Kinetics of Templated Grain Growth of 0.65Pb(Mg1/3Nb2/3)O3·0.35PbTiO3

Single-crystal layers of 0.65Pb(Mg_1/3Nb_2/3)O₃·0.35PbTiO₃ (PMN-35PT) were grown heteroepitaxially on {001}-BaTiO₃ template crystals. A {001}-BaTiO₃ crystal was embedded in a fine-grained matrix of PMN-35PT containing excess PbO and heated between 950° and 1150°C for 0–5 h. The initial growth of the PMN-35PT on the {001} surface and the growth of the matrix grains both displayed a t ^1/3 dependence which is characteristic of diffusion-controlled growth. Growth was limited to ∼100–150 μm due to the significantly reduced driving force at longer times because of matrix coarsening and porosity evolution.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

Effect of Seeding Sr2KNb5O15 on the Phase Formation and Microstructural Development in Reactive Sintering of Sr2NaNb5O15 Ceramics

The phase formation, densification behavior, and microstructure development of Sr₂NaNb₅O₁₅ (SNN) ceramics in both 10 wt% acicular Sr₂KNb₅O₁₅ (SKN) seed-containing and unseeded systems were investigated in this work. SNN ceramics were reactively sintered from SrNb₂O₆ and NaNbO₃ powders. The results show that the acicular SKN seeds not only accelerate SNN phase formation but also promote the densification at lower temperature. In reactive sintering, the acicular SKN seeds prepared by the molten salt synthesis method can give rise to the formation of a liquid phase and provide the structural framework for the grain growth of ceramics, leading to the formation of large anisotropic grains (>80 μm) in ceramics sintered at 1340°C. However, there are no such large anisotropic grains obtained in the SKN-free system. Observation of the large anisotropic grain growth is explained by the liquid-phase-assisted growth mechanism. For comparison, the microstructure evolution in the system with 10 wt% SKN seed, which was prepared by the conventional mixed-oxide method and without acicular morphology, was also investigated to further support the new growth mechanism.     

Microstructure Development in Reactive-Templated Grain Growth of Bi1/2Na1/2TiO3-Based Ceramics: Template and Formulation Effects

"Reactive-templated grain growth" (RTGG) processing of Bi_1/2Na_1/2TiO₃ (BNT)-based ceramics is reported. Molten salt synthesis was used to prepare platelike (∼0.2 μm × 5 μm × 5 μm) Ruddlesden–Popper (Sr₃Ti₂O₇ (ST)) and Aurivillius (BaBi₂Nb₂O₉ (BBN)) phases which were used as "templates" in studies of RTGG with BNT-based matrixes. A "citrate-gel" route was designed to produce intimately mixed, fine-grain matrixes for these studies. The analytical techniques used were powder X-ray diffraction and microstructural examination of dry-pressed and fired compacts. For mixtures templated with BBN, single-phase perovskite readily formed, and an initially heterogeneous microstructure evolved toward a dense assemblage of anisometric, micrometer-scale grains. Perovskite formation was more sluggish in the mixtures templated with ST, and the final sintered microstructure featured larger, porous grains in an equiaxed, micrometer-scale matrix. A qualitative model, which examined the excess constituents in the matrix after formation of stoichiometric ABO₃ perovskite, is proposed to explain the observations. The model predicted an excess of Na₂O and TiO₂ in the matrix in the case of BBN templates and only excess TiO₂ in the case of ST templates. The results indicate that careful examination of matrix and template chemistry could be important in the selection of systems for RTGG processing.     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

Textured PMN–PT and PMN–PZT

A promising way to improve the performance of piezoelectric ceramics is grain orientation by templated grain growth. In this work lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN–32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN–37PT–21PZ) ceramics were textured via templated grain growth process. For texturization (001)-oriented BaTiO₃ (BT) platelets (approximately 10 μm × 10 μm × 2 μm) were utilized as templates. The texturized ceramics were accomplished by aligning the templates by tape casting. The template growth into the matrix resulted in textured ceramics with Lotgering factors between 0.94 and 0.99 for both compositions. Consequences of the texture are enhanced dielectric and piezoelectric properties. Unipolar strain-field measurements of textured ceramics showed 0.25% strain s ₃₃ at 3 kV/mm. Large signal d ₃₃^* of up to 878 pm/V were determined directly from strain measurements. Compared with randomly oriented ceramics in texturized samples unipolar strain s ₃₃ and large signal d ₃₃^* was enhanced by a factor of up to 1.8.     

