Anisotropic Grain Growth in Diphasic-Gel-Derived Titania-Doped Mullite

Densification and anisotropic grain growth in diphasic-gel-derived, titania-doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania-doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final-stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania-doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania-doped diphasic mullite gels followed the empirical equation Gⁿ - G ₀ ⁿ = Kt , with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.     

Interactions between Ruthenia-Based Resistors and Cordierite–Glass Substrates in Low-Temperature Co-fired Ceramics

Low-temperature co-fired ceramics (LTCCs) that are composed of a RuO₂-based resistor and a cordierite–glass substrate have been sintered at temperatures of 850° and 900°C. The microstructure of the resistor/substrate interface has been investigated using scanning and transmission electron microscopy, and its correlation to the overall resistance has been discussed. X-ray diffractometry has revealed that lead ruthenate pyrochlore (Pb₂Ru₂O_6.5) in peak-fired thick-film resistors (TFRs) disappears and the co-fired samples contain only RuO₂ in the resistor film when sintered at 900°C. The overall resistance of the LTCC resistors is increased by a factor of ∼3 when temperature is increased from 850°C to 900°C. The cordierite–glass composition of the initial substrate reacts with glass in the resistor film. The greatly extended layer of the resistor/substrate interface that contains the conductor particles is either broad or diffuse, which contrasts the abrupt interface that often is observed in conventional TFRs. This layer contains predominantly faceted platelike crystals of anorthite, in addition to other phases (such as diopside, sapphirine, and cristobalite) that apparently crystallize during co-firing as vitrification and chemical reactions between glass compositions of the substrate and the resistor occur. The increase in the resistance of the LTCC resistors is attributed to the interruption of the conducting path by platelike anorthite crystals that are produced in the resistor/substrate interface when subjected to co-firing.     

The influence of firing procedures on strain sensitivity of thick-film resistors

Thick-film resistors are electrical composites containing particles of ruthenate/ruthenium oxide conductor distributed in a highly modified silicate glass. Thick-film technology is widely used for the fabrication of force and pressure sensors for its virtues of good piezoresistivity and reliability. This paper reports a systematic study on the influence of firing conditions on the gauge factor (GF) of thick-film resistors. The relationship between sheet resistivity and GF of thick-film resistors with various concentrations of the conductive phase (10–30?wt% RuO₂) fired through different firing procedures was studied. The results show that the GF and the sheet resistivity of the thick -film resistor will decrease with the increase of conductive phase concentration, peak firing temperature and dwell time. When resistors of the same sheet resistivity are considered, the increase of the firing temperature will lead to a decrease of GF. The sheet resistivity and GF decrease simultaneously with the increase of dwell time and move along the straight line. The results indicate that the influence of firing conditions on the microstructure of thick-film resistors comprises two aspects. The first one is the increase in the number of conductive chains, and the second is the changes in the distribution of conductive chain particles.     

Shape of MgAl2O4 Grains in a CaMgSiAlO Glass Matrix

When sintered Al₂O₃ is annealed with CaMgSiAlO glass at 1600°C, polyhedral MgAl₂O₄ grains form and glass pockets are entrapped within the grains. After annealing for 13 h at 1600°C, the liquid pockets show a regular octahedron shape which is expected to represent the equilibrium shape. All grain surfaces in contact with the glass matrix show the same shape. The small grains, which must be shrinking, thus have the equilibrium shape, because their shrinkage shape is is identical to the equilibrium and growth shape. However, the octahedral shape also represents the growth shape for the growing large grains. The grains also form grain boundaries with neighboring grains.     

Grain Boundary Films in Rare-Earth-Glass-Based Silicon Nitride

The thickness of the intergranular films in Si₃N₄ densified with lanthanide oxides has been systematically investigated using high-resolution transmission electron microscopy. Four lanthanide oxides—La₂O₃, Nd₂O₃, Gd₂O₃, and Yb2O3—as well as Y₂O₃ are chosen so that the results will reflect the overall trend in the effect of the lanthanide utilized. The film thicknesses increase with increasing ionic radius of the lanthanide. In addition, Si₃N₄ particles flocculated into isolated clusters in the lanthanide-based glasses are also characteristically separated by an amorphous film whose thickness is similar to that in the comparable polycrystalline ceramics, demonstrating that the film thickness is dictated entirely by the composition and not by the amount of the glass phase present.     

