Seebeck effect
Contenido
El efecto Seebeck es la conversión de diferencias de temperatura directamente a electricidad.
Seebeck descubrió que la aguja de una brújula se desviaba cuando se formaba un circuito cerrado de dos metales unidos en dos lugares con una diferencia de temperatura entre las uniones. Esto se debe a que los electrones se ven excitados a niveles energéticos de manera diferente dependiendo del material, provocando una diferencia de potencial en la unión de estos y, consecuentemente, creando una corriente de circuito, que produce un campo magnético. Seebeck, aun así, en ese momento no reconoció allí una corriente eléctrica implicada, así que llamó al fenómeno el efecto termomagnético, pensando que los dos metales quedaban magnéticamente polarizados por el gradiente de temperatura. El físico Danés Hans Christian Ørsted jugó un papel vital en la explicación y concepción del término “termoelectricidad”.
El efecto es que un voltaje, la FEM termoeléctrica, se crea en presencia de una diferencia de temperatura entre dos metales o semiconductores diferentes. Esto ocasiona una corriente continua en los conductores si ellos forman un circuito completo. El voltaje creado es del orden de varios microvoltios por kelvin de diferencia. Una de esas combinaciones, cobre-constantán, tiene un coeficiente Seebeck de 41 microvoltios por kelvin a temperatura ambiente.
En el circuito:.
(que puede estar en varias configuraciones diferentes y regirse por la misma ecuación), el voltaje obtenido puede ser derivado de:.
S y S son los coeficientes Seebeck de los metales A y B en función de la temperatura, y T y T son las temperaturas de las dos uniones. Los coeficientes Seebeck no son lineales en función de la temperatura, y dependen de la temperatura absoluta, material y estructura molecular de los conductores. Si los coeficientes Seebeck son efectivamente constantes para el rango de temperatura medido, la fórmula anterior puede aproximarse como:.
El efecto Seebeck se usa comúnmente en dispositivos llamados termopar (porque está hecho de un acople o unión de materiales, generalmente metales) para medir una diferencia de temperatura directamente o para medir una temperatura absoluta colocando un extremo a una temperatura conocida. Una sonda metálica mantenida a una temperatura constante en contacto con un segundo metal de composición desconocida puede clasificarse por este efecto TE. Instrumentos de control de calidad industriales usan este efecto Seebeck para identificar aleaciones metálicas. Esto se conoce como clasificación Termoeléctrica de aleación.
Varios termopares cuando se conectan en serie son llamados termopila, la cual se construye a veces para aumentar el voltaje de salida ya que el voltaje inducido sobre cada acople es bajo.
Este es también el principio de trabajo detrás de los diodos térmicos y generadores termoeléctricos (tales como los generadores termoeléctricos de radioisótopos o GTR) los cuales se usan para crear potencia a partir de la diferencia de calor.
El efecto Seebeck se debe a dos efectos difusión de portador de carga y arrastre de fonones (descritos abajo). Si ambas conexiones se mantienen a la misma temperatura, pero una conexión se abre y cierra periódicamente, se mide un voltaje AC, el cual es también dependiente de la temperatura. Esta aplicación de la sonda Kelvin") a veces se usa para demostrar que la física subyacente solo necesita una unión. Y este efecto se ve aún si los alambres solo se acercan, pero no se tocan, así no se necesita difusión.
Seebeck coefficient
The Seebeck Coefficient of a material measures the magnitude of a thermoelectric voltage induced in response to a temperature difference across that material. The Seebeck coefficient has units of (V/K), although in practice it is more common to use microvolts per kelvin. Values in the hundreds of V/K, negative or positive, are typical of good thermoelectric materials. The term thermopower is a misnomer since it measures the voltage or electric field induced in response to temperature difference, not electrical power. An applied temperature difference causes charged carriers in the material, whether there are electrons or holes, to diffuse from the hot side to the cold side, similar to classical gas expanding when heated. Charged mobile carriers migrate to the cold side, leaving behind their immobile core oppositely charged to the hot side, thus giving rise to thermoelectric voltage (thermoelectric refers to the fact that voltage is created by a temperature difference). Since a charge separation also creates an electric potential, the accumulation of charged carriers on the cold side eventually ceases at some maximum value since there is a quantity of derived charged carriers moved to the hot side as a result of the electric field in equilibrium. Only an increase in the temperature difference can resume an accumulation of more charge carriers on the cold side and thus lead to an increase in the thermoelectric voltage. Coincidentally, the Seebeck coefficient also measures the entropy "Entropy (thermodynamics)") per charge carrier in the material. To be more specific, the partial molar electronic heat capacity is said to be equal to the absolute thermoelectric power multiplied by the negative of Faraday's constant.
