4.5 Thermoelectric effects
7 min read•august 20, 2024
Thermoelectric effects involve the conversion between thermal and electrical energy in materials. These phenomena arise from the coupling of heat and charge transport, enabling the development of devices that can generate electricity from heat or provide cooling through electrical input.
The Seebeck, Peltier, and Thomson effects form the foundation of thermoelectric phenomena. By understanding and optimizing these effects, researchers aim to improve the efficiency of thermoelectric materials and devices for applications in energy harvesting, cooling, and thermal management.
Thermoelectric phenomena
- Thermoelectric phenomena involve the direct conversion between thermal and electrical energy
- These effects arise due to the coupling of heat and charge transport in materials
- Understanding thermoelectric phenomena is crucial for developing efficient energy conversion devices
Seebeck effect
- Discovered by in 1821
- Occurs when a temperature gradient is applied across a material, generating a voltage
Seebeck coefficient
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- Quantifies the magnitude of the
- Defined as the voltage generated per unit temperature difference ()
- Depends on the material properties and temperature
- Higher Seebeck coefficients indicate better thermoelectric performance
Temperature gradient
- The difference in temperature between two points in a material
- Drives the flow of charge carriers (electrons or holes) from the hot side to the cold side
- Creates a built-in electric field that opposes further charge carrier diffusion
Voltage generation
- The Seebeck effect generates a voltage across the material due to the temperature gradient
- The generated voltage is proportional to the temperature difference and the
- Can be harnessed to generate electrical power in
Peltier effect
- Discovered by in 1834
- Occurs when an electric current is passed through a junction between two dissimilar materials
Peltier coefficient
- Quantifies the magnitude of the
- Defined as the heat absorbed or emitted per unit electric current ()
- Related to the Seebeck coefficient through the Kelvin relations
Current flow
- An electric current is passed through a junction of two dissimilar materials
- The direction of the current determines whether heat is absorbed or emitted at the junction
- Reversing the current direction reverses the heat absorption/emission process
Heat absorption/emission
- At the junction where current flows from material A to material B, heat is absorbed (cooling effect)
- At the junction where current flows from material B to material A, heat is emitted (heating effect)
- The Peltier effect is the basis for thermoelectric cooling and heating devices
Thomson effect
- Discovered by William Thomson (Lord Kelvin) in 1851
- Describes the heat absorbed or generated when an electric current flows through a material with a temperature gradient
Thomson coefficient
- Quantifies the magnitude of the
- Defined as the heat absorbed or generated per unit current density and unit temperature gradient ()
- Related to the Seebeck coefficient through the Kelvin relations
Current density
- The amount of electric current flowing per unit cross-sectional area of the material
- Higher current densities lead to more pronounced Thomson effect
Heat generation/absorption
- When an electric current flows from a hot region to a cold region, heat is absorbed (cooling effect)
- When an electric current flows from a cold region to a hot region, heat is generated (heating effect)
- The Thomson effect is typically smaller than the Seebeck and Peltier effects
Figure of merit
- A key parameter used to evaluate the performance of thermoelectric materials
- Combines the Seebeck coefficient, , and
Thermoelectric efficiency
- The efficiency of a thermoelectric device depends on the figure of merit ()
- Higher values indicate better thermoelectric efficiency
- The maximum efficiency is given by the Carnot efficiency multiplied by a factor related to
Dimensionless quantity
- The figure of merit is a dimensionless quantity
- Defined as , where is the Seebeck coefficient, is the electrical conductivity, is the thermal conductivity, and is the absolute temperature
- Allows for comparison of thermoelectric performance across different materials and temperatures
Optimization strategies
- Maximizing the figure of merit requires optimizing the Seebeck coefficient, electrical conductivity, and thermal conductivity
- Strategies include doping, nanostructuring, and band structure engineering
- Trade-offs exist between these properties, making optimization challenging
Thermoelectric materials
