Directional Solidification of the ZrO2-MgO Eutectic

The eutectic between ZrO₂ and MgO was located at 27 wt% MgO. Directional solidification of this composition yields MgO rods in a cubic ZrO₂ matrix. Plane front growth occurred at solidification rates of <2.0 cm/h. The interfiber spacing was proportional to the inverse square root of the solidification rate. In samples where cellular growth occurred, the colony size was not affected by the solidification rate. The room-temperature fracture strength, ∼23,000 psi, which was not affected by the MgO fiber size, was controlled by the colony size. Annealing for 24 h at 1200°C increased the fracture strength by 50% as a result of a compressive stress in the matrix caused by the cubic-to-monoclinic transformation in the ZrO₂.     

Formation of a High-Temperature Liquid Phase During the Sintering of PbFe2/3W1/3O3

A high-temperature liquid phase (rather than a low-temperature liquid phase at 690°C as reported recently) has been demonstrated to form at 860°C on heating and to solidify at 840°C on cooling in PbFe_2/3O₃. This liquid phase not only promotes densification, but also induces the formation of rounded PbFe_2/3W_1/3O₃ grains during sintering at 870°C. Through slow cooling at a rate of 25°C/h after sintering, platelike grains, designated G phase, are found to form in a thin surface layer of specimens. This formation of platelike G phase is considered to be related to the solidification and recrystallization of the liquid phase exuded from the interior. The amount of the G phase on the surfaces decreases with the increase of cooling rates, indicating that fast cooling will lead the liquid phase to be solidified in the bulk of specimens. These results reveal that the microstructure of PbFe_2/3W_1/3O₃ is greatly affected by the high-temperature liquid phase; additionally, the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PbFe_2/3W_1/3O₃.     

Oriented Structure and Crystallography of Directionally Solidified LaB6—ZrB2 Eutectic

Directional solidification of LaB₆—ZrB₂, by use of an electron beam heating technique, yielded oriented ZrB₂ fibers in a LaB₆ matrix. The average diameter of the ZrB₂ fibers was ∼0.2–1.2 µm, with fiber lengths up to 100 µm. Primary platelike LaB₆ dendrites formed upon the solidification of an ingot with a composition of LaB₆—18 wt% ZrB₂. LaB₆ was the first phase to nucleate when eutectic growth occurred, and ZrB₂ showed nonfaceted growth. For the ingot solidified with planar growth the orientation relations of the phases were as follows: growth direction, [001]_LaB6∥[00.1]_ZrB2; interfacial plane, (11.0)_LaB6∥(11.0)_ZrB2.     

Characterization of Melt-Quenched Quasibinary II-IV-V2 Chalcopyrite Semiconducting Alloys

Experimental work on melt synthesis, solidification behavior, and characterization of alloys in mixed II-IV-V₂ chalcopyrite semiconducting systems is reported. Melting relations diagrams and transformations in solidified alloys in the pseudo-binary systems ZnGeAs₂-ZnSnAs₂ and ZnGeAs₂-CdGeAs₂ were studied by thermal analysis. Decomposition of a 10 wt% ZnGeAs₂-90 wt% CdGeAs₂ glass, characterized by transmission electron microscopy, revealed the onset of phase instability at ≥400°C.     

Effects of Annealing on the Microstructure and Phase Chemistry of Directionally Solidified Bi2Sr2CaCu2O8

The effects of annealing on the microstructure and phase chemistry of directionally solidified Bi₂Sr₂CaCu₂O₈ (2212) were studied. Boules grown at fast growth rates are multiphase, contain no 2212, and exhibit some preferred orientation; Sr-deficient 2201 exhibits a (220) texture. These samples, however, do show a significant amount of 2212 after annealing, with (220) texture similar to the texture of the preexisting 2201 phase in the as-grown boules. In contrast, 2212 grown directly from the melt at slow growth rates has (200) texture. Boules grown at moderate growth rates are multiphase and contain 2212 as one of the phases; these boules show an increase in 2212 volume fraction after annealing. Microstructural observations in both cases indicate solidstate formation of 2212 occurs topotactically on or within the preexisting 2201 phase. Boules grown at fast growth rates are oxygen-deficient, since they show an increase in mass with annealing, whereas boules grown at moderate growth rates show no appreciable mass increase with annealing. An estimate of the Cu(I) concentration in the liquid zone during crystal growth at fast growth rates was obtained from gravimetric analysis.     

