Showing papers in "Journal of the American Ceramic Society in 1977"
TL;DR: In this article, it was shown that partially stabilized zirconia containing homogeneously nucleated precipitates of tetragonal ZrO/sub 2/ of low solute content in a cubic matrix of high solute contents can change to the stable monoclinic structure within the stress field of the crack.
Abstract: Recent discovery is announced that partially stabilized zirconia containing homogeneously nucleated precipitates of tetragonal ZrO/sub 2/ of low solute content in a cubic matrix of high solute content, the primary contribution to the toughness is associated with particles within a ''transformation zone'' adjacent to a propagating crack. These particles change to the stable monoclinic structure within the stress field of the crack. (GHT)
386 citations
TL;DR: In this article, an analysis of the rate at which a cubic array of cylinders densifies by viscous flow driven by surface energy reduction is presented, and the cubic array is proposed as a model for the microstructure of low-density, open-pore materials such as gels and flame-hydrolysis preforms.
Abstract: An analysis is presented which describes the rate at which a cubic array of cylinders densifies by viscous flow driven by surface energy reduction. The cubic array is proposed as a model for the microstructure of low-density, open-pore materials such as gels and flame-hydrolysis preforms. The parameters of the model structure (cylinder length and radius) can be deduced from Hg penetration porosimetry.
331 citations
245 citations
TL;DR: In this article, high-resolution electron microscopy lattice imaging studies of an MgO fluxed hot-pressed silicon nitride were conducted to show that intergranular second phases do indeed exist, but they are heterogeneously distributed.
Abstract: Although high-temperature strength loss in ceramics such as silicon nitride has been attributed to the presence of intergranular amorphous phases, until now no direct proof has been offered. The present paper describes high resolution electron microscopy lattice imaging studies of an MgO fluxed hot-pressed silicon nitride. These studies indicate that intergranular second phases do indeed exist, but they are heterogeneously distributed, appearing especially at multiple grain junctions. These room temperature observations are compatible with the microstructures expected at elevated temperatures.
233 citations
TL;DR: In this article, the effects of porosity and grain size on the magnetic properties of NiZn ferrites were examined on groups of specimens with controlled porosity, and both the permeability and the B-H loop were measured.
Abstract: The effects of porosity and grain size on the magnetic properties of NiZn ferrites were examined on groups of specimens with controlled porosity and grain size; both the permeability and theB-H loop were measured. The demagnetizing factor, which is proportional to porosity, was estimated on the basis of the permeability-porosity relation determined. The discussion of the permeability-grain size relation takes into consideration the dependence of domain wall spacing on grain size. The experimental results for the B-H loops indicate that the remanent magnetic flux density is independent of grain size and the coercive field is independent of porosity. Both the remanent magnetic flux density-porosity and the coercive field-grain size relations are explained via the formulas proposed.
219 citations
TL;DR: In this article, the phase relations in the Si/sub 3/N/sub 1/-SiO/sub 2/Y/sub O/sub 4/SiO 2/O/Sub O/Sub 2/2/O-O-Y/O 2 O 7/O 7/N 2 O/O 3/O 4/O 5/O 6/O 8/O 9/O 10/O 12/O 14/O 15/O 16/O 17/O 18/O 19/O 20/O 21/O 22/
Abstract: Composite powders were hot-pressed to determine the phase relations in the Si/sub 3/N/sub 1/-SiO/sub 2/-Y/sub 2/O/sub 3/ pseudoternary system. Four quaternary compounds, Si/sub 3/Y/sub 2/O/sub 3/N/sub 4/, YSiO/sub 2/N, Y/sub 10/Si/sub 7/O/sub 23/N/sub 4/, and Y/sub 4/Si/sub 2/O/sub 7/N/sub 2/, were identified. Studies of polyphase and single-phase materials in this system showed that these 4 compounds are unstable under oxidizing conditions. Materials within the Si/sub 3/N/sub 4/-Si/sub 2/N/sub 2/O-Y/sub 2/Si/sub 2/O/sub 7/ compatibility triangle precluded the unstable compounds, and are extremely resistant to oxidation.
212 citations
TL;DR: In this paper, the strength of single-phase polycrystalline mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C.
Abstract: The high strength potential of single-phase mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C. In single-crystal experiments, no plastic deformation and, hence, no dislocation glide was observed. Polycrystalline mullite was deformed via the Nabarro-Herring mechanism accompanied by grain-boundary sliding and some cavitation. In stress-strain tests, deformation was enhanced by intergranular separation.
