Electrical and humidity sensing properties of lead(II) tungstate–tungsten(VI) oxide and zinc(II) tungstate–tungsten(VI) oxide composites

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Abstract

Lead(II) tungstate and zinc(II) tungstate were prepared by a solution route and sintered at 973 K in the form of cylindrical discs. Experimental results on PbWO4 (PW) and WO3 (WO) composites for humidity sensing are described. Sintered polycrystalline discs of PbWO4 (PWWO-10), WO3 (PWWO-01), ZnWO4 (ZWWO-10) and composites of PW or ZW and WO in the mole ratios 8:2, 6:4, 4:6, 2:8 designated as PWWO and ZWWO-82, 64, 46 and 28, respectively and doped with 2 mol% of Li+ were studied. The composites were subjected to dc conductance measurements over the temperature range 373–673 K in air atmosphere from which activation energies were determined. The activation energy values for dc conductance were found to be in the range of 1.09–1.30 eV. The composites were identified by powder XRD data. The scanning electron microscopy (SEM) studies were carried out to study the surface and pores structure of the sensor materials. The composites were subjected to dc resistance measurements as a function of relative humidity in the range of 5–98% RH, achieved by different water vapor buffers thermostated at room temperature. The sensitivity factor (Sf=R5%/R98%) measured at 298 K revealed that PWWO-28 and ZWWO-46 composites have the highest humidity sensitivity factor of 17 615±3000 and 2666±550, respectively. The response and recovery time for these humidity sensing composites were good.

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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