A Thermoelectric Cat Warmer from Microprocessor Waste Heat

1
A Thermoelectric Cat Warmer from Microprocessor
Waste Heat

• Simha Sethumadhavan
• Doug Burger
• Department of Computer Sciences
• The University of Texas at Austin

2
Motivation

• Hot laptops
• Cold cats
• Frozen whiskers
• Reduced pest control

3
Solution
Purr
On chip Thermoelectric Generator
Heat
Current
This talk
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Thermoelectricity

• Thermoelectricity Electricity produced from heat
• First observed by Seebeck in 1822

Replica of the apparatus
Wire
V S.?T
Thomas Seebeck
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Traditional Uses
Seiko Thermic watches 5C body heat,
60?W Doped Poly Si, .3 efficiency
Cassini space probe 32.8Kg radioactive plutonium
fuel, InGaAs thermocouple, 628 Watts, 3-4
efficiency
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Cat Mutator
Docile Cat
Radioactive Plutonium Pellet
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The Physics
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Hot end
Cold end

• When a wire is heated electrons and phonons
  diffuse
• Electrons
• Higher electron diffusion ? 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)

8
Traditional Materials

• Ideally for large thermoelectric current
• Low phonon flow
• Const temperature difference ? Low thermal
  conductivity
• Many high energy electrons
• Small resistance ? High electrical conductivity
• Many phonon electron collisions
• Large voltage per unit temperature difference ?
  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
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New Thin-film Wires
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Cold end
Hot 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

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Generator Efficiency
Maximum theoretical efficiency of any generator

• Chip temperatures
• Cold end (Tc)
• 27C
• Hot end (TH)
• 77 C, 52 C
• M for Bismuth Telluride
• 6x better

Temperature Difference Max. efficiency of a Bismuth Telluride Generator
50 7.1
25 3.7
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Horizontal Generator
Horizontal Generator (nanowire bundles)
Hot end
Cold end
Wiring Layers
Die

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

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Vertical Generator
Wiring Layers
Hot surface
Die
Cold surface
Vertical Generator

• Run a bundle of Bismuth Telluride nanowires from
  logic level to the heat spreader
• Temperature difference 20 degrees

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Multiple Generators
Vertical Generator
Purr
Cold surface
Die
Hot surface
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Rough Estimates
Parameters Horizontal Vertical
Length 1mm .25mm
Area 300nm x 300nm 1cm x 1cm
Resistance 13M? .3 ??
Temp Diff 50 25 (50)
Real Power .13?W .15W (.6W)
Theoretical 7.1W 3.7W

• For Bismuth Telluride
• Seebeck coefficienct 243?V/K
• Resistivity 1.2 x 10-5 ohm/meter

15
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!