193 citations
TL;DR: In this paper, the principles and controlling parameters of directional solidification (DS) eutectic growth are described, and the criteria for plane-front growth and the effect of growth rate on interlamellar or interfiber spacing are discussed.
Abstract: The aligned structures which result from the directional solidification (DS) of ceramic eutectics are of interest because of their potential for use in electronic devices and as structural materials. Techniques for growing DS ceramic eutectics are briefly discussed. The principles and controlling parameters of DS eutectic growth are described. The criteria for plane-front growth and the effect of growth rate on interlamellar or interfiber spacing are discussed. Examples of the effect of growth parameters on the alignment of the microstructure are given. Examples of the mechanical properties of directionally solidified oxide-oxide ceramics are also cited.
152 citations
TL;DR: In this article, the Raman spectra of ZrO/sub 2/ were measured in the range 300 to 1600/sup 0/K. The temperature dependence of the spectra shows a hysteresis.
Abstract: Raman spectra of ZrO/sub 2/ were measured in the range 300 to 1600/sup 0/K. Irreducible components of the Raman spectra were obtained from polarization measurements at room temperature, and the symmetries of the Raman-active modes in the monoclinic phase were determined. The temperature dependence of the spectra shows a hysteresis. In the temperature range of phase change, two kinds of Raman lines coexist. Six Raman-active mode frequencies were found in the tetragonal phase at high temperatures. Analysis of the spectra showed that the Raman-active modes in the tetragonal phase are affected by strain introduced into the crystals during the phase transformation.
148 citations
TL;DR: In this paper, the effects of Dy doping and sintering parameters on the properties of BaTiO3 ceramics were studied, and the average grain size decreases with increasing Dy content and is controlled at ∼1.5 μ m by 0.8 at.% Dy.
Abstract: The effects of Dy doping and sintering parameters on the properties of BaTiO3 ceramics were studied. The average grain size decreases with increasing Dy content and is controlled at ∼1.5 μ m by 0.8 at.% Dy. The Curie temperature change, associated with ≤1.2 at.% Dy, is <3°C. The dielectric constant is ∼2600 for specimens doped with 0.8 at.% Dy, calcined at 1200°C, and sintered at 1450°C. The dielectric constant variation with ambient temperature is much less than that of conventional BaTiO3 ceramics. Lattice constant c decreases with increasing Dy concentration whereas a increases slightly. The effects of grain size on dielectric constant, lattice parameters, and linear thermal expansion coefficient are more pronounced than the chemical effects of Dy doping in the ferroelectric state, whereas in the paraelectric state, these characteristics are almost independent of grain size as well as Dy concentration. The decrease in the frequency of occurrence of 90° twins with decreasing grain size implies that internal stress, which develops when BaTiO3 ceramics are cooled below Tc, strongly influences the effects of grain size.
132 citations
TL;DR: In this paper, the system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies.
Abstract: The system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies. Results reveal the monoclinic α1 phase (0 to 2 mol% Sc2O3), the tetragonal α2’phase (5 to 8% Sc2O3), the rhombohedral β phase (9 to 13% Sc2O3), the rhombohedral γ phase (15 to 23% Sc2O3), the rhombohedral δ phase (24 to 40% Sc2O3), and the cubic % phase (77.5 to 100% Sc2O3). The monoclinic α1 phase and the tetragonal α2’phase were found to transform to the tetragonal α2 phase over a wide temperature range depending on composition. The β, γ, and α phases transformed to a cubic phase at temperatures of %600%, 1100%, and 1300%C, respectively. A maximum melting point of %2870%C was found at %10% Sc2O3 and a eutectic at %2400%C at 55% Sc2O3.
TL;DR: Tritium diffusion in aluminum and beryllium oxides was studied by recoiling tritium into specimens and measuring the time rate of tritium release during postirradiation heating as mentioned in this paper.
