Piezoelectric thick films are of real interest for devices such as ceramic Micro-ElectroMechanical Systems (MEMS) because they bridge the gap between thin films and bulk ceramics. The basic design of MEMS includes electrodes, a functional material, and a substrate, and efforts are currently focused on simplified processes. In this respect, screen-printing combined with a sacrificial layer approach is attractive due to its low cost and the wide range of targeted materials. Both the role and the nature of the sacrificial layer, usually a carbon or mineral type, depend on the process and the final device. First, a sacrificial layer method dedicated to screen-printed thick-film ceramic and LTCC MEMS is presented. Second, the recent processing of piezoelectric thick-film ceramic MEMS using spark plasma sintering combined with a protective layer approach is introduced. Whatever the approach, the focus is on the interdependent effects of the microstructure, chemistry, and strain/stress, which need to be controlled to ensure reliable and performant properties of the multilayer electroceramics. Here the goal is to highlight the benefits and the large perspectives of using sacrificial/protective layers, with an emphasis on the pros and cons of such a strategy when targeting a complex piezoelectric MEMS design.
Interpenetrating polymer network (IPN) hydrogel based on crosslinked natural rubber (NR) latex and crosslink cassava starch (St) was successfully prepared by a solution blending technique. Crosslinked NR latex was firstly prepared in emulsion state at 70 °C by using potassium persulfate as initiator and N, N'-methylene-bis-acrylamide (MBA) as crosslinker. Secondly, the crosslinked NR latex was mixed then with gelatinized cassava starch and maleic acid (MA) as crosslinker for starch at ambient temperature by using mechanical stirrer. Finally, the mixture was casted on a glass mould to form IPN NR/St sheet via crosslinked reaction at 120 oC in oven for 1.5 h. The water absorption, tensile properties, gel and soluble fraction in water as well as biodegradation of IPN NR/St hydrogels were investigated. It was found that water absorption, soluble fraction, biodegradation and tensile properties decreased with increasing NR contents, while gel fraction in water and elongation at break increased. This was because the rubber molecules prevented the hydrogel from dissolution in water and degraded by microorganism. Furthermore, by immersion of IPN NR/St for long time, water absorption of hydrogel increased with increasing rubber contents due to the effect of elasticity of rubber molecule to easy expansion of INP NR/St hydrogel.
Ces dernieres annees, des procedes de serigraphie associes a la technique de la couche sacrificielle ont ete developpes pour preparer des microsystemes piezoelectriques a base de couches epaisses de titanate de zirconate de plomb (PZT PbZrTiO3). Cependant, les choix des materiaux et les conditions de traitement (telles que la composition des encres de serigraphie, les conditions d'impression et le traitement thermique) pourraient affecter la microstructure et par consequent les proprietes electromecaniques des transducteurs a couches epaisses imprimes. Avec des films epais de PZT deposes sur une couche sacrificielle composite a base d'epoxy et de SrCO3, une porosite residuelle est observee au sein des films de PZT. Ainsi, les films de PZT obtenus ont des proprietes piezoelectriques inferieures a celles d'un PZT massif commercial. Ce travail met en evidence l'amelioration possible des proprietes des films piezoelectriques imprimes par une preparation appropriee de l’encre de PZT et le choix des materiaux des electrodes et de la couche sacrificielle. Avec le depot sur une couche sacrificielle de polyester et en utilisant une encre de PZT a base des nanoparticules de PZT et avec 3 % en poids d'aide de frittage LBCU (Li2CO3, Bi2O3 et CuO), les microdisques de PZT liberes ont montre une meilleure densification apres une cuisson a 900°C (≈7,4 g/cm3). En outre, les electrodes en Ag/Pd ont permis d'obtenir une meilleure interface PZT/electrodes par rapport aux electrodes en Au. En consequence, des valeurs plus elevees du coefficient de couplage electromecanique effectif (≈45%) et de permittivite relative (≈1200) ont ete obtenues. Dans la deuxieme partie de ce travail, l’encre optimisee a ete utilisee