Showing papers in "Journal of the American Ceramic Society in 2007"

Refractory Diborides of Zirconium and Hafnium

Abstract: This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

On the Vickers Indentation Fracture Toughness Test

Abstract: The Vickers indentation fracture toughness test, or VIF, is addressed by considering its origins and the numerous equations that have been applied along with the technique to estimate the fracture resistance, or the KIc of ceramics. Initiation and propagation of cracks during the VIF test are described and contrasted with the pre-cracking and crack growth for internationally standardized fracture toughness tests. It is concluded that the VIF test technique is fundamentally different than standard fracture toughness tests. The VIF test has a complex three-dimensional crack system with substantial deformation residual stresses and damage around the cracks. The VIF test relates to an ill-defined crack arrest condition as opposed to the rapid crack propagation of the standardized fracture toughness tests. Previously published fracture toughness results employing the VIF technique are reviewed. These reveal serious discrepancies in reported VIF fracture toughness values. Finally, recent fracture resistance measurements by the VIF technique for the Standard Reference Material SRM 2100 are presented. These are compared with standardized test results for the same material. It is concluded that the VIF technique is not reliable as a fracture toughness test for ceramics or for other brittle materials. What the VIF actually measures in terms of fracture resistance cannot be readily defined. It is recommended that the VIF technique no longer be acceptable for the fracture toughness testing of ceramic materials.

Statistical indentation techniques for hydrated nanocomposites: concrete, bone, and shale

Abstract: Concrete, bone and shale have one thing in common: their load-bearing mineral phase is a hydrated nanocomposite. Yet the link between material genesis, microstructure, and mechanical performance for these materials is still an enigma that has deceived many decoding attempts. In this article, we advance statistical indentation analysis techniques that make it possible to assess, in situ, the nanomechanical properties, packing density distributions, and morphology of hydrated nanocomposites. These techniques are applied to identify intrinsic and structural sources of anisotropy of hydrated nanoparticles: calcium–silicate–hydrate (C–S–H), apatite, and clay. It is shown that C–S–H and apatite, the binding phase in, respectively, cement-based materials and bone, are intrinsically isotropic; this is most probably due to a random precipitation and growth process of particles in calcium oversaturated pore solutions, which can also explain the nonnegligible internanoparticle friction. In contrast, the load-bearing clay phase in shale, the sealing formation of most hydrocarbon reservoirs, is found to be intrinsically anisotropic and frictionless. This is indicative of a ‘smooth’ deposition and compaction history, which, in contrast to mineral growth in confined spaces, minimizes nanoparticle interlocking. In all cases, the nanomechanical behavior is governed by packing density distributions of elementary particles delimitating macroscopic diversity.

Elastic Properties and Short-to Medium-Range Order in Glasses

Abstract: Very different materials are named “Glass,” with Young's modulus (E) and Poisson's ratio (ν) extending from 5 to 180 GPa and from 0.1 to 0.4, respectively, in the case of bulk inorganic glasses. Although glasses have in common the lack of long-range order in the atomic organization, they offer a wide range of structural features at the nanoscale and we show in this analysis that beside the essential role of elastic properties for materials selection in mechanical design, the elastic characteristics (E, ν) at the continuum scale allow to get insight into the short- and medium-range orders existing in glasses. In particular, ν, the atomic packing density (Cg) and the glass network dimensionality appear to be strongly correlated. Maximum values for ν and Cg are observed for metallic glasses (ν∼0.4 and Cg>0.7), which are based on cluster-like structural units. Atomic networks consisting primarily of chains and layers units (chalcogenides, low Si-content silicate, and phosphate glasses) correspond to ν>0.25 and Cg>0.56. On the contrary, ν<0.25 is associated with a highly cross-linked network, such as in a-SiO2, with a tri-dimensional organization resulting in a low packing density. Moreover, the temperature dependence of the elastic moduli brings a new light on the structural changes occurring above the glass transition temperature and on the depolymerization rate in the supercooled liquid. The softening rate depends on the level of cooperativity of atomic movements at the source of the deformation process, with an obvious correlation with the “fragility” of the liquid.

Thermodynamic Analysis of ZrB2–SiC Oxidation: Formation of a SiC-Depleted Region

Abstract: A thermodynamic model was developed to explain the formation of a SiC-depleted layer during ZrB2–SiC oxidation in air at 1500°C. The proposed model suggests that a structure consisting of (1) a silica-rich layer, (2) a Zr-rich oxidized layer, and (3) a SiC-depleted zirconium diboride layer is thermodynamically stable. The SiC-depleted layer developed due to active oxidation of SiC. The oxygen partial pressure in the SiC-depleted layer was calculated to lie between 4.0 × 10−14 and 1.8 × 10−11 Pa. Even though SiC underwent active oxidation, the overall process was consistent with passive oxidation and the formation of a protective surface layer.

