Toxic Gas Detectors Based on a MnSb₂O₆ Oxide Chemical Sensor

Abstract

We synthesized the semiconductor oxide MnSb2O6 through a wet chemical process assisted by low-power microwave radiation. A gas-sensitive sensor was elaborated from the MnSb2O6 powders obtained by calcination at 600°C. The sensor was electrically characterized in static CO and C3H8 atmospheres by measuring direct current signals at 100, 200, and 300°C. The toxic gases’ concentrations were 1, 5, 50, 100, 200, 300, 400, and 500 ppm of C3H8; and 1, 5, 50, 100, 200, and 300 ppm of CO. From the MnSb2O6’s electrical resistance results, a sensor’s operational point and a low-cost analog circuit were proposed, obtaining two new prototypes: one for detecting C3H8 and a second one for detecting CO. We selected the response at 200°C and 5 ppm for both cases. Notably, this concentration (5 ppm) is selectable with a calibration resistance, generating an alarm signal of ≈11.3V at a supply voltage of 120 V AC. The toxic gas detectors showed excellent functionality. The resistive sensor showed high sensitivity and good electrical response, while the analog circuit presented a rapid response. Due to the operating temperature employed (200°C), these devices could find practical applications, for example, exothermic generators and heaters.

1. Introduction

The detection of polluting gases is of great importance worldwide because an alarming increase in respiratory, ocular, skin, and lung diseases has been detected [1, 2]. There is no adequate control on the emission of toxic gases into the atmosphere by many large industries and transportation systems [2]. Therefore, a huge effort in research and synthesis of new materials capable of detecting different levels of toxic gases has been undertaken. The current aim is the development of new inexpensive, reliable, sensitive, efficient, and thermally stable detectors in toxic atmospheres (like in CO, CO₂, C₃H₈, NO₂, etc.) [1, 2, 3, 4]. The most widely studied semiconductors for their application as gas detectors are the p and n-type binary oxides (like SnO₂, ZnO, NiO, CuO, In₂O₃, WO₃, Fe₂O₃, etc.) [4, 5, 6], as well as the ternary perovskite-type oxides (like YCoO₃ and LaFeO₃) [7, 8], and the oxides with spinel-type structure (like CoFe₂O₄ and ZnFe₂O₄) [9, 10]. However, recent reports informed that other transition-metal ternary semiconductors with a trirutile-type crystalline structure (AB₂O₆, where A and B possess divalent and pentavalent bonds, respectively) are alternative materials for toxic-gas sensors [11, 12]. Among these materials are the CoSb₂O₆ [11], the NiSb₂O₆ [12], the MgSb₂O₆ [13], and the ZnSb₂O₆ [14]. These oxides have been studied in CO₂, CO, C₃H₈, LPG, H₂S, and NO₂ atmospheres [11, 12, 13, 14, 15, 16, 17].

The MnSb₂O₆ oxide (manganese antimonite, where in this case the Mn ion substitutes the A) can show a hexagonal crystalline structure with the spatial group P₃₂₁ [18] or a trigonal chiral-type crystalline structure [19]. The MnSb₂O₆ possesses good catalytic activity in toxic atmospheres (such as CO) at temperatures above 100°C [20, 21]. In Ref. [20], it was reported that the MnSb₂O₆ showed a maximum sensitivity magnitude (S) of ∼8.98 at 300 ppm of CO and 300°C. At 500 ppm of C₃H₈ (propane), the response was ∼0.439 at the same temperature. In Ref. [21], the MnSb₂O₆ was synthesized by the colloidal method and it showed increased sensitivity as a function of the increase in the operating temperature and the gases’ concentrations. The response in 500 ppm of propane had a maximum of ∼165.66 at 300°C. In 300 ppm of CO, the maximum response was ∼14.929, also at 300°C. The good MnSb₂O₆’s electrical response was attributed to the nanometric particle’s microstructure (porosity, morphology, and size) obtained during the synthesis process.

We studied the MnSb₂O₆ regarding its application as a gas detector and found little information on the application of the oxide in devices for detecting C₃H₈ and CO, which form atmospheres with a high risk of explosion (propane) or intoxication (CO). In this work, we designed two novel gas detectors: one of them detects C₃H₈ concentrations and the second one detects CO concentrations. Both are analog detectors and were designed based on the oxide’s electrical response. They are capable to detect concentrations of 5 ppm, which can be modified through a calibrating resistance. They generate an alarm signal at $\sim 11.3\mathrm{V}$, use a supply voltage of 120 V in alternate current (AC), and an operating voltage of ${\mathrm{V}}_{\mathrm{cc}}=12\mathrm{V}$. They possess rapid response, and are easy to build, install, and repair. Both are ideal for industrial applications where combustion is involved.

