- Email: [email protected]
PDMS flexible sensor: A new entry
Accepted Manuscript Title: Moisture sensitive inimitable Armalcolite/PDMS flexible sensor: A new entry Authors: Ashis Tripathy, Priyaranjan Sharma, Narayan Sahoo, Sumit Pramanik, Noor Azuan Abu Osman PII: DOI: Reference:
S0925-4005(18)30231-4 https://doi.org/10.1016/j.snb.2018.01.207 SNB 24056
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
26-10-2017 19-1-2018 25-1-2018
Please cite this article as: Ashis Tripathy, Priyaranjan Sharma, Narayan Sahoo, Sumit Pramanik, Noor Azuan Abu Osman, Moisture sensitive inimitable Armalcolite/PDMS flexible sensor: A new entry, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.01.207 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Moisture sensitive inimitable Armalcolite/PDMS flexible sensor: A new entry Ashis Tripathy 1*, Priyaranjan Sharma 2, Narayan Sahoo 3, Sumit Pramanik 4, Noor Azuan Abu Osman 5
2 3 4 5
Department of Electronics and Communication Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Mechanical Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Electrical Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Mechanical Engineering, Faculty of Engineering, SRM University, Tamil Nadu, India. Centre for Applied Biomechanics, Department of Biomedical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.
IP T
1
Highlights
A rare lunar earth material, Armalcolite was synthesized first time in the laboratory environment. Armalcolite/PDMS nanocomposite based flexible humidity sensor was developed for biomedical and flexible electronics application. Newly developed flexible humidity sensor shows excellent linearity, extremely low hysteresis, high sensitivity and, faster response and recovery time.
CC E
PT
ED
M
A
N
U
SC R
Graphical Abstract
A
*Corresponding author, Tel.: +91-8763578440; E-mail address: [email protected] (A.Tripathy)
Abstract Armalcolite, a pioneering material which is commonly available on the moon surface, was synthesized first time in the laboratory environment using innovative solid-state-step sintering process. Afterwards, Armalcolite/Polydimethylsiloxane (PDMS) nanocomposite was prepared using ball milling technique and then spin coated on interdigitated customized gold (Au) electrode on a polyimide substrate in order to develop a high sensitive and fast-response
1.
IP T
flexible humidity sensor. To evaluate the performance of developed flexible humidity sensor, the electrical characterization was performed at room temperature of 25°C in a humidity environment of 33−95% RH using alternating current. The developed sensor exhibits an exceptional performance in terms of high sensitivity, good linearity, negligible hysteresis (<1%), better response and recovery time of 10 s and 15 s respectively, which is outstanding when compared with existing ones within the same category. The long-term stability of flexible sensor was confirmed upon when it was put into test for 30 consecutive days. Due to its grater flexibility, low hysteresis, high stability and the fast response/recovery time, it is intended to use for various application in modern industry, agriculture, and medical care where humidity plays a crucial role. Keywords: Armalcolite; nanocomposite; flexible film; relative humidity; porosity; hysteresis. Introduction
M
A
N
U
SC R
Humidity plays an important role round the year in deciding our comfort level including preserving food crops, controlling industrial processes and health care application [1]. Thus it’s a big challenge for the entire research community to develop a suitable humidity sensor. For the last decade, oxide based ceramic such as MgO [2, 3], iron oxide (Fe2O3) [4], zinc oxide (ZnO) [5], aluminum oxide (Al2O3) [6], titanium oxide (TiO2) [7], tin oxide (SnO2) [8] played a key role in the development of humidity sensor. High thermal, chemical, and mechanical stability [9, 10] of the ceramic nanocomposite make them suitable for humidity sensing application, however inherent brittleness and the undesired morphological structures creates hindrance for various sensing applications. Apart from these concerns, inhomogeneous dispersal of pores and inadequate porosity supplements to the misery which reduces the hydrophobicity and water absorption capability leads and amplify the issue of brittleness. Hence, it reduces the sensing properties of humidity sensor by reducing the electrical conductivity between the sensing electrodes.
PT
ED
To overcome the above stated issues of ceramic nanocomposite, a novel Armalcolite nanocomposite was synthesized for the humidity sensor application. This unique material has never been successfully synthesized in a laboratory environment till date. The hidden potential of Armalcolite nanocomposite has never been explored for humidity sensing application even after many decades of its discovery on moon.
A
CC E
The wonder material Armalcolite was processed by using inexpensive in situ step sintering technique to control the morphology, porosity and crystalline phases. As most of the ceramics and their composites have congenital brittleness. To overcome this physical abnormality, elastomers and polymers are added to enhance their flexibility [11-13]. However, controlling the aspect of hydrophilicity in such flexible composite materials is a great challenge for researchers. There is a drought of study on the flexibility of composites [11, 14]. Therefore, there is lots of scope for the development of flexible humidity sensor for various industrial and biomedical applications. In the current study, an extremely sensitive and flexible humidity sensor having excellent linearity, low hysteresis, rapid response and recovery time was successfully developed. The developed flexible sensor consists of Armalcolite/PDMS composite as humidity sensing layer on interdigitated gold electrode followed by Polymide (PI) substrate. Due to inherent flexibility, nontoxicity and biocompatibility of PDMS, metal oxide deposition and patterning on PDMS are gaining more and more interest of researchers. Moreover, PDMS based
nanocomposites are more stretchable and most suited for biomedical applications due to their easy skin attachment and implant into human body. The incorporated innovative ideas presented in this article paves path for designing an ultrasensitive flexible humidity sensor and also creates a new direction of its application in the field of medical science. 2.
Experimental
2.1
Preparation of Armalcolite nanocomposite
ED
M
A
N
U
SC R
IP T
To prepare the Armalcolite nanocomposites, calcium oxide (CaO), magnesium carbonate (MgCO3), hematite (Fe2O3), and titania (TiO2) powders of 99.9% purity were procured from ‘Fisher Scientific Ltd., Selangor, Malaysia’ [15]. Solid state step-sintering technique was used for synthesis of this wonder material Armalcolite nanocomposite having empirical formula CaMgFe1.33Ti3O12. The Almalcolite nanocomposite was synthesised under following stages, In the first stage all the raw powders were poured into a ball milling machine (PM200, Retsch, Düsseldorf, Germany) with alumina balls of 10 mm diameter then, 70% ethyl alcohol was added to the mixture. The mixing process was carried out at 300 rpm for a time duration of 3 days under room temperature. In the second stage, resultant slurry was kept for 6 h in an oven (OF-11E, Lab Companion) at 120°C to eliminate the moisture completely. In the third stage, the dry composite powder was transformed into pallets of diameter of 10 mm and thickness of 2.75 mm using uniaxial hydraulic press (GS15011, Graseby Specac, Kent, UK). Finally, the developed pallets were solid state step-sintered at a temperature of 1050°C in a programmable furnace under following steps: (i) 350°C for 60 min at ramp rate of 5°C/min, (ii) 550°C for 210 min at ramp rate of 10°C/min, and (iii) 1050°C for 78 min at heating rate of 15°C/min, and then cooling at (iv) 750°C for 180 min at heat rate of 20°C/min and (v) furnace cooling. The solid-state step-sintering has played an important role to minimize the particle size by increasing grain boundary with optimum porosity. A whole chemical reaction of this material is presented in Eq. (1).
2.2
PT
4Fe2O33 4TiO2 CaO MgCO 0.5O2 Fe2 MgTi3O10 CaTiO3 2Fe3O4 O2 CO2
(1)
Synthesis of Armalcolite/PDMS Based Composite Flexible Film
A
CC E
The Armalcolite/PDMS flexible composite film was prepared under following stages [15]. In first stage, sintered pallets were re-pulverized into fine powder to mix with PDMS-gel (Sylgrade184 Silicon Elastomer Base, Dow Corning) in an optimized ratio. The mixing was done in a ball-mill machine for 240 min at 300 rpm by using alumina balls. To enhance the cross-linking in PDMS chain, a curing agent (Dow Corning, curing agent: PDMS of 3:25 (w/w)) was added to Armalcolite/PDMS mixture. The ceramic-polymer composite slurry was then spin coated to prepare a thin film on a glass petri-dish, subsequently the micro bubbles present in the thin film was removed by using self-drying vacuum pump (PM200, Memmert, Camberley, UK). The final form of the flexible film was obtained after heating it for 300 min at 60°C in an oven (OF-11E, Lab Companion). Moreover, a schematic illustration of synthesis procedure of Armalcolite nanocomposite and its flexible film was shown in the Fig.1.
IP T SC R
Fig. 1. Flow chart for the preparation of flexible Armalcolite/PDMS composite film.
2.3
Characterizations
ED
M
A
N
U
The XRD peaks were analysed with the help of X-ray diffractometer (Empyrean, PAN analytical, Cu-Kα radiation, = 1.54056 Å). SEM (AURIGA, Carl Zeiss) was used for morphological study of thin film. Archimedes’ principle [16, 17] was used to obtain density, open porosity and water absorption for Armalcolite and its PDMS based composite film. Sessile contact angle meter (OCA15E, Data Physics Instruments GmbH, Filderstadt, Germany) was used for measuring water contact angle. Fourier transforms infrared (FTIR) spectroscopy was recorded by using Perkin-Elmer spectrometer (400, Perkin Elmer,Waltham, UK). Finally, universal testing machine (5848, Instron Micro Tester) was used for mechanical flexibility study. 2.4 Flexible sensor fabrication and measurements
CC E
PT
The flexible humidity sensor was fabricated using spin coating method by depositing the Armalcolite/PDMS cured gel on interdigited gold electrode followed by polymide (PI) substrate. The developed sensor was kept inside a vacuum oven at 90°C for 60 min and flexible film of thickness of 0.7 mm was obtained. The stability and durability of the fabricated flexible sensor was improved by maintaining the maximum relative humidity of 95% at supplied voltage and frequency of 1 V and 100 Hz respectively for 24 h.
A
Impedance spectroscope (3532-50 LCR Hi tester, Hioki) was used for electrical characterization of humidity sensors. For this aim, frequency range of 102-106 Hz and AC voltage of 1 V were used. The sensor response was measured at 25°C in a temperature controlled chamber (Memmet, Naluri Scientific, Schwabach, Germany) having resolution of 5°C. Different humidity environment were produced using saturated salt solution such as magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2), sodium chloride (NaCl), potassium chloride (KCl) and potassium nitrate (KNO3) within a resolution of ±1% RH at 25°C [18]. The humidity level of various salt solutions was shown in Table 1. A schematic illustration of developed flexible humidity sensor, corresponding measurement technique and humidity sensing equipment were depicted in Fig. 2.
Table 1. Humidity generation for different salts at room temperature. Salt Humidity (% RH) MgCl2 33 Mg(NO3)2 55
Temperature 25°C 25°C
75 85
25°C 25°C
KNO3
95
25°C
ED
M
A
N
U
SC R
IP T
NaCl KCl
PT
Fig. 2. A schematic representation of the measurement system of Armalcolite/PDMS based humidity sensor.
