Journal of the American Ceramic Society

Wyss, Hans M.; Tervoort, Elena V.; Gauckler, Ludwig J.

doi: 10.1111/j.1551-2916.2005.00622.xpmid: N/A

It is often assumed that the viscoelastic properties of dense colloids are determined by the colloid volume fraction, the interaction potential, as well as the particle size distribution and shape. The dependence of the viscoelastic behavior of particle suspensions and gels on these parameters has been widely studied, and is well understood in many cases. In contrast, our knowledge on the influence of microstructure on mechanical and rheological properties, in particular for high solid loading suspensions as used in ceramic processing, is much less developed. This aspect has been the focus of recent experiments, which show that small changes in microstructure can have dramatic effects on the mechanics and dynamics of concentrated colloidal gels. In this article, we attempt to give an overview of the influence of microstructure on the mechanical and rheological properties of colloidal systems. Particular attention is given to colloidal particle gels at high volume fractions.

Hwang, K. S.; Hsieh, C. C.

doi: 10.1111/j.1551-2916.2005.00370.xpmid: N/A

It is generally accepted that MgO additions enhance the sintering of alumina. The distribution of the low concentration required is, however, difficult to attain and critical to the final properties and microstructures. To improve the MgO distributions in ceramic injection‐molded (CIM) alumina, Mg‐containing precursors, Mg‐stearate and Mg‐acetate, were added during the kneading step of the CIM process, and the results were compared with those from adding MgO particles. Of the three additives, the Mg‐stearate showed the highest sintered density and bending strength. This was attributed to its high molecular weight and it being able to mix with other binder components homogeneously in the liquid state, which leads to a better distribution of the MgO. In contrast, the MgO powder was kneaded in the solid particle form, and its distribution was not uniform. The Mg‐acetate also imparted poorer MgO distribution because the liquid Mg‐acetate dehydrated in the early stage of the kneading and formed solid lumps before it was well mixed into the binder system. The resulting sintered densities and bending strength using these two additives were, therefore, inferior to those for specimens in which Mg‐stearate was used.

Cock, Alex M.; Shapiro, Ian P.; Todd, Richard I.; Roberts, Steve G.

doi: 10.1111/j.1551-2916.2005.00443.xpmid: N/A

Alumina and alumina‐based “nanocomposites” with 2 and 5 vol% silicon carbide and varying amounts of yttria (0–1.5 wt%) have been prepared by pressureless sintering in the temperature range 1450°–1650°C. The effects of composition and sintering temperature on density and microstructure are reported. Yttria inhibited sintering in alumina, but enhanced the sinterability of the nanocomposites. It also induced abnormal grain growth in both alumina and nanocomposites, but strongly bimodal grain size distributions could be prevented by careful choice of the composition and the sintering temperature. Fully dense (>99%), fine‐grained alumina–5 vol% SiC–1.5 wt% yttria nanocomposites were produced from uniaxially pressed powders with a yttria content of 1.5 wt% and a sintering temperature of 1600°C. Reasons for this behavior are discussed, and it is suggested that the enhancement of sintering in the alumina–SiC materials is because of the reaction of silica on the surface of the silicon carbide particles with alumina, yttria, and possibly magnesia, modifying the grain boundary composition, resulting in enhanced grain boundary diffusion. scanning transmission electron microscopy/energy‐dispersive X‐ray data show that such co‐segregation does occur in the yttria‐containing nanocomposites.

Babilo, Peter; Haile, Sossina M.

doi: 10.1111/j.1551-2916.2005.00449.xpmid: N/A

The influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr0.85Y0.15O3–δ was readily sintered to above 93% of theoretical density at 1300°C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO2 showed ZnO‐modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H2O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ∼0.9 at 600°C. The combination of electrical and chemical properties and good sinterability render ZnO‐modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications.

