Trystan Casey
The thermoelectric effect is a physical phenomenon that involves the direct conversion of temperature differences into electric voltage and vice versa. It encompasses three effects: the Seebeck effect, the Peltier effect, and the Thomson effect, which are all thermodynamically reversible. This effect has been studied in relation to solid-liquid equilibrium under an electric field, where it can impact freezing and precipitating processes. The thermoelectric effect is also widely used in temperature sensors and has applications in powering devices, with potential in biomedical and wearable electronics. Thus, the thermoelectric effect provides a link between electric effects and thermodynamics, offering a unique interplay between heat and electricity.
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What You'll Learn
The thermoelectric effect and thermocouples
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, heat is transferred from one side to the other, creating a temperature difference. This effect can be used to generate electricity, measure temperature or change the temperature of objects.
The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, the Peltier effect, and the Thomson effect. The Seebeck effect is the emergence of electromotive force (emf) that develops across two points of an electrically conducting material when there is a temperature difference between them. The emf is called the Seebeck emf (or thermo/thermal/thermoelectric emf). The ratio between the emf and temperature difference is the Seebeck coefficient. A thermocouple measures the difference in potential across a hot and cold end for two dissimilar materials. This potential difference is proportional to the temperature difference between the hot and cold ends.
The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors. When a current is made to flow through a junction between two conductors, heat may be generated or removed at the junction. The Thomson effect is an extension of the Peltier-Seebeck model and is credited to Lord Kelvin.
Thermoelectric coolers are trivially reversible, in that they can be used as heaters by simply reversing the current. Unlike ordinary resistive electrical heating (Joule heating) that varies with the square of the current, the thermoelectric heating effect is linear in current (at least for small currents) but requires a cold sink to replenish with heat energy. This rapid reversing heating and cooling effect is used by many modern thermal cyclers.
In practice, thermoelectric effects are essentially unobservable for a localized hot or cold spot in a single homogeneous conducting material, since the overall EMFs from the increasing and decreasing temperature gradients will perfectly cancel each other out. Thermocouples involve two wires, each of a different material, that are electrically joined in a region of unknown temperature. The loose ends are measured in an open-circuit state (without any current).
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Electric fields and solid-liquid equilibrium
The link between electricity and thermodynamics is undeniable, despite traditional pundits limiting their schools of training in either electricity or thermodynamics, failing to see the connection between the two. The thermoelectric effect, discovered by Thomas Johann Seebeck, is a prime example of the intersection of these two fields.
Electric fields have a significant influence on the solid-liquid equilibrium, which can be studied using Gibbsian composite-system thermodynamics. This approach allows us to derive the effect of electric fields on the thermal, chemical, and mechanical equilibrium of a composite system. The general conditions for solid-liquid equilibrium under an electric field involve modifying the Gibbs free energy to include the effect of the electric field.
The application of an electric field can alter the composition-dependent freezing and precipitating processes, affecting the eutectic point temperature and mole fraction. For instance, in a water/glycerol system, the eutectic point temperature increases with stronger electric fields, reaching 230.9 K at 3 x 10^8 V/m.
The dielectric behaviour of water also changes under an electric field. The dielectric constant of water in the Stern layer (first layer of the electric double layer) is approximately 6, while in the diffuse layer (second layer) it is estimated to be 12, both lower than that of bulk water. This inhomogeneity in dielectric behaviour has implications for the solid-liquid equilibrium.
Furthermore, the electric field's strength and nature (static vs. pulsed) play a role in solid-liquid equilibrium. A static electric field can increase nucleation temperature and growth time, while a pulsed electric field at an equal voltage results in decreased electric field effectiveness.
Overall, the presence of an electric field can modify the thermodynamic equations governing solid-liquid equilibrium, providing control over the shape, aggregation phase, and properties of certain materials.
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Electric fields and freezing/precipitating processes
Electric fields have been shown to have an impact on freezing and precipitating processes. The application of an electric field can affect the composition-dependent freezing and precipitating processes, changing freezing and precipitating temperatures and the eutectic point temperature.
For example, the eutectic point temperature of the water/glycerol system changes noticeably as the electric field strength increases. When the electric field strength increases up to 108 V/m, the eutectic point temperature does not change significantly. However, when the electric field strength is increased further, up to 3 × 108 V/m, the eutectic point temperature increases to 230.9 K.
The effect of an electric field on freezing processes is related to the dipole behaviour of water molecules. Water molecules are dipoles, meaning they have a negative and positive end. When an electric field is applied, the molecules rotate so that the negative end is oriented toward the positive end of the field. This can be used to manipulate the structure of water in both its liquid and solid states.
