Al2O3/Ni Laminar Ceramics Shaped by Tape Casting and Electroless Plating

Tape casting and electroless plating were used to fabricate Al₂O₃/Ni laminar ceramic composites with close control of the thickness of the Al₂O₃ and Ni layers. Ninety-seven percent relative density, macrodefect-free composites were obtained by spark plasma sintering. In electroless plating solutions, the stable potential of grain boundary led to the first deposition of nickel on the grain boundary of Al₂O₃. Scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction were used to analyze the structure, elements distribution, and phase composition of the Al₂O₃/Ni laminar composites.     

Precipitation of Magnesium Aluminum Spinel from Alumina-Matrix Solid Solution: I, Fundamental Concept and Precipitation Behavior

Al₂O₃ ceramics with magnesium aluminum spinel dispersion particulates were produced by a precipitation treatment of Al₂O₃-matrix solid solution, (Al_1—2x,Ti _x ,Mg _x )₂O₃. We successfully constructed the precipitation process for synthesizing the nanocomposite, using the solubility dependence of titanium and magnesium in Al₂O₃ on the valence of titanium. Changing of the valence of titanium from 4+ to 3+ was accomplished by controlling the heating atmosphere, and, thereby, magnesium precipitation was promoted. The precipitation behavior was characterized using X-ray diffractometry, and the microstructure was observed using transmission electron microscopy. We confirmed that magnesium aluminum spinel nanosized particulate were precipitated in the Al₂O₃ grain.     

Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures

High-temperature plastic flow in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) has been examined at temperatures between 1400° and 1500°C. Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al₂O₃. The suppression of grain growth with ZrO₂ dispersion into Al₂O₃ is not effective for improving the tensile ductility. The limited ductility in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) is associated with the increment of flow stress caused by ZrO₂. The ZrO₂ dispersion or segregation in Al₂O₃/Al₂O₃ boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

Novel Fabrication Route to Al2O3–TiN Nanocomposites Via Spark Plasma Sintering

A novel method for the preparation of Al₂O₃–TiN nanocomposites was developed. A mixture of TiO₂, AlN, and Ti powder was used as the starting material to synthesize the Al₂O₃–TiN nanocomposite under 60 MPa at 1400°C for 6 min using spark plasma sintering. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used for detailed microstructural analysis. Dense (up to 99%) nanostructured Al₂O₃–TiN composites were successfully fabricated, the average grain size being less than 400 nm. The fracture toughness ( K _{I C} ) and bending strength (σ_b) of the nanostructured Al₂O₃–TiN composites reached 4.22±0.20 MPa·m^1/2 and 746±28 MPa, respectively.     

Microstructures of SrTiO3 Internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties

The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO₂, Al₂O₃, and SiO₂ have a completely wetting liquid phase at the sintering temperature; during cooling Ti_nO_{2 n −1}, Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi₂O₃, PbO, CuO, and B₂O₃) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling.     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

Elastic Properties and Microcracking Behavior of Particulate Titanium Diboride–Silicon Carbide Composites

The spontaneous microcracking of particulate TiB₂–SiC composites is studied as a function of TiB₂ volume fraction. The degree of microcracking was examined by measuring elastic properties from room temperature to 1300°C. The results showed that only one composition contains microcracks. All other compositions did not microcrack regardless of TiB₂ volume fraction. This was attributed to the difference in the sintering aids. In particular, the Al₂O₃ sintering aid needed in these compositions had reacted with SiO₂ to form an amorphous grain boundary phase that allowed residual stresses to relax by viscous flow at moderate to high temperatures. The existence of this amorphous grain boundary phase was directly observed by transmission electron microscopy.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

Synthesis and Properties of Porous Single-Phase β'-SiAlON Ceramics

Single-phase β'-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0–4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of á-Si₃N₄, AlN, and Al₂O₃ of the SiAlON compositions. A solution of AlN and Al₂O₃ into Si₃N₄ resulted in the β'-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%–90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase β'-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength.     

Microstructure of 96% Alumina Ceramics: III, Crystallization of High-Calcia Boundary Glasses

The crystallization characteristics of a group of 96% Al₂O₃ ceramics containing a high-CaO boundary glass were examined using a combination of electron microscopy techniques. As many as six crystalline phases formed at the boundaries during anneals between 1000° and 1350°C. Devitrification was rapid, and little glass remained following a 1-h anneal. Despite the extensive changes in grain-boundary microstructure, heat treatment had little effect on the room-temperature mechanical properties of these Al₂O₃ materials; however, high-CaO bodies fractured intergranularly, whereas high-MgO bodies fractured transgranularly.     

High-Resolution Electron Microscopy of Silicon Carbide-Whisker-Reinforced Alumina Composite Interfaces in Specimens Subjected to Elevated Temperatures

Interfaces of silicon carbide-whisker-reinforced alumina (SiC( w )/Al₂O₃) composites were examined using high-resolution electron microscopy (HREM). HREM specimens were prepared from the bulk of samples that were previously tested for fracture toughness at 25°, 1000°, 1200°, or 1400°C, in ambient air. The test temperature history served as an independent variable. It was found that the as-received material did not possess a distinct interfacial layer and that the test temperature history (which included a 30°C/min heating and cooling rate, a 30-min soak prior to specimen loading, and a typical test duration of 5–10 min) did not appreciably change the interface thickness at any of the elevated test temperatures.     