Abstract: Tritium diffusion in aluminum and beryllium oxides was studied by recoiling tritium into specimens and measuring the time rate of tritium release during postirradiation heating. Results were consistent with classical diffusion solutions and gave single values for the diffusion activation energy over the temperature range of measurement for single-crystal, sintered, and powdered specimens. Varying amounts of tritium were released as tritiated water even after attempts to remove most of the water absorbed by the specimens. Condensable amounts of tritium ranged from approximately 20% for single-crystal specimens to as high as 90% for some powders. Alumina containing 0.2% MgO gave tritium diffusion coefficients 4 to 5 orders of magnitude higher than those for undoped Al/sub 2/O/sub 3/, implying a significant impurity effect on tritium diffusion. Measured diffusion coefficients were many orders of magnitude lower than hydrogen diffusion coefficients in metals. When a simple equilibrium permeation model was used which neglected tritium partition effects at interfaces and enhanced diffusion along grain boundaries, it was shown that thin coatings of beryllium or aluminum oxides on metal substrates could markedly reduce tritium permeation rates through metals. The present results may be applied in the future to fusion reactors where tritium inventories will be highmore » and tritium diffusion within the blanket region undesirable.« less
TL;DR: In this paper, the phase diagram of the ZrO2-CaO system was determined by using a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique.
Abstract: The phase diagram of the system ZrO2-CaO for the region ZrO2-CaZrO3 was redetermined. The extent of the cubic solid solution field was ascertained by using a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. At 850 ± 150°C, the cubic solid solutions decompose by a eutectoid reaction. A reasonable agreement for the position of the eutectoid in this system was obtained by comparing the extrapolated data from high temperatures with the data obtained by the hydrothermal technique. Only one ordered compound, CaZr4O9, with monoclinic symmetry, was found; this compound is unstable above 1310 ± 40°C.
TL;DR: In this paper, the nature and extent of degradation incurred by glass surfaces impacted with spheres of steel and tungsten carbide were studied and the essential features of the degradation were predicted by a theoretical analysis of residual strength as a function of impact velocity as derived from indentation fracture mechanics.
Abstract: The nature and extent of degradation incurred by glass surfaces impacted with spheres of steel and tungsten carbide were studied. The residual strength after impact depends on the velocity, radius, and density of the projectile; on the toughness and (indirectly) the hardness of the target; and, to a lesser degree, on the preexisting mechanical condition of the surface. The damage morphology involves modification of the basic Hertzian cone crack pattern by median (radial) cracks and crushed glass at the impact site. The essential features of the degradation may be predicted by a theoretical analysis of residual strength as a function of impact velocity as derived from indentation fracture mechanics. This study accounts, in particular, for a threshold velocity for significant strength loss, above which further strength decrease is relatively slight. Small, but significant, discrepancies between observed and predicted degradation characteristics are attributed to the departure from ideal Hertzian fracture geometry and to the dynamic nature of the contact. However, it is suggested that quasi-statically based theory may be used for estimating the strength of structural ceramics in small-particle impact situations.
TL;DR: The melting, eutectic, peritectic, solidus, and liquidus temperatures in the system Zr-O have been measured directly by a simple optical pyrometric technique requiring only a few hundred milligrams of sample.
Abstract: The melting, eutectic, peritectic, solidus, and liquidus temperatures in the system Zr-O have been measured directly by a simple optical pyrometric technique requiring only a few hundred milligrams of sample. The saturation solubility of oxygen in α-Zr(s) between 1270° and 1980°C and the lower phase boundary of the ZrO2–α phase between 1900° and 2400°C have been measured by an isopiestic equilibration method. The oxygen solubility limit in α-Zr(s) agrees well with previous low-temperature studies and reaches a maximum solubility of 35°1 at.% O at the eutectic temperature, 2065°°5°C. The maximum melting temperature of α-Zr(ss) is 2130°°10°C and corresponds to a composition of 25°1 at.% O. Both of these temperatures are approximately 150° higher than previously reported. Liquidus compositions above the eutectic temperature were obtained via mass spectrometry from the kinetic behavior of the liquid solution-ZrO2–x(s) mixture as it approached equilibrium at 2125°°5°C. The lower phase boundary or solidus of the ZrO2–x phase departs appreciably from ideal stoichiometry above 1900°C and smoothly reaches its most reduced composition, 61 at.% (ZrO1.56), near 2300°C. The solidus is retrograde at higher temperatures. The melting temperature of the stoichiometric dioxide is 2710°°15°C.
TL;DR: In this paper, the authors combined fracture mechanics with fracture surface analysis to analyze brittle failure of glass bars which were tested relative to the direction of grinding and found that the fracture mirror radius, r, occurs at a constant stress-intensity level independent of flaw geometry.
