JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ADVANCED MATERIALS Previous Question Paper (September - 2020 - R16)

Nanomaterials are classified based on the number of dimensions that are outside the nanometer range (typically less than 100 nm). The classification is as follows:

1. Zero-dimensional (0D) nanomaterials: All three dimensions are in the nanoscale. Examples include quantum dots, nanoparticles, and nanoclusters.
2. One-dimensional (1D) nanomaterials: Two dimensions are in the nanoscale, while the third dimension is larger. Examples include nanotubes, nanowires, and nanorods.
3. Two-dimensional (2D) nanomaterials: One dimension is in the nanoscale, while the other two are larger. Examples include nanofilms, nanolayers, and graphene sheets.
4. Three-dimensional (3D) nanomaterials: None of the dimensions are in the nanoscale, but they are composed of nanostructured building blocks. Examples include bulk materials with nanostructured grains, such as polycrystalline materials or nanocomposites.

Carbon-Carbon (C-C) composites are typically prepared using the Chemical Vapor Infiltration (CVI) method. This process involves the deposition of a carbon matrix into a porous carbon fiber preform using gaseous precursors.

1. Preform Preparation: Carbon fibers are woven or arranged into the desired shape to create a porous skeleton.
2. Gas Introduction: The preform is placed in a reaction chamber and heated to high temperatures (typically 900°C to 1100°C). A hydrocarbon gas, such as methane or propane, is introduced into the chamber.
3. Thermal Decomposition: The hydrocarbon gas undergoes thermal decomposition (pyrolysis) upon contact with the hot fiber surfaces, depositing solid carbon atoms onto the fibers.
4. Densification: Over time, the carbon deposits fill the voids between the fibers. The process is continued until the desired density is achieved.

CVI is preferred for C-C composites because it allows for the fabrication of complex shapes with high purity and excellent mechanical properties without subjecting the fibers to the high pressures associated with liquid-phase processing.