Phonon tunnels from a sonic black hole

We investigate the phonon radiation from a spherically symmetrical, stationary, viscid-free son-ic black hole by using a semi-classical method. The backreaction of the radiated phonon is taken into account. We obtain the phonon emission temperature, and it is consistent with the Hawking's formula

Mechanical and tribological performance of (TiNbTaWMo)C high-entropy ceramics in a wide temperature range

High-entropy carbide ceramics have attracted extensive attentions for potential applications as high temperature structural materials, the mechanical and tribological performance of (TiNbTaWMo)C high-entropy carbides over wide temperature range combined with the oxidation behavior were investigated in the present study. (TiNbTaWMo)C high-entropy carbides were successfully prepared from five kinds of carbide powders by spark plasma sintering at 2000 °C. The mechanical properties of (TiNbTaWMo)C and the tribological performance at room temperature –800 °C were studied. The Vickers hardness, bending strength and fracture toughness of the sintered (TiNbTaWMo)C high-entropy carbides are 21.31 ± 0.43 GPa, 477.15 ± 34.17 MPa, and 6.41 ± 0.85 MPa m1/2, respectively. At room temperature, the friction coefficient is stable at 0.35, and the wear rate is as low as 3.39 × 10−7 mm3/Nm. With temperature increasing, friction coefficient increases firstly and decreases above 600 °C, which is contributed by the formation of oxide films at high temperatures. The wear mechanisms of (TiNbTaWMo)C high-entropy ceramics at different temperatures were discussed in detail by microstructures and phase analysis of the worn surfaces together with the oxidation behaviors of (TiNbTaWMo)C at 600 °C and 800 °C