Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Mullite—Alumina Composites by Extrusion

The rheological properties of a paste containing chopped alumina fiber and particulate silica suspended in a gelled boehmite liquid phase have been evaluated using a physically based extrusion model. When sintered, the paste formed a mullite-alumina fiber composite. Extrudates with fiber volumes up to 30% in the sintered product were prepared. During extrusion, the pressure drop was largely independent of extrudate velocity, fiber length, and the fiber concentration. All pastes showed significant yield behavior leading to good postextrusion shape retention. For any given fiber length, it was shown that there exists a critical volume fraction above which fiber-fiber interactions are so great that both yield and wall shear stresses increase. At these high concentrations of fiber, inhomogeneities also increase. Up to the critical volume fraction, dispersed wet fibers produced lower extrusion parameters than when dry fibers were used as the starting material. The observed behavior is explained in terms of low viscosity liquid formation above the yield point of the boehmite gel.     

Strain-Induced Deformation in Magnesia–Alumina Layered Composites

Ceramic beams are induced in situ to form complex shapes at elevated temperature without the application of an external stress. This process has been demonstrated for thin alumina substrates coated with a layer of magnesia. The internal strain causing the substrates to deform at elevated temperature arises as a consequence of strain mismatch accompanying the penetration of the coating into the substrate. The magnitude of the deformation depends on the amount of coating applied, on the thickness of the substrate, on the density of the substrate, and on the temperature. During exposure of the beams to elevated temperature, the magnesia coating reacts with the alumina substrate to form the spinel phase; the resulting volume change accompanying the phase transformation is likely the predominant driving force for deformation.     

Multifunctional Alumina Composites with Toughening and Crack‐Healing Features Via Incorporation of NiAl Particles

In this research, NiAl particles modified Al₂O₃ composites with toughening and crack‐healing performances were prepared by spark plasma sintering. The toughening effects as a function of the NiAl particle size and content were systematically investigated. The fracture toughness was increased by 80% compared with pure alumina after incorporation of 1 μm NiAl particles at 15 wt%. A self‐healing process of damaged composites induced by indentation was found to occur, mediated by oxidation of NiAl at elevated temperatures with the size of the NiAl particles. Atomic force microscopy was adopted to characterize the crack‐healing performance and to explore the healing mechanism under each healing condition. The best crack‐healing efficiency of 99.2% was found in the composite containing 1 μm NiAl at 15 wt% that was annealed at 1300°C for 5 h. In addition, the fracture toughness of the composite was further enhanced by 40% after the crack‐healing process. These multifunctional ceramic composites may have promising and broad application in areas such as thermal barrier coatings, fuel cell membranes, etc.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

High-Temperature Creep Deformation of Alumina — SiC-Whisker Composites

The creep resistance at temperatures between 1200° and 1300°C in air of alumina—SiC-whisker composites was investigated via four-point flexure to examine (1) the effect of whisker content and (2) the influence of densification additives (i.e., Y₂O₃ (plus MgO)). The creep resistance of polycrystalline alumina is greatly improved with the addition of ≤ 20 vol% SiC whiskers. The interlocking/pinning of grains by whiskers which limits grain-boundary sliding contributes to the improvement in creep resistance. However, the creep rates of alumina composites in air increase at whisker contents ≥ 30 vol%. Electron microscopy observations suggested that the degradation in creep resistance for whisker content ≥ 30 vol% originated from (1) the promotion of creep cavitation and subsequent microcrack generation from the higher number density of nucleation sites and (2) more extensive formation of grain-boundary amorphous phase(s) associated with an observed increased oxidation rate. Along this one, the excellent creep resistance of alumina composites containing 20 vol% SiC whiskers was significantly degraded by the presence of the intergranular amorphous phases introduced by the addition of the Y₂O₃ densification additive.     

Ceramic Composites of Monazite and Alumina

A new family of high-temperature, oxidation-resistant, ceramic composites, with (La)-monazite (LaPO₄) serving as a weakly bonded interphase, is proposed. Monazite is stable and phase-compatible with alumina at temperatures at least as high as 1750°C in air. Especially important for use in high-toughness composites, the monazite-alumina interface is shown to be sufficiently weak that interfacial debonding prevents cracks from growing from monazite into alumina. Observations of fracture responses of fibrous and laminar reinforcements are presented.     

Alumina and Alumina/Zirconia Multilayer Composites Obtained by Slip Casting

The slip casting technique has been revealed as a powerful method to obtain multilayer composites close to theoretical density. From zeta potential and viscosity measurements of Al₂O₃ and Al₂O₃/ZrO₂ (4 vol% ZrO₂) suspensions, the corditions for the preparation of multilayer composites by slip casting have been determined. A microstructural analysis of the different layers by scanning electron microscopy is also reported.     

