Showing papers in "Journal of the American Ceramic Society in 1992"

Apatite Formation Induced by Silica Gel in a Simulated Body Fluid

Abstract: It has been confirmed that the essential condition for glasses and glass-ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body. It has been proposed that a hydrated silica formed on the surfaces of these materials in the body plays an important role in forming the surface apatite layer, but this has not yet been proved. In the present study, it is shown experimentally that a pure hydrated silica gel can induce apatite formation on its surface in a simulated body fluid when its starting pH is increased from 7.2 to 7.4.

Rapid and Continuous Hydrothermal Crystallization of Metal Oxide Particles in Supercritical Water

Abstract: Hydrolysis of 10 metal salt aqueous solutions of 6 metal oxides was conducted in supercritical water. Continuous and rapid production of metal oxide fine particles was achieved by mixing a metal salt aqueous solution with preheated water fed from another line. The reaction time required was less than 2 min. Particle size, morphology, and crystal structure of the obtained metal (hydrous) oxides were examined. Particle size (20 to 600 nm) was different among the systems but the size range was relatively narrow in all the cases.

Heavy-Metal Oxide Glasses

Abstract: Heavy-metal oxide glasses might arbitrarily be defined as those containing over 50 cation percent of bismuth and lead which participate in the glass structure as network formers. Glass systems based on these heavy-metal oxides in silicates, borates, phosphates, germanates, and ferrites are surveyed. Gallate systems are emphasized since they have a unique ability to form stable glasses while maintaining key properties, such as the best infrared transmission and the highest nonlinear optical susceptibilities and diamagnetic Verdet constants for oxide glasses.

Interpenetrating Phase Composites

Abstract: In this paper an assessment of the processing and properties of interpenetrating phase composites is presented. Such materials, defined as multiphase composites in which each phase is topologically interconnected throughout the microstructure, were the subject of a DOE Basic Energy Sciences sponsored workshop on research needs and opportunities held in July 1989 in Snowmass, Colorado. In place of a report representing the proceedings of the workshop, this paper provides a review of the field in its present, nascent state and identifies some of the critical scientific and technological issues posed by development of this class of material. The emphasis is on processing approaches and properties of interpenetrating phase composites. A list of the participants contributing to the workshop is given.

Combined-Stage Sintering Model

Abstract: By focusing on the similarities between the three stages of sintering, a single equation is derived that quantifies sintering as a continuous process from beginning to end. The microstructure is characterized by two separate parameters representing geometry and scale. The dimensionless geometry parameter, denoted T, comprises five scaling factors that relate specific microstructural featuers (e.g., surface curvature) to the scale (grain diameter). Calculations of T from experimental data show (a) agreement with computer simulations of initial-stage sintering, (b) the effect of surface diffusion on T, and (c) changes in T with microstructural evolution during sintering. Application of the model to the design of firing schedules and the study of microstructural geometry effects on sintering is discussed.

Grain-Size-Dependent Transformation Behavior in Polycrystalline Tetragonal Zirconia

Abstract: In this paper experimental observations of the tetragonal phase transformation behavior in polycrystalline zirconias and the related toughening contribution are presented. An analysis which considers transformation thermodynamics and residual stresses is developed to describe the influence of grain size on tetragonal-to-monoclinic transformation temperature. The model is based on the promotion of the transformation by local internal tensile stress concentrations whose effects scale with grain size. The analysis is supported by observations of the martensite start temperature---grain size behavior in polycrystalline tetragonal zirconia containing 12 mol% ceria (12 CeTZP). Next, the analysis considers the grain-size-dependent behavior of the transformation-toughening contribution, {Delta}K{sup T}, and the transformation zone size, r{sup T}.

Thermodynamic Assessment of the System MgO–Al2O3

Abstract: The system MgO–Al2O3 was assessed with the CALPHAD technique using a computerized optimization procedure called PARROT. Two of the solid phases, MgO and spinel, showed extensive solid solubility at high temperature and were modeled with the compound energy model. The third solid phase, α-Al2O3, was modeled as a stoichiometric phase. For the liquid phase, an ionic two-sublattice model was used. In total, 13 adjustable parameters were optimized: 3 for the MgO, 8 for the spinel, and 2 for the liquid. This resulted in a consistent thermodynamic description for most of the available experimental data points on the phase diagram as well of the thermodynamic properties.