2. Materials and methods

2.1 Synthesis of powders MnSb₂O₆

MnSb₂O₆ powders were synthesized by the wet chemistry method assisted by microwave radiation reported in the literature [21]. In particular, for this work, a synthesis of the oxide (MnSb₂O₆) at room temperature, in presence of ethylenediamine was developed. For the synthesis of the MnSb₂O₆ 5 mmol of Mn (NO₃)₂•4H₂O (Sigma-Aldrich), 10 mmol of SbCl₃ (Sigma-Aldrich), 8 mmol of ethylenediamine (Sigma-Aldrich), and ethyl alcohol (Golden Bell) were used. These were dissolved separately in 5 ml of ethyl alcohol, except for ethylenediamine, which was added to 10 ml of the same solvent. The three solutions obtained were transparent and stirred for a period of 30 min. Subsequently, the solutions with ethylenediamine were added dropwise to the solution with manganese nitrate without stopping stirring. Then, antimony chloride was also added dropwise to the mixture formed, obtaining a white solution with very fine particles dispersed throughout the solution (colloidal dispersion). The resulting solution was left stirring under stirring for 24 h at room temperature (at 370 rpm). The evaporation of the solvent from this solution was carried out by applying microwave radiation, using a General Electric model JES769WK domestic device operating at a power of 130 W. The microwave applications made to the solution were made in steps of 60 s until reaching a time of 160 min. The precursor material obtained from evaporation (a white paste) was heated at 200°C (drying process) for 8 h and later it was calcinated at 600°C with increments of 100°C/h for 5 h in static air. The thermal treatment applied to the composite was carried out with a muffle (Novatech) with programmable control.

2.2 Characterization equipment

To know the purity and crystallinity of the MnSb₂O₆,X-ray diffraction was used in the powder. For this study, a Panalytical Empyrean equipment with CuKα radiation and a wavelength λ of 1.540598 Å by a continuous 2θ scanning from 10 to 70°, with 0.026°-steps at a rate of 1 second per step. The microstructure of the powders of the compound at the micrometric scale was analyzed with field emission scanning electron microscopy (FE-SEM) that uses a system Tescan MIRA 3 LMU with an acceleration voltage of 10 kV in a high vacuum. To determine the morphology and particle size at the nanometric scale, a transmission electron microscope was used (TEM), model JEM-2100 with an acceleration voltage of 200 kV, in image mode. It is important to mention that to study the individual nanoparticles by TEM, the powders of the compound (0.01 g) were placed inside a vial that was previously provided with 1 ml of ethyl alcohol. The vial with the powders was dispersed with an ultrasonic generator for 5 min. Later a drop with the material was extracted and deposited on a 300-mesh copper grid that had a Formvar/carbon membrane.

2.3 Materials

In Figure 1a, an electronic diagram is depicted of the propane gas detector. In Figure 1b, the carbon monoxide detector is shown. As can be noted, the devices consist of a Wheatstone bridge for adapting the electric signal of the gas sensor, a circuit based on three operating amplifiers for performing a comparison between the Wheatstone bridge exit signals and an instrumental circuit for amplifying the signal generated by the Wheatstone bridge when the sensor detects the presence of the toxic gas. The complete list of materials is shown in Table 1.

Figure 1.

Electronic diagrams: a) propane detector; b) carbon monoxide detector.

