Pyroelectricity is a scientific phenomenon that allows special materials to create electricity when they change temperature. Minerals and crystals are the most common substances to produce this effect. These materials are commonly used to build temperature sensors, such as passive infrared detectors used in security systems.
The word pyroelectricity comes from the Greek term "pyr," which means fire. The earliest observation of the pyroelectric effect occurred in 314 B.C., when the ancient philosopher and scientist Theophrastus noticed that tourmaline minerals create electrical attraction when they are heated. Several scientists studied this phenomenon during the 18th century, but it was not until the 19th century that researchers fully understood the cause.
All crystals have electrical, mechanical, and thermal properties. These three properties are able to interact. In piezo crystals, for instance, physical force such as bending a mineral will generate electricity. Similarly, pyroelectricity results from the interaction of the thermal and electrical properties of a crystal.
When a crystal is heated or cooled, electrical charges form on opposite sides of the mineral. These charges can be harnessed as electrical current by placing electrodes on the crystal surfaces. Electricity flows in one direction when heat increases, and in the opposite direction when heat decreases.
The amount of electricity produced by the pyroelectric effect is typically not enough to power other devices. This means that pyroelectricity is not a practical power generation method. The small electrical signal from a crystal is very useful in sensors, however. Pyroelectric materials can be combined with other electronics to indicate when a temperature change has occurred.
Passive infrared detectors are one common application of pyroelectric signals. These devices are also known as motion detectors, and are frequently placed in homes and businesses as part of a security system. Each detector contains a wide-angle lens and a pyroelectric crystal. When a person walks past the sensor, their body heat causes the crystal to generate pyroelectricity and the alarm is activated.
Pyroelectric sensors can be used to detect other sources of heat, such as fire. Unlike traditional smoke detectors, a sensor that uses pyroelectricity is able to detect an actual flame even if smoke is not present. These types of sensors are useful in applications such as gas heaters, where they can detect if a pilot light has been properly ignited.
A collection of these sensors can be combined to detect temperature changes over a wide area. An array of these sensors can function as a thermal camera, and is able to show temperature variations caused by people or vehicles. This technology is often used by the military and law enforcement as a form of night vision.