Metal–Ceramic Composite Actuators

Abstract: A new type of actuator composed of metal (brass) end caps and piezoelectric ceramics has been developed as a displacement transducer. Shallow cavities positioned between the metal caps and the central ceramic disk convert and amplify the radial displacement of the piezoelectric ceramic into a large axial motion of the metal end caps. Large d33 coefficients exceeding 2500 pC/N are obtained with the composite actuators. The behavior of the electrically induced strain with geometric variables, such as the thickness of the metal end caps, and with pressing force and driving frequency has been evaluated. Sizeable strains are obtained with both PZT (piezoelectric lead zirconate titanate) and PMN (electrostrictive lead magnesium niobate) ceramics.

Carbon Monoxide and Hydrogen Detection by Anatase Modification of Titanium Dioxide

Abstract: From the viewpoint of industrial and automobile exhaust pollution control, a sensor capable of detecting and measuring the concentration of harmful gases such as carbon monoxide and hydrogen in the ambient is desired. This paper describes the development and use of a titania (TiO2)-based semiconducting oxide material as a reliable and rugged CO and H2 gas sensor in the temperature range of 773 to 1073 K. Significant change in the sensing characteristic of the anatase modification of TiO2 was observed when admixed with an insulating second phase, such as alumina or yttria. In the case of TiO2–10 wt% Al2O3, the sensor response was found to be exclusively dependent on the hydrogen concentration alone; the presence of CO or CO2 did not affect the sensitivity. On the other hand, the sensor based on TiO2–10 wt% Y2O3 showed increased sensitivity to CO and decreased sensitivity to H2, compared to that of the undoped TiO2. Addition of elemental iron, in small concentration, to the two-phase mixture of titania and yttria seemed to further improve the sensitivity and selectivity of the latter to CO.

Rapid and Continuous Hydrothermal Synthesis of Boehmite Particles in Subcritical and Supercritical Water

Abstract: Hydrothermal synthesis of AlOOH particles from an Al(NO 3 ) 3 aqueous solution was examined in subcritical and supercritical water. Continuous and rapid production of AlOOH fine particles was achieved by mixing a metal salt aqueous solution with preheated water fed from another line. The effects of temperature, pressure, and initial concentration of solution on the perticle size, morphology, and crystal structure of the metal (hydrous) oxides were examined. These properties are strongly affected by slight changes in pressure and temperature.

Microwave Sintering of Solid Oxide Fuel Cell Materials: I, Zirconia‐8 mol% Yttria

Abstract: This paper reports that the successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack-free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced hot spots in the parts, which led to differential sintering and subsequent cracking. To produce crack-free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 1375{degrees} to 1200{degrees}C by microwave processing and that the resulting grain size was finer.

Low-temperature nucleation of rutile observed by Raman spectroscopy during crystallization of TiO2

Abstract: The thermal evolution of amorphous TiO2 powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy. On calcination at 350°C, the solid crystallized, giving anatase as a major phase. A small amount of rutile was also detected and attributed to small seeds localized at the particle outlayer. The nucleation of rutile at so low a temperature was ascribed to the presence of organic impurities in the powders. The transformation of anatase into rutile was clearly observed after heating at 660°C.

Consolidation Behavior of Flocculated Alumina Suspensions

Abstract: The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φm, obtained, with φm∼ 0.54 for the most strongly flocculated suspension to φm∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Abstract: A review of experimental results and theoretical models for thermal conductivities of ceramic materials with porosity less than 30% is given. It is shown that the abnormal non-monotonic pressure and temperature dependences of thermal conductivity arise from the effects of microcracks and porous grain boundaries, characterizing many industrial refractories, and from the competitive influences of classical and novel mechanisms of heat transfer in composite multiphase materials. The latter mechanisms include segregation and surface diffusion of impurities and defects in crystal structure, and the mechanism arising from chemical conversion and gas emission, occurring within pores of ceramic materials. A fractal model of porous materials' structure is proposed and used for analysis, explanation, and classification of thermophysical properties of ceramic materials measured in various thermodynamic conditions.

Unique Opportunities for Microstructural Engineering with Duplex and Laminar Ceramic Composites

Abstract: Duplex (two-phase) microstructures and laminar composites offer some unique opportunities for improving the room-temperature mechanical reliability (e.g., flaw tolerance) and the high-temperature microstructural stability (e.g., resistance to grain growth and creep damage) of structural ceramic materials. Examples illustrating the approach to designing novel multiphase microstructures and laminar composites with enhanced structural reliability are given. These are based on current work on various alumina-based ceramics as well as zirconia-, silicon nitride-, and silicon carbide-containing ceramics. Critical issues and areas for future work are discussed.

