Highly Ordered Nanoporous Alumina Films: Effect of Pore Size and Uniformity on Sensing Performance
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Dawei Gong, Maggie Paulose, and Keat G. Ong, Craig A. Grimes Department of Electrical Engineering & Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
Elizabeth C. Dickeya) Department of Materials Science & Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 1 November 2001; accepted 22 February 2002)
The effect of pore size and uniformity on the humidity response of nanoporous alumina, formed on aluminum thick films through an anodization process, is reported. Pore sizes examined range from approximately 13 to 45 nm, with a pore size standard deviations ranging from 2.6 to 7.8 nm. The response of the material to humidity is a strong function of pore size and operating frequency. At 5 kHz an alumina sensor with an average pore size of 13.6 nm (standard deviation 2.6 nm) exhibits a well-behaved change in impedance magnitude of 103 over 20% to 90% relative humidity. Increasing pore size decreases the humidity range over which the sensors have high sensitivity and shifts the operating range to higher humidity values. Cole–Cole plots of 5 to 13 MHz measured impedance spectra, modeled using equivalent circuits, are used to resolve the effects of water adsorption and ion migration within the adsorbed water layer. The presence of impurity ions within the highly ordered nano-dimensional pores, accumulated during the anodization process, appear highly beneficial for obtaining a substantial variation in measured impedance over a wide range of humidity values.
I. INTRODUCTION
Humidity sensors have attracted considerable attention for many years due to their great importance in applications ranging from monitoring food quality to meteorological studies.1– 4 Most studies have focused on the use of polymer1,5–8 and ceramic materials3,9–20 for humidity detection due to their low cost and excellent performance. Ceramic humidity sensors are commercially available and offer major advantages because of their high resistance to chemical attack and their thermal stability, mechanical strength, and quick response. However, ceramic humidity sensors still suffer from insufficient sensitivity over wide humidity ranges, as well as lack of reversibility, and the drift in base resistance with time due to the chemisorption of water molecules. Our interest is in developing reliable, highperformance ceramic materials with large surface areas for humidity and gas sensing. Therefore, we revisited the
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rK =
2␥M . RT ln共PS Ⲑ P兲
(1)
P is the water-vapor pressure, PS the water vapor pressure at saturation, ␥ the surface tension, R the universal gas constant, T the temperature in Kelvin, and and M
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use of nanoporous alumina for humidity sensing, a topic of considerable interest since the discovery of its moisturesensitive properties almost fifty years ago.21–32 The operation of ceramic humidity sensors is based on either electronic or ionic conductivity. The ability of alumina to sense hum
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