Oxidation Behavior of the Si-B-X (X = Mo, Cr, or Ti) Alloys in the Temperature Range of 1000-1400 °C.

Low-melting-point silicon-boron system alloys are promising for low-temperature reactive melt infiltration to reduce high-temperature damage to silicon carbide fibers during the densification of SiC/SiC composites. Meanwhile, the oxidation resistance of the alloys will have a large impact on the intrinsic oxidation resistance of the composite. Herein, three alloys, Si-14.88B-7Mo, Si-14.88B-7Ti, and Si-14.88B-7Cr, were fabricated to investigate the oxidation behavior in air at 1000-1400 °C. The results showed that the oxidation weight gains of the Si-B-Mo alloy after oxidation at 1400 °C for 100 h were 0.9 mg/cm-2, which were only 50 and 1.5% of those of Si-B-Ti and Si-B-Cr alloys, respectively. The excellent oxidation resistance of Si-B-Mo alloys at 1000-1400 °C was attributed to the formation of glassy-surface layers and the dense internal oxide layer. The dense oxide layer and the low solubility of Mo ions in SiO2 inhibit the volatilization of MoO3 and the oxidation reaction, reducing the oxidation rate of the Si-B-Mo alloy. The difference in the coefficients of thermal expansion for SiO2 and TiO2 led to penetrating cracks in the oxide layer of the Si-B-Ti alloy during cooling, thereby reducing the oxidation resistance. In addition, the rate of volatilization of Cr2O3 as CrO3 in an oxidation atmosphere above 1200 °C increased significantly in the Si-B-Cr alloy. The simultaneous volatilization of B2O3 and CrO3 resulted in the formation of loose oxide layers in the CrB2 region of the Si-B-Cr alloy, leading to severe oxidation.

Effect of heat treatment on the microstructure and ablation performance of C/C-SiC composites containing ZrSi2-Si.

Low-temperature reactive melt infiltration (LRMI) is advantageous for the fabrication of ceramic matrix composites (CMCs). However, residual metal in CMCs prepared by LRMI deteriorates the high-temperature properties. In this study, C/C-SiC composites containing ZrSi2-Si were prepared using LRMI at 1400 °C, and the effect of heat treatment at 1400 °C for 10, 20, or 30 h on the microstructure and ablation properties of the composites fabricated using three different ternary alloys (e.g., Zr0.047B0.0378Si0.9152, Zr0.0724B0.0366Si0.891, and Zr0.1B0.05Si0.85) was investigated. The results show that the residual Si in the composites can be decreased by volatilisation and the reaction between Si and C during heat treatment, resulting in a decrease in the density and an increase in the SiC content. The ablation rates of the composites after heat treatment for 20 h were lower than those of the samples after heat treatment for 30 and 10 h. Among the three alloys, the composites prepared using Zr0.0724B0.0366Si0.891 demonstrated the best ablation performance. Their linear and mass ablation rates at 1911 °C were -0.11 µm s-1 and 1.82 µg s-1, respectively.

Successful restoration of a tropical shallow eutrophic lake: Strong bottom-up but weak top-down effects recorded.

Fish manipulation has been used to restore lakes in the temperate zone. Often strong short-term cascading effects have been obtained, but the long term-perspectives are less clear. Fish manipulation methods are far less advanced for warm lakes, and it is debatable whether it is, in fact, possible to create a trophic cascade in warm lakes due to the dominance and high densities of fast-reproducing omnivorous fish. However, removal of benthic feeding fish also reduce disturbance of the sediment, which not only affects the nutrient level but also the concentration of suspended organic and inorganic matter with enhanced water clarity and potentially better growth conditions for submerged macrophytes. We conducted a biomanipulation experiment in one of the basins in Chinese Huizhou West Lake that have remained highly turbid after extensive nutrient loading reduction. Another basin was used as control (control-treatment pairing design). Removal of a substantial amount of plankti-benthivorous fish was followed by planting of submerged macrophytes and stocking of piscivorous fish. We found strong and relatively long-lasting effects of the restoration initiative in the form of substantial improvements in water clarity and major reductions in nutrient concentrations, particularly total phosphorus, phytoplankton and turbidity, while only minor effects were detected for crustacean zooplankton grazers occurring in low densities before as well as after the restoration. Our results add importantly to the existing knowledge of restoration of warm lakes and are strongly relevant, not least in Asia where natural lakes frequently are used extensively for fish production, often involving massive stocking of benthivorous fish. With a growing economy and development of more efficient fish production systems, the interest in restoring lakes is increasing world-wide. We found convincing evidence that fish removal and piscivores stocking combined with transplantation of submerged macrophytes may have significant effects on water clarity in warm shallow lakes even if the zooplankton grazing potential remains low, the latter most likely as a result of high predation on the zooplankton.