Microstructural Development During Gas‐Pressure Sintering of α‐Silicon Nitride

Abstract: Gas-pressure sintering of α-Si3N4 was carried out at 1850 ° to 2000°C in 980-kPa N2. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3N4 powder was similar to that in the materials from β-Si3N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3N4 ceramics.

Processing Effects in the Sol–Gel Preparation of PZT Dried Gels, Powders, and Ferroelectric Thin Layers

Abstract: This paper reports on infrared spectroscopy that was used to monitor and characterize solutions used in the preparation of PZT 53/47 powders, dried gels, and thin layers deposited on platinized silicon substrates. It was found that careful control of the chemistry of the precursor solutions, in particular the control of the formation of esters and their elimination, resulted in reproducible solutions which could be used to prepare thin layers with superior properties. In addition, it was found that phase development and microstructure were related to the precursor chemistry. Single-phase perovskite layers with uniform microstructures resulted from ester-free solutions and were suitable for ferroelectric applications (P[sub r] [approximately] 22 [mu]C/cm[sup 2], E[sub c] [approximately] 80 kV/cm, K [approximately] 800, tan [delta] [approximately] 0.01).

Dopants in Vapor‐Phase Synthesis of Titania Powders

Abstract: Gas-phase synthesis of titania by TiCl4 oxidation in the presence of dopants (SiCl4, POCl3, and BCl3) was investigated in an aerosol reactor as a function of temperature (1300–1700 K) and dopant concentration (0–25 mol% of TiCl4). The addition of dopants, most notably silicon and phosphorus, drastically altered the morphology of titania particles from polyhedral to spheroidal, increased the extent of aggregation, increased the specific surface area, reduced the primary particle size, and decreased the rutile content. The observed morphology/crystallinity changes were explained in terms of ionic radii and valence of the dopant element.

Damage and Failure in Unidirectional Ceramic–Matrix Composites

Abstract: A study of the mechanical characteristics of a unidirectional fiber–reinforced calcium aluminosilicate matrix composite has been conducted. The properties have been related to the individual properties of the matrix, the fibers, and the interfaces, as well as the residual stress, using available models of matrix cracking and fiber fracture. Comparisons have been made with lithium aluminosilicate matrix composites. Predictions of initial matrix cracking and of ultimate strength using the models are found to correlate well with the measured values. However, deficiencies have been noted in the ability of the models to predict the evolution of matrix cracks, plus associated changes in the modulus.

Cyclic Fatigue-Crack Growth in a SiC-Whisker-Reinforced Alumina Ceramic Composite: Long- and Small-Crack Behavior

Abstract: The ambient-temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-crack experiments using compact C(T) specimens, cyclic fatigue-crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack-tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic-matrix composite materials are discussed.

Combustion Synthesis of Metal Chromite Powders

Abstract: Fine-particle metal chromites (MCr2O4, where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn) have been prepared by the combustion of aqueous solutions containing the respective metal nitrate, chromium(III) nitrate, and urea in stoichiometric amounts. The mixtures, when rapidly heated to 350°C, ignite and yield voluminous chromites with surface areas ranging from 5 to 25 m2/g. MgCr2O4, sintered in air at 1500°C for 5 h, has a density of 4.0 g/cm3.

Effect of Reinforcement Particle Size on the Thermal Conductivity of a Particulate‐Silicon Carbide‐Reinforced Aluminum Matrix Composite

Abstract: The thermal conductivity of a 40 vol% silicon carbide-particulate-reinforced aluminum matrix composite was determined as a function of silicon carbide mean particle size ranging from 0.7 to 28 μm. A size dependence was found consisting of a decrease in thermal conductivity with decreasing SiC particle size. This effect is in accordance with theoretical expectations for composites with an interfacial thermal barrier at the dispersion–matrix interface. At the finest particle size of the silicon carbide, the composite thermal conductivity approached the value for the matrix with pores, as expected from theory. Only at the largest SiC particle size did the composite thermal conductivity exceed the value for the matrix. These results suggest that in maximizing the thermal conductivity of composites with an interfacial thermal barrier, the reinforcement particle size should be as large as practically possible.

