Studies on the oxidation behaviour of intermetallic compounds

The oxidation behaviour of the AB₅ type intermetallic compounds CaNi₅ and LaNi₅ has been studied at different temperatures ranging from 100 to 800° C. The kinetic results indicate that the element A (Ca, La) is first oxidized rapidly followed by slower oxidation of nickel. X-ray diffractograms of the oxidized LaNi₅ show the formation of ternary phases like LaNiO₃ and La₂NiO₄ at temperature as low as 400° C. A comparison between the oxidation behaviour of the two compounds reveals that CaNi₅ is more resistant to oxidation and decomposition than the LaNi₅ system.     

Characterization of Synthetic and Naturally Occurring Clays by 27Al and 29Si Magic-Angle Spinning NMR Spectroscopy

Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (²⁷Al) detected the coordination of Al in clays containing as little as 0.26% Al₂O₃. The ²⁹Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad ²⁹Si resonances indicative of short-range disorder. A saponite showed four ²⁹Si resonances. Considerable insight into the short-range ordering of clays can be gained by ²⁷Al and ²⁹Si MASNMR.     

Titania gel spheres by a new sol-gel process

A new technique is devised for the preparation of solution-derived titania gel spheres. The gel sphere preparation process involves the dispersal of the polymerized titanium ethoxide solution into a two-phase liquid suspension in kerosene followed by heating in a microwave oven. This is a simple and fast method for the preparation of xerogel spheres with diameter in the range of tens of micrometers which may be useful in coating technology by plasma-spraying.     

A comparison of alternative kanban control mechanisms. II. Experimental results

This paper continues our study of the design of inventory control policies for serial systems. Using simulation, we compare alternate control policies where a target of mean throughput has to be met under three different objectives - minimize the maximum inventory, minimize the average inventory and minimize the variance of the output - for balanced lines and lines with one bottleneck,

The main objectives of this paper are: (1) to study several objectives not amenable to sample path analysis and (2) to show some sets of conditions and objectives under which Constant-Work-in Progress (CONWIP) is preferred over the traditional kanban mechanism, and vice-versa.     

Selective Cation Exchange in Substituted Tobermorites

Selective cation exchange in tobermorites with three levels of Al³⁺ and Na⁺ substitution for Si⁴⁺ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al³⁺ and 0 mol% Na⁺ substitution increased as follows: Mg²⁺ > Ba²⁺ > Sr²⁺. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca²⁺ sites. Tobermorites with 10 and 15 mol% Al³⁺ (and Na⁺) substitution have additional Na⁺ exchange sites in the interlayers and show the following selectivity: Ba²⁺ > Sr²⁺ > Mg²⁺. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.     

Non-dissociative adsorption on activated carbon: A statistical rate theory of interfacial transport

The statistical rate theory of interfacial transport, describing the non-dissociative adsorption of gases onto solid adsorbents is evaluated. Transient experimental adsorption data of acetylene, vinyl coloride and hydrogen chloride onto activated carbon are compared with the model predictions. The statistical rate theory is found to be much superior to the conventional Elovich equation. Equilibrum thermodynamic properties for gas-solid system have also been estimated suing the model.     

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl₂ in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ?1000°C, however, PtAl₂ became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni₃Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni₃Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.     

Effect of porous properties on self-cooling of fired clay plate by evaporation of absorbed water

Porous ceramic plates were prepared from clay and wood charcoal powder at 900 and 1100?°C and their porous properties, water absorption and the cooling effect of porous plates were investigated to produce eco-friendly porous ceramics for cooling by the evaporation of absorbed water. Porous properties were dependent on the firing temperature, and total pore volume, average pore size and porosity, which were 0.38–0.39 cm³/g, 0.15–0.17 μm and 49–50%, respectively at 900?°C and 0.31–0.33 cm³/g, 2.47–2.59 μm and 43–44%, respectively at 1100?°C. By the addition of wood charcoal powder, the cooling rate of porous plate fired at 1100?°C was 1.7 times faster than that of the plate fired at 900?°C and the cooling temperature difference (?T) was around 2.3?°C at 22.5?°C and 52–54% of relative humidity and around 3.2?°C at 29?°C and 77–80% of relative humidity. The porous ceramic plates developed here are potential materials for cooling buildings.