Iron and Spinel Precipitation in Iron-Doped Sapphire

Single crystals of Al₂O₃ containing 0.5 wt% Fe were exposed to low p o₂ atmospheres at 1500°C to produce precipitate phases. Analytical TEM identified the precipitate phases as spinel (hercynite) and iron, with the following orientation relationships: 〈001〉_Fe‖〈2 2 01〉_s with {1 1 0}_Fe‖{11 2 0}_s, 〈111〉_Fe‖〈10 1 0〉_s with {0 1 1}_Fe‖ (0001)_s, and 〈1 1 0〉_H‖〈01 1 0〉_s with {111}_H‖ (0001)_s. The three phase fields observed — (Fe, Al)₂O₃, spinel +α-Al₂O₃, and iron +αAl₂O₃— are in accordance with phase stability diagrams. Precipitation kinetics indicate that oxygen is mobile in the reduced region of the crystal.     

Dislocation-Containing Planar Faults in Metastably Retained Hexagonal Barium Titanate

Planar faults, designated F ₁– F ₆ and F _a , with intermixed dislocations in metastably retained hexagonal-BaTiO₃ ceramics, were found and analyzed by transmission electron microscopy. Only faults with one, two, and four extra c -layers parallel to the basal plane have been identified. Fault vectors R_F1=R_Fa=1/6[02 2 1], R_F2=R_F3=1/3[0 1 11], R_F4=1/3[01 1 2], and R_F'1=R_F5= R_F6=R_F'a=1/6[0 2 21] were determined adopting the 2πg·R=0 (or 2 n π) criteria in combination with high-resolution imaging. Further, the embedded dislocations were half-partials with Burgers vectors b=1/3〈10 1 0〉 determined by the g·b=0 effective invisibility criteria in conjunction with the eligible fault vectors. A rotation by 60° about the c -axis was found between fault segments with R_F1=1/6[02 2 1] and R_F'1=1/6[20 2 1] located on either side of a partial. Basal dislocations with b_B=1/3〈1 2 10〉 have dissociated into two prism-plane Shockley half-partials with b_Pr=1/3〈1 1 00〉 by glide in the fault plane (0002) according to 1/3〈1 2 10〉→1/3〈1 1 00〉+1/3〈0 1 10〉. The fault segment F'₁ encompassed by two half-partials is an extrinsic complex stacking fault.     

Vacancy-Ordered Structure of Cubic Bismuth Oxide from Simulation and Crystallographic Analysis

A judicious combination of molecular dynamics simulations, electronic-structure calculations, and crystallographic analysis is used to fully characterize the structure of cubic δ-Bi₂O₃. It is found that the intrinsic oxygen vacancies in the structure align in a coordinated manner along the 〈110〉 and 〈111〉 directions to form a defect-fluorite superstructure containing eight nonprimitive cubic cells, for a total of 80 atoms. This structure is consistent with that determined previously from neutron scattering.     

A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite

(Na_1/2La_1/2)(Mg_1/3Nb_2/3)O₃ undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (ɛ_r∼ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.     

π-Rotation Faults in Hexagonal BaTiO3

Planar defects in the metastably retained h-BaTiO₃ exhibiting α-fringe pattern have been characterized via transmission electron microscopy (TEM). The eligible fault vectors were determined by adopting the invisibility criteria of 2πg·R = 0 or 2 n π augmented by high-resolution imaging. Three stacking faults, F₁, F₂, and F₃, of the extrinsic nature have been fully analyzed. The eligible fault vectors for faults F₁ and F₃ contained a basal component respectively of ⅓[0001] and ⅙[0001] and a common prismatic component of ⅓〈10[1-macr]0〉. However, only three of the 〈10[1-macr]0〉 vectors are the eligible prismatic component for the fault vectors R_F1=⅓[0[1-macr]11], ⅓[10[1-macr]1], and ⅓[[1-macr]101], and R_F3=⅙[02[2-macr]1], ⅙[2[2-macr]01], and ⅙[[2-macr]021] that have fulfilled the invisibility criteria. On the other hand, all fault vectors R_F21=⅙〈[4-macr]223〉 for fault F₂, containing six vectors of the 〈[2-macr]110〉 family, is eligible. Unlike the faults of πR_F=⅙〈[2-macr]203〉 found in the D0₁₉ intermetallics of Ni₃Sn and Co₃W, neither fault F₁ nor F₃ is the π-rotation type. Fault F₂, however, is a π-rotation fault since a 60°-rotation clockwise about [0001] has produced another eligible fault vector.     

