X-ray Absorption Near-Edge Structure of Grain-Boundary-Segregated Y and Zr in Creep-Resistant Alumina

X-ray absorption near-edge structure (XANES) depends on stereochemical features of coordination polyhedra around probe atoms. K -edge XANES of Y and Zr segregated in alumina grain boundaries has been obtained and analyzed using metallic Y and Zr, Y₂O₃, YAG, and monoclinic ZrO₂ as standards. Grain-boundary-segregated Y and Zr show a positive chemical shift, and the magnitude of the shift, as compared with that of Y₂O₃ and ZrO₂, respectively, is different for Y and Zr, indicating that, relative to Y₂O₃ and ZrO₂, charge transfers for the grain-boundary-segregated Y and Zr are different. This result is also supported by the strength of the threshold resonance. A pre-edge shoulder is seen in K -edge XANES for grain-boundary-segregated Y and Zr but not for Y₂O₃ and ZrO₂. This shoulder is attributed to the 1 s → 4 d transition, which is normally forbidden, but can occur because of d – p mixing, which is favored by a tetrahedral coordination configuration. These results suggest that some of the grain-boundary-segregated Y and Zr have coordination configurations with a well-defined tetrahedral symmetry. The XANES results are compared with those obtained from EXAFS. Implications of these results for understanding of the enhanced creep resistance in alumina are also discussed.     

Defect and Electronic Structures of Calcium-Doped Lanthanum Cuprate

Quenched calcium-doped lanthanum cuprate (La₂CuO₄) samples were examined at room temperature using extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge spectroscopy (XANES) near the Ca K edge, and via atomic- and electronic-structure modeling. Calculations and experiments show that the Ca atom, in a manner similar to that of Sr and Ba atoms, replaces the La atom and maintains its nine-fold coordination above the center of a CuO₄ square, which is the nominal position of the La atom. In contrast to the Sr and Ba atoms, the distances from Ca to its nearest neighbors are similar to the size of the La³⁺ cation. The distances that are obtained from the simulation are in reasonable agreement with the experimental results for calcium, as well as previous studies that involved strontium and barium. The binding of a calcium dopant to vacancies on the O(1) sites is stable, but only slightly; therefore, there are probably many different types of such clusters. This calculated binding also is supported by the EXAFS data. The overall electronic structure is similar to that observed in calculations for pure La₂CuO₄: the LaO planes remain ionic, even with the presence of the calcium dopant, and the Cu-O(1) bond is covalent. The calcium is ionic in character, with a net charge of 1.6, and the narrow Ca 3 d band lies ∼6 eV above the Fermi energy level.     

X-ray Absorption Studies of Ceria with Trivalent Dopants

The local structural environments of Sc³⁺ and Gd³⁺ in ceria were studied for the first time by using EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopy. Our results indicate that Sc³⁺ in 10-mol%-Sc₂O₃-doped ceria has a 7-fold-coordinated Sc-ordered structure with two different Sc─O distances (2.13 and 2.26 Å). An octahedrally coordinated Sc─O cluster is present in 5 mol% Sc₂O₃─5 mol% Gd₂O₃─CeO₂, because of the strong scavenging effect of Sc³⁺ ions on oxygen vacancies created by Gd³⁺. The Gd³⁺ in this sample is largely unassociated. Sc₂O₃ (5 mol%) and Gd₂O₃ (5 or 10 mol%) form random solid solutions with CeO₂. These results are consistent with the current concept of defect structures in this structural family.     

Optical Properties of Zinc Aluminate, Zinc Gallate, and Zinc Aluminogallate Spinels

The optical spectra of zinc aluminate (ZnAl₂O₄), zinc gallate (ZnGa₂O₄), and zinc aluminogallate (ZnAlGaO₄) spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy. The ZnAl₂O₄ and ZnGa₂O₄ powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M₂O₃ (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X-ray diffractometry. The ZnAl₂O₄ powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al₂O₃ (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl₂O₄. The ZnGa₂O₄ powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO₄ showed optical spectra that were intermediate to that of the endpoint compositions.     

Study by Extended X-ray Absorption Fine Structure Technique and Microscopy of the Amorphous State of Titanium Diboride Thin Films

The local atomic structure and the stability of amorphous titanium diboride (TiB₂) thin films, prepared by electron beam vaporization (EBV) of the crystalline compound onto liquid-nitrogen-cooled substrates, are studied using extended X-ray absorption fine structure (EXAFS) and extended energy-loss fine structure (EXELFS). From a comparison of the extended fine structure spectra of the amorphous films with corresponding spectra of crystalline TiB₂, accurate information is derived on the nature of local structure, or short-range order, and on the type and degree of nanostructural atomic disorder σ² in amorphous TiB₂. Subsequent controlled heating experiments and observation and imaging by transmission electron microscopy to determine the crystallization temperature of the amorphous phase show that the crystallization temperature and, thus, the stability of the amorphous phase are strongly dependent on σ². It is shown that minimizing σ² is crucial in enhancing the amorphicity. Finally, EXAFS and EXELFS are employed to examine in detail the structural changes induced in amorphous TiB₂ by variations in the EBV deposition parameters.     

