Synthesis of Nanostructured TiN Using a Two-Step Transition Metal Halide Approach

A two-step transition metal halide approach has been developed for synthesizing uniform ultra-fine titanium nitride (TiN) nano-crystallites (<10 nm) at relatively low temperatures of 400°C. The nano-sized TiN crystallites were obtained by reacting liquid titanium tetrachloride dissolved in anhydrous chloroform with anhydrous NH₃ gas at room temperature followed by heat treatment under ultra-high-purity Ar, N₂, and anhydrous NH₃ atmosphere. The synthesized TiN nano-crystallites were characterized by X-ray diffraction, Brunauer–Emmett–Teller, pycnometer, differential thermal analysis/thermo-gravimetric analysis, and high-resolution transmission electron microscopy. The effect of the heat treatment temperature and atmosphere (Ar, N₂, and NH₃) on the specific surface area, density, crystal structure, and composition is discussed.     

Thermal Decomposition in the System Si-Y-AI-O-N

The limited transparency of high N (13.3 at.%) glasses in the system Si-Y-AI-O-N was found to be due to Si precipitates that resulted from thermal decomposition during melting. Thermodynamic analysis supported experimental findings that decomposition in N₂ at 200 kPa resulted from oxide-nitride reactions and not from simple dissociation of Si₃N₄. The Si content of the glass was reduced much more by substitution of AIN for Si₃N₄ in the glass batch and melting at low N₂ pressure (200 kPa) than it was by merely melting in N₂ at high pressure (1.6 MPa). The thermodynamic analysis suggested that decomposition could be minimized effectively by melting under a modest partial pressure of SiO(g), but this could not be verified experimentally.     

Effect of Melting History on the Mechanical Properties of Glass: I, Role of Melting Time and Atmosphere

The pristine strength of 0.5Li₂O·0.5K₂O·2SiO₂ glass measured in 3-point bending under liquid nitrogen was studied as a function of melting time and atmosphere at a constant melting temperature. The atmosphere over the melt was one of the most important parameters affecting the pristine strength. The most significant time-dependent melting effects and the highest strengths resulted from melting in a dry atmosphere of O₂ or air. In contrast, vacuum-melted and Ar-melted glasses had the lowest strengths. Melting in N₂ or CO₂ led to intermediate strengths. The presence of water vapor in the atmosphere during melting was detrimental to strength in those cases where very high strengths had been obtained with dry gases. The introduction of water vapor into those gases which in the dry state had led to low-strength glasses did not significantly affect the strength. The effects of the atmosphere in leading to oxygen-deficient or excess-oxygen glasses on the formation or dissolution of microheterogeneous regions were correlated with the pristine strengths of the glasses.     

Surface and Bulk -OH Infrared Absorption in ZrF4- and HfF4-Based Glasses

Infrared absorption at the 3400 cm⁻¹ -OH peak has been measured as a function of thickness for several ZrF₄-BaF₂-LaF₃ and HfF₄-BaF₂LaF₃ glasses to separate contributions from bulk and surface -OH. For glasses melted under CCl₄ reactive atmosphere, the peak is due almost entirely to surface -OH. In ambient atmosphere the -OH peak exhibited no time dependence over a 30 d period, indicating a very small rate of surface attack by atmospheric H₂O.     

Yttrium-Silicon-Aluminum Oxynitride Glass Fibers

Fibers were made from Y-Si-Al-O-N glasses previously shown to have excellent mechanical properties and outstanding water corrosion resistance. Fibers of glasses containing, respectively, 3.2 and 6.6 wt% N were pulled free-hand in air and from glass rods in N₂ Continuous fibers (up to several kilometers long) of the former glass were melt-drawn in N₂ while being wound in air outside of the glass-melting furnace. The fibers, some as small as 10 μm in diameter, retained the desirable properties of the bulk glass.     

Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses

Iron exists in Fe²⁺ and Fe³⁺ states in CaO–P₂O₅–Fe₂O₃ glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.     

Influence of As2O3 on the Optical Properties of Ultrapure Multicomponent Silicate Glasses and Optical Fibers

To obtain low transmission loss optical fibers from ultrapure multicomponent silicate glasses, it is necessary to add small quantities of As₂O₃ (or Sb₂O₃). In optical fibers prepared from glasses without these agents, a significant increase in loss is observed. To investigate this effect, the influence of As₂O₃ (added to the batch and present in the glass as As₂O₅) on the optical properties of ultrapure silicate glasses was studied. These properties are the Rayleigh scattering loss coefficient, transition metal absorption, and position of the uv absorption edge. This study showed that the increase in loss of As₂O₃-free glass cannot be assigned to any of these contributions and was attributed to absorption by electrons, trapped in relatively shallow traps in the glass network. The As⁵⁺ ions serve as deep traps and therefore remove the additional absorption. The same phenomenon, although much more pronounced, was observed in optical fibers prepared from alkali borosilicate glasses.     