TEM of Dislocations in AIN

Sintered AIN specimens were deformed by Vickers hardness (HV) indentations. Compared with Al₂O₃ the HV hardness values indicate a much higher plasticity of AIN at room temperature, but above 600°C a higher ductility for Al₂O₃. Deformed AIN specimens were examined by transmission electron microscopy. Basal and prismatic glide with the slip systems (0001) 〈1120〉 and {1 1 00}〈11 2 0〉 were frequently observed. This results in four linearly independent slip systems. The critical resolved shear stress for single prismatic slip seems to be even smaller than for basal slip. However, thermally activated dislocation reactions are frozen up to at least 1000°C. Thus, prismatic slip is suppressed as soon as more than one slip direction is activated.     

Compressive Creep of Beryllium Oxide-Uranium Dioxide Mixtures

Beryllium oxide-uranium dioxide mixtures were deformed in compression in the region 1375° to 1540°C. The average apparent activation energy for creep of the oxide mixtures containing up to 10 wt% uranium dioxide is 95.1 kcal/mole. The activation energy is not sensitive to the applied stress and does not vary with urania additions. Creep rate is linearly dependent on the applied stress to 6000 psi. At constant stress and temperature, creep rate dependence on grain size for BeO-10 wt% UO₂ specimens can be described by the relation ɛ∼ 1/2². The creep rate dependence on the applied stress and grain size is consistent with the Nabarro-Herring mechanism. Creep behavior of the oxide mixtures is ascribed to the deformation of the beryllium oxide matrix.     

High-Temperature Precipitation Hardening of Two-Phase Y2O3-Partially-Stabilized ZrO2 Single Crystals: A First Report

Precipitation hardening was observed in two-phase (cubic plus tetragonal) Y₂O₃-partia1ly-stabilized ZrO₂ single crystals deformed at 1400°C. Slip was activated on (001) 〈110〉, primarily in Luders bands.     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Preparation of 〈110〉-Textured BaTiO3 Ceramics by the Reactive-Templated Grain Growth Method Using Needlelike TiO2 Particles

Dense, highly 〈110〉-textured BaTiO₃ ceramics were prepared by the reactive-templated grain growth method. Needlelike TiO₂ (rutile) particles with their needle axis parallel to 〈001〉 were used as reactive template particles. Slurry containing an equimolar mixture of TiO₂ and BaCO₃ was tape cast to form a green compact, in which TiO₂ particles were aligned with their needle axis parallel to the casting direction. Calcination of the green compact changed TiO₂ particles into BaTiO₃ grains with their 〈110〉 direction parallel to the casting direction, for which the topotaxial relation of was responsible. Sintering yielded a dense, highly textured BaTiO₃ compact.     

Microstructural Evolution during Pressureless Sintering of Lead Lanthanum Zirconate Titanate Ceramics with Excess Lead(II) Oxide

The early stages of sintering of lead lanthanum zirconate titanate (PLZT) 9/65/35 ^† ceramics prepared with 10 wt% excess PbO were monitored by quenching uniaxially pressed pellets after a variety of heat treatments. TEM revealed a PbO-rich amorphous film covering the particle surfaces and interparticle porosity in pellets quenched after 5 min at 1125° to 1180°C. This amorphous phase is indicative of the presence of a high-temperature liquid phase with composition approximately Pb_0.87Zr_0.15Ti_0.04O_1.19. The liquid composition moves toward the PbO-TiO₂ eutectic as sintering progresses. After 10 to 30 min at 1180°C, the liquid composition was approximately Pb_0.9Zr_0.04Yi_0.06O_l.1 and crystallized on quenching. High densities of dislocations with = 1/2〈110〉 and low-angle boundaries were observed in samples quenched from 1180°C after 10 to 30 min. Mechanisms for the formation of these dislocations are suggested.     

