Lecture 07

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Lecture 07 Powered By Docstoc
					Today’s objectives-Powder and advanced
    ceramics processing techniques
 1. Cements
 2. Powder processing
    a. Pressing
    b. Sintering
 3. Other processing Techniques
    a. Sol-gel
    b. Bio mimetic
    c. Single crystals
    d. Thermal oxidation
                            Know the basic concepts
    e. Sputtering          behind fabrication and use
    f. PLD                      of three of these

    g. CVD
    h. MEMS
• Prepare by mixing clay and lime (CaO)
• Calcinate at 1400C
• Grind into a fine powder.
   – Higher surface area
   – More reactive
• Add water=paste->sets and hardens->structural element
  (especially in compression).
• With cements, the bonding process is a chemical hydration
  reaction between the water that has been added and the
  various cement constituents, not a drying event.
        2Ca2 SiO4  5H 2O  3CaO * 2 SiO2 * 4 H 2O  Ca OH 2  58 .7kJ

• The cement particles are bonded together by reactions that
  occur at the particle surfaces. Smaller particles=more surface
  area=better response.
                      More cement
  • The reaction is complicated, is exothermic, and also
    exhibits swings in ph as a function of time.

•During the original moon shots, NASA tested for and found
that it would be possible to make portland cement on the
moon using the particles there. Construction! But, where do
we get the water?
•If lime could be mined and refined, then it has a cementious
reaction that doesn’t require water, but rather CO2.
                     Powder processing

• Procedure:
 --grind to produce ceramic and/or glass particles
 --inject into mold
 --press to improve density by better packing (ideally close packed)
 --sinter at elevated temperature to further densify by shrinking pores

• Sintering: useful for both clay and non-clay compositions.

• Compare powder processing to working with clay:
                                      •Less uniform
                                                 •Less dense
                                                 •Has to be dried
                                                 •No need to press

• Pressing minimizes pores
   – Uniaxial
   – Hydrostatic (all axes)
• Either can be at elevated temperatures
   – “H Uniaxial P, HUP” or “HIsostaticP, HIP”
   – Improve density without growing grain
   – Usually only for high temperature materials
   – Costly
                   Stages of sintering

                   • Egb<Esurface
                         – Thus, if atoms are given enough energy to
                           move (i.e. during sintering), then grain
                           boundaries grow and pores shrink.
                         – Eventually, pores are pseudo spherical and
                           grain boundaries exist at every initial particle
                           junction. Why? Is this important?
                         – Process is atomic diffusion, not liquid flow.
                         – Introducing glass formers (TiO2, Bi2O3) will
                           improve pore reduction and boundary growth
                           as atomic diffusion is faster in the liquid, and
                           the liquid flows. Now we are combining
                           techniques, though.
        Y 2 o 
K Ic  
              a 3
        t                        Somewhat like clays, but little if any glass.
       Powder processing microstructure
• Some pores remain
• Many particles have fused.
• Pores are obvious crack propagation points (cracks)
                   Pressing Applications
• Ball bearings of SiN instead of steel
   –   Lighter
   –   Higher elastic modulus (320 instead of 200) so less deformation
   –   Better compressive strength (3000 MPa vs 900 MPa) so less wear
   –   Corrosion resistant
• Piezoelectrics (sonar, telephone vibration, inkjet printer
• Dielectrics (cell phones)
• Ceramic armor
• Used in:
    –   Paint
    –   Paper
    –   Inks
    –   Soap
    –   Electronics (cell phones)
    –   Plastics
    –   Cosmetics
    –   Suntan lotion
    –   Oreos
    –   Surge protectors
• 4 Million tons worldwide–11000 tons/day (=96% of total Ti use)
• 68% of worldwide TiO2 production capacity is in the hands of
  five US-based companies: Dupont (1 million metric tons
  annually), Millennium Chemicals, Huntsman Tioxide, Kerr-
  McGee Chemical and Kronos Inc.  

