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Inconel 625 superalloy samples were fabricated using wire arc additive manufacturing (WAAM). The phase composition, microstructure, anti-corrosion, and mechanical properties of the Inconel 625 WAAM samples were analyzed. The microstructure of the Inconel 625 WAAM alloy showed good forming quality, no defects, and good metallurgical bonding within the specimens. The metallographic structure exhibited primarily γ-Ni and granular precipitated phases; the average microhardness of the transverse and longitudinal cross-sections of the sample was 243.5 and 243.3 HV0.1, respectively. Yield and tensile strength as well as elongation, decrease in area, and the room-temperature impact values of this alloy were equal to 450 and 736 MPa, 38% and 52%, and 152 J, respectively. The intergranular corrosion test results indicated that the average corrosion rate of the sample is 0.609 mm/year, indicating excellent resistance to intergranular corrosion.

The gas sensing properties of NASICON-based CO 2 sensor attached with a composite of Bi 8 Nb 2 O 17 plus small amount of Pt as a solid-reference electrode has been investigated for application of the sensor at high CO 2 concentration. Compared with conventional sensor coated with Pt reference electrode, the sensor attached with solid-reference electrode was found to efficiently improve the detection limit due to the presence of solid-reference electrode prevented NASICON reacting with CO 2 and water to form carbonate or bicarbonate. A good bonding interface between the solid-reference electrode and NASICON was formed as revealed by the scanning electron spectroscopy (SEM), which provided a powerful guarantee for the thermal stability of the sensor. Furthermore, the as-fabricated sensor also exhibited fast response time, small cross-sensitivity to humidity and little interference from the coexistent oxygen partial pressure at 500 °C. All the present results suggest that using the composite of Bi 8 Nb 2 O 17 plus Pt as a solid-reference electrode is a promising candidate for NASICON-based CO 2 sensor.

A planar-type NASICON thick-film CO2 sensor attached with integrated auxiliary electrode composed of NASICON, Li2CO3, and a little Pt paste is investigated. The electromotive force (EMF) values of the device in the CO2 concentration (300-750 ppm) are measured at 300-500°C under dry as well as humid conditions. The relationship between EMF values and logarithm of CO2 concentration is linear with the number of reaction electrons n=1.9, 2.0 at 400 and 500°C, respectively, which are excellent agreement with the theoretical values (n=2.0). Compared with the conventional sintered pellet sensor fitted with binary carbonate (Li2CO3-BaCO3) as auxiliary electrode, the sensor showed smaller cross-sensitivity against humidity and faster CO2 response time as short as 50 s for 90% saturation. And most important, the EMF of the sensor is more stable than that of the conventional sensor during the heating-cooling process and without any degradation. The long-term stability of the sensor is also perfect.