3 Results and discussion
CC E
3.1 Structural and morphological and mechanical analysis
A
The XRD patterns of Armalcolite nanocomposite and Armalcolite/PDMS based film has been shown in Fig. 3(a) and 3(b) respectively. As a result of sintering of raw material (CaMgFe1.33Ti3O12) at 1050°C, two new phases were detected, which are matching with the standard XRD pattern of orthorhombic Armalcolite (Fe2MgTi3O10, 101) and perovskite (CaTiO3, 440) as shown in Fig. 3(a). Moreover, change in peak intensity was also detected for CaTiO3 (440) and Fe2MgTi3O10 (101) which exhibit different crystal sizes of 5.9 nm and 20.7 nm respectively. Moreover, the prominence of these two phases was also confirmed by scanning electron micrograph as shown in Fig. 4(a). Remarkably, another peak of Fe3O4 (311) was detected at 2θ = 35.72° which clearly indicates the conversion of Fe3+ into Fe2+. At the position of 2θ = 12°, a wide peak of PDMS was observed which indicate the semi-crystalline nature as shown in Fig. 3(b). Additionally, peaks of Fe2MgTi3O10 and CaTiO3 were also
N
U
SC R
IP T
detected along with PDMS peak which indicate the homogeneous nature of composite. A peak shift was also observed in Fig. 3(a-b) where Fe2O3 is transformed into Fe3O4 and could be beneficial for remote sensing application [19].
A
Fig. 3. XRD (CuKα1, λ=1.54056 Å) patterns of, (a) Armalcolite nanocomposite; (b) Armalcolite/PDMS composite film.
A
CC E
PT
ED
M
The morphology of Armalcolite and flexible film has been examined with SEM as shown in Fig. 4(a-c) where yellow circle indicate pores; blue arrow indicate Armalcolite phase; red arrow indicate peroskite phase; yellow arrow indicate particles. In Fig. 4(b), pink arrow indicate grain and white arrow indicates grain boundary. An average particle size and pore size of the Armalcolite nanocomposite were found to be 630 nm and 850 nm, respectively. Different sizes of particle was observed in microscopic investigation of Armacolite nanocomposite which indicates the formation of two distinct phases (Fe2MgTi3O10 and CaTiO3) having average grain size between 100 nm to 685 nm. From the SEM analysis, it was found that larger size particle typically belongs to Armalcolite phase (Fe2MgTi3O10) however smaller size particle belongs to perovskite phase (CaTiO3) and also confirmed through XRD analysis. From the nanoscopic investigation of Armalcolite nanocomposite as shown in Fig. 4(b) clearly indicates the existence of grain and grain boundaries. Fig. 4(c) indicates the microscopic image of Armalcolite/PDMS flexible film which indicates the uniform distribution of Armalcolite particles in PDMS. Moreover, the developed flexible film exhibits the better bonding at interface of Armalcolite particle and polymer matrix which was also confirmed through FTIR analysis as shown in Fig. 5(c).
IP T SC R U N A M ED
Fig. 4. SEM images of the (a) Armalcolite nanocomposite; (b) Magnified image of Armalcolite nanocomposite; and (c) Armalcolite/PDMS composite film.
A
CC E
PT
Once Armalcolite nanocomposite was developed, the formation of new bond was examined using FTIR spectra as shown in Fig. 5(a-c). From the FTIR analysis that it could be identified that all FTIR peaks of PDMS polymer such as Si–O–Si (650-720 cm-1), Si–C (602 cm-1) and Si–CH3 (842 cm-1) were also present in the Armalcolite/PDMS composites. Moreover, FTIR peaks of the Armalcolite ceramic such as Ca–O–Ti (470 cm-1) and Ca–O–Ca (754 cm-1) were related to titanate vibration and stretching vibration respectively [20, 21]. Remarkably, three new peaks such as C–Si–O (600 cm-1) from Armalcolite/PDMS, Ca–O(Si) (754 cm-1)) and Ca– O (910 cm-1) from PDMS and titanate reaction were also perceived in Armalcolite/PDMS nanocomposite.
IP T SC R U N A M
ED
Fig. 5. FTIR spectra of the (a) Armalcolite nanocomposite; (b) Pristine Polydimethylsiloxane (PDMS) film, and (c) Armalcolite/PDMS composite film.
A
CC E
PT
In the Fig. 6(a-d), a comparison of physical properites (i.e., water contact angle, water absoption, density and open porosity) of PDMS, Armalcolite/PDMS and Armalcolite were presented. In this study, open porosity of materials was determined by measuring the bulk density. The experimental investigation revealed that Armalcolite nano composite has lower density in comparison to raw materials which confirms the high level of porosity within Armalcolite nano composite. The porosity of material can also be confirmed by calculating the amount of water absorbed by the material. High water absorption capability (~68%) of Armalcolite nano composite confirms the uniform porosity throughout the materials which has already been confirmed in microscopic investigation (Fig. 4). In Armalcolite/PDMS composite film, low water contact angle (WCA) is desirable as it is mainly responsible for changing the nature of PDMS from hydrophobic (WCA=107°) to hydrophilic (WCA=88.1°) [20]. This hydrophilic nature of Armalcolite/PDMS composite film make it suitable for various remote control humidity sensing applications such as remote sensing of soil moisture, remote sensing of structural health monitoring to avoid corrosion, monitoring of human health by measuring sweating and breathing etc. [13, 20, 22, 23].
IP T SC R U
N
Fig. 6. Physical properties of PDMS, Armalcolite/PDMS, Armalcolite, (a) Open porosity; (b) Water absorption; (c) Density; (d) Water contact angle.
A
CC E
PT
ED
M
A
The comparison of Armalcolite/PDMS composite film and PDMS film was presented in Fig. 7 which revealed that Armalcolite/PDMS film has higher flexiblity compared to PDMS film. In order to confirm the flexibility, tensile test was conduted up to 90% of the elongation. The stress-strain relationship is the ratio of stress and strain which also called as Young’s modulus (E). In the current study, Young’s modulus was calculated from the highlighted portion of stress-strain curve as shown in Fig. 7. The stress-strain relationship (E = Stress/Strain) revealed that Armalcolite/PDMS composite film has the Young’s modulus of 0.80 0.21 MPa, which is considerably higher than that of PDMS film of 0.27 0.08 MPa. Moreover, both the film has indicated the elongation of 90%. The experimental results has confirmed that Armalcolite/PDMS composite films has more flexiblity compared to PDMS film. Further, it was also confirmed by the modulus value which indicated that Armalcolite/PDMS composite films has lower modulus value than the other PDMS composites leading to the grater flexibility [20].
IP T SC R
Fig. 7. Static tensile properties of PDMS and Armalcolite/PDMS composite film.
U
3.2 Humidity sensing properties
Δ Impedance Δ % RH
PT
Sensitivity S
ED
M
A
N
In order to determine the optimum value of impedance, humidity and frequency for developed humidity sensor, the impedance as a function of relative humidity was measured at different frequency under room temperature. From the Fig. 8, it can be clearly observed that the impedance value of humidity sensor reduces significantly with increase in frequency at low relative humidity. Moreover, the difference in impedance value become gradually smaller at two consecutive frequency while increasing the relative humidity. This behaviour could be explained by the fact that at high frequency, the water absorbed by the thin film can’t be polarized and hence, dielectric phenomenon won’t occur [24]. Further, the sensitivity of Armalcolite/PDMS humidity sensor was determined using following expression [25]. (1)
A
CC E
The sensitivity in the range of 33–95% RH at 102, 103, 104, 105, and 106 Hz is found to be 0.072, 0.066, 0.049, 0.037 and 0.025 respectively.
IP T SC R
Fig. 8. Dependence of impedance on relative humidity for the sensor based on Armalcolite/PDMS nanocomposite measured at various frequencies at 1 V and 25°C.
A
CC E
PT
ED
M
A
N
U
In order to measure the linearity index of humidity sensor, log-impedance as a function of relative humidity was measured at frequency of 103 Hz and at temperature of 25°C as shown in Fig. 9. The experimental investigation of plotted data revealed that there is significant linearity (Y = −0.0633X + 9.196; R2 = 0.9994) between log impedance and relative humidity within the 33–95% RH. Further, high RH sensitivity and better linearity could be obtained by applying a low operating frequency within the range of 102 to 103 Hz. For extremely high RH sensitivity and better linear response, 103 Hz could be selected as optimized operating frequency in the current experimental setup.
Fig. 9. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at 1 V, 1 kHz and 25°C.
The voltage dependency of developed flexible sensor was shown in Fig. 10 where impedance was measured In order to understand the voltage dependency of the developed flexible sensor, the impedances of the sensor are measured at different voltages (i.e., 0.2 V, 1 V and 2 V). From the analysis of Fig. 10, it was obtained that there is slight deviation in impedances– relative humidity sensing for the lower operating voltages of less than 1 V. However, no significant
SC R
IP T
deviation in humidity sensing was detected at operating voltage of 1-2 V within the humidity range of 33-95%. Further, better linearity, good stability and high sensitivity, AC voltage of 1 V and frequency of 103 Hz were designated as optimum operating parameters.
U
Fig. 10. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different applied voltages (0.2 V, 1.0 V, and 2.0 V) at 1 kHz and 25°C.
A
CC E
PT
ED
M
A
N
The temperature dependency of developed flexible humidity sensor was investigated by measuring the sensor impedances at different temperature ranges starting from 15°C to 55°C as shown in Fig. 11. The result indicates that there is a negligible variation in impedance with respect to temperature at lower humidity range of 33-55% RH. This behavior could be explained by fact that at lower humidity range, temperature variation is not capable enough to ionize the water molecule. Further, the sensor impedances get decrease minutely at higher humidity (75-95% RH). This is occurred due to improved ionization phenomena of water molecules. The experimental results indicate that humidity sensing characteristics is slightly influenced by temperature which is quite considerable.
Fig. 11. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different temperatures (15°C, 25°C and 35°C) at 1 V, 1 kHz.
The performance of humidity sensors can be evaluated by measuring the response and recovery characteristics. In Fig. 12(a), adsorption and desorption of water molecules for developed
M
A
N
U
SC R
IP T
humidity sensor was measured by analyzing the response and recovery curves for one cycle. Form the Fig. 12(a), it was observed that when sensor exposed from lower humidity (33% RH) to higher humidity (95% RH), the sensor impedance decreases rapidly to end with a constant value. Afterward, when the sensor was exposed from higher humidity (95% RH) to lower humidity (33% RH), the sensor impedance increases abruptly to end with a stable value. Basically, response and recovery time is defined as the time required for the sensor to achieve 90% of the final value corresponding to adsorption and desorption process as shown in Fig. 12(a). Within the humidity range of 33-95% RH, response time and recovery time were found to be 10 s and 15 s respectively for developed humidity sensor which is comparatively faster than the other reported sensor results as presented in Table 2. Further, response and recovery curve for seven cycle has been illustrated in Fig. 12(b). Even though there is a slight fluctuation in impedance response however it indicates the good reproducibility of the developed flexible sensor. The faster response, recovery time and good reproducibility characteristics of developed humidity sensor proved that it is best suited for high performance humidity sensing application.
ED
Fig. 12. Response and recovery characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C for, (a) 1 cycle and (b) 7 cycles (Note: Response time was measured by varying the relative humidity from 33% RH to 95% RH at the time of adsorption whereas recovery time was measured by varying relative humidity form 95% RH to 33% RH at the time of desorption).