Processing and Microstructure Development in Alumina–Silicon Carbide Intragranular Particulate Composites

Al₂O₃–SiC particulate composites were fabricated by hot-pressing mixtures of 5–30 vol% SiC with either α-Al₂O₃, γ-Al₂O₃, or boehmite (γ-AlOOH) to determine whether grain growth or the α-alumina phase transformation could be used to fabricate intragranular particulate composites. Samples starting with α-alumina resulted in primarily intergranular SiC of 0.3 μ and an alumina grain size of 1.5–4.1 μm. Heat treatments resulted in SiC coarsening but no entrapment of SiC by grain boundary breakaway. The α-alumina transformation in the samples starting with γ-alumina resulted in the entrapment of ∼48% of the 5 vol% of SiC added whereas 79% of the SiC was entrapped in the α-alumina grains in samples starting with boehmite. Only SiC particles ≤0.2 SmUm were entrapped in the α-alumina grains during the phase transformation. With increasing SiC content, the relative volume of intragranular SiC decreased, but the amount of intragranular SiC was constant and independent of the amount of SiC added before transformation. The formation of intragranular composites from γ-alumina and boehmite samples was explained with a model that attributes particle entrapment to the vermicular growth of α-alumina into the transition alumina matrix during the α-alumina phase transformation. Seeding the boehmite-based samples did not affect the concentration of entrapped SiC, but did lower the hot-pressing densification temperature by as much as 150°C.     

Effect of Inclusions on Densification: I, Microstructural Development in an Al2O3 Matrix Containing a High Volume Fraction of ZrO2 Inclusions

The microstructural changes produced by large (38 to 53 μ m), single-crystal ZrO₂ inclusions (0, 0.09, 0.30 volume fractions, based on solid volume) within an Al₂O₃ powder matrix were detailed as a function of constrained densification. Composite powder compacts were produced by pressure filtration for conditions where the Al₂O₃ slurry was either flocced or dispersed. For both conditions, the ZrO₂ inclusions constrained densification. Microstructural observations for all composites revealed (1) the presence of cracks with large opening displacements between inclusions and (2) large density variations within the matrix. The cracks were most frequent at high volume fraction of inclusions in composites produced from flocced slurries and apparently originated during specimen preparation. Their large opening displacment was a result of matrix densification. Fewer cracks were observed in composites produced from dispersed slurries. Instead, these microstructures were dominated by large variations in matrix density, viz., dense regions surrounding low-density regions, not consitent with the initial packing density of the matrix powder. The denser regions were formed early in the densification schedule. The lower-density regions eventually developed into regions containing large, elongated voids as the Al₂O₃ matrix grains became larger with heat-treatment time. This pore enlargement process was shown to result from the disappearance of necks between originally sintered grains and appeared similar to the thermodynamic instability observed in thin films and constrained fibers.     

Rapid Hot-Pressing of Ultrafine PSZ Powders

The process of compaction and densification of ultrafine (40- to 60-nm grain size) powder of partially stabilized zirconia with 3 mol% of Y₂O₃ (Y3-PSZ) during rapid hot-pressing was investigated. A special apparatus was designed to allow rapid application of 1.6 GPa of quasi-isostatic pressure at temperatures of 1100° to 1300°C to powder compacts encapsulated in glass under vacuum. Pressure was applied for 10 s, then the samples were rapidly cooled to room temperature, removed from the encapsulating glass, and characterized using SEM, TEM, and X-ray diffraction. Density and mechanical properties of the prepared materials were measured and compared with those of similar materials fabricated using conventional hot-pressing. SEM and TEM observations revealed that the ultrafine grains of the starting powder coarsened rapidly during the initial heating, and the compacts developed large (> 10 μm) and small (< 1 μm) pores. The process of densification under pressure consisted of closing of the large pores, whereas the small pores were relatively unaffected by the application of pressure at all investigated temperatures. The major mechanism of densification during the rapid pressing appears to be rearrangement and sliding of grains around the large pores. The material prepared by rapid pressing at 1300°C had higher hardness ( H _v= 1400 versus 1300 kg/mm²) but somewhat lower fracture toughness ( K _{I C} = 5.5 versus 6.0 MPa · m^1/2) compared with the conventionally hot-pressed Y3-PSZ. Density of the material pressed at 1300°C was 97% of theoretical density.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Mechanism of Texture Development in Bi0.5(Na,K)0.5TiO3 prepared by the Templated Grain Growth Process

The mechanisms of texture evolution in Bi_0.5(Na,K)_0.5TiO₃ (BNKT) prepared by the templated grain growth method are examined using platelike SrBi₄Ti₄O₁₅ and Al₂O₃ powders and SrTiO₃ and Al₂O₃ single crystals as templates. These templates give rise to a 〈100〉 texture in the BNKT matrix. The mechanism of texture evolution is dependent on the template species. When SrBi₄Ti₄O₁₅ and SrTiO₃ are used, a new grain (terrace) forms between the matrix and the template grains. The terrace has the same crystallographic orientation as the template. The terrace grows at the expense of the matrix grains, resulting in texture evolution. For the Al₂O₃ template, no terrace forms between the matrix and the template grains. Instead, the matrix grains directly attach to the template surface. The formation of a phase boundary with a specific orientation gives rise to texture evolution for this template.     

Effects of Temperature and Reagent Concentration on the Morphology of Chemically Vapor Deposited β-Ta2O5

Crystalline β-Ta₂O₅ coatings were deposited on hot-isostatically-pressed Si₃N₄ by reacting TaCl₅ with H₂ and CO₂ in the temperature range of 1000°–1300°C and at a pressure of 660 Pa. The Ta₂O₅ coatings generally consisted of wellcoalesced 2–3 μm grains, resulting in the formation of a nonporous coating morphology. However, the presence of microcracks on the as-deposited surface was consistently observed. The surface morphology, texture, and growth rate of the coatings were examined as a function of deposition parameters.     

