Structure, electric and thermoelectric properties of binary ZnO-based ceramics doped with Fe and Co

Abstract

We have studied the interconnection between structure (grain sizes, chemical and phase composition, porosity) and some electric properties (resistivity, Hall and Seebeck coefficients, as well as power factor, concentration and mobilities of carriers) in composite ceramics (ZnO)z[(TM)xOy]1-z (TM = Fe and Co – transition metals, 0 ≤ x ≤ 3; 1 ≤ y ≤ 4, 0.5 ≤ z ≤ 10 wt%), prepared by one-step and/or two-step annealing on air of powder mixtures of ZnO and TM oxides. The structure of ceramic samples was studied by X-ray diffraction (XRD), Mossbauer (MS) and Raman (RS) spectroscopies, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and magnetometry. It was proved that phase composition of (ZnO)z[(TM)xOy]1-z ceramics depends on the type of doping agents. Addition of CoO to ZnO results in the formation of solid solutions with wurtzite structure after two-step annealing independently on Co concentration z ≤ 10 wt%. Addition of FexOy to ZnO results in the formation of the samples with three phase components after synthesis: submicron grains of wurtzute-like structure with large grains of ZnFe2O4 ferrite with spinel structure and residual oxides FexOy used as dopant in powder mixture. SEM measurements evidence that size of the wurtzite phase grains decreases from several tens of micrometers when using the one-step synthesis to a submicron level for the case of the two-step technology. Temperature dependences of electrical resistivity ρ(Т), as well as Hall and Seebeck coefficients in the undoped ZnO in th range of 6–500 K have shown a competition of the Mott and Shklovsky-Efros variable range hopping conductance (due to disordering), percolative one (due to formation of large-scaled potential relief) and a standard electron transport by Cband. It was found that Cband contribution to the carrier transport in ceramic samples (ZnO)z[(TM)xOy]1-z above 100 K was provided by both low and deep donor centers which have been formed in the wurtzite phase Zn1-δ(TM)δO. For example, in the TM doped samples we observed intrinsic shallow levels with ionization energies of about ΔE1 = (0.04–0.05) eV as well as extrinsic deep levels with ΔE2 = (0.24–0.37) eV. The iron doping of ZnO-based ceramic samples increased the room temperature Seebeck coefficient S(300 K) in average. In so doing, the S(300 K) dependence on electron concentration n of the studied samples (ZnO)z(FexOy)1-z approached maximal values (up to 1000 µV/K) at n = 1021 m−3. At the same time ZnO doping with Co resulted in more weak increase (up to 1.3–1.5 times) of S(300 K) values. Practically threefold growth of S(300 K) doe to Fe doping was attributed by us to the influence of particles of ferrite) formed in wurtzite matrix. The power factor P = (S2/ρ), estimated for the studied samples, evidenced that for both dopants it’s the highest values ( = 10−5 W/K·m) were approached in the (ZnO)z[(TM)xOy]1-z ceramics with the lowest resistivity values.