Thermoelectric materials. Hyo-Jeong Moon

Transcription

1 Thermoelectric materials Hyo-Jeong Moon

2 Electrical conductivity Thermoelectric materials Ratio of current density to electric field, when no temperature gradient is present. Thermal conductivity Ratio of heat flow per unit area to the temperature gradient, when no electric current is allowed to flow. They are bulk properties of any individual substance. There is, however, a third phenomenon which, at least until rather recently, has received less detailed attention. Thermoelectricity Temperature difference creates an electric potential or an electric potential creates a temperature difference.

3 Timeline of TE and related research Carnot cycle Headt diffusion theory Voltaic cells Galvanometer Kelvin temperature scale Ohm s law Joule heat Entropy formalized Debye specific heat Thermionic Third law of emission thermodynamics Umklapp Isotope effect on heat flow Phonon theory quantization Band theory Size effect on heat flow Quantum well superlattices Minimum thermal conductivity TE quantum well expt. Heterostructure thermionics Seebeck effect Thermocouple thermometry Peltier effect Kelvin relations Thomson effect TE efficiency derived PbS based TE generation Solid solutions TE quantum Promising data on well theory Bi Useful cooling Vacuum thermionic 2 Te 3 Vacuum with Bi 2 Te 3 cooling thermoelements Glass-likee heat flow(skutt.) Seebeck/Peltier/Thomson effect Phonon quantization PbS based TE generation Useful cooling with Bi 2 Te 3 Quantum well superlattices TE quantum well theory TE quantum well expt.

4 3 Important Effects in TE Seebeck effect Peltier effect Thomson effect

5 Seebeck-Effect found in 1821 by Thomas Johann Seebeck Conversion of temperature difference to electric current Quelle: A conductor with an applied temperature gradient. Electrons will thermally diffuse from the hot end to the cold end. This charge builds up on the cold end and creates an electric field inside the sample. Absolute Seebeck effect.

6 Seebeck-Effect Dissimilar metals A and B Their two junctions are at different temperatures The voltage difference produced across the terminals is directly proportional to the temperature difference. The factor of proportionality is called the relative Seebeck coefficient. (= thermoelectric power or thermopower) Seebeck coefficient, S AB or α AB = Δ V / Δ T Th: hot temperature, Tc: cold temperature, Δ T : temperature difference, Δ V : Votage difference

7 Seebeck-Effect Seebeck's instrument Thermomagnetic effect by Seebeck

8 Seebeck-Coefficient The thermopower, or Seebeck coefficient, can be thought of as the heat per carrier over temperature or, more simply, the entropy per carrier where C is the specific heat and q is the charge of the carrier For the case of a classical gas, each particle has an energy of 3/2(k B T), where k B is the Boltzmann constant. The thermopower is thus approximately k B /e, where e is the charge of the electron. For metals, the heat per carrier is essentially a product of the electronic specific heat and the temperature divided by the number of carriers (N), that is,, and then α is approximately

9 Seebeck-Coefficient In a semiconductor, a charged particle must first be excited across an energy Gap Eg. the thermopower is approximated by The thermopower for different carrier types is given by a weighted average of their electrical conductivity values (σ n and σ p ): It is necessary to dope the semiconductors with either donor or acceptor states to allow extrinsic conduction of the appropriate carrier type, electrons or holes, respectively.

10 Temperature dependence of the Seebeck coefficient Si containing different kinds of impurity some chosen metals S for metals ~1 to 10 μv/k S for semiconductors ~ 10 2 to 10 3 μv/k Typical thermopower Values required for good TE performance are on the order of V/K or greater. (Nolas)

11 Peltier-Effect Found in 1834 by Jean Charles Athanase Peltier Conversion of electric current to temperature difference (Reversion of Seebeck-effect) Peltier coefficient, п AB = Q / I Q: heat absorbed or rejected I : electrical current п : Peltier coefficient A junction between two different materials at a constant temperature. Consider passing a current through a junction. Thermal energy leaves or is required. Heat is generated or absorbed at the junction.