pour la fabrication d'une micro-poutre dediee a la detection des composes organiques volatils (COVs). Le principe de ce capteur est base sur la mesure des decalages de frequence de resonance de la micropoutre en vibration lorsqu'elle est exposee a des especes cibles. Avec la micropoutre recouverte d'une couche sensible presentant une capacite de sorption elevee, ces decalages peuvent etre negatifs en raison d'une augmentation de la masse. Pour cette application, differentes geometries de cantilevers en PZT ont ete fabriquees (de 3x1x0,1 mm3 a 6x2x0,1 mm3). Ensuite, la micro-poutre de taille 3x2x0,1 mm3 a ete choisie pour l'application de detection parce qu'elle donnait des valeurs acceptables de sensibilite massique (≈27 Hz/µg) et qu'elle presentait une surface suffisante pour le depot de la couche sensible. La silice mesoporeuse (MCF-Si) est un type de materiau qui est interessant pour etre utilise comme recepteur en raison de la grande taille de ses pores et de leurs volume. Ici, elle a ete synthetisee par la methode sol-gel avant d'etre fonctionnalisee par l'hexamethyldisilazane (HMDS) pour etre plus hydrophobe. Les MCF-Si fonctionnalises ont une surface, une taille de pores et un volume de pores de ≈484 m2/g, 6,59 nm et 0,906 cc/g, respectivement. Enfin, ce materiau poreux (≈50 µg) a ete depose sur l'extremite libre d’une micro-poutre de PZT avant la detection des COVs a temperature ambiante. De faibles reponses sous des vapeurs d'eau, d'ethanol et de benzene ont ete constatees alors que le capteur a montre une tres bonne sensibilite a la vapeur de toluene. De plus, les decalages de frequence se sont averes positifs ou negatifs en raison d'une competition entre les effets de masse et de rigidite. Une dependance avec la nature de la surface de la micropoutre (avec ou sans couche sensible) et avec la composition et concentration du gaz a ete notee. Avec les valeurs de sensibilite au toluene (≈ 0,24 Hz/ppmV) et la limite de detection (25 ppm), le capteur serait benefique pour la surveillance de l'environnement.
Mesoporous silica (MPS) is a porous silica material with various pore structures. In this study, mesocellular foam silica (MCF) was synthesized and functionalized by hexamethyldisilazane (HMDS) to study effects of surface chemistry on benzene adsorption capability. Physical and chemical properties of pristine and functionalized MCFs were characterized and compared. Scanning and transmission electron microscopy showed that the complex pore structures of the MCFs were retained after the functionalization at relatively high temperature (573K). TGA and FTIR results showed that the functionalization led to a reduction of water adsorbed on the surfaces of the MCF. The functionalization improved adsorption of benzene compared to the pristine MCF and the optimum HMDS:SiO 2 molar ratio was 1.5. The amount of benzene adsorbed has a linear relationship with the concentration of benzene in the environment. This relationship enables quantitative benzene detection by using the functionalized MCF as sensing materials in resistive-type or gravimetric-type benzene gas sensors.
In order to improve compatibility and composite properties of pineapple leaf fibers/polyamide 6 composite, alkaline and silane treatments were conducted for fiber modification. Effect of fiber surface treatment on the properties of the composites was investigated. Untreated and treated fibers were raw (R-PALF), alkaline treated (Na-PALF) and silane treated pineapple leaf fibers (Si-PALF). Fiber loading was varied in polyamide 6 based composite. Morphology and fiber-matrix interfacial adhesion, thermal and mechanical properties were examined. Surface treatments can improve interfacial adhesion between these two phases. All PALFs have enhanced thermal stability of all composites. However, thermal characteristics of the composites, i.e. Tc, Tm and crystallinity, have not been affected by PALF types. Na-PALF and Si-PALF help improving mechanical properties of the composites. Young's modulus and tensile strength have been increased whereas elongation decreased by both treated PALFs which indicates that the composites become stiffer and stronger but less flexible. From the results, it could be stated that alkali treatment is sufficient to improve compatibility and properties of the PALF/polyamide 6 composites at fiber loading of 30%wt.
