Densification Energy during Nanoindentation of Silica Glass

Load/unload displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5–1.2 μm using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, H _u, and elastic recovery, E _R, at the penetration depth, h _t, of 1.0 μm are H _u= 4.1 GPa and E _R= 0.7. The following energies for total deformation, U _t, elastic deformation, U _e, and plastic deformation (i.e., densification during loading), U _p, are obtained: U _t=190, U _e=135 and U _p= 55 kJ/mol at h _t= 0.5 μm and U _t= 139, U _e= 96 and U _p= 43 kJ/mol at h _t=1.0 μm. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38–55 kJ/mol for 0.5 < h _t < 1.2 μm are very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass, but are very small compared with the single bond strength (443 kJ/mol for Si—O bond) of SiO₂.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Sulfate Decomposition and Sodium Oxide Activity in Soda–Lime–Silica Glass Melts

Reaction equilibrium constants for the sulfate decomposition process, which releases oxygen and sulfur oxide gas in soda–lime–silica glass melts, have been determined. The chemical solubility of SO₂, probably in the form of sulfite ions in soda–lime–silica melts, has also been determined. The chemical solubility value of SO₂, dissolving as sulfite, ranges between 0.02 and 0.06 wt% SO ₃ ²⁻ at 1 bar SO₂ pressure in the temperature range of 1600–1800 K. Results of square-wave-voltammetry studies and measurements of the temperature-dependent sulfur retention after the fining process of commercial float glass melts and a model soda–lime–silica melt, with 74 wt% SiO₂, 16 wt% Na₂O, and 10 wt% CaO, are presented. The measured sulfur retention data and the results of the square-wave-voltammetry studies are used to determine the equilibrium constant of the sulfate decomposition reaction in the temperature range of 1600–1800 K. The thermodynamic relations and properties found for sulfate decomposition are used to derive activities of sodium oxide in soda–lime–silica melts. Literature values for sodium oxide activities in these glass melts are rare. In this study, these activities have been determined by a method, based on the measurement of sulfate decomposition equilibrium constants and the residual sulfate concentrations in glass melts, equilibrated with almost pure sodium sulfate galls.     

Effects of Bi2O3 and Na2O on the Thermal and Dielectric Properties of Zinc Borosilicate Glass for Plasma Display Panels

An approach to select appropriate network modifiers to tailor the thermal and dielectric properties of zinc borosilicate (ZBS) glass has been explored to apply the glass composition to a dielectric layer of plasma display panels. Based on ionic field strength of the modifiers and the ionic polarizability of the corresponding crystalline form, Bi₂O₃ and Na₂O modifiers have been selected to modify the thermal and dielectric properties of the glass toward the required properties for the application (440< T _g<460°C, 7.5 × 10⁻⁶−6/K, and 10< K <15). Bi₂O₃ addition to ZBS could meet all the required properties simultaneously at a given addition range (8.5–14 mol%), while the addition of Na₂O could not meet all the properties at a single composition range.     

Electrochemical Studies in Glass: I, The System NiO-Na2Si2O5

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO _{a =1}∥ZrO₂+7.5% CaO∥Ni:NiO _{a <1}+glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na₂Si₂O₅ and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na₂Si₂O₅ proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.     

Nucleation and Crystallization Kinetics in Fragile Glass-Forming Liquids

Isothermal kinetics of crystallization in the "fragile" Ca(NO₃)₂─KNO₃ melts and in AgI─Ag₂SO₄─Ag₂WO₄ melts of intermediate fragility have been investigated using singlestep and multistep calorimetric techniques. Time-temperature-transformation curves for crystal nucleation and growth have been delineated and the temperature for maximum nucleation rate ( T _NN) identified. The results are compared with the kinetics of nucleation observed in other fragile systems, such as fluoride glass melts, and with classical oxide melts (such as Li₂Si₂O₅) which have "strong" liquid characteristics. Reduced-temperature presentations of nucleation-rate data show qualitative correlations between T _NN/ T _L ( T _L is liquidus temperature) and liquid fragility. These correlations show that strong-liquid glass formers survive much larger supercoolings without nucleation than do fragile liquids.     

Structural Similarities Between a Glass and Its Melt

It is suggested that the large expansion coefficient increase that occurs near T _g, for mixed oxide glasses is related to cooperative thermal displacements involving polyanionic clusters in the liquid state. This is illustrated by the striking relation between melt isoexpansion coefficient contours and structure for these ternary oxide systems. It is also shown that the molar volume dependence on composition for a series of oxide glasses can be quite similar to that observed for their liquid state analogs. The adjusted melt partial molar volume models can be used to develop realistic glass models, suggesting structural similarities between an oxide glass and its corresponding high-temperature melt. Recently acquired evidence for alkali borogermanate compositions indicates a shift toward the right for the GeO₆⇄GeO₄ equilibrium in GeO₂₋rich melts at relatively high temperatures. Thus, an exact structural correlation between an oxide melt and its corresponding glass may, in some cases, be limited to temperatures below T _g+Δ T (where ΔT ~300° to 400°C).     