Gamma-Ray Induced Coloring of Some Phosphate Glasses

The changes in gamma-ray induced optical absorption in phosphate glasses resulting from changes in composition, conditions during melting, and additions of small amounts of some oxides are discussed. A resolution of the induced spectra showed that the observed absorption is due to the superposition of three bands at 2.3, 2.9, and 5.5 e.v. (540, 425, and 225 mü) and to a fourth band whose absorption peak is beyond 6 e.v. The ultraviolet induced absorption increases, whereas the visible absorption decreases in glasses melted under reducing conditions as opposed to those melted under normal conditions in air and on replacement of K⁺ by Na⁺ or Li⁺. A similar effect is produced on replacement of Ba⁺⁺ by Pb++ and on the addition of T1₂O to a CaO-P₂O₅ glass or the addition of As₂O₃ to a CaOP₂O₅, GeO₂ glass. Additions of large amounts of GeO₂ are accompanied by a decrease in the number of nonbridging oxygens and a decrease in the visible induced absorption. Replacement of Ca ⁺⁺ by Ba⁺⁺ ions showed an over-all decrease in the induced absorption. The addition of small amounts of the oxides of germanium, titanium, iron, thallium, niobium, and arsenic showed an appreciable effect in inhibiting the visible induced coloration.     

Coalescence of Thin Films of Gold Condensed on Glass

Thin films of gold were deposited by evaporation onto glass surfaces generated by slow fracture in a vacuum. Such films characteristically show an agglomerated or coalesced structure. Deliberate contamination of the substrate and growing films with O₂, CO₂, H₂O, and Ar by exposure to a pressure of 0.0013 Pa (1×10⁻⁵ torr) of the gas had no effect on the structure of films of an average thickness of 5 nm (50 Å). Similar treatment of the films with N₂ resulted in a decrease in the extent of agglomeration of the gold. Treatment of the substrate with N₂ prior to the deposition caused an even greater decrease in agglomeration. Films of an average thickness of 2 nm (20 Å) were not affected detectably by the N₂ treatment. The available evidence indicated that the N₂ acted directly on the glass and that the structure of the films was unaffected by minor variations in the age of the surface or by differences in the speed of the fracture which generated the surface. The film structure was not changed by extended exposure to the atmosphere.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Pyrolysis of Titanium-Metal-Filled Poly(siloxane) Preceramic Polymers: Effect of Atmosphere on Pyrolysis Product Chemistry

Pyrolysis of unfilled as well as Ti-metal-filled poly(siloxane) preceramic polymers, heated at 1350^deg;C in N₂, Ar, or NH₃, was evaluated in terms of the effect of atmosphere on pyrolysis product chemistry. Pyrolysis of the unfilled poly(siloxane) in N₂ or Ar resulted in the formation of a metastable, amorphous SiO_xC_y phase with a residual turbostratic carbon phase, with the evolution of CH₄ and H₂ as the main pyrolysis products. However, pyrolysis of the unfilled poly (siloxane) in NH₃ resulted in a partial substitution of nitrogen from the atmosphere for network carbon to form a binary mixture of amorphous SiO_xC_y and SiO_xN_y phases. Pyrolysis of the Ti-filled poly(siloxane) in NH₃ resulted in the reaction of the Ti particles with the atmosphere to form nearly stoichiometric TiN. In Ar, the Ti particles react with either gaseous hydrocarbon, i.e., CH₄, or the carbon pyrolysis products of the poly(siloxane), to form slightly nonstoichiometric TiC along with a partial reduction of the SiO_xC_y matrix. Finally, in N₂, the Ti particles react with both carbon from the poly(siloxane) and nitrogen from the atmosphere to form a solid solution of TiC and TiN.     

Optical Absorption of Lead in Glass

The uv absorption spectra caused by Pb in bulk silicate glasses and in a film sputtered from 1.08 PbO·SiO₂ glass are measured. Lead causes a strong absorption band between 5 and 6 eV. Vacuum uv measurements on the sputtered film reveal a second, much larger, absorption band that has a maximum near 7.6 eV and that is tentatively attributed to excitation of an O ion bonded to a Pb²⁺ ion.     

Concentrations of Residual Carbonate Ions in the Bi-Sr-Ca-Cu-O Glasses

It was found that the IR absorption bands appearing at 600 to 1600 cm⁻¹, which had been previously assigned to the fundamental vibrations of [BiO₃] or [BiO₆] polyhedra, are due to residual carbonate ions (CO²⁻₃) dissolved in Bi-Sr-Ca-Cu-O glasses. The concentrations of the remaining CO²⁻₃ in the Bi_2.2Sr₂Ca₁Cu₂O _x glasses that melted at 1100° and 1400°C are 170 × 10⁻⁵ mol/cm³ (3.3 mol%) and 3.2 × 10⁻⁵ mol/cm³ (0.25 mol%), respectively. The apparent activation energy for the dissociation of the carbonates was approximately 220 kJ/mol. The CO²⁻₃ content in the precursor glasses did not significantly affect the superconducting properties of the resulting glass–ceramics.     