Abnormal Grain Growth and Intergranular Amorphous Film Formation in BaTiO3

The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO₃. 0.4 mol% TiO₂-excess BaTiO₃ powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape.     

Characterization and Electrophoretic Deposition of Poly(Phenylsilsesquioxane)–Titania Hybrid Particles Prepared by the Sol–Gel Method

Poly(phenylsilsesquioxane)–titania (PhSiO_3/2–TiO₂) hybrid particles were prepared from phenyltriethoxysilane and titanium tetra- n -butoxide by the sol–gel method. Fourier transform infrared spectra showed that PhSiO_3/2 and the TiO₂ components were hybridized through Si–O–Ti bonds. The refractive index of the particles was monotonically increased from 1.57 to 1.62 with an increase in the TiO₂ content. The PhSiO_3/2–TiO₂ particles were electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates to form opaque, thick films about 3 μm in thickness. When the mole ratio x in (1− x )PhSiO_3/2· x TiO₂ was equal to or less than 0.05, the deposited PhSiO_3/2–TiO₂ films became transparent with a heat treatment at 400°C because of the thermal sintering of the particles.     

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)).     

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.     

Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO₂) and doped TiO₂ ceramic thin films were prepared on a glass substrate by a sol–gel and dip-coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of TiO₂ and doped TiO₂ thin films sliding against Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO₂ films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO₂ films are inferior to the TiO₂ films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO₂ thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO₂ thin films show lower plasticity than the TiO₂ thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.     

Crack Growth and Damage in Constrained Sintering Films

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al₂O₃ films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al₂O₃ films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al₂O₃ films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.     

Effect of Initial Microstructure on Final Intergranular Phase Distribution in Liquid-Phase-Sintered Ceramics

The glass-melt penetration of dense TiO₂ polycrystals (where the grains are initially in contact) and the sintering of SiO₂-coated TiO₂ powders (where the grains are initially separated) have been used to investigate the influence of initial particle separation on final microstructures. Grains that were initially in sintered contact resisted penetration by the liquid and retained crystalline boundaries. However, grains that were initially separated by liquid had a tendency to form a final state where they were separated by a glass film ∼1 nm thick. The results imply that crystalline grain boundaries and those that contain a thin amorphous film represent two local thermodynamic minima that are separated by an energy barrier. These observations are in agreement with a thermodynamic model that predicted such a barrier for this system, and these observations show that the stable phase distribution in liquid-phase-sintered ceramics can be dependent on the path.     

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si₃N₄ composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si₃N₄ prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si₃N₄ ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO₂ content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si₃N₄, causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si₃N₄ grains under greater local stress.     

Effects of Individual Layer Thickness on the Microstructure and Optoelectronic Properties of Sol–Gel-Derived Zinc Oxide Thin Films

Zinc oxide (ZnO) thin films were prepared under different conditions on glass substrates using a sol–gel process. The microstructure of ZnO films was investigated by means of diffraction analysis, and plan-view and cross-sectional scanning electron microscopy. It was found that the preparation conditions strongly affected the structure and the optoelectronic properties of the films. A structural evolution in morphology from spherical to columnar growth was observed. The crystallinity of the films was improved and columnar film growth became more dominant as the zinc concentration and the substrate withdrawal speed decreased. The individual layer thickness for layer-by-layer homoepitaxy growth that resulted in columnar grains was <20 nm. The grain columns are grown through the entire film with a nearly unchanged lateral dimension through the full film thickness. The columnar ZnO grains are c -axis oriented perpendicular to the interface and possess a polycrystalline structure. Optical transmittance up to 90% in the visible range and electrical resistivity as low as 6.8 × 10⁻³·Ω·cm were obtained under optimal deposition conditions.     

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

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.     

