Journal of the American Ceramic Society

doi: 10.1111/j.1151-2916.2002.tb00468.xpmid: N/A

Jose, Rajan; John, Asha Mary; Koshy, Jacob

doi: 10.1111/j.1151-2916.2002.tb00469.xpmid: N/A

Barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized through liquid‐phase sintering for the first time. The conventional solid‐state reaction method using constituent oxides and carbonates was found to be inadequate for the synthesis of Ba2HoZrO5.5 material. During high‐temperature annealing, the development of stable BaZrO3 and BaHoO2.5 phases prevented the formation of Ba2HoZrO5.5 as a single‐phase material, even at 1650°C. However, an addition of a small amount of CuO (1 wt%) in the reaction mixture has resulted in the formation of an ordered complex perovskite Ba2HoZrO5.5 phase during the heating process. The structure of Ba2HoZrO5.5 was studied by X‐ray diffraction and found to have a cubic perovskite structure with a lattice constant of a= 8.482 Å. Dielectric constant and loss factor values of Ba2HoZrO5.5 are also in the range suitable for use as a substrate for microwave applications. The X‐ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction in YBa2Cu3O7−δ–Ba2HoZrO5.5 and Bi(2212)–Ba2HoZrO5.5 composites, even under extreme processing conditions. Dip‐coated and melt‐textured YBa2Cu3O7−δ and Bi(2212) thick films developed on polycrystalline Ba2HoZrO5.5 gave zero‐resistivity transition temperatures of Tc(0) = 92 and 85 K, respectively.

Jose, Rajan; John, Asha Mary; Divakar, Ramachandran; Koshy, Jacob

doi: 10.1111/j.1151-2916.2002.tb00470.xpmid: N/A

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 Ba2HoZrO5.5 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 Ba2HoZrO5.5 obtained by the present method could be sintered to 98% theoretical density at 1500°C.

Ghosh, Abhijit; Upadhyaya, Dharni D.; Prasad, Ram

doi: 10.1111/j.1151-2916.2002.tb00471.xpmid: N/A

A major component of the driving force for crystallization of hydrous zirconia–yttria powders comes from their noncrystallinity. Subsequent to dehydration of coprecipitated gels, the amorphous‐to‐crystalline phase transformation is the regime of primary crystallization. For the present study of 3.0‐, 5.0‐, and 9.0‐mol%‐yttria‐stabilized zirconia, the powders were produced as fine aggregates of ∼300 nm median diameter. The activation energy in all three cases was determined by the hot‐stage X‐ray diffraction technique where the powders were heated isothermally in the temperature range of 500°–600°C. The Avrami equation was used to calculate the rate constant value and the Arrhenius relationship was utilized to determine the value of the activation energy for primary crystallization.

Andersson, Karin M.; Bergström, Lennart

doi: 10.1111/j.1151-2916.2002.tb00472.xpmid: N/A

We used the colloidal probe technique with atomic force microscopy to study the interactions between a tungsten oxide sphere, WO3, and flat oxidized tungsten and cobalt surfaces in aqueous electrolytes. We investigated the effects of adsorption of cobalt ions to tungsten oxide surfaces and adsorption of polyethyleneimine (PEI). Low concentrations of cobalt ions added to a WO3 system resulted in extended hydration forces and a lowering of the absolute value of the surface potential. PEI was shown to adsorb to the WO3 surfaces and induce an electrosteric repulsion in both the symmetric (WO3–WO3) and asymmetric (WO3–CoOOH) systems. The possible complexation of cobalt ions with PEI does not significantly influence the thickness of the adsorbed layer.

Martinez, Carlos J.; Lewis, Jennifer A.

doi: 10.1111/j.1151-2916.2002.tb00473.xpmid: N/A

The rheological, structural, and stress evolution of aqueous alumina (Al2O3):latex tape‐cast layers of varying composition were studied by shear rheology, direct visualization, and a controlled environment stress measurement device. Their low shear viscosity was nearly independent of the alumina:latex ratio for binary mixtures whose particle size ratio (λ=D̄alumina:D̄latex) approached unity, but varied over an order of magnitude for systems with particle size asymmetry. Direct visualization of these mixtures revealed that particle flocculation occurred as their total solids loading increased. Their structure was characterized at intermittent points during the drying process by imaging freeze‐dried samples using scanning electron microscopy (SEM). Their corresponding stress histories exhibited three distinct regions: an initial period of stress rise, followed by a stress maximum, and, finally, a period of stress decay. Pure alumina layers exhibited a maximum stress of ∼1 MPa and a residual stress below 0.01 MPa. Pure latex films exhibited a maximum stress of ∼0.1 MPa and only a slight stress decay. The ceramic phase dominated the initial period of stress rise, while the latex phase strongly influenced the residual stress of composite layers cast from alumina:latex suspensions. Their maximum drying stress increased with decreasing Al2O3 particle size, whereas their residual stress increased with increasing latex Tg.

