Thermoelectric Energy Harvesting

Transcription

1 Thermoelectric Energy Harvesting Prof Douglas Paul Director: James Watt Nanofabrication Centre University of Glasgow U.K.

2 The University of Glasgow Established in Nobel Laureates, 2 SI units, ultrasound, television, etc... 16,500 undergraduates, 5,000 graduates and 5,000 adult students 186M research income pa 400 years in High Street Moved to Gilmorehill in 1870 Neo-gothic buildings by Gilbert Scott

3 Famous Glasgow Scholars William Thomson (Lord Kelvin) James Watt William John Macquorn Rankine Rev Robert Stirling Rev John Kerr Joseph Black John Logie Baird Adam Smith

4 James Watt Nanofabrication Vistec VB6 750m 2 (+150 m 2 ) cleanroom - pseudo-industrial operation 15 technicians + 4 PhD research technologists E-beam lithography Processes include: MMICs, III-V, Si/SiGe/Ge, integrated photonics, metamaterials, MEMS (microfluidics) Part of EPSRC III-V National Facility & STFC Kelvin-Rutherford Facility Commercial access through Kelvin NanoTechnology Süss MA6 optical lith 14 RIE / PECVD / ALD 6 Metal dep tools 4 SEMs: Hitachi S4700 Veeco: AFMs

5 Electron Beam Lithography Capability 30 years experience of e-beam lithography Sub-5 nm single-line lithography for research Measured linewidth vs dose HSQ Dose (µc cm-2) 10nm Vistec VB6 Penrose tile: layer-to-layer alignment 0.46 nm rms Alignment allows 1 nm gaps between different layers: Vistec EBPG5 > nanoscience: single molecule metrology

6 200 nm gate length 10 nm wide, 50 nm tall nanowire 10 nm Width Si Nanowire FET

7 10 nm Wide Si Nanowire SET Depletion mode nanowire 1.4 K

8 Micro and Nanotechnology from Glasgow 1 nm nanogaps Nanoelectronics: 10 nm T-gate HEMT Optoelectronics: 1.55µm DFB laser Manufacture: AFM probes Hydrophobic patterns Healthcare: STEM cell interrogation metres Sensing: Si nanowires III-V CMOS Environment: Microfluidics MEMS: THz optics 100 µm 2 mm

9 Thermoelectrics History History: Seebeck effect 1822 heat > electric current Peltier (1834): current > cooling Thomson effect: Thomson (Lord Kelvin) 1852

10 Thermoelectric Applications NASA Voyager I & II Peltier cooler: telecoms lasers Cars: replace alternator Micropelt BMW Temperature control for CO2 sequestration Buildings / industry temperature control autonomous sensing

11 Energy Harvesting for Remote Sensing Sports performance sensors Flood sensors Weather monitoring Aged well being sensors Radio Processor Sensor Energy harvester > 10 mw > 100 µw > 10 µw < 100 µw/cm 2!!! Battery free autonomous sensors: ECG, blood pressure, etc.

12 Background Thermal Physics Fourier thermal transport Joule heating Q = appleart Q = I 2 R Area, A hot side, Th Q = heat (power i.e energy / time) Heat (energy/t) = Q L resistance, R I cold side, Tc Q = applea T c T h L

13 The Peltier Effect heat transfer, Q Hot reservoir Th Material 2 Material 1 electrons Material 2 Cold reservoir Tc I Peltier coefficient, = Q I units: W/A = V Peltier coefficient is the heat energy carried by each electron per unit charge & time

14 The Seebeck Effect Hot reservoir Th Material 2 Material 1 α => I V Material 2 Cold reservoir Tc Open circuit voltage, V = α (Th Tc) = α ΔT Seebeck coefficient, = dv dt units: V/K Seebeck coefficient = 1 x entropy ( Q transported with electron q T )

15 The Physics of the Thermoelectric Effect If we ignore energy dependent scattering (i.e. τ = τ(e)) then from J.M. Ziman = q 2R 3 (E) 2 (E) g(e) de df de E g(e) EF EF g(e) f(e) df(e) de (E EF) df(e) g(e) de = q 2 3T R (E) 2 (E) g(e) df de (E EF )de Thermoelectric power requires asymmetry in red area under curve

