This work has focused on the electrical and microstructural properties of Nb2O5-doped SnO2-MnO2 ceramics. The pellets were prepared by solid-state reaction method according to the system (99.5-x) SnO2 - 0.5 MnO2 - x Nb2O5, on the following molar ratio x = 0.05; 0.10; 0.15; 0.20 and 0.25. Scanning electron microscopy and electrical measurements (ac and dc) were used to study the materials properties. The results showed that the increase of Nb2O5 content in the SnO2-MnO2 matrix has led to an increase in the varistor properties. A detailed study of the electrical properties of materials was conducted using impedance spectroscopy and the results indicated that Nb2O5 has a tendency to form an electron trapping level near the conduction band. In order to explain these findings, a parallel equivalent circuit model was used to simulate the values of the grain boundary resistance. A close relationship was found between the value of the nonlinear coefficient and the grain boundary resistance for the studied varistors, where both values increased with increasing the amount of Nb2O5, which is important for practical applications.

Tin oxide is an n type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of aliovalent oxide doping. The sintering and grain grow of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO2 powder the surface state depends only in the interaction of atmosphere molecules with the SnO2 surface. Inert atmosphere like argon promotes oxygen vacancy formation at the surface due to the reduction of SnO 2 to SnO at surface and liberation of oxygen molecules forming an oxygen vacancy. As a consequence, surface diffusion is enhanced leading to grain coarsening, but no densification. Oxygen atmosphere inhibits the SnO2 reduction decreasing the surface oxygen vacancy concentration. Additions of dopants with lower valence at sintering temperature create extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide. Cobalt and niobium doped SnO2 ceramics exhibit varistor behavior, which can be applied in electronics. Moreover, SnO2 ceramics are chemically inert and can be applied in form of crucibles to melt some optical glasses.

Tin oxide is an n type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of aliovalent oxide doping. The sintering and grain growth of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO2 powder the surface state depends only on the interaction of atmosphere molecules with the SnO2 surface. Inert atmosphere like argon or helium promotes oxygen vacancy formation at the surface due to reduction of SnO2 to SnO at the surface and liberation of oxygen molecules forming oxygen vacancies. As a consequence surface diffusion is enhanced leading to grain coarsening but no densification. Oxygen atmosphere inhibits SnO2 reduction by decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at the sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at the grain boundary leading to densification and grain growth of this polycrystalline oxide.

Montmorillonite (Mt) clay was pillared with SnO2 and tested as an adsorbent for the removal pharmaceuticals trimethoprim (TMP) and sulfamethoxazole (SMX) frequently present in wastewaters. Adsorption process was carried out in dynamic conditions and the effects of light exposure were investigated. The initial and modified clays were characterized using XRD, FTIR, SEM, potentiometric titration, TA/MS and adsorption of nitrogen. The XRD analysis revealed an enlargement of the clay’s interlayer space as a consequence of SnO2 particles intercalation into the clay interlayer spaces. A marked increase in the volume of mesopores was also noticed. It indicated the precipitation of tin species on the flake surface. These changes resulted in a five-fold increase in TMP adsorption. Upon exposure to light a 30% increase in TMP adsorption was found in comparison to the performance in the dark. The good performance of the modified clay was linked to specific acid–base and electrostatic interactions of TMP with the Lewis acid sites of SnO2. Since exposure to solar light resulted in the decomposition of some TMP adsorbed on the surface, those smaller molecules formed were able to relocate to small pores of higher adsorption energy, which were initially inaccessible for TMP. SMX was not adsorbed on the surface of the modified clay owing mainly to its acidic character.

Pós nanométricos SnO2.Nb2O5 foram estudados para o desenvolvimento de sensores de etanol. Estes pós foram preparados pelo método Pechini, caracterizados quanto à sua morfologia por difração de raios X, determinação de área superficial específica por BET e Microscopia Eletrônica de Transmissão e foram submetidos a testes de sensibilidade ao vapor de etanol. Foi estabelecida uma correlação entre a microestrutura do material, os efeitos do dopante e a resposta do sensor.