Radiation Heat Transfer

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View Factor Orientation (or View factor or shape factor) plays an important role in radiation heat transfer. View factor is defined as, "fraction of radiation leaving surface 'i' and strike 'j' ". Summation Rule (View Factor) If there is are similar surfaces 'i' and 'j' , then: Blackbody Radiation Exchange Radiation Exchange between Opaque, Diffuse, Gray surfaces in an Enclosure 1. Opaque 2. Surfaces 3. Two surface enclosure Radiation Shield It is used to protect surfaces from radiation act like a reflective surface. References: Material from Class Lectures + Book named Fundamentals of Heat and Mass Transfer by Theodore L. Bergman + My knowledge.  Photoshoped pics  are developed.  Some pics and GIF from Google.   Videos from YouTube ( Engineering Sights ).
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Nano-Materials

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Nano-Materials 

It is defined as, "materials having size of individual building blocks less than 100 nm".
  • Bulk gold  ↔  have no catalytic properties but Au-Nanocrystal  ↔  excellent low temperature catalyst.
Why Nano-materials?
At nanoscale strange things happen to materials and their properties may change.
  1. Reactivity  ↔  smaller particles react differently with their environment than larger particles.
  2. Size  ↔  have different optical and reflection properties.
  3. Magnetism  ↔  have different magnetic properties.

Nanomaterials Synthesis Technique

It includes design and preparation of nanomaterials with high functionality (i.e. to fabricate nanoparticles with suitable properties).
Synthesis involves the control of size, shape, structure and composition and purity of constituents.
  1. Method of synthesis
  2. Growth Kinetics

1. Method of Synthesis

Two approaches are used:

a. Top-Down Approach

  • Refers to slicing or cutting of bulk materials.
  • Biggest problem is imperfection of structure surface.
  • It involves cutting, grinding, ball milling of solid state.

b. Bottom-Up Approach

  • Refers to build-up of material from bottom, atom-by-atom, molecule-by-molecule or cluster-by-cluster.
  • Better chance to obtain nanostructures with less defects, more homogenous chemical composition.
  • Physical Vapor Disposition Technique (PVD) ↔ involves evaporation, sputtering, plasma arcing, laser ablation.

2. Growth Kinetics

Synthesis of nanoparticles is a combination of two stages: Nucleation and Growth.
  • Nucleation  ↔  in which nuclei (seeds) act as templates for crystal growth.

Properties of Nano-Particles

  1. Catalytic  ↔  Better catalytic efficiency through higher surface to volume ratio.
  2. Electrical  ↔  Increase electrical conductivity in Ceramics. Increase electrical resistance in Metals.
  3. Magnetic  ↔  Increase magnetic coercivity up to a critical grain size, super paramagnetism.
  4. Mechanical  ↔  Improve hardness and toughness in metals and alloys and ductility and super-plasticity in Ceramics.
Note: 

References:

  • Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
  • Pics and GIF from Google Images.  
  • Videos from YouTube (Engineering Sights).

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