Creep in TiO2 doped Zirconia: Implications for high strain rate superplasticity

In ceramics, dopants offer the possibility of higher creep rates by enhancing diffusion. The present study examines the potential for high strain rate superplasticity in a TiO2 doped zirconia, by conducting creep experiments together with microstructural characterization. It is shown that both pure and doped zirconia exhibit transitions in creep behaviour from Coble diffusion creep with n similar to 1 to an interface controlled process with n similar to 2. Doping with TiO2 enhances the creep rate by over an order of magnitude. There is evidence of substantial grain boundary sliding, consistent with diffusion creep.

Combustion synthesis of oxide materials for nuclear waste immobilization

Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450 degrees C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and Si-29 MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85-95% dense ceramics in the temperature range 1000 degrees-1300 degrees C.

Precipitation in small systems-I. stochastic analysis

Precipitation in small droplets involving emulsions, microemulsions or vesicles is important for Producing multicomponent ceramics and nanoparticles. Because of the random nature of nucleation and the small number of particles in a droplet, the use of a deterministic population balance equation for predicting the number density of particles may lead to erroneous results even for evaluating the mean behavior of such systems. A comparison between the predictions made through stochastic simulation and deterministic population balance involving small droplets has been made for two simple systems, one involving crystallization and the other a single-component precipitation. The two approaches have been found to yield quite different results under a variety of conditions. Contrary to expectation, the smallness of the population alone does not cause these deviations. Thus, if fluctuation in supersaturation is negligible, the population balance and simulation predictions concur. However, for large fluctuations in supersaturation, the predictions differ significantly, indicating the need to take the stochastic nature of the phenomenon into account. This paper describes the stochastic treatment of populations, which involves a sequence of so-called product density equations and forms an appropriate framework for handling small systems.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Coarse (BOn/2)-O-n+/xH(2)O (10ceramics in terms of operational cost and procedural simplicity.

Generalization of Abrams' law

Considering cement based composites as chemically bonded ceramics (CBC) the consequent strength development with age is essentially a constant volume solidification process, such that the hydrated gel particles fill the space resulting in the compatible gel space ratios. Analysis has been done of the extensively used graphical method of mix design (British method of mix design) i.e., the relation between the compressive strength and the free water - cement ratio. By considering the strength (S) at w/c 0.5 (S-0.5) as the reference state to reflect the synergetic effects between constituents of concrete a generalized relationship obtained is of the form {S/S-0.5} = a + b {1/(w/c)}.

Microstructure control and wear of Al2O3-SiC-(Al,Si) composites made by melt oxidation

Ceramic matrix composites of Al2O3-SiC-(Al,Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of Al2O3, alloy, and porosity in the composite can be controlled by proper selection of SLC particle size and the processing temperature. The wear resistance of composites was evaluated in pin-on-disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.

Pyroelectric and Electrostrictive Properties of (1 –x–y) PZN ·xBT ·yPT Ceramic Solid Solutions

The pyroelectric and electrostrictive properties of lead zinc niobate-lead titanate-barium titanate (PZN-BT-PT) ceramic solid solution were investigated. These properties of the (1 - x)PZN.xBT series were qualitatively explained with a composition fluctuation model. The pyroelectric depolarization temperatures of (1 - x - y)PZN.xBT.yPT ceramics were utilized to select compositions for room-temperature electrostrictive applications. Among them, 0.85PZN.0.10BT.0.05PT ceramic with Q11 = 0.018 m4/C2, Q12 = -0.0085 m4/C2, S2 at 25 kV/cm = -6.1 x 10(-4), T(max) = 75-degrees-C at 1 kHz, and T(t) = 27-degrees-C shows optimum properties for micropositioner applications.

Reactivity Of Silica From Rice Husk With Lime

The paper correlates the reactivity of rice husk ash with its physicochemical properties such as crystallinity, surface area, microstructure, particle size distribution, porosity and solubility. These properties, in tum, are dependent on the time-temperature conditions under which the ash is prepared. It is found that the reactivity of the ash cannot be quantified by any one of these parameters alone, though they all indicate it qualitatively. Therefore, a method for quantifying this property was developed, by which the Reactivity Index is obtained. There is only a gradual change in the reactivity index of RHA with ashing temperature, as in many other properties, like surface area, porosity and total volume of gas absorbed by unit mass of the silica ash. This reactive index is found to be useful in determining the optimum ash/lime ratios required to give the best performance for RHA-lime composites.

Nucleation and growth characteristics of cavities during the early stages of tensile creep deformation in a superplastic zirconia-20 wt% alumina composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.

