TALC-IMPREGNATED POLYIMIDE FOR HUMIDITY SENSORS WITH IMPROVED HYSTERESIS B. Serban1, V. Avramescu1, M. Brezeanu1, R. Gavrila2, A. Dinescu2, O. Buiu1, C. Cobianu1, S. Beck3, B. Moffat4 1
Honeywell Romania SRL, ACS Sensors & Wireless Lab Bucharest, 3 George Constantinescu, Entrance A, 4th Floor, Sector 2, 020339, Bucharest, Romania 2 National Institute for R&D in Microtechnology, IMT-Bucharest, 126A Erou Iancu Nicolae, 077190, Voluntari, Romania 3 Honeywell, Sensing and Control, TX 75094, Richardson, USA 4 Honeywell, Sensing & Control, ML1 5SB, Newhouse, Scotland
[email protected]
(CAB) [9], poly(methyl methacrylate) [10], polyethersulfone, [2]poly(ethyleneterephatalate) [11]. The main drawback of capacitive RH sensors is the fact that they exhibit hysteresis. Its main cause relates to the clusters of absorbed water, localized in the bulk of sensitive polymeric film. A way to reduce the hysteresis is by increasing its hydrophobicity. This can be obtained by decreasing the number and size of the voids in the polymeric sensing film [1]. One route for performing this process is via cross-linking [12, 13]. Another option is the impregnation of the polymeric film with either a hydrophobic and dense inorganic material (such as carbon black [14]) or with an organic material (such as lignin [15]). Talc, a hydrated magnesium covered silica having the chemical formula Mg3Si4O10(OH)2, is another example of hydrophobic inorganic filler that can be blended and dispersed within organic media, including polymers, to amplify their hydrophobicity [16]. A direct and simple mean to evaluate the hysteresis of the polymeric sensing layer employed by a capacitive RH sensor is by depositing it on a quartz crystal microbalance (QCM) substrate. In a humid environment, the polymer gets loaded with a mass proportional with RH. In turn, this mass-change alters the QCM resonant frequency, with the associated frequency shift being proportional with RH [17]. At the same time, the dielectric constant (ε) of the polymeric sensing layer changes when absorbing the water molecules [18]. The change in ε is proportional with RH. When employed in an RH capacitive sensor, the polymer ε variation is translated in a capacitance variation.
Abstract – This paper reports on the design and synthesis of a talc–impregnated polyimide film exhibiting reduced relative humidity (RH) hysteresis. The morphology of both simple polyimide and talc–impregnated polyimide RH sensing layers are investigated by means of Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). When deposited on quartz crystal microbalance (QCM) substrates, the talc-impregnated polyimide layer yields a highly linear response and a resonant frequency hysteresis improvement as high as 36% with respect to simple polyimide. These experimental results recommend the proposed layer as suitable for high performance RH capacitive sensors. Keywords: Relative Humidity Sensor; QCM; Hysteresis; Talc; Polyimide
1.
Introduction and Motivation
Relative humidity (RH) sensors have received increasing attention in the last decades due to their high importance in many domestic and industrial applications: control of the living environment in buildings, textile and paper manufacturing, food processing, medical field, automotive industry, pharmaceutical processing, agriculture, chemical gas purification, etc. [1]. Among the various RH sensing structures reported in literature, capacitive sensors are an attractive solution due to their highly linear response [2]. Their operating principle is rather simple: the amount of water absorbed by the sensing layer, which is proportional with the RH level, induces a proportional variation of the layer dielectric constant, thus of the sensor output capacitance [3]. Among the insulating polymers employed for manufacturing capacitive RH sensors one can enumerate: polyimides (P84, Matrimid 5218, Kapton, Upilex R, Upilex S) [47], polysulfones [8], cellulose acetate butyrate 978-1-4799-8863-1/15/$31.00 © 2015 IEEE
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Therefore, when assessing the suuitability of a polymer as RH sensing layer, the ressults obtained when measuring the resonant frequenncy hysteresis of a QCM structure employing the targeted polymer are similar to that obttained when measuring the capacitance hysteeresis of a capacitor employing the same polymeer. In this paper, we demonstratte how the hysteresis of an RH sensor is reeduced when impregnating its polymeric sensing layer (polyimide) with talc. For our study,, we use twoQCM-based sensing structures, onne employing polyimide as sensing layer, the other, talcimpregnated polyimide [19].
M) image (Fig. 2) Electron Microscopy (SEM indicates that, while some talc particles are incorporated in the polyimidee layer, others are to be found at its surface. Taalc particles in the synthesized talc-based RH sensing layer have dimensions varying from to 4 µm to 10 µm.
2. Experimental 2.1 Sensing Layers Preparation Polyimide, talc powder, gamma bbutyrolactone (GBL), deionizated water, dimethhylformamide (DMF) and methanol were purcchased from Sigma-Aldrich and used as purchaased. Quartzcrystal microbalances were purcchased from International Crystal Manufacturing (ICM). Prior to any deposition, the QCMs werre cleaned in methanol and deionized water andd dried in an oven at 150 oC for 30 minutes. Two sensing solutions were prepaared and then deposited on QCMs. The simplle polyimide solution (10%) was prepared by the ddissolution of polyimide powder in DMF and GB BL (1:3 v/v). Talc slurry (1%) was prepared from m talc powder (average particle size < 10 µm)) and DMF, ultrasonically stirred (at room tempeerature, for 6 hours), then mixed with simple poolyimide and again ultrasonically stirred (at room m temperature, for 24 hours) for full dispersion. C Consequently, both sensing solutions were spin-ccoated (6.000 rpm for 30 seconds) onto the QCMs and placed in an oven, where they endured the folllowing curing process in air: at 85 oC for 30 minuttes, at 150 oC for 30 minutes, at 300 oC for 30 miinutes, and at 400 oC for 30 minutes. The temperatture was then slowly ramped down to room temperaature.
