Showing papers in "Journal of the American Ceramic Society in 2003"

Transparent Alumina: A Light-Scattering Model

Abstract: A model based on Rayleigh-Gans-Debye light scattering theory has been developed to describe the light transmission properties of fine-grained, fully dense polycrystalline ceramics consisting of birefringent crystals. This model extends light transmission models based on geometrical optics, which are only valid for coarse-grained microstructures, to smaller crystal sizes. We verify our model by measuringthe light transmission properties of fully dense (>99.99%) polycrystalline alpha-alumina (PCA) with mean crystal sizes ranging from 60 mm down to 0.3 mm. The remarkable transparency exhibited by PCA samples with small crystal sizes (< 2 mm) is very well explained by thismodel.

Novel Fast Lithium Ion Conduction in Garnet-Type Li5La3M2O12 (M = Nb, Ta)

Abstract: Lithium metal oxides with the nominal composition Li5La3M2O12 (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10−6 S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li5La3Nb2O12 and Li5La3Ta2O12, respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li14ZnGe4O16. Among the investigated materials, the tantalum compound Li5La3Ta2O12 is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

Abstract: To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.

Structure of Calcium Silicate Hydrates Formed in Alkaline-Activated Slag: Influence of the Type of Alkaline Activator

Abstract: The influence of the alkaline activator (NaOH, waterglass, or Na2CO3) on the structure of the hydrated calcium silicate formed in alkali-activated slag (AAS) cement pastes has been investigated by FTIR, 29Si and 27Al magic-angle scattering nuclear magnetic resonance, and TEM/EDX techniques. In all cases, the main product formed after 7 d of activation, with activators giving an Na2O concentration of 4%, is a semicrystalline calcium silicate hydrate with a dreierkette-type anion. In these structures, linear finite chains of silicate tetrahedra (Q2 units) are linked to central Ca-O layers, and tetrahedral aluminum occupies bridging positions in the chains. The main chain length and the amount of aluminum incorporated in the tetrahedral chains depend on the activator used. The detection of Q3 silicon entities in alkaline-activated slags is discussed in relation to the possible formation of cross-linked structures that may be responsible for increased flexural and compressive strengths in AAS mortars.

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Abstract: Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3-2SiO2) or mullite + BSAS (1-xBaO-xSrO-Al2O3-2SiO2) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, silica volatility, phase stability, and thermal conductivity. Variants of this family of EBCs were applied to monolithic SiC and melt infiltrated SiC/SiC composites. Reaction between BSAS and silica results in low melting (approx. 1300 C) glasses at T > 1400 C, which can cause the spallation of the EBC. At temperatures greater than 1400 C, the BSAS top coat also degrades by formation of a porous structure, and it suffers significant recession via silica volatilization in water vapor-containing atmospheres. All of these degradation mechanisms can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. There was evidence of rapid sintering at temperatures as low as 1300 C, as inferred from the sharp increase in thermal conductivity.

Transparent Sintered Corundum with High Hardness and Strength

Abstract: Commercial corundum powder and a liquid-shaping approach are used for manufacturing complex hollow components and large flat windows of sintered and hot isostatically pressed Al2O3 ceramics having grain sizes of 0.4–0.6 μm at relative densities of >99.9%. High macrohardness (HV10 = 20–21 GPa) and four-point bending strength (600–700 MPa; 750–900 MPa in three-point bending) are associated with a real in-line transmission of 55%–65% through polished plates. The submicrometer microstructure and the optical properties can be retained for use at >1100°C using dopants that shift the sintering temperature to high values without additional grain growth.

Blocking Grain Boundaries in Yttria-Doped and Undoped Ceria Ceramics of High Purity

Abstract: CeO 2 samples doped with 10, 1.0, and 0.1 mol% Y 2 O 3 and undoped CeO 2 samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain-boundary resistance was negligible. Nevertheless, the specific grain-boundary conductivities were still 2-7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain-to-grain contacts, whose properties were then determined by the space-charge layer. The space-charge potential in acceptor-doped CeO 2 was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space-charge layer. The very low grain-boundary conductivities can be accounted for by the oxygen-vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO 2 at high temperatures and under low oxygen partial pressures.

Chemically bonded phosphate ceramics: I, A dissolution model of formation

Abstract: This is the first of three papers in which the kinetics of formation of chemically bonded phosphate ceramics is discussed. A literature survey indicates that the formation of such ceramics is a three-step process. First, oxides dissolve in a phosphoric acid or an acid phosphate solution and metal ions are released into the solution. The aquoions formed from these cations then react with phosphate anions and form a gel of metal hydrophosphates. In the last step, the saturated gel crystallizes into a ceramic. In this paper, we have proposed that the dissolution is the controlling step and developed a general dissolution model of the kinetics of formation of these ceramics. As an example, the model is used to discuss the kinetics of formation of magnesium phosphate ceramics in detail. In the second and third papers, the model has been used to develop processes to form ceramics of alumina and iron oxides.

