MgAl2O4 films have been grown epitaxially on both Si(100) and MgO(100) by a novel single source metal-organic chemical vapor deposition (MOCVD) process. A single molecular source reagent [magnesium dialuminum isopropoxide, MgAl2(OC3H7)8] having the desired Mg: Al ratio was dissolved in a liquid solution and flash-vaporized into the reactor. Both thermal and plasma-enhanced MOCVD were used to grow epitaxial MgAl2O4 thin films. The Mg: Al ratio in the deposited films was the same as that of the starting compound (Mg: Al = 1:2) over a wide range of deposition conditions. The deposition temperature required for the formation of crystalline spinel was found to be significantly reduced and crystallinity was much improved on Si by using a remote plasma-enhanced MOCVD process. The epitaxial nature of the MgAl2O4 films was established by x-ray pole figure analysis.

YBa2Cu3O7−x (YBCO) superconducting thin films that show no x-ray diffraction peaks due to any other non-superconducting phases have been synthesized by annealing Y-Ba-Cu-O amorphous precursors at 750 °C. The Y-Ba-Cu-O precursors have been fabricated by oxidizing Y-Ba-Cu metallic precursors coevaporated from Y, Ba, and Cu metallic sources under ultrahigh vacuum conditions. Crystallization behavior from the Y-Ba-Cu-O precursor to YBCO films drastically depends on an oxidation temperature for the Y-Ba-Cu metallic precursor. YBCO thin film synthesized from the precursor oxidized at an optimum temperature shows a zero resistance temperature of over 80 K and a very smooth surface.

The variation of grain morphology in silver-doped YBa2Cu3O7−x was investigated as a function of sintering temperature, atmosphere, and amount of Ag addition. In the presence of the liquid phase formed at 925 °C for undoped samples, and at 910 °C for silver-doped samples, the grain shape and size changed drastically from small and nearly equiaxed to large and elongated. The anisotropy in the grain shape was sensitive to both the silver content and the atmosphere. On the other hand, while the grain size was generally insensitive to the atmosphere, it decreased as the silver content increased. The silver phase, if sufficiently large, would block the grain growth in the matrix. The amount of silver for effective blocking was predicted from a microstructural model, and the prediction was in agreement with experimental results.

The unidirectional solidification by a zone-melting method was performed to investigate the growth anisotropy of YBa2Cu3O6+x (123) superconductive oxides. Large faceted interfaces were obtained at growth rates lower than 2 mm/h. The three-dimensional interface morphology and the onset of growth were observed. The crystal faces toward the growth direction were revealed to be close to the {112} face, which was selected as the result of the preferential growth. The growth rate of the {100} faces was evaluated to be about 1.5 times faster than that of the {001} faces from the steady growth condition.

A simple method for the bulk synthesis of 80 K bismuth-based superconducting phase (2212) is described. It employs a modified citrate gel route giving rise to an auto-ignited combustion process. The precursor ash obtained after combustion is further calcined at 800 °C to produce the desired superconducting powder with excellent homogeneity and fineness, leading to good sinterability and fairly sharp transition temperature. For a particular citrate-nitrate ratio, the nature of combustion of the gels containing two different bases has been compared. The thermal decomposition characteristics of the gels and the plausible mechanism of formation of the superconducting phase are also described.

A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is oxygen release which causes effluence of the oxide materials from the Ag sheath. Higher oxygen partial pressure during the melt process cuts the oxygen release. We investigated the effect of the oxygen partial pressure, melting temperature, and cooling rate on superconducting properties of Ag-sheathed wires. It was found that (i) the formation of the Bi-2212 phase under a higher oxygen partial pressure of 1 atm is slower than that with a lower oxygen partial pressure of 0.2 or 0.5 atm; therefore, the cooling rate should be as slow as 0.25 °C/min to get homogeneity of the intragrain superconductivity as well as formation of the Bi-2212 phase, and (ii) melting temperature should be just above the incipient melting point; otherwise, Jc significantly decreases due to the formation and growth of voids together with degradation of superconductivity.