Boron Nitride Interphase in Ceramic-Matrix Composites

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF₃─NH₃, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH₃SiCl₃─H₂ at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF₃─NH₃ gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO₂/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO₂ interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.     

Insight into solute-solute and solute-solvent interactions of semicarbazide hydrochloride in water and D-glucose/D-sucrose+water solutions at temperatures (293.15 to 318.15) K

The solute-solute and solute-solvent interactions of drug semicarbazide hydrochloride with carbohydrates (D-glucose/D-sucrose) are investigated by using volumetric, viscometric and acoustic properties. The measurements of the densities ρ, ultrasonic speeds u, and viscosities η. of semicarbazide hydrochloride in 5% and 10% D-glucose/D-sucrose+water (w/w) solutions were carried out at temperatures (293.15-318.15) K and at pressure, p=101 kPa. The apparent molar volumes, V_ϕ, limiting apparent molar volumes,V_ϕ°, apparent molar compressibilities, K_{s, ϕ}, limiting apparent molar compressibilities, K_s,ϕ°, partial molar expansibilities, E_ϕ°, transfer volumes, V_ϕ,tr° and transfer compressibilities, K_s,ϕ,tr° have been calculated from the experimental data. The viscosity data were examined by using the Jones-Dole equation and the viscosity A and B coefficients were evaluated. The results are discussed in terms of solute-solute and solute-solvent interactions in these solutions. The structure making/breaking ability of semicarbazide hydrochloride is examined using the sign of temperature derivative of B-coefficient, dB/dT.     

Oriented Structure and Crystallography of Directionally Solidified LaB6—ZrB2 Eutectic

Directional solidification of LaB₆—ZrB₂, by use of an electron beam heating technique, yielded oriented ZrB₂ fibers in a LaB₆ matrix. The average diameter of the ZrB₂ fibers was ∼0.2–1.2 µm, with fiber lengths up to 100 µm. Primary platelike LaB₆ dendrites formed upon the solidification of an ingot with a composition of LaB₆—18 wt% ZrB₂. LaB₆ was the first phase to nucleate when eutectic growth occurred, and ZrB₂ showed nonfaceted growth. For the ingot solidified with planar growth the orientation relations of the phases were as follows: growth direction, [001]_LaB6∥[00.1]_ZrB2; interfacial plane, (11.0)_LaB6∥(11.0)_ZrB2.     

Solid Solubility and Precipitation in a Single-Crystal Alumina–Zirconia System

Solid solubility was examined in Zr-doped sapphire and Al-doped yttria-stabilized zirconia (YSZ) single crystals from 1200° to 1600°C. Specimens were fabricated via ion implantation of single crystals, followed by annealing in air. Secondary ion mass spectroscopy (SIMS) was used to quantify solute redistribution during annealing. Comparison of SIMS results with analytical electron microscopy (AEM) revealed an alumina solubility of 0.2–0.3 wt% in zirconia, and a zirconia solubility of 0.004–0.027 wt% in alumina. Direct imaging of zirconia precipitates revealed that tetragonal zirconia precipitates from supersaturated sapphire with the following orientation relationship: (100)_tetragonal‖ (0001)_sapphire and [01¯1]_tetragonal‖ [12¯10]_sapphire.     

Microstructural Evolution in Near-Eutectic Yttrium Silicate Compositions Fabricated from a Bulk Melt and as an Intergranular Phase in Silicon Nitride

Near-eutectic composition Y₂O₃─SiO₂ melts were formed as bulk samples or as an intergranular phase in Si₃N₄. Upon cooling to room temperature the bulk material partially crystallized to γ-Y₂-Si₂O₇ whereas the intergranular phase was glass. On heat-treating at 1500°C the bulk material transformed to γ-Y₂Si₂O₇ whereas the intergranular glass crystallized first to γ-Y₂Si₂O₇ and then to β-Y₂Si₂O₇. Possible reasons for the different behavior are discussed.     

Iron and Spinel Precipitation in Iron-Doped Sapphire

Single crystals of Al₂O₃ containing 0.5 wt% Fe were exposed to low p o₂ atmospheres at 1500°C to produce precipitate phases. Analytical TEM identified the precipitate phases as spinel (hercynite) and iron, with the following orientation relationships: 〈001〉_Fe‖〈2 2 01〉_s with {1 1 0}_Fe‖{11 2 0}_s, 〈111〉_Fe‖〈10 1 0〉_s with {0 1 1}_Fe‖ (0001)_s, and 〈1 1 0〉_H‖〈01 1 0〉_s with {111}_H‖ (0001)_s. The three phase fields observed — (Fe, Al)₂O₃, spinel +α-Al₂O₃, and iron +αAl₂O₃— are in accordance with phase stability diagrams. Precipitation kinetics indicate that oxygen is mobile in the reduced region of the crystal.     

