Isothermal Oxidation Resistance of Zr3[Al(Si)]4C6-Based Composite Ceramics at 1000-1300°C in Air

The isothermal oxidation behavior of Zr3[Al(Si)]4C6-ZrB2-ZrC composite ceramics at 1000-1300 °C in air has been investigated. The oxidation kinetics of the composites and generally follows a parabolic law. At the same oxidation temperature and time, the weight gain per unit surface area, oxidation rate constant and oxide thickness of the composites are higher than those of monolithic Zr3[Al(Si)]4C6 ceramic. With the incorporation of ZrB2 and ZrC, the oxidation resistance of the composites becomes poor. The surfaces of the oxide layer have a loose and porous structure, consisting of mainly ZrO2 and little mullite, and there are no dense oxide films preventing the inward diffusion of oxygen element effectively.

Effects of the chemical characteristics and concentration of inorganic suspended solids on nitrification in freshwater

The effect of inorganic suspended solids (ISS) on nitrification in freshwater samples has been described inconsistently and remains unclear. This study therefore investigated the effects of the chemical characteristics and concentration of ISS on the nitrification rate by focusing on Nitrosomonas europaea and Nitrobacter winogradskyi as the two most dominant nitrification species in freshwater. Batch-wise experiments were conducted using three chemically well-characterized ISS (i.e. the clay minerals montmorillonite, sericite, and kaolinite in the concentration range 0–1,000 mg L−1). The results show that the ammonium oxidation rate constant (kNH4) was significantly affected by the ISS type, whereas changes in the ISS concentration had an insignificant effect on kNH4, except for kaolinite. The highest kNH4 was observed in samples containing sericite (kNH4, 0.067 L mg−1 day−1), followed by samples containing montmorillonite (kNH4, 0.044 L mg−1 day−1). The ammonium oxidation rate was low in the control and kaolinite samples. Nitrite oxidation was enhanced in the presence of all types of ISS. The rate constants of ISS-mediated nitrite oxidation (kNO2, 0.13–0.21 L mg−1 day−1) were not significantly different among the three types of ISS, but kNO2 was significantly affected by ISS concentration. Overall, our study indicated various effects of the ISS type and concentration on nitrification and, in particular, a notable positive effect of sericite.

The Study on Oxidation Resistance of SiC Composites Used for Solar Absorber Coating

The SiC composites with good oxidation resistance were fabricated via semi-dry pressing and pressureless firing in the air. The SiC, talc, kaolin, alumina, BN and H3BO3were used as starting materials. Sample B2 fired at 1280 °C had the optimal physical properties with a bending strength of 85.84 MPa. Sample B3 contained 1 wt% H3BO3and fired at 1300 °C for 2 h had the best oxidation resistance. After oxidized at 1000 °C for 50 h, the weight gain and oxidation rate constant of sample B3 were 3.3990 mg/cm2and 0.52×10-4g2/(cm4·s) respectively. The oxidation products of SiC, BN and H3BO3could generate SiO2-B2O3film and improve the oxidation resistance effectively. It is believed that these SiC composites should be a promising candidate in solar absorber coating.

Calculation of Boundary Conditions between Internal and External Oxidation of Silicon or Chromium Containing Steels

The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were calculated in order to clarify the formation mechanism of fayalite scale (Fe2SiO4) or chromite scale (FeCr2O4), which can form as a “sub-scale” in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, NO(s), which are constituents of the internal oxidation rate constant, (2DONO(s)/NB(O)n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2DONO(s)/NB(O)n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were also calculated by substituting the calculated values of DO and NO(s) into the rate equation.

Oxidation Behavior of an Al-Modified Silicide Coating on an Nb-Silicide-Based Ultrahigh-Temperature Alloy

The oxidation behavior of an Al-modified silicide coating prepared through a siliconization and then an aluminization pack cementation process on an Nb-silicide-based ultrahigh-temperature alloy was investigated in air at 1,250°C. The coating consisted of a 50-μm-thick (Nb,X)Si2 (X represents Ti, Cr, and Hf elements) outer layer with (Nb,Ti)3Si5Al2 distributed discontinuously in its superficial part and a 90-μm-thick (Nb,Ti)Al3 inner layer containing Si-rich precipitates. The oxidation of this coating was controlled primarily by the preferential oxidation of Al with the formation of α-Al2O3 scale. The (Nb,Ti)Al3 inner layer could act as an Al reservoir for forming scale and maintaining the existence of (Nb,Ti)3Si5Al2 phase in the zone just beneath the scale. The parabolic oxidation rate constant of this coating was about 1.72 μm2/h, lower than that of the bare alloy by about 4 orders of magnitude.

Influence of ageing on the catalytic activity of ferric sludge for oxidation of fe(II)

The catalytic effect of freshly formed or added ferric iron on the Fe(II) oxidation has been demonstrated by previous studies. High Fe(III) concentrations significantly accelerate the oxygenation rate. High Fe(III) concentrations can be maintained only by sludge recycle which eventually leads to sludge ages as high as 10 days. Therefore, the aim of this study was to determine whether ageing of ferric hydroxide sludge affects its catalytic effect on the oxidation of ferrous iron by aeration. In order to reach this aim four different groups of experiments were carried out using ferric hydroxide sludge aged for a period of 0 to 10 days. Initial Fe(III) concentrations were varied within the range of 50 mg/l to 200 mg/l. It has been demonstrated that the catalytic effect of ferric hydroxide sludge on the oxidation of ferrous iron by aeration increases with increasing sludge age contrary to what is commonly expected. It has been concluded that, catalytic oxidation rate constant kcat obtained from the batch systems can safely be used in design of continuous flow iron oxidation reactors with sludge recycle.