In the paper we describe the influence of doping with 1 mol% CoO and different amounts of Nb2O5 (0.1 - 2 mol%) on the formation of twins, the development of microstructure and electrical properties of cassiterite-based ceramics (rutile-type structure). The addition of Nb2O5 shifts the densification to 1430 ° C and triggers the development of a microstructure consisting of twinned cassiterite grains. At the lowest addition (0.1 mol%) of Nb-oxide, the final grains of cassiterite are the largest due to the fastest diffusion processes along the grain boundaries (DIGM). With higher additions, the average grain size decreases, and twins are observed in higher fraction of the grains. In addition to contact twins, cyclic twins are common. HAADF-STEM analysis of binary boundaries showed local enrichment with Co and Nb along the twin boundaries, however the dopants are not ordered within the contact plane. This suggests that twinning is not triggered by the presence of the dopants but is a result of yet unexplained sequence of topotaxial reactions. Based on EDS grain analyzes, the charge compensation mechanism in cassiterite doped with aliovalents dopant Co2 + and Nb5 + was determined. The dopants are present in cassiterite in a ratio of 1: 4, which indicates that the charge is compensated by the following mechanism 6Sn (IV) (-) Sn (II) + Co (II) + 4 Nb (V). Optimal varistor properties were determined at the addition of 1 mol% Nb2O5.
COBISS.SI-ID: 30844711
We investigated the effect of pentavalent donor dopant Ta2O5 on microstructure development, the formation of twin boundaries and functional properties of cassiterite based ceramics. Already low additions of tantalum oxide (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO2 growth rate. Rietveld analysis shows that the amount of Co2SnO4 spinel phase drops with the addition of Ta2O5 due to incorporation of Co2+ and Ta5+ into SnO2 structure. With higher additions, however, Ta2O5 segregates to the grain boundaries and hinders SnO2 grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta2O5 the Co:Ta ratio in SnO2 grains is constant 1:2, which means that a twice lower amount of Ta5+ is incorporated in the SnO2 structure compared to the Nb2O5-doped SnO2–CoO system. Accordingly, the following charge compensation mechanism is proposed 3Sn(IV) (-) Co(II) + 2Nb(V).
COBISS.SI-ID: 32686887
Based on the charge compensation mechanism in cassiterite-based ceramics doped with Co- and Ta-oxide with a high fraction of twinned cassiterite grains. We successfully improved densification, electric and dielectric properties of the system. Quantitative TEM/EDS analysis confirmed a constant Co:Ta (1:2) ratio in cassiterite grains, with a small amount of tantalum accumulated at the GBs. Electric and dielectric properties are strongly influenced by Ta2O5 doping. With 1 mol% of Ta2O5 doping, the obtained ceramics possess dense, fine-grained microstructure, where nonlinearity coefficient ? reaches the highest value of 40 with exceptionally low leakage currents (IL=1.2 mikroA) and threshold voltages of ~270 V/mm. Oppositely, dielectric permittivity decreases with the addition of Ta2O5 and has the highest value of 6525 at 0.10 mol% Ta2O5, where coarse-grained microstructure is obtained.
COBISS.SI-ID: 13414147