Showing papers in "Journal of the American Ceramic Society in 1970"

Stress Corrosion and Static Fatigue of Glass

Abstract: Stress corrosion cracking of six glasses was studied using fracture mechanics techniques. Crack velocities in water were measured as a function of applied stress intensity factor and temperature, and apparent activation energies for crack motion were obtained. Data were consistent with the universal fatigue curve for static fatigue of glass, which depended on glass composition. Of the glasses tested, silica glass was most resistant to static fatigue, followed by the low-alkali aluminosilicate and borosilicate glasses. Sodium was detrimental to stress corrosion resistance. The crack velocity data could be explained by the Charles and Hillig theory of stress corrosion. It is probable that stress corrosion of glass is normally caused and controlled by a chemical reaction between the glass and water.

Initial Sintering with Constant Rates of Heating

Abstract: Initial sintering of several materials was studied by measuring powder compact densification at constant rates of heating (CRH). The CRH technique was extremely sensitive to the particle size distribution and other characteristics of the compacts. Although the CRH method circumvents several problems encountered in isothermal studies, it cannot be used to identify the mechanism of diffusion. Using the method on carefully prepared alumina powder compacts and assuming a grain-boundary diffusion mechanism, an activation energy of 115±10 kcal/mol was obtained. Zirconia (yttria-stabilized) and titania also exhibited a single densification mechanism with diffusion coefficients which correlate well with values obtained by isothermal measurements.

Magnetic Properties of Barium Ferrite Formed by Crystallization of a Glass

Abstract: A glass with composition 0.265 B2O3-0.405 BaO-0.33 Fe2O3 (mole ratio) was prepared by a fast-quenching technique. When it is heat-treated, this glass exsolves up to ∼45 wt% BaFe12O19 as the only magnetic phase. Magnetic measurements of glasses heated at various temperatures show that superparamagnetic, single-domain, or multidomain magnetic behavior is present, depending on the thermal history. The volume of a typical superparamagnetic particle (calculated from the magnetic data) is equivalent to that of a sphere 47 A in diameter. The intrinsic coercive forces of two heat-treated glasses were independent of temperature at high levels of Hci (2600 and 2900 Oe) from 77° to 300°K. Another heat-treated glass has an Hci of 5350 Oe at 300°K. Apparently, the coherent rotation model of Stoner and Wohlfarth describes the magnetic behavior of BaFe12O19 very well. The single-domain critical size for BaFe12O19 was ∼0.5 μm. An attractive feature of this system is that the BaFe12O19 powder can be recovered from the barium-borate-rich matrix by leaching with a weak acid.

Strength Behavior of Polycrystalline Alumina Subjected to Thermal Shock

Abstract: Theoretical predictions of crack propagation behavior in brittle solids under conditions of thermal shock were verified by water quenching of cylindrical polycrystalline alumina rods followed by strength testing. The calculated quenching temperature difference (ΔTO) required to initiate thermal-stress fracture agreed fairly well with experiment. When fracture was initiated, strength decreased catastrophically, in agreement with theory. An expression for the strength remaining after thermal stress fracture was derived in terms of the pertinent physical parameters. Values of surface fracture energy similar to those reported in the literature agreed with experiment. Strength after thermal shock was predicted to be inversely proportional to the 1/4 power of the rod diameter; this prediction was supported by experimental data for two rod sizes. Over a range of quenching temperature differences ΔTΔT0 strength remained constant, in agreement with the theoretical expectation that the newly formed cracks were subcritical. Only at the highest quenching temperature differences could further decreases in strength be observed; the quantitative changes, however, were masked by nonlinear deformation (evidenced by permanent crack opening). It was concluded that, although thermal shock behavior of brittle ceramics can be approximated fairly well, reliable quantitative estimates require considerably more information about strength and surface fracture energies as a function of environment, stress distribution, strain rate, and temperature and specimen size effects.

