The effect of uv irradiation on the material properties of AlN containing low oxygen content (<0.3 wt. %) has been investigated. Upon irradiation with a 150 W Hg-Xe arc lamp, increases in dielectric loss and thermal conductivity drops were observed. Changes in the optical properties, including formation of an absorption band at 650 nm, and photodarkening were also observed. All samples were characterized for trace impurities including carbon, calcium, and oxygen. The magnitude of the uv-induced drops in dielectric loss and the absorption at 650 nm of samples tested were found to depend on the presence of trace levels of residual carbon. UV-induced increases in dielectric loss show an inverse relationship, and formation of an absorption band shows a direct relationship to carbon content.

The effect of Ga doping on the grain boundary properties of a ZnO varistor has been presented in this note. Within the range of doping level studied, it is shown that Ga acts as a donor at the grain boundary and behaves similar to that of Al.1 The grain boundary doping behavior of both Ga and Al, when acting as donors, can therefore be represented by the same defect model.

We report the observation of fractal patterns in C60-tetracyanoquinodimethane thin films. The fractal patterns and their microscopic features are described and characterized. The fractal dimension was determined to be 1.69 ± 0.07. According to the characterization results, the observed fractals are compared to the cluster-diffusion-limited-aggregation model. The growth of the fractal patterns in the thin films is also in terms of the existing long-range correlation.

Heteroepitaxial growth of Ba2YCu3O7−x (BYC) thin films prepared by postdeposition annealing on (001) LaAlO3 was characterized by TEM and x-ray diffraction studies of specimens rapidly cooled from various points in the growth heat treatment. Heteroepitaxial nucleation of BYC occurred between 720 and 770 °C during heating at 25 °C/min to the annealing temperature of 830 °C. The c-axis normal BYC rapidly coalesced into a continuous film with nearly complete coverage of the substrate surface after growth of a film of several unit cells thickness. The experimental results were not consistent with purely solid phase heteroepitaxial nucleation and growth or epitaxial grain growth, mechanisms for microstructural evolution of other chemically derived epitaxial oxide thin films. The nature of the transformation and the microstructure of the final superconducting films were consistent, instead, with growth of epitaxial BYC from a liquid phase that is present transiently during the anneal. This hypothesis was supported by thermal analysis results obtained from the precursor material of which the films are composed prior to transformation to BYC.

Following our recent report on the successful fabrication of “stationary” MTG Bi1.6Pb0.4Sr2Ca2Cu3Oy pellets in a simple tube furnace, we have tried many heating programs with various “crystal growth starting temperatures”, Ts, and sintering temperatures, Te; the samples are sintered for 20–40 h before quenching in air. It is found that Ts-Te should be rather narrow to produce good MTG samples, and the typical value of Jc is enhanced from 100 A cm−2 (for a polycrystalline sample) to about 3 × 103 A cm−2 for our good ones. SEM and TEM results indicate that the samples are full of packs which contain highly oriented mica-like layers.

Internal cavities often develop during the deformation of superplastic alloys. Experiments were conducted to determine whether a nondestructive photoacoustic method may be used to detect the presence of internal cavities in two different commercial Al-based alloys. An analytical procedure was developed to provide quantitative information on the volume fraction of cavitation. The results confirm that the photoacoustic signal can be used to detect the presence of cavities, and it is demonstrated that the quantitative measurements derived from the photoacoustic data are consistent with observations obtained by sectioning and standard metallographic techniques.

The joints formed by brazing Al2O3 to itself or to a Ti-alloy (Ti-6Al-4V) with a Ag–Cu–Ti braze filler were investigated. In the brazing process, Ti in the braze alloy reduces Al2C3 to form a series of reaction products which have a layered morphology. The formation of M6X-type compounds, Ti4Cu2O or Ti3Cu3O, at the interface is characteristic of these joints. The other reaction products also belong to the Ti–Cu–O system (with the reduced Al in solution), and hence this subsystem was chosen to assess the thermodynamic stability of the joints. The Ti–Cu–O section was established experimentally at 945 °C, and activities of elements in three of the three-phase regions were estimated based on the phase boundaries of the ternary section and available binary thermodynamic data. The estimated free energies of formation of the two M6X-type compounds, Ti4Cu2O and Ti3Cu3O, are −120 kcal/mol and −122 kcal/mol, respectively. The highly negative values for the free energies of formation suggest that these compounds are thermodynamically stable. The activity data were also used to generate activity diagrams for Ti–Cu–O system. The layer sequences at the joints satisfied the stability requirements based on the ternary section and the activity diagrams, indicating that even though the interfaces formed in a matter of minutes, they were at local thermodynamic equilibrium.

