Functional coatings

Question 1

Why is TiO2 brilliant?

Answer 1

• 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

Question 2

Hiding power

Answer 2

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

Question 3

Two routes of manufacture of TiO2

Answer 3

• Sulphate process
• Chloride process

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

Question 4

Sulphate process

Answer 4

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

3. 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
4. Calcination of this yields anatase(800-850°C) or rutile (900-930°C)
   TiO2.nH2O -> TiO2 + nH2O

Question 5

Chloride process

Answer 5

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

Question 6

Differences between sulphate and chloride processes

Answer 6

• 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

Question 7

3 main forms of titanium dioxide

Answer 7

• Anatase
• Rutile
• Brookite

Question 8

Photoactivity of TiO2

Answer 8

TiO2 + hv –> TiO2 (e- + h+)

Question 9

Wetting

Answer 9

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

Question 10

Poor wetting

Answer 10

• Hydrophobic

- Wetting angle of around 150’

Question 11

Normal wetting

Answer 11

90’

Question 12

Good wetting

Answer 12

• Hydrophilic

- 10’

Question 13

Components of paint

Answer 13

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

Question 14

Three approaches for self-cleaning surfaces

Answer 14

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

Question 15

Photocatalytic surfaces reactions

Answer 15

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

Question 16

Electron reaction to form radical

Answer 16

e- + O2 –> •O2-

•O2- + H2O –> OH- + •O2H

Question 17

Hole reaction to radical

Answer 17

h+ + OH- –> •OH

Question 18

Water purification reaction

Answer 18

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

Question 19

Anti fog

Answer 19

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

Question 20

Transparent conducting oxides

Answer 20

• Fluorine dope tin oxide (FTO)

- Indium Tin oxide (ITO)

Question 21

Manufacture of FTO

Answer 21

• 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

Question 22

Manufacture of ITO

Answer 22

-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

Question 23

Sputtering yield

Answer 23

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

Question 24

Challenges of scale

Answer 24

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

Question 25

Challenges of scale (Glass)

Answer 25

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.

Question 26

The band gap problem

Answer 26

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

Question 27

Dye-sensitized solar cell electron transfer processes

Answer 27

- 7 electron transfer processes - 4 good ones - 3 bad ones

Question 28

Good transfer processes

Answer 28

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

Question 29

Bad transfer processes

Answer 29

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

Question 30

Dye vs Perovskite differences

Answer 30

(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

Question 31

Advantages of BIPV

Answer 31

- 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

Question 32

Micro encapsulation for self healing

Answer 32

- 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

Question 33

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

Answer 33

In book.

Question 34

Resistivity heated floor tile

Answer 34

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

Question 35

Transpired solar collector

Answer 35

- 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

Question 36

Electroluminescent lighting system

Answer 36

- 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

Question 37

Low emissivity glazing

Answer 37

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