Pressureless Sintering of Boron Carbide

Abstract: B4C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B4C–B4C contact between particles permitted via volatilization of B2O3 particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B4C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B4C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B2O3 to form B4C and CO gas, thereby extracting the B2O3 coatings, permitting sintering to start at ∼1350°C.

Optimization of Reactive-Element Additions to Improve Oxidation Performance of Alumina-Forming Alloys

Abstract: It is well-known that the addition of reactive elements (including yttrium, zirconium, lanthanum, cerium, and hafnium) improve the high-temperature oxidation performance of alumina-forming alloys. Less studied are strategies for optimizing these additions for developing a high-performance, wrought FeCrAl alloy with maximum oxidation-limited component life. Results from the literature are summarized regarding potential improvements. One promising strategy is the addition of two reactive elements, such as yttrium and hafnium, which has been effective in commercial and laboratory nickel-based alloys and appears to impart the expected benefits, such as reducing the scale growth rate while minimizing detrimental effects, such as the formation of reactive element-rich oxides in the scale and internal oxidation. Although there are promising data, the long-term studies are not yet complete, and it is difficult to predict if “co-doped” FeCrAl alloys will produce superior oxidation-limited lifetimes in high-temperature environments.

Hydration Products and Reactivity of Blast‐Furnace Slag Activated by Various Alkalis

Abstract: Pastes of blast-furnace slag were cured for up to 90 d using sodium silicate (waterglass), NaOH, and three different mixtures of Na2CO3–Na2SO4–Ca(OH)2 to activate reactions. The highest slag reactivity was observed for NaOH activation and the least for waterglass, although nonevaporable water indicated similar amounts of hydration products formed. The main hydration products found using X-ray diffractometry in all systems were calcium silicate hydrate (C-S-H) and a hydrotalcite-type phase. Microanalysis was performed on pastes activated using 50% Na2CO325% Na2SO425% Ca(OH)2, NaOH, and waterglass; the chemical composition of the C-S-H in the waterglass case was different relative to the other two alkalis. For all alkaline agents used, the C-S-H seemed finely intermixed with a hydrotalcitetype phase of Mg/Al 1.82, on average.

Synthesis of Highly Porous Yttria‐Stabilized Zirconia by Tape‐Casting Methods

Abstract: Porous ceramics of Y2O3-stabilized ZrO2 (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.

Processing of High‐Density Submicrometer Al2O3 for New Applications

Abstract: Sintered corundum components with submicrometer grain sizes exhibit properties which enable numerous new applications. Wet powder processing is developed to associate minimum grain sizes at highest densities with the lowest population of macrodefects. A closest ratio of powder particle size and sintered grain size is important for obtaining most fine-grained microstructures. This target was approached best by using powders with particle sizes in the range of 100-200 nm rather than with smaller nanoparticles.

Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

Abstract: The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10−8 m2·K·W−1. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T= 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10−8 m2·K·W−1 in dense alumina and 2.2 × 10−8 m2·K·W−1 in alumina containing a pore volume fraction of 0.3.

Microwave sintering of alumina at 2.45 GHz

Abstract: The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.

Fabrication and Evaluation of Porous Piezoelectric Ceramics and Porosity-Graded Piezoelectric Actuators

Abstract: Porous ceramics of lead zirconate titanate (PZT) were prepared by sintering powder compacts consisting of PZT and stearic acid powders in an air atmosphere; stearic acid was added as a pore-forming agent (PFA). The dielectric, elastic and piezoelectric properties of uniformly porous PZT ceramics were investigated as a function of the porosity volume fraction. Furthermore, a beam-shaped PZT actuator sample with a graded porosity content across its thickness was fabricated by sintering PFA-graded powder compacts. The electric-field-induced bending displacement characteristics of the actuator samples were measured by using strain gauges and were found to be in good agreement with the theoretical predication based on a classical lamination theory.

Impurity Effects on Alumina Scale Growth

Abstract: Most high-temperature-resistant alloys oxidize to form an external alumina layer, or scale, whose slow growth protects the underlying alloy from continued aggressive oxidation. The growth of the Al 2 O 3 scale is controlled by the transport of oxygen inward and aluminum outward through it, with the rate dominated by the fastest diffusing species down the fastest path. Components in the alloy can be incorporated into the growing Al 2 O 3 layer, hence affect the transport rates of oxygen and/or aluminum. This paper summarizes existing experimental data to assess the possible effect of these incorporated impurities on the growth rate and transport properties of Al 2 O 3 scales formed on Fe-, Ni-, and Pt-based alloys. The amount and distribution of the alloy base metal, sulfur impurity, and reactive elements, such as Hf, Y, Zr, and Ce, in the alumina scale are evaluated. Their effect on the oxidation and transport rates through the scale are discussed and compared with Al and O diffusion rates deduced from creep studies.

