Journal of the American Ceramic Society

Shin, Jason; Ahn, Do‐Hwan; Shin, Mee‐Shik; Kim, Yong‐Seog

doi: 10.1111/j.1151-2916.2000.tb01324.xpmid: N/A

The synthesis of aluminum nitride (AlN) via self‐propagating high‐temperature synthesis (SHS) was attempted, using aluminum powder that was mixed with AlN powder as a diluent. The AlN content in the reactant was varied over a range of 30%–70%, and the nitrogen pressure was varied over a range of 0.1–1.0 MPa. The SHS reaction that was performed using a reactant that contained 50% AlN diluent, under a nitrogen‐gas pressure of 0.8 MPa, yielded the highest conversion ratio of aluminum powder to AlN powder. A mechanism for the reaction of aluminum with nitrogen gas during the SHS process was discussed, based on observations of the microstructures of the reaction zone and products.

Durán, Pedro; Moure, Carlos; Villegas, Marina; Tartaj, Jesús; Caballero, Amador C.; Fernández, José F.

doi: 10.1111/j.1151-2916.2000.tb01325.xpmid: N/A

This paper describes the preparation of homogeneous Bi7Ti4NbO21 single‐phase ceramic powders of ∼55 nm crystallite size, at temperatures as low as 400°–500°C using a metal citrate complex method based on the Pechini‐type reaction route. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, and SEM, led to the formation of a well‐defined orthorhombic Bi7Ti4NbO21 compound with lattice parameters a= 0.544, b= 0.540, and c= 2.905 ± 0.0005 nm. Reaction takes place through an intermediate binary phase with a stoichiometry close to Bi20TiO32 which forms between 300° and 375°C. The metal‐organic precursor synthesis method, where Bi, Ti, and Nb ions are first chelated to form metal complexes and then polymerized to give a gel, allows control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of nanosized bismuth titanate niobate (Bi7Ti4NbO21) ceramic powders, at temperatures much lower than usually needed by conventional processing of mixed‐oxide powders.

Agrafiotis, Christos; Tsetsekou, Athena; Leon, Ioanna

doi: 10.1111/j.1151-2916.2000.tb01326.xpmid: N/A

Recently, a novel automotive catalyst design, based on the use of palladium supported on yttria‐stabilized zirconia (YSZ), was proposed. In the present work, the coating of cordierite honeycomb samples with YSZ slurries for the preparation of such washcoats was investigated. The loading percentage, homogeneity, and reproducibility were found to depend strongly on the slurry viscosity. Parameters such as the slurry‐solids content, pH, type of powder used, and use and quantity of the dispersants were optimized for the preparation of stable, low‐viscosity YSZ slurries, adjustments that could lead to an efficient coating process for honeycomb structures.

Clegg, William J.

doi: 10.1111/j.1151-2916.2000.tb01327.xpmid: N/A

The effect of carbon on the sintering of boron‐doped SiC was studied. The free carbon present in the green compact was found to react with the SiO2 covering the surfaces of the SiC particles; however, even if no carbon was added, the surface SiO2 reacted with the SiC itself at a slightly higher temperature. This latter reaction was associated with the onset of substantial pore growth in the shrinking green body, which, as the pores continued to grow at higher temperatures, prevented complete densification. Therefore, the reaction of the SiC with the SiO2 may have led to the fracture of interparticle contacts, resulting in the onset of coarsening. Thus, the role of the carbon was to prevent reaction between the SiC and the surface SiO2, by removing the SiO2 at a temperature below that at which this reaction could occur.

Hirata, Yoshihiro; Matsura, Tsuyoshi; Hayata, Kazunori

doi: 10.1111/j.1151-2916.2000.tb01328.xpmid: N/A

Incorporating Si‐Ti‐C‐O fabric into a mullite matrix is expected to increase the fracture energy of mullite ceramics. The present paper describes the processing of an Si‐Ti‐C‐O fabric/mullite/polytitanocarbosilane composite. A polytitanocarbosilane (a precursor of Si‐Ti‐C‐O fiber)/xylene solution was infiltrated into a laminated porous mullite composite with 35–37 vol% fabric and thermally decomposed to an amorphous solid at 1000°C, in an argon atmosphere, to decrease the porosity and residual stress induced by the difference in thermal and mechanical properties between the Si‐Ti‐C‐O fabric and the mullite. The decrease in porosity of the composite with pyrolysis of the precursor polymer was analyzed theoretically, and those results were used to control the effective experimental parameters. The infiltration/pyrolysis process was repeated eight times to produce a composite of 90.4% theoretical density. The composite exhibited significant pseudoductility, with a fracture energy of 11.4 kJ/m2 and a flexural strength of 290 MPa.