Deformation and Cracking in Ge–Sb–Se Chalcogenide Glasses During Indentation

Deformation and cracking behavior of Ge–Sb–Se binary and ternary chalcogenide glasses of varying average covalent coordination number, 〈 r 〉, was studied by indenting with Vickers and Brinell microindenters using static and recording machines, and subsequent analysis using a non-contact profilometer. Vickers-produced cracks were the smallest around the GeSe₄ composition (〈 r 〉=2.4) after unloading, hence the indentation toughness was a maximum and the brittleness a minimum at 〈 r 〉=2.4. Brinell-created pond (crater) depth, the mound (pile-up) height, and the radial fractures originating from the mounds displayed minima in the binaries, presumably due to maximized elastic recovery around 〈 r 〉=2.4. Consequently, Brinell hardness computed from the unloaded depth ( H _BD) showed a maximum around GeSe₄. The maximized elastic recovery around GeSe₄ is consistent with Phillips' optimized connectivity arguments. GeSe₄ resembles the "anomalous" SiO₂ glass for deformation and cracking behavior. Surprisingly, many of the extrema were nearly non-existent in the ternary glasses. The apparent contrast to the binary glasses is not understood.     

Nanobelt Structure in Perovskite–Spinel Composite Thin Films

We report a nanobelt structure along the 〈110〉 direction in perovskite–spinel (BiFeO₃–CoFe₂O₄ or BFO–CFO) composite epitaxial thin films deposited on SrTiO₃ (STO) substrates by pulsed laser deposition. The results reveal the nucleation and growth mechanism of a nanostructure: first nucleation of perovskite phase regions on STO, followed by segregation of spinel out of the perovskite phase, and finally by evolution of spinel nanobelts that are elongated along the 〈110〉. The reason for control of the nanostructure evolution is a ledge growth mechanism of spinel along the 〈110〉 direction.     

Dissociation of the 〈001〉 Dislocations and Their Interactions with Dislocation Loops in Tetragonal BaTiO3

Dislocations in pressureless-sintered BaTiO₃ ceramics have been analyzed using transmission electron microscopy. Subjected to effective sintering stresses, dislocations were generated and multiplied in plastically deformed BaTiO₃ crystals by the Frank–Read mechanism from both single- and double-ended sources. This is represented by dislocations encompassing a series of square-like borders that shared a common center. All border dislocations exhibited the characteristic scallop shape. True dislocation line directions ( u ) were determined by trace analysis and Burgers vectors ( b ) by contrast analysis for the dislocations dissociated from b =〈001〉 into two half-partials following the type (I) reaction of     by climb on {001}. Dislocation interactions between the main dislocations created from plastic deformation and dislocation loops of b =〈100〉 or 〈110〉 forming condensation of intrinsic Schottky vacancies were also found to obey the type (IV) reaction of     , the type (V) reactions of     . Migrating dislocations and loops interacting mutually in several stages, illustrated schematically, before arriving at the configuration described by types (IV) and (V) were observed and discussed.     

Texture Development in Barium Titanate and PMN–PT Using Hexabarium 17-Titanate Heterotemplates

Bulk BaTiO₃ ceramics with 〈111〉-texture have been prepared by the modified templated grain growth method, using platelike Ba₆Ti₁₇O₄₀ particles as templates, and the mechanism of texture development is examined. The Ba₆Ti₁₇O₄₀ particles induce the abnormal growth of BaTiO₃ grains, and a structure similarity between {001} of Ba₆Ti₁₇O₄₀ and {111} of BaTiO₃ gives 〈111〉-texture to abnormally grown BaTiO₃ grains. Thus, the 〈111〉-texture develops in the BaTiO₃ matrix. The use of platelike Ba₆Ti₁₇O₄₀ particles has been extended to a 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ matrix, but the matrix phase is decomposed by extensive chemical reactions between the matrix and template phases.     

