Shape Dependence of the Coarsening Behavior of Niobium Carbide Grains Dispersed in a Liquid Iron Matrix

In niobium carbide–iron (NbC-Fe) specimens where the grains were faceted, abnormally large grains appeared during coarsening. Normal and uniform grain growth occurred when the grain shape was changed to a spherical morphology by the addition of a small amount of boron. The results have been discussed, in terms of a coarsening mechanism, depending on the atomic structure of the interface. For faceted grains with an atomically smooth interface structure, the coarsening was suggested to occur via two-dimensional nucleation and a lateral-growth mechanism. For spherical grains with an atomically rough interfacial structure, diffusion was suggested to control the coarsening process.     

Effect of Grain Shape on Abnormal Grain Growth in Liquid-Phase-Sintered Nb1−xTixC–Co Alloys

The effect of titanium substitution for niobium on the grain shape change, grain growth inhibition, and abnormal grain growth during liquid-phase sintering of Nb_{1− x} Ti_xC–Co alloy was studied. With increased titanium substitution, the shape of the Nb_{1− x} Ti _x C grains in the liquid cobalt matrix was changed from a cube with round corners to a cube with angular corners, which implied increased edge energy. As the grain corners became more angular, the grain growth became markedly inhibited, and abnormal grain growth occurred. The results could be best explained by the increased edge energy of the interface of the Nb_{1− x} Ti _x C grains, which increased the barrier for the growth by two-dimensional nucleation.     

Temperature Dependence of the Coarsening Behavior of Barium Titanate Grains

The coarsening behavior of large seed particles during the sintering of BaTiO₃ ceramics has been investigated. At 1350°C, the grains are faceted, and the seed particles grow extensively. At 1380°C, however, the grains are spherical, and coarsening of the seed particles is limited. The observed difference is discussed in terms of the growth mechanism and the atomic structure of the interfaces.     

Effect of Liquid Volume Fraction on Grain Growth of Magnesium Oxide Grains in Molten Calcium Magnesium Silicate Matrix

The grain-growth kinetics of fully dense MgO compacts containing various amounts of CaMgSiO₄ from 1 to 20 wt% was investigated. The relationship between grain size, volume fraction of liquid phase, and annealing time was determined. The exponent of grain growth ( n ) was 3, independent of the volume fraction of the liquid phase, and the rate constant ( k ) was inversely proportional to the volume of liquid. The overall grain-growth kinetics was governed by mass transport through liquid pockets at grain corners, which provided the longest diffusion paths between the grains. This result was modeled after a solid-state system containing isolated pores in which the pores move by vapor-phase diffusion.     

Effect of Magnesium Oxide Addition on Surface Roughening of Alumina Grains in Anorthite Liquid

Alumina sintered with 5 wt% anorthite at 1620°C for 48 h has grains with flat boundaries and a size distribution representing abnormal grain growth. TEM observations of the grain triple junctions show flat grain surfaces, some of which are the (0001), ([Onemacr]012), and (1[Onemacr]01) planes. HRTEM observations confirm these surfaces to be atomically flat and hence singular. When sintered further after embedding in MgO powder, the {0001} and { 01[Onemacr]2} planes remain flat, but curved surface segments also appear. These curved surface segments are confirmed to be atomically rough by HRTEM. They are connected to the flat segments with discontinuously changing slopes. Thus, when MgO is added, the singular and rough surface phases coexist.     

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.     

Habits of Grains in Dense Polycrystalline Solids

We show here that the boundaries of individual grains in dense polycrystals prefer certain crystallographic habit planes, almost as if they were independent of the neighboring crystals. In MgO, SrTiO₃, MgAl₂O₄, TiO₂, and aluminum, the specific habit planes within the polycrystal correspond to the same planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations decrease the apparent complexity of interfacial networks and suggest that the mechanisms of solid-state grain growth may be analogous to conventional crystal growth. The results also indicate that a model for grain-boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.     

Coarsening Behavior of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) Grains Dispersed in a Clinker Melt

Microstructural evolution during the heat treatment of cement clinker was investigated. Two model specimens, which consisted of faceted tricalcium silicate (C₃S) and spherical dicalcium silicate (C₂S) grains dispersed in a liquid matrix, were prepared with 5 wt% of large seed particles. The seed particles of faceted C₃S grains grew extensively, whereas those of the spherical C₂S grains grew rather slowly, relative to the matrix grains. As a consequence, C₃S grains exhibited a bimodal size distribution that was typical of exaggerated grain growth, whereas C₂S grains retained a uniform and normal size distribution. These results suggest that the growth of faceted C₃S grains was controlled by the interface atomic attachment, such as two-dimensional nucleation, and that of spherical C₂S grains was controlled by diffusion through the liquid matrix. The dependence of growth mechanisms on grain morphology has been explained in terms of the atomistic structure of the solid/liquid interface.     

