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Lecture 05

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  • 1. Today’s objectives-Diffusion and Phase Diagrams
    • How do diffusion mechanisms differ for metals vs. ionic ceramics?
    • Why is diffusion important in ceramics?
    • How do we interpret phase diagrams?
    • How does diffusion modify our interpretation?
    • How do we use phase diagrams to our advantage?
  • 2. Diffusion
    • Diffusion in ceramics differs from in metals because there are multiple species with varying charges.
    • Since charge must balance, diffusion is often limited to the slowest moving species.
    • Vacancy mechanisms usually dominate (easier for them to move).
  • 3. Diffusion-why it matters
    • Mechanical properties (tailoring) (Ge on Si sites)
    • Current carrying (the motion of ionic species in an electric field yields an electronic current).
      • Batteries
      • Fuel cells
    • Electronic properties (IC feature size and capabilities)
    bulk Compressive at surface - + Mg i 2+ Li Mg - O i 2- Al Mg + V O 2+ V Mg 2-
  • 4. Transistors
    • Transistors must be uniform across the entire 12” wafer.
    • The dopant levels are on the order of 10 21 to depths of less then 50 nanometers.
    http://developer.intel.com/technology/itj/q31998/articles/art_3.htm
  • 5. Si,Ge multilayers—dopant mediated diffusion Ranade et. al., Electrochemical and Solid State Letters, 5 (2), 2002, p. G5.
    • Diffusion profile after annealing just 1 minute makes or breaks a device.
    • In order to understand the composition of the final structure, the phase diagram for the material must be known.
  • 6. Binary Phase Diagram with Complete SS (Cu-Ni)
  • 7. Phase diagrams: refractory: Al 2 O 3 -Cr 2 O 3
    • Exhibits a solid solution over a wide range of temps (that is, Al and Cr easily substitute for one another). Why?
    • Al and Cr ions have the same charge (3+).
    • Approximately same radii (.053 and .062, respectively).
    • Isomorphic--same crystal structure for Al 2 O 3 and Cr 2 O 3 .
    • Note temperatures—HIGH!
    Pure Cr 2 O 3 Pure Al 2 O 3 If we know temp and composition with given starting materials, then we know the phase or phases at equilibrium .
  • 8. Phase diagram-interpretation: MgO-Al 2 O 3
    • 2) Use the Lever Rule:
    73% spinel (MgAl 2 O 4 (ss)) and 27% MgO(ss). At 72 wt% Al 2 O 3 , there is a distinct compound (spinel) up to 1400 ° C and a nonstoichiometric solid solution or liquid above that T. At 50:50 wt% and 1900 ° C, what is the equilibrium concentration? Note there is a difference between mol% and wt% . 1) Identify T and phases 10% 65% 50%
  • 9. Diffusion and phase diagrams
    • Diffusion is still relatively slow in the solid vs. the liquid—therefore cooling can lead to:
      • Varying compositions
      • Varying stress
      • Varying properties
    • Note that unlike in the last phase diagrams, there are 2-phase regions instead of a solid solution between 2 compounds.
    Fe 3 C
  • 10.
    • The SiO 2 phase is ‘polymorphic:’
      • Depending on the temperature, multiple different phases will form.
    Si and O
  • 11. Si and Cu
    • What happens if we anneal to 460 ° C a Si device with Cu lines fabricated on top?
    • Upon cooling from above 802 ° C?
    • Imagine the interface composition is 50/50 wt%. What if we now heat up beyond 802 °C?
    Si Cu Cu Cu Si Cu Cu Cu η ” Si Cu Cu Cu L and Si Si Cu η η ’ η ” Cu η η ’ η ” Cu η η ’ η ”
  • 12. SiO 2 -Al 2 O 3 Adding alumina leads to mullite (ss). Mullite is 3Al 2 O 3 .2SiO 2 (60 mol % Al2O3 and is an important constituent in porcelain. Clays with <60 % Al 2 O 3 convert to mullite, or porcelainite ,
  • 13. Temperature/Pressure phase diagram for Silica
    • Don’t forget that phase diagrams do not only indicate temperature vs. composition.
    • Pressure can be an important parameter.
  • 14. ZrO 2 and CaZrO 2
    • It turns out that there is a large volume change for the tetragonal to monoclinic transition upon cooling, starting at about 1150 ° C.
    • This leads to cracking throughout a ZrO 2 component and thus total mechanical failure.
