JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech IV Year CERAMICS SCIENCE AND TECHNOLOGY Previous Question Paper (February/March - 2022 - R18)

Glass is an amorphous, non-crystalline solid material, typically produced by the rapid cooling of molten silica. Its primary composition is based on a network-forming oxide, most commonly silica (SiO₂). Commercial soda-lime glass typically consists of 70-75% SiO₂, 12-15% Na₂O (soda) to lower the melting temperature, and 8-12% CaO (lime) to improve chemical durability and prevent water solubility.

Glasses are categorized based on their chemical composition and application:

• Soda-Lime-Silica Glass: The most common type used for windows, bottles, and jars.
• Borosilicate Glass: Contains B₂O₃, providing high thermal shock resistance and chemical stability (e.g., Pyrex laboratory glassware).
• Lead Glass: Contains PbO, which increases the refractive index and density, used for optical lenses and decorative crystal.
• Aluminosilicate Glass: Contains Al₂O₃, offering high mechanical strength and thermal resistance, used for high-temperature applications.

The mechanism of glass formation is primarily governed by the Zachariasen's Rules for random network theory. Glass formation occurs when a melt is cooled rapidly enough to bypass the crystallization temperature, preventing the long-range ordered arrangement of atoms. Instead, the material retains a disordered, liquid-like structure at room temperature.

1. Network Formers: Oxides like SiO₂, B₂O₃, and P₂O₅ form the backbone of the glass structure. These atoms coordinate with oxygen to create a three-dimensional, non-periodic network of polyhedra.
2. Network Modifiers: Oxides like Na₂O, K₂O, and CaO break the bridging oxygen bonds in the silica network. This reduces the viscosity and melting point of the glass, making it easier to process.
3. Intermediate Oxides: Oxides like Al₂O₃ or ZnO can act as either formers or modifiers depending on the overall composition, helping to stabilize the glass structure.

As the melt cools, the viscosity increases exponentially until the atoms are locked into a rigid, disordered state at the glass transition temperature (T_g), resulting in a metastable solid.

Superconductors are materials that exhibit zero electrical resistance and the expulsion of magnetic fields (the Meissner effect) when cooled below a specific critical temperature (T_c). Below this temperature, the material allows electrical current to flow without any energy dissipation.

Materials used for superconductors include:

• Metallic superconductors: Pure metals like mercury, lead, and aluminum, and alloys such as niobium-titanium (NbTi) or niobium-tin (Nb₃Sn).
• Ceramic superconductors: Complex metal oxides, most notably Yttrium Barium Copper Oxide (YBCO) and Bismuth Strontium Calcium Copper Oxide (BSCCO).

The key properties of superconducting materials are:

1. Zero Electrical Resistance: The material allows current to flow indefinitely without power loss due to heating.
2. Meissner Effect: The material expels internal magnetic fields when transitioning into the superconducting state, acting as a perfect diamagnet.
3. Critical Temperature (T_c): The specific temperature below which the material transitions from a normal conductor to a superconductor.
4. Critical Magnetic Field (H_c): The maximum magnetic field strength a material can withstand before superconductivity is destroyed.
5. Critical Current Density (J_c): The maximum current density the material can carry without losing its superconducting state.

Ceramic based superconductors, also known as high-temperature superconductors (HTS), are typically cuprate-based perovskite structures. Unlike metallic superconductors that require liquid helium (4 K), these ceramics can transition to a superconducting state at temperatures above the boiling point of liquid nitrogen (77 K). Their structure consists of copper-oxide planes separated by charge-reservoir layers, which facilitate the movement of Cooper pairs. While they offer the advantage of higher operating temperatures, they are inherently brittle, difficult to fabricate into wires, and possess complex grain boundary sensitivities that limit their current-carrying capacity in polycrystalline forms.