Carrier Generation in Wide-Gap Conductor, Zinc Antimonate

Mechanisms for carrier generation in zinc antimonate (ZnSb₂O₆) with a wide band gap were examined in relation to its crystallinity. ZnSb₂O₆ prepared by the reaction of Sb₂O₅ sol and 3ZnCO₃·4Zn(OH)₂ was sintered at 893, 1173, and 1393 K, and characterized by X-ray diffraction (XRD), electrical conductivity and diffuse reflectance measurements. A higher conductivity was found for ZnSb₂O₆ sintered at lower temperatures. Rietveld analysis of XRD patterns showed that the cations in conductive ZnSb₂O₆ occupied irregular sites, similar to the random rutile structure. Electron carriers were also considered to be generated by oxygen vacancies in ZnSb₂O₆ samples which have a structure similar to the random rutile structure.     

Preparation of Gadolinium Gallium Garnet [Gd3Ga5O12] by Solid-State Reaction of the Oxides

We have prepared dense polycrystalline gadolinium gallium garnet (GGG) by solid-state reaction of the oxides. The oxides were prereacted at 1350°C, ground, pressed, and sintered at 1650°C, yielding 97% dense samples. Ga₂O₃ evaporated from the sample surface leaving Gd₄Ga₂O₉ that could spall off the sample. For the short times needed to sinter samples, the bulk composition of the material remained essentially constant. The microhardness of the GGG was 11.8 ± 1.2 GN · m⁻².     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Effect of Bismuth of Oxide Content on the Sintering of Zinc Oxide

Densification and microstructure de velopment in Bi₂O₃-doped ZnO have been studied with a special emphasis on the effect of the Bi₂O₃ content. A small amount of Bi₂O₃ in ZnO (0.1 mol%) retarded densification, but the addition of Bi₂O₃ to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi₂O₃ content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Synthesis and Evaluation of Doped Y2O3-Stabilized ZrO2 for the Production of Hydrogen

The preparation and characterization of single-phase sintered ceramic materials based on Y₂O₃-stabilized ZrO₂ with oxides of Ti, Zn, Ce, Cr, Mn, and U/Fe as third-component additives are described. These materials were prepared for a process to produce high-purity hydrogen and were evaluated for this process via measurements of electrical conductivity, stability, and transport number. Compositions with oxides of Ce and Cr in Y₂O₃-stabilized ZrO₂ are shown to be the most promising candidates.     

Scanning Force Microscopy Study of Ceramic Fibers for Advanced Composites

The surface roughness and topography of three different Al₂O₃ fibers were evaluated using atomic force microscopy (AFM). The fibers used were as-received, refractory-metal coated, and Y₂O₃/refractory-metal duplex-layer coated. The refractory-metal coating and Y₂O₃ coating on the Al₂O₃ fibers increased the average surface roughness from 13.3 to 17.1 and 18.8 nm, respectively. The topographic image of the fibers evaluated by AFM was compared to that obtained by scanning electron microscopy. Lateral force microscopy (LFM) was used to measure the distribution of the friction force on the refractory-metal-coated Al₂O₃ surfaces, with friction coefficients ranging from 0.2 to 0.8; the average friction coefficient was 0.38. Tailoring the mechanical properties at fiber/matrix interfaces by surface modification of Al₂O₃ fibers to improve the overall mechanical properties of the composites also was proposed.     

Machining-Induced Surface Residual Stress Behavior in Al2O3–SiC Nanocomposites

The machining and subsequent annealing behavior of an Al₂O₃-SiC nanocomposite (A1₂O₃+ 5 vol% 0.2 μm SiC particles) was compared to that of single-phase A1₂O₃. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be ∼ 10 μm for the A1₂O₃ and ∼12 μm for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al₂O₃, whereas only partial stress relaxation occurred for the nanocomposite.     

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.     

Diffusionless Cubic-to-Tetragonal Phase Transition and Microstructural Evolution in Sintered Zirconia–Yttria Ceramics

Microstructural changes associated with the diffusionless cubic-to-tetragonal phase transition ( c–t ') in sintered ZrO₂–Y₂O₃ ceramics with 4 to 20 mol% Y₂O₃ are investigated. Ceramics containing 4 to 7 mol% Y₂O₃ that are quickly cooled from high temperatures experience cubic-to-tetragonal transformation by a diffusionless mechanism. The TEM diffraction pattern of the transformation product, t '-ZrO₂, reveals (112) reflections that are suppressed in the c -ZrO₂ phase. The microstructural features of the t '-ZrO₂ phase include antiphase boundaries and a twinned substructure in the (112) dark-field images. The (112) reflections result from the displacement of oxygen atoms in the lattice during phase transformation. With increasing Y₂O₃ content, the antiphase domain size, the tetragonality of the lattice, and the intensity of the (112) reflections decrease. When the Y₂O₃ content reaches 8 mol% or more, the (112) reflections appear, but the dark-field image shows no antiphase boundary, and the tetragonality decreases to 1. In ceramics with 14 to 20 mol% Y₂O₃, no suppressed reflection appears because the c–t transformation is fully suppressed. A model of the lattice structure is calculated and the intensity of diffraction determined experimentally; on this basis, the mechanism of diffusionless c–t 'transformation is discussed.     

