This work presents an application of mass-sensitive microcantilevers coated with a new alternative of sensitive materials that is hexamethyldisilazane functionalized mesocellular foam silica (HMDS-MCF-Si) for detection of solvent vapors. The microcantilevers (3 mm $\times2$ mm $\times0.1$ mm) based on a lead zirconate titanate (PZT) beam sandwiched by thin gold (Au) electrodes were fabricated by screen-printing. Mesocellular foam silica (MCF-Si) was synthesized and used as the sensitive material due to its high specific surface area and pore volume. This material was functionalized by hexamethyldisilazane (HMDS) to enhance hydrophobicity. By a small deposition of HMDS-MCF-Si, the obtained devices demonstrated longitudinal vibration mode with the quality factor of approximately 200 at ambient temperatures. Responsiveness of the microcantilever coated with HMDS-MCF-Si to each solvent vapor at room temperature was compared. The largest response was obtained from exposure to toluene vapor with the sensitivity of 0.85 ppm/ppmV (218 mHz/ppmV) and a limit of detection (LOD) of 28 ppmV. The sensitivity dropped by almost six times when exposed to the higher polar vapor of ethanol. Very low response to water vapor (0.05 ppm/ppmV) confirmed the hydrophobic behavior of HMDS-MCF-Si. Detectable performances under different vapors of our device can be utilized as a guide for developing sensitive materials and resonant microcantilever-based vapor sensors.
In recent years, screen-printing process associated with sacrificial layer technique have been developed to prepare piezoelectric MEMS (MicroelectroMechanical Systems) based on lead zirconate titanate (PZT PbZrTiO3) thick films. However, material choices and process conditions (such as pastes compositions, printing conditions and thermal treatment) could affect microstructure and consequently electromechanical properties of printed thick films transducers. With PZT thick films deposited on a composite sacrificial layer based on epoxy and SrCO3, a residual porosity within PZT films is observed. Thus, the obtained PZT films have lower piezoelectric properties compared to a bulk commercial PZT. This work points out improvements of the properties of the printed piezoelectric films by appropriate PZT paste preparation and differences in choices of electrode and sacrificial layer materials. With deposition on a polyester sacrificial layer and using PZT paste containing nano-PZT particles and 3 wt% LBCU sintering aid (Li2CO3, Bi2O3, and CuO), free-standing PZT micro-disks showed improved densification after firing at 900°C (≈7.4 g/cm3). Furthermore, Ag/Pd electrodes led to better PZT/electrode interface compared to Au electrodes. As a result, higher values of effective electromechanical coupling coefficient (≈45%) and relative permittivity (≈1200) were obtained. In the second part of this work, the optimized paste was used for processing of a micro-cantilever dedicated to Volatile Organic Compounds (VOCs) detection. The principle of this sensor is based on the measurement of resonant frequency shifts of the vibrating cantilever when exposed to target species. When the cantilever coated with a sensitive coating presented high sorption capacity, these shifts could be negative due to a mass increase. For this application, various geometries of PZT cantilevers were fabricated (from 310.1 mm3 to 620.1 mm3). Then, the cantilever in size of 320.1 mm3 was chosen for sensing application because it gave acceptable values of mass sensitivity (≈27 Hz/µg) and had enough surface area for receptor depositing. Meso-cellular foam silica (MCF-Si) is one type of mesoporous materials which is attractive to be used as receptor (sensitive layer) due to its large pore size and pore volume. Here, it was synthesized by sol-gel method before functionalization by hexamethyldisilazane (HMDS) to be more hydrophobic. HMDS functionalized MCF-si’s surface area, pore size and pore volume were ≈484 m2/g, 6.59 nm and 0.906 cc/g, respectively. Finally, this porous material (≈50 µg) was dropped onto the free-end of the PZT cantilever before detection of VOCs at room temperature. Low responses under vapors of water, ethanol and benzene were noticed whereas the sensor showed a very good sensitivity to toluene vapor. Also, frequency shifts could be positive or negative because of a competition between mass and stiffness effects. These effects depended on cantilever surface (uncoated or coated with sensitive layer) and on the target vapor nature and concentration. With the acceptable values of sensitivity to toluene (≈ 0.24 Hz/ppmV) and the limit of detection (≈25 ppm), the sensor would be benefit to environmental monitoring field.
Attractive for MEMS, PZT thick films are often microstructured on Si supporting platforms to span the gap between ceramics and thin film technologies. Printing process might lead to lower cost than ceramic process to open routes for MEMS applications. In this paper processing by screen-printing of Au/PZT/Au thick-films supported on alumina or completely released from the substrate are described. Investigations of the film microstructures nevertheless show lower densification than those of bulk ceramics. Prior to selective coating deposition, routes to improve the reduction of the film’s porosity are proposed.
