- Open Access
Methane and hydrogen sensing properties of catalytic combustion type single-chip micro gas sensors with two different Pt film thicknesses for heaters
© The Author(s) 2018
- Received: 12 September 2018
- Accepted: 8 November 2018
- Published: 15 November 2018
A catalytic combustible type single-chip micro gas sensor was fabricated by MEMS technology and responses with input powers and methane and hydrogen gas concentrations were characterized. The ranges of responses at Pt thickness of 450 nm and input power of 128 mW were 1.076–2.433 mV for methane concentrations of 2315–5787 ppm, and 0.965–2.514 mV for hydrogen concentrations of 282–706 ppm, respectively. The ranges of responses at Pt thickness of 150 nm and input power of 112 mW were 0.192–0.438 mV for methane concentrations of 2315–5787 ppm and 0.949 mV to 2.496 ppm for hydrogen concentrations of 282–706 ppm, respectively. The responses to H2 concentration ratios were 3.65 mV/103 ppm for a micro gas sensor with a 450 nm thick heater and 3.81 mV/103 ppm for a micro gas sensor with a 150 nm thick heater. But in the case of methane gas response, the response to concentration ratios of the micro gas sensor using the 150 nm thick Pt heater was remarkably different from the case of the 450 nm thick Pt heater. The ratios for CH4 were 3.51 mV/104 ppm for the micro gas sensor with a 450 nm thick heater and 0.6 mV/104 ppm for the micro gas sensor with a 150 nm thick heater, respectively. From these results, the micro gas sensor that has the thicker heater with a thickness of 450 nm showed higher sensitivity to methane gas than the micro gas sensor with a thinner heater with a thickness of 150 nm.
- Micro gas sensor
- Methane and hydrogen gases
- Pt heater
It is expected that the production of natural gas will inevitably increase for a while until the switch to sustainable and eco-friendly energy sources. Natural gas, a “bridge fuel,” is known to have environmental credentials but the benefits are marred by leaks from human activities and natural sources [1–4]. A micro-scale gas sensor will likely be able to quantitatively measure leaks with low power consumption and speediness. And along with quantity, another topic for the micro gas sensor, is recognizing the type of gas. The response of a micro gas sensor might be suitable for a location but it is usually hard to recognize what type of gas it is. The main component of natural gas, methane, is sometimes blended with hydrogen, and there is commercial interest due to the effectiveness and low emissions of NOx . However, the gas sensing of microchips have limits of selectivity and stability to exceed the current level [6, 7]. A variety of ways of enhancing gas selectivity have been tried. The Cu-BTC metal–organic framework was used as a sensing layer and measured work function . In the field of semiconducting types, working temperature-modulating frequencies, grain size of the sensing layer film, filter use, and the facet properties of the surface of sensing materials have shown potential as methods to obtain selectivity [9–11]. Besides chemical approaches, the refractive index changes of the sensing layers  and the resonance frequency of the sensor for gas type  have been investigated. The combustible type gas sensor is a transducer that converts the heat of a combustion reaction into an electric signal. The reactivity leads to a response that varies with different gases in the equal condition of intensive property, and a catalytic combustible type gas sensor could be able to harness this variety to discriminate chemicals.
In this work, the micro gas sensor platforms embedding thin film micro-heaters with a meander structure were realized by the MEMS process. The film heater has less power consumption than the coil of the traditional pellistor type by between one order and two orders of magnitude . The catalytic combustible type micro gas sensors were measured and how to discriminate between hydrogen and methane gas was studied.
Design and fabrication
Temperature characteristics of the micro heaters using two different Pt film thicknesses
Gas sensing measurement
The gas sensing performances of the fabricated micro gas sensors were studied with the following steps. The micro gas sensor package was connected to the socket of a 1728 cc static acrylic container. The socket was connected to a Wheatstone bridge circuit outside the container by cable. As for gases for the test, 10% methane was balanced with argon and 1.22% hydrogen was balanced with nitrogen. After electric power was supplied, in operation mode, the output voltage was regulated to nearly 0 V and a certain amount of gas extracted from the cylinder was injected into the container. It was assumed that the injected gases were most immediately diffused throughout the container, and uniform concentrations were made. The micro gas sensor responded to the gas and the output voltage was reached within 25 s in all cases. After 1 min, the mixed gas of air and test gas was released outside by a rotary pump connected to the container. Fresh air was replaced through an inlet at the same time, and then the output voltage recovered to the initial value. When the signal levelled off, the measuring steps were repeated. The output voltage values were automatically recorded every 5 s using a multi-meter (Model: Fluke 287) and PC.
Responses to methane and hydrogen gas concentrations of fabricated catalytic combustible type single-chip micro gas sensors with two different Pt heater thicknesses and input powers
Pt heater thickness (nm)
Input power (mW)
CH4 concentration response
H2 concentration response
Catalytic combustible type single-chip micro gas sensors were designed and fabricated with two different heater thicknesses of 150 nm and 450 nm, and responses to methane and hydrogen gases were measured. The responses to H2 concentration ratios were quite similar, such as 3.65 mV/103 ppm for the micro gas sensor with a 450 nm thick heater and 3.81 mV/103 ppm for the micro gas sensor with a 150 nm thick heater. Meanwhile, the responses to CH4 concentration ratios were remarkably different between 3.51 mV/104 ppm for micro gas sensors with 450 nm thick heaters and 0.6 mV/104 ppm for the micro gas sensor with the 150 nm thick heater, respectively. From these results, it was seen that the micro gas sensor with the thicker heater with a thickness of 450 nm showed higher sensitivity to methane gas than the micro gas sensor with a thinner heater with a thickness of 150 nm. The catalytic combustible type single-chip micro gas sensor that was developed could be used for the leakage detection of methane and hydrogen gases after a long-term reliability test.
WJ and JSP made substantial contributions to conception, fabrication, acquisition of data, and analysis. WJ and JSP were involved in drafting the manuscript and revising it critically for important intellectual content. KWL and YR gave approval for the version. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This work was supported by Project No. SD122962 of the SMBA and project No. N0001711 of an international collaboration program by the KIAT and MOTIE in Korea. And J-S Park was partially supported by project CAP-13-1-KITECH of the Ministry of Science, ICT and NST. We thank the government for research funding.
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