A Simple Direct Casting Route to Ceramic Foams

Abstract: A simple direct foaming and casting process using ovalbumin-based aqueous slurries for fabricating ceramic and metal foams is demonstrated. Foaming of aqueous ceramic slurries and the foam microstructure were seen to be a strong function of slurry rheology. Setting of foams with ceramic solids loading above 20 vol% was achieved by addition of acid, which also prevented binder migration. Acid addition resulted in excessive shrinkage, causing cracking of foams with ceramic loading below 20 vol%. Addition of sucrose to the slurries suppressed shrinkage leading to defect-free foams with porosity exceeding 95%. Overall porosity and foam microstructure could be controlled through ceramic solids loading, ovalbumin–water ratio, foaming time and sucrose amount, and sintering temperature. The ceramic foams fabricated by the process were strong enough to be green machined to different shapes.

Cubic-formation and grain-growth mechanisms in tetragonal zirconia polycrystal

Abstract: The microstructure in Y2O3-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y3+ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y2O3 concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y3+ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y3+ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y3+ ions segregating along the grain boundary, respectively.

Fabrication of Dense Nanostructured Silicon Carbide Ceramics through Two‐Step Sintering

Abstract: SiC powder compacts were prepared with Al2O3, Y2O3, and CaO powders. By two-step sintering, fully dense nanostructured SiC ceramics with a grain sizes of ∼40 nm were obtained. The grain size–density trajectories are compared with those of conventional sintering processes.

Preparation of SiC/SiC Composites by Hot Pressing, Using Tyranno-SA Fiber as Reinforcement

Abstract: The development of advanced Tyranno SA SiC fiber with a near-stoichiometric composition and a well-crystallized microstructure has made it possible to prepare SiC/SiC composites even under harsh conditions. To assess the reinforcing effectiveness of Tyranno SA fiber at high temperature under pressure, unidirectional SiC/SiC composites were prepared by hot pressing, using pyrolytic carbon (PyC)-coated Tyranno SA fiber as a reinforcement and nanopowder SiC with sintering additives for matrix formation. The effects of sintering conditions on the microstructural evolution and mechanical properties of the composites were characterized. As the sintering temperature increased (from 1720° to 1780°C) and the sintering pressure increased (from 15 to 20 MPa), the density of the composites gradually increased. Simultaneously, the elastic modulus, the proportional limit stress, and the strength, under both bend and tensile tests, also improved. At lower temperature and/or pressure, long fiber pullout was a predominant fracture behavior, indicating relatively weak fiber/matrix bonding. However, at high temperature and/or pressure, short fiber pullout became a main fracture characteristic, indicating relatively strong fiber/matrix bonding. These phenomena were also confirmed by the characteristics of the hysteresis loops derived from the stress–strain curves produced by a tensile test with unloading–reloading cycles. In the present investigation, the reinforcement of Tyranno SA fiber is effective for providing noncatastrophic fracture behavior to composites.

Physicochemical Degradation of Titania‐Stabilized Soluble Phosphate Glasses for Medical Applications

Abstract: Four different formulations of phosphate glasses in the system P2O5–CaO–Na2O–TiO2 were developed. Their physicochemical, morphologic, and structural evolution was analyzed during in vitrodegradation in SBF (simulated body fluid) at 37°C up to 16 weeks. The results showed that the addition of TiO2 into the glass system enhanced both the elastic modulus and the chemical durability of the glasses. Indeed, the elastic modulus increased from 66.6 to 75.95 GPa and the weight loss percentage diminished from 1.6% to 0.3% with the addition of 8 mol% TiO2. A uniform and superficial degradation mechanism could be observed throughout the dissolution time by means of environmental scanning electron microscopy (ESEM), inductively coupled plasma mass spectroscopy (ICP-MS), and Raman spectroscopy. The degradation process undergone by these glasses allows them to maintain their mechanical properties during the degradation process. Therefore, these materials offer an interesting choice for slowly resorbable materials in biomedical applications.

Synthesizing and Comparing the Photocatalytic Properties of High Surface Area Rutile and Anatase Titania Nanoparticles

Abstract: Rutile titania nanocrystalline particles with high specific surface areas were directly prepared by thermal hydrolysis of titanium tetrachloride aqueous solution. The as-prepared rutile titania powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis, and Fourier transform Raman and IR spectroscopies. Neither anatase nor amorphous titania could be detected in this titania powder by XRD, Raman spectroscopy, and high-resolution TEM. In the phenol degradation reaction, the rutile titania powder with an initial crystalline size of 7 nm was found to have higher photocatalytic activity than that of anatase titania with the same specific surface area. The rutile titania powders calcined at 300° and 450°C also showed a relatively high photocatalytic property. The high activity of the as-prepared rutile titania was attributed to the abundance of hydroxy groups in the powder, as was proven by thermogravimetric analysis data, which provided more active sites for the degradation reaction.