Influence of Rare-Earth Dopants on Barium Titanate Ceramics Microstructure and Corresponding Electrical Properties

In this article, ytterbium and erbium oxides are used as doping materials for barium titanate (BaTiO₃) materials. The amphoteric behavior of these rare-earth ions leads to the increase of dielectric permittivity and decrease of dielectric losses. BaTiO₃ ceramics doped with 0.01–0.5 wt% of Yb₂O₃ and Er₂O₃ were prepared by conventional solid-state procedure and sintered at 1320°C for 4 h. In BaTiO₃ doped with a low content of rare-earth ions (0.01 wt%) the grain size ranged between 10 and 25 μm. With the higher dopant concentration of 0.5 wt%, the abnormal grain growth is inhibited and the grain size ranged between 2 and 10 μm. The measurements of capacitance and dielectric losses as a function of frequency and temperature have been carried out in order to correlate the microstructure and dielectric properties of doped BaTiO₃ ceramics. The temperature dependence of the dielectric constant as a function of dopant amount has been investigated.     

Factors Affecting the Formation of Ba2Ti9O20

The phase development sequence based on a composition equivalent to Ba₂Ti₉O₂₀ during heating is found to be in the following order: BaTi₅O₁₁ > BaTi₄O₉ > Ba₂Ti₉O₂₀. The lowest rate of formation of Ba₂Ti₉O₂₀ is caused by its high surface energy and interface energy, which result in a low nucleation rate. The existence of BaTi₅O₁₁ in calcined powder helps to form Ba₂Ti₉O₂₀ in sintered compacts. The effect of BaTi₅O₁₁ on Ba₂Ti₉O₂₀ formation can be explained by their similar oxygen packing and by reduced volume change during transformation. The amount of BaTi₅O₁₁ formed during heating depends greatly on the compositional homogeneity of powders. The addition of SnO₂ aids the formation of Ba₂Ti₉O₂₀ by reduced strain energy at transformation and reduced surface energy.     

Polishing Behavior and Surface Quality of Alumina and Alumina/Silicon Carbide Nanocomposites

The response of Al₂O₃ and Al₂O₃/SiC nanocomposites to lapping and polishing after initial grinding was investigated in terms of changes in surface quality with time for various grit sizes. The surface quality was quantified by surface roughness ( R _a ) and by the relative areas of smooth polished surfaces as opposed to rough as-ground areas. Polishing behavior of the materials was discussed in terms of SiC content and grain size. It was concluded that nanocomposites are more resistant to surface damage than Al₂O₃, and this behavior does not depend on the amount of SiC in the range 1–5 vol%. SiC addition ≥1 vol% is enough to produce a noticeable improvement in surface quality during lapping and polishing.     

Thermal Expansion of Substituted Copper Aluminosilicate Glasses

The effects of various components on the thermal expansion coefficient of low-expansion Cu₂O-Al₂O₃-SiO₂ glasses were examined. When a component of glass was substituted by another oxide, the expansion coefficient always increased, except for substitution of Al₂O₃ by B₂O₃. This result indicates that the essential components to maintain the low expansivity are Cu₂O, such trivalent oxides as Al₂O₃ and B₂O₃, and tetravalent oxide SiO₂. Glasses of the systems Cu₂O-Al₂O₃-B₂O₃, Cu₂O-Al₂O₃-GeO₂, and Cu₂O-Al₂O₃-P₂O₅, which were derived by replacing SiO₂ by other network-forming oxides, showed fairly low expansion coefficients compared with other conventional borate, germanate, and phosphate glasses. It was also found that the valence state of copper ions is important for the thermal expansion characteristics of these glasses; Cu⁺ ions contribute to the low expansion coefficient.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Mechanochemical Polishing of Silicon Nitride

Two oxides of iron, Fe₂O₃ and Fe₃O₄, were identified as suitable soft abrasives for mechanochemical polishing of Si₃N₄. Removal rates up to 1.6 μ.m /h were observed when hot-pressed Si₃N₄ samples were mechanochemically polished using these abrasives on a linen plastic lap. The polished surfaces were flat and scratch-free, with a peak-to-valley roughness of <20 nm. Auger electron spectroscopy of these surfaces revealed a thin layer (≤10 nm) of a silicon oxynitride that contained carbon and ∼0.5 at.% iron.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Microstructure of Varistors Prepared with Zinc Oxide Nanoparticles Coated with Bi2O3

Zinc oxide (ZnO) nanoparticles coated with 1–5 wt% Bi₂O₃ were prepared by precipitating a Bi(NO₃)₃ solution onto a ZnO precursor. Transmission electron microscopy showed that a homogeneous Bi₂O₃ layer coated the surface of the ZnO nanoparticles and that the ZnO particle size was ∼30–50 nm. Scanning electron microscopy showed that ZnO grains sintered at 1150°C were homogeneous in size and surrounded by a uniform Bi₂O₃ layer. When the ZnO grains were surrounded fully by Bi₂O₃ liquid phases, further increases in the ZnO grain size were not affected by the Bi₂O₃ content. This predesigned ZnO nanoparticle structure was shown to promote homogeneous ZnO grains with perfect crystal growth.