    • Avoid this by doping with Calcia from 3-7% to form cubic and monoclinic (and no tetragonal about 1000 °C).
    • Below this T diffusion is too slow to form enough monoclinic to generate the unwanted cracks.
      • “ Partially Stabilized Zirconia”
  • 15. Yttria Stabilized Zirconia
    • The monoclinic transition can be suppressed even further by stabilizing zirconia with yttria from 3-8%.
    • Retains cubic and tetragonal phases (avoiding monoclinic) down to roughly 700 ° C.
    • Yttria, partially, and cubic stablized zirconia (CZ) are increasingly common commercially.
  • 16. Si and P (common dopant)
    • Often, closeup phase diagrams are available for detailed industries.
      • A bit of P in Si forms a solid solution, but beyond 2% a solid solution of Si and SiP forms.
  • 17. Melting point for very small concentrations of N in Si
    • Si devices are so highly engineered that precise studies (and control) are necessary for proper IC processing.
  • 18. Phase change materials
    • Not all modern materials are 2-component systems.
    • Intel and Ovonyx are redesigning flash memory.
      • Based on a phase change alloy—GeSbTe—similar to optical recording media, but transistor written (electrons instead of photons) so possibly:
        • more robust, uniform, and smaller.
    http://www.ovonyx.com/tech_html.html
  • 19. Phase change materials
    • A Chalcogenide glass is a glass containing a sulphur, selenium or tellurium as a substantial constituent.
    • The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity values, and this forms the basis by which data is stored. The amorphous, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1.
    • Chalcogenide is the same material utilized in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's refractive index also changes with the state of the material.
    • Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. It is heated to a high temperature (over 600°C), at which point the chalcogenide becomes a liquid. Once cooled, it is frozen into an amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a temperature above its crystallization point, but below the melting point, it will transform into a crystalline state with a much lower resistance. This phase transition process can be completed in as quickly as five nanoseconds (modern DRAM cells have a switching time on the order of two ns).
  • 20. How to read a ternary phase diagram
    • Draw a tie line from the point to the axis defining the content of material A (ie. BA).
      • The line must be parallel to the axis opposite the ‘pure material A’ corner (ie. CB).
      • The intercept yields the % of A in the ternary composition (75% A).
    Pure A i Pure C Weight % C Pure B Weight % B 0% 50% 75% 25% 0% 50% 75% 25%
    • Draw a tie line from the point to the B axis (ie. CB).
      • Line is parallel to AC axis.
      • Intercept on B yields 20% B.
    Weight % A
    • Draw tie line from point to C axis (AC).
      • Parallel to AB.
      • Intercept: 5% C.
  • 21. Ternary diagrams II
    • What is the composition for the points shown (% A,B,C):
    • i) 75, 20, 5
    • ii) 60, 25, 15
    • iii) 25, 75, 0
    • iv) 20, 50, 30
    • v) 5, 5, 90
    • Note the composition always sums to 100%
    Pure A i iii ii iv Pure C Weight % C Pure B Weight % B 0% 50% 75% 25% 0% 50% 75% 25% Weight % A v
  • 22. Ternary diagrams III
    • The ternary is for a single temperature.
    • Any line connecting 2 points sharing a common concentration of 1 of the 3 components can be simplified to a binary phase diagram.
    Pure A Pure C Weight % C Pure B Weight % B 0% 50% 75% 25% 0% 50% 75% Weight % A Pure C Pure B 0% A throughout Liquid 50% A throughout Pure C Pure B Liquid
  • 23. Hydroxyapatite Ca 5 (PO 4 ) 3 (OH)
    • Hydroxyapatite is based on calcia, phosphorous pentoxide, and water.
    • To make bioactive HA, the composition must be fairly close to pure HA.
    Individual phases are also shown: Hydroxyapatite (HA), Monetite (M), Brushite (B) also shown. Calcia Phosphorous pentoxide
  • 24. SUMMARY
    • Diffusion
      • How does diffusion differ in ceramic vs. metallic systems?
      • Name 2 systems where diffusion is crucial.
    • Phase diagrams
      • Know how to read binary and ternary phase diagrams.
      • Be able to identify where compounds, phases, and solid solutions exist.
      • Be able to use the lever rule.
      • Be able to apply a phase diagram to understanding dopants and impurities.
    Reading for next class Next class: Glass and Clay products and processing Chapter sections 13.1-3, 13.8-9