Nucleation Sites of Calcium Phosphate Crystals during Collagen Mineralization

Abstract: The nucleation sites of calcium phosphate crystals during collagen mineralization were studied by Fourier transform infrared spectrometry and transmission electron microscopy. It was found for the first time that there is another nucleation site, i.e., carbonyl (>C=O) on collagen, besides the previous reported nucleation site of carboxyl (-COOH). By comparing the IR spectra of collagen not only with collagen/calcium phosphate but also with collagen/Ca 2+ , it was observed that the peak intensities of amides I, II, and III of collagen decreased significantly after mineralization. The decrease of the amide I peak intensity was mainly due to blockage of the C=O stretch. Furthermore, the peak for amide I gradually shifted to a lower wavenumber during collagen mineralization. This shift indicated that chemical interaction between carboxyl groups and Ca 2+ ions formed in the mineralization.

Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

Abstract: The 1.5- to 3-mol%-Y2O3-stabilized tetragonal ZrO2 (Y-TZP) and Al2O3/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa·m1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6–14.8 MPa·m1/2 (2Y-TZP) and 11.9–13.9 MPa·m1/2 (1.5Y-TZP).

Numerical Simulation of Solid-State Sintering: I, Sintering of Three Particles

Abstract: A kinetic, Monte Carlo model, capable of simulating microstructural evolution sintering in a two-dimensional system of three particles, has been presented. The model can simulate several mechanisms simultaneously. It can simulate curvature-driven grain growth, pore migration and coarsening by surface diffusion, and densification by diffusion of vacancies to grain boundaries and annihilation of these vacancies. Morphologic changes and densification kinetics are used to verify the model.

Weibull Strength Scaling for Standardized Rectangular Flexure Specimens

Abstract: Formulas for the effective volumes and effective surfaces for rectangular bars loaded in flexure are reviewed. The ratio of strengths from any two configurations, such as three-point flexural to four-point, is independent of whether the flaws are volume- or surface-distributed, if the cross-sectional sizes are the same. Discrimination between volume- or surface-distributed flaw types, based on the strength ratios, is not possible in such cases. On the other hand, conversion of strengths from one configuration to another is very easy if testing is performed in accordance with one of the current world standard test methods.

Synthesis of Hydroxyapatite Nanopowders via Sucrose‐Templated Sol–Gel Method

Abstract: The present research describes synthesis of hydroxyapatite (HAp) nanopowders using a sol–gel route with calcium nitrate and ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively. Sucrose is used as template material, and alumina is added as a dopant to study its effects on particle size and surface area. Synthesized powders are characterized using X-ray diffractometry, BET surface-area analysis, and transmission electron microscopy. Results show that alumina stabilizes the HAp crystalline phase. Average particle size of mesoporous HAp samples is between 30 and 50 nm with surface area of 51–60 m2/g.

Characterization and High-Temperature Mechanical Behavior of an Oxide/Oxide Composite

Abstract: An oxide/oxide ceramic fiber-matrix composite (CMC) has been extensively characterized for high-temperature aerospace structural applications. This CMC is called GEN-IV™, and it has a porous and cracked aluminosilicate matrix reinforced by 3M Nextel 610™ alumina fibers woven in a balanced eight harness weave (8HSW). This CMC has been specifically designed without an interphase between the fiber and matrix, and it relies on the porous matrix for flaw tolerance. Stress-strain response is nearly linear to failure and without a well-defined proportional limit in tension and compression. In-plane shear and interlaminar strength increases with increasing temperature. The 1000°C fatigue limit in air at 105 cycles is 160 MPa, and the residual tensile strength of run-out specimens is not affected by the fatigue loading. The creep-rupture resistance above 1000°C is relatively poor, but it can be improved with a more-creep-resistant fiber.

Fabrication of Macroporous Alumina with Tailored Porosity

Abstract: Macroporous alumina materials were fabricated via colloidal processing using polymer spheres as the template and ceramic particles as the building blocks. The influence of the suspension conditions and volume ratio of the polymer/ceramic particles on the formation of the pore structure has been investigated. The results showed that the suspension conditions have a significant effect on the pore morphology. A well-defined three-dimensional, ordered porous structure with a controllable pore size and porosity could be obtained through the hetero-coagulation, self-assembled processing of the polymer/ceramic particles. The pore size and porosity could be easily tailored by varying the polymer size and the volume ratio of the polymer/ceramic particles.