Materialen op een koolstofbasis

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Materialen op een koolstofbasis

• Focus on synthesis and properties of three
  classes of materials
• Graphite and pyrolytic carbon
• Carbon nanotubes and fullerenes
• Conjugated polymers
• Properties and structure of carbon materials are
  a result of the covalent C-C bonds

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Carbon allotropic forms
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Pyrolytic carbon

• Belongs to the special family of turbostratic
  carbons
• Structure related to that of graphite
• Graphite carbon atoms are covalently bonded in
  planar hexagonal arrays that are stacked in
  layers with relatively weak interlayer bonding.
• Turbostratic carbons stacking sequence is
  disordered and wrinkles or distortions may exist
  within each layer.

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Pyrolytic carbon

• Excellent mechanical properties
• Good inherent cellular biocompatibility with
  blood and soft tissue.
• Durable, strong, and resistant to wear
• Highly thromboresistant (resistant to blood
  clotting)

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Pyrolytic carbon
Large differences in texture possible
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Pyrolytic carbon
Atomic force microscopy (AFM) / Scanning
tunneling microscopy (STM)
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Pyrolytic carbon

• Manufacturing
• Small fluidized bed reactor (1000-2000 C)
• Coatings of up to 500mm
• Fine-grained isotropic graphite most common
  substrate
• Machining and polishing required

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Pyrolytic carbon
For heart valves, a silicon-alloyed pyrolytic
carbon coating is used. Silicon is added to
improve mechanical properties such as stiffness,
hardness, and wear resistance. Fabrication
involves co-depositing of carbon and silicon
carbide on the graphite substrate
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Pyrolytic carbon

• Blood compatibility likely is believed to be a
  result of its apparent inertness and ability to
  absorb proteins
• However.
• Blood compatibility is not perfect.
• The mechanism for blood compatibility is not well
  understood.
• Pyrolytic carbon is not inert
• Anticoagulant therapy remains required.
• Still one of the best materials currently
  available.

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Carbon allotropic forms
Carbon nanotubes
Pyrolytic carbon
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Carbon nanotubes
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Carbon nanotubes

• Characteristics
• Between 1 and 50 concentric tubes
• Layer separation 3.4 Å
• Smallest possible diameter 7.1 Å (C60)
• Five-rings present for curvature (reactivity!)

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Carbon nanotubes
Structural direction (chirality!) important for
electronic properties (semiconductor or metal)
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Carbon nanotubes

• Synthesis
• Electrical arc discharge
• Catalytic decomposition of hydrocarbons or
  Catalytic Chemical Vapor Deposition (CCVD)
• Laser ablation
• Microwave plasma chemical vapor deposition
• Polymer pyrolysis
• Etc.

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Carbon nanotubes

• Synthesis using Electrical arc discharge
• Typical current of 100 A at 18 V in Helium
  atmosphere
• Graphite electrodes (consumptionof positive
  electrode)
• Usually generates mixturesof fullerenes,
  SWNTsand MWNTs
• Maximum yield ofnanotubes ca. 60

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Carbon nanotubes

• Synthesis using Catalytic Chemical Vapor
  Deposition
• In situ formation possible
• Large yields
• Comparatively purecompounds
• Not suitable for fullerenes

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Carbon nanotubes

• Synthesis using Catalytic Chemical Vapor
  Deposition
• Decomposition of carbon rich gases at a catalysts
• Typical catalysts Co, Fe and Ni
• Typical gases CH4,CO, C2H2
• High temperatures(500 1500 C)
• 2 CO ? CO2 C

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Carbon nanotubes
Tip growth model Growth by insertion of
smaller carbon species into the graphitic network
of a smaller, closed nanotube. The catalyst will
remain in the tip.
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Carbon nanotubes
Base growth model Growth by addition of smaller
carbon species to the open end of a nanotube. The
catalyst will remain at the bottom. More
generally accepted
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Carbon nanotubes