Abstract: Fracture mechanics is combined with fracture surface analysis to analyze brittle failure of glass bars which were tested relative to the direction of grinding. Grinding essentially produces two sets of flaws from which failure occurs. In the most severe set, formed basically parallel to the grinding direction, the ratio of the average depth (a) to the half-width (b) is 0.5. In the less severe set, formed perpendicular to the grinding direction, the average a/b ratio is 1.6. In both sets the most severe flaws are generally associated with a particularly deep grinding groove or gouge. The strength reduction resulting from testing perpendicular to the grinding direction results from the larger flaw size and slightly higher stress-intensity factor resulting from the greater ellipticity of the flaws formed parallel to the grinding grooves and perpendicular to the tensile axis. Detailed analysis of these 2 sets of flaws causing failure of appropriately oriented specimens shows that (1) the fracture mirror radius, r, occurs at a constant stress-intensity level independent of flaw geometry; (2) unsymmetric fracture mirrors result from unsymmetric, irregular flaws leading to unsymmetric stress-intensity distributions; (3) is constant for semielliptical flaws; and (4) fracture energy calculated from an expression including mirror constants, the flaw-to-mirror size ratio, and the flaw geometry agrees with measured values over a wide range of a/b values.
TL;DR: In this article, the ionic product for calcium hydroxide (CH) was calculated as a function of time and correlated with calorimetric measurements and crystal growth data.
Abstract: The concentration of ionic species in the solution in contact with hydrating tricalcium silicate (C3S) has been studied as a function of time and in the presence of admixtures. The ionic product for calcium hydroxide (CH) was calculated as a function of time and correlated with calorimetric measurements and crystal growth data. The results support the hypothesis that nucleation of CH crystals controls the induction period and that their subsequent growth controls the second heat peak. The influence of admixtures can be predicted from their influence on CH crystal growth. However, solution data indicate that the system cannot properly be compared with the growth of pure CH; the combined role of C-S-H and CH must be considered. Some implications for C3S hydration are discussed.
TL;DR: In this paper, densification rates for three classes of porous glass materials are reported for soot preforms made by flame hydrolysis, silica gel, and phase-separated and leached borosilicate glass.
Abstract: Densification rates are reported for 3 classes of porous glass materials: soot preforms made by flame hydrolysis, silica gel, and phase-separated and leached borosilicate glass. Viscosity values obtained by fitting the densification data to the theory of Part I of the present study are in reasonable agreement with viscosity measured by beam bending using fully sintered glass of the same type.
TL;DR: In this article, a calculated metastable liquid miscibility gap with a consolute temperature of approx. = 36 mol % Al/sub 2/O/sub 3/ was considered to be thermodynamically most probable.
Abstract: Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO/sub 2/--Al/sub 2/O/sub 3/ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO/sub 2/- and Al/sub 2/O/sub 3/-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of approx. = 1540/sup 0/C at a critical composition of approx. = 36 mol % Al/sub 2/O/sub 3/ was considered to be thermodynamically most probable; the gap extended from approx. = 11 to approx. = 49 mol % Al/sub 2/O/sub 3/ at 1100/sup 0/C. SiO/sub 2/-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.
TL;DR: In this paper, a double-torsion specimen was analyzed with a 3D elastic finite-element stress analysis and a crack-front configuration where the stress intensity is constant over ≅ 1/2 of the specimen thickness was determined.
Abstract: The double-torsion specimen was analyzed with a 3-dimensional elastic finite-element stress analysis. A crack-front configuration where the stress intensity is constant over ≅ 1/2 of the specimen thickness was determined. That value of the stress intensity is nearly equal to the “strength-of-materials” value. The stress intensity is nearly constant for crack lengths > 0.55 × specimen width (W) and ligament lengths > 0.65W.
TL;DR: In this article, the thermal expansion coefficient and aqueous dissolution rate were measured for alkali phosphate glasses containing a third and fourth component, and the observed expansion coefficients were in the range 99 to 286 × 10−7°C−1.
Abstract: The thermal expansion coefficient and aqueous dissolution rate were measured for alkali phosphate glasses containing a third and fourth component. For a given series, both properties increased monotonically as the radii of the alkali ions increased. The observed expansion coefficients were in the range 99 to 286 × 10−7°C−1. A small mixed-alkali effect was observed for both properties.