Deformation Microstructure in (001) Single Crystal Strontium Titanate by Vickers Indentation

Recent interests on the plastic deformation of strontium titanate (SrTiO₃) are derived from its unusual ductile-to-brittle-to-ductile transition (DBDT). The transition is divided into three regimes (A, B, and C) corresponding to the temperature range of 113–1053 K (−160° to 780°C), 1053 to ∼1503 K (780° to ∼1230°C), and ∼1503–1873 K (∼1230° to 1600°C), discovered by Sigle and colleagues in the MPI-Stuttgart. We report the dislocation substructures in (001) single crystal SrTiO₃ deformed by Vickers indentation at room temperature, studied by scanning and transmission electron microscopy. Dislocation dipoles of screw and edge character are observed and confirmed by inside–outside contrast using ± g -vector by weak-beam dark field imaging. They are formed by edge trapping, jog dragging, and cross slip pinching-off. Similar to dipole breaking off in deformed sapphire (α-Al₂O₃) at 1200°C and γ-TiAl intermetallic at room temperature, the dipoles pinch off at one end, and emit a string of loops at trail. Two sets of slip systems {110}〈     〉 and {100}〈011〉 are activated under both 100 g and 1 kg load. The suggestion is that plastic deformation has reached the stage II work hardening, which is characterized by multiplication of dislocations through cross slip, interactions between dislocations, and operating of multiple slip systems.     

Growth and Morphology of Magnesium Oxide Whiskers

Magnesium oxide crystals and whiskers were grown by vapor phase reactions between MgO and tungsten, hydrogen, and carbon. The large number of morphologies obtained are described in terms of two, three, and fourfold growth symmetry and are related to the growth conditions by current heterogeneous nucleation theory. Threefold (or 〈111〉) growth occurred principally with MgO + H₂ reactions, and in all experiments 〈100〉 and 〈110〉 growth occurred under closely similar conditions of temperature and vapor supersaturation. Observations of growth steps suggest that {100} surfaces are favored only at the highest vapor supersaturation. Tensile strength measurements of whiskers with l/d ratios of about 300 show increasing strength with decreasing cross section. A value of 8.13 × 10⁴ psi was observed for a whisker 9.76μ in diameter. The results are of particular interest to the formation of refractory oxides crystallizing in the simple cubic NaCl structure.     

Strengthening of Alkali Halides by Divalent-Ion Additions

Single crystals of NaCl, NaBr, KCl, and KBr containing divalent additions of Ca²⁺, Sr²⁺, and Ba²⁺ were tested mechanically. In the solution-treated condition, the yield strength, σ _v , as determined from compression testing in a 〈100〉 direction is essentially dependent on the concentration of the dopant only and is independent of the species of either the dopant or of the host material. All crystals soften on aging, with the exception of the NaCl:Ca²⁺, NaBr:Ca²⁺, and NaBr:Sr²⁺ systems. In addition, correlation was good between σ _v , and the Knoop hardness number, H , obtained by indentation with the long axis of the indenter aligned in 〈100〉. The equation is of the form σ _v = C ( H–H ₀), where C ≅ 0.21 for all four halide families and H₀ is near the hardness value of the pure halides. Furthermore, H ₀≅5×10⁻³E₁₁₁, the Young's modulus in the 〈111〉 direction. Hence σ _v ≅0.21 H –10⁻³E₁₁₁.     

Formation of Acicular Monoclinic Zirconia Particles under Hydrothermal Conditions

Acicular monoclinic ZrO₂, particles were prepared by hydrothermal treatmentat 250°C using sulfuric acid solutions containing zirconium ions. The formation process and morphology of the ZrO₂, particles were investigated. Two types of acicular monoclinic Zr0₂, particle morphologies were obtained, both elongated in the 〈001〉 direction, and the range of acicular ZrO₂, particle sizes changedfrom 0.3 to 1.3 μ m with hydrothermal conditions. Addition of MgSO₄, to the starting solution promoted the crystallization of the monoclinic ZrO₂, particles.     

Knoop Microhardness Anisotropy of Single-Crystal Stoichiometric MgAl2O4 Spinel

Microhardness anisotropy profiles for the (100) and (111) planes of single-crystal stoichiometric MgAl₂O., spinel were determined at room temperaturé. The (100) microhardness profile has ahardness maximum in tiie [001] and a minimum in the [O11], which supports the previous suggestion that the primary slip system is the {111}〈11¯0〉. The microhardness of the (111) plane is independent of indenter orientation, also consistent, with a {111}〈11¯0〉 primary slip system. It is concluded that these microhardness profiles are in accord with other experimental observations that the {111}〈11¯0〉 is the primary slip system in stoichiometric MgAl₂O₄ spinel.