Introduction Humidity control is a real need in different sectors for a variety of reasons. For example, in the food industry, too high levels can lead to product deterioration whereas in the medical sector, low levels can cause a deterioration in respiratory exchanges. Humidity sensors using different principles are therefore very widespread [1]. Also, for gas sensor applications, humidity appears as an interfering factor that must be taken into account. In this context, resonant beam-based sensors developed for BTEX detection [2] have been studied under various humidity conditions. Made with piezoelectric layers exhibiting residual porosity, a comparison of the sensor responses with and without sensitive coating was conducted. Discussion on competitive stiffness and mass effect governing the cantilever resonance shift is proposed. Piezoelectric cantilever fabrication and experimental set-up for humidity tests The piezoelectric cantilever sensors fabricated using screen-printing associated with sacrificial layer technique used for humidity measurement are clamped on a ceramic substrate. The process consists of printing and drying, successively, the anchor, the sacrificial layer, the PZT (PbZrTiO 3 ) beam and the Au electrodes. All layers are then co-fired at 900°C (Fig.1a). Although the sacrificial layer nature and the sintering aid strongly contribute to densification improvement, a ≈12% residual porosity is observed in the PZT beam [3]. For comparison of the responses of the coated and uncoated cantilevers, some transducers are covered with a drop coated mesoporous silica (with a type of mesocellular foam functionalized by hexamethyldisilazane MCF-HMDS) [2]. The cantilevers are piezoelectrically self-actuated and read-out with a longitudinal vibration mode due to the symmetry of the PZT layer sandwiched between the 2 gold electrodes. Admittance measurement and resonance frequency as a function of time are respectively carried out with an Agilent E5063B network analyser and extracted using a polynomial fit. Low levels of relative humidity (<16%rH) are sent in an home-made PTFE cell (volume 17cm 3 ) using a PULL110 vapor generator, whereas a climatic chamber is used for higher rH levels (Fig.1b). Results and Discussions The cantilever sensitivity is correlated to both the humidity range and the coating type (Table 1) [5-8]. Firstly, phenomena occurring for ranges rH>70% with irreversibility is probably linked to a complete filling of the PZT porosity where H 2 0 vapor condensates (Fig.2). For lower %rH and uncoated cantilever (Fig.2), the negative shifts may be explained by a predominance of mass effect because of residual porosity in the cantilever. These results are also observed with similar printed cantilevers [4-5], with, as expected, increasing sensitivity for smaller size [3]. On the other hand, Wasisto et al showed that uncoated Si cantilever showed lower negative shifts, due to denser Si beam [6]. These shifts were attributed to the additional mass created by the formation of isles (rH<50%) or of a water film (rH>50%) [6]. The rH stages also modify the uncoated PZT cantilever responses with a sensitivity ≈5 times higher at rH >40% than the one at rH<16%. Our hypothesis is that, at higher concentration of vapor, the mass effect should become predominant, whereas at low concentration, surface stress which increases the cantilever stiffness should also be taken into account [9]. Finally, considering coated cantilevers, positive or negative shifts can be observed in Table 1. Here, phenomena are quite complicated because, in addition to the expected mass effect due to sorption in the coating, stiffness of porous material may vary during humidity sorption [10]. With Si cantilevers coated with microporous zeolite CBV100, dominant stiffness effect can be seen by Huber et al. [7]). However, for the PZT cantilever coated with mesoporous MCF-HMDS silica (Fig.3), negative shifts of a few Hz /%rH are observed, similarly to Xu et al [8]. The fact that the sensitivity is stable on the rH whole range let us think that mass effect in the MCF-HMDS coating is predominant, with a mass uptake of ≈0.26µg for a transducer sensitivity of ≈27Hz/µg [2]. This sensitivity to humid vapor is still low enough to predict VOCs detection [2]. References [1] Humidity Sensors: A Review Chia-Yen Lee and Gwo-Bin Lee, Sensor letters, Vol.3, 1–14, 2005 [2]BTEX detection using piezoelectric cantilever with mesocellular foam silica functionalized with hexamethyldisilazane sensitive coating, H.Debéda et al. 2020 Meet. Abstr. MA2020-01 2325 [3] Fabrication and characterisation of piezoelectric screen-printed in plane resonant microcantilevers used as gravimetric sensors, Simon Grall et al, 2019, Smart Material and structures Volume 28 , Number 10 [4] Screen-Printed Microcantilevers for Environmental Sensing, Simon Grall et al MDPI Proceedings 2018, 2, 722; [5] Inorganic and organic screen-printed cantilever-based gas sensors, H.Debéda et al., Sensors and Transducers, Vol. 173, Issue 6, June 2014, pp. 215-223 [6] Airborne engineered nanoparticle mass sensor based on a silicon resonant Cantilever, H- S Wasisto et al, Sensors and Actuators B 180 (2013) 77– 89 [7]A Multiparameter Gas-Monitoring System Combining Functionalized and Non-Functionalized Microcantilevers C. Huber et al., Micromachines 2020, 11, 283; doi:10.3390/mi11030283 [8] Functionalized Mesoporous Silica for Microgravimetric Sensing of Trace Chemical Vapors P. Xu et al. Anal. Chem. 2011, 83, 3448–3454 [9] Unusual behavior of uncoated thick-film PZT cantilevers towards fluid phases sensing. Application to water and ethanol detection, R. Lakhmi et al, doi: 10.1109/Transducers.2013.6626974 , 2013 [10] Mesoporous TiO2 Sensitive Films for Love Wave Humidity Detection: Origins of Stress Release Induced by Sorption, L. Blanc et al., DOI: 10.5162/IMCS2012/2.4.1 Conference: Proceedings IMCS 2012 , pp. 205-208, 2012. Figure 1