X-ray photoelectron spectroscopy (XPS) was performed in as-received, thermally annealed, and laser-irradiated sapphire and alumina specimens in order to study the effects of the different treatments on surface chemistry and properties. Laser irradiations with a 308 nm wavelength laser were performed in air and in a reducing atmosphere consisting of a mixture of Ar and 4% of hydrogen. The atomic percentages of carbon, aluminum, and oxygen were measured in all the specimens. Particular attention was paid to the percentages of oxygen in the oxide and in a hydroxyl state. The XPS analyses clearly established that a very thin film of metallic aluminum is formed on the surface of both alumina and sapphire substrates when they are irradiated under a reducing atmosphere. However, the film is discontinuous because it is electrically insulating. Substrates irradiated in air have metallic aluminum only for fluences below 0.4 J/cm2. The valence band photoemission spectra of as-received, annealed, and laser-irradiated specimens were measured. In irradiated specimens, the width of the valence band spectra was found to decrease by ∼10%. One possible cause of this decrease is the generation of point defects during laser irradiation. Electroless copper deposition occurs on sapphire and alumina substrates if their surface has been activated by laser irradiation. The time required for copper deposition was monitored by measuring the electrical resistivity in the irradiated area while the substrates were immersed in an electroless bath. The kinetics of deposition on laser-activated substrates and the XPS results show that the presence of metallic aluminum accelerates the deposition process. However, the presence of aluminum is not the sole reason for laser activation in alumina. Very strong metal-ceramic bonding is produced after thermal annealing of samples having preirradiated substrates. This result is explained in terms of the excess oxygen that is present at the ceramic surface after irradiation.

C-axis LiNbO3 epitaxial films have been grown on c-plane sapphire substrates by solid source metal-organic chemical vapor deposition (MOCVD) using the tetramethylheptanedionate sources, Li(thd) and Nb(thd)4. Stoichiometric LiNbO3 films were deposited from Li(thd)-rich source compositions. Rocking curve FWHM values as low as 0.044°were measured on films grown at 710 °C. Rocking curve peak widths became broader as films were grown at progressively lower substrate temperatures. Single prism coupling experiments revealed clearly visible optical waveguiding, with optical attenuation values as low as 2 dB/cm in the best films.

Epitaxial VO2 thin films grown on (1120) sapphire (α-Al2O3) substrates by MOCVD at 600 °C have been characterized by conventional electron microscopy and high resolution electron microscopy (HREM). Three different epitaxial relationships between the monoclinic VO2 films and sapphire substrates have been found at room temperature: I. (200) [010] monoclinic VO2 ‖ (1120) [0001] sapphire, II. (002) [010] monoclinic VO2 ‖ (1120) [0003] sapphire, and III. (020) [102] monoclinic VO2 ‖ (1120) [0001] sapphire. Expitaxial relationships II and III are equivalent to each other when the film possesses tetragonal structure at the deposition temperature; i.e., they can be described as (010) [100] tetragonal VO2 ‖ (1120) [0001] sapphire and (100) [010] tetragonal VO2 ‖ (1120) [0001] sapphire. HREM image shows that the initial nucleation of the film was dominated by the first orientation relationship, but the film then grew into the grains of the second and the third (equivalent to each other at the deposition temperature) epitaxial relationships. Successive 90°transformation rotational twins around the a-axis are commonly observed in the monoclinic films.

