec,1 using multilayer adsorption data; D values obtained for the various gas-solid systems investigated cover the range 2.1–3.0. A systematic comparison between D values inferred from N2 and CCl4 adsorption data shows that the CCl4 molecule probes the surface roughness in a more discriminate fashion than N2. In most cases, the differences between D(N2) and D(CCl4) appear to reflect changes due to sample preparation more than intrinsic differences amongst the various solids.