- Materials that exhibit strong thermoelectric properties
- Typically semiconductors or heavily doped semiconductors
Semiconductors
- Semiconductors are the most common thermoelectric materials
- Examples include (Bi2Te3), (PbTe), and silicon germanium (SiGe)
- Offer a good balance between electrical and thermal properties
Thermal conductivity
- Low thermal conductivity is desirable for efficient thermoelectric performance
- Reduces the amount of heat that flows through the material without generating useful electrical power
- Can be reduced through phonon scattering, nanostructuring, or introducing defects
Electrical conductivity
- High electrical conductivity is necessary for efficient thermoelectric performance
- Allows for the flow of charge carriers with minimal resistive losses
- Can be increased through doping or optimizing carrier concentration
Seebeck coefficient optimization
- A high Seebeck coefficient is crucial for thermoelectric efficiency
- Depends on the material's band structure and carrier concentration
- Can be enhanced through band structure engineering, quantum confinement, or energy filtering
Applications of thermoelectrics
- Thermoelectric devices have a wide range of applications in energy conversion and thermal management
Thermoelectric generators
- Convert waste heat into useful electrical power
- Used in automotive exhaust systems, industrial processes, and space missions
- Example: Radioisotope thermoelectric generators (RTGs) power NASA's deep space probes
Thermoelectric coolers
- Use the Peltier effect to provide solid-state cooling
- Used in small-scale refrigeration, temperature control, and optoelectronic device cooling
- Example: Thermoelectric coolers maintain stable temperatures in laser diodes and infrared detectors
Waste heat recovery
- Thermoelectric generators can recover waste heat from various sources
- Examples include industrial furnaces, power plants, and geothermal sources
- Improves overall energy efficiency and reduces greenhouse gas emissions
Space exploration
- Thermoelectric devices are reliable and have no moving parts, making them suitable for space applications
- RTGs provide long-lasting power for deep space missions (Voyager probes, Curiosity rover)
- Thermoelectric coolers regulate temperatures of sensitive instruments and electronics in satellites and spacecraft
Measurement techniques
- Accurate measurement of thermoelectric properties is essential for material characterization and device optimization
Seebeck coefficient measurement
- Typically measured using a differential method
- A temperature gradient is applied across the sample, and the voltage difference is measured
- The Seebeck coefficient is calculated from the slope of the voltage vs. temperature gradient plot
Electrical conductivity measurement
- Can be measured using the four-point probe technique
- Four equally spaced probes are placed on the sample surface
- A current is passed through the outer probes, and the voltage drop is measured across the inner probes
- Electrical conductivity is calculated from the sample geometry and measured resistance
Thermal conductivity measurement
- Several methods exist, including the laser flash method and the 3ω method
- The laser flash method measures the thermal diffusivity of a sample by analyzing its temperature response to a laser pulse
- The 3ω method uses a metal strip as both a heater and a thermometer to measure the thermal conductivity of thin films
Challenges in thermoelectrics
- Despite progress in thermoelectric materials and devices, several challenges remain
Material optimization
- Simultaneous optimization of the Seebeck coefficient, electrical conductivity, and thermal conductivity is difficult
- Trade-offs exist between these properties, limiting the maximum achievable figure of merit
- New materials and strategies are needed to overcome these limitations
Thermal management
- Efficient heat transfer is crucial for thermoelectric device performance
- Thermal interfaces and heat exchangers must be designed to minimize parasitic losses
- Thermal stress and reliability issues arise from the large temperature gradients in thermoelectric devices
Cost-effectiveness
- Thermoelectric materials often contain rare or expensive elements (tellurium, germanium)
- Material processing and device fabrication costs can be high
- Improving the cost-effectiveness of thermoelectric devices is necessary for widespread adoption
Future prospects
- Advances in materials science and nanotechnology offer new opportunities for thermoelectric research
Nanostructured materials
- Nanostructuring can enhance thermoelectric properties by reducing thermal conductivity and increasing the Seebeck coefficient