The biodegradable thermoplastic poly(lactic acid) (PLA) that has restricted its application due to its high brittleness and poor crystallization behavior. Toughness properties of PLA can be developed by blending with natural rubber (NR) and epoxidized natural rubber (ENR). Blending PLA with NR and ENR were prepared at various compositions from 0-30% by weight. Morphology, crystallization behavior, thermal stability and mechanical properties of blends were investigated. The rubber phase of NR was dispersed in the continuous PLA matrix with small droplet. Increasing of NR content, the large droplet size of rubber will be resulted. However, the partially compatible between PLA and ENR was responsible for coarse surface, i.e. very fine particles of ENR dispersion. Incorporation of NR would enhance the crystallization ability of PLA better than ENR but thermal stability was decreased with both rubbers. The ductility of PLA has been significantly improved by blending with NR. The amount of NR at 10% weight seems to give optimum property. At high content of NR, it seems to suffer tensile properties. In the case of the addition of ENR, it reduced crystallization ability, thermal resistance and tensile properties of the blend.
Abstract The design and processing of vibrational energy harvester based on screen‐printed piezoelectric lead zirconate titanate (Pb(ZrxTi1‐x)O3 (PZT)) are described here. Two different structures, a simple cantilever and a complex zig‐zag geometry made of PZT layer sandwiched between gold electrodes and supported on a metallic stainless steel substrate have been successfully fabricated by screen printing thick film technique. Compared to bulk PZT ceramics, the main limiting features at different scales are porosity, interfaces, and bending issues. The microstructural analysis of the interfaces in the cantilever has highlighted the formation of an interface between the substrate and the bottom electrode which ensures cohesion of the structure but can limit its dynamic. Bending has shown to be dependent on the thickness of the active piezoelectric layer. Dielectric and electromechanical characterizations performed on multilayers, bulk ceramics, and free‐standing screen‐printed disks are compared and discussed on the basis of interface issues.
Introduction Gas detection with gravimetric sensors such as SAW, MEMS, FBAR or QCM is based on a frequency shift when the target specie is (ad)sorbed on a sensitive layer covering the sensor [1]. Sensor’s sensitivity and reliability depend on transducer type and sensitive coating, while selectivity and reversibility are correlated with the sensitive coating’s nature. Research studies are still active on the development of transducers to reduce their cost and power consumption but sensitive coating optimization remains a hot topic for sensitive and selective sensor achievements. Among attractive sensitive coating for gravimetric sensors, high specific area coatings such as CNTs, graphene oxide, microporous/mesoporous materials or MOFs are nowadays majorly studied one [2-3]. Their performances in gas detection can be modified by tuning of pores or surface functionalization. In this work we are interested in applying mesoporous foam silica (MCF), a type of mesoporous silica, as a base for hydrophobic hexamethy-disilazane (HDMS) functionalization, and tested for BTEX detection. Also, low cost printed piezoelectric MEMS cantilevers are used as gravimetric transducers. Piezoelectric cantilever sensor fabrication and tests One type of inorganic mesoporous silica (MPS), mesocellular foam (MCF), is synthesized by using P123 as a template and 1,3,5-trimethylbenzene (TMB) as a pore expander as described in [4]. After the template removal by ethanol extraction, the template removed MCFs are then functionalized by hexamethyldisilazane (HDMS) following protocol described in [5]. For the piezoelectric PZT based transducer fabrication, the process consists of screen-printing of all the layers before a co-sintering at 900°C allowing releasing of the PZT