Core-Shell Structure of Acceptor-Rich, Coarse Barium Titanate Grains

Heavily doped barium titanate (BaTiO₃) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO₃ under the conventional sintering conditions_.     

Effect of Twin-Plane Reentrant Edge on the Coarsening Behavior of Barium Titanate Grains

When BaTiO₃ ceramics were sintered at relatively low temperatures (≤1250°C), the grains with reentrant edges caused by a (111) double twin grew exclusively. As a result, a microstructure with a bimodal grain-size distribution composed of platelike large grains and fine matrix grains was obtained. In contrast, at the usual sintering temperature between 1250° and 1350°C, grains containing a (111) double twin did not exhibit any growth advantage. In this case, a coarse and uniform microstructure was obtained. When this coarse-grained specimen was further heat-treated at 1365°C, the grains possessing a double twin were observed to grow exclusively again. The results were explained in terms of a coarsening process controlled by two-dimensional nucleation.     

Effect of Nb2O5/ZnO Addition on Microwave Properties of (Zr0.8Sn0.2)TiO4 Ceramics

The effects of Nb₂O₅ and ZnO addition on the dielectric properties, especially the quality factor, of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were investigated in terms of the sintered density acquired by the zinc. For ZST ceramics with 2 mol% added ZnO, the relative density of the samples decreased with >0.5 mol% addition of Nb₂O₅. On the other hand, for samples with 6 mol% added ZnO, the relative density remained >97%, even when the amount of Nb₂O₅ was increased to 2.0 mol%. When >0.5 mol% Nb₂O₅ was added, both the quality factor and the dielectric constant exhibited similar trends with sintered density. The ZST ceramics with 6 mol% added ZnO, especially, still manifested a quality factor >40 000 and a dielectric constant of 37, even when the amount of Nb₂O₅ was increased, values that are not explainable by the previously suggested electronic defect model.     

Dielectric and Structural Characteristics in Barium Lanthanum Magnesium Niobate

The dielectric properties and their related microstructural characteristics in solid solutions of (1 — x )Ba(Mg_1/3Nb_2/3)O₃ (BMN)— x La(Mg_2/3Nb_1/3)O₃ (LMN) (BLMN) were investigated by measuring the relative permittivity (ɛ_r), Q value, and temperature coefficient of resonator frequency (τ_f), and by observing the microstructure using transmission electron microscopy. The trend of variation of the temperature coefficient of the dielectric permittivity (τ_ɛ) was the same for our solid solutions as that reported by Reaney et al . When the tolerance factor ( t ) was >1.01 in BLMN with composition x = 0 to 1.0, where the tilting of oxygen octahedra was not involved, the components of the microstructure included a disordered and transition phase as well 1:1 and 1:2 ordered phases. In the region where 1.01 < t < 0.96 with x = 0.2 to 0.7, the 1:1 order, the disorder, and the phase due to the antiphase tilting of oxygen octahedra were present. Finally, in the region where t < 0.96 with x = 0.7 to 1.0, the microstructure of BLMN was the same as that of the pure LMN, including the 1:1 order and the antiphase, inphase tilting of oxygen octahedra, and the antiparallel shift of A-site cations.     

Microstructure and Dielectric Properties of Barium Strontium Magnesium Niobate

Dielectric properties and their related microstructural characteristics in solid solutions of (1 – x )Ba(Mg_1/3Nb_2/3)O₃– x Sr(Mg_1/3Nb_2/3)O₃ (BMN–SMN, or BSMN) were investigated by measuring the relative permittivity (ɛ_r), Q values, and temperature coefficient of resonator frequency (τ_f), and by observing microstructure using transmission electron microscopy. When the tolerance factor ( t ) was >0.99 in BSMN with composition 0 < x < 0.5, where the tilting of oxygen octahedra was not involved, the microstructure included only 1:2 ordered phase. In the region where 0.99 > t > 0.97 with 0.7 < x < 1.0, the phase due to the antiphase tilting of oxygen octahedral, the disordered phase, and the 1:2 ordered phase were also present. In a few of the grains, core–shell-type structures, whose main components were dislocations and stacking faults, were found in the solid solution of BSMN.     

