2.1 Thermodynamic principles in thermoelectric materials
3 min read•august 9, 2024
Thermodynamic principles are key to understanding how thermoelectric materials work. They explain energy conversion, , and limits in these systems. Knowing these basics helps us grasp the bigger picture of thermoelectric phenomena.
The Seebeck, Peltier, and Thomson effects are the main thermoelectric phenomena. These effects describe how heat and electricity interact in materials, forming the basis for thermoelectric devices like generators and coolers.
Thermodynamic Laws
Fundamental Principles of Energy and Heat
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- states energy cannot be created or destroyed, only converted between different forms
- Total energy of an isolated system remains constant over time
- Mathematically expressed as ΔU = Q - W, where ΔU represents change in internal energy, Q denotes heat added to the system, and W signifies work done by the system
- Applies to various processes (adiabatic, isothermal, isobaric)
- Crucial for understanding energy transformations in thermoelectric devices
Entropy and Irreversibility
- introduces concept of entropy, measure of disorder or randomness in a system
- Entropy of an isolated system always increases over time, never decreases
- Formulated mathematically as ΔS ≥ Q/T, where ΔS represents change in entropy, Q denotes heat transferred, and T signifies absolute temperature
- Explains why certain processes occur spontaneously (heat flows from hot to cold objects)
- Imposes limitations on efficiency of heat engines and thermoelectric devices
Thermodynamic Potentials and Efficiency
- Free energy quantifies available energy in a system to perform useful work
- Gibbs free energy (G) defined as G = H - TS, where H represents , T denotes temperature, and S signifies entropy
- Helmholtz free energy (F) expressed as F = U - TS, where U represents internal energy
- Carnot efficiency sets theoretical maximum efficiency for heat engines
- Calculated using formula η = 1 - Tc/Th, where Tc represents cold reservoir temperature and Th denotes hot reservoir temperature
- Serves as benchmark for evaluating performance of real thermoelectric devices
Thermoelectric Effects
Seebeck and Peltier Effects
- describes generation of voltage difference in a conductor or semiconductor subjected to
- (S) quantifies magnitude of effect, measured in V/K
- Utilized in thermoelectric generators for direct conversion of heat into electricity
- represents inverse of Seebeck effect, involves heat absorption or release when current flows through junction of dissimilar materials
- Peltier coefficient (Π) measures strength of effect, expressed in W/A
- Applied in thermoelectric coolers and heat pumps
Thomson Effect and Figure of Merit
- Thomson effect describes heat absorption or release in a current-carrying conductor with temperature gradient
- (τ) quantifies effect, measured in V/K
- Relates Seebeck and Peltier coefficients through Kelvin relation: τ = T(dS/dT)
- Thermoelectric (ZT) evaluates overall performance of thermoelectric materials
- Calculated using formula ZT = S²σT/κ, where S represents Seebeck coefficient, σ denotes electrical conductivity, T signifies absolute temperature, and κ represents
- Higher ZT values indicate better thermoelectric performance (current goal ZT > 2)
Applications and Material Considerations
- Thermoelectric generators convert waste heat into electricity (automotive exhaust, industrial processes)
- Thermoelectric coolers provide solid-state cooling for electronics and small appliances
- Material selection crucial for optimizing thermoelectric performance
- Semiconductors (bismuth telluride, lead telluride) commonly used due to favorable electrical and thermal properties
- Nanostructuring and doping techniques employed to enhance ZT values
- Ongoing research focuses on developing new materials with improved efficiency and cost-effectiveness
Key Terms to Review (21)
Carnot Cycle: The Carnot cycle is an idealized thermodynamic cycle that provides a standard for evaluating the efficiency of heat engines. It consists of four reversible processes: two isothermal and two adiabatic, showcasing the maximum possible efficiency that any heat engine can achieve when operating between two temperature reservoirs. This theoretical model underscores the fundamental limits of efficiency in real-world thermoelectric materials and devices.
Efficiency: Efficiency in thermoelectric systems refers to how effectively these materials and devices convert thermal energy into electrical energy, or vice versa, with minimal energy loss. High efficiency is crucial because it determines the overall performance of thermoelectric applications, influencing their viability for energy conversion and waste heat recovery.
Enthalpy: Enthalpy is a thermodynamic property that represents the total heat content of a system, defined as the sum of the internal energy and the product of pressure and volume. It plays a crucial role in thermodynamics, particularly in processes involving heat transfer during chemical reactions and phase changes. Understanding enthalpy helps in analyzing energy transformations and efficiencies in thermoelectric materials and devices.
Entropy: Entropy is a measure of the disorder or randomness in a system, reflecting the number of microscopic configurations that correspond to a thermodynamic state. It plays a vital role in understanding energy distribution, spontaneous processes, and the efficiency of thermoelectric materials. In thermoelectric effects, entropy influences how heat and electrical energy convert, impacting device performance and stability.
