Anisotropic Calcium Segregation to the Surface of Al2O3

The surface enrichment of Ca on various crystalloraphic planes of CaO-doped sapphire has been measured as a function of annealing temperature using Auger electron spectroscopy (AES). Strong Ca enrichment was found on the (10 1 0) prism plane at ≥1300°C, whereas no evidence or Ca segregation could be observed on the (0001) basal plane up to 1500°C. The surface enrichment factor estimated at 1300°C was at least 3 × 10³ for the prism plane, while that for the basal plane was below 50, which corresponds to the detectability limit of AES. The Ca segregation to the prism plane was uniform and limited to the surface monolayer; an apparent heat of segregation determined in the range of 1300° to 1500°C was–169 kJ/mol. Low-energy electron diffraction studies of annealed surfaces revealed that Ca segregation was concurrent with a two-dimensional surface phase transition. The anisotropic segregation behavior of Ca may explain why CaO is not an effective additive for alumina sintering.     

Segregation of Mg to the (0001) Surface of Doped Sapphire

Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were used to study the segregation of Mg to the surface of a sapphire crystal doped with 40 ppm of Mg. In situ vacuum annealing and AES measurements at 900° to =1800°C failed to reveal Mg segregation on the (0001) plane above the detectability limit of the cylindrical mirror analyzer. However, when identical crystals were placed together to simulate a "closed system" and annealed in air, surface segregation of Me was detected above 1200°C. The variation of the equilibrium surface concentration of Mg with temperature in the range 1300° to =1500°C, gave an effective Langmuir heat Of segregation of Mg of =–146 kJ/mol. Evidence for the formation of a two-dimensional ordered structure on the sapphire surface was also obtained through LEED studies. The implications of these segregation results for our understanding of the sintering behavior of magnesia-doped alumina are discussed.     

Melt Processing of YBa2Cu3O7-x

Sintering YBa₂Cu₃O_{7- x} bulk forms at 1050°C followed by annealing at 980°C causes the development of a thick oriented surface layer (Lotgering factor = 0.7). The thickness of the layer depends on the thermal treatment, which is a two-step sintering process. Firing at 1050°C for 2.5 h followed by 30 h at 980°C leads to the development of a 0.1-mm-thick surface layer, with clear indication that longer annealing would result in a thicker film. Some orientation develops during un-axial compaction of the powders. Lotgering orientation factor calculation from X-ray diffraction analysis. SEM, and TEM were used to characterize the microstructure of these samples. T _c was similar to that of conventionally processed high-density samples, between 83 and 87 K. Some thermal treatments resulted in samples that displayed high resistivity above T _c , possibly caused by segregation of Cu to the grain boundaries.     

Controllable growth of single-layer graphene on a Pd(1 1 1) substrate

Using a surface segregation technique, single-layer graphene can be grown on a carbon-doped Pd(1 1 1) substrate. The growth was monitored and visualized using Auger electron spectroscopy, X-ray photoelectron spectroscopy, Raman microscopy, atomic force microscopy and scanning tunneling microscopy. Appropriate adjustment of annealing parameters enables controllable growth of single-layer graphene islands and homogeneous, wafer-scale, single-layer graphene. The chemical state of the C 1s peak from X-ray photoelectron spectroscopy indicates there is almost no charge transfer between graphene and the Pd(1 1 1) substrate, suggesting weak graphene–substrate interaction. These findings show surface segregation to be an effective method for synthesizing large-scale graphene for fundamental research as well as potential applications.     

Nucleation and Crystal Growth of a MgO–CaO–Al2O3–SiO2Glass with Added Steel Fly Ash

The nucleation and growth of diopside Ca(Mg,Al)(Si,Al)₂O₆crystals on the free surface of a 24 wt% MgO, 14 wt% CaO, 9 wt% Al₂O₃, and 53 wt% SiO₂glass, with a 2 wt% addition of steel fly ash, were investigated through DTA, XRD, SEM, and optical microscopy. Crystallization was complete at ∼920°C with an activation energy of 589 kJ/mol. Samples with polished free surfaces were nucleated at selected temperatures in the range of 730° to 820°C, and then heat-treated at 870°C for 15 min for crystal growth. Nucleation was predominantly observed at the surface, and the number of diopside crystals per unit of area and the mechanism of crystallization were determined. It was concluded that nucleation reaches a maximum at 750°C, corresponding to an average density of diopside crystals of 8.4 × 10⁶ nuclei/cm², and that between 900° and 1100°C, a uniformly crystallized layer is formed at an exponential rate. The crystallized volume fraction increased significantly in the 880°–890°C growth range, and remained almost constant at higher temperatures. In the 860°–910°C range, the size of the diopside crystals formed in the samples nucleated at the temperature of the maximum nucleation rate, and linearly increased, reaching values between 1.0 and 3.0 μm at 870° and at 910°C, respectively.     

Oxidation Behavior of Hot-Pressed Si3N4

The high-temperature chemical stability of hot-pressed Si₃N₄ was studied between 600° and 1450°C. Reactions were followed by X-ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO₂ surface layer results after 24 h at 750°C-Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance of Si₃N₄ in air.     