Dielectric polarizability of SiO2 in niobiosilicate glasses

Understanding the mechanisms contributing to dielectric properties of glasses is critical for designing new compositions for microwave frequency applications. In this work, dielectric permittivity was measured using a cavity perturbation technique at 10 GHz for a series of niobiosilicate glasses with the compositions (100-2x)SiO₂- xNb₂O₅- xLi₂O where x = 32.5, 30, 25, and 15 mol%. Permittivity measurements and glass compositions were used to calculate the polarizability of each cation-anion unit in the glass network using the Clausius-Mossotti equation. The SiO₂ polarizability in niobiosilicates was calculated to be 6.16 ų, which is much higher than the SiO₂ polarizability in fused silica glass (5.25 ų), alkali modified silicates (5.37 ų), and aluminosilicates (5.89 ų). The increasing trend in SiO₂ polarizability is attributed to the disruption in the connectivity of the SiO₄ tetrahedral network as it accommodates different network formers. The high SiO₂ polarizability of 6.16 ų accurately predicts measured dielectric permittivity when Nb₂O₅ = 25, 30, and 32.5 mol%, but overpredicts measured permittivity when Nb₂O₅ ≤ 15 mol%, which is attributed to a decrease in SiO₂ polarizability as the percentage of corner sharing SiO₄ tetrahedra with NbO₆ octahedra goes down. This work demonstrates that SiO₂ polarizability depends on chemistry and connectivity of the glass, which has important implications in designing glass compositions for microwave frequency applications.     

Synthesis of Lead Nickel Niobate–Lead Zirconate Titanate Solid Solutions by a B-site Precursor Method

A modified processing method for lead nickel niobate–lead zirconate titanate (Pb(Ni_1/3Nb_2/3)O₃–Pb(Zr,Ti)O₃, PNN–PZT) solid solutions is presented. This method is based on the high-temperature synthesis of a precursor that contains all the B-site cations (Ti, Zr, Ni, and Nb). This synthesis yields a diphasic mixture that contains a ZrTiO₄-like phase and a rutile-like phase. Both phases exhibit a cationic valence of 4; thus, it is concluded that the mixing of Ni and Nb cations is adequate for the preparation of PNN–PZT solid solutions. Indeed, a pure perovskite phase has been obtained after calcination with lead oxide for compositions that contain 40 and 50 mol% PNN. Moreover, their electromechanical properties have been shown to be superior to values reported for standard columbite routes. This conclusion has been interpreted in terms of enhanced chemical homogeneity.     

Synthesis of Niobium(V)-Stabilized Tetragonal Zirconia Nanocrystalline Powders

The polymer precursor method is very useful to prepare Nb⁵⁺-stabilized nanocrystalline powders of t -ZrO₂. The precursor solution is composed of zirconium oxalate, niobium tartrate, and poly(vinyl alcohol), which help to form a network matrix to disperse the metal ions homogeneously. Nb⁵⁺ is an effective agent to stabilize t -ZrO₂, and ease of formation of the tetragonal phase increases with increasing dopant concentration. Thermal stability of t -phase is found up to 1700°C having 15 mol% Nb⁵⁺, prepared at 600°C with particle sizes of 35 ± 5 nm.     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

Crystallization of Lead Niobate Glass by Mechanical Activation

Mechanical activation-triggered crystallization in PbNb₂O₆-based glass was dependent on the initial presence of nuclei. The crystallization cannot be initiated by mechanical activation in a highly amorphous glass composition quenched from 1350°C where PbNb₂O₆ nuclei did not exist. The steady growth of nanocrystallites of PbNb₂O₆ was observed with an increasing degree of mechanical activation in the glass quenched from 1300°C, where a density of PbNb₂O₆ nuclei existed before mechanical activation. The inability to nucleate in the highly amorphous oxide glass by mechanical activation is consistent with the much higher structural stability as compared with that of metallic glasses, such as Fe-Si-B. The mechanical activation-grown PbNb₂O₆ nanocrystals were 10–15 nm in size as observed using HRTEM and their crystallinities were further improved by thermal aging at an elevated temperature in the range of 550° to 650°C.     

Synthesis and Characterization of Nanocrystalline Niobium Nitride Powders

Nanocrystalline NbN powder was synthesized by the direct nitridation of amorphous Nb₂O₅ powder with high BET surface area. X-ray diffractometry analysis indicated that the cubic-phase NbN powder could be obtained by nitridation at 650°–800°C for 3–8 h. Transmission electron microscopy images showed that the particle sizes were in the range of 15–40 nm. The effect of the nitridation temperature and holding time on the powder properties was discussed.