Surface Tension of ZrF4-BaF2-LaF3 Glass

The surface tension of a 62ZrF₄.33BaF₂.5LaF₃ (mol%) glass was measured by a solidified pendant drop technique. The surface tension at 550°C is 0.174 ± 0.005 J/m².     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Mineralizing Magnesium Aluminate Spinel Formation With B2O3

B₂O₃ mineralizes spinel formation from stoichiometric (1:1 mole ratio) calcined magnesia and alumina. After 3 h at 1100°C, X-ray diffraction (XRD) shows the mineralization effect of B₂O₃ is limited to 1.5 wt% additions with higher B₂O₃ contents leading to Mg₃B₂O₆ formation and reduced spinel content. Boron nuclear magnetic resonance, electron probe microanalysis (EPMA), scanning electron microscopy, transmission electron microscopy (TEM) and XRD reveal formation of a boron-containing liquid. Energy dispersive spectroscopy in the TEM and EPMA of the glassy phases formed from solidification of the liquid reveal that initially it is Mg borate, later becoming a magnesia-modified boroaluminate, composition suggesting dissolution–precipitation as opposed to templated growth as the mechanism of this liquid phase mediated mineralization.     

Interfaces in Oriented Al2O3-ZrO2 (Y2O3) Eutectics

Oriented samples of Al₂O₃-ZrO₂ (Y₂O₃) eutectics consisting of an alumina matrix with zirconia dispersoids were grown by directional solidification. Preferred growth directions and epitaxial relations were determined from X-ray and electron diffraction analyses. Imaging of interfaces was performed by high-resolution transmission electron microscopy on oriented platelets. Semicoherent interfaces were observed with faceting along crystallographic planes of both phases.     

Formation of Pores and Y2BaCuO5-Free Regions during Melt Processing of Y1.6 Ba2.3Cu3.3Ox

The formation of spherical pores and regions free of Y₂BaCuO₅ (2-1-1) has been studied by melt processing Y_1.6Ba_2.3Cu_3.3O _x: in two different atmospheres (air and oxygen). When the sintered Y_1.6Ba_2.3Cu_3.3O _x specimens are melted at 1050°C, many spherical pores form in the melted specimens. During the subsequent cooling, the pores are filled by liquid flow and finally solidified to Y₂BaCuO₅-free regions. Melt processing in an oxygen atmosphere produces more pores and regions free of 2-1-1 than in air. Because peritectic melting of YBa₂Cu₃O_7-y in an oxygen atmosphere produces more oxygen gas than that in air, the formation of the pores and Y₂BaCuO₅-free regions is suggested to be attributed to the oxygen evolution during the peritectic melting of YBa₂Cu₃O_7−y     

Effect of the Melt Temperature on the Solidification Process of Liquid Garnets Ln3Al5O12 (Ln = Dy, Y, and Lu)

The solidification process of liquid garnets Ln₃Al₅O₁₂ (Ln = Dy, Y, Lu) was probed by differential thermal analysis. A critical temperature T _clg was evidenced in the liquid. Cooling from temperatures under T _clg leads to the crystallization of the single garnet phase. Cooling from temperatures above T _clg leads to the solidification of a mixture of perovskite LnAlO₃ and aluminum oxide α-Al₂O₃. For Ln = Y, the results are in agreement with stable and metastable phase diagrams previously proposed. The YAIO₃ melt was shown to display a critical temperature T _clp also. Its metastable solidification from temperatures above T _clp leads to the formation of the perovskite at around 1550°C.     

Fracture Behavior of Directionally Solidified Y3Al5O12/Al2O3 Eutectic Fiber

The strength and fracture of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth technique. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The tensile strength and Weibull's modulus of the eutectic fibers were determined in the as-fabricated state and after extended thermal exposure at 1460°C in air. Fractographic analysis was used to identify and classify the strength-limiting mechanisms. The fracture toughness and crack growth behavior were characterized by an indentation technique. A fracture mechanics analysis was also used to establish the relationships between surface flaw size, tensile strength, and fracture toughness of the fiber.     

Local Dendrite Development during Seeded Peritectic Solidification of Large YBa2Cu3O7−δ Grains

The growth morphology of large YBa₂Cu₃O_7−δ grains during peritectic solidification has been reported to be responsible for the generation of processing defects, such as platelets, and an inhomogeneous distribution of 211 particles, both of which influence significantly the superconducting properties of the fully processed material. The present study demonstrates both experimentally and theoretically the formation of local dendrites at macroscopically planar YBa₂Cu₃O_7−δ growth fronts which propagate along different crystallographic directions and identifies these as key growth features of the peritectic solidification process.     

