1. IntroductionLanthanoid-based perovskite-type oxides, such as LnMeO3 (Ln: lanthanoids, Me: transition metals), have been well-known as functional inorganic materials having a wide range of applications for electrode materials of the alkaline fuel cell [1], gas sensor [2-10], ion sensor [11], and for high-performance catalysts for the complete oxidation of hydrocarbons or CO, and NO reduction [12]. Among the lanthanoid-transition metal perovskite-type oxides, Sm-based oxides seem to be interesting materials as they have the largest amount of adsorbed oxygen [13]. For example, the Sm-based perovskite-type oxide sensors have been reported to detect NOx [14], volatile organic compounds [15], ethanol [16] and so on. It is also wellknown that the oxide thin-film devices have good properties as electrochemical devices. So far, oxide thin-film with a perovskite-type structure have been prepared by dry processes such as sputtering and electron-beam deposition methods [17,18], as well as the wet processes of the sol-gel method mainly starting from metal alkoxides or organic acid salts [19,20]. They can field high-quality oxide thin-films; however, they still have some problems, such as relatively low cost performance and lack of handling of the chemicals using the sol-gel method. Consequently, in this work it is focused attention on a wet process to evade such problem, and perovskite-type oxide could be synthesized by a polymer precursor with metal nitrates contained constituent elements [21,22]. By the way, acetylene (C2H2) is widely used as the fuel for cutting and welding metals, so there are also strong needs to detect acetylene as combustible gas. Recently, it has known that small amount of acetylene is to be generated from depleted insulating oils of an oil-immersed transformer. Thus, the acetylene gas sensor could be applicable as a new type of maintenance’s marker of the transformers, especially for the large sized transformers set in remote areas.
The conventional chromatographic method for acetylene detection has high accuracy and is widely used, but it is not suitable for on-site monitoring because of the limited portability as well as the high operating cost. So far, considerable efforts have been directed to develop high performance gas sensors for monitoring acetylene, such as electrochemical sensors [23], and semiconductor type sensors [24,25], however, the sensor for detection acetylene have been seldom reported.
In this study, the Sm-based perovskite-type oxide thinfilm as the material of an acetylene sensor was picked up and systematically evaluated about wet-chemical synthesize of perovskite-type oxide thin-film [26] and the C2H2 sensing properties of the prepared oxide thin-film.
2. Experimental2.1. Synthesis of Perovskite-Type Oxide Thin-FilmsPerovskite-type oxide (SmMeO3: Me = Cr, Mn, Fe, Co) thin-films were synthesized by a polymer precursor method [26] as shown in Figure 1. Metal nitrates were dissolved in Ethylene Glycol (EG) solvent with Polyvinylpyrrolidone (PVP) (3.75 wt%) and acetylacetone (AcAc), as a polymer additive and a coordination agent,
respectively. The solution thus prepared was spin-coated on an alumina substrate with Au interdigitated electrodes at 4000 rpm, and finally sintered at 750˚C in air. The spin-coating and sintering processes were repeated several times to adjust the thickness.
The samples were analyzed by X-ray diffraction using CuKα radiation (XRD: JEOL JDX3500K), field emission type scanning electron microscope (FE-SEM: JEOL JSM- 6500F/III), and thermo gravimetric-differential thermal analysis (TG-DTA: Rigaku 8120H). Electrical conductivities of the thin-films were measured in air (PO2 = 0.21 atm) at the temperature range between 200˚C and 500˚C in the frequency range from 50 Hz to 5 MHz with applied voltage of 0.5 V by AC impedance method (LCR meter: HIOKI 3532-50).
2.2. Fabrication of Sensor DevicesFigure 2 shows schematic diagram of the measurement apparatus. The perovskite-type oxide thin-film sensor device was connected to LCR meter with Au lead wires attached with a silver paste covered with an inorganic adhesive. Gas sensing properties were investigated by AC impedance method using the LCR meter at 400˚C - 500˚C. Sample gases, containing C2H2 were prepared from a parent gas, i.e., 2 - 80 ppm C2H2 diluted with nitrogen, by mixing with nitrogen and/or oxygen, were flowed at a total flow rate of 100 cm3/min. The oxygen partial pressure of the sample gases was fixed at 0.21 atm. Sensitivities of the responses of the sensors were defined as Equation (1);