Humidity Sensor Based on Alum–Fly Ash Composite
Amit Sachdeva, Shri Prakash Pandey and Pramod K. Singh
Introduction
Solid electrolytes characterized by exceptionally high ionic conductivity and relatively small electronic conductivity have attracted a great deal of attention because of their unique transport properties and potential applications in batteries, sensors, etc. [1,2]. Although intensive research in the past decade has already resulted in a number of good solid electrolyte materials, efforts continue to discover new materials and methods for increasing the level of ionic conduction.
A number of recent investigations have reported significant enhancement in ionic conductivity by the dispersion of fine, insulating, insoluble fly ash (FA) particles; importantly, this does not appreciably alter the electronic conductivity. These so- called dispersed solid electrolyte systems (DSES) have become increasingly important to both experimentalists and theorists [3-5]. A comparison of the pure system and the two-phase mixture (containing FA) reveals that the two-phase mixtures and/or DSES in general exhibit a higher conductivity than the starting materials. In this paper we report electrical and structural data on potash alum dispersed with FA. Additionally we have tried to develop a humidity sensor based on this composite electrolyte.
FA is a waste product produced by coal-fired thermal power stations during the combustion of coal. The many coal-fired power plants all over the world produce a large quantity of FA, causing serious environmental problems, as less than half of the ash is used as a raw material for concrete manufacturing and construction; the rest is directly dumped as landfill or simply piled up. Owing to environmental concerns, new ways of using FA have to be explored. Hence, there is considerable interest in FA as a raw material. Recently, materials scientists and engineers have suggested and devised various methods to use FA for the synthesis of some useful composites [6-8].
There have been many experimental analyses of FA’s basic compositional, physical, and chemical properties for technical studies and applications. Raw FA consists of quartz and mullite as crystalline phases and some quantity of glassy phase. It is a gray, alkaline powder with pH 9-9.9.
The most common alum is the double sulfate of potassium and aluminum, KiAl,(S04)4.24H;!0, a white crystalline powder that is readily soluble in water. It is used in curing animal skins. Other alums are used in papermaking and to fix dyes in the textile industry. The raw material of manufacture of common alums is alum rock, composed chiefly of alunite or alum stone. Alum is also made from alum shale, which is either allowed to decompose by exposure, or roasted. During the process, free sulfuric acid is formed, which acts upon the clay, producing aluminum sulfate, which is then dissolved out. Potassium sulfate or ammonium sulfate is added to the solution to produce potash alum or ammonia alum.
Through this work we have been successful in preparing potash alum-FA composites. We have developed a humidity sensor that gives a new, better way to use FA.
Experimental
Potash alum was purchased from market and used without further purification; FA of unknown purity and composition was collected from a local supplier. Appropriate amounts of potash alum and FA were weighed and thoroughly mixed in an agate mortar and pestle (~2 hours) followed by pulverization and pelletization in a nickel- plated steel die at pressures of 2.5 tons using a hydraulic pelletizer machine. The circular pellets thus obtained were 0.15 cm2 in area and 4-6 mm in thickness. Silver paste was coated onto both surfaces of the pellets and dried under room environment to produce pellets that were ready for electrical measurement.
Complex Impedance Spectroscopy
The electrical conductivity of the composite pellets was measured at 1 KHz frequency using a Hioki 3522-50LCR Hi Tester (Hioki, Japan). The electrical conductivity (o) was evaluated using the formula
where о is ionic conductivity, Rb is the bulk resistance, l is the thickness of the pellet, and A is the area of the sample.
Results and Discussion
Electrical
Complex Impedance Spectroscopy
The calculated values of ionic conductivity are listed in Table 9.1 and plotted in Figure 9.1. It is clear that the electrical conductivity of the potash alum-FA system increases initially with increasing content of FA to a conductivity maximum obtained at 65% FA, then decreases. To interpret the results of conductivity measurements in dispersed solid electrolytes, several theoretical models have been proposed. For ionic transport in dispersed ionic conductors, a percolation model was proposed, assuming that highly conducting paths are created along the interface between the host electrolyte and the dispersoid. Electrical conductivity enhancement in two-phase composites was shown to be strongly dependent on sample preparation conditions, and higher conductivities are expected if a better contact between the solid electrolyte and dispersoid can be obtained [9].
The high ionic conductivity can be quantitatively explained by a space-charge effect induced by internal cation adsorption at the FA surfaces [ 10-14]. The observed conductivity maximum could be explained by the percolation model as water of crystallization present in alum is adsorbed on the surface of the composite, where movement of H+ and OH- ions are responsible for the increase in conductivity. The decrease in conductivity after 65% FA composition was due to achievement of the percolation threshold [15, 16].
Temperature Dependence of Conductivity
In DSES the temperature dependence can either be Arrhenius (linear plots) or Vogel- Tammann-Fulcher type. Figure 9.2 shows the temperature dependence of the electrical conductivity of a typical composite system (maximum conductivity composition).
