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Functional coating > Functional coatings > Flashcards

Functional coatings Flashcards

1
Q

Why is TiO2 brilliant?

A
  • high refractive index
  • high photo catalytic stability in the visible region
  • physiological harmlessness (low toxicity)
  • resistant to high temperatures
  • low reactivity, insoluble in water, organic solvents , alkalis and most inorganic acid
  • UV Photoactivity
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2
Q

Hiding power

A

-Paint property related to how much pigment is required for complete opacity.

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3
Q

Two routes of manufacture of TiO2

A
  • Sulphate process
  • Chloride process

-Essentially converts impure TiO2 into another chemical, separate the impurities and then convert back into pure TiO2 .

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4
Q

Sulphate process

A
  1. Slags are ground, dried and classified
  2. Dissolving the titanium rich ore ilmenite(FeTiO3) in hot concentrated sulphuric acid This produces titanylsulphate( TiO(SO4) ). Addition of Fe to ensure Fe2+ not Fe3+ (Fe2+ is more soluble to hydrolysis) –this is comproportionation Fe + Fe3+ -> Fe2+

FeTiO3 + 2H2SO4 -> FeSO4 + TiO(SO4) + 2H2O

  1. This is then hydrolysed thermally to precipitate titanium oxyhydroxide. Seed crystals are added in order to orientate the crystal formation towards anatase or rutile
    TiO(SO4) + (n+1)H2O -> TiO2.nH2O + H2SO4
  2. Calcination of this yields anatase(800-850°C) or rutile (900-930°C)
    TiO2.nH2O -> TiO2 + nH2O
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5
Q

Chloride process

A
  1. Carbothermal chlorination
    The ore (containing at least 70% TiO2) is mixed with pulverized coke and passed into a chlorinator at elevated temperatures (900 -1700°C) . This produces titanium tetrachloride.
    TiO2 + C(s) + 2Cl2 -> TiCl4 + CO2
  2. The TiCl4 is then purified and passed into an oxygen flame. The chlorine released during oxidation is collected and recycled into the chlorinator.
    TiCl4 + O2 -> TiO2 + 2Cl2
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6
Q

Differences between sulphate and chloride processes

A
  • Sulphate is batch, chloride is continuous
  • Sulphate uses lower grade staring materials but chloride produces purer products
  • Sulphate creates large amounts of waste
  • Sulphate lower capital investment
  • Sulphate produces anatase and rutile, where as chloride produces on rutile
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7
Q

3 main forms of titanium dioxide

A
  • Anatase
  • Rutile
  • Brookite
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8
Q

Photoactivity of TiO2

A

TiO2 + hv –> TiO2 (e- + h+)

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9
Q

Wetting

A

Ability of a liquid to wet to a smooth surface can be determined by the contact angle

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10
Q

Poor wetting

A
  • Hydrophobic

- Wetting angle of around 150’

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11
Q

Normal wetting

A

90’

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12
Q

Good wetting

A
  • Hydrophilic

- 10’

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13
Q

Components of paint

A

Binders –organic macromolecular compounds. Anchor pigments and extenders in the film

Solvents –used to dissolve the natural or synthetic resins used as binders. Influence the flow properties

Pigments –the essential colouring components of the film

Extenders –additions that make formulations more economical. Influence in-service properties

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14
Q

Three approaches for self-cleaning surfaces

A
  • hydrophobic -Cleans by rolling droplets
  • hydrophilic - sheeting water carries away dirt
  • photocatalytic - UV activated
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15
Q

Photocatalytic surfaces reactions

A

TiO2 + hv -> TiO2 (e-+ h+)
forms an electron and a hole
Electron -reducing agent, forms perhydroxyl radical with oxygen and water
Hole - oxidizing agent , forms hydroxyl radicals with OH

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16
Q

Electron reaction to form radical

A

e- + O2 –> •O2-

•O2- + H2O –> OH- + •O2H

17
Q

Hole reaction to radical

A

h+ + OH- –> •OH

18
Q

Water purification reaction

A

•OH + dye molecule → intermediates → CO2↑ + H2O + mineral salts.

