Mixed-gas CH4/CO2/CO detection based on linear variable optical filter and thermopile detector array
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This paper presents the design, fabrication, and characterization of a middle-infrared (MIR) linear variable optical filter (LVOF) and thermopile detectors that will be used in a miniaturized mixed gas detector for CH4/CO2/CO measurement. The LVOF was designed as a tapered-cavity Fabry-Pérot optical filter, which can transform the MIR continuous spectrum into multiple narrow band-pass spectra with peak wavelength in linear variation. Multi-layer dielectric structures were used to fabricate the Bragg reflectors on the both sides of tapered cavity as well as the antireflective film combined with the function of out-of-band rejection. The uncooled thermopile detectors were designed and fabricated as a multiple-thermocouple suspension structure using micro-electro-mechanical system technology. Experimentally, the LVOF exhibits a mean full-width-at-half-maximum of 400 nm and mean peak transmittance of 70% at the wavelength range of 2.3~5 μm. The thermopile detectors exhibit a responsivity of 146 μV/°C at the condition of room temperature. It is demonstrated that the detectors can achieve the quantification and identification of CH4/CO2/CO mixed gas.
KeywordsLinear variable optical filter Tapered cavity Multi-layer dielectrics Thermopile detector Mixed gas detectors
Linear variable optical filter
Fourier transform infrared
Micro-Electro Mechanical Systems
Gas sensors have a great demand in many industrial and real-life applications. In many of these applications, multiple gases must be monitored simultaneously over a long period of time with minimal maintenance and in different locations . Taking natural gas for example, it contains a mixture of a large amount of methane (CH4) and small amount of various hydrocarbon gas (e.g., CxHy), which has emerged as a major energy source. However, when natural gas burns openly, the use of natural gas has been found to increase the risk of human health and environment. It produces a great deal of water vapor and a mixture of compounds, e.g., nitrogen oxides (N2O), carbon dioxide (CO2), and even carbon monoxide (CO) and fumes caused by the incomplete-combustion of natural gas . Some toxic chemicals emitted by natural gas are not just harmful to residents, but the leaked natural gas also can cause an explosion. Over the last decades, the requirement for safety monitoring on natural gas and its combustion products is continuously increasing, resulting in a great amount of demand for the miniaturized mixed gas detectors . The miniaturizations of gas detectors can bring about the low-cost and large-scale manufacturing processes as well as low power consumption. Meanwhile, it could also result in degraded analytical capabilities or reduced flexibility in multi-parameter measurement.
Gas detectors based on chemiresistive gas-sensing materials (e.g., metal-oxide semiconductors (MOSs), polymers, carbon nanotubes (CNTs), and moisture-absorbing materials) have been widely developed and applied due to its small size and low cost, but it is not satisfying because each detector detects only one type of gas with qualitative information regarding the gas concentration [4, 5, 6, 7]. Moreover, the high operating temperature and the requirement for calibration and readjustment after a short period limit their application and increase the maintenance cost . For these reasons, some gas analysis techniques have been developed for fabricating the miniaturized mixed gas sensors. Micro-gas chromatography (μGC) based on micro-electro mechanical systems (MEMS) technology has made a significant progress in recent decades . A μGC system is a hybrid integration of several MEMS devices (e.g., injector, separation column, gas detector, micro-valves, and micro-pumps), which can provide an accurate analysis of complex gas mixtures [9, 10]. However, up to now, the handheld μGC instruments for on-site analysis are still not commercially available . Optical sensing technique is another alternative solution for gas measurement [11, 12]. Fourier transform infrared (FTIR) spectrometer is a good example of an instrument that can measure mixed gas through analyzing specific spectral response in IR region. However, FTIR spectrometers are usually a bulky instrument, which is not suitable for gas monitoring due to its high cost and the lack of portability. MEMS-based scanning mirror (Michelson interferometer) is a recently emerging solution for the miniaturized FTIR spectrometers, which are capable of providing a set of continuously changing wavelengths across Near-IR (NIR) or Middle-IR (MIR) band [13, 14, 15, 16]. However, the use of fast-response IR laser and detectors (e.g., the cooled PbSe or the HgCdTe photoconductive detector) will increase the cost and system size of spectrometer . Another effective mixed gas measurement method based on IR absorption spectrum technology is the non-dispersive infrared (NDIR) gas detection, which can be realized by using multiple IR filter channels or using single gas channel with a spinning multi-filter chopper system . Doubtlessly, both techniques will inevitably result in the increase of detector size and cost. For these reasons, many micro-optics devices have been used to construct the miniaturized NDIR multi-gas sensors, e.g., MEMS-based Fabry-Pérot (F-P) filters [18, 19], photonic crystal filters [20, 21], and linear variable optical filter (LVOF) [22, 23]
In this work, a miniaturized mixed gas (e.g., CH4/CO2/CO) detector based on NDIR gas detection mechanisms was fabricated using a MIR linear variable optical filter (LVOF) and MEMS-based uncooled thermopile detector array. The designs, fabrications, and characterizations of micro-devices and integrated gas detectors were presented in detail, respectively. The usages of these micro-devices make a compact integration of multiple gas detectors, which have significant advantages in small size as well as low cost and power consumption by using a light source, a gas cell, and a data process element when compared with the traditional NDIR gas detectors.
