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Section 2. Biotechnology
https ://doi.org/10.29013/ESR-19-11.12-7-11
Abdurakhmanov Ilkhom Ergashboevich, Assistant of the Department of Inorganic Chemistry and Materials Science, Samarkand State University E-mail: ergash50@yandex.ru Kuchkarov Otabek Artikovich, Postgraduate Student, Department of Analytical Chemistry,
Samarkand State University E-mail: o-kuchkarov2018@mail.ru Abdurakhmanov Ergashboy, Professor, Department of Analytical Chemistry, Samarkand State University E-mail: ergash50@yandex.ru
SELECTIVITY AND STABILITY OF SIGNAL OF AMMONIA
SEMICONDUCTOR SENSORS WITH MEMBRANE COATINGS BASED ON SiO2/TiO2 + Fe2O3 STRUCTURE
Abstract. A selective semiconductor method for determining ammonia in atmospheric air and a mixture ofvarious process gases has been developed. A sensor has been developed that provides selectivity for the determination of ammonia in multicomponent gas-air mixtures, which simultaneously contain ammonia, hydrogen, carbon monoxide and methane (natural gas). In all cases, the value of the relative standard deviation (Sr) due to unmeasured components does not exceed 0.03. The output signal of the sensors also does not depend on the location in space and the angles of inclination, which allows us to classify the developed sensors (according to GOST-13320-82) as independent.
Keywords: calibration characteristics of the sensor, selectivity, stability, signal, ammonia, sensor, semiconductor, catalyst, titanium oxide, iron(lIl) oxide.
Introduction
Ammonia is a colorless gas with a pungent production of HNO3, ammonium salts and coke
characteristic odor, formed as a result of the de- plants [1].
composition of amino acids, proteins and other The maximum permissible concentration of am-
nitrogen-containing organic compounds. The monia in the air of settlements 0.2 mg/m3, in the
main sources of ammonia emission are refrig- working area of industrial premises is 20 mg/m3, in
eration units, livestock farms, enterprises for the the water of reservoirs 2 mg/m3. The odor perception
threshold is 0.5 mg/m3 [2]. At concentrations of 4080 mg/m3, there is a sharp irritation of the eyes, upper respiratory tract, headache, with 1200 mg/m3 cough, pulmonary edema is possible. Fatal concentrations of 1500-2700 mg/m3, acting for 0.5-1 hours. Forms explosive mixtures with air within 15-28 vol.% ammonia [2]. The most available and correct solutions to the problems ofexpress and accurate determination of the content of gases (in particular ammonia) in air and process gases is the use of simple and affordable semiconductor sensors [3]. Semiconductor ammonia sensors are becoming the main devices that can quickly monitor the environment and process gases [4]. In this regard, the development of highly efficient methods and semiconductor sensors for determining ammonia in environmental objects is becoming an actual task of safety engineering, analytical chemistry and ecology.
Research methods and results
The experimentally established dependences of the sensor signal on the content of the detected impurity are calibration characteristics of the sensor. The calibration characteristic of the ammonia sensor was determined by passing a vapor-gas mixture
tions through a developed sensor. The experiments were carried out under ordinary conditions (temperature 20 °C, ambient pressure 745 mm Hg and an relative humidity of 60%). In the experiments performed, each test point in the measuring range was characterized by six values: three for the forward and three for the inverse measurement cycle. The analytical signal of the sensors was monitored by a B7-35 digital voltmeter after establishing a constant value (at least 1 min after supplying a standard mixture to the device). During the experiments, the calibration characteristic of the ammonia sensor based on titanium oxide was studied. To increase the sensitivity to ammonia, iron oxide was deposited on the titanium oxide film, which is an active and selective catalyst for the oxidation of ammonia by atmospheric oxygen [5]. The results of determining the dependence of the resistance of the GSM on the ammonia content in the gas mixture are shown in (Table 1). As follows from the data given in (Table 1), depending on the content of the alloying component (Fe2O3), the properties of the GSM as a whole change. With increasing concentration of the alloying component, the resistance of the films decreases.