A
CC E
PT
The humidity hysteresis is another important figure of merit of a humidity sensor. The humidity hysteresis is defined as the maximum difference between the humidification and desiccation curve. It was measured by switching the sensor between the chambers of 33%, 55%, 75%, 85% and 95% RH, and then transferring back. The impedance of desorption process is slightly lower than that of adsorption process. According to the adsorption and desorption processes of the adsorbed water in the humid membrane are corresponding to the exothermic and endothermic processes, and the different reaction speeds at the same RH lead to the impedance on desorption process slightly lower than that on adsorption process. The humidity hysteresis characteristics of Armalcolite/PDMS nanocomposite based sensor are shown in Fig. 13. It is found that the maximum hysteresis is < 1% RH at the range 11–95% RH, and it indicates a good reliability of the humidity sensor.
IP T SC R
Fig. 13. Humidity hysteresis characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C.
A
CC E
PT
ED
M
A
N
U
In order to examine the stability of flexible sensor, it was kept in open air environment at room temperature for one month. Then, impedance was measured at interval of 3 days at different relative humidity environment. During the measurement, AC voltage (1 V) and frequency (f = 103 Hz) were kept constant as shown in Fig. 14. The experimental results of sensor revealed that there is a negligible change in the impedance response which was continuously monitored in a time span of 30 days and hence, confirms the long term stability of developed humidity sensor. Based on the above mentioned points, it can be state that the sensor has excellent stability and quite suitable for various humidity sensing application.
Fig. 14. Impedance stability of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C.
Table 2. Comparison of the performance of Armalcolite-PDMS based sensor with other flexible humidity sensors. Sensing Material
Humidity range (% RH)
Sensitivity (log Z/%RH)
Response time (s)
Recovery time (s)
Hysteresis (% RH)
Reference
Armalcolite/PDMS
33-95
0.072
10
15
<1
PAMPS doped salts PMMA/PMAPTAC PAMAM-AuNPs TiO2 NPs/PPy/PMAPTAC
20–90 30–90 30–90
0.026 0.033 0.045
60 45 40
70 150 50
<8 <6 <2
Current work [26] [27] [28]
30–90
0.065
30
45
<2
[29]
3.3 Discussion on sensing principles
M
A
N
U
SC R
IP T
In this section, humidity sensing principle of Armalcolite/PDMS nanocomposite materials was demonstrated. The mechanism of improved conductivity of Armalcolite/PDMS nanocomposite along with relative humidity could be explained by the Grotthuss phenomena, which indicate that this behaviour is almost similar to the tunnelling of proton from one water molecule to another [30]. There are two different phenomena of water absorption depending upon the humidity level. At lower humidity, chemisorption phenomena occurs which is based on chemical adsorption of water molecules on the activated side of Armalcolite/PDMS nanocomposite. During chemisorption process, two hydroxyls group will be bonded with one water molecule via dissociative phenomena which means that chemisorption already initiated at 33% RH. At lower humidity condition, the current conduction is mainly occurs due to the hopping of H+ ion on the surface of Armalcolite/PDMS nanocomposite. Due to the formation of double hydrogen bonding at low humidity conditions, water molecules can’t move freely. Thus, it requires more energy for hoping of H+ ion in their adjacent hydroxyl group which increases the value of sensor impedance. Further, in Fig. 16(a-b), a semicircle was detected at low humidity condition which is mainly responsible due to intrinsic impedance of the material. Since, vapour pressure is low at this stage and intrinsic electrons in the material are actively participating in conduction. Therefore, it could be confirmed that at low humidity condition (33-55% RH), electrolytic conduction is mainly responsible for current conduction.
A
CC E
PT
ED
As the humidity goes on increase, chemisorption phenomena shifted to physisorption where water molecules are physically absorbed. Basically, Physisorption is a multilayer phenomenon in which first layer of physically adsorbed water is due to double hydrogen bonding which makes the layer immobile. From the succeeding physisorbed layers, water molecules are absorbed physically through single hydrogen bond as a result the water molecules become mobile and thus H+ ion would be available for current conduction as demonstrated in Fig. 15. Then, more number of physisorbed layers are formed when humidity increases and thus, excessive number of protons would be available for current conduction. Then, the cations (Mg2+,Ti2+,Ca2+,Fe2+) of the Armalcolite/PDMS nanocomposite are electrostatically attached to the negatively charged oxygen because of polar nature of water molecule. The external relative-humidity environment also affects the physisorption phenomena. With increase in relative humidity, continuous layer of water molecules will be formed where H+ ion can move freely. The physisorbed water molecules (2H2O) were dissociated into hydronium (H3O+) and OH− ions. According to Grotthuss chain reaction [31, 32], when H3O+ releases a proton to adjacent water molecules, proton transport take place. Due to further increase in relativehumidity, electrolytic conduction [33] will be more prominent due to excessive proton transfer. Further, in Fig. 16(c-e), it could be seen that a straight line appears after semicircle at higher humidity range. This straight line formation is mainly responsible due to the conduction of
U
SC R
IP T
ions. Therefore, it could be confirmed that at high humidity condition (75-95% RH), ionic conduction is mainly responsible for current conduction.
A
N
Fig. 15. Schematic representation of the sensing mechanism of the Armalcolite/PDMS based nanocomposite humidity sensor.
A
CC E
PT
ED
M
Further, the humidity sensing mechanism of Armalcolite/PDMS based nano composite thin film was investigated using complex impedance plots (CIP) having frequency range between 102-106 Hz at different RH value. In Fig. 16(a-e), Z' and –Z'' represents the real and imaginary parts of the CIP respectively. At low RH (33%, and 55%), a formation of semicircle was detected which confirms a “non-Debye” behavior. Earlier investigations have reported that it is due to the polarization effect which can be modeled using an equivalent circuit of parallel capacitor (Cf) and resistor (Rf) [34-36]. At higher RH (75%, 85%, and 95%), the radius of semicircle get decreases and a line formation was detected in the low-frequency range. With further increase in RH value, the line becomes longer whereas semicircle becomes smaller. The formation of line is mainly due to the diffusion phenomena at the electrodes which represent ‘Warburg impedance’ [9, 37-39]. The equivalent circuits of CIP have been demonstrated in Table 3 where Rf and Cf are resistance and capacitance of the Armalcolite/PDMS film respectively. The impedance at the electrode/sensing film interface was denoted by Zi. According to the CIP, at low RH value (Fig. 16(a-b)), the change in impedance of the sensor is mainly due to the Rf because at low RH, Rf << Zi. However, at high RH (Fig. 16(c-e)), impedance variation of the sensor is evaluated by Rf and Zi because both Rf and Zi have the same magnitude.
IP T SC R U N A M
ED
Fig. 16. Complex impedance plots of Armalcolite/PDMS based sensor obtained from 10 2 Hz to 106 Hz, at 25°C.
A
CC E
PT
Table 3. Equivalent circuit of CIP at various RH value
4
Conclusions
In the resent research work, a novel Armalcolite/PDMS based flexible resistive humidity sensor was fabricated and designed. With the help of solid-state step-sintering technique, different morphology of the Armalcolite and perovskite phases were obtained, and confirmed by XRD and SEM analysis. The newly developed flexible sensor has indicated the excellent impedance sensing characteristics through the physisorption mechanism. From the complex impedance
SC R
IP T
response analysis, two distinct water absorption mechanism (i.e., at lower humidity range of 33-55% RH and at higher humidity range of ≥ 75% RH) were observed however critical relative humidity was observed at 75% RH. The impedance of the flexible sensor was decreased from 1.23×107 ohm to 1.5×103 ohm in a humidity range of 33−95% RH at the frequency of 103 Hz. Moreover, it has indicated a greater sensitivity, a weak dependence on temperature, rapid response time of 10 s and fast recovery time of 15 s, and extremely low hysteresis (< 1%) which is far better than the result reported in previous literature based on flexible resistive humidity sensor. All improved sensing features along with improved linearity and high stability of the Armalcolite/PDMS based humidity sensor confirm that it could be an effective and efficient flexible humidity sensor for advanced electronics and biomedical applications.
M
A
N
U
Dr. Ashis Tripathy is an Assistant Professor in the Department of in Electronics and Communication Engineering, in JK Lakshmipat University, Jaipur, India. He has obtained his Ph.D. degree in Biomedical Engineering from University of Malaya, Kuala Lumpur, Malaysia. His research interest is Fabrications and characterizations of Micro/Nano electronics devices, Sensor Design, Flexible electronics, advanced ceramics nano-composite synthesis and characterization, thin film characterization and Bio-medical application.
A
CC E
PT
ED
Dr. Priyaranjan Sharma is an Assistant Professor in the Department of Mechanical Engineering at JK Lakshmipat University, Jaipur, India. He did his Ph.D in Advanced Manufacturing from National Institute of Technology Karnataka (NITK), Surathkal. His area of interest is materials and manufacturing, conventional/non-conventional machining, manufacturing of micro/nano components for MEMS, aerospace and bio-medical application.
Mr. Narayan Sahoo is an Assistant Professor in the Department of Electrical Engineering at JK Lakshmipat University, Jaipur, India. He has obtained his MTech degree in Electrical Engineering from National Institute of Technology, Silchar, India. His area of research is sensor technology, control and automation, material synthesis and characterization.
IP T
Dr. Sumit Pramanik is a Research Associate Professor in SRM University, Chennai, India. He has obtained his Ph.D and M.Tech degree in Materials Science, Indian Institute of Technology (IIT) Kanpur, India. His area of interest is Materials science & Biomedical engineering: Bioceramics, Tissue engineering scaffolds, Polymers, Biodegradable polymers, Carbon nanostructures, Nanotechnology, Advanced ceramics, Porous ceramic catalyst, Surface coating, Tribology, Sensors, BioMEMS/NEMS, Functional graded material (FGM), Advanced polymers and composites.
SC R
Prof. Noor Azuan Bin Abu Osman is a Professor in Department of Biomedical Engineering, Dean of Engineering and also the Deputy Director of Centre for Applied Biomechanics, Faculty of Engineering, University of Malaya, Malaysia. He has graduated from University of Bradford, UK with a B.Eng. Hons. in Mechanical Engineering, followed by MSc. and Ph.D. in Bioengineering from University of Strathclyde, UK. His area of interest is measurements of human movement, development of instrumentation for forces and joint motion, and the design of prosthetics and orthopaedic
U
implants.
A
N
Conflicts of Interest: The authors declare no conflict of interest.
M
Reference
ED
Acknowledgment:;1;;1; This study was supported by UM/MOHE/HIR, University of Malaya, project No.: D000014-16001.
2.
