Statistical analysis of Sr substituted NiFe2O4 thin films for liquefied petroleum gas sensor applications

In this work, the synthesis of SrxNi(1_ x)Fe2O4 thin films and their performance in the detection of Liquefied Petroleum Gas (LPG) are presented. These films were synthesized through a combination of Sol-Gel and SpinCoating techniques and their LPG gas sensing response was analyzed using a 32 x 23 complete factorial design of experiments with 4 central points. The factors used in the design of experiments were Sr percentage, LPG concentration, heat treatment temperature, number of spin coated layers and testing temperature; sensor response was the main response variable. The morphological and structural properties of the coatings were characterized by Atomic Force Microscopy, Scanning Electron Microscopy and X-ray Diffraction. For the synthesis conditions evaluated, the films presented a spinel structure, and their grain size was found to vary between 20 and 50 nm. Their surface roughness (Sa) was found to be between 0.2 and 2.7 nm. The films were strongly affected by the heat treatment temperature, with higher temperatures leading to higher levels of crystallinity and larger grain size. For the experimental conditions considered in this work, films with one spin coated layer, 20% Sr and treatment at 650 degrees C showed the best performance in terms of response (28%) against 2000 ppm of LPG. In general, a low heat treatment temperature and a high concentration of Sr led to lower LPG sensing capabilities of the films.