Doh-Yeon Kim

Analysis of the proportionality constant correlating the mean intercept length to the average grain size

The proportionality constant, K, which relates the mean intercept length measured on a polished section to the average grain size has been reanalyzed. It has been shown that the value of K varies with both grain shape and grain size distribution. An expression of K including the grain shape and the size distribution has been proposed.

Determination of three-dimensional grain size distribution by linear intercept measurement

Abstract For grains of any shape, a computer simulation technique to calculate the probability of finding a certain intercept length is suggested and the results are given for cubic, tetragonal and hexagonal grains. The distribution of linear intercepts measured is then converted into that of a three-dimensional (3D) size. The developed method is applied to determine the 3D distribution of cubo–spherical grains observed in the TiC–Ni cermet.

Effect of SiO2 and TiO2 addition on the exaggerated grain growth of BaTiO3

Abstract The exaggerated grain growth (sometimes referred to as secondary recrystallization) during sintering of BaTiO 3 was studied with two different liquidforming additive oxides, TiO 2 and SiO 2 . The liquid phase was distributed inhomogeneously by placing a small amount of TiO 2 and SiO 2 , respectively, on top of the BaTiO 3 powder compact. The exaggerated grain growth was observed to occur more rapidly in the SiO 2 -containing specimen than in the TiO 2 -containing specimen. Once the initial fine-grained structure was completely replaced by the exaggeratedly grown grains, the TiO 2 -containing specimen exhibited practically no more micro structural evolution. For the SiO 2 -containing specimen, on the other hand, a few grains continued to grow without limit. The observed unusual growth has been explained in terms of twin-assisted growth mechanism.

Ostwald ripening kinetics of angular grains dispersed in a liquid phase by two-dimensional nucleation and abnormal grain growth

Abstract Based on the fact that the angular shape of solid grains dispersed in the liquid matrix indicates a singular interface, the coarsening kinetics of angular grains was formulated based on 2-dimensional (2-D) nucleation and solved numerically. For comparison, diffusion-controlled coarsening of grains with a spherical shape was also solved numerically. The solutions showed that coarsening by 2-D nucleation induced abnormal grain growth whilst diffusion-controlled coarsening did not. This result agrees with the general experimental observation that the abnormal grain growth in liquid phase sintering takes place exclusively in the system with angular grains. The ratio of the largest grain size to the average increased monotonously with time in coarsening by 2-D nucleation whilst it decreased in diffusion-controlled coarsening. The artificially-added large grain (10 times larger than the average) became the abnormal grain in 2-D nucleation controlled coarsening but did not in diffusion-controlled coarsening.

Space-charge concepts on grain boundary impedance of a high-purity yttria-stabilized tetragonal zirconia polycrystal

A detailed impedance analysis using the brick-layer model is performed on a high-purity yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). Space-charge impedance is generally formulated and expressions for the respective space-charge models are therefrom derived depending on whether dopant ions are mobile or immobile. Pronounced yttrium segregation in Y-TZP is also considered in the analysis in that the dopant profile is assumed to be frozen from a high-temperature equilibrium distribution. Comparison with experimental observations shows that the electrically measured grain-boundary thickness corresponds to the Schottky-barrier width, slightly modified by the dopant segregation. The grain-boundary resistance is not consistent with any space-charge models and the strong defect interaction due to the yttrium enrichment is suggested to be mainly responsible.

Fabrication of BaTiO3 single crystals by using the exaggerated grain growth method

Abstract The exaggerated grain growth which can occur during the sintering of BaTiO3 was implemented for the fabrication of single crystals. A very small amount of SiO2 was placed on top of a BaTiO3 powder compact to form seed grains composed of twin lamellae. The seed grains continued to grow without limit and single crystals of up to a few centimetres in size were obtained.

Charged clusters in thin film growth

Abstract A cauliflower structure is a granular film composed of spherical particles similar in size, each with numerous nanoscale nodules on its surface. The structure is produced during certain chemical vapour deposition (CVD) processes for diamond and silicon thin film growth. A classical account in terms of atomic unit deposition fails to explain the growth of such a cauliflower structure, as it requires a gas phase of much higher supersaturation than for onset of diffusion controlled growth. Another interesting and somewhat puzzling phenomenon encountered during a diamond CVD process is that while diamond is depositing on a graphite substrate, carbon atoms in the graphite itself are etched away into the vapour phase; that is, experience evaporation. Again, an elementary kinetic barrier mechanism fails to explain such CVD deposition of a less stable diamond phase combined with simultaneous evaporation of a stable graphite phase. In order to account for such puzzling CVD phenomena and others, a theory of charged clusters has been developed over the past decade as a new paradigm for thin film growth. The theory and its applications are reviewed in this work.

Effect of anorthite liquid on the abnormal grain growth of alumina

A small amount of anorthite (2CaO·Al2O3SiO2) powder was placed on the top-center of the pure Al2O3 powder compact and then sintered at 1600°C. During sintering some grains grew extensively along the radial direction from the liquid source. By considering an atomically smooth solid–liquid interface structure of alumina, grain growth controlled by 2-dimensional (2-D) nucleation was suggested. The elongation has been explained in terms of nucleation advantage of grain-boundary re-entrant edges (GBRE) formed dominantly at non-basal planes.

Microstructural evidence of abnormal grain growth by solid-state wetting in Fe-3%Si steel

In this investigation, the mechanism of abnormal grain growth in Fe-3%Si steel was based on the microstructure evolution at the growth front of grains undergoing the abnormal growth. The most striking feature in the growth of abnormal grains was the penetration along the grain boundary of neighboring grains. This is energetically possible if the energy of the penetrated grain boundary is higher than the sum of the energy of two other grain boundaries shared by the penetrating abnormal grain. Along the growth front of an investigated abnormally growing grain, 15 out of 1381 triple junctions showed the clear microstructural evidence of the grain boundary penetration by the abnormal grain. Misorientation measurements of 34 penetrated grain boundaries using electron backscattered diffraction showed that not a single boundary has a low angle, implying that the penetrated grain boundaries have the high energy. These results are best explained by the abnormal grain growth with solid-state wetting.

Effect of sintering temperature on the secondary abnormal grain growth of BaTiO3

Abstract When BaTiO3 specimens containing a small amount of excess TiO2 were sintered for more than 15 h, some grains grew abnormally to several millimeters in size. This phenomenon may be referred to as the secondary abnormal grain growth (SAGG) because it occurred from a large and uniform grain structure (average grain size: 70 μm) after completion of primary abnormal grain growth. SAGG was observed only at a very narrow temperature range between 1360 and 1370°C, where the solid–liquid interface structure was atomically smooth. Almost all the secondary abnormal grains contained the (111) double twin, which provides the persistent twin-plane re-entrant edge (TPRE). During SAGG, the growth of matrix grains was strongly suppressed and material transfer occurred preferentially to the grains with a (111) double twin.