Wu, Suxing; Chan, Helen M.; Harmer, Martin P.

doi: 10.1111/j.1551-2916.2005.00450.xpmid: N/A

Tantalum (V) oxide (Ta2O5) has potential applications as part of an environmental barrier coating system for Si3N4‐based turbine components. However, at elevated temperatures, Ta2O5 undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta2O5 microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X‐ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta2O5, transformation of the low‐temperature β‐phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta2O5 was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO4 second and phase particles were seen to be highly effective at inhibiting grain growth.

Buscaglia, Maria Teresa; Bassoli, Marta; Buscaglia, Vincenzo; Alessio, Rocco

doi: 10.1111/j.1551-2916.2005.00451.xpmid: N/A

Barium titanate has been prepared by solid‐state reaction of nanocrystalline TiO2 (70 nm) with BaCO3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO3 in the solid‐state synthesis of barium titanate. The use of nanocrystalline BaCO3 already leads to formation of the single‐phase BaTiO3 after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d50=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid‐state reaction of nanocrystalline BaCO3 and TiO2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.

Seok, Sang Il; Lim, Mi Ae; Ju, Jung‐Jin; Lee, Myung‐Hyun

doi: 10.1111/j.1551-2916.2005.00454.xpmid: N/A

Er‐doped Al2O3–SiO2 (1/9 in mol ratio of Al2O3/SiO2) thin films were prepared by using a modified sol–gel process. The modified process entails the precipitation and digestion of Er(OH)3, obtained from the reaction between Er ions and NH4OH in solution. Thin films were deposited on Si wafers by using a spin coating technique (3000 rpm) and the coated films were heat treated at different temperatures for 1 h in an oxygen‐purged furnace. All the films were structurally characterized by the X‐ray diffraction technique using CuKα radiation. Refractive indices and the morphologies of the films were studied using a spectroscopic phase modulated ellipsometer and atomic force microscopy, respectively. It was observed that the films were crack free and of about 0.4 μm thickness in a single spin coating and both the lifetime and the photoluminescence intensity of Er ions increased with increasing the annealing temperature. The luminescence properties of the Er‐doped Al2O3–SiO2 made by a conventional and our modified doping process were compared and discussed from the stand point of peak intensities and lifetimes as a function of annealing temperatures. It is to be noted here that our modified process was found to be more effective in reducing the clustering of Er ions in Al2O3–SiO2 materials as compared to that of the conventional method.

Shim, Seung Hwan; Shim, Kwang Bo; Yoon, Jong‐Won; Kawaguchi, Kenji; Koshizaki, Naoto

doi: 10.1111/j.1551-2916.2005.00465.xpmid: N/A

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.

Wildhack, Stefanie; Rixecker, Georg; Aldinger, Fritz

doi: 10.1111/j.1551-2916.2005.00467.xpmid: N/A

The impact of different additives on the hydrolysis of AlN powder in aqueous suspensions at room temperature was studied. The results show that citric acid and polyacrylic acid are most effective in chemically protecting AlN against hydrolysis. The protected powder is hydrophilic, which facilitates aqueous processing, and the chemical stability is retained when basic dispersing agents are added. Based on these results, the solid loading of the aqueous slurries was maximized by utilizing bimodal particle size distributions. Combining bimodal powders with the dispersants Dolapix and citric acid, colloidally stable slurries with solid loadings in excess of 50 vol% were obtained.

Srikanth, Srinivasan; Ray, Ajay Kumar; Bandopadhyay, Amitava; Ravikumar, Bandari; Jha, Animesh

doi: 10.1111/j.1551-2916.2005.00471.xpmid: N/A

The phase constitution during the sintering of pure red mud and red mud–fly ash mixtures was studied in the temperature range of 900°–1250°C. The phases formed at different sintering temperatures were analyzed by X‐ray powder diffraction. The phases that are likely to form at equilibrium at any isotherm were predicted using the Gibbs free energy minimization technique and the databases provided in the FactSage software. Although the thermodynamic prediction is in reasonable agreement with the experimental results for the major phases, there is some disagreement regarding the minor phases, especially the more complex phases. The major limitation of the thermodynamic approach presently is the non‐availability of thermodynamic data for several complex and multi‐component solution phases.