Detecting NO.sub.x using thin film zinc oxide semiconductor

A method for detecting NO.sub.x species in an oxygen-containing gas comprises measuring the electrical resistance of a thin film of oxygen-deficient zinc oxide exposed to the sample.

BACKGROUND OF THE INVENTION 
This invention relates to measuring the concentration of nitrogen oxide 
compounds (NO.sub.x) in a gaseous mixture using a solid state sensing 
element comprising a semiconductive zinc oxide thin film. 
The emission of gaseous nitrogen oxide compounds, particularly from 
automotive internal combustion engines, is a major environmental concern. 
Several nitrogen oxide compounds have been identified and are generally 
referred to as a group by the symbol "NO.sub.x ". The most significant are 
nitric oxide (NO) and nitrogen dioxide (NO.sub.2). 
To better study and monitor NO.sub.x emissions, an instrument is desired 
that provides accurate, continuous readings directly from a sample gas 
without pretreatment or reagents and without interference from other 
sample constituents. It is known that the electrical resistance of a thin 
film of a semiconductive material exposed to a gaseous mixture may be 
affected by the presence of certain species, depending upon the material, 
the species and the composition of the mixture. U.S. Pat. No. 4,169,369, 
issued to the present inventor and coassigned, describes a NO.sub.x sensor 
having a semiconductive tin oxide thin film. Zinc oxide, iron oxide, lead 
oxide and cadmium oxide also form N-type thin film semiconductors that are 
sensitive to common reducing and oxidizing gaseous species in an inert 
gas. However, it is not possible to predict the effect of a particular 
species upon the thin film in the presence of a relatively high 
concentration of another species to which the film is sensitive. In 
contrast to tin oxide, it has been found that iron oxide, lead oxide and 
cadmium oxide are essentially insensitive to NO.sub.x in air. Apparently, 
the relatively high oxygen concentration saturates the thin film surface 
so that any interaction with NO.sub.x species is too small for detection. 
In view of this, it is totally unexpected that a zinc oxide sensor could 
be adapted to measure NO.sub.x concentration in the presence of a 
relatively high oxygen concentration, such as in air, or other oxidizing 
or reducing species. 
It is an object of this invention to provide a method for detecting 
NO.sub.x in a gaseous mixture utilizing a semiconductive zinc oxide thin 
film to make relatively rapid, direct readings without sample pretreatment 
or reagent additions, which readings produce an accurate measurement of 
the NO.sub.x concentration despite a relatively large concentration of 
oxygen in the mixture and further despite the presence of other oxidizing 
and reducing species, including hydrogen and propylene, in the mixture. 
SUMMARY OF THE INVENTION 
In a preferred embodiment, NO.sub.x is quantitatively detected in a gaseous 
sample by measuring the resistance of a thin film of semiconductive zinc 
oxide exposed to the sample. The thin film is prepared by RF sputtering 
onto an inert substrate from a sintered zinc oxide target and thereafter 
heat treating in air between 400.degree. and 500.degree. C. When exposed 
to the sample, the film is heated between about 270.degree. to 300.degree. 
C. and the resistance is measured between spaced electrodes using a 
suitable ohmmeter. It has been found that the logarithm of the resistance 
is directly proportional to the logarithm of the NO.sub.x concentration in 
the sample. Thus, the NO.sub.x concentration of an unknown sample can be 
determined by comparing the measured value with values obtained from known 
mixtures. Further, the sensor responds to the NO.sub.x concentration even 
in a sample having a substantial oxygen concentration and optionally 
containing other common oxidizing and reducing gases. Thus, the sensor is 
useful for measuring NO.sub.x in air or particularly in automotive exhaust 
gases diluted with air. 
While I do not wish to be limited to any particular theory, it is believed 
that the NO.sub.x -sensitive zinc oxide has an oxygen to zinc ratio less 
than the ZnO stoichiometric ratio, that is, less than one. This oxygen 
deficiency results in defects in the zinc oxide crystalline structure, 
which supply electrons for conduction through the crystal. Thus, zinc 
oxide is an N-type semiconductor. It is also believed that sensor 
operation is based upon a chemisorption phenomenon. NO.sub.x species are 
absorbed onto the zinc oxide film surface and alter the electron 
conduction properties of defects near the surface. For a thin film, this 
NO.sub.x -defect interaction has a significant and observable affect upon 
bulk electrical properties, particularly upon resistance. The amount of 
gas absorbed, and thus the effect upon resistance, depends upon the 
NO.sub.x concentration. The sensor responds to the gas until a 
concentration is reached at which the chemisorption sites are 
substantially saturated and a maximum number of defects are effected. 
