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This review paper encompasses a study of metal-oxide and their composite based gas sensors used for the
detection of ammonia (NH3) gas. Metal-oxide has come into view as an encouraging choice in the gas sensor industry.
This review paper focuses on the ammonia sensing principle of the metal oxides. It also includes various approaches
adopted for increasing the gas sensitivity of metal-oxide sensors. Increasing the sensitivity of the ammonia gas sensor
includes size effects and doping by metal or other metal oxides which will change the microstructure and morphology of
the metal oxides. Different parameters that affect the performances like sensitivity, stability, and selectivity of gas sensors
are discussed in this paper. Performances of the most operated metal oxides with strengths and limitations in ammonia gas
sensing application are reviewed. The challenges for the development of high sensitive and selective ammonia gas sensor
are also discussed.
The various gas sensors were designed for detection of different gases in the air using different oxides and impurities [1-3]. For example the manufacturing of ammonia sensors on the basis of CuxS-micro-porous-Si structure includes manufacture of micro-porous silicon, drawing on it of SiO2 isolating layer, and then the CuxS layer [4, 5]. The special equipment for all these processes is needed. More usable method for sensor production is so-called soft chemistry or sol–gel synthesis [6, 7].
Polyaniline (PANI) nanostructures are successfully prepared and deposited by in-situ and drop-coating on glass substrates without using any template. By changing synthesis and deposition method, a new morphology of nanostructures, “the cauliflower-like structure”, is developed. These nanostructures were then tested as optical ammonia gas sensors by measuring the optical absorbance variations at 632 nm at different NH3 concentrations. The results show a strong dependence of the morphology on the deposition method. The in-situ one leads to better performances compared to the drop coated one. Protonation /deprotonation is the mechanism of interaction between NH3 molecules and PANI nanostructures.
A new kind of gas sensors were prepared via chemical polymerization of polypyrrole (PPy) as sensitive layer on base of Mg2SiO4 porous ceramics. Thermal stability of the PPy sensitive layer was characterized with thermogravimetric analysis, including TG and DTA. Chemical structure of the samples was identified with X-ray diffraction (XRD) analysis. Microstructure of the PPy film was observed by a scanning electron microscope (SEM). Chemo-electrical relationship of the gas sensors was investigated with the resistance variation of PPy films at different gas concentrations. Based on series of experimental results, linear behavior of the chemo-electrical relationship was obtained for the gas sensor, y=0.00344x+1.33754, with R2=0.9734.
High-Performance Wireless Ammonia Gas Sensors Based on Reduced Graphene Oxide and Nano-Silver Ink Hybrid Material Loaded on a Patch Antenna