Design and Experimental Methods
Design and Fabrication of LVOF
where m = k + (φ1 + φ2)/2π. From Eq. (10), it can be seen that the peak wavelength is a linear dependence on the thickness of cavity.
In this study, Si and SiO2 were selected as high and low refractive index materials, and the SiO2 was used to fabricate tapered cavity. The Si was used as substrate material. These materials are transparent in the MIR band, and they are MEMS compatible in fabrication process. The refractive index of Si and SiO2 is 3.43 and 1.42 in the wavelength range of 2.3~5.0 μm, respectively. The layer configuration of LVOF was designed as Si/(LH)n(xL)(HL)nH/Air, where H and L represent high and low refractive index layer, respectively, n is the number of LH pairs, and x is the changing factor of cavity thickness. It is noted that the reflectors will obtain the maximum reflectivity when the outmost layer of reflectors uses the high refractive index of Si material.
Fabrication parameters of LVOF
Design and Fabrication of IR Thermopile Detectors
Thermopile detectors have several advantages for the application of IR gas detecting. Firstly, it does not need a power supply, and thus it rejects the noise voltage against the power source. Secondly, because the current flowing through the thermopile detector is very small, a low-frequency noise (1/f noise) caused by the driving current can also be ignored. Finally, the thermopile detectors can be used without any chopper to detect infrared DC and AC radiation . By contrast, the pyroelectric IR detectors have higher responsivity and signal-to-noise ratio (SNR) than thermopile detectors, but they require a chopper to detect the incident radiation. This will result in the increase of detector size as well as the application cost. Therefore, the thermopile detectors are more suitable for the application of the low-cost and miniaturized gas detectors.
Design and Fabrication of Miniaturized Mixed Gas Detectors
Figure 7 b and c show the photographs of the miniaturized LVOF-based spectrometer and the thermopile chip array packaged in socket, respectively. A total of 12 thermopile chips were integrated as a linear array and installed side by side in socket, above which is the LVOF window. Such design will operate IR wavelength from 2.3 to 5.0 μm, with an excellent linear dependence of ~ 156 nm/mm over 16 mm. The concentration of each gas in gas mixture can be detected separately by controlling a switch array to sweep-read and process data from each thermopile chip.
Results and Discussion
In order to verify gas analysis capacity of mixed gas detectors, some standard gases with strong and wide absorption peaks were selected as the measured gases. The characteristic absorption peaks of gases used in our experiment are CH4/~ 3.3 μm, CO2/~ 4.3 μm, and CO/~ 4.6 μm, respectively. The single gas at different concentrations and the mixed gas at different mixing ratios were measured, respectively. The gas flows getting in and out gas housing were controlled through the mass flowmeter, and some commercial standard gas detectors were used to calibrate the gas concentrations.
In conclusion, the design, fabrication, and characterization of a MIR LVOF and a MEMS-based infrared thermopile detector were presented, respectively. The LVOF was designed as a linear array of F-P type resonators to transform MIR continuous spectrum into multiple narrow band-pass spectra, separately corresponding to each filter channel with peak wavelength in linear variation. A Si/SiO2 multi-layer structure was used to fabricate the Bragg reflectors on the both sides of SiO2 tapered cavity, and a Ge/SiO multi-layer structure on the backside of Si substrate was used to achieve both functions of antireflection and out-of-band rejection. The MEMS-based thermopile detector was designed and fabricated to generate the amplified Seebeck voltage by connecting multiple pairs of p- and n-poly-Si/Al thermocouple elements in series to form a compact structure. The LVOF was installed above a linear array of MEMS-based thermopile detectors to form a miniaturized MIR spectrometer, which can be used to detect mixed gases and was experimentally verified by the quantification and identification of CH4/CO2/CO mixed gases.
This work was supported by the National Natural Science Foundation of China (Grant No. 61574117 and 61574119) and the Natural Science Foundation of Guangdong Province (Grant No. 2018B030311002).
SZ, BW, and XZ performed the simulation, fabrication, and tests of devices; BH performed the device package; HS, BX, and WH analyzed the data and wrote the paper; and XL and BC provided part technique supports in the fabrication and test of devices. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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