Wetting of Ceramics by Liquid Aluminum

Abstract: The wettability of AlN, TiB2, TiN, and TiC by liquid aluminum was investigated by the sessile drop technique at 2×10-7 torr or less. The contact angle decreased linearly with increasing temperature in all cases. Variation of the contact angle with temperature is correlated with the surface tension of the drop, and the Zisman-type relation is established. The critical surface tensions (Rγc) of AlN, TiB2, TiN, and TiC are 664, 700, 713, and 725 dynes/cm, respectively. These values are correlated with the melting point of the ceramics. The Zisman relation is discussed and applied to calculation of the work of adhesion. The surface tension of liquid aluminum varies linearly with temperature: γLv (dynes/cm) = 948–0.202T (°K).

Metastable Immiscibility Surface in the System Na2O‐B2O3‐SiO2*

Abstract: Opalescence and clearing techniques were used to determine the metastable immiscibility surface for sodium borosilicate solutions. These results indicate that a three-liquid region, which may or may not be metastable to two-liquid regions, underlies the immiscibility surface.

Thermal Expansion of Rutile and Anatase

Abstract: Lattice parameters of rutile and anatase were determined from 30° to 650°C and from 28° to 712°C, respectively, using a high-temperature powder camera These data were used to evaluate the coefficients of thermal expansion of rutile and anatase The temperature dependence of the coefficients, α11 and α1, parallel and perpendicular to the principal axis, respectively, are represented by the equations: Rutile: Anatase: The relative magnitudes of the coefficients of thermal expansion of these polymorphs are explained in terms of the interionic distances

Effect of TiO2 on Initial Sintering of Al2O3

Abstract: Alpha alumina with additions of TiO2 sintered more rapidly than “pure” alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO2 in Al2O3. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The “solubility limit” increased with decreasing particle size, indicating an excess surface concentration of TiO2. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO2 concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.

Crystal Structure of Monoclinic Hafnia and Comparison with Monoclinic Zirconia

Abstract: The crystal structure of monoclinic HfO2 was determined using Weissenberg techniques. Data were obtained from a needlelike single crystal which was grown from a lithium molybdate melt. The crystal was rotated about the α axis, and reflection data were obtained for nine levels. A three-dimensional least-squares refinement confirms that monoclinic hafnia is isomorphous with monoclinic zirconia. The atomic coordinates in the structures agree within one standard deviation. Thus, the fractional coordinates for the metal atom differ by less than 0.0005 and those for the oxygen atoms by no more than 0.01. Whereas three of the seven Hf–O distances are larger than the corresponding Zr–O distances, the average value is approximately 0.01 A smaller. The average metal-metal distance is approximately 0.02 A less in HfO2 than in ZrO2.

Stress and Volume Relaxation in Annealing Flat Glass

Abstract: Laboratory simulation of the industrial process of annealing sheet glass has yielded data on the genesis of stresses in initially stress-free glass. The experimental results differed from expectations based on classical annealing theory in that stresses began to develop in the annealing range even when the glass was being cooled at a constant rate, i.e. even in the absence of any changes of temperature gradients within the glass. Typically, these stresses account for 40% of the total residual stress in glass annealed according to a linear schedule. The remaining 60% are the well-known thermoelastic stresses that arise later in the annealing process from the decay of temperature gradients in the glass. The stresses observed to arise in glass as it is being cooled at a constant rate are attributed to volume relaxation effects which, in parts of the annealing range, generate stresses rapidly enough that they are not dissipated by stress relaxation. A mathematical model of annealing is proposed that takes account of both stress and structural relaxation. The model fits the experimentally observed evolution of stresses during linear cooling. It also suggests that (with the activation energies of stress and structural relaxation about the same) the actual rate, at any given temperature, of structural relaxation is about 4 times lower than that of stress relaxation.

