Crucible is a key vessel in superalloy smelting, and high-performance crucible is quite important to the development of superalloy. Pointed at the easy cracking and poor thermal-shock stability of Al 2 O 3 crucibles for superalloy smelting, particle gradation was managed in slip casting of Al 2 O 3 crucibles. The ceramic slurry in good dispersion was prepared by optimizing particle gradation, with solid content of 86 wt% and viscosity of lower than 0.5 Pa s. For the crucible with optimized particle gradation, the density was 2.37 g/cm 3 , the linear shrinkage was less than 2%, the porosity was 18.72%, the bending strength was close to 22 MPa and the thermal expansivity was 2.77 × 10 −6 /°C. Strength retention rate was raised to evaluate the thermal shock stability of crucibles, which was up to 73.5% after five-time thermal shock, well meeting the requirements of crucibles for superalloy smelting, and the superior thermal-shock stability was attributed to the uniform network microstructure and low thermal expansivity. The optimal particle gradation in experiments was explained in terms of Dinger-Funk model. This work provides a reference for preparing high-performance Al 2 O 3 crucibles for superalloy smelting with low thermal expansivity as well as excellent thermal-shock stability.
Lei Xiang,Li-Juan Zeng,Pei-Pei Du,Xiao-Dan Wang,Xiao-Lian Wu,Binoy Sarkar,Huixiong Lü,Yan-Wen Li,Hui Li,Ce-Hui Mo,Hailong Wang,Quan-Ying Cai
Sorption of organic contaminants by biochar greatly affects their bioavailability and fate in soils. Nevertheless, very little information is available regarding the effects of biochar on sorption and desorption of organic contaminants in different soil particle-size fractions. In this study, di- n -butyl phthalate (DBP), a prevalent organic contaminant in agricultural soils, was taken as a model contaminant. The effects of biochar on DBP sorption and desorption in six particle-size fractions (i.e., coarse sand, fine sand, coarse silt, fine silt, clay, and humic acid fractions) of paddy soil were investigated using batch sorption-desorption experiments. A straw-derived biochar with high specific surface area (116 m 2 /g) and high content of organic matter (OM) rich in aromatic carbon (67%) was prepared. Addition of this biochar (1% and 5%) significantly promoted the sorption and retention of DBP in all the paddy soil particle-size fractions at environmentally relevant DBP concentrations (2–12 mg/L) with 1.2–132-fold increase of the K d values. With increasing addition rates of biochar, DBP retention by the biochar enhanced. The biochar’s effectiveness was remarkably influenced by the physicochemical properties of the soil particle-size fractions, especially, the OM contents and pore size showed the most striking effects. A parameter ( r kd ) reflecting the biochar’s effectiveness showed negative and positive correlations with OM contents and pore size of the soil particle-size fractions, respectively. Accordingly, strong effect of the biochar was found in the soil fractions with low OM contents and high pore size. The findings of this study gave insight into the effects and influencing factors of biochar on sorption and desorption of organic contaminants in soils at scale of various particle-size factions.
Porous gelcasted and solid-state-sintered SiC (S-SiC) ceramics with significantly improved connectivity were successfully prepared through the synergistic pore-forming mechanism of direct foaming and particle packing, using fine (D 50 = 0.5 μm) and coarse (D 50 = 5.0 μm) graded SiC powders as the main raw materials. As the proportion of coarse SiC powder increased, the viscosities of aqueous SiC-B 4 C-C slurries sharply decreased, which thus flourished foaming and enlarged bubble-derived-pore diameter in porous S-SiC ceramics, accompanied with the increased amount and size of window. Moreover, abundant particle-packing pores in struts of bubble-derived pores were also formed and even interconnected with each other at 60 wt% of coarse SiC powder. Besides, there was a remarkable increase in porosity from 61.0% to 81.7% and accordingly a significant decline in density from 1.24 g cm −3 to 0.58 g cm −3 as the content of coarse SiC powder increased. The optimized multiple pore microstructures resulted in an amazing nitrogen gas permeability of 1.78×10 −11 m 2 under the condition of an excellent compressive strength of 10.8 ± 2.7 MPa.
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