12 Peltier-Effekt

13 Thermo-couple The most common devices use 254 alternating p and n type TE devices. The devices can operate at V at 4-5 amps.

14 Thomson-Effect Found in 1851 by William Thomson (Lord Kelvin) Heating or Cooling of a conductor with a temperature gradient. The gradient of the heat flux dq dt = τ I ds ds 1st Thomson (Kelvin) relation. 2nd Thomson(Kelvin) relation. s: spatial coordinate τ : Thomson coefficient Π = αt d τ = T α dt Quelle: digitalcollections/hst/scientific-identity/

15 Figure of Merit Efficiency of thermoelectric solid is found to depend on material properties through the dimensionless parameter ZT Figure of Merit: Z = S 2 σ / κ [1/K] Dimensionless Figure of Merit: ZT = (S 2 σ / κ) T (Goldsmid 1986) S: Seebeck coefficient, σ : Electrical conductivity κ : Thermal conductivity, T: Temperature High electrical- and low thermal conductivity are required for high figure of merit. These values are temperature dependent therefore, the figure of merit is temperature dependent.

16 Figure of Merit

17 ZT and classes of TE materials Semimetal ZT = (S 2 σ / κ) T Semimetals : they have both holes and electrons as carriers el.& hole have different signs of S the effects will cancel out in the cooling process. So, the edge of semiconductors has been focused. -> property improvement : alloy

18 Thermopower and Electronic structure ( Mott Equation ) σ(e) is the electrical conductivity determined as a function of band filling or Fermi energy, E F. If the electronic scattering is independent of energy, σ(e) is just proportional to the density of states (DOS) at E F. For maximun S, a large asymmetry in the DOS and/or scattering within a few kt above and below the Fermi energy is required.

19 Nanotech thermoelectricity Science 303, 777 (2004) at Room Temperature

20 Thermoelectric Materials Narrow band-gap semiconductors For operation at room temperature Heavy elements High mobility, low thermal conductivity Large unit cell, complex structure Low thermal conductivity Highly anisotropic or highly symmetric Complex compositions Low thermal conductivity, electronic structure

21 Thermoelectric Materials The most commonly used semiconductor for electronics cooling applications is Bi 2 Te 3 because of its relatively high figure of merit. However, the performance of this material is still relatively low and alternate materials are being investigated with possibly better performance. Materials currently under investigation Skutterudites Intermetallic clathrates Low-dimensional systems Artificial superlattice thin-film structure (CVD, MBE) etc.

22 Bulk binary semiconductor (classic) Te Bi Rhombohedral-hexagonal symmetry Space group: Rm(D53d) Hexagonal unit cell dimensions at RT: a=3.8 Å and c=30.5 Å Layers stacked along the c-axis : Te Bi Te Bi Te Te Bi Te Bi Te thermal conductivity: c-axis 0.7 and a-aixs 1.5W/mK

23 Seebeck coefficient of Bi 2 Te 3 Seebeck coefficient plotted against electrical conductivity of bismuth telluride at 20 o C (Nolas et. al, 2001)

24 Band structure of Bi 2 Te 3 Band structure of Bi2Te3 without spin-orbit interaction.

25 Temperature variation of Bi 2 Te 3 material properties (Yazawa, 2005)

26 p- and n-type Semiconductors ZT for p-type thermoelectric materials ZT for n-type thermoelectric materials

27 Electron crystals and Phonon glasses The best themoelectric material should posses: (1995 Slack) thermal properties similar to that of a glass electrical properties similar to that of a perfect single-crystal material i.e. poor thermal- and good electrical-conductor ZT = (S 2 σ / κ) T Skutterudites are materials that appear to have the potential to fulfill such criteria