Barium hexa-aluminate (BaO · 6Al2O3) aerogels were prepared using a supercritical drying method and their properties examined. A barium hexa-aluminate aerogel prepared from a double alkoxide of barium and aluminum showed a high specific surface area of 421 m2/g. Monolithic barium hexa-aluminate formed. No BaO · Al2O3 or alumina was observed, as is often the case in powder processing. The specific surface area of the monolithic barium hexa-aluminate fired at 1400 °C for 2 h was 12 m2/g, while that of the barium hexa-aluminate xerogel was only 0.8 m2/g. A barium hexa-aluminate aerogel was also prepared using barium by using a chelating agent. This aerogel showed a specific surface area of 454 m2/g as-dried. In the case of the chelating agent, BaO · Al2O3 was also detected along with barium hexa-aluminate after firing.

Barium metazirconate (BaZrO3) fine powder has been produced by thermally decomposing a molecular precursor, barium bis(citrato)oxozirconate(IV) tetrahydrate at about 700 °C. The precursor, Ba[ZrO(C6H6O7)2] · 4H2O (BZO) has been synthesized and characterized by employing a combination of spectroscopic and thermoanalytical techniques. The precursor undergoes thermal decomposition in three major stages: (i) dehydration to give an anhydrous barium zirconyl citrate, (ii) decomposition of the anhydrous citrate in a multistep process to form an ionic oxycarbonate intermediate, Ba2Zr2O5CO3, and (iii) decomposition of the oxycarbonate to produce BaZrO3 fine powder. The particle size of the resultant BaZrO3 is about 0.2 μm, and the surface area is found to be 4.0 m2 g−1.

The purpose of this investigation was to study the effect of changes in composition and thermal history on some properties of potential leucite-glass composites made by a coprecipitation process. Four compositions from the K2O–Al2O3−SiO2 system were investigated for differences in phases, density, thermal expansion, and strength. Strength, density, and thermal expansion increased with increasing leucite content. Strengths of up to 140 MPa were obtained with air-fired specimens. Composites with different thermal expansion coefficients could be obtained by altering SiO2 content in a predictable way. Leucite-glass composites offer potential for improved dental porcelains.

Fine β-spodumene-type amorphous powders were obtained through sol-gel techniques using Si(OC2H5)4, Al (OC2H5)3, LiOCH3, and Ti(OC2H5)4 as the starting metal alkoxides. Differential thermal analysis (DTA), x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction (ED) analysis were utilized to study the phase transformation behavior of the LAST gels. The viscosity of the LAST solution increased abruptly at longer time when the TiO2 content was increased. As the TiO2 content was increased, the peak position of β-spodumene phase formation in DTA curves was shifted to a lower temperature. For calcination of LAST gels at 800°-1200 °C, the crystallized phases are composed of the major phase of β-spodumene and a minor phase of rutile (TiO2). Unlike earlier studies, heating the dried LAST gels from 800 °C to 1200 °C did not show β-eucryptite, nor found γ-spodumene.

The upper and lower bounds of the bulk, shear, and Young's moduli are calculated point-by-point along the radius of SCS-6 silicon carbide fibers using the Hashin-Shtrikman equations from considerations of the Auger spectra along the fiber radius. The calculated values are in agreement with measurements of the average Young modulus obtained over relatively large radial regions of the fiber using scanning acoustic microscopy (SAM). The validity of the Hashin-Shtrikman bounds calculations is enhanced by the agreement found between experimental SAM V(z) curves for fixed z along the fiber radius and the theoretical bounds of such curves obtained from the bounds of the calculated bulk and shear moduli.

Seven batches of a high-lead glass were used for the preparation of RuO2-based thick film resistors. Investigation of their electrical properties showed a lack of reproducibility of results whose origin was related to changes of the physicochemical properties of the glassy matrix. A systematic investigation of the glass batches, both in form of frit powders and screen printed and fired layers, was carried out with several spectroscopies to detect changes in composition and structure. The spectroscopic methods included x-ray Energy Dispersive Fluorescence (EDS), Scanning Electron Microscopy (SEM), Atomic Absorption (AA), diffuse optical reflection of the powders and specular reflection of the layers, optical transmission, and other complementary methods. The dissolution of Al, due to interaction between the glasses and the alumina substrate, as well as the diffusivity and solubility of Ag due to interaction with the Ag-bearing terminations were measured. The results demonstrated that, apart from small compositional differences, the various batches were characterized by differences in residual stresses, redox reactions, and “microstructure.” The latter was responsible for very notable differences in the optical properties of the glasses, which in turn are closely related with the difference in atomic solubility and diffusivity. Optical spectroscopies have been found to be a suitable means for testing reproducible preparation methods of glass frits for thick-film hybrid microelectronics.

