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Reaction-bonded B4C-SiC composites are highly promising materials for numerous advanced technological applications. However, their microstructure evolution mechanism remains unclear. Herein, B4C-SiC composites were fabricated through the Si-melt infiltration process. The influences of the sintering time and the B4C content on the mechanical properties, microstructure, and phase evolution were investigated. X-ray diffraction results showed the presence of SiC, boron silicon, boron silicon carbide, and boron carbide. Scanning electron microscopy results showed that with the increase in the boron carbide content, the Si content decreased and the unreacted B4C amount increased when the sintering temperature reached 1650°C and the sintering time reached 1 h. The unreacted B4C diminished with increasing sintering time and temperature when B4C content was lower than 35wt%. Further microstructure analysis showed a transition area between B4C and Si, with the C content marginally higher than in the Si area. This indicates that after the silicon infiltration, the diffusion mechanism was the primary sintering mechanism of the composites. As the diffusion process progressed, the hardness increased. The maximum values of the Vickers hardness, flexural strength, and fracture toughness of the reaction-bonded B4C-SiC ceramic composite with 12wt% B4C content sintered at 1600°C for 0.5 h were about HV 2400, 330 MPa, and 5.2 MPa·m0.5, respectively.

Based on the first 13.4 yr of Fermi science data in the energy range from 300 MeV to 500 GeV, we discovered a bright GeV gamma-ray source with a $\sim$5.64 $\sigma$ detection, named Fermi J1242.5+3236, which has an offset of about 0fdg0996 from a nearby star-forming galaxy NGC 4631. When using the 12-yr data, the detection significance of Fermi J1242.5+3236 is about 4.72 $\sigma$. Fermi J1242.5+3236 is a steady point source without significant temporal variability and has a hard gamma-ray photon index of about -1.60 $\pm$0.24. The spatial offset and the hard gamma-ray spectrum disfavor this source as the diffuse gamma-ray emission from this galaxy. This new source might have a possible origin of an unidentified background blazar, which is more likely a high-synchrotron-peaked blazar for its hard gamma-ray photon index. A follow-up optical observation would help distinguish origin of Fermi J1242.5+3236., Comment: 8 pages,6 figures, published in Research in Astronomy and Astrophysics

Among thermoelectric materials, tin telluride has been attracting significant interest for its nontoxic and eco-friendly nature. Meanwhile, band engineering and ultrathin film technologies have bee...

In this paper, to identify the order-disorder structure and performance of (Gd, Sm)2(Zr, Ce)2O7 ceramic as the host matrices for immobilizing actinides, various contents of Sm/Ce co-incorporated (Gd, Sm)2(Zr, Ce)2O7 ceramics were designed and fabricated. The influences of various Sm/Ce doping contents on the order-disorder structure of the samples were investigated systematically by XRD and Raman analysis. The microstructure, elemental composition and distribution of the samples were analyzed by SEM-EDX approach. The aqueous durability of the specimens was also evaluated by a static leaching experiment. The results showed that the Gd/Zr-sites of Gd2Zr2O7 pyrochlore could be concurrently substituted by various contents of Sm/Ce. The phase structure of the as-prepared (Gd, Sm)2(Zr, Ce)2O7 ceramics were transformed from the ordered pyrochlore phase to the disordered defect-fluorite phase with decreasing the average r[(Gd, Sm)3+]/r[(Zr, Ce)4+] radius ratio after substitution. And the ceramics fabricated in this study presented high densification and excellent aqueous durability.

In order to ascertain the structural relationship of zirconolite and pyrochlore for their potential application in HLW immobilization, the Gd-doped zirconolite-pyrochlore composite ceramics (Ca 1- x Zr 1- x Gd 2 x Ti 2 O 7 ) were systematically synthesized with x = 0.0–1.0 by traditional solid-phase reaction method. The phase evolution and microstructure of the as-prepared samples have been elucidated by XRD and Rietveld refinement, Raman spectroscopy, BSE-EDS and HRTEM analysis. The results showed that zirconolite-2M, zirconolite-4M, perovskite and pyrochlore, four phases were identified in Ca 1- x Zr 1- x Gd 2 x Ti 2 O 7 system and could be coexisted at x = 0.4 composition. With the increase of Gd 3+ substitution, the phase evolution was followed by zirconolite-2M→zirconolite-4M→pyrochlore. It is illustrated that the phase transformation from zirconolite-2M to zirconolite-4M was promoted by the preferential substitution of Gd 3+ for Ca 2+ . And the solubility of Gd 3+ in zirconolite-2M, zirconolite-4M and pyrochlore increased in sequence. The chemical stability test was also measured by the PCT leaching method. The normalized elemental release rates of Ca, Zr, Ti and Gd in Ca 1- x Zr 1- x Gd 2 x Ti 2 O 7 system were fairly low and in the range of 10 −6 −10 −8 g m −2 d −1 , which indicated a potential ceramics composite ensemble of CaZrTi 2 O 7 -Gd 2 Ti 2 O 7 system for nuclear HLW immobilization.

