Journal of the American Ceramic Society

Jo, Wook; Kim, Doh‐Yeon; Hwang, Nong‐Moon

doi: 10.1111/j.1551-2916.2006.01160.xpmid: N/A

The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid‐phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion‐controlled, because there is little energy barrier for atomic attachments. On the other hand, kink‐generating sources such as screw dislocations or two‐dimensional (2‐D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on a 2‐D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that a densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain‐boundary structural transition from an ordered to a disordered open structure.

Funakoshi, Kunio; Nonami, Toru

doi: 10.1111/j.1551-2916.2006.01011.xpmid: N/A

The crystallization of anatase titanium dioxide (TiO2) was achieved by a hydrolysis reaction of titanium alkoxide without annealing. The hydrolysis reaction rates of tetraethyl orthotitanate were indicated by a function of the concentration of acetylacetone added. The degree of crystallinity of the product particles was influenced by the amounts of acetylacetone and seed crystals. Anatase TiO2 was crystallized by restraining the rapid increase in supersaturation of TiO2 and the consequent nucleation of amorphous TiO2. The degree of crystallinity of the product particles also changed with the types of seed crystals used, and was strongly influenced by the specific surface areas of the seed crystals.

Etin, Aleksey; Shter, Gennady E.; Baltianski, Sioma; Grader, Gideon S.; Reisner, George M.

doi: 10.1111/j.1551-2916.2006.01078.xpmid: N/A

Elemental depth profiles of PZT films prepared by two sol–gel formulations, differing in the zirconium precursor stabilization, were investigated by SIMS analysis. Early decomposition of the zirconium precursor yielded opposing gradients of zirconium and titanium, while simultaneous late decomposition of zirconium and titanium precursors provided profile uniformity. The gradients formed during initial crystallization are irreversible. Both types of films showed excellent hysteresis; however, uniform films exhibited a much higher dielectric constant, indicating superior piezoelectric properties. Non‐uniform films displayed a complex CV pattern, consistent with an inhomogeneous structure. Finally, thermal decomposition of the individual metal precursors is crucial for controlling film uniformity.

Moritz, Tassilo; Richter, Hans‐Jürgen

doi: 10.1111/j.1551-2916.2006.01081.xpmid: N/A

Ceramic bodies with a complex shape and closed ceramic shells encapsulating other components like steel parts were fabricated by the freeze‐casting technique using ice as a mold and as core material. The ice molds and cores were simultaneously removed with the frozen suspension liquid of the ceramic slip by subsequent freeze drying. In this way, cores can be eliminated from a closed shell by sublimation through the porosity of the shell. Moreover, the ice cores allowed to transfect other components into porous ceramic bodies. Complex dental parts such as steel rods encapsulated in porous zirconia shells achieved by this ice mold freeze casting are represented in this article.

Venet, M.; Vendramini, A.; Garcia, D.; Eiras, J. A.; Guerrero, F.

doi: 10.1111/j.1551-2916.2006.01082.xpmid: N/A

The processing of a PbNb2O6 (PN) ceramic was investigated and optimized. Through thermal gravimetric and X‐ray diffraction analysis the optimal calcination condition and the starting firing temperature were estimated. The powders were submitted to thermal treatments to obtain PN ceramic powders with an orthorhombic ferroelectric phase. The effect of PbO excess addition to compensate the PbO losses, during the calcination and the firing, was also studied. The highest density value was found to be 96% of the theoretical density. The temperature dependence of the dielectric properties, for samples sintered at different temperatures, allowed the study of the sintering kinetic.

Basu, Bikramjit; Venkateswaran, T.; Kim, Doh‐Yeon

doi: 10.1111/j.1551-2916.2006.01083.xpmid: N/A

In a recent work,1 we have reported the optimization of the spark plasma sintering (SPS) parameters to obtain dense nanostructured 3Y‐TZP ceramics. Following this, the present work attempts to answer some specific issues: (a) whether ZrO2‐based composites with ZrB2 reinforcements can be densified under the optimal SPS conditions for TZP matrix densification (b) whether improved hardness can be obtained in the composites, when 30 vol% ZrB2 is incorporated and (c) whether the toughness can be tailored by varying the ZrO2–matrix stabilization as well as retaining finer ZrO2 grains. In the present contribution, the SPS experiments are carried out at 1200°C for 5 min under vacuum at a heating rate of 600 K/min. The SPS processing route enables retaining of the finer t‐ZrO2 grains (100–300 nm) and the ZrO2–ZrB2 composite developed exhibits optimum hardness up to 14 GPa. Careful analysis of the indentation data provides a range of toughness values in the composites (up to 11 MPa·m1/2), based on Y2O3 stabilization in the ZrO2 matrix. The influence of varying yttria content, t‐ZrO2 transformability, and microstructure on the properties obtained is discussed. In addition to active contribution from the transformation‐toughening mechanism, crack deflection by hard second phase brings about appreciable increment in the toughness of the nanocomposites.

Kumar, Vinod; Khamba, J. S.

doi: 10.1111/j.1551-2916.2006.01085.xpmid: N/A

The aim of this article is to present the ultrasonic machining of an alumina‐based engineering ceramic composite. The machining mechanism and the behavior of certain tool material and work material combinations with respect to penetration rate and tool‐wear rate under the influence of different parameters in ultrasonic machining are discussed in this paper. The shocking movement and cutting mechanics of the abrasive particle under the tip of the vibrating tool have been observed using high‐speed photographic equipment. The machined surface has been analyzed by using a scanning electron microscope. Surface roughness values were measured and plotted at entry and at exit of the holes for different types and sizes of holes using the surface roughness‐measuring instrument “Perthometer.”

Studart, André R.; Amstad, Esther; Antoni, Mathieu; Gauckler, Ludwig J.

doi: 10.1111/j.1551-2916.2006.01087.xpmid: N/A

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.

Wu, Jeremy P.; Rawlings, Rees D.; Boccaccini, Aldo R.; Dlouhy, Ivo; Chlup, Zdenek

doi: 10.1111/j.1551-2916.2006.01089.xpmid: N/A

A novel glass–ceramic material was developed from the melt of a TiO2‐containing iron‐making slag with additional waste glass. The high percentage (∼20 wt% TiO2) of this network‐modifying oxide has promoted a crystallization of the parent glass, resulting in a fine‐grained, homogeneous polycrystalline material with high mechanical properties (E=120 GPa, flexural strength=∼180 MPa, and Vickers hardness=7 GPa) after a heat treatment at 1100°C for 2 h. The room temperature and elevated temperature fracture toughness were also studied. The main crystalline phases of the glass–ceramic material were of the pyroxene series until heat‐treatment temperature reached 1000°C, at which titanium‐rich perovskite and armalcolite crystals became the dominant phases. The end material is high‐strength, aesthetically acceptable (metallic gray or opaque brown colored), and suitable for structural and architectural applications.

Dudley, Shannon; Kalem, Tugba; Akinc, Mufit

doi: 10.1111/j.1551-2916.2006.01091.xpmid: N/A

Diatom frustules were used as bio‐templates to synthesize functional ceramics via solid–gas displacement reactions. Silica‐based frustules were exposed to TiF4 at 330°C to form TiOF2, which was later converted to TiO2 (anatase) by heat treatment in air at 600°C. The TiO2 frustules were then exposed to molten Ba(OH)2 or Sr(OH)2 to form BaTiO3 or SrTiO3, respectively. In both cases, near‐complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO3 and SrTiO3 frustules exhibit nearly phase pure, nanocrystalline perovskite structure.