Development of Humidity Sensor Using Nanoporous Polycarbonate ...

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 13, pp. 2666–2670. © Pleiades Publishing, Ltd., 2017.

PHYSICAL CHEMISTRY OF SURFACE PHENOMENA

Development of Humidity Sensor Using Nanoporous Polycarbonate Membranes1 Sunil Kumara, H. C. Jeona, T. W. Kanga, Rajesh Kaliab, J. K. Sharmab, Sanjay Panwarb, Sapna Kaliab, Vandana Sharmab, and R. K. Choubeyc,* a

Quantum Functional Semiconductor Research Center, Dongguk University, Seoul-100715 South Korea Department of Physics, Maharishi Markandeshwar University, Mullana, Ambala–133203, Haryana, India c Department of Applied Physics, Amity Institute of Applied Sciences (AIAS), Amity University, Noida Campus, Sector-125, Noida–201313, (U.P.), India *e-mail: [email protected]

b

Received June 8, 2016

Abstract—In the present work, the effect of temperature and moisture has been studied on nano-porous polycarbonate membranes of size 15, 50, and 80 nm, respectively to formulate the calibration curves for the future development of humidity sensors. These studies were conducted for virgin as well as humid samples while the temperature range was kept from room temperature to 160°C. Interesting variations in the capacitance with pore size and temperature have been observed. Calibration curves demonstrate that nanoporous polycarbonate samples are one of the important candidates for developing humidity sensors. In addition surface behavior has also been studied for the selective samples to better understand the dielectric behavior for nascent as well as moisturized samples. Keywords: nano-porous, polycarbonate, humidity sensor, capacitance, dispersion DOI: 10.1134/S0036024417130192

1. INTRODUCTION Both humidity sensors and nano-porous membrane have attracted considerable attention over many years due to their great importance in applications such as meteorological services, air conditioning and electronics processing [1–3]. The most common humidity sensors are resistive and capacitive humidity sensors [4]. The humidity sensors based on resistivity usually consist of an metal electrode on a moisturesensitive substrate such as porous ceramic [5]. On the other hand, the capacitive humidity sensors are based on non-conductive materials such as polymers [6]. In short, most humidity detection studies have focused on the use of polymer and ceramic materials due to their low cost and excellent performance [7–9]. Ceramic humidity sensors are easily available, and have major advantages such as good thermal stability, mechanical strength, high resistance to chemical attack and quick response. However, ceramic humidity sensors have some serious disadvantages such as insufficient sensitivity over wide humidity ranges, low reversibility and brittleness [10], which limits the use of ceramics in certain applications. Polyimide film has 1 The article is published in the original.

also been extensively used as sensing material [11–15] but it also suffers from poorer sensitivity. Till now, some work has also been done using alumina and other ceramic nanopore materials for humidity sensing [16, 17]. Taguchi [18] first made a commercial device using the sensitivity of semiconductors to adsorbing gases, with SnO2 as the semiconductor, to avoid oxidation in air and other reactions. Ammonia sensors based upon polyaniline [19, 20], nafion [21], and polypyrrole-poly (vinyl alcohol) [22] thin film coatings have been reported. From the literature survey, it is found that insufficient work has been done on development of humidity sensors using nanoporous membranes and to the best of our knowledge, nobody has reported the use of nanoporous polycarbonate membrane. Nanoporous membrane is a type of porous nanomaterial, which usually has pore diameters between 1–100 nm with a high porosity. The very high surface-area-to-volume ratio is probably the most important property of nanoporous materials. Nanopore based humidity nanosensors can detect both resistance as well as capacitance change with a very high sensitivity [15]. Keeping this fact in mind, present work is focused on the development of calibration curves by observing the effect of temperature on the capacitance of humid and non-humid 15,

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Since capillary condensation enhances the sensing capabilities of a porous material, consequently pore size distribution has been widely considered to be an important parameter in determining the sensitivity in a particular humidity range. The membrane pore size should be comparable to r for enhanced sensing. The capillary condensation might cause hysteresis in the characteristics [23]. As P increases, wider and wider pores will be filled, as P decreases, the pores get empty. The value of P for a given value is less during desorption than adsorption causing the hysteresis.

Fig. 1. Setup for dielectric measurements using PID controlled oven.

50, and 80 nm nanoporous polycarbonate samples. Surface topology has also been studied to develop the perception for the use of high temperature humidity sensors, which demonstrates the novelty of the work. 2. SENSING MECHANISM OF HUMIDITY SENSOR Humidity sensor operation is based on either electronic or ionic conductivity. The ability of nanoporous membrane is based upon ionic conduction [10]. The presence of an adsorbed layer of water at the surface reduces the total sensor impedance due to the increase in the ionic conductivity, as well as the capacitance due to the high dielectric constant of water. An additional advantage of porosity is that at a particular temperature and relative humidity (RH), water condensation occurs in pores up to rk in radius given by Kelvin’s relation [3]

rk (nm) =

2γ M ⎛ ⎞ ρ RT log ⎜ P ⎟ ⎝ PS ⎠

.