Investigation of Annealing Induced Yttria Segregation in Sputtered Yttria‐Stabilized Zirconia Thin Films

8 mol% yttria‐stabilized zirconia (8YSZ) is an extensively studied solid electrolyte. But there is no consistency in the reported ionic conductivity values of 8YSZ thin films. Interfacial segregation in YSZ thin films can affect its ionic conductivity by locally altering the surface chemistry. This article presents the effects of annealing temperature and film thickness on free surface yttria segregation behavior in 8YSZ thin film by Angle Resolved XPS and its influence on the ionic conductivity of sputtered 8YSZ thin films. Surface yttria concentration of about 32, 20, and 9 mol% have been found in 40 nm 8YSZ films annealed at 1273, 1173, and 1073 K, respectively. Yttria segregation is found to increase with increase in annealing temperature and film thickness. Ionic conductivities of 0.23, 0.16, and 0.08 Scm^?1 are observed at 923 K for 40 nm 8YSZ films annealed at 1073, 1173, and 1273 K, respectively. The decrease in conductivity with increase in annealing temperature is attributed to the increased yttria segregation with annealing. Neither segregation nor film thickness is found to affect the activation energy of oxygen ion conduction. Target purity is found to play a key role in determining free surface yttria segregation in 8YSZ thin films.     

Grain-Boundary and Surface Segregation of Ba-Ti-O Phases in Rutile

The solubility of barium in rutile and the formation and morphology of second phase at external and internal surfaces of single-crystal and polycrystalline rutile (TiO,₂) have been studied for various heat treatments up to 1450°C and in several configurations of the barium source. It has been found that barium is not incorporated into the rutile lattice but instead forms an amorphous, quasi-liquid film of Ba-Ti-O at high temperature (=1400°C) which covers the grain boundaries and the external surfaces. On cooling or annealing at lower temperature (=1100°C) the continuous film breaks up to form isolated segregation patterns. Transport of the second phase by surface and grain-boundary diffusion is rapid and reversible by heat treatment.     

Laser-Assisted Growth of Superconducting MgB2 Films in an In Situ Annealing Process Using a Stoichiometric Target

A superconducting MgB₂ film with a superconducting transition temperature of ∼36 K was successfully prepared on an MgO substrate by pulsed-laser ablation from a stoichiometric MgB₂ target. A multilayer deposition process involving interposed Mg-rich layers was performed at 200°C by controlling the laser energy density in order to maintain the correct stoichiometry, and followed by an in situ annealing process at temperature ranging from 700° to 900°C. It was found that smooth, fine-grained films with superconducting transition temperatures of 24–36 K could be obtained from in situ annealing of the as-deposited multilayer at 900°C without excess Mg addition.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Calcium Segregation to a Magnesium Oxide (100) Surface

The segregation of calcium to a (100) cleavage surface of an MgO crystal, with bulk calcium concentration of 200 ppm was measured in situ at T =900° to 1450°C in ultrahigh vacuum, using Auger and low-energy ion-scattering spectroscopies. A measured heat of segregation of approximately -50.3 kJ/mol (-12 kcal/mol) is in favorable agreement with a value of -58.7 kJ/mol (-14 kcal/mol) determined using solute strain energy and surface free energy criteria. The equilibrium value for the calcium segregation between 950° and 1000°C is estimated to correspond to a 20% occupation of the surface cation sites.     

Segregation in Titanium Dioxide Co‐Doped with Indium and Niobium

This work reports the effect of oxygen activity on surface segregation for TiO₂ co‐doped with two cations, indium and niobium (0.076 at.% In + 0.103 at.% Nb). In this work, we studied the effect of annealing at 1273 K in the gas phase of controlled oxygen activity on surface segregation of both ions. The applied oxygen activity included pure oxygen, p(O₂) = 100 kPa, and argon, p(O₂) = 10 Pa. The segregation‐induced concentration gradients were determined using both secondary ion mass spectrometry and X‐ray photoelectron spectroscopy. The obtained results indicate that annealing of the studied TiO₂ specimens in argon results in cooperative segregation of both ions leading to the formation of a surface structure involving comparative concentrations of both cations. However, annealing in oxygen results in preferential segregation of indium leading to the formation of a In₂TiO₅‐type surface structure. The obtained results are considered in terms of the effect of multicomponent segregation on processing of the surface layer with controlled properties that are desired for specific applications. The present work indicates that oxygen activity may be used as the parameter in surface engineering of the solid solution.     

2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: III, The Role of Atmosphere

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤ T < 830°C. A Ca₂PbO₄-like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)₁₄Cu₂₄O₄₁ phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca₂PbO₄-like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa₂Cu₃O _x superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen.     

TEM Characterization of Nanostructured MgAl2O4 Synthesized by a Direct Conversion Process from γ-Al2O3

Nanostructured MgAl₂O₄ spinel was synthesized by a direct conversion process from cubic γ-Al₂O₃. The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl₂O₄ phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl₂O₄ spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78.     

