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 Heat On chip Thermoelectric Generator Current This talk

Thermoelectricity Thermoelectricity: Electricity produced from heat First observed by Seebeck in 1822 Thomas Seebeck Replica of the apparatus Hot End Cold End THTH TcTc i Wire V = S.  T

Traditional Uses Cassini space probe 32.8Kg radioactive plutonium fuel, InGaAs thermocouple, 628 Watts, 3-4% efficiency Seiko “Thermic” watches 5°C body heat, 60  W Doped Poly Si,.3% efficiency

Cat Mutator Radioactive Plutonium Pellet Docile Cat

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) p e pp e e p e pp e e e ee p e Phonons: heat flow Electrons: current flow Hot endCold end

Traditional Materials ConstantMetalsInsulatorsSemiconductors SeebeckSmallHighAcceptable ElectricalHighVery LowVariable ThermalHighX MediumHigh 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

p e pp e e p e pp e e e ee Hot end Cold end Thin 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 = seebeck 2. 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 Temperature Difference Max. efficiency of a Bismuth Telluride Generator 507.1% 253.7% Chip temperatures Cold end (T c ) –27°C Hot end (T H ) –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 endCold end Horizontal Generator (nanowire bundles) Wiring Layers

Vertical Generator Die Vertical Generator 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 Vertical Generator Cold surface Hot surface Purr

Rough Estimates For Bismuth Telluride: Seebeck coefficienct 243 V/K Resistivity: 1.2 x ohm/meter ParametersHorizontalVertical Length1mm.25mm Area300nm x 300nm1cm x 1cm Resistance 13M.3  Temp Diff5025 (50) Real Power.13W.15W (.6W) Theoretical7.1W3.7W

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!