Concurrent Measurement of Volume, Grain-Boundary, and Surface Diffusion Coefficients in the System NiO-Al2O3

Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al₂O₃ system were measured concurrently by using a diffusion couple consisting of an A1₂O₃ bicrystal and an NiO single crystal. The A1₂O₃ bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H₂ atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D _s = 7.41×10^-2 exp (-35,200/ RT ), D _gb = 2.14×10^-1 exp (-63,100/ RT ) (tilt angle =30°), and D _v = 1.26×10⁴ exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.     

Grain Boundary Evolution in Hot-Pressed ABC-SiC

Silicon carbide with aluminum, boron, and carbon additions (ABC-SiC) was hot-pressed to full density. The samples were examined by transmission electron microscopy (TEM), with an emphasis on high-resolution electron microscopy (HREM). Amorphous grain boundary interlayers, typically less than 2 nm wide, were formed between SiC grains. Heat-treating the ABC-SiC at temperatures as low as 1100°C in Ar crystallized the grain boundary interlayers completely without significantly changing the dominant chemical constituents. Chemical microanalyses demonstrated Al and O enrichment for all examined grain boundaries in both as-prepared and annealed samples. Quantitative EDS analyses revealed Al₂OC- and Al₂O₃-related species (with Si, C, B, or S substitutions) as two of the most likely grain boundary interlayer materials, both before and after heat treatment. Al₂O₃, and (Al_{1− x} Si _x )₂OC with a 2H-type wurtzite structure, were identified as grain boundary films by HREM images. The structural evolution in the grain boundary phases during the hot pressing and postannealing is discussed.     

Effect of Temperature and Water-Solid Ratio on Growth of Ca(OH)2 Crystals Formed During Hydration of Ca3SiO5

The growth behavior, time of nucleation, and morphology of Ca(OH)₂ crystals formed during the hydration of Ca₃SiO₅, at 15°, 25°, and 35°C at water-solid ratios ( w/s ) from 0.3 to 5.0 were studied by optical microscopy. In samples with w/s >0.5 growth of Ca(OH)₂ in the c -axis direction is initially dominant. Growth in this direction ends after a few hours, but growth perpendicular to the c axis continues for several days and produces a dendritic morphology. Growth behavior is not so well defined for w/s <0.5, in part because of the large number of unhydrated particles engulfed. Increasing temperature resulted in an increase in the number of Ca(OH)₂ nuclei and a decrease in nucleation time and crystal size. Increasing the w/s ratio improved the euhedral character of the Ca(OH)₂ crystals, decreased the number of engulfed Ca₃SiO₅ particles, and increased the nucleation time. Dendritic morphology was most pronounced in the samples for which w/s = 1. Growth rates and the ultimate size of the Ca(OH)₂ crystals varied within a given sample. The effects of temperature and the w/s ratio on the heat evolved during the hydration were studied by isothermal calorimetry. The times of nucleation of crystalline Ca(OH)₂ estimated from calorimetry were similar to those derived from growth curves determined by optical microscopy.     

Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Comparison of Sizes of Magnesia Crystals Calculated from X-ray Diffraction Line-Broadening and Transmission Electron Microscopy Observations

When MgO is formed by decomposition of MgCO₃ under vacuum, crystal sizes calculated from XRD line breadths are in reasonable agreement with direct TEM observations, 4.4 ± 0.6 nm compared with 3.2 ± 0.8 nm. For aligned MgO crystallites from Mg(OH)₂, the sizes calculated are discordant, 14.0 ± 3.0 nm and 2.6 ± 0.7 nm. The sizes calculated from XRD data are too large because the MgO crystallites form from Mg(OH)₂ in near-perfect alignment.     

Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)₂ were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X-ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting-sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting-sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)₂ underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO₃ precipitated on the surface of disrupted Mg(OH)₂ crystals acting as a kinetic barrier to both the outward diffusion of H₂O and the inward diffusion of CO₂.     