In this paper, blended Ti/SiC powder was processed by selective laser melting (SLM) method to prepare a novel kind of Ti-Si-C composite. Microstructure, elements distribution and phase evolution of Ti-Si-C composites fabricated by SLM were studied. SEM, EDS, TEM and XRD were performed to evaluate the microstructure and phase evolution during the SLM process. The results showed that processing parameters significantly affected the microstructure and element distribution of the samples. It was found that higher laser energy density could promote the densification process and hence suppress balling phenomenon of Ti-rich phases. At the micro-level, the microstructure could be classified into the dense area and the porous area, which was closely related to the redistribution of elements. The phases obtained by the SLM process were composed by TiCx, TiSi2, Ti5Si3, and Ti5Si4, which appeared in different microstructures. The results indicated that composites with different phases and microstructure could be obtained by merely modifying laser processing parameters, which could give experimental guidance for the fabricating of gradient Ti-Si-C composites by selective laser melting without additional apparatus on powder feeding.

The detoxification and disposal of hexavalent chromium [Cr(VI)] is of real importance to the ecological environment protection and human being health. In this paper, Cr(VI) was in advance reduced to trivalent chromium [Cr3+] by Fe2+ and was immediately immobilized in a “fly ash-metakaolin” based (FMB) geopolymer through alkaline activation and hydrothermal processing. The phase structure and microstructure of the as-prepared detoxified Cr-added FMB geopolymer were investigated by XRD, FT-IR and SEM. The compressive strength and Cr(VI) leaching toxicity of the samples were also systematically investigated. The results showed that the phase compositions of the samples were mainly composed of amorphous phase, several weak crystalline phases of quartz, gismondite and sodium chloride after hydrothermal processing. The detoxified Cr can be fixed as the form of chemical bonding in the skeleton structure of FMB geopolymer. In addition, the compressive strength test results revealed that the compressive strength was mainly affected by the contents of Cr addition and reducing agent. Importantly, the extremely low leaching concentration of Cr(VI) suggested that the Cr-added FMB geopolymer prepared by hydrothermal processing possessed excellent Cr(VI) leaching toxicity, and the reduced Cr3+ was difficultly reoxidized to Cr(VI) after being immobilized in the FMB geopolymer.

Long alumina fibers were prepared by sol-gel method. The spinning sol was obtained by mixing aluminum nitrate, tartaric acid, and polyvinylpyrrolidone with a mass ratio of 10:3:1.5. Thermogravimetry-differential scanning calorimetry (TG-DSC), Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the properties of the gel and ceramic fibers. A little of α-Al2O3 phase is observed in the alumina precursor gel fibers sintered at 1273 K. The fibers with a uniform diameter can be obtained when sintered at 1473 K, and its main phase is also indentified as α-Al2O3.

The precursor sol of alumina was prepared by sol-gel method with aluminum nitrate and malic acid as raw materials. The effects of content of malic acid and polyvinylpyrrolidone (PVP) on sol spinnability were explored. The gel fibers with above 80 cm in length were obtained by mixing aluminum nitrate, malic acid and PVP on mass ratio of 10:3:1.5. Thermogravimetry-differential scanning calorimetry (TG-DSC), Fourier transform infrared (FTIR) spectrum, X-ray diffractometry (XRD), and scanning electron microscopy (SEM) were used to characterize the properties of the gel and ceramic fibers. The alumina fibers with a smooth surface and about 20 μm in diameter were obtained by sintering at 1 200 °C, and their main phase was indentified to be α -Al 2 O 3 .

The transformation of tempering for quenched steel corresponded to complicated process of phase transformation, and mechanical properties of quenched-and-tempered steel were related to the phase transformation. In practice, hardness test was adopted to judge whether the properties of tempered-parts qualified because of its facility. Numerous researches indicated that, there existed correlativity expressed by different function forms between tempering hardness of quenched-steel and its tempering parameters. However, considering physical metallurgy of tempering process, the adoption of double-exponential function would help to describe regularity of hardness changing more exactly for quenched-steel during tempering process. Additionally, results of hardness tests for isothermal tempering and molding/simulation researches have shown that, the model of double-exponential function, which can reflect decline law of tempering hardness for quenched-steel, would provide basis for optimization design of tempering parameters, performance prediction of tempered-parts, and energy-saving heat-treatment on tempering process.