cat_warmer by liwenting

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									  A Thermoelectric Cat Warmer
from Microprocessor Waste Heat



           Simha Sethumadhavan
               Doug Burger



          Department of Computer Sciences
          The University of Texas at Austin
                Motivation
• Hot laptops




• Cold cats
  – Frozen whiskers
  – Reduced pest control
            Solution

                             Purr

               On chip
     Heat   Thermoelectric
              Generator




This talk     Current
                     Thermoelectricity
  • Thermoelectricity: Electricity produced from heat
  • First observed by Seebeck in 1822

           Hot End       Wire   Cold End           Replica of
                                                   the apparatus
             TH                   Tc
V = S.DT                  i




                      Thomas
                      Seebeck
             Traditional Uses
                               Seiko “Thermic” watches

                               5°C body heat, 60mW
                               Doped Poly Si, .3% efficiency




     Cassini space probe
32.8Kg radioactive plutonium
fuel, InGaAs thermocouple,
628 Watts, 3-4% efficiency
           Cat Mutator

Docile
Cat      Radioactive
         Plutonium Pellet
                     The Physics
 e Electrons: current flow             e e         e e
                                              e
                                   p   p￿ p       p p￿ p
 p Phonons: heat flow
                                     e      e      e  e

                             Hot end                       Cold end


When a wire is heated electrons and phonons diffuse

• Electrons
   – Higher electron diffusion Þ more current (good)

• Phonons
   – Collide with other phonons and increase heat flow (bad) or
   – Either transfer their momentum to electrons (good) or
   – Lose their momentum due to boundary collisions (good)
            Traditional Materials
Ideally for large thermoelectric current
• Low phonon flow
    – Const temperature difference Þ Low thermal conductivity
• Many high energy electrons
    – Small resistance Þ High electrical conductivity
• Many phonon electron collisions
    – Large voltage per unit temperature difference Þ High
      Seebeck constant
    Constant        Metals       Insulators    Semiconductors
    Seebeck          Small          High          Acceptable
    Electrical       High         Very Low         Variable
    Thermal          High             X          Medium®High


Nanotech allows constants be controlled independently & precisely
            New Thin-film Wires
                                e  e    e        e e
                             p   p￿ p           p p￿ p
                               e      e          e  e


                    Hot end                                 Cold end
                               Thin film (few nanometers)



• Thin film and metal boundary do not align
  – More phonon boundary collisions
  – More electron phonon collisions
• Figure of Merit (M =                 seebeck2. elec/therm)
  – Traditional Poly Si is 0.4
  – Thin-film Bismuth Telluride is 2.38
  –   [Venkatasubramanium et al. Nature 2001]
            Generator Efficiency
                   Maximum theoretical
                   efficiency of any generator




Chip temperatures
• Cold end (Tc)                   Temperature
                                  Difference
                                                 Max. efficiency of a
                                                 Bismuth Telluride
   – 27°C                                        Generator
• Hot end   (TH)                        50              7.1%
   – 77° C, 52 ° C
                                        25              3.7%
• M for Bismuth Telluride
   – 6x better
       Horizontal Generator

                      Horizontal Generator
          Hot end     (nanowire bundles)     Cold end

                                                   Wiring
                                                   Layers



                    Die




• Run a bundle of Bismuth Telluride nanowires
  from processor hot spot to cold spot
• Temperature difference: ~50 degrees
             Vertical Generator

                                           Wiring
                                           Layers

   Hot surface

   Cold surface    Die

                               Vertical
                               Generator




• Run a bundle of Bismuth Telluride nanowires
  from logic level to the heat spreader
• Temperature difference: ~20 degrees
               Multiple Generators
                        Vertical
                        Generator
                                    Purr



Cold surface

Hot surface       Die
                   Rough Estimates

     Parameters        Horizontal      Vertical
     Length            1mm             .25mm
     Area              300nm x 300nm   1cm x 1cm
     Resistance        13MW            .3 mW
     Temp Diff         50              25 (50)
     Real Power        .13mW           .15W (.6W)
     Theoretical       7.1W            3.7W



For Bismuth Telluride:
• Seebeck coefficienct 243mV/K
• Resistivity: 1.2 x 10-5 ohm/meter
                    Conclusions
• Limitations
   – Manufacturing
   – Engineering: Hinders cooling, peripheral circuitry overheads
   – Only cats are supported

• Final thoughts
   – Thermoelectric heat extraction looks interesting
   – Newer materials can improve power output further
   – How far can this be pushed?
   – When does this become interesting to architects?



                        Thank You!

								
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