Polishing Behavior and Surface Quality of Alumina and Alumina/Silicon Carbide Nanocomposites

The response of Al₂O₃ and Al₂O₃/SiC nanocomposites to lapping and polishing after initial grinding was investigated in terms of changes in surface quality with time for various grit sizes. The surface quality was quantified by surface roughness ( R _a ) and by the relative areas of smooth polished surfaces as opposed to rough as-ground areas. Polishing behavior of the materials was discussed in terms of SiC content and grain size. It was concluded that nanocomposites are more resistant to surface damage than Al₂O₃, and this behavior does not depend on the amount of SiC in the range 1–5 vol%. SiC addition ≥1 vol% is enough to produce a noticeable improvement in surface quality during lapping and polishing.     

Electrokinetic Properties of Nanosized SiC Particles in Highly Concentrated Electrolyte Solutions

In this research, the electrokinetic behavior and stability of nanosized SiC particles suspended in various electroplating solutions were studied. Analyses were performed using electrophoretic mobility photometry and streaming current (SC) techniques. The electrolytes included NiCl₂, Ni(SO₃NH₂)₂, and Na₃Co(NO₂)₆, which are currently used in composite plating solutions with concentrations as high as 0.5 M . The results showed that the adsorption of dissolved Ni²⁺ ions onto the surface of the SiC in the pH range 4–8 changed the sign and magnitude of the surface potential. Moreover, trivalent complex species Co(NO₂)₆³⁻ replaced nickel species on the SiC surface and decreased the surface charge of SiC to between pH 3 and pH 5. Even in a highly concentrated electrolyte solution, the SiC particles still maintained a positive charge in a Ni(SO₃NH₂)₂ suspension with nickel coplating on the cathode. The difference between the SC reading and the zeta potential, as well as the surface adsorption of various species onto the SiC, are discussed here.     

Pressureless Sintering of SiC

Pressureless sintering of SiC was accomplished at 2100°C with oxide additives. These additives were the products of the reaction of Al(OH)₃ with HCl and of Y(OH)₃ with HCOOH. These reaction products were dissolved in water and mixed with submicrometer β-SiC. A mixture of equal weights of these additives was effective for the sintering of SiC.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al₂O₃-based porous ceramics fabricated from pure Al₂O₃ powder and the mixtures with Al(OH)₃ were investigated. The fracture strength of the porous Al₂O₃ specimens sintered from the mixture was substantially higher than that of the pure Al₂O₃ sintered specimens because of strong grain bonding that resulted from the fine Al₂O₃ grains produced by the decomposition of Al(OH)₃. However, the elastic modulus of the porous Al₂O₃ specimens did not increase with the incorporation of Al(OH)₃, so that the strain to failure of the porous Al₂O₃ ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)₃ particles. Fracture toughness also increased with the addition of Al(OH)₃ in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.     

Thermal Decomposition of Europium Hydroxide

The thermal decomposition of europium hydroxide in an air atmosphere was investigated by means of weight-loss measurements, infrared spectroscopy, X-ray diffraction analysis, and electron microscopy. These studies showed that EU(OH)° decomposed at temperatures between 225° and 300°C into EuOOH, which was stable up to about 425°C. Between 435° and 465°C this compound decomposed into cubic Eu₂O₃, which was stable until its inversion to the high-temperature monoclinic form. X-ray diffraction data were collected for Eu(OH)₃ and EuOOH and showed that the trihydroxide has a hexagonal crystal structure and the oxyhydroxide is possibly orthorhombic. The Eu(OH)₂, EuOOH, and cubic EunOa powders contained particles up to several microns in size consisting of agglomerates of crystallites in the size range 200 to 400 A. The single monoclinic Eu₂O₃ sample studied contained crystallites whose average size was greater than 2000 A.     

Hydrolytic Condensation of Tin(IV) Alkoxide Compounds to Form Particles with Well-Defined Morphology

The sol-gel-type condensation of tin(IV) ethoxide [Sn(OEt)₄] n (where OEt is ethoxide) under basic conditions produced spherical, submicrometer-sized tin(IV) oxide (cassiterite) particles. Transmission electron microscopy and powder X-ray diffraction data indicated that the grain size was approximately 20 to 30 Å (2 to 3 nm). The mixed-metal alkoxide compound [ZnSn(OEt)₆] was hydrolyzed under analogous conditions to give either spherical or octahedral submicrometer-sized crystalline particles of ZnSn(OH)₆ depending on the solvents used. These data demonstrated that the stoichiometry of the mixed-metal alkoxide precursor was retained during condensation. Thermal treatment of ZnSn(OH)₆ resulted in crystallization of ZnSnO₃ at approximately 676°C. At neutral pH, hydrolysis of [ZnSn(OEt)₆] resulted in formation of a high surface area (261 m²/g) amorphous powder.     

