Paraelectric
A paraelectric is a non-conductive material that has no permanent electrical dipole moments aligned in parallel . The term is mainly used for substances that are ferroelectric at a lower temperature (i.e. for ferroelectrics above their Curie temperature ) or that become a ferroelectric through small modifications, for example of the chemical composition. The term is therefore in direct contrast to the term ferroelectric, in analogy to the pair of terms paramagnetic / ferromagnetic .
Paraelectrics (in the narrower sense) are already close to the formation of an electrical polarization and are therefore characterized by a high permittivity . There are also non-linear effects in them. The permittivity depends on the electrical field strength and the mechanical pressure. This effect can be exploited in high-frequency technology : by changing the permittivity via an applied direct voltage, a controllable phase shift of a high-frequency ( microwave ) signal can be achieved and tunable filters can be produced.
Well-known paraelectrics are SrTiO 3 , Ba x Sr 1 − x TiO 3 (x <0.5) and KTaO 3 . These materials are sometimes referred to as STO, BSTO, and KTO for short. STO16 called "ordinary" SrTiO 3 with the oxygen isotope 16 O and STO18 the same compound with the oxygen isotope 18 O. STO16 is paraelectric at all temperatures up to the absolute zero point and has a dielectric constant of ε r = 300 at room temperature and 20,000 at a very low temperature (below approx. 5 K ). On the other hand, STO18 becomes ferroelectric below approx. 30 K. A transition from para- to ferroelectric behavior can be brought about by changing the mass of the oxygen atoms.
Individual evidence
- ↑ M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y.-J. Shan, T. Nakamura: Ferroelectricity Induced by Oxygen Isotope Exchange in Strontium Titanate Perovskite . In: Physical Review Letters . tape 82 , no. 17 , 1999, p. 3540-3543 , doi : 10.1103 / PhysRevLett.82.3540 .