16 Semiconductor Example: SiGe Alloys Seebeck coefficient, α (µv K 1 ) Mott criteria ~ 2 x cm 3 Degenerately doped p-si0.7ge0.3 α decreases for higher n For SiGe, α increases with T = 8 2 k 2 B 3eh 2 m T 3n 2 3 J.P. Dismukes et al., J. Appl. Phys. 35, 2899 (1964)

17 The Thomson Effect Hot reservoir Th dx Q I Cold reservoir Tc T dt α is temperature dependent dq dx = I dt dx x Thomson coefficient, β: dq = IdT units: V/K

18 The Kelvin Relationships Derived using irreversible thermodynamics = T = T d dt These relationships hold for all materials Seebeck, α is easy to measure experimentally Therefore measure α to obtain and

19 Carnot Efficiency for Thermal Engines heat input, Q1 turbine Wt Efficiency = net work output = heat input = W t Wcom Q1 water boiler Power station steam 1 st law thermodynamics (Q1 Q2) (Wt Wcom) = 0 Wcom compressor Condenser = Q 1 Q 2 Q 1 heat recovered, Q2 = 1 Q 2 Q 1

20 Carnot Efficiency Efficiency = = net work output heat input Δ = 1 Q 2 Q 1 Carnot: maximum η only depends on Tc and Th c = 1 T c T h Tc = 293 K = 20 C Higher temperatures give higher efficiencies

21 Peltier Effect, Heat Flux and Temperature If a current of I flows through a thermoelectric material between hot and cold reservoirs: Area, A hot side, Th Heat flux per unit area = ( = Peltier + Fourier ) Heat (energy/t) = Q current, I Q A = J but = T and applert J = I A cold side, Tc Q = IT appleart

22 Semiconductors and Thermoelectrics Seebeck effect: electricity generation heat source metal n p Th metal metal heat sink Tc Heat transfer Q Peltier effect: electrical cooling i.e. heat pump heat source metal n p Th metal metal heat sink Tc I I + Load Battery

23 Conversion Efficiency heat source metal n p Th η = power supplied to load heat absorbed at hot junction Power to load (Joule heating) = I 2 RL Rn Rp metal metal heat sink Tc I Heat absorbed at hot junction = Peltier heat + heat withdrawn from hot junction Peltier heat = I = IT h Load, RL I = (T h T c ) R+R L (Ohms Law) R = Rn + Rp Heat withdrawn from hot junction = applea (T h T c ) 1 2 I2 R NB half Joule heat returned to hot junction

24 Thermoelectric Conversion Efficiency η = = = power supplied to load heat absorbed at hot junction power supplied to load Peltier + heat withdrawn I 2 R L IT h +applea(t h T c ) 1 2 I2 R heat source metal n p Rn Rp Th metal metal heat sink Load, RL Tc I For maximum value d d( R L R ) = 0 T = 1 2 (T h + T c ) T h T c T h p p 1+ZT 1 1+ZT+ Tc T h max = Z = 2 where RappleA = 2 apple = Carnot x Joule losses and irreversible processes

25 Thermodynamic Efficiency Figure of merit 2 ZT = T Efficiency, T H T T H Solar Stirling ZT-=-5 ZT-=-20 ZT-=-10 Nuclear Rankine Solar Rankine ZT-=-2 10 Today-ZT-=

26 Power Generation From Macro to Micro Efficiency (%) Illustrative schematic diagram cross over Power level (W e ) Engines TE (ZT=2) TE today At large scale, thermodynamic engines more efficient than TE ZT average for both n and p over all temperature range Diagram assumes high ΔT At the mm and µm scale with powers << 1W, thermoelectrics are more efficient than thermodynamic engines (Reynolds no. etc..) C.B. Vining, Nature Mat. 8, 83 (2009)

