Transmission electron microscopy (TEM) techniques have been employed to study the room temperature solid state form of chromatographically purified C70. Tilting and electron diffraction experiments in three-dimensional reciprocal space, on samples prepared by crystallization from several different solvents, show that C70 crystallites adopt hexagonal close packed (hcp) structure with a = 1.01 ± 0.05 nm and c = 1.70 ± 0.08 nm. The extinctions and observed reflections conform to the P63/mmc space group. High resolution TEM images reveal the molecular order and periodicity associated with C70 crystallites in real space. The experimental results are in agreement with the preliminary computations of crystal structure within acceptable error limits.

The dynamic behavior of a novel chemical vapor infiltration (CVI) technique called pulsed-power volume-heating CVI is investigated using a diffusion-reaction model. In this technique, a volume-heating source (e.g., RF or microwave) is used to heat the preform. The source power is modulated in time (e.g., square-wave modulation) with a specific period and duty cycle. During the low-power part of the cycle, the temperature of the composite drops, reducing the reaction rate and thus allowing the precursor gas to diffuse into the composite, essentially “refilling” the composite. This alleviates reactant concentration gradients within the composite minimizing density nonuniformities. The high-power part of the cycle is used to achieve rapid reaction rates, thereby minimizing processing time. CVI of a carbon fiber preform with carbon resulting from methane decomposition is taken as an example to illustrate the technique. The results reveal the dependence of density uniformity and processing time on relevant variables such as pulse period, duty cycle, power density level, and methane mole fraction. It is shown that pulsed-power volume-heating CVI can provide a window of operating conditions leading to rapid and complete densification.

Oxide films are grown on a silicon wafer at low temperatures through the catalytic action of a thin gold film. Our results indicate that the oxide film thickness and morphology vary with the initial gold film thickness but do not significantly depend on the temperature. A Fourier analysis of the film structure suggests that the growth mechanism may include a spinodal decomposition where a binary alloy undergoes a phase separation. It is argued that gold silicide is the most likely candidate for spinodal decomposition.

This paper deals with the electrical characterization of thin layers of cubic SiC, grown on (100) Si substrates. The resistivity and Hall coefficient for undoped SiC layers were measured between 10 K and 500 K. The influence of inhomogeneities on the electrical properties of the as-grown films has been established. The Hall data show a clear sign of a transition to impurity band conduction. The donor concentrations studied are in the “intermediate” range. The donor activation energy has been shown to decrease with increasing nitrogen concentration. The nitrogen concentration was measured by SIMS. The variation in nitrogen concentration is also seen in changes in the shape of the ESR spectrum. The presence of nitrogen in the intermediate concentration range is the most likely reason for the conflicting values reported for the donor ionization energy as measured by Hall and PL measurements.

Amorphous silicon carbide films (a–SixC1−x :H) deposited by the argon- or helium-diluted PECVD technique were studied as a function of their composition. Microstructural investigations were mainly achieved by means of FTIR and XPS techniques. Nuclear techniques were used to obtain precise information on the film hydrogen content. The Si–H IR-absorption band was deconvoluted in different monohydride and dihydride silicon environments. The existence of SiH2 bonds in the Si-rich composition was evidenced. From the analysis of the C–H and Si–H absorption bands it is shown that hydrogen atoms are preferentially bonded to carbon atoms. The deconvolution of the Si2p core level peak suggests that above a composition of x ∊ 0.5, the noncarburized (Si, Si, H) local environment contribution increases to the detriment of the hydrocarburized (Si, C, H) environments. From the evolution of the C1s peak, it can be deduced that there is a change in the carbon atom bonding states when the film composition is varied. These results are correlated and discussed in terms of the local bonding environments and their evolution with film composition.

Indium tin oxide (ITO) films were prepared on a glass substrate by thermal decomposition of ethylene glycol (EG) solutions which were prepared by dissolving indium and tin chlorides into ethylene glycol and then heating the solution at 200 °C. Homogeneous ITO film with a thickness of 1 μm without any cracks was obtained from the EG solution with a viscosity of about 50 cSt by spin coating, with the rotation of the substrate at 2000 rpm. The crystallization of the film thus obtained occurred above 400 °C. A well-crystallized ITO film was obtained by heat-treating at 600 °C. The resistivity of the ITO film obtained on a glass substrate showed a minimum value of 1.1 × 10−4 ohm cm at about 7 at.% Sn.

The thermal conductivities of LaAlO3 and NdAlO3 have been measured for the first time between 10 and 300 K. These substrates can be classified as intermediate thermal conductors. The measured room temperature values of 9 W m−1 K−1 for LaAlO3 and 7.5 W m−1 K−1 for NdAlO3 are close to those predicted from the crystal structure of these materials, despite the fact that the crystals are twinned.

Titanium dioxide thin films were grown on sapphire (11$\overline 1$0) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 °C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 °C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 °C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.