Melting Apparatus for the Synthesis of Bulk ZrF4-Based Fluoride Glass

Heavy-metal fluoride glasses are currently prepared by melting of the solid fluoride precursors. We have constructed a melting facility for the synthesis of ZrF₄-based fluoride glass cullet. This stand-alone system provides a controlled (dry and reactive) atmosphere and in situ quenching of the melts. The glass is melted in a vitreous carbon crucible, which is mounted in a silica tube flow reactor. The reactor is resistively heated with a tube furnace and purged with He and SF₆ or Cl₂. It was found that of the latter two halogenating species only Cl₂ prevents effectively the formation of black inclusions in the melt (ZrF₃ and ZrF₂ particles). With this melting apparatus we have routinely melted 250-g batches of optically clear ZrF₄–BaF₂–LaF₃–AlF₃–NaF glass samples.     

Solubilities of Ar, N2, CO2, and He in Glasses at Pressures to 10 Kbars

Glasses containing up to several percent of Ar, N₂, and CO₂ were prepared at 2000 to 150,000 psi and up to 950°C and retained under ambient conditions. The solubilities are presented as a function of pressure, temperature, and composition of glass. Solubilities of O, He, and H₂O were also investigated in various glass compositions, especially K₂O–4SiO₂ and B₂O₃. The evolution of the gases at atmospheric pressure was followed by electron microscopy and density measurements.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part III, Effect of Nitrogen-Gas Sintering Atmosphere

Effects of N₂ sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid-phase-sintered (LPS) SiC were studied. It was found that, for the β-SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar-atmosphere case. Chemical analyses of the specimens sintered in N₂ atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β-SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N₂ atmosphere. For specimens sintered in N₂ atmosphere, a remarkable shift of the grain-boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α-SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N₂ atmosphere open the possibility of tailoring their microstructures for room-temperature mechanical properties and for making high-temperature materials that are highly resistant to coarsening and creep.     

Preparation of Titanium Nitride Films from Amide Precursors Synthesized by Electrolysis

A TiN precursor solution was synthesized by galvanostatic electrolysis of Ti metal and isopropylamine at a current density of 50 mA·cm⁻² at room temperature. TiN films were prepared by dip-coating of the precursor solution on a Si wafer, followed by two-stage heat treatment at 400°C and a fixed temperature of 800–1200°C in flowing N₂, N₂/NH₃, or NH₃ gas. The TiN films were characterized by XRD, chemical analysis, XPS, and electrical resistivity measurements. The TiN films were composed of uniform grains 20 to 200 nm in size with thicknesses ranging from 300 to 400 nm at temperatures of 800–1200°C. The effect of the heat treatment atmosphere (N₂ and NH₃) on the impurity content, crystallinity, particle size, and electrical resistivity is discussed.     

Fabrication of Continuous-SiC-Fiber-Reinforced SiAlON-Based Ceramic Composites by Reactive Melt Infiltration

A technique for fabrication of β'-SiAlON-based ceramics in three-dimensional woven fabrics of BN-coated SiC (Hi-Nicalon™) fibers was developed by reactive melt infiltration in a controlled N₂ atmosphere. β'-SiAlON was produced in situ by the reaction of β-Si₃N₄, AlN, and Y-Al-Si-O molten glass. The wettability of the fibers with the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane, resulting in fully dense matrix composites. The reaction between the fiber and molten glass could be depressed by increasing the N₂ partial pressure during the melt infiltration. The inhibition of the interfacial reaction may be related to the formation of carbon and oxynitride on the SiC fiber, in agreement with thermodynamic calculations as a function of N₂ partial pressure. The fabricated composites had a high ultimate flexure strength and a large work of fracture at room temperature. Degradation of the mechanical performance of the composites was small, even at 1773 K in an argon atmosphere.     

Dependence of Sodium Ion Mobility upon Melt Atmosphere in GeO2 Glasses

Sodium ion mobilities in high-purity GeO₂ glasses were examined for glasses melted under varying oxygen partial pressure. Increased oxygen partial pressure during melting increases the sodium ion mobility for glasses melted at the same temperature. This effect is attributed to the effect of oxygen partial pressure upon intrinsic defect concentrations within the glass.     

Phototropy of Reduced Silicate Glasses Containing the 570 m°Color Center

Simple silicate glasses of high purity and melted under strong reducing conditions develop a characteristic visible color center on absorbing ionizing radiation. This center peaks at 570 mμ in soda-silica glasses but changes position depending on the specific modifier ions. The center grows to relatively large concentrations in glass irradiated by long wavelength ultraviolet light but also decays rapidly at room temperature, imparting phototropic response to the glass. The 570 mμ center probably does not require specific ion impurities; however, minor additions of Ce, Eu, and Zr enhance its formation in certain base compositions. Transition series elements, even in small concentrations, inhibit the center. Sustained irradiation by ultraviolet light leads to lower equilibrium concentrations of the center and hence to fatigue of phototropic response.