Bi Jia

In this paper, we estimated the temperature-dependent critical inclusion size for microcracking under residual stress and applied stress for particulate-reinforced ultra-high-temperature ceramic matrix composites. The critical flaw size and applied stress for the stable growth of radial cracks under different temperatures were also estimated. It was found that under a lower applied stress, the critical inclusion size was sensitive to the temperature. Under higher applied stresses, the sensitivity became smaller. For ceramic materials with pre-existing microcracks, the crack resistance could be improved by increasing the service stress when the service stress was low. As the temperature increased, the critical flaw size of the materials decreased; the applied stress first increased and then decreased. Finally, a temperature-dependent fracture strength model of composites with a pre-existing critical flaw was proposed. A good agreement was obtained between the model prediction and the experimental data. In this work, we show a method for the characterization of the effects of temperature on the fracture behavior of ceramic-based composites.

In this work, alumina-graphene (Al₂O₃-G) composites with graphene contents ranging from 0.5 to 3% were prepared by stepwise feeding ball milling and hot pressing. The influences of graphene content on the microstructure and mechanical properties of Al₂O₃-G composites were investigated. Results showed that the densification, grain sizes, flexural strength, fracture toughness and Vickers hardness of materials increased firstly and then decreased with increasing graphene contents. When the graphene content was 1%, the value of each performance parameter reached the maximum. The average grain size of material decreased from 991 to 551 nm as the graphene content increased from 0 to 1%, but it increased to 863 nm when the graphene content was 3%. The flexural strength, fracture toughness and Vickers hardness of composites with graphene content of 1% increased to 763.5 MPa, 7.4 MPa m^1/2 and 21.28 GPa. Compared with the Al₂O₃, the fracture strength and toughness of the composites increased by up to 54.63 and 65.54%. Analysis suggested that the strength of Al₂O₃-G composites was mainly related to the grain size and preexisting microflaws.

Recent years have witnessed a growing research interest in graphene-reinforced alumina matrix composites (Al₂O₃-G). In this paper, to better achieve the dispersion of graphene in composites, a ball milling method for adding raw materials step by step, called stepwise feeding ball milling, was proposed. The Al₂O₃-1.0 wt % graphene composites were prepared by this stepwise feeding ball milling and hot pressing. Then, the effects of sintering temperature and sintering pressure on the microstructure and mechanical properties of composites were studied. Results showed that the bending strength, fracture toughness and Vickers hardness of composites increased firstly and then decreased with increasing sintering temperature. The mechanical properties of composites were all at their maximum with the sintering temperature of 1550 °C. For example, the bending strength of composites reached 754.20 MPa, which was much bigger than 478.03 MPa at 1500 °C and 364.01 MPa at 1600 °C. Analysis suggested that the strength of composites was mainly related to the grain size, microflaw size and porosity.

Monodispersed Pt nanoparticles supported on a TiO₂ and nitrogen-doped carbon composite (TiO₂/NDC) were successfully synthesized via an efficient in situ self-assembly strategy and microwave-assisted polyol process. The Pt/TiO₂/NDC catalyst exhibited superior electrocatalytic activity toward the methanol oxidation reaction (MOR). The electrochemically active surface area of the Pt/TiO₂/NDC catalyst was 2-fold higher than that of the Pt/C/NDC catalyst. In addition, the Pt/TiO₂/NDC catalyst revealed a better electrocatalytic activity and CO-tolerance as well as stability toward MOR. The combined characterization from FTIR, BET, TEM, EDS, TGA, ICP, XRD and XPS analyses demonstrated that the superior catalytic performance and stability of the Pt/TiO₂/NDC catalysts likely arose from the synergistic effect of their unique morphology and composition as well as the electronic effect between the TiO₂/NDC and Pt. This electrocatalyst holds great promise for application in direct methanol fuel cells.

Suppressing charge recombination and improving carrier transport are key challenges for the enhancement of photocatalytic activity of heterostructured photocatalysts. Here, we report a ferroelectric polarization-enhanced photocatalysis on the basis of BaTiO₃-TiO₂ core-shell heterostructures synthesized via a hydrothermal process. With an optimal weight ratio of BaTiO₃ to TiO₂, the heterostructures exhibited the maximum photocatalytic performance of 1.8 times higher than pure TiO₂ nanoparticles_. The enhanced photocatalytic activity is attributed to the promotion of charge separation and transport based on the internal electric field originating from the spontaneous polarization of ferroelectric BaTiO_3. High stability of polarization-enhanced photocatalysis is also confirmed from the BaTiO₃-TiO₂ core-shell heterostructures. This study provides evidence that ferroelectric polarization holds great promise for improving the performance of heterostructured photocatalysts.