containing ammonia in a wide range of its concentra-
Table 1. - The dependence of the resistance of the GSM on the content of ammonia, and the gas mixture
No Ammonia content in the mixture, mg/m3 Composition GSM
SiO TiO 2/ 2 SiO2/TiO2+ +1%Fe2O3 SiO2/TiO2+ +5%Fe2O3 SiO2/TiO2+ +10%Fe2O3
Resistance < 5SM, kOhm
1. Air 2768 2465 1980 1665
2. 100 2590 2219 1440 1100
3. 200 2515 2145 1233 917
4. 300 2480 2095 1143 815
5. 400 2450 2043 1072 737
6. 500 2430 1997 1011 670
7. 600 2410 1953 960 613
8. 700 2387 1908 910 566
9. 800 2363 1865 866 527
10. 900 2346 1821 826,6 492
11. 1000 2328 1779 789 461
More sensitive ammonia sensors are formed using mixed oxides of titanium and iron. The Ti02+10% Fe2O3 deposited on the surface of the film at an ammonia concentration of 1000 mg/m3 in the mixture leads to a 5-fold decrease in the resistance of the GSM (from 2328 to 461). In the studied concentration range, the dependence of the resistance of the semiconductor sensor on the amount of ammonia in the mixture is, as a rule, nonlinear (table 1). The most noticeable decrease in the resistance of GSM is observed at initial concentrations of ammonia in
the mixture. With increasing concentration, the resistance of the GSM decreases. This makes it difficult to use the developed sensors to create gas analytical instruments. In fig. Figure 1 shows the dependence of the signal of semiconductor sensors with various GSMs on the concentration of ammonia. As follows from the above data, in a wide range of concentrations (20-1000 mg/m3), the dependence of the signal of the semiconductor sensor on the concentration of ammonia in the GSM has a straightforward character.
Figure 1. The dependence of the signal (Act foair) of semiconductor sensors on the
content of ammonia in the air. 1 - SiO2/TiO2, 2 - SiO2/TiO2+1%Fe2O3, 3 - SiO2/TiO2+5%Fe2O3, 4 - SiO2/TiO2+10% Fe2O3; experience temperature - 350 °C
Dependencies similar to those presented in (Fig. 1), they allow us to compare the sensitivity (signal magnitude) of various sensors in a wide range of gas impurity concentrations, extrapolate to a low concentration range and evaluate the possibility of sensors measuring threshold concentrations of individual components in air.
The analysis presented in Fig.1. The data shows that GSM based on Si02/Ti02 and Si02/Ti02-1%Fe203 are characterized by a low sensitivity to ammonia. The sensitivity threshold of sensors based on thin-film un-doped titanium dioxide is 0.01% NH3 in air. A sharp increase in gas sensitivity is observed for a sample with an iron oxide content of 5-10%. Semiconductor sen-
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sors based on Si02/Ti02-5% Fe203 and Si02/Ti02-10%Fe203 can detect NH3 gas impurities at the permissible and lower. The minimum ammonia concentration that can be detected by a semiconductor sensor based on Si02-Ti02+10% Fe203 is about 5.0 mg/m3. The dependence of the sensor signal on the concentration of ammonia in air at the test temperature of 350 °C was studied. A linear section of the signal was detected that provides the determination of ammonia content in a wide range of its concentration. The curve of gas sensitivity (Ao'/o' ) of this sensor versus ammonia concentration in the concentration range ofthe latter from 10 to 1000 mg/m3 is straightforward (Fig. 1), and the S value varies in the range from 0.01 to 0.17.
Selectivity of a semiconductor ammonia sensor.
