Metals are often soft while ceramics are often hard. To make metals as hard as ceramics has been long-awaited in metals and ceramics communities. In this study, bulk nanocrystalline (NC) tungsten-1 wt% yttrium-0.7 wt% titanium (WYT) was fabricated via mechanical alloying followed by high-temperature/high-pressure sintering. The high pressure directly inhibits grain growth by suppressing atomic diffusion and indirectly inhibits grain growth by lowering sintering temperature, enabling the synthesis of dense NC WYT bulks with an average grain size of ∼ 9.0 nm. The high temperature causes annealing- and segregation-induced structural relaxation of grain boundaries (GBs) in sintered bulk NC WYT, resulting in GBs with a small excess volume of only ∼ 0.005 nm, one fifth that of bulk NC tungsten. The small grain size and the relaxed GBs in the bulk NC WYT make it hard for GBs to emit dislocations, resulting in extremely high hardness of 23.2 ± 0.3 GPa, yield strength of 9.4 ± 0.3 GPa, and ultimate compressive strength of 13.1 ± 1.1 GPa. The hardness of metallic bulk NC WYT is comparable to that of ceramic tungsten carbide (WC) whereas the wear rate of bulk NC WYT is even lower than that of WC. The structure-relaxed GBs lower the thermodynamic driving force for grain growth and result in an extremely high thermal stability - no grain growth occurs in bulk NC WYT annealed at a high temperature of 1500 °C, which is above the recrystallization temperature of tungsten. The present strategy - multifunctional treatments of NC alloy powders by high-temperature/high-pressure - provides a huge potential to manufacture dense bulk NC materials with exceptional mechanical property and thermal stability for practical applications.
Lead-free dielectric ceramics with ultrahigh energy storage performance are the best potential stocks used in next-generation advanced pulse power capacitors. Here, an ultrahigh recoverable energy storage density W rec of ≈7.57 J cm −3 and a large efficiency η of ≈81.4% are first realized in (Bi 0.5 K 0.5 )TiO 3 (BKT)-based relaxor ferroelectric ceramics with an ultrahigh Vickers hardness H v ≈ 8.63 Gpa by adding BaTiO 3 and NaNbO 3 in order to synergistically design the domain and microstructure in multiscale, leading to the existence of ultrasmall polar nanoregions, ultrafine grain size, compact grain boundaries, dense microstructure, and large band gap E g simultaneously. Encouragingly, an excellent energy storage temperature stability ( W rec ≈ 4.31 ± 0.25 J cm −3 , η ≈ 86 ± 5%, 20–200 °C), frequency stability ( W rec ≈ 5.14 ± 0.12 J cm −3 , η ≈ 81.3 ± 1.2%, 5–100 Hz), and excellent charge/discharge performance (power density P D ≈ 103.2 MW cm −3 , discharge energy density W D ≈ 2.4 J cm −3 , discharge rate t 0.9 ≈ 130 ns) are also achieved in BKT-based ceramics. The results demonstrate that BKT-based ceramics can be very competitive lead-free relaxors for energy storage capacitors in pulsed power devices.
危险品化学品经营许可证（带存储）