Electrical Conductivity and Thermal Expansion of Spinels at Elevated Temperatures

Abstract: Binary oxide spinels composed of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, and Zn were synthesized. Electrical conductivity was measured in air at 500°–800°C. Thermal expansion was measured from room temperature to 1000°C. Ferrite spinels have thermal expansion coefficients of 11–12 ppm/K, compared with 7–9 ppm/K for other spinels except Cu–Mn and Co–Mn which show anomalous behavior. The highest electrical conductivity among transition metal spinels was found for MnCo2O4 (60 S/cm at 800°C) and Cu1.3Mn1.7O4 (225 S/cm at 750°C).

Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part II: Carbonated Phases

Abstract: The effects of carbonation of mechanochemically prepared C-S-H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X-ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO2 (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S >= 0.67 and aragonite at C/S <= 0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium-rich phases polymerize rapidly to yield Q(2) silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C-S-H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q(2) silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short-range order of poorly crystalline materials is ideal for investigations of C-S-H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C-S-H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO2 gel in the aged samples.

Ceramics for Prosthetic Hip and Knee Joint Replacement

Abstract: The most commonly used bearing couple in prosthetic hip or knee joint replacements consists of a cobalt–chrome (CoCr) metal alloy articulating against ultrahigh-molecular-weight polyethylene. Ceramics have been used as an alternative to metal-on-polyethylene in joint replacement surgery of arthritic hips and knees since the 1970s. In prosthetic hip and knee bearings, ceramic surfaces offer a major benefit of drastically reduced wear rates and excellent long-term biocompatibility, which can increase the longevity of prosthetic hip and knee joints. This benefit is important clinically because hip and knee replacement has become a very common surgical procedure, particularly in the United States, and because these procedures are being increasingly performed in younger patients who place greater demands on the prosthetic bearings. However, ceramics are brittle and the risk of catastrophic bearing failure in vivo, while rare, is a major concern. Improvements in material quality, manufacturing methods, and implant design have resulted in a drastic reduction of the incidence of such failures, so that modern ceramic bearings are safe and reliable if used with components of proven design and durability. Future material improvements are actively being investigated to reduce the risk of ceramic-bearing failures even further. The purpose of this article is to review the structure, properties, applications, and limitations of the ceramics that have been used in orthopedic bearings, and to describe the new ceramic composite materials and surface treatments that will be available for joint replacement surgery in the near future.

Ab Initio Calculation of Elastic Constants of Ceramic Crystals

Abstract: An effective computational scheme to calculate the complete set of independent elastic constants as well as other structural parameters including bulk modulus, shear modulus, Young's modulus, and Poisson's ratio for crystals is reported. The scheme is based on the stress–strain analysis approach with the appropriate selection of strain governed by symmetry consideration. The first principles Vienna ab initio simulation package (VASP) is used in stress calculations. Comprehensive tests were performed for α-SiO2 and spinel MgAl2O4 with different exchange-correlation potentials, and different sets of computational parameters to investigate the relative accuracies of the calculations. A wide range of oxides, nitrides, and carbonate crystals with different crystal symmetries were chosen to test the scheme under both LDA and GGA approximations at zero temperature and pressure. Some of these calculations for large complex crystals are believed to be attempted for the first time. The calculated elastic constants show quite good agreement with the existing experimental data for almost all the examined systems with the exception of the relatively soft material such as α-SiO2 and the C14 parameter of some trigonal crystals expressed in the hexagonal form such as in α-Al2O3. Other structural properties derived from the elastic constants also show good agreements with the measured values.

In Vitro Bioactive Characteristics of Borate‐Based Glasses with Controllable Degradation Behavior

Abstract: Silicate-based bioactive glasses undergo incomplete conversion to a calcium phosphate material after in vivo implantation, which severely limits their biomedical application. In this communication, novel borate-based glasses with controllable degradation behavior were developed and their bioactive potential was investigated in vitro. When immersed in a 0.02M K2HPO4 solution at 37°C, these glasses reacted to form a carbonate-substituted hydroxyapatite (c-HA) on their surfaces, indicating their bioactive potential. The conversion rate to c-HA was controlled by adjusting the B2O3/SiO2 ratio in the glass composition. The results indicate the potential application of the borate-based bioactive glass as scaffold materials for bone tissue engineering.