Ambient-Temperature Mechanical Response of Alumina–Fluoromica Laminates

Flexural delamination experiments were used to evaluate the mechanical performance of thermochemically stable alumina–fluoromica laminates. Hot-pressed, precracked laminate specimens, in which two MgAl₂O₄-spinel-coated alumina substrates were separated by a thin layer of fluorophlogopite (KMg₃(AlSi₃)O₁₀F₂), were tested in fourpoint flexure at room temperature. Two types of mechanical response were observed: steady-state delamination and brittle failure. Microstructural analysis showed that the delamination response was associated with fine (≤5 μm) grains of the mica; the brittle response occurred when the mica interphase consisted of large (>30 μm) grains that bridged the interphase. The steady-state strain-energy release rate ( G _ss) measured on the graceful, delaminating beams was 9.1 ± 0.4 Jm^–2 for randomly oriented ∼ 5–μm grains but only 2.8 ± 0.2 Jm^–2 for ∼1–μm grains that were aligned with easy-cleavage planes parallel to the laminate interfaces. The results suggested that debonding of the specimens occurred via cleavage of the mica grains. Observation of delamination cracks confirmed this point: propagation occurred within the fluoromica interphase rather than along the spinel/alumina or spinel/fluorophlogopite interfaces. The mechanical feasibility of laminate specimens without the protective spinel coating on the substrate containing the notch was also tested to address an issue related to the preparation of alumina fiber/mica interphase/alumina matrix composites. The delamination response again occurred for the case of a fine-grained mica interphase.     

Comparative Electrical Behavior at Low and High Current of SnO2- and ZnO-Based Varistors

The complete I–V characteristics of SnO₂-based varistors, particularly of the Pianaro system SCNCr consisting in 98.9%SnO₂+1%CoO+0.05%Nb₂O₅+0.05%Cr₂O₃, all in mol%, have been seldom reported in the literature. A comparative study at low and high currents of the nonohmic behavior of SCNCr- and ZnO-based varistors (modified Matsuoka system) is proposed in this work. The SCNCr system showed higher nonlinearity coefficients in the whole range of measured current. The electrical breakdown field ( E _b) was twice as high for the SCNCr system (5400 V/cm) than for the ZnO varistor (2600 V/cm) due to a smaller average grain size of the former (4.5 μm) with respect to the latter (8.5 μm). Nevertheless, we consider that another important factor responsible for the high E _b in the SCNCr system is the great number of electrically active interfaces (85%) as determined with electrostatic force microscopy (EFM). It was also established that the SCNCr system might be produced in disks of smaller dimensions than that of commercial ZnO-based product, with a 5.0 cm⁻¹ minimal area–volume ( A/V ) ratio. The SCNCr reached the saturation current in a short time because of the high resistivity of the grains, which is five times higher than that of the grains in ZnO-based varistors.     

Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics

Different microstructures in SiC ceramics containing Al₂O₃, Y₂O₃, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated α-SiC grains with aspect ratio >4, length >2 μm, and grain thickness ( t ) <3 μm (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 μm < t < 3 μm and length >2 μm (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPa·m^1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes.     

Growth of BaTiO3 Seed Grains by the Twin-Plane Reentrant Edge Mechanism

Two different types of BaTiO₃ seed particles, normal and twinned seeds of ∼30 μm on the average, were prepared from crushed sintered specimens. Normal seeds were obtained from the usual BaTiO₃ sintered compacts, while twinned seeds containing a double twin were obtained from BaTiO₃ compacts sintered with 2 mol% of SiO₂. The BaTiO₃ powder compacts were again prepared with 5 wt% of seed grains and sintered under various conditions. The microstructural evolution was quite different in the two cases: the growth of normal seed grains was ultimately limited but that of the twinned seeds continued extensively. The observed difference is discussed in terms of the growth mechanism and the atomic structure of interfaces.     

Microstructure Development in Low-Antimony Oxide-Doped Zinc Oxide Ceramics

The grain growth of ZnO ceramics sintered with low additions of Sb₂O₃ (<500 ppm of Sb) was investigated. Additions of Sb<250 ppm resulted in a coarse-grained microstructure with large ZnO grains (55–70 μm), much larger than the grain size of ZnO ceramics without any Sb₂O₃ addition (45 μm). The addition of 500 ppm of Sb resulted in a fine-grained microstructure with an average ZnO grain size of about 12 μm. The results are explained by an inversion-boundary (IB) -induced grain-growth mechanism. The grain-growth exponent has a value of about 2 as long as the grains containing IBs grow at the expense of IB-free grains. It increases to about 4 after the IB-containing grains impinge on each other, and achieves values above 10 for additions of 500 ppm of Sb when IBs nucleate in nearly all the ZnO grains so that grains with IBs prevail in the microstructure at an early stage in the grain-growth process.