Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Preparation and Properties of Porous α-Al2O3 Membrane Supports

Strong and permeable macro-porous α-Al₂O₃ membrane supports are made by colloidal filtration of 20 vol% dispersions of α-Al₂O₃ with an average particle size of 600 nm. Intact compacts with very good surface quality were obtained at an optimum pH of 9.5 and dosage of 0.2 wt% ammonium aurintricarboxylate (Aluminon), based on dry alumina. The colloidal stability of the aluminon-stabilized slurries is confirmed by ξ potential measurements. Slight sintering of dense-packed α-Al₂O₃ compacts was found to result in >67% packing density and a bimodal pore-size distribution as derived from shrinkage behavior and gas adsorption studies. Non-stationary single gas permeation measurements showed improved gas permeability, compared with α-Al₂O₃ compacts prepared using powder with a smaller particle size (300 nm). The strength of the disk-shaped alumina compacts within the porosity range of 30%–20% increased from 100 to 300 MPa with a standard deviation of 20 and 50 MPa, respectively.     

Particle Size Control and Dependence on Precursor pH: Synthesis Uniform Submicrometer Zinc Aluminate Particles

Zinc aluminate (ZnAl₂O₄) particles have been synthesized by the hydrothermal method using NH₃·H₂O as a pH adjustment mineralizer. Experimental results showed that ZnAl₂O₄ particle size was dependent on the precursor pH, and could be controlled through pH adjustment. It was 5.5, 11.5, and 27 nm when the precursor pH was 8.2, 9.3, and 10.5, respectively. On the other hand, the particle size distribution changed broader with increase in pH. These differences were attributable to the different NH₃·H₂O function. NH₃·H₂O was mainly used as a base at lower pH (<9.0), while its complex function predominated at higher one (>9.5). From thermodynamic viewpoint, the rate-limiting steps were dissolution of Al(OH)₃ and γ-AlO(OH) to Al(OH)₄⁻ at lower and higher pH, respectively. The newly formed γ-AlO(OH) with high reactivity was the critical factor in the synthesis of bimodal particles. Higher temperature treatment of γ-AlO(OH) could decrease the reactivity, and could be used as an aluminum source for synthesis uniform ZnAl₂O₄ particles.     

Evaluation and Statistical Analysis of Dielectric Permittivity of BaTiO3 Powders

A slurry method was used to determine the dielectric permittivity (ɛ_r) of BaTiO₃ powders with different characteristics, such as tetragonality ( c / a ratio), density, particle size, and specific surface area. The ɛ_r of powders highly depended on their characteristics. In order to extract the effect of each characteristic, a statistical analysis was carried out to represent the ɛ_r of powders with an empirical formula. A fairly good agreement was obtained between observed data and those estimated from the formula. The ɛ_r decreased with particle size because of the size effect of BaTiO₃ and high tetragonality and density was essential to obtain high ɛ_r of powders.     

Pulsed Electric Current Sintering of Silicon Nitride

Pulsed electric current sintering (PECS) has been used to densify α-Si₃N₄ powder doped with oxide additives of Y₂O₃ and Al₂O₃. A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS.     

Effect of Particle Size on the Room-Temperature Crystal Structure of Barium Titanate

The room-temperature tetragonal-to-cubic transformation in BaTiO₃ powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO₃ powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO₃ powders with room-temperature ( c/a ) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO₃ particles with a ( c/a ) > 1.008 at room temperature. BaTiO₃ powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO₂ particles, is consistent with the experimental data.     

Effect of Particle Size on Colloidal Zirconia Rheology at the Isoelectric Point

This paper examines the effects of particle concentration and size on the yield stress of ZrO₂ suspensions at a well-defined surface chemistry condition of the isoelectric point (IEP). At the IEP, the relationship between yield stress τ_Ymax and particulate volume fraction φ_s and mean particle size d was evaluated to be τ_Ymax = K φ_s^4.0/ d ^2.0. The difference in size distribution of the various ZrO₂ suspensions examined causes some degree of scatter in the data used to establish the τ_Ymax, φ_s, and d relation. The use of particle concentration n_t based on the fine size fraction instead of volume fraction φ_s provided a better correlation, because the fine particles govern the properties of the flocculated network structure.     

Synthesis of (La,Sr)MnO3–YSZ Composite Particles by Spray Pyrolysis

(La_0.8Sr_0.2)_0.9MnO₃–YSZ composite particles were synthesized by spray pyrolysis. The mean particle size of the synthesized powders was about 1 pm and the particle size distribution was very narrow. The synthesized powders were composed of the perovskite (La,Sr)MnO₃ and cubic phase YSZ. Each particle synthesized consisted of uniform and well-dispersed line primary particles of (La,Sr)MnO₃ and YSZ (0.1 μm particle size).     

Metal Nitrate-Urea Decomposition Route for Y-Ba-Cu-O Powder

Y-Ba-Cu-O powder, having desirable composition, particle size, compaction, and sintering properties, has been prepared by a novel combustion process involving metal nitrate-urea decomposition. Single-phase Y-Ba-Cu-O is obtained by reacting the mixture of yttrium, barium, and copper nitrates in 123 stoichiometry with urea at 900°C for a period of 1 h. Following grinding in acetone the powder possessed an average particle size of 1 μm, a surface area of 42.2 m²/g, a bulk density of 2.7 g/cm³, and could be sintered to 92% theoretical density at 930°C with the resulting material having a T _c of 90 K.     