High‐Alumina Boron‐Containing Refractory Castables

This work addresses the optimization of alumina‐based castables designed for petrochemical applications by adding an additive able to speed up the samples' densification at lower temperatures. Hot elastic modulus, thermal shock, hot modulus of rupture, and erosion resistance measurements were carried out to evaluate these castables. The E profiles confirmed the transient feature of the generated liquid phase, as it was crystallized throughout subsequent heating cycles. Thermal fatigue effect was also observed for the compositions with higher boron content. Nevertheless, the boron‐containing castables showed outstanding properties, which could be attained only with using suitable amounts of this engineered additive.     

Alumina Agglomerate Effects on Toughness-Curve Behavior of Alumina–Mullite Composites

Toughness-curve ( T -curve) behavior of composites of spherical, polycrystalline, coarse-grained, alumina agglomerates dispersed throughout a constant-toughness, fine-grained, 50–50 vol% alumina–mullite matrix has been evaluated as a function of agglomerate content for the range 15 to 45 vol%. T -curve behavior was evaluated using the indentation-strength method. Increasing alumina agglomerate content resulted in a progressive increase of large indentation load strengths with negligible change of plateau strength levels at small indentation loads. This behavior is consistent with underlying T -curves that rise to greater values and are shifted toward longer crack lengths with increasing agglomerate content, suggesting that both bridge spacing and bridge potency increase with increasing agglomerate content over the range tested. The proposed relationships between bridge spacing and agglomerate content, and bridge potency and agglomerate content, are rationalized in terms of residual stress considerations. The indentation-strength data also demonstrated that the composite containing the greatest alumina agglomerate content, 45 vol%, exhibited the greatest flaw tolerance.     

Fabrication of Graded Nickel–Alumina Composites with a Thermal-Behavior-Matching Process

Composites of nickel and Al₂O₃ with compositionally graded microstructures were fabricated from powders through an empirically determined thermal-behavior-matching process that was designed to minimize processing-induced stresses. Compositions ranged from pure Al₂O₃ to pure nickel. Specimen geometries included round disks 25 mm in diameter and 5–25 mm thick, as well as rectangular bars 25 mm × 25 mm in cross section and 75 mm long. Several different gradients were produced, including samples with single interlayers. Compacts were formed by cold uniaxial pressing in a die, followed by consolidation through sintering at 1 atm or hot isostatic pressing. Several different particle sizes of nickel and Al₂O₃ comprised the composite interlayers. The compaction behavior, sintering start temperature, sintering rate, and total linear shrinkage of each composition were evaluated. Careful data analysis, coupled with sintering theory, led to a layer configuration with matched green density and sintering behavior. Thermomechanically matched layers allowed large, crack-free, graded composites to be produced.     

UV‐Patterning of Anti‐Biofouling Zwitterionic Copolymer Layer with an Aromatic Anchor Group

A thin layer (around 2 nm in thickness) composed of a random copolymer of zwitterionic carboxymethyl betaine (CMB) and p‐trimethoxysilylstyrene (STMS) (9:1) is constructed on a glass substrate or a silicon wafer. The copolymer layer is highly resistant against nonspecific adsorption of bovine serum albumin (BSA). However, on UV irradiation at 193 nm, the layer becomes hydrophobic and BSA is significantly adsorbed on the substrate. Upon UV irradiation through a photomask, a patterning of the fluorophore‐labeled protein with a resolution of about 1 µm can be clearly observed. On the other hand, the copolymer layer of CMB and 3‐methacryloxypropyltrimethoxysilane (MPTMS) (9:1) without an aromatic group exhibit less distinct pattern. Further, the poly(CMB‐r‐STMS) layer decomposes more quickly compared with the poly(CMB‐r‐MPTMS) layer at low irradiation dose. The zwitterionic polymer layer with an aromatic anchor group will be used at the modification of substrates applicable to sensing devices.

     

Sulfobutyl Ether‐β‐Cyclodextrins: Promising Supramolecular Carriers for Aqueous Organometallic Catalysis

The potentialities of sulfobutyl ether‐β‐CDs derivatives as supramolecular carrier in a biphasic Tsuji–Trost reaction catalyzed by a water‐soluble palladium complex of trisulfonated triphenylphosphine have been investigated. The efficiency of these cyclodextrins (CDs) strongly depends on the average molar substitution degree of cyclodextrin and the highest rate enhancements were obtained with cyclodextrins containing about 7 sulfobutyl ether groups. This result was attributed to the absence of a strong interaction between this cyclodextrin and the trisulfonated triphenylphosphine used to dissolve the catalyst in the aqueous phase and to the presence of an extended hydrophobic cavity allowing a better molecular recognition between the substrate and the cyclodextrin. This constitutes the first example of a non‐interacting β‐cyclodextrin/phosphine couple with high catalytic activities.     