27 Application Reality Check NASA with finite Pu fuel for RTG requires high efficiency Automotive requires high power (heat is abundant) Industrial sensing requires high power (heat is abundant) Autonomous sensing requires high power (heat is abundant) As heat is abundant the issue is how to maximise power output NOT efficiency for most applications Power α 2 σ

28 Maximum Temperature Drop As the system has thermal conductivity apple a maximum ΔT can be sustained across a module limited by heat transport T max = 1 2 ZT2 c The efficiency cannot be increased indefinitely by increasing Th The thermal conductivity also limits maximum ΔT in Peltier coolers heat source metal n p Th metal metal heat sink Tc Higher ΔTmax requires better Z materials

29 Thermoelectric vs Doping of Semiconductors apple ZT 2 ZT apple el apple ph Carrier density (cm 3 ) Electrical and thermal conductivities are not independent Wiedemann Franz rule: electrical conductivity thermal conductivity at high doping

30 Bulk Thermoelectric Materials Performance zt 1.4 n-type zt 1.4 p-type zt TAGS Bi SiGe Te Sb 3 2 Te 3 Yb 14 MnSb PbTe CoSb 3 CeFe 4 Sb PbTe SiGe , ,000 Temperature ( C) Temperature ( C) zt Nature Materials 7, 105 (2008) Bulk n-bi2te3 and p-sb2te3 used in most commercial thermoelectrics & Peltier coolers But tellurium is 9 th rarest element on earth!!! Bulk Si1-xGex (x~0.2 to 0.3) used for high temperature satellite applications

31 Main Strategies for Optimising ZT Reducing thermal conductivity faster than electrical conductivity: e.g. skutterudite structure: filling voids with heavy atoms Low-dimensional structures: Increase α by enhanced DOS h i 3q k2 B T dln(µ(e)g(e)) de ( = 2 ) E=E F Make apple and σ almost independent Reduce apple through phonon scattering on heterointerfaces Energy filtering: = k B q apple E c E F k B T + R 1 0 (E Ec) k B T R 1 0 (E)dE (E)dE enhance Y.I. Ravich et al., Phys. Stat. Sol. (b) 43, 453 (1971)

32 Seebeck Enhancement at Low Dimensions Increase α through enhanced DOS: h i = 2 3q k2 B T dln(µ(e)g(e)) de E=E F 3D bulk 2D quantum well 1D quantum wire 0D quantum dot g(e) g(e) g(e) g(e) E E EF EF EF EF E E α increasing M. Cutler & N.F. Mott, Phys. Rev. 181, 1336 (1969)

33 Phonons: Lattice Vibration Heat Transfer Frequency (THz) TO LO LA SiGe TA Energy (mev) optic modes - neighbours in antiphase δ+δ LO TO NB acoustic phonons transmit most thermal energy acoustic modes - neighbours in phase LA 0 Γ Δ wavenumber X 0 TA The majority of heat in solids is transported by acoustic phonons

34 Phonon Wavelengths that Carry Heat 1.0 Germanium Silicon Cumulative distribution Wavelength [nm] Greater than 95% of heat conduction in Si / Ge from phonons with wavelengths between 1.2 and 3.5 nm

35 Phonon Enhancements Phonon scattering: Require structures below the phonon mean free path (10s nm) Phonon Bandgaps: Change the acoustic phonon dispersion > stationary phonons or bandgaps Require structures with features at the phonon wavelength (< 5 nm) Phonon group velocity de dkq Frequency (THz) Γ Δ wavenumber TA X Energy (ev)

36 ZT versus Temperature p-type n-type Nanostructures can improve Seebeck coefficient and/or decrease thermal conductivity

37 q z (a+ Thermoelectric Low Dimensional Structures 378 QWs Lateral superlattice heat source metal Th Vertical superlattice heat source metal Th n p n p metal heat sink metal Tc metal heat sink metal Tc 2 nm Quantum Dots heat source metal Th Nanowires heat source metal Th n p n p metal heat sink metal Tc metal heat sink metal Tc

