Gas Sensors Using Solid Electrolytes
- PDF / 1,798,645 Bytes
- 7 Pages / 604.8 x 806.4 pts Page_size
- 57 Downloads / 227 Views
Gas Sensors Using Solid Electrolytes
Equilibrium-Potential Sensors
Noboru Yamazoe and Norio Miura Introduction Some metal oxides, inorganic salts, and polymer electrolytes are known to exhibit significant ionic conductivities over normal temperature ranges. When their electronic conductivities can be disregarded, these ionic conductors are called solid electrolytes (SEs). A variety of SEs with conducting ions such as H + , Li + , Na + , K + , Ag + , F", C P , and O 2 " have been developed and tested, particularly for electrochemical applications. An electrochemical cell is easily constructed when a membrane (diaphragm) of a chosen SE is attached to a pair of electrodes. The resulting cell is all solid State and thus compatible with elevated tempera tures. Various high-temperature devices have been developed, such as the Na-S battery, based on Na-/3-alumina (Na + conductor); the H 2 - 0 2 fuel cell, based on yttria-stabilized zirconia (YSZ, O 2 - con ductor); and the oxygen concentration sensor, based on YSZ. See the article in this issue by Schwank and DiBattista for applications of oxygen sensors. The required characteristics of SEs differ depending on the nature of the envisaged devices. For energy conversion devices such as batteries and electrolytic cells, high ionic conductivity is one of the most important requirements for the achievement of high power or high efficiency. The available SEs are therefore rather limited. On the other hand, for sensors that convert chemical information into physical quantities, high ionic conductivity is not always important. In an equilibrium-potential gas sensor, for example, the open-circuit electromotive force (EMF) is the quantity measured, and ionic conductivity must be just sufficient enough to keep the EMF measurement undisturbed by the leakage current flowing through the external circuitry. This Situation allows use of familiär SEs as well as unfamiliar ones such as K 2 C0 3 (K + conductor).1 Gas sensing consists of recognizing a gas from a chemical or electrochemical interaction (gas recognition) and transduc-
MRS BULLETIN/JUNE 1999
ing the interaction into a physical (usually electrical) Output signal. For example, consider the following Potentiometrie oxygen sensor System: 0 2 , Pt11 YSZ(0 2_ conductor)| "Pt, 0 2 . (1) (reference) (sensing) Oxygen is recognized by an electrochemi cal reaction taking place via three-phase contact with the YSZ, the Pt electrode, and the gas phase as follows: l / 2 0 2 + 2e(Pt) ^ O2' (YSZ).
(2)
Identical reactions on the reference and sensing sides cause the sensor to generate an EMF of the following Nernstian form: E = (RT/4F)ln (Po2"/Po2').
(3)
Here F, R, and Tare the Faraday constant, gas constant, and temperature, respectively, and Po2' and Po2" are the partial pressures of oxygen at interfaces I and 11, respectively. So, when Po,1 is known, Po," can be found from E. Obviously the threephase contact at interface II executes the recognition funetion, whereas the rest of the device performs the transduetion. Also, the SE an
Data Loading...