Degradation During Aging of Transformation-Toughened ZrO2-Y2O3 Materials at 250°C

The detrimental aging phenomenon observed in ZrO₂-Y₂O₃ materials, which causes tetragonal ZrO₂ to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y₂O₃ and SiO₂ content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO₂ to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)₃. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y₂O₃) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.     

Preparation by a Sol-Gel Process of Borosilicate Glasses Containing Microcrystals of Tellurium and Zinc Telluride

Borosilicate glasses, 5B₂O₃· 95SiO₂ (mol%), containing TeO₂ and ZnO nominally equivalent to 10 wt% Te and ZnTe were prepared by a solgel method from Si(OC₂H₅)₄, B(OCH₃)₃, H₆TeO₆, and Zn(NO₃)₂. A study by electron spectroscopy for chemical analysis (ESCA) showed that glasses heated at high temperature (450°C) in air contained both Te⁶⁺ and Te⁴⁺ ions on the surface layer, but that mainly Te⁴⁺ ions occurred inside the bulk glass. When solgel-derived borosilicate glasses containing the TeO₂ compound were reduced at elevated temperature in a hydrogen atmosphere, Te crystallites ranging in size from 4 to 15 nm were produced at a lower temperature, between 200° and 250°C. The absorption edge moved from the infrared to the visible wavelength region as the particle size decreased to about 4 nm. For glasses containing both TeO₂ and ZnO, ZnTe crystallites formed at high temperature—over 300°C—and existed along with the Te phase.     

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)₃) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl₃ solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO₂ flakes, or α-Al₂O₃ flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al₃O(OH)₃(OH₂)₉]⁴⁺ polycation is promoted, leading to the formation of the (Al₄) tetramer ([Al₄O(OH)₁₀(OH₂)₅]^o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al₃O(OH)₃(O₂H₃)₃(OH₂)₉]⁺) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al₂O₃ layers, while calcination of gibbsite leads to δ-Al₂O₃ layers.     

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si₃N₄/SiC) were studied by combined high-resolution transmission electron microscopy and electron energy-loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si₃N₄ matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si₃N₄, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si₃N₄ grain boundaries and at phase boundaries between Si₃N₄ and SiC. The Si₃N₄ grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω-Si₃N₄ grains are discussed on the basis of these experimental results.     

AI2O3 Coatings as Diffusion Barriers Deposited from Particulate-Containing Sol-Gel Solutions

A procedure for the formation of A1₂O₃ coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi₂ and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A1₂O₃ particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A1₂O₃ particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A1₂O₃ and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.     

Dispersion of Alumina and Silicon Carbide Powders in Alumina Sol

Dispersion of Al₂O₃ and SiC particles in an alumina sol has been investigated through determination of particle-size distribution, zeta potential, and agglomerate morphology. The particle size of Al₂O₃ and SiC (as determined by the particle-size analyzer) is strongly affected by the presence of alumina sol in the solution. The average agglomerate size is decreased by at least 50%. The zeta potential of Al₂O₃ in 1 M alumina sol increases slightly, whereas that of SiC reverses its sign over a wide range of pH values. It is proposed that these effects are caused by AlO₄Al₁₂(OH)₂₄(H₂O)⁷⁺₁₂ sol clusters (1-2 nm in size) that are absorbed on the surface of the large (1-5 µm) ceramic particles. The electrostatic and steric effects of the cluster absorption help to control the dispersion and stabilize the suspension of ceramic particles in the alumina sol during wet processing. It is expected that the alumina-sol clusters can be used as an efficient, clean dispersant for single-phase and composite ceramic powders.