ß→α Phase Transformation and Properties of Solid-State-Sintered SiC Ceramics with TaC Addition.

Dense SiC-based composite ceramics were fabricated by means of the ex situ addition of TaC using solid-state spark plasma sintering (SPS). Commercially available ß-SiC and TaC powders were chosen as raw materials. Electron backscattered diffraction (EBSD) analysis was conducted to investigate the grain boundary mapping of SiC-TaC composite ceramics. With the increase in TaC, the misorientation angles of the α-SiC phase shifted to a relatively small range. It was deduced that the ex situ pinning stress from TaC greatly suppressed the growth of α-SiC grains. The low ß→α transformability of the specimen with the composition of SiC-20 vol.% TaC (ST-4) implied that a possible microstructure of newly nucleated α-SiC embedded within metastable ß-SiC grains, which could have been responsible for the improvement in strength and fracture toughness. The as-sintered SiC-20 vol.% TaC (ST-4) composite ceramic had a relative density of 98.0%, a bending strength of 708.8 ± 28.7 MPa, a fracture toughness of 8.3 ± 0.8 MPa·m1/2, an elastic modulus of 384.9 ± 28.3 GPa and a Vickers hardness of 17.5 ± 0.4 GPa.

Improved Near-Infrared Up-Conversion Emission of Tm3+ Sensitized by Yb3+ and Ho3+ in LuF3 Nanocrystals.

In the present work, mono-disperse and uniform orthorhombic lutetium fluoride (LuF3) nanocrystals with an average size of about 35 nm have been successfully synthesized by a simple ionothermal method without any template. The infrared (IR) to visible up-conversion (UC) photoluminescence of LuF3 doped with Yb3+, Tm3+, and Ho3+ under 980 nm excitation was systemically studied. The intensity of near infrared (NIR) to visible up-conversion emission of Tm3+ was improved efficiently by adding Yb3+ and Ho3+ in LuF3, especially for the broad NIR emission band located at 812 nm. Meanwhile, compared to the Yb3+ and Tm3+ co-doped LuF3, the ratio of red to green emission in the Yb3+, TmS+, and HoS+ co-doped LuF3 changed greatly, and a bright yellowish-green emission was observed under 980 nm laser excitation. It shows that Yb3+, Tm3+ and Ho3+ co-doped LuF3 nanocrystals provided a potential application in vitro and in vivo bio-imaging, color displays and optical storage.