Humidity Responses of Resonant Piezoelectric Ceramic Cantilever: A Comparison between Uncoated and Mesocellular Foam Silica Coated Sensors

Abstract

IntroductionHumidity 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 (PbZrTiO3) 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 17cm3) 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 H20 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].