Materials for the propane detector
Gas	Reactants	Wheatstone bridge	Instrumental circuit	Comparator	Supply source
Propane $\left(C_3{H}_8\right)$	1.255 g of; Mn (NO3)2•4H2O (Sigma-Aldrich); 2.28 g of SbCl3 (Sigma-Aldrich); 0.5 mL de ethylenediamine; 20 mL of; ethyl alcohol; (Golden Bell)	$R_c=570\mathrm{\Omega }$; $R_1=570\mathrm{\Omega }$; $R_2=570\mathrm{\Omega }$	3; Operating amplifiers; 2 resistances $R_3=1K\mathrm{\Omega }$; 2 resistances; $R_f=100K\mathrm{\Omega }$	1; Operating amplifier	1 transformer; 120/32; 1 diode bridge KBL610; 1 regulator LM7812; 1 regulator LM7912; 4 capacitors $C_1=C_2=C_3=C_4=\mathrm{2,200}\mathrm{\mu F}$; 2 capacitors $C_5=C_6=0.01\mathrm{\mu F}$
Materials for the carbon monoxide detector
Carbon monoxide $\left(\mathit{CO}\right)$	1.255 g of; Mn (NO3)2•4H2O (Sigma-Aldrich); 2.28 g of SbCl3 (Sigma-Aldrich); 0.5 mL of ethylenediamine; 20 mL of ethyl alcohol; (Golden Bell)	$R_c=582\mathrm{\Omega }$; $R_1=582\mathrm{\Omega }$; $R_2=582\mathrm{\Omega }$	3; Operating amplifiers; 2 resistances $R_3=1K\mathrm{\Omega }$; 2 resistances; $R_f=100K\mathrm{\Omega }$	1; Operating amplifier	1 transformer; 120/32; 1 diode bridge KBL610; 1 regulator LM7812; 1 regulator LM7912; 4 capacitors $C_1=C_2=C_3=C_4=\mathrm{2,200}\mathrm{\mu F}F$; 2 capacitors $C_5=C_6=0.01\mathrm{\mu F}$

Table 1.

Materials employed for the construction of the ${\mathrm{C}}_3{\mathrm{H}}_8$ and CO detectors.

As can be seen in Table 1, both detectors employ nearly the same materials. The only difference is the Wheatstone bridge resistances because the gas sensor produces different resistive responses for each gas.

2.4 Process

In Figure 2, we show schematically the construction of the gas detectors. It consisted of three stages: 1) fabrication and characterization of the gas sensor, 2) analog electronic circuit for the adaptation, and 3) voltage source for the electronic circuit and the sensor.

Figure 2.

Schematic diagram for the fabrication of the toxic gas detectors.

In stage 1, the resistive sensor consisted of pellets made with the oxide. It was exposed to different gas concentrations and three operating temperatures: 100, 200, and 300°C. The resistivity was measured with a digital multimeter. In stage 2, we performed a signal adaptation of the gas sensor using a Wheatstone bridge, an instrumental circuit, and a comparator circuit. The exit signal was an alarm signal produced when the system detected the presence of a gas. In stage 3, the supply source had an exit voltage of ${\mathrm{V}}_{\mathrm{cc}}$ = $\pm 12\text{Volts}$ for the sensor, the Wheatstone bridge, the instrumental circuit, and the comparator circuit.

2.5 Stage 1: Sensor construction and electrical characterization

We elaborated pellets from MnSb₂O₆ powders calcinated at 600°C. For that, we weighed 0.3 g of MnSb₂O₆ powder and deposited them in a circuit breaker box. The breaker box was placed in a hydraulic press (Simplex Ital Equip-25-ton-brand equipment) and applied 10 tons of pressure for 1 min. The dimensions of the pellets were 12 mm in diameter and 0.5 mm in thickness. We placed two colloidal-silver paint (Alfa Aesar ˃ 99%) ohmic contacts on the pellets’ surface to maintain a good connection between the electrodes and the pellets’ surface. The pellets were placed inside a high vacuum chamber with a capacity of 10⁻³ Torr. The test gases were injected individually and then removed from the chamber with a vacuum pump. The gas concentration and the partial pressure were constantly monitored and controlled with a Leybold detector (model TM20). The changes in the electrical resistance were recorded with a digital multimeter (Keithley 2001) as a function of operating temperature (100–300°C) and gas concentration (CO: 1−300 ppm, C₃H₈: 1−500 ppm). Pellets’ resistivity was calculated with equation [20]:

$$\rho =\frac{\mathit{RA}}{t}$$E1

where R is the pellets’ electrical resistance in the tested gas, A is the pellets’ area (12 mm in diameter), and t is the thickness (0.5 mm).

2.6 Stage 2: Gas sensor’s signal adaptation

Electronic circuits, comprising a Wheatstone bridge, an instrumental circuit, and a comparator circuit, were designed (Figure 1) to generate an alarm signal when the resistive sensor detected a gas. These circuits were supplied by a ${\mathrm{V}}_{\mathrm{cc}}=\pm 12\mathrm{V}$ voltage source, which is described in Section 2.6.1. Whereas, in Section 2.6.2., we describe the functioning of the gas detector.

