The internal tensile stress in heteroepitaxial films of cubic (β) SiC grown on Si substrates by chemical vapor deposition at 1520–1650 K has been determined from the resonant frequencies of membrane samples formed by removal of a central circular region of the substrate. At 293 K, the stress falls in the range 250–500 MPa, and tends to be larger for samples grown at higher temperatures. The temperature dependence of the stress has been studied over the range 90–670 K and found to exhibit a reversible, mild, but convoluted variation that includes a minimum near 160 K and a maximum near 400 K. Analysis of this behavior reveals that the thermal expansion of β–SiC is remarkably close to that of Si, with an expansion curve that intersects that of Si near 80 K and 500 K. This behavior is different from that obtained from available handbook data for α–SiC, and suggests that data for α–SiC should not be relied on to estimate the overall magnitude of the thermal stress component in β–SiC films. Annealing studies carried out in the range 673–1473 K on as-prepared membranes indicate a high degree of membrane stability, with no evidence of stress relaxation. It has also been observed that annealed membranes may exhibit very low levels of mechanical damping, corresponding to Q-values in the range 106−107.

In this paper a model is presented for predicting the structural changes that occur in a metaphosphate glass during nitriding in anhydrous ammonia. Predicted values for the nitrogen content, the mole fraction of phosphate groups, the bridging to nonbridging oxygen ratio, and the ratio of the structural types for nitrogen are compared to the values obtained in earlier experimental work. A plausible correlation is found between the values predicted from this structural model and those observed experimentally. In addition, a maximum nitrogen content (N/P = 0.9) is predicted and the lower nitrogen content normally observed is explained.

The mullite formation process in both single phase and diphasic sol-gel precursors to mullite was studied using dynamic x-ray diffraction (DXRD). A metastable, tetragonal-like mullite phase was observed in all the single gels at temperatures from 980 °C to 1200 °C, but not in any of the other precursors. The tetragonal to orthorhombic mullite transformation was very slow as the lattice parameters, a and b, split and moved gradually away from each other as a result of a gradual decrease of alumina content in the mullite solid solution with increasing temperature from 1100 °C to 1200 °C. The formation of tetragonal mullite coincides with that of the Al–Si spinel. The occurrence of tetragonal mullite or the spinel (or both) is determined mainly by the processing conditions of the sol-gel precursors.

The oxidation of liquid Al–Mg–Si alloys at 900–1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl2O4 allows the continuous formation of an Al2O3-matrix composite containing an interpenetrating network of metal microchannels at 1000–1350 °C. An initial incubation period precedes bulk oxidation, wherein Al2O3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ∼1300 °C, with a nominal activation energy of 270 kJ/mole for an Al−2.85 wt. % Mg−5.4 wt. % Si alloy in O2 at furnace temperatures of 1000–1300 °C. An oscillatory rate regime observed at 1000–1075 °C resulted in a banded structure of varying Al2O3-to-metal volume fraction.

Smooth, continuous TiB2 films were deposited on TiB2−AlN composites and Si substrates by sputtering in high vacuum. Excellent adhesion between films and substrates was indicated; no film delamination was induced by Vickers indentation at high loads. Analyses by secondary ion mass and x-ray photoelectron spectroscopies showed that the films were of high purity and contained H, C, O, N, and Na elements as trace impurities. Quantitative depth profile by the elastic recoil detection (ERD) nuclear scattering technique using 30 MeV 35Cl beam revealed that the Ti/B atomic ratio was very close to 0.5 while the 10B/11B isotopic ratio was 0.250.

Two sets of submicronic mullite whiskers, which have potential applications for fiber reinforced composites or as a thermal insulator, have been characterized to be tetragonal or orthorhombic using x-ray and electron microscopy techniques. The whiskers decompose upon heating under vacuum with a continuous loss of silicon and reduction in oxygen content up to the limit for which pure aluminum metal and α-alumina are formed.