cantilever with Au electrodes. Details on the process can be found in [6]. Photographs of the fired cantilever (~ 3x2x0.08 mm 3 ) are shown Fig. 1. The HDMS functionalized MCF (F_MCF) powder is dispersed in deionized water with the ratio of 100 mg/1 mL before depositing by drop-coating with 1µl of this suspension, followed by a drying . The cantilever is self-actuated and self-readout thanks to piezoelectric effect. Admittance circle B(G) (Fig.3a and Fig. 3b)) shows the piezoelectric effect for the 31 in-plane longitudinal mode. Conductance ( G(f)) curve before and after F_MCF deposit (weight ~0.05mg) shows the damping effect due to the additional mass (Fig.3c). Based on the measured dimensions of the cantilever, its density and its resonance, frequency f 0 , a theoretical sensitivity S=-f 0 /2m of ~23Hz/µg is calculated. Finally, the cantilever is tested under gas. A PULL110 vapor generator controls N 2 flow (100ml/min) and toluene concentration sent in a home-made PTFE cell. Temperature is controlled during the measurements, and acquisition data ( f 0 as a function of time) are extracted from G measurement using a polynomial fit described in [6]. Results and Conclusions After washing and drying of the MCF, interconnected spherical pore structure are observed by TEM analysis even after functionalized by HMDS (Fig.2). The analysis results of the N 2 adsorption-desorption isotherms of the MCFs are shown in Table 1. Pore and window sizes are not affected by functionalization whereas BET surface areas and BJH pore volumes decrease with the increasing HMDS ratio. Also, after functionalization, as expected, reduction of water adsorbed on the surface of the MCF has been observed by TGA and FTIR analysis, and the optimum HDMS:SiO 2 for VOCs detection has been shown to be 0.15 (for benzene detection in fact) [5]. This composition is selected for further experiment under gas. Tests under toluene with F_MCF show a sensitivity of 241 mHz/ppm at f 0 ~ 256kHz (Figs. 4a and 3b). The blank cantilever also demonstrates slight sensitivity to toluene (~6 mHz/ppm) (Fig. 4b), attributed to density/viscosity changes of the surrounding fluid and cantilevers’s stiffness. In previous work using polymer PEUT coating, silicon cantilever (0.15x0.15x0.095mm 3 ) showed sensitivity of 125mHz/ppm at 350 kHz (out of plane mode), whereas similar printed PZT cantilever of size 8x2x0.08mm 3 gave sensitivity of 15mHz/ppm at 70kHz (in plane vibration mode) [7]. Size reduction leading to higher sensitivity because of the increased f 0 and lower m , combined with the F_MCF coating clearly improves the sensor performances. Good reversibility is also observed. The noisy curve is mainly due to temperature fluctuation which is not precisely controlled in the cell (±0.2°C). The sensor is indeed sensitive to temperature as shown Fig. 3c. Tests under humidity and benzene are ongoing and temperature compensation with another dummy cantilever fabricated in the same conditions could be implemented. References [1] Gas sensors based on gravimetric detection—A review S. Fanget et al . Sensors and Actuators B 160 (2011) 804– 821 [2] Ni-MOF-74 as sensing material for resonant-gravimetric detection of ppb-level CO, Lv Y et al ., Sensors and Actuators B 262 (2018) 562–569 [3] Zeolite-modified cantilevers for the sensing of nitrotoluene vapors, Urbiztondo et al . Sensors and Actuators B: Chemical, vol. 137, pp. 608-616, 2009. [4] A Comparative Study on Mesocellular Foam Silica with Different Template Removal Methods and their Effects on Enzyme Immobilization, N. Thananukul et al ., Journal of Porous Materials , Volume 26 (4) – Dec 6, 2018. [5] Characterization of Mesoporous Silica and its Application as an Adsorbent for Benzene Sensing, Bralee Chayasombat et al . to appear solid state phenomena, 2020 [6]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 [7] Chapter Resonant Micro-cantilever devices for gas sensing, H. Debéda and I. Dufour in Book Advanced Nanomaterials for Inexpensive Gas Microsensors, Editor Eduard Llobet, Elsevier Figure 1
