Introduction to Thermoelectric Materials and Devices 4th Semester of 2012 2012.03.29, Thursday Department of Energy Science Sungkyunkwan University Radioisotope Thermoelectric Generator (PbTe) Space probe to the Jupiter From JPL, NASA
Plan 1 Thermoelectric Phenomena and Conversion Efficiency 2 Thermoelectric Transport Theory I : Electrical Properties 3 Thermoelectric Transport Theory II : Thermal Properties 4 Measurement of Thermoelectric Properties 5 Materials Preparation : Bulk and Film 6 Thermoelectric System : Current and Future of Modules 7 Applications : Power Generation and Heat Cooling 8 Mid-term Exam 9 Thermoelectric Materials : State-of-the-art 10 Thermoelectric Materials : Intermetallics 11 Thermoelectric Materials : Oxides 12 Thermoelectric Materials : Phonon Glass and Electron Crystal (PGEC) Materials 13 Theory and Modeling in Nanostructured Thermoelectrics 14 High efficiency in Low Dimensional Materials 15 Hybrid Energy Conversion Systems of Thermoelectrics 16 Final Exam
Thermoelectric Energy Conversion Efficiency Dimensionless Figure of Merit, ZT ZT 2 T or S : Seebeck Coefficient (Thermoelectric Power) : Electrical Resistivity : Thermal Conductivity 2 / : Power Factor lat el lat : Lattice Thermal Conductivity el : Electronic Thermal Conductivity
Measurement : Quiz Prof. M at a conference talked a bulk material with ZT over 2.5 was obtained at 1000 K. Mainly due to the high Seebeck coefficient Quasi 4-point probe Specimen Furnace Tc = 300K I Th = 1000K Why not at 350K??? If a material with 100 V/K : Acceptable range of Temperature difference??? V
Measurement : Seebeck Coefficient 1. Integral method 2. Differential method 1. Integral method : Measurement of V generated by two thermocouples consisting of a sample wire and a reference material wire. Known Seebeck coefficient of a third wire. But, Who makes wire??? Nobody use now.
Measurement : Seebeck Coefficient 2. Differential method : When the net current in the sample is ZERO, T and V are measured. Due to the electrical field in the sample is due to the E= T. The sample should be homogeneous!!!
Measurement : Electrical Conductivity V = I R Contact resistance should be small To Minimize the Joule heating on the resistivity measurement, the current density should be lower. The electromagnetic noise gives a random contribution and can be reduced -By appropriate shielding of the circuit, -By using low-noise measuring equipment -By averaging the measured values DC : Direct Current AC : Alternating Current Spurious voltage originating from thermoelectric effect can be eliminated -By additional measurement with the current set to zero -With different current value -By reversing the current flow direction
Measurement : Electrical Conductivity 2-point probe 4-point probe Applying Voltage, Measuring Current Thickness, t << distance of electrode, s Simple Not correct, unreliable Bulk
Measurement : Seebeck Coefficient and Electrical conductivity
Measurement : Seebeck Coefficient and Electrical conductivity Apparatus for Seebeck coefficient and electrical conductivity at 100K to 1300K
Measurement : Seebeck Coefficient in Magnetic Field 1: sample, 2:sample pressing and sample-supporting plates, 3: AlN plates, 4: Cu plate, 5: heater, 6: heatsink (Cu-Be alloy), 7: temperature sensor, 8: alumina tube holding thermocouple, 9: spring (Cu-Be alloy)
Measurement : Seebeck Coefficient and Electrical conductivity Apparatus for Seebeck coefficient and electrical conductivity at 300K to 2000K 1: heatsink, 2: Mo tubing, 3: sample, 4, 5: sample supports, 6: Mo stopper, 7: alumina insulating ring, 8: Mo pressing rod, 9 stainless pressing rod, 10: alumina tube
Measurement : Mistakes or Ignorance Prof. M at a conference talked a bulk material with ZT over 2.5 was obtained at 1000 K. Mainly due to the high Seebeck coefficient Quasi 4-point probe Specimen Furnace Tc = 300K I Th = 1000K Why not at 350K??? If a material with 100 V/K : Acceptable range of Temperature difference??? V
Thermal Conductivity : Steady-State Technique Q T A L T 0 Q T : heating power through sample L 0 : length between thermocouple (0.004 in) (Heater, I 2 R) (0.001 in) (Conducting Epoxy) Adiabatic Condition!!! At low Temp, Cernox (resistance temperature sensors) Substantial Errors : Radiation loss or gain, Convection, Conduction through lead wires
Thermal Conductivity : Steady-State Technique at low Temp. PPMS (Quantum Design Inc.)