The outgassing behavior and mechanical properties of polysiloxane based and phosphorus doped silicate based films as planarization candidates for device processing were evaluated using various analytical techniques. After curing between 370 °C and 450 °C, a high temperature rebake above 410 °C caused twice the weight loss in polysiloxane based films as in silicate films. This means that further outgassing, which could occur to a greater degree from polysiloxane than from silicate, could lead to a more probable blistering within the interlayer of the sandwiched spin-on-glass (SOG) during subsequent thermal processing. However, a well-cured polysiloxane would be a better candidate for planarization applications because the film was found to absorb less moisture and had lower stress than the silicate. Due to high silanol content and high porosity in silicate, it was found to absorb six times more water than polysiloxane. When water evolved, significantly higher stress levels were observed in silicate than in polysiloxane during thermal cycle tests. Infrared spectroscopic analysis revealed that polysiloxane contained Si–O–CH3 moiety, which rendered the film flexible, while silicate contained near-stoichiometric SiO2 bonds, which made for a more rigid and dense structure. This difference in the film structures translated to three times higher stress in silicate than in polysiloxane. During device processing, it was seen that silicate films were more prone to cracking than polysiloxane films. The components of the outgassing materials were either volatile organic species from residual solvents not completely burned out during cure, or carbon dioxide and water vapor as by-products from further cure. Gas chromatography indicated that both types of films contained volatile organic residues when cured at 370 °C. However, at 410 °C, volatile organic species were present in the polysiloxane but not in the silicate. A 30 to 60 min cure at temperature greater than 410 °C was then found to adequately cure polysiloxane. It was concluded that a “well cured” polysiloxane based spin-on-glass (SOG) would be more suitable than a silicate based SOG for planarization application.

A general method that predicts the thermal behavior of optical fibers during the cooling stage of the drawing process was developed. The method can be used for thin diameter D < 200 μm, medium (200 μm < D < 500 μm), and thick (0.5 mm < D < 2 mm) single as well as core-clad fibers. A two-dimensional analysis implementing a finite difference method combined with the Kármán–Pohlhausen technique was performed to obtain the temperature profiles in thick fibers. This method accounted for axial and radial heat conduction, and can also be applied to thin and medium fibers. The case of the core-clad fibers was investigated to obtain the temperature profiles both in the radial and the axial directions. All results are presented in graphical form and can be used for optimization of the drawing process.

Characterizations of side chain liquid crystalline polymers and their monomers point toward chemical structures and processing techniques which optimize their suitability for nonlinear optical applications. Polymers with methacrylate backbones and nitrostilbene or nitrobiphenyl side groups are studied; they tend to form smectic phases, but no solid crystallinity. By copolymerizing with nonmesogenic backbone units, the smectic-isotropic transition temperature can be controlled and may fall below the glass transition temperature. There is evidence for a significant degree of pretransitional alignment due to the surface fields, and mesogen ordering perpendicular to flow-induced backbone alignment. Very rapid cooling can suppress the highly scattering polydomain smectic phase.

Phosphor packages for a laser-heat thermoluminescence radiation-dosimetry system have been analyzed with Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy. Samples consist of 20–50 μm diameter powder grains of CaSO4: Tm and LiF: (Mg, Ti) phosphor embedded in a transparent silicone matrix about 60 μm thick. Our principal finding with regard to layer morphology indicates an inhomogeneous outer layer of areal density at least ∼300 μg/cm2 depleted of phosphor and contaminated with boron.

Bridged quasi–one-dimensional macrocyclic transition metal complexes [MacM(L)]n using phthalocyanine (Pc), tetrabenzoporphyrine (TBP), 1,2- and 2,3-naphthalocyanines (1,2-Nc; 2,3-Nc) as macrocycles (Mac) with M = e.g., Fe, Ru, Co and L = e.g., pyrazine (pyz), 1,4-diisocyanobenzene (dib), tetrazine (tz) exhibit interesting electrical properties. Regardless of the size of the bridging ligand after either chemical or electrochemical doping stable semiconducting compounds [MacM(L)Xy]n (X = e.g., I2 BF4−, ClO4−…) are formed. For the first time the complete characterization of soluble oligomers [MacM(L)]n (Mac = t-Bu4Pc, M = Ru, L = dib, me4dib) by 1H-NMR spectroscopy is reported. Some of these shish kebab complexes, e.g., [PcM(tz)]n, (M = Fe, Ru) or [MacMCN]n; (Mac = Pc, TBP; M = Co, Fe) exhibit good semiconducting properties without additional external doping. Whether or not these compounds are intrinsic semiconductors will be discussed. The synthesis of oligomeric bridged mixed valence compounds [PcM(III)LPcM(II)L]n, (M = e.g., Fe) is another target of our work. We report here on first results of these oligomers.