Nanocomposites involving iron particles in silica glass matrix have been synthesized by the hot pressing of suitably reduced precursor gel powders. The metal particles have diameters in the range 3.8 to 10.2 nm. An almost four orders of magnitude resistivity range at room temperature has been obtained by such changes in particle diameters. The resistivity in the temperature range 200-340 K shows a fractional temperature dependence with an average value of n ∼ 0.69. The resistivity changes in this temperature region can be explained on the basis of an electron tunneling mechanism.

The optical absorption edge of the sol-gel derived borosilicate, silica, and phosphosilicate glasses containing heavy-metal oxides PbO and TeO2 exhibits a strong dependence on the temperature and composition. As the temperature is varied, a significant reversible thermochromic effect is observed in these glasses with a temperature coefficient as high as 10−3 eV/°C for most compositions, which is in general one magnitude of order greater than those of conventional semiconductor materials. The absorption edge of the glasses containing 10.24 wt. % PbTeO3 at room temperature was located at 3.76, 3.86, and 4.33 eV for the borosilicate, silica, and phosphosilicate glasses, respectively. With an increase in the heavy-metal concentration from 5.36 up to 18.62 wt. % in the borosilicate glasses, the absorption edge moved from 3.80 to 3.31 eV. On further increasing the concentration, the absorption edge no longer showed obvious red-shift. The observed optical characteristics were explained in view of the bond polarizability and structural coordination.

Fine KSbOSiO4 (KSS) powders have been prepared by a sol-gel method starting from the system of TEOS, KSb(OH)6, H2O, C2H5OH, or mechanochemical mixing method with a stoichiometric mixture of amorphous silica and KSb(OH)6. The KSS crystallized sluggishly into a tetragonal symmetry in low temperature and transformed into an orthorhombic one in higher temperature, indicating the amorphous-like powder patterns in the intermediate step. This amorphization process could be related to some redistribution of the polysilicate network structures. The mechanochemical activation energy stored in the powder surface, as well as the coexisting matrix (KSbO3), retards the transformation. It was concluded that the KSS powders composed of high molecular weight siloxane polymers, which were formed by hydrolysis in the presence of a small amount of NH3 aqueous solution, showed higher sinterability. The sintering, where the calcining process was omitted, enabled the dense tetragonal-type ceramics stabilized at lower temperature, rather than higher temperature, to fabricate.

The chemical composition of amorphous SiOx has been analyzed by oxidation studies and is found to be SiO1.7. SiO1.7 appears to be a monophasic amorphous material on the basis of 29Si nuclear magnetic resonance, high resolution electron microscopy, and comparative behavior of a physical mixture of Si and SiO2. Carbothermal reduction and nitridation reactions have been carried out on amorphous SiO1.7 and on amorphous SiO2 obtained from oxidation of SiO1.7. At 1623 K reactions of SiO1.7 lead exclusively to the formation of Si2N2O, while those of SiO2 lead exclusively to the formation of Si3N4. Formation of copious fibers of α-Si3N4 was observed in the latter reaction. It is suggested that the partial pressure of SiO in equilibrium with reduced SiO1.7 and SiO2 during the reaction is the crucial factor that determines the chemistry of the products. The differences in the structures of SiO2 and SiO1.7 have been considered to be the origin of the differences in the SiO partial pressures of the reduction products formed prior to nitridation. The effect of the ratios, C:SiO1.7 and C:SiO2, in the reaction mixture as well as the effect of the temperature on the course of the reactions have also been investigated.