- Examples include quantum dots, nanowires, and superlattices
- Nanostructured materials can decouple the optimization of electrical and thermal properties
High-temperature thermoelectrics
- Developing thermoelectric materials that operate efficiently at high temperatures (>1000 K) is an active area of research
- High-temperature applications include from industrial waste heat and concentrated solar power
- Materials such as skutterudites, clathrates, and half-Heusler alloys show promise for high-temperature thermoelectrics
Flexible thermoelectrics
- Flexible thermoelectric devices can conform to curved surfaces and adapt to dynamic environments
- Potential applications include wearable electronics, personalized temperature control, and energy harvesting from body heat
- Polymer-based thermoelectric materials and composites are being developed for flexible applications
Organic thermoelectrics
- Organic semiconductors and conducting polymers are emerging as potential thermoelectric materials
- Advantages include low cost, easy processing, and the ability to tune properties through molecular design
- Challenges include improving the electrical conductivity and stability of organic thermoelectric materials
Key Terms to Review (20)
Bismuth Telluride: Bismuth telluride is a semiconductor material composed of bismuth and tellurium, widely recognized for its excellent thermoelectric properties. This compound is primarily used in thermoelectric devices that convert temperature differences into electrical voltage and vice versa, making it an essential component in energy conversion applications and refrigeration technologies.
Charge Carrier Mobility: Charge carrier mobility is a measure of how quickly charge carriers, such as electrons or holes, can move through a semiconductor or conductive material in response to an electric field. This mobility is crucial because it directly influences the electrical conductivity of the material and impacts the efficiency of thermoelectric devices, where charge transport and thermal management play vital roles.
Electrical Conductivity: Electrical conductivity is a measure of a material's ability to conduct electric current, often represented by the symbol $$
ho$$ (sigma). It is influenced by factors such as temperature, the presence of impurities, and the structure of the material. Understanding electrical conductivity is crucial for exploring properties of materials, energy transfer, and electronic applications.
Figure of merit (zt): The figure of merit, often denoted as zt, is a dimensionless quantity that measures the efficiency of thermoelectric materials in converting heat to electricity. It is defined as the ratio of the material's Seebeck coefficient squared to its electrical resistivity multiplied by its thermal conductivity. A higher zt value indicates better thermoelectric performance, making it crucial for applications such as power generation and refrigeration.
Four-probe method: The four-probe method is a technique used to measure the electrical resistance of materials with high accuracy by eliminating the effects of contact resistance. This method involves placing four separate probes in contact with a material, where two probes inject current and the other two measure the resulting voltage drop. By using this configuration, the four-probe method provides a clearer understanding of a material's intrinsic properties, especially in the context of thermoelectric effects.
Hot-wire method: The hot-wire method is a technique used to measure thermal conductivity by observing the temperature change of a heated wire placed in contact with a material. This method is particularly effective for determining thermal properties of solids, liquids, and gases, and it relies on the heat transfer between the wire and the surrounding material to assess thermal characteristics.
Jean Charles Athanase Peltier: Jean Charles Athanase Peltier was a French physicist best known for discovering the Peltier effect in 1834, which describes the heating or cooling of a material when an electric current passes through a junction of two different conductors. This phenomenon is fundamental to thermoelectric effects, enabling applications like thermoelectric cooling and power generation by utilizing temperature differences.
Lead Telluride: Lead telluride (PbTe) is a semiconductor material known for its excellent thermoelectric properties, making it a popular choice in thermoelectric devices. It has a high figure of merit (ZT), which indicates its efficiency in converting heat into electricity and vice versa. This property connects lead telluride to various applications in energy conversion and refrigeration technologies.
