Based on literature, thermionic emission of tungsten filament is considered as one of the most important topics during formation of electron source for bombardment purposes 25. This system also plays role as a simple and controllable thermal sources for different kinds of thermo-reactions, especially those deal with the endothermic processes 25. One of the reactions is the chemical interaction between tungsten and water molecules 26,27. This interaction is often accompanied with formation of thermionic emission (radiation) 25. The selectivity of this radiation is therefore related to that of the reaction. About some physical processes such as thermal sources that are operated based on the electrical current flow from the filament, formation of radiation is not so selective 25; whereas when these processes are originated from an electrochemical phenomenon, at constant physical conditions, the selectivity of this process seems to be more acceptable 27. Based on these phenomena, hereby in this report, for the first time, the electrochemical interaction between the tungsten filament and water molecules is evaluated based on the driving force of the electron radiation. This interaction has therefore resulted in introducing a reliable optical RH sensor
3.1. Structure characterization
Effective interaction between tungsten filament and water molecules were evaluated via spectroscopic methods such as FT-IR spectrometry. It should be noted that, for the preparation of the FT-IR samples, the tungsten element was introduced to a dry KBr powder (Analytical grade, Merck Company) for mechanically contacting with the solid powder. After that, the powder was pressed and analyzed by the FT-IR spectrometer. Based on the FT-IR spectra (Figure 2), the peaks positioned at 3440 and 1604 cm-1 were attributed to the formation of adsorbed water 28. In addition, the peak situated at frequency of 445 cm-1 was related to the formation of O-W-O bond 29, as the product during reaction between tungsten filament and water vapor. In addition, major difference was observed between the water interacted W-based filament before (A) and after (B) applying the electrical potential to the system at the optimum conditions. Consequently, significant interaction was evidenced between tungsten and water vapor during the humidity detection and measurement process.
Formation of this reagent was evidenced via surface analyses of the tungsten filament before and after reaction with the water molecules using the driving force of the thermionic electrical current by the XRD spectrometry. The XRD patterns were shown in Figure 3. According to the XRD pattern (Figure 3.A), the peaks positioned at 2θ= 28 and 55 o were related to the W filament. No changes were observed between the fresh W-filament and that interacted with H2O vapor (RH: 40-45 %), in the absence of any thermionic electrical current flow. Whereas, after applying the electrical potential (i.e., 110 ± 1 V, AC ,vs. GND) and visualization of the thermionic radiation, appearance of partially sharp peaks (Figure 3.B) at 2θ= 26, 28, 51 and 60 o, clearly, pointed to the formation of almost WO3 with (020), (200) (4112) and (311) lattices as the product of the reaction, which agreed with those reported in the literature 30. In addition, no significant change was observed between the fresh tungsten filament (A) and that interacted with water, in which the memory effect was eliminated by the recommenced procedure (Figure 3.C).
3.2. Optimization of effective parameters
Parameters having strong influence for the humidity measurement purposes included red, green and blue components, length (resistance) of the filament, applied electrical potential, pressure and the flow rate of Ar as carrier gas. The optimization process was discussed in detail in the following sections.
3.2.1. Selection of RGB components
To reach the highest sensitivity (light intensity) during reaction between tungsten and water molecules, one of the most important parameters was the selection RGB components for having maximum sensitivity and light intensity. As explained before, summation of each R, G and B components of each photographic image was estimated independently, pixel-by-pixel, using the VB6 program. To optimize this factor, electrical potential ranged the same as 110 ± 1 V (AC, vs. GND) was applied to the tungsten filament with 6.00 ± 0.01 mm length (measured using a micrometer) at 25-30 torr pressure during introduction of standard solution of 40-45% RH. The photographic image during applying 110 ± 1 V (AC, vs. GND) electrical potential has been shown in Figure 4.A. The summations of red, green and blue components of the thermionic radiation during introduction of 40-45 % RH and at the other previously mentioned conditions have been shown according to the histogram shown in Figure 4.B.
Based on the results, maximum sensitivity was observed for the blue component. Therefore, this component was selected as optical probe (analytical signal) during humidity measuring process.
3.2.2. Electrical potential
To optimize the electrical potential, different electrical potentials, ranged between 10 to 220 (± 1) V (AC, vs. GND) were applied to the tungsten filament with 6.00 ± 0.01 mm length and at 25-30 torr pressure. The correlation between the blue component and the electrical potential has been shown in Figure 5.
