16.1 Novel materials and nanostructures for thermoelectrics

Novel materials and nanostructures are revolutionizing thermoelectric technology. Skutterudites, half-Heusler alloys, clathrates, and topological insulators offer unique crystal structures and electronic properties that boost performance. These materials provide new ways to optimize electrical conductivity while minimizing thermal conductivity.

Nanostructured thermoelectrics like quantum dots, superlattices, nanowires, and 2D materials take advantage of quantum effects and increased phonon scattering. These approaches, combined with phonon and band engineering strategies, are pushing the boundaries of thermoelectric efficiency and opening up new applications.

Novel Thermoelectric Materials

Skutterudites and Half-Heusler Alloys

Top images from around the web for Skutterudites and Half-Heusler Alloys

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  Thermoelectric properties of n-type half-Heusler NbCoSn with heavy-element Pt substitution ... View original

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  The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal ... View original

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  Thermoelectric properties of n-type half-Heusler NbCoSn with heavy-element Pt substitution ... View original

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  The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal ... View original

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Top images from around the web for Skutterudites and Half-Heusler Alloys

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  Thermoelectric properties of n-type half-Heusler NbCoSn with heavy-element Pt substitution ... View original

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  The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal ... View original

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  Thermoelectric properties of n-type half-Heusler NbCoSn with heavy-element Pt substitution ... View original

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  The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal ... View original

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• Skutterudites comprise a family of thermoelectric materials with a cubic crystal structure
  • General chemical formula: MX3, where M represents a metal atom and X represents a pnictogen atom (phosphorus, arsenic, or antimony)
  • Contain large voids in their crystal structure allowing for "rattling" atoms to be inserted
  • Rattling atoms disrupt phonon propagation, reducing thermal conductivity
• Filled skutterudites incorporate guest atoms into the voids to further enhance thermoelectric properties
  • Guest atoms (lanthanides, alkaline earths, or alkali metals) occupy the voids
  • Improve phonon scattering while maintaining good electrical conductivity
• Half-Heusler alloys consist of three interpenetrating face-centered cubic sublattices
  • General formula: XYZ, where X and Y are transition metals, and Z is a main group element
  • Exhibit high power factors due to their favorable electronic structure
  • Possess relatively high thermal conductivity, which can be reduced through alloying or nanostructuring

Clathrates and Topological Insulators

• Clathrates form cage-like structures with guest atoms trapped inside
  • Two main types: Type I (X2Y6E46) and Type II (X8Y16E136), where X and Y are guest atoms, and E is the framework atom (usually silicon, germanium, or tin)
  • Guest atoms act as rattlers, reducing thermal conductivity
  • Framework structure provides good electrical properties
• Topological insulators exhibit unique electronic properties
  • Behave as insulators in their bulk but conduct electricity on their surface
  • Surface states protected by time-reversal symmetry, resistant to non-magnetic impurities
  • Potential for high electrical conductivity while maintaining low thermal conductivity
• Both materials offer promising avenues for thermoelectric applications
  • Clathrates excel in low to moderate temperature ranges
  • Topological insulators show potential for room temperature and above applications

Nanostructured Thermoelectrics

Quantum Dots and Superlattices

• Quantum dots consist of nanoscale semiconductor particles
  • Typical size range: 2-10 nanometers in diameter
  • Exhibit quantum confinement effects, leading to discrete energy levels
  • Allow for precise tuning of electronic properties by adjusting size and composition
• Quantum dot superlattices form ordered arrays of quantum dots
  • Create artificial crystal structures with tailored electronic and thermal properties
  • Enable manipulation of both electron and phonon transport
• Superlattices comprise alternating layers of different materials
  • Layer thicknesses typically range from a few to tens of nanometers
  • Create periodic potential wells for charge carriers
  • Enhance electron mobility while impeding phonon transport
• Both structures offer opportunities for improving thermoelectric efficiency
  • Quantum dots increase phonon scattering and modify electronic density of states
  • Superlattices allow for independent optimization of electrical and thermal properties

Nanowires and 2D Materials

• Nanowires form one-dimensional nanostructures with high aspect ratios
  • Typical diameters range from 1-100 nanometers
  • Exhibit quantum confinement effects in two dimensions
  • Provide enhanced phonon scattering at surfaces and interfaces
• Various materials can be synthesized as nanowires for thermoelectric applications
  • Silicon nanowires show significantly reduced thermal conductivity compared to bulk
  • Bismuth telluride nanowires demonstrate improved thermoelectric performance
• 2D materials consist of atomically thin layers with unique properties
  • Include graphene, transition metal dichalcogenides (TMDs), and phosphorene
  • Exhibit strong anisotropy in thermal and electrical transport
  • Allow for precise control of material properties through layer stacking and functionalization
• Both nanowires and 2D materials offer advantages for thermoelectric devices
  • Nanowires provide opportunities for phonon engineering and bandgap tuning
  • 2D materials enable the creation of van der Waals heterostructures with tailored properties

Strategies for Enhancing Performance

Phonon Engineering Techniques

• Phonon engineering aims to reduce lattice thermal conductivity without significantly affecting electrical properties
• Introduce point defects to scatter high-frequency phonons
  • Incorporate impurity atoms or create vacancies in the crystal structure
  • Alloying introduces mass and strain fluctuations, enhancing phonon scattering
• Utilize nanostructuring to target mid-frequency phonons
  • Create grain boundaries, interfaces, and nanoparticle inclusions
  • Phonons with mean free paths comparable to nanostructure dimensions experience strong scattering
• Employ hierarchical structuring to address phonons across multiple length scales
  • Combine point defects, nanostructures, and mesoscale features
  • Achieve broadband phonon scattering for comprehensive thermal conductivity reduction
• Exploit anharmonic phonon scattering to limit thermal transport
  • Design materials with strong anharmonic interatomic potentials
  • Increases phonon-phonon interactions, reducing phonon mean free paths

Band Engineering Strategies

• Band engineering focuses on optimizing electronic properties to enhance thermoelectric performance
• Increase the density of states near the Fermi level
  • Utilize materials with complex band structures or multiple degenerate bands
  • Resonant levels created by impurities can enhance the density of states
• Employ energy filtering to selectively scatter low-energy carriers
  • Introduce potential barriers at interfaces or grain boundaries
  • Improves the average energy of conducting electrons, enhancing the Seebeck coefficient
• Manipulate band convergence to achieve high valley degeneracy
  • Align multiple electron or hole pockets in the Brillouin zone
  • Increases the effective mass without significantly reducing carrier mobility
• Exploit quantum confinement effects in low-dimensional structures
  • Create sharp features in the density of states (quantum wells, wires, or dots)
  • Enhances the Seebeck coefficient through modified electronic structure
• Implement modulation doping to separate carriers from ionized impurities
  • Introduce dopants in a layer adjacent to the conducting channel
  • Reduces ionized impurity scattering, improving carrier mobility