MOX Resistive Microsensors for Low Concentration Methane Detection ^†

Abstract

A series of MOX resistive sensors with CuO and CoO-sensitive films were prepared using an eco-friendly technique (sol-gel). The sensor transducers are based on a custom-made alumina wafer with gold (Au) or platinum (Pt) interdigital electrodes (IDE) printed onto the alumina surface. The sensors’ responses (sensor electrical resistance variations, measured at the IDE’s contact pads) were recorded under lab conditions (dried target and carrier gas from gas cylinders) in a constant gas flow and with a 1.5 Volts direct current (DC) being applied to the IDE as sensor operating voltage.

1. Introduction

Methane is an odorless and colorless gas with a major greenhouse effect. It can accumulate gradually, up to explosive concentrations (50,000 ppm or 5% volume), so its detection is very important for safety reasons. Human activities that emit methane include mining industry or LPG refining. Methane is also emitted from natural sources, such as natural wet areas (swamps). NIOSH (National Institute for Occupational Safety and Health’s) established a maximum limit of 1000 ppm [1] for an exposure time of 8 h in the workplace. The detection of methane by humans without specialized equipment (sensors) is impossible because, as it was previously mentioned, methane is odorless and colorless.

The most widely spread gas detectors are the MOX-based ones (contain a form of metal oxide as sensitive material in contact with electrodes). For MOX gas sensors, the principle of detection is based on the electrical resistance change which is caused by the surface reaction between the target gas and the metal oxide (acts as catalyst) deposited on the surface of the sensor (in this case, it is called a chemiresistor), upon the sensor’s exposure to different gaseous atmospheres [2].

Although CuO and CoO, in different combinations, were previously used as sensitive materials for different gases (the detection for VOC’s, NH₃, carbon oxides, H₂S are summarized in [2]), methane detection using these oxides has very rarely been reported, and usually, high-cost preparation techniques [3,4], high working temperature, or with incomplete sensor characterization [5] (e.g., influence of humidity, cross-response measurements) are reported.

In this paper, we report preliminary results regarding methane detection, within the NIOSH recommended limits, using abundant sensitive oxide materials (films of CuO and Co), which were obtained via an eco-friendly low-cost technique (sol-gel), at low sensor operating temperatures with fast response/recovery sensor characteristics.

2. Materials and Methods

The sensitive films were obtained using the sol-gel spinning method (1000 rotations/min). As precursors, the basic carbonates of the respective metals were used (Cu(CO₃)₂Cu(OH)₂ for CuO and Co(CO₃)Co(OH)₂ for CoO).

The deposited films were stabilized by a thermal treatment after the deposition stage.

Our self-designed alumina transducers were used with the following dimensions: 5 mm × 10 mm × 0.6 mm. The transducers contained Pt or Au IDE’s on one side, and a Pt heater on the opposite side of the transducer.

All of the sensor determinations were carried out under laboratory conditions using dry, high purity gases. The sensors’ operating voltage was a 1.5 V direct current (DC), the tested working temperatures (T_w) were in the range situated between room temperature and 220 °C (specific for each sensor used), and the sensing experiments were carried out with a continuous flow of gas (max. 180 mL/min). The target gas concentrations were achieved using a calibrated system of mass-flow controllers (MFC). The two separate gas flows were mixed inside a special glass vessel, which is shown in the scheme of the experimental installation (Figure 1), using an on–off valve system.

The mixed gases (the carrier and the target gas) were then inserted into a self-designed sensor cell, which contained the investigated sensor. A chemical reaction took place (between the sensitive oxide material and the target gas molecules) on the surface of the sensor, which leads to a change in its electrical resistance, a variation which was recorded by the RLC bridge connected to the sensing cell.

3. Results and Discussion

Gas Sensing Experiments

The following sensors were prepared (listed in

Table 1. The investigated sensors and their composition.

Sensor Abbreviation *	Sensitive Film	Transducer (IDE/Wafer)
S3	CuO	Au/Al2O3
S4	CuO	Pt/Al2O3
S5	CoO	Pt/Al2O3

* As denoted during sensing experiments.

):

It can be observed that the sensors having CuO-sensitive films were available with two IDE types, gold (Au) or platinum (Pt), to investigate the influence that the IDE material may have. Figure 2 evidences the response/recovery characteristics of the sensors that are presented in Table 1. The S4 sensor has a slightly higher working temperature—T_w (220 °C in comparison with 210 °C for the other two sensors: S3 and S5). The sensors’ responses are comparable when we were using different noble metals such as Pt or Au as IDE (S4, S3). The cobalt-based sensor S5 seems to have performed slightly better than the copper based sensors did: S3 and S4.

The responses of the sensors were fast (250 s), and their recovery was complete (250 s), thus making it possible to resume the sensing experiments after the corresponding recovery cycle without a sensor replacement being conducted. Further investigations regarding sensitive surface characterization, sensor sensitivity, selectivity and stability are currently undergoing, but based on the preliminary results obtained so far, we can state that the obtained sensors are promising candidates for the development of a new commercial methane detection sensor.

4. Conclusions

Sensors with CuO and CoO-sensitive films have been prepared via an eco-friendly low-cost technique (sol-gel), and they were tested for methane detection. The preliminary test results showed that the obtained sensors have successfully detected methane with a fast response (250 s) and a full recovery (250 s). The tested concentration of 1000 ppm CH₄ in air represents the NIOSH maximum allowed limit for an exposure time of 8 h in the workplace. Based on the reported preliminary results, we can conclude that the obtained sensors are promising candidates for a new MOX-based methane detection resistive sensor (MOX chemiresistor).