🔋Thermoelectric Materials and Devices Unit 8 – Nanostructured Thermoelectrics

Key Concepts and Definitions

• Nanostructured thermoelectrics involve materials with features on the nanoscale (1-100 nm) that exhibit unique properties compared to bulk materials
• Thermoelectric effect converts temperature differences into electrical voltage (Seebeck effect) or electrical current into temperature differences (Peltier effect)
• Figure of merit ($ZT$) measures the efficiency of a thermoelectric material, defined as $ZT = \frac{S^2 \sigma T}{\kappa}$, where $S$ is the Seebeck coefficient, $\sigma$ is electrical conductivity, $T$ is absolute temperature, and $\kappa$ is thermal conductivity
  • Higher $ZT$ values indicate better thermoelectric performance
• Phonons are quantized vibrations in a crystal lattice that contribute to thermal conductivity
• Charge carriers (electrons or holes) transport electrical current and heat in thermoelectric materials
• Energy filtering selectively allows high-energy charge carriers to pass while blocking low-energy ones, enhancing the Seebeck coefficient
• Decoupling electrical and thermal transport involves strategies to maintain high electrical conductivity while reducing thermal conductivity

Nanostructuring Techniques

• Size reduction techniques (ball milling, wet chemical synthesis) create nanoscale features by breaking down bulk materials or building up from atomic or molecular precursors
• Nanoinclusions introduce nanoscale particles or phases into a matrix material to scatter phonons and reduce thermal conductivity
• Superlattices are alternating layers of different materials with nanoscale thicknesses that can enhance electrical conductivity and reduce thermal conductivity through interfaces
• Nanowires and nanotubes are one-dimensional nanostructures that exhibit quantum confinement effects and can be used as building blocks for thermoelectric devices
• Hierarchical nanostructuring combines multiple length scales of nanostructures to simultaneously optimize electrical and thermal properties
  • Example: nanoinclusions embedded in a nanocrystalline matrix
• Atomic-scale engineering (doping, alloying) modifies the electronic structure and transport properties of thermoelectric materials
• Surface engineering (functionalization, passivation) controls the surface chemistry and charge transport at interfaces

Quantum Confinement Effects

• Quantum confinement occurs when the size of a material is comparable to the wavelength of electrons, leading to discrete energy levels and modified electronic properties
• Density of states (DOS) describes the number of electronic states per unit energy and is altered by quantum confinement
  • Quantum wells (2D), wires (1D), and dots (0D) exhibit different DOS profiles
• Bandgap engineering involves modifying the bandgap of a material through quantum confinement to optimize electronic properties
  • Example: increasing the bandgap can enhance the Seebeck coefficient
• Carrier mobility is affected by quantum confinement due to changes in scattering mechanisms and effective mass
• Enhanced Seebeck coefficient results from sharp features in the DOS near the Fermi level, which can be achieved through quantum confinement
• Quantum size effects can also influence phonon transport, leading to reduced thermal conductivity
• Modulation doping spatially separates dopants from the conduction channel to reduce impurity scattering and enhance carrier mobility

Thermal and Electrical Transport in Nanostructures

• Phonon scattering mechanisms in nanostructures include boundary scattering, defect scattering, and phonon-phonon interactions
  • Boundary scattering dominates at low temperatures and small feature sizes
  • Defect scattering (impurities, dislocations) is important at intermediate temperatures
  • Phonon-phonon interactions (Umklapp processes) dominate at high temperatures
• Thermal conductivity reduction in nanostructures is achieved by increasing phonon scattering rates and reducing phonon mean free paths
• Electrical conductivity in nanostructures depends on carrier concentration, mobility, and effective mass
  • Carrier concentration can be optimized through doping and modulation doping
  • Mobility is influenced by scattering mechanisms (impurity, phonon, interface) and quantum confinement effects
• Electron energy filtering selectively transmits high-energy electrons while blocking low-energy ones, enhancing the Seebeck coefficient
• Interfacial effects (energy barriers, charge accumulation) can influence electrical and thermal transport in nanostructured systems
• Grain boundaries in nanocrystalline materials can scatter phonons and affect carrier mobility
• Anisotropic transport properties may arise in nanostructures due to different confinement effects along different directions