19
Q

Anti fog

A

Achieved using a very hydrophilic surface, forms a film over surface

20
Q

Transparent conducting oxides

A
  • Fluorine dope tin oxide (FTO)

- Indium Tin oxide (ITO)

21
Q

Manufacture of FTO

A
  • Made via a float line using chemical vapour deposition
  • Precursors in vapour form are decomposed into atoms or molecules
  • React with each other at the hot substrate surface to form non-volatile product coating
  • Decomp triggered by high temp, plasma or light
  • Does not need a vacuum
  • SnCl4+ 2(H2O) -> SnO2 +4(HCl)
  • Desorption of volatile surface products
  • Barrier layer (SiO2) deposited before SnO2 to stop diffusion of ions from soda lime glass into coating which would disrupt electrical properties
22
Q

Manufacture of ITO

A

-Made via physical vapour deposition (sputtering)
-System consists of vacuum, cathode assembly including target material for sputtering, working gas (Ar) at low pressure and substrate to be coated
• Establishment of a glow discharge gas in the gas to create Ar+ ions
• Application of negative bias
to the target to attract the ions
• Ion bombardment of the target and ejection of atoms at the target

23
Q

Sputtering yield

A

depends on:
• Energy and angle of incidence of the ion
• Mass of the ion and target atoms
• Binding energy of the surface atom

24
Q

Challenges of scale

A
  1. Suitability for manufacturing in a roll to roll process (related to flexibility of the coating).
  2. Earth abundancy. Are the materials able to transfer to scale, is there enough mine-able material for that sort of manufacturing level.
  3. Does the material have a chemical interaction with the steel? It needs to be inert to the steel or have an additional coating for chemical isolation.
  4. Can it be manufactured at speed or in a continuous process?
  5. Are the materials sets cheap enough?
  6. Does the performance translate at scale?
  7. Ability to handle a rough surface
  8. Durability and lifetime
25
Q

Challenges of scale (Glass)

A
  1. Roll to roll processing no longer an issue. The material no longer requires a level of flexibility.
  2. Deposition possible on a sheet process .
  3. Flatness of the substrate dictates how thin the material might be.
  4. The substrate is far smoother so this will have a greater effect.
  5. Opacity is critical for glazing products.
  6. Ensuring the coating does not have too much influence on the solar gain of the window itself (possible conflict).
  7. Tighter temperature limitations for glass products than for some metal products.
26
Q

The band gap problem

A

TiO2 absorbs in the UV region but we need visible

The solution is to cover the TiO2 in a dye that absorbs in the visible region

27
Q

Dye-sensitized solar cell electron transfer processes

A
  • 7 electron transfer processes
  • 4 good ones
  • 3 bad ones
28
Q

Good transfer processes

A

Firstly there is photon absorption of the sensitizer S + hv -> S*

1) Charge Injection
2) Diffusion through the TiO2
3) Reduction of Triiodide
4) Regeneration of the dye

29
Q

Bad transfer processes

A

5) excited state decay
6) Recombination from the TiO2 to the dye
7) Recombination from the TiO2 to the electrolyte

30
Q

Dye vs Perovskite differences

A

(i) A hole transport material is used instead of a liquid redox couple
(ii) The use of a blocking layer is required due to shorting between the HLT and the working electrode substrate
(iii) A metal contact is required as an electrical contact
(iv) A thinner layer of TiO2 is used, typically 500nm instead of 10µm
(v) PEROVSKITE material is used instead of a dye

31
Q

Advantages of BIPV

A
  • There is no additional requirement for land
  • The cost of the PV can be offset against the cost of the building
  • Power is generated onsite (otherwise purchased at commercial rates)
  • Grid connection avoids the high cost of storage
  • Public expression of environmental commitment
32
Q

Micro encapsulation for self healing

A
  • Micro-capsules filled with polymerisible healing agents and catalyst embedded in a polymer matrix
  • Crack ruptures capsule, healing agent leaks out and comes in contact with the catalyst
  • Healing agent polymerises, seals the crack and prevents crack propagation
33
Q

Draw an energy level diagram illustrating all electron transfer processes with arrows

A

In book.

34
Q

Resistivity heated floor tile

A
  • The process by which the passage of an electric current through a conductor releases heat
  • Conventional current flows from positive to negative terminal
35
Q

Transpired solar collector

A
  • A micro-perforated solar collector that can act as an active solar air heater
  • Functions as a third skin with optimised perforations to absorb and trap solar energy and convert it into heat
  • Perforations allow a boundary layer of heat to be captured and drawn uniformly into an air cavity behind to exterior facade
  • This is then drawn into the buildings ventilation system
  • Southerly facing wall mounted
36
Q

Electroluminescent lighting system

A
  • Printed Thick Film Electroluminscence (TFEL)
  • Phosphor layer that emits light when a big enough electrical field is applied
  • phosphor film requires such a high level of energy that there is a potential for damaging short circuit through imperfections
  • Insulating layers are used between electrode and phosphor on both sides to limit current
37
Q

Low emissivity glazing

A

The requirements are transparent in the visible region and high infrared reflectivity

Functional coating (1 decks)

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