The Seebeck coefficient of a material represented by (or sometimes by ), depends on the temperature and crystalline structure of the material. Metals typically have low Seebeck coefficients because most have half-filled bands. Both electrons (negative charges) and holes (positive charges) contribute to the induced thermoelectric voltage thus canceling each other's contribution to the voltage and making it small. On the other hand, semiconductors can be doped "Doping (semiconductors)") with an excess amount of electrons or holes and thus can have large positive or negative values of the Seebeck coefficient depending on the charge of the excess carriers. The sign of the Seebeck coefficient can define which charged carriers dominate electrical transport in both metals and semiconductors.
If the temperature difference between the two extremes of a material is small, then the Seebeck coefficient of a material is defined (approximately) as:.
and a thermoelectric voltage is seen at the terminals.
Thus, a relationship between the electric field and the temperature gradient can be written by approximating the equation:
In practice the absolute Seebeck coefficient of the material of interest is rarely measured. Because the electrodes connected to the multimeter can be placed in the material to measure the thermoelectric voltage. The temperature gradient also induces a thermoelectric voltage across one of the electrode tips. Therefore the measured Seebeck coefficient includes a contribution from the Seebeck coefficient of the material of interest and the material of the measurement electrodes.
Charge carrier diffusion
Charge carriers in materials (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) will diffuse when one end of a conductor is at a different temperature from the other. Hot carriers will diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor. Cold carriers will diffuse from the cold end to the hot end for the same reason.
If the conductor were allowed to reach thermodynamic equilibrium, this process would result in the uniform distribution of heat throughout the conductor (see heat transfer). The movement of heat (in the form of charged carriers) from one end to the other is called heat flow. Just as charge carriers move, it is also an electric current.
In a system where both ends are maintained at a constant temperature difference (a constant flow of heat from one end to the other), it is a constant diffusion of carriers. If the rate of diffusion of hot and cold carriers in opposite directions is equal, there would be a net change in charge. But, charge diffusion disperses "Dispersion (physics)") with impurities, imperfections, and crystal lattice vibrations (phonons). If scattering depends on energy, hot and cold carriers will diffuse at different rates. This creates a greater density of carriers at one end of the material, and the distance between the positive and negative charges produces a potential difference; an electrostatic voltage.
This electric field, however, opposes the unequal dispersion of carriers, and an equilibrium is reached where the net number of carriers diffused is canceled by the net number of carriers moving in the opposite direction from the electrostatic field. This indicates that the Seebeck coefficient of a material depends greatly on impurities, imperfections, and structural changes (which often vary among themselves with temperature and electric field), and the Seebeck coefficient value of a material is the collection of many different effects.
At first thermocouples were metallic, but more recently thermoelectric devices are developed from alternating p-type and n-type semiconductor elements connected by metallic interconnectors as drawn in the figure below. Semiconductor junctions are common especially in power generation devices, while metallic junctions are more common in temperature measurements. Charge flows through the n-type element, crosses a metallic interconnection, and passes to the p-type element. If a power source is supplied, the thermoelectric device can act as a cooler, as in the left figure below. This is the Peltier effect, described in the next section. The electrons in the n-type element will move in the opposite direction of the current and the holes in the p-type element will move in the direction of the current, both removing heat from one side of the device. If a heat source is supplied, the thermoelectric device can function as a power generator, as in the right figure below. The heat source will drive electrons in the n-type element toward the cooler region, thus creating a current through the circuit. The holes in the p-type element will then flow in the direction of the current. Current can be used to drive a load, thus converting thermal energy into electrical energy.
Phonon drag
Phonons are not always in local thermal equilibrium; They move against the thermal gradient. They lose momentum due to interaction with electrons (or other carriers) and imperfections in the crystal. If the phonon-electron interaction predominates, the phonons will tend to push the electrons to one end of the material, losing momentum in the process. This contributes to the electric field already present. This contribution is the most important in the temperature region where phonon-electron scattering predominates. This happens because:
where: is the Debye temperature. At lower temperatures there are fewer phonons available to drag, and at higher temperatures they tend to lose momentum in phonon-phonon scatterings rather than phonon-electron scatterings.
This region of the Seebeck coefficient versus temperature function is highly variable under a magnetic field.
Seebeck effect of spin and magnetic batteries
Physicists have recently discovered that heating one side of a magnetized nickel-iron rod allows electrons to rearrange their spins. This so-called “spin Seebeck effect” could lead to batteries that generate magnetic currents, rather than electrical current. A magnetic current source could be useful especially for the development of spintronic devices, which use magnetic currents to reduce overheating in computer chips, since, unlike electrical currents, magnetic currents do not generate heat.