28 Skutterudites Name from a naturally occurring mineral, skutterudite or CoAs 3 Co Binary skutterudite compounds, CoSb 3. Sb The structure: MX 3 with M=Co, Rh, Ir, Fe, Ru and X=P, As, Sb Cubic Conaining 32 atoms per unit cell 8 M atoms and 24 X atoms Space group Im3 Planar X 4 rings are mutually orthogonal and parallel to the cubic crystallographic axes. CoSb3 (Cobalt antimonide) good electronic properties but, κ L = 10 W/m-K (too large for good TE)

29 Filled Skutterudites Compounds can be formed with atoms filling the voids of the skutterudite structure. Phonon-scattering -> maximize ZT LaFe 3 CoSb 12 : ZT>1 at more than 700K CeFe 3.5 Co 0.5 Sb 12 : ZT=1.35 at near 900K K L =1/3*(V S CL ph ) K L : lattice thermal conductivity Vs : velocity C : heat capacity L ph : mean free path of the phonons The rattlers should therefore dramatically lower κ L. What was not clear, however, was how the rattlers would alter the electronic conduction. Although the electronic properties of the skutterudite antimonides were somewhat degraded by the presence of various rattlers, there was an overall increase in ZT

30 Skutterudites Compounds can be formed with atoms filling the voids of the skutterudite structure. Phonon-scattering -> maximize ZT skutterudite CoSb3 has good electronic properties and can be doped n or p type However, the RT κ L of CoSb3 is 10 W/m-K, which is too large for a good TE material. Slack (1995) suggested filling the voids in the skutterudite structure with weakly bound atoms that rattle about their equilibrium positions. He reasoned that heavy rattlers with low Einstein temperatures would be effective in scattering the low frequency acoustic phonons that carry most of the heat in a solid. The rattlers should therefore dramatically lower κ L. What was not clear, however, was how the rattlers would alter the electronic conduction. Although the electronic properties of the skutterudite antimonides were somewhat degraded by the presence of various rattlers, there was an overall increase in ZT

31 Skutterudites The skutterudite material system possesses the basic conditions for high ZT values. Large unit cell Heavy constituent atom masses Low electronegativity differences between the constituent atoms Large carrier mobilities Compounds can be formed with atoms filling the voids of the skutterudite structure. Phonon-scattering -> maximize ZT

32 Skutterudites Structural parameters of known binary skutterudites, 1999

33 Oftedal Relation Oftedal relation y+z = 0.5

34 Filled Skutterudites Atomic Displacement Parameter The skutterudite unit cell centered at the voidfiller atom, which is enclosed in an irregular dodecahedral (12fold coordinated) cage of pnicogen (filled circles) atoms Temperature dependence of the isotropic displacement parameters measurde on a single crystal of La 0.75 Fe 2.74 Co 1.26 Sb 12.

35 Clathrates compound These compounds have cage-like crystal structures in which the spaces are filled with atoms that can effectively rattle around. For example Ba 8 Ga 16 Ge 30 ZT>1 at 900 K

36 Low-Dimensional TE materials 2D: Superlattice thin film structure 1D: Nano-Wires 0D: Quantum-Dots

37 Superlattice thin-film structure L el <Superlattice properties dimensions <L ph phonon-blocking (scattered at the interface)/electron-transmitting structure <MIT> 1993 Dresselhaus <RTI> 2001, Venkatasubramanian Group Bi2Te3 (1nm)/Sb2Te3 (5nm) Multi-layers ZT~ 2.4 at RT In Cross-plane: phonons are scattered more than in In-plane electron transport should not be hindered!!

38 Superlattice thin-film structure The difference of energy levels of two films should be small in order that the electrons can overcome it only with their thermal energy <RTI> 2001 Band diagram of Superlattice: Short period/shallow potential <RTI> Thin-film TE cooling device: Refrigeration from RT until 32.2K

39 Superlattice techniques MOCVD ( Metalorganic Chemical Vapor Deposition) : Bi2Te3/Sb2Te3 MBE ( Molecular beam epitaxy ) : PbSe0.98Te0.02

40 Nano- Wires Insulator Conductor electron phonon Bismut-nanowires TE nanowires by electrochemical deposition in nano scale Al 2 O 3 -pore structures. The Oxide matrix is removed by selective wet chemical etching. Nanowires Superlattice 2002 Dresselhaus Calculation of ZT values for nanowires and superlattice of Bi.

