A Multiparameter Gas-Monitoring System Combining Functionalized and Non-Functionalized Microcantilevers

Abstract

The aim of the study is to develop a compact, robust and maintenance free gas concentration and humidity monitoring system for industrial use in the field of inert process gases. Our multiparameter gas-monitoring system prototype allows the simultaneous measurement of the fluid physical properties (density, viscosity) and water vapor content (at ppm level) under varying process conditions. This approach is enabled by the combination of functionalized and non-functionalized resonating microcantilevers in a single sensing platform. Density and viscosity measuring performance is evaluated over a wide range of gases, temperatures and pressures with non-functionalized microcantilevers. For the humidity measurement, microporous Y-type zeolite and mesoporous silica MCM48 are evaluated as sensing materials. An easily scalable functionalization method to high-throughput production is herein adopted. Experimental results with functionalized microcantilevers exposed to water vapor (at ppm level) indicate that frequency changes cannot be attributed to a mass effect alone, but also stiffness effects dependent on adsorption of water and working temperature must be considered. To support this hypothesis, the mechanical response of such microcantilevers has been modelled considering both effects and the simulated results validated by comparison against experimental data.

Keywords: microcantilever; nanoporous functional coatings; ppm of water content; welding gas monitoring.

Figure A1

Cantilever model used in this study.

Figure 1

Top: top and side view of the silicon microcantilevers from SCL-Sensor.Tech. (PRSA-L300-F50-TL-PCB) [7] used in this study. The cantilevers have a length of 300 µm a width of 110 µm and a thickness between 2.5 to 4 µm. Bottom: scanning electron microscope (SEM) images of the cantilever top surface and details of the heater coil.

Figure 2

Top: General view of the measuring chamber with sensor printed circuit board (PCB), gas cylindrical chamber with inner diameter of 30 mm, fluidic connections and dew point sensor. Zoom: top view of the sensor PCB with the pressure and temperature sensor [11] and a first microcantilever in front of a permanent magnet. Bottom left: the second cantilever is placed on the backside of the PCB. Actuation occurs by supplying a small alternating current (AC) intensity over the metal coil on the cantilever tip. Bottom right: sensor PCB and dew point sensor mounted in a pressure tight measuring chamber (numbers in the scale correspond to cm).

Figure 3

Schematic representation of the measurement setup.

Figure 4

(a) SEM images and (b) digital image processing analysis for size distribution of synthetic MCM-48 spherical particles used for the functionalization of the cantilevers.

Figure 5

SEM images of the CBV100 crystals used for the functionalization of the cantilevers.

Figure 6

Water sorption isotherms at 40 °C and 60 °C for MCM-48 type material.

Figure 7

SEM images of the mesoporous silica coating on the top of the microcantilever obtained by self-assembly of MCM-48 nanoparticles onto plasma activated surfaces (cantilever #166).

Figure 8

SEM images of microporous silicalite coating on the top of the microcantilever obtained by self-assembly of CBV100 crystals (2% wt. aqueous suspension) onto plasma activated surfaces (a–d) and onto poly-(diallyldimethylammonium chloride) (PDDA) activated surfaces (e–h) (cantilever #181).

Figure 9

Optical microscope images of cantilevers coated by spotting of a suspension of nanoparticles directly onto the cantilever top surface according to method 2. (a) MCM-48 on chip #188. (b) MCM-48 on chip #144. (c) PDDA and MCM-48 on chip #163. (d) CBV100 on chip #162.

Figure 10

Density and viscosity measuring data of cantilever #149 calibrated according to the model described in Appendix A.1. Measurements for 4 gases (N₂, CO₂, Ar and He) at temperatures between 4.5 and 60 °C and pressures from 1 to 10 bar are shown. Deviations of the model estimations from the theoretical values from NIST Refprop Database [19] are plotted. Viscosity data not includes the measurements with helium at pressures lower than 6 bar, where the proposed model for viscosity does not work.

Figure 11

Estimation of the binary gas composition from density and viscosity values given by the sensor model described in Appendix A.1. and the NIST Refprop Database [19].

Figure 12

Top: Frequency responses of the MCM-48 coated cantilever #166 and a pristine cantilever # 167 exposed to 30 scc/min of Ar with 100 ppmV H₂O at 29 °C and 990 mbar. Bottom: CBV100 coated Cantilever # 181 exposed to argon with 100 ppmV H₂O at 990 mbar, 29 °C and 39 °C.

Figure 13

Frequency response of #166 functionalized microcantilever at 27 °C and 970 mbar exposed to 100 scc/min of air with a vapor concentration of 190 ppmV with two different model fittings both according to approach 1 Equation (2). Top: model fit 1 assuming a complete degassing of MCM-48 at $t_0$ = 93 min. Bottom: model fit 2 assuming a complete degassing of MCM-48 a $t_0$ = 0 min.

Figure 14

Frequency response of #162 functionalized microcantilever exposed to humid air (up to 1000 ppmV of water) and model fit with the parameters shown in Table 4 Set 2. Temperatures 16 °C (0 to 4100 s), 25 °C (5000 to 9000 s), 45 °C (10,000 to 15,500 s) and 55 °C (16,000 to 19,600 s). Pressure values between 970 and 1100 mbar.