Structure and electrical properties of zinc oxide base iron doped ceramics

The search for new economically advantageous technologies of new zinc oxide based composite ceramic materials and the study of their structure and properties attract special attention today. These ceramics have a number of advantages as compared with materials prepared by more expensive technologies, due to the possibility to fabricate items having different shapes and sizes and particularly to vary their morphology, structure and phase composition. This allows controlling their functional properties by varying the powder particle size in charge, the temperatures, durations and atmospheres of synthesis and heat treatment, and the types of doping impurities in the ceramics. The structure and electrical properties of (Fe_xO_y)₁₀(ZnO)₉₀ ceramics (0 ≤ x ≤ 3; 1 ≤ y ≤ 4) synthesized in air using single- and two-stage synthesis methods have been studied. FeO, α-Fe₂O₃ and Fe₃O₄ powders or (α-Fe₂O₃ + FeO) mixture have been used for ZnO doping. X-ray diffraction, gamma-ray resonance spectroscopy and Raman spectroscopy data suggest that at average iron concentrations of 1–3 at.% the ceramic specimens contain at least three phases: the Zn_1-δFe_δO solid solution with a wurtzite structure, the ZnFe₂O₄ ferrite phase with a spinel structure and Fe_xO_y residual iron oxides which were used as doping impurities. Scanning electron microscopy and energy dispersion X-ray analysis have shown that the wurtzite phase grain size in the ceramic specimens decreases from several decades of microns for single-stage synthesis to submicron sizes for two- stage synthesis. We show that iron addition to ZnO induces a compression of the wurtzite phase crystal lattice, the compression of lattice magnitude being proportional to the oxygen content in the Fe_xO_y iron oxide doping agent. The temperature dependences of the electrical resistivity suggest that deep donor centers with an activation energy of about 0.37 eV are formed in the Zn_1-δFe_δO wurtzite phase. The temperature dependences of the electrical resistivity of electrons for undoped ZnO in the 6–300 K range and for doped (FeO)₁₀(ZnO)₉₀ ceramic synthesized in one stage exhibit a variable activation energy below 50 K which indicates a heavily disordered structure.