When poled, PVDF is a ferroelectric polymer, exhibiting efficient piezoelectric and pyroelectric properties. All known pyroelectric materials are also piezoelectric. Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. Ferroelectric materials are required by symmetry considerations to be also piezoelectric and pyroelectric. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well. Among pyroelectric materials, some of them are ferroelectric. A Nanogenerator has three typical approaches&colon; piezoelectric, triboelectric, and pyroelectric nanogenerators. The PIR sensor consists of a pyroelectric sensor and a Fresnel lens. In IR spectrometers the pyroelectric detectors are the most widespread. The pyroelectric effect converts a temperature change into electric current or voltage. U.S. DOE Research on pyroelectric refining and fast neutron reactors was stopped in 1994. KI100 crystals have no pyroelectric effect. Nuclear D-D fusion driven by pyroelectric crystals was proposed by Naranjo and Putterman in 2002. Very small changes in temperature can produce a pyroelectric potential. It is often considered to be the physical inverse of the pyroelectric effect.