Dynamic and Static Fatigue of Silicate Glasses

Abstract: Fatigue of silicate glasses was studied using both static and dynamic tests. Static fatigue data for acid-etched soda-lime silicate glass determined at 74°F in 50 and 100% rh can be represented by a single universal fatigue curve (UFC). The UFC for acid-etched glass does not lie on the UFC of Mould and Southwick, which was determined for abraded soda-lime silicate glass; the acid-etched glass was less susceptible to static fatigue. The susceptibility of acid-etched soda-lime silicate glass to static fatigue differed little from that of pristine E-glass and fused SiO2 fibers. Dynamic fatigue data for soda-lime, E, borosilicate, and fused quartz glasses agreed well qualitatively and quantitatively with fundamental crack velocity data for these glasses; the dynamic fatigue theory of Charles was used in the comparison. The theoretical implications of these results are discussed.

Crack Healing in Glass

Abstract: Cracks in soda-lime-silica glass specimens closed spontaneously; the recovery in strength was determined by fracture mechanics techniques. Approximately 80% strength was recovered in cracks formed by mechanical shock, whereas approximately 20% was recovered in cracks that closed after being held open to the atmosphere for several minutes. The high strength recovery in the mechanically shocked specimens is attributed to the very active surface formed during fracture. If the surface is allowed to adsorb O2 or H2O vapor, the activity is reduced, and healing is less complete. Crack healing can introduce surface flaws into glass that cannot be detected by current methods of nondestructive testing.

Elastic Properties of Oxide Solid Solutions: The System Al2O3−Cr2O3

Abstract: Sound velocities, densities, and lattice parameters of hot-pressed bodies of the solid-solution series Al2O3−Cr2O3 were investigated. Measured lattice parameters deviated slightly from Vegard's law. The compositional dependence of the longitudinal sound velocity agreed well with the predictions of Birch; the observed dependence of the bulk modulus on ionic volume did not agree with the relation of Anderson and Nafe.

Direct‐Current Polarization During Field‐Assisted Glass‐Metal Sealing

Abstract: Thin plates of borosilicate glass were polarized near the annealing point with a dc voltage of typically 600 V. From the measurements of capacitance and current, the width of the polarized region and the electric field within the region were deduced. The results indicate that the glass layer adjacent the anode becomes depleted of positive sodium ions and of compensating negative charge. As a result, the electric field in the layer acquires a maximum value of the order of 106 V/cm. Some implications of the process with regard to field-assisted glass-metal sealing are discussed.

A General Model for the Intermediate Stage of Sintering

Abstract: A model proposed for intermediate-stage sintering relates the densification rate to the instantaneous geometry in the compact and to the volume and grain-boundary diffusion coefficients, despite concurrent surface diffusion, vapor transport, and/or grain and pore growth. Quantitative stereological methods can be used to measure the necessary geometric parameters. The suggested experiments would provide a detailed description of the evolution of the microstructure and calculated values for the grain-boundary and/or volume diffusion coefficients.

Diffusion and Internal Friction in Na-Rb Silicate Glasses

Abstract: The internal friction and self-diffusion coefficients of sodium and rubidium ions for (l-X)Na2O-XRb2O-3SiO2 glasses were measured. Diffusion was measured by radioactive isotopes and a thin-sectioning technique at 350° to 500°C. Internal friction was measured at −140° to 500°C and 0.05 to 6000 Hz. The maximum height for the mixed-alkali internal friction peak occurs at the composition at which the Na and Rb diffusion coefficients are equal. It is concluded that the mechanism responsible for the mixed-alkali peak is a coupled, cooperative rearrangement of sodium and rubidium ions, with the slower moving ion controlling the rate of rearrangement. This mechanism should also apply to other mixed-alkali silicate glasses since the internal friction for these systems is similar.