The selectivity ofdetermining the individual components of the analyzed gas mixture is the most important characteristic of semiconductor sensors. In the sensors developed by us, the selectivity of determination is ensured by the selection of optimal temperatures and the composition of the catalytic coating of GSM. The study of ammonia selectivity by the developed sensors was carried out using certified gas
mixtures according to the requirements ofGOST, presented to gas analytical devices for closed ecological systems and chemical industry facilities. The selectivity of a semiconductor ammonia sensor was determined in the presence of hydrogen, carbon monoxide, and methane. The ammonia selectivity of the sensors was determined at a sensor temperature of 350 °C and a pressure of 730-10 mm Hg. using standard gas mixtures, the composition of which is given in (table 2).
Table 2.- Composition and parameters of calibration gas mixtures used to determine the selectivity of the ammonia sensor
No. Composition of SGS Component content, mg/m3.
NH, H, CO ch„
1. NH3+ air 356.0 ± 0.6 —
2. NH3+H2+ air 356.0 ± 1.0 460.0 ± 1.8 - —
3. NH3+CO+ air 356.0 ± 0.8 - 380.0 ± 2.5 —
4. NH3+CH4+ air 356.0 ± 1.0 - — 450.0 ± 1.5
At the SCS input, mixture No 1(NH3 + air) was fed for 5 min, the readings were recorded with a digital voltmeter, then mixture No 2(NH3 + H2 + air) was fed, and after 5 minutes the readings of the digital voltmeter were re-recorded. Mixture No 1(NH3+air) was fed to the SCS input for 5 min, the readings were recorded with a digital voltmeter, then mixture No
3,0-i
« 2,5-
l/2
Ö 2,0 —I
2(NH3+H2+air) was fed and the digital voltmeter readings were re-recorded after 5 min. Similarly, signals were obtained for a mixture of No 3(NH3+CO+air) and No 4(NH3+CH4+air). The number of repeated measurements for each standard gas mixture is 5. The average results obtained when establishing the selectivity of SCS-NH3 are presented in (Fig. 2).
ü
1,5 H
t>
I i-o H
m 1—i o
S 0,5-1
u
M
£ O -1- <J
O „ <-> I u
□
Q *
" U I □□□
I
4
Figure 2. The results of the study of the selectivity of sensors based on titanium and iron oxides for ammonia. 1 - SiO2/TiO2, 2 - SiO2/TiO2+1%Fe2O3, 3 - SiO2/TiO2+5%Fe2O3, 4 - SiO2/TiO2+10% Fe2O3
As follows from the given experimental data (Fig. 2), when determining ammonia in the presence ofC0, H2 CH4, the most selective ofthe studied GSM is a sensor based on Si02/Ti02+10%Fe203. In the presence of Si02/Ti02+10%Fe203 at a temperature of 350 °C, the presence of carbon monoxide (380 mg/m3), hydrogen (460 mg/m3) and methane (450 mg/m3) in the analyzed mixture does not affect the value of the output signal of the ammonia sensor. From the above data it follows that the developed sensor in the studied concentration range allows the selective determination of
NH3.
Thus, as a result of the experiments, a selective semiconductor sensor was developed that provides the rapid determination of ammonia in atmospheric air and process gases in the presence of CO, H2, CH4 in a wide range of their concentrations.
The developed semiconductor ammonia sensors are not inferior to the known foreign analogues in selectivity and reproducibility, while maintaining the following characteristics: expressness, portability, ease of operation and manufacture.
Conclusion
A selective semiconductor method has been developed for the determination of NH3 in atmospheric air and a mixture of various gases. A sensor has been developed that provides selectivity for the determination of NH3 in multicomponent mixtures where simultaneously with NH3 H2, C0 and CH4 are contained. Such mixtures include gaseous emissions from industrial enterprises producing mineral fertilizers, paints, sugar, asphalt, air from livestock breeding complexes, sewer pipes, etc. In all cases, the value of the relative standard deviation (Sr) due to unmeasured components does not exceed 0.03.
References:
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3. 0bvintseva L. A. Semiconductor metal oxide sensors for determining chemically active gas impurities in the air // Russ. Chem. g. (J. Russ. Chem. Society named after D. I. Mendeleev), 2008.- Vol. LII.-No. 2.- P. 113-121.
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