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.
Ammonia Detection
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.
Figure 1. The vertical arrangement of the films dictates the cell s behavior. The horizontal geometry can be varied greatly
to suit applications.
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).
