Lesson 8 & 9 - Glass Flashcards
What are some general properties of Ceramics?
- Density greater than polymers but often less than metals
- High compressive strength but weak in tension and bending
- Rigid
- Brittle
- Hard
- High melting point
- Very good thermal insulators
- Very good electrical insulators
- Durable and resistant to degradation
What is glass? And its basic ingredients?
- An amorphous solid
- Transparent -> no grain boundaries to scatter light
- Does not crystallise
- Basic ingredients are silicon and oxygen which form silica when combined
How is glass made easier to form at lower temperatures?
Other compounds are added in to lower the melting temp. and viscosity.
What are the 5 thermal properties points of glass?
Melting point
Working point
Softening point
Annealing point
Strain point
What is the melting point?
Temperature at which viscosity is low enough for glass to be considered a liquid
What is the working point?
Temperature at which glass is easily deformed
What is the softening point?
Maximum temperature at which glass can be handled without deforming
What is the annealing point?
If heated to this temperature residual stresses can be removed
What is the strain point?
Below this temperature the glass exhibits no plastic deformation prior to fracture
What are a few types of glass and their application?
Soda-Lime Glass: Windows in residential properties, containers
Coloured glass: Traffic lights, sunglasses
Obscured/translucent glass: Where privacy is required (e.g. meeting room, bathroom)
Toughened/tempered glass: Glass doors, shower cubicles
Laminated glass: Vehicle windscreens, bullet proof screens
Optical glass: Lenses, prisms
Borosilicate glass: Kitchenware, lighting, wind turbine blades
Glass Ceramics: Cladding, kitchenware, military
Photochromic glass: Commercial buildings, photochromic spectacles
Photovoltaic glass: Windows in residential and commercial buildings
Lead crystal glass: Expensive crystal glassware, electric light fitting
What is the most common method for mass production of sheet glass?
Float Glass Process
How does the float glass process work?
Stage 1: Melting and refining
- Process can take up to 50 hours
Stage 2: Float bath
- Molten glass flows over molten tin
Stage 3: Coating
- Chemical coatings applied to improve thermal/
optical properties
Stage 4: Annealing
- Slow cooling to reduce stresses in the glass
Stage 5: Inspection
- Inspection at every stage to identify flaws
Stage 6: Cutting to order
- Diamond wheel used to cut glass to size
What are some methods of strengthening glass?
- Heat treatment
- Annealing
- Tempering
- Laminating
- Chemical toughening
Describe the tempering process and how the properties change when it is tempered.
The process: The glass is heated then rapidly cooled, the inside cools slower than the outside so it creates compressive stresses on the surface and tensile stresses on the inside.
Properties: Increases strength and makes it shatter into small blunt fragments that are less dangerous.
What is structural glazing?
- Forms major elements of the structure
- Takes some of the loads that the structure is subject to
What are the 2 types of structural glazing and what are their characteristics?
Patent Glazing:
- Supported by aluminium frames at edges
- Frames are attached to the primary structure
- Used for roof glazing but also ‘curtain walling’
- ‘Dry’ glazing system
Pilkington ‘Planar’:
- Frameless
- Flush glass surface
- Steel fittings attach to the primary structure or glass fins
- A silicone seal between panels
- A wide range of glazing types and thicknesses
What is thermal shock?
- A way in which ceramics and glass can degrade
- Caused by rapid temperature change or uneven heating/cooling
- Induced stress can cause:
- Strength reduction
-Cracking or shattering
- Strength reduction
What are some other types of degradation?
- Chemical degradation: hydrofluoric acid can dissolve glass
- Weathering: bricks and other ceramics used in structures can degrade due to the accretions of wind, rain, ice, heat etc.
- Glass disease: Caused by salts that leach out of the glass and form a crust on the surface
- Extreme heat: Furnaces, ovens, fireplaces, etc. can cause internal brick lignin to wear out and crack