2.6.1 Voltage source

We were interested in developing toxic gas detectors based on the antimoniate manganese oxide and analog electronic circuits from an economic perspective (they should not be very expensive). Thus, we developed a voltage source for feeding our prototypes. Its electronic diagram is shown in Figure 3. The functioning of each stage is as follows:

Figure 3.

Resistive sensor’s connection to the Wheatstone bridge and diagram of the voltage source.

Voltage reduction is done with the 120 to 32 V reducer transformer. When the transformer is connected to the 120 V alternate current signal, the primary coil generates a magnetic field that reaches the secondary coil. According to the number of wire rotations in the secondary coil, the transformer generates an output of 32 V.

Alternate current signal rectification is done by a KBL610 diode bridge, which consists of four rectifier diodes. This circuit converts the alternate current signal into a direct current signal. However, the diode bridge’s signal contains very loud noise components, thus requiring a third filtering stage.

Filtering of the signal is done through capacitors C₁, C₂, C₃, and C₄, which should possess a sufficiently large capacitance value to eliminate to a certain degree the voltage of the curling produced by the alternate current rectification.

Voltage regulation by the regulators LM7812 and LM7912. LM7812 is a 12 V positive voltage regulator and LM7912 is a − 12 V negative voltage regulator. Both regulators require 0.1 μF capacitors (C₅ and C6, respectively).

The voltage source supplies the device using a linear behavior. Its construction is easy and very cheap.

2.6.2 Operating principle

The operation consists of two stages: Calibration and detection. During the calibration, the device does not detect the presence of gas and the alarm signal is set to zero, $V_{\mathit{\text{Alarm}}}=0$. To achieve this, we followed these steps: a) the resistive sensor was placed in an atmosphere free of the test gas, and its terminals were connected to a Wheatstone bridge’s arm; b) the variable resistance $R_c$ changed its value until the Wheatstone bridge’s exit voltage was equal to zero:

$$V_A-V_B=\frac{R_1}{R_1+R_s}{V}_{\mathit{cc}}-\frac{R_2}{R_2+R_c}{V}_{\mathit{cc}}=0$$E2

where $R_1$ and $R_2$ are the precision resistances, $R_s$ is the sensor’s resistance at initial conditions, $R_c$ is the variable resistance, and ${\mathrm{V}}_{\mathrm{cc}}$ is the supply voltage; c) $V_A\text{and}{V}_B$ are simultaneously the Wheatstone bridge exit voltages, and the entry and exit voltages for the follower circuits (see Figure 1); d) the adder-subtracter yields the difference between V_A and V_B, and amplifies the entry signal based on the ratio $\frac{R_f}{R_3}$:

$$V_{\mathit{\text{OUT}}}=\frac{R_f}{R_3}\left(V_A-V_B\right)=0.$$E3

If $V_A-V_B=0$(see Eq. 2), the voltage $V_{\mathit{\text{OUT}}}$ is also equal to zero; and e) the exit signal of the comparator circuit is the device’s alarm signal and is determined by

$$V_{\mathit{\text{Alarm}}}=V_{\mathit{Sat}}=A_{\mathit{ol}}{V}_{\text{OUT}}=100000\left(0\right)=0$$E4

where $A_{\mathit{ol}}\approx 100000$ is the gain of the operating amplifier’s open loop. In this case, the detector does not generate the alarm signal $\left(V_{\mathit{\text{Alarm}}}\approx 0\right)$ because the sensor does not detect the gas. In the detection phase, the device detects the gas. The alarm signal is equal to the operating amplifier’s saturation voltage. In this stage, we followed these steps: a) the resistive sensor was placed in an atmosphere for monitoring the possible presence of the test gas; b) the sensor’s terminals were connected to a Wheatstone bridge’s arm (see Figure 1); c) when the device detected the gas, the Wheatstone bridge was not balanced, satisfying following condition:

$$V_A>V_B=\frac{R_1}{R_1+R_s-\mathrm{\Delta }{R}_s}{V}_{\mathit{cc}}>\frac{R_2}{R_2+R_c}{V}_{\mathit{cc}},$$E5

where $\mathrm{\Delta }{R}_s$ is the sensor’s resistance variation due to the presence of the gas, the adder–subtractor circuit’s exit voltage is then greater than zero:

$$V_{\mathit{\text{OUT}}}=\frac{R_f}{R_3}\left(V_A-V_B\right)>0;$$E6

and d) if $V_{\mathit{\text{OUT}}}>0$, the comparator circuit’s amplifier is saturated and the device’s exit voltage is the alarm signal ${\mathrm{V}}_{\text{Alarm}}={\mathrm{V}}_{\mathrm{Sat}}$, that is, the device has detected the presence of toxic gas in the atmosphere.