Synthesis and characterization of novel reactive ladder‐like polysilsesquioxanes with side‐chain ester groups (Ester‐Ts)

Two kinds of structure‐ordered reactive ladder‐like polysilsesquioxanes with different side‐chain ester groups (Ester‐Ts) have been synthesized successfully by a new route – stepwise coupling polymerization – with three steps including preaminolysis, hydrolysis and polycondensation. Two monomers, 2‐trichlorosilylethyl acetate (M₁) and 3‐trichlorosilylpropyl propionate (M₂), were first synthesized by hydrosilylation with dicyclopentadienylplatinum (II) chloride (Cp₂PtCl₂) as a catalyst. Monomer and polymer structures were characterized by FTIR, ¹H NMR, ¹³C NMR, ²⁹Si NMR, X‐ray diffraction (XRD), differential scanning calorimetry (DSC) and vapour pressure osmometry (VPO). Characterization data indicate that these two polymers have ordered ladder‐like structures. © 2000 Society of Chemical Industry     

Synthesis,Oxygenation and Catalytic Oxidation Performance of Crown Ether‐Containing Schiff Base‐Transition Metal Complexes

Mono‐Schiff bases containing a crown ether ring ( HL ¹ , HL ² , HL ³ and HL ⁴ ) and their transition metal complexes were synthesized and characterized by ¹H NMR, IR, and mass spectroscopy as well as elemental analysis. The oxygenation constants (K) of Schiff base‐Co(II) complexes were measured over the range of −5 to +25 °C, and the values of ΔH° and ΔS° were calculated based on these K values. Using Mn(III)‐Schiff base complexes, the biomimetic catalytic oxidation of styrene to benzaldehyde was carried out with 100% selectivity. Comparison of this complex with analogues not containing a crown ether moiety clearly demonstrated the influence of crown ether ring on the dioxygen affinities and biomimetic catalytic oxidation performance of the Schiff base complexes.     

Dispersion and Consolidation of Alumina Using a Bis-Hydrophilic Diblock Copolymer

A water-soluble diblock copolymer, poly[(methacrylic acid)- b -(ethylene oxide)], has been used to formulate alumina slurries that are dispersed over a wide range of pH through a combination of electrostatic and steric repulsion. The anionic poly(methacrylic acid) block, a short chain ( M _W∼700 g/mol), interacts with the amphoteric alumina surface at pH 3 and acts as an anchor block. The nonionic poly(ethylene oxide) (PEO) block ( M _W= 3000 g/mol) only appears to have a steric functionality. At high pH, the polymer coated surface has a net negative charge due to the excess negative sites associated with the highly dissociated methacrylic acid (MAA) units; this provides electrostatic repulsion at, for example, pH 9, which is the isoelectric point for pure alumina. The PEO chains extend from the surface and stabilize the alumina particles between pH 5 to 9 via an entropy-driven, steric repulsion. The PEO chains provide the dominating dispersive force at low pH, where very little electrostatic repulsion is possible because the positive surface charge of alumina has been neutralized by the negative sites of the adsorbed polyacid. On consolidation through pressure filtration at low pressures (2 to 5 MPa), the saturated, consolidated bodies formulated at pH 5 have a relative density of ∼0.56. These consolidated bodies initially exhibit a flow stress, but then fluidize after an extended strain (up to 0.5). The fluidization phenomenon is believed to be due to the unraveling of the PEO chains that appear to entangle between adjacent particles during consolidation. The unraveling was directly observed with an atomic force microscope between two BaTiO₃ surfaces coated with the same diblock copolymer used for the alumina powder.     

Indentation Fatigue in Random and Textured Alumina Composites

The effect of grain orientation on contact fatigue behavior has been investigated by using alumina with elongated grains as a model system. Two kinds of composite microstructures, textured and random, were prepared by controlling the processing conditions. The textured material has the platelets aligned parallel to the surface, and the random material has the platelets randomly oriented. The Hertzian indentation results show that, although both materials exhibit damage accumulation with increasing number of cycles due to frictional degradation at the microstructural level, the damage evolution rate is much lower for the textured material. This suppression of fatigue damage in the textured material appears to result from the lower shear stress concentration along the textured weak interfaces between elongated alumina grains. The implication of the present results for structural design in improvement of contact-fatigue resistance is also addressed.     

Aging and Thermal Stability of the Mixed Product of the Ether‐Soluble Fraction of Bio‐Oil and Bio‐Diesel

The storage and thermal stability of blends of the ether‐soluble fraction of bio‐oil (ES) and bio‐diesel are reported. Fuel properties such as viscosity, water content, acid number and average molecular weight of the ES/bio‐diesel blends were measured before and after aging. Compared to the aging properties of bio‐oil alone, very small changes in water content and viscosity were shown for the blends aged at 80 °C for 180 h. Chemical changes were characterized using gel permeation chromatography, showing a slight increase in the molecular weight over time. Further confirmation of the changes was provided through Fourier transform infrared spectrometry, thermal decomposition analysis using a thermogravimetric analyzer, proton assignment using proton nuclear magnetic resonance, and carbon assignment using carbon nuclear magnetic resonance. Overall, the study indicates that ES/bio‐diesel blends are stable as fuel under the conditions tested in this paper.