Formation of Hexagonal Metastable Phases from an Undercooled LuFeO3 Melt in an Atmosphere with Low Oxygen Partial Pressure

Controlling the oxygen partial pressure ( P _O2) in the ambient atmosphere is an important parameter for material processing because the transition metal ion changes its valence depending on P _O2. In the present study, containerless solidification of the LuFeO₃ melt, where the undercooling level can be treated as another experimental parameter, was carried out in order to explore the unidentified metastable phases under the controlled P _O2 using an aerodynamic levitator. Decreasing P _O2 down to 1 × 10³ Pa, the unidentified phase was solidified from the undercooled melt. The X-ray diffractometry results after annealing at 1 × 10³ Pa showed the peak profile of the stable perovskite LuFeO₃ phase, suggesting that this unidentified phase was thermodynamically metastable. Thermogravimetric analysis showed that the mass of the sample solidified at P _O2= 1 × 10³ Pa significantly increased, suggesting that the formation of the metastable phases might be related to the presence of Fe²⁺ ions.     

Strontium Dialuminate SrAl4O7: Synthesis and Stability

Up to now, strontium dialuminate, SrAl₄O₇ (SA₂), could be synthesized only by solidification from the high-temperature liquid state. We describe its synthesis from a spray-dried amorphous precursor, and specify its stability domains. Its kinetics of formation is very low. It can be crystallized in the 900–1000°C temperature range either directly with a low heating rate or via two metastable solid solutions—hexagonal strontium monoaluminate (SrAl₂O₄ (SA)) and γ-alumina—by annealing at 950–1000°C. As the temperature is raised beyond 1100°C, SA₂ becomes metastable, its formation is no longer possible, and the crystallization of Sr₄Al₁₄O₂₅ (S₄A₇) is favored. The latter compound, whose composition is close to that of SA₂, is stable up to 1500°C. At higher temperature it decomposes into SA and SA₂, which in its turn decomposes into SA and SA₆ (SrAl₁₂O₁₉). There is again another stability domain for SA₂, restricted to a narrow temperature scale close to its melting point (∼1800°C). The behaviors at crystallization from amorphous precursors at low temperature and from liquid at very high temperature are symmetrical: low heating or cooling rates produce pure SA₂ while too rapid kinetics result in mixtures of phases.     

Microwave Dielectric Properties and Low-Temperature Sintering of Ba0.60Sr0.40TiO3 Ceramics

In the present work, the sintering behaviors and dielectric properties of Ba_0.60Sr_0.40TiO₃ (BST) ceramics with the addition of BaCu(B₂O₅) were investigated in detail. The results indicated that the addition reduced the sintering temperature of BST by about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) assisted the densification of BST ceramics at lower temperatures. For a low-level BaCu(B₂O₅) addition (2.0 mol%), the BST sample sintered at 950°C for 5 h displayed good dielectric properties, with a moderate dielectric constant (ɛ=2553) and a low dielectric loss (tan δ=0.00305) at room temperature and at 10 kHz. The sample showed 45.9% tunability at 10 kHz under a dc electric field of 30 kV/cm. At the frequency of 0.984 GHz, BST-added 2.0 mol% BaCu(B₂O₅) possessed a dielectric constant of 2204 and a Q value of 146.7.     

Transparent Nano-Composites Ceramics by Annealing of Amorphous Phase in the HfO2-Al2O3-GdAlO3 System

We have succeeded in fabricating transparent nano-structured ceramics by annealing of the amorphous phase obtained by the solidification of the eutectic melt in the ternary system HfO₂− Al₂O₃− GdAlO₃. The ceramics annealed at 1273 K for 6 hr contained 5-10-nm cubic hafnia grains, and those annealed at 1273 K for 72 hr contained both cubic hafnia and gadolinium aluminum garnet grains 5-10 nm in size. They showed high transparency. Annealing at 1473 K, however, resulted in grain growths that brought about non-transparency.     

Fast Li+ Ion Conduction in Li2O-Al2O3-TiO2-SiO2-P2O5 Glass-Ceramics

Fast lithium ion conducting glass-ceramics have been successfully prepared from the pseudobinary system 2[Li_{1+ x} Ti₂Si _x P_{3− x} O₁₂]-AlPO₄. The major phase present in the glass-ceramics was LiTi₂P₃O₁₂ in which Ti⁴⁺ ions and P⁵⁺ ions were partially replaced by Al³⁺ ions and Si⁴⁺ ions, respectively. Increasing x resulted in a considerable enhancement in conductivity, and in a wide composition range extremely high conductivity over 10⁻³ S/cm was obtained at room temperature.