In The Name of God

1
In The Name of God
GAS PHASE SYNTHESIS
OF NANOCRYSTALS
Hajar Ebrahimi
by
hajar_at_mehr.sharif.edu
2
Outline

• Introduction
• Gas phase synthesis methods
• - gas condensation processing
• - chemical vapor condensation
• - expansion cooling
• Difference in products
• Advantages

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Introduction

• Synthesis methods of nanostructure materials is
  divided into two different groups
• - top-down methods
• - bottom-up methods
• gas phase synthesis

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Gas phase synthesis methods

• Inert gas condensation
• - gas condensation processing (GPC)
• - chemical vapor condensation (CVC)
• Expansion cooling

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Gas condensation processing

• Inert gas pressure
• 1 - 50 mbar
• Glass evacuated
• Crucible temperature
• 500-1000 c

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Growth of nanoparticles
Collisions with inert gas atoms
Losing energy

Metal vapor becomes cold
supersaturation
Homogeneous nucleation
coalescence
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Changes in size of nanoparticles

• higher source temperature
• higher evaporation rate
• higher inert gas pressure
• heavier inert gas atoms
• Longer residence time in growth regime
• larger
  particles

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Chemical vapor condensation method

• GPC evaporative source is replaced by a hot-wall
  tubular reactor.
• CVC instead of GPC for
• - high melting points material
• - low vapor pressure

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Schematic diagrams of

• CVC apparatus reactor

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CVC process

• A low concentration of precursors in the carrier
  gas
• Rapid expansion of the gas stream through the
  uniformly heated tubular reactor
• Rapid quenching of the gas phase clusters in a
  low pressure chamber

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Expansion-cooling

• Expansion of a condensable gas through a
  nozzle leads to cooling of the gas and a
  subsequent homogeneous nucleation and
  condensation.

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Changes through nozzle

• change in velocity change in pressure
  
• 
  temperature

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Size distribution

• change in pressure change in the
• length
  of nozzle

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Difference in products

• Expansion cooling is used more for metals.
• CVC is used to produce a variety of ceramics more
  than GPC.

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Advantages

• high purity
• ability to create complex chemical structures
• Products are loosely agglomerated.

• continuous process
• controllable process and products

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THANK YOU
THE END
hajar_at_mehr.sharif.edu