Thermal Conductivity : Laser Flash method, = C p One surface of a disc sample is irradiated by a short pulse of heat from a laser times being 1 msec. The resultant temperature rise of the opposite surface is recorded, from which the thermal diffusivity is computed from temperature rise vs. time data. Maximum temperature rise of rear surface T m Q CL is the density of specimen with dimension of g/cm 3. C is the specific heat with dimension of J/gK.
Thermal Conductivity : Laser Flash a 0.1388 L t 2 1/ 2 Only Bulk Sample available Impossible for Low temperature measurement Steady state
Thermal Conductivity : in-plain of thin film Anisotropy Problem : Out-plain of thin film (Direction to thickness)???
Thermal Conductivity : 2 (3 ) method for thin film Thin metal strip evaporated on the sample acts as heat source and a thermometer The heater is driven with AC current at frequency ω, which causes heat source to oscillate at frequency 2ω Thermal conductivity is evaluated along the thickness direction T P C 2 lk ln Penetration depth : Thermal diffusivity ( 2 1/ 2 ) d Films are neglected ( 2 1/ 2 ) d Confined to the films
Thermal Conductivity : 2 (3 ) method for thin film We will have in 3 months.
Z Meter, Haman Technique Direct method for measuring ZT of a material and device Under steady-state or adiabatic condition, the heat pumped by the Peltier effect will be equal to heat carried by the thermal conduction; IT( Q P A T ) ( ) L ZT VIR VTE ( 1) V Valid for Ideal case : Contact, Radiation, loss IR Reference material with ZT of 0.1 is necessary We will have in 3 months too.
V between Probes Z Meter, Haman Technique Thermocouples Wire +Q Q Electrode Electrode l Q IR Probes Time +Q Electrode l Wire Electrode
Seebeck Coefficient Presentation by Group 1 Variable-Range Hopping conduction Electron-Electron Interaction Electron Localization Sign Change of Seebeck coefficient
Crystal Structure of 12CaO 7Al 2 O 3 (C12A7) 1 Molecule = 12CaO 7Al 2 O 3 1 Unit Cell = 2 molecules = [Ca 24 Al 28 O 64 ] 4+ 2O 2 Lattice: Positive framework (12 cages per UC) Free O 2 ions (2 per UC) Charge per framework cage: + 1/3 Three dimensionally connected subnanometer-sized cages Cage: 5 Å wide
Crystal Structure of 12CaO 7Al 2 O 3 (C12A7) monovalent anion (4X ) 1 Molecule = 12CaO 7Al 2 O 3 1 Unit Cell = 2 molecules = [Ca 24 Al 28 O 64 ] 4+ 2O 2 1 Unit Cell = 2 molecules = [Ca 24 Al 28 O 64 ] 4+ 4X Lattice: Positive framework (12 cages per UC) X ions (4 per UC) X = O, H, e, etc Charge per framework cage: + 1/3 Three dimensionally connected subnanometer-sized cages
Metal Insulator Transition Thermal Annealing with metal Ti Band Conduction Nc = ~1 10 21 cm 3 Metal Insulator Transition Concentration of electrons Hopping Conduction Polaron : electron localized by lattice distortion Metal composed of typical insulators, lime and alumina!
Thermoelectric Properties of C12A7:e C12A7:O 2 + Ti C12A7:e + TiO X Treatment Temperature, 700 1100 o C Time, 12 24 hr
Thermoelectric Properties of C12A7:e S decreases with N e T 1/2 dependence : Variable range hopping Sign change with N e : Impurity-band-conduction like Si
Electronic Structure of C12A7:e DOS of FVB is mainly formed by the O atoms of lattice framework cages DOS of FCB and CCB is mainly formed by the component of Ca atoms The DOS at E F decreases with N e m* of C12A7 electride Semiconducting : 1.1 1.5m 0 Metallic : 0.8m 0 m* of SrTiO 3 Nb-doped STO : 7.3 7.7m 0
Density of State (DOS) [Ca 24 Al 28 O 64 ] 4+ 4e DOS of FVB is mainly formed by the O atoms of lattice framework cages DOS of FCB and CCB is mainly formed by the component of Ca atoms
Thermoelectric Properties of C12A7:e Thermal Conductivity Acoustic Properties CaO : ~ 15 mw/k, Al 2 O 3 : ~ 30 mw/k Amorphous-like thermal conduction Varshni s equation Phonon Mean Free Path : 0.7 nm
Seebeck Coefficient Presentation by Group 1
Presentation Articles by Group 1 Presentation by Group 1
Presentation Articles by Group 1 Presentation by Group 1