(1)

Here, P is the water-vapor pressure, PS the water vapor pressure at saturation, γ the surface tension, R the universal gas constant, T the temperature in Kelvin, and ρ and M are, respectively, the density and molecular weight of water. Equation (1) can be written as [4]

rk (nm) =

2γ V ≈ 1.08 . ⎛ ⎞ ⎛ ⎞ RT log ⎜ P ⎟ log ⎜ P ⎟ P ⎝ S⎠ ⎝ PS ⎠

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

(2)

3. EXPERIMENTAL Track etched Polycarbonate nanoporous membranes having pore-size of 15, 50, and 80 nm have been procured from Nuclepore, USA. Microporous and nanoporous polymer membranes are commercially available dielectric material, which are prepared by the track-etch method. A broad range of pores diameters (down to 10 nm) is available, and pore densities approaching 109 pores cm–2 can be obtained. Both the sides of the polycarbonate samples were coated with a thin (100 Å) gold layer with the sputtering system. The polycarbonate samples were inserted between two electrodes, thus the sample acted as parallel plate capacitor. The capacitance of the paper sample with variable temperature was measured with the help of specially designed setup having a PID controlled oven as shown in Fig. 1. The surface topology of one of the polycarbonate membranes, i.e., 15 nm (nascent, moisturized, and after dielectric studies) was analyzed by optical microscope and the images are shown in Fig. 4. 4. RESULTS AND DISCUSSION The term Polycarbonate describes a polymer which is composed of units of bis-phenol A, connected by carbonate-linkages in its backbone chain. Chemically, a carbonate group is a di-ester of carbonic acid. The result is a polymeric chain as can be seen in Fig. 2. During dielectric measurements nanoporous polycarbonate membrane connected to two electrodes generates a graph when recorded as a function of temperature. Figures 3a–3c show the change in capacitance with temperature for moisture and without moisture nanoporous membrane of sizes 15, 50, and 80 nm. We found that capacitance of all the nanoporous membrane (15, 50, and 80 nm) decreases with increase in temperature in case of without moisture condition. In moisture condition, capacitance remains almost same by increase in temperature for 15 nm nanoporous membrane while capacitance increases with increasing temperature of larger pore size i.e., 50 and 80 nm nanoporous membrane. Capacitance change depends on the dissipation factor also. The change in capacity depends on adsorption of water on surface of grain and pores, it follows

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O O

O O

O

O O

O

Carbonate group

O

Bisphenol A

Fig. 2. Structure of the polycarbonate chain.

(a)

120

250 Capacitance, pF

Capacitance, pF

Capacitance with moisture Capacitance without moisture

Capacitance with moisture Capacitance without moisture

110 100 90 80 70

200 150 100

60 50

(b)

300

40

60

80

100

50

120 140 160 Temperature, °C

40

60

80

100


(c) Capacitance with moisture Capacitance without moisture

Capacitance, pF

300 250 200 150 100 50 40

60

80

100


Fig. 3. Variation of capacitance with temperature of polycarbonate membranes of (a) 15, (b) 50, and (c) 80 nm.

Onsager’s model [24]. The model relates the molecular polarizability and number density to permittivity for condensation with permanent dipoles. It is suitable since water molecules are polar and condensable. As the pores in the membranes are randomly distributed across the membrane surfaces with uniform diameters and due to the random nature of the pore production process, the pores have tilt with respect to the surface normal. Hence, a number of pores may actually intersect with in the membrane, the properties of polycarbonate are bound to change with pore

size and temperature. It appears that with the increase in temperature, pores are healing with heating effect leading to increase in dielectric area and corresponding decrease in capacitance values. Figures 4a–4c show the surface microstructures for the 15 nm nanoporous membranes in nascent condition, moisturized condition and after the dielectric studies. As the polycarbonate sample thickness is 7– 11 μm, it is observed by the optical microscope that the surface texture remains same for nascent as well as for moisturized samples, but it changes after the dielectric studies. The result clearly indicates that the


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(b)

(c) Fig. 4. Optical micrographs of 15 nm polycarbonate membranes (a) nascent, (b) moisturized, and (c) after dielectric studies.

humidity calibration curves are reproducible with the same pore-size membrane if it is used once or it is replaced by a same new membrane for the same poresize. For industrial application, the nanoporous membranes with thickness greater than 11 μm will be more preferable to retain the surface texture for repeated cycles or one has to use a new nanoporous membrane every time. 5. CONCLUSIONS In the present work, we have studied change in capacitance with temperature for moisture and without moisture nanoporous membrane of sizes 15, 50, and 80 nm. Interesting variations in the capacitance with pore size and temperature have been observed and the calibration curves were also formulated for the future development of humidity sensors using nanoporous polycarbonate membranes. Surface behavior has also been studied for the 15 nm nanoporous membranes and found that the surface texture remains same for nascent and moisturized samples, but it changes after the dielectric studies. ACKNOWLEDGMENTS This research was partially supported by the National Research Foundation of Korea (NRF) grant RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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