A New Approach to Modeling the Nucleation and Growth Kinetics of Tricalcium Silicate Hydration

Abstract: The hydration kinetics of tricalcium silicate (C3S), the main constituent of portland cement, were analyzed with a mathematical “boundary nucleation” model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C3S particle surfaces. This model more closely approximates the C3S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C3S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C3S pastes in the temperature range of 10°–40°C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the “induction period” is not a separate chemical process as has often been hypothesized.

Macroporous Ceramics from Particle-Stabilized Wet Foams

Abstract: We present a novel direct-foaming method to produce macroporous ceramics using particles instead of surfactants as stabilizers of the wet foams. This method allows for the fabrication of ultra-stable wet foams that resist coarsening upon drying and sintering. Macroporous ceramics of various chemical compositions with open or closed cells, average cell sizes ranging from 10 to 300 μm and porosities within 45% and 95%, can be easily prepared using this new approach. The sintered foams show high compressive strengths of up to 16 MPa in alumina foams with porosities of 88%.

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

Abstract: The interaction of water and the alumina surface is comprehensively reviewed. Water can be incorporated in the alumina crystal structure resulting in the formation of aluminum hydroxides such as gibbsite. Alumina dissolves into water to an extent that depends primarily upon the solution pH and temperature. The soluble Al (III)aq species (hydrolysis products) likewise depend upon the solution pH, temperature, aluminum, and other salt concentrations. The development of charge on the surface of alumina is controlled by amphoteric surface ionization reactions. The charging behavior of both alumina powders and single crystal faces is compared. The differences can be explained by the reactivities of different types of surface hydroxyl groups. The substantial difference in surface charging behavior of single crystal sapphire and alumina powders indicates that experiments and modeling conducted on single crystals is of limited use in predicting suspension behavior. The atomic scale structure of the hydroxylated sapphire (0001) basal plane is nearly identical to the gibbsite (001) basal plane. The observed surface structures are consistent with the charging behavior of the surfaces. The role of surface charge on the adsorption of processing additives is briefly discussed. How surface charge and processing additives at the alumina aqueous solution interface influence surface forces between particles is reviewed. The influence of these forces on suspension properties such as rheological behavior is outlined. The importance of controlling these behaviors to improve colloidal ceramic powder processing is stressed.

Oxide materials with low thermal conductivity

Abstract: A heuristic approach to identifying candidate materials with low, temperature-independent thermal conductivity above room temperature is described. On the basis of this approach, a number of compounds with thermal conductivities lower than that of 8 mol% yttria-stabilized zirconia and fused silica have been found. Three compounds, in particular, the Zr 3 Y 4 O 12 delta phase, the tungsten bronzes, and the La 2 Mo 2 O 9 phase, exhibit potential for low thermal conductivity applications. As each can exhibit extensive substitutional solid solution with other, high atomic mass ions, there is the prospect that many more compounds with low thermal conductivity will be discovered.

Resistance to Thermal Shock and to Oxidation of Metal Diborides–SiC Ceramics for Aerospace Application

Abstract: Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm2. Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB2+HfB2)–SiC and the ZrB2–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB2+HfB2)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB2+HfB2–SiC and ZrB2–SiC, respectively) as the specimen size increases.

Lithium Ion‐Conducting Glass–Ceramics of Li1.5Al0.5Ge1.5(PO4)3–xLi2O (x=0.0–0.20) with Good Electrical and Electrochemical Properties

Abstract: NASICON-type structured Li1.5Al0.5Ge1.5(PO4)3–xLi2O Li-ion-conducting glass–ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li2O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass–ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 × 10−3 and 7.25 × 10−4 S/cm, respectively. In addition, the Li1.5Al0.5Ge1.5(PO4)3–0.05Li2O glass–ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP–0.05LO glass–ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.

Near Infrared Absorption of Tungsten Oxide Nanoparticle Dispersions

Abstract: Homogeneous dispersions of reduced tungsten oxide and tungsten bronze nanoparticles with ternary additives Na, Tl, Rb, and Cs have been prepared in the wet process and examined for optical properties. The dispersions of reduced tungsten oxide and tungsten bronze nanoparticles are found to show a remarkable absorption of near infrared light while retaining a high transmittance of visible light. This property is highly suitable for solar control filters in automotive and architectural windows.

Two‐Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Abstract: Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density ( > 98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼ 4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.

