Effect of the Atomic Ratio of Ba to Ti on Optical Properties of Gold-Dispersed BaTiO3 Thin Films

Gold-dispersed BaTiO, thin films were prepared by the rf magnetron sputtering method. The atomic ratio of Ba to Ti in the films was varied and the effects on the linear and nonlinear optical properties were investigated. As the atomic ratio increased, the value of χ⁽³⁾/α₅₃₂ for the gold-dispersed BaTiO₃ thin films slightly increased. It was found that the increase in the value of χ⁽³⁾/α₅₃₂ is due mainly to a change in the crystalline state of the BaTiO₃ matrix. However, it was also found that the atomic ratio had a smaller effect on the value of χ⁽³⁾/α₅₃₂ than did the refractive index of the matrix.     

Synthesis of TiB2 by High-Dose Implantation of Boron Ions in Titanium Films

Boron ions were implanted at room temperature in Ti films at a high dose (7.1 × 10^I7 and 2.3 × 10¹⁸ ions/cm²), The formation of TiB₂ films was confirmed by X-ray diffraction. Boron concentration profiles in implanted films were studied by secondary-ion mass spectrometry.     

Chemical Speciation of Trace Zinc in Ordinary Portland Cement Using X-ray Absorption Fine Structure Analysis

Chemical change of trace zinc in ordinary portland cement (205.1 ppm) was investigated in hydration process using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Intensities of the peaks appearing at the same energy of ZnO in XANES spectra were decreased with cement hydration. The interatomic distances and the coordination numbers of the first and the second shells calculated from EXAFS spectra indicated that ZnO hydrolyzed to zincate ion [Zn(OH)₄]²⁻ with cement hydration keeping their fundamental structure of ZnO₄ tetrahedra.     

ON THE CONSISTENCY OF LEAST SQUARES ESTIMATORS FOR A THRESHOLD AR(1) MODEL

Abstract. For the SETAR (2; 1,1) model

where {a_t(i)} are i.i.d. random variables with mean 0 and variance σ²(i), i = 1,2, and {a_t(l)} is independent of {a_t(2)}, we consider estimators of φ₁, φ ₂ and r which minimize weighted sums of the sum of squares functions for σ²(1) and σ²(2). These include as a special case the usual least squares estimators. It is shown that the usual least squares estimators of φ₁, φ₂ and r are consistent. If σ²(1) ≠σ²(2) conditions on the weights are found under which the estimators of r and φ₁ or φ₂ are not consistent.     

Early-Stage Thermal Oxidation of Carbothermal β-Sialon Powder

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state ²⁹Si and ²⁷Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but ²⁹Si and ²⁷Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol⁻¹, suggesting the rate-limiting step to be due to dissolution of O₂ in an amorphous silica surface layer on the β-sialon particles.     

Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Open-circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO₂. The open-circuit emf data were used to delineate the low- p o₂ ionic domain boundary for "pure" ThO₂, which is presented as a log P_θ line on a log Po₂-1/ T diagram. In addition the ionic conductivity, σ_ion, and the high-Po₂ log P_θ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po₂^I/4 dependence of σ_p, the p-type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 10³/4.575T, log σ_ion= 1.9−44.3×10³/4.575T, and log P_θ=−1.0−31.4 × 10³/4.575T; for the second batch, log Pθ=11.2−219.7 × 10³/4.575T, log σ_ion= 1.7−41.6 × 10³/4.575T, and log Pθ=0.6−40.4 × 10³/4.575T. The oxygen permeability of ThO₂ tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.     

Crystallization Behavior of Amorphous Silicon Carbonitride Ceramics Derived from Organometallic Precursors

The crystallization behavior of organometallic-precursor-derived amorphous Si-C-N ceramics was investigated under N₂ atmosphere using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si-C-N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane-polysilazane blends, single-source polysilazanes, and single-source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si₃N₄ increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon-free amorphous Si-N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si-C-N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si-C-N network.     