TL;DR: In this article, the adsorption of N2 and water vapor was studied in completely hydrated tricalcium silicate and in fully hydrated Tricalcium Silicate from which Ca(OH)2 had been extracted.
Abstract: Adsorption of N2 and water vapor was studied in completely hydrated tricalcium silicate and in fully hydrated tricalcium silicate from which Ca(OH)2 had been extracted. Compared with results obtained using N2, water vapor adsorption led to increased values for small-pore volume, peak shifts to smaller sizes, and decreased values for large-pore volume. Marked hysteresis was observed in the case of water vapor adsorption; the resorption branch apparently represents the true pore structure. Extraction of Ca(OH)2 from the paste increased the calculated volume of small pores strikingly, suggesting that adsorption is hindered by Ca(OH)2; this tendency is more obvious in water vapor adsorption. The adsorption measurements indicate the existence of two kinds of pores, i.e. a wider intergel-particle pore and a smaller pore existing within the gel particle. The latter pore was further classified into intercrystallite and intracrystallite pores.
TL;DR: The effective medium theory for diffusion in random composite media was developed in this paper, and the theoretical diffusivity depends crucially on whether concentration or chemical potential is taken to be the field variable for describing transport.
Abstract: The effective medium theory is developed for diffusion in random composite media. It is demonstrated that the theoretical diffusivity depends crucially on whether concentration or chemical potential is taken to be the field variable for describing transport.
TL;DR: In this paper, the temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAlx) in the forbidden gap and the temperature dependent of the mobility of the native ionic defects (Al vacancies, VAlm).
Abstract: Titanium-doped α-Al2O3 exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAlx) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, VAlm). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of po2 and is used to determine concentrations of Ti3+ and Ti4+. Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al2O3:Ti and undoped Al2O3 is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively.
TL;DR: The Young's and shear moduli of polycrystalline monoclinic hafnium oxide were determined by the sonic resonance method at room temperature as a function of volume fraction porosity.
Abstract: The Young's and shear moduli of polycrystalline monoclinic hafnium oxide were determined by the sonic resonance method at room temperature as a function of volume fraction porosity. The Spriggs equation empirically described the data and the zero porosity moduli as 283.6 GN/m/sup 2/ and 109.2 GN/m/sup 2/ for Young's and shear moduli, respectively. The Debye temperature calculated from the elastic constants was 484/sup 0/K. Several specimens had anomalously low moduli values, attributed to microcracking. Grain size and internal friction measurements showed that the microcracking occurred in specimens with grain sizes >2 to 3 ..mu..m and was characterized by high internal friction.
TL;DR: In this paper, the dehydroxylation phases of a kaolinite from Zettlitz, Czechoslovakia, were investigated by radial electron density distribution (RED) and X-ray spectroscopy.
Abstract: Dehydroxylation phases of a kaolinite from Zettlitz, Czechoslovakia, were investigated by radial electron density distribution (RED) and X-ray spectroscopy. Special emphasis was placed on the 2 amorphous or poorly crystallized transition phases, i.e. metakaolinite (600 degrees to 800 degrees C) and the spinel structure (900 degrees C). The RED diffraction method, which is particularly suited to the study of such materials, indicated (1) a particularly expanded tetrahedral configuration for the Al atoms in metakaolinite and (2) a pure Al/sub 2/O/sub 3/ spinel structure. The first result confirms earlier observations; the second eliminates, for the experimental conditions and kaolinite studied, the possibility of the formation of an Al-Si spinel structure with the Al and Si occupying the octahedral and tetrahedral interstices, respectively. Complementary measurements on the Al/b K/ alpha line shifts for the same samples support the present interpretation. Determination of the apparent Si coordination number by the same spectroscopy procedure revealed a regular 4 coordination for all phases except the end-member mullite.
TL;DR: In this article, the theory of structural stability of ceramics in severe thermal environments was explored via the use of precracking, and it was shown that structural stability should be possible when an array of prec-acks is used, but the precracks required for this purpose are relatively large.
Abstract: The theory of structural stability of ceramics in severe thermal environments was explored via the use of precracking. It was shown that structural stability should be possible when an array of precracks is used, but that the precracks required for this purpose are relatively large. The approach is thus limited to systems which are not required to sustain a significant mechanical stress. Preliminary quenching experiments on short cylinders confirmed that stabilization can be achieved by precracking.