Peltier coolers: Peltier coolers are thermoelectric devices that create a heat flux between two different materials when an electric current is applied, enabling them to transfer heat from one side to the other. This unique effect is based on the thermoelectric principle known as the Peltier effect, which allows for efficient cooling without the need for moving parts, making them ideal for applications in refrigeration, electronic cooling, and temperature control systems.
Peltier Effect: The Peltier Effect is a thermoelectric phenomenon where a temperature difference is created at the junction of two different conductors when an electric current passes through them. This effect allows for the transfer of heat, making it crucial in applications like thermoelectric cooling and heating devices. By utilizing this principle, engineers can design systems that either absorb heat from one side while releasing it on the other, or vice versa, enabling temperature control in various electronic and industrial applications.
Power Generation: Power generation refers to the process of converting various forms of energy into electrical energy that can be used for various applications. In the context of thermoelectric effects, power generation often involves the conversion of temperature differences directly into electricity through thermoelectric materials, which can create a flow of electric current when subjected to a temperature gradient. This process is crucial for developing energy-efficient systems and harvesting waste heat.
Seebeck Coefficient: The Seebeck coefficient is a measure of the thermoelectric effect, defined as the voltage developed across a material in response to a temperature difference. It is a key parameter in thermoelectric devices, determining their efficiency and effectiveness in converting thermal energy into electrical energy. A high Seebeck coefficient indicates a greater ability to generate voltage from temperature gradients, which is essential for applications like power generation and refrigeration.
Seebeck effect: The Seebeck effect is the phenomenon where a voltage is generated in a circuit made of two different conductors or semiconductors when there is a temperature difference between the two junctions. This effect is a fundamental principle behind thermoelectric devices, converting thermal energy into electrical energy. Understanding the Seebeck effect is crucial for applications in power generation, refrigeration, and temperature measurement.
Temperature measurement: Temperature measurement is the process of determining the thermal state of an object or environment, typically expressed in degrees Celsius (°C), Fahrenheit (°F), or Kelvin (K). Accurate temperature measurements are crucial in understanding thermoelectric effects, as they influence the behavior of materials and their charge carriers under varying thermal conditions.
Thermal Conductivity: Thermal conductivity is a physical property of materials that indicates their ability to conduct heat. It plays a crucial role in understanding how heat flows through solids, liquids, and gases, and is influenced by factors such as the material's atomic structure, temperature, and the presence of defects.
Thermal efficiency: Thermal efficiency refers to the ratio of useful work output to the heat input in a thermodynamic system. It indicates how effectively a system converts heat energy into work, often expressed as a percentage. High thermal efficiency means that a larger portion of the input energy is transformed into useful work, while lower thermal efficiency implies more energy is wasted as heat.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It provides a set of principles that govern the behavior of systems in thermal equilibrium and describes how energy is transferred and transformed within those systems. Understanding thermodynamics is essential for analyzing phenomena such as heat engines, refrigeration, and the thermoelectric effects, where temperature differences can generate electric voltage.
Thermoelectric generators: Thermoelectric generators (TEGs) are devices that convert heat directly into electricity through the thermoelectric effect, exploiting the temperature difference across thermoelectric materials. These devices are particularly useful for power generation in situations where waste heat is available, as they can operate without moving parts, making them reliable and maintenance-free. Their efficiency and potential applications in energy harvesting make them an important area of study.
Thomas Johann Seebeck: Thomas Johann Seebeck was a German physicist known for discovering the thermoelectric effect, where a temperature difference between two different conductive materials generates an electric voltage. This principle underpins the functioning of thermoelectric devices, which can convert heat energy into electrical energy and vice versa, forming a crucial part of thermoelectric applications.
Thomson Effect: The Thomson Effect refers to the phenomenon where a temperature difference along a current-carrying conductor results in a transfer of heat, either absorbing or releasing thermal energy depending on the direction of current flow. This effect is significant in understanding thermoelectric effects and is tied closely to concepts like the Seebeck and Peltier effects, which are crucial for applications in thermoelectric devices.