Preparation of long alumina fibers by sol-gel method using tartaric acid

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.

Microstructure and mechanical properties of reaction-bonded B_(4)C–SiC composites

Reaction-bonded B_(4)C–SiC composites are highly promising materials for numerous advanced technological applications.However,their microstructure evolution mechanism remains unclear.Herein,B_(4)C–SiC composites were fabricated through the Si-melt infiltration process.The influences of the sintering time and the B_(4)C 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 B_(4)C amount increased when the sintering temperature reached 1650°C and the sintering time reached 1 h.The unreacted B_(4)C diminished with increasing sintering time and temperature when B_(4)C content was lower than 35wt%.Further microstructure analysis showed a transition area between B_(4)C 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 B_(4)C–SiC ceramic composite with 12wt%B_(4)C content sintered at 1600°C for 0.5 h were about HV 2400,330 MPa,and 5.2 MPa·m^(0.5),respectively.

Possible Origin of a Newly Discovered GeV Gamma-Ray Source Fermi J1242.5+3236

Based on the first 13.4 yr of Fermi science data in the energy range from 300 Me V to 500 Ge V,we discovered a bright Ge V gamma-ray source with a ~5.64σ detection,named Fermi J1242.5+3236,which has an offset of about0°.0996 from a nearby star-forming galaxy NGC 4631.When using the 12 yr’ data,the detection significance of Fermi J1242.5+3236 is about 4.72σ.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 ± 0.24.The spatial offset and the hard gammaray 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.