3. Experimental results

3.1 XRD analysis

Once the experimental process to obtain the powders of the MnSb₂O₆ through a wet chemistry method, in Figure 4a diffractogram of MnSb₂O₆ powders calcinated at 600°C is shown. In this X-ray diffraction pattern, it is identified that the largest peaks belong to the crystalline phase of MnSb₂O₆. These high-intensity reflections were compared taking as reference the file PDF # 84–1237 of the database. According to this file, the oxide MnSb₂O₆ presents a crystalline hexagonal structure with spatial group P321 [18, 22] and cell parameters a = 8.8054 Å and c = 4.7229 Å [19, 20]. According to these values and the major diffraction peaks shown in Figure 4, this phase corresponds to materials known in the literature as type antimonates ASb₂O₆ [23, 24] where A is a divalent ion as Co, Zn, Mi, and Mn (Mn, in our case) [20, 24]. In addition, it is observed that the width and height of the peaks presented by the diffractogram of MnSb₂O₆ indicate good purity [20], and the relatively low noise shows a high crystallinity [25, 26]. However, even after having achieved a good crystallinity of the oxide at 600°C, small portions of the inorganic material SbO₂ (PDF#11–0694) and Sb₂O₄ (PDF#78–2067) were identified, which agree with what was reported in the reference [20, 21], in which the same (MnSb₂O₆) was prepared.

Figure 4.

Diffractogram of the MnSb₂O₆ oxide (at 600°C) prepared by a microwave radiation-assisted wet chemistry process.

In the literature, the preparation of the oxide MnSb₂O₆ has been reported using the solid state reaction method at a temperature of 900°C and 1100°C [27]. While in Ref. [20], they mention that they synthesized MnSb₂O₆ at a temperature of 800°C applying a colloidal method. These temperatures reported by these authors are high compared to those obtained in this work, which was able to obtain the crystalline phase of MnSb₂O₆ at 600°C by using an alternative method of synthesis.

3.2 SEM analysis

In Figure 5, scanning electron microscopy (SEM) images of the surface of the calcined MnSb₂O₆ oxide at 600°C are shown. To analyze the microstructure of the oxide in detail, it was necessary to use the following three magnifications: (a) 3.05kx, (b) 5.72kx, and (c) 9.72kx, respectively. Regarding the low magnification of 3.05kx (Figure 5a), we observe a large number of individual microrods, constituted by the agglomeration of very fine and irregular particles. These microrods appear to grow dispersedly oriented in different directions until they form micro bases of different sizes. The length of the microrods was calculated in the range of 1 to 6 μm, with an average of ∼2.8527 μm and a standard deviation of ∼0.8770 μm (Figure 6a).

Figure 5.

SEM photomicrographs of the MnSb₂O₆ powders analyzed at magnifications of (a) 3.05kx, (b) 5.72kx, and (c) 9.72kx.

Figure 6.

Histograms of the distribution of microrod-like morphologies of MnSb₂O₆ calcined at 600°C, (a) microrod length distribution and (b) microrod diameter distribution.

In agreement with the literature, the growth of these microcolumns is associated with the increase in temperature and the effects that ethylenediamine produces on the morphology of the compound [20, 28, 29]. In addition, in this same image, plates composed of irregular and very fine particles can also be seen. At a magnification greater than 5.72kx (Figure 5b), we corroborate that the microrods obtained at a temperature of 600°C are composed of the agglomeration of irregular and very small particles. The average diameter of the microrods analyzed was estimated to range from 0.1 to 1.2 μm, with a mean of ∼0.3425 μm and a standard deviation of ∼0.1689 μm (Figure 6b). The granular surface shown by the microcolumns in Figure 5b is due to the removal of organic material that was present during the oxide synthesis process.

Observing another zone of the surface of the material, Figure 5c shows an image analyzed at a magnification of 9.72kx. According to the study carried out in this area of the material, particles with different morphologies to those described in Figure 5a and b were found. In this case, a particle that grows with an octahedral geometry (with a size of 1.74 μm) was identified. In the same image, the growth of microrods and very fine particles can be observed that, when agglomerated by the effect of the calcination temperature, form micro bases where microcolumns and other particles without apparent shape grow. The morphologies described in Figure 5c are formed due to the fact that these microstructures are nucleated, taking as raw material the finest particles that are found surrounding the microrods [29, 30].