The effects of milling and Ag addition on the decomposition of single-phase LaBa2Cu3Oy have been evaluated. It was found that the decomposition of milled single-phase LaBa2Cu3Oy powders when sintered in pure N2 is attributed to the introduction of strain in the lattice which causes the instability of the structure. The possible reasons why single-phase LaBa2Cu3Oy could be synthesized by sintering in pure N2 at high temperatures and its transition width is always broadened are proposed. The decomposition of compacts of unmilled powders of single-phase LaBa2Cu3Oy when sintered in pure N2 for a long time is due to the fact that oxygen diffuses along the grain boundaries and evolves through the surface of the specimens. Silver might segregate to the grain boundaries and prevent decomposition. It is suggested that to obtain a high-quality LaBa2Cu3Oy, sintering in the reduced atmosphere to achieve a proper oxygen content is required. Reduced atmosphere and Ag addition could enhance the densification rate. For Ag-doped specimens, Tc is highest for x = 0.0001, but decreases for x > 0.0001.

The growth morphology and interface structure of Nd1.85Ce0.15CuO4−y (NCCO) films grown by pulsed laser deposition on two different types of substrates, “perovskite” LaAlO3 (LAO) and SrTiO3 (STO) and “fluorite” Y2O3-stabilized ZrO2 (YSZ), were studied using cross-sectional electron microscopy. Structurally, the NCCO films are different when grown on the two types of substrates in three aspects: (i) epitaxy, (ii) substrate-film intermixing, and (iii) substrate-film interface roughness. In general, films deposited on “fluorite” substrates showed better superconducting properties than the films grown on “perovskite” substrates, especially for thinner films. Lattice mismatch considerations are not sufficient to explain the observed differences since films grown on the YSZ substrate showed sharp substrate-film interface in spite of their large lattice misfit. The atomic arrangements at the interface were analyzed in terms of electrostatic energy (charge balance) and matching of the oxygen sublattices in order to account for the experimental results.

Grain sizes were measured after various heat treatments in three Fe3Al alloys having similar composition that were fabricated using the techniques of ingot metallurgy (cast and wrought), hot extrusion of prealloyed powder, and hot isostatic pressing (HIP) of elemental powders. The ingot metallurgy (I/M) material exhibited normal grain growth behavior at temperatures above 750 °C, in agreement with previous observations. Both powder metallurgy (P/M) materials displayed unusual resistance to grain growth compared to the I/M alloy. In the case of the prealloyed P/M material, the initial (recrystallized) grain size was larger than the initial grain size of the I/M material, although little grain growth was observed for heat-treatment temperatures up to 1100 °C. At higher temperatures grain growth occurred at a rate comparable to that observed to the I/M alloy. The elemental powder P/M material exhibited similar grain growth behavior to the prealloyed P/M material, although the initial (as-HIPed) grain size was considerably smaller. Transmission electron microscopy (TEM) indicated that the grain growth resistance of the P/M materials could be attributed to grain boundary pinning by oxide particles presumed to originate from the powder particle surfaces. The difference in the stable grain size between the prealloyed and elemental powder P/M materials was attributed to the nature of the particle dispersions resulting from processing.

A Mg-6.5 wt. % Li alloy containing 80% hep alpha phase and 20% bcc beta phase was processed to achieve an average grain size of 5.9 μm. Strain-rate-change tests were performed in the temperature range from 398 K to 573 K. Two types of creep behavior were observed. A stress exponent of five, obtained at low temperatures and high stresses, is attributed to a diffusion-controlled dislocation creep process in the alpha matrix. A stress exponent of three, obtained at high temperatures and low stresses, is attributed to a solute-drag controlled dislocation creep process in the alpha matrix.