41 Quantum Dots 2000 Harman in MIT Schematic cross section of the quantum-dot superlattice structure by MBE Field-emission SEM image of quantum-dot superlattice structure.

42 Multilayer- und Quantum-Dot-Structure

43 Quantum-Dot Superlattice nanowires 2003 Dresselhaus reported Theoretical model of superlattice nanowires. ZT values higher than 4 and 6 are predicted for 5-nm-diameter PbSe/PbS and PbTe/PbSe superlattice nanowires at 77 K, respectively. These ZT values are significantly larger than those of their corresponding alloy nanowires

44 Quantum-Dot Superlattice nanowires

45 AgPb m SbTe m+2 (LAST) Bulk, classic: Bi2, PbTe, SiGe Filled Skutterudites: Yb 0.19 Co 4 Sb 12,CeFe 4-x Co x Sb 12, Pnictide: Yb 14 MnSb 11 Nanocomposites CsBi4Te6 : ZT=0.8 at 225K Pb-Sb-Ag-Te (Lead-Antimony-Silver-Tellurium) =LAST High ZT = 2.2 at 800 K with m=18 of AgPb m SbTe 2+m

46 AgPb m SbTe m+2 (LAST) NaCl structure (Fm3m symmetry) Metal Ag, Pb and Sb are disordered in the structure on Na sites, whereas the chalcogen atoms occupy the Cl sites. The formula is charge-balanced, because the average charge on the metal ions is 2+ and on the chalcogen ions 2- LAST materials are derived by isoelectronic Substitution of Pb2+ ions for Ag+ and Sb3+ in the lattice. The Ag-Sb rich area forms. This generates local distortions, both structural and electronic. Quantum nanodots

47 Applications TE Generators TE Coolers TE Sensors

48 Thermoelectric Generator Heat to Electric Energy Up to 20% conversion efficiency with right materials Electrical Power Generation [ Telluxrex corp.] But currently, bulk application ~4%. Multi-layer TE ~ more than 10% nanotechnology under investigation ~7-12%

49 Themoelectric coolers CPU in PC Air conditioner etc. Refrigerator Optical communication device DNA analysis currently ~8% conversion efficiency

50 Themoelectric coolers Compared with mechanical refrigeration, thermoelectric cooling offers the following advantages: -No moving parts -Environmentally friendly -No loss of efficiency with size reduction -Can be integrated with electronic circuits (e.g. CPU) -Localized cooling with rapid response However, the current ZT value (~1) for TE cooling still lags behind that of mechanical refrigeration (ZT~3).

51 Themoelectric coolers For certain electronic and optoelectronic application, The desired operating temperature cannot be obtained with a single-stage TE cooler under steady-state operation. Two options to increase the maximum temperature difference 1.Tansient cooling, which utilizes the fact that there is a time delay in the propagation of the volumetric Joul heating to the cold junction whereas the Peltier cooling effect occurs nearly instantaneously. This effect can be exploited for the pulsed operation of certain devices. 2. Multistage module, also called a cascaded cooler. A typical multistage bulk TE cooler is a pyramid stack of single-stage coolers, because the lower stage must pump the heat dissipated by the upper stages in addition to the cooling target heat load on the top stage.

52 Thermoelectric Sensors The Thermocouple is a thermoelectric temperature sensor which consisits of two dissimilar metallic wires. These two wires are conncected at two different junctions, one for temperature measurement and the other for refoerece. The temperature difference between the two junctions is detected by measuring the change in voltage across the dissimilar metals at the temperature measurement junction. Advantage High sensitivity Disadvantage Limit to a smaill measuring area and from it non-linear line of values

53 Thermoelectric application Waste Heat Recovery!! Danke Schön für ihre Aufmerksamkeit!