Densification of raw coke powder compacts

The effects of grinding on densification of a petroleum raw coke were examined. The compact of the powder ground for a short time showed large puffing during calcination below 400 °C, and no densification by subsequent heat treatment. It was found that the puffing could be suppressed by long grinding of the powder. In the compact of the powder subjected to prolonged grinding, the pore volume decreased in two temperature ranges of calcination, at 400–500 and 600–700 °C. The first decrease seems to be attributed to softening and volume reduction due to carbonization of quinoline-soluble components among the coke grains. The second decrease suggests that the sintering due to solid state material transport, such as viscous flow and plastic deformation, takes place in addition to gas-phase material transport. The increase in bulk density at temperatures over 1000 °C was mainly due to contraction of constituent grains in which micropores decreased, not a decrease in pore volume of the compact. The most remarkable effect of grinding on densification of the raw coke was the suppression of puffing during the calcination process.     

A mechanism for the nitridation of silicon powder compacts

A mechanism for the nitridation of silicon powder is proposed, based on an interpretation of the microstructure of partially reacted compacts. It is observed that the reaction does not occur at the solid-state interface between the silicon and the nitride product layer. Both silicon and nitrogen are transported through this layer and the removal of silicon results in the formation of pores in the silicon crystals at the nitride-silicon interface. The nitridation reaction takes place within these pores, which subsequently migrate into the silicon grains, and within the original voidage of the compact.     

Rearrangement phenomenon developed inside powder compacts: theoretical model expressing the associated shrinkage

A distinct new Stage 0 has been resolved which precedes the normal Stage 1 in the sintering curves of iron powder composed of smooth spherical particles. It can, therefore, be resolved only under conditions of high-speed dilatometry, where the errors induced by heating are minimized. The temperature and pressure dependence of Stage 0 suggests that this kinetic regime is related to the relaxation of tangential interparticle stresses caused by incomplete compaction. To take into account the experimental results a model is proposed. The agreement between the dilatometric curves calculated from this model and the experimental ones is good.     

Sintering behaviour of powder compacts with multiheterogeneities

The sintering behaviour of a powder compact containing uniformly distributed heterogeneities has been analysed. The results reveal a strong retardation of sintering in the presence of non-sinterable agglomerates, due to the development of a uniform hydrostatic tensile stress in the powder matrix. The variation in stresses and sintering rate depends strongly on the total volume fraction of heterogeneities especially when the volume fraction is small, but is insensitive to the actual number of heterogeneities. Specific results are calculated using densification and deformation laws pertinent to Al₂O₃.     

Hot isostatic pressing of Y-TZP powder compacts

A fundamental study of hot isostatic pressing (HIPing) was performed on Y-TZP powder compacts, and the effect of the various HI Ping process variables (time, temperature and pressure) was examined on the densification behaviour. The results were analysed using Ashby's HIPing model, which was found to be applicable for this system. The densification was found to be governed by the grain-boundary diffusion of cations. The use of grain-boundary transport characteristics determined in this study enabled HIP maps to predict the densification behaviour of HIPing within an accuracy of a factor of three for all stages of densification, except for the results at low HIPing pressure and the region of near full density.     

Electrical and thermal tortuosity in powder compacts

A simple equation to calculate the effective mean path to be travelled by the heat flow or electrical current in a porous powdered material is proposed. In this equation, the effective mean path is a function of the sample porosity degree and the tap porosity of the original powder. This latter parameter determines to a great extent the compact pore structure, since it depends on the powder particle size, shape and distribution. The proposed equation has been validated using real metallographic images from sintered powders compacts. For measurements, a computer program, based on the Pathfinding A* Algorithm, has been developed. Results are in very good agreement with theoretical predictions. The proposed equation can be useful to model the electrical and thermal conductivity of sintered compacts.     

Processing defects in ceramic powders and powder compacts

1. Download : Download full-size image

     

Thermal expansion behaviour of nanocrystalline titanium powder compacts

This investigation is an attempt to understand the expansion behaviour of compacts of nanocrystalline titanium powder during thermal treatments. Entrapped gases cause excessive expansion in the green compacts. The dilatometric study reveals that there is no significant difference in the expansion coefficient of sintered nanocrystalline titanium samples, compared to their micron-size counterpart.     

Characterization of internal structure in Y-TZP powder compacts

The internal structure of yttrium-doped tetragonal zirconia powder compact was characterized using the immersion liquid technique, mercury porosimetry, scanning electron microscopy, and a compaction test. The specimens examined were prepared by pressing spray-dried granules at various pressures from 5–600 M Pa. The immersion liquid technique had numerous potential advantages, over the other methods for evaluating defects of 1–10 μm in size. With this technique, intergranular interfaces and pores were found to be present even in the specimen prepared at 600 M Pa. A feature, which is considered to be an agglomerate, was also found. The significance of the large defects on ceramic processing, as well as a comparison of the technique with the other methods, is presented.     

