FIM C, Si (9)

What allotropes of carbon exist?

graphite (glassy C, graphene)

diamond (DLC, diamond-like carbon)

fullerene

carbon nanotubes

What is graphene?

a single atomic layer of carbon atoms arranged in a hexagonal (“honeycomb”) lattice

What properties does graphene have?

extremely high electrical conductivity

very high thermal conductivity

exceptional mechanical strength

What is graphite?

a stack of graphene layers held together by weak covalent bonds

What does a layered structure of graphite lead to?

strongly anisotropic (direction-dependent) mechanical, thermal and electrical properties

What is the relationship between tensile strength of graphite and T?

increases slowly to a peak at 2500 degrees and rapidly decreases

What is the relationship between electrical resistivity of graphite and T?

decreases until 500 degrees and then increases slightly

What is the relationship between thermal conductivity of graphite and T?

decreases with T

How resistant is graphite against HF acid?

resistant until concentration above 60 wt.%

How resistant is graphite against HCl and H₃PO₄?

at all concentrations

How resistant is graphite against H₂SO₄?

at 0-80 wt.% until 160 degrees

at 80-90 wt.% untl 140 degrees

What is graphite used for?

40% for electrodes

alloying element in steel

solid state lubricant

filler for polymers for electrical conductivity

What are some examples of graphite electrodes?

electrodes for arc melting furnaces

pre-baked anodes for Al production

carbon brushes for electric motors

anode material in LIBs

How can graphite be produced?

natural graphite mined by open pit and underground methods

synthetic graphite is produced by graphitization of petroleum coke

Where does graphite occur naturally?

metamorphic rocks as a result of reduction of sedimentary carbon during metamosphism

What are advantages and disadvantages of synthetic graphite?

superior consistency and purity

very expensive and high T production route

costs up to 10 times more

Where can synthetic graphite be produced?

acheson graphitization furnace (rods perpendicular)

castner graphitization furnace (rods horizontal)

What is carbon black?

synthetic paracrystalline form of graphite particles with high SA/V

How is carbon black synthesized?

partial combustion of aromatic heavy HCs from petroleum or coke (furnace black method)

What can carbon black be used for?

rubber reinforcement

colours and pigments (newspaper ink)

conductive component for ceramics, batteries, ink

UV protection of plastics

How can carbon black is used for rubber reinforcement?

mostly as reinforcing filler in automotive tires to increase tensile strength and roadwear resistance

What are the properties of diamond?

high hardness

high thermal conductivity

high optical transparency and wide bandgap

large electrochemical window

good p-dopability

What can diamond be used for?

cutting tools

abrasion resistant layers

polishing applications

optical components

diamond electrodes

How is synthetic diamond produced?

chemical vapor deposition for films (CDV)

high p - high T synthesis (HPHT)

detonation synthesis of nanodiamond

Describe CVD

chemical vapour deposition

carbon source with H₂/CH₄ mixture in 99/1 ratio is passed to the excitation zone at gas phase T —> diamond forming C radicals and graphite etching H radicals form

the radicals travel to the deposition zone on the substrate (e.g. Mo, W) surface

the substrate is heated so the radicals adsorb and C atoms rearrange into diamond lattice allowing layers to grow one by one

What is the main idea of CVD?

carbon-containing gas is activated into reactive radicals, which then deposit diamond onto a heated surface

How can the energy be supplied to the excitation zone?

externally by hot wire resistive heating or by plasma generator

internally by combustion (flame)

What can be used as a nucleation aid in the deposition zone?

diamond seeds

cubic boron nitride seeds

What is HPHT method?

a method that simulates diamond growth inside the Earth

graphite/carbon is placed in a growth cell

high pressure + high temperature stabilize diamond and diamond seeds to help crystal growth, resulting in large synthetic diamond crystals

What is the detonation synthesis of nanodiamonds?

use of an explosion with hexogen to briefly create extreme condition

Why is Si never found in elemental state?

it has a very high affinity for oxygen

Describe the properties of Si

hard, brittle mettaloid

alpha-Si: diamond like crystal structure

semiconductor

forms oxide passive layer

How can elemental Si be used?

deoxygenation of liquid steel

Al-Si alloys

semiconductor industry

How is Si produced?

carbothermal reduction of quartz using high purity coke

reduction in an electric arc furnace with an excess of SiO₂

What happens after liquid Si is produced?

it is casted into large flat ingots and crushed after cooling

What is the purity of liquid Si produced?

98.5-99.7%

How can produced Si be used?

directly for Al-Si alloys

further purification for Si solar cells or integrated circuits

How pure does metallurgical grade Si have to be?

98-99%

How pure does solar grade Si have to be?

99.99%

How pure does semiconductor grade Si have to be?

less than 1 ppb impurities

What can be used to purify metallurgical grade Si?

Siemens process

How does Siemens process work?

treatment of powdered metallurgical grade Si with blowing HCl at 300 degrees to get HSiCl₃

thermal decomposition of HSiCl₃ on silicon seed rods at 1000 degrees in H atmosphere to produce polycrystalline silicon

Why is there a need for monocrystalline silicon?

semiconductor industry for VLSI manufacture

What is monocrystalline silicon?

it has a single continuous crystal lattice without grain boundaries, impurities or lattice defects

How can monocrystalline Si be produced?

Czochralski process

What does Czochralski process involve?

precisely oriented seed crystal to initiate the formation of a continuous single crystal

How is silicon purified to obtain monocrystalline semiconductor grade Si?

zone melting process

Why does Si need to be doped?

increasing conductivity by providing mobile charge carriers

influencing type of carriers used for current transport (e.g. p-doping for hole transport, n-doping for electron transport)

What elements can be used for p-doping?

B

Al

Ga

What elements can be sued for n-doping?

P

As

Sb

What are silicon carbides used for?

abrasive and curring tools

caron-fibre reinforced SiC disc brakes

Why are SiC used?

high hardness

high melting T

low price and readily available

How are silicon nitrides produced?

nitridation process

carbothermal reduction of SiO₂ in N₂ atmosphere

diimide process via reaction of ammonia with SiCl₄

What is nitridation process?

heating of powdered Si in a nitrogen atm at 1300-1400 degrees

What are silicon nitrides used for?

silicon nitride ceramic for ceramic hybrid bearings, cutting tools, AFM tips

What are some properties of Si₃N₄?

high hardness and wear resistnace

good thermal and mechanical shock resistance