A. V. Galakhov

Powder Compact Structure. Part 1. Particle Packing Inhomogeneity

Powder compact characteristics, on which its sintering kinetics, grain formation, and structural oxide ceramic mechanical properties depend, are considered. Powder compact inhomogeneity criteria are formulated. Reasons are analyzed for forming pore space structure inhomogeneity. Experimental data are provided for the effect of powder compact structure inhomogeneity on compaction kinetics and grain growth within it during sintering.

α-Al2O3 powders from amorphous alumina gel

We have studied the properties of α-Al2O3 powders prepared from amorphous alumina gel. The powders have a small particle size owing to the low synthesis temperature. The sintering temperature of compacts produced by pressing the powders is lower than that in the case of powders prepared from hydroxide precursors. The sintered material has a homogeneous, fine-grained microstructure, which ensures excellent mechanical properties.

Powders for fabricating polycrystalline optical ceramics: Synthetic procedures (an overview)

Overviewed are existing synthetic procedures for preparation of powders used as raw materials in fabricating optical polycrystalline ceramics such as aluminum oxynitride 9Al2O3 · 5AlN (AlON), yttrium aluminum garnet 5Al2O3 · 3Y2O3 (YAG), and magnesium aluminate spinel Al2O3 · MgO (MAS). Most promising for practical implementation seems to be solution-combustion synthesis from dehydrated gels containing appropriate inorganic salts and organic fuels.

Powder Compact Structure. Part 3. Theoretical Analysis of Sintering in Powder Compacts with Inhomogeneous Porosity

Publications devoted to theoretical analysis of powder compact sintering are reviewed taking account of particle packing inhomogeneity within it. The methods used for resolving the problem are conditionally divided into analytical and numerical. It is shown that in order to obtain results agreeing with experimental observations more extensive possibilities may be realized using numerical methods in combination with computer modeling of particle packing. Examples are provided of implementing the method applied to oxide ceramic material sintering.

Effect of Water on α-Al2O3 Crystallization in Alumogels

Results are presented for a study of phase transformations during synthesis of α-Al2O3 from dehydrated alumogel. It is shown that water removal from an original precursor has a marked effect on subsequent phase transformations in the synthesis temperature range. By comparison with the crystallization temperature for α-Al2O3 precipitated from aqueous solution, for gel this is reduced by 300°C from 1200 to 900°C.

Effect of Seeding on α-AL2O3 Crystallization in Alumogels

Results are presented for a study of phase transformations during synthesis of α-Al2O3 from dehydrated alumogel. It is shown that introduction of α-Al2O3 seeding crystals into gel does not affect the temperature for initiation of transformation of γ-Al2O3 into α-phase. However, presence of seeding markedly increases the transformation rate, which under certain conditions leads to a reduction in α-phase total recrystallization temperature.

Powder Compact Structure. Part 2. Methods for Increasing Particle Packing Uniformity1

Methods are considered for increasing powder compact structure uniformity, conditionally classified as processing and structural. In considering processing methods main attention is devoted to an original approach such as working a compact by deformation and two-stage sintering. Among structural methods those noted increase compact uniformity by organizing ordered structures of agglomerated powders and use of powders with an internal nanostructure.

Aerosol powders with fractal structure

The scanning electron microscopy of ZrO2-Y2O3 (3 mol %) powder obtained by combustionspray pyrolysis revealed particles of unusual morphology: hollow thin-walled balls filled with solid spherical particles. A feasible mechanism for the appearance of such fractal structures is proposed.

Distribution of particles in sizes and sintering: A numerical analysis

On the basis of a physical model of sintering, a numerical analysis of the temporal form change in the simulative powder compacts from particles with various distribution widths is carried out. It is shown that, provided the average particle size and initial density of packing are the same, the average grains size in the sintered material and the grains size distribution width both increase with growth of particle-distribution width. The best results from the point of view of productivity of the process (time of sintering) and the qualities of the final product (the average size of the grain) are demonstrated by monodisperse powders. The pro-posed numerical technique may be useful for a qualitative analysis of sintering of the powder compacts, for the reason to optimize the technological parameters and the quality of the material.

Effect of compaction pressure on the sinterability of submicron powders of tetragonal zirconium dioxide

ConclusionsWe studied the effect of compaction pressure on the pore structure of the paniculate compacts obtained using two types of agglomerated submicron powders of tetragonal zirconium dioxide, on the structure evolution during the sintering process, and on the strength of the obtained material. It was established that the characteristics of the agglomerates present in the powders have a significant effect on their behavior during compaction and sintering. At a given compaction pressure, the powders having weaker agglomerates densify up to a higher density and give a more uniform distribution of pores in the preform. The low-density compacts obtained using agglomerated powders having a high specific surface area sinter faster and attain high strength levels at a lower temperature; however, the sintered materials obtained from such compacts contain several structural defects in the form of large pores and have a lower strength. The uniformity of the distribution of pore volume with respect to size (or the specific content of the interagglomerate pores) forms the main criterion of the quality of particle packing in the compacts obtained from agglomerated powders. The compacts having a low content of the interagglomerate pores give a defect-free dense and strong material after sintering. The presence of the anion impurities in the original powders leads to a decrease of density during the sintering process after the attainment of a threshold density at which formation of closed porosity occurs. Pressure sintering (HIP) forms an effective method of suppressing the decrease of density.