Estimation in nonstationary random coefficient autoregressive models

Abstract.  We investigate the estimation of parameters in the random coefficient autoregressive (RCA) model X _k  = ( φ  +  b _k ) X _{k −1} +  e _k , where ( φ ,  ω ²,  σ ²) is the parameter of the process,     ,     . We consider a nonstationary RCA process satisfying E  log | φ  +  b ₀| ≥ 0 and show that σ ² cannot be estimated by the quasi-maximum likelihood method. The asymptotic normality of the quasi-maximum likelihood estimator for ( φ ,  ω ²) is proven so that the unit root problem does not exist in the RCA model.     

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)₃) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl₃ solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO₂ flakes, or α-Al₂O₃ flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al₃O(OH)₃(OH₂)₉]⁴⁺ polycation is promoted, leading to the formation of the (Al₄) tetramer ([Al₄O(OH)₁₀(OH₂)₅]^o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al₃O(OH)₃(O₂H₃)₃(OH₂)₉]⁺) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al₂O₃ layers, while calcination of gibbsite leads to δ-Al₂O₃ layers.     

Crystallization of Electrostatically Seeded Lanthanum Hexaluminate Films on Polycrystalline Oxide Fibers

Lanthanum hexaluminate (LaAl₁₁O₁₈), which features easily cleavable atomic planes in the magnetoplumbite (MP) structure, is a good candidate material for the fiber/matrix interface in oxide/oxide-fiber-reinforced composites. Difficulties that are encountered when using sol–gel synthesis to produce this material include the high crystallization temperature and the occurrence of an intermediate phase. This paper presents a method to overcome these difficulties by introducing crystalline seeds into the LaAl₁₁O₁₈ sol during the fiber coating. Seed particles are deposited initially on the fibers via electrostatic forces. Then, the sol is dip-coated on the preseeded fibers, and the final LaAl₁₁O₁₈ coating is achieved via the use of heat treatments. The crystallization temperature can be reduced by at least 50°C, in comparison with that for the unseeded case, according to differential scanning calorimetry measurements, and no intermediate LaAlO₃ phase is formed. The estimated seed numbers are on the order of 10⁹ seeds/cm³ in the preseeded sol-coated fiber surfaces and 10¹¹ seeds/cm³ in the 4-wt%-seeded gels. The crystallization and structure of the La_{1− x} Al_{12− y} O_{19− z} MP phase in the 4-wt%-seeded gels that have been heated at 400° and 1200°C have been studied using powder neutron diffraction.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

Thermal Expansion of the Hexagonal (6H) Polytype of Silicon Carbide

The thermal expansion of the hexagonal (6H) polytype of α-SiC was measured from 20° to 1000°C by the X-ray diffraction technique. The principal axial coefficients of thermal expansion were determined and can be expressed for that temperature range by second-order polynomials: α¹¹= 3.27 × 10^–6+ 3.25 × 10^–9T – 1.36 × 10^–12 T ² (1/°C), and ş₃₃= 3.18 × 10^–6+ 2.48 × 10^–9 T – 8.51 × 10^–13 T ² (1/°C). The σ₁₁ is larger than α₃₃ over the entire temperature range while the thermal expansion anisotropy, the δş value, increases continuously with increasing temperature from about 0.1 × 10^–6/°C at room temperature to 0.4 × 10^–6/°C at 1000°C. The thermal expansion and thermal expansion anisotropy are compared with previously published results for the (6H) polytype and are discussed relative to the structure.     

Electrical Resistivities of Monocrystalline and Polycrystalline TiB2

The electrical resistivity of monocrystalline and polycrystalline TiB₂, was measured under an inert atmosphere by a four-point ac impedance technique over the range 298 to 1373 K. The results are expressed in the form ρ-ρ₂₉₈= m(T -298). The following values of ρ₂₉₈ (μω.cm) and m (nω.cm-K^-1) were determined: for polycrystalline TiB₂ (69% dense) 18.2 and 95; for polycrystalline TiB₂ (99% dense) 7.4 and 42; and for monocrystalline TiB₂, 6.6 and 34.9.     

