活性填料在先驱体转化法纤维增强陶瓷基复合材料中的应用II——复合材料的制备及其表征

将活性填料Al应用到吉林碳纤维(JC)和M40JB纤维增强先驱体转化SiC陶瓷基复合材料的制备中。研究表明,经过7个周期的致密化处理,当复合材料素坯中不含活性填料时,JC增强复合材料比M40JB增强复合材料有更高的弯曲强度,因此,JC纤维更适合用作先驱体转化陶瓷基复合材料的增强纤维;当复合材料素坯中含有活性填料Al时,由于Al与碳纤维发生碳化反应,使纤维受损,在纤维与基体之间形成不良的界面结合,导致复合材料的强度发生退化。图象分析表明,M40JB与Al的反应层厚度约为0.94μm。为了防止碳纤维与Al发生反应,应对碳纤维进行适当的表面处理。     

基于固体聚硅氧烷的前驱体浸渍裂解法(PIP)制备C/SiOC复合材料及其微结构与力学性能研究

以熔融的MK树脂(聚甲基倍半硅氧烷)为前驱体, 采用改进的前驱体浸渍裂解法(PIP)制备了致密的C/SiOC复合材料。为了降低MK树脂的固化温度, 选择有机磺酸作为交联剂, 并采用红外光谱分析仪(FT-IR)和热重分析-差热分析仪(TG-DTA)研究了MK树脂的固化机理和陶瓷化行为。研究表明: MK树脂的陶瓷产率高达85wt%, 其裂解得到的SiOC陶瓷自由碳含量低于3wt%, 有利于提高陶瓷的高温稳定性。经过8次PIP制备的C/SiOC复合材料的密度可达1.82 g/cm ³。对得到的C/SiOC复合材料进行三点弯曲测试, 其弯曲强度为(312±25) MPa, 表现出明显的非脆性断裂行为。     

低分子量聚碳硅烷制备3D-Cf/SiC复合材料

研究了低分子量聚碳硅烷 (PCS) 通过先驱体浸渍裂解 (PIP) 工艺制备C_f/SiC复合材料。分析表明:PCS的数均分子量为400,活性较强,陶瓷化产率为70％左右,在1200℃基本转化为微晶态的β-SiC。分别通过3种不同升温速率制备了3D-C_f/SiC复合材料试样,其弯曲强度分别为745.2MPa、686.7MPa和762.5MPa,明显高于文献报道3D-C_f/SiC复合材料弯曲强度300~500MPa的水平。试样断口的SEM照片均显示长的纤维拔出,有良好的增韧效果,低分子量PCS裂解得到的基体比较致密。实验结果说明,低分子量PCS适合于制备3D-C_f/SiC复合材料,并且提高升温裂解速率对材料性能影响很小。      

Pressure infiltration processes to synthesize metal matrix composites – A review of metal matrix composites,the technology and process simulation

Metal matrix composites (MMCs) acquire their improved physical and mechanical properties through the careful reinforcement of their matrices by a variety of light but strong and stable reinforcement materials. The pressure infiltration process (PIP) is one of the most important techniques used for making MMCs with a high reinforcement content in which a molten metal or alloy is injected and solidified in a mold packed with continuous or discontinuous reinforcement materials. Several factors affect the quality of MMCs made by this process. These include, but are not limited to, the reinforcement type, preform geometry, applied pressure and pressure control, as well as the transport phenomena of the molten metal. This paper presents a review of the various aspects of MMCs, the process in terms of the technological details, the latest developments in the reinforcement materials used and the simulation models developed for pressure infiltration manufacturing of MMCs.     

Natural phytochemicals and probiotics as bioactive ingredients for functional foods: Extraction,biochemistry and protected-delivery technologies

BackgroundThe well-known correlation between diet and physiology demonstrates the great possibilities of food to maintain or improve our health, increasing the interest in finding new products with positive physiological effects. Nowadays, one of the top research areas in Food Science and Technology is the extraction and characterization of new natural ingredients with biological activity that can be further incorporated into a functional food, contributing to consumer's well-being. Furthermore, there is a high demand for effective encapsulation methodologies to preserve all the characteristics of bioactive compounds until the physiological action site is reached.Scope and approachIn this review, the relevance of developing standard approaches for the extraction of the highly diverse bioactive compounds was described, as it defines the suitability of the following steps of separation, identification and characterization. Special attention was also dedicated to the encapsulation techniques used on hydrophilic and/or lipophilic compounds (e.g., emulsification, coacervation, supercritical fluid, inclusion complexation, emulsification-solvent evaporation and nanoprecipitation).Key findings and conclusionsSome useful conclusions regarding the selection of the best extraction methodology (Soxhlet extraction, ultrasound-assisted extraction, supercritical fluid extraction, accelerated solvent extraction, or shake extraction) were achieved, considering important aspects such as cost, required technical skills, extract integrity, green chemistry principles, solvent type, sample size, pH, temperature and pressure. In addition, this comprehensive review allowed defining the best protective approach to solve the limitations related to the extremely low absorption and bioavailability of bioactive phytochemicals, overcoming problems related to their low solubility, poor stability, low permeability and metabolic processes in the GI tract.     