As shown, due to the flickering effect as well as small lifetime of the tungsten filament during applying high AC voltage, the middle region of the stable region between 100 to 115 (± 1) V (AC, i.e., 110 ± 1 V, vs. GND) was selected as optimum applied voltage.
3.2.3. Effect of pressure
Another factor having important influence on the thermionic radiation was the pressure. To optimum this factor, the system was set during individually setting the pressure of the cell between 5 to 90 torr throughout applying 110 ± 1 V (AC, vs. GND) potential. The results have been shown in Figure 6.
According to the results, maximum sensitivity was observed during applying vacuum condition between 25-30 torr. Therefore, this range was selected as the optimum pressure.
3.2.4. Effect of different lengths of tungsten filament
At constant electrical potential, the length of the tungsten filament directly affected the electrical resistivity and the electrical current flew through the tungsten filament. To optimize this factor, different lengths of the tungsten filament, ranged between 2.00 to 8.00 (±0.01) mm were tested during introduction of 40-45 % RH at the optimum condition. The results have been shown in Figure 7.
Based on the results, a tungsten filament with a thickness of 6.00 ± 0.01 mm was selected as optimum length.
3.2.5. Flow rate of Ar as carrier gas
To optimize the flow rate of the carrier gas, different flow rates of Ar gas were tested through the introduction of 40-45 % RH at the optimum condition such as length of filament 6.00 ± 0.01 mm, RH: 40-45 %, applied voltage: 110 ± 1 V (AC, vs. GND) at 25 °C. Based on the results, the little fluctuation was observed in the vacuum condition at flow rates larger than 5.00 ± 0.03 mL min-1. Therefore, this flow rate was selected.
3.2.6. Effect of DC and AC electrical potentials
Effect of type of electrical potential on analytical signal (blue component of RGB) was also investigated (Figure 8).
This result indicated that, maximum blue component was provided when AC voltage was applied, versus the DC potential (under similar conditions) at the optimum conditions. This process was almost attributed to the i) segmented thermionic radiations during the applying the AC potential and ii) periodic resting the tungsten figment during the AC alternates.
3.2.7. Effect of temperature
Linear stability was also observed for the fabricated sensor during providing reverse changes between RH % and temperature ranging between 20 to70 oC for RH 40-45 % (Figure 9).
According to the results, thermal stability of the fabricated sensor at different temperatures was analyzed for a humidity sensing purposes. As maximum sensitivity was observed at temperature between 30 to 35 °C, therefore this thermal range was selected as optimum temperature of the water vapored prior introduction to the analyzing system.
3.3. Calibration of relative humidity and stability study
The calibration curve of the tungsten-based optical sensor ranging from 2 to 98 % RH has been shown in Figure 10.
The rate of the change in the humidity of the chamber was controlled for having enough time to stabilize the response of the sensor during sweeping the humidity. Based on 90 % of maximum response time (i.e., t90), the response time of the fabricated RH sensor was estimated to be maximum 4.5 s. In addition, the recovery time of the sensor based on 90 % of minimum response (t90) was found to be maximum 5.0 s. The hysteresis during rapid and alternative contacting the optical RH sensor to two sequential conditions such as 20 and 80 % RH during three sequential analyses has been shown in Figure 11.
Satisfied results were observed that revealed the stability as well as the reproducibility of the fabricated RH sensor at the hard conditions. The results were also compared to that of referenced. RH sensor. Minimum difference (<4.0 %, n = 3) was observed between these two RH sensors that revealed the acceptable and stable behavior of the introduced RH sensor during sensing RH % at different real environments.
In this study, detection limit was defined as three folds of the standard deviation of blank to the calibration sensitivity. This value was estimated to less than 0.5 % RH. Due to the hard instrumental conditions of this system, it was not possible to estimate the accurate value for the detection limit. More improved detection limit was evaluated for the tungsten-based humidity optical sensor, in comparison with other types of optical sensors 31–33.
3.4. Selectivity and interference studies
The probable interfering effect of different foreign gaseous species was investigated in the room temperature. For this purpose, the humidity sensor was placed in the chamber and enough excess (at least 100-fold excess) of foreign gases such as CO, CO2, C2H2, CH4, Ar, He, and volatile organic compounds (VOCs) like ethanol and acetone as well as vapor of acids for instance HCl vapors, individually introduced to the cell at RH = 40-45%. The results are shown in Figure 12.