Suppression of Grain Growth in Sol–Gel-Derived Tin Dioxide Ultrathin Films

Tin dioxide thin films of various thicknesses up to 150 nm were prepared on quartz glass substrates from a sol solution of SnO₂ (particle size 3 nm) by a spin-coating method and subjected to calcination at different temperatures up to 800°C for 30 min. The grain size of SnO₂ was found to be far smaller than those obtained from the SnO₂ sol-derived powder under the same calcination conditions. The suppression of grain growth of SnO₂ was more conspicuous as the film thickness decreased so that in the thinnest film (20 nm thick) the SnO₂ grain size remained as small as 6 nm after calcination at 800°C. It is suggested that the SnO₂ grains in the ultrathin film deposited on the substrate are restrained from moving and coalescing with each other, resulting in the suppression of grain growth.     

Thermal Stability and Acid Resistivity of Glasses Based on ZnO-Al2O3-SiO2

The glass-forming region in the ZnO-Al₂O_3-SiO₂ system was studied, and the acid resistivity of the glass stabilized by PbO addition was measured. Glass containing more than 50 at.% SiO2 had a markedly increased acid resistivity. At about 50 at.% SiO₂, the crystallization of the glass caused its resistivity to decrease.     

Polar-Oriented Crystallization of Fresnoite (Ba2TiSi2O8) on Glass Surface Due to Ultrasonic Treatment with Suspensions

Improved polar ( c -axis) oriented growth of fresnoite (Ba₂TiSi₂O₈) was observed for surface crystallization of a glass 33.3BaO· 16.7TiO₂· 50SiO₂ (in mol%) due to ultrasonic surface treatment with an aqueous suspension of Ba₂TiSi₂O₈, and an oriented film more than 50 μm in thickness was prepared. The effect of ultrasonic treatment on the polar orientation depends on the suspending particles; of these, foreign particles of a phase other than the crystallizing Ba₂TiSi₂O₈ give poorer efficiency than the Ba₂TiSi₂O₈ particles. It is assumed that fresnoite has a tendency toward preferred polar growth on the glass, and seeding of a large number of fine particles through ultrasonic bombardment realizes the tendency. The polar-oriented growth kinetics of Ba₂TiSi₂O₈ were also discussed.     

Amorphous Intergranular Films in Silicon Nitride Ceramics Quenched from High Temperatures

High-temperature microstructures of an MgO-hot-pressed Si₃N₄ and a Yb₂O₃+ Al₂O₃-sintered/annealed Si₃N₄ were obtained by quenching thin specimens from temperatures between 1350° and 1550°C. Quenching materials from 1350°C produced no observable changes in the secondary phases at triple-grain junctions or along grain boundaries. Although quenching from temperatures of ∼ 1450°C also showed no significant changes in the general microstructure or morphology of the Si₃N₄ grains, the amorphous intergranular film thickness increased substantially from an initial ∼ 1 nm in the slowly cooled material to 1.5–9 nm in the quenched materials. The variability of film thickness in a given material suggests a nonequilibrium state. Specimens quenched from 1550°C revealed once again thin (1-nm) intergranular films at all high-angle grain boundaries, indicating an equilibrium condition. The changes observed in intergranular-film thickness by high-resolution electron microscopy can be related to the eutectic temperature of the system and to diffusional and viscous processes occurring in the amorphous intergranular film during the high-temperature anneal prior to quenching.     

Positive Temperature Coefficient of Resistivity Effect in Highly Donor–Doped Barium Titanate

BaTiO₃ ceramics doped with different La concentrations (0–12 mol%) were prepared by sintering under the reducing conditions of a nitrogen atmosphere containing 1% hydrogen. The critical donor concentration that causes blocking of the exaggerated grain growth was observed to be ∼10 mol% La. The samples, which were semiconducting after sintering under reducing conditions, were subsequently reoxidized by annealing in air to induce the positive temperature coefficient of resistivity (PTCR) effect. After reoxidation at 1150°C a noticeable PTCR effect was observed in the samples doped with La concentrations as high as 2.5 mol%. The room-temperature resistivity after reoxidation was found to increase with increasing donor concentration due to an increase in the thickness of the insulating layers at the grain boundaries. TEM analysis showed that reoxidation of the samples caused precipitation of the Ti-rich compound Ba₆Ti₁₇O₄₀ inside the doped BaTiO₃-matrix grains.