Fig. 1a. AFM of simple po olyimide layer.
Fig. 1b. AFM of talc-impregnatted polyimide layer.
Fig. 2. SEM of talc-impregnaated polyimide layer.
3. RH Measureements
2.2 Sensing Layers Characterizattion
3.1 Experimental Set-Up
Atomic Force Microscopy (AF FM) analysis performed on the sensing layers (Figgs. 1a and 1b) show that the talc-impregnated pollyimide layer has a slight increase in roughness. T The Scanning
s was employed A complex experimental setup for measuring the RH respo onse of the sensing structures (Fig. 3). The setup p comprises a small 110
testing chamber (5 mm x 5 mm x 5 m mm) with gas inlet/outlet and electrical connectionss, a system of mass flow controllers (Brooks MFC 4800 Series), two glass bubblers (for RH control), a PicoLog, an Agilent 8753ES network analyzer annd a computer for data readout. The testing chamber accommoodates 3 RH detectors in parallel: the two QCM-bbased sensing structures which are the subject oof this paper (depicted as DUT – Devices Under T Test in Fig. 3) ® and a Sensirion RH sensor (Fig. 44). The latter was used for double-checking the RH value indicated by the MFC-system. The simultaneous measurement of the polyimide-basedd QCM and of the talc-doped polyimide-based QCM M eliminates all potential environmental differeences which might occur should the experiments bbe carried out sequentially. At the same time, byy positioning them both close to each other and to the gas inlet, the two QCMs were subject to the same amount of gas flow. Therefore, the experimental conditions for the two devices undder test were identical, leading to reliable results.
Fig. 3. Experimental set-up employed for RH H measurements.
midity from 0% up to when varying the relative hum 90% and then back down to 0%. As depicted in Figs. 5a and 5b, both QCM Ms, either employing simple polyimide or talc-based polyimide as sensing layer, exhibited a lineaar RH response. As expected, both structurees yielded hysteresis, which was calculated accordin ng to equation 1: %
%
% %
(1)
Fig. 5a. RH response of simple po olyimide-based QCM.
Fig. 5b. RH response of talc-impreg gnated polyimide-based QCM.
where Sens (x%) is the sensitivity s at RH = X%, RF (x%DOWN) is the value v of the resonant frequency at RH = x% when n RH is decreasing from 90% to 0%, while RF (x% %UP) is the value of the resonant frequency at RH H = x% when RH is increasing from 0% to 90%. Since the resonant frequency exhibits a higher deegree of linearity for RH in the range 25% - 75% %, the sensitivity is defined as follows: Fig. 4. Testing chamber for RH deteectors.
3.2 RH Response and Hysteresis In order to assess the RH behavioor of the two QCM-based sensing structures ddescribed in Section 2, their resonant frequency w was measured 1
%
%
% %
%
(2)
The hysteresis was measurred for 7 RH values: 0%, 10%, 25%, 40%, 50% %, 60%, 75%. As expected, both sensing structures exhibit
Sensirion® is a registered trademark of S Sensirion AG Aktiengesellschaft. All other trademarks useed herein are the property of their respective owners. 111
hysteresis. Results presented in Fig. 6 show slightly larger hysteresis of thee talc-doped polyimide for RH lower than 25% and significantly smaller hysteresis forr larger RH values. The hysteresis improvement iis particularly significant (larger than 24%) for RH H in the 40%60% range, with a maximum as highh as 36% for RH = 40% (Fig. 7). This is the RH range where the state-of-the-art capacitive sensorrs exhibit the largest hysteresis. On the other haand, the talcimpregnated polyimide QCM show ws a slightly lower sensitivity to RH compared to tthe one based on simple polyimide due to thhe increased hydrophobicity of the sensing layer. Adding talc to the polyimide increeases both the sensing layer hydrophobicity and rouughness (Figs. 1). For this reason, two types of interraction can be anticipated. For low RH (0% – 25%), the predominant interaction is water – polyimide, giving higher hysteresis for the talcc-impregnated polyimide layer due to its increaased specific surface. At higher RH values (25% - 75%), water - talc interaction becomes significantt. In this case, the talc – impregnated polyimide bennefits from its reduced number and size of voids, giving lower hysteresis.
4. Conclusio ons Two QCM-based relativee humidity sensing structures, one employing, as sensing layer, simple polyimide, the otheer talc-impregnated polyimide, were manufactured d and experimentally tested. Both exhibited linear resonant frequency response to RH variation, whiile the one based on talc-impregnated polyimide showed significantly reduced hysteresis (up to 36% % improvement) for RH larger than 25% and slightly lower RH sensitivity. Future work will investigate the talc loading of the polyimide fillm required for an optimum trade-off betweeen the hysteresis improvement and the sensitiivity decrease. The talc-impregnated reported results qualify polyimide as a polymer high hly suitable for RH capacitive sensors with low hy ysteresis. References [1] [2] [3] [4] [5] [6]
[7]
[8] [9] [10]
Fig. 6. Hysteresis comparison between two Q QCM-based RH sensors: one employing polyimide as sensingg layer, the other talc-impregnated polyimide.
[11] [12] [13] [14] [15] [16] [17] [18] [19]
Fig. 7. Improvement in RH hysteresis obtainned when using talc-impregnated polyimide as instead of sim mple polyimide.
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