Ceramics with Special Porous Structures Fabricated by Freeze-Gelcasting: Using tert-Butyl Alcohol as a Template

Abstract: By a novel freeze-gelcasting technique, ceramic bodies with unidirectional ordered and gradient porous structures were fabricated, using tert-butyl alcohol (TBA)/acrylamide (AM)/alumina (Al2O3) slurries. TBA, which can freeze below 25 °C and volatilize rapidly above 30 °C, was used as a template for forming pores. The porous structures could be controlled by the temperature conditions, resulting in special unidirectional and gradient porous structures over a long range of several millimeters. At the same time, gelation of AM was successfully introduced in this process, and played an important role in strengthening the green bodies (with compression strength over 10 MPa). Sintered Al2O3 with a high porosity showed high compression mechanical properties, which contributed to the high density of ceramic walls. This technique combines the two processes of cold freezing and thermal gelation in one procedure, and is considered to be potentially useful in many applications.

Preparation of Monolithic Silica Aerogel of Low Thermal Conductivity by Ambient Pressure Drying

Abstract: Monolithic silica aerogels with thermal conductivity as low as 0.036 W·(m·K)−1 and porosity as high as 97% were successfully prepared by ambient pressure drying through a multiple modification approach. This approach may replace the more costly and dangerous operation of supercritical drying. The tetraethoxysilane (TEOS)-derived wet gel was made hydrophobic with multiple treatments of trimethylchlorosilane and dried under ambient pressure. The multiple treatments were found to be necessary to achieve sufficient modification of the wet gel for reduction in drying-induced surface tension force to maintain product integrity and high porosity. Comparisons in nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy for surface bonding and contact angle measurement for hydrophobicity between the no, single, and multiple surface modification (MSM) samples were conducted to reveal the difference in the extent of the resulting surface modification. In conclusion, the MSM procedure reduced the volume shrinkage, increased the monolithicity and porosity, and lowered the thermal conductivity of the resulting aerogels.

Phase Evolution in Yttria-Stabilized Zirconia Thermal Barrier Coatings Studied by Rietveld Refinement of X-Ray Powder Diffraction Patterns

Abstract: One failure mechanism of thermal barrier coatings composed of yttria-stabilized zirconia (YSZ) has been proposed to be caused, in part, by the transformation of the tetragonal phase of YSZ into its monoclinic phase. Normally, studies of phase evolution are performed by X-ray diffraction (XRD) and by evaluating the intensities of a few diffraction peaks for each phase. However, this method misses some important information that can be obtained with the Rietveld method. Using Rietveld's refinement of XRD patterns, we observed, upon annealing of YSZ coatings, an increase of cubic phase content, a reduction in as-deposited tetragonal phase content, and the appearance of a new tetragonal phase having a lower yttria content that coexists with the as-deposited tetragonal phase of YSZ.

Fabrication of Functionally Graded and Aligned Porosity in Thin Ceramic Substrates With the Novel Freeze–Tape-Casting Process

Abstract: Functionally graded and continuously aligned pore structures have been fabricated by a modified tape-casting process for use as solid oxide fuel cell electrodes, catalysts, sensors, and filtration/separation devices. Pore gradients from <5 to 100 μm and aligned pore tubules have been directly fabricated in various ceramic materials with thin substrate sections approximately 500-1500 μm utilizing both low-toxicity aqueous-based slips and organic solvents. This process allows for the generation of pores without the use of thermally fugitive pore formers in a single processing step with no need for tape lamination. The incorporation of tape casting, unidirectional solidification, and the freeze-drying process results in uniformly acicular pores aligned with the direction of the moving carrier film. Processing and microstructure variability will be discussed as it pertains to the effects of solids loading, freezing temperatures, and solvent type. Applications for this ceramic processing technology will also be discussed.

Pressureless Sintering of Zirconium Diboride Using Boron Carbide and Carbon Additions

Abstract: The synergistic roles of boron carbide and carbon additions in the enhanced densification of zirconium diboride (ZrB2) by pressureless sintering have been studied. ZrB2 was sintered to >99% relative density at 1900°C. The combination of 2 wt% boron carbide and 1 wt% carbon promoted densification by removing surface oxide impurities (ZrO2 and B2O3) and inhibiting grain growth. Four-point bending strength (473±43 MPa), Vickers' microhardness (19.6±0.4 GPa), fracture toughness (3.5±0.6 MPa·m1/2), and Young's modulus (507 GPa) were measured. Thermal gravimetry showed that the combination of additives did not have an adverse effect on the oxidation behavior.

Two‐Stage Sintering of Alumina with Submicrometer Grain Size

Abstract: This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.

