Riassunti VERIFICATO
riassunti lezioni - glass science and technology
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- Definition of Glass: Glass is an amorphous solid exhibiting a glass transition, differing from crystalline solids due to its disordered structure, short-range order, variable bond lengths/angles, and isotropy. Key characteristics include disorder (not random), solid-like structure of a liquid, fragility, free volume, resistance, transparency, rheology, and chemical inertness.
- Glass Formation:
- Structural Point of View: Based on Zachariasen's network theory (1932), oxides are classified as formers (e.g., SiO2), modifiers (e.g., Na2O), or intermediates (e.g., Al2O3).
- Kinetic Point of View: Requires a cooling rate faster than the critical crystallization rate. Devetrification is unwanted crystallization.
- Phase Separation: Occurs during cooling, influenced by thermodynamics. Can be liquid-in-liquid (opaque glass) or glass-in-glass (transparent, microscopic separation) via spinodal decomposition or nucleation and growth.
- Key Properties:
- Viscosity (η): Resistance to flow, decreases with temperature. Defined points (fusion, working, softening, annealing, strain) characterize processing stages. Influenced by bond strength, water content, and modifiers.
- Coefficient of Thermal Expansion (CTE, α): Isotropic in glass, influenced by bond strength, NBO content, and thermal history. Small CTE improves thermal shock resistance.
- Optical Properties: Explores refraction (n), dispersion (Vd), absorption (UV by electrons, IR by atomic bonds, visible as optical window), and scattering. Coloration can be ionic, colloidal, or due to redox reactions.
- Chemical Durability: Resistance to chemical attack (acid, alkaline, neutral, atmospheric). Acid attack involves ion exchange and is auto-limiting; alkaline attack involves network dissolution and is thermally activated; neutral/atmospheric attack is a combination.
- Glass Manufacturing Processes:
- Batch Preparation: Mixing raw materials (sand, soda, dolomite, sulfates, cullet) with controlled composition.
- Melting and Fining: Eliminating moisture and gas bubbles, forming the liquid phase, homogenization.
- Forming:
- Flat Glass: "Float glass" (molten glass on molten tin for flat, parallel surfaces) or "fusion process" (for thin glass).
- Hollow Glass (Containers): Automated 2-stage processes (press-blow, blow-blow) using molds, with viscosity being crucial.
- Post-Forming: Includes surface treatments (e.g., SnO coating, organic waxes), annealing (to relieve stresses), and quality control.
- Pharmaceutical Glass: Must have high chemical stability, mechanical/thermal resistance, light protection, and reasonable cost. Classified into types based on composition and treatment (e.g., alumino-borosilicate, treated SSL).