Since semiconductive zinc oxide thin films are sensitive to oxygen, a high 
concentration of oxygen, such as in air, saturates the chemisorption sites 
and would be expected to have such a substantial effect upon the film that 
any interaction with a small amount of a second oxidizing species would be 
masked. Surprisingly, a substantial NO.sub.x -film interaction has been 
found that is clearly observable even when oxygen saturated. For example, 
less than about 4 ppm nitrogen dioxide in air increases the resistance by 
an order of magnitude. Thus, the method of this invention is capable of 
detecting trace NO.sub.x levels (ppm) in air (21 percent oxygen).

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a sensing element 10 comprising a semiconductive zinc 
oxide thin film 12 is illustrated for measuring NO.sub.x in a gaseous 
mixture in accordance with a preferred embodiment of this invention. 
Sensor 10 comprises an alumina body 14 having dimensions of approximately 
5 mm.times.5 mm.times.0.5 mm. Alumina is preferred because it is a good 
electrical insulator, but has adequate thermal conduction to provide 
uniform heating of sensor 10. Body 14 has a first major surface 16 having 
a surface finish (the perpendicular distance between the levels of the 
highest peak and lowest valley) of approximately 400 A.degree.. It is 
believed that the roughness of surface 16 increases the surface area of an 
applied film and enhances defects in the film to improve NO.sub.x 
sensitivity. 
Two opposite, parallel gold-glass electrodes 18 having dimensions of 1 
mm.times.2 mm are applied to surface 16 using silk screen technology and 
fired. Electrodes 18 are spaced apart by approximately 1.5 mm. Electrodes 
18 are connected to a low power ohmmeter 20 that measures the resistance 
of film 12 utilizing a substantially constant current of about one 
microampere. Maintaining sensor 10 at a constant temperature is critical 
to making accurate measurements. Since current passing through a resisting 
material generates heat, the use of a small constant current is preferred 
to avoid temperature fluctuations. 
A resistance heater 22 is applied to body 14 opposite surface 16. The 
heater material displays a relatively constant and temperature-independent 
resistance at NO.sub.x -sensing temperatures. Adjacent corners of heater 
22 are connected to a conventional DC power source 24. A 
chromel-constantan thermocouple (not shown) is attached to the heater 
surface to provide a suitable signal for electrically controlling power 
source 24 and thereby maintaining sensor 10 at a desired temperature. 
Zinc oxide film 12 is applied to surface 16 over electrodes 18 and is 
continuous between the electrodes. Film 12 is deposited by RF sputtering 
from a sintered zinc oxide target in a low pressure oxygen-argon 
atmosphere. The target is prepared by pressing zinc oxide powder with a 
suitable organic binder and sintering to vaporize the binder and fuse the 
powder. A mask is used to control the region of deposition. Within a RF 
discharge plasma apparatus, substrate 14 is positioned on one electrode 
with surface 16 facing the target positioned upon the second electrode. 
The substrate-target distance is about 7.6 cm. The atmosphere contains 
approximately 8 millitorr partial pressure argon and approximately 2 
millitorr partial pressure oxygen. The RF plasma is generated with a 
forward power of 400 watts and an accelerating voltage of 2.2 kilovolts 
(the target being cathodic). During sputtering, the temperature of 
substrate 14 does not exceed 200.degree. C. Under these conditions, a 
suitable film is deposited after about 10 minutes. The deposited zinc 
oxide film is stabilized by heating in air at between 400.degree. to 
500.degree. C. for two hours to optimize NO.sub.x sensitivity. The product 
zinc oxide thin film 12 is approximately 1000 A.degree. thick. 
A sensor 10, prepared as described hereinabove, was tested by exposing to a 
gaseous sample in an airtight container. The samples consisted of room 
temperature air to which known nitric oxide (NO) additions were made. 
