Gas analyzer
Our studies of samarium sulfide thin films deposited on a dielectric substrate with derived metal contacts for conductivity measurements, allowed us to confirm the earlier hypothesis arose about the possibility of using this material as an oxygen analyzer.
We found out that annealed films of samarium sulfide in comparison with semiconductors on metal oxides have significantly higher values of ionized donor impurities. These values of samarium sulfide are about 1022 cm-3 while those of metal oxide are 1017 сm-3.
Furthermore, dielectric permittivity of samarium sulfide is considerably higher than of the semiconductors based on metal oxide and has a value of about 18. This affects on thermodynamic stability of formation of surface oxygen chemical compounds with adsorption center.
Therefore, we are convinced that the use of samarium sulfide films as the sensitive layer of oxygen analyzers will significantly improve the sensitivity of the device in comparison with devices that use semiconductors on metal oxides.
The main novelty of developed oxygen analyzers is that there is used a technique of gas express analysis with the help of samarium sulfide films. We developed a new method for detecting gas components. Its essence lies in finding the optimal temperature sensitivity of a semiconductor element surface to a concentration of a particular trace contaminant with the possibility of obtaining a high-speed response of the sensor in real time.
SmS gas analyzer is a fundamentally new class of analyzers that differs from analogues with a user-friendly interface, self-diagnosis availability for basic parameters of the measuring system, possibility for remote control from a PC desktop or integrated PC and, moreover, new portable devices will achieve mobility criteria of analytic system, operational reliability and promptness of service.
Constructively the detector is made of two identical temperature-controlled reaction cells - comparative and measuring in which the heating elements are placed - concertina wires incorporated to the adjacent measuring circuit of bridge Winston. The operation principle of the sensor is based on the fact that the heated body loses heat at a rate dependent on composition of the ambient gas, therefore the rate of heat transfer can be used to determine gas composition. Heating elements in working and comparative cells are heated by constant electric current from a battery or from a special stabilized power source.
The thermal conductivity of the gas surrounding the heating elements determines the temperature and, consequently, the resistance of heating elements. When through both sensor cell flows clean carrier gas the temperature of the heating elements is the same. If pure carrier gas runs through a comparative cell and through a measuring cell runs carrier gas with a component coming from the chromatographic column, then the temperature and resistance of heating elements are different which disrupts the balance of the measuring bridge. The output signal from the sensor is compared with conventional heating elements.
The figure below shows the construction of an oxygen sensor, where
- Dielectric substrate
- A layer of polycrystalline material on the basis Sm1-xLnxS
- Metal electrodes
Developed compact analyzer has the following features:
- High sensitivity (10-4-10-5)
- Selectivity to different gases (О2 and others)
- High stability to external conditions (temperature, radiation, mechanical stress)
- Low power consumption
- Ability to work from portable power sources (batteries)
- High manufacturability
- Competitive cost
Developed SmS sensors will make a breakthrough in production and use of semiconductor sensors in gas analysis of a number of air pollutants and will be successfully applicatied in monitoring and process control.
Basic properties of semiconductor SmS that provide competitive advantages of developed SmS analyzers:
| High concentrations of ionized donors (Nd ~ 1019-1020 pcs/cm3) | |
| Provide electronic n-type conductivity in SmS which is close to the theoretically possible Weitz limit for a surface (Ns ~ 1012 -1013 pcs/cm3) and which is almost impossible in other semiconductors. This ensures high sensitivity to a range of gases and reduces detection temperature of impurities (i.e. operating temperature of a sensor). | |
| Low operating range of detection temperatures | |
| For example for air oxygen ~130 - 150°C | |
| High dielectric permittivity (ε ≈ 18) | |
| Considerably lowers energy characteristics as compared to one of metal oxides. For example, the effective conductivity activation energy Ei ≈ 0.042 eV, which makes activation processes low-energy and, in some cases, leads to nonactivated processes (e.g. hopping conductivity which is not observed in typical semiconductors). | |
| Melting temperature of SmS is 2300°C | |
| This allows to attribute this semiconductor to high-temperature materials by its applicability | |
| Record high radiation resistance | |
| Among known semiconductors SmS provides high thermodynamic stability which makes it possible to use it in atomic industry and rocket-space technology | |
| High sensitivity of films to adsorption | |
| High sensitivity of films to adsorption of a number of gases does not dramatically affect the reproducibility of their electrophysical characteristics as from sample to sample, as well as in small-scale batch. |
The main advantages of the developed SmS gas analyzer are:
- Portability of measuring devices
- Ability to monitor air quality
- Procedures of service maintenance are significantly facilitated
- It is easy to replace the sensor modules
- Calibration system of measuring devices is made at a level consistent with international standards
- Small sizes of sensors based on thin films make them cost-attractive for serial and small batch production
Comparison table presented below demonstrates superiority of SmS serving as an active material in a sensor over other materials
| Active material of the sensor | Detected gas |
Operating temperature of the |
Sensitivity, volume fraction % |
|
SnO2
|
CO, CH4, C2H5OH, H2, NH3
|
100 - 150 | 5*10-4 – 5*10-2 |
|
ZnO
|
NO2; О3, CO, CH4, C2H5OH, H2, O2
|
150 - 400 | |
|
Ga2O3
|
CO, CH4 | 500 - 800 | |
|
WO3
|
NOx, O3 | 400 - 500 | |
|
WO3-Bi2O3
|
NO | 350 - 400 | |
|
GeO2
|
CH4 и H2 | 600 - 1000 | |
| BaSnO2, (Ba, Sr)TiO2, CdS | NOx | 400 - 900 | |
| SmS | O2 and others | 140 - 160 | 10-4 - 10-5 |
Technology developed by our company reduces the cost of gas analyzers and minimizes their sizes. The technology allows to significantly expand the scope of application of the devices. Thus, the use of analyzers will be possible in the following areas:
- Petroleum and oil refining industry
Leak detection of explosive gases at production and transportation of petroleum products.
- Mining and Coal Industry
Gas analyzers might be used as portable and stationary sensors to methane, carbon dioxide and some other gases.
- Ecological monitoring
In ecological monitoring analyzers of a considered type are usually used to detect small concentrations. Moreover they can determine concentration of carbon dioxide though it is more complicated.
Since atmospheric air consists of many components problems with selectivity come in the forefront. Extraneous gases may cause incorrect operation of a device. Organic matter and some other gases can greatly affect the readings. Humidity can also affect operation of a device due to hydroxyl ion.
- Housing and communal services
Safety and security arrangements in residential areas where liquefied gas is used.
- Medicine, biology, chromatography, household use, food industry and other fields
It is important to note that due to small size of a developed semiconductor gas-sensitive element it can be successfully used for quality control of products right at production process (technological control of production processes).