TABLE 9.1
Room Temperature Conductivities in the Potash Alum-fly Ash Composite System
|
Sr No |
Fly Ash (wt%) |
Potash Alum (wt%) |
Conductivity (S/cm) |
|
1 |
95 |
5 |
3.2 x 10-6 |
|
2 |
85 |
15 |
5.3 x 10-6 |
|
3 |
75 |
25 |
9.2 x 10-6 |
|
4 |
65 |
35 |
1.5 x 10-5 |
|
5 |
60 |
40 |
1.2 x 10-5 |
|
6 |
40 |
60 |
5.4 x 10-6 |
|
7 |
10 |
90 |
4.6 x 10-6 |
FIGURE 9.1 Variation in conductivity with concentration of fly ash in the potash alum-fly ash composite system
FIGURE 9.2 Variation of conductivity with temperature (maximum conductivity)
It follows Arrhenius behavior in which conductivity can be explained using the formula o=oncxp(-E./kT), where о is the conductivity, o„ the pre-exponential factor that depends upon the concentration, the attempt frequency, and the jump distance of the atomic defects, E the overall activation energy, к the Boltzmann constant, and T the absolute temperature (in K).
From the figure we can clearly observe that conductivity rises with increasing temperature up to 48°C and then falls before attaining a constant value. This is because physisorbed water present in potash alum, which plays an important role in conductivity enhancement, is lost between 45 and 50°C.
Structural
Scanning Electron Microscopy
Scanning electron microscopy (SEM) has been used to examine the surface morphology of composite electrolytes. We have recorded SEM micrographs using a Hitachi S-570 SEM instrument, which are shown in Figure 9.3.
FIGURE 9.3 SEM micrographs of (a) Pure Potash Alum (b) Potash Alum Flyash composite and (c) pure Flyash
It is clear from the figure that the uncrushed potash alum shows a needle-like morphology (Figure 9.3a) whereas the FA of unknown purity shows an irregular structure (Figure 9.3b). In the composite, morphology is mixed: white FA is uniformly mixed with darker potash alum grains (Figure 9.3c).
Infrared Spectroscopy
Infrared spectroscopy was carried out to study the nature and functional groups present in the composite. Figure 9.4 shows the recorded infrared spectra (Perkin Elmer 883) of pure potash alum, pure FA. and potash alum doped with FA (maximum conductivity composition).
It is obvious that the IR spectrum of composite materials (Figure 9.4c) contains the peaks from pure potash alum (Figure 9.4a) or FA (Figure 9.4b). The absence of any new peaks clearly confirms its composite nature.
The characteristic broad band at 3422 cm-' in the composite spectrum is due to N-H stretching of aniline. The peak observed at 1094 cnr' in the spectrum of FA (Figure 9.4b) may be attributed to the presence of silica [17]. This peak confirms the highest % of alum in the FA, which is confirmed by chemical analysis [8]. Several peaks are observed between 1400 and 1300 cm-1, which correspond to various metal oxides present in the FA. Absorptions at 1297.49 ± 10, 1139.71 ± 10, 1089.48 ± 10, and 794.81 ± 20 cm-1 correspond to FA constituents in the potash alum-FA composite.
Figure 9.4a shows the spectrum of potash alum. Infrared spectra of alums based on one monovalent and one trivalent cation have been published [18]. Ross interpreted the infrared spectrum of potassium alum as 981 cm-1, 465 cnr1, 1200 cm-1, 1105 cm4,618 cm-1, and 600 cm-1 for (S04)2-. Water stretching modes were reported at 3400 and 3000 cm-1, bending modes at 1645 cm-1, and librational modes at 930 and 700 cm-1 [19].
X-Ray Diffraction
To further investigate the composite, we recorded its X-ray diffraction (XRD) patterns along with those of the host materials (Figure 9.5). Peaks in the XRD pattern of
FIGURE 9.4 Infrared spectra of (a) pure potash alum, (b) pure fly ash (c) potash alum-fly ash composite
FIGURE 9.5 X-ray diffraction patterns of (a) pure fly ash, (b) pure potash alum, and (c) potash alum-fly ash composite
the composite (Figure 9.5c) also appear in the XRD patterns of FA (Figure 9.5a) or potash alum (Figure 9.5b), which affirms its composite nature, supporting the IR data.
Humidity Sensor
Based on the maximum electrical conductivity sample, we tried to fabricate a humidity sensor in our laboratory (Figure 9.6). We developed a finger-type electrode on the surface of a composite pellet using vacuum-coated silver paint as electrode material, deposited using a vacuum-coating unit (Hind High Vac, India) at 10~3 Torr pressure. Conducting copper wire was used for contact and sensing behavior. To measure the
FIGURE 9.6 Photograph of humidity sensor based on potash alum-fly ash composite
FIGURE 9.7 Experimental setup to maintain constant humidity
response of the humidity sensor we have also developed a constant humidity chamber in our laboratory (Figure 9.7), as designed by Chandra et al. [20].
A very simple approach was adopted for creating different constant humidities for in situ measurement. It is known that the water vapor pressure of over-saturated solutions of different salts gives different relative humidities.
The response of the sensor (voltage vs. humidity) is shown in Figure 9.8. It is clear that the sensor works well and has a quick response, rapidly showing an exponential decrease in voltage with increase in humidity level.
FIGURE 9.8 Response of humidity sensor based on potash alum-fly ash composite (maximum conductivity sample)
Conclusion
A solid-state composite based on potash alum and FA has been developed and well characterized using various techniques. Electrical conductivity measurement shows that the addition of FA enhances electrical conductivity to a maximum 1.5 x 10~5 S/ cm at 65% FA composition, then decreases. Infrared spectroscopy as well as XRD confirmed the composite nature of the material while SEM affirmed the homogeneous mixing. A humidity sensor has been fabricated (with maximum a sample) that shows stable and good performance.
Acknowledgment
This work was supported by the DST project. Government of India (SR/S2/ CMP-0065/2010).
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