Densification, Sintering Reactions, and Properties of Titanium Diboride With Titanium Disilicide as a Sintering Aid

In the present investigation, we explore the feasibility of using TiSi₂ as a sintering aid to densify titanium diboride (TiB₂) at a lower sintering temperature (<1700°C). The hot-pressing experiments were conducted in the temperature range of 1400°–1650°C for 1 h in an argon atmosphere and TiSi₂ addition to TiB₂ was restricted up to 10 wt%, with an overall objective to densify the materials with a fine microstructure as well as to assess the feasibility of enhancing the mechanical and electrical properties. When all the materials were hot pressed at 1650°C, the hot-pressed TiB₂– X % TiSi₂ ( X =0, 2.5, 5, 10 wt%) composites were found to be densified to more than 98%ρ_th (theoretical density), except monolithic TiB₂ (∼94%ρ_th). An interesting observation is the formation of a Ti₅Si₃ phase and this phase formation is described by thermodynamically feasible sintering reactions. Our experimental results suggest that the optimal TiB₂–5 wt% TiSi₂ composite can exhibit an excellent combination of properties, including a high hardness of 25 GPa, an elastic modulus of 518 GPa, an indentation toughness of ∼6 MPa·m^1/2, a four-point flexural strength of more than 400 MPa, and an electrical resistivity of 10 μΩ·cm.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

Structural Equilibria in Silicate Glass Melts Investigated by Raman Spectroscopy

Structural equilibria of alkali silicate glasses and melts are investigated based on the Q ⁿ fractions ( n =number of bridging oxygen in SiO₄ tetrahedron) obtained from high-temperature Raman spectra. A mathematical treatment of the entropy of mixing, Δ S _mix, of structural groups of Q ⁿ is conducted and applied to the investigation of phase transformation and phase separation. Characteristics of Q ⁿ traces and the counter-map of Δ S _mix on Q ⁿ diagram are found to possess useful information to understand their mechanisms. An extended Q ⁿ diagram is also proposed to understand the melt structure and properties for the wide composition range.     

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI₂O₃ made from AI-isopropoxide (AI₂O₃(II)). The undoped material was found to contain 1.5×10¹⁸ cm⁻³ fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving V _AI"in donor-, AI," in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (O_i^p). The p_O2 dependence of σ _t and σ _h are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.     

29Si and 27Al MAS-NMR of NaOH-Activated Blast-Furnace Slag

b²⁹Si and ²⁷Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad ²⁹Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q ¹], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new ²⁹Si peaks with widths of ca. 2 ppm are formed at -78.5 Q ¹, –81.4 [ Q ²(1Al)J, and -84.3 [ Q ²]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, ²⁷Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)₄AH₁₃. ²⁹Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.     

Transparent Conducting Oxides Based on the Spinel Structure

The structural characteristics of spinel (from both theoretical and experimental viewpoints) that are advantageous for producing transparent conducting oxide (TCO) materials are considered. A working hypothesis is presented for finding new TCO materials with spinel structure. Results of tight-binding energy-band calculations reveal the formation of extended conduction bands in selected TCOs. This is a necessary criterion for n -type TCOs containing cations with d ¹⁰ s ⁰ electronic configuration. Discovery of some transparent conducting spinels, namely MgIn₂O₄, CdGa₂O₄, and ZnGa₂O₄, provides experimental confirmation for the formation of extended conduction bands. Li⁺-, He⁺-, and H⁺-ion implantations into MgIn₂O₄ films have been performed to explore a new technique to generate electron carriers. Carrier generation in implanted MgIn₂O₄ films has been verified from electrical conductivity and optical absorption measurements. Implanted Li⁺ ions have been observed to occupy vacant tetrahedral cation interstices in the spinel lattice. Each Li⁺ ion releases an electron to the conduction band upon occupying this tetrahedral vacancy. Observations suggest that the ion implantation technique is effective and feasible for carrier generation in TCO materials.     

Recrystallization of Oxygen Ion Implanted Ba0.7Sr0.3TiO3 Thin Films

The crystallization behavior of chemical-solution-deposited and amorphous Ba_0.7Sr_0.3TiO₃ (BST) thin films was analyzed with respect to the evolution of the structural and dielectric properties of the films as a function of the annealing temperature. The amorphous films were produced by oxygen ion implantation into crystalline BST thin films. In the amorphous thin films, the crystallization to the perovskite phase occurred at T = 550°C, whereas the as-deposited CSD films showed the first crystalline XRD-reflex only after annealing at T = 650°C. Here a carbon-rich intermediate phase delayed the crystallization process to higher temperatures.