Factors inducing degradation of properties after long-term oxidation of Si3N4–MoSi2 electroconductive composites

The effects of heat treatments on strength and electrical conductivity after 100 h in air up to 1500 °C were evaluated on hot-pressed Si3N4–35 vol% MoSi2 composite. The long-term oxidation involves microstructural changes at the material surface and subsurface, such as the formation of oxide scales and of a multilayered microstructure. At T ⩾ 1200 °C, a glassy silicate phase is formed, which embeds cristobalite grains and highly textured Y2Si2O7 crystals. At the same time, MoSi2, assisted by oxygen, reacts with Si3N4 forming Mo5Si3, Si2N2O, and SiO2. The decrease of the room temperature flexural strength reached about 25% in the samples exposed at 1000 °C for 100 h, compared to the as-produced materials. On the contrary, after treatments at higher temperatures, the strength decrease is lower at 1500 °C, the residual strength is 836 ± 62 MPa with a strength decrease of about 8%. The surface oxide scale is an insulator and, consequently, the electrical resistivity of the composite rises from 10-3 to 107–109 Ωcm.

Isotope geochemistry and origin of illite-smectite and kaolinite from the Seilitz and Kemmlitz kaolin deposits, Saxony, Germany

Residual clays that developed on Permian and Carboniferous glass-rich silicic volcanic rocks (pitchstones, ignimbrites) at the Seilitz and Kemmlitz kaolin deposits, Saxony, Eastern Germany, contain locally abundant lath-shaped illite-rich illite-smectite mixed-layer minerals (I-S). Analyses by XRD and TEM-AES reveal a large illite percentage (>∼90%) and R3 ordering in I-S from Seilitz (>∼90%) and smaller illite percentage (∼70%) and R1 ordering in I-S from Kemmlitz. The clays never suffered a deep burial and there is no geological, petrographic or fluid inclusion evidence for aeolian input or hydrothermal origin of I-S at either deposit. The I-S formed exclusively at the expense of volcanic glass and not from K-feldspar. Residual quartz phenocrysts in the clays still preserve primary glassy silicate melt inclusions and lack secondary aqueous fluid inclusion trails. The dD and δ18O values of kaolinite and I-S are suggestive of low formation temperatures (<40ºC). Rb-Sr and K-Ar dating of I-S-bearing clay separates yield Lower Cretaceous ages at Seilitz and indicates the presence of excess or inherited 40Ar in illite-rich I-S. In contrast, Triassic to Jurassic Rb-Sr ages are obtained for I-S from the Kemmlitz kaolin deposit.