Li, M., et al., Humidity sensitive properties of pure and Mg-doped CaCu 3 Ti 4 O 12. Sensors and Actuators B: Chemical, 2010. 147(2): p. 447-452. Wang, R., et al., The humidity-sensitive property of MgO-SBA-15 composites in one-pot synthesis. Sensors and Actuators B: Chemical, 2010. 145(1): p. 386-393. Adhyapak, P.V., et al., Influence of Li doping on the humidity response of maghemite (γ-Fe 2 O 3) nanopowders synthesized at room temperature. Ceramics International, 2013. 39(7): p. 8153-8158. Gu, L., et al., Humidity sensors based on ZnO/TiO 2 core/shell nanorod arrays with enhanced sensitivity. Sensors and Actuators B: Chemical, 2011. 159(1): p. 1-7. Tai, W.-P., J.-G. Kim, and J.-H. Oh, Humidity sensitive properties of nanostructured Al-doped ZnO: TiO 2 thin films. Sensors and Actuators B: Chemical, 2003. 96(3): p. 477-481. Qi, Q., et al., Influence of crystallographic structure on the humidity sensing properties of KCldoped TiO 2 nanofibers. Sensors and Actuators B: Chemical, 2009. 139(2): p. 611-617. Tai, W.-P. and J.-H. Oh, Preparation and humidity sensing behaviors of nanocrystalline SnO 2/TiO 2 bilayered films. Thin Solid Films, 2002. 422(1): p. 220-224. Kulwicki, B.M., Humidity sensors. Journal of the American Ceramic Society, 1991. 74(4): p. 697-708.
CC E
3.
PT
1. Song, X., et al., A humidity sensor based on KCl-doped SnO 2 nanofibers. Sensors and Actuators B: Chemical, 2009. 138(1): p. 368-373.
4.
A
5. 6. 7. 8. 9.
17. 18. 19.
20.
21. 22. 23.
24.
IP T
CC E
25.
SC R
16.
U
15.
N
14.
A
13.
M
12.
ED
11.
Traversa, E., Ceramic sensors for humidity detection: the state-of-the-art and future developments. Sensors and Actuators B: Chemical, 1995. 23(2): p. 135-156. Su, P.G. and C.P. Wang, Flexible humidity sensor based on TiO2 nanoparticles-polypyrrolepoly-[3-(methacrylamino) propyl] trimethyl ammonium chloride composite materials. Sensors and Actuators B: Chemical, 2008. 129(2): p. 538-543. Kar, K.K. and S. Pramanik, Hydroxyapatite poly (etheretherketone) nanocomposites and method of manufacturing same. 2014, Google Patents. Tripathy, A., et al., Role of Morphological Structure, Doping, and Coating of Different Materials in the Sensing Characteristics of Humidity Sensors. Sensors (Basel, Switzerland), 2014. 14(9): p. 16343. Nayak, S., et al., Development of polyurethane–titania nanocomposites as dielectric and piezoelectric material. RSC Advances, 2013. 3(8): p. 2620-2631. Tripathy, A., et al., Synthesis and characterizations of novel Ca-Mg-Ti-Fe-oxides based ceramic nanocrystals and flexible film of polydimethylsiloxane composite with improved mechanical and dielectric properties for sensors. Sensors, 2016. 16(3): p. 292. Pramanik, S., B. Pingguan-Murphy, and N.A. Abu Osman, Developments of immobilized surface modified piezoelectric crystal biosensors for advanced applications. Int. J. Electrochem. Sci, 2013. 8: p. 8863-8892. Pramanik, S., et al., Design and development of potential tissue engineering scaffolds from structurally different longitudinal parts of a bovine-femur. Scientific reports, 2014. 4. Greenspan, L., Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards, 1977. 81(1): p. 89-96. Morris, R.V., et al., Spectral and other physicochemical properties of submicron powders of hematite (α‐ Fe2O3), maghemite (γ‐ Fe2O3), magnetite (Fe3O4), goethite (α‐ FeOOH), and lepidocrocite (γ‐ FeOOH). Journal of Geophysical Research: Solid Earth, 1985. 90(B4): p. 3126-3144. Ataollahi, F., et al., Endothelial cell responses in terms of adhesion, proliferation, and morphology to stiffness of polydimethylsiloxane elastomer substrates. Journal of Biomedical Materials Research - Part A, 2015. 103(7): p. 2203–2213. Lozano-Sánchez, L., et al., Practical microwave-induced hydrothermal synthesis of rectangular prism-like CaTiO3. CrystEngComm, 2013. 15(13): p. 2359-2362. Chuang, S.H., et al., Synthesis and Characterization of Ilmenite-Type Cobalt Titanate Powder. Journal of the Chinese Chemical Society, 2010. 57(4B): p. 932-937. Yazawa, Y., A. Yamaguchi, and H. Takeda, Lunar Minerals and Their Resource Utilization with Particular Reference to Solar Power Satellites and Potential Roles for Humic Substances for Lunar Agriculture, in Moon. 2012, Springer. p. 105-138. Bondarenka, V., et al., Thin films of poly-vanadium-molybdenum acid as starting materials for humidity sensors. Sensors and Actuators B: Chemical, 1995. 28(3): p. 227-231. Imran, Z., et al., Excellent humidity sensing properties of cadmium titanate nanofibers. Ceramics International, 2013. 39(1): p. 457-462. Su, P.-G., et al., Fully transparent and flexible humidity sensors fabricated by layer-by-layer self-assembly of thin film of poly (2-acrylamido-2-methylpropane sulfonate) and its salt complex. Sensors and Actuators B: Chemical, 2011. 153(1): p. 29-36. Su, P.-G. and C.-S. Wang, Novel flexible resistive-type humidity sensor. Sensors and Actuators B: Chemical, 2007. 123(2): p. 1071-1076. Su, P.-G. and C.-C. Shiu, Electrical and sensing properties of a flexible humidity sensor made of polyamidoamine dendrimer-Au nanoparticles. Sensors and Actuators B: Chemical, 2012. 165(1): p. 151-156. Su, P.-G. and C.-P. Wang, Flexible humidity sensor based on TiO 2 nanoparticles-polypyrrolepoly-[3-(methacrylamino) propyl] trimethyl ammonium chloride composite materials. Sensors and Actuators B: Chemical, 2008. 129(2): p. 538-543. Chen, Z. and C. Lu, Humidity sensors: a review of materials and mechanisms. Sensor letters, 2005. 3(4): p. 274-295. Anderson Jr, J.H. and G.A. Parks, Electrical conductivity of silica gel in the presence of adsorbed water. The Journal of Physical Chemistry, 1968. 72(10): p. 3662-3668.
PT
10.
26.
27.
A
28.
29.
30. 31.
33. 34. 35. 36. 37. 38.
A
CC E
PT
ED
M
A
N
U
SC R
39.
Ernsberger, F.M., The nonconformist ion. Journal of the American Ceramic Society, 1983. 66(11): p. 747-750. Kulwicki, B.M., Humidity sensors. Journal of the American Ceramic Society, 1991. 74. Yeh, Y. and T. Tseng, Analysis of the dc and ac properties of K 2 O-doped porous Ba 0.5 Sr 0.5 TiO 3 ceramic humidity sensor. Journal of Materials Science, 1989. 24(8): p. 2739-2745. Traversa, E., et al., Study of the conduction mechanism of La2CuO4—ZnO heterocontacts at different relative humidities. Sensors and Actuators B: Chemical, 1995. 25(1-3): p. 714-718. Traversa, E., et al., Ceramic thin films by sol-gel processing as novel materials for integrated humidity sensors. Sensors and Actuators B: Chemical, 1996. 31(1-2): p. 59-70. Feng, C.-D., et al., Humidity sensing properties of Nation and sol-gel derived SiO 2/Nafion composite thin films. Sensors and Actuators B: Chemical, 1997. 40(2): p. 217-222. Casalbore-Miceli, G., et al., Investigations on the ion transport mechanism in conducting polymer films. Solid State Ionics, 2000. 131(3): p. 311-321. Quartarone, E., et al., Investigations by impedance spectroscopy on the behaviour of poly (N, N-dimethylpropargylamine) as humidity sensor. Solid State Ionics, 2000. 136: p. 667-670.
IP T
32.
Figure caption
Fig. 10
Fig. 11
Fig. 12
Fig. 13 Fig. 14 Fig. 15
IP T
CC E
Fig. 16
SC R
Fig. 9
U
Fig. 7 Fig. 8
N
Fig. 6
A
Fig. 5
M
Fig. 4
ED
Fig. 3
Figure Heading Flow chart for the preparation of flexible Armalcolite/PDMS composite film. A schematic representation of the measurement system of Armalcolite/PDMS based humidity sensor. XRD (CuKα1, λ=1.54056 Å) patterns of: (a) Armalcolite nanocomposite; (b) Armalcolite/PDMS composite film. SEM images of the (a) Armalcolite nanocomposite; (b) Magnified image of Armalcolite nanocomposite; and (c) Armalcolite/PDMS composite film. FTIR spectra of the (a) Armalcolite nanocomposite; (b) Pristine Polydimethylsiloxane (PDMS) film, and (c) Armalcolite/PDMS composite film. Physical properties of PDMS, Armalcolite/PDMS, Armalcolite, (a) Open porosity; (b) Water absorption; (c) Density; (d) Water contact angle. Static tensile properties of PDMS and Armalcolite/PDMS composite film. Dependence of impedance on relative humidity for the sensor based on Armalcolite/PDMS nanocomposite measured at various frequencies at 1 V and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at 1 V, 1 kHz and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different applied voltages (0.2 V, 1.0 V, and 2.0 V) at 1 kHz and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different temperatures (15°C, 25°C and 35°C) at 1 V, 1 kHz. Response and recovery characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C for, (a) 1 cycle and (b) 7 cycles. Humidity hysteresis characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C. Impedance stability of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C. Schematic representation of the sensing mechanism of the Armalcolite/PDMS based nanocomposite humidity sensor. Complex impedance plots of Armalcolite/PDMS based sensor obtained from 102 Hz to 106 Hz, at 25°C.
PT
Figure No. Fig. 1 Fig. 2
Table caption
A
Table No. Table 1 Table 2 Table 3
Table Heading Humidity generation for different salts at room temperature. Comparison of the performance of Armalcolite-PDMS based sensor with other flexible humidity sensors. Equivalent circuit of CIP at various RH value
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-1
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-2
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-3
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-4
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-5
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-6
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-7
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-8
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-9
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-10
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-11
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-12
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-13
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-14
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-15
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-16
Table 1. Humidity generation for different salts at room temperature Humidity(%RH)
Temperature
MgCl2 Mg(NO3)2
33 55
25 °C 25 °C
NaCl KCl
75 85
25 °C 25 °C
KNO3
95
25 °C
A
CC E
PT
ED
M
A
N
U
SC R
IP T
Salt
Table 2. Comparison of the performance of armalcolite-PDMS based sensor with other flexible humidity sensors. Humidity range (% RH)
Sensitivity (log Z/%RH)
Response time (s)
Recovery time (s)
Hysteresis (% RH)
Reference
Armalcolite/PDMS
33-95
0.072
10
15
<1
PAMPS doped salts PMMA/PMAPTAC PAMAM-AuNPs TiO2 NPs/PPy/PMAPTAC
20–90 30–90 30–90
0.026 0.033 0.045
60 45 40
70 150 50
<8 <6 <2
Current work [26] [27] [28]
30–90
0.065
30
45
<2
[29]
A
CC E
PT
ED
M
A
N
U
SC R
IP T
Sensing Material
A ED
PT
CC E
IP T
SC R
U
N
A
M
S0925-4005(18)30231-4 https://doi.org/10.1016/j.snb.2018.01.207 SNB 24056
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
26-10-2017 19-1-2018 25-1-2018
Please cite this article as: Ashis Tripathy, Priyaranjan Sharma, Narayan Sahoo, Sumit Pramanik, Noor Azuan Abu Osman, Moisture sensitive inimitable Armalcolite/PDMS flexible sensor: A new entry, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.01.207 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Moisture sensitive inimitable Armalcolite/PDMS flexible sensor: A new entry Ashis Tripathy 1*, Priyaranjan Sharma 2, Narayan Sahoo 3, Sumit Pramanik 4, Noor Azuan Abu Osman 5
2 3 4 5
Department of Electronics and Communication Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Mechanical Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Electrical Engineering, IET, JK Lakshmipat University, Jaipur, Rajasthan, India. Department of Mechanical Engineering, Faculty of Engineering, SRM University, Tamil Nadu, India. Centre for Applied Biomechanics, Department of Biomedical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.
