Atomistic Structure of Sodium and Calcium Silicate Intergranular Films in Alumina

Abstract

Sodium silicate intergranular films (IGF) in contact with the [0001] basal plane of α-alumina were studied using the molecular dynamics computer simulation technique. The results were compared to previous simulations of calcium silicate and sol-gel silica IGF's in contact with alumina. An ordered, cagelike structure was observed at the interface. Sodium ions segregated to the cages at the interfaces. Calcium and hydrogen ions were also observed to segregate to the cages in the previous simulations. The modifier ions were surrounded by more oxygen ions in the cages at the interface than in the bulk of the IGF. This explains the segregation of modifiers at the interface. Interface energy decreased as the sodium content of the IGF increased. Interface energy decreased faster as a function of Na2O content than as a function of CaO content. However, interface energy decreased slower as a function of Na+ content than as a function of Ca2+content.

Molecular Dynamics Simulations of the Effect of the Composition of Calcium Alumino-Silicate Intergranular Films on Alumina Grain Growth

“…24,25 This is again the case for most of the compositions studied here, as shown on the left side of Figure 3. At the basal side, for all but the 141 composition, the first adsorbed Al peak from the IGF is much smaller than the terminal Al crystal peak on this side and the first adsorbed Ca peak is present at the same position as the first O peak (indicative of the cage structure shown in Figure 2).…”

“…However, the presence of the crystal surfaces modifies this behavior, and as seen in all of the compositions, a very strong second adsorbed Al peak forms on the basal side (which is at the "top" of the cage structure). 25 The Al atoms in this second peak are predominantly four coordinated, as shown in the 334 system in Figure 4, thus requiring charge compensation by the Ca. So the first adsorbed Ca layer charge compensates the tetrahedral Al in the terminal plane of the original crystal and the undercoordinated O in the crystal surface 23,26,28 as well as the Al in the second adsorbed peak.…”

“…The parameters, shown in Table 2, were also developed in previous studies. 25,30,32,33 Previous simulations using this potential and parameters generated stable crystals of both R-Al 2 O 3 and γ-Al 2 O 3 , 30 where the coordination of the Al ions differs. Surface energies were calculated, with results providing an explanation for the stability of small crystals of γ-Al 2 O 3 in comparison to R-Al 2 O 3 .…”

“…[23][24][25][26][27][28] Recent highresolution TEM (HRTEM) studies observe such density oscillations. 20 These density oscillations decay within 1 nm of the interface, attaining a glassy structure in the interior of the thicker films.…”

A comparison of the microstructure and mechanical properties of two liquid phase sintered aluminas containing different molar ratios of calcia–silica sintering additives

“…9 Subsequent simulations of calcium silicate intergranular films 10,11 have suggested that for concentrations of up to 12 at.%, calcium cations prefer to segregate to these cage-like structures where they lower the glass/grain interfacial energy and promote ordering of the interfaces. Interestingly fracture simulations of these model interfaces revealed that, for low levels of calcia addition, fracture occurred only at the glassy film/ alumina grain interface plane.…”