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Lecture 07


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  • 1. Today’s objectives-Powder and advanced ceramics processing techniques
    • Cements
    • Powder processing
      • Pressing
      • Sintering
    • Other processing Techniques
      • Sol-gel
      • Bio mimetic
      • Single crystals
      • Thermal oxidation
      • Sputtering
      • PLD
      • CVD
      • MEMS
    Know the basic concepts behind fabrication and use of three of these
  • 2. Cements
    • 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.
    • The cement particles are bonded together by reactions that occur at the particle surfaces. Smaller particles=more surface area=better response.
  • 3. 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 CO 2 .
  • 4. 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: PARTICULATE FORMING
    • Less uniform
    • Less dense
    • Has to be dried
    • No need to press
  • 5. Pressing
    • 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 boundaries
      • Usually only for high temperature materials
      • Costly
  • 6. Stages of sintering
    • E gb <E surface
      • 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 (TiO 2 , Bi 2 O 3 ) 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.
    Somewhat like clays, but little if any glass.
  • 7. Powder processing microstructure
    • Some pores remain
    • Many particles have fused.
    • Pores are obvious crack propagation points (cracks)
  • 8. 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 heads)
    • Dielectrics (cell phones)
    • Ceramic armor
  • 9. TiO 2 -Dupont
    • 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 TiO 2 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.,1846,115639,00.html
  • 10. TiO 2 - 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.
  • 11. Refractories
    • Want a high temperature before even partial melting (liquid composition=0). Need chemical insensitivity.
    • Improve properties further by including pores
      • 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.
  • 12. 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.
    • Annealing at lower temperatures allows porous inorganic membranes Thickness=C/speed ½
  • 13. Biomimetic
    • 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 SiO x .
        • The protein R5 is especially good at converting Si and O from solution into crystalline SiO x .
        • Applying the protein to a dental surface and exposing to a Si rich solution offers the possibility of low temperature SiO x 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 SiO x containing fibers. Harvest these for mechanical or fiber-optic applications.
  • 14. 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.
  • 15. Single crystal from polycrystalline bulk.
    • Float zone
      • Carrier concentration can be improved three orders of magnitude over CZ methods.
    Polycrystalline raw materials Ultra pure single crystal
  • 16. SUMMARY
    • Cements- strength and temperature vs time
    • Powder processing-distinction between clays and cements
    • Sintering-procedure
    • Pressing-types and advantages
    • Concepts and applications of three of the following
      • Sol-gel
      • Bio mimetic
      • Single crystals
      • Thermal oxidation
      • Sputtering
      • PLD
      • CVD
      • MEMS
    Reading for next class Optical Fiber processing Chapter sections: Handout Know the basic concepts behind fabrication and use of three of these