IP T
1
Highlights
A rare lunar earth material, Armalcolite was synthesized first time in the laboratory environment. Armalcolite/PDMS nanocomposite based flexible humidity sensor was developed for biomedical and flexible electronics application. Newly developed flexible humidity sensor shows excellent linearity, extremely low hysteresis, high sensitivity and, faster response and recovery time.
CC E
PT
ED
M
A
N
U
SC R
Graphical Abstract
A
*Corresponding author, Tel.: +91-8763578440; E-mail address: [email protected] (A.Tripathy)
Abstract Armalcolite, a pioneering material which is commonly available on the moon surface, was synthesized first time in the laboratory environment using innovative solid-state-step sintering process. Afterwards, Armalcolite/Polydimethylsiloxane (PDMS) nanocomposite was prepared using ball milling technique and then spin coated on interdigitated customized gold (Au) electrode on a polyimide substrate in order to develop a high sensitive and fast-response
1.
IP T
flexible humidity sensor. To evaluate the performance of developed flexible humidity sensor, the electrical characterization was performed at room temperature of 25°C in a humidity environment of 33−95% RH using alternating current. The developed sensor exhibits an exceptional performance in terms of high sensitivity, good linearity, negligible hysteresis (<1%), better response and recovery time of 10 s and 15 s respectively, which is outstanding when compared with existing ones within the same category. The long-term stability of flexible sensor was confirmed upon when it was put into test for 30 consecutive days. Due to its grater flexibility, low hysteresis, high stability and the fast response/recovery time, it is intended to use for various application in modern industry, agriculture, and medical care where humidity plays a crucial role. Keywords: Armalcolite; nanocomposite; flexible film; relative humidity; porosity; hysteresis. Introduction
M
A
N
U
SC R
Humidity plays an important role round the year in deciding our comfort level including preserving food crops, controlling industrial processes and health care application [1]. Thus it’s a big challenge for the entire research community to develop a suitable humidity sensor. For the last decade, oxide based ceramic such as MgO [2, 3], iron oxide (Fe2O3) [4], zinc oxide (ZnO) [5], aluminum oxide (Al2O3) [6], titanium oxide (TiO2) [7], tin oxide (SnO2) [8] played a key role in the development of humidity sensor. High thermal, chemical, and mechanical stability [9, 10] of the ceramic nanocomposite make them suitable for humidity sensing application, however inherent brittleness and the undesired morphological structures creates hindrance for various sensing applications. Apart from these concerns, inhomogeneous dispersal of pores and inadequate porosity supplements to the misery which reduces the hydrophobicity and water absorption capability leads and amplify the issue of brittleness. Hence, it reduces the sensing properties of humidity sensor by reducing the electrical conductivity between the sensing electrodes.
PT
ED
To overcome the above stated issues of ceramic nanocomposite, a novel Armalcolite nanocomposite was synthesized for the humidity sensor application. This unique material has never been successfully synthesized in a laboratory environment till date. The hidden potential of Armalcolite nanocomposite has never been explored for humidity sensing application even after many decades of its discovery on moon.
A
CC E
The wonder material Armalcolite was processed by using inexpensive in situ step sintering technique to control the morphology, porosity and crystalline phases. As most of the ceramics and their composites have congenital brittleness. To overcome this physical abnormality, elastomers and polymers are added to enhance their flexibility [11-13]. However, controlling the aspect of hydrophilicity in such flexible composite materials is a great challenge for researchers. There is a drought of study on the flexibility of composites [11, 14]. Therefore, there is lots of scope for the development of flexible humidity sensor for various industrial and biomedical applications. In the current study, an extremely sensitive and flexible humidity sensor having excellent linearity, low hysteresis, rapid response and recovery time was successfully developed. The developed flexible sensor consists of Armalcolite/PDMS composite as humidity sensing layer on interdigitated gold electrode followed by Polymide (PI) substrate. Due to inherent flexibility, nontoxicity and biocompatibility of PDMS, metal oxide deposition and patterning on PDMS are gaining more and more interest of researchers. Moreover, PDMS based
nanocomposites are more stretchable and most suited for biomedical applications due to their easy skin attachment and implant into human body. The incorporated innovative ideas presented in this article paves path for designing an ultrasensitive flexible humidity sensor and also creates a new direction of its application in the field of medical science. 2.
Experimental
2.1
Preparation of Armalcolite nanocomposite
ED
M
A
N
U
SC R
IP T
To prepare the Armalcolite nanocomposites, calcium oxide (CaO), magnesium carbonate (MgCO3), hematite (Fe2O3), and titania (TiO2) powders of 99.9% purity were procured from ‘Fisher Scientific Ltd., Selangor, Malaysia’ [15]. Solid state step-sintering technique was used for synthesis of this wonder material Armalcolite nanocomposite having empirical formula CaMgFe1.33Ti3O12. The Almalcolite nanocomposite was synthesised under following stages, In the first stage all the raw powders were poured into a ball milling machine (PM200, Retsch, Düsseldorf, Germany) with alumina balls of 10 mm diameter then, 70% ethyl alcohol was added to the mixture. The mixing process was carried out at 300 rpm for a time duration of 3 days under room temperature. In the second stage, resultant slurry was kept for 6 h in an oven (OF-11E, Lab Companion) at 120°C to eliminate the moisture completely. In the third stage, the dry composite powder was transformed into pallets of diameter of 10 mm and thickness of 2.75 mm using uniaxial hydraulic press (GS15011, Graseby Specac, Kent, UK). Finally, the developed pallets were solid state step-sintered at a temperature of 1050°C in a programmable furnace under following steps: (i) 350°C for 60 min at ramp rate of 5°C/min, (ii) 550°C for 210 min at ramp rate of 10°C/min, and (iii) 1050°C for 78 min at heating rate of 15°C/min, and then cooling at (iv) 750°C for 180 min at heat rate of 20°C/min and (v) furnace cooling. The solid-state step-sintering has played an important role to minimize the particle size by increasing grain boundary with optimum porosity. A whole chemical reaction of this material is presented in Eq. (1).
2.2
PT
4Fe2O33 4TiO2 CaO MgCO 0.5O2 Fe2 MgTi3O10 CaTiO3 2Fe3O4 O2 CO2
(1)
Synthesis of Armalcolite/PDMS Based Composite Flexible Film
A
CC E
The Armalcolite/PDMS flexible composite film was prepared under following stages [15]. In first stage, sintered pallets were re-pulverized into fine powder to mix with PDMS-gel (Sylgrade184 Silicon Elastomer Base, Dow Corning) in an optimized ratio. The mixing was done in a ball-mill machine for 240 min at 300 rpm by using alumina balls. To enhance the cross-linking in PDMS chain, a curing agent (Dow Corning, curing agent: PDMS of 3:25 (w/w)) was added to Armalcolite/PDMS mixture. The ceramic-polymer composite slurry was then spin coated to prepare a thin film on a glass petri-dish, subsequently the micro bubbles present in the thin film was removed by using self-drying vacuum pump (PM200, Memmert, Camberley, UK). The final form of the flexible film was obtained after heating it for 300 min at 60°C in an oven (OF-11E, Lab Companion). Moreover, a schematic illustration of synthesis procedure of Armalcolite nanocomposite and its flexible film was shown in the Fig.1.
IP T SC R
Fig. 1. Flow chart for the preparation of flexible Armalcolite/PDMS composite film.
2.3
Characterizations
ED
M
A
N
U
The XRD peaks were analysed with the help of X-ray diffractometer (Empyrean, PAN analytical, Cu-Kα radiation, = 1.54056 Å). SEM (AURIGA, Carl Zeiss) was used for morphological study of thin film. Archimedes’ principle [16, 17] was used to obtain density, open porosity and water absorption for Armalcolite and its PDMS based composite film. Sessile contact angle meter (OCA15E, Data Physics Instruments GmbH, Filderstadt, Germany) was used for measuring water contact angle. Fourier transforms infrared (FTIR) spectroscopy was recorded by using Perkin-Elmer spectrometer (400, Perkin Elmer,Waltham, UK). Finally, universal testing machine (5848, Instron Micro Tester) was used for mechanical flexibility study. 2.4 Flexible sensor fabrication and measurements
CC E
PT
The flexible humidity sensor was fabricated using spin coating method by depositing the Armalcolite/PDMS cured gel on interdigited gold electrode followed by polymide (PI) substrate. The developed sensor was kept inside a vacuum oven at 90°C for 60 min and flexible film of thickness of 0.7 mm was obtained. The stability and durability of the fabricated flexible sensor was improved by maintaining the maximum relative humidity of 95% at supplied voltage and frequency of 1 V and 100 Hz respectively for 24 h.
A
Impedance spectroscope (3532-50 LCR Hi tester, Hioki) was used for electrical characterization of humidity sensors. For this aim, frequency range of 102-106 Hz and AC voltage of 1 V were used. The sensor response was measured at 25°C in a temperature controlled chamber (Memmet, Naluri Scientific, Schwabach, Germany) having resolution of 5°C. Different humidity environment were produced using saturated salt solution such as magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2), sodium chloride (NaCl), potassium chloride (KCl) and potassium nitrate (KNO3) within a resolution of ±1% RH at 25°C [18]. The humidity level of various salt solutions was shown in Table 1. A schematic illustration of developed flexible humidity sensor, corresponding measurement technique and humidity sensing equipment were depicted in Fig. 2.
Table 1. Humidity generation for different salts at room temperature. Salt Humidity (% RH) MgCl2 33 Mg(NO3)2 55
Temperature 25°C 25°C
75 85
25°C 25°C
KNO3
95
25°C
ED
M
A
N
U
SC R
IP T
NaCl KCl
PT
Fig. 2. A schematic representation of the measurement system of Armalcolite/PDMS based humidity sensor.