The mechanisms for fiber damage in single crystal Al2O3 fiber-reinforced composites were investigated. Both fiber fragmentation and fiber strength degradation were observed in composites with a variety of matrix compositions. Four mechanisms that may be contributing to the fiber strength loss have been proposed and include matrix reaction, reaction with binders, residual stress-induced damage, and pressure from hot pressing. The effect of matrix reaction was separated from the other three effects by sputter-coating the matrices on cleaned fibers and annealing with a temperature profile that simulates processing conditions. These experiments revealed that Y and Cr in FeCrAlY base alloys and Zr in NiAl alloys reacted with the fiber, and grooves and adherent particles were formed on the fiber surface which were responsible for the strength loss. The effects of the matrix reaction appeared to dominate over the other possible mechanisms, although evidence for reaction with binders was also found. Ridges on the fiber surface, which reflected the grain boundaries of the matrix, were also observed. In order for single-crystal Al2O3 to be used as a fiber in MMC's and IMC's, a matrix or protective coating which minimizes matrix reaction during processing will be necessary. Of the matrices investigated, the Thermo-spanTM alloy was the least damaging to fiber properties.

Theoretical and empirical considerations of the work-of-fracture, γwof, of brittle materials are reviewed. The energy principle of the work-of-fracture provides a modified Irwin similarity relationship in the nonlinear fracture mechanics regime. Various microscopic deformation and fracture processes in the crack wake and the crack-face contact regions contribute to the rising R-curve behavior of brittle materials, and then significantly affect the work-of-fracture, resulting in the work-of-fracture that is dependent on the dimension and geometry of test specimens as well as test methods. The requisite for the work-of-fracture to be a material characteristic resistance to failure is discussed in relation to the R-curve behavior. Some examples of the work-of-fracture test results demonstrate the usefulness of the work-of-fracture for designing brittle materials with improved toughness.

MgAl2O4 thin films, to be studied as active elements for humidity sensors, were deposited on Si/SiO2 substrates by radio-frequency sputtering. This paper discusses the x-ray photoelectron spectroscopy (XPS) investigation of these films. XPS demonstrated that the thin films had a stoichiometry close to that of MgAl2O4. The evaluation of the modified Auger parameter α' for Al gave structural information about the order of the crystalline structure of the thin films. The combination of Ar+ ion etching and XPS analysis showed the simultaneous presence of Mg, Al, and Si at the film-substrate interface. The thicknesses of the interfaces were calculated between 7 and 10 nm. The analysis of the binding energy (b.e.) values of the XPS peaks at different etching depths showed that O 1s and Si 2p b.e. values were characteristic of a silicate at the interface, whereas in the substrate they were typical of silica. This suggests a chemical interaction took place between film and substrate with the formation of a silicate layer at the interface, which may be the cause of the good adhesion of MgAl2O4 films to silica, as observed by peel tests with Scotch tape.

We performed a series of glancing angle and reflection x-ray diffraction experiments to study both the in-plane and out-of-plane structure of epitaxial YSi2−x films grown on Si(111), with thicknesses ranging from 85 Å to 510 Å. These measurements allowed us to characterize the mean film lattice constants, the position correlation lengths of the film, and the presence and extent of strain as a function of film thickness. We find that the strain along the basal plane increases as a function of increasing thickness to approximately 1% in the 510 Å film; the corresponding out-of-plane strain is such that the film unit cell volume increases as a function of thickness. The corresponding in-plane microscopic strain varies from 0.5% for the 85 Å film to 0.3% for the 510 Å film. We relate our results to the mode of film growth and the presence of pinholes in the films.

Studies of high-temperature alloys in the Ti-Sn system based on the intermetallic compound Ti3Sn have identified alloys that damp strongly both at low frequencies (0.1 to 10 Hz) and high frequencies (5 to 20 MHz). The low frequency damping behavior shows loss factors as high as 0.04 at room temperature and Young's moduli that rise with temperature from 40 °C to 100 °C for two alloys. Although the basic mechanism or mechanisms of energy dissipation are presently unknown, the alloys are notable for unusual shapes of microhardness indentations. The deformations imply that large reversible strains can occur at temperatures from 23 °C to 1150 °C.

