High temperature colloidal behavior: Particles in liquid silicates

A provisional framework for comprehending many trends in the evolution of solid-liquid systems at high temperature emerged from synthesizing aspects of theories for interfacial wetting and stable grain boundary films with those for sintering and microstructural evolution of materials with low dihedral angles. The energetic factors that dictate the morphology and connectivity of solid particle networks that may provide a basis to describe and understand colloidal systems under conditions wherein particles can change shape. The stability of such networks plus the role of interfacial adsorption layers are elucidated. A critique of reported behavior of several ceramic systems yields some support for these ideas but identifies discrepancies. Experiments were performed involving Si(3)N(4) and other powders dispersed into glass forming silicates, and also involving grain boundary penetration by similar liquids into dense, liquid phase sintered Si(3)N(4). Several general trends emerged in terms of flocculation behavior of Si(3)N(4) particles. The existence of equilibrium boundary films implies that particles being generally attractive should tend to form stable networks with relatively high density. However, two processes eventually disrupt well packed flocs and even dense polycrystalline material upon exposure to the liquids. One ensues from a transient wetting of grain boundaries that are not pre-equilibrated with the liquid. Secondly, anisotropic grain growth can generate sufficient forces to rupture a flocculated, partially sintered particle network. This can lead to a suspension of very low particle density that yet contains small clusters of flocculated particles. With MgO and Al(2)O(3), an observed tendency to form small flocs or chains is interpreted to mean that for certain special particle alignments, low energy boundaries are formed. This may also lead to loose, interconnected networks. With metallic particles, open networks quickly formed, which were stabilized by growth of necks with dry boundaries which did not readily admit particle rearrangement.