In the literature, results like those shown in Figure 5a-c have been reported. For example, Michel et al. synthesize the CoSb₂O₆ oxide using the colloidal route and using low concentrations of ethylenediamine (0.5 mL) [31]. According to the results reported by this author, he obtains the formation of microcolumns and other irregular particles due to the use of ethylenediamine during the synthesis of the compound [31]. In Ref. [20], they prepare the oxide MgSb₂O₆ applying the colloidal method in the presence of ethylenediamine (4 mL). In this report, they mention that with the use of ethylenediamine in the preparation of trirutile-type oxides, microrods composed of irregular nanoparticles are obtained and that they agglomerate due to the effect of temperature until elongated morphologies (microrods) are obtained [13]. The authors cited above apply these microstructures for their study as potential gas sensors [20, 21]. In our case, it was possible to synthesize the MnSb₂O₆ by a simple, economical, and easy-to-control wet chemical process in the presence of ethylenediamine, obtaining microrods, and other irregular particles for their study as potential sensors of C₃H₈ and CO atmospheres. Figure 6 shows the measured particle size.

3.3 TEM analysis

In order to have a clearer idea of the individual morphology and to make a more precise estimation of the particle size of the material, four TEM images (one of them in high resolution; HRTEM) acquired from the surface of the MnSb₂O₆ oxide are shown in Figure 7. calcined at 600°C. The dark areas observed in these photomicrographs are due to the poor transmission of electrons to pass through the thickness of the particles. Looking at Figure 7a, we can see the formation of a large agglomeration of dispersed particles on the surface of the material. The scattering of the particles that are observed in this image is due to the fact that, in the process of preparation for its study by TEM, the MnSb₂O₆ powders were dispersed with the purpose of analyzing the individual particles and giving the appearance shown in Figure 7a. The average size of these particles was calculated as an average of 150 nm, with a range of 100 to 220 nm. For Figure 7b, elongated morphologies were found and identified as microrods. In this case, the studies carried out by scanning electron microscopy (SEM) are confirmed, where the growth of microrods was recorded (see, Figure 5). The length of the microrod in Figure 7b was estimated as 438 nm in length and 140 nm in diameter. In Figure 7c, an individual particle is presented, with this image, we also corroborate that the MnSb₂O₆ powders are made up of irregular nanoparticles (see also Figure 7a).

Figure 7.

TEM images of MnSb₂O₆ powders calcined at 600°C showing (a) particle dispersion, (b) microrod growth, (c) individual morphology of a nanoparticle, and (c) a nanoparticle where observes the crystalline planes of the compound observed in high resolution (HRTEM).

The size of the analyzed nanoparticle was estimated to be approximately 55 nm in size. Referring to Figure 7d, a high-resolution TEM (HRTEM) image of the surface of an individual nanoparticle is shown. Of this particle, an enlargement (zoom) of the surface was made in order to identify the crystalline planes formed by the effect of the calcination temperature (600°C). According to this analysis, the presence of the crystalline planes of the compound is observed, indicating its crystalline nature. The distance between the (d) planes was calculated to be 4.01 Å, which corresponds to the (101) plane of the hexagonal structure. The diffraction angle for the found plane is 2θ = 22.12°. These results can be verified in the diffractogram of Figure 4.

