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This paper presents two types of MIR (mid-infrared) gas sensors namely chalcogenide- based gas IR sensor and spectroscopy-based gas IR sensor. These sensors offer a sensitive measurement of gases in industrial and chemical environments. These are based on the principle of absorption in the mid-infrared range.
The interaction between MIR photons and organic molecules provides particularly sharp transitions, which despite the wide variety of organic molecules provide unique MIR absorption spectra reflecting the molecularly characteristic arrangement of chemical bonds within the probed molecules via the frequency position of the associated vibrational and mixed rotational vibrational transitions.
Both the sensors have been briefly discussed along with their applications and a conclusion has been drawn on the basis of their advantages and disadvantages.
Infrared (IR) gas detectors are now at the peak of routine applications in air pollution sensing, chemical sensing, industrial process and other gas analysis. With the increasing applications these sensors have slowly reduced from a macro size to a much smaller size expanding its use from being stationary sensors to becoming hand-held mobile sensors.
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Infrared sensors are work on the phenomenon of Infrared absorption. Usually an infrared source illuminates a volume of a gas that has entered inside a chamber.
Absorption takes place based on the wavelength of the molecules. The amount of absorption is directly proportional to the concentration of the gas which is measured by a set of optical detectors. There are mainly two kinds of IR gas sensors namely Near-Infrared (NIR) and Mid-Infrared (MIR). Most of the gas sensors are based on MIR sensing technique because fundamental vibrating binds of most chemicals are under this range.
This paper deals with two kinds of MIR gas sensors:
Theory
Optical methods are very promising with respect to applications in security systems since they often allow rapid, sensitive and selective detection of substances without sample preparation. As contact- free methods, optical technologies can be used for the setup of stand- alone systems. For different applications a stand-off configuration is helpful and the demand for a safety system which works autonomously. [1]
An optical absorption-based gas sensor offers fast response, very minimal drift and high gas specificity with zero cross sensitivity to others gases. [2] This type of sensors has proven to be most suitable in terms of detection limit, lifespan, susceptibility to cross-sensitivity. It also has a capability of lower detection limit. Absorption spectroscopy techniques offer direct, rapid, and often highly selective means of accurately measuring gas concentration [3]. However, to use absorption spectroscopy technique, the gas detected must have significant distinct absorption, emission or scattering in a convenient region of the optical spectrum. Different gas molecules will absorb radiation at different wavelengths, as each gas species has their own individual absorption spectrum.
Applications
Two of the many applications of this technique have been discussed below:
1. Highly responsive detection of CO2: The sensing principle is based on open path direct absorption spectroscopy in the mid-infrared region. A reflective type of gas cell offers long path length incorporating more compact gas cell design than that of the transmissive type. It is proven from Beer-Lambert law in which absorption is directly dependent on path length of the gas cell [5]. Longer optical path length offered by the reflective gas cell than transmissive type by which indirectly raising the level of absorption by the gas species, subsequently improve the sensitivity of the system [4]. In the optical carbon dioxide gas sensor, the infrared radiation from optical source transmitted through the detected gas twice and doubled the optical path length, thus improve the detection accuracy [6].
In comparison, a reflective gas cell is not only compact in size but also offers better sensitivity due to its longer optical light path. The sensitivity of an open path absorption-based sensor can simply be improved by increasing the path length of the gas cell. However, this will decrease the signal detected at the detector circuitry subsequently reducing the signal-to-noise ratio (SNR) of the sensor system which may affect the minimum detection limit of the sensing system. Hence, the optical path length of the gas cell should not be increased to a point where it may decrease the overall minimum detection limit of the sensing system [7]. The optimized CO2 sensor using single-input and single-output (SISO), 2-input and single-output (2-MISO), 4-input and single-output (4-MISO), and 8- input and single- output (8-MISO) was designed and simulated using ZEMAX®12 software to get the optimum radius of reflective curve surface. [8]
2. Detection of Explosives: Not only is the reliable detection of explosives of great importance but also the safety of personnel operating the devices. Therefore, a stand-off remote system where the operator and the device are located at a safe distance from the object to be investigated is chosen for most applications. In stand-off geometry hazardous materials can be detected in a contact-free mode which is realized in the present investigation by recording absorption features of back- reflected and/or back-scattered light intensities. The sample to be examined in a second step. Back scattered and back-reflected light of the probe laser is collected by a telescope system consisting of two gold coated mirrors.
The signals are recorded by a LN2 cooled detector and analysed via LabVIEW® standard software. The emission of the fragmentation is aimed through a hole within the collection mirror onto the sample. There, molecules from the surface layer are fragmented forming a plume very close to the surface. The synchronized probe beam of the QCL is adjusted collinear to the PLF beam and part of it is back reflected and back-scattered. The plume generated by PLF contains NOx molecules and our results indicate that information about the surface contamination can be gained from measuring the amount of generated NOx in the vicinity of the sample surface. Spectroscopic analysis of the NOx in the plume shows that explosive and non-explosive surface contamination can be distinguished simply by measurement of the NO/NO2 ratio.