3 Results and discussion
CC E
3.1 Structural and morphological and mechanical analysis
A
The XRD patterns of Armalcolite nanocomposite and Armalcolite/PDMS based film has been shown in Fig. 3(a) and 3(b) respectively. As a result of sintering of raw material (CaMgFe1.33Ti3O12) at 1050°C, two new phases were detected, which are matching with the standard XRD pattern of orthorhombic Armalcolite (Fe2MgTi3O10, 101) and perovskite (CaTiO3, 440) as shown in Fig. 3(a). Moreover, change in peak intensity was also detected for CaTiO3 (440) and Fe2MgTi3O10 (101) which exhibit different crystal sizes of 5.9 nm and 20.7 nm respectively. Moreover, the prominence of these two phases was also confirmed by scanning electron micrograph as shown in Fig. 4(a). Remarkably, another peak of Fe3O4 (311) was detected at 2θ = 35.72° which clearly indicates the conversion of Fe3+ into Fe2+. At the position of 2θ = 12°, a wide peak of PDMS was observed which indicate the semi-crystalline nature as shown in Fig. 3(b). Additionally, peaks of Fe2MgTi3O10 and CaTiO3 were also
N
U
SC R
IP T
detected along with PDMS peak which indicate the homogeneous nature of composite. A peak shift was also observed in Fig. 3(a-b) where Fe2O3 is transformed into Fe3O4 and could be beneficial for remote sensing application [19].
A
Fig. 3. XRD (CuKα1, λ=1.54056 Å) patterns of, (a) Armalcolite nanocomposite; (b) Armalcolite/PDMS composite film.
A
CC E
PT
ED
M
The morphology of Armalcolite and flexible film has been examined with SEM as shown in Fig. 4(a-c) where yellow circle indicate pores; blue arrow indicate Armalcolite phase; red arrow indicate peroskite phase; yellow arrow indicate particles. In Fig. 4(b), pink arrow indicate grain and white arrow indicates grain boundary. An average particle size and pore size of the Armalcolite nanocomposite were found to be 630 nm and 850 nm, respectively. Different sizes of particle was observed in microscopic investigation of Armacolite nanocomposite which indicates the formation of two distinct phases (Fe2MgTi3O10 and CaTiO3) having average grain size between 100 nm to 685 nm. From the SEM analysis, it was found that larger size particle typically belongs to Armalcolite phase (Fe2MgTi3O10) however smaller size particle belongs to perovskite phase (CaTiO3) and also confirmed through XRD analysis. From the nanoscopic investigation of Armalcolite nanocomposite as shown in Fig. 4(b) clearly indicates the existence of grain and grain boundaries. Fig. 4(c) indicates the microscopic image of Armalcolite/PDMS flexible film which indicates the uniform distribution of Armalcolite particles in PDMS. Moreover, the developed flexible film exhibits the better bonding at interface of Armalcolite particle and polymer matrix which was also confirmed through FTIR analysis as shown in Fig. 5(c).
IP T SC R U N A M ED
Fig. 4. SEM images of the (a) Armalcolite nanocomposite; (b) Magnified image of Armalcolite nanocomposite; and (c) Armalcolite/PDMS composite film.
A
CC E
PT
Once Armalcolite nanocomposite was developed, the formation of new bond was examined using FTIR spectra as shown in Fig. 5(a-c). From the FTIR analysis that it could be identified that all FTIR peaks of PDMS polymer such as Si–O–Si (650-720 cm-1), Si–C (602 cm-1) and Si–CH3 (842 cm-1) were also present in the Armalcolite/PDMS composites. Moreover, FTIR peaks of the Armalcolite ceramic such as Ca–O–Ti (470 cm-1) and Ca–O–Ca (754 cm-1) were related to titanate vibration and stretching vibration respectively [20, 21]. Remarkably, three new peaks such as C–Si–O (600 cm-1) from Armalcolite/PDMS, Ca–O(Si) (754 cm-1)) and Ca– O (910 cm-1) from PDMS and titanate reaction were also perceived in Armalcolite/PDMS nanocomposite.
IP T SC R U N A M
ED
Fig. 5. FTIR spectra of the (a) Armalcolite nanocomposite; (b) Pristine Polydimethylsiloxane (PDMS) film, and (c) Armalcolite/PDMS composite film.
A
CC E
PT
In the Fig. 6(a-d), a comparison of physical properites (i.e., water contact angle, water absoption, density and open porosity) of PDMS, Armalcolite/PDMS and Armalcolite were presented. In this study, open porosity of materials was determined by measuring the bulk density. The experimental investigation revealed that Armalcolite nano composite has lower density in comparison to raw materials which confirms the high level of porosity within Armalcolite nano composite. The porosity of material can also be confirmed by calculating the amount of water absorbed by the material. High water absorption capability (~68%) of Armalcolite nano composite confirms the uniform porosity throughout the materials which has already been confirmed in microscopic investigation (Fig. 4). In Armalcolite/PDMS composite film, low water contact angle (WCA) is desirable as it is mainly responsible for changing the nature of PDMS from hydrophobic (WCA=107°) to hydrophilic (WCA=88.1°) [20]. This hydrophilic nature of Armalcolite/PDMS composite film make it suitable for various remote control humidity sensing applications such as remote sensing of soil moisture, remote sensing of structural health monitoring to avoid corrosion, monitoring of human health by measuring sweating and breathing etc. [13, 20, 22, 23].
IP T SC R U
N
Fig. 6. Physical properties of PDMS, Armalcolite/PDMS, Armalcolite, (a) Open porosity; (b) Water absorption; (c) Density; (d) Water contact angle.
A
CC E
PT
ED
M
A
The comparison of Armalcolite/PDMS composite film and PDMS film was presented in Fig. 7 which revealed that Armalcolite/PDMS film has higher flexiblity compared to PDMS film. In order to confirm the flexibility, tensile test was conduted up to 90% of the elongation. The stress-strain relationship is the ratio of stress and strain which also called as Young’s modulus (E). In the current study, Young’s modulus was calculated from the highlighted portion of stress-strain curve as shown in Fig. 7. The stress-strain relationship (E = Stress/Strain) revealed that Armalcolite/PDMS composite film has the Young’s modulus of 0.80 0.21 MPa, which is considerably higher than that of PDMS film of 0.27 0.08 MPa. Moreover, both the film has indicated the elongation of 90%. The experimental results has confirmed that Armalcolite/PDMS composite films has more flexiblity compared to PDMS film. Further, it was also confirmed by the modulus value which indicated that Armalcolite/PDMS composite films has lower modulus value than the other PDMS composites leading to the grater flexibility [20].
IP T SC R
Fig. 7. Static tensile properties of PDMS and Armalcolite/PDMS composite film.
U
3.2 Humidity sensing properties
Δ Impedance Δ % RH
PT
Sensitivity S
ED
M
A
N
In order to determine the optimum value of impedance, humidity and frequency for developed humidity sensor, the impedance as a function of relative humidity was measured at different frequency under room temperature. From the Fig. 8, it can be clearly observed that the impedance value of humidity sensor reduces significantly with increase in frequency at low relative humidity. Moreover, the difference in impedance value become gradually smaller at two consecutive frequency while increasing the relative humidity. This behaviour could be explained by the fact that at high frequency, the water absorbed by the thin film can’t be polarized and hence, dielectric phenomenon won’t occur [24]. Further, the sensitivity of Armalcolite/PDMS humidity sensor was determined using following expression [25]. (1)
A
CC E
The sensitivity in the range of 33–95% RH at 102, 103, 104, 105, and 106 Hz is found to be 0.072, 0.066, 0.049, 0.037 and 0.025 respectively.
IP T SC R
Fig. 8. Dependence of impedance on relative humidity for the sensor based on Armalcolite/PDMS nanocomposite measured at various frequencies at 1 V and 25°C.
A
CC E
PT
ED
M
A
N
U
In order to measure the linearity index of humidity sensor, log-impedance as a function of relative humidity was measured at frequency of 103 Hz and at temperature of 25°C as shown in Fig. 9. The experimental investigation of plotted data revealed that there is significant linearity (Y = −0.0633X + 9.196; R2 = 0.9994) between log impedance and relative humidity within the 33–95% RH. Further, high RH sensitivity and better linearity could be obtained by applying a low operating frequency within the range of 102 to 103 Hz. For extremely high RH sensitivity and better linear response, 103 Hz could be selected as optimized operating frequency in the current experimental setup.
Fig. 9. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at 1 V, 1 kHz and 25°C.
The voltage dependency of developed flexible sensor was shown in Fig. 10 where impedance was measured In order to understand the voltage dependency of the developed flexible sensor, the impedances of the sensor are measured at different voltages (i.e., 0.2 V, 1 V and 2 V). From the analysis of Fig. 10, it was obtained that there is slight deviation in impedances– relative humidity sensing for the lower operating voltages of less than 1 V. However, no significant
SC R
IP T
deviation in humidity sensing was detected at operating voltage of 1-2 V within the humidity range of 33-95%. Further, better linearity, good stability and high sensitivity, AC voltage of 1 V and frequency of 103 Hz were designated as optimum operating parameters.
U
Fig. 10. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different applied voltages (0.2 V, 1.0 V, and 2.0 V) at 1 kHz and 25°C.
A
CC E
PT
ED
M
A
N
The temperature dependency of developed flexible humidity sensor was investigated by measuring the sensor impedances at different temperature ranges starting from 15°C to 55°C as shown in Fig. 11. The result indicates that there is a negligible variation in impedance with respect to temperature at lower humidity range of 33-55% RH. This behavior could be explained by fact that at lower humidity range, temperature variation is not capable enough to ionize the water molecule. Further, the sensor impedances get decrease minutely at higher humidity (75-95% RH). This is occurred due to improved ionization phenomena of water molecules. The experimental results indicate that humidity sensing characteristics is slightly influenced by temperature which is quite considerable.
Fig. 11. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different temperatures (15°C, 25°C and 35°C) at 1 V, 1 kHz.
The performance of humidity sensors can be evaluated by measuring the response and recovery characteristics. In Fig. 12(a), adsorption and desorption of water molecules for developed
M
A
N
U
SC R
IP T
humidity sensor was measured by analyzing the response and recovery curves for one cycle. Form the Fig. 12(a), it was observed that when sensor exposed from lower humidity (33% RH) to higher humidity (95% RH), the sensor impedance decreases rapidly to end with a constant value. Afterward, when the sensor was exposed from higher humidity (95% RH) to lower humidity (33% RH), the sensor impedance increases abruptly to end with a stable value. Basically, response and recovery time is defined as the time required for the sensor to achieve 90% of the final value corresponding to adsorption and desorption process as shown in Fig. 12(a). Within the humidity range of 33-95% RH, response time and recovery time were found to be 10 s and 15 s respectively for developed humidity sensor which is comparatively faster than the other reported sensor results as presented in Table 2. Further, response and recovery curve for seven cycle has been illustrated in Fig. 12(b). Even though there is a slight fluctuation in impedance response however it indicates the good reproducibility of the developed flexible sensor. The faster response, recovery time and good reproducibility characteristics of developed humidity sensor proved that it is best suited for high performance humidity sensing application.
ED
Fig. 12. Response and recovery characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C for, (a) 1 cycle and (b) 7 cycles (Note: Response time was measured by varying the relative humidity from 33% RH to 95% RH at the time of adsorption whereas recovery time was measured by varying relative humidity form 95% RH to 33% RH at the time of desorption).
A
CC E
PT
The humidity hysteresis is another important figure of merit of a humidity sensor. The humidity hysteresis is defined as the maximum difference between the humidification and desiccation curve. It was measured by switching the sensor between the chambers of 33%, 55%, 75%, 85% and 95% RH, and then transferring back. The impedance of desorption process is slightly lower than that of adsorption process. According to the adsorption and desorption processes of the adsorbed water in the humid membrane are corresponding to the exothermic and endothermic processes, and the different reaction speeds at the same RH lead to the impedance on desorption process slightly lower than that on adsorption process. The humidity hysteresis characteristics of Armalcolite/PDMS nanocomposite based sensor are shown in Fig. 13. It is found that the maximum hysteresis is < 1% RH at the range 11–95% RH, and it indicates a good reliability of the humidity sensor.
IP T SC R
Fig. 13. Humidity hysteresis characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C.
A
CC E
PT
ED
M
A
N
U
In order to examine the stability of flexible sensor, it was kept in open air environment at room temperature for one month. Then, impedance was measured at interval of 3 days at different relative humidity environment. During the measurement, AC voltage (1 V) and frequency (f = 103 Hz) were kept constant as shown in Fig. 14. The experimental results of sensor revealed that there is a negligible change in the impedance response which was continuously monitored in a time span of 30 days and hence, confirms the long term stability of developed humidity sensor. Based on the above mentioned points, it can be state that the sensor has excellent stability and quite suitable for various humidity sensing application.
Fig. 14. Impedance stability of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C.
Table 2. Comparison of the performance of Armalcolite-PDMS based sensor with other flexible humidity sensors. Sensing Material
Humidity range (% RH)
Sensitivity (log Z/%RH)
Response time (s)
Recovery time (s)
Hysteresis (% RH)
Reference
Armalcolite/PDMS
33-95
0.072
10
15
<1
PAMPS doped salts PMMA/PMAPTAC PAMAM-AuNPs TiO2 NPs/PPy/PMAPTAC
20–90 30–90 30–90
0.026 0.033 0.045
60 45 40
70 150 50
<8 <6 <2
Current work [26] [27] [28]
30–90
0.065
30
45
<2
[29]
3.3 Discussion on sensing principles
M
A
N
U
SC R
IP T
In this section, humidity sensing principle of Armalcolite/PDMS nanocomposite materials was demonstrated. The mechanism of improved conductivity of Armalcolite/PDMS nanocomposite along with relative humidity could be explained by the Grotthuss phenomena, which indicate that this behaviour is almost similar to the tunnelling of proton from one water molecule to another [30]. There are two different phenomena of water absorption depending upon the humidity level. At lower humidity, chemisorption phenomena occurs which is based on chemical adsorption of water molecules on the activated side of Armalcolite/PDMS nanocomposite. During chemisorption process, two hydroxyls group will be bonded with one water molecule via dissociative phenomena which means that chemisorption already initiated at 33% RH. At lower humidity condition, the current conduction is mainly occurs due to the hopping of H+ ion on the surface of Armalcolite/PDMS nanocomposite. Due to the formation of double hydrogen bonding at low humidity conditions, water molecules can’t move freely. Thus, it requires more energy for hoping of H+ ion in their adjacent hydroxyl group which increases the value of sensor impedance. Further, in Fig. 16(a-b), a semicircle was detected at low humidity condition which is mainly responsible due to intrinsic impedance of the material. Since, vapour pressure is low at this stage and intrinsic electrons in the material are actively participating in conduction. Therefore, it could be confirmed that at low humidity condition (33-55% RH), electrolytic conduction is mainly responsible for current conduction.
A
CC E
PT
ED
As the humidity goes on increase, chemisorption phenomena shifted to physisorption where water molecules are physically absorbed. Basically, Physisorption is a multilayer phenomenon in which first layer of physically adsorbed water is due to double hydrogen bonding which makes the layer immobile. From the succeeding physisorbed layers, water molecules are absorbed physically through single hydrogen bond as a result the water molecules become mobile and thus H+ ion would be available for current conduction as demonstrated in Fig. 15. Then, more number of physisorbed layers are formed when humidity increases and thus, excessive number of protons would be available for current conduction. Then, the cations (Mg2+,Ti2+,Ca2+,Fe2+) of the Armalcolite/PDMS nanocomposite are electrostatically attached to the negatively charged oxygen because of polar nature of water molecule. The external relative-humidity environment also affects the physisorption phenomena. With increase in relative humidity, continuous layer of water molecules will be formed where H+ ion can move freely. The physisorbed water molecules (2H2O) were dissociated into hydronium (H3O+) and OH− ions. According to Grotthuss chain reaction [31, 32], when H3O+ releases a proton to adjacent water molecules, proton transport take place. Due to further increase in relativehumidity, electrolytic conduction [33] will be more prominent due to excessive proton transfer. Further, in Fig. 16(c-e), it could be seen that a straight line appears after semicircle at higher humidity range. This straight line formation is mainly responsible due to the conduction of
U
SC R
IP T
ions. Therefore, it could be confirmed that at high humidity condition (75-95% RH), ionic conduction is mainly responsible for current conduction.
A
N
Fig. 15. Schematic representation of the sensing mechanism of the Armalcolite/PDMS based nanocomposite humidity sensor.
A
CC E
PT
ED
M
Further, the humidity sensing mechanism of Armalcolite/PDMS based nano composite thin film was investigated using complex impedance plots (CIP) having frequency range between 102-106 Hz at different RH value. In Fig. 16(a-e), Z' and –Z'' represents the real and imaginary parts of the CIP respectively. At low RH (33%, and 55%), a formation of semicircle was detected which confirms a “non-Debye” behavior. Earlier investigations have reported that it is due to the polarization effect which can be modeled using an equivalent circuit of parallel capacitor (Cf) and resistor (Rf) [34-36]. At higher RH (75%, 85%, and 95%), the radius of semicircle get decreases and a line formation was detected in the low-frequency range. With further increase in RH value, the line becomes longer whereas semicircle becomes smaller. The formation of line is mainly due to the diffusion phenomena at the electrodes which represent ‘Warburg impedance’ [9, 37-39]. The equivalent circuits of CIP have been demonstrated in Table 3 where Rf and Cf are resistance and capacitance of the Armalcolite/PDMS film respectively. The impedance at the electrode/sensing film interface was denoted by Zi. According to the CIP, at low RH value (Fig. 16(a-b)), the change in impedance of the sensor is mainly due to the Rf because at low RH, Rf << Zi. However, at high RH (Fig. 16(c-e)), impedance variation of the sensor is evaluated by Rf and Zi because both Rf and Zi have the same magnitude.
IP T SC R U N A M
ED
Fig. 16. Complex impedance plots of Armalcolite/PDMS based sensor obtained from 10 2 Hz to 106 Hz, at 25°C.
A
CC E
PT
Table 3. Equivalent circuit of CIP at various RH value
4
Conclusions
In the resent research work, a novel Armalcolite/PDMS based flexible resistive humidity sensor was fabricated and designed. With the help of solid-state step-sintering technique, different morphology of the Armalcolite and perovskite phases were obtained, and confirmed by XRD and SEM analysis. The newly developed flexible sensor has indicated the excellent impedance sensing characteristics through the physisorption mechanism. From the complex impedance
SC R
IP T
response analysis, two distinct water absorption mechanism (i.e., at lower humidity range of 33-55% RH and at higher humidity range of ≥ 75% RH) were observed however critical relative humidity was observed at 75% RH. The impedance of the flexible sensor was decreased from 1.23×107 ohm to 1.5×103 ohm in a humidity range of 33−95% RH at the frequency of 103 Hz. Moreover, it has indicated a greater sensitivity, a weak dependence on temperature, rapid response time of 10 s and fast recovery time of 15 s, and extremely low hysteresis (< 1%) which is far better than the result reported in previous literature based on flexible resistive humidity sensor. All improved sensing features along with improved linearity and high stability of the Armalcolite/PDMS based humidity sensor confirm that it could be an effective and efficient flexible humidity sensor for advanced electronics and biomedical applications.
M
A
N
U
Dr. Ashis Tripathy is an Assistant Professor in the Department of in Electronics and Communication Engineering, in JK Lakshmipat University, Jaipur, India. He has obtained his Ph.D. degree in Biomedical Engineering from University of Malaya, Kuala Lumpur, Malaysia. His research interest is Fabrications and characterizations of Micro/Nano electronics devices, Sensor Design, Flexible electronics, advanced ceramics nano-composite synthesis and characterization, thin film characterization and Bio-medical application.
A
CC E
PT
ED
Dr. Priyaranjan Sharma is an Assistant Professor in the Department of Mechanical Engineering at JK Lakshmipat University, Jaipur, India. He did his Ph.D in Advanced Manufacturing from National Institute of Technology Karnataka (NITK), Surathkal. His area of interest is materials and manufacturing, conventional/non-conventional machining, manufacturing of micro/nano components for MEMS, aerospace and bio-medical application.
Mr. Narayan Sahoo is an Assistant Professor in the Department of Electrical Engineering at JK Lakshmipat University, Jaipur, India. He has obtained his MTech degree in Electrical Engineering from National Institute of Technology, Silchar, India. His area of research is sensor technology, control and automation, material synthesis and characterization.
IP T
Dr. Sumit Pramanik is a Research Associate Professor in SRM University, Chennai, India. He has obtained his Ph.D and M.Tech degree in Materials Science, Indian Institute of Technology (IIT) Kanpur, India. His area of interest is Materials science & Biomedical engineering: Bioceramics, Tissue engineering scaffolds, Polymers, Biodegradable polymers, Carbon nanostructures, Nanotechnology, Advanced ceramics, Porous ceramic catalyst, Surface coating, Tribology, Sensors, BioMEMS/NEMS, Functional graded material (FGM), Advanced polymers and composites.
SC R
Prof. Noor Azuan Bin Abu Osman is a Professor in Department of Biomedical Engineering, Dean of Engineering and also the Deputy Director of Centre for Applied Biomechanics, Faculty of Engineering, University of Malaya, Malaysia. He has graduated from University of Bradford, UK with a B.Eng. Hons. in Mechanical Engineering, followed by MSc. and Ph.D. in Bioengineering from University of Strathclyde, UK. His area of interest is measurements of human movement, development of instrumentation for forces and joint motion, and the design of prosthetics and orthopaedic
U
implants.
A
N
Conflicts of Interest: The authors declare no conflict of interest.
M
Reference
ED
Acknowledgment:;1;;1; This study was supported by UM/MOHE/HIR, University of Malaya, project No.: D000014-16001.
2.
