🔋Thermoelectric Materials and Devices Unit 1 – Thermoelectrics: Intro and History

Key Concepts and Definitions

• Thermoelectrics involves the direct conversion between thermal and electrical energy
• Seebeck effect generates an electrical potential difference from a temperature gradient across a material
• Peltier effect describes the heating or cooling that occurs when an electrical current passes through the junction of two dissimilar materials
• Thomson effect relates to the heating or cooling of a current-carrying conductor with a temperature gradient
• Figure of merit ($ZT$) quantifies the efficiency of a thermoelectric material, with higher values indicating better performance
• Thermoelectric generators (TEGs) convert heat directly into electricity using the Seebeck effect
  • TEGs have no moving parts, making them reliable and low-maintenance
• Thermoelectric coolers (TECs) use the Peltier effect to create a temperature difference for cooling applications
• Thermoelectric materials are typically semiconductors or semimetals with high electrical conductivity and low thermal conductivity

Historical Background

• In 1821, Thomas Johann Seebeck discovered the Seebeck effect, laying the foundation for thermoelectrics
• Jean Charles Athanase Peltier discovered the Peltier effect in 1834, demonstrating the reversibility of thermoelectric phenomena
• William Thomson (Lord Kelvin) described the Thomson effect in 1851, relating the Seebeck and Peltier effects
• Early thermoelectric devices were inefficient and had limited practical applications
• Space missions in the 1960s, such as the Transit and Nimbus satellites, employed radioisotope thermoelectric generators (RTGs) for reliable, long-lasting power
• The oil crisis of the 1970s sparked renewed interest in thermoelectric energy conversion as an alternative to fossil fuels
• Advances in materials science and nanotechnology in the late 20th and early 21st centuries have led to significant improvements in thermoelectric performance

Fundamental Principles

• Thermoelectric effects arise from the coupling of thermal and electrical transport in materials
• The Seebeck coefficient ($S$) relates the voltage difference to the temperature gradient in a material
  • $S = -\Delta V / \Delta T$, where $\Delta V$ is the voltage difference and $\Delta T$ is the temperature difference
• The Peltier coefficient ($\Pi$) quantifies the heat absorbed or released at a junction per unit electric current
  • $\Pi = Q / I$, where $Q$ is the heat and $I$ is the electric current
• The Thomson coefficient ($\tau$) relates the rate of heating or cooling to the electric current and temperature gradient
  • $\tau = \rho J \cdot \nabla T$, where $\rho$ is the resistivity, $J$ is the current density, and $\nabla T$ is the temperature gradient
• The figure of merit ($ZT$) is a dimensionless quantity that characterizes the efficiency of a thermoelectric material
  • $ZT = (S^2 \sigma / \kappa) T$, where $S$ is the Seebeck coefficient, $\sigma$ is the electrical conductivity, $\kappa$ is the thermal conductivity, and $T$ is the absolute temperature
• Optimizing thermoelectric performance requires maximizing the power factor ($S^2 \sigma$) while minimizing the thermal conductivity ($\kappa$)

Major Discoveries and Milestones

• In 1911, Edmund Altenkirch developed the theory of thermoelectric generation and refrigeration, introducing the concept of the figure of merit
• Maria Telkes and Abram Ioffe made significant contributions to thermoelectric materials research in the 1930s and 1940s
• The development of bismuth telluride (Bi2Te3) in the 1950s led to the first practical thermoelectric cooling devices
• In 1993, Hicks and Dresselhaus proposed that quantum confinement in nanostructured materials could enhance thermoelectric performance
• The discovery of high-ZT materials, such as lead telluride (PbTe) and silicon germanium (SiGe) alloys, in the 1990s and 2000s expanded the range of thermoelectric applications
• Recent advancements in materials engineering, such as nanostructuring and band structure engineering, have pushed $ZT$ values above 2 in some materials

Applications and Technologies

• Thermoelectric generators convert waste heat from various sources (automotive exhaust, industrial processes, etc.) into useful electricity
  • TEGs can improve energy efficiency and reduce greenhouse gas emissions
• Thermoelectric coolers provide precise temperature control for applications such as electronics cooling, medical devices, and scientific instruments
  • TECs offer compact, silent, and vibration-free cooling solutions
• Radioisotope thermoelectric generators (RTGs) power spacecraft and remote sensors, utilizing the heat from radioactive decay
• Wearable thermoelectric devices can harvest body heat to power small electronics or provide personalized heating and cooling
• Thermoelectric air conditioners and refrigerators offer energy-efficient and environmentally friendly alternatives to conventional vapor-compression systems
• Thermoelectric sensors, such as thermocouples and infrared detectors, rely on the Seebeck effect for accurate temperature measurement

Theoretical Framework

• The Boltzmann transport equation describes the transport of charge carriers and phonons in thermoelectric materials
  • The Boltzmann equation relates the distribution function of particles to external fields and scattering mechanisms
• Density functional theory (DFT) is used to calculate electronic band structures and predict thermoelectric properties of materials
• The phonon glass-electron crystal (PGEC) concept guides the design of thermoelectric materials with high electrical conductivity and low thermal conductivity
  • PGEC materials have a crystal-like electronic structure for high carrier mobility and a glass-like phonon structure for low lattice thermal conductivity
• Nanostructuring strategies, such as quantum dots, superlattices, and nanoinclusions, can enhance thermoelectric performance by reducing thermal conductivity and increasing the power factor
• Band structure engineering techniques, such as doping, alloying, and strain engineering, can optimize the electronic properties of thermoelectric materials

Challenges and Limitations

• The efficiency of thermoelectric devices is limited by the figure of merit ($ZT$) of available materials
  • Current commercial thermoelectric materials have $ZT$ values around 1, limiting their widespread adoption
• The cost and scarcity of some thermoelectric materials, such as tellurium and germanium, hinder their large-scale deployment
• The thermal stability and mechanical properties of thermoelectric materials can limit their operating temperature range and durability
• The low power output of thermoelectric generators compared to other energy conversion technologies restricts their applications
• The need for a heat sink to maintain the temperature gradient in thermoelectric devices can increase their size and complexity
• The interdependence of thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) makes simultaneous optimization challenging

Future Directions and Research

• Developing new thermoelectric materials with higher $ZT$ values, such as complex chalcogenides, skutterudites, and half-Heusler alloys
• Exploring novel nanostructuring techniques, such as hierarchical architectures and topological materials, to further enhance thermoelectric performance
• Investigating the thermoelectric properties of organic and hybrid materials, which offer the potential for low-cost, flexible, and environmentally friendly devices
• Developing advanced manufacturing techniques, such as 3D printing and roll-to-roll processing, for the scalable production of thermoelectric devices
• Integrating thermoelectric devices with other energy conversion technologies, such as photovoltaics and fuel cells, for improved overall efficiency
• Exploring the use of machine learning and high-throughput computational screening to accelerate the discovery and optimization of thermoelectric materials
• Studying the fundamental transport mechanisms in thermoelectric materials using advanced characterization techniques, such as ultrafast spectroscopy and scanning probe microscopy
• Investigating the thermoelectric properties of materials under extreme conditions, such as high pressure or strong magnetic fields, to uncover new phenomena and potential applications