Figure of Merit: The figure of merit, often represented as ZT, is a dimensionless parameter that quantifies the efficiency of thermoelectric materials and devices. It combines the material's Seebeck coefficient, electrical conductivity, and thermal conductivity to assess how effectively it can convert temperature differences into electrical power.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle is fundamental to understanding how energy flows within systems, especially in the context of thermoelectric materials, where heat energy is converted into electrical energy and vice versa, emphasizing the conservation of energy in processes like thermoelectric generation and refrigeration.
Fourier's Law of Heat Conduction: Fourier's Law of Heat Conduction states that the rate of heat transfer through a material is proportional to the negative gradient of temperature and the area through which the heat flows. This principle is essential for understanding how thermal energy moves within thermoelectric materials and devices, impacting their efficiency and performance.
Heat Transfer: Heat transfer is the process of thermal energy moving from one material or substance to another, typically occurring from a region of higher temperature to one of lower temperature. This concept is vital in understanding how thermoelectric materials function, as they rely on efficient heat transfer to convert temperature differences into electrical voltage and vice versa, thereby playing a crucial role in energy conversion processes.
Joule Heating: Joule heating, also known as resistive heating, is the process where electric current passing through a conductor generates heat due to the resistance of the material. This phenomenon is crucial in thermoelectric materials and devices, as it directly impacts energy conversion efficiency, temperature gradients, and performance characteristics essential for cooling applications and understanding thermoelectric effects.
N-type semiconductors: N-type semiconductors are materials that have been doped with elements that provide extra electrons, which are the charge carriers. This doping process enhances the electrical conductivity of the semiconductor by increasing the number of negatively charged carriers, making it essential for thermoelectric applications where efficient charge transport is crucial.
Nernst Equation: The Nernst Equation is a fundamental equation in electrochemistry that relates the reduction potential of a half-cell at non-standard conditions to the standard electrode potential, temperature, and the reaction quotient. This equation is crucial in understanding how electrochemical cells function, particularly in thermoelectric materials, as it helps predict the voltage produced by a thermoelectric device based on temperature differences and concentration gradients.
P-type semiconductors: P-type semiconductors are materials that have been doped with elements that create an abundance of holes, or positive charge carriers, which allows for the conduction of electricity. These semiconductors play a crucial role in thermoelectric applications by improving the efficiency of devices through their interaction with electrons and holes, ultimately influencing thermodynamic principles, power generation, cooling efficiency, and the theoretical limits of device performance.
Peltier Effect: The Peltier Effect is a thermoelectric phenomenon where heat is absorbed or released when an electric current passes through a junction of two different conductors or semiconductors. This effect is fundamental in thermoelectric cooling and heating applications, as it enables the transfer of thermal energy in response to electrical energy, creating temperature differences.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time, and it can only remain constant in ideal cases where the system is in equilibrium. This principle explains why energy conversions are not 100% efficient and highlights the direction of spontaneous processes, which tend to increase the overall disorder or entropy in a system. Understanding this law is crucial in the study of thermoelectric materials, as it governs the efficiency and limitations of energy conversion processes.
Seebeck Coefficient: The Seebeck coefficient is a measure of the thermoelectric voltage generated in response to a temperature difference across a material. It indicates how effectively a material can convert heat energy into electrical energy and is fundamental to understanding the performance of thermoelectric devices.
Seebeck Effect: The Seebeck effect is the phenomenon where a voltage is generated in a circuit made of two different conductive materials when there is a temperature difference between the junctions. This effect is fundamental in understanding how thermal energy can be converted into electrical energy, impacting various thermoelectric applications.
Temperature Gradient: A temperature gradient is a measure of the rate of temperature change in a given direction, indicating how temperature varies across space. It plays a crucial role in various heat transfer processes, influencing thermal conduction, convection, and radiation, and is foundational for understanding thermoelectric materials and their performance.
Thermal conductivity: Thermal conductivity is a measure of a material's ability to conduct heat. It plays a crucial role in thermal transport processes, as it directly influences the efficiency of heat transfer in thermoelectric materials and devices, impacting their performance in energy conversion applications.
Thermal diffusion: Thermal diffusion refers to the process by which thermal energy spreads from regions of higher temperature to regions of lower temperature, driven by the temperature gradient. This movement of heat is a fundamental aspect of thermodynamics and plays a critical role in the behavior of thermoelectric materials, influencing their efficiency and performance in energy conversion applications. Understanding thermal diffusion helps in grasping how temperature differences can be harnessed to generate electrical energy in thermoelectric devices.
Thermal Equilibrium: Thermal equilibrium refers to the state in which two or more bodies in thermal contact with each other cease to exchange heat energy, resulting in them having the same temperature. In this state, there is no net flow of thermal energy between the bodies, indicating that they are at a stable thermal condition. This concept is crucial when discussing thermoelectric materials, as it helps in understanding how temperature gradients can be utilized to generate electric power or achieve cooling effects.
Thomson Coefficient: The Thomson coefficient is a measure of the thermoelectric effect that describes how an electrical current flowing through a conductor affects its temperature in a temperature gradient. It relates to the ability of a material to convert heat into electrical energy or vice versa and is crucial in understanding thermodynamic principles, coupled transport phenomena, and the performance of thermoelectric devices.