Room Temperature Hydrogen Gas Sensitivity of Nanocrystalline-Doped Tin Oxide Sensor Incorporated into MEMS Device
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Room Temperature Hydrogen Gas Sensitivity of Nanocrystalline-Doped Tin Oxide Sensor Incorporated into MEMS Device Satyajit Shukla1, Rajnikant Agrawal1, Lawrence Ludwig2, Hyoung Cho1, Sudipta Seal1 1
University of Central Florida (UCF) Mechanical Materials Aerospace Engineering (MMAE) Department and Advanced Materials Processing and Analysis Center (AMPAC) Engineering # 381 4000 Central Florida Blvd. Orlando, FL 32816 Phone: (407) 823-5227 Fax: (407) 823-0208 E-mail(s): [email protected], [email protected] 2
Kennedy Space Center (KSC-NASA), FL 32899
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To whom the correspondence should be addressed
ABSTRACT Nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin film sensor has been sol-gel dip-coated on a microelectromechanical systems (MEMS) device. The micro-sensor device is successfully utilized to sense ppm level H2 at room temperature with high sensitivity. The chamber pressure has no pronounce effect on the room temperature H2 sensitivity. INTRODUCTION Nanocrystalline tin oxide (SnO2) is a well known n-type semiconductor oxide, used in doped [1,2] and undoped [3,4] forms, for gas sensing application. The mechanism of gas sensing using n-type semiconductor oxides, such as SnO2, is now well established and involves decrease in the electrical resistance of the thin film due to the reaction of the reducing gas, such as hydrogen (H2), with the surface-adsorbed oxygen-ions (O2-ads or O-ads species), which releases the electrons into the conduction band [5]. The gas sensitivity (S) of the thin film, defined as the ratio Rair/Rgas or Ggas/Gair (where, Rair and Rgas are the resistances, and Gair and Ggas are the conductances of the thin film gas sensor in air without and with the reducing gas respectively), can be well predicted using the constitutive equation [6-8], which is of the form,
2 ⎡ ⎡ O − ⎤ ⎤⎥ ⎢ ⎢⎣ ⎥⎦ ⎥ 2d q2 • • exp ⎢ • S = A • (1) 1 D n ⎢ 2 ε ε k [V ]T ⎥ b o ⎢ r o ⎥ ⎣ ⎦ -3 where, A1 is a constant (m ), d the space-charge-layer thickness, D the nanocrystallite size, C the reducing gas concentration (ppm), n the gas concentration exponent, nb the bulk charge-carrierconcentration, q the electronic charge, εrε0 the permittivity of the sensor, k the Boltzmann’s Cn
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constant, [O-] the surface-density of states, [V0] the oxygen-ion vacancy concentration, and T the sensing temperature. On the other hand, there is an increased demand for H2 (in liquid and gaseous forms) as a cheap replacement fuel for automobiles, power generation using solid oxide fuel cells (SOFCs) and launching the space-shuttles into space. As a result, much attention has been recently given for the production, storage, and transportation of H2 [9-11] Since, H2 is the smallest gaseous molecule, it is more susceptible for leakage through the existing system of pipelines and storage devices. As H2 can catch fire easily in the presence of oxygen when present in critical amount (4 %), detecting H2 leakage is vital for safety concerns. Since H2 gets combusted easily at higher temperatures, sensing H2 at
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