                   TiO2- from WikiPedia (!)
• As a pigment of high refringence Titanium dioxide is the most widely
  used white pigment because of its brightness and very high refractive
  index (n=2.4), in which it is surpassed only by a few other materials.
  When deposited as a thin film, its refractive index and color make it an
  excellent reflective optical coating for dielectric mirrors. TiO2 is also an
  effective opacifier in powder form, where it is employed as a pigment to
  provide whiteness and opacity to products such as paints, coatings,
  plastics, papers, inks, foods, and most toothpastes. Used as a white
  food coloring, it has E number E171. In cosmetic and skin care
  products, titanium dioxide is used both as a pigment and a thickener. It
  is also used as a tattoo pigment and styptic pencils.

• This pigment is used extensively in plastics and other applications for
  its UV resistant properties where it acts as a UV reflector.

• In ceramic glazes titanium dioxide acts as an opacifier and seeds
  crystal formation. In almost every sunblock with a physical blocker,
  titanium dioxide is found both because of its refractive index and its
  resistance to discoloration under ultraviolet light. This advantage
  enhances its stability and ability to protect the skin from ultraviolet light.
• Want a high temperature before even
  partial melting (liquid composition=0).
  Need chemical insensitivity.
• Improve properties further by including
   – less thermal expansion/contraction upon
     thermal cycling
   – Resistance to thermal shock
   – Increased insulation
   – Lighter
• But, some disadvantages:
   – Worse resistance to
     chemical attack
   – Weaker load bearing
     capability (probably not a
     problem since refractories
     can be used to protect load-
     bearing structures).
   – Some liquid is ok at max T.
                          Sol-Gel processing
• Prepare a liquid “sol” that is mostly colloidal
    – Inorganic metal salts or metal
      alkoxides (metal organics)
    – Hydrolize and polymerize to form
      a colloidal suspension (“sol”):
      like glass, but not viscous & in solution
    – Spray, dip coat, or spin coat
      onto a substrate
    – The cast sol now forms a “gel”
    – Further drying converts to a loose ceramic
    – Anneal to burn off the polymer
    – Anneal at higher temperatures to densify
• This is a primary process for preparing small, pure
  particles of most performance ceramics.
• If the sol is cast before the gel has set, then a porous,
  low density material can be made (“aerogel”).
    – Excellent thermal insulator
    – Lousy mechanical strength
• Fibers can be drawn from the sol as well under
  certain conditions.                                                        Thickness=C/speed½
• Annealing at lower temperatures allows porous
  inorganic membranes
• Snails, seashells, krill, etc, have evolved methods for
  fabricating phenomenally complicated ceramic structures
  (mostly silica and calcia), usually at ambient conditions.
• Research continues into trying to understand how this is
  done, and especially how we can mimic it.
   – Proteins have been harvested, and even improved, for
     low temperature fabrication of SiOx.
      • The protein R5 is especially good at converting Si and O from
        solution into crystalline SiOx.
      • Applying the protein to a dental surface and exposing to a Si
        rich solution offers the possibility of low temperature SiOx
        deposition and thus simple tooth regeneration.
   – Opposite halves of DNA base pairs are attached to a
     substrate and a ceramic particle, respectively. The
     particles can then be selectively bound to a surface in a
     cost effective aqueous procedure, avoiding the
     complexity of high energy and vacuum based deposition
     techniques like CVD, PLD, and sputtering.
   – Spiders are great at forming SiOx containing fibers.
     Harvest these for mechanical or fiber-optic applications.
                 Single Crystal processing
• Czochralski growth
   –   Melt starting materials in a crucible
   –   Introduce a seed crystal
   –   Rotate and slowly extract the seed crystal
   –   Large diameter, dislocation free

  Most common method for semiconductor substrates, but also optical materials.
       Single crystal from polycrystalline bulk.
 • Float zone
     – Carrier concentration can be improved three orders of
       magnitude over CZ methods.

Polycrystalline                                       Ultra pure
raw materials                                       single crystal
•   Cements- strength and temperature vs time
•   Powder processing-distinction between clays and
•   Sintering-procedure
•   Pressing-types and advantages
•   Concepts and applications of three of the following
    –    Sol-gel
    –    Bio mimetic
    –    Single crystals             Know the basic concepts
    –    Thermal oxidation          behind fabrication and use
    –    Sputtering                      of three of these
    –    PLD
    –    CVD
    –    MEMS

                  Reading for next class
        Optical Fiber processing
        Chapter sections: Handout

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