The reaction between tantalum ethoxide and an inorganic, silicon-carbon based polymer known as polycarbosilane resulted in a modified polymer that could be thermally converted into a binary ceramic of SiC and TaC. In this report, the initial reaction of the precursors and the high temperature transformations that resulted in the mixed ceramic carbide are discussed. The synthesis of this modified polymer was characterized using 29Si, 13C NMR, and infrared spectroscopy. The reaction involved cross-linking of polycarbosilane through bridging carbon bonds and the formation of Si–OCH2CH3 ligands. According to these data and to the low-angle x-ray diffraction data, the structure of the reaction product can be described as a network of modified polycarbosilane with intimately dispersed tantalum oxide particles. The structural transformations that occurred during inert atmosphere pyrolysis of the polymer product were determined using 29Si, 13C MAS NMR, infrared and x-ray diffraction spectroscopy. Inert atmosphere pyrolysis at temperatures below 500 °C involved continued cross-linking of polycarbosilane through the endothermic formation of bridging carbon bonds. During pyrolysis at 500 °C, an exothermic reaction between the modified polycarbosilane and the intimately dispersed tantalum oxide particles was observed. This reaction involved the formation of an inorganic, amorphous oxycarbide phase that can be described as a continuous network of C–Si–O and C–Ta–O bonds. At pyrolysis temperatures exceeding 1000 °C, carbothermal reduction of the oxide constituents initiated. Further pyrolysis at temperatures exceeding 1200 °C resulted in the crystallization of zinc-blend β–SiC and NaCl structured TaC.

Acoustic energy has been primarily used in materials research to disaggregate solids and/or disperse them in fluids. It has also been used in a few studies to influence or cause new reactions in organic and organometallic systems. However, the potential of acoustic energy in the area of inorganic reactions and materials science has been virtually unexploited. We report herein new effects of acoustic waves in catalyzing reactions in the systems Al2O3 + H2O and Al2O3 + P2O5 + H2O. In this study the effect of different alumina precursors and their particle size on their reactivity with water and H3PO4 was investigated when exposed to 20 kHz waves.

In this paper, we address some fundamental questions regarding the response of a crack to externally generated dislocations. We note that since dislocations that formed at external sources in the material must be in the form of loops or dipoles, the theory must be couched in terms of crack shielding in a plastically polarizable medium. There are strong analogies to dielectric theory. We prove two general theorems: (1) Dipoles formed in the emission geometry relative to a crack tip always antishield the crack and (2) when dipoles are induced during uniform motion of a crack through a uniformly plastically polarizable material, then the net shielding is always positive. We illustrate these general theorems with a number of special cases for fixed and polarizable sources. Finally, we simulate the self consistent time dependent response of a crack to a polarizable source as the crack moves past it. The results show that the crack is initially antishielded, but that positive shielding always dominates during later stages of configuration evolution. The crack may be arrested by the source, or it may break away from it, depending upon the various parameters (source strength and geometry, dislocation mobility, Griffith condition for the crack, etc.). The results indicate that the time dependence of crack shielding in the presence of a nonuniform density of sources will be very important in practical cases of brittle transitions in materials.