Microscopic Calculation of Dielectric Loss at Microwave Frequencies for Complex Perovskite Ba(Zn1/3Ta2/3)O3

Abstract: The dielectric loss tangent at microwave frequencies for the complex perovskite Ba(Zn1/3Ta2/3)O3 was calculated with respect to the degree of structural disorder on B sites. Starting out from the equations of ion motion, dielectric loss was expressed in terms of the pair-correlation functions corresponding to the ordering of Zn and Ta ions on B sites. The characteristic length included in the pair-correlation functions corresponds to the average size of the region containing disorder in ion arrangements on B sites; thus the relation between the structural disorder on the B site and the dielectric loss tangent at microwave frequencies was clarified theoretically. The numerical results show that the microwave loss tangent values change their power from – 3 to – 6 with increasing degree of order on the B site, which agrees well with the experimental observations. Results obtained here confirm the physical origin of the microwave loss of complex perovskite Ba(Zn1/3Ta2/3)O3.

Comparative Measurement of Indentation Fracture Toughness with Berkovich and Vickers Indenters

Abstract: Measurements of the load dependence of the radial crack size with Vickers and Berkovich indenters are compared for a range of materials. It is found that the extent of radial cracks was slightly larger for the Berkovich than for the Vickers indenter. The observations reveal that cracks from a Berkovich indenter are best described by an expression developed by Laugier combined with a modification proposed by Ouchterlony to account for the number of radial cracks. It was also found that the Berkovich indenter, which offers advantages for ultramicroindentation, gave more consistent toughness values at lower loads than a Vickers indenter.

Experimental Observations of Frictional Heating in Fiber‐Reinforced Ceramics

Abstract: The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber-reinforced ceramics are also discussed.

Fabrication and Secondary‐Phase Crystallization of Rare‐Earth Disilicate–Silicon Nitride Ceramics

Abstract: The fabrication and intergranular-phase devitrification of silicon nitride densified with rare-earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3 in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrification.

Influence of Silica Fume on the Early Hydration of Portland Cements Using Impedance Spectroscopy

Abstract: Electrical properties of hydrating portland cements (PC) and portland cements containing silica fume were studied from 5 min to 90 days Cement pastes with water to solids ratios (w/s) of 030, 035, and 040, as well as silica fume to portland cement ratios (s/c) of 005, 010, and 020, were made and impedance was measured within the frequencies of 13 MHz to 5 Hz The impedance spectra exhibit electrode arcs at low frequencies and bulk material arcs at high frequencies The bulk resistance of the paste increases with increasing silica fume content and/or decreasing water content The conductivity of pore fluid from PC pastes increases rapidly with time during the early stages and then remains constant, while that of the silica fume pastes increases then decreases sharply

Mechanism of Reaction between Lanthanum Manganite and Yttria‐Stabilized Zirconia

Abstract: The reaction of La1-xCaxMnO3 (x= 0, 0.1, 0.2) with ZrO2-8 mol% Y2O3 (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO3 depresses the reactivity with YSZ. A layer of La2Zr2O7 is formed at the La1-xCaxMnO3/YSZ interface after an induction period, and its formation is accelerated when the La1-xCaxMnO3 phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO3/single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed.

Production of Fine, High-Purity Beta Silicon Carbide Powders

Abstract: Submicrometer SiC (β-form) powders were synthesized by reacting silica and carbon black at temperatures between 1450° and 1800°C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400°C), the dominant mechanism for silicon carbide formation involves the solid-state reaction of silica and carbon. At higher temperatures (above approximately 1400°C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400°C, the rate of SiC formation is controlled by the rate of SiO formation. In as-synthesized form, the SiC powders typically contain < 0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large-scale commercial production of SiC, requiring no postfabrication acid leaching or major milling.

Objective Evaluation of Short‐Crack Toughness Curves Using Indentation Flaws: Case Study on Alumina‐Based Ceramics

Abstract: An objective methodology is developed for evaluating toughness curves (T-curves) of ceramics using indentation flaws. Two experimental routes are considered: (i) conventional measurement of inert strength as a function of indentation load; (ii) in situ measurement of crack size as a function of applied stress. Central to the procedure is a proper calibration of the indentation coefficients that determine the K-field of indentation cracks in combined residual-contact and applied-stress loading, using data on an appropriate base material with single-valued toughness. Tests on a fine-grain alumina serve to demonstrate the approach. A key constraint in the coefficient evaluation is an observed satisfaction of the classical indentation strength–(load)−1/3 relation for such materials, implying an essential geometrical similarity in the crack configurations at failure. T-curves for any alumina-based ceramic without single-valued toughness can then be generated objectively from inert-strength or in situ crack-size data. The methodology thereby circumvents the need for any preconceived model of toughening, or for any prescribed analytical representation of the T-curve function. Data on coarse-grained aluminas and alumina-matrix material with aluminum titanate second-phase particles are used in an illustrative case study.