3.4 Electrical response

The electrical resistivity variations of MnSb₂O₆ pellets in CO and C₃H₈ atmospheres, at the operating temperatures of 100, 200, and 300°C, and concentrations of 1, 5, 50, 100, 200, and 300 ppm of CO and 1, 5, 50, 100, 200, 300, 400, and 500 ppm of C₃H₈, are depicted in Figure 8 as resistivity (ρ) vs. test gas concentration and operating temperature. As can be observed in Figure 8a and b, at 100°C, no changes in electrical resistivity were detected, regardless of the increase in gas concentration. That is because, at this temperature, the thermal energy is not enough to drive a reaction of CO or C₃H₈ with the pellets’ surface [13], causing poor mobility of the charge carriers (in this case, electrons) on the surface [20]. Additionally, that temperature causes the oxygen adsorption and desorption processes not to take place [26]. According to the literature, when a semiconductor is employed as a gas sensor in atmospheres similar to those studied in this work, at temperatures below 150°C, the available oxygen species are of type ${\mathrm{O}}_2^{-}$ [8, 11, 13, 20], which are low reactive at such temperatures. Therefore, the oxygen desorption process does not take place, regardless of the increase in CO and C₃H₈ concentrations [13, 20, 21]. On the other hand, by raising the operating temperature to 200 and 300°C, the electrical resistivity diminishes with the increasing concentration and operating temperature. The decrease in both atmospheres was more evident at 300°C (see Figure 8b and d). That is attributed to the fact that, as the temperature increases from 100 to 300°C, the dynamic activity of the charge carriers rises [11, 12, 14], contributing to the increase of the material’s conductivity and the adsorption and desorption processes [4, 14, 21, 32] at 200 and 300°C. The excellent pellets’ response at those temperatures is due to the strong chemical reaction of the oxygen species present on the pellets’ surface and to the increased concentration of the gases [12, 26]. We observed that when the temperature increased from 100 to 200°C, the electrical resistivity variations were more abrupt, as can be observed in Figure 8c and d. An inflection point, due to the high diffusion of the gases that reacted with the oxygen can be seen [20, 21, 32]. Also, we corroborated that the best pellets’ response at 300°C was due, in great measure, to the increase of the thermal energy, which provoked better oxygen species’ (${\mathrm{O}}^{-}\text{and}{\mathrm{O}}^{2-}$ -ionic forms) absorption on the surface [8, 13, 21, 26], thus causing higher velocity of the charge carriers and leading to an increase in the material’s conductivity (or decreased resistivity) [12, 31]. In agreement with the literature, the different oxygen species (such as ${\mathrm{O}}_2^{-},{\mathrm{O}}^{-},\text{or}{\mathrm{O}}^{2-})$that react at temperatures above 150°C [8, 26, 33] are the most probable responsible for causing a rise of the electrical response as a function of the increase in gas concentration and operating temperature (see Figure 8a-d). The values of the electrical resistivity at 300°C were 90.40, 87.46, 70.51, 53.78, 32.31and 5.67 Ωm for CO and 42.71, 42.03, 38.42, 33.44, 27.57, 17.24, 5.16 and 0.25 Ωm for C₃H₈. This trend, depicted in Figure 8a-d, is normally shown in semiconductor oxides employed as toxic-gas sensors.

Figure 8.

Electrical response of MnSb₂O₆ pellets as a function of (a, b) CO and C₃H₈ concentrations, and (c, d) operating temperature.

3.5 Gas-detection devices

The Wheatstone bridge previously discussed was calculated when the sensor’s resistance had a value of ${\mathrm{R}}_{\mathrm{s}}=570$ Ω (in C₃H₈) and ${\mathrm{R}}_{\mathrm{s}}=582$ Ω (in CO). These resistance values were calculated using the Eq. 1: $\rho =\frac{\mathit{RA}}{t}\to R=\frac{\mathit{\rho t}}{A}$ and the experimental measurements were shown in Figure 8a and c. Now, according to Figures 1, 3, and 8, it is possible to establish three operating parameters for our devices: an operating temperature of 200°C, an operating gas concentration of 5 ppm, and the initial sensor’s resistance $R_s$, mentioned above. On the other hand, another three operating parameters can be established for the electronic circuits: a supply voltage of 120 V AC, an operating voltage $V_{\mathit{cc}}=\pm 12V,$and an exit signal (or alarm signal) ${\mathrm{V}}_{\text{Alarm}}\approx 11.3\mathrm{V}$. Figure 9 shows our devices: Figure 9a depicts the C₃H₈ detector device, and Figure 9b shows the CO detector device.

Figure 9.

Low-cost toxic gas detectors: a) for C₃H₈, b) for CO.

Already manufactured the gas sensor and the electronic device according to the description of the previous sections, to apply the gas detection device (C₃H₈, CO), the sensor must be installed in the atmosphere to be monitored whose temperature must be 200°C, while its terminals must be connected to a pair of terminals of the electronic card developed for the detection of toxic gas (C₃H₈ or CO). The electronic card is manufactured using a copper PCB with the size of 10 cm by 10 cm, its design was made with the program Proteus®, it operates at room temperature and its supply voltage is through a plug (see Figure 5b), which is connected to a 110 V alternating current voltage source. Finally, using a multimeter, the alarm signal voltage V_Alarm produced by the electronic card is measured: If the alarm voltage is $\approx 11.3V$, the device is detecting the presence of gas in the monitored atmosphere. But if $V_{\mathit{\text{Alarm}}}\approx 0V$ then the device is not detecting the presence of gas.