Formation Mechanisms of Tetragonal Barium Titanate Nanoparticles in Alkoxide–Hydroxide Sol-Precipitation Synthesis

The early stage of barium titanate (BaTiO₃) nanoparticle formation is investigated by in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) using synchrotron radiation. BaTiO₃ nanoparticles are synthesized via dissolution of barium hydroxide octahydrate and hydrolysis of titanium (IV) isopropoxide in isopropanol. In the course of raising the temperature of the alkoxide–hydroxide mixture solution to 80°C, in situ synchrotron XRD reveals that BaTiO₃ nanocrystals smaller than 6 nm begin to nucleate at 50°C without intermediate TiO₂ anatase formation, and Ti K edge absorption spectra also confirm the formation of corner-sharing TiO₆ octahedra at 60°C. The average size of BaTiO₃ precipitates increases to about 7.5 nm at 80°C. The synthesized nanopowders show an anomalously high tetragonality according to the Rietveld refinement of synchrotron XRD data.     

Determination of Valence States of Chromium in Calcium Chromates by Using X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

Calcium chromates with the empirical formulas Ca₁₀Cr₆−O₂₅, Ca₃Cr₂O₈, and Ca₅Cr₃O₁₂, which form at temperatures >900°C at CaO:Cr₂O₃ molar ratios of 3 in an oxidation atmosphere have been synthesized in the pure state. X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the average valence state of chromium in the samples. The presence of unusual chromium valence states, 4+ and 5+, which was proposed via X-ray diffractometry studies, is strongly supported.     

Characterization of Hot-Pressed Cement Pastes

Very-high-strength cement pastes (compressive strengths ∼100,000 psi) were prepared using hot-pressing techniques at temperatures up to 250°C. The results of characterization studies summarized include X-ray diffraction, DTA, ir spectra, and microstructural investigations. Much less Ca(OH)₂ is formed than in normally hydrated pastes. Some well- crystallized hydroxyl-containing hydrates, C₅S₂H, C₃SH_1.5, C₃AH₆, and C₄A₃H₃, as well as poorly crystalline CSH, are formed. The structure of the CSH, as revealed by ir absorption studies, does not appear to be very different from that in low- temperature hydrated cement pastes. Microstructures are extremely dense and, in some cases, the sulfate-containing phase concentrates along original grain boundaries.     

Calculation of XANES/ELNES Spectra of All Edges in Si3N4 and Si2N2O

Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si- L _2,3, Si- K , and N- K edges in α-Si₃N₄, β-Si₃N₄, spinel c -Si₃N₄, and Si₂N₂O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β-Si₃N₄ and c -Si₃N₄. The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ELNES spectral calculation.     

Synthesis of Zinc Aluminate Spinel Film through the Solid-Phase Reaction between Zinc Oxide Film and α-Alumina Substrate

We synthesized spinel ZnAl₂O₄ film on α-Al₂O₃ substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al₂O₃ substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl₂O₄ was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl₂O₄ was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl₂O₄ film became disoriented from the single (111) orientation. The ZnAl₂O₄ (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl₂O₄ was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl₂O₄ orientation.     

Encapsulation of Sn(II) and Sn(IV) Chlorides in Composite Cements

The hydration of two high replacement composite cements (3:1 blast furnace slag:ordinary Portland cement (BFS:OPC), and 3:1 pulverized fuel ash:OPC (PFA:OPC)) with the addition of both SnCl₂ and SnCl₄ has been investigated and the results from X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) are presented. Adding 5% or 1% SnCl₂·2H₂O or SnCl₄·5H₂O to the mix water resulted in the formation of Friedel's salt, Ca₃Al₂O₆.CaCl₂·10H₂O, and calcium hydroxo-stannate CaSn(OH)₆, which also involved the consumption of calcium hydroxide. After 90 days hydration at lower levels of addition (i.e., 1%) there was no longer evidence for CaSn(OH)₆, indicating that it too had been consumed in the pozzolanic reaction due to the lack of calcium hydroxide present. Results from SEM and EDS showed that bright regions between the BFS or PFA grains were tin containing and they were incorporated into the hydrated cement matrix. The tin was, therefore, localized rather than spread throughout and intimately incorporated into the microstructure.     

Structural Relaxation around Substitutional Cr3+ Ions in Sapphire

The local structural arrangement of ions around a substitutional Cr³⁺ ion in sapphire (ruby) has been studied experimentally using extended X-ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott-Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α-Cr₂O₃. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around the substituted Cr³⁺ ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self-similarly and in proportion to the macroscopic strain.     