Heater 22 maintained the sensor temperature at about 300.degree. C. The 
film resistance in the blank air sample (no NO added) was about 
1.75.times.10.sup.6 ohm. FIG. 2 shows the film resistance as a function of 
the NO concentration, plotted on log-log coordinates. As seen, up to at 
least 1000 ppm, the logarithm of the sensor resistance increases linearly 
with the logarithm of the nitric oxide concentration. Further, an increase 
in NO concentration from 100 to 1000 ppm increases the sensor resistance 
by about 5.times.10.sup.7 ohms, a substantial increase in view of the 
blank resistance. In these experiments, the sample was prepared and the 
sensor then introduced into the container. Response time was typically 
about 2 minutes. The recovery time; i.e., the time after the container was 
vented until the thin film sensor returned to the blank value, was 
typically 8 minutes. 
A sensor was similarly tested by heating at 300.degree. C. and exposing to 
room air samples containing known nitrogen dioxide (NO.sub.2) additions. 
The results are depicted in FIG. 3. The film resistance in the blank air 
sample (no NO.sub.2) was about 1.25.times.10.sup.6 ohms. The logarithm of 
the sensor resistance increases linearly with the logarithm of the 
nitrogen dioxide concentration. An increase from 5 ppm to 50 ppm increases 
the resistance by about 4.5.times.10.sup.7 ohms. It is noted that the film 
is substantially more sensitive to nitrogen dioxide than to nitric oxide. 
A resistance increase of one order of magnitude above the blank value 
requires about 3 ppm nitrogen dioxide, compared to about 175 ppm nitric 
oxide. 
Sensor response to common reducing gases was also determined. A sensor was 
exposed to 20 ppm nitrogen dioxide and was essentially unaffected by 
additions of up to 250 ppm of hydrogen and propylene. The effect was 
estimated to be less than 1%. Water vapor affects the reading, but this 
effect is believed to be small in comparison to the NO.sub.x response. 
In the described embodiment, the zinc oxide film thickness was 1000 
A.degree. and the substrate finish was about 400 A.degree.. The roughness 
of the substrate surface enhances defects in the film that interact with 
chemisorbed NO.sub.x species. Film thickness and surface finish are 
interrelated parameters. Preferably, the film is sufficiently thick and 
the surface finish sufficiently smooth to produce a continuous film. 
However, a substrate surface that is too smooth provides insufficient 
lattice defects. If the film is too thick, the NO.sub.x -defect 
interactions, which occur near the film surface, do not have a measurable 
effect upon the overall film resistance. In general, substrates having 
surface finishes ranging between 300 to 4000 A.degree. are suitable for 
use with zinc oxide film ranging between 600 and 10,000 A.degree.. 
The NO.sub.x sensitive zinc oxide film is preferably deposited upon the 
substrate by sputtering from a zinc oxide target in an argon-oxygen 
atmosphere. NO.sub.x sensitivity is enhanced by heating the sputtered film 
in air prior to its use. This heat treatment stabilizes the oxygen content 
and also its resistance. Suitable film stabilization is afforded by 
heating the sensor to a temperature preferably between 400 to 500.degree. 
C. Although a sputtered film is preferred, films having similar NO.sub.x 
characteristics may be produced by metal vapor deposition followed by 
oxidation or by the application of a suitable slurry followed by 
evaporation. 
Materials other than those mentioned above may be used to manufacture the 
substrate and the electrodes without significantly affecting the 
performance of the zinc oxide thin film. For example, other inert, 
refractory materials, such as steatite, are good electrical insulators and 
form suitable sensor bodies. Any good electrical conductor may be used for 
the electrodes. The heater need not be attached to the sensor. Attaching 
the heater as in the preferred embodiment permits the sensor to be 
maintained at a desired temperature without heating the entire sample. It 
has been found that the most accurate readings are obtained by operating 
the sensor between about 250.degree. to 325.degree. C., preferably between 
270.degree. to 300.degree. C. Operated above that temperature range, the 
sensor responds faster, but typically overshoots. When the sensor is 
operated below that range, a slow response is obtained that typically 
represents too low a concentration. 
Although this invention has been described in terms of certain embodiments 
thereof, it is not intended that it be limited to the above description 
but rather only to the extent set forth in the claims that follow.