BASIC THEORY

Thermistor structure

Most of the NTC thermistors exhibiting semiconductor characteristics that resistance decreases significantly when the temperature rises have a spinel or rock salt crystal structure. Chemically, it can be expressed as A2+B2+3O4

  • A is an AO type, a divalent cation oxide (eg NiO, CoO, etc.), B is a B03 type, a trivalent cation oxide (Mn2O3, Fe2O3, etc.), and O is oxygen.
  • The crystal structure of A2+B2+3O4 is composed of a tetrahedron (A cation) with 4 oxygen vertices and an octahedron with 6 oxygens (B cation).
  • This A site and B site are made into a set (unit cell) to make 8 sets of unit cells, and 32 oxygen atoms are contained in the center of the unit cell, and the B site cations are placed side by side on the diagonal of the unit cell. In this way, a spinel structure in which a divalent cation is contained in the A site and a trivalent cation in the B site is called a normal spinel, and a spinel in which a trivalent cation is contained in the A site and a divalent cation in the B site is called an inverse spinel.
  • Also, as in the case of 2O3.Fe2O3, a mixed spinel with a mixture of normal spinel and reverse spinel actually exists.
  • In the case of MnFe2O4 mixed spinel, in which divalent Mn+2 does not exist only at the A site, only about 80% remains in the normal position and 20% exists at the B site at the MnO.82+2 FeO.23+3(MnO .22+2Fe 1.8+3) inverse position.