Los materiales piroeléctricos se constituyen en un grupo especial de materiales piezoeléctricos que se polarizan naturalmente en las condiciones naturales de temperatura y presión. Sin embargo, el grado de polarización del material cambia sensiblemente con la temperatura, de ahí su denominación ("piros" significa fuego en griego).

Un material común que presenta propiedades piroeléctricas es la turmalina. Ver que, mientras que en los materiales piezoeléctricos el campo eléctrico se manifiesta solamente cuando ocurre la deformación, en los materiales piroeléctricos el campo está siempre presente. En los materiales piroeléctricos, el campo eléctrico interno presente es muy intenso, de modo que su polarización no puede ser alterada fácilmente por medios externos.

De acuerdo con las teorías del electromagnetismo, una región en la que los átomos están con momentos magnéticos orientados igualmente o paralelos, se denomina dominio. Este mismo término puede ser aplicado a la electrostática en el caso de la región en que los momentos eléctricos de los dipolos elementales sean paralelos.

Existen diversas sustancias que, de la misma forma que son los imanes para el magnetismo, presentan múltiples dominios eléctricos, por lo que se denominan (como analogía ) de "ferroeléctricas" (en contrapartida a las sustancias ferromagnéticas, que presentan las mismas propiedades pero en relación a los campos magnéticos), como muestra la figura 1.


 

 

 

Lo interesante es que la analogía entre las sustancias ferroeléctricas y ferromagnéticas va más lejos: ambas poseen una histéresis elevada, constante dieléctrica muy alta y, además, poseen un punto Curie, o sea, existe una temperatura límite que superada, hace que ellas pierden sus propiedades. En el caso de las sustancias que presentan las propiedades piroeléctricas podemos citar el Titanato de Bario (BaTi3), que a los 120 grados centígrados pierde estas propiedades (punto Curie), convirtiéndose en un dieléctrico común.

Arriba de los 120 grados el cristal tiene un dominio único con el mismo, el átomo de Titanio en el centro del cubo como se muestra en la figura 2.


 

 

 

Cuando la temperatura cae a menos de 120 grados, ocurre una transición de fase que cambia esta disposición. Una de las dimensiones del cubo se vuelve 1% más corta que la otra y con ello la figura se transforma en un tetraedro. Con ello, el átomo de titanio es desplazado levemente en el interior pasando a ocupar una posición fuera de su centro.

Diversas regiones del material se convierten en ámbitos con diferentes orientaciones. Esto es suficiente para crear un vector de polarización que se manifiesta en la forma de un campo eléctrico natural en el interior del material. Si el cristal es enfriado por debajo de los 10 grados centígrados, nuevamente tenemos otra transformación de fase y el material deja de ser piroeléctrico.

 

 

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