Compositional Inhomogeneity in Li‐ and Ta‐Modified (K, Na)NbO3 Ceramics

Abstract: Reference LC-ARTICLE-2008-016doi:10.1111/j.1551-2916.2007.01962.xView record in Web of Science Record created on 2008-04-11, modified on 2017-05-10

Processing of Particle‐Stabilized Wet Foams Into Porous Ceramics

Abstract: Direct foaming of colloidal suspensions is a simple and versatile approach for the fabrication of macroporous ceramic materials. Wet foams produced by this method can be stabilized by long-chain surfactants or by colloidal particles. In this work, we investigate the processing of particle-stabilized wet foams into crack-free macroporous ceramics. The processing steps are discussed with particular emphasis on the consolidation and drying process of wet foams. Macroporous alumina ceramics prepared using different consolidation and drying methods are compared in terms of their final microstructure, porosity, and compressive strength. Consolidation of the wet foam by particle coagulation before drying resulted in porous alumina with a closed-cell structure, a porosity of 86.5%, an average cell size of 35 μm, and a remarkable compressive strength of 16.3 MPa. On the other hand, wet foams consolidated via gelation of the liquid within the foam lamella led to porous structures with interconnected cells in the size range from 100 to 150 μm. The tailored microstructure and high mechanical strength of the macroporous ceramics can be of interest for the manufacture of bio-scaffolds, thermal insulators, impact absorbers, separation membranes, and light weight ceramics.

Highly Aligned Porous Silicon Carbide Ceramics by Freezing Polycarbosilane/Camphene Solution

Abstract: We fabricated highly aligned porous silicon carbide (SiC) ceramics with well-defined pore structures by freezing a polycarbosilane (PCS)/camphene solution. In this method, the solution prepared at 60°C was cast into a mold at temperatures ranging from 20° to -196°C, which resulted in a bicontinuous structure, in which each phase (camphene or PCS) was interconnected in a regular pattern. After the removal of the frozen camphene network, the samples showed highly porous structures, in which long straight and short elongated pore channels were formed parallel and normal to the direction of freezing, respectively. Thereafter, porous SiC ceramics were produced by the pyrolysis of the porous PCS objects at 1400°C for 1 h in a flowing Ar atmosphere, while preserving their mother pore structures having aligned pore channels.

Physicochemical Mechanism for the Continuous Reaction of γ-Al2O3-Modified Aluminum Powder with Water

Abstract: Previous experiments showed that γ-Al2O3-modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH− ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH− ions react with Al and release H2. Because of the limited H-soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H2 molecules accumulate and form small H2 gas bubbles at the Al:Al2O3 interface. When the reaction equilibrium pressure in H2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as-received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H2 bubbles decrease as well.

Crystallization Maps for SiCO Amorphous Ceramics

Abstract: Amorphous silicon–oxycarbide (SiCO) can retain large mole fractions of carbon when it is made from controlled pyrolysis of silicon-based polymers. The crystallization resistance of these ceramics, which is quite remarkable, varies with the carbon content. In high-carbon materials, crystallization is usually accompanied by weight loss (resulting from the carbothermal reduction of silica), whereas phase separation can lead to crystallization without significant weight loss in the low-carbon materials. A simple graphical method is developed to estimate the composition of the crystallized material, and the concomitant weight loss. The results are presented as maps for a quick estimate of crystallization and weight loss for any composition. Experiments with a medium–high carbon SiCO are used to quantify the degree of crystallization and the associated weight loss at 1300°C and at 1350°C; these results show that, in the case of medium high carbon content, crystallization begins with phase separation but becomes quickly dominated by weight loss.

Ionic Substitutions in Biphasic Hydroxyapatite and β-Tricalcium Phosphate Mixtures: Structural Analysis by Rietveld Refinement

Abstract: The structural information on the influence of ionic additions in biphasic (hydroxyapatite (HAP) and β-tricalciumphosphate (β-TCP)) mixtures ranging from single ionic substitutions to combined ionic substitutions of most of the essential ions embedded in biological apatite was analyzed through the Rietveld refinement technique. The results have proved that the determined quantitative phase composition of HAP and β-TCP in biphasic mixtures was dependent on the initial calcium (Ca) deficiency of the precursor powders precipitated from the different molar concentrations used in the synthesis. The substitution of cations (Na + , Mg 2+ , and K + ) improved the stabilization of the β-TCP structure whereas anions (F - and Cl - ) were found incorporated at the OH site of the HAP phase. Rietveld analysis of X-ray powder diffraction data from the present study proved to be a powerful technique to describe the position and occupancy of certain ions like Mg 2+ and Cl - in the biphasic mixtures. However, it has also shown limitations in tracking back other ions like Na + , K + , and F - , which require the use of other complementary characterization methods.