Fang, Jiye; Wang, John; Gan, Leong‐Ming; Ng, Ser‐Choon; Ding, Jun; Liu, Xiangyuan

doi: 10.1111/j.1151-2916.2000.tb01329.xpmid: N/A

A fine strontium ferrite powder with high coercivity was successfully prepared by forming hydroxide precursor particles in the continuous ethanol‐based phase of a microemulsion consisting of iso‐octane, NP9, and an ethanol solution containing Sr2+ and Fe3+ cations at a molar ratio of 1:12. The microemulsion‐derived hydroxide precursor was calcined at various temperatures, ranging from 600° to 1100°C, to develop the hexagonal strontium ferrite phase. X‐ray diffractometry and infrared characterizations revealed that the formation mechanisms of strontium ferrite in the microemulsion‐derived precursor differed from those of the precursor derived by conventional coprecipitation. The microemulsion resulted in a strontium ferrite of finer particle size and better magnetic properties than those of the conventionally coprecipitated strontium ferrite. The microemulsion‐derived strontium ferrite exhibited an intrinsic coercivity of 6195 Oe and a saturation magnetization of 58.28 emu/g when calcined at 900oC. The saturation magnetization increased further, to 69.75 emu/g, when the microemulsion‐derived precursor was calcined at 1100oC.

Kozuka, Hiromitsu; Kajimura, Masahiro

doi: 10.1111/j.1151-2916.2000.tb01330.xpmid: N/A

BaTiO3 coating films were prepared from Ba(CH3COO)2–Ti(OC3H7i)4–H2O–CH3COOH–C3H7iOH solutions containing poly(vinylpyrrolidone) (PVP) via single‐step, nonrepetitive dip coating. The critical thickness—i.e., the maximum film thickness achieved without crack formation via nonrepetitive dip coating—was successfully increased by incorporation of PVP in the precursor solution. Relatively dense, crack‐free BaTiO3 films >1 μm in thickness were achieved via single‐step deposition using a solution containing PVP of average molecular weight of 630 000. Incorporation of an excess amount of PVP, however, led to a decrease in the critical thickness. Higher‐molecular‐weight PVP was more effective in increasing the critical thickness, whereas N‐vinyl‐2‐pyrrolidone monomers did not affect the critical thickness. Stepwise heating of the gel films resulted in increased optical transmittance of the films, accompanied by film densification.

Takeda, Michio; Saeki, Akinori; Sakamoto, Jun‐ichi; Imai, Yoshikazu; Ichikawa, Hiroshi

doi: 10.1111/j.1151-2916.2000.tb01331.xpmid: N/A

SiC‐based fibers with various chemical compositions were synthesized using an irradiation‐curing process. Polycarbosilane (PCS) fibers were cured by irradiation with an electron beam in a helium atmosphere. The cured PCS fibers were pyrolyzed at 1300°C under controlled hydrogen or argon atmospheres, and SiC fibers with C/Si of 0.84 to 1.56 were obtained. The fibers consisted of <1.0 wt% O, <0.2 wt% N, <0.1 wt% H, with the balance being Si and C. The mechanism of pyrolytic transformation of cured PCS to SiC‐based ceramics was investigated using TG/DTA analysis. Greater mass losses were observed during pyrolysis in a hydrogen atmosphere than in argon. This result suggests that the hydrogen atmosphere suppresses H2 evolution and helps to remove excess carbon as CH4 during pyrolysis. The microstructure and mechanical properties of the resulting SiC‐based fibers were found to be very dependent on their C/Si chemical compositions.

Cazalla, Olga; Rodriguez‐Navarro, Carlos; Sebastian, Eduardo; Cultrone, Giuseppe; Torre, Maria Jose

doi: 10.1111/j.1151-2916.2000.tb01332.xpmid: N/A

The influence of storing slaked lime under water for extended periods of time (i.e., aging) on Ca(OH)2 crystal morphology, texture, and carbonation evolution of various lime mortars has been studied by the combined use of X‐ray diffractometry, phenolphthalein tests, porosity measurements, electron microscopy, and ultrasonic wave propagation analyses. Mortars prepared using traditional aged lime putties (up to 14 years storage under water) show rapid, extensive carbonation, resulting in porosity reduction and ultrasonic speed increase. The aged hydrated lime mortar carbonation reaction (i.e., Ca(OH)2+ CO2= CaCO3+ H2O) follows a complex diffusive path, resulting in periodic calcite precipitation as Liesegang rings. In this case, binder:aggregate ratios >1:4 result in crack development. Nonaged commercial hydrated lime mortars show slower carbonation and need a higher binder:aggregate ratio (1:3). The carbonation of nonaged lime mortars follows a normal diffusion‐limited continuous path progressing from the mortar sample surface toward the core. Differences between aged and nonaged lime mortar carbonation evolution are explained considering Ca(OH)2 crystal shape changes (from prisms to platelike crystals) and size reduction that occurs on aging of lime putty. Implications of these results on historic building conservation using traditional lime mortars are discussed.

Xia, Bin; Duan, Lianyun; Xie, Youchang

doi: 10.1111/j.1151-2916.2000.tb01333.xpmid: N/A

ZrO2 powder is prepared by low‐temperature vapor‐phase hydrolysis of ZrCl4. TG‐DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as‐prepared ZrO2 powder is characterized by large surface area (150 m2/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as‐prepared ZrO2 powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.