We have studied the oxidation behaviors of aluminum (Al) nanopowders with different particle sizes using a real-time synchrotron X-ray scattering during annealing in air. The Al nanopowders with small particle size of 78 nm at room temperature (RT) were a single crystal. The surface of the nanopowders was first oxidized to amorphous Al oxide near 450 °C, and then crystallized to γ-Al₂O₃ phase at 550 °C. The inside of the nanopowders existed as crystal Al phase at 680 °C, high compared to the melting temperature of Al bulk, 660 °C. In contrast, the Al nanopowders with large particle size of 816 nm at RT have multi grains inside a particle. The surface and grain boundary of the powders were first oxidized to amorphous Al oxide near 470 °C, and then crystallized to γ-Al₂O₃ phase at 550 °C. The inside of the powders existed as amorphous Al phase at 620 °C, melted at 656 °C, and then oxidized gradually above 656 °C.
The zirconia used in dental implants requires excellent mechanical and chemical properties such as high strength, high biological performance, corrosion resistance, and phase stability. In this study, after we prepared a highly fluidized solution of calcium phosphate, we fabricated a hydroxyapatite (HA) coating layer on a zirconia substrate using the sol-gel method to enhance its biocompatibility and bone-bonding ability. We dipped the zirconia substrate into the calcium phosphate sol to obtain the HA-coated film, which was dried at room temperature. The phase change and microstructural evolution were examined while the coating dried and during heat treatment. The biological activity of the coated and as-received substrates was evaluated using an in vitro experiment and the results were compared. The HA-coated film showed a highly dense and uniform layer structure, while its physical and biological properties depended on the starting substrate, coating times, and processing conditions.
Dramatic increases in fossil fuel consumption inevitably led to the emission of huge amounts of CO₂ gas, causing abnormalities in the climate system. Despite continuous efforts to resolve global atmospheric problems through CO₂ capture and separation, success has been limited by poor CO₂ selectivity in the CO₂/N₂ mixture. Herein, we demonstrate the fabrication of a three-dimensional (3D) nanostructure from two-dimensional transition metal carbides (Ti₃C₂Tx, MXene), and assess its utility as an adsorbent in a CO₂ capture system. Through structural and textural analysis, we confirm that the as-prepared MXene possesses high size uniformity with a thickness of ~2.5 nm, and that an MXene aerogel interconnected by MXene nanosheets has a 3D porous architecture with micro/nano porosity (Barrett-Joyner-Halenda (BJH) pore size = 11.4 nm). Moreover, the MXene aerogel exhibits favorable adsorption behavior for CO₂, due to the high-quality MXene nanosheets even with a low specific surface area. Our approach could lead to significant advances in CO₂ capture by adsorbents and open up new opportunities for mass production.
In thermoelectric modules, multiple n-type and p-type thermoelectric elements are electrically connected in series on a Cu electrode that is bonded to a ceramic substrate. Defects in the bond between the thermoelectric elements and the Cu electrode could impact the performance of the entire thermoelectric module. This study investigated the effect of plating layers on the bonding strength of p-type Bi-Te thermoelectric elements. Ni and Pd electroplating was applied to Bi-Te thermoelectric elements; further, electroless Ni-P immersion gold (ENIG) plating was applied to Cu electrodes bonded to ceramic substrates. Forming a Pd/Ni electroplating layer on the surface of thermoelectric elements and an ENIG plating layer on the surface of the Cu electrode improved the bonding strength by approximately 3.5 times. When the Pd/Ni and ENIG plating layers were formed on Bi-Te elements and Cu substrates, respectively, the solderability greatly increased; as the solderability increased, the thickness of the diffusion layer formed with the solder layer increased. The improved bonding strength of the Pd/Ni plated thermoelectric element bonded on the ENIG plated substrate is attributed to the enhanced solderability due to the rapid inter-diffusion of Pd and Au into the solder layer and the formation of a stable and non-defected solder reaction interface layer.
A dense nanostructured 2HfB₂-SiC composite was simultaneously synthesized and consolidated by the pulsed current activated sintering method in one step within very short time (two minutes) from mechanically activated 2Hf, B₄C and Si powders. Simultaneous combustion synthesis and consolidation were achieved through the combination of the effects of the pulsed current and mechanical pressure. A highly dense 2HfB₂-SiC composite with 97.5% relative density was achieved under the simultaneous application of a pressure of 80 MPa and the pulsed current. The fracture toughness of the 2HfB₂-SiC composite was higher than that of monolithic HfB₂.