Growth of Sapphire Disks from the Melt by a Gradient Furnace Technique

Abstract: : Single crystal Al2O3 (sapphire) is a candidate material for various window applications. A gradient furnace technique has been developed for the growth of large sapphire disks, 7.3 cm in diameter and 1.5 cm thick, from the melt in molybdenum crucibles. The disks are free of gas bubbles and inclusions and consist of several large pie-shaped grains. Within the grains dislocation densities of 100,000/sq.cm. have been measured. The disks are essentially transparent to radiation from wavelengths of 0.25 micrometers in the ultraviolet spectrum to 4.5 micrometers in the infrared spectrum.

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Abstract: Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO2 and HfO2 lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO2 is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO2 at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 A is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.

Effects of lithium and oxygen losses on magnetic and crystallographic properties of spinel lithium ferrite

Abstract: Effects of lithium and oxygen losses on magnetic and crystallographic properties of spinel lithium ferrite

Phase Separation in Borosilicate Glasses as Seen by Electron Microscopy and Scanning Electron Microscopy

Abstract: Microstructures of two sodium borosilicate glasses were examined. Electron micrographs of replicas of water-etched fracture surfaces show that the microphases increase in size with the temperature and time of heat treatment. The relative amounts of soluble and insoluble microphases remain essentially unchanged. Scanning electron micrographs of completely leached glasses show that the insoluble phase is randomly interconnected. Scanning electron micrographs of colloidal deposits in the pores of the leached glass, which result from acid leaching of the heat-treated base glass, are shown. Electron micrographs showing re-solution of the phases are also included.

Properties of Soda Aluminosilicate Glasses: V, Low-Temperature Viscosities*

Abstract: The low-temperature viscosities of two series of sodium aluminosilicate glasses were measured by the fiber elongation method. The results suggested that a modification of the Day and Rindone structural model for sodium aluminosilicate glasses was necessary. The modified model suggests that additional (AlO½)− groups form at Al/Na ratios >1.

Interpretation of the Infrared Spectra of Fused Silica

Abstract: : A vibrational analysis was performed for fused silica assuming the existence of short range order in the glassy network. This short range order was described by an average structural unit based on the model of alpha-quartz. The analysis consisted of using a group theoretical method and the Wilson FG matrix techniques to predict sixteen normal mode vibrational frequencies based on a diagonal force field. The most probable value of the three force constants, which were obtained by making the best fit between the calculated vibrational frequencies and the measured spectral values, is: K sub r = 480,000 dynes/cm Si-0 stretching; K sub alpha = 35,000 dynes/cm 0-Si-0 bending; K sub beta = 5,000 dynes/cm Si-0-Si bending. Although adherence to the selection rules was not observed, fifteen out of the sixteen predicted frequencies were within the experimental spectra of fused silica reported in the literature. (Author)

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

Abstract: High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO3 or SrTiO3 powders of high surface activity. As-prepared helium-dried SrTiO3 is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO3 crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.

Mechanism of Formation of Zircon Stains

Abstract: The mechanism of formation of iron-zircon pink and vanadium-zircon blue stains was studied, primarily by the marker technique. Reaction occurred at the site of the reactant ZrO2. The primary function of the mineralizer in these stains is to react with SiCK and to assist in its transport to the reaction site. Although the basic mechanism of formation of both stains is the same, important differences result from the physical properties of the dopants at reaction temperatures.

Wetting of AlN and TiC by Liquid Ag and Liquid Cu

Abstract: The wetting of AlN and TiC by liquid Ag and liquid Cu was investigated by the sessile drop technique at 10−6 torr or less. An empirical relation was established between the cosine of the contact angle and the temperature or surface tension of the liquid drop. The critical surface tension for spreading and its physical significance are discussed. A method for estimating the surface energies of ceramics is proposed. The surface tension of TiC is estimated to be 1242±158 dynes/cm and that of AlN 990±110 dynes/cm. The surface tension of liquid Ag followed the equation γLv (Ag) (dynes/cm) = 1092–0.14T (°C) and that of liquid Cu γLV (Cu) (dynes/cm) = 1462–0.27T (°C).