M.I. Pech-Canul

Microstructure and electrical properties in SnO2 ceramics with sequential addition of Co, Sb and Ca

The effect of sequential additions of Co3O4 ,S b 2O5 and CaCO3 on the microstructure and electrical behaviour of SnO2-based ceramics was investigated. Although without conferring varistor properties, Co3O4 additions promote densification in the material. Successive addition of Sb2O5 provides the sample varistor properties but with low nonlinearity coefficient (α = 5.7) and high electric field (E1 = 2022 V cm −1 ) at current density 10 −3 Ac m −3 . Moreover, antimony oxide addition degrades the microstructure condition by inducing porosity. Subsequent addition of CaCO3 promotes densification and grain growth. However, it does not lead to the increase in the nonlinearity coefficient. It only lowers the electric field, thus making the material suitable for lower voltage applications. Observed significant increase in the relative dielectric permittivity up to a factor of about 60 and 200 in the cases of antimony or antimony and calcium addition to SnO2–Co3O4 (accompanied by the appearance of varistor effect) is due to the formation of barrier depletion layers at grain boundaries.

Effect of processing parameters on the degree of infiltration of SiCp preforms by Al-Si-Mg alloys

The effect of the following processing parameters on the degree of infiltration of SiCp preforms by Al–Si–Mg alloys was investigated and quantified: SiC particle size, SiC type and Mg content in the aluminum alloy. These parameters were studied at two levels: 20 and 75 μm, GC and C, and, 3% and 6% Mg, respectively. The contribution of each of the aforementioned parameters and their interactions was determined using analysis of variance and the effect of the levels of each parameter was examined using surface response analysis. Analysis of variance (ANOVA) results show that particle size is the parameter that most significantly impacts the degree of infiltration, followed by magnesium content in the alloy. Surface response analysis shows that regardless of the type of SiC, the degree of infiltration increases with increase in magnesium content and that the increase rate in the degree of infiltration is higher for GC-SiC than that for C-SiC. For both SiC particle sizes, the type of SiC has no significant effect on the degree of infiltration; however, GC-SiC enhances infiltration. When using an alloy 3 wt.% Mg in the alloy, SiC particle size slightly affects the degree of infiltration. In contrast, an alloy 6 wt. % Mg significantly affects the degree of infiltration. An increase in the degree of infiltration with increase in Mg content is observed for both particle sizes; however, in preforms with 75-μm powders, such increment is negligible compared to that for preforms with 20-μm powders. Accordingly, the optimum parameters for maximum degree of infiltration are: 20-μm powders, 6 wt.% Mg in the alloy and GC-SiC.