Theory
Optical methods are very promising with respect to applications in security systems since they often allow rapid, sensitive and selective detection of substances without sample preparation. Analysis of the composition and concentration of chemical vapor mixtures is important in many fields including environmental monitoring, forensic analysis, and medical diagnoses. The measurement of characteristic infrared absorption spectra has been widely used to gather such information. Traditional IR optical sensors use free-space geometry to measure transmission through a gas chamber. With the development of micro- photonic fabrication technology, bulky free-space structures can be replaced by their on-chip counterparts enabling smaller footprints and reduced power consumption. Near-infrared (NIR) on-chip spectroscopy devices exist for various applications. However, similar devices for the mid-infrared (MIR) regime are required because fundamental vibrational modes of most chemical bonds are located in this regime. [9] A number of material systems are currently being actively investigated for MIR sensing applications. In particular, chalcogenide glasses (ChGs) have been proposed as attractive material candidates due to their wide transparency range (from visible to far infrared), low processing temperature, and large capacity of compositional alloying. [10]
Applications
1. On chip MIR gas detection using Chalcogenide glass wavelength: In this application the design and fabrication of an on-chip chalcogenide glass mid- infrared gas sensor, whose performance is then quantified using methane-nitrogen gas mixtures. The light source is a broadly tunable laser (Firefly, M Squared Laser Ltd.), which is coupled to a ZrF4 MIR single mode fiber. Light from the fiber output is collected by a MIR camera for imaging and intensity measurements. Methane and nitrogen gas flows are controlled by individual mass flow controllers. After mixing, the gas mixture is delivered from the top of the sample into a polydimethylsiloxane (PDMS) chamber. The total pressure in the chamber is 1 atm and the measurement is conducted at room temperature. Fig 3: A schematic representation of the experimental set-up used to characterize the performance of fabricated MIR gas sensors. [11]
2. Infrared monitoring of underground CO2 storage using chalcogenide glass fiber: An optical-fiber- based system suitable for monitoring the presence of carbon dioxide, so-called ‘‘greenhouse gas”, is developed with the help of Chalcogenide glass fiber. Since each pollutant gas shows a characteristic optical absorption spectrum in the mid-infrared, it is possible to detect selectively and quantitatively the presence of gases in a given environment by analyzing mid-IR spectra. The main infrared signature of carbon dioxide gas is a double absorption peak located at 4.2 lm (luminous flux) Chalcogenide optical fibers, which can transmit light in the 1–6 lm range, are well-adapted for CO2 analysis.
In this wavelength range, they show attenuation losses that compare favorably with other types of fiber such as silver halide fibers. Chalcogenide glasses are well known for their transparency in the infrared optical range and their ability to be drawn into fiber. Such optical fibers can transmit light from 2 to 20 lm, in function of the composition of the glass constituting the fiber. The experimental set-up is shown schematically in Fig. 3. It consists in a cylindrical gas cell in which two fibers are aligned thanks to a pierced iron tube. The input fiber aims at transporting the infrared beam from the spectrometer to the cell, and the output fiber picks up the beam to bring it back to the detector. Argon gas is used to purge the cell, on the one hand, and is mixed with CO2 for concentration- dependent experiments, on the other hand. Gases to be analyzed flow between the two fibers before being collected by a CO2 analyzer, which gives the exact concentration of carbon dioxide in the gas flux. [12] Fig 3: Experimental setup of CO2 sensor using chalcogenide glass fiber. [12]
In this paper we broadly discussed two kinds of IR gas sensors in the mid-infrared range. There have been technological advancements to the above-mentioned methods but with the context to this paper we will discuss the advantages and disadvantages of Chalcogenide based IR sensor and Spectroscopy based IR sensor. Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is also used in forensic analysis in both criminal and civil cases, for example in identifying polymer degradation. It can be used in determining the blood alcohol content of a suspected drunk driver.
Hence, it is used in quality control, dynamic measurement, and monitoring applications such as the long-term unattended measurement of CO2 concentrations in greenhouses and growth chambers by infrared gas analysers. MIR spectroscopy also has a very accurate frequency scale due to internal laser reference beam which facilitates co-addition of data to improve signal- to-noise ratio. The High optical throughput enables plenty of radiation available to interact with sample.
Detector receives all wavelengths at once giving a signal-to-noise benefit ((i.e. multiplexing). However, some absorption bands are so strong in the infrared (O- H in water, for instance) that sample thicknesses of more than a few microns lead to complete absorption and no spectrum is obtained. This makes transmission sampling practically impossible for many samples, and reflectance methods must be used instead which is one of the disadvantages of the technique. [13] Whereas, chalcogenide glass (ChG) materials which are suitable for multi-spectral chemical and biological sensing have been established which allow extension of the operating wavelength range for devices from the UV to visible to far infrared.
ChGs enable use of a low temperature process to manufacture an ultra-high-Q optical resonant cavity with an atomically smooth surface. Although being widely used as phase change materials for optical disks and non-volatile random- access memories, ChG films are also good for planar integrated nonlinear optical devices due to their high nonlinearities and low linear and nonlinear loss, their fabrication and structural flexibility, a diverse range of optical and electrical properties, large capacity for doping, and tailorable photosensitivity. [14] It would certainly be appropriate to say that both the approaches are useful in different applications based on the strengths and weaknesses of the individual methods. They differ vastly in terms of materials used, methodology and applications. There is still plenty of opportunity for more research and advancement in each of the above discussed approaches.
Advancements in Mid-Infrared Gas Sensors: A Comparative Analysis. (2024, Feb 21). Retrieved from https://studymoose.com/document/advancements-in-mid-infrared-gas-sensors-a-comparative-analysis
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