Sintering Of SnO2 Research Articles

Twinning in SnO2-based ceramics doped with CoO- and Nb2O5: morphology of multiple twins revealed by electron backscatter diffraction.

Electron backscatter diffraction (EBSD) was used for the analysis of multiple cyclic twins in cassiterite (SnO2), which form during sintering of SnO2 with small additions of CoO and Nb2O5. Grain misorientation analysis has shown that about one third of all grains contain {101} twin boundaries (TBs). The majority of these grains are contact twins, whereas a small fraction of grains are multiple, mainly cyclic twins. A procedure was developed in MTEX [Bachmann, Hielscher & Schaeben (2010). Solid State Phenom. 160, 63-88] for automated identification of crystallographically different types of cyclic twins and found two main types: coplanar twins composed of three or four domains with a common [010] axis and alternating twins composed of three to seven domains oriented along the [111] axis. Both types of cyclic twins have a characteristic common origin (nucleus) of all TBs, which is positioned eccentric relative to the grain section and the cycle is closed with a shorter non-crystallographic contact between the first and the last twin domain. The morphology of cyclic twins suggests that they form by nucleation in the initial stages of grain growth. The average size of twinned grains increases with the number of twin domains indicating the influence of TBs formation on the growth of composite grains.

Optimal Doping for Enhanced SnO2 Sensitivity and Thermal Stability

AbstractTin oxide nanocrystals (5–10 nm) doped with silica (0–15 wt %) were made by flame‐spray‐pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO2‐doping reduced the SnO2 crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO2 content. The thermal stability and morphology of SiO2‐doped SnO2 nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO2 content, sintering of SnO2 was prevented only partially resulting in small sinter necks (bottlenecks) between SnO2 primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO2SiO2 heterojunctions rather than the decreasing SnO2 crystallite and grain sizes with increasing SiO2 doping. At high SiO2 contents, SnO2 sintering was inhibited as its grains were separated effectively by dielectric SiO2; this resulted in isolated SnO2 nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.

NonlinearI-V characteristics study of doped SnO2

When tin oxide is doped with Sb2O3 and CoO, it shows highly nonlinear current (I)-voltage (V) characteristics. Addition of CoO leads to creation of oxygen vacancies and helps in sintering of SnO2. Antimony oxide acts as a donor and increases the conductivity. The results are nearly the same when antimony oxide is replaced by tantalum oxide. The observed nonlinear coefficient, α = 30 and the breakdown voltage is 120 V/mm.

Varistors Based on Doped SnO2

When tin oxide is doped with either Sb2O3 and CoO or Ta2O5 and CoO, it shows highly nonlinear current (I)–voltage (V) characteristics. Addition of CoO leads to creation of oxygen vacancies and helps in sintering of SnO2. Both tantalum and antimony act as donors and increases the conductivity. The change in nonlinear coefficient (α) and the breakdown voltage (E B) are studied as function of the quantity of the donor introduced. The magnitudes of grain size and grain boundary barrier height (Φ B ) are calculated as a function of duration of sintering. The observed nonlinear coefficient (α) is in the range of 3–34 and the breakdown voltage varies from 110 V/cm to 4400 V/cm. Incorporation of 5 mole% titanium leads to increase in both α and E B values. All the samples show single phase as found by X-ray diffraction. These samples exhibit low breakdown voltages as compared to commercial ZnO varistors.

Effect of atmosphere and dopants on sintering of SnO2

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.

Effect of atmosphere and dopants on sintering of SnO2

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 a non-isovalent 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 consequence surface diffusion is enhanced leading to grain coarsening but no densification. Oxygen atmosphere inhibits the SnO2 reduction decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide.

Mechanism of activated sintering of SnO2

Mechanism of activated sintering of SnO2

Effect of oxide additives on the sintering of cassiterite refractories

An investigation was carried out of the effect of added MnO2, ZnO, CuO, and Sb2O5 on the structure and sintering of SnO2 at 1300–1400°C in which it was shown that the SnO2 forms solid solutions of substitution and compounds with these additives. The addition of large amounts of MnO2, ZnO, CuO, and Sb2O5 impairs the sintering of SnO2; small quantities of these oxides, more especially in combinations with each other, promote the sintering of tin oxide ceramics at 1400–1550°C to a near-theoretical density. The improvement in the sintering process is the result of the formation of solid solutions and the development of a liquid phase. It was established that the sintering additives exert a mutual influence on their solubility in SnO2.

Effect of V<sub>2</sub>O<sub>5</sub> Additive on the Sintering of SnO<sub>2</sub>

The effect of V2O5 additive on the sintering of SnO2 was studied by measuring relative density and isothermal shrinkage in the temperature range 800 to 1400°C. The optimum amount of V2O5 added was 10wt%, which might be the solid solubility limit of vanadium ion into SnO2 lattice. With this amount of V2O5, the shrinkage behavior agreed with a model based on liquid-phase sintering at lower temperature. At higher temperature above 1250°C, the major parts of the densification occurred very rapidly and was followed by a much slower sintering process typical of volume diffussion. The fast early shrinkage might be caused by the capillary forces of liquid and the rapid formation of solid solution.Grain growth followed the expression G3-G03=At with an apparent activation energy of 104kcal/mol. The volume diffusion of oxygen ion was rate determining step for grain growth in this experiment.