Microstructure and Grain Boundary Structure of Na+-Diffused (Sr,Ca)TiO3 Capacitor-Varistor Ceramics

The microstructure of (Sr,Ca)TiO₃ capacitor-varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO₃ grains (∼30 μm) having perovskite crystal structure with domains, a Na⁺-diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple-grain junctions in which the Ti _n O_2n–1 Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na⁺are discussed.     

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al₂O₃ at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al₂O₃, AI₂O₃ with anorthite, AI₂O₃ with anorthite and MgO, and Al₂O₃ with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al₂O₃.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

Fracture Strength and Microstructure of β-SiAlON with Hafnia Addition

The influence of HfO₂ addition on the fracture strength and microstructure of ß-SiAlON ceramics sintered from Si₃N₄ and Al₂O₃ powders was investigated. The strength was increased by the addition of HfO₂, from ∼500 MPa to 700 MPa, and was almost constant from ambient temperature to 1300°C. Monoclinic HfO₂ grains that were distributed in the SiAlON grain boundaries had a flat shape (∼20 nm thick) and were surrounded by an amorphous phase. The aluminum concentration in ß-SiAlON in the samples with an Al₂O₃ starting composition of 15 wt% was decreased by the addition of HfO₂. The amount of secondary phases was very small at grain boundaries between the SiAlON grains; amorphous phases were observed infrequently at the triple points but were very small (∼20 nm). The effects of HfO₂ addition on the mechanism of the microstructure development and fracture strength are discussed.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

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.     

Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B₂O₃, 2 wt% P₂O₅, 2 wt% Cr₂O₃, and (1 wt% P₂O₅+ 1 wt% Cr₂O₃) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al₂O₃ or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B₂O₃ and P₂O₅ increased the temperature of spinel formation and that B₂O₃ significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B₂O₃ was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B₂O₃ had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B₂O₃ was the most volatile additive.     

Transmission Electron Microscopy Characterization of Hot-Pressed ZrB2 with MoSi2 Additive

Microstructure of the hot-pressed ZrB₂ with MoSi₂ additive was investigated by transmission electron microscopy (TEM). The effect of MoSi₂ addition on the microstructure of the ceramic was assessed. For the pure ZrB₂, the microstructure consisted of the equiaxed ZrB₂ grains and a few elongated ZrB₂ grains. For the ZrB₂ with MoSi₂ additive, the microstructure consisted almost entirely of equiaxed ZrB₂ grains. A few dislocations were present in the ZrB₂ grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB₂ grain boundaries in the ZrB₂ with MoSi₂ additive.     

Evaluation of Grain-Growth Parameters

The complicated texture of a thin sheet of MgO and MgO-doped Al₂O₃ (∼15 pm) was statistically resolved into four typical grain geometries. Grain size distribution was related to the ratio of expressions describing the growth (or shrinkage) of individual grains and the growth of grains of average size R. Grain-growth data for MgO indicate that the mobility of a grain boundary is independent of R (parabolic-grain-growth behavior) but depends strongly on the ratio of individual grain size to average grain size. When mobility is proportional to 1/R, the time dependence of grain growth is <1/2.     

Grain Boundaries in Titania-Doped α-Alumina with Anisotropic Microstructure

The microstructure of sol-gel-derived alpha-alumina (Al₂O₃) doped with 0.6 wt% titania, sintered at 1450°C for 1 h, consisted of thin platelets with (0001) faces in a matrix of equiaxed grains. Short facets at the edges of the platelets developed primarily parallel to the {10     2} planes, while some were parallel to the {11     3} planes; other edges showed irregular, curved boundaries. The basal surfaces of the platelets were coated with thin layers (0.5-6 nm) of an amorphous titanium-containing aluminosilicate phase, which also was present at triple points. No amorphous phase was found on the short faceted boundaries, on curved boundaries at platelet edges, or at grain boundaries of equiaxed, matrix grains. However, titanium enrichment was observed at all examined boundaries, suggesting that titanium segregation alone did not account for the development of anisotropic microstructure. Curved incursions on basal facets were associated with occasional particles of aluminum titanate (Al₂TiO₅).     