Extended X-ray Absorption Fine Structure (EXAFS) Study on the local Environment around Copper in Low Thermal Expansion Copper Aluminosilicate Glasses

The local environment around copper in the Cu₂O·Al₂O₃·4SiO₃ and CuO·Al₂O₃·4SiO₂ glasses was investigated using the extended X-ray absorption fine structure (EXAFS) technique. It has been found that each Cu(I) in the former glass is coordinated to two oxygens through covalent Cu(I)-O bonds. This is the primary reason for its low thermal expansion coefficient (∼10 × 10⁻⁷ K⁻¹). In the latter glass, which was made by heating the former glass at 600°C in air, each Cu(II) is coordinated to four oxygens and the bonds are ionic in character, contributing to the increase in the thermal expansion coefficient to ∼30 × 10⁻⁷ K⁻¹.     

Optical Spectra of Cu2+ and Ni2+ in Intermediate Aluminate Spinels

Optical absorption spectra were determined for well characterized intermediate spinels, NiAl₂O₄ and CuAl₂O₄, equilibrated over a range of temperatures. The absorption spectra were interpreted satisfactorily in terms of crystal field theory and were consistent with results of detailed X-ray analyses. Covalent bonding caused some band shifts and changes in the ratio Dq (tetr. coord.)/ Dq (oct. coord.) and the net stabilization energies of the transition metal cation.     

Synchrotron Radiation Ti-k XANES Study of TiO2─Y2O3-Stabilized Tetragonal Zirconia Polycrystals

Valence state and site symmetry of Ti ions in TiO₂–Y₂O₃–ZrO₂ powders with 2 mol% Y₂O₃ and 5, 10, 15, and 20 mol% TiO₂, respectively, are studied by X-ray absorption near-edge spectroscopy (XANES). Tetravalent Zr⁴⁺ ions are replaced predominantly by Ti⁴⁺ ions. Within the solubility region of Ti ions, a subsequent displacement of Ti ions from the center of symmetry is observed with increasing TiO₂ content in TiO₂–Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Ti-Y-TZP) under investigation. This behavior cannot be interpreted with a random substitution of Ti⁴⁺ ions on Zr⁴⁺ lattice sites. On the contrary, this correlation between the TiO₂ content in Ti-Y-TZP and the shift of Ti ions indicates an increasing interaction between the Ti ions with growing TiO₂ content, caused by a subsequent clustering of Ti ions.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

Sodium Coordination Environments in Silica Films

The local structure centered on sodium after diffusion in silica (Na-SiO₂ samples) has been determined by means of extended X-ray absorption fine structure (EXAFS) studies. The Na-SiO₂ samples are of particular interest because (i) their sodium content can be varied over a wide range of concentration and (ii) their local structure is representative of that of soda–silica glass. EXAFS analyses reveal the existence of a well-defined local structure involving oxygen, sodium, and silicon neighbors. The Na-O, Na-Na, and Na-Si bonds lengths, which amount to 0.23, 0.30, and 0.38 nm, respectively, do not depend on sodium concentration. This environment closely resembles that found in soda–silica glass. Moreover, it is compatible with the "target site" and "the site memory effect" suggested by recent theories of the ionic conductivity in oxide glasses.     

Physical Properties and Structure of rf-Sputtered Amorphous Films in the System Al2O3–Y2O3

Amorphous films in the system Al₂O₃–Y₂O₃ were prepared by the rf sputtering method in the range of 0–76 mol% Y₂O₃, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al₂O₃–Y₂O₃ films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y₂O₃ content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al₂O₃–Y₂O₃ films, the Al K α X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y₂O₃ content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y₂O₃, the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y₂O₃ content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al₂O₃–Y₂O₃, the change of the coordination state of aluminum ions has an important effect on physical properties.     

The Activation Effect of K2SO4 on the Hydration of Gypsum Anhydrite, CaSO4(II)

The effect of K₂SO₄ activator on the hydration of chemical anhydrite obtained from burned FGD-gypsum has been studied by different experimental techniques. Results obtained show that the degree of hydration increases when the K₂SO₄ concentrations increase from 0.5 to 3.3 wt%. Their heat evolution rate and maximum value also increase with the increase of K₂SO₄ concentration. The highest values were obtained when the hydration degree was about 50%. Also a correlation between the hydration degree and the total heat evolved was obtained. The X-ray and SEM/EDX studies have shown that K₂SO₄ is adsorbed at the surface of CaSO₄ even within 5 min of hydration and a syngenite—a double salt K₂SO₄·CaSO₄·H₂O is formed (even in the presence of 1.0 wt% K₂SO₄). Also, important changes in the morphology of the dihydrate crystal are detected. Finally, in the presence of 1.0 wt% K₂SO₄ (water/anhydrite ( W / A ) ratio=0.33), it was found that the resonance frequency, modulus of elasticity, compressive strength, and tensile strength increase with the degree of hydration whereas the total porosity decreases.