As shown, no noticeable change in blue parameter clearly revealed the reliability of the fabricated sensor for the trustworthy humidity sensing purpose. This result was not comparable with the thermionic electrical current. In addition, selectivity of proposed sensor was investigated against foreign gases and VOCs. Results exhibited a good selectivity for developed sensor (Figure 13).
3.5. Reusability of the sensor
The reproducibility of fabricated humidity sensor is also shown in Figure 14 at 45-50 % RH. The results revealed acceptable relative standard deviation (RSD %, repeatability as large as 4.18 % (n = 5) for fabricated optical sensor. More reproducibility of the optical imaging process, relative to the thermionic electrical current (7.46 %, n=5), under similar conditions, again pointed to the importance of the image processing for the RH detection purpose.
Also, the RSD % (reproducibility) during analyses of five RH standard samples during at least five replicated analyses was estimated to be 6.05 % (n = 3), again pointed to the acceptable reproducibility of the sensor for RH sensing purposes. However, pressure dependency of the desorption of this thin layer from the surface of the W filament causes to have reusable (renewability) of the RH optical sensor without any memory effect(s), which was considered as the noticeable feature of this introduced sensing device.
3.6. Real sample analysis
The reliability of the introduced was evaluated via determination of the RH % in different kinds of real gaseous sample, along with comparison with the results analyzed by the reference RH probe, under similar conditions. The results pointed to have relative error percentages as maximum as ±2.31 %, which pointed to the applicability of this RH sensor for the analysis of different real samples.
3.7. Comparison
Comparison between the developed sensor and other sensors, reported in the literature for the RH sensing has been summarized in Table 1. As clearly exhibited, significant figures of merit were estimated for the introduced RH optical sensor, versus different types of RH sensing probes. This comparison therefore approved that, the fabricated RH sensor was considered as satisfactory humidity probe with high accuracy and precision.
Table 1. Comparison between the developed sensor and other RH sensing probes, reported in the literature
|
Sensor
|
LDR (% RH)
|
Response time (s)
|
Ref.
|
Depositing the hydrogels poly-hydroxyethyl methacrylate, poly-acrylamide, poly-N-vinyl pyrrolidinone and agarose on optical fiber in order to study their behavior with humidity.
|
10- 100
|
90.00
|
31
|
Highly porous nanostructured titanium dioxide thin film as an optical interference filter
|
13- 71
|
0.27
|
34
|
Spin coated films of Co-Polyaniline nanocomposite for their transmission properties using He–Ne laser
|
20- 95
|
8.00
|
35
|
A hetero-core optical fiber structure that was coated with hygroscopic polymer layers by a layer-by-layer technique, producing a [poly-glutamic acid/poly-lysine] nanostructured overlay
|
20-90
|
0.40
|
32
|
Microrings assembled with polyacrylamide (PAM) microfibers
|
5-71
|
0.12
|
36
|
An optical fiber Fabry–Perot interferometric sensors to detect humidity by depositing a hydrophilic coating material on the optical fiber tip
|
22-80
|
0.24
|
33
|
Thermionic emission of tungsten filament
|
2-98
|
≤ 5 s
|
This work
|
3.8. Proposed behavior of the fabricated RH sensor
Based on the following evidences, the probable behavior (mechanism) of the RH behavior of the fabricated sensor was attributed to the reaction between the tungsten filament and water vapors. The evidences related to this claim have been summarized as follows:
According to these evidence, the most probable proposed behavior of the fabricated RH sensor were evaluated using spectroscopic techniques such as Fourier transform- infrared spectrometry (FT-IR spectra, Figure 2), X-ray diffraction spectroscopy (XRD patterns, Figure 3). As explained in detail, these analyses pointed to the formation of WO3 during interaction between the H2O molecules and tungsten filament.
The chemical reaction between tungsten and water molecules has also been evaluated thermodynamically. Based on the chemical physics studies (53-56), the reactions between the tungsten and the water molecule were endothermic process with apparent activation energy was 132.7 ± 1.1 kJ mol⁻¹ [56] .The thermodynamic reactions are therefore as follows (Eqs. 1-3).
![](https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1619199922.jpg)
As shown, both the thermodynamic and spectroscopic results pointed to the formation of very thin layers of the WO3.xH2O as the electrochemical production of the H2O molecules and tungsten filament at the optimum condition by the driving force of the thermionic current (radiations).