Randomly oriented microstructure

Solidification. When the ingot or casting solidifies, there are three main possible microstructures that form (see Figure 7.5). We will describe here only the final structures the thermodynamics of the liquid-solid phase transformation have been described previously in Chapter 2. The outside layer of the ingot is called the chill zone and consists of a thin layer of equiaxed crystals with random orientation. 

Cottin (C50) earlier studied pastes of Ciment Fondu, largely by the TEM replica method. The microstructures of pastes of low w/c ratios hydrated for 7 days at 12°C were too compact to show more than occasional detail, but with pastes of w/c = 0,7 much detail could be observed in cavities exposed by fracture, which were lined with small crystals of CAH,o and sometimes contained plates of C,AHg. The crystals of CAH[q, which were hexagonal prisms up to 1 pm long and 0.2 pm wide, were present in rounded aggregates. Hydrous alumina was seen in a paste of w/c = 1.0 hydrated at 30"C it occurred in the lining of a cavity, mainly as rounded masses of randomly oriented platelets. 

On the microstructural level, several types of pyrolytic carbons may be deposited each with one of four distinctly different structures, ranging from layered, highly anisotropic forms to structures with very small, randomly-oriented crystallites with no preferred orientation. All of these structural variations are a result of modifications in processing conditions. In this particular study, only the isotropic forms of both pure LTI carbon and co-deposited LTI carbon-silicon alloyed carbon (Pyrolite registered trademark of Carbo-Medics, Inc., San Diego, California) were investigated. 

As discussed in the previous chapter, the network of the inter-connected membrane pores formed during preparation and fabrication may be tortuous or nearly straight depending on the synthesis and subsequent heat treatment methods and conditions. The microstructure of a membrane, particularly the type with tortuous pores, is too complicated to be described by a single parameter or a simple model. Due to the relatively poor knowledge of flow through porous media, an empirical term called " tortuosity" has been introduced and used by many researchers to reflect the relative random orientation of a pore network and is based on the Kozeny-Carmen equation for the membrane flux, J ... 

Microstructural Study of Randomly Oriented Polycrystalline Samples... 

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