Figure 10a and b depict the operating principle of the C₃H₈ and CO detectors. It is as follows: If the concentration of the test gas is equal or greater than 5 ppm, the sensor’s resistance diminishes, thus unbalancing the Wheatstone bridge, and causing the devices’ exit voltage to be equal to that of the alarm signal $V_{\mathit{\text{Alarm}}}\approx 11.3V$ (alarm state: “On”). However, if the gas concentration is below 5 ppm, Eqs. (5) and (6) are not satisfied. Consequently, the alarm signal is not active (alarm state: “Off”), that is, the devices have an exit voltage of approximately zero V. It is worth mentioning that, for our devices, the operating threshold value is selectable with the variable resistance $R_c$. For the C₃H₈ detector, if $R_c>570\mathrm{\Omega }$ (see Figure 1a and 9a), the device will detect concentrations lower than 5 ppm. On the other hand, for the CO detector, if $R_c>582\mathrm{\Omega }$ (see Figure 1b and 9b), the device will detect concentrations lower than 5 ppm. If $R_c<570\mathrm{\Omega }$ or $R_c<582\mathrm{\Omega }$, respectively, the devices will be able to detect concentrations higher than 5 ppm.

Figure 10.

Operating principle of our gas detectors: (a) propane detector and (b) carbon monoxide detector.

4. Discussion

According to our results, our C₃H₈ and CO-detector devices possess good features, which include low cost, high sensitivity, rapid response, good behavior, ability to select the operating concentration through a variable resistance, an operating temperature of 200°C, dimensions of 10 cm x 10 cm, a supply voltage of 120 V AC, an exit voltage of $V_{\mathit{\text{Alarm}}}\approx 11.3V$, ease of construction, ease of repair, and ease of use.

We previously proposed a CO₂ gas detector based on an analog circuit and on the dynamic response to the impedance of the oxide CoSb₂O₆ [22]. Such a device detected a gas concentration of 100 ppm with an operating temperature of 250°C. On the other hand, we also studied theoretically the dynamic electrical response of the oxide ZnAl₂O₄ and proposed a propane-gas detector [23]. That device detected gas concentrations of 1000 ppm with an operating temperature of 250°C. For both detectors, the design of the analog electronic circuits possessed high complexity since the analysis of the signal was conducted on complex planes and based on the sensors’ dynamic response. In this work, the MnSb₂O₆ oxide was applied also in the detection of C₃H₈ and CO. However, its characterization and signal adaptation were done using DC currents, thus simplifying its analysis and implementation. Therefore, in comparison, our new proposal facilitates the construction of the device, lowering the operating temperature (from 250 to 200°C), the test-gas concentration threshold (from 100 ppm to 5 ppm for C₃H₈ and from 1000 ppm to 5 ppm for CO), and the device dimensions (from 15 cm x 15 cm to 10 cm x 10 cm). The overall components were also optimized.

Devices for the detection of C₃H₈ and CO find practical applications as explosion and intoxication-prevention measures, respectively. Our future work will be aimed at designing and developing gas detectors based on digital technology and quasi-distributed systems for the detection.

5. Conclusions

The reproducibility of the MnSb₂O₆ oxide was excellent. It was electrically characterized through static direct-current (DC) tests, obtaining its resistance-gas concentration behavior. Based on these results, the electronic prototypes for two toxic-gas-detection devices were designed: one of them for C₃H₈-detection, and the other one for CO-detection. Both prototypes have an operating temperature of 200°C and an operating concentration of 5 ppm. They can produce an alarm signal of approximately 11.3 V. Their supply and operating voltages are 120 V AC and ± 12 V, respectively. They possess fast response, ease of construction, ease of operation, very low cost, and ease of repair. The detectors can find rapid application in processes involving combustion like, for example, boilers, smelting furnaces, and exothermic generators.

Acknowledgments

The authors thank Mexico’s National Council of Science and Technology (CONACyT) and the University of Guadalajara for their support. We also thank María de la Luz Olvera Amador and Jaime Santoyo Salazar for their technical assistance. This research was carried out following the research-line “Nanostructured Semiconductor Oxides” of the academic group UDG-CA-895 “Nanostructured Semiconductors” of CUCEI, University of Guadalajara.

Conflict of interest

The authors declare that there are no conflicts of interest regarding the publication of this article.