A Thermoelectric Cat Warmer from Microprocessor Waste Heat

Simha Sethumadhavan Doug Burger

Department of Computer SciencesThe University of Texas at Austin

Motivation• Hot laptops

• Cold cats– Frozen whiskers– Reduced pest control

Solution

Purr

HeatOn chip

Thermoelectric Generator

CurrentThis talk

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

ThomasSeebeck

Replica ofthe apparatus

Hot End Cold End

TH Tci

Wire

V = S.T

Traditional Uses

Cassini space probe32.8Kg radioactive plutonium fuel, InGaAs thermocouple, 628 Watts, 3-4% efficiency

Seiko “Thermic” watches

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

Cat Mutator

Radioactive Plutonium Pellet

DocileCat

The Physics

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)

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p pe

ep

ep p

e

eee ep

e Phonons: heat flow Electrons: current flow

Hot end Cold end

Traditional Materials

Constant Metals Insulators SemiconductorsSeebeck Small High AcceptableElectrical High Very Low VariableThermal High X MediumHigh

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

Nanotech allows constants be controlled independently & precisely

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p pe

ep

ep p

e

eee e

Hot end Cold endThin film (few nanometers)

New Thin-film Wires

• 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

€

Efficiency = Th - Tc

Th• 1+ M −1

1+ M + TcTh

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⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

Maximum theoretical efficiency of any generator

Temperature Difference

Max. efficiency of a Bismuth TellurideGenerator

50 7.1%25 3.7%

Chip temperatures• Cold end (Tc)

– 27°C• Hot end (TH)

– 77° C, 52 ° C• M for Bismuth Telluride

– 6x better

Horizontal Generator

• Run a bundle of Bismuth Telluride nanowires from processor hot spot to cold spot

• Temperature difference: ~50 degrees

Die

Hot end Cold endHorizontal Generator (nanowire bundles)

Wiring Layers

Vertical Generator

DieVerticalGenerator

Wiring Layers

Cold surface

Hot surface

• Run a bundle of Bismuth Telluride nanowires from logic level to the heat spreader

• Temperature difference: ~20 degrees

Multiple Generators

Die

VerticalGenerator

Cold surface

Hot surface

Purr

Rough Estimates

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

Parameters Horizontal VerticalLength 1mm .25mmArea 300nm x 300nm 1cm x 1cmResistance 13M .3 Temp Diff 50 25 (50)Real Power .13W .15W (.6W)Theoretical 7.1W 3.7W

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!