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    Design of Novel Non-equiatomic Cu-Ni-Al-Ti Composite Medium-Entropy Alloys
    (Springer, 2020) Polat, Gokhan; Erdal, Ziya Anil; Kalay, Yunus Eren
    There has been great attention on high-entropy alloys (HEAs) over the past decade. Unlike conventional alloy systems, HEAs commonly include at least five principal elements with equiatomic or near-equiatomic ratio. HEAs with their superior mechanical, magnetic, and thermal properties are promising materials for critical engineering applications. Medium-entropy alloys (MEAs), which consist of less than five principal elements, have very similar structural features with HEAs such as robust thermodynamic stability and exceptional mechanical performance. The insights of MEAs have not been fully revealed yet. In the present study, novel MEAs (Cu20Ni20Al30Ti30, Cu25Ni25Al25Ti25, Cu34Ni22Al22Ti22, and Cu35Ni25Al20Ti20) have been designed using thermo-physical calculations and Thermo-Calc software. These MEAs were then produced using copper heart arc melting and suction cast into cylindrical rods with 3 mm diameters. X-ray diffraction (XRD), optical microscope (OM), transmission electron microscope (TEM), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS) were used for structural characterization. The corresponding results reveal that the Cu20Ni20Al30Ti30, MEA, consists of a body-centered cubic (BCC-B2) phase with intermetallic compounds (ICs), whereas Cu25Ni25Al25Ti25 has single BCC-B2 phase. When the amounts Cu and Ni are increased, system drives itself toward a face-centered cubic (FCC) structure. A dual BCC and FCC composite Cu35Ni25Al20Ti20 has been detected as the most promising MEA among the others with 820 and 1338 MPa measured yield and compressive strength, respectively.
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    Highly stable silver-platinum core-shell nanowires for H2O2 detection
    (Royal Soc Chemistry, 2021) Koylan, Serkan; Tunca, Sensu; Polat, Gokhan; Durukan, Mete Batuhan; Kim, Dongkwan; Kalay, Yunus Eren; Ko, Seung Hwan
    Silver nanowire (Ag NW) networks have great potential to replace commercial transparent conducting oxides due to their superior properties in conjunction with their competitive cost, availability and mechanical flexibility. However, there are still challenges to overcome for the large scale utilization of Ag NWs in devices due to oxidation/sulfidation of NWs, which leads to performance loss. Here, we develop a solution-based strategy to deposit a thin platinum (Pt) shell layer (15 nm) onto Ag NWs to improve their chemical, environmental and electrochemical stabilities. Environmental and thermal stabilities of the core-shell NW networks were monitored under different relative humidity conditions (RH of 43, 75 and 85%) and temperature settings (75 degrees C for 120 hours and 150 degrees C for 40 hours) and compared to those of bare Ag NWs. Afterwards, stability of core-shell NW networks in hydrogen peroxide was investigated and compared to that of bare Ag NW networks. The potential window for electrochemical stability of the Ag NW networks was broadened to 0-1 V (vs. Ag/AgCl) upon Pt deposition, while bare Ag NWs were stable only in the 0-0.6 V range. Moreover, Ag-Pt core-shell NWs were used for the detection of hydrogen peroxide, where a high sensitivity of 0.04 mu A mu M-1 over a wide linear range of concentrations (16.6-990.1 mu M) with a low detection limit (10.95 mu M) was obtained for the fabricated sensors. All in all, this highly effective and simple strategy to improve the stability of Ag NWs will certainly open new avenues for their large-scale utilization in various electrochemical and sensing devices.