Reformulation of an Aqueous Alumina Slip Based on Modification of Particle-Size Distribution and Particle Packing

Six alumina casting slips with particle-size distributions varying from 44 to 0.1 μm were examined. Particle packing was calculated using the approach of Andreasen. Viscosity, green density, and pore-size distribution were measured. It was found that contouring the intermediate size distribution for particles finer than 15 μm provided the most desirable viscosity for slips composed of wide size distributions. For slips containing 50 vol% solids, the lowest viscosity obtained was 196 × 10⁻³ N · s/m² (with a two-component size distribution), and a green density of 2.52 g/cm³ (65% of theoretical) was achieved with a ternary system. These casts had bimodal pore-size distributions centered around approximately 1 and 0.1μm.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Preparation and Characterization of Samaria-Doped Ceria Electrolyte Materials for Solid Oxide Fuel Cells

The microstructure, thermal expansion, mechanical property, and ionic conductivity of samaria-doped ceria (SDC) prepared by coprecipitation were investigated in this paper. The results revealed that the average particle size ranged from 10.9±0.4 to 13.5±0.5 nm, crystallite dimension varied from 8.6±0.3 to 10.7±0.4 nm, and the specific surface area distribution ranged from 62.6±1.8 to 76.7±2.2 m²/g for SDC powders prepared by coprecipitation. The dependence of lattice parameter, a, versus dopant concentration, x , of Sm³⁺ ion shows that these solid solutions obey Vegard's rule as a ( x )=5.4089+0.10743 x for Ce_{1− x} Sm _x O_{2−1/2 x} . For SDC ceramics sintered at 1500°C for 5 h, the bulk density was over 95% of the theoretical density; the maximum ionic conductivity, σ_800°C=(22.3±1.14) × 10⁻³ S/cm with minimum activation energy, E _a=0.89±0.02 eV, was found in the Ce_0.80Sm_0.20O_1.90 ceramic. A dense Ce_0.8Sm_0.2O_1.9 ceramic with a grain size distribution of 0.5–4 μm can be obtained by controlling the soaking time at 1500°C. When the soaking time was increased, the microhardness of Ce_0.8Sm_0.2O_1.9 ceramic increased, the toughness slightly decreased, which was related to grain growth with the soaking time.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.     

Barium Holmium Zirconate, A New Perovskite Oxide: II, Synthesis as Nanoparticles through a Modified Combustion Process

Nanoparticles of barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized using a modified self-propagating combustion process. The solid combustion products obtained were characterized by X-ray diffraction (XRD), electron diffraction, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, particle size analysis, surface area determination, and high-resolution transmission electron microscopy. The XRD and electron diffraction studies have shown that the as-prepared powder is phase pure Ba₂HoZrO_5.5 and has a complex cubic perovskite (A₂BB'O₆) structure with a lattice constant a = 8.428 Å. The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 4–16 nm with a mean grain size of 8.2 nm. The nanoparticles of Ba₂HoZrO_5.5 obtained by the present method could be sintered to 98% theoretical density at 1500°C.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

Reactive Laser Ablation Synthesis of Nanosize Alumina Powder

An aluminum (Al) target was laser ablated in an oxygen (O₂) atmosphere, producing nanosize alumina (Al₂O₃) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m²/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X-ray diffraction. These powders showed a direct transformation from γ- to α-alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing at 40 MPa.     

Dispersion Properties of BaTiO3 Colloids with Amphoteric Polyelectrolytes

An amphoteric water-soluble copolymer, i.e., poly(acrylamide/(α- N,N -dimethyl- N -acryloyloxyethyl) ammonium ethanate) (PAAM/DAAE), was evaluated as a novel dispersant for aqueous BaTiO₃ (BT) slurries. The dispersing property of this copolymer was examined by means of rheology, sedimentation, particle size, green density, zeta potential, and leached Ba²⁺ concentration measurements. The results indicate that PAAM/DAAE could reduce the viscosity of slurries greatly, cause BT particle sizes to shift to smaller values, and make green compacts more consolidated. Compared with a commercial dispersant, ammonium salt of poly(methacrylic acid) (PMAA-NH₄), it is as effective or even better in preparing stabilized suspensions. More importantly, PAAM/DAAE could lessen the leached Ba²⁺ concentration. This is related to the adsorption behavior of this copolymer onto BT particles, and the interaction between the adsorbed dispersant and dissolved barium ions.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

Shear Modulus and Yield Stress Measurements of Attractive Alumina Particle Networks in Aqueous Slurries

The shear modulus and yield stress of attractive alumina particle networks in aqueous slurries was determined as a function of volume fraction (0.1 to 0.5), pH (2, 4, 5, 6, and 9), and salt (NH₄l) concentration (0.25M to 2.34) using both vane and couette rheological tools. Consistent with previous observations concerning the relative strength of attractive particle networks, the shear modulus increased to a plateau value with salt concentration. In this work we have shown that the salt concentration at which this plateau value is achieved is a function of the pH, and thus, the surface charge density. The values of the shear modulus [G'], yield stress [τ_y], and yield strain [γ_y] of the attractive networks can be described with power law functions for particle volume fraction [φ] (G'∝φ^4.75, τ_y∝φ^3.6, and γ_y∝φ^−1.1) and salt concentration [c] (G'∝ [c]^2.0, τ, ∝ [c]^1.15, and γ_y∝ [c]^−0.85).