6.2 Factors influencing ZT

Thermoelectric materials' performance hinges on a delicate balance of electrical and thermal properties. The figure of merit, ZT, depends on the Seebeck coefficient, electrical conductivity, and thermal conductivity. Optimizing these factors is key to boosting efficiency.

Strategies to enhance ZT include tuning carrier concentration, doping, and nanostructuring. These techniques aim to increase the power factor while reducing thermal conductivity. Understanding and manipulating these factors is crucial for developing high-performance thermoelectric devices.

Material Properties

Electrical and Thermal Characteristics

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• Electrical conductivity determines charge carrier movement through material
• Seebeck coefficient measures voltage generated per unit temperature difference
• Thermal conductivity influences heat transfer rate across material
• Electronic thermal conductivity relates to charge carrier heat transport
• Lattice thermal conductivity arises from phonon vibrations in crystal structure

Band Structure and Energy Levels

• Energy band gap separates valence and conduction bands
• Narrow band gaps enhance thermoelectric performance
• Band convergence increases density of states near Fermi level
• Effective mass of charge carriers affects mobility and Seebeck coefficient
• Band alignment impacts carrier scattering and transport properties

Lattice Thermal Conductivity Management

• Phonon mean free path determines heat conduction through lattice vibrations
• Umklapp scattering reduces lattice thermal conductivity at high temperatures
• Point defects create mass fluctuations and disrupt phonon propagation
• Grain boundaries act as phonon scattering centers in polycrystalline materials
• Nano-inclusions introduce additional phonon scattering mechanisms

Optimization Techniques

Carrier Concentration Tuning

• Optimal carrier concentration balances electrical conductivity and Seebeck coefficient
• Carrier concentration affects position of Fermi level within band structure
• Heavy doping increases electrical conductivity but may reduce Seebeck coefficient
• Light doping enhances Seebeck coefficient at the cost of electrical conductivity
• Modulation doping creates charge carrier reservoirs for improved performance

Doping Strategies and Effects

• N-type doping introduces excess electrons as majority carriers
• P-type doping creates excess holes as majority carriers
• Isovalent doping modifies band structure without changing carrier concentration
• Co-doping combines multiple dopants for synergistic effects
• Resonant doping enhances density of states near Fermi level

Nanostructuring Approaches

• Quantum confinement effects alter electronic properties in low-dimensional structures
• Superlattices create periodic potential barriers for selective carrier filtering
• Nanowires and nanotubes offer enhanced phonon scattering and quantum confinement
• Nanocomposites combine bulk and nanostructured phases for optimized properties
• Hierarchical structuring introduces multi-scale phonon scattering mechanisms

Performance Metrics

Power Factor Optimization

• Power factor combines electrical conductivity and Seebeck coefficient ($S^2σ$)
• Increasing power factor improves thermoelectric conversion efficiency
• Weighted mobility concept relates band structure to power factor
• Energy filtering enhances power factor through selective carrier transmission
• Density of states engineering aims to maximize power factor near Fermi level

Phonon Scattering Mechanisms

• Rayleigh scattering occurs when phonon wavelength exceeds defect size
• Resonant bonding induces anharmonic lattice vibrations for phonon scattering
• Interfacial phonon scattering occurs at grain boundaries and heterostructures
• Alloy scattering arises from mass and bond strength fluctuations in solid solutions
• Phonon-electron scattering contributes to thermal resistance in heavily doped materials