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu-Al₂O₃) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al₂O₃/Cu/Al₂O₃ triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al₂O₃ grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al₂O₃ interfaces, whereas the Al₂O₃ boundaries remained dry. Detailed high-resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al₂O₃ interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al₂O₃ grain, and its value varied from 0.4 nm for Al₂O₃ rhombohedral-plane termination ((1¯012)) up to 1 nm for Al₂O₃ basal-plane termination ((0001)).     

Effects of Zirconium Doping on Grain-Boundary Bonding in Alumina-Silicon Carbide Composites

In this work, 800 ppm of Zr⁴⁺ dopants were added to Al₂O₃-5 vol% SiC particle composite. Zr⁴⁺ doping led to a weak Al₂O₃ grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr⁴⁺-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr⁴⁺ and Si⁴⁺ ions at the Al₂O₃ grain boundary.     

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al₂0₃ second-phase particles and their interactions with grain boundaries in two high-conductivity Y₂0₃/Yb₂0₃ stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al₂0₃ particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al₂O₃ particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al₂0₃ acts as a scavenger for SiO₂, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO₂-Al₂O₃-SiO₂ interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al₂O₃ additions, as reported in the literature.     

Microstructural and Microchemical Characterization of a Calcium Aluminate—Polymer Composite (MDF Cement)

The microstructure of a macrodefect-free (MDF) cement has been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution electron microscopy (HREM). The microchemistry of ultramicrotomed samples has been studied by energy dispersive spectrometry (EDS) and parallel electron energy loss spectrometry (PEELS). MDF cement consists of CaAl₂O₄ and CaAl₄O₇ grains randomly distributed in a polymer matrix. The ceramic/polymer interface contains an amorphous interphase inside of which are distributed very fine crystallites of the metastable hydration product Ca₂Al₂O₅·8H₂O. PEELS analyses of the interphase revealed the presence of carbon, indicating that Ca₂Al₂O₅·8H₂O was most likely stabilized by the intercalation of polymeric chains into its basal interlayers. The polymer phase preferably cross-links with Al. In situ environmental cell electron microscopy showed that moisture uptake of MDF cements occurred by polymer swelling and interphase dissolution. The role of the interfacial interphase in dry and wet mechanical properties is discussed.     

Orientation and Grain Boundary Microstructure of Alumina in Al/Al2O3 Composites Produced by Reactive Metal Penetration

The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al₂O₃ composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al₂O₃. The interconnected Al₂O₃ in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al₂O₃ are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.     

Forming and Sintering of in Situ Alumina Composite with Hydraulic Inorganic Binder

Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl₂O₄), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl₂O₄·H₂O), and BaAl₂O₄. The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl₂O₄ powders to Al₂O₃ powders improved the bending strength of Al₂O₃ matrix green bodies. The synthesized BaAl₂O₄ powders led to the in situ forming of barium hexaaluminate (BaO· x Al₂O₃, x = 6.9: BA6) platelets in the matrix by reacting with Al₂O₃ during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al₂O₃ grains.     

Crystallization of Pseudo-orthorhombic Anorthite on Basal Sapphire

Anorthite-glass films were grown on basal Al₂O₃ substrates using pulsed-laser deposition. The substrates were cleaned and annealed in air at 1400°C to produce crystallographically flat (0001) terraces. The films were deposited in an oxidizing environment. X-ray microanalysis confirmed the composition of the glass films to be close to that of anorthite (CaO·Al₂O₃·2SiO₂). Although anorthite usually has triclinic symmetry, subsequent crystallization of these films in air at 1200°C resulted in the formation of pseudo-orthorhombic CaAl₂Si₂O₈ ( o -anorthite), a known metastable form of the mineral. Microstructural characterization was performed using visible-light microscopy, scanning electron microscopy, and transmission electron microscopy. The films dewetted the substrate either before or after crystallization to form o -anorthite islands which had strong orientation relationships to the Al₂O₃ substrate. The epitaxy of the o -anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. The formation of three orientational variants is consistent with the symmetry of the basal Al₂O₃ surface. The dislocation network observed at the o -anorthite/Al₂O₃ interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacial amorphous layer. The study of anorthite-glass films is important because they are present in liquid-phase-sintered Al₂O₃, and may be devitrified by postsintering heat treatments.     

Influence of Dopant Concentration on Creep Properties of Nd2O3-Doped Alumina

The microstructural features and tensile creep behavior of Al₂O₃ doped with Nd₂O₃ at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd³⁺ ions were strongly segregated to the Al₂O₃ grain boundaries. Microstructural observations revealed that the solubility of Nd₂O₃ was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al₂O₃. The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism.