A novel humidity sensor based on alumina nanowire films

Abstract

Alumina nanowire (ANW) films were prepared by etching porous anodic aluminium oxide (AAO), and then humidity sensors with coplanar interdigitated electrodes based on ANWs were fabricated. The ANWs not only present tremendous surface area for water molecule adsorption but also provide efficient sites for attracting water molecules at low relative humidity (RH) levels. The sensors based on the particular morphology of ANWs with large open voids show high sensitivity and small hysteresis, and have fast response and recovery time to humidity. The capacitance rises slowly at a lower RH and increases rapidly after 70% RH, which is associated with the humidity mechanism of chemisorption at lower RH and physisorption at higher RH levels. The impendence analysis suggests that the ANWs are the main factor for sensing humidity, and AAO also contributes to humidity sensing. This study demonstrates that ANWs have promising applications in humidity monitoring.

Sensitivity Enhancement of a PPM Level Capacitive Moisture Sensor

“…This difference of the base value is due to the fact that the effective cross-sectional area of each sensor is different. The pore morphology also influences the capacitance value but all the four sensors have identical morphology [1,2].It is observed in Figure 6 that as the moisture concentration increases, the capacitance value increases but the change in capacitance for CP1 and CP4 for the same moisture range is significantly different. The highest (46.4%) and the minimum sensitivities (2.9%) are shown by Cp1 and Cp4 respectively.…”

“…Comparison of the characteristics of the single sided and double sided sensor with the reading of the commercial dew point meter is shown in Table 3. Investigation is needed to study the drift due to aging [2]. Our experience shows that if the sensor is kept in drying agent (molecular sieve) as practiced in a commercial dew point meter (SHAW), the reading will be stable for a long time [10,11].…”

“…However, if there is a drift in the output, it can be reduced by heating the sensor at a temperature of around 400 • C. Drift is caused due to deposition of residual impurity from the N 2 gas or due to gradual change in γ phase to γ-Al 2 O 3 .H 2 O (boehmite). This causes volume expansion of the oxide, resulting in a gradual decrease in surface area and porosity [2]. The sensor does not show any drift due to the change in working temperature [9,18,19,23].…”

“…The sensor should be sensitive to sudden and drastic change of moisture and available at low cost with a relatively long calibration period. The dynamic range of humidity measurement, which extends from %RH to ppb level, is extremely large [1][2][3][4][5]. Capacitive sensors are widely used for the measurement of humidity over a wide range.…”

“…Among the different methods of alumina film preparation, the sol-gel method is a simple, less expensive method to prepare a pure ceramic thin film of desired thickness. The film can be formed on the ceramic substrate by the dip coating, spin coating or spray coating method [1,2,[6][7][8][9][10]. Incomparison to other ceramic materials, aluminum oxide (Al 2 O 3 ) is highly hydrophilic and it is one of the most useful materials for moisture measurement in most industrial gases [4,10,11].…”