Introduction Nowadays, air pollution is becoming more and more critical in huge overcrowded towns with large industrial activities. This is a societal problem causing more than 3 million deaths/year according to WHO [1]. As a result, monitoring of the concentration of gases or Particulate Matter (PM) is more and more expected in polluted areas. Large equipment is usually installed for this purpose, but increasingly sophisticated research is being conducted to use sensitive and reliable sensors of low cost and small size with reduced power. Among the existing sensors, resonant MEMS sensors are known for their high sensitivities to gas and PM [2]. Previous work already showed that printed self-actuated and self-readout piezoelectric cantilevers using longitudinal vibration mode are good candidates for mass detection [3]. In this work, feasibility of piezoelectric transducers for PM detection, more precisely cigarette smoke particles, is shown. Transducer processing and experimental set-up The cantilever-based piezoelectric transducers are fabricated using screen-printing technology associated with sacrificial layer technique. In this process, all the layers are successively printed before a co-firing at 900°C, leading to free-standing microcantilevers. The final transducers are made of a PZT active layer sandwiched between to Au electrodes, the clamping part being attached to an alumina ceramic substrate. Details on the process can be found in [3]. Figure 1 shows the design of all layers and photographs of dried and fired cantilevers. After polarization of the PZT, determination of the resonant frequency f0 of the in-plane longitudinal axial first resonant mode is led using an Agilent Impedance analyzer. This frequency f0 is the maximum of the conductance curve extracted using a polynomial fit [3]. High quality factor (highest 1700), low noises (lowest 0.5Hz) and theoretical sensitivities between 50 and 740Hz/µg are obtained for resonators of dimensions 4x1x0.1mm3 to 1x1x0.1mm3, respectively. The smallest resonator 1×1×0.1mm3is finally placed in the test bench shown Fig. 2 allowing PM deposition on the surface of the PZT cantilever. The PM are either calibrated silica powders generated by stirring (in the container with a magnetic agitator) or cigarette particles produced by a cigarette burning (Fig. 3). During PM particle deposition, temperature (T) and humidity (RH) are measured thanks to sensors placed as close as possible to the resonator. These values are stabilized before the PM generation thanks to a 20 min N2 flow. Last, a Fidas Frog optical particle analyzer placed at the end of the gas line gives information on the PM generation. Results and Conclusions Feasibility of silica powder detection with the experimental bench was demonstrated in a previous work [4] with estimated mass of 23ng. As expected, the PM particles are detected by the analyzer when cigarette smoke is released (Fig. 4c). Resonant frequency curve is a function of T, RH and PM variations (Fig. 4a). Indeed, previous humidity tests showed that the resonant frequency of the microcantilever decreased with both temperature and humidity [4]. Here, three distinct behaviors can be discussed. In fact, the overshoot (labeled 1 in Fig 4a) may be attributed to an important RH change as seen Fig. 4b. The corresponding calculated sensitivity (~7Hz/%RH) proves that the second frequency shift (~200Hz ) is too high to be only due to RH variation (5%). This clearly illustrates that other contributions have to be taken into account such as T variations and/or PM deposition. Once this transitory state is achieved, signal stabilization indicates that the cigarette particles are immobilized on the cantilever top electrode. SEM analysis (Fig. 4d, 4e and 4f) confirm the PM cigarette deposition. The corresponding frequency shift is estimated to 110Hz. Considering the theoretical sensitivity of 740Hz/µg estimated for this device, deposited cigarette particle mass can be estimated to 0,46µg. Then, using values given by the PM analyzer and the cell volume (2cm3) 128 µg is considered to be flowing around the sensor. Hence, a collection efficiency of 0.36% can be deduced. With this first bench test, it has been shown that printed microcantilevers can be used as fine particle sensors. Temperature and relative humidity are interfering on the measurement, clearly showing the necessity of temperature compensation and humidity detection. Knowing the complex composition of the cigarette, other pollutants may affect the resonance behavior. Also, collection efficiency appears to be an improvement route, as well as considering sensor cleaning after several trials [5] .
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Jun 12, 2020
Le Van Cuong + 10
Open Access