Riley, Daniel P.; Kisi, Erich H.; Hansen, Thomas C.; Hewat, Alan W.

doi: 10.1111/j.1151-2916.2002.tb00474.xpmid: N/A

In situ neutron diffraction at 0.9 s time resolution was used to reveal the reaction mechanism during the self‐propagating high‐temperature synthesis (SHS) of Ti3SiC2 from furnace‐ignited stoichiometric 3Ti + SiC + C mixtures. The diffraction patterns indicate that the SHS proceeded in five stages: (i) preheating of the reactants, (ii) the α→β phase transformation in Ti, (iii) preignition reactions, (iv) the formation of a single solid intermediate phase in <0.9 s, and (v) the rapid nucleation and growth of the product phase Ti3SiC2. No amorphous contribution to the diffraction patterns from a liquid phase was detected and, as such, it is unlikely that a liquid phase plays a major role in this SHS reaction. The intermediate phase is believed to be a solid solution of Si in TiC such that the overall stoichiometry is ∼3Ti:1Si:2C. Lattice parameters and known thermal expansion data were used to estimate the ignition temperature at 923 ± 10°C (supported by the α→β phase transformation in Ti) and the combustion temperature at 2320 ± 50°C.

Aguilar‐Santillán, Joaquín; Cuenca‐Alvarez, Ricardo; Balmori‐Ramírez, Heberto; Bradt, Richard C.

doi: 10.1111/j.1151-2916.2002.tb00475.xpmid: N/A

The influence of attrition milling on the thermal decomposition of kyanite (Al2O3·SiO2) to mullite (3Al2O3·2SiO2) and SiO2, and its subsequent sintering, was studied. A commercial kyanite was attrition‐milled for times up to 12 h. Dilatometry confirmed that as‐received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one‐half of the decomposition occurs in the last 35°C. For the attrition‐milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition‐milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering.

Santacruz, Isabel; Nieto, M. Isabel; Moreno, Rodrigo

doi: 10.1111/j.1151-2916.2002.tb00476.xpmid: N/A

Previous works have shown that carrageenan can be successfully used in the aqueous gelforming of powders, because carrageenan forms a firm gel, similar to that formed by agarose, but at a much lower cost. In this work, the synergistic effect of carrageenan with locust bean gum is studied. The rheological behavior of 2 wt% solutions of these polysaccharides and their mixtures are measured under mixing conditions (60°C) and by recording the viscosity and elastic modulus on cooling. The effect of the addition of these solutions to 50 vol% alumina slurries up to a concentration of 0.5 wt% is studied. Although gelling time increases, the resulting gels are stronger than for carrageenan alone. Gelcast alumina bodies with green and sintered densities of 57% and 97.6% of theoretical have been obtained.

Goswami, Ramasis; Herman, Herbert; Sampath, Sanjay; Parise, John; Zhu, Yimei; Welch, David

doi: 10.1111/j.1151-2916.2002.tb00477.xpmid: N/A

Shock synthesis of cubic BN (c‐BN) was accomplished using high‐velocity thermal spray. Al‐8Si‐20BN (hexagonal BN, h‐BN) composite powders were injected into a high‐energy flame where the particles partially melted and accelerated to impact on steel substrates. The shock wave generated by the sudden impact of the droplets propagated through the underlying deposit, which experienced a polymorphic transition to high‐pressure forms. Transmission electron microscopy revealed that the deposits contained platelike c‐BN embedded in h‐BN. The c‐BN formed with a specific orientation relation, with (0001) h‐BN parallel to (111) c‐BN and [11–20] h‐BN nearly parallel to [2–20] c‐BN. As a result of shock propagation, a number of half‐Frank loops with Burgers vector ½[0001] were introduced and thin (∼3 nm) layers of amorphous BN (a‐BN), parallel to the (0002) plane, were formed. Significant shearing of the (10–10) plane with respect to the c‐axis, delamination, and kink bands were also observed in most of the h‐BN grains. The Hugoniot pressure calculation suggested that the impact pressure was sufficient to trigger the nucleation of c‐BN.