• Applications
• Patterned growth from surfaces
• FET and SET structures

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Carbon nanotubes
Biomedical applications are thus far
limited Exception Nanotubes as chemically and
biologically-sensitive probes
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Conductive polymers
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Conductive polymers
Nobel prize chemistry 2000 Discovery of electric
conductivity in polymers by Alan J. Heeger, Alan
G. MacDiarmid and Hideki Shirakawa
Conductivity Polyacetylene 10-8
Sm-1 Copper 108 Sm-1 Silicon 10-3
Sm-1 Teflon 10-16 Sm-1
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Conductive polymers
Conductivity U R I (Ohms law) R r l / A r
resistivity r 1/s s conductivity s q n
m (q charge n number m mobility) Conductiv
ity is determinedby charge carrier densityand
mobility
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Conductive polymers
Conductivity lt10-8 Sm-1 Insulators 10-8 102
Sm-1 Semiconductors 102 108 Sm-1 Metallic
conductors gt108 Sm-1 Super conductors For
Example GaAs µ 104 cm2V-1S-1 PPV µ
10-6 cm2V-1S-1 Amorphous silicon µ 10-8
cm2V-1S-1
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Conductive polymers

• High Conductivity
• High charge carrier density
• High charge carrier mobility

• However. Conductive polymers
• Good charge carrier mobility
• Very low intrinsic charge carrier density

Polyacetylene
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Conductive polymers

• An increased charge carrier density is needed!
• Two methods
• Injection of additional charge carriers High
  currents
• Doping Generation of extrinsic charge carriers
  at low currents by addition of chemicals

Total charge carrier density ? ? ?int ?ex
?inj
Small in conductive polymers
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Conductive polymers
(oxidative doping or p-doping) Polymer
conductivity after doping 102-107 Sm-1
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Conductive polymers
Why do we want to use conductive polymers? SIZE
3 Å
Compare AMD Athlon / Intel Pentium ? 0.13 mm
technology (Best available microlithography circa
0.05 mm) Conductive polymer devices can be a
factor of 100 smaller!
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Conductive polymers

• FLEXIBILITY
• Twisting and bending without any effect on
  electrical characteristics
• Light Emitting Foils
• Electronic Labels
• Smart Windows
• Implantable flexible biosensors

Dupont/Uniax
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Conductive polymers

• LOW COST
• Polymers are easy to process (plastic technology)
• Small size
• Low weight
• Not much waste
• SMALL AMOUNT OF MATERIAL HIGH ECONOMIC
  IMPACT/VALUE
• AN IDEAL SPECIALTY POLYMER

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Conductive polymers
Current engineering challenges STABILITY Intrinsi
c instability Slow degradation over time in a dry
and oxygen free environment, caused by
irreversible chemical reactions between charged
sites of the polymer Extrinsic instability Polymer
vulnerability to external attack by compounds
such as O2 and H2O, which is problematic during
production and processing
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Conductive polymers

• PROCESSIBILITY
• The rigid conjugated backbone of conductive
  polymers, which is essential for the
  conductivity, gives problems during processing.
• Polymers are often insoluble
• Polymers have high melting points
• SCALE
• Most polymers are only available in milligram or
  gram laboratory quantities new chemical
  processes are essential.

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Conductive polymers

• Applications
• Light emitting diodes
• Solar cells
• Anti-static coatings
• Electromagnetic shielding
• Sensors
• Displays
• Plastic electronics
• Biosensors

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Sources of Pictures and Text
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Forms_of_Carbon.html http//www.swri.org/3pubs/tto
day/summer99/valve.htm http//www.rz.uni-karlsruhe
.de/lem/forschung/pyrocarbon/Infiltrated_carbon.h
tm http//www.eng.odu.edu/interaction/e-Interactio
n20Nov2024,202000.htm http//www.chem.qmw.ac.uk
/surfaces/scc/ http//www.mcritx.com/valvedesign.h
tm http//wwwrsphysse.anu.edu.au/admin/php/photos_
materials.php http//www.astrosurf.com/lombry/seti
-civilisations-avancees2.htm http//pages.unibas.c
h/phys-meso/Education/Teaching/teaching.htmlCNT h
ttp//data.engin.umich.edu/umseds/kc135/nanotubes/
report/FinalReport.htm http//nanotube.msu.edu/syn
thesis/cvd.html