Gas Sensors Based on Ceramic p-n Heterocontacts
- Iowa State Univ., Ames, IA (United States)
Ceramic p-n heterocontacts based on CuO/ZnO were successfully synthesized and a systematic study of their hydrogen sensitivity was conducted. The sensitivity and response rates of CuO/ZnO sensors were studied utilizing current-voltage, current-time, and impedance spectroscopy measurements. The heterocontacts showed well-defined rectifying characteristics and were observed to detect hydrogen via both dc and ac measurements. Surface coverage data were derived from current-time measurements which were then fit to a two-site Langmuir adsorption model quite satisfactorily. The fit suggested that there should be two energetically different adsorption sites in the system. The heterocontacts were doped in an attempt to increase the sensitivity and the response rate of the sensor. First, the effects of doping the p-type (CuO) on the sensor characteristics were investigated. Doping the p-type CuO with both acceptor and isovalent dopants greatly improved the hydrogen sensitivity. The sensitivity of pure heterocontact observed via I-V measurements was increased from ~2.3 to ~9.4 with Ni doping. Dopants also enhanced the rectifying characteristics of the heterocontacts. Small amounts of Li addition were shown to decrease the reverse bias (saturation) current to 0.2 mA at a bias level of -5V. No unambiguous trends were observed between the sensitivity, the conductivity, and the density of the samples. Comparing the two phase microstructure to the single phase microstructure there was no dramatic increase in the sensitivity. Kinetic studies also confirmed the improved sensor characteristics with doping. The dopants decreased the response time of the sensor by decreasing the response time of one of the adsorption sites. The n-type ZnO was doped with both acceptor and donor dopants. Li doping resulted in the degradation of the p-n junction and the response time of the sensor. However, the current-voltage behavior of Ga-doped heterocontacts showed the best rectifying characteristics with very high forward currents. Ga doped heterocontacts showed the highest sensitivity observed during current-time measurements as well, even though the sensor response was rather slow. Finally, a possible synergistic effect of doping both p and n-sides was studied by utilizing current-time measurements for 1.5 mol% Ni-CuO/1.5 mol% Ga-ZnO heterocontact. A sensitivity value of ~5.1 was obtained with the fastest response among all the samples. The time needed to reach 90% coverage was lowered by a factor of 4 when compared to the pure heterocontact and the time needed to reach 70% coverage was just over one minute. Heterocontact gas sensors are promising candidates for high temperature sensor applications. Today, Si-based microelectromechanical system (MEMS) technology has shown great promise for developing novel devices such as pressure sensors, chemical sensors, and temperature sensors through complex designs. However, the harsh thermal, vibrational, and corrosive environments common to many aerospace applications impose severe limitations on their use. Sensors based on ceramic p-n heterocontacts are promising alternatives because of their inherent corrosion resistance and environmental stability. The other advantages include their inherent tuning ability to differentiate between different reducing gases and a possible cost efficient production of a wireless sensor. Being a capacitive type sensor, its output can be transformed into a passive wireless device by creating a tuned LC circuit. In this way, the sensor output (the capacitance) can be accessed remotely by measuring the resonant frequency. The relatively simple structure of heterocontacts makes it suitable for thick film fabrication techniques to make sensor packages.
- Research Organization:
- Ames Lab., Ames, IA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- W-7405-Eng-82
- OSTI ID:
- 837264
- Report Number(s):
- IS-T 2498; TRN: US200506%%91
- Resource Relation:
- Other Information: TH: Thesis (M.S.); Submitted to Iowa State Univ., Ames, IA (US); PBD: 19 Dec 2004
- Country of Publication:
- United States
- Language:
- English
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