PIP结合CVI制备氧化铝-莫来石陶瓷基复合材料

通过PIP结合CVI法制备了C纤维增初三维氧化铝-莫来石陶瓷基复合材料,采用CVD法制备了防氧化涂层,研究了复合材料致密化过程、复合材料的物相、微观结构、力学性能和抗氧化性能。结果表明,CVI能够将氧化硅引入到多孔氧化铝基体内部,1400℃处理后氧化硅与氧化铝完全反应生成莫来石,显著提高了仅以PIP法制备的多孔氧化铝基复合材料的力学性能,CVD制备的氧化硅涂层有效阻止了氧气的侵入,复合材料在1200℃大气环境下保温50h后,试样三点弯曲强度保持率为70％。     

Preparation and mechanical properties of the 2.5D ASf/ZrO2 composites after high-temperature heat treatment

In the paper, the aluminosilicate fiber-reinforced zirconia (AS_f/ZrO₂) ceramic composites were successfully fabricated by polymer impregnation and pyrolysis (PIP) method. The microstructure and high-temperature mechanical properties of the original composites were well studied. The results revealed that the composites could maintain the stability of microstructure at 1000 °C. The flexural strength increased from 58.82 ± 2.83 MPa to 88.74 ± 6.20 MPa and the flexural modulus increased from 29.26 ± 4.67 GPa to 40.76 ± 8.76 GPa. The thermal exposure improved the interfacial bonding and made the load transfer more effective. After heat treatment from 1200 °C to 1400 °C, the flexural strength gradually declined due to the crystallization of the AS fibers and ZrO₂ matrix, while the flexural modulus increased in a completely different trend. After heat treatment at 1400 °C, the composites could maintain a flexural strength of 66.95 ± 4.24 MPa with a flexural modulus of 60.42 ± 7.25 GPa. But the fracture mode gradually evolved to brittleness.     

Pressureless fabrication of dense monolithic SiC ceramics from a polycarbosilane

This paper presents the pressureless preparation of dense and crack-free near stoichiometric SiC monoliths via cross-linking and pyrolysis of a polycarbosilane, followed by polymer-infiltration-pyrolysis cycles. The composition and the porosity of the samples strongly depend on the processing temperature. Thus, at 1050–1100 °C, the SiC monoliths are X-ray amorphous and exhibit low amounts of oxygen and excess carbon; their porosity was rather high (>10%). Higher processing temperatures induced the crystallization of β-SiC. The removal of oxygen and excess carbon due to CO release allowed for obtaining near-stoichiometric compositions at 1700 °C. However, the residual porosity of the samples increased. The use of the PIP technique led already after six cycles to dense monoliths (residual porosity ca. 0.5%).The present study emphasizes the potential of the polymer processing technique for the fabrication of near stoichiometric and dense SiC monoliths, which might be used for structural applications in harsh conditions.     

Long-term oxidation behaviors and strength retention properties of self-healing SiCf/SiC-SiBCN composites

The self-healing SiC_f/SiC-SiBCN composites with various boron contents in SiBCN were prepared, and their long-term oxidation behaviors and strength retention properties were investigated. The 100 h oxidation at 1200–1350 °C leads to parabolic mass gain of the obtained composites. With the oxidation temperature increased from 1200 °C to 1350 °C, the oxidation rate constants increase from 5.91 × 10^?8 mg²/(mm⁴ h) to 9.31 × 10^?7 mg²/(mm⁴ h) for the boron-lean (3.14%) composites, and from 2.57 × 10^?7 mg²/(mm⁴ h) to 6.04 × 10^?7 mg²/(mm⁴ h) for the boron-rich (7.18 wt%) composites. Correspondingly, the oxidation activation energy decreases from 363 kJ/mol to 112 kJ/mol due to the low initial oxidation temperature of boron-rich SiBCN. All the composites exhibit the higher strength retention rates after 1350 °C oxidation due to the enhanced self-healing performance. The boron-rich composites show a high strength retention rate of up to 104% due to the good self-healing capacity of the boron-rich SiBCN as well as the high CVI-SiC content.