Electrical and humidity sensing properties of lead(II) tungstate–tungsten(VI) oxide and zinc(II) tungstate–tungsten(VI) oxide composites
Introduction
The sensors based on resistance and capacitance changes are widely investigated because of their small size and compatibility with electronic circuits. The effects of water vapor on SnO2, In2O3, ZnO and WO3 thick films have been investigated by the measurement of resistance. The electrical conductivity of the semiconductors is influenced by change in humidity. Among the different types of humidity sensors, the ones based on metal oxides are more promising owing to their high physical and chemical stability [1], [2], [3], [4], [5]. The sensors based on tungsten oxides were reported to be potential materials from both fundamental and technological point of view [6], [7], [8], [9], [10], [11]. Electrical conductivity and defect structures of PbWO4 have also been reported [12]. As a part of our study to develop smart materials, we here by report the humidity dependent electrical resistance characteristics of PbWO4–WO3 and ZnWO4–WO3 composites.
Section snippets
Experimental
Lead tungstate and zinc tungstate were prepared by precipitation method and sintered at 973 K in the form of cylindrical discs. The composite sensors were synthesized from different mol% of PbWO4 and WO3 or ZnWO4 and WO3; the exact details are presented in Table 1. As lithium was shown to enhance the sensitivity of humidity sensors [13], the base matrices were doped with 2 mol% of Li+ which amounted to 80, 60, 40 and 20% of pure PbWO4 (PWWO-10) and ZnWO4 (ZWWO-10) to 20, 40, 60 and 80% of pure WO3
Results and discussion
The powder XRD patterns (Fig. 1, Fig. 2) of the composites correspond to PbWO4–WO3 and ZnWO4–WO3 only implying that there are no impurity peaks. The samples showed the linear current–voltage curves (Fig. 3a–c) and thus the electrical conductivity was calculated from the slope by curve fitting using the least square method. Since dc mode is used for resistance measurements at various relative humidities, the activation energy for electrical conduction was determined in air atmosphere in the
Conclusions
Composites with different mol% of PbWO4–WO3 and ZnWO4–WO3 were fabricated and studied for humidity sensing applications. The scanning electron microscope revealed that PWWO-28 and ZWWO-46 composites have larger and greater number of microscopic pores, hence are good candidates for humidity sensors. This was further evidenced by humidity sensor studies which show a sensitivity factor higher than 1.7×104 and 2.6×103, respectively. The good response and recovery characteristics even at 298 K are
Acknowledgements
The Department of Atomic Energy, India, through BRNS Grant No. 99/37/14/BRNS Cell/276 supports this work. The authors are thankful to Rev. Dr. John Pragasam, Director and Dr. K. Swaminathan of LIFE for their support.
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