Effects of Magnesium Oxide on Grain-Boundary Segregation of Calcium During Sintering of Alumina

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.     

Segregation Isotherms at the Surfaces of Oxides

A non-Arrhenius segregation isotherm is derived which includes the change in the heat of segregation with surface coverage due to impurity—impurity interactions. It is shown that a linear dependence of log ( X_s ) on the reciprocal temperature, where X_s is the surface atomic ratio, can derive either from a constant heat of segregation, i.e., Arrhenius behavior, or from a heat of segregation that varies as X⁻¹_s . This isotherm is then used to calculate the equilibrium surface coverages of Ca at the {001} surface of MgO₁ Mg at the {0001} surface of α-Al₂O₃, γ at the {1012} and {1120} surfaces of α-Al₂O₃, and Na at the {111} and {110} surfaces of Li₂O from the calculated heats of segregation. Where possible, comparisons are made with experiment. The more useful operational definition of the heat of segregation, namely, that derived from the measured coverage or that defined atomistically and obtained by calculation, is discussed.     

Highly Reactive β-Dicalcium Silicate: III, Hydration Behavior at 40–80°C

The effect of curing temperature (40°, 60°, 80°C) on the hydration behavior of β-dicalcium silicate (β-C₂S) was investigated. The β-C₂S was obtained by decomposition of hillebrandite, Ca₂(SiO₃)(OH)₂, at 600°C, has a specific surface area of about 7 m²/g, and is in the form of fibrous crystals. The dependence of the hydration reaction on temperature continues until the reaction is completed. The hydration is completed in 1 day at 80°C and in 14 days at 14°C. The hydration mechanism is different above and below 60°C, but at a given temperature, the reaction mechanism and the silicate anion structures of C-S-H do not change significantly from the initial to the late stages of the reaction. High curing temperature and long curing times after completion of reaction promote silicate polymerization. The Ca/Si ratio of C-S-H shows high values, being almost 2.0 above 60°C and 1.95 below 40°C.     

Yttrium Segregation and YAG Precipitation at Surfaces of Yttrium- Doped α-A12O3

Auger electron spectroscopy and low-energy ion scattering were used to assess surface composition of yttrium-doped polycrystalline and single-crystal α-aluminas which were heated under vacuum to temperatures above 1500°C. Extensive surface and intergranular precipitation of yttrium aluminum garnet was observed in a compact of nominal Y content of 210 wt ppm, while apparent Y segregation to surface cation sites predominated in a second compact with a Y content of 160 wt ppm. In the latter case, the segregation enthalpy of yttrium to the alumina surface in the temperature range 1700° to 1900°C was estimated to be of the order –44 kJ/mol with a maximum surface cation site occupancy of ∼13%.     

Surface Domain Reorientation Produced by Abrasion and Annealing of Polycrystalline BaTiO3

The abraded surface of polycrystalline BaTiO₃ was examined by X-ray diffraction methods. The distorted layer produced by abrasion on 320-grade SiC consists of (1) a region extending to the bottom of the deepest scratches in which the domains tend to be oriented with their c axes perpendicular to the surface and (2) a transition layer below this in which the orientation varies with depth. The orientation changes little on annealing until recrystallization begins at #900°C; however, annealing at 1100° to 1400°C produces an α-domain reorientation. Possible explanations for these effects are discussed in terms of dislocations introduced by deformation during abrasion and grain size in the recrystallized surface layer.     

Early Stages of Diamond-Film Formation on Cobalt-Cemented Tungsten Carbide

The surface composition of cemented tungsten carbide (WC-5.8 wt% Co) was studied by X-ray photoelectron spectroscopy (XPS), during the early stages of diamond-film deposition, by hot-filament chemical vapor deposition (HFCVD). The nucleated diamond films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and automatic image analysis (AIA). The evolution of the surface composition of cemented tungsten carbide during the early stages of diamond-film deposition was strongly dependent on the substrate temperature. At relatively low temperature (750°C), cobalt-rich particles started to segregate at the substrate surface after a few minutes of diamond deposition. The conspicuous segregation of the binder partly inhibited the formation of stable diamond nuclei, through intense carbon dissolution or carbon segregation at the binder surface, but did not affect nucleic growth. At higher temperatures (940°C), no cobalt-rich particles formed at the substrate surface, even after 2 h of deposition. However, XPS results demonstrated the presence of cobalt in a surface layer, although in a lower amount than at 750°C. Nevertheless, the nucleation density of diamond at 940°C was much lower than at 750°C. Gaps between WC grains formed within 10 mins. Therefore, intergranular cobalt was removed at 940°C, a finding attributed to the etching performed by monohydrogen, rather than to binder evaporation. The time evolution of the substrate area fraction covered by diamond islands, S ( t ), was well described by Avrami kinetics for two-dimensional phase transformations, suggesting that diamond formation took place via a heterogeneous nucleation process. The S ( t ) functions exhibited a similar trend at 750° and 940°C, because the higher growth rate of diamond crystallites at higher temperature counteracted the slower nucleation rate at the higher temperature.