Single crystals of mixed mercurous halides Hg2X2 (where X = Cl and Br) were grown by the physical vapor transport process. Structural parameters and optical properties were determined to examine the suitability of mixed crystals for acousto-optic devices. Mixed crystals grown in the low concentration region did not show striations or banding. Crystals grown in the high concentration region showed considerable inhomogeneity in the refractive index. Since we had used identical thermal conditions, inhomogeneity was attributed to the solutal convection caused by the difference in densities of mercurous chloride and bromide vapors. A low concentration of dopant did not change the acoustic velocity of the parent components.

A new NH4Bi3F10 compound was prepared, which results from a reaction between Bi2O3 and NH4F in an argon stationary atmosphere. It has a cubic fluorite-related structure isotypic with KBi3F10 with space group Fm3m (a = 11.974 Å). Further heating leads to decomposition to BiF3. IR spectra at different temperatures show the disappearance of NH4 group characteristic vibrations. Indexed x-ray diffraction pattern and IR spectra at different decomposition temperatures are given.

Reaction kinetics between borosilicate glass (BSG) and sapphire has been studied at temperatures from 850 °C to 950 °C. Microstructural and chemical analyses show that the nonporous interdiffusion layer is formed with Al+3 ion dissolving from sapphire and K+ diffusing from BSG onto the interface of sapphire/BSG, and that both ions are always coupled together in the reaction layer. The interdiffusion layer moves toward BSG with time and the reaction starts immediately at temperatures investigated without incubation period. The growth kinetics for the interdiffusion layer follows a parabolic rate law in the temperature range investigated, and shows an apparent activation energy in the range of 176 k–/mol. The diffusion coefficient of aluminum ion is determined from EDX analysis, and the values range from 0.7–1.4 × 10−12 at 850 °C to 3.0–6.0 × 10−12 cm2/s at 950 °C. The above results also show an activation energy close to that determined from the parabolic growth rate constants, suggesting that the mass-transport kinetics of aluminum ion from sapphire into the interdiffusion layer controls the formation process.

Lead zirconate titanate (PZT) thin films were prepared by excimer laser ablation on platinum coated silicon substrates. The composition of the films showed dependence on the fluence at low energy densities (<2 J/cm2), and less dependence on the ablation fluence was observed beyond a fluence of 2 J/cm2. A correlation among the fluence, ablation pressure, and substrate temperature has been established. Crystalline perovskite PZT films showed a dielectric constant of 800–1000, a remnant polarization of 32 μC/cm2, and a coercive field of 130 kV/cm. Films showed fatigue behavior that may be used in a device, and a close comparison of fatigue behavior between the films deposited at different energy densities indicated a better fatigue behavior for a fluence of 4 J/cm2.

The sequence of structural evolution in the gelation of vanadic acid to form vanadium pentoxide gels was studied using cryogenic transmission electron microscopy (cryo-TEM) and scanning tunneling microscopy (STM). Small whiskers form from initially homogeneous solutions, and then grow into crystalline ribbon-like colloidal particles. It is proposed that the whiskers form from polymerization of dioxovanadium cations. The ribbons then grow by continued addition of dioxovanadium cations which are supplied by the decomposition of decavanadate ions. In solution, the ribbon-like particles have dimensions of approximately 25 nm × 3 nm × over 1 μm. These ribbons are flexible perpendicular to the plane of the ribbon. Upon drying, a flat rigid mass of ribbon-like particles is formed. The ribbons examined by STM showed striations 3 nm wide, a value that corresponds with the width of the unit cell proposed by J. Legendre and J. Livage [J. Colloid. and Interf. Sci. 94, 75 (1983)].

Uniform α–Fe2O3 particles of varying axial ratios have been prepared from hydrolyzed ferric chloride solutions at 100 °C. In the absence of phosphate anions, spherical particles were obtained by a mechanism that follows the classical LaMer and Dinegar scheme. However, in the presence of phosphates ellipsoidal particles were observed, with their formation taking place through an aggregation process from smaller primary particles of α–Fe2O3. It is also shown that all particles are monocrystalline irrespective of their formation mechanism.

An aqueous, metallo-organic precursor for PbTiO3 (PT), Pb(Zr, Ti)O3 (PZT), and (Pb1−x, Lax) (Zry, Ti1−y)1−x/4O3 (PLZT) ceramic powders has been developed employing the acetate complexes of Pb2+, La3+, Zr4+, and Ti4+. The crystallization behavior (e.g., degree of crystallization and crystallite size) was examined over the temperature range of 400–700 °C with concentrations of excess lead varying from 0–5% using x-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy techniques. It was found that PT, PZT, and PLZT crystallize at temperatures as low as 450 °C, 500 °C, and 500 °C, respectively, and the excess lead required for crystallization of the perovskite phase is ⋚1%. Intermolecular mixing among acetate precursors via chelating and bridging structures was evident and appeared to be responsible for the low crystallization temperatures. The process is compared to the current sol-gel methods for its possible use in thin film ferroelectric manufacture.