Preparation of Cobalt- and Fluorine-Doped Barium Hexaferrite by the Glass Synthesis Method

Doped barium hexaferrite (BaFe_12-xCo_xO_19-xF_x) was prepared by the glass synthesis method. The fluorine improves the pouring of the melt and the crystallization of the glass, while the cobalt decreases the coercive force. Addition of Al₂O₃ does not significantly alter the properties of barium hexaferrite, but it increases the crystallization. Doped products are attractive for high-density vertical magnetic recording (1400 < H _c < 2300 Oe, 60 < σ_s < 68 (emu cgs)/g, and σ_r/σ_s≃ 0.5).     

Consolidation of Participate Layers in the Fabrication of Optical Fiber Preforms

Viscous sintering is experimentally identified through electron microscopy as the mechanism of consolidation of thin particulate layers in the fabrication of optical fibers. The rate of consolidation is primarily a function of the capillary number, C=[νl₀(1 -ε₀)^1/3/(σ t _s)], where ν is the glass viscosity, l ₀ the size of the initial void regions, ε₀ the initial void fraction, σ the surface tension, and t_s the sintering time. Numerical results indicate a sensitivity of the sintering rate to temperature, chemical composition of the particles, and gas thermal properties, primarily through the strong dependence of glass viscosity on these variables. These results are fully supported by the available experimental evidence. Under the conditions of the experiments, gas bubbles were occasionally produced; the causes are discussed.     

Toughening of a Particulate-Reinforced Ceramic-Matrix Composite by Thermal Residual Stress

The micromechanics involved in increased crack growth resistance, K _R, due to the addition of TiB₂ particulate in a SiC matrix was analyzed both experimentally and theoretically. The fractography evidence, in which, the advancing crack was attracted to adjacent particulates, was attributed to the tensile region surrounding a particulate. Countering this effect is the compressive thermal residual stress, which results in the toughening of the composite, in the matrix. This thermal residual stress field in a particulate-reinforced ceramic-matrix composite is induced by the mismatch in the coefficients of thermal expansion of the matrix and the particulate when the composite is cooled from the processing to room temperature. The increase in K _R of the composite over the monolithic matrix, which was measured by using a hybrid experimental-numerical analysis, was 77%, and compared well with the analytically predicted increase of 52%. The increase in K _R predicted by the crack deflection model was 14%. Dependence of K _R on the volume fraction of particulates, f _p, and of voids, f _v, is also discussed.     

Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass

A SiO₂─Al₂O₃─CaO─CaF₂ ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.     

Effect of Nature of Surfaces on Wetting of Sapphire by Liquid Aluminum

Contact angles of aluminum drops on sapphire measured under vacuum conditions from 660° to 1250°C generally fell into three ranges. Large obtuse contact angles indicating interfacial energies greater than either of the two surface energies were obtained up to about 900°C; van der Waals bonding then existed at a compound interface. In the intermediate range, contact angles were 90° or slightly greater indicating a common interface with an energy, _sγ_l, greater than _sγ_g but less than lγ_g. Acute contact angles indicating a _sγ_l less than _sγ_g and greater than _lγ_g occurred above about 950°C because of the formation of a high temperature complex surface structure with _sγ_g > _lγ_g. A hydroxylated sapphire surface has a lower _sγ_g which increases with gradual dehydroxylation and conversion to the high temperature surface structure with a corresponding change in contact angle through the three ranges. Chemical bonding existed in the latter two ranges. Reactions occurred between Al and the sapphire surface to form volatile species at contact angles less than 90°. Molten Al normally has an oxide coating the effect of which appears to be removed at about 870° C.     

Use of Fracture Mirrors to Interpret Impact Fractures in Brittle Materials

The impact resistances and fracture mirror radii ( r_m ) of rods of several ceramic materials were measured. The fracture stresses (σ _f ) were determined from σ _f vs r_m^−1/2 curves obtained from fiexural strength tests. An analysis, based on the assumption that the principal factor contributing to the impact energy absorbed is the energy ( U_e ) required to deflect the specimen to the fracture stress, indicated that the impact energy absorbed ( U ) per unit of specimen cross-sectional area ( A ) increased in proportion to the square of the maximum stress. The analysis also indicated that the slopes of the curves of U/A vs σ _f ² are proportional to the reciprocal of Young's modulus. Experimental data for several materials are consistent with this analysis.