Piezoelectrically stimulated dielectric dispersion in KAP single crystals

The frequency response of the dielectric constant (epsilon(r)), the loss tangent (tan delta) and impedance Z of potassium acid phthalate (KAP) single crystals, monitored along the polar axis, exhibit strong resonances in the frequency range 50-200 kHz, depending on the dimensions of the sample. The observed resonance effect, which is strongly dependent on the geometric shape and size of the sample, is attributed to its piezoelectric nature. The resonance peak positions have been monitored as a function of both temperature and uniaxial pressure. The stiffness coefficient (C), computed based on the resonance data, is found to decrease with increasing temperature and increase with increasing pressure. The electro-mechanical coupling coefficient (k), obtained by resonance-anti-resonance method, has also been found to increase with rise in temperature. The epsilon(r) behaviour along the polar axis, as a function of temperature is consistent with that of k. The preliminary results on the influence, of partial replacement of K+ ions in the KAP crystal by Cs+ and Li+ ions, on the observed piezoelectric resonance effects are also included.

Some microbiological aspects of bauxite mineralization and beneficiation

A microbial survey of Jamnagar bauxite mines in Gujarat, India, revealed the indigenous presence of a variety of autotrophic and heterotrophic bacteria and fungi associated with the ore body and water ponds in the vicinity. Among these, bacteria belonging to the genera Thiobacillus, Bacillus and Pseudomonas are implicated in the weathering of aluminosilicates; the precipitation of iron oxyhydroxides; the dissolution and conversion of alkaline metal species; and the formation of alumina, silica and calcite minerals. Fungi belonging to the genus Cladosporium can reduce ferric iron and dissolve alumina silicates. Biogenesis thus plays a significant role in bauxite mineralization. Various types of bacteria and fungi, such as Bacillus polymyxa, Bacillus coagulans and Aspergillus niger, were found to be efficient in significant calcium solubilization and partial iron removal from bauxite ore. Probable mechanisms in the biobeneficiation process are analyzed. Biobeneficiation is shown to be an effective technique for the removal of iron and calcium from bauxite ores for use in refractories and ceramics.

Preparation and characterization of nanocrystalline powders of bismuth vanadate

The influence of mechanical activation on the formation of Bi2VO5.5 bismuth vanadate (BiV) phase, was investigated by ball-milling a stoichiometric mixture of bismuth oxide and vanadium pentoxide. The structural evolution of the desired BN phase, via an intermediate BiVO4,phase, was investigated using X-ray powder diffraction; (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). Milling for 54h. yielded monophasic gamma-BiV powders with an average crystallite size of 30 nm. The electron paramagnetic resonance (EPR) peaks associated with the V4+ ions are stronger and broader in nanocrystalline (n) BN than in the conventionally prepared microcrystalline (m) BN, suggesting theta significant portion of V5+ has been transformed to V4+ during milling. The optical bandgap of n-BiV was found to be higher than that of m-BiV. High density (97% of the theoretical density), fine-grained (average grain-size of 2 tun) ceramics with uniform grain-size distribution could be fabricated using n-BiV powders. These fine-grained ceramics exhibit improved dielectric, pyre and ferroelectric properties. (C) 1999 Elsevier Science S.A. All rights reserved.

Structural, dielectric and optical properties of lithium borate-bismuth tungstate glass-ceramicsc

Glasses in the system (1 - x)Li2B4O7-xBi(2)WO(6) (0.1 less than or equal to x less than or equal to 0.35) were prepared by splat quenching technique. Powder X-ray diffraction (XRD) and differential thermal analysis (DTA) were employed to characterize the as-quenched glasses. High-resolution transmission electron microscopy (HR TEM) revealed the presence of fine, nearly spherical crystallites of Bi2WO6 varying from 1.5 to 20 nm in size, depending on x in the as-quenched glasses. The glasses (corresponding to x = 0.3) heat-treated at 723 K for 6 h gave rise to a clear crystalline phase of Bi2WO6 embedded in the Li2B4O7 glass matrix, as observed by X-ray studies. The dielectric constants of the as-quenched glasses as well as the glass-ceramics decreased with increase in frequency (40Hz-100 kHz) at 300 K, and the value obtained for the glass-ceramic (x = 0.2) is in agreement with the values predicted using Maxwell's model and the logarithmic mixture rule. The dielectric constants for both the as-quenched glass and the glass-ceramic increased with increase in temperature (300 - 873 K) and exhibited anomalies close to the onset of the crystallization temperature of the host glass matrix. The optical transmission properties:of these glass-ceramics were found to be compositional dependant. (C) 2000 Elsevier Science Ltd.

Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SIC,) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading. and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.

Wear of Al2O3-SiC-(Al,Si) melt oxidised ceramic composites

Al2O3-SiC-(Al,Si) cermets are fabricated using the melt oxidation route. The tribological properties of the composites are tested under adhesive sliding and two body abrasion conditions. Under adhesive conditions, the network of residual aluminium in the matrix plays a role in the formation of a thin tribofilm on the interface while in abrasion the hardness of the composite plays a prominent tribological role. The work suggests that microstructural control can make this low temperature composite competitive with the conventional high temperature monolithic ceramics. (C) 1999 Published by Elsevier Science S.A. All rights reserved.