This work describes the formation of stoichiometric AlN films by single ion-beam sputtering of Al, using an ionized N2 (75%) + H2 (25%) mixture, onto substrates heated to 200 °C or above. The role of substrate temperature on film composition and properties is followed in the substrate temperature range between ambient and 250 °C. Infrared spectra of freshly prepared and 2 month old (aged in air) films demonstrate that substrate heating significantly affects the chemical nature of the resulting films. SEM and STM data, combined with IR and UV-visible spectral results, indicate that films formed at a substrate temperature of ≥200 °C are very smooth and highly resistant to attack by atmospheric gases. X-ray diffraction data show no diffraction peaks, indicating that the film is either amorphous or crystalline on a scale of less than 4 nm.

Epitaxial TiN/V0.3Nb0.7N superlattices with a 1.7% lattice mismatch between the layers were grown by reactive magnetron sputtering on MgO(001) substrates. Superlattice structure, crystalline perfection, composition modulation amplitudes, and coherency strains were studied using transmission electron microscopy and x-ray diffraction. Hardness H and elastic modulus were measured by nanoindentation. H increased rapidly with increasing Λ, peaking at H values ≍75% greater than rule-of-mixtures values at Λ ≍ 6 nm, before decreasing slightly with further increases in Λ. A comparison with previously studied lattice-matched TiN/V0.6Nb0.4N superlattices, which had nearly identical composition modulation amplitudes, showed a similar H variation, but a smaller H enhancement of ≍50%. The results suggest that coherency strains, which were larger for the mismatched TiN/V0.3Nb0.7N superlattices, were responsible for the larger hardness enhancement. The results are discussed in terms of coherency strain theories developed for spinodally decomposed materials. Nanoindenter elastic modulus results showed no significant anomalies.

Titanium di- and sesquioxide films were epitaxially grown on the (001) surface of sapphire single-crystalline substrates by an activated reactive evaporation method. Formation range for each titanium oxide was determined as a function of oxygen pressure (Po2) by means of x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Films prepared at Po2 ≥ 2.0 × 10−4 Torr were stoichiometric (100)-oriented rutile of TiO2, and with decreasing Po2 they would accommodate more and more Ti3+ ions in the rutile structure. At Po2 = 0.6 × 10−4 Torr, on the other hand, (001)-oriented Ti2O3 was formed and an electrical transition was clearly detected at about 400 K. However, the large lattice mismatch between the substrate and these films leads to a periodic introduction of misfit dislocations in the case of the TiO2 films and a mixing of stacking sequences for the Ti2O3 films.

The chemical vapor deposition (CVD) of MoS2 by reaction of H2S with molybdenum halides was determined to be thermodynamically favored over a wide range of temperature, pressure, and precursor concentration conditions as long as excess H2S was available. The thermochemical stability of H2S, MoF6, and MoCI5 was also assessed to address their suitability as precursors for the CVD of MoS2. The results from the thermodynamic analysis were used as guidance in the deposition of MoS2 thin films from MoF6 and H2S. The (002) basal planes of MoS2 films deposited above 700 K were preferentially oriented perpendicular to the substrate surface.

Room-temperature current-voltage dependence of ultrafine-grained ferroelectric Pb(Zr, Ti)O3 thin films has been investigated. Both strong varistor type behavior and space charge limited conduction (SCLC) were observed. Differences in the current-voltage characteristics are attributed to differences in the nature of the grain boundaries resulting from variations in processing conditions. The strong varistor type behavior is believed to be due to the presence of highly resistive grain boundaries and thus may be termed grain boundary limited conduction (GBLC). A double-depletion-layer barrier model is used to describe the origin of high resistivity of the grain boundaries. It is suggested that the barrier height varies significantly with the applied field due to the nonlinear ferroelectric polarization, and that the barrier is overcome by tunneling at sufficiently high fields. In some other cases, the resistivity of the grain boundaries is comparable to that of the grains, and therefore the intrinsically heterogeneous films degenerate into quasi-homogeneous media, to which the SCLC theory is applicable. As such, a unified grain boundary modeling reconciles different types of conduction mechanisms in the ultrafine-grained ferroelectric thin films. This grain boundary modeling also well accounts for some other dc-related phenomena observed, including abnormal current-voltage dependencies, remanent polarization effect, electrode interface effect, and unusual charging and discharging transients. In addition, many other electrical properties of the ferroelectric films may be better understood by taking the effect of grain boundaries into account.