The structure of amorphous tungsten trioxide films was investigated with IR and Raman spectroscopic analyses and an XRD method. The films were prepared by ion exchange from sodium tungstate as a starting material. Films consisted of microclusters of 10–30 Å diameter, in which the networks are formed with WO6 octahedra sharing their corners and edges. The networks in the as-prepared samples consisted of WO6 units with low symmetry, in which termination by W=O and W—OH2 groups was common. As the post-annealed temperature became higher, the symmetry of WO6 was improved and the edge-sharing octahedra disappeared.

GeS2 is known to be a good chalcogenide glass former with a transmission cutoff at 11 μm and has been studied for fiber optic application in the mid infrared region. The rare earth sulfides, oxysulfides, and oxides (La–Er) form reasonably good and stable glasses when mixed with chalcogenides such as Ga2S3. In this work, glass formation was studied in the GeS2−La2S3 system. Two compositions containing 60 mol % and 92.5 mol % GeS2, respectively, were analyzed, and the effects of composition on the microstructure and thermal stability of these glasses were investigated. Microstructural studies were conducted on the as-prepared and heat-treated glasses using TEM and SEM/EDXA. Glasses rich in GeS2 exhibited primary (6–88 nm) and secondary (3–13 nm) phase separation at the molecular level. Differential thermal analysis performed on these glasses indicated glass transition temperatures (Tg) of 510 °C and 420 °C for the two compositions studied. The glasses were stable and the (Tg) was observed to decrease with increasing contents of GeS2 in these glasses.

TiO2 rutile thin films grown on (110) sapphire (α−Al2O3) by the MOCVD technique have been characterized by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The TiO2 rutile thin films grew on the sapphire with two epitaxial orientations. The epitaxial orientation relationships between the rutile films (R) and the sapphire substrate (S) were found to be (1) (101)[010]R ‖ (110)[0001]s. Detailed atomic structures of near-interface regions have been investigated by HREM, providing a clear picture of the initial stage of film growth. HREM images show that about 70% of the nuclei at the interface are the (101) rutile, but most of them are very small, about 5 nm (or 2% of the film thickness) in the growth direction. The film growth was dominated by the (200) orientation. Nucleation and growth of the films will be discussed in terms of the lattice mismatch at the interface and growth rates along the two orientations. Planar defects such as twin boundaries and special grain boundaries are commonly observed in the films, especially in regions close to the substrate. The twin plane and twinning direction are {101} and 〈101〉, respectively. Special grain boundaries are found to be correlated with nucleation and twinning.

NbVO5 is characterized by 51V and 93Nb NMR resonance frequencies strongly upfield shifted when compared to those in model compounds V2O5 and LiNbO3. The chemical shift anisotropy dominates the 51V observed spectrum in a magnetic field of 11.7 T. The asymmetry parameter ηc is 0.2 and the quadrupole coupling constant is relatively small (1 MHz). The quadrupolar Hamiltonian overwhelmingly dominates the 93Nb spectrum (ηQ = 0.9) and the quadrupole coupling constant is huge (16.5 MHz). In agreement with the structure obtained from the x-ray powder diagram the isotropic chemical shift of 51V suggests that NbVO5 is indeed an orthovanadate. Interestingly, in NbVO5 the isotropic chemical shift of 93Nb reveals a better shielding of the 93Nb nucleus and a lower electric field gradient than in LiNbO3. Nb octahedra in NbVO5 are sharing corners whereas they share edges in LiNbO3.

Phase transformations of Al2O3 and Na2O · 6Al2O3 prepared by the gel route have been investigated for the first time by 27Al MAS NMR spectroscopy in combination with x-ray diffraction. Of particular interest in the study is the kinetics of the γ → α and γ → β transformations, respectively, in these two systems. Analysis of the kinetic data shows the important role of nucleation in both these transformations.

A method for making glass microspheres by the sol-gel process has been developed. Porous silica microspheres were produced at temperatures as low as 500 °C and densified silica microspheres were prepared at 800 °C. The size of the microspheres was controlled by adjusting the feed rate of the raw materials and the frequency of the droplet generator so that uniform spheres were obtained. Glass microspheres prepared by this method were characterized by Nuclear Magnetic Resonance (NMR), Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and optical microscopy.