The Role of Excess Magnesium Oxide or Lead Oxide in Determining the Microstructure and Properties of Lead Magnesium Niobate

Near-phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb₂O₆ at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X-ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very "clean" or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

A Test of the Second-Phase and Impurity-Segregation Models for MgO-Enhanced Densification of Sintered Alumina

To determine if MgO added within the solubility limit in Al₂O₃ is sufficient to suppress discontinuous grain growth, an undoped alumina pellet was sintered next to a preequilibrated, two-phase mixture of spinel (MgAl₂O₁) and alumina. The MgO-doped outer surface of the undoped pellet did sinter to full density; metallography confirmed that the sample was free of second phase. Grain boundaries were analyzed with a scanning Auger microprobe, which showed that the grain boundaries of the inner porous region had approximately the same Ca segregation as those in the dense outer shell region. Therefore, it was concluded that the beneficial effect of magnesium is due neither to second-phase pinning nor to Ca impurity segregation.     

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ_s, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al₂O₃, and MgO-doped Al₂O₃. The values of Ψ_s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al₂O₃ at 1870 K, and 90° to 139° for MgO-doped Al₂O₃ at 1870 K. For all three systems, the median Ψ_s values are 105° to 113°, implying that the median γ_gb/γ_s is 1.1 to 1.2, in contrast to metals where γ_gb/γ_s ranges typically from 1/4 to 1/2. The widths in Ψ_s distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al₂O₃, undoped Al₂O₃. For all three materials, the grainboundary grooves and their corresponding Ψ_s were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψ_s, with the maximum inclination angles of ∼13°, in quantitative agreement with theory.     

Grain Growth of Silica-Added Zirconia Annealed in the Cubic/Tetragonal Two-Phase Region

The grain growth in silica-doped 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (SiO₂-doped 3Y-TZP) and undoped 3Y-TZP has been examined in the temperature range of 1400°-1800°C. The presence of a SiO₂ phase inhibits rather than promotes the grain growth in 3Y-TZP, particularly at high temperatures. During the grain growth in 3Y-TZP, yttrium ions are partitioned between grains, and the grain growth mechanism can be understood from Ostwald ripening dominated by lattice diffusion of cations. In SiO₂-doped 3Y-TZP, an amorphous SiO₂-rich phase exists only in the grain-boundary corners or junctions, not in the grain-boundary faces. The grain growth in SiO₂-doped 3Y-TZP is controlled by using different mechanisms below and above the eutectic temperature of the zirconia-silica (ZrO₂-SiO₂) system. The glass phase does not have a major role in grain growth below the eutectic temperature, and the grain growth is dominated by a similar mechanism in undoped 3Y-TZP. The grain growth is more effectively retarded by the presence of a SiO₂ phase above the eutectic temperature and is likely to be controlled by a solution-reprecipitation process in the amorphous phase at the grain-boundary corners or junctions.     

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI₂O₃ made from AI-isopropoxide (AI₂O₃(II)). The undoped material was found to contain 1.5×10¹⁸ cm⁻³ fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving V _AI"in donor-, AI," in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (O_i^p). The p_O2 dependence of σ _t and σ _h are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.     

Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: II, Multicomponent Systems

The glass melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, TGA, and XRD interactively. The first-formed liquid phase occurred at 700°C from eutectic melting among CaCO₃, Na₂CO₃, and MgO. Further liquid phase formed at the CaCO₃–Na₂CO₃, eutectic at 785°C and a fusion reaction among SiO₂, CaO, and the molten phase at 812°C. Reactions between molten soda ash and silica grains to form a sodium disilicate coating also occurred in this temperature range. The effects of reaction accelerant additions (Na₂SO₄, NaNO₃, NaCI) on batch fusion were analyzed. Sodium chloride was found to be the most effective melting accelerant due to the formation of a NaCI–Na₂CO₃ eutectic liquid phase at ∼636°C, which effectively attacked the silica relic. CO₂ gas release terminated ∼80°C earlier with 1 wt% NaCI additions to the base glass.     

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.