Flexible Ammonia Detection with Voltammetric MicrosensorsFebruary 1, 2005 By: Michael C. Vogt, Laura R. Skubal, Argonne National Laboratory Sensors
Ammonia (NH3) is a common reagent for and byproduct of numerous processes. Nearly 80% of all ammonia produced in the U.S. is used for agricultural applications including liquid fertilizer, protein for livestock feeds, and antifungal agents for fruit and corn. The petroleum industry uses ammonia to neutralize acids in crude oil; the mining industry, to help extract copper, nickel, and molybdenum from ore. Other applications include use in metal nitriding and annealing processes and as a reagent for manufacturing nitric acid, dyes, commercial household cleaners, detergents, pharmaceuticals, vitamins, and synthetic textiles.
The average human nose can detect ammonia concentrations on the order of 50 ppm or less; 100 ppm significantly irritates human olfactory passages. Because the odor of ammonia is so potent, concentrations well below 300 ppm (the concentration immediately dangerous to life and health) can be detected before harm ensues.
Common ammonia detection instruments and technologies include UV absorption; NIR spectroscopy; color-changing cards capable of detecting concentrations in the 1–5 ppm range; and metal oxide, electrochemical, and polymer film sensors. UV sensors typically require high sample volumes and long warm-up periods for accurate measurements. NIR sensors are expensive; their cost limits the placement and coverage area of networks. Metal oxide sensors typically detect at 30 ppm thresholds and require extremely accurate temperature control for acceptable speciation. Electrochemical sensors require frequent electrolyte replacement, need stable oxygen concentrations for operation, and respond to interferent gases such as carbon monoxide and chlorine. Polymer-resistive devices are available but are still maturing and suffer from memory effects.
Continued demand for an inexpensive, rugged, sensitive ammonia detector capable of tolerating various gaseous interferents has encouraged the development of voltammetric cermet microsensors. These experimental devices are fabricated from thick films and extremely durable materials. The measurement technique is flexible for various gases and gas concentrations, and can measure both the target species and the interferents in a sample. An embedded microcontroller provides device intelligence including automated new sample learning, remote communication, and smart sensor standards such as the IEEE 1451.4 smart transducer interface.
Cermet Sensing Element
The sensing elements in the prototypes being developed at Argonne National Laboratory (ANL) consist of thick monolithic metal oxide and metallic films screen-printed on an alumina (Al2O3) substrate. The layered arrangement of the films forms an electrochemical cell, with a nickel oxide (Ni-NiO) film providing a source of oxygen ions for operation in extremely oxygen-deficient environments and a catalyst for ammonia reactions. Sensing electrode materials have included platinum, palladium, ruthenium, gold, and iron oxide films. Specificially selected film materials enhance some chemical reactions and inhibit others, tailoring the response of devices. Miniature arrays of various sensors further improve species identification.
Individual electrodes are 10–15 µm thick and sandwich a 25–30 µm solid electrolyte. Several electrolytes, designed to enhance specific chemical reaction, are used in fabricating the sensors. An yttria-stabilized zirconium oxide (YSZ) electrolyte has been ideal for small hydrocarbons, carbon monoxide, and ammonia. A YSZ electrolyte combined with tungsten bismuth oxide (WBO) suppresses many hydrocarbon reactions and provides reaction sites for carbon dioxide and chlorinated compounds. Both YSZ and YSZ/WBO (hereafter referred to as the WBO sensor) electrolytes limit chemical reactions by conducting specific ions. Alternative electrolytes, such as lanthanum fluoride, can be used to further partition the different species and aid in isolating compounds from mixtures. The thickness of the electrolyte influences the ideal sensor operating temperature and can be increased or decreased to respond best in a given environment (thicker electrolyte films for higher environment temperatures). A sample vertical cross-sectional arrangement of a Pt/Ni, NiO/YSZ/Pt sensor is shown in Figure 1).
Figure 1. The vertical arrangement of the films dictates the cell s behavior. The horizontal geometry can be varied greatly to suit applications.
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