Zoran S. Nikolic

Computer simulation of grain growth by grain boundary migration during liquid phase sintering

From many experiments with mixtures of small and large particles, it can be concluded that during liquid phase sintering, smaller particles partially dissolve and a solid phase precipitates on the larger particles. Therefore, the number of smaller particles decreases due to coarsening. The growth rate can be controlled either by the solid-liquid phase boundary reaction or by diffusion through the liquid phase. This dissolution-reprecipitation process leads to further densification by rearrangement of smaller and larger particles. The microstructure may change either by larger particles growing during the Ostwald ripening process or by shape accommodation. In this study, two-dimensional simulation of grain growth by grain boundary migration based on such a physical and corresponding numerical modeling of liquid phase sintering was considered. The simulation method developed is based on the defined submodels for model system definition, for solution-precipitation, and for grain coarsening process.

Computer simulation of chemically driven grain growth during liquid phase sintering

Abstract A simulation of solution reprecipitation during liquid phase sintering was carried out on a computer whereby compositional differences between dissolving and growing grain were assumed as the driving force and diffusion through the liquid as rate controlling mechanism. The equations developed are for the two-dimensional case, i.e. they describe the behaviour of plates and cylindrical wires separated from each other by liquid phase. Slightly different results were obtained for the two initial conditions, i.e. whether it was assumed that no solid was dissolved in the liquid initially or whether supersaturation of the liquid was assumed. In the latter case the calculated microstructural changes of the wire models are in good agreement with earlier experimental observations on W-spheres sintered in liquid Ni.

Distribution of time delay of electrical breakdown in nitrogen

Gives the distribution of experimentally obtained values of time delay of electrical breakdown (td). Measurements of td were made on a specially made nitrogen-filled diode at 13.33 mbar pressure. The results show that the distribution form is easily identifiable after about 500 successively measured td values, and that its form may be estimated after about 100 successively measured td values. These distributions were fitted using the least-squares polynomial approximation method. It was found that the best approximation is obtained by the seventh-order polynomial. Laues distributions for three groups of successively measured td values, in the range of one millisecond to several seconds, have the same form as those previously obtained by Meek and Craggs (1978), where td values were of the order of one microsecond.

SEM investigation of domain structure in (Ba,Ca,Pb)TiO3

In the present paper the microstructure and domain structure in modified BaTiO3 with Pb and Ca as additives have been investigated using SEM technique. The (Ba,Pb)TiO3 and (Ba,Ca,Pb)TiO3 ceramics show a slight diAerence in grain size, being smaller in composites with Ca additives which acts as grain growth inhibitor. The domain configuration is almost the same. The small grain microstructure with tiny domains have been observed in specimen sintered at 1300C and the average grain size is in the range 1‐3mm. For those specimens sintered at 1320C the homogenous microstructure is also obtained with grain size around 2‐4mm. For both types of specimens, the single domain structure is associated with grain which size is lower than 2mm. The banded domain structure could be observed in grains with size bigger than 3mm. The bar shape grains and elongated grains together with some large region in microstructure are free of domain structure. The observed domain patterns reveal mainly the straight domain boundary lines with 90 domains walls. The wall thickness ranged from 0.03m mt o 0.15mm, while the domain width is in the range of 0.1mm‐1mm. # 1999 Elsevier Science Limited. All rights reserved

A model for 3-D study of rearrangement in liquid phase sintering

Abstract The rearrangement process assumes that if there is good wetting between liquid and solid phase, solid particles will rearrange themselves under the action of surface tension forces, producing more stable green packing. This paper outlines a computer-based method for calculation of the capillary force during liquid phase sintering based on a two-spherical-particle model and circle approximation for computation of the liquid bridge meniscus. The method was applied to a 3-D study of the rearrangement during liquid phase sintering using a multi-particle model. The results obtained have shown that rearrangement is essentially governed by geometrical factors where uniformity of initial particle packing and a high degree of mixing homogeneity are prerequisites for effective densification.

Three-dimensional computer study of rearrangement during liquid phase sintering

Abstract The rearrangement process during liquid phase sintering is generally accepted as being driven by the capillary forces between solid grains embedded in liquid. This process assumes that if there is good wetting between liquid and solid phases, solid grains will rearrange themselves under the action of surface tension forces, producing more stable packing. This paper outlines a computer-based method for three-dimensional computer simulation of rearrangement during liquid phase sintering. The theoretical models dealing with the fundamental interaction forces that exist between grains attached by liquid bridges will be outlined and the development from these pair-wise interactions to multi-grain models will be described.

Numerical simulation of gravity induced skeletal settling during liquid-phase sintering

In this paper we have investigated numerically the phenomenon of gravity induced skeletal settling during liquid-phase sintering, where the solid-skeleton formation was introduced through the definition of skeleton units and their time evolution during the formation of a large solid skeleton. Since it was assumed that under gravity, Stokess law settling usually dominates microstructure formation, the settling time was used for computation of the average migration distance during the time interval @Dt. Thus gravity induced solid-phase evolution was simulated by means of simultaneous computation of the displacement of the center of mass and mass transport due to dissolution and precipitation at the interfaces between solid-phase domains and the liquid matrix. The new methodology is illustrated by application to a regular multi-domain model including both solid-phase and pore-phase domains. The microstructures computed for tungsten heavy alloy substantiate previous observations that the settled solid-volume fraction can be directly related to the solid-liquid density difference, but is dictated by the formation of a solid skeleton.

A three-dimensional computer study of gravity induced skeletal structure evolution during liquid phase sintering

Abstract In this paper we will investigate numerically gravity induced skeletal structure evolution during liquid phase sintering. Applying three-dimensional domain methodology, solid skeleton evolution will be introduced by the definition of skeleton units determined by the equilibrium dihedral angle and the formation of large solid skeletons arranged in a long chain of connected solid phase domains. The settling procedure will be simulated by using two general submodels: for free settling, in which solid phase domains fall under gravity over domains that have already settled, and for extended settling, in which settled domains continue their motion until they reach a position of local equilibrium. The same submodels will be applied for free settling and extended settling of solid skeletons. It will be assumed that under gravity conditions, Stokes’s law settling usually dominates microstructure formation, where the settling procedure will be simulated by computation of the settling time and average migration distance during a defined time interval. Thus gravity induced skeleton structure evolution will be simulated by simultaneous computation of the displacement of the center of mass.

Determination and modification of the thermal conditions in a double ellipsoid mirror furnace

A crucible-less double ellipsoid mirror furnace was used for growing beryllium single crystals with certain defect structures. The temperature field is not rotationally symmetrical in the double ellipsoid mirror, hence inhomogeneous configurations of dislocations and even subgrains can be formed. Investigation of the temperature fields during solidification and their manipulation was therefore necessary. By controlled defocusing of the lamps, a significant reduction of the temperature gradients in azimuthal and axial direction can be achieved.

Numerical simulation of pore evolution during liquid-phase sintering

Pore evolution by pore filling and pore growth during liquid-phase sintering was investigated numerically. Pore growth by pore coalescence was treated as the nearly instantaneous formation of one large pore upon contact of two smaller pores. In that sense, three different types of pore replacement, irregular (pore skeleton), circular and ellipsoidal, were defined and applied for the study of diffusional effects on the microstructural development of the porous W-Ni system.