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

Chemical Vapour Deposition (CVD) is a bottom-up synthesis process used to produce high-quality nanomaterials, such as carbon nanotubes, nanowires, and thin films. The process involves the chemical reaction of gaseous precursors on or near a heated substrate surface to form a solid deposit.

The fundamental steps of the CVD process include:

1. Transport of reactant gases into the reaction chamber.
2. Diffusion of reactants through the boundary layer to the substrate surface.
3. Adsorption of reactants onto the substrate.
4. Surface chemical reactions (decomposition or reduction) to form the solid nanomaterial.
5. Desorption of gaseous by-products from the surface.
6. Removal of by-products from the reaction chamber.

The typical setup consists of a gas delivery system, a mass flow controller, a quartz tube furnace (reaction chamber), and a vacuum pump. The substrate is placed inside the furnace, heated to the required temperature, and the precursor gases are introduced. For nanomaterial growth, a catalyst (typically transition metal nanoparticles like Fe, Co, or Ni) is often deposited on the substrate to act as a nucleation site.

Diagram description: A horizontal tube furnace contains a substrate with catalyst particles. Precursor gases (e.g., methane for carbon nanotubes) flow through the tube. The furnace provides thermal energy for the catalytic decomposition of the gas, leading to the growth of nanostructures on the substrate surface, while exhaust gases exit through a vacuum pump system.

Advantages of CVD include:

• High purity and excellent control over the stoichiometry of the deposited material.
• Ability to produce uniform films and nanostructures over large surface areas.
• High deposition rates and the capability to coat complex, non-planar geometries.
• Flexibility in choosing precursors, allowing for the synthesis of a wide range of materials including metals, ceramics, and semiconductors.
• The process can be operated at atmospheric or reduced pressures, providing control over the microstructure and crystallinity of the synthesized nanomaterials.

Filament winding is a fabrication technique used to manufacture open or closed-end composite structures by winding continuous resin-impregnated fibers onto a rotating mandrel. The process is highly automated and produces components with high strength-to-weight ratios, such as pressure vessels, pipes, and rocket motor cases.

The process involves the following key components and steps:

1. Mandrel: A rotating form that defines the internal geometry of the final component. It can be collapsible for removal or permanent if it becomes part of the structure.
2. Fiber Creels: Continuous fiber tows (e.g., glass, carbon, or aramid) are pulled from multiple spools.
3. Resin Bath: The fibers are passed through a resin tank where they are thoroughly impregnated with a thermosetting matrix, typically epoxy or polyester.
4. Delivery Head: A carriage moves along the length of the mandrel, guiding the fibers onto the rotating surface. The orientation of the fibers is controlled by the ratio of the mandrel rotation speed to the carriage traverse speed.
5. Winding Patterns: Fibers can be wound in helical, circumferential (hoop), or polar patterns depending on the structural requirements of the component.
6. Curing: Once the desired thickness is achieved, the mandrel is placed in an oven or autoclave to cure the resin.
7. Extraction: After curing, the mandrel is removed, leaving the finished composite part.

Diagram description: The setup consists of a rotating mandrel mounted between two headstocks. A carriage moves parallel to the mandrel axis, carrying a fiber delivery eye. Fibers are drawn from a creel, passed through a tensioning device, dipped into a resin bath, and then deposited onto the mandrel surface in a precise, controlled pattern determined by the synchronized motion of the mandrel and carriage.