Pulsed electric field (PEF) technology has emerged as a promising pre-treatment for enhancing freezing processes and improving the quality of frozen foods. PEF involves applying short-duration, high-voltage pulses to food, creating micropores in cellular membranes. This increases the permeability of the membranes, allowing for more efficient freezing and dehydration processes. PEF has been shown to improve the texture, colour, and nutritional retention of frozen foods, as well as extend their shelf life.
In addition to its applications in freezing processes, electric fields have also been studied in relation to the crystallization of proteins. An external electric field can affect the crystallization process by producing large protein concentration gradients, leading to local supersaturation in the crystallization solution. This results in a smaller number of larger crystals, which tend to grow near the cathode. The interaction of charged molecules with an external electric field can also influence their distribution inside the solution, potentially promoting the formation of high-quality crystals.
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The Seebeck effect and electromotive forces
The Seebeck effect is a classic example of an electromotive force (EMF) that leads to measurable currents or voltages. It is named after the Baltic German physicist Thomas Johann Seebeck, who discovered it in 1821. The Seebeck effect occurs when there is a temperature difference between two dissimilar conductors or semiconductors, resulting in a voltage difference between them. This phenomenon can be used to generate electricity, measure temperature, or change the temperature of objects.
The Seebeck effect is the emergence of an electromotive force (EMF) that develops across two points of an electrically conducting material when there is a temperature difference between them. The ratio between the EMF and the temperature difference is the Seebeck coefficient, which is a property of the local material. The Seebeck coefficient generally varies as a function of temperature and depends strongly on the composition of the conductor. For ordinary materials at room temperature, the Seebeck coefficient may range in value from -100 to +1000 μV/K.
The Seebeck effect can be understood by considering the behaviour of electrons in a material when it is subjected to a temperature difference. In a thermoelectric material, a temperature gradient will cause the carriers in the material (conductors and semiconductors) to move, resulting in the transition from thermal energy to electric energy. Under the action of a temperature gradient, holes in the hole-rich material (p-type) diffuse toward the electron-rich material (n-type), and electrons in the electron-rich material (n-type) diffuse into the hole-rich material (p-type), thus forming an electromotive force.
The Seebeck effect is one of the three separately identified effects that make up the thermoelectric effect, along with the Peltier effect and the Thomson effect. The Peltier effect occurs when a temperature difference is created between junctions by applying a voltage difference across the terminals, while the Thomson effect is an extension of the Peltier-Seebeck model and describes how the Seebeck coefficient varies with temperature. These effects are different manifestations of the same physical process and are thermodynamically reversible.
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The Peltier effect and thermocouples
The Peltier effect is a temperature difference created by applying a voltage between two electrodes connected to a sample of semiconductor material. It is the basis for many modern-day refrigeration devices. The effect is even stronger in circuits containing dissimilar semiconductors. The Peltier effect is the reverse of the Seebeck effect, which states that by applying a temperature gradient across a conductor, a voltage can be generated. This is the basis for the thermocouple sensor.
Thermocouples are the most widely used temperature sensors on the planet. They are indispensable to humanity, just like mobile phones. A thermoelectric device creates a voltage when there is a different temperature on each side. When a voltage is applied, heat is transferred from one side to the other, creating a temperature difference. This effect can be used to generate electricity, measure temperature, or change the temperature of objects.
The Thomson effect, the Seebeck effect, and the Peltier effect are all different manifestations of the same physical process. They are thermodynamically reversible, unlike Joule heating, which is the heat generated when a current is passed through a conductive material. Thermoelectric materials can harvest waste heat and directly convert it to electricity. This is known as the Seebeck effect. The reverse process, the Peltier effect, can lead to efficient solid-state cooling, potentially replacing refrigerants or allowing active cooling of microdevices.
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Frequently asked questions
The thermoelectric effect is the direct conversion of temperature difference into electric voltage and vice versa.
Thermoelectric generators (TEGs) are solid-state devices that generate electricity by exploiting the temperature differences between the two sides of the device. TEGs are used to power low-power devices such as watches, light diode bulbs, and vacuum-tube radios.
The Seebeck effect, the Peltier effect, and the Thomson effect. The Seebeck and Peltier effects are different manifestations of the same physical process, and the Thomson effect is an extension of the Peltier-Seebeck model.
The thermoelectric effect lies beyond the scope of equilibrium thermodynamics as it involves continuous flows of energy. However, it can be described by a quasi-thermodynamic technique proposed by Lord Kelvin.
Metals are good conductors of both electricity and heat. Iron, for example, conducts well for both.