The evolution of microstructure as a function of firing temperature in sol-gel derived porous titania xerogels was investigated by small-angle x-ray scattering (SAXS). SAXS curves for xerogels fired below 550 °C exhibit a well-defined structure peak. This peak indicates the presence of a high degree of order in the electron density correlations associated with the interparticle structure factor. Results from electron microscopy and SAXS give a primary particle mean diameter of 50 Å, while scaling analysis of the scattered intensity at large momentum transfer values yields the Porod exponent 4, indicating a sharp transition zone between the solid and void phases. The pore volume fraction of the unfired xerogel is consistent with random close-packing of spheres. The internal surface area decreases almost linearly with increasing firing temperature. Rapid grain growth and pore coarsening begin near 90% of theoretical density, and lead to a breakdown in pore interconnectedness and the development of isolated pores. Observed enhanced sintering properties may be attributed primarily to the large surface-to-mass ratio of the sol-gel particles. The SAXS curves were adequately fit using a bicontinuous phase model developed for late-stage spinodal decomposition structures. Alternatively, the microstructure can be described by a hierarchical close-packing of spheres model. SAXS results are compared with data from gas adsorption and electron microscopy.

Rheological behavior, properties of colloidal solids consolidated by filtration, and their structure change during drying were studied with aqueous suspensions of a mullite powder of nanometer size and two kinds of alumina powders of submicrometer sizes. The rheological properties of non-Newtonian flow suspensions were analyzed by a power law equation of S = K γn, where S is the shear stress, γ the shear rate, and K and n (O ⋚ n ⋚ 1) constants. The critical solids content (Vc) at n = O (indicating colloidal solids) depended greatly on the zeta potential of particles in suspensions, and dominated the densities of dried green compacts (Vg) of submicrometer sized powders. In a nanometer sized powder, the densities of dried green compacts were dependent on both Vc value and the solid contents of suspensions. Phase diagrams of one-component colloidal systems were constructed by plotting the Vc and Vg values against the zeta potential of particles. These phase diagrams indicate that the colloidal solids range (surrounded by Vc and a minimum Vg lines) is narrow for nanometer sized powder and wide for submicrometer sized powder. The solids content range of dried green compacts was very narrow for submicrometer sized powder but relatively wide for nanometer sized powder due to the low flexibility of colloidal structure during drying.

A series of investigations were carried out to evaluate the topographic and structural effects of ion implantation on monolithic Si3N4 ceramic surfaces. Implantations were performed with N2, Ne, or Ar in the fluence range of 2.0 × 1016 to 4 × 1017 particles/cm2 and implant voltages of 125 to 200 keV, depending on the mass of the implanted species. Single crystal Si and SiC were also examined for comparative purposes. Noble gases produced blisters on Si3N4 and significant increases in surface expansion. TEM examination of the Si3N4 blister shell showed a distribution of small bubbles, ranging in size from 5 to 300 nm, depending upon the type of Si3N4, and a transformation of the original crystalline structure into an amorphous phase. Analysis of the blister shell, using electron energy loss spectrometry (EELS) and energy dispersive x-ray spectrometry (EDXS) showed that a significant quantity of the implanted Ar was still present in the blister (skin).

Composites of SiC–Al2O3 and SiC–mullite are unstable at high temperatures. The reactions occurring within the composites between 1700 and 1850 °C in stagnant inert atmospheres were characterized. Gaseous products cause excessive weight losses which cannot be attributed to passive oxidation. These losses can be successfully retarded by processing under high pressures. Compatible phases were determined by x-ray analysis for mixtures lying in the section SiC–Al4C3−Al2O3−SiO2. The reactions produced condensed phases of Al2OC and Al4O4C as well as gaseous SiO and CO. The condensed phases have high vapor pressures above 1700 °C. The effect of these reactions on densification of composites by firing at different temperatures for various periods under different pressures was studied. Dense materials prepared under high pressures at 1825 °C were tested at 1700 °C under normal pressure in argon, where active oxidation is expected, and weight losses were insignificant.

Modification of substrates by controlled deposition of nanometer-size particulates (nanoclusters) is an efficient means of fabricating materials designed for applications in which specific surface interactions play a vital role (e.g., molecular catalysis and microelectronics). We have found that highly dispersed nanoclusters form on thin films of poly(siloxaneimide) (PSI) copolymers supported on copper transmission electron microscopy (TEM) grids when subjected to long anneals at elevated temperatures. In this note, we report on the composition and source of these anomalous nanoclusters, as determined by a variety of electron microscopical techniques. Spectra obtained with parallel electron energy-loss spectroscopy (PEELS) indicate that these particulates, which typically measure 4–18 nm in diameter, are composed of copper with a mean valence of +1. Electron microdiffraction patterns reveal that the nanoclusters are polycrystalline, possessing lattice spacings similar to those of Cu2O. Mechanistic routes of formation are suggested based on experimental design, and factors influencing formation are also described.