Li, M., et al., Humidity sensitive properties of pure and Mg-doped CaCu 3 Ti 4 O 12. Sensors and Actuators B: Chemical, 2010. 147(2): p. 447-452. Wang, R., et al., The humidity-sensitive property of MgO-SBA-15 composites in one-pot synthesis. Sensors and Actuators B: Chemical, 2010. 145(1): p. 386-393. Adhyapak, P.V., et al., Influence of Li doping on the humidity response of maghemite (γ-Fe 2 O 3) nanopowders synthesized at room temperature. Ceramics International, 2013. 39(7): p. 8153-8158. Gu, L., et al., Humidity sensors based on ZnO/TiO 2 core/shell nanorod arrays with enhanced sensitivity. Sensors and Actuators B: Chemical, 2011. 159(1): p. 1-7. Tai, W.-P., J.-G. Kim, and J.-H. Oh, Humidity sensitive properties of nanostructured Al-doped ZnO: TiO 2 thin films. Sensors and Actuators B: Chemical, 2003. 96(3): p. 477-481. Qi, Q., et al., Influence of crystallographic structure on the humidity sensing properties of KCldoped TiO 2 nanofibers. Sensors and Actuators B: Chemical, 2009. 139(2): p. 611-617. Tai, W.-P. and J.-H. Oh, Preparation and humidity sensing behaviors of nanocrystalline SnO 2/TiO 2 bilayered films. Thin Solid Films, 2002. 422(1): p. 220-224. Kulwicki, B.M., Humidity sensors. Journal of the American Ceramic Society, 1991. 74(4): p. 697-708.
CC E
3.
PT
1. Song, X., et al., A humidity sensor based on KCl-doped SnO 2 nanofibers. Sensors and Actuators B: Chemical, 2009. 138(1): p. 368-373.
4.
A
5. 6. 7. 8. 9.
17. 18. 19.
20.
21. 22. 23.
24.
IP T
CC E
25.
SC R
16.
U
15.
N
14.
A
13.
M
12.
ED
11.
Traversa, E., Ceramic sensors for humidity detection: the state-of-the-art and future developments. Sensors and Actuators B: Chemical, 1995. 23(2): p. 135-156. Su, P.G. and C.P. Wang, Flexible humidity sensor based on TiO2 nanoparticles-polypyrrolepoly-[3-(methacrylamino) propyl] trimethyl ammonium chloride composite materials. Sensors and Actuators B: Chemical, 2008. 129(2): p. 538-543. Kar, K.K. and S. Pramanik, Hydroxyapatite poly (etheretherketone) nanocomposites and method of manufacturing same. 2014, Google Patents. Tripathy, A., et al., Role of Morphological Structure, Doping, and Coating of Different Materials in the Sensing Characteristics of Humidity Sensors. Sensors (Basel, Switzerland), 2014. 14(9): p. 16343. Nayak, S., et al., Development of polyurethane–titania nanocomposites as dielectric and piezoelectric material. RSC Advances, 2013. 3(8): p. 2620-2631. Tripathy, A., et al., Synthesis and characterizations of novel Ca-Mg-Ti-Fe-oxides based ceramic nanocrystals and flexible film of polydimethylsiloxane composite with improved mechanical and dielectric properties for sensors. Sensors, 2016. 16(3): p. 292. Pramanik, S., B. Pingguan-Murphy, and N.A. Abu Osman, Developments of immobilized surface modified piezoelectric crystal biosensors for advanced applications. Int. J. Electrochem. Sci, 2013. 8: p. 8863-8892. Pramanik, S., et al., Design and development of potential tissue engineering scaffolds from structurally different longitudinal parts of a bovine-femur. Scientific reports, 2014. 4. Greenspan, L., Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards, 1977. 81(1): p. 89-96. Morris, R.V., et al., Spectral and other physicochemical properties of submicron powders of hematite (α‐ Fe2O3), maghemite (γ‐ Fe2O3), magnetite (Fe3O4), goethite (α‐ FeOOH), and lepidocrocite (γ‐ FeOOH). Journal of Geophysical Research: Solid Earth, 1985. 90(B4): p. 3126-3144. Ataollahi, F., et al., Endothelial cell responses in terms of adhesion, proliferation, and morphology to stiffness of polydimethylsiloxane elastomer substrates. Journal of Biomedical Materials Research - Part A, 2015. 103(7): p. 2203–2213. Lozano-Sánchez, L., et al., Practical microwave-induced hydrothermal synthesis of rectangular prism-like CaTiO3. CrystEngComm, 2013. 15(13): p. 2359-2362. Chuang, S.H., et al., Synthesis and Characterization of Ilmenite-Type Cobalt Titanate Powder. Journal of the Chinese Chemical Society, 2010. 57(4B): p. 932-937. Yazawa, Y., A. Yamaguchi, and H. Takeda, Lunar Minerals and Their Resource Utilization with Particular Reference to Solar Power Satellites and Potential Roles for Humic Substances for Lunar Agriculture, in Moon. 2012, Springer. p. 105-138. Bondarenka, V., et al., Thin films of poly-vanadium-molybdenum acid as starting materials for humidity sensors. Sensors and Actuators B: Chemical, 1995. 28(3): p. 227-231. Imran, Z., et al., Excellent humidity sensing properties of cadmium titanate nanofibers. Ceramics International, 2013. 39(1): p. 457-462. Su, P.-G., et al., Fully transparent and flexible humidity sensors fabricated by layer-by-layer self-assembly of thin film of poly (2-acrylamido-2-methylpropane sulfonate) and its salt complex. Sensors and Actuators B: Chemical, 2011. 153(1): p. 29-36. Su, P.-G. and C.-S. Wang, Novel flexible resistive-type humidity sensor. Sensors and Actuators B: Chemical, 2007. 123(2): p. 1071-1076. Su, P.-G. and C.-C. Shiu, Electrical and sensing properties of a flexible humidity sensor made of polyamidoamine dendrimer-Au nanoparticles. Sensors and Actuators B: Chemical, 2012. 165(1): p. 151-156. Su, P.-G. and C.-P. Wang, Flexible humidity sensor based on TiO 2 nanoparticles-polypyrrolepoly-[3-(methacrylamino) propyl] trimethyl ammonium chloride composite materials. Sensors and Actuators B: Chemical, 2008. 129(2): p. 538-543. Chen, Z. and C. Lu, Humidity sensors: a review of materials and mechanisms. Sensor letters, 2005. 3(4): p. 274-295. Anderson Jr, J.H. and G.A. Parks, Electrical conductivity of silica gel in the presence of adsorbed water. The Journal of Physical Chemistry, 1968. 72(10): p. 3662-3668.
PT
10.
26.
27.
A
28.
29.
30. 31.
33. 34. 35. 36. 37. 38.
A
CC E
PT
ED
M
A
N
U
SC R
39.
Ernsberger, F.M., The nonconformist ion. Journal of the American Ceramic Society, 1983. 66(11): p. 747-750. Kulwicki, B.M., Humidity sensors. Journal of the American Ceramic Society, 1991. 74. Yeh, Y. and T. Tseng, Analysis of the dc and ac properties of K 2 O-doped porous Ba 0.5 Sr 0.5 TiO 3 ceramic humidity sensor. Journal of Materials Science, 1989. 24(8): p. 2739-2745. Traversa, E., et al., Study of the conduction mechanism of La2CuO4—ZnO heterocontacts at different relative humidities. Sensors and Actuators B: Chemical, 1995. 25(1-3): p. 714-718. Traversa, E., et al., Ceramic thin films by sol-gel processing as novel materials for integrated humidity sensors. Sensors and Actuators B: Chemical, 1996. 31(1-2): p. 59-70. Feng, C.-D., et al., Humidity sensing properties of Nation and sol-gel derived SiO 2/Nafion composite thin films. Sensors and Actuators B: Chemical, 1997. 40(2): p. 217-222. Casalbore-Miceli, G., et al., Investigations on the ion transport mechanism in conducting polymer films. Solid State Ionics, 2000. 131(3): p. 311-321. Quartarone, E., et al., Investigations by impedance spectroscopy on the behaviour of poly (N, N-dimethylpropargylamine) as humidity sensor. Solid State Ionics, 2000. 136: p. 667-670.
IP T
32.
Figure caption
Fig. 10
Fig. 11
Fig. 12
Fig. 13 Fig. 14 Fig. 15
IP T
CC E
Fig. 16
SC R
Fig. 9
U
Fig. 7 Fig. 8
N
Fig. 6
A
Fig. 5
M
Fig. 4
ED
Fig. 3
Figure Heading Flow chart for the preparation of flexible Armalcolite/PDMS composite film. A schematic representation of the measurement system of Armalcolite/PDMS based humidity sensor. XRD (CuKα1, λ=1.54056 Å) patterns of: (a) Armalcolite nanocomposite; (b) Armalcolite/PDMS composite film. SEM images of the (a) Armalcolite nanocomposite; (b) Magnified image of Armalcolite nanocomposite; and (c) Armalcolite/PDMS composite film. FTIR spectra of the (a) Armalcolite nanocomposite; (b) Pristine Polydimethylsiloxane (PDMS) film, and (c) Armalcolite/PDMS composite film. Physical properties of PDMS, Armalcolite/PDMS, Armalcolite, (a) Open porosity; (b) Water absorption; (c) Density; (d) Water contact angle. Static tensile properties of PDMS and Armalcolite/PDMS composite film. Dependence of impedance on relative humidity for the sensor based on Armalcolite/PDMS nanocomposite measured at various frequencies at 1 V and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at 1 V, 1 kHz and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different applied voltages (0.2 V, 1.0 V, and 2.0 V) at 1 kHz and 25°C. Impedance vs. relative humidity for the sensor based on Armalcolite/PDMS nanocomposite at three different temperatures (15°C, 25°C and 35°C) at 1 V, 1 kHz. Response and recovery characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C for, (a) 1 cycle and (b) 7 cycles. Humidity hysteresis characteristic of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C. Impedance stability of the Armalcolite/PDMS nanocomposite based humidity sensor measured at 1 V, 1 kHz and 25°C. Schematic representation of the sensing mechanism of the Armalcolite/PDMS based nanocomposite humidity sensor. Complex impedance plots of Armalcolite/PDMS based sensor obtained from 102 Hz to 106 Hz, at 25°C.
PT
Figure No. Fig. 1 Fig. 2
Table caption
A
Table No. Table 1 Table 2 Table 3
Table Heading Humidity generation for different salts at room temperature. Comparison of the performance of Armalcolite-PDMS based sensor with other flexible humidity sensors. Equivalent circuit of CIP at various RH value
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-1
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-2
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-3
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-4
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-5
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-6
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-7
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-8
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-9
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-10
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-11
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-12
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-13
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-14
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-15
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-16
Table 1. Humidity generation for different salts at room temperature Humidity(%RH)
Temperature
MgCl2 Mg(NO3)2
33 55
25 °C 25 °C
NaCl KCl
75 85
25 °C 25 °C
KNO3
95
25 °C
A
CC E
PT
ED
M
A
N
U
SC R
IP T
Salt
Table 2. Comparison of the performance of armalcolite-PDMS based sensor with other flexible humidity sensors. Humidity range (% RH)
Sensitivity (log Z/%RH)
Response time (s)
Recovery time (s)
Hysteresis (% RH)
Reference
Armalcolite/PDMS
33-95
0.072
10
15
<1
PAMPS doped salts PMMA/PMAPTAC PAMAM-AuNPs TiO2 NPs/PPy/PMAPTAC
20–90 30–90 30–90
0.026 0.033 0.045
60 45 40
70 150 50
<8 <6 <2
Current work [26] [27] [28]
30–90
0.065
30
45
<2
[29]
A
CC E
PT
ED
M
A
N
U
SC R
IP T
Sensing Material
A ED
PT
CC E
IP T
SC R
U
N
A
M