Grain growth in sintering of clustered powder compacts

A theoretical model of grain growth in sintering of clustered powder compacts is outlined, showing that the microstructure evolution is stepwise continuous in time and its general trends can be predicted independently of the particular system and process being considered. The dependence of coarsening on densification can be accounted for by introducing a densification-scaled time variable (intrinsic time). The theory is successively applied to systems where particular local mechanisms of matter transport are supposed to operate, respectively in the initial/intermediate and the intermediate/final stage of sintering. The obtained mathematical models are solved numerically to follow the evolution of three regularly packed clusters. The model predictions are in good agreement with experimental data obtained by other researchers.     

Solid-state reaction behavior of Al-Ti-C powder mixture compacts

The solid-state reaction behavior of the Al-Ti-C powder mixture compacts was investigated. The results showed that the TiC/Al composite was synthesized by the solid-state diffusion reaction in a vacuum electric resistance furnace at a heating temperature of 600 ± 2 °C, with a holding time for 3 h. The TiC particulate was fine and irregular, with a mean size of 2 μm.     

Measurement of activation energy values for sintering of active urania powder compacts

Abstract

Wide variations in sintering behaviour have been observed for uranium dioxide powder compacts prepared from ammonium diuranate (ADU) powder precursor and calcined at various temperatures. In general, it is observed using X-ray diffraction that powders calcined below 800°C are non-crystalline and they are also highly sinterable (active), whereas powders calcined above 800°C have poor sinterability and are designated inactive. A model has been proposed for measurement of the activation energy values for sintering of compacts of urania powder based upon a modification derived from Frenkel's sintering model as applicable to non-crystalline powders, from which the shrinkage rate at zero time (t → 0) can be obtained. Moreover, the associated effect on sintering due to the non-crystalline to crystalline transition with progressive thermal treatment is eliminated using this model. The activation energy values for sintering of the non-crystalline powders at various calcination temperatures are in good agreement and, below 800°C, values of ~167·5 kJ mol^?1 are obtained. For powders calcined above 800#x00B0;C, values of ~314·1 kJ mol^?1 are obtained.

MST/812     

Joining of iron powder compacts by an infiltration method

Joining of iron powder compacts by an infiltration method was attempted. The joining strength of the compacts was investigated with respect to variations in roughness of the surface, applied pressure, relative density of preforms, infiltration temperature and the time. The joining strength was found to be almost equal to those of monobloc ompacts by conventional infiltration when the joining was done under the optimum conditions. Joining of the iron powder compact and wrought steel samples such as S10C and S55C was also carried out and high strength could be obtained by this method. Samples with complicated shapes or with holes in the parts could be easily made with the infiltration technique proposed.     

Tensile fracture characteristics of double convex-faced cylindrical powder compacts

Doubly-convex cylindrical tablets compacted uniaxially from different particle-size fractions of Di-Pac-Sugar powder, have been fractured under diametral loading conditions employing the standard diametral compression test. The ratio of cylinder length to diameter (W/D) ranged from 0.476–1.06; the ratio of cylinder diameter to radius of curvature of the tablet faces (D/R) was varied from 0.0–1.184. An equation based on geometrical volume equivalence conditions, of a doubly-convex cylinder to that of a plane-faced cylinder, relating the tensile strength of the material to the fracture load and dimensions of doubly-convex cylindrical specimens, has been developed. This equation is valid for any compacted cylindrical tablet. Using this equation, it was possible to assess the tensile strength of doubly-convex tablets, and a shape factor has been defined. Also, the predicted fracture loads obtained by this equation are found to compare well with those values determined by utilizing a stress factor based on the photoelastic stress analysis. For higher ratios of W/D and smaller diameter values, the variation in the results calculated is found to be less than 5%. Meanwhile, for higher values of W/D, the geometrical end effect is reduced and the tensile fracture stress tends to become similar to that of a plane-faced specimen.     

Thermal debinding of Fe3Al-X metal powder compacts

The process of removing a multicomponent binder from a metal powder compact has been investigated. Model experiments of debinding were performed on compounds consisting of less than 40 vol% binders (low molecular weight polyethylene, paraffin and Carnauba waxes) and more than 60 vol% metal content. As typical representatives for injection moulding morphology and meeting all other requirements for optimal powder characteristics, elemental powders of the Fe-Al system were used. Viscosity results over a wide range of shear rates for various plastisols are presented as functions of binder system composition and metal powder content. Based on the rheological response, an optimization of plastisol formulation was performed. Results are reported on three series of debinding modes using heat and fluid wicking in air and in nitrogen. The time dependence of fractional debinding, x(t), during wicking has been estimated using a model. Direct observation by SEM of binder distribution and pore structure evolution at different stages of the debinding process was made. Wick-assisted thermal debinding in nitrogen proved to be an effective debinding method in terms of shape preservation and the absence of defects in the studied material.