Degradation Flashcards
1
Q
Glass
A
- only material that can be recycled an infinite number of times
- An amorphous brittle solid which is made up of a network of covalent atoms (silica tetrahedron)
- Brittle
- Has a Tg
2
Q
Why is glass transparent
A
- Has a band gap that is longer than the wavelength so electrons do not have enough energy to be excited and are not absorbed
- No grain boundaries so there is nothing that will cause scattering of light
3
Q
Q
A
- Qn has n bridging atoms. If something is Q4, all 4 oxygen atoms are bonded to a central Si atom
- Q refers to the number of bridging oxygens
4
Q
Glass Transition Temperature
A
- Below Tg, glass is solid
- After Tg, flow begins to occur and movement of atoms occur so that the atoms can rearrange
5
Q
Crystallisations Temperature
A
- At Tc onset, crystals start to form
6
Q
Sol-gel Derived glass
A
- Occurs at a lower temperature than melt derived glass (can occur at room temperature)
- sodium is not needed as it is not a melting process
- Creates a nano-porous texture with high specific surface area but it is hard to make in large monoliths as capillary stresses are high so it may crack in larger monoliths
- Sol-gel is made up of dispersions of colloidal particles in a liquid (solid particles which float around int he solution)
- Gels are made up of long polymer chains that are bonded together and water gets in between the chain which causes swelling, gel must then be dried to be left with only the network
7
Q
Drying Sol-Gel
A
- Ambient drying (normal method - oven) Creates a Xerogel
- Critical point Drying (sublimation) which creates an aerogel which has 99% porosity
- Water is replaced with CO2 in the drying process which becomes liquid in a chamber. The CO2 in the pores gets sublimed off so there is no time for shrinkage and gel is kept in its original volume
8
Q
Sol-Gel process
A
- MIXING: water + alkoxide + catalyst are mixed at room temperature
- SOL PRODUCTION: colloidal solution of silica nanometers
- GELATION: agglomeration and gelation of nanoparticles where a network starts to grow so viscosity increases as those bonds are formed.
9
Q
Glass Corrosion
A
- Glass is highly resistant to corrosion but when it interacts with the environment through ion exchange and dissolution it can still occur
- Corrosion is caused by hydrofluoric acid, concentrated alkali solutions and superheated water
- Alkalis attach the silica network directly causing the surface to dissolve. If the supply of alkali is sufficient, corrosion is at a uniform rate
- Acids will dissolve the alkali in the glass composition and a porous surface is left which consists of the silica network with holes where the alkali has been removed. The porous surface will slow down the rate of attack since the acid must penetrate this surface layer to find more alkali to dissolve
10
Q
Common composition of melt-derived glass
A
- Soda lime silica (SiO2-Na2O-CaO)
- Consists of SiO2 network former and sodium and calcium network modifiers.
- The network modifiers lower the melting temperature to make the glass easier to produce
- It is unstable in aqueous environments especially at high temperatures making it prone to corrosion
- Corrosion is more likely in Q2/Q3 structures as they have non-bridging oxygen which are more reactive
11
Q
Corrosion of water
A
- Works similarly to acid where alkali is removed from the glass surface but at a much slower rate
- At high temperatures, water corrosion becomes significant because glass is hydrophilic which holds and attracts moisture
- Glass has a molecular layer of moisture on the surface which increases with humidity or rainfall contributing to the destruction of the surface of the glass
- Ion exchange occurs between H+ (in water) and ions such as Na+ which will leave behind a hydrated silica layer
- In alkaline environments (pH > 9), complete dissolution of surface layers will occur
- In water, there will be a rapid cation exchange due to the presence of Na+ to form a silica rich layer. pH will increase rapidly which will cause corrosion in the silica-rich layer. This makes it become a type IV glass when a type II is what is wanted
12
Q
Network Former
A
- May include SiO2, B2O3, P2O5
- Borate and phosphate are very non-corrosion resistant because max Q structure is 3
- Na+ is easy to do cation exchange so the higher content means that there is less corrosion resistance (sodium is needed to melt the glass at lower temperatures)
- Adding CaO helps with corrosion resistance as water has a harder chance to do cation exchange since calcium is 2+ compared to Na+ (10% CaO is optimum)
13
Q
Commercial Glass
A
71.5% SiO2, 15.2% Na2O which is used to reduce melting temperature (also does not crystallise so glass stays transparent), 10.4% CaO which aids in corrosion resistance, 1.16% Al2O3 & 0.57% MgO which is used to slow down ion exchange since it can form covalent bonds with the network formers
14
Q
Float Glass Process
A
- SO2 gas is passed over the glass sheet after cooling
- Sodium in the surface layers will react with the SO2, to form sodium sulphate
- Deposit is washed before packing. The glass goes through reaction 1 to create silica rich layer but the reaction will be stopped as there is less alkali near the surface (Type II surface). The Silica-rich layer will prevent further corrosion when water hits the glass
15
Q
Gorilla Glass
A
- Made up of alkali aluminosilicate which has silica network former and alumina network intermediate.
- Has inherent high hardness and corrosion resistance as it undergoes chemical tampering
- Chemical tampering is where ion exchange process by replacing sodium with potassium
- K+ diffuses into the surface and gives a layer of high compressive strength
- Potassium ions are bigger than sodium so by forcing it in, the whole surface will be under stress.
- Stressed surface creates a hardness which makes gorilla glass scratch resistant
- Critical crack size for brittle failure is much longer in gorilla glass than window glass