38 Vertical (Cross-plane) Superlattice TEs Use of transport perpendicular to superlattice quantum wells Vertical superlattice heat source metal Th Higher α from the higher density of states Lower electron conductivity from tunnelling n metal heat sink p metal Tc Lower apple ph from phonon scattering at heterointerfaces Able to engineer lower apple ph with phononic bandgaps Overall Z and ZT should increase Figure of merit ZT = 2 T

39 p-type Wafer Designs SL1 to SL4: 922 x 5 nm 2.85 ± 1.5 nm p-ge QW 1.1 ± 0.6 nm p-si0.5ge0.5 Si0.175Ge0.825 SL5: 2338 x 1.1 ± 0.2 nm p-ge QW 0.5 ± 0.1 nm p-si0.5ge0.5 Si0.175Ge nm

40 Electron and Phonon Dispersion Holes (Electronic Dispersion) Phonon Dispersion Distance (nm) HH1 dispersion Rytov model Energy (ev) LH1 dispersion HH2 dispersion HH LH Energy (mev) 10 5 SL5-0.4 SL k z ( /L) SO [001] q k z (a+b) [001] S.V. Rytov, Sov. Phys. Acoustics 2, 67 (1956)

41 Thermal Parasitic Removal Isotropic structure 1/2 lateral parasitic contribution half structure Half structure allows parasitics to be measured and removed for more accurate heat flux determination

42 Thermal Measurements Measured 41% of heat in vertical transport

43 Bulk SiGe ZT Comparisons MIT, Nano Lett. 8, 4670 (2008) GREEN Si p-type SiGe J.P. Dismukes et al., J. Appl. Phys. 35, 2899 (1964)

44 Nanowire Fabrication on Suspended Hall Bar heater thermometer thermometer heater 100 x 45 nm wide Si nanowires with integrated heaters, thermometers and electrical probes

45 Si Nanowires: How many atoms wide? Pt coat for TEM silicon Si SiO2 SiO2

46 45 nm Wide n-silicon 300 K: σ = 20,300 S/m 4 terminal κ = 7.78 W/mK α = 271 µv/k ZT = ZT enhanced by x117 α 2 σ = 1.49 mw m 1 K 2 What enhancements with SiGe?

47 Micropelt Microfabrication of BiTe Alloys n-bi2te3 p-sb2te3 20 µm Bi2Te3

48 System Design: Power Output l c Ll hot side electrical insulator, T H metal metal metal n p n p n p A = module leg area L = module leg length N = number of modules l c metal metal metal cold side electrical insulator, T c metal apple c = thermal contact conductivity ρc = electrical contact resistivity P = 2 AN T 2 2( c +L)(1+2 applel c D.M. Rowe & M. Gao, IEE Proc. Sci. Meas. Technol. 143, 351 (1996) applecl ) System: power in BiTe alloys limited by Ohmic contacts 2 Power density (mw/cm 2 ) K data BiTe Micropelt ZT = 0.6 SiGe Dismukes ZT = 0.11 Nano SiGe ZT= 0.26 Nano BiTe ZT = 0.9 ρc (Bi2Te3) 1 x 10 7 Ω-cm ρc (Si1-xGex) = 1.2 x 10 8 Ω-cm Δ temperature (K or C)

49 122 Leg Modules n-type p-type Process tested and works well SOI growths now in progress for final modules

50 Thermoelectric References D.M. Rowe (Ed.), Thermoelectrics Handbook: Macro to Nano CRC Taylor and Francis (2006) ISBN G.S. Nolas, J. Sharp and H.J. Goldsmid Thermoelectrics: Basic Principles and New Materials Development (2001) ISBN X M.S. Dresselhaus et al. New directions for low-dimensional thermoelectric materials Adv. Mat. 19, 1043 (2007) D.J. Paul, "Thermoelectric Energy Harvesting" Intech Open Access from "ICT - Energy - Concepts Towards Zero - Power Information & Communication Technology " (2014) - DOI: /57347

51 Further Information Contact: Prof Douglas Paul Tel: Address: School of Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow, G12 8LT, U.K.