Materials Selection and Design

• Thermoelectric materials selection considers intrinsic material properties (bandgap, carrier concentration, mobility) and compatibility with nanostructuring techniques
• Bismuth telluride (Bi2Te3) and its alloys are commonly used for near-room-temperature applications due to their high $ZT$ values
• Lead telluride (PbTe) and its alloys are suitable for mid-temperature range (500-900 K) applications
• Silicon-germanium (SiGe) alloys are used for high-temperature (above 900 K) thermoelectric generators
• Nanostructured silicon exhibits reduced thermal conductivity while maintaining high electrical conductivity, making it a promising thermoelectric material
• Organic and polymer-based thermoelectric materials offer advantages such as flexibility, low cost, and low thermal conductivity
• Doping optimization involves selecting the appropriate dopant type (n-type or p-type) and concentration to maximize $ZT$
  • Example: heavy doping can increase electrical conductivity but may also increase thermal conductivity
• Band structure engineering (alloying, solid solutions) can be used to modify the electronic properties and enhance thermoelectric performance

Fabrication Methods

• Top-down approaches start with bulk materials and introduce nanostructures through techniques such as ball milling, nanolithography, and etching
  • Ball milling mechanically grinds bulk materials into nanoscale powders
  • Nanolithography (electron beam, focused ion beam) creates nanoscale patterns on surfaces
• Bottom-up approaches build nanostructures from atomic or molecular precursors using techniques such as chemical vapor deposition (CVD), atomic layer deposition (ALD), and solution-based synthesis
  • CVD involves the reaction of gaseous precursors on a substrate to form nanostructured films
  • ALD deposits thin films one atomic layer at a time, enabling precise control over thickness and composition
• Spark plasma sintering (SPS) is a rapid consolidation technique that uses pulsed electric current to sinter nanopowders into dense bulk materials while preserving nanostructures
• Molecular beam epitaxy (MBE) grows single-crystal thin films with precise control over composition and thickness, suitable for fabricating superlattices and quantum well structures
• Electrochemical deposition can be used to fabricate nanowires, nanotubes, and other nanostructures by reducing ions from a solution onto a substrate
• Inkjet printing and roll-to-roll processing enable scalable and low-cost fabrication of flexible thermoelectric devices
• Microwave-assisted synthesis uses microwave heating to rapidly synthesize nanostructured thermoelectric materials with controlled morphology and composition

Characterization and Measurement Techniques

• Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provide high-resolution imaging of nanostructures and enable analysis of morphology, size distribution, and crystal structure
• X-ray diffraction (XRD) characterizes the crystal structure, phase composition, and grain size of nanostructured materials
• Atomic force microscopy (AFM) maps the surface topography and can be used to study the morphology and mechanical properties of nanostructures
• Raman spectroscopy probes the vibrational modes of materials and can be used to characterize phonon transport and thermal properties
• Electrical characterization techniques (Hall effect, four-point probe) measure the electrical conductivity, carrier concentration, and mobility of thermoelectric materials
• Seebeck coefficient measurement involves applying a temperature gradient and measuring the resulting voltage to determine the Seebeck coefficient
• Thermal conductivity measurement techniques include the 3ω method, time-domain thermoreflectance (TDTR), and the laser flash method
  • The 3ω method uses an AC current to generate a temperature oscillation and measures the resulting voltage to determine thermal conductivity
  • TDTR uses a pulsed laser to create a temperature gradient and measures the reflectance change to extract thermal properties
• Density functional theory (DFT) simulations predict the electronic structure, transport properties, and thermoelectric performance of materials

Applications and Future Prospects

• Thermoelectric generators (TEGs) convert waste heat into electricity for power generation in automobiles, industrial processes, and space applications
• Thermoelectric coolers (TECs) provide solid-state cooling for electronic devices, sensors, and medical applications
• Wearable thermoelectric devices harvest body heat to power sensors and electronics for health monitoring and personal comfort
• Flexible thermoelectric materials enable conformal and adaptable energy harvesting and cooling solutions
• Thermoelectric energy harvesters can be integrated with solar cells, fuel cells, and other energy sources for improved efficiency and reliability
• Nanostructured thermoelectric materials have the potential to achieve $ZT$ values greater than 3, significantly enhancing the efficiency of thermoelectric devices
• Phonon engineering strategies, such as coherent phonon effects and phononic crystals, offer new avenues for reducing thermal conductivity and improving thermoelectric performance
• Machine learning and high-throughput computational screening can accelerate the discovery and optimization of new thermoelectric materials
• Transient thermoelectric devices exploit the time-dependent response of materials to temperature changes for sensing and energy harvesting